Combustion,Explosion, and Sltock Waues,VoI. 47, No. 6, pp. 639456, 2005
High-Temperature Combustion Synthesis: Generation of Electromagnetic Radiation and the Effect of External Electromagnetic Fields (Review) I. A. Filimonovl
and N. I. Kidin2
138 uDc 536.46 I 541. I 541.135.4
Tlanslated from Fizika Goreniya i, Vzryua,Vol. 41, No. 6, pp. 34-53, November-December,2005. Original article submitted March 30, 2005. A critical review of recent papers on mechanisms of generation of internal electrofields on self-propagating magnetic fields and the action of external electromagnetic high-temperature synthesis of heterogeneous systems is presented. Generation of an internal electromagnetic field is caused by different rates of diffusion of charged dereactions. fects through the layer of the growing product in strongly nonequilibrium of magnetic Possible emergence of residual magnetic fields is related to orientation fields of the synthesis domains in the arising internal thermal and electromagnetic wave. The external electromagnetic action is characterized by thermal, magneto- and electrodynamic, and kinetic factors. The thermal factor is caused by the Joule effect, whereas the electro- and magnetodynamic influence is caused by electromigration, of the condensed phase, magnetic compression and changes in electrical conductivity and ion wind in the gas in pores. The kinetic factor is caused by generation of superequilibrium charge carriers in condensed particles (defects) and by emission of high-energy electrons into the gas surrounding the particles. field. I{ey words: combustion, high-temperature synthesis, electromagnetic
INTRODUCTION It is well known [1-3J that the influence of an electromagnetic field on gas flames is exerted through three basic mechanisms: thermal, electrodynamic, and kinetic ones. As high-temperature synthesis, Iike the gas flame, is a diffusion-thermal wave, naturally, it involves the same mechanisms of the external electromagnetic action [4, 5]. The presenceof a large amount of the condensed phase and numerous interfaces in synthesis, however, introduces some specific features into this action. For instance, the thermal (or Joule) action of the external electromagnetic field in this case seems to be much more effective, because it is in the condensed phase that field absorption and the corresponding heating of the reacting mixture occur. Gases do not practically absorb llnstitute
of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka L 42432; fi I - limonov@hotmail. com. 2lnstitute of Problems in Mechanics, Russian Academy of Sciences, Moscow IL7526;
[email protected].
electromagnetic waves of moderate frequency, and the thermal effect of the field in the gas flame is manifested indirectly, via collisions with charged particles accelerated in the field. Vice versa, it has been believed until recently that high-temperature synthesis of many condensed systems proceeds with a negligibly small effect of the gas phase. Correspondingly, if the fraction of the melt is low and mobilities of charged defects in the condensed phase a.re small, the electrodynamic action of the external field can be expected to vanish in such s1stems. The action of the external electromagnetic field on the kinetic mechanism of high-temperature combustion synthesis should also be more effective than the action on the gas flame, because of a more developed reaction surface and the presence of numerous interfaces between the phases, which are the main sources and sinks of charge carriers in the condensed phase. In the present review, we do not mean to cover all papers published in this field, moreover, many of them were disclosed in the monograph [4] and revie* [5]. We only mention those papers that are of principal impor-
001G5082 /05/4L06-0639 @ 2005 Springer Science* BusinessMedia, Inc.
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tance and briefly formulate their arhievements and the questions left open for resea.rchersin these papers. The main attention will be paid to new researchespublished a.fter the last review in this field [5].
INTRINSIC
ELECTRIC
AND MAGNETIC FIELD Depending on the ionization degree, the internal electric field of the gas flame can strongly afiect its thermal and.difiusion-structue [6, 7]. In i weakly ionized flame [6], the Coulomb bonds'between fons and electrons a.reweak, the cha.racteristic Debye screening length is large as compared to the thermal scale, a.nd the chaxgesdiffuse independent of each other. Correspondingly, the intrinsic electric fie]d of such a flame can be found independent of the thermal a.nd diffusion fields in this flame. Vice versa, in a strongly ionized flame {7], the characteristic Debye length is- compa,ratively imall, ambipola.r difiusion Lf cnalges occurs, and the thermal-diffusion structure of the flame should be determined simultanmusly with its electric structl.lle. A simila.r situation is observed in combustion of a small particle of a metal, such as Zr or Ti, in o:cygen. In this case, the characteristic thermal scale is normally greater than the particle size, whereasthe Debye screeninglength is estimated in [8] as t0-2 to t0-l of the particle radius. As a result, w; ;an naturally expect that the intriasic electric 6.eldof such a paxticle is self-consistent and exerts a strong efiect on the thermal and concentration fields irside and around the pa.rticle. The same conclusion refers to synthesis of a la.rgemass of a powder or a pressedsample. Therefore, the study of intrinsic electric and magnetic fields developing during high-temperature combustion synthesis seemslo be I'ery importa,nt.
