Journal of Alloys and Compounds Synthesis and ...

Journal of Alloys and Compounds 479 (2009) 863–869

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Synthesis and magnetic properties of quasi-single domain M-type barium hexaferrite powders via sol–gel auto-combustion: Effects of pH and the ratio of citric acid to metal ions (CA/M) Liu Junliang, Zhang Wei, Guo Cuijing, Zeng Yanwei ∗ School of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China

a r t i c l e

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Article history: Received 4 July 2008 Received in revised form 20 January 2009 Accepted 22 January 2009 Available online 4 February 2009 Keywords: Barium hexaferrite Auto-combustion Phase formation Magnetic properties

a b s t r a c t Quasi-single magnetic domain M-type barium hexaferrite powders have been synthesized via sol–gel auto-combustion route, followed by secondary heating treatment at 800 ◦ C for 4 h, using barium nitrate, ferrite nitrate, ammonium nitrate, citric acid, and ammonia solution as the starting materials. The autocombustion producing powders were ␥-Fe2 O3 and BaCO3 with aid of additional ammonium nitrate to increase the combustion temperature. The influences of the citric acid to metal ions (CA/M) and pH values on the gel auto-combustion, the phase compositions of the synthesized powders and their magnetic properties have been studied. The results showed that the chelation of barium and ferrite ions were important for the phase formation of barium hexaferrite: the phase compositions of the synthesized powders changed from a multi-phased mixture to a single phase of M-BaFe12 O19 for the gradually complete complexing of barium and iron ions with citrate as the pH values and CA/M increased. The resulting powders demonstrated that various magnetic properties mainly depended on the variation in phase composition, sintering adhesion between grains and grown-up of crystalline sizes. With CA/M = 1.5, pH 7, the synthesized powders had a particle size distribution in the range of 100–200 nm and a saturation magnetization of 58.0 emu/g at 12 kOe. © 2009 Elsevier B.V. All rights reserved.

1. Introduction M-type barium hexaferrite (BaFe12 O19 ) has been well-known as a permanent magnetic material for its relatively large B–H magnetic energy, high coercive field and low cost since it was first synthesized by Went et al. [1]. Over the past decades, its applications were also expanded as microwave absorbing materials [2] and perpendicular recording materials [3]. With the rapid development of radar electronics and wireless technologies, the millimeter wave devices such as isolators, filters, phase shifters, circulators, etc. will be of planar structure, self-biased, low loss, and well performance over today’s devices [4]. M-type barium hexaferrite is a potential candidate as the gyromagnetic ferrite in the millimeter wave devices for its large uniaxial magnetocrystalline anisotropy as 17 kOe to partially or totally replace the external static magnetic field [5]. The single crystal barium hexaferrite has low FMR loss (H < 100 Oe [6]) but its growth is rather difficult and costly, while the polycrystalline barium hexaferrite can be easily obtained, but their large FMR line width (H > 2000 Oe [7]) prevents their practical applications in the

∗ Corresponding author. Tel.: +86 25 83587254; fax: +86 25 83588316. E-mail addresses: [email protected], junliang [email protected] (Z. Yanwei). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.081

devices. To overcome this problem, one feasible way is to make barium hexaferrite grains in polycrystalline material highly oriented with their crystallographic c-axis in parallel and to eliminate all kinds of defects like porosity, impurities at grain boundaries as much as possible to form a quasi-single crystal material [5]. Obviously, the realization of such a well-organized microstructure will be based on the availability of high quality barium hexaferrite powders, which should be composed of particles with precisely controlled chemical compositions, single/quasi-single magnetic domain structure and narrow particle size distribution. As is well known, the conventional solid state reaction technology for barium hexaferrite usually requires a high temperature (1200–1300 ◦ C) firing and repeated ball-millings, which inevitably results in wide size distributions, ill-controlled domain structure and often introduces impurities and lattice strain into the materials. Nevertheless, many unconventional methods based on the co-precipitation [8–10], the reverse microemulsion [11–13], the glass crystallization [14], the hydrothermal process [15,16], the sol–gel route [17–19], the sol–gel auto-combustion [20–25], etc. have been developed and used in recent years to synthesize nanosize barium hexaferrite with single domain structure. Of them, sol–gel auto-combustion method seems to be exceptionally interesting for its proper utilization of the heat released from the in

