B O L E T Í N DE LA S O C I E D A D E S P A Ñ O L A DE

Cerámica y Vidrio A R T I C U L O

Production and characterization of strontium ferrite powder from the citrate-nitrate gel process

J. BASABE*, F. SALE**, J.A. LLAUGER* * Universidad Pontificia Católica del Ecuador, Quito, Ecuador

** Departament of Metallurgy and Material Science, University of Manchester, England

A study has been made of the application of the citrate-nitrate gel process to the production of strontium hexaferrite. The airas of the work were to understand the mechanism for decomposition of the gel and to relate it to the properties and subsequent proces­sing of the ferrite powder. The process was shown to give a homogeneous stoichiometric form of strontium ferrite (SrFe220|9) with a controlled particle size and morphology. The thermal decomposition/oxidation of the ferrite precursor was studied using Differential Thermal Analysis (DTA), Thermogravimetry (TG) and Evolved Gas Analysis (EGA), to provide information on the mechanism of decomposition of the citra­te-gel to yield the crystalline ferrite. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were used to characterise the crystalline phases and morphology of the products of partial and complete decomposition of the gel. Dilatometry (TMA) and Optical Microscopy have shown that it is necessary to use a two stage firing process to control the exother­mic nature of the gel decomposition and hence control de particle size and stoichiometry of the oxide product.

Key words: strontium ferrite, citrate-nitrate gel-process, morphology, sintering, thermal analysis, microscopy, magnetic ceramics.

Producción y caracterización de f erritas de estroncio en polvo a partir del citrato-nitrato por sol gel

Se ha realizado un estudio sobre la aplicación del proceso gel citrato-nitrato para producir hexaferrita de estroncio. Los objetivos de este trabajo consistieron en comprender el mecanismo de descomposición del gel y en relacionarlo con las propiedades y el subsi­guiente procesamiento de la ferrita en polvo. Se mostró que el proceso proporcionaba una forma estequiométrica homogénea de ferrita de estroncio (SrFe220i9) con un tamaño de partícula y morfología controlados. La descomposición/oxidación térmica del precursor de la ferrita fue estudiada utilizando Análisis Térmico Diferencial (ATD) Termogravimetría (TG) y Análisis de Emanación de Gases (EGA), con el fin de procurar información sobre la forma y el mecanismo de descomposición del proceso gel-citrato hasta dar la ferrita cristalina. Se utilizaron Difracción de Rayos X (DRX) y Microscopía Electrónica de Barrido (MEB), para caracterizar las fases cristalinas y la morfología de los productos de la descomposición parcial y completa del gel. Mediante la Dilatometría (TMA) y la Microscopía Optica se ha mostrado la necesidad de usar un proceso térmico de descomposi­ción en dos etapas con el fin de controlar la naturaleza exotérmica de la decomposición del gel y por consiguiente el tamaño de par­tícula y la estequiometría del óxido resultante.

Palabras clave: ferrita de estroncio, citrato-nitrato por sol gel, morfología, sinterización, análisis térmico, microscopía, cerámica magnética.

1. INTRODUCTION

Ferrite ceramics, at present, are still being investigated, improved and increasingly applied. They are essential to modern life as components of a wide variety of electromecha­nical and electronic devices. The apphcations of ferrites as per­manent magnets range from loudspeakers, small electric motors and generators, door latches and toys, to ore separa­tors, water filters, electric watches and microwave tubes (1).

Ferrimagnetic oxides, commonly known as ferrites, include a whole range of magnetic ceramics based on Fe203 as a major compositional component. The ferrites which are magnetically hard have a hexagonal crystal structure and have been used as permanent magnets in electric engineering and magnet mecha­nics for the last 45 years (2).

Strontium ferrite (SrFe|20|9) forms the basis of may hard ferrite magnets, together with barium ferrite (BàFei20ig) and lead ferrite (PhFei20i(^). BaFe220|9 was the first hard ferrite produced on an industrial scale but SrFe220|9 has been pre­dicted to take over some of the applications of barium ferrite. PbO. 6Fe203 is used only as an additional material for oxide magnet purposes at present.

Control of the particle size and morphology, along with purity, of the hexaferrites is important for the optimization of magnetic properties. Consequently, a number of novel synt­hesis methods have been studied for their production (3). This paper reports a study of the use of the citrate gel process for the production of strontium hexaferrite which has been carried out as part of a series of investigations on the novel synthesis of magnetic ceramics.