Generation of Magnetic Fields Apparently, the intrinsic magnetic field was registered ior the drst time Uv , .t"iar"J ioa*il |.';netomet€r in products of higrt-t.*pai"t*" ryrtL*i. lr ferrites 19, 10]. Owing to re-grouping of domains in the sample, a rather powerful (r10 pT) residual field is formed, which is independent of the sample orientation in the magnetic field of the Ea.rth. The use of a highly sensitive superconducting quantum interference device (SQID) made it possibleto find [11-14] e:cimum transport velocity of gaseousoxygen to the particle surface affects the burning time and the distribution of charge carriers at the late stage of oxidation. Lower characteristic velocities favor an increase in the maximum potential and burning time. Calculations by the full nonisothermal model [27] allowed understanding the relation between the generated voltage pulse and the current particle temperature
(Fig. ). It turned out that a change in thermophysical pa^rameters exerts a significant effect on the mutual dynamics of the electric pulse and/or particle temperature in the course of oxidation. For instance, all Ieads not only to an inincrease in the ratio D+IDcrease in the pulse amplitude and to a decreasein the burning time, but also to a decreasein the delay of ap peaxance of the temperature ma>cimum after generation of the maximum electric potential on the particle surface. An increase in heat-transfer coefficients (radiative and/or convective) also decreases the delay though simultaneously decrea^sesthe maximum potential being generated. According to [27], the equilibrium concentrations of charge carriers are determined by the difference between the activation energies of adsorption and desorption processes. The calculations of [27] suggested an important conclusion that the surface concentrations of charge carriers reach equilibrium rialues by the moment the maximum particle temperature ir reached. Equilibrium values inside the particle and al the metal-oxide interface are reached much later'
EXTERNAL ELECTROMAGNETIC ACTIOI\ Thermal
Effect
The overwhelming majority of technologies of high temperature combustion synthesis, which include exter nal electromagnetic actions, involve the thermal (Joule effect. The fact that the currents and powers of electri heating are extrenaell'trigh and almost cancel the mai:
High-Temperature
Combustion
Synthesis
(Review)
advantage of self-propagating high-temperature synthesis (low energ"ycosts) is absolutely ignored. As the estimates of [33] show, for an electric current density of ry103-104 Af cm2, the Joule heat-releaserate becomes commensurable with the heat-release rate in the reaction in the wave front and has a significant effect on temperature and synthesis-wave velocity. Under these conditions, the synthesis becomes much more energyconsuming, which restricts the practical application of the above-mentioned technologies. Thus, in the electrothermal explosion (ETE) method [34-38], currents of the indicated density are used to heat the examined samples up to spontaneous ignition and sometimes even to a higher temperature (the heating time is =10-2 to 10 sec). Though the chemical heat-release rate at the final (quasi-adiabatic) stage can substantially exceed the intensity of the Joure heating, the electric energy spent becomes significant already at the stage of inert heating. The ETE method is used to obtain refractory materials [39], intermetalIides and iolid alloys [40, 41], and. ceramics and cermets [42], as well as to perform SHS welding [48-4bJ. The technology of SHS welding is used for wear-resistant cermets [43] and for application of coatings onto metallic substrates [46]. In all these applications, direct nonIocalized transmission of current allows rapid heating of the samiole before the beginning of the reaction and substaur:an speedup of the technological process of obtarn,iao cbe .final product. It should be always borne in' nund, how-ever, that the use of ETE technologies requu€ hrigher energy costs (=100 kUg) than the traditional SHS. Currents used in electric pulse thermosynthesis (EPTS) are not lower than those used in the ETE method 147-491. Owing to a very short period of the electric action (=t0-a sec), nevertheless, EpTS ensures significant saving of energy used; the energy costs are predicted to be very low (tO-s to 10 J/cms). Because of such low energy costs, EPTS can be used to synthesize powders and fails to synthesize compact and wellpressed materials. The reason can be easily found: this is an increase in the number of conducting contacts on the reacting particle, which not only decreasesthe electric current through an individual contact (locar Joule heat release) but also favors a faster sink of heat from the reacting particle. Based on the data of [50], the electrical conductivity of titanium and, hence, the number of contacts per one particle in densely compressed compact samples are much higher than the corresponding quantities in powders of bulk density. Therefore, the specific feature of EPTS mentioned above seems to be fairly reasonable. It is assumed in [b, b1] that low expenditures of electric energy indicate that EpTS has
