Structural characteristics of ZrO2 powders prepared from acetates

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Structural Characteristics of ZrO2 Powders Prepared from Acetates

E. B e r n s t e i n , a M . G . B l a n c h i n a & A . S a m d i b

a D6partement de Physique des Mat6riaux (U.A. CNRS No. 172). b Laboratoire de Chimie Min6rale 1II (U.A. CNRS No. 116), Universit6 Claude Bernard 69622 Villeurbanne C6dex, France

(Received 27 October 1988; accepted 12 January 1989)

Abstraet: Submicron Z r O 2 powders were prepared by thermal decomposition of two zirconium acetates of different compositions and crystallographic natures. In both cases, the phase transformations were the same, i.e. crystalline or amorphous acetate ~ amorphous ZrO 2--* tetragonal ZrO 2 ~ monoclinic ZrO2; however, important morphological differences between the two powders were observed by transmission electron microscopy. The carbon content of the precursor seemed to play an important role in determining the morphology of the powders. Samples with increased and with reduced carbon content respectively, were studied to clarify this effect.

1 I N T R O D U C T I O N

Few materials show as much potential as Z r O 2- based ceramics, for such a wide range o f advanced engineering ceramic applications. To p r o d u c e these ceramics, high-purity powders with uniform particles o f submicron size and good sintering properties are required. 1

Although much w o r k has been done on preparation methods o f ultrafine Z r O 2 powders in recent years,2-6 there is n o w special interest in finding simple, low-temperature, inexpensive and reprodu- cible m e t h o d s for preparation o f powders which have such characteristics. Thermal decomposition o f zirconium acetates could fulfil these requirements.

Nevertheless, little w o r k has been done on Z r O 2 powders prepared from acetates. 7'8

The aim o f this study was to determine the effect o f composition and crystalline nature o f the precursor on the final characteristics o f the Z r O 2 powder.

Samples obtained from two different acetates were compared. The c a r b o n content resulting from the decomposition o f acetates seems to play an important role in determining the morphological character-

istics o f particles (size, aggregation, state o f surface).

Both the addition o f carbon, as sugar, and the reduction o f c a r b o n content by calcination at 400°C were analysed to clarify this effect.

2 E X P E R I M E N T A L P R O C E D U R E 2. 1 Powder preparation

Submicron Z r O 2 powders were prepared by thermal decomposition o f two zirconium acetates o f different compositions and crystalline natures. Both acetates were obtained by different techniques from the same commercially available zirconyl acetate solution, which was m a n u f a c t u r e d by Riedel-de H a e n and contained 22 g l - 1 o f Z r O 2.

(a) A crystallized acetate (C) was p r e p a r e d by crystallization o f the zirconyl acetate solution, in pure acetic acid. Chemical d e t e r - mination o f the concentration o f carbon, hydrogen and zirconium gave a ratio C H a C O O - / Z r ~" o f 2"5. The chlorinecontent, determined by potentiometry, was < 4 x 1 0 - 4 % . 9

337

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338 E. Bernstein, M. G. Blanchin, A. Samdi (b) An amorphous acetate (A) was prepared by

partial hydrolysis o f the same zirconyl acetate solution, and was evaporated on a sand bath at 70-80°C. In this case, the ratio C H 3 C O O - / Z r ~ was 1.2.1°

Both acetates were calcined up to total elimination of carbon, i.e. to 850°C for sample C and 950°C for sample A respectively.

2.2 Characterization techniques

Both samples were analysed by differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) at a heating rate of 600 and 150°C/h respectively. Carbon content was determined at each temperature by chemical analysis. 9'1°

The crystallographic phases present were determined by X-ray diffractometry (XRD) and crystallite sizes were calculated using Sherer's equa- tion 11 for the (111) line o f tetragonal Z r O 2 and the (111) line of monoclinic Z r O 2 respectively.

The residual carbon content was investigated by electronic paramagnetic resonance. 9

The specific surface area o f the powders was measured by the multi-point BET method.

The morphological characteristics, size distribution and aggregation state of the powder particles were observed by transmission electron microscopy (TEM). The T E M samples were prepared by direct deposition of powder onto a carbon film on copper T E M grids; other techniques, such as dispersion in different solvents, resulted in highly agglomerated samples.

3 R E S U L T S

3. 1 Thermal behaviour

The D T A and T G A curves corresponding to both samples were published elsewhere. 9'1° The carbon content and the crystallographic phases present were determined at temperatures corresponding to the conspicuous points on those curves, and are given in Tables I and 2.