Bol. Soc. Esp. Cerám. Vidrio, 35 [6] 453-459 (1996) 453

A. IBAÑEZ, F. SANDOVAL

2. EXPERIMENTAL PROCEDURE

2.1. Production of strontium ferrite

2.1.1. GEL

"Analar Grade'' citric acid, iron (III) nitrate and strontium nitrate were used to prepare individual solutions, so that when mixed they gave a solution in the proportion of:

112.99x10"^ mol iron (III) nitrate: 9.42x10"^ mol strontium nitrate: 119.68x10"^ mol of citric acid.

Each calculated quantity of substance was dissolved in approximately 150ml of distilled water. The iron (III) nitrate solution was then mixed with the strontium nitrate solution. The solution of citric acid was then added. The amount of citric acid used was the minimum necessary to bind the metal ions, if all the nitrate ions were to be replaced by citrate groupings. The solution was evaporated under vacuum at 70°C±5°C in a rotary evaporator to produce the gel, i.e. a viscous liquid. Immediately, the gel was transferred to an evaporating basin and then dried in a vacuum oven at 70°C±5°C and 10'^ torr for 12-24 hours. This gave the solid, reddish brown precursor, in powder form.

2.1.2. PRECURSOR

The thermal stability of the precursor was studied using TG, DTA and EGA in order to gain an understanding of its mecha­nism of decomposition.

A small amount of sample, approximately 10 mg, was hea­ted at the rate of 10°C min"^ in a platinum crucible in air using calcined alumina powder in the reference crucible. The gas flow through the system was 100 ml min"^, except when EGA was performed when the flow was reduced to 15 ml min"^ to prevent excessive dilution of the evolved gases. The thermal analyses were carried out from room temperature to 800°C.

2.1.3. POWDER

400 Temperature, "C

Fig. 1. Simultaneous thermal analysis of the precursor.

It can be seen that the DTA curve shows four exotherms and one endothermic peak.

The first sharp exotherm at 137°C is recorded as a rapid reac­tion with 62% weight loss in the TG curve and in addition the EGA (table I) shows the evolution of water vapour, carbon dio­xide and nitrogen oxides.

TABLE I. MASS SPECTROSCOPY DATA FROM THE EGA

TEMPERATURE ( X ) MOLECULAR WEIGHT (g/mol) and FORMULA MASS

120-220

230-360

550-610

17(0H), 18(H20), 30(NO) 44 (CO2, N2O), 46 {NO2)

44 (CO2, N2O)

30 (NO)

Each thermal event in the DTA was investigated by conduc­ting a calcination of the precursor at a temperature before and after each significant change.

Each calcined sample was then characterized by a study of: the crystalline phases present by using XRD and the morpho­logy by using SEM

Further studies were conducted by calcining the precursor at each of the significant temperatures and then pressing samples in pellet form. Each pressed pellet sample was subjected to dilatometric studies and then the study of the microstructure of the sintered samples.

3. RESULTS AND DISCUSSION

3.1. Thermal stability of precursor and oxide powder

3.1.1. SIMULTANEOUS THERMAL ANALYSIS (TG/DTA) AND EGA

Figure 1 shows TG/DTA data up to 800°C for the strontium ferrite precursor.

The second less sharp exothermic effect at 195°C followed by two wide exotherms at 265°C and 330°C are recorded by the TG as 16.2% weight loss. This stage is accompanied by the detection of CO2 and N2O in the EGA.

The endothermic peak at 566°C is associated with a 3.2% weight loss and by the evolution of NO as can be seen from table I.

As shown in table II the theoretical percentage weight loss of the precursor is 82.55% which is consistent with the one obtai­ned in the TG study, that is 81.40%. The shghtly lower weight loss determined experimentally is because some nitrate ions are removed from the gel during drying. These were detected as ''nitric acid'' odour present in the vacuum oven and its cold trap after the drying process was complete.

