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Solid solution formation in alumina—chrome refractories
Pdyhedron Vol. 8, No. 13/14, pp. 176H766, Printed in Great Britain
1989
SOLID SOLUTION
Maxwell
0277-5387/89 $3.00+.00 Pergamon Macmillan plc
FORMATION IN ALUMINA-CHROME REFRACTORIES
T. J. DAVIES,* H. G. EMBLEM, C. S. NWOBODO, V. TSANTZALOU
A. A. OGWLJ and
Manchester Materials Science Centre, University of Manchester/UMIST, Manchester, U.K. Abstract-Preparative procedures, densification, grain growth and change in corundum lattice dimensions in sintered compacts made from chromium(III)oxide/aluminium(III) oxide mixtures were evaluated in a study of alumina-chrome refractories. High energy milling using A- 17 “reactive” alumina gave densification at low temperature and the most rapid grain growth, also (by Weibull analysis) the least variability in modulus of rupture, 7% W/W chromium(III)oxide giving minimum strength. XRD confirmed solid solution formation, the corundum lattice dimensions contracting with 7% W/W chromium(III)oxide, other compositions giving lattice expansion. The modulus of rupture at 1150°C for a series of ethyl-silicate-bonded alumina refractories containing 5-12% W/W chromium(III)oxide and fired at 1700°C was also a minimum at 7% W/W chromium(III)oxide.
measurements were used to evaluate changes in the corundum lattice dimensions. For both aluminium(III)oxide materials, better blending was obtained with increasing chromium(III)oxide content, the highest “green” density being observed with 14 wt% chromium(II1) oxide, because the fine chromium(III)oxide particles fill the spaces between the aluminium(II1) oxide particles. As expected, high compaction pressure gave high green density. The microstructure observed in the sintered compact depends on (i) composition, (ii) the milling process, (iii) sintering temperature and (iv) sintering time. At a short sintering time, the method of mixing is not important. High energy milling using Al7 “reactive” material gave densification at low temperatures and the most rapid grain growth during sintering. For MA95 material the milling process is important, probably because this material has a larger primary particle size. In general, this material gives sintered compacts with lower density. XRD studies of compacts prepared from Al7 “reactive” material with 14 wt% chromium(II1) oxide, sintered at 1600°C for 0.5 h, show them to contain free chromium(III)oxide. Other sintered compacts do not contain free chromium(III)oxide. The corundum peak width increases as the chromium(III)oxide content increases. The corundum *Author to whom correspondence should he addressed. lattice parameters, a and c, for sintered compacts
Resistance to thermal shock and reduction of slag attack can both be achieved by incorporating chromium(III)oxide in alumina or mullite bodies. Examples are the addition of fine chrome to coarse alumina or mullite to reduce slag attack’ and mullite-chrome systems2 in which an iron-chromite ore is the source of chrome. Incorporating fine chromium(III)oxide into ethyl-silicate-bonded refractories3 reduces slag attack and improves resistance to thermal shock. In the present work, preparative procedures, densification, grain growth and change of corundum lattice dimensions in sintered compacts, made from chromium(III)oxide/aluminium(III)oxide mixtures, were evaluated in a study of alumina-chrome refractories.“6 The aluminium(III)oxide materials were MA95 (BA Chemicals Ltd, average particle size 4 p angular) and Al7 “reactive” alumina (Alcoa, average particle size 2~, round). The chromium(III)oxide (British Chrome and Chemicals Ltd) had an average particle size of 0.4 p. Materials containing 3, 7 or 14 wt% chromium(III)oxide were prepared by ball-milling, high energy milling, or grinding in a tungsten carbide mill. These mixtures were compacted at 97.5, 120 or 310 MPa and sintered for various times at 1600, 1650 or 1700°C. XRD
