AEROSOL NANOPARTICLES OF METALS,ALLOYS

Chapter

AEROSOL NANOPARTICLES OF METALS, ALLOYS AND COMPOUNDS: SYNTHESIS, PROPERTIES AND POTENTIAL APPLICATIONS E. A. Shafranovsky* and Yu. I. Petrov Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin str. 4, 119991, GSP-1, Moscow, Russia

ABSTRACT Production of aerosol nanoparticles of metals, alloys, and their compounds by gas evaporation technique has been considered. Development of the latter has been briefly presented. The features of nucleation and growth of aerosol particles have been discussed. The structure of particles and its temperature transformations have been examined. The results of optical, electric and magnetic measurements for different samples in the form of powders and thin deposits have been presented. A number of properties being of both fundamental and applied interest have been observed.

1. INTRODUCTION Conditions of preparation and physicochemical properties of aerosol particles of metals and their compounds have been systematically studied only in recent decades (see [1-5]). In the mid 1950s the Laboratory of molecular physics in the Institute of Chemical Physics, Academy of Sciences of the USSR (nowadays Semenov Institute of Chemical Physics, Russian Academy of Sciences) has proceeded to systematic studies of methods for preparing ultrafine metal aerosol particles and their physicochemical properties. Most results obtained for a half century have been published in Russian and unknown to the world scientific community. However, some of them are unique and evidently of interest to everybody who is involved in studying clusters and small particles. Critical, sufficiently complete reviews of *

Author to whom correspondence should be addressed: e-mail: [email protected]; tel.: 007(495)9397462; fax: +7(499)1378318.

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E. A. Shafranovsky and Yu. I. Petrov

works on the subject under discussion carried out all over the world before 1983 have been presented in monographs [2, 3] every of which contains over 1100 references. Later works of Petrov with his coworkers fulfilled till 2000 have been considered in [4]. All most significant results taken by his team up to 2010 are generalized below.

2. GAS EVAPORATION TECHNIQUE 2.1. Polyatomic Complexes in a Real Gas Many scientists (Boltzmann, Clausius, van der Waals, Nernst etc.) connected deviations from the laws of ideal gas with formation of associated complexes in real gases. In subsequent experiments existence of such associates in vapors of almost all investigated substances has been established. An appreciable part of vapors of carbon and metals (Bi, Cu, Ag, and Au) makes up dimers a fraction of which increases with temperature. It is interesting that, when evaporating AgAu, AgCu, AuCu, AgSn, AuSn, CuSn alloys, stable symmetric (Cu2, Ag2, Au2, Sn2) and asymmetric (AgAu, AgCu, AuCu, AgSn, AuSn, CuSn) dimers have been found in addition to atoms of components. Presence of polynuclear complexes (As 4, Se6, Se8, Sb 4) in vapor has been shown in many works. A noticeable concentration of trimers and tetramers in Cu, Ag, Au vapors has been revealed also. A large number of polymeric molecules has been recorded in mass spectra of C, Be, Mg, Ti, Fe, Cu vapors, and, at that, molecular ions up to C31+, Be25+, Mg5+, Ti5+, Fe6+, Cu5+ have been observed. Most authors have come to conclusion that polynuclear complexes detected by mass spectrometry stem from evaporation of structural groups of liquid metal. It is corroborated with a low probability of double and triple collisions of atoms at low pressure of evaporated metal which could lead to occurrence of polymeric aggregations. Development of the adequate theory of gas requires the account of presence of the associated complexes. The well-known group theory of Mayer based on the rough estimate of statistical integral of states drew up the first step in this direction. The basic premise of calculation scheme was in presentation of real gas in the form of fast changeable ensemble of non-interacting groups consisting of 1, 2, 3 etc. molecules. An approximate kind of the general function for complex size distribution versus pressure and gas temperature has been almost simultaneously and independently obtained in 1939 by Band [6], Frenkel [7, 8], as well as Vukalovich and Novikov [9, 10]. Physical conceptions of their theories are sufficiently analogous. They have been most completely formulated by Vukalovich and Novikov [9, 10] as follows. 1. Association of molecules (atoms) is the basic process inherent to any state of gas. 2. The reason of association is in van-der-Waals interaction of gas molecules. In view of weakness of these forces compared to a chemical bond, simple molecules, being a part of the complex aggregate, keep their structure and properties. 3. Formation of the complex aggregates occurs by colliding at least three molecules. As a result, a part of colliding molecules are united forming relatively stable aggregates, and the rest carry away the heat of condensation in the form of translation energy.


Aerosol Nanoparticles of Metals, Alloys and Compounds

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4. Associated complexes are supposed to be in the thermodynamic equilibrium with separate molecules and to have the same translation energy. The average life time of such complexes surpasses many times over duration of simple collision of molecules. At temperatures less than the critical temperature of liquid-vapor transformation the bond energy in molecules is much higher than the thermal one, therefore dimers are quite stable. As a complex grows, the bond energy of every molecule entering into it increases. 5. Real gas is considered as a mixture of the simple gases consisting of separate molecules, dimers, and trimers etc. If the system is in the thermal equilibrium, then decomposition of complexes of any simple gas is accurately compensated by their formation from more simple complexes of other gases. The proceeding processes are similar to reversible chemical reactions. Frenkel [7, 8] has developed the theory of heterophase fluctuations, according to which the equilibrium distribution of associated complexes in gas is established in accordance with the general formula of probability of fluctuations N r ∼ const. × exp [– ∆Φ (r) / kT] , where Nr is a number of complexes with a radius r; ∆Φ(r) stands for a fluctuation of the thermodynamical potential of vapor due to arising a complex with the radius r. The calculation shows that, in addition to complexes of small size, there is a significant part of aggregations containing more than 1000 molecules in a saturated vapor. Unlike mental shortterm molecular groups in the Mayer theory atomic complexes in the theories of Band [6], Frenkel [7, 8] as well as Vukalovich and Novikov [9, 10] are thermodynamically stable. The presence in vapor of equilibrium polyatomic complexes compels to critically revise both the very concept of vapor phase and the widespread idea of "oversaturation" which was still applied only to a nonassociated vapor. In this connection the question naturally arises to what size the associations can be attributed to the gas phase in sense of creating pressure, density, and hence the degree of oversaturation. When changing the pressure and the vapor temperature a new complex size distribution is attained, with the macroscopic fluid being considered simply as the largest drop in the given complex size distribution .

2.2. Condensation of Metallic Vapor within an Inert Gas 2.2.1. Substance Evaporation Methods Metals, alloys and compounds are evaporated in different ways, using (1) the melt in a suitable crucible heated by ac power or in a graphite furnace; (2) samples locating on W or Mo filament as well as inwards Ta capsules heated by ac power; (3) rapid discharge of capacitor through the evaporated metal (method of “explosive wire"); (4) arc, electron, magnetron sputtering, plasma or laser heating. Each of these methods has its advantages and shortages. However, on the basis of long-term comparative studies, we came to conclusion that the tendency to increase productivity of the method is always associated with the deterioration of


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product quality. For example, using arc or plasmatron sputtering of metal, one can fast produce appreciable amounts of high-dispersed powder, which, however in some cases, have to be completely rejected because of its contamination with large sprayed drops. Molten metals, especially Al, easy impregnate the porous walls of Be, Ti, and/or Al oxide crucibles bypassing the heating coil. This process is accelerated with temperature. Generally speaking, the application of ceramic crucibles at temperatures above 10000C is at a loss for constructive restrictions of furnaces and active destruction of the crucible material with the molten metal. Therefore, we used in our experiments a quiet evaporation of the substance mainly from W wire or Ta capsules. The main drawback of this method is alloying of different metals with W or Ta giving rise to a lower melting point and greater fragility in the solid state. As a result, the evaporator is rapidly destroyed and cannot be repeatedly used. The W wire filament is sometimes covered with Al oxide for increasing its service life. However, a method of “sliding drop” [11] appears to be more efficient for melting, for instance Al. Its principle is in feeding the Al wire to the upper end of a vertically heated W wire. A piece of aluminium quickly melts and shrinks to a drop. Wetting the W filament, the drop slides down and practically instantly evaporates. By this way one manages to increase many times the efficiency of the set-up and reduce the W consumption approximately by factor of 20 (up to 1 g per 1 g of evaporated Al). When evaporating powdered materials (CdSe, CdS, and ball milled FeNi, FeMn, FeCu, FeCo, FePt, and FePd alloys) one should take special measures to prevent scattering of the powder particles by jets of gases dissolved in the sample. So one can put the powder into a helically twisted Ta(W) foil or, in advance, slightly sinter the powder in a vacuum of 2.5×10 -3 Pa at T = 500-600 0C. Before evaporating samples they are always outgassed in a vacuum of 2.5×10-3 Pa at a temperature close to the melting point of substance.

2.2.2. Early Observations of Solid Aerosol Particles The presence of an inert gas or solid surfaces capable to dissipate the heat of formation of polyatomic complexes can very effectively influence on both the complex size distribution and the process of vapor condensation as a whole. Formation of atomic complexes of the metal vapor is the dominant process in the volume of inert gas. The first attempt to establish quantitative relationships of this process has been likely undertaken in 1912 by Kohlschütter et al. [12, 13]. Volatile substances (Zn, Cd, Se, As) were slowly sublimated in a vacuum or various gases (H2, N2, CO2), a pressure of which can be varied. The size and density of the particles deposited on a certain area of glass or quartz were studied with an optical microscope having magnification of 56–300. It was not found any essential difference between the results at different rates of evaporation and when changing the location of substrate in the device. All particles had a shape close to spherical. In Se deposits spherulites and dendritic crystals were sometimes observed. The key findings are as follows. 1. With increasing gas pressure from 7.3×103 to 1.1×10 5 Pa the particle size decreases from tenths to thousandths of millimeter. 2. At equal pressures, the particle size and density of the deposit depend on the gas species. The average particle size decreases when going from H2 to N2 and CO2.