645 a non-thermal nature and that the charge transfer in gas pores makes a significant contribution. The EPTS nature can be finally elucidated in special experiments, which would allow a comparison of the amplitudes of local electric currents in the gaseous and condensed phases of a reacting powder mixture. In addition to the time constriction, the EPTS electromagnetic action is also characterized by spatial localization of currents used, at least, at the scale of one particle, i.e., at the microscale. A similar specific feature at the macroscopic scale (at the scale of the heating zone in the SHS wave) is typical of the Field Activated Combustion Synthesis (FACS) method 152-541developedas a method for supporting high-temperature combustion synthesis in weakly exothermal compositions. The authors of the method put into practice the idea of [33, 55] about macroscopic localization of transmitted current in the vicinity of the hot front in the case of transverse superposition of the electric field with respect to the combustion-wave propagation direction and the energy benefits associated with this localization. The possibility of providing such localization is ensured by a drastic (by several orders) increase in conductivity of the reacting mixture with increasing temperature of the mixture. In terms of energy, the FACS method requires higher expenditures than EPTS, but it is substantially more beneficial than ETE. Despite the relative benefits, the density of current used in FACS remains at the ETE level (=103 A7cm2), ild the intensities of the Joule and chemical heat release in the front are of the same order. It has been discovered recently that, in addition to the purely thermal effect, the FACS method can be used to modify the material properties, a.ffect the composition [56-60] and size of crystallites in synthesis products [57-59], and alter interaction between the phases [56]. Moreover, in combination with mechanical activation of the raw mixtures and application of uniocial pressure in the course of the process, the FACS method allows [58, 59] the synthesis of dense nanometric materials with the grain size smaller tha.n 100 nm. The dynamics of temperature, current, and pressure for typical synthesis conditions [59] is shown in Fig. 5. Mechanical activation was performed to obtain a mixture of initial powders with nanometric particles. The FACS method was used to synthesize rather dense samples. As a result, Gras et al. [59] managed to synthesize volume samples of MoSi2 with a dimensionless density of 82-93% and a grain size of 58-75 nm. In [58], a fine crystalline (30-55 nm) powder of TaC was obtained without mechanical activation on a usual FACS setup. It was found [fa] that the external electric field can have a significant effect on the size distribution of crystallites being formed (Fie. 6). In [56, 60], it was suggested that FACS can be used as an
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Filimonov
T,K 1400
I/10,A;
MPa 180 160
+T - - t/10 ,,.,.p
1200 1000
1q
.........:
800
120 100
600 400 200t 0
100
300
0 t, sec
500
Fig. 5. Dynamics of the mold temperature (?), alternating current amplitude (I110), and pressure(p) in the FACS process[59].
7 a o
O = 30.33Vlcm a
a
..'
a a a
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a
a
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29.95
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70 d,nm
Fig. 6. Sizedistribution of crystallites of the TaC powder synthesizedin [58] with different valuesof electric field strength (/ is-thb dimensionlessfrequencyof observanceof crystallites of the indicated size in the sample).
effective method of drastic termination of motion of the reaction wave. Obviously, a sudden and drastic termination of electric current supporting combustion leads to much faster termination of front motion, as compared to the traditional method of wedge hardening.