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situ reactions of fuels (hydrocarbon species from chelating agents) and oxidants (nitrate radicles) leading to the rapid in situ formation of target nanosize powders. Since Huang et al. [20] first reported the work on the synthesis of ultrafine BaFe12 O19 powders using citric acid–nitrate sol–gel auto-combustion followed by a secondary heat-treatment, many investigations have been committed to the sol–gel auto-combustion synthesis of BaFe12 O19 , with efforts in probing the influences from the ratios of Fe3+ to Ba2+ , citric acid/metal ions and citric acid/nitrate as well as the pH values of the starting solutions [20–25]. Obviously, the homogeneities of the metal ions in the gels are extremely important for the phase formation of barium hexaferrite, which relies on the complex of metal ions (Ba2+ and Fe3+ ) with citrates. So, many investigations have been committed to the sol–gel auto-combustion synthesis of BaFe12 O19 , with efforts in probing the influences from the ratios of Fe3+ to Ba2+ , citric acid/metal ions and citric acid/nitrate as well as the pH values of the starting solutions. Nevertheless, it still seemed to be ambiguous for most of their discussion directly related the starting materials to the resulting powders after secondary heat-treatment without sufficient attention on the producing intermediate phase from sol–gel auto-combustion, which contains one of the important phases, ␥-Fe2 O3 , for barium hexaferrite easily to form [17,18]. In this paper, the distributions of Ba2+ and Fe3+ in the gels have been evaluated based on the calculation of their complex with citrates. Additional ammonium nitrate has been added as the compensative oxidant to increase the combustion temperature to promote the formation of important phase ␥-Fe2 O3 . Barium hexaferrite powders with quasi-single/single magnetic domain have been synthesized via sol–gel auto-combustion plus with secondary heat-treatment, and the influences of pH values and CA/M on the phase forma-

tion and their magnetic properties of the synthesized powders have been discussed.

As starting materials, Ba(NO3 )2 and Fe(NO3 )3 ·9H2 O with A.R. grade were weighed in the stoichiometry Ba/Fe = 1/12 and dissolved together to form a clear aqueous solution. Then citric acid monohydrate (CA·H2 O) was added into the solutions according to different CA/M ratios of 1.0, 1.2, 1.5 and 2.0. A proper amount of ammonium nitrate (NH4 NO3 ) according to oxidizing degree Q of 0.45 was used to increase the combustion temperature and promote the formation of highly reactive ultrafine ␥-Fe2 O3 and BaCO3 during the combustion process. The Q is defined as the molar ratio of the providing oxygen by nitrates to the required oxygen completely oxidizing the fuels to CO2 , H2 O and N2 . The solutions were adjusted using ammonia of 28% to reach a pH value of 0.1 (without ammonia), 3, 7 and 9, and continuously stirred for 2 h to ensure the complete chelation of metal ions at room temperature. Subsequently, they were transformed into brown wet gels after evaporation at 80 ◦ C with continuous stirring for gelation and dried at 80 ◦ C for another 48 h. The freshly obtained brown gels were auto-ignited after several minutes in a muffle at 300 ◦ C and automatically burnt out within 1 min. After autocombustion, the combustion product powders were calcined at 800 ◦ C for 4 h to form the desired phase. The temperatures during the gel auto-combustion process were monitored by a thermocouple. The phase compositions were analyzed by X-ray diffraction method with CuK␣ radiation (ARL XTRA), the morphology of the particles was observed by TEM (JEOL, JEM-200CX). All the resulting powders were pressed into compacts with the same pressure of 25 MPa and their magnetic properties were measured by a vibrating sample magnetometer (VSM, HH-15) with a maximum applied field of 12 kOe. The procedure was as illustrated in Fig. 1.