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3. Deposits produced in H2 are more dense than that in CO2; in vacuum and at very low gas pressure, reflex deposits arise, and a fine dust with particles being far apart each other is formed at atmosphere pressure. For a long time, these results were considered as the basic regularities for aerosol particles (e.g. see [14, 15]). Meanwhile, it is absolutely evident that Kohlschütter et al. recorded only the large secondary conglomerates in their experiments. Subsequent electron microscopic studies of deposits showed that a function of the degree of dispersion versus pressure and nature of the carrier-gas for the primary aerosol particles was opposite to conclusions of Kohlschütter et al. It has been first established by Guen, Ziskin, and Petrov in 1959 [16]. Early experiments concerned mainly the possibility of formation of primary aerosol particles of metals. Guen, Zel’manov, and Shal'nikov [17] studied condensation of sodium vapor in the vapors of xylene and butadiene. They have developed an industrial method for manufacturing organosols by radio frequency evaporation of sodium directly inside the liquid. The method of dispersion of metal using an electric arc in liquid proposed by Tikhomirov and Lyadov [18] and then improved by Bredig [19] and Svedberg [14] is also reduced to condensation of the metal in a vapor of liquid. Aerosol particles of metals produced by one or another way have already long been an object of electron microscopic observations (see, for example [20, 21]). However, the first attempt to correlate a particle size with the conditions of their formation was apparently undertaken in [22]. The authors evaporated Au from a heated W filament in N2 atmosphere and studied the deposits on a collodion film. If the evaporation rate was maintained constant (3 mg/s), then the smallest spherical particles with diameter of 15-100 Å (average size ~40 Å) were obtained at a pressure of 40 Pa, whereas the particle size was 125-200 Å at N2 pressure of 130-400 Pa. Increasing the evaporation rate from 0.5 to 15 mg × s-1 (N2 pressure was 70 Pa) resulted in a strong growth of the amount of deposited substance and did not give rise to a noticeable change in the particle size. It was shown that the deposits of the high-dispersed Au powder well absorb the IR radiation with a very weak reflection [23]. According to Pfund [24], Bi and Zn condensates obtained in an inert atmosphere or in a low vacuum, had a black color. Levinstein [25] showed the universality of this phenomenon and explained it with formation of aerosol particles by condensation of metal vapor in the presence of residual gas in the set-up. He found that with increasing a residual gas pressure from 1.3×10-3 to 13 Pa an electron microscopic picture of metal condensate on the collodion film varies greatly. In a high vacuum (1×10-3 Pa) the deposits were obtained which consisted of individual crystallites with a specific facet (e.g. hexagonal lamels of size ~10 -5 cm in the case of Zn). At the same time at a pressure of ~13 Pa, condensates of many metals were very similar to each other. They had a black color and consisted of many small particles with a size of ~10-6 cm. The most striking result was obtained in the case of evaporation of Zn and Cd at a relatively high residual gas pressure (1.3 Pa) when the forming films surprisingly had a uniform structure and reflex surface. Levinstein gave the following explanation of the observed phenomena. In the presence of residual gas the metal vapor consists of a cloud of particles, which are apparently formed near the evaporator. Reaching the substrate, these particles stay long in the adsorbed state, acting as condensation nuclei. The adsorbed single atoms have time to re-evaporate from the surface of the substrate prior to merging with each other. Deposits of (Au, Ag, Cu, Ni, Fe, Co, Cr, Mn, Ti, Be, Pb, Sn, Pd, Pt) metals, when their


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vapor is condensed in a high vacuum, yield sharp lines in the electron-diffraction patterns (EDP). At the same time the deposits of these metals prepared at a residual gas pressure of ~13 Pa show diffuse rings in EDP. On the other hand, the diffuse rings arise in EDP of W, Ta, Ir, Nb, Rh, Ge, and Si deposits regardless of the preparation conditions (high vacuum or pressure of ~13 Pa). Based on these results, Levinstein thought that the particles formed near the evaporator are not much different in size from crystallites arising at the substrate surface in the case of refractory metals (W, Ta, etc.) but much smaller than the latter in the case of more fusible metals (Au etc.). Works [26, 27] have also confirmed that the black deposits obtained by spraying a number of metals (Au, Ag, Bi, Pb, Cd, Zn, Sb, Sn) in different gases (air, N2, Ar) at pressures s2). In this case the solidification temperature of a small drop appears to be higher than T∞ with the excess of TR over T∞ increasing in inverse proportion to R. A paradoxical possibility arises to preserve the liquid state of any mentally singled out small part of the bulk melt after its solidification. Moreover, a principle discrepancy of Thomson formula and its bad applicability in the case of solid-liquid equilibrium are established [2, 3]. This formula is approximately valid only for the system including a vapor phase. It is of interest to determine the melting point Tr of a small particle at a constant external pressure. Thermodynamical calculation shows that in this case Tr = T∞ , i.e. the melting points for particles and massive substance coincide each other [135–139]. We should deduce Wullf’s rule as well. We adhere, with some alterations, to the approach of Volmer [156] and Stranski [157]. Let us first consider the growth of the individual ith face of a small crystal under the equilibrium vapor press p. We transfer, under reversibly isothermal conditions, ∆Ni molecules from the bulk liquid onto the given face with an area Ai and on a distance hi from the crystal center. For this purpose, we evaporate ∆Ni molecules from the liquid at the vapor pressure p∞, separate the vapor from the liquid, and reduce its volume from V∞ to V by raising the pressure to p and making the work

∫

V∞ V

∆N i kT V p dV = ∆N i kT ln ∞ = ∆N i kT ln , V V p∞

where the equation of state for an ideal gas is used. Returning ∆Ni molecules condensed on the crystal back into the liquid we perform a negative work (–γi dAi) against the molecular forces of particle. The balance of works in a closed cycle yields γi dAi == ∆Ni kT ln (p / p∞) = (dVi / V) kT (p / p ∞),

(16)

where v is the volume accounting for one molecule in the crystal and dVi = v∆N i. From geometric consideration dAi = χ 2h i dhi and dVi = χ (hi)2dhi where χ is the proportionality factor. Therefore, we have an important relationship: dAi = (2 / h i) dVi.

(17)

Substituting it into (16) we get Wulff’s rule [158], namely, (kT / 2v) ln(p / p∞) = γ1 / h 1 = γ2 / h 2 = ·· = γn / h n .


(18)


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It says that the equilibrium pressure p and the ratio of surface energy γi to the distance hi between the face and the particle center are the same for all the crystal faces. In the case of a cubic particle hi = h = R where R is the radius of the inscribed sphere, and Exp. (18) is converted into Kelvin formula. The quantity 2γ / R arises in the Kelvin formula not because of the surface curvature which for the faces of a cube is equal to zero and owing to the universal geometric relationship (17). This quantity has nothing to do with the excess pressure in a crystal but serves as the measure of the pressure rise of saturated vapor over a small spherical or faceted particle.

6. OPTICAL TRANSMISSION, ELECTRIC CONDUCTION, AND PHOTOCONDUCTION OF HIGH-DISPERSED THIN FILMS Figure 36 shows transparency of thin deposits on glass of ultrafine Al, Ag, and Se particles with 2-3 nm in diameter at a normal incidence of light [159]. All deposits have been produced at the same conditions (a distance from the evaporator was about 7 cm; Ar pressure, 20 Pa; the evaporation duration, several seconds). Condensates consisted of aerosol particles of less than 2 nm in size grouped to aggregates of various shape and size. Unlike Al and Se deposits the Ag precipitates show a strong increase of transparency at λ = 0.32 µm followed a resonance light absorption at λ = 0.36 µm. Figure 37 illustrates transparency of Ag deposits of various configuration and concentration [159] on a cover glass located at a distance 15-20 cm apart of the evaporator. At Ar pressure 13 Pa (curves 1, 2) the deposits consist of 3 nm particles united in groups with an average size of 120-150 nm. With increasing Ar pressure up to 2.7×103 Pa (curves 3, 4) the configuration of deposits changed. They acquired a shape of open-chain and mesh configurations consisting of particles with an average size of about 25 nm. Most particles had a spherical shape, and some large particles showed a weak hexagonal faceting. Curve 7 is referred to a colloid suspension in water of aerosol Ag particles with an average size of 18 nm. A powder was aged in air for more than 5 years prior to introducing it into water. There was first detected an existence of two types of selective absorption, called afterwards as plasma resonance (PR) and resonance of optical conductivity (ROC) [160-162]. Since the photon energy (hωo ≈ 3.45 eV) in absorption peak at λ ≈ 0.36 µm is close to energies of volume (hωo ≈ 3.78 eV) and surface (hωo ≈ 3.52 eV) plasmons, the given peak can be referred to PR. This peak does not depend on size, degree of aggregation, and concentration of particles. Specific optical resonance (ROC) is caused with collective excitation of a large group of particles in plane of the film, and it is shifted to a long wavelength direction as a density of deposits enlarges (curves 5, 6 in Figure 38). One succeeded in a simultaneous observation of PR and ROC peaks in aerosol Ag particles (Figures 38 and 39) [163].


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Figure 36. Transparency (T) of aerosol Ag, Al, Se deposits on a glass base versus light wavelength (λ) [159].

Figure 37. Transparency (T) of aerosol Ag deposits on a glass base of different structure and density versus light wavelength (λ) [159]. 1, 2 – deposits obtained at Ar pressure 13 Pa; 3, 4 – deposits obtained at Ar pressure 266 Pa; 5, 6 – a thin film prepared at a residual air pressure 10 -2 Pa; 7 – suspension of Ag particles of 18 nm size in water.

Transparency of Cu deposits is given in Figure 40. Curve 1 shows the data for a suspension in water of spherical ~50 nm particles aged in air for more than 7 years. Curves 2, 3 show a transparency of deposits on glass of particles with a size less than 3 nm. A dip in transparency at λ ≈ 0.65 µm is seen in curves 1–3 that is specific to copper and explains its color.