and Kidin
On the other hand, it was frequently noted [ ] that the results obtained in 152-54,56-60] are hard to analyze in terms of understanding the mechanism of the action. No measures were taken in those experiments to stabilize the electric parameters: currents, voltages, and resistances changed permanently during the synthesis and did not reach any steady'state values. NevertheIess, based on these experimental results, a conclusion was drawn [56-50] about a dependence (in most cases, a linear dependence) of the synthesis-wave parameters on the applied voltage; no explanations were given why these and not other values of voltage were used in each experiment. Clearly, the reliability of such conclusions is doubtful and does not allow one to determine factors (apart from the thermal effect) of the external electromagnetic action associated with the phenomena observed. A numerical model of FACS was constructed in [60-64]. In this model, only the thermal mechanism of the action of the transmitted current was considered. The above-indicated drawbacks of FACS experiments, apparently, did not allow any meaningful conclusions from the numerical results [60-64]. The main conc]usion about current localization in the vicinity of the high-temperature synthesis wave front is fairly obvious and was predicted many years ago [33, 55]. The conclusion made in [6a] about the influence of the sample size and about the transition from the volume to the wave regime of synthesis in the case of large.size samples is also obvious. Generally, the theory of propagation of a hightemperature synthesis wave under conditions of an additional Joule heating by an external electric field originated in works on the influence of currents with a constant density and intensity [33, 55, 65, 66]. Two methods of application of the external field were considered: along the wave direction (with a constant current density) and across the wave direction (with a constant voltage); quasi-steady regimes of synthesis propagation, where the wave front has enough time to fit the time-dependent temperatures of the products and initial mixture, were examined. Stability of the quasisteady regimes was studied. In the case of low intensity of the Joule dissipation of energy in the front, a*scompared to chemical heat release, it turned out [33] that the temperature of the synthesis wave remains almost unchanged and is a weakly growing quadratic function of the current density or voltage, respectively. Yet, even a weak Joule action can significantly affect the coefficient of temperature sensitivity of the front-propagation velocity and, thus, stability of the combustion mode. It was found [33, 55, 65, 66] that, other conditions being identical, the Joule heat release localized in reaction
High-Temperature
Combustion
Synthesis
(Review)
products and/or in the front stabilizes propagation of the high-temperature synthesis wave, whereas the Joule heat release localized in the initial mixture initiates instability development and can Iead to the thermal explosion. The results of [33, 55, 65, 66] on stabi]ity are in agreement with the concepts of the classical theory of combustion [67, 68]. Two important conclusions were made: first, it is principally possible to control the synthesis, and such control does not require high powers of electric heat release, as in the ETE, EPTS, and FACS methods; second, the greatest effect of the action and, hence, the possibility of control occur at the Iimit of propagation of the synthesis wave. Like FACS, the Microwave Combustion (MICROCOM) method is based on the idea of macroscopic localization of the thermal effect from an external electromagnetic action [69-71]. In this case, however, the external action is applied in a contactless manner: through an inductor irradiating a powerful electromagnetic wave.. It was found [72] that the region of the non-reacted, weakly conducting initial mixture does not absorb such a wave; in the MICROCOM method, therefore, additional heating is mainly localized in the region of synthesis products and the combustion front. As a result, automatic localization of external heating (electric energy saving) and associated improvement of stability of combustion-front propagation occur simultaneously. An additional advantage of MICROCOM is a possibility of its more effective application in hotspot regimes of propagation (e.g., spin) and in regimes with a curved combustion front. The experiments of 172-741and the estimates based on parameters of these experiments showed [72] that zones of heat release due to chemical and electromagnetic sources are commensurable in thickness (=1 cm) and overlap in space. If we use the estimate of the skin-layer thickness [72], we can conclude on the basis of the data of 172-Talthat the density of induced eddy eiectric currents in the MICROCOM method is not lower than the current density in the ETE or FACS methods. The data on setup pov/ers 175-771 also indicate significant specific energy expenditures (=10 kJ/g). Another point is that significant saving of energy in the entire reacting volume is reached owing to localization of electromagnetic heating both in combustion zones (in the products and reaction front) and in the surface skin layer, as compared to ETE and FACS. The large induced current allows synthesizing very dense materials by the MICROCOM method. In theoretical research, in contrast to the abovementioned technologies, the external electromagnetic field and the heat release due to this field are mainly considered as control factors (with a lower dimensionless intensity) rather than as the main sources of heat.
