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«Oxalate-precursor processing for high quality BaZrO3 Corresponding Author Nigel M. Kirby* Curtin University of Technology Department of Applied ...»

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DTA/TGA analysis of precursors dried at 120ºC was conducted on a Setaram TAG24 instrument using a heating rate of 10ºC per minute to 1300ºC in air. Specimens were analysed using Pt crucibles using alumina as a reference. Multipoint N2 BET analysis was conducted using an ASAP 2400 surface area analyser (Micromeritics Inc.) on samples vacuum dried at 200ºC. Powder specimens for TEM analysis were dispersed in water using ammonium polyacrylate dispersant (Dispex A-40, Allied Colloids) and dried on holey carbon grids. TEM analysis was conducted using a JEOL JEM-2011 operated at 200kV.

2. Results and Discussion

2.1 Barium Chloride, Zirconium Oxychloride, Oxalic Acid System Powders produced in preliminary experiments using BaCl2, zirconium oxychloride and oxalic acid with a solution mole ratio of 1.00:1:2.20 Ba:[Zr+Hf]:C2O4 were severely barium deficient irrespective of reaction conditions (Table 1). A 10% excess of oxalic acid was not sufficient for stoichiometric precipitation when using a 1.00 : 1 BaCl2 : zirconium oxychloride solution mole ratio as has previously been claimed 8.

Table 1 HERE A second series of experiments was performed to investigate the use of larger excesses of oxalic acid. because preliminary results showed that a severely barium deficient product was produced at solution mole ratios of 1.00:1:2.20 BaCl2:Zr:H2C2O4. Figure 1 shows the effect of oxalate excess on product stoichiometry for the BaCl2, zirconium oxychloride and oxalic acid system using a 1.00 : 1 Ba:Zr+Hf solution mole ratio. The addition of greater than 10% excess of oxalic acid was useful in controlling product stoichiometry in the BaCl2, zirconium oxychloride, oxalic acid system at both ambient and elevated temperature. In all processes investigated, the stoichiometry of the product was dependent on the initial mole ratio of the solution used for precipitation.

Barium chloride, zirconium oxychloride and oxalic acid was a poor system for controlling product stoichiometry, due to the relationship between product stoichiometry, solution ratios and temperature. Unless the ratio of H2C2O4 : Zr was approximately 2.6:1 and not 2.2:1 as implied by Reddy and Mehrohtra 14, a stoichiometric product could not be obtained at elevated temperature without a significant excess of barium or the addition of ammonia. The sensitivity of product composition to solution ratios at ambient temperature made the barium chloride, zirconium oxychloride and oxalic acid system unsuitable for high quality process control. The system was also sensitive to acid/base additions: addition of either HCl or NH3 caused reduced and increased product Ba:[Zr+Hf] mole ratios, respectively.

Figure 1 HERE The chemical properties of zirconium salts are affected by their processing history, for example, speciation prior to crystallization, thermal history, extent of drying etc. This is primarily due to differences in olation and oxolation. In order to confirm that the chemical properties of Millennium zirconium oxychloride were not responsible for the higher oxalic acid addition required for stoichiometric precipitation than previously reported 14, reactions were also conducted using Riedel-de Haën zirconium oxychloride. Figure 1 shows there is effectively no difference between the results obtained with Millennium or Riedel-de Haën zirconium oxychloride. At near boiling temperature, the flatter region of the 95ºC curve in Figure 1 suggests the system may be capable of stable production. However, the requirement for elevated temperature control adds undesired complexity to the process.

Fresh zirconium oxychloride – oxalic acid solutions are strongly acidic at ambient temperature and heating to 95ºC and cooling back to ambient temperature lead to irreversible precipitation and pH change. For example, a freshly mixed solution containing 0.15 M zirconium oxychloride and

0.36 M oxalic acid had a pH of 0.63 ± 0.05. Heating this solution for 30 minutes at 95ºC caused the formation of a large amount of a white colloidal precipitate, and after cooling to ambient temperature the pH dropped to 0.30 ± 0.05. This precipitate did not dissolve within several weeks at ambient temperature, thus the effect of heating acid zirconium oxalate solutions appeared to be permanent. At a mole ratio of 2.60:1 H2C2O4 : Zr, precipitation during heating began at 74ºC, and the amount of precipitate increased as the temperature was raised to 90ºC. At a mole ratio of 2.20:1 H2C2O4 : Zr, the solution was cloudy at ambient temperature, and the quantity of precipitate increased as the slurry was heated. The precipitation of zirconium oxalates under acidic conditions at elevated temperatures makes such solutions undesirable for barium zirconate processing by inhibiting the formation of a single phase precursor. Qualitative testing showed that as the H2C2O4 : Ba solution mole ratio increased, the barium concentrations of the supernatant solutions decreased, and the zirconium concentration increased. Under such conditions, zirconium was solubilised by an excess of oxalic acid, resulting in a zirconium deficient precipitate above a 2.60:1 H2C2O4 : Zr solution mole ratio.