Previous work (4) had recognised two types of pyrolysis of citrate precursors. Type I was characterized by a continuous and vigorous reaction and occurred with precursors contai­ning Fe, Ni, Ag, Cu and Co, which have a strong catalytic acti­vity in oxidation processes. Type II was a two stage process that was characterized by an intermediate decomposition step involving a mixed citrate salt. It is apparent that the decom-

454 Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 35 Num. 6 Noviembre-Diciembre 1996

LA COCCIÓN RÁPIDA

TABLE IL THEORETICAL WEIGHT LOSSES FOR I O G PRODUCT

Product

SrFei20i9

(10g)

wt change (g) caused by loss from dried precursor of:

%wt loss attributed to: Product

SrFei20i9

(10g)

N0¡ Citric Acid N0¡ Citric Acid

Product

SrFei20i9

(10g) 22.2 25.1 38.7 43.9

Product

SrFei20i9

(10g) 22.2 25.1

TOTAL 82.6

position of the strontium ferrite precursor is an example of a Type II pyrolysis. The rapid decomposition, v^ith the v^eight loss of 62.00% over the temperature range 110-150°C, is follo-w ed by a steady state situation up to approximately 170°C where a slower second weight loss starts and finishes at approximately 300°C. It is evident that a ''transiently - stable'' intermediate phase exists over the temperature range 150-170°C. The reason why these TG data do not agree exactly with those from EGA is that there is a lag time associated with the removal of evolved gases from the furnace surrounding the sample. Hence, as the furnace is being heated conti­nuously there appears to be a wider range of temperature over which the gas is evolved.

In addition to the decomposition of the amorphous gel, there is one extra weight loss around 565°C which is associated with the thermal decomposition of a small amount of precipitated strontium nitrate, as discussed in the following section. In order to test whether this higher temperature weight loss was induced by the strontium nitrate, a sample of pure strontium nitrate was analysed by TG/DTA. Figure 2 shows the data obtained, from which it is clear that the nitrate decomposes over the temperature range 550°C to 680°C.

It is interesting to note from figure 2 that the TG curve and its derivative indicate a fairly smooth decomposition of the nitrate. However, the DTA data clearly show the existence of two endothermic events within the temperature range 550°C and 680°C. It is quite likely that the small event at 580°C,

which is superimposed on the overall endothermic peak, is associated with a fusion process. Similar events have pre­viously been observed for anhydrous cadmium nitrate (5).

3.1.2. XRDANDSEM

As can be seen from table III that the strontium ferrite pre­cursor shows the presence of strontium nitrate. It is apparent, therefore, that some strontium nitrate is separated out from the mixed nitrate-citrate solution during dehydration to produce the gel. This illustrates that the precursor is not amorphous, as is the ideal case. Previous studies* have shown similar results with nitrate salts of low solubility (6-8).

TABLE III . IDENTIFIED PHASES BY X R D

TEMPERATURE OF CALCINATION {"C)

MATERIAL MATCHING PATTERN

1 160 Sr(N03)225-746Dl and FejOs 391346DI

230 Sr(N03)225-746DI

300 Sr(N03)225-746DI

450 Sr(N03)2 25-746DI, SrOs 7-234DI and Fe203 391346DI

600 FezOg 33-664DI and SrCOs 5-418DI (except one peak)

700 Fe203 33-664DI and SrCOs 5-418D1 (except one peak)

900 FesOs 33-664DI, SrFei20i933-1340DI and SrFeOa-x 34-638DI (except one peak)

1100 FeaOg 33-664DI, SrFei2Oi933-1340DI and SrFeOa-x 34-638DI (except one peak)

exo

V »ndo

1 •/« / m(n 1 0 %

t

L \ \ \ \ \

\ i !

V ; .•\ ; :\ : ; \ ; • \: - \; ; \; / L TG J

V ; .•\ ; :\ : ; \ ; • \: - \; ; \; / L

DTAj

\ ^

DTG ... ... A.,.:f.M. .. A A A t • ..i^l'K- A . . . - . . . , . . i-,,.iU.. — 4 ^ Ä . 1 JUi. W.-i.A¿¿r"iJ

400

Tamperoturo, •€

Fig. 2. Simultaneous thermal analysis of strontium nitrate.

The precursor was found to be in the form of large, flat glassy particles, as shown in figure 3. It is evident that the usual glass-like precursor morphology is obtained despite the presence of strontium nitrate. Previous studies have shown a similar morphology of citrate nitrate precursors for both soft ferrites and superconductors (9).

During the thermal treatment of the strontium ferrite pre­cursor at temperatures up to 450°C the major crystalline phase present is strontium nitrate. This can be designated as Stage One. Heating the precursor at 600°C and 700°C produces iron III oxide as well as strontium carbonate. This can be termed as Stage Two. The final stage. Stage Three, gives the formation of the required ferrite on heating the sample to temperatures of 900°C and 1100°C.

Table III summarises the XRD patterns of the products obtai­ned for each of the three stages (10).

