Effect of Yttria on Thermal Dissociation

EFFECT OF YTTRIA ON THERMAL DISSOCIATION OF ZIRCON

V. N. Antsiferov, V. B. Kulmetyeva, S. Ye. Porozova Scientific Centre of Powder Materials Science of SEI HPE PSTU, Perm, Russia Abstract

Effect of yttria on thermal dissociation of zircon was studied by X-ray diffraction and Raman scattering spectroscopy. Addition of up to 1.5 to 3.0 wt. % of yttria, depending on isothermal conditioning mode, restrains zircon decomposition.

Introduction

Zircon is zirconium orthosilicate, its chemical formula is: ZrO2·SiO2. Despite its favourable thermal properties, good heat resistance and dielectric properties, it has still not found extensive application in engineering. It is mainly used for refractory fabrication in production of glass and steel, as an additive in various stocks and in zirconia production (1, 2). One peculiar feature of zircon is its solid-state thermal dissociation into constituent oxides. Data concerning ZrSiO4 thermal stability vary considerably among most publications both as to decomposition pattern and as to decomposition temperature. According to various authors, zircon decomposes at 1676°C into zirconia and cristobalite (2). The paper (3) states that decomposition occurs about 1800°C, ZrO2 being produced in cryptocrystalline form, SiO2 in amorphous. “Диаграммы состояния силикатных систем” by N. A. Toropov et al. gives data by Curtis and Sowman according to which zircon was decomposed at temperature as low as about 1540°C (4). In the paper (5) V. Eitel states that sintering at 1600°C produces a uniform structure and conditioning at 1650°C in enough to cause about 10% of zircon to be decomposed. Authors (6) refined the state diagram given in (4) and pointed out that zircon begins decomposing at 1931 K to 1960 K when silica-based melt is produced containing 4 to 5 wt. % of ZrO2. The differences noted may be attributed to the decomposition reaction occurring at the temperature close to that of eutectic melt formation. Studying decomposition of zircon of various purity and known grain size as well as of individual crystals, the authors (7) found that thermal dissociation of ZrSiO4 occurs at 1673±10°C. ZrSiO4 is decomposed in the course of solid-state reaction into discrete metastable intermediate SiO2 phases of superstoichiometric silicon content. Considerable influence of even slight impurity amounts on decomposition temperature and kinetics may be attributed to low-melting liquid phase formation which has the following composition, wt. %, in particular, in presence of alumina: 62.6 to 81.1 SiO2; 27.1 to 13.5 ZrO2; 10.3 to 5.4 Al2O3. A number of additives, such as calcium oxide, iron oxide, reduce zircon decomposition temperature dramatically (3). Additives promoting zircon decomposition also include magnesia in presence of which zircon decomposition signs were found in specimens burnt at 950°C (8). In order to obtain mullite-zirconia based ceramic composite materials, reactive zircon sintering is often used, alumina added in various proportions (9, 10). According to (11), it is difficult to find any substances impeding zircon

204 decomposition, since silica incorporated in it has high capability of forming compounds at high temperatures, often forming low-melting eutectics. The work was aimed at studying the effect yttria introduction has on thermal dissociation of zircon. Yttria introduction was selected in view of state diagram studies (4). Yttria forms compounds with silica found in zircon concentrate in free state in small amounts, producing liquid phase above 1660°C. With alumina, similar compounds are formed above 1760°C, although, according to (11), low-melting compounds may be formed below 1400°C in Y2O3-Al2O3-SiO2 system. Yttria may also stabilize zirconia formed during zircon decomposition.

Experimental

Zeta Zircon Superfine zircon concentrate (Europe Minerals) was used in the work, its chemical composition, wt. %, according to the certificate, is as follows: 66.2 ZrO2+HfO2; 32.6 SiO2 (total); 0.4 SiO2 (free); 0.08 Fe2O3; 0.11 TiO2; 1.2 Al2O3; 0.05 CaO; 0.01 Cr2O3. Yttria content was varied from 1 to 10 wt. %. Yttria was introduced into the concentrate as aqueous solution of yttrium nitrate Y(NO3)3·6H2O. Specimens were molded from powders baked at 600°C for 1 h in a steel die by cold uniaxial pressing, at compaction pressure of 200 MPa. 4% aqueous solution of ПВС 7/2 ГОСТ 10779-69 vinyl alcohol was used as a temporary process binder, introduced at 10% of powder weight. Temporary process binder was removed by pre-baking in СНОЛ- 1.6.2.5.1/11-М1 furnace at 600°C. Sintering was carried out in СВК 5163 resistance furnace at 1600 and 1650°C in air atmosphere at constant heating rate, isothermal conditioning lasted for 0 to 3 h. Apparent density was determined by measuring weight and geometrical dimensions of sintered specimens. Phase composition was studied by X-ray techniques by means of Shimadzu XRD-6000 diffractometer using СuК-radiation and by Raman spectroscopy using SENTERRA multifunctional Raman spectrometer (Bruker). Measurements were made at room temperature.