647 The conclusion drawn in [33, 55, 65, 66] about the greatest controlling Joule effect at the limit of propagation of the synthesis wave initiated the study [78, 79] of the spin combustion mode heated locally by external electromagnetic radiation of moderate power. Radiation localization was caused by an exponential growth of conductivity with temperature owing to dissolution or evaporation of oxide films from the medium particles. It turned out [79] that, owing to the high sensitivity of the spin to the heat flux toward the sample center and to the possibility of easily decreasing this heat flux (providing its moderate magnitude) under the thermal action of external radiation, it is possible to significantly expand the limits of spin combustion, substantially increase its temperature and velocity, and convert combustion to the steady-state mode. Unfortunately, wide possibilities of spin controlling have not found practical applications yet. Zakiev and Shkadinskii [80] and Zakiev [81] studied the thermal action of external electromagnetic radiation from a plane inductor, Iocalized in the reaction zone, on the high-temperature synthesis wave with a plane front. Similar to [78, 79], quasi-steady absorption of radiation by the reacting medium was assumed, i.e., a limited range of frequencies was considered. We should note the contradiction of the additive representation of conductivity [80, 81], which implies that the contributions to the total conductivity at an axbitrary point of the reacting medium are made by both the products and the raw mixture. It seems that each of these terms shouid be the limit of a certain single functional dependence for conductivity in the corresponding zone of the combustion wave. The universality of the conclusion drawn in [80, 81] on the stabilizing action of electromagnetic heating independent of whether the main Joule heating occurs ahead of the front or behind it seems also doubtful. Otherwise, it contradicts the results of [33, 55, 65-68] on the influence of additional distributed heat sources on stability of the plane combustion front. Zakiev and Shkadinskii [82J did not use the quasi-steady approximation and proposed a more generic approach, based on the splitting method, to the Macwell equations. They concluded that unsteady receptivity of the medium can change the position and width of the Joule heating zone in the combustion v/ave. One of the drawbacks of all theoretical papers [33, 55, 65-68, 78-82] is the use of the continuous medium approximation and consideration of only one phase, namely, solid condensed phase, in the description of combustion in powders and pressed samples. This is not always justified, especially if the motion of the gas phase or appearance of the melt, and also transfer processesin pores exert a significant effect on combustion. FYom the
648
Filimonov
viewpoint of the electric cha^rgetransfer proper, how_ ever) such an approach seems to be fairly admissible, at least at the initial stage of research. The reason is that the electric currents in the gas phase experimentaily found [20] during combustion are approximately two orders of magnitude lower than the currents in the condensed conducting particle. This fact is supported by the estimates of typical concentrations of cha,rgecarriers in the gas [83] (=1010 to 1013 cm-3) and, e.g., metal oxides [84] (=1015 to L01ecm-3). Therefore, the approach used in [33, bb, Ob-68, Z8-82], which involves a continuous single-phase medium, nevertheless, covers the transfer of the overwhelming majority of electric charge carriers and takes into account the most important (thermal) effect associated with this transfer.
and Kidin
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Magneto-
and Electrohydrodynamics
The iufluence of a constant external magnetic field on the combustion synthesis was first observed in [gb] Iong ago. Since that time, various effects caused by the action of the field have been observed [8b-92]: structuring of the product, changes in its composition, magnetic properties, and burning rate, increase in front temperature and conversion depth, expansion of flammability limits, etc. Nevertheless, the decrease in the burning rate for some systems, noticeable structuring in densely packed samples, and some other effects are hard to explain from the viewpoint of formation of conducting chains of particles and increasing effective thermar conductivity in the magnetic field [8b]. On the other hand, nev/ papers have been published, where some of these difficulties have been resolved. Thus, the theoretical studies [93, 94] demonstrated that the induced or directly applied electric current exerts a significant effect on phase equilibrium and dynamics of phase formation in the reacting medium. In the absence of the Joule effect, the action of the current reduces to changing thermodynamic pressure in the conductor and/or induction when a new phase appears. The first factor is caused by the changes in the pressure itself owing to the field energy (magnetic compression), and the second one is caused by the appearance of ponderomotive forces generated by the difference in conductivity in the existing and new phases (os and a;). If the density changesinsignificantly in the course of the phase transition, like that in the course of melting, magnetic compression has almost no effect on the temperature of such a transition. If the change in density is substantial, Iike that in the course of evaporation, the contribution of compression is commensurable with the contribution of the ponderomotive forces to changes in the phase-
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Fig. 7. Shift in temperature in the phase.transition front versus the volume fraction of the hightemperature (e < 1) and low-temperature (ae > 1) phases[9a] (a and b, respectively).
transition temperature. Depending on the relation between the conductivities of the phases (* - o y f o s), the ponderomotive forces can either increase or decreasethe phase-transition temperature ?$ ("$ > ?o or Tfl < To) (see Fig. 7, where ?o is the initial temperature of the medium, ,\ is the latent heat of the phase transition, pis the characteristic pressure of ponderomotive forces caused by transmission of the electric current, and ue is the specific volume of the initial phase) and affect the stability of the existing phase. For instance [94], there is a range of parameters where both phases become simultaneously metastable under the action of weak current (FiS. 8). In addition to the ohmic decreasein voltage in the conductor, the voltage on the varying inductance .L also decreases(Fig. 9). If the difference in conductivities of the phases is rather large, the decreasein voltage on the inductance can be of the same order as the differ-
High-Temperature Combustion Synthesis (Review) h
649 c2L^L/l 1.4
1.2 1.0
&
1) a = 0.60 2) e = 0.4O q a = 0.20 q e=0.05
.