Fig. 1. The flow chart of synthesizing barium hexaferrite via sol–gel autocombustion.

Fig. 2. The relative concentrations of Fe+3 /Ba2+ -containing species in solution as a function of pH value.

2. Experimental


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Fig. 3. The relative concentrations of metal-citrate complexes in the solution as a function of pH with different CA/M.

3. Results and discussions 3.1. The complexing of metal ions with citric acid As referred above, the homogeneities of Ba2+ and Fe3+ in the gels are the key factor for the phase formation of barium hexaferrite, which relies on the complex of metal ions with citric acid. In order to make clear the metal ions’ distributions in the gels and help the further discussion, the relative concentrations of iron and barium citrates in the solution under different pH values and CA/M have been theoretically calculated on the basis of the work by Lee and Fang [19]. The relative concentrations of metal-citrate complexes in the solution as a function of the pH values with CA/M = 1.5 were shown in Fig. 2. In Fig. 2(a), it is demonstrated that Fe3+ easily complexed with citric acid: as the pH value approached to 3, Fe3+ -citrate complexes in the form of Fe(HCit)+ increased to the local maximum about 0.65 at pH 1 and then decreased near to 0 at pH 3. As the pH value further went up, the Fe-citrate complexes existed in the stable form of FeCit and their relative concentration dramatically increased near to 1, which meant Fe3+ has achieved complete complexing with citrates as pH > 3. In Fig. 2(b), it is shown that Ba2+ citrate complexes in different forms varied with the pH values: the relative concentrations of Ba(H2 Cit)+ initially increased from pH 1 and achieved to the local maximum concentration about 0.20 at pH 3, then gradually went down near to 0 at pH near to 5. Similarly, Ba(H2 Cit)+ increased from pH 2 and achieved to another local maximum concentration about 0.4 at pH 4, then gradually decreased near to 0 at pH 7. However, the relative concentrations of Ba(Cit)− increased at pH > 4 and achieved to the maximum concentration

about 0.97 at pH 7. It maintains at a high value without any decrease even if the pH value of the solution approached to 9. It seemed that Ba2+ cannot completely chelate with citrate for there was still about 3% dissociation even if the pH has achieved to as high as 9. Fig. 3 shows the relative concentrations of major metal-citrate complexes in the solution as a function of pH values with different CA/M. In Fig. 3(a), FeCit appeared nearly to have the same changing trend with different CA/M ratios, which indicated Fe3+ had the good chelating ability with citrates independent of different CA/M. In Fig. 3(b) and (c), as CA/M increased from 1.0 to 2.0, the concentrations of Ba2+ decreased much faster and approached to the lower values, while Ba(Cit)− increased faster and approached to the relatively high values as the pH values increased (as shown in Fig. 3(d)). As CA/M >1.5, the concentrations of Ba-citrate complexes as a function of pH values appeared to be independent of the CA/M. The calculation shows that Fe3+ ions have achieved complete complexing with citrate as pH > 3 and the relative concentrations of Fe-citrate complexes as a function as pH values appeared to be independent of CA/M. However, Ba2+ ions cannot be completely chelated by citrate for there was still about 2% barium ions left even if CA/M was as large as 2.0 and the pH value approached to 9, which may also lead to a little surplus of barium in preparing barium hexaferrite by using citrate process [20,22]. With the same CA/M of 1.5: barium and iron ions were nearly dissociative as pH was 0.1; most of the barium ions were not chelated by citrates while iron ions were nearly completely chelated as pH was 3; both barium and iron ions were completely chelated by citrates as pH ≥ 7. With the same pH of 7: barium ions were partially chelated by citrates and iron ions were nearly completely chelated as CA/M < 1.5 while both barium and iron ions were completely chelated by citrates as

