CERAMICS INTERNATIONAL, Vol. 9, n. 2, 1983 59

Influence of Precipitation Procedure on Sinterability of Y203 Prepared

from Hydroxide Precursor M.D. RASMUSSEN, G.W. JORDAN, M. AKINC, 0. HUNTER, JR. and M.F. BERARD

Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA

The sinterabiiity of YzOl prepared from reverse strike hydrox- ide precursors was found to be highly dependent on final pH and precursor dewatering procedure, high pH precursors yielding poorly-sinterabie oxides if oven dried, but quite sinterabie ox- ides if dewatered either by acetonatoiuene-acetone or controi- led-humidity treatments. Direct strike hydroxide precursors yielded highly-sinterabie Y203 regardless of final pH or dewater- ing procedure.

1 - INTRODUCTION

The most frequently used technique for preparing high- purity oxide powders for fabrication into ceramic specimens is by calcination of a precursor compound that has been precipitated from an acidic solution of the oxide cation(s). The precursors most used are oxalates, carbonates and hydroxides.

In the batch precipitation of precursors of a single oxide, one has a choice as to whether to add the precipitation solu- tion (i.e,, one rich in oxalate, carbonate or hydroxide ions) to the acidic cation solution (the so-called adirect striken method), or to add the acidic cation solution to the precipita- tion solution (the so-called *reverse strikem method) . The en- vironment in which nucleation and growth of the insoluble precursor precipitate occurs is different in the two cases, and it is to be expected that major differences in precursor mor- phology’*’ and other characteristics may thereby result.

The purpose of this study was to investi ate differences in sinterability of YZOS powders produced a rom direct and reverse strike precipitated hydroxide precursors. As part of this study, several different precursor dewatering pro- cedures were investigated which had previously been reported2-’ to have dramatic effects on the behavior of reverse strike precursors of rare-earth oxides and rare-earth stabilized hafnia and zirconia.

14 ( I I I 1 ------ - REVERSE STRIKE

12 - DIRECT STRIKE

z ‘\ ; IO-‘----__,

2 ------____ a 3 0-

w

: 6-

P n 4- 3 n

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0 50 100 150 200

ml of REAGENT ADDED

FIGURE 1 - pH history for final pH of 9.0.

2 - EXPERIMENTAL PROCEDURES

An acidic cation solution of Y was prepared by dissolving 99.9% Y203* in 4.0 N HNOs to produce a concentration of 50 g of Y203 per liter at pH << 1. The precipitation solution (base) consisted of 2.5 N NH4 OH with a pH of 11.85.

Reverse strike precipitations were carried out by adding the Y(NO& solution at the rate of 12 mllmin to 192 ml of the base using a peristaltic pump to control the addition. The pH of the solution containing the precipitate was continuously monitored with a digital pH meter (Markson, Model 88). Direct strike precipitations were carried out by metering the base at a rate of 13 mllmin into 80 ml of the acidic solution. Both direct and reverse strike precipitations were carried to final pH values of 7.0, 9.0, and 10.5. Figure 1 shows pH histories for direct and reverse strike precipitations with a final pH of 9.0.

After precipitation, the precursors were washed twice with approximately 180 ml of distilled water, with excess water being removed by vacuum filtration on coarse paper in a Buchner tunnel.

Three different methods were employed to remove the considerable amount of retained water from the somewhat gelatinous precursor precipitates. The first method, designated as oven drying (OD), consisted in placing the water-washed precipitate in an uncovered beaker in a cir- culating oven at 11 O°C for at least three hours. The second method, designated as acetone-toluene-acetone (ATA) dewatering used a series of washes, first acetone, then toluene, then acetone, to strip the retained water from the precipitates. Vacuum filtration in the Buchner funnel foilow- ed each wash, and the dewatered powder was stirred in a warm (150°C) mortar and pestle to drive off the acetone after the final wash. The ATA method has previously been reported2*3’5 to yield quite sinterable oxide powders of a variety of types from reverse strike hydroxide precursors. The third dewatering method, designated as controlled- humidity drying (CHD), consisted of placing the water- washed precipitates in a chamber maintained at 95% relative humidity and 90°C for 100 hours. The CHD method has previously been reported4 to yield sinterable rare-earth oxides from reverse strike hydroxide precursors.

Following dewatering by one of the three methods, all precursors were subjected to identical subsequent treat- ment This began with calcination to Y2C3 in air at 1000°C for two hours. The calcined powders were ground to pass a 80-mesh sieve (all of the ATA-derived powders passed 80 mesh without grinding) and were pressed (without binder) in- to cylinders approximately 0.95 cm diam. by 0.5 cm long, first in a double-acting die at 50 MPa followed by isostatic pressing at 207 MPa. Three to five specimens were prepared for each combination of precipitation and dewatering methods. The specimens were sintered in vacuum in covered Y203 crucibles at 1 880°C with a rate of temperature rise of 20°C/min and a one-hour hold at sintering

a Ames Laboratory Rare-Earth Separation Group.

60 M.D RASMUSSEN, G.W. JORDAN, M. AKINC, 0. HUNTER, JR. and M.F. BERARD

temperature. Following sintering, all specimens were heated in air for two hours at 1 100°C to offset slight reductions that may have occurred during the vacuum sintering. The bulk density of each specimen was determined by a saturation and immersion technique that has been shown to have a precision of +0.013 g/cc’. Using an x-ray density of 5.031 g/cc as calculated from the iattice parameter for pure Y203’, measured bulk densities were converted into % theoretical density values as an index of the sinterability of the oxide powders.