\
0.8
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0.6
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'/ 9-'
0.4
2
0.2 Fig. 8. Diagram of coexistenceof the phasesin the caseof weak electric currents A: 1) region of simultaneous metastability of both phases;2) region of the stable positive phase;3) region of the stable negative phase; the curves h+ and h- indicate the binoidal lines of the phase transitions * * - and - =+ +, respectively[94].
ence in potentials on the ohmic resistance R (Fig. 10). In this case, it may turn out that the major part of the conductor volume B is already melted at a temperature Iower than its melting point (ZF < ?o), and the melt phase exists in the form of a stable local small region of the melt inside the bulk of the conductor (see Fig. 7a). As the thermal and electrical conductivities of metals are normailly much higher than the corresponding quantities in the melt (at the same temperature) [8 ], the effective thermal and electrical conductivities of the conductor decrease as soon as the rnelt appears. For example, the conductivity of the melt of Ni is almost two times lower than the conductivity of the solid phase. Therefore, under the action of electric current, up to 60% of the nickel particle can be melted at a temperature substantially lower than the temperature of bulk melting equal to 1455"C (seeFig. 7a). It is the decrease in effective thermal conductivity of the mixture because of partially melted nickel that can be responsible for the decreasein the burning rate of the Ni + AI mixture with rather coarse nickel particles, which was observed in [85]. A decrease in the local melting point and possible appearance of a large number of local zones of the melt can be also responsible for structuring in densely packed samples [85]. Bertolino et al. [95] considered the influence of a constant (=103 Alcm2) electric current on mass transfer in the condensed phase (the so-called phenomenon of electromigration). A multilayer Al*Au system reacting by the mechanism of solid-phase diffusion wa^sexamined. To eliminate the influence of the Joule heating and separate the effect of temperature from the effects
0
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1 )E = 1 . 5 2) & =5.0 3 )a = 1 0 4).8=20 0.6
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Fig. 9. Induction of the current-carryingconductorversus the fraction of the high-temperature (e < 1) and low-temperature(e > 1) phases[9a] (a and b, respectively); A,L is the change in induction owing to the phasetransition, I is the conductor length, and c is the velocity of light.
(R+L)/R 1.5 1.0 0.5 0
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0.8 p
1.0
Fig. LO. Parameter (R + AL/AI)/R in the currentcarrying conductor as a function of the volume fraction of the melt [93].
650
Filimonov
5,
100
*Au r- Au5,Al2 '*.- AuzAl .x' AuAl+AuAl2 0 6.0 d', pm
0
7.5
2
4
6
t,h g.o
t,h
Fig. 11. Evolution of intermetallide phasesfor the sam_ ple annealedat T : 723 K [gS]without current (a) and with current of density 1.019. 103 A/crn, (U).
associated with current transmission through the interface, a special setup was developed, which allowed the reaction to proceed at the same temperature with and without current application. significant accereration of phase-formation processes in the Al-Au system under the action of the current was observed (Fig. 11) [gb]. Yet, the current did not change the sequence of phase formation in the system, and the acceleration observed was independent of the electric field direction. Correspondingly, Bertolino et al. [g5] concluded that the electric field made an insignificant contribution to the diffuse flux of charge carriers and exerted a substantial effect on formation of defects. unfortunatery, the experiments of [95] were conducted at only one, rather high temperature and for very high current densities actually corresponding to the conditions of the ETE or FACS methods. Under conditions of such powerful electric fields, the limiting stage of charge transfer in the system can be generation of new defects in the volume of the phase being formed rather than diffusion of the already existing defects. In this case, naturalry, the flux of charge carriers formed is independ.ent of the field direction. Experiments performed. at red.ucedtemperatures and substantially Iower values of the external
and Kidin
electric field, if successful, could confirm the assumption made in [95] about the kinetic action on synthesis at rather high electric fields and temperatures. At the moment, this assumption is indirectly evidenced by the results of [96, 97]. Chen et al. [g6] found a dependence of the electric action effect on temperature and current direction in an Ag + Sn mixture. Asoka-Kumar et al. [97] considered the dynamics of formation of vacancies in a two-layer Al-Cu system under the action of a constant current applied (8 . t04 A/cm2). The irz situ depth-resolved positron annihilation spectroscopy (PAS) revealed a more than tenfold increase in the concentration of vacancies (up to 4 . 1018"*-3) at 2T5oC and allowed estimation of their formation energy. A theoretical study of electromigration effects in a diffuse pair (two plane metallic layers with a common boundary) [98] and sets of pairs [gg, 100] revealed a significant effect of the external field direction on charge diffusion in the growing intermediate phase (intermetallide). Yet, the results of [99, 100] should be treated rather cautiously. The assumptions of a uniform temperature in the diffusion pair and an identicai constant strength of the field in all pairs have to be severely justified and are invalid in practice for most systems. The theory developed in [33, 55, Gb*08,Z8-82] primarily refers to purely thermal regimes of combustionwave propagation. Results obtained in these regimes should be applied to filtration combustion with certain caution (only with the Semenov number Se (( 1 and filtration Peclet number Per ) 1 [101, 102]). Apparently, the theoretical problem of the influence of an external electromagnetic field on the filtration combustion regime will have to be solved in the near future. In addition to the thermal effect, the issues of hydre and gas-dynamics under conditions of an external field will be very important. At the moment, oniy some experimental results and estimates are available L23,241. Among others, the conclusion of [23] about the importance of charge transfer through the gas phase and through the pores in the mixture seemsto be rather disputable if we recall the following facts. First, in accordance with experiments on oxidation of single metallic particles [20], electric currents in the gas phase (=1 mA) are substantially lower than currents in the condensed conducting particle (=100 mA). Second, the estimated current density for the condensed phase on the basis of the data of [20] (J >- 1-10 Alcm2) is at least two orders of magnitude higher than the values found in [23]. Third, as v/as noted above, typical concentrations of charge carriers in metal oxides [8a] a"re again at least two orders of magnitude higher than the concentration of charged particles in the gas surrounding the burning sample [83]. Therefore, the main charge transfer