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Table 1 The relative concentrations of metal-citrate complexes in the starting solution and the phase compositions of combustion products and resulting powders with different pH and CA/M. CA/M

pH

Ba2+ -citrate/Ba0 2+

Fe2+ -citrate/Fe0 2+

Combustion products

Resulting powders

1.5 1.5 1.5 1.5 1.0 1.2 2.0

0.1 3 7 9 7 7 7

0.00 0.28 0.97 0.97 0.79 0.94 0.98

0.02 0.98 1.00 1.00 1.00 1.00 1.00

␥-Fe2 O3 , BaCO3 , ␣-Fe2 O3 ␥-Fe2 O3 , BaCO3 ␥-Fe2 O3 , BaCO3 ␥-Fe2 O3 , BaCO3 ␥-Fe2 O3 , BaCO3 , ␣-Fe2 O3 ␥-Fe2 O3 , BaCO3 ␥-Fe2 O3 , BaCO3

␣-Fe2 O3 , BaFe2 O4 , BaFe12 O19 ␣-Fe2 O3 , BaFe12 O19 BaFe12 O19 BaFe12 O19 ␣-Fe2 O3 , BaFe2 O4 , BaFe12 O19 ␣-Fe2 O3 , BaFe2 O4 , BaFe12 O19 BaFe12 O19

CA/M ≥ 1.5, also the participation of Ba2+ as Ba(NO3 ) is suspended as CA/M increased. All the relative metal-citrate complexes with the selected pH values and CA/M were listed in Table 1. 3.2. The combustion process of the dried gels The combustion temperatures of the dried gels were measured by a thermocouple and shown in Fig. 4. In Fig. 4(a), as CA/M increases from 1.0 to 2.0, the auto-igniting time of the dried gels increased from 180 s to 320 s due to the gradual formation of 3D net-structured gel, the maximum combustion temperature was all around 700 ◦ C and the combustion times were near no obvious different after adding ammonium nitrate. In Fig. 4(b), the auto-igniting time in the case of pH 0.1 was shorter than those of pH ≥ 3 while the

maximum combustion temperature achieved near to 600 ◦ C with comparison to those of about 700 ◦ C as pH ≥ 3. These differences are obvious because of poor chelation of metal ions with citrates which lead to the incomplete 3D net-structured gel and therefore the oxidant nitrate containing in the gel was easily decomposed both in drying and auto-igniting process, which made the oxidant decomposition before the combustion and resulted in the decreasing of the maximum combustion temperature. The relatively long auto-igniting times and the slight decreasing of the combustion temperatures of dried gels in the cases of CA/M = 1.5 and pH ≥ 3 ascribed to the adding ammonia which released from the gels and consumed some combustion energy during combustion process. Almost all the maximum combustion temperatures were adjusted to a little higher than the formation temperature of ␥-Fe2 O3 (about 700 ◦ C) after adding proper ammonium nitrate from the combustion temperature curves. 3.3. The XRD patterns of combustion powders and resulting powders varied with pH values Fig. 5(a) shows the XRD patterns of the combustion powders and the obtained powders with the same CA/M of 1.5 and pH varying with 0.1 (without ammonia solution), 3, 7 and 9. The XRD patterns of the burnt powders were well-matched with ␥-Fe2 O3 (PDF# 251402) and BaCO3 (PDF# 41-0373), which indicated that ␥-Fe2 O3 and BaCO3 were the main crystalline phases in the burnt powders. This result demonstrated that after adding a mount of ammonium nitrate to adjust the adiabatic combustion temperature, the combustion powders have been of good control into ␥-Fe2 O3 and BaCO3 as expected. The phase ␣-Fe2 O3 was also detected as pH 0.1, which ascribed to the low maximum combustion temperature resulting from the poor chelation. Fig. 5(b) shows the X-ray patterns of the resulting powders after calcined at 800 ◦ C for 4 h with different pH values. The resulting powders under the conditions of CA/M of 1.5 and pH of 0.1 mainly consisted of intermediate phases, ␣-Fe2 O3 and BaFe2 O4 , as well as only scarce target phase BaFe12 O19 for the poor chelation of barium and iron ions with citrates. BaFe12 O19 became the main phase and there was still a few mount of ␣-Fe2 O3 existing as pH increased to 3 according to partial chelation of barium ions. In the case of pH 7 and 9, both barium and iron ions had nearly complete chelation with citrates, so the resulting powders were single phase of BaFe12 O19 and no other phases were detected. 3.4. The XRD patterns of combustion powders and resulting powders varied with CA/M