Explanations of the reaction mechanisms involved in each of the three stages from the thermal decomposition/oxidation are given below.

Stage One: XRD from 160°C to 450°C

Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 35 Num. 6 Noviembre-Diciembre 1996 455

A. IBAÑEZ, F. SANDOVAL

O • Imm

Fig. 3. Scanning electron micrograph of the precursor.

The powder calcined at 160°C, which has shown a large weight loss by TG, shows strontium nitrate as the main crysta­lline phase and iron III oxide as traces and so indicates that crystallization of major new phases has not begun. It is inte­resting to note that iron III oxide was not detected in each of the gels that were produced. It is anticipated that its occurrence is possibly related to the precipitation of a hydrated iron oxide, as extremely fine particles, in some of the original gels. Such pre­cipitation of iron oxides/hydroxides is known to occur if pH control is not achieved. It is clear that further work needs to be carried out in this area. In samples produced at 230, 300 and 450°C the presence of the strontium nitrate is still evident. In the case of material made at 450°C there are traces of strontium oxide (Sr02) and iron III oxide, which indicate the beginning of decomposition of the strontium nitrate.

Stage Two: XRD at 600°C and 700°C. The XRD analysis of samples produced by decomposition at

600°C and 700°C showed the presence of iron oxide (Fe203) and the formation of strontium carbonate (SrCOß). At 600°C strontium nitrate has disappeared totally form the product and it seems that strontium oxide produced from the nitrate reac­ted with carbon dioxide produced either by combustion of the citrate or from the air to yield the carbonate. Previous studies on the production of BSCCO superconductors (11) and of con­ducting oxides for fuel cell applications showed similar results (6, 7). Figure 4 shows the morphology of the calcined product obtained at 700°C. It can be seen that the increase in the decomposition/oxidation temperature yields an irregular crac­ked material in which there is evidence of the formation of a newly crystallized phase.

Stage Three: XRD at higher temperatures, 900°C and 1100°C. In products obtained at 900 and 1100°C there is still the pre­

sence of iron III oxide as a trace but the major diffraction pat­terns give evidence of the formation of SrFe22029 mixed with SrFe03_-^. It is evident that at these higher temperatures the strontium carbonate has decomposed according to:

SrC03 (s) -^ CO2 (g) + SrO (s)

The SrO has subsequently reacted with the iron oxide to pro­duce the required ferrite.

A way of increasing the yield of the required ferrite and pos­sible allowing its production at lower temperatures would be to by-pass the carbonate formation. A possible method for doing this has been given by Gholinia and Sale (11) who sho­wed that during the heat treatment for the decomposition of the precursor when calcination is carried out in two stages at 300°C and 700°C continuously, i.e. without cooling between the stages, carbonate formation does not occur. FTIR was used to show the absence of carbonates. Figure 5 shows the morp­hology of a calcined product obtained at 900°C. There is clear evidence of the formation of individual grains which is asso­ciated with the initial stage of the formation of the ferrite.

10 um Fig. 5. Scanning electron micrograph of the calcined product at 900°C.

Figure 6 shows the morphology of a calcined product obtai­ned at 1100°C, clearly showing the formation of long flat plate­let crystals, which illustrates the hexagonal micro crystalline structure of the required ferrite.

10 um

10 un

Flg. 6. Scanning electron micrograph of the calcmed product at 1 100°C.

Fig. 4. Scanning electron micrograph of the calcined product at 700°C.

456 Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 35 Num. 6 Noviembre-Diciembre 1996

LA COCCIÓN RÁPIDA

3.2. SINTERING AND CHARACTERIZATION OF THE SINTERED PRODUCT

3.2.1. TMA

The dilatometric curves for the sintering in air of powders obtained by two stage decomposition of the precursor at 300-700°C, 300-900°C and 300-1100°C are shown in figure 7.

The extent of shrinkage observed for the various pow^ders may be explained by the green density of he initial powder compacts. With gel-processed material, as particle size beco­mes smaller it is frequently harder to press the powder with the result that a lower green density is obtained. So the pow­der produced at lower decomposition temperatures, gives a lower green density than the higher temperature products, table IV.

' " " ^ x

\, \

10 V.

\ \

\\

\ ^

L J L J

TABLE IV. G R E E N DENSITY D A T A

800

Temperature, ' C

Fig. 7. Dilatometric curves for the sintering in air of powders obtained by two stage

decomposition of the precursor at 300-700°C, 300-900°C and 300-1.100°C.