Altogether, these results showed that, in both cases, independently of the crystalline nature of the acetate, the phase transformation was the same, i.e.

crystalline or a m o r p h o u s acetate ~ a m o r p h o u s ZrO2 --* tetragonal ZrO2 ~ monoclinic ZrO2.

The thermal decomposition of the acetate was slower for sample A than for sample C. It took place between 50 and 500°C for A and between 40 and

Table 1. Carbon content and crystallographic phases present in the samples prepared from the amorphous acetate (A) calcined at temperatures corresponding to the conspicuous points on the DTA

and TGA curves (150°C/h)

Temperature Carbon Crystallographic (°C) content (%) phase (XRD) Before

calcination 13.4 Amorphous acetate

345 7.4 Amorphous ZrO=

570 2.0 Tetragonal ZrO=

750 0.6 Tetragonal +

monoclinic ZrO=

350~00°C for C. Crystallization of the tetragonal phase occurred with a difference of 100°C between the samples (at 530°C for A and at 435°C for C).

Carbon was eliminated more rapidly in sample C than in sample A, especially at the beginning of heat treatment. For C the carbon content changed from 22 to 2"5% when calcined up to 345°C at a rate of 150°C/h, whereas the change for the same thermal treatment for A was from 13.4 to 7"4%.

The relative content of tetragonal ZrO2 versus temperature for both samples is shown in Fig. l(a).

Transformation from the tetragonal to the monoclinic phase was completed at 850°C for sample C, whereas 30% of the tetragonal phase remains in sample A at this temperature, and the transformation in this case was completed at 980°C.

RPE experiments revealed symmetric signals, with g = 2 . 0 0 2 6 typical of carbonaceous impuri- ties. 8'9 Signal disappearance, which indicated the elimination of residual carbon, took place at 840°C for sample C and at 980°C for A; this coincides with the complete transformation from the tetragonal to the monoclinic phase.

Table 2. Carbon content and crystallographic phases present in the samples prepared from the crystallized acetate (C) calcined at temperatures corresponding to the conspicuous points on the DTA

and TGA curves (150°C/h)

Temperature Carbon Crystallographic (°C) content (%) phase (XRD) Before

calcination 22.0 Crystallized acetate

345 2.5 Amorphous ZrO=

450 1.2 Tetragonal ZrO=

750 0"5 Tetragonal +

monoclinic ZrO=

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,-~100

0

C O

~ 50

b

C ~ Cm

Aq

g00 500

600

5 0 0 "~

z, O0 ~ 300 m N

_ 200

600 700 800 900 1000 T(*C)

Fig. 1. (a) Relative content oftetragonal-ZrO2 as a function of temperature for samples prepared from the amorphous acetate (A) and from the crystallized acetate (C). (b) Thermal evolution of crystallite size for tetragonal phase (AqA, C q O ) and

monoclinic phase (A m/k, Cm O).

The evolution of crystallite size (measured by XRD) depending on temperature, is shown for both samples in Fig. l(b). For all temperatures consid- ered, the crystallites of sample A were smaller than those of sample B. Nevertheless, some similarities were observed: (1) the crystallite size of the tetragonal phase increased slowly with temperature;

(2) when both phases coexisted, crystallites of the monoclinic phase remained slightly smaller than those of the tetragonal phase; and (3) once the transformation was completed, the size of monoclinic crystallites increased sharply.

typical distribution of particles in those samples. The particles of powder A (950°C) were easily dispersed and formed agglomerates with size between 0-05 and 0.75~m (Fig. 2(a)). The particles of powder C (850°C) seemed very charged and were more heavily agglomerated. Two different populations of particles could be seen in this sample: agglomerates of almost the same sizes as those of the previous sample (for instance a and b in Fig. 2(b)), and large agglomerates of a few micrometres (for instance, c and d in the same figure).

Observations at higher magnification revealed important morphological differences between both samples, as shown in Fig. 3. Sample A is composed of loose agglomerates of particles, which have sizes ranging from 20 to 250nm. Some of the smallest particles (e.g. those marked a, b and c in Fig. 3(a)) were monocrystalline and their size was in good

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~, ^~ ^" ^l~m

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3.2 Microstructural studies

Both samples were calcined up to the temperature at which the transformation from the tetragonal to the monoclinic phase was completed, i.e. 850°C for C and 950°C for A, and they were then characterized by BET and TEM. The resulting specific surface areas were: 12.2m2/g for C (850°C) and 6.2m2/g for A (950°C).