Fig. 4. The auto-combustion temperature curves of dried gels prepared under different conditions.

Fig. 6(a) demonstrated the XRD patterns of the combustion powders with pH 7 and CA/M varying with 1.0, 1.2, 1.5 and 2.0. The XRD patterns of the combustion powders also indicated the phase compositions were ␥-Fe2 O3 and BaCO3 , which again confirmed that the formation of ␥-Fe2 O3 during the sol–gel auto-combustion was in good control by adding additional ammonium nitrate. Fig. 6(b) shows the X-ray patterns of the resulting powders at 800 ◦ C for 4 h


Fig. 5. The XRD patterns of the combustion powders (a) and the resulting powders (b) with CA/M of 1.5 and different pH values.

with pH of 7 and different CA/M. As CA/M = 1.0 and 1.2, the synthesized powders consisted of ␣-Fe2 O3 and BaFe12 O19 for there were also partial barium ions uncomplexed, while as CA/M = 1.5 and 2.0, there was single phase of BaFe12 O19 existing without any other crystalline phases detected in the resulting powders by XRD method. The decrease of phase content of ␣-Fe2 O3 in the resulting powders as CA/M increased ascribed to the improvement of barium ions’ homogeneity in the gels. As listed in Table 1, it was clear that the homogeneities of barium and iron ions in the gels were the key factor which dominated the phase formation of the target phase of barium hexaferrite. The pH values and CA/M took effects on its phase formation through the adjustment of chelation of barium and iron ions with citrates. As pH < 3, the 3D net-structured gel did not completely formed, the oxidant decomposed during the gelation process and the distribution of ions in the gel were inhomogeneous. As pH ≥ 7 and CA/M ≥ 1.5, the barium and iron ions were nearly completely chelated by citrate and the homogenous distributions of metal ions in the gel were achieved, the phase of barium hexaferrite was easy to form.

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Fig. 6. The XRD patterns of the combustion powders (a) and resulting powders (b) with pH of 7 and varied CA/M.

3.5. TEM analysis of the synthesized powders The TEM images and the electronic diffraction pattern of the synthesized powders were shown in Fig. 7. The particle sizes of the synthesized powders with CA/M = 1.5 and pH 7 were in the range of 100–200 nm and the adhesion between the particles ascribed to the magnetic attraction and additional heat-treatment. Its electronic diffraction has the hexagonal symmetry axis, which is in concordance with hexagonal structure of M-type barium hexaferrite. The weak diffraction ring may result from the agglomeration of the magnetic particles or the slight sintering between the crystalline particles. With combination of the fact that the critical size of single magnetic domain is about 460 nm [26], we can conclude that the synthesized powders under the condition of CA/M = 1.5 and pH 7 were single/quasi-single domain powders. Also, the particle size of synthesized powders with CA/M = 1.5 and pH 9 was in the range of 100–200 nm and has an uneven distribution. The adhesion between the particles was much deteriorative than that of the case of CA/M = 1.5 and pH 7. The particle size of the synthesized powders with CA/M = 2.0 and pH 7 was between 300 nm and 500 nm, which

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Fig. 7. The TEM images and electronic diffraction of the synthesized powders: (a) CA/M = 1.5, pH 7; (b) CA/M = 1.5, pH 9; (c) CA/M = 2.0, pH 7.

was larger than that of CA/M = 1.5 for the large crystallites of precursor powders forming during the combustion process in the fuel rich case.