Rapid addition of reagents in the barium chloride, zirconium oxychloride, oxalic acid system at elevated temperature caused difficulties in filtration and washing. Precipitates formed by rapid reagent addition settled much more slowly and had much more severe particle losses during filtration, leading to poor repeatability of product stoichiometry. Slow rates of reagent addition were required to produce a washable product, adding undesired complexity to process control.

2.2 Barium Acetate, Zirconium Oxychloride, Ammonium Oxalate System As zirconium oxychloride and ammonium oxalate solutions were mixed to form pH neutral (7.02 ± 0.05) solutions at 25ºC, an unstable precipitate formed at the contact zone of the two solutions. The precipitate rapidly re-dissolved with stirring. No precipitate formed within 10 minutes of boiling. Zirconium oxychloride / ammonium oxalate solutions were much more resistant to precipitation during heating than zirconium oxychloride / oxalic acid solutions. Slight opalescence of the solutions was observed only after cooling to ambient temperature. However, the pH dropped to 6.45 ± 0.05 indicating chemical change as a result of heating. Zirconium oxychloride / sodium oxalate mixtures formed stable clear solutions between 50ºC and 100ºC. The stability of zirconium oxychloride / oxalate solutions upon heating was clearly pH dependent, with resistance to precipitation upon heating increasing with initial pH.

The barium acetate, zirconium oxychloride, ammonium oxalate system proved to be far more suitable for stoichiometric precipitation than oxalic acid systems because the product composition could be varied and controlled by changing the Ba:Zr solution mole ratio. This could be carried out at ambient temperature with low sensitivity to excess oxalate addition above a 2.4:1 C2O4 : Ba solution mole ratio. A slight excess of barium acetate (2.7%) was required for stoichiometric precipitation in small scale experiments (Figure 2). A slightly greater Ba excess (6%) was required for stoichiometric precipitation when the volume of solution was increased to 20L to produce powder for crucible fabrication. Other than correcting for minor scale-up effects, product stoichiometry was controlled simply by the Ba:[Zr+Hf] solution ratio in the presence of a suitable excess of ammonium oxalate. For routine production of BaZrO3 we used a (NH4)2C2O4 : barium acetate mole ratio of 2.4, though a greater excess of oxalate can be used if desired because excess ammonium oxalate does not solubilise zirconium under the conditions studied. Alternatively, near stoichiometric precursors can also be produced using a large excess of barium acetate instead of an excess of ammonium oxalate, i.e. using solution mole ratios of 2.0:1:2.0 barium acetate: zirconium oxychloride : ammonium oxalate. However the waste barium causes unnecessary disposal problems.

Figure 2 HERE

2.3 Calcination and Ceramic Production Powders derived from barium acetate, zirconium oxychloride and ammonium oxalate were calcined at temperatures up to 1500 ºC although DTA/TGA analysis (Figure 3) indicated weight loss was complete by 1100 ºC. The phase purity of powders observed by XRD increased upon calcination at higher temperatures suggesting high phase purity was achieved by solid-state reaction, or that decomposition kinetics for bulk powders were sluggish compared to 50 mg specimens for DTA/TGA analysis. Near phase-pure BaZrO3 was produced a temperature of 1300ºC. BaCO3 and ZrO2 were not detectable by XRD for the sample with Ba : Zr + Hf mole ratio of 0.986 ± 0.002 (Figure 4). However, small amounts of BaCO3 (approximately 2 wt.%) were detected in calcined samples after exposure to air, particularly for samples with an excess of barium. Barium carbonate measured by quantitative XRD could be detected in powders after a few hours exposure to air, with BaCO3 increasing to a maximum within approximately 24 - 48 hours.