3.2.1 Transmission electron microscopy observations Low-magnification micrographs (Fig. 2) showed the

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Fig. 2. Transmission electron micrographs showing typical distribution of particles in (a) ZrO 2 powder prepared from the amorphous acetate calcined at 950°C: A (950°C); (b) ZrO 2 powder prepared from the crystallized acetate calcined at 850°C:

c (850°c).

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340 E. Bernstein, M. G. Blanchin, A. Samdi

Illl

(a) (b)

Fig. 3. Transmission electron micrographs showing morphology of particles and electron diffraction patterns from (a) ZrO2 powder prepared from the amorphous acetate calcined at 950°C: A (950°C); (b) ZrO2 powder prepared from the crystallized acetate calcined at

850°C: C (850°C).

agreement with the crystallite median size determined for this sample by X R D (30 nm). The largest particles probably originated from many crystallites, and in some cases (e.g. d and e) grain boundaries were visible. The presence of these relatively large particles would explain the fact that the specific surface area determined by BET for this sample was 6.2 m2/g and corresponds to an equivalent spherical diameter of 0.16 #m.

Agglomerates in powder C were formed by thin platelike particles (e.g. those shown in Fig. 3(b)) with sizes ranging from 130 to 500 nm, i.e. they are larger than particles of the previous sample, but are much thinner. Their thickness, determined by tilting the platelets in the microscope, was approximately 30 nm, which was about the median size of crystallites for this sample, as determined by XRD.

Grain boundaries were clearly visible (see arrows), and indicated that the particles were formed by internal sintering of a large number of crystallites.

This sintering seems to occur preferentially into a plane, leaving many pores (see a and b in Fig. 3(b)).

The surface seems to present many small cavities.

The specific surface area for this sample is higher than for the previous one, i.e. 12.2 m2/g. This could be explained by the reduced thickness of these particles. This value would correspond to a powder formed by parallelepipeds, of approximately

300 nm x 300 nm and 30 nm thick. The equivalent spherical diameter has less physical meaning in this case.

The corresponding selected area diffraction patterns (SADP) for both samples are shown in Fig.

3(a, b); they have similar characteristics, as they are formed by a great number of spots which correspond to many well-crystallized particles of monoclinic structure, with different orientations. No preferential orientation of the particles was revealed by the diffraction patterns.

3.3 Effect of carbon content on internal sintering during calcination

The more rapid elimination of carbon in sample C than in sample A suggests that the presence of carbonaceous substances during calcination could play an important role in determining the morphological characteristics of the powders, through inhibition of sintering for example. To clarify this effect, we have studied the behaviour of a sample prepared from the crystallized acetate with addition of carbon.

The solid acetate was mechanically mixed with about the same amount of sugar, both as powders, before being calcined up to 950°C and maintained at this temperature for 12h. To provide a basis of

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comparison, a Similar thermal treatment was carried out on samples o f b o t h acetates C and A, without addition o f sugar.

T r a n s m i s s i o n electron m i c r o g r a p h s o f these samples are shown in Figs 4 and 5. Figure 4 shows b o t h samples without sugar, after the calcination for 12h. Sample A (Fig. 4(a)) is c o m p o s e d o f small c o m p a c t aggregates formed by a few particles sintered together without any preferential directions. In contrast, sample C (Fig. 4(b)), is formed o f very large aggregates, in which sintering seems to have occurred in preferential directions (see a), leaving, in some cases, very large pores as shown in (b).

Figure 5 shows the behaviour o f sample C after

!i i !iiiiiii !!i!iiill , !i i ¸ !!i iiiiii !i ii i! . . .

Fig. 5. Transmission electron micrograph of ZrO 2 powder prepared from the crystallized acetate (C) mixed with sugar

before being calcined for 12h at 950°C.

the addition o f sugar. The size o f aggregates has been much reduced, c o m p a r e d with those in the sample prepared from the same crystallized acetate without sugar (see Fig. 4(b)). Also, sintering seems to have lost its directionality and particles look more similar

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(b)

Fig. 4. Transmission electron micrographs of powders calcined for 12 h at 950°C. (a) ZrO 2 prepared from the amorphous acetate (A). (b) ZrO2 prepared from the crystallized acetate (C).

A

(a)

~ m

~ n m

(b)

Fig. 6. Transmission electron micrographs of ZrO 2 powder prepared from the amorphous acetate (A) maintained for 4 h at

400°C before being calcined up to 950°C.

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342 E. Bernstein, M. G. Blanchin, A. Samdi to those prepared from the amorphous acetate (Fig.