Figure 38. Plasma resonance and resonance of optical conductivity in Ag particles produced in a residual air at pressure 13 Pa [163]. Evaporation duration 2-3 s; a distance from an evaporator 100 cm. λ1 - transparency maximum; λ2 plasma resonance; λ3 -resonance of optical conductivity.

Figure 39. Plasma resonance and resonance of optical conductivity in Ag particles produced in Ar at pressure 53 (1) and 66.5 (2) Pa [163]. Evaporation duration 10 s; a distance from an evaporator 70 mm. λ1 - transparency maximum; λ2 plasma resonance; λ3 -resonance of optical conductivity.


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Figure 40. Transparency (T) of Cu deposits having a different structure and concentration [159]. 1 – suspension of particles in water; 2, 3 – deposits on a cover glass prepared at Ar pressure 20 Pa; 4 – a thin film precipitated on a glass at pressure 10 -2 Pa.

Figure 41. Temperature dependent conductivity of Al film with particle size of ~7 nm on a glass base [164]: 1 – sample with 0.8 cm2 area; film thickness 1.6 µm; applied voltage 3 V; 2 – sample of the same area; film thickness 12 µm; applied voltage 4 V.

An important feature of the considered dispersed systems is their color. Ag and Cu films obtained at low Ar pressure (10 -2 Pa) have a bluish and greenish color in transmitted light. Precipitates of aerosol Al, Ag and Cu particles pr An important feature of the considered dispersed systems is their color. Ag and Cu films obtained at low Ar pressure (10 -2 Pa) have a bluish and greenish color in transmitted light. Precipitates of aerosol Al, Ag and Cu particles prepared at a higher Ar pressure appear black. Se condensates are bright red and have a break



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in a transparency curve (Figure 36) although the evaporated sample is dark gray. A colloid solution of silver has a dark color. Cu particles with a diameter of ~50 nm look dark crimson in a powder and reddish in an aqueous suspension. The dark color of the aerosol particles of metals is due to frequent collisions of electrons with a particle surface which oscillate under the influence of light. The color of Cu and Se particles is caused with transitions of electrons from one energy level to another. A comprehensive review on optical properties of highdispersed films and small particles is given in [2].epared at a higher Ar pressure appear black. Se condensates are bright red and have a break in a transparency curve (Figure 36) although the evaporated sample is dark gray. A colloid solution of silver has a dark color. Cu particles with a diameter of ~50 nm look dark crimson in a powder and reddish in an aqueous suspension. The dark color of the aerosol particles of metals is due to frequent collisions of electrons with a particle surface which oscillate under the influence of light. The color of Cu and Se particles is caused with transitions of electrons from one energy level to another. A comprehensive review on optical properties of high-dispersed films and small particles is given in [2]

Figure 42. Dependence of a dark current IT (1, 3) and integral photocurrent IФ(2, 4) on annealing duration of CdS deposits [168]: 1, 2 – sample of mass 600 µg/cm2 doped with 0.5 wt.% Cu, annealing in O2 atmosphere at pressure 400 Pa and temperature 2500С, voltage applied to the electrodes 10 V; 3, 4 – sample of mass 200 µg/cm2 doped with 1.0 wt.% Cu, annealing in air at temperature 4500С, voltage applied to the electrodes 100 V.

A typical peculiarity of thin deposits of aerosol particles is the temperature dependent behavior of electric conduction. All measurements of electric conduction were carried out in a high vacuum (10 -4 Pa). A sample was prepared of “sandwich” type with the layer a few micrometers thick from incompletely oxidized Al particles with 7 nm in diameter (Al content in deposit was 8%). The measurements showed at low temperatures a predominance of direct electron tunneling from one metallic core into another through oxide layers between them, and the electric conduction did not depend on temperature (Figure 41) [164]. At high temperatures the activation process with low activation energy 0.69 eV dominates.


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Figure 43. Spectral distribution of photocurrent at CdS samples heated for 30 min in air at 4500C [168]. 1 – pure CdS; 2 – CdS doped with 0.2 wt.% Cu; 3 – CdS doped with 0.5 wt.% Cu. .

It is difficult to explain such a low activation energy by decreasing a work function of electron both due to the electron affinity of the oxide, since the bandgap in Al2O3 is about 7 eV, and accounting of mutual image forces when approaching the electrodes because this effect is completely negligible for a sandwich with a thick (12 µm) insulating layer. Electrical conductivity of sediment at high temperatures can be attributed to the presence of defects in the oxide being oxygen vacancies. Relatively weakly bound valence electrons of Al, appearing owing to defects, can be moved by thermal excitation into the conduction band of the oxide. Electrons participating in the conductivity of the dielectric are captured by deep oxide traps having several equilibrium energy states. The transition from one state to another through an energy barrier, separating them, proceeds via thermal excitation. When applying an electric field, a number of electron jumps along the field direction and against it changes, giving rise to a relaxation polarization of the dielectric [167]. At high temperatures there is a strong increase in its dipole moment due to decreasing a relaxation time, on one hand, and increasing the number of filled traps owing to the gain of conduction current, on the other side. Indeed, the capacitance of sandwich increases from 200 pF to about 2 µF at frequency 40 Hz when the temperature changes from room one to 3200C. Rapid cooling of the sample leads to freezing the electrons in traps and reducing the dipole moment of dielectric in agreement with the experiment. The sample returns with time to the equilibrium state.



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Figure 44. Optical absorption spectra of CdS deposits [168]: 1 – a film thick 1 µm heated for 30 min at 3500С; 2 –a sample heated for 30 min in air at 4500C; 3 – a sample doped with 1.0 wt.% Cu heated for 30 min in air at 4500С.

Similar behavior of the temperature dependence of electric conduction showed deposits of aerosol CdS [165] and CdSe [166] particles of 10-20 nm size. Aluminum electrodes were previously in vacuum deposited on the cover glass in the form of interpenetrating combs with a gap width of 0.5 mm. Over the electrodes having a total gap length 70 mm, a layer of CdS particles, forming in Ar at pressure 13 Pa, was deposited. In experiments with CdSe the particles appeared in Ar at pressure of 8.6 Pa, and they were initially deposited on the cover glass over the electrodes of 5-9 mm in width with a gap of 0.8 mm. However, it soon became clear that after opening the vacuum system, the deposited layer sometimes was broken and there was a violation of contact with the electrodes. To prevent this, the electrodes were deposited on top of a CdSe layer in all experiments. Aerosol CdS and CdSe particles had a high-temperature wurtzite structure unlike the sphalerite structure of the initial powders. It is important to emphasize that the aerosol particles of compounds and alloys hold a composition of initial substance evidenced by X-ray diffraction for CdS [165] и X-ray analysis with CAMEBAX for CdSe [67]. The samples were heat treated at different temperatures and in various gas atmospheres. In the course of annealing the dark current, IT, and photocurrent, IФ, were periodically measured for what the samples were cooled to room temperature. To prevent the influence of adsorbed moisture, the measurements of IT and IФ were always made in vacuum with a special electrometric device.


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Figure 45. Temperature dependent dark current of CdS deposits [168]. 1 – a film thick 0.8 µm heated for 30 min in 10–4 Pa at 3500С, voltage applied to the electrodes 5 V; 2 – a film thick 2 µm heated for 30 min in air at 4500С, voltage applied to the electrodes 50 V.

To measure integral photoresponse one illuminated the sample (CdS sediments) with an incandescent lamp creating a luminous flux ~1300 lm and being at a distance of 15 cm. In a case of CdSe sediments a thermal radiation of the incandescent lamp was cut off with glass filters. A filament of the lamp was at a distance of 22 cm from the sample. The spectral distribution of photocurrent was studied using different monochromators. The results are shown in Figures 42-45 for CdS, and in Figures 46-48 for CdSe. When measuring an optical transmission with spectrophotometer, a width of energy gap in high-dispersed semiconductor CdS (2.4 eV) [168] and CdSe (1.65 eV) [169] layers appeared near identical to that of a bulk substance. Electric phenomena in a system of semiconducting contacting particles are mainly controlled with intercrystallite barriers and surface states generating a change of Fermi level and curvature of energy zones near the particle surface. Authors of studies of photoconduction of high-dispersed CdS [168] and CdSe [169] films came to the following conclusions. Photoeffect is observed even in as-prepared films without any special treatment for their sensitization, with a ratio of integral photocurrent to dark current IФ / IТ equals about 2.5·102. The ratio increases to a record value IФ/ IТ ≈107 after annealing the film in air for 1 h at 450 0C. Additive of 1 wt. % Cu by vacuum deposition before heat treatment of samples results only in a fall of ratio IФ / IТ in case of CdSe films compared to similarly prepared and annealed non-doped films. Meanwhile, Cu additive of up to 0.5 wt. % in high-dispersed CdS films shifts a maximum of photosensitivity from green to red wave range of visible light and increases the integral photocurrent by a few dozens of times. It is significant that a sharp increase of the ratio IФ / IТ as a result of heat treatment of samples occur mainly due to growth IФ whereas IТ change essentially weaker. Probably, intercrystallite potential barriers in CdS and CdSe films are formed basically by a capture of



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holes with surface states of particles. As result, the surface layer of grains acquires a positive charge and forms n-p transition with its interior which is supposed in the barrier Slater theory [170]. According to it, appearing during light absorption electron-hole pairs are pulled out by the electric field of barrier and neutralize contrary charges of minority carriers that results in reducing a height of barrier. The amount of surface traps evidently depends on heat treatment of samples. In addition to this mechanism, the integral photocurrent grows also owing to two circumstances. First, a capture of holes by surface traps increases a lifetime of free electrons till their recombination. Second, a strong electric field of n-p transitions results in washingout of the absorption edge that makes possible absorption of light quanta with the energy less than a width of forbidden band.

