Preparation of polycrystalline yttrium iron garnet ceramics

iii i

i

POWDF.R TECHNOLOGY E LS EV I E R

Powder Technology 93 (1997) 247-251

Preparation of polycrystalline yttrium iron garnet ceramics P. Grosseau a.., A. Bachiorrini b, B. Guilhot

a

" Ecole Nationale Sap~rieure des Mines, 158 Cours Fauriel, 42023 Saint-Etienne Cedex 2, France b Universita di Udine, DSTC. via del Catonificio 108, 33100 Udine, Italy

Received 30 July 1996: revised 4 April 1997; accepted 27 May 1997

Abstract The production of polycrystalline ceramics from YIG powders, obtained by reacting Y203 and Fe203 oxides, is a well understood and controlled industrial process. However, methods of production via chemical pathways, such as coprecipitation, possess the advantage that the garnet powder is more reactive and no longer requires prolonged grinding which can affect its purity. In this paper, we will show that the mastering of these two processes enables the production of ceramics possessing good magnetic characteristics for microwave applications. © 1997 Elsevier Science S.A. Keywords: Ceramics: Chemical processing; Coprecipitation

1. Introduction Due to their microwave properties, YIG and its substituted derivatives are of primary interest for use in the conception of microwave equipment ( insulators, circulators, dephasers). In most cases, the materials used are sintered polycrystalline ceramics. In order that the materials developed possess the required magnetic characteristics they must be pure, homogeneous and have a density very close to the theoretical value. Industrially, YIG powder used for the preparation of these ceramics is obtained by reacting Y203 and Fe20.~ oxides at high temperature. Preparing homogeneous powders by this method which are suitable for sintering requires prolonged grinding which is prone to reduce the purity of the material. In general, production processes via chemical pathways result in the formation, at low temperatures, of pure, homogeneous and reactive powders without requiring grinding. Thus, these methods seem to be possible alternatives to the classical process of ceramic production but their use must be limited to advanced materials because of their cost. Few studies have been carded out on the synthesis of YIG by coprecipitation l 1-31, although this method was sometimes used during the initial studies on the magnetic properties of rare earth iron garnets [4]. The experimental results which we present here are relevant to the research we have completed on the production process by the reaction between oxides and by a non-conventional method of coprecipitation, * Corresponding author. Tel.: + 33 4 77 42 0123; fax: + 33 4 77 42 0000. 0032-5910/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved PIIS0032-5910(97)03279-8

2. YIG synthesis by reaction between solids

2.1. Principle The method consists of reacting Y203 and Fe20~ oxides together: 3Y203+5Fe203~2Y.~FesOIz (YIG)

(I)

This transformation results in the appearance of an intermediate compound with a perovskite structure and chemical formula YFeO3 (YIP): Y203 + Fe20~ --->2YFeO3

(2)

and 3YFeO3 + Fe203 ~ Y3FesOt 2

(3)

The temperatures corresponding to reactions (2) and ( 3 ) are significantly different depending on the author [5-71. The values given vary from 600 to 900°C for reaction (2) and from 700 to 1100°C for reaction (3).

2.2. ExT~erimentai results The reactants used for this study are commercial haematite powder (Merck) and 99.999% yttrium oxide (Aldrich). Agitation of a suspension of the powders in ethanol with simultaneous evaporation of the solvent is used to obtain a stoichiometric homogeneous mixture. in order to avoid the reduction of iron oxide, all high temperature thermal treatments are carded out under oxygen.

248

P. Grossea,I et al /Powder Technology 93 (1997) 247-251

Miss fractions , F%03 ---e-- Y203

1

[ Mimg [

//

I

vv, o, o. jI 0.8 - - .•- - v#

/

1 or 2 times

I Compacling I 0.2 Fig. 3. The operating protocol used for the preparation of YIG via reaction between solids.

o 400

600

800

1000

1200

1400

Fig. I. Evolution of the composition of5Fe20~ + 3Y20~ mixture as a function of temperature for a thermal treatment of 3 hours. M~ 1

fractions J t

I

I

I

I

CI

i'l

0. 8

0. 6

•

F%03 Y203

A

VFeOj

o

vpe~o.

(i) the use of two calcination-grinding cycles allows an increase in the degree of advancement of reaction ( I ), in addition to an improvement in the sinterability of the powder; (ii) this operating procedure allows the improvement of the densification, better than when only one cycle is used with prolonged grinding; (iii) contrary to the observation of Young et al. [ 81 a high degree of transformation of the powder seems to increase its sinterability in the context of our operating protocol.

0.4

3. YIG synthesis by coprecipitation 0.2

3. !. Principle of the method 0 0 2 4 6 8 10 12 14 Fig. 2. Evolution of the composition of5Fe:O~ + 3Y.,O~mixture as a function of time at a calcination temperature of 1300°C.