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containing 3 or 14 wt% chromium(III)oxide, increase by 0.02-0.45%, depending on the sintering temperature and the sintering time. However, for sintered compacts containing 7 wt% chromium (1II)oxide the parameters decrease, especially in compacts fired at 1650°C where the a parameter decreases by 2.13%. Compacts prepared from MA95 material behave similarly. The results confirm the formation of a chromium(III)oxide/ aluminium(III)oxide solid solution. Compacts prepared with MA95 material lose chromium on sintering at high temperature for long periods, suggesting -that -chromium volatilization is faster than solid solution formation, which would be expected because of the large primary particle size of the MA95 material. The modulus of rupture of sintered compacts was determined at ambient temperature (three point loading ; Instron testing machine). Weibull analysis7 showed that compacts prepared from high energy milled materials had the highest Weibull modulus (ca 17), hence the least variability in strength. Weibull modulus values of 5-20 are common for ceramic materials,8 the higher values indicating less strength variability. A typical value for the modulus of rupture of a compact prepared from material high energy milled and sintered at 1650°C for 5 h would be 205 MPa. The modulus of rupture of a series of ethylsilicate-bonded alumina refractories prepared’ from coarse alumina grain, and a “fines” fraction comprising fine aluminium(III)oxide plus fine chromium(III)oxide, was determined at 1150°C. The
refractories contained from 5 to 12 wt% chromium(III)oxide and were fired at 1700°C prior to determining the modulus of rupture, which was a minimum at 7 wt% chromium(III)oxide. The occurrence of the minimum strength at this level of chromium(III)oxide coincides with the change in aluminium(III)oxide lattice parameters found in aluminium(III)oxide/chromium(III)oxide compositions containing 7 wt% chromium(III)oxide. This change would alter the elasticity of the refractory body, which could reduce the modulus of rupture.
REFERENCES 1. Kyushu Taika-Renga and Kabushiki Kaisha, Bit&/r Patent 1,533,890 (1978). 2. C. Taylor’s Sons Company, British Patent 1,421,418 (1976). 3. R. D. Shaw and C. Shaw, British Patent 1,313,498 (1973). 4. C. S. Nwobodo, MSc dissertation. UMIST, Manchester (1984). 5. A. A. Ogwu, MSc dissertation. UMIST, Manchester (1985). MSc thesis. UMIST, Manchester 6. V. Tsantzalou, (1984). 7. W. A. Weibull, J. Appl. Mech. 1951,18,293. 8. R. W. Davidge, Mechanical Behaviour of Ceramics, pp. 133-139. Cambridge University Press, London (1980). 9. H. G. Emblem, Trans. J. Brit. Cerum. Sot. 1975, 74, 223.
1989
SOLID SOLUTION
Maxwell
0277-5387/89 $3.00+.00 Pergamon Macmillan plc
FORMATION IN ALUMINA-CHROME REFRACTORIES
T. J. DAVIES,* H. G. EMBLEM, C. S. NWOBODO, V. TSANTZALOU
A. A. OGWLJ and
Manchester Materials Science Centre, University of Manchester/UMIST, Manchester, U.K. Abstract-Preparative procedures, densification, grain growth and change in corundum lattice dimensions in sintered compacts made from chromium(III)oxide/aluminium(III) oxide mixtures were evaluated in a study of alumina-chrome refractories. High energy milling using A- 17 “reactive” alumina gave densification at low temperature and the most rapid grain growth, also (by Weibull analysis) the least variability in modulus of rupture, 7% W/W chromium(III)oxide giving minimum strength. XRD confirmed solid solution formation, the corundum lattice dimensions contracting with 7% W/W chromium(III)oxide, other compositions giving lattice expansion. The modulus of rupture at 1150°C for a series of ethyl-silicate-bonded alumina refractories containing 5-12% W/W chromium(III)oxide and fired at 1700°C was also a minimum at 7% W/W chromium(III)oxide.