3 - RESULTS AND DISCUSSION

Hydroxide precipitations of Y (and the heavy rare-earth oxides) are quantitative only for final pH values above about 8. Consequently, the precipitations carried out at a final pH of 7.0 were incomplete with about 50% yield.

The effects on sinterability of precipitation method, final pH and precipitate dewatering method are summarized in Figure 2 which shows average sintered bulk densities (% of theoretical) as a function of these variables.

For direct strike precipitations (Fig. 2a), quite sinterable powders are produced regardless of final pH or dewatering method utilized. Minor differences suggest that higher final pH is desirable, that CHD offers no advantage over OD dewatering, and that ATA dewatering actually reduces the sinterabiliy of oxides derived from direct strike precipitates.

The picture is quite different for reverse strike precipitates (Fig. 2b). For OD dewatering, the sinterability of powders is very dependent on final pH of precipitation. A final pH of 7.0 yields highly-sinterable material, but it should be recalled that precipitation is far from complete at this pH. Sintered density drops drastically as final pH is increased for OD-dewatered materials. When a reverse strike precipitation of Y or a heavy rare-earth is carried out simply using concen- trated NHdOH in excess, the final pH will typically be bet- ween 10 and 11. The present results clearly show why such uncontrolled precipitations usually yield powders that are onnrlv sinterahln when thg nrecursnr is dnwntnrd hv hnatino r__.., _...__.__._ r. _ _ _ _ _ . . - _ _ . . _. _ _ _ , . _ _. . . = In an oven. Long experience at our laboratory has also shown that high pH reverse strike hydroxide precipitates give great variation in oxide sinterability from batch to batch when oven dried.

FIGURE 2 - Slntered densltles of Y203 as function of preclpltatlon method, final pH and dewaterlng method. (a-Direct Strlke, b- Reverse Strike).

If a reverse strike precipitation has been carried out at a high final pH, Figure 2b indicates that quite sinterable oxide powders can nevertheless be produced if, instead of oven drying, the precipitate is dewatered by either ATA or CHD methods. The results for CHD are slightly better than for ATA, but the CHD process is much lengthier and requires a special cpntrolled-humidity chamber.

2.

3.

4. While the findings of this study should be applicable to

the preparation of all heavy rare-earth oxides, care must be taken-in generalizing them to systems in which other cations are also present. Co-precipitation of hydroxide precursors of a rare-earth oxide with even small amounts of ZtQ or Ht02 must be carried out as a reverse strike to avoid segregation problems. (If performed as a direct strike, precipitation of hydroxides of Zr or Hf would begin at a pH of about 3 and would be essentially completed before reaching a pH at which the rare-earth hydroxide would begin to precipitate5). Such reverse strike hydroxide precipitates containing Zr or Hf will not yield sinterable oxides either by oven drying or by any combination of conditions of CHD dewatering. Only ATA dewatering has proven effective in producing sinterable materials from hydroxide amounts of either Zr or Hf 5.

precursors containing significant

5.

6.

7.

8.

REFERENCES 9.

1. D.T. RANKIN and G.A. BURNEY, #Particle Size of 23*Pu02 Ob- tained by Oxalate Precipitation and Calcination-. Am. Ceram. Sot. Bull.54(1975)1061. Received August 23,1982; accepted September 29,1982.

ATA CHD

8.0g

i

1 3.0 I

8.E

0.:

9 7

T

I 1 I 9.c

FINAL pH

OD ATA 9.‘

i ‘

I I

7.0 7.0

33.t

69.4

II 3.0 10.5

-L FINAL pH

s.L. DOLE, i3.w. SCHE~DECKER, L.E. SHIERS, M.F. BERARD and 0. HUNTER. Jr., *Technique for Preparing Highly Sinterable Oxide Powders*, Mat. Sci. Eng. 32 (1978) 277. R.J. TESCH, CD. WIRKUS and M.F. BERARD, *Selfdiffusion of Er and Hf in Polycrystalline Er203-stabilized Hf02*, J. Am. Ceram. Sot. 65 (1982) 511. G.W. JORDAN and M.F. BERARD, *Production of Highly Sinterable Oxide Powders by Controlled Humidity Dewatering of Precursorsm, Presented at the 83rd Annual Meeting of the American Ceramic Socie cepted for Publication in 2

Washington, DC., May 5, 1981 (Ac- eramics International).

G.W. JORDAN, M.F. BERARD and CD. WIRKUS, &intering Behavior of Er203-stabilized HfOl Powders Produced by Dif- ferent Processing Methods., Proceedings of the American Ceramic SocietyAlME Powder Processing Symposium, Louisville, KY, October 1 l-15,1981 (in press). 0. HUNTER, Jr., M. AKINC and M.D. Rasmussen, -Effect of Processing Variations on the Sinterability of YzOjm, Presented at the 83rd Annual Meeting of the American Ceramic Society, Washington, D.C., May 6,198l. M.D. RASMUSSEN. M. AKINC and 0. HUNTER. Jr.. ??Dewater- ing of Yttrium Hydrhxynitrate Precipitates by Organic Liquids,, Proceedings of the American Ceramic Society-AIME Powder Processing Symposium, Louisville, KY, October 11-15, 1981 (in

- press R.J. 4. ESCH, &elf-diffusion of Hf in ErzOsdoped Polycrystalline Hf02*. M.S. Thesis. Iowa State Universitv. Ames, Iowa (1981). V.B. GLUSHKOVA and A.G. BOGAN-?)V, *Polymorphism of Rare-earth Sesquioxides-, Academy of Sciences U.S.S.R., Chemistry Series Bulletin 7 (1965) 1101.