High-Temperature
Combustion
Synthesis
(Review)
still proceeds through the condensed phase. Obviously, observance of these or other processesin the course of combustion does not give grounds to judge about their importance for the synthesis wave and the mechanism of its propagation. For instance, the fact of afterburning is weil known, but it is also well known that this phenomenon does not exert any significant effect on combustion characteristics. An attempt was made l2{l1,o explain the effects observed in [23] by convective motion of the gas filtered in pores, which entrains charge carriers, over the reacting powder. Smolyakov et al. [24] suggested that there are several factors that support the validity of the assumption on transfer of charged particles by the gas flow in pores. The main reasons are as follows: the difference in potentials in combustion of heterogeneous systems with condensed products arises on a scale greater than the heating-zonewidth (=tO-t cm) and the initial powder mixture is practically electro-non-conducting at the macroscopic level (at the scale of the combustion wave). Though the first fact is difficult to argue, there are data contradicting the assumption of [24] about the importance of gas fi.ltration in terms of charge transfer. An example is generatlon of large currents and voltages [20] in single metallic particles 0.5-0.8 mm in diameter during their combusti.on in oxygen. It makes no sense to speak about filtration for such particles, and the registered signals exceed the voltages and currents generated in conobustlon of the same metals in a powdered form 122,26-.. The second fact contradicts the data of Smolyakov et al. themselves [24]. Indeed, in accordance w"iih ihe estimate of [24], the ma>rimum electrical conductivity of the gas (with a typical concentration of electrons [83] of =1010 to 1013 cm-3 and a minimuul pressure of =1 torr) is at a level of =5 . 10-4 to 10-1 O-1. cm-l. This does not exceed the electrical conductivity of moderate'quality semiconductors, Ieaving aside metals. At atmospheric pressure, the corresponding estimate decreasesby another three orders of magnitude. Hence, there are no reasons to believe that a significant portion of electric current is conducted by the gas.
Kinetic
Factor
As was already mentioned, it is often difficult to distinguish one effect of the external electromagnetic action from another in an experiment. This refers not only to the Joule heating or to hydro- and gas-dynamic influence of the field but also to the kinetic effect of the action. An individual study of this effect requires special (rather severe) test conditions. In the absenceof
651 these conditions, the kinetic action is manifested only indirectly (via hydrodynamics) and is complicated by the Joule effect. There are few works where these special conditions are ensured; studies that indirectly testify to the kinetic effect of the external electric field are more frequent. We can mention the papers [103, 104] on conditions of initiation of high-temperature synthesis of intermetallides by external corpuscular fluxes and on the characteristics of charged particles emitted by the condensed phase in the course of the reaction. The experiments of [103, 104] showed that the temperature of spontaneous ignition of the Ni+31.5% Al mixture decreasesto 420K under bombardment by an external flux of electrons from the zone of a specially organized glow discharge. At the same time, spontaneous ignition is not observed within the entire examined range of temperatures (up to 670 K) under bombardment by positive ions from the same zone. The effect of the decrease in temperature of spontaneous ignition is recovered if field-accelerated emission electrons from another burning sample are used and is expiained in [103, 104] by the influence of bombardment on the kinetics of the synthesis reaction under conditions of the powder contact with the glowdischarge plasma. The necessary energy expenditures are ry6.4 . 10-7 to 10-a J f cm3, which is lower than energy consumption in EPTS, which is the most economical existing technology of high-temperature combustion synthesis. Based on the estimates in [104], the specific Joule heat release in these experiments corresponds to the upper boundary of energy costs in EPTS and is independent of frequency in the range examined. Therefore, the non-thermal effect of the alternating electric field on the burning rate is attributed in [104] io an increase in electron energy in the plasma in the pores of the reacting mixture and to a possible resonance with the process of electron emission into the pores. The composition and energy characteristics of the emission current v/ere determined in [103, 104] by the voltage-current curves of the electric probe (Fig- 12). Based on the registered current, the presence of free electrons in the emission flux was determined. Interpolation of the voltage-current curves with subse quent double differentiation of the interpolation function allowed obtaining qualitative distributions of emitted electrons in terms of their energy. The resultant distributions [103, 104] were multimodal and of a nonMaxwellian type, with a significant fraction of highenergy electrons (see Fig. 12). This was explained in [103, 104] by the non-thermal mechanism of formation of such electrons and by strong nonequilibrium of physical and chemical processesin the emission zone of the synthesis v/ave.