X-ray diffraction is used to determine the composition of the mixture as a function of the treatment temperature, and for a duration of 3 hours. The curves in Fig. I show the perovskite is formed between 800 and 900°C, the formation of YIG occurring between I000 and 1100°C. To obtain a pure garnet phase, temperatures of the order of 1400°C are required. At 1300°C, even for a long treatment time the material contains only 90 wt.% of the garnet phase (Fig. 2). At this temperature, however, it is possible to obtain pure YIG by performing two consecutive 6 hour treatments with a 5 minute grinding operation between them. These findings have led us to define an operating protocol which is described by the diagram illustrated in Fig. 3. In order to prepare powders with good sinterability it is important to control the tbllowing parameters: • the temperature and duration of the treatment; • the grinding time; • the number of calcination-grinding cycles. Several samples have been elaborated by varying these parameters; their characteristics are listed in Table I. The comparison of the densities after sintering leads us to make several remarks:

The method of coprecipitation we have developed consists of introducing a solution of Y( NO3),~ and Fe( NO.O,~, with iFe.~+ ]/[y.a+ ] =5/3, into an ammoniacal solution of pH 10-1 !, in order to precipitate all the ions simultaneously. After this stage, the coprecipitate is washed several times with 3000 ml of deionized water or an ammoniacal solution of pH 10.5, then dried at 65°C for 24 hours and ground for several seconds in a crushing grinder. A more detailed description of this method is given by Grosseau et al. [91. In this paper we present the experimental results concerning the influence of the following parameters on the sinterability of the powders prepared: • the number of washings; • the nature of the washing solvent; In all cases the coprecipitate leads to the formation of a garnet without any impure phases for a treatment temperature of more than 1100°C. 3.2. Experimental res,dts The sinterability of the powders is characterized using diiatometry. After sintering, the ceramics are analysed using mercury porosimetry (open porosity) and helium pycnometry (closed porosity). In order to study the effect of washing, the coprecipitates are synthesized at pH 10.5 and washed from one to five times

P. Grosseau et al. /Powder Technology 93 (I 997) 247-251

249

Table i Characteristics of the samples prepared via reaction between solids Sample

1

2

3

4

5

6

7

Calcination temperature (°C) Calcination time (h) Number of treatments Grinding time ( min ) Mass percentage of YIP % of theoretical density

1200 6 I 5 17.48 95.6

1200 1 2 5 9.56 96. I

1200 6 2 5 9.16 97. I

1300 6 I 5 1.5 97.4

1300 6 I 20 1.5 98. I

1300 3 2 5 0.67 97.5

1300 6 2 5 0 98.3

with deionized water o r with an ammoniacal solution o f pH 10.5. After grinding, the coprecipitates are calcined under oxygen at 875°(= for six hours. Then a compact o f 1 g is formed by uniaxial compacting at 220 MPa. The derivative o f the dilatometric curve with respect to temperature yields a relative minimum corresponding to a m a x i m u m for the rate o f shrinkage (Fig. 4 ) . It is generally considered that the corresponding temperature, TM, is characteristic of the size o f the pores eliminated during sintering

[101. The evolution o f TM as a function of the number o f washings for both o f the solvents used is represented in Fig. 5. These curves show that only deionized water has a positive effect on the sinterability o f the powder. This phenomenon At/to O.OS 0

-0.0! -0,;

is confirmed by the increase in density after sintering (Fig. 6 ) . in the same way the negative effect of washing with the ammoniacal solution may be observed from the curves in Figs. 5 and 7. The analysis o f the porous texture o f the compacted p o w d e r before sintedng, using porosimetry, shows that washing the coprecipitate leads to a reduction in the size o f the aggregates formed during drying, irrespective of the solvent used (Figs. 8 and 9). Contrary to what one might expect from the previous results, the ammoniacal solution is more efficient than deionized water in reducing the size o f the aggregates. 20

Porosity (%)

/q" ~ o p e n porosit~ ( ~ ) / • closed-porosity (%) L : total porosity (%)

IS I

I

I

I

I

I

I

:=I ....z.'.,.,.o.,.,.,.,.l................ 10

S

-0.1 -0,2

0

r.

.0.2.4

0

I

I

I

I

200

400

600

800

I

I

lOOO 1200

Number of washings 0

T (*C) -0,3

-\

I

1400 1600

1

2

3

4

S

6

Fig. 6. Evolution of the porosity after sintering as a function of the number of H_,Owashings carried out on the coprecipitate.

Fig. 4. Typical form of a dilatometric curve. 20

Porosity (%)

T M (°C)

1480

,

,

,

,

I

I

I

I

I

I

1

2

3

4

S

,

'J --o---open porosit~ (%) ] • closed porosity (%) y (~)

lS

1460 1440

J - - 4 - - Ammonia solution

I 10

1420 1400 1380 Number

Number of washings 1360 0

i

I

I

1

2

3

I

4

I

$

0 6

Fig. 5. Evolution of the temperature TM as a function of the number of washings.

0

of

washings 6

Fig. 7. Evolution of the porosity after sintering as a function of the number of NH~OH ( pH 10.5} washings carried out on the coprecipitate.

