Preparation of ZrO2 powders by oxalate precipitation

Solid State lonics 32/33 (1989) 544-549 North-Holland. Amsterdam

PREPARATION OF ZrO2 POWDER BY OXALATE PRECIPITATION J.L. SH1 and Z.X. LIN Shanghai Institute qf ('eramic's, Academia Smica, 865 Chang-ning Road, Shanghai 200050, P.R. China

Received 16 May 1988; accepted for publication 19 September 1988

Ultrafine ZrO2 powder was prepared by the decomposition of zirconium oxalate which was precipitated from soluble zirconium salts with proper amount of H2C204 or from zirconium oxalate solution with NH4OH. It was found that the powders obtained could be amorphous, microcr)'stalline and crystalline Zr(C204)'4H20, according to the different procedures. Mctastable tetragonal ZrO2(t-ZrO2) formed when differently treated precipitates were calcined at 420 C, but the temperatures at which t-ZrO: transformed to monoclinic ZrO2(m-ZrO2) were different from each other for differently obtained powders. Particle sizes, morphology and agglomeration states were also process dependent. Non-agglomerated ZrO2 powder was obtained with alcohol washed powder with its particle size equal to 20 nm at 600°C.

1. Introduction

Chemical methods like coprecipitation, freeze drying, sol-gel and thermal d e c o m p o s i t i o n of salts or organic alkoxides can be used for the p r e p a r a t i o n of high purity, ultrafine powders [1] which can be sintered to high densities at relatively low temperature. In the case o f stabilized ZrO2, several m e t h o d s of ultrafine ZrO2 powder preparation have been developed [ 2 - 8 ] . Haberko [2] prepared Y203-stabilized ZrO2 powder by the coprecipitation o f hydroxides followed by water and ethyl alcohol washing. Mazdiyashi et al. [4,5] obtained very, active ZrO2 from metal alkoxides by thermal d e c o m p o s i t i o n or hydroxide gel precipitation. The p r e p a r a t i o n from organic alkoxides is expensive, so the coprecipitation method is widely used. Besides hydroxide coprecipitation, oxalate precipitation or coprecipitation can also be used for ultrafine ceramic powder p r e p a r a t i o n [9]. Unfortunately, the work on zirconium oxalate preparation has not been done so far. Most recently, the present authors have prepared pure and stabilized ZrO2 by oxalate precipitation as the first step. This paper mainly concerns the results o f the chemical preparation of pure ultrafine zirconia powders using the oxalate precipitation m e t h o d and the characteriza-

tion of the powders by X R D , TEM and D T A - T G techniques.

2. Experiments Zr(NO3)4.5H~O was used as raw material. 80 ml H2C204 solution of 2.73 m o l / ~ was a d d e d to 140 ml Z r ( N O 3 ) 4 - 5 H 2 0 solution with Z r O : content equal to 0.436 mol/~, and thus obtained precipitates were then treated with different processes before being dried at about 100 *C. The powder denoted P- 1-1 was o b t a i n e d by direct drying of the precipitate; P-l-2 was obtained after washing the precipitate with distilled water: P- 1-3 with dilute NH4OH washing: P- 14 with absolute ethyl alcohol washing; and finally, P1-5 with alcohol washing under basic condition. P-2 powder was prepared by the addition of ammonia to a complex c o m p o u n d solution of zirconium oxalate which was made by the solution of newly precipitated Z r O y n H 2 0 in excess a m o u n t of oxalic acid, with p H value greater than 9. As-precipitated P-2 powder was not washed because there are no other anions involved in the precipitates except C20~-. As-obtained and calcined powders at different temperatures were analysed by X R D (X-ray diffraction), D T A - T G (differential thermal analysis-

0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division )

545

J.L. Shi, Z.X. Lin / Preparation of ZrOe powderby oxalate precipitation thermal gravity) and TEM (transmission electron microscopy). The t ( l l l ) , m ( l l l ) and m ( l l i ) peaks in the X R D spectra were used to calculate the content of tetragonal phase [10]. The X R D peak broadening method was employed to calculate the grain sizes of zirconia using the t ( 1 11 ) and m ( 11 i ) peaks. D T A - T G experiments were conducted under a rate of 10°C/rain.