4(a)).

Slightly different calcination conditions were used to study the effect of a reduction in carbon content before crystallization of the tetragonal ZrO2 in sample A: the amorphous acetate was heated to 400°C at a rate of 150°C/h, maintained at this temperature for 4 h and then calcined up to 950°C, at a rate of 900°C/h. This treatment gave rise to larger and denser aggregates (Fig. 6(a)) compared with the sample calcined directly to 950°C (Fig. 2(a)). The effect of internal sintering becomes important, as shown in Fig. 6(b), but it does not show any directionality, as was the case for sample C (Fig.

4(b)).

4 D I S C U S S I O N

Thermal decomposition o f zirconium acetates produces submicronic monoclinic Z r O 2 powders.

Independently of the nature of the acetate, the formation of monoclinic Z r O 2 is preceded by the existence o f an amorphous phase and then by the crystallization o f metastable tetragonal Z r O 2.

Nevertheless, the temperatures at which these transformations occur are different and the monoclinic powders obtained have quite different mor- phologies, depending on the acetate precursor.

The same transformation sequence has already been observed in many investigations, 7'a'12 and seems to be independent of the precursor used.

Livage et al. 12 have reported that the a m o r p h o u s

Z r O 2 s o far obtained is similar in all preparations and presents the same Z r - - Z r and Z r - - O distances as the tetragonal phase. Osendi et al. a suggested that the observed differences in crystallization temperature and intensity of the corresponding exothermic effect result from the influence of impurity molecules left over from the preparation method.

In the present case, the carbon and/or the carbonaceous substances appear to play a major role. Even if the crystallized acetate has a higher carbon content before calcination, the carbon is eliminated more easily in that case. This suggests a different interaction between the carbonaceous substances and Z r O 2 from both precursors.

When the tetragonal phase crystallizes, the carbon content in sample A (at 570°C) is twice as large as that in sample C (at 450°C). At these temperatures crystallites are larger in sample C than in A, even if the temperature is higher in the latter, which suggests that carbon inhibits crystal growth. In both cases, the average size of tetragonal phase crystallites does

not increase further than the critical size predicted by Garvie la ( - 300 nm). As the evolution o f size with temperature is similar for both samples, sample C reaches this critical values at a lower temperature.

This explains the difference in tetragonal stability as a size effect. However, this is probably not the only reason for this effect, because in both samples complete transformation to the monoclinic phase coincides with the total elimination of carbon traces.

Carbonaceous substances seem to have a great influence not only on crystallite size but also on their morphology. The addition of carbon as sugar to sample C has shown that carbon partially inhibits internal sintering. Even when all carbon has been eliminated, the thermal behaviour o f the powders are different, as indicated by the comparison between A and C calcined for 12 h. This is probably caused by differences in the structure of the particles' surface, determined by different interaction with carbonaceous material.

The fact that the crystallite size of monoclinic

Z r O 2 is slightly smaller than that oftetragonal Z r O 2

in the earlier stage of the transformation, i.e. when both phases coexist, was already observed by Murase & Kato, 7 who suggested that there might be a transformation to twinned monoclinic crystallites or a lattice strain o f the monoclinic crystallites under the conditions o f mutual restriction in powder aggregates and volume change in the transformation.

5 C O N C L U S I O N

The crystalline nature of the precursor does not affect the phase transformations observed during the preparation o f ZrO2 by thermal decomposition of acetates, but T E M observations have revealed that the carbon content o f t h e precursor acetate and its ability to be eliminated greatly influence the m o r p h o l o g y o f the powder particles. As such morphological differences could give rise to very different behaviours during further sintering o f the powders, this work has emphasized the need for microstructural studies at a resolution which cannot be achieved by optical or scanning electron microscopy. Further studies o f the structure of the particle surface by T E M at higher resolution are now under way.

A C K N O W L E D G E M E N T S

The authors are grateful to Th. Grollier Baron, B.

Durand and M. Roubin (Laboratoire de Chimie

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Min6rale III) for their contributions to powder synthesis and characterization and for stimulating discussions.

R E F E R E N C E S

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4. TANI, E., YOSHIMURA, M. & SOMIYA, S., Hydro- thermal preparation of uitrafine monoclinic ZrO 2 powder.

J. Am. Ceram. Soc., 64(12) (1981) C-181.

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12. LIVAGE, J., DOI, K. & MAZIEREZ, C., Nature and thermal evolution of amorphous hydrated zirconium oxide.

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