Discussion

Superfine zircon concentrate is notable for its high dispersity, specific surface of the powder as delivered determined by BET method was 6.4 m²/g, average particle size 1.25 µm (determined using Analysette 22 laser diffraction-type particle size analyzer). X-ray phase analysis of Superfine zircon powder as delivered showed that peak locations corresponded to tetragonal ZrSiO4 lines, no other compounds were found, Fig.1. After heat treatment of powder at 1600 and 1650°C without isothermal conditioning, zircon was found to dissociate at temperature as low as 1600°C, Fig.2. A maximum peak at 1000 cm−¹, and two less intense peaks at 400 cm−¹ are clearly detected in zircon Raman spectrum, Fig. 2.1. After heat treatment at 1600°C zirconia peaks, in the range of 600 to 400 cm−¹, and an amorphous halo attributable to glass phase, are found in Superfine zircon powder. As temperature increases to 1650°C, zircon decomposition degree is increased considerably. Distinct zircon decomposition at temperature as low as 1600°C may be attributed to high enough alumina content of Superfine zircon concentrate which results in formation of low-melting phases (4, 7).

205 Fig.1. Zeta Zircon Superfine zircon concentrate powder diffraction pattern.

Fig.2. Zircon Raman spectrum from ОПУС 65 (1) library and Superfine zircon powder Raman spectra after heat treatment: 2 – 1600°C; 3 – 1650°C.

Figure 3 gives Superfine zircon Raman spectra after heat treatment at 1600°C for 1 h. From the Figure we can see that if Y2O3 is added in amount as small as 0.3%, amorphous halo intensity decreases with respect to original zircon, Fig. 2. As yttria content is increased, peaks in the range a of 600 to 400 cm−¹ attributable to zirconia disappear from Superfine zircon Raman spectra. Further increase in Y2O3 content over 3.0% results in amorphous halo formation, whose intensity increases dramatically if 10% of yttria are added.

206 Fig.3. Superfine zircon Raman spectra after heat treatment at 1600°C for 1 h.

As sintering temperature is increased to 1650°C, zirconia peaks in Superfine zircon Raman spectra are not detected only at 1.5% of Y2O3, Fig. 4.

Fig.4. Superfine zircon Raman spectra after heat treatment at 1650°C for 1 h.

According to Z-ray phase analysis, thermal dissociation of zircon occurs in spite of yttria introduction. Phase composition of Superfine zircon after sintering at 1600°C for 1 h comprises mainly ZrSiO4, monoclinic and tetragonal ZrO2, No Z-Ray

207 amorphous phase was detected. If yttria is added, monoclinic-to-tetragonal ZrO2 ratio changes. Only if 10% of Y2O3 are introduced, α-cristobalite (dα = 4,0174 nm) was observed in the diffraction pattern, while zircon persists in cubic modification, its lines becoming much more intense, Fig. 5. These changes in zircon phase composition are reflected in considerably reduced material density, its structure becoming porous.

Fig.5. Superfine zircon diffraction patterns after heat treatment at 1600°C for 1 h: a – without Y2O3; b – 1.5 % of Y2O3; c – 10% of Y2O3.

208 For kinetic assessment of phase formation in zircon, depending on yttria content, intensity ratios of X-ray peaks (111) – m-ZrO2, (111) – t-ZrO2 и (101) – ZrSiO4, Fig. 6. As yttria content increases, monoclinic ZrO2 is observed to decrease, being undetectable at Y2O3 content as low as 1.5% for 1600°C (Fig. 5, b) and 3.0 Y2O3 for 1650°C. At the same time, tetragonal ZrO2 increases, total zircon amount of both modifications not exceeding the initial value. From 5% Y2O3, tetragonal ZrO2 is observed to increase dramatically. As isothermal conditioning time at sintering temperature is increased, the pattern of changes in Superfine zircon phase composition remains the same.

Fig.6. Superfine zircon decomposition degree as ZrO2/ZrSiO4 X-ray peak intensity ratio measurement, depending on Y2O3 content: 1, 3 - m-ZrO2; 2, 4 - t-ZrO2; 1, 2 – 1600°C; 3, 4 – 1650°C, conditioning for 1 h.

During sintering at temperature close to that of thermal dissociation of zircon, a small amount (up to 1.5 to 3.0%) of yttria added to Superfine zircon concentrate appears to suppress the decomposition process partially and stabilize monoclinic zirconia formed to tetragonal zirconia. Further increase in yttria content results in its interaction with zircon producing Y2O3·2SiO2 compound, α- cristobalite (4) and cubic zirconia. Thus, small additions of yttria to zircon containing impurities, which promote its thermal dissociation, restrain its development. The amount of yttria added varies depending on sintering temperature and isothermal conditioning time.

References

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