measurements were used to evaluate changes in the corundum lattice dimensions. For both aluminium(III)oxide materials, better blending was obtained with increasing chromium(III)oxide content, the highest “green” density being observed with 14 wt% chromium(II1) oxide, because the fine chromium(III)oxide particles fill the spaces between the aluminium(II1) oxide particles. As expected, high compaction pressure gave high green density. The microstructure observed in the sintered compact depends on (i) composition, (ii) the milling process, (iii) sintering temperature and (iv) sintering time. At a short sintering time, the method of mixing is not important. High energy milling using Al7 “reactive” material gave densification at low temperatures and the most rapid grain growth during sintering. For MA95 material the milling process is important, probably because this material has a larger primary particle size. In general, this material gives sintered compacts with lower density. XRD studies of compacts prepared from Al7 “reactive” material with 14 wt% chromium(II1) oxide, sintered at 1600°C for 0.5 h, show them to contain free chromium(III)oxide. Other sintered compacts do not contain free chromium(III)oxide. The corundum peak width increases as the chromium(III)oxide content increases. The corundum *Author to whom correspondence should he addressed. lattice parameters, a and c, for sintered compacts
Resistance to thermal shock and reduction of slag attack can both be achieved by incorporating chromium(III)oxide in alumina or mullite bodies. Examples are the addition of fine chrome to coarse alumina or mullite to reduce slag attack’ and mullite-chrome systems2 in which an iron-chromite ore is the source of chrome. Incorporating fine chromium(III)oxide into ethyl-silicate-bonded refractories3 reduces slag attack and improves resistance to thermal shock. In the present work, preparative procedures, densification, grain growth and change of corundum lattice dimensions in sintered compacts, made from chromium(III)oxide/aluminium(III)oxide mixtures, were evaluated in a study of alumina-chrome refractories.“6 The aluminium(III)oxide materials were MA95 (BA Chemicals Ltd, average particle size 4 p angular) and Al7 “reactive” alumina (Alcoa, average particle size 2~, round). The chromium(III)oxide (British Chrome and Chemicals Ltd) had an average particle size of 0.4 p. Materials containing 3, 7 or 14 wt% chromium(III)oxide were prepared by ball-milling, high energy milling, or grinding in a tungsten carbide mill. These mixtures were compacted at 97.5, 120 or 310 MPa and sintered for various times at 1600, 1650 or 1700°C. XRD
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containing 3 or 14 wt% chromium(III)oxide, increase by 0.02-0.45%, depending on the sintering temperature and the sintering time. However, for sintered compacts containing 7 wt% chromium (1II)oxide the parameters decrease, especially in compacts fired at 1650°C where the a parameter decreases by 2.13%. Compacts prepared from MA95 material behave similarly. The results confirm the formation of a chromium(III)oxide/ aluminium(III)oxide solid solution. Compacts prepared with MA95 material lose chromium on sintering at high temperature for long periods, suggesting -that -chromium volatilization is faster than solid solution formation, which would be expected because of the large primary particle size of the MA95 material. The modulus of rupture of sintered compacts was determined at ambient temperature (three point loading ; Instron testing machine). Weibull analysis7 showed that compacts prepared from high energy milled materials had the highest Weibull modulus (ca 17), hence the least variability in strength. Weibull modulus values of 5-20 are common for ceramic materials,8 the higher values indicating less strength variability. A typical value for the modulus of rupture of a compact prepared from material high energy milled and sintered at 1650°C for 5 h would be 205 MPa. The modulus of rupture of a series of ethylsilicate-bonded alumina refractories prepared’ from coarse alumina grain, and a “fines” fraction comprising fine aluminium(III)oxide plus fine chromium(III)oxide, was determined at 1150°C. The
refractories contained from 5 to 12 wt% chromium(III)oxide and were fired at 1700°C prior to determining the modulus of rupture, which was a minimum at 7 wt% chromium(III)oxide. The occurrence of the minimum strength at this level of chromium(III)oxide coincides with the change in aluminium(III)oxide lattice parameters found in aluminium(III)oxide/chromium(III)oxide compositions containing 7 wt% chromium(III)oxide. This change would alter the elasticity of the refractory body, which could reduce the modulus of rupture.
REFERENCES 1. Kyushu Taika-Renga and Kabushiki Kaisha, Bit&/r Patent 1,533,890 (1978). 2. C. Taylor’s Sons Company, British Patent 1,421,418 (1976). 3. R. D. Shaw and C. Shaw, British Patent 1,313,498 (1973). 4. C. S. Nwobodo, MSc dissertation. UMIST, Manchester (1984). 5. A. A. Ogwu, MSc dissertation. UMIST, Manchester (1985). MSc thesis. UMIST, Manchester 6. V. Tsantzalou, (1984). 7. W. A. Weibull, J. Appl. Mech. 1951,18,293. 8. R. W. Davidge, Mechanical Behaviour of Ceramics, pp. 133-139. Cambridge University Press, London (1980). 9. H. G. Emblem, Trans. J. Brit. Cerum. Sot. 1975, 74, 223.