652
Filimonov
and Kidin
mA 2 I 0 -1 2
-1 I, mA 1.0
I, mA
60
Ee,eY
f (8")/f^n(8") 1.0
-0.5
-1.0 -1 50
-1 0 0
-50
0
u"v50
0
50
100
Ee,aY
Fig. L2. Voltag*current curves for the emission current and corrssponding distributions of electrons in terms of energy [103, 104]: the mixtures used are Ni+31.5% Al (1-3) and Mo* 10.2% B (4 and 5); curve 2 shows the static construction regime (po : 1.8 Pa); curves 1 and 3-5 show the dynamic construction regime (po : 100 Pa); curves 1, 4 and 3, 5 correspond to the times of 1.0 and 3.0 sec from the beginning of current registration, respectively; the asterisks indicate the points of the zero potential of the probe with respect to the undisturbed plasma; M is the calculated band of electron energy under equilibrium conditions.
Though the Joule dissipation of energy was at a low Ievel, its effect in the experiments of [103, 104] cannot be completely neglected: in initiation of the reaction by particle fluxes, the temperature of the mixture increased to 670 K owing to electric heating by the glow discharge. The contribution of the thermal effect to the increase in the linear burning rate is estimated in [104] to be Iower than 5-10%. The hydrodynamic factor also plays its role because it affects the energy of charged particles bombarding the surface of the condensed phase. Therefore, the kinetic action on the high-temperature combustion synthesis observed in [103, 104], as in many other investigations, is manifested via hydrodynamics and is complicated by the Joule effect.
CONCLUSIONS Let us note the most important, in our opinion, trends observed in studying the external and internal electromagnetic fields in high-temperature combustion synthesis. The first one is a rather obvious and longtime desire to stridy non-thermal mechanisms of the electromagnetic action. This does not mean, certainly, that everything is clear concerning the Joule effect. For instance, the self-consistent problem about the zone of intense electric heating in the synthesis wave under the action of external electromagnetic radiation, which couples thermodiffusion processesand absorption of radiation by the reacting medium, is still to be solved. Apparently, researchers' attention will now be riveted to situations where the thermal factor is veiled by the elec-
High-Temperature
Combustion
Synthesis
(Review)
653
trohydrodynamic or kinetic action and it is difficult to estimate the contribution of each factor to the resultant effect. The second trend is associated with the desire to better study electromagnetic fields generated by the mere combustion of heterogeneous systems. A certain progress at the scale of individual particles has been achieved here, but there is no clear understanding of the role of various chemical and physical processesand their contributions at the scale of the thermal wave. Clearly, the better the internal fields are studied, the deeper the understanding of the mechanisms of the external electromagnetic action. Finally, the third trend is the desire to discover the role of the gas phase in charge transfer and the activation reaction in the heterogeneous medium. Until recently, the answer to this question seemedobvious. Now data on high-energy electrons in the pores of the reacting mixture and their possible contribution to combustion activation are available. Unfortunately as in the case with the Joule effect, the final answer to this question can be given only by a comprehensive study with detailed consideration of not only the influence on heterogeneouskinetics and gas dynamics but also on the thermal effect of bombardment of the condensed surface bv such electrons.
10. P. B. Avakyan, M. D. Nersesyan,and A. G. Merzhanov, "New materials for electronic engineering," Amer. Ce-
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