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(a}

3. R e s u l t s

3.1. Phase analysis of powder before calcination Powder obtained by the direct drying o f the precipitate ( P - l - 1 ) is microcrystalline, as shown in fig. l a. There are several broad peaks at about 16.5 °, 20 o, 27 ° and 31.5 ° (20) in the X R D spectrum. When the precipitate is washed with distilled water (P-l-2), the microscrystalline character is virtually unchanged (fig. lb); when the precipitate is washed with dilute a m m o n i a (pH 9) (P-l-3), the resulting powder is amorphous to XRD, as shown in fig. l c; and the alcohol washed powder (P-1-4) is also ap-

30'

20' I0° 4 2t3 Fig. 1. XRD spectra of differently treated powders before calcination. See text for details.

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Fig. 2. DTA-TG curves for differently treated powders.

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T¢O

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J.L. Shi, Z..¥. Lin / Preparation ql ZrO, powder In" oAa/ate precipztamm

546

8o •~ ~0o N =

pq-4 \ \P-l-5 . P-I-4\ ~ ' ~

/ /

20 @ 400

500

600

700

803

900

10(7)0

TI'E}

Fig. 3. Dependence of the content of metastable tetragonal ZrO: on calcination tcmeprature.

proximately amorphous (fig. ld ). On the other hand. crystalline zirconium oxalate Z r ( Q O 4 ) 2 ' 4 H , O formed, as indicated in fig. le, for alcohol washed powder under basic condition (P-I-5) by adding ammonia. The X R D phase analysis of powder P-2 without any washing shows that crystalline Z r ( C e O a ) e ' 4 H : O formed as indicated in fig. lf.

3.2. Thermal analysis o/the precipitates Fig. 2 shows the DTA.-TG curves of various pow-

ders obtained. From the figure it can be seen that the weight losses (WE) of all powders are basically completed before about 360:C, but the quantities are different from each other. For powder P-l-3 it is less than 25%, while for powders P-I-2 and P-l-4 they are 51-53%, and the WL of crystalline Z r ( C ~ O 4 ) 2 4 H 2 0 (P-I-5) reach 68-69% of the total weight (P-2 is not shown).

3,3. l'/7ase transformation qf powder during, calcination All powders were calcined from 420 C to 1000: C for 25 rain. 11 is known that the calcination at relatively low temperature of zirconium salts or zirconium hydroxide can lead to the formation of metastable tetragonal ZrO,(t-ZrO2) and such metastable tetragonal ZrOe will transform to monoclinic ZrO2 ( m-ZrO2 ) at relative higher temperature [ 10 ]. Generally speaking, the temperature (7]) at which t-ZrO: transform to monoclinic ZrO~ is less than or approximately equal to 800 ~C [ 10-12] for pure ZrO2 powder. This is also true for the powders prepared

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Fig. 4. TEM pictures of powders P-l-l, P-I-3 and P-1-5 betbre calcination.

J.L. Shi, Z.X. Lin /Preparation of ZrO: powderby oxalateprectpitation in our experiments. Fig. 3 shows the dependence of t-ZrO2 content on calcination temperature. It can be seen that the t-ZrO2 content decreases as the temperature increases, but the I] for different powders are different. The T~ of powder P-l-1 is above 750°C and only less than 20% oft-ZrO2 exists above 520°C for P-l-3; for powders P-l-4 and P-1.5, t-ZrO, transform to m-ZrO2 between 520 and 750°C. 3.4. Particle size and morphology There are different particle morphologies for differently washed powders. Fig. 4a is a TEM picture for powder P-l-l, in which very solid agglomerates can be seen. If the precipitate is washed with dilute ammonia (P-I-3), looser agglomerates can be obtained as shown in fig. 4b. The particles of both powders P-l-1 and P-l-3 are irregularly shaped. Particles of crystalline Zr (C204) 2"4H20 of powder P- 1-5 are rod-like with an average size of about 0.13 X 0.3 gin. Calcination of differently washed powders at 600 :C leads to the formation of ZrO2 but with different particle size, morphology and agglomeration

