Self-propagating High-Temperature Synthesis of

Inorganic Materials, Vol. 33, No. 10, 1997, pp. 1058–1060. Translated from Neorganicheskie Materialy, Vol. 33, No. 10, 1997, pp. 1249–1251. Original Russian Text Copyright © 1997 by Kuznetsov, Morozov, Nersesyan.

Self-propagating High-Temperature Synthesis of Alkali Ferrites M. V. Kuznetsov, Yu. G. Morozov, and M. D. Nersesyan Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia Received February 14, 1996; in final form, April 25, 1996

Abstract—The main parameters of self-propagating high-temperature synthesis of alkali ferrites are studied. The synthesized ferromagnetic ferrites exhibit good magnetic properties.

INTRODUCTION Some alkali ferrites exhibit good magnetic properties in the megahertz range [1]. Iron oxide and the corresponding alkali oxides form solid solutions having various magnetic characteristics; in particular, they can be ferromagnetic or nonferromagnetic ferrites. Therefore, it is interesting to study phase formation in alkali ferrites during their self-propagating high-temperature synthesis (SHS). Simple ferrites of alkali metals appear to be most appropriate for this purpose. The composition of ferromagnetic ferrites with inverted spinel structure is MFe5O8 , where M = Li, Na, or K. The composition of nonferromagnetic ferrites is M2O ⋅ Fe2O3 . None of the nonferromagnetic ferrites have a spinel structure. The difference in crystal structure between the ferromagnetic and nonferromagnetic ferrites leads to formation of two-phase products exhibiting decreased saturation magnetization and increased coercivity. Conventional synthesis of alkali ferrites requires high temperatures (1200°C), which cause an appreciable loss of volatile alkali oxides. In view of this, the charge should be prefired at a temperature 200–300°C lower than the synthesis temperature. At the same time, SHS allows one to prepare alkali ferrites very rapidly and at reduced temperatures [2]. In this work, the SHS technique was used to prepare alkali ferrites. EXPERIMENTAL Synthesis was carried out in air with alkali peroxide compounds as oxidizers and iron powder as a fuel: MxO2 + Fe + Fe2O3

MFe5O8 (M2O ⋅ Fe2O3).

In our experiments, we used peroxides and superoxides of sodium (x = 1, 2), potassium (x = 1), and lithium (x = 2). The choice of oxidizers was dictated by the following factors: (1) The R-10 iron powder was found to become reactive at ~200°C (Fig. 1). In the Fe–O2 system, active oxidation of iron powder occurs between 260 and 650°C, with the highest rate at 450°C. The process is

strongly exothermic and is accompanied by an increase in sample weight (29%) due to the reaction Fe + 0.5O2 FeO. (2) Earlier [3], we showed, by the example of copper, that mixed oxides involving alkali metals can be formed. We also studied oxygen desorption from alkali superoxides and peroxides. Typical temperatures of the desorption processes are 200–300°C for 2NaO2 Na2O2 + O2 , 125–300°C for 2KO2

K2O2 + O2 ,

330–400°C for Li2O2 >630°C for Na2O2 >550°C for K2O2

Li2O + 0.5O2 , Na2O + 0.5O2 , and K2O + 0.5O2 .

SHS is characterized by a substantial temperature gradient in the system, since the temperature rises by a few hundred degrees Celsius during the first 1–2 s. This ensures fast decomposition of peroxides, accompanied by evolution of large amounts of oxygen. The temperature range of peroxide decomposition overlaps the range of iron oxidation. Oxygen is evolved largely in the heating zone ahead of the combustion front, which ensures oxygen supply to the reaction zone and selfpropagation of the process.

t, °C 1400 1200

450

650

1000 800 600 400 200

200 0 1000

2000

U, mV ∆m, % t 35 TG 30 25 20 1 15 10 DTA 5 0 0 –5 3000 4000 τ, s

Fig. 1. TG and DTA curves for oxidation of the R-10 iron powder in oxygen.

0020-1685/97/3310-1058 $18.00 © 1997 åÄàä ç‡Û͇ /Interperiodica Publishing

SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS OF ALKALI FERRITES t, °C 1100 1000 900 800 700 600 500 400 300 200 100 0

U, mV ∆m, % 0.2 6 TG 4 2 0.1 0 –2 0 DTA –4 –6 –8 –10 –0.1 –12 –14

t 550

1200

2400

3600

4800

τ, s

σ, J/(T kg) 30

3 2 1

10 0 –10

–30 –800

–400

Fig. 2. TG and DTA curves of the Li0.5Fe2.5O4 powder obtained by SHS.

Combustion was initiated by local electrical heating and occurred in a self-propagating mode owing to reaction of iron with evolved oxygen. Dilution of the system by Fe2O3 (average particle size d = 1.3 µm) enabled optimization of the combustion parameters tmax and v, which were monitored by microthermocouples.