Previous experiences in the production of oxide powders from EDTA and citrate precursors have shown that two-stages decomposition is preferable to single-stage decomposition. This is in order to avoid a single vigorous heat evolution as well as the formation of carbonates during decomposition and oxidation (9, 11). It is necessary to avoid the single vigorous evolution of heat, so that the temperature of decomposition can be regulated with the subsequent control on the particle size of the product.

As can be seen from figure 7, that the lower temperature decomposition/oxidation of the precursor allows the produc­tion of powder which starts to sinter at lower temperatures. This behaviour is due to the small particle size of the powder obtained at low temperature. Similar behaviour has been reported (8) for MgO-based soft ferrites. The fine grain size gives rise to a high surface area, and hence a high surface free energy is available for sintering with the low temperature pro­duct. In the sintering process, the driving force is the overall reduction in surface free-energy that accompanies the removal of surface area as a dense body is formed (12). For a bigger ini­tial surface area (finer powder) there is a bigger driving force", as is the case of the powder produced with heat treatment at 300-700°C. Hence, sintering begins at lower temperature as is evident by the start up of the sintering at a temperature of 576°C for the powder produced at 300-700°C. The powder obtained by heat treatment at 300-900°C, which had a finer grain size than the one obtained at 300-1100°C, can be seen to begin to sinter at a considerable lower temperature than that for the 300-1100°C powder. The temperatures at which the shrinkage begins are 783°C for the 300-900°C powder and 982°C for the 300-1100°C powder. The temperatures selected as the onset of sintering are those at which the first shrinkage traces were observed on the dilatometric curves.

MATERIAL 300-700''C 300-900°C 300-1 lOO' C

MASS (g) 0.128 0.125 0.127

LENGTH (mm) 4.48 3,88 3.45

PRESSED DENSITY

1 (g cnf)._ . 0.8351 0.9417 1.0759

This difference in the green densities explains the higher shrinkage observed on heating pressed pellets up to 1100°C and 1200°C for the lower temperature powder (see table V). For a given mass of powder pressed, the low temperature pro­ducts give a longer green pellet as can be seen in table VI. As a result once sintering takes place to obtain a given final den­sity more shrinkage must occur, as is the case of the powder produced by heat treatment at 300-700°C. Previous studies on soft ferrites and oxide superconductors have shown similar results (9).

TABLE V. S H R I N K A G E DATA

MATERIAL 300-700°C 300-900°C 300-1100°C

% Shrinkage . at '

1100°C -7.676 -7.492 1.000 % Shrinkage .

at ' 1200°C -21.500 -15.500 -0.500

TABLE VI . S H R I N K A G E DATA

MATERIAL 300-700°C 300-900°C 300-1 lOO^C

MASS (g) 0.128 0.125 0.127

LENGTH (mm)

before shrinkage 4.93 4.46 4.10 LENGTH (mm)

after shrinkage 4.48 3.88 3.45

% LENGTH LOSS 9 13 16

3.2.2. OPTICAL MICROSCOPY

Figures 8,9 and 10 show optical micrographs from pellets which have been heated to 1240°C in the dilatometer at a rate

Boletín de la Sociedad Española de Cerámica y Vidr io . Vol . 35 Num. 6 Nov iembre-Dic iembre 1996 457

A. IBAÑEZ, F. SANDOVAL

of 10°C min" . The samples were furnace cooled to room tem­perature over a period of approximately 3 hours immediately following the attainment of the maximum temperature of 1240°C.

0 » 2mm Fig. 8. Optical micrograph of the calcined powder by two stage heat treatment at 300-700°C.

1

T 1%/min

^

10%

\

V TG

• • ' - " — — i-::: . . : . - i , . • • • • ! — 1

400 600

Temperature, ^C

Fig. 11. Thermogravimetric analysis of strontium carbonate.

0 . 2mm Fig. 9. Optical micrograph of the calcined powder by two stage heat treatment at 300-900°C.

i^^S o . 2mm

Fig. 10. Optical micrograph of the calcined powder by two stage heat treatment at 300-1.100°C.

The sample form the 300-700°C calcination treatment (see figure 8) shows very irregular and loosely agglomerated parti­cles with an extremely large amount of voids after sintering.

Figure 9, shows the optical micrograph of a pellet made form the 300-900°C powder. It can be seen that the particles have become sintered together to form a dense mass with a lower porosity.