Figure 46. Current-voltage curves of CdSe deposits [169]. Electrode widths 6 mm, a gap between them 0.8 mm. As-prepared sample, thickness of deposit d =6 µm. 1 – dark current IT; 1,а – integral photocurrent IФ. Samples annealed at 450 0С for a time t : t = 15 min, d = 5.2 µm, 3 – IФ.; t = 45 min, d = 4.6 µm, 4 – IФ.; t = 60 min, d = 3.9 µm, 2 – IT; 2,а – IФ ; t = 75 min, d = 2 µm, 5 – IФ; t = 90 min, 6 – IФ.

Band bending near the particle surface makes easier tunneling of charge carriers from impurity levels of the band gap of one particle into the conduction band of a neighboring particle. Frantz-Keldysh effect is realized by this way. It is in that the electron ejected from a


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valence band with a quantum of light with the energy less than the band gap can then tunnel into the conduction band of the same or neighboring particle. The magnitude of band bending in the case of CdS particles is varied with a change in their surface coverage. Addition of Cu 1 wt.% is sufficient to cover all the particle with a monolayer.

Figure 47. Temperature dependent dark (IT ) and integral photocurrent (IФ ) of CdSe deposits after different heat treatment [169]. (Labels are the same as in Figure 46). Electrode widths 9 mm, a gap between them 0.27 mm. As-prepared sample, thickness of deposit d =0.27 µm: 1 – IT; 1,а – IФ. Deposits annealed for t = 60 min. at T =3500С, d = 0.27 µm, 2 – IT , 2,а – IФ; t = 90 min, T=4500С, d = 0.27 µm, 3 – IT ; Samples doped with 1 wt.% Cu and annealed for t = 60 min at T = 3000С, d = 0.5 µm, 4 – IT , 4,а – IФ; t = 90 min., 350 0С, d = 0.4 µm, 5 – IT , 5,а – IФ.

Compounds of copper with O2, Cd, and S arising at high temperature treatment in air apparently cause such a strong bending of the bands and the reduction of barriers which dramatically increases the quantum yield of photocurrent due to the deeper impurity levels. The latter is filled by electrons ejected from the valence band by the "soft” photons. However, the addition of copper to the CdSe particles causes the opposite effect. As seen in Figure 47, in the high-temperature range, the curves for IT and IФ merge, and above the merging point the thermal excitation of charge carriers dominates over the



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photoelectric effect. At room temperature the curves tend to a limiting value of current equal approximately 10 -4 A. This effect can be explained with a current restriction by a space charge being accumulated in the traps of intercrystallite regions. Upon subsequent lowering of temperature the attained limiting current does not fall down at once. For example, as shown by the arrow in Figure 47, a 10-fold current decrease occurred at room temperature for 19 h.

Figure 48. Spectral photocurrent distribution of CdSe deposits [169]. (Labels are the same as in Figure 46; k – coefficient taking the spectral distribution of the light source radiation into consideration, d – a thickness of deposit). The electrode widths 5 mm, a gap between them 0.8 mm. As-prepared samples: 1,а – d = 0.7 µm; 2,а – d = 0.45 mm.Annealed samples: 1,b – t = 60 min , 3000С, d = 0.4 mm; 1,c – t = 60 min , 4500С, d = 0.5 mm; 2,b – t = 30 min , 300 0С, d = 0.3 mm; 2,c – t = 30 min , 4500С, d = 0.6 mm; 2,d – t = 60 min , 450 0С, d = 0.4 mm.

7. ARTIFICIAL DIELECTRICS. FERROMAGNETIC RESONANCE Electromagnetic properties of suspension of aerosol Ni particles with an average size 50 nm in paraffin were studied in [171]. Complex dielectric ε =ε ′- jε ″ and magnetic µ =µ ′- jµ ″


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constants of samples were measured at a fixed frequency 9370 MHz with the waveguide method when one placed samples by turn in loops of magnetic and electric standing waves by changing the static magnetic field H from 0 to 8000 Oe. The set-up of wavelength 3cm was applied for these experiments. It was shown that as the volume concentration of particles ξ increases and attains the limiting value 30%, ε′ grows near proportionally from value ε ′≈2 (pure paraffin) to ε ′≈13, and ε″ keeps its value close to zero. At the same time, µ’ does not practically change, whereas µ ″ first rises proportionally from zero to µ ″ ≈ 4.5 at ξ =22%, thereafter it does not practically change. Such behavior of ε and µ is characteristic to so called “artificial dielectrics”, that is media dielectric constant of which grows with introduction of small well regulated conductors. A resonance character of magnetic losses µ ″ induced a special study of ferromagnetic resonance (FMR) by measuring coefficients of transmission (t) and reflection (ρ) of a super-high-frequency (SHF) wave. Actually, a relative absorbed by sample energy, W, is linked with t and ρ by relation W = 1 – (ρ2 + t2), which displayed a broad resonance peak nearby a magnetic field strength Ho=2500 Oe [171].

7.1. FMR Features in Aerosol Fe, Ni, and Co Particles A ferromagnetic resonance of suspension in paraffin of spherical Ni and Fe particles with about 50 nm in diameter and Co particles of size about 120 nm was studied in [2, 3, and 95]. The particles prepared with the levitation-flow generator were directly introduced in melted paraffin. The measurements were carried out in a center of the SHF magnetic field loop in a rectangular resonator of Н106 type with a frequency 9320 MHz at a temperature range from 77 K to 293 K. The magnetic field H was smoothly changed from zero to 8000 Oe and measured by Hall sensor with error of 1.5%. SHF frequency was set with a klystron generator. It was measured with accuracy of ± 200 kHz. The results of measurements are given in Figures 4954. The magnitude [(V ∞ / V) –1] proportional to the relative change of Q-factor of the resonator due to FMR is plotted everywhere along the ordinate; an index ∞ refers to the case of sufficiently large magnetic field (8000 Oe) when one can neglect the losses in the resonator due to FMR. A magnitude V ∞ / V stands for the ratio of the electric field of the wave in the line after the resonator. The procedure of work was the following. For each fixed value of the magnetic field the resonator with a sample was tuned to a maximal transmission of SHF power by changing the frequency of klystron generator (the power of the incident wave was kept constant).The resonance frequency ν r and a relative power at the output of the tuned resonator P /P∞ was directly measured. The increase of losses in the resonator due to FMR was calculated by the formula (V ∞/ V ) = (P ∞/P )1/2 . The magnitudes ∆ν =(ν r∞−ν r) and [(V ∞/ V) –1] are proportional to the components of complex magnetic susceptibility χ = χ′ − i χ″. The wide lines of FMR absorption were detected (Figures 49-51). So the broad lines are usually associated with multidomain construction of particles and movement of domain walls. In fact, when increasing the resonance frequency ωres, it is necessary to strengthen the magnetic field Н0 according to a formula [3]



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ωres = γ (H0 m 2K1 / Is), where γ is a gyromagnetic ratio, K1, a constant of magnetocrystalline anisotropy; Is, a saturation magnetization; a sign minus in parenthesis is taken for K1 < 0. Increase of ωres by a factor of 4 gives rise to so significant growth of Н0 that the particles become single domain with a narrow FMR peak [3]. However, the curves in Figures 49-50 could not be explained by a simple movement of walls of magnetic domains because, according to all estimations, Ni, Co and Fe particles under study were single domain.

Figure 49. Ferromagnetic resonance (FMR) at Ni particles suspended in paraffin or oil with a different concentration at room temperature [95].

О – spherical samples of 3.2 mm in diameter from paraffin; • – a glass capillary of 2 mm in diameter filled with a suspension of Ni particles in oil with a concentration ξ =3%.


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Figure 50. Ferromagnetic resonance (FMR) at Co particles suspended in paraffin with a different concentration at room temperature [95].

О – spherical samples of 3.2 mm in diameter from paraffin; • – a cylindrical sample of 3 mm in diameter and 9 mm in height (ξ =10%).

Since the magnitude of the FMR linewidth, ∆Η, was the same for Ni suspensions both in paraffin and in oil, where the particles could align along the field lines of the constant magnetic field, the supposed reason for broadening the FMR line due to different orientations of anisotropy axes of single-domain particles fixed in a dielectric is negligible. Moreover, the increase by near three times of width of resonance Ni line, as temperature goes down from 293 to 77 K (Figure 52), contradicts to all existing theories (Figure 54). It can be understood, if one takes into account a magnetic interaction of structural groups constituting the particles and making the rotating vibrations [2, 3]. In this case a temperature dependence of FMR linewidth can be explained by movement of structural groups just as the effect of NMR line constriction is due to enlarging mobility of atomic nuclei [178]. An estimation of the mean time τ (T) between two successive changes of dipole interacted groups relative to the fluctuating field, which was made by the random walk method [119], gave the following values: τ = 4.6 × 10-12 s at T = 631 K (Curie point) and τ = 1.8 × 10-11 s at T→ 0 [2]. These values τ appear much less than a period of Larmour precession (1.05 × 10 -10 s) of the vector of saturation magnetization for particles that justifies the application of the random walk method.



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Figure 51. Ferromagnetic resonance (FMR) at Fe (ξ =19%) particles suspended in paraffin at different temperature [95]. × – a cylindrical sample of 3 mm in diameter and 9 mm in height, Т = 230С; + and ∆ – a spherical sample of 3.8 and 3.3 mm in diameter, respectively, Т = 230С; О and • – a spherical sample of 3 mm in diameter, Т = 1960С (О, increase and •, decrease of magnetic field H).

Figure 52. Change of FMR linewidth when cooling Ni suspension [95]. A sample of 3.2 mm in diameter (ξ =30%). О, increase and •, decrease of magnetic field H.