Table 2 The magnetic properties of the resulting powders with different pH and CA/M. CA/M

pH

Coercive force Hc (Oe)

Magnetization M1.2T (emu/g)

Remanent magnetization Mr (emu/g)

3.6. The magnetic properties of the resulting powders

1.5 1.5 1.5 1.5 1.0 1.2 2.0

0.1 3 7 9 7 7 7

5247 3749 4142 4489 3540 4340 3981

30.9 44.3 58.0 54.5 48.5 53.2 55.5

17.9 25.5 31.5 32.6 28.5 30.8 31.2

All the magnetic properties of the resulting powders were used by VSM with a maximum applied field of 12 kOe at room temperature (as shown in Fig. 8 and listed in Table 2). From Fig. 8(a), with the sample CA/M of 1.5, M1.2T was as low as 30.9 emu/g and Hc is as high as 5247 Oe corresponding to the existence of BaFe2 O4 and

Fig. 8. The hysteresis loops of the resulting powders with different pH values and CA/M.


␣-Fe2 O3 except BaFe12 O19 as shown in the XRD pattern as pH 0.1; M1.2T increased to 44.3 emu/g and Hc was 3749 Oe for the major phase composition was BaFe12 O19 as pH 3; the slight decrease of the M1.2T between pH 7 and 9 ascribed to the severe sintering adhesion between the grains and uneven particle distribution. From Fig. 8(b), with the same pH of 7, the hysteresis loops were almost the same for the main composition of the resulting powders was BaFe12 O19 . M1.2T increased as CA/M increased from 1.0 to 1.5 for the phase composition content of BaFe12 O19 in the synthesized powders increased. The slight decrease of M1.2T as CA/M increased from 1.5 to 2.0 mainly resulted from the grown-up of the grains for the fuel content (CA) increased, which was a little high than the critical size of barium hexaferrite. The maximum saturation magnetization in 12 kOe and the coercive force with the experimental condition of CA/M = 1.5, pH 7 have achieved to 58.0 emu/g and 4142 Oe, respectively, which is near to the value of the intrinsic magnetic properties of the powders with single magnetic domain. 4. Conclusions Quasi-single magnetic domain M-type barium hexaferrite nanopowders have been synthesized via sol–gel auto-combustion approach with the citric acid as the fuel and the nitrate as the oxidant. The pH values of the starting solutions and the malar ratios of the citric acid to metal ions take effect on the homogeneities of the metal-citrate complexes distributions in the gels and therefore have influences on the combustion behaviors of the gels and the phase compositions of the synthesized barium hexaferrite powders as well as their magnetic properties: (1) M-type barium hexaferrite powders with quasi-single magnetic domain and particle sizes of 100–200 nm were synthesized with CA/M of 1.5 and pH of 7 and they had fine magnetic properties of M1.2T of 58.0 emu/g and Hc of 4142 Oe. (2) With adding additional ammonium nitrate as the oxidant, the combustion temperature was of good control and almost all the combustion powders were ␥-Fe2 O3 and BaCO3 . (3) The chelation of metal ions with citrate has the influences on the auto-combustion behaviors including the auto-igniting time, maximum combustion temperature and the combustion time: as the pH value and CA/M increased, the metal ions had fine chelation with citrate, the auto-igniting time and combustion time prolonged while the maximum combustion temperature slightly decreased for more ammonia gas were released. (4) The phase compositions changed from the mixtures of BaFe oxidants to single phase of M-BaFe12 O19 as pH and CA/M increased. They became single phase of M-BaFe12 O19 in the case of pH ≥ 7 and CA/M ≥ 1.5 according to the gradually complete chelation of metal ions with citrates.

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