Finally, the micrograph of pellets from the 300-1100°C calci­nation treatment shows flat platelet crystals of approximately 0.1mm in size surrounded by small agglomerated particles. The porosity is even lower that in the previous sample (see figure 10).

It can be seen that the increased temperature of production of the initial powder has resulted in reduced porosity of sam­ples and hence increased density. This is probably due to the presence and gradual decomposition of strontium carbonate in the pretreated mixtures. The X-ray analyses from the powder calcined at 600°C and 700°C have shown there to be a forma­tion of strontium carbonate. This strontium carbonate will decompose between 800 and 1000°C as shown in the TG of pure strontium carbonate (see figure 11). If this occurs within the pressed material voids will result as gas evolution occurs.

Thus, in the 300-700°C calcined product it may be predicted that many voids would be present, with less in the 300-900°C material and even less in the 300-1100°C sample. This was observed in the optical micrographs. In addition, the differen­ces in the initial green densities must also contribute to the resi­dual porosity after sintering.

Further studies of the sintering phenomena and the preven­tion of the formation of strontium carbonate in the gel proces­sed powders are now in progress following the present preli­minary work.

4. CONCLUSIONS

Strontium ferrite can be produced form citrate-nitrate gel precursors. However, without pH control a true amorphous gel is not produced and a fine precipitate of strontium nitrate is obtained within the gel as a result of the low solubility of the strontium nitrate.

On thermal decomposition, the strontium nitrate yields

458 Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 35 Num. 6 Noviembre-Diciembre 1996

LA COCCIÓN RÁPIDA

strontium carbonate as the oxide reacts with carbon dioxide and other carbonaceous material from the gel. The presence of the carbonate, as this intermediate phase, means that calcina­tion at temperatures of the order of 900-1100°C is required before the hexaferrite can be produced. These temperatures are necessary for decomposition of the carbonate and to allow further reaction with iron oxide, which is produced at lower temperature, such that the hexaferrite is obtained. The hexafe­rrite product has a characteristic platelet morphology.

5. ACKNOWLEDGEMENT

The authors wish to thank the British Council in Quito for funding this project.^

6. REFERENCES

1. K.S. Talbot, "International Market Assessment through the year 2000" in "Magnetic Ceramics", Edited by B.B. Ghate and J.J. Simmins, Ceramic Trans. 47,133-137,1995.

2. D. Jiles, "Introduction to Magnetism and Magnetic Materials", Chapman and Hall, London, 315-319,1991.

10,

11

12.

3. H. Hibst, "Hexagonal Ferrites from Melts and Aqueous Solution, Magnetic Recording Materials", Angew. Chem. Lnt. Engl. Ed., 21, 270-282,1982.

4. P. Courty, H. Ajot and C. Marcilly, "Oxides mixtes ou en solution solide sous forme très divisée obtenus par décomposition thermique de précurseurs amorphes". Powder Technology, 7, 21-38,1973.

5. D.J. Anderton and F.R. Sale, "Partial phase diagrams for the water-cadmium nitrate, water-silver nitrate and water-cadmium nitrate-silver nitrate sys­tems", Thermochim. Acta, 30, 263-271,1979.

6. D.J. Anderton and F.R. Sale, "Production of conducting oxide powders by amorphous citrate process". Powder Metallurgy, 1,14-21,1979.

7. M.S.G. Baythoun and F.R. Sale, "Production of strontium-substituted lantha­num manganite perovskite powder by the amorphous citrate process", J. Mater. Sei., 17, 2757-2769,1982.

8. F.R. Sale and F. Mahloojchi, "Citrate Gel Processing of Oxide Superconductors", Ceramic Internat., 14, 229-237,1988.

9. F. Mahloojchi, "Production, characterization and sintering of MgO-based ferrite powders", Ph.D. Thesis, University of Manchester, Institute of Science and Technology, Department of Metallurgy and Materials Science, 1987. J. Basabe, "Gel-processing of strontium ferrite", M.Sc. Thesis, University of Manchester and UMIST, Manchester Materials Science Centre, 1993. A. Gholinia and F.R. Sale, "Synthesis and characterization of BSCCO high Tc superconductors by EDTA process" in "Euro-Ceramics Il-vol.3" Edited by G. Ziegles and H. Hausner, Deutsche Keramische Gesellschaft e.V., Köln, 2239-2243,1993.

W.D. Kingery, "Introduction to Ceramics", J. Wiley and Sons, New York, 370-373,1963.

Recibido: 21-5-96. Aceptado: 12-10.96.

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