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The interesting effect was observed for suspensions of Co particles in paraffin at 77 K (Figure 53). The curves of direct and reverse changes of magnetic field formed a hysteresis loop of the absorbed energy, with a FMR maximum being shifted to a range of higher magnetic fields on the curve of reverse movement. In addition, when H went down from 8000 Oe, an unusual step of magnetic susceptibility χ′ of sample was seen. Appearance of the hysteresis loop can be explained by interaction of the ferromagnetic particle core with antiferromagnetic oxide shell (the Neel point is 270 K). The acquired in the high magnetic field range ordering of spin system of antiferromagnetic keeps also in the weak magnetic field range at 77 K. This results in appearance of a single-axis exchange anisotropy which increases a “rigidity” of spin system in the ferromagnetic particle core causing the resonance frequency and, respectively, the resonance field Ho to increase. In experiments with suspensions of Ni and Fe particles the hysteresis phenomena were not observed.

Figure 53. FMR in Co suspension (ξ =30%) at Т = –1960С [95]. A sample of 3.2 mm in diameter (ξ =30%). О, increase and •, decrease of magnetic field H.

An absorption peak width on its half-height at room temperature appeared equal ∆H = 7000 Oe for Fe, Co, and ∆H = 1800 Oe for Ni. Similar experiments with a rather perfect threadlike single crystal gave significantly lower values (∆H = 87 Oe for Fe, ∆H = 110 Oe for Co, and ∆H = 120 Oe for Ni, see [95]). Such a substantial broadening of FMR lines for small aerosol particles distinctly indicates their complex structure.



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Figure 54. Comparison of theoretical and experimental results for a relative FMR linewidth at Ni [95]. ⋅⋅⋅⋅⋅⋅⋅⋅⋅ theory of Ament and Rado [172]; − − − − − − theory of Landau and Lifshits [173]; ▪ ▬ ▪ ▬ ▪ theory of Kittel and Mitchell [174, 175]; • – aerosol particles of 59 nm size; × – colloid particles [176]; + – data for scaly crystals and whiskers thick ~1 µm [177]; ∆ – data for a scaly crystal thick 60 nm [177].

8. CORRELATION OF STRUCTURE AND SPIN ORDERING IN SMALL PARTICLES OF FE AND ITS ALLOYS 8.1. Mössbauer Effect in Aerosol Particles. Influence of High-Temperature γ- Fe Modification on the Particle Properties Combined studies of aerosol particles by Mossbauer, structural, and magnetic methods allows one to shed light on fine interactions of spin and structural ordering of the core and oxide shell in particles. Resonance scattering of gamma radiation which occurs without the recoil γ-quanta energy loss in the Mossbauer effect allows one to measure very weak changes in the magnetic state of the sample. Nanoparticles of alloys of 5-20 nm in a size were prepared mainly inside the stainless steel cylinder with a diameter of 10 cm in Ar atmosphere at pressures of 13 to 1.3×10 3 Pa. In most cases one applied Ar pressure of 400 Pa and prepared particles with an average size 13-15 nm. All as-prepared particles contained up to 30-40% of X-ray amorphous oxide which was displayed in the central part of Mössbauer spectra as a superparamagnetic doublet. At low temperatures, the latter transformed to a sextet with the hyperfine field (46-49) T. The Mössbauer measurements were carried out at different temperatures using a conventional constant-acceleration spectrometer with 57Co(Cr)


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or 57Co(Rh) sources. Since as-fresh aerosol particles are weakly linked with each other (porous powders), losses of the recoil γ-quanta energy become involved, reducing the probability of the Mössbauer effect. However, prepressing the powders placed between two Ta plates with a hammer prior to Mössbauer measurements increased the Mössbauer effect by several times and entirely suppressed superparamagnetism of nanoparticles. X-ray patterns of the powders after such treatment did not reveal any changes in structure as well as in a lattice parameter. The obtained spectra were processed on computer with the NORMOS program [211]. The gas evaporation technique has unique peculiarities making it possible to get information about high-temperature states of metals and alloys at room temperature. The aerosol particles during their preparation are rapidly cooled from high (~20000C) down to room temperature with a very high rate 10 4-10 6 degree × s-1. In a number of cases, for example, in Co, CdS, and CdSe particles, a stable high-temperature structural modification was observed at room temperature. In Fe powders [96, 155] it was not managed to retain, and a stable bcc phase (α-Fe) dominated in along with a very small impurity of high-temperature fcc phase (γ-Fe). A study of thermal expansion of Fe particles with an average size ≈30 nm [96] showed that it is almost identical to that for a bulk bcc Fe (Figure 28). Though a temperature of α→γ transition in particles (796 0C) is below than that at a bulk phase (910 0C), the reverse γ → α transition of particles begins at near 700 0C, and lines of the fcc phase in X-ray patterns continue to retain with a further reduction of temperature, though they strongly fall (Figure 55). Lattice constant of γ-Fe at elevated temperatures is significantly larger for particles than for a bulk Fe. However, an extrapolated value of the lattice constant for bulk γ-Fe at 230C agrees with that for particles (Figure 55) [155].

Figure 55. Thermal expansion of γ–Fe particles of ~30 nm size [155]. Line 1 presents data of Basinski et al. for a massive iron [179]. Line 2 is related to aerosol Fe particles.



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Figure 56. Mössbauer spectra of Fe particles of ~50 nm size [155]. (a) initial powder; (b) the same powder after heating for 1.5 h at 8000С and subsequent quenching to room temperature.

The striking results gave Mössbauer spectra of Fe particles with an average size 50 nm (Figure 56) [155]. It is seen that these spectra can be decomposed on two components corresponding to hyperfine fields on Fe nuclei H1=366 kOe and H2=330 kOe (Figure 56, a). Annealing of particles at 8000C in Ar atmosphere for 30 min followed by quenching down room temperature resulted in a significant growth of intensity of the component with the high hyperfine field H1 (Figure 56, b). These results have been obtained only with powders previously prepared by late M.Ya. Guen at the levitation-flow generator in Ar at pressure of about 1 atm. The powders were kept in air for several decades. Slight traces of oxide were observed only in the Mossbauer spectra and absent in X-ray patterns. Auger spectra recorded after etching the samples by argon ions did not manifest any impurity elements in the particles. Later, X-ray analysis showed a slight admixture of cobalt. However, a specially prepared powder of iron doped with cobalt showed the typical Mossbauer spectrum with nonsplitting sextet lines. There was no splitting of spectral lines also in spectra of 7 nm and larger Fe particles, produced by the levitation-flow generator. One can assume that the observed effect arose probably due to the extremely high cooling rate in the course of particles preparation. A certain role may be played by a prolonged exposure of powder in air. At 800 0C the Fe particles acquire the fcc structure which transits into the bcc structure in the course of cooling. Therefore the single explanation of observed spectra can be in availability in the bcc lattice of regions inherited a ferromagnetic ordering of spins of the hightemperature fcc Fe modification. It is the ferromagnetic state of the fcc structure which results in growth of lattice constant of particles compared to that of bulk metal at high temperatures. As temperature goes down, a constriction of lattice constant down a = 3.57 Å occurs at 230C, and a ferromagnetic state of the fcc structure simultaneously gradually transits in a


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paramagnetic state as demonstrated by an availability of a little singlet peak in the central part of Mössbauer spectrum (Figure 56). It was earlier shown that the small γ-Fe particles precipitated from an oversaturated solution of Fe in Cu [180] or epitaxially grown on Cu single crystal [181] are antiferromagnetic with the Neel point between 67 K and 80 K. Experiments with aerosol particles give a direct indication on existence of ferro- and antiferromagnetic states in the fcc Fe lattice in accordance with the Weiss’ hypothesis stated still in 1963 [182]. Weiss supposed that a high-temperature γ-Fe could exist in two-spin states. One of these states, high-spin, is ferromagnetic and has a lattice constant а = 3.64 Å and a magnetic moment µ ≈ 2.8 µB (µB is Bohr magneton), and the other, low-spin, antiferromagnetic with а = 3.54 Å and µ ≈ 0.5 µB. The energy difference of these two states is very little. Therefore transition of one state to another is easy realized by thermal excitation. At any temperature both spin states are mixed at different proportion in accordance with Boltzmann law. By this hypothesis, both states of the fcc Fe lattice must display also in Fe-rich alloys. The ground state of alloy at room temperature depends on its composition. For example, in a case of fcc FeNi alloy with Ni content exceeding 30% the ground state is ferromagnetic, and when Ni content below 30% it is antiferromagnetic. Weiss supposed that fcc FeMn alloys should behave similarly except that at room temperature the ground state for them should be antiferromagnetic which is gradually replaced by ferromagnetic one with decreasing Mn content and increasing temperature. But so far this hypothesis was not confirmed experimentally because of martensite transformation of austenite in ferrite. Asano [183] showed that to suppress martensite transformation fcc → bcc and retain the fcc phase at rather low temperatures FeNi particles should have a size ≤4 µm. However, experiments with aerosol FeNi(30.3%, 35%, and 52%) particles gave unexpected results [184-186]. The very smallest FeNi(30.3% and 35%) particles with 5-8 nm size appeared to have the bcc structure which transits into the fcc one after high temperature treatment or with doubling of their size. At the same time even the very smallest FeNi(52%) particles with a size 7 nm have the fcc structure as in a bulk alloy. Lattice constants of particles including the very smallest ones did not differ from those for bulk alloys within a measurement error (±0.002 Å). For particles having the bcc structure, the lattice constant was practically identical to that of bulk α-Fe. Mössbauer spectra of particles having the fcc structure show a narrow paramagnetic singlet whereas spectra of particles with the bcc structure contain a sextet of broad lines with a little admixture of superparamagnetic doublet referred to oxide (Figure 57). The sextet can be decomposed on two components corresponding to hyperfine fields H1 = 345 kOe and H2 =318 kOe. As in case of pure Fe, the hyperfine field H1 should be related to a remnant of ferromagnetic order of the high-temperature fcc lattice in the bcc structure, and H2 belongs to the bcc lattice itself. Attention is drawn to the fact that after quenching from high temperature, the particles transited almost entirely from the initial bcc phase to a paramagnetic fcc phase (Figure 57, c, d)



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Figure 57. Mössbauer spectra of FeNi alloys [185]: (a) FeNi (30.3 wt.%) foil; (b) FeNi(30.3 wt.%) powder with a mean particle size of 15 nm; (c) FeNi (35 wt.%) powder with a mean particle size of 8 nm; (d) the same powder as the previous one heated at T=9000C for half an hour followed by quenching to room temperature.