BaCO3 levels are often interpreted as indicating phase impurity and hence calcination temperatures around 1300ºC were used to produce powders for crucible fabrication even though the TGA results (Figure 3) indicated weight loss was essentially complete by approximately 1100 ºC. However, our recent work has suggested BaZrO3 reacts with atmospheric CO2 to form BaCO3 at low levels according to the phase purity and surface area of the material 25. BaCO3 was observed in BaZrO3 close to phase equilibrium with either excess Ba or Zr after grinding to a fine powder only after air exposure, with BaCO3 levels increasing with increased grinding time. BaCO3 levels of BaZrO3 powders may be caused by incomplete phase formation, surface carbonation of BaZrO3 and off-stoichiometric Ba-rich phases. It is not clear that a method for separating the contributions of phase impurity and surface areas of powders to observed BaCO3 levels is available. The relatively high surface areas achievable using the oxalate process make such powders prone to reaction with air after calcination.

Figure 3 HERE Figure 4 HERE Figure 5 shows that crystallite size increased with calcination temperature as observed by XRD.

Crystallite sizes were estimated by Voigt function profile fitting of the (100) diffraction peak of BaZrO3 using the method of de Keijser et al. 23. These results were consistent with decreasing N2BET surface areas with calcination temperature and can be seen directly in the TEM images (Figure 6). Crystallite sizes measured by XRD were in good agreement with primary particle sizes observed by TEM. Agglomeration in powders is clearly shown in Figure 6c. The powders required de-agglomeration after calcination, which was conveniently performed by brief milling in an ethanolic solution containing cetyl alcohol used as a solid lubricant to assist isostatic pressing. The minor increase in crystallite size above 1300ºC clearly occurs by solid-state diffusion because decomposition of the oxalate is complete at approximately 1100ºC as shown by the TGA data (Figure 3). The DTA/TGA results are similar to those reported by Gangadevi et al. 8 and Potdar et al. 13. As for other processes, the calcination temperature should be kept to the minimum required to achieve adequate phase purity, in order to provide high surface areas for solid-state densification during sintering.

Figure 5 HERE Figure 6 HERE The sintering properties of powders were affected by chemical purity. For example, contamination by aluminosilicates from process water of inadequate quality dramatically improved sinterability.

Powders produced using high purity water were resistant to crystallite growth under severe calcination conditions up to 1500ºC. Using high purity water is essential for melt corrosion resistance because uncontrolled contamination may cause grain boundary defects or secondary phases that are readily corroded. For example, ~30 nm barium aluminosilicate precipitates were observed at triple points in contaminated sintered samples by STEM-EDS analysis. High purity water was used for all ceramic materials made for density and corrosion studies.

2.4 Ceramic Properties High density ceramics produced from oxalate derived powders had a fine grain size of approximately 5µm (Figure 7). During YBCO melt exposure at 1050ºC crucibles with a sintered density of 6.07 g/mL (97.4% theoretical density), the first sign of melt percolation through the wall section was observed after 60 hours of exposure. The rate of melt percolation was low and crucibles were still approximately half full of melt after 6 days of corrosion exposure which is sufficient time to complete high quality single crystal growth experiments. The melt viscosity did not change significantly during 6 days of corrosion exposure and remaining melt was readily decanted from the crucible.

Figure 7 HERE

3. Conclusions The barium zirconium oxalate system may not inherently produce a strictly stoichiometric product which decomposes to phase-pure BaZrO3 as readily as those of more expensive processes. Some of the experimental difficulties of oxalate processing are illustrated in this study, including effects of temperature, pH, solution ratios and types of reagents used. These difficulties were largely overcome using the barium acetate, zirconium oxychloride, ammonium oxalate system, which is shown to be a practical process using inexpensive reagents and very simple processing equipment.

Using accurate control over Ba:Zr solution ratios, the process allows precise control of product stoichiometry without requiring large excesses of barium or zirconium reagents, precipitation at elevated temperature or slow addition of reagents. The precipitate can be converted to near phasepure ultrafine BaZrO3 using a brief calcination above 1300ºC, and particle sizes and surface areas can be adjusted by varying the calcination temperature. These attributes make the process a potential candidate for industrial application. Whilst many reports have claimed to have synthesised BaZrO3 powders of high quality, this is the first process using a chemically derived precursor to demonstrate the capability of producing BaZrO3 ceramics able to provide sustained BaCuO2-CuO melt containment, for use in YBCO single crystal growth.

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