states. For unwashed powder, the particles are large, irregularly shaped and aggregated, as shown in fig. 5a. Washing with dilute ammonia does only slightly improve the situation as indicated in fig. 5b. In fig. 5c, a TEM picture of an alcohol washed powder under basic conditions is shown, and it can be seen that basically non-aggregated fine particles formed when it was calcined at 600°C, and even some lattice stripes can be seen on some grains (as arrowed). These well-crystallized fine particles are about 20 lain or less in size. 3.5. Stabilized ZrO, with Y,O~ by oxalate coprecipitation Y203-stabilized ZrO2 powder could also be prepared by oxalate coprecipitatiom by adding H2C204 solution to a mutual solution of zirconium and yttrium nitrate [ 13 ]. The coprecipitate is washed with alcohol several times and dried at 100°C. Such prepared powder could be sintered to high density at 1400°C (13].

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m

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Fig. 5. TEM pictures of powders P-l-l, P-I-3 and P-I-5 after calcination at 600°C.

548

.I.L. Shi. Z.X. Lin / Preparation qf ZrO: powder bj, o.valaleprecipitation

4. Discussion

4.2. 1"he stability q f metastable t-ZrO: in calcination

4. l.Analvsis o f precipitation products The differently washed powders have different phase and different weight losses on heating, as noted above. In table 1, the weight loss, yield of Z r O : and the phase of the powders obtained are listed. For powders P-l-5 and P-2, X R D analysis has confirmed that the precipitates are crystalline Zr (C204) 2'4H20 with their W L = 6 8 - 6 9 % on heating. On the D T A T G spectra, it can he found that there are two endothermal peaks during heating, one at 100-120~C and another at 3 3 4 - 3 3 5 ° C . It is estimated that the first peak ( < 2 0 0 ° C ) corresponds to the vaporization of free water and some structure water (about 2H20 in Zr(C~Oa)2"4HeO) which cause about 9 10% weight loss; and the second peak ( > 3 0 0 : C ) which leads to about 58-59% weight loss corresponds to the d e c o m p o s i t i o n of C20~ and the remaining structure water. Similar p h e n o m e n a were observed for powders P1-2 and P-l-4, as shown in fig. 2. However, dilute a m m o n i a washing leads to an unusual behavior. The total weight loss is less than 25% and no higher-temperature endothermal peak and corresponding weight loss are observed. With the help o [ X R D analysis (fig. l c), it is speculated that the zirconium oxalate precipitate has converted to zirconium hydroxide during washing with NHaOH solution, and the total weight loss of 24.7% is basically the same as the water content in Z r ( O H ) 4 (22.6%). Such a conversion is a precipitation conversion from oxalate to hydroxide in the presence of O H - .

Tetragonal ZrO2 can exist as a metastable phase at room temperature when its grain size is less than a critical value [ 11 ]. Many authors [ 10-12] think that this is because the surface energy of tetragonal ZrO~ is lower than that of the monoclinic form. The critical size means that at this size tetragonal ZrO2 and monoclinic ZrO2 have the same total energy (including Gibbs energy and surface energy) when dynamic factors are not considered. When the grain size is less than the critical value, tetragonal ZrO2 will be more stable than the monoclinic form because the former has lower total energy than the latter: but as the grain size increases, the difference in surface energy between the two phases decreases and the metastable tetragonal phase will transform to the monoclinic one. In fig. 6, the correlation between grain size and calcination temperature is plotted. The figure shows the grain size change as temperature increases for the powders obtained from oxalate precipitation, but it seems that there was no fixed critical size or transformation temperature for different powders (see fig. 3). For as-precipitated powder ( P - I - 1 ) , t-ZrO: is the main phase at 750:C, but the grain size is larger than 40 nm (fig. 6). For alcohol washed powder under basic conditions ( P - I - 5 ) , the content o f m-ZrO2 is larger than 75% at 600°C with the grain size less than 22.5 nm; and for dilute a m m o n i a washed powder ( P1-3) the content of m-ZrO is greater than 95% at 600~C but the grain size is less than 30 nm. So, in addition to the grain size, there must be other factors