Similar results were obtained in experiments with aerosol FeMn(32% and 34.65%) [187, 188, and 190] and Fe3Pt [189-191] particles with the average size 7-20 nm. The following complicating circumstances have taken place in these cases, namely, an appearance of intermediate hcp structure in case of FeMn system, and a transition of ordered state in disordered one and vice versa for Fe3Pt. Such transition in a bulk Fe3Pt alloy was specially studied by us [191]. For all particles the lattice constants of bcc, fcc, hcp phases were identical to those for bulk alloys. Mössbauer spectra of FeMn particles contained a sextet which was decomposed on two components with hyperfine fields H1= 3 50 kOe and H2 = 310 kOe belonging to the bcc phase and an intense singlet referring to the hcp phase. In case of Fe3Pt system, decomposition of the Mössbauer sextet on two components with hyperfine fields H1=350 kOe and H2=326 kOe was observed even in the bulk bcc phase. Quenching of Fe3Pt foil from 640 0C down room temperature gives the fcc phase, the Mössbauer spectrum of which has a single paramagnetic singlet. However, a long-standing (15 years) ageing of this fcc foil in air transforms again its structure in the bcc lattice [189-190]. Structure and Mössbauer spectra of small Fe3Pt particles are similar to that for the bulk alloy.


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Appearance of the bcc structure in small (5-8 nm) aerosol particles of Fe-rich alloys with the lattice constant like that at pure α-Fe is difficult to consider as martensite transition. It can easier be explained with decomposition of the primary fcc phase of particles with their cooling during preparation on two phases in according to the constitution diagram. One of the phases must be a solid solution of Ni, Mn or Pt in bcc iron, whereas another fcc one has been enriched with the dilute dope. Probably, due to kinetic reasons the first phase rapidly spreads through the particle volume, leaving a little space for the second one. However, a solubility of Ni, Mn or Pt in α-Fe does not exceed several percentages, therefore the lattice constant of this phase is close to that of pure α-Fe. Meanwhile, the retained fcc phase appears X-ray amorphous. A volume relation of two phases changes after heat treatment or increase of the particle size when X-ray lines of the fcc structure of the second phase arise and intensify. Now it is not sufficiently clear why such different, concerning their magnetic behavior, metals (Ni is ferromagnetic, Mn antiferromagnetic, Pt paramagnetic) give near identical Mössbauer spectra of aerosol particles of Fe-rich alloys. In this connection we carried out Xray diffraction and Mössbauer studies of aerosol particles of ternary Fe65(Nix Mn1-x)35 (0 ≤x ≤1) alloy with successive substitution of Mn atoms by Ni [192, 193]]. Reduction of Ni content leads to increase of a low-spin state fraction with lower values of the magnetic moment and lattice parameter. The powders were prepared by evaporation of foil containing from 4.98 to 35 wt.% Ni in Ar at pressure 70 and 400 Pa. The particle size was about 5-8 nm and 10-15 nm, respectively. In contrast to massive samples having the fcc structure, the smallest particles had the bcc structure. In addition, with increasing their size or Ni content an admixture of a fcc structure arose. The appearance of these two structures is due to decomposition of the initial fcc structure into two (bcc and fcc) phases while rapid cooling of the particles during their preparation. The analysis of results indicates the existence of two spin states at high-temperature fcc structure of Fe65(Nix Mn1-x)35 particles. A singlet in the Mossbauer spectra can be attributed to the fcc phase although it cannot be detected by X-ray diffraction because of its amorphous state. By analogy with the binary alloys, one of the sextet components with a high hyperfine field of about 34.5 T is attributed to the residual magnetic ordering of the high-temperature fcc modification in the resulting bcc structure. Another sextet component with the lower hyperfine field of about 29.5 T belongs to the bcc phase itself. The results of explorations showed that substitution of Mn atoms by Ni ones or vice versa results only in a change of relation between ferro- and antiferromagnetic states of the high-temperature fcc phase of limiting binary Fe65Ni35 or Fe65Mn 35 alloys. So, by combining the results of X-ray diffraction and Mössbauer spectroscopy studies at room temperature, the direct evidence for the two-spin state in the high temperature fcc structure of fine Fe, FeNi, FeMn, Fe3Pt and Fe65(Ni1–xMnx)35 (0 ≤ x ≤ 1) particles has been obtained. One of them is ferromagnetic with a hyperfine field H1 ≈ 35 T. It is observed as a remnant of the magnetic order of the high temperature fcc phase in the resulting bcc phase which is stable at room temperature with hyperfine field H1 ≈ 33 T. Another state of the fcc phase appears paramagnetic. The behavior of the two states of the fcc lattice in the aerosol FeNi and FeMn particles at low temperatures has been discussed in a work [194]



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Figure 58. Concentration dependent hyperfine field at 57Fe nuclei in FeCo alloys produced by different methods [196]. Alloying components: О – according to [197],+ – for the disordered and ■ – for the ordered alloys [198]; ▼ – for the disordered and ∇ – for the ordered alloys [199]; × – electrolytic precipitation [200]; ▲ – ball milling [201]; ◊ – precipitation of small particles in Cu matrix [202]; □ – aerosol particles [196].The curves correspond to (1) disordered alloy; (2) ordered alloy; (3) electrolytic precipitated films; (4) aerosol particles.

Concentration dependence of the Mössbauer effect in FeCo particles of 10-15 nm size was examined in [196]. The spectra consisted of a sextet with broadened lines and a wide central peak. Estimates based on the Mössbauer data showed that oxide Fe3O4 was less than 34% in the Fe65Co35 particles. With increasing Co content, the oxide fraction decreased. In some samples of the FeCo(85%) particles the spectrum could be decomposed into two components with hyperfine fields H1 = 36.1 T and H2 = 33.6 T which indicate the presence of two spin states in fcc Co-rich FeCo alloys. With further increase of Co content up to 90-95% a part of the iron oxide in the central peak of the spectrum has almost disappeared. Two separated singlets are mainly kept, one of which belongs to the paramagnetic fcc alloy, while the other, apparently, is due to the presence of small Fe aggregates in the Co matrix. The concentration dependent hyperfine field for FeCo particles presented in Figure 58 appeared the same as that at the bulk alloy. It has a maximum between Fe80Co20 and Fe65Co35, and then it drops to a value of the hyperfine field for pure iron (33 T) for FeCo particles containing >80 at.% Co. To clarify such behavior of the concentration dependent hyperfine field, X-ray examination of aerosol FeCo particles was thoroughly fulfilled [203]. The results are shown in Figure 59. It is seen that with increasing Co content to ~81 at.% the iron based bcc structure retains. Then the bcc→fcc phase transition occurs. The alloy is partially


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decomposed onto small clusters of pure Fe and Co particles doped with smaller (dimers, trimers) Fe aggregates which increase to some extent a fcc-Co lattice constant. At the same time, small Fe clusters have a reduced lattice parameter (~2.834 Å) compared to that for the massive Fe (2.866 Å).

Figure 59. Concentration dependent lattice constants of aerosol FeCo particles [203]. Aerosol particles were prepared at Ar pressure from 68 to 1.3×103 Pa to follow influence of the particle size. ■ – lattice constant for aerosol fcc Co [114].

It is noteworthy a similarity of curves in Figures 58 and 59. The existing theories consider the magnetic interaction between neighboring atoms with Heisenberg exchange integral J [204]. With increasing the interatomic distance with respect to the radius of the unfilled shell, a negative value J describing the region of antiferromagnetism at first increases rapidly, getting over to the positive region of ferromagnetism. Then J reaches a maximum value followed a smooth decrease. The average hyperfine field at Fe100-xCox nanoparticles shows a similar behavior in the ferromagnetic region. Ferromagnetism of the FeCo alloy increases slightly to the maximum value at a little Co addition which increases the bcc lattice parameter. The further Co content increase, however, leads to decreasing the latter and its ferromagnetism. Thus, it is observed that the lattice parameter and magnetization behave symbatically. This is confirmed also by the fact that at the bcc→fcc phase transition, the magnetization (the average hyperfine field) of particles and the lattice parameter change similarly (the former changes by a factor 36.6 / 33 = 1.11, while the latter by a factor 2.875 / 2.845 = 1.01). The interesting results have been obtained when studying aerosol Fe1-xCrx(0 ≤ x ≤ 0.83) particles of ~14 nm size by X-ray diffraction and Mössbauer spectroscopy techniques [205208]. Within the whole range of Cr content X-ray patterns of FeCr particles showed a