1059

0 H, kA/m

400

800

Fig. 3. Magnetization curves for SHS products in the Li–Fe– O system: (1) LiFeO2 , (2) Li0.5Fe1.5Ox, (3) Li0.5Fe2.5O4 .

σ, J/(T kg) 10

1

5

RESULTS AND DISCUSSION The combustion parameters for the systems under study are listed in the table. According to XRD data, the products contained the following phases: (1) cubic K2FeO4 (a = 0.799 nm), (2) orthorhombic β-NaFeO2 (a = 0.567 nm, b = 0.715 nm, c = 0.527 nm), (3) cubic α-LiFe5O8 (a = 0.833 nm) and LiFeO2 (a = 0.414 nm). No traces of other phases, except for starting reagents in case of incomplete reaction, were detected. TGA of SHS-Li0.5Fe2.5O4 (Fig. 2, TG curve) shows 3% weight gain at 550°C caused by the reaction 3LiFeO2 + 4Fe3O4 + O2 6Li0.5Fe2.5O4 owing to the presence of unreacted iron oxide and orthoferrite phase in the SHS product. The process causes no changes in the ferromagnetic properties of the product. Contrary to Takeshi [4], we did not detect tetragonal LiFeO2 in the SHS product. The measured lattice parameters were close to the reported values [5]. All of the materials under study are ferromagnetic ferrites having saturation magnetization in accordance with reference data [6] and differing in coercive force owing to different crystal structures. Figure 3 shows typical magnetization curves for different Li–Fe–O compositions. The lower value of saturation magnetization in the LiFeO2 powder is likely to be related to tetragonal distortions resulting in nonferromagnetic phases. Combustion in the Na–Fe–O system was performed using both NaO2 and Na2O2 as oxidizers. The use of NaO2 resulted in considerably higher saturation magnetization (Fig. 4). This can be tentatively attributed INORGANIC MATERIALS

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2 0

–5 –10 –800

–400

0 H, kA/m

400

800

Fig. 4. Magnetization curves for SHS-NaFeO2 obtained with (1) NaO2 and (2) Na2O2 as oxidizers.

to the higher synthesis temperature for the reaction with NaO2 , which probably increases the content of γ-Fe2O3 , a material with a higher saturation magnetization. This suggestion is supported by the presence of XRD lines due to the high-temperature phase. One cannot, however, exclude the effect of morphology of sodium ferrite, whose magnetic properties have not yet been studied adequately. In the potassium system, saturation magnetization does not exceed 0.2 A J/(T kg); the coercive force and remanent magnetization differ little from those in the Na system. The Curie points tC were determined from the temperature dependences of magnetization (table). Comparison of tmax and tC shows that the formation of LiFe5O8 occurs below the Curie points of both the starting components and the final product. This suggests that the process of structure formation and, consequently,

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KUZNETSOV et al.

Combustion temperature tmax, combustion velocity v, magnetic ordering temperature tC, specific saturation magnetization σs, and coercive force Hc of alkali ferrites Composition

tmax , °C

σs , Hc , v, t , °C mm/s C J/(T kg) kA/m

LiFeO2

990

1.8

667

12.3

11.1

Li 0.5Fe 1.5Ox

750

1.1

–

45.5

18.3

Li 0.5Fe 2.5O4

580

0.4

670

57.0

18.3

NaFeO2 (Na2O2)

690

1.2

597

10.0

17.5

NaFeO2 (NaO2)

720

2.0

597

2.9

14.3

K2 FeO4

640

1.5

532

0.2

17.5

the properties of the products in this system can be controlled by applying a magnetic field to the reaction zone. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 96-02-17104.

REFERENCES 1. Smit, J. and Wijn, H.P.J., Ferrites: Physical Properties of Ferromagnetic Oxides in Relation to Their Technical Applications, Eindhoven: Philips Technical Library, 1959. Translated under the title Ferrity, Moscow: Inostrannaya Literatura, 1962. 2. Merzhanov, A.G. and Nersesyan, M.D., Self-Propagating High-Temperature Synthesis of Oxide Materials, Zh. Vses. Khim. O–va im. D. I. Mendeleeva, 1990, vol. 35, no. 6, pp. 700–707. 3. Kuznetsov, M.V., Morozov, Yu.G., and Nersesyan, M.D., The Self-Propagating High-Temperature Synthesis of Alkali Cuprates, Inorg. Mater., 1995, vol. 31, no. 2, pp. 237–239. 4. Takeshi, T., Ferrity (Ferrites), Moscow: Metallurgizdat, 1964. 5. Rabkin, L.I., Soskin, S.A., and Epshtein, B.Sh., Ferrity, stroenie, svoistva, tekhnologiya proizvodstva (Ferrites: Structure, Properties, and Technology), Leningrad: Energiya, 1968. 6. Tablitsy fizicheskikh velichin: Spravochnik (Tables of Physical Quantities: A Handbook), Kikoin, I.K., Ed., Moscow: Atomizdat, 1976.

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1997