Table 1 Anal>sisof precipitation products. Powder

P-I-I P-I-2 P-I-3 P-I-4 P-l-5 1"-2

WL

ZrO:

(%)

(%)

61.0 50.5 24.7 53.0 68.4 69.2

39.0 49.5 75.3 47.0 31.6 30.8

Phase

microcrystalline microcwstalline amorphous amorphous cwstalline crystalline

Estimated products molecular formula

ZrO~ (%)

Zr(OH)a

77.4

Zr(C~O4)r4H20 Zr(QO4 k,'4H:O

36.3 36.3

J.L. Shi. ZX. Lin / Preparation of ZrO: powder by o.valate precipitation

Am-Zr02 • t-ZrO2

9 .g

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/p

-I-I 3 ~_.1._~,-~-l
I000

Fig. 6. Dependence of the grain size of powders on calcination temperature.

that affect the phase relations of ZrO2 powders obtained. As has been shown in figs. 5 and 6, powder P- 1-1 has a larger grain size and the grains are strongly aggregated when calcined at 600 ° C; but smaller and basically non-aggregated grains were obtained for powder P-l-5. It migh be speculated that the strong aggregation of grains could cause a compressive stress on grains, and such a compressive stress would exert a resistance to the transformation of t-ZrO2 to mZrO2 because the cell volume of the monoclinic phase is larger than that of the tetragonal phase. So the difference in temperature and critical size between tZrO2 and m-ZrO2 is considered to be closely related to the different aggregation states of differently treated powders. Higher temperature and larger critical size are needed for strongly aggregated powders for the t-ZrO2 to m-ZrO2 transformation.

5. Conclusion

Ultrafine ZrO2 powder (pure or Y203 stabilized) can be prepared by oxalate precipitation or coprecipitation by adding H2C204 tO proper amount of zirconium salt solution or to a mutual solution of zirconium and yttrium salts, or adding NH4OH solu-

549

tion to a complex compound solution of zirconium oxalate. The differently treated powders have different states of crystallization and particle morphology. All the powders decompose below 400°C and lead to the formation of pure ZrO> but grain size and the aggregation state of Z r O 2 after calcination are also process dependent. Metastable tetragonal ZrO2 occurs on low-temperature calcination and transforms to the monoclinic phase as the temperature increases, but the transformation temperature and the critical size for the different powders are different, which is considered to be related to the different aggregation states of the powders after calcination.

References [ 1 ] D.W. Johnson and P.K. Gallagher, in: Ceramic processing before firing, eds. G. Onoda and L.L. Hench (Wiley, New York, 1978) p. 125. [2] K. Haberko, Ceram. Intern. 5 (1979) 148. [ 3 ] M.A.C.G. van de Graaf, K. Keizer and A.J. Burggraaff, Sci. Ceram. 10 (1979) 83. [4 ] K.S. Mazdiyashi, C.T. Lynch and J.S. Smith, J. Am. Ceram. Soc. 48 (1965) 372. [ 5 ] K.S. Mazdiyashi, C.T. Lynch and J.S. Smith, J. Am. Ceram. Soc. 50 (1967) 532. [6] A. Roosen and H. Hausner, in: Ceramic powders, ed. P. Vincenzini (Elsevier, Amsterdam, 1983 ) p. 773. [7] B. Fegley Jr., P. White and H.K. Bowen, Am. Ceram. Soc. Bull. 64 (1985) 1115, [8] J.L. Shi, J.H. Gao and Z.X. Lin, Chinese J. Silicate Soc. ( 1988 ), to be published. [9] W.S. Clabaugh, E.M. Swiggard and R. Gillchrust, J. Res. Nat. Std. 56 (1956) 289. [10] M,I. Osendi, J.S. Moya, C.J. Serna and J. Soria, J. Am. Ceram. Soc. 68 (1985) 135. [ 11 ] R.C. Garvie, J. Phys. Chem. 82 (1978) 218. [ 12 ] T. Mitsuhashi, M. lchihara and U. Tatsuka, J. Am. Ceram. Soc. 57 (1974) 97. [ 13] J.L. Shi and Z.X. Lin, in preparation.