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presence of the bcc phase. In addition, beginning from 23.4 at.% Cr lines of the tetragonal structure (σ phase) were also observed, the intensity of which first increased, attaining a maximum at 47.7% at.% Cr, and then fell away, becoming very weak at 83.03 at.% Cr. In quenched massive alloys, σ phase cannot be observed due to the very slow rate of its formation and a strong deceleration of the atom diffusion at room temperature. When cooling nanoparticles during their preparation, mixing of atoms occurs for a small fraction of a second. Therefore they have time to attain close to the equilibrium state of σ phase. The new result is also a significant increase in the lattice parameter at FeCr(≤ 8.86 at.%) particles compared with that at a bulk alloy. This can be explained by freezing the ferromagnetic ordering of the high-temperature fcc modification having stronger magnetism than that at the bcc phase in some parts of the resulting bcc phase of particles. As a result, an additional expansion of lattice arises due to the repulsion of parallel spins. In FeCr alloys containing Cr >14 аt.%, which is beyond the limits of existence for a high-temperature fcc Fe modification in the phase diagram, its influence is excluded. Therefore the bcc lattice parameters of particles and a massive alloy do not differ any more. Owing to decomposition of the homogeneous high-temperature structure in FeCr system onto equilibrium phases, a transition from ferromagnetism of the Fe-rich phase to paramagnetism of σ phase and a Cr-rich phase is accurately traced in Mössbauer spectra of FeCr particles. The massive quenched alloys exist at room temperature in a metastable state, keeping a homogeneous high-temperature structure. However, with increasing Cr content their spectra changed by such a way that the ferromagnetic sextet transformed to a paramagnetic singlet, with ferromagnetism smoothly disappearing near FeCr(≈ 68 аt.%) composition. On the other hand, in particles there was a sharp transformation from ferromagnetic to a paramagnetic state already at Cr content ~35 at. %. Studies of Fe, FeNi, FeMn, FeNiMn, FePt, and FeCo particles by X-ray diffraction and Mössbauer spectroscopy techniques have been summarized in a review [209]. Structure and magnetic state of aerosol FeCu nanoparticles of 10-30 nm size with Cu content of 0.6 to 92.1 at.% have been examined in a recent work [210]. A feature of this system is in a practically complete mutual immiscibility of both components. The FeCu particles have been shown to consist of an iron core surrounded by a copper and Fe oxide shell. With increasing Cu content the iron core having a bcc structure is reduced down to its complete disappearance followed by vanishing ferromagnetism of the particles. Within the copper content from 4.9 to 74.3 at.% the bcc and fcc phases coexist, with the fcc phase having a lattice constant close to that of pure copper and the bcc lattice constant being slightly higher than that for pure Fe due to embedding Cu atoms into the Fe lattice. At Fe-rich FeCu samples a presence of two spin (ferromagnetic and paramagnetic) components of the fcc Fe is also observed. In a thin copper shell, surrounding the iron core, there is only the ferromagnetic fcc Fe, whereas with further thickening of the shell both spin states of the fcc Fe appear to exist up to a 20% Cu content. For FeCu samples with a higher Cu content they disappear due to oxidation of the copper grains. The Cu-rich samples with Cu content higher 80 at.% have a fcc structure, with the lattice constant being slightly higher than that of copper and they are paramagnetic. A slight increase of the lattice constant is due to the penetration of small iron clusters into the Cu grains. In contact with air, the FeCu particles become covered with Fe3O4 and Cu2O. Their long-term exposure to ambient conditions leads to further oxidation process of Cu2O to Cu O.


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8.2. Discontinuity in a Hyperfine Field Distribution at 57Fe Nuclei 8.2.1. Extraction of Hyperfine Parameters from Mössbauer Spectra One of the prime goals for Mössbauer spectroscopy of iron contained substances is in determining a hyperfine field distribution (HFD) at 57Fe nuclei. For this purpose one applies a variety of codes, for instance the NORMOS program [211] which offers fitting either by a superposition of components (SITE) or by a hyperfine distribution (DIST). In the last case spectra can be presented by superposition of up to 50 sextets. A contribution of every sextet is defined by finding least-squares deviation of the fitted spectrum with respect to the measured one (χ2). As a rule, the hyperfine field range is chosen quite wide. Therefore a step for change of hyperfine field between the adjacent trial sextets (∆) appears to be relatively large that might result in a loss of details in HFD. On the other hand, if one would wish to reveal a fine structure in HFD for some magnetic component separated in a particular Mössbauer spectrum it would be reasonable to apply all resources of the NORMOS code available for HFD analysis (up to 50 trial sextets) for describing it by superposition of trial sextets within a narrow hyperfine range. In this way one might set a comparatively small step ∆ that would allow revealing a plausible fine structure in HFD. But for all that a question arises if the program NORMOS can generally distinguish two closely spaced narrow peaks in HFD. If so, what restrictions should be imposed to the defined parameters? If one takes into account overlapping of lines of the 57Fe source and absorber, then a natural linewidth in Mössbauer spectra of pure iron can easy be estimated as ~0.2 mm × s-1. A linewidth observed in our measurements typically was about 0.35 mm × s-1 that is by factor about 1.8 larger than a total natural linewidth. This permits to suppose that the spectra may be a superposition of several sextets. The proper hyperfine field characterizes every sextet. The linewidths increase to some extent due to instrumental effects but they are evidently kept for every sample examined in standard conditions. Different mathematical approaches to extract hyperfine fields from Mössbauer spectra are considered in reviews [212–215]. The NORMOS code summarizes positive results of the previous theoretical works. It permits, in particular, to introduce an arbitrary smoothing parameter λ; and to evaluate a goodness of fitting with the Hesse-Rübartsch criterion [216]. As noted above, a feature of the applied fitting procedure to reveal a fine structure in HFD for some magnetic component separated in a particular Mössbauer spectrum was that all resources of the NORMOS code available for HFD analysis (up to 50 trial sextets) were applied to describe it by superposition of trial sextets within a narrow hyperfine range. For this one set in advance a magnitude of the hyperfine field for the first trial sextet (Hoi), its step pro subspectrum (∆), a linewidth (l) as well as areas of lines 1 and 6 referenced to lines 3 and 4 (A13). A magnitude ∆ was usually set equal 0.1-0.2 T so that it would be less than the expected space between peaks in HFD (0.5-1.0 T). In all cases the smoothing parameter λ was taken equal zero because, for only under such condition, it is possible to separate adjacent, closely located peaks in HFD. The latter has been proved by direct testing and agrees with a work of Le Caer and Dubois [217]. In addition, one set the initial values of the isomer shift, quadrupole line splitting and areas of lines 2 and 5 referenced to lines 3 and 4 (A23). When fitting the spectra, magnitudes of isomer shift and quadrupole splitting, A23, isomer shift change pro trial sextet were optimized and a least-squares deviation (χ2) of the fitted spectrum with respect to the measured one was defined. A fitting quality was also



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monitored by the Hesse-Rübartsch parameter (HR). To attain the best fitting, the program was run several times with different values of linewidth and A13, with the initial value of the linewidth being set to 0.25 mm × s-1, i.e. approximately a double value of linewidth of source and absorber, and A13 = 3.0 specific to an isotropic sample. The optimization procedure was completed when the absolute minimal χ2 and HR values had been obtained. Fittings with much larger χ2 and HR values were rejected. A similar application of the NORMOS code was briefly described before [218–222].

8.2.2. Discrete Hyperfine Fields in a Massive Substance and Aerosol Particles The given technique was applied to pure Fe[218, 219] and FePd [220], Fe-Co [221], and FeCr [222] alloys. In all cases there was a discrete HFD at Fe nuclei. To establish if the NORMOS program operates correctly, its testing has been carried out for a thin Fe foil applied for the set-up calibration [219]. Probation of the code operation was carried out by four ways: 1) effect of a smoothing parameter on the HFD shape with changing a value of λ of zero to 0.1; 2) variation of a step for change of hyperfine field between the adjacent trial sextets (∆) within 0.05-0.5 Т; 3) change of the hyperfine field value for the first trial sextet (Hoi) of 30.5 to 31.55 Т; 4) change of the trial sextet linewidth (l) of 0.1 to 0.25 mm × s–1. It has been found out that HFD for Fe foil shows three main completely separated peaks (Figure 60, а). The central peak is related to the field of 32.9 Т and two others 32.44 and 33.60 Т. There were no oscillations of the base line specific for the initial Hesse-Rübartsch method. The location of these peaks did not change when varying parameters Hoi and ∆ within the above given range. However, decrease of l of 0.25 mm × s–1 to 0.1 mm × s–1 has resulted in appearance of unrealistic “paling” corresponding to very large values of HR = 54.18 and χ2 = 70.23 (Figure 60, b). Three peaks (Figure 60, a) are transforming to one peak in HFD (Figure 60, c) when applying even a weak magnetic field (0.03 T). With increase ∆ of 0.05 to 0.5 T these three peaks are gradually broadened and finally merged (Figure 61). The influence of λ on HFD shape in iron foil has been also examined in [219]. Three main peaks were well separated at λ = 0 and λ = 10 –5 (Figure 62). However with increasing the smoothing parameter by two orders (λ = 10 –3) two overlapped humps appeared. At λ = 10–2 a degree of overlapping of humps was growing. Finally, at λ = 10–1 only one wide hump remained. Note that within the whole range of change λ the quantities χ2 and HR retained and a choice of λ = 0 was preferred as it permitted to reveal a discrete structure in HFD. Comparison of Mössbauer spectra fitted using the SITE subprogram for different Fe foils made it apparent that the isomer and quadrupole splitting are small and close to each other. It is worth noting that for every foil the average hyperfine field at Fe nuclei was accepted equal to 33.0 T. Nevertheless, due to different thermal and mechanical treatment of the foils the sextet lines somewhat vary in width and intensity relationship. The latter, as known, is caused with the direction of the foil magnetization. When the foil is demagnetized, there is an intensity ratio of 3:2:1:1:2:3 for absorption lines. When the magnetization is in the foil plane, then there is a 3:4:1:1:4:3 ratio, and when the foil magnetization is parallel to gamma-ray, the 2nd and 5th lines generally disappear. Other orientations of the magnetization vector with respect to the foil plane result in different ratios for the absorption lines.


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Figure 60. HFD for Fe foil spectrum at different linewidths of trial sextets (a), (b) and in an external magnetic field [216]. (a) l = 0.25 mm × s–1, ∆ = 0.1 T, HR = 7.277, χ2 =9.354; (b) l = 0.10 mm × s–1, ∆ = 0.1 T, HR = 54.18, χ2 =70.23; (c) l = 0.26 mm × s–1, ∆ = 0.1 T, HR = 5.475, χ2 =7.031.

Depending on prehistory of Fe foils and by applying external magnetic field there were also spectra for which the HFD contained only one or two peaks. For example, the foil having initially 3 peaks in HFD (Figure 60, a) showed only one peak when the sample was subjected to a small external magnetic field of 0.03 T within its plane resulting in a drastic increase of the intensity for the 2nd and 5th lines in the sextet (Figure 60, c). One may say that the multidomain structure of a foil leads to a set of peaks in HFD whereas the magnetic saturation in an external field transforms the foil into a single-domain state which is characterized only by one peak in HFD.