P. Grosseau et aL / Powder Technology 93 (1997) 247-251

250

200

Cumulativepore volume(mm3/g) r

250

: unwashed • I washing o 2 washings ---e--- 3 washings gs

150

Cumulative pore volume (mma/g) , 0

,

'0

200

~ N )

100 100

SO

. *

SO t Pore radius (Ixm)j

0,001

0,01

~,~,,.

0,1

5 washings with H-'--~O $ washinp with HzO

I%,~

+3 with Ethanol

I ~'N

0 0,001 1

Fig. 8. Evolution of porous texture of the calcinated powder compact as a function of the number of H,O washings carried out on the coprecipitate.

Pore radius (j~m) ~ i I 0,01 0,1

1

Fig. 10. The effect of washing with ethanol on the porous texture of the calcinated powder compact.

4. Magnetic characterization of the materials Cumulative pore volume (mmSlg) 250

T

T

/

*

1 washing

~\

[

*

l

2 washings

a 3 washings

~

t.

.

. 200

IS0

-

,

~

J

~b'% | - .

,

unwashed

'

1

$ washings . . ~ . , ~ . ~ ¢ ~ ,

!00 SO Pore

r a d i u s ~

0.0l 0,1 Fig. 9. Evolutk)nof poroustexture of the calcinatedpowdercompactas u function of the numberof NH~OH(pH 10.5) washingsCalTiedout on the copreeipitate, 0.001

This result shows, in this particular case, that the temperature TM cannot be related simply to the size of the pores eliminated, but that it is necessary to take into account the purity of the powder. In spite of the positive effect that washing with deionized water has on the densification of the ceramic, the reduction in size of the aggregates is slight, even after five washes, and the density after sintering is still insufficient (about 96.5% of the theoretical density). According to Kaliszewski and Heuer, it is possible to eliminate this problem by washing with ethanol. The replacement of the OH groups on the surface of the solid by CH3CH20 groups leads to an intraparticular thermal decomposition rather than the interparticular decomposition during drying

Samples elaborated by each of these methods are characterized by their resonance line widths (A H). These are measured on a I mm diameter ball by the perturbation method at 9.3 GHz in TEto6 mode with an accuracy of 2% at room temperature. Principal applications of YIG ceramics require a value of 3.6+0.7 kA m - ' for AH. Among all the materials prepared by solid state reaction, only sample 7 (Table 1) satisfies this requirement with A H = 3.98 kA m - ' . The material obtained with the powder elaborated by coprecipitation and washed three times with ethanol presents a resonance line width of 3.90 kA m- ~. So it appears that YIG ceramics produced by coprecipitation, as well as those elaborated by solid state reaction, are suitable for microwave applications. 5. Conclusions Each of the two methods presented here enables the production of ceramics which have suitable microwave properties. This result is very important because although the quality of production by means of chemical pathways is renowned, it is considered that obtaining a satisfactory density after sintering is not easy and remains to be proven. This study shows that the rigorous control of the production parameters permits the preparation, using the simple method of coprecipitation, of magnetic ceramics possessing satisfactory characteristics.

[!11.

In carrying out a series of three ethanol washings following five washings with water, we can verify the efficiency of this procedure by analysing the porous texture of the compacts (Fig. 10), After sintering, the ceramics obtained have a density greater than 98% of the theoretical value.

Acknowledgements The authors wish to thank Mr P. Filhol from Tekelec Microwave for the experimental determination of the resonance line widths AH.

P. Grosseau et al. /Powder Technology 93 (1997) 247-251

References

[ I ] K.R. Nair, Ceram. Bull., 60 ( 1981 ) 626. [2] A. Bachiorrini and B. Guilhot, ler Congr. Transfrontalier sur les C~ramiques Avanc~es, Turin, Italy, 1989. 131 A. Bachiorrini, Silic. Ind., 55 (1990) 121. [ 4 ] W.P. Wolf and G.P. Rodrigue, J. Appl. Phys.. 29 ( 1958 ) 105. 151 E.V. Tkaehenko, P.P. Pazdnikov, V.M. Zhulovskii, A. Ya. Neiman, A.G. Shapovalov, M.N, Rivkin and T.G. Ve~lovskaya. Neorg. Mater., 16 (1980) 2034.

251

[61 N.R. Holmquist, C.F. Kooi and R.W. Moss, J. Am. Ceram. Soc., 44 (1961) 194. [ 71 A. Sztaniszlav, E. Sterk, L. Fetter, M. Farkas-Jahnke and J. Labar, J. Magn. Magn. Mater., 41 (1984) 75. [ 8 ] R.J. Young, T.B. Wu and l.N. Lin, J. Mater. Sci., 25 (1990) 3566. 191 P. Grosseau, B. GuUhol and A. Bachiorrini, J. Therm. Anal., 4611996 ) 1633. [ IOl B. Dubois, F. Cabannes, D. Ruffler and P. Odier, Euro-Ceramics, Vol. I. Elsevier, London. 1989, p. 1431. [ I 11 M.S. Kaliszewski and A.H. Heuer, J. Am. Ceram. Soc., 73 (1990) 1504.