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Figure 61. HFD for Fe foil with different field step pro subspectrum [216]. (1) ∆ = 0.05 T; (2) ∆ = 0.1 T; (3) ∆ = 0.5 T.

Figure 62. Smoothing parameter λ dependent HFD for foil [216]. Hoi = 30.48 T; ∆ = 0.1 T; l = 0.29 mm × s–1.m (a) λ = 0; (b) λ = 10–5; (c) λ = 10 –3; (d) λ = 10–2; (e) λ = 0.1.


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Unlike Fe foil HFD for small 13 nm aerosol Fe particles at room temperature always showed only one peak though the linewidth in spectrum was somewhat larger than in a case of Fe foil (Figure 63) [218, 219]. The similar testing was carried out also in this case by varying parameters Hoi, ∆, and λ.

Figure 63. Mössbauer spectra of small (~13 nm) aerosol Fe particles at different temperatures [216]. (a) T=293 K (l = 0.21 mm × s–1, ∆ = 0.01 T, HR = 1.484, χ2 = 1.902); (b) T=293 K (l = 0.39 mm × s–1, ∆ = 0.1 T, HR = 1.877 χ2 = 2.407); (c) T=80 K (l = 0.30 mm × s–1, ∆ = 0.12 T, HR = 2.085 χ2 = 2.727); (d) T=4.2 K (l = 0.34 mm × s–1, ∆ = 0.12 T, HR = 0.8188 χ2 = 1.069).

The results exhibited that a quite broad sampling of these parameters did not change a HFD pattern similar to that for the Fe foil. Meanwhile, HFD for small Fe aerosol nanoparticles had a number of peculiarities. First of all, HFD extracted from their room temperature Mössbauer spectra showed always only one peak [218, 219]. The peak was



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constituted by, at least, two adjacent trial sextets separated by ∆ = 0.1 T. This suggests the possibility of further reduction of ∆. Changing a step of trial sextets from ∆ = 0.01 Т to ∆ = 0.5 Т when fitting the room temperature Mössbauer spectrum for small Fe particles gave rise to the following results. For describing this spectrum at ∆ = 0.01 Т it has taken only one trial sextet with H ≈ 32.6 T. The latter is apparently close to the actual field in the sample. With increase ∆ the code selects several trial sextets for better fitting the Mössbauer spectrum, with the mean field keeping the value 32.6 T independently of change Hoi between 20.1 and 32.4 T. The fitting goodness in all cases is approximately the same and quite high. Since an increase ∆ results in farther removal of the adjacent trial sextet fields from the actual one in the sample, the code has to vary the available parameters including a contribution of the trial sextets. As a result, the peak in HFD respective to these trial sextets is broadening. Such spread has artificial character. To get idea concerning influence of variable parameters on two close peaks in HFD it is necessary to fit the low temperature Mössbauer spectra of Fe (Figure 63). HFD extracted from the spectra taken at 80 and 4.2 K have shown two individual intense peaks. The location of the peaks (Hi = 33.9 and 34.9 T) was kept invariable with decrease of Hoi (see curves 1-3 in Figure 64). These curves are characterized with the good fitting quality that evidences the values of HR (0.8085–0.8432) and χ2 (1.056–1.101).

Figure 64. HFD for small Fe particles (4.2 K) at different Hoi (∆ = 0.15 T). (a) Hoi = 30.08 T (λ = 0, l = 0.34 mm × s-1, HR = 0.8241, χ2 = 1.076); (b) Hoi = 25.3 T (λ = 0, l = 0.31 mm × s-1, HR = 4.707, χ2 = 6.160); (c) Hoi for curves:1) 33.7 T (λ = 0, l = 0.33 mm × s-1, HR = 0.8085, χ2 = 1.056); 2) 30.08 T; 3) 27.7 T (λ = 0, l = 0.36 mm × s-1, HR = 0.8432, χ2 = 1.101); 4) 25.3 T

When a hyperfine field range of trial sextets is leaving the region where intense peaks are located (curve 4 in Figure 64), the code tries to compensate their loss by emergence of several new peaks being not relevant to the reality. This case is accompanied with a bad agreement


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between calculated and measured Mössbauer spectra (Figure 64, b). The mismatch is characterized by large values of χ2 = 6.160 and HR = 4.707 at which the results of fitting procedure were rejected.

Figure 65. Effect of value ∆ on HFD shape extracted from 4.2 K Mössbauer spectrum for small Fe particles. (a) 0.05 T (λ = 0, Hoi = 33.3 T, l = 0.36 mm × s-1, HR = 0.8414, χ2 = 1.099); (b) 0.15 T (λ = 0, Hoi = 30.3 T, l = 0.34 mm × s-1, HR = 0.8204, χ2 = 1.071); (c) 0.5 T (λ = 0, Hoi = 22.0 T, l = 0.33 mm × s-1, HR = 0.7795, χ2 = 1.012); (d) 0.8 T (λ = 0, Hoi = 15.0 T, l = 0.31 mm × s-1, HR = 0.7593, χ2 = 0.9917).

Figure 65 presents a change of HFD pattern when ∆ grows from 0.05 Т (Figure 65, а) to 0.8 Т (Figure 65, d). Note that a space between two peaks remains equal about 1 T. At the same time a number of trial sextets within this range (1 T) diminish with increase of ∆. At



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∆ =0.8 T (Figure 65, d) the both peaks fall on two adjacent trial sextets and look like as if they would merge with each other. To attain the best fitting of Mössbauer spectrum (Figure 64, a) the code selects the appropriate parameters and the contributions of three nearest trial sextets. Note that with change Hoi from 33.4 to 15 T (Figure 64) the fitting goodness of the simulated and experimental spectra appear very good with a slight variety of χ 2 and HR (see captions to Figure 64). Meanwhile, at ∆ = 0.8 T there is a loss of information when two actual hyperfine fields cease to be distinguished (Figure 65). The influence of the smoothing parameter λ on a shape of HFD for the 80 K Mössbauer spectrum of small Fe nanoparticles is shown in Figure 66. It is seen that two peaks are clearly separated at values λ of zero to 10 –3 whereas at λ ≥10 –2 the peaks merged with each other. In all cases the high fitting goodness has been attained (see captions to Figure 66) but two peaks with ∆ =1 T appear to be separated well only at λ = 0.

Figure 66. Smoothing parameter λ dependent HFD (right) extracted from 80 K Mössbauer spectrum (left) for small Fe particles (Hoi = 31.47 T, l = 0.31 mm × s-1, ∆ = 0.15 T). (a) λ = 0 (HR = 2.085, χ2 = 2.727); (b) λ = 10 –3 (HR = 2.088, χ2 = 2.731); (c) λ = 10 –2 (HR = 2.092, χ2 = 2.736); (d) λ = 0.05 (HR = 2.095, χ2 = 2.740).

The lines of sextet in the Mössbauer spectrum of Fe nanoparticles appeared to be asymmetrically broadened at the base (Figure 63) [218, 219]. Jing et al. [223], examining Fe particles of 6 and 10 nm at temperatures below 75 K, supposed that such broadening was caused with the presence of two components, one of which was related to the particle core with the hyperfine field Hi = 34.2 Т and the other was ascribed to two surface particle layers with a larger hyperfine field (Hi = 35.9 Т for 6 nm particles and Hi = 35.8 Т for 10 nm ones). The Fe particles under study were suspended in silicon oil or paraffin and this fact, according to the authors’ opinion, did not allowed one to interpret unambiguously the nature of the small increase of hyperfine fields. The authors [223] supposed that this was due to difference of electron localization inwards the particles and at their surface. Meanwhile, with cooling small aerosol Fe particles to 80 K and 4.2 K, there was simultaneously, in addition to appearance of two peaks (Hi =33.9 and 34.9 T), development of sextet in spectra related to


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Fe3O4 (a respective hyperfine field Hi = 46-49 T) (Figure 63, с,d) [218, 219]. In this case one may suppose that one of two peaks (Hi = 34.75 T) is caused with interaction between surface Fe atoms and oxide shell of the nanoparticles because this peak disappears after warming up the particles of 4.2 K to room temperature when magnetic oxide Fe3O4 becomes again superparamagnetic [195, 218, 219]. Though the hyperfine field in the core of Fe nanoparticles (33.9 Т) appeared close to that (34.2 Т) given in [223], the hyperfine fields Hi = 35.8 Т and Hi = 35.9 Т attributed in [223] to two surface layers of particles were not observed. Basing on the results obtained for Fe foils and nanoparticles one may conclude that the increase of step ∆ between trial sextets to about 0.5 – 0.8 T and smoothing parameter λ to 0.1 results in a complete smearing of detailed structure in HFD at 57Fe nuclei. Note that the suggested technique to elicit a fine structure of HFD is based not only on broadening lines in some Mössbauer spectrum but taking into account a characteristic shape of the lines. This, as well as the possibility to detect quite weak (230С and high-ohmic resistance independent of temperature at Т < 230С. Simple production of photoconductive films by precipitation of aerosol CdS and CdSe particles without the laborious procedure for their sensitizing opens up wide possibilities for their application. In particular, pilot samples of photoresistors and vidicon targets were produced [224]. Evaporation of copper and silver in the tetrafluoroethylene atmosphere in the glow discharge gives rise to the formation of Teflon films on the vessel walls that can be used for covering some component part by Teflon [230].

Figure 67. Chains of aerosol Fe particles prepared in Ar at pressure 160 Pa [226]. A magnetic field of 1000 Oe is applied in the collodion film plane. The chains of particles hang in vacuum beyond the broken film.

ACKNOWLEDGEMENT The authors express their sincere gratitude to O.V. Skvortsova for the preparation of illustrative material to this review.


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