Preparation of mullite by the reaction sintering of kaolinite
and alumina
C.Y. Chen, G.S. Lan, W.H. Tuan *
Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, ROC Received 9 December 1999; received in revised form 6 April 2000; accepted 11 April 2000
Abstract
In the present study, mullite specimens and mullite/alumina composites are prepared by reaction sintering kaolinite and alumina at a temperature above 1000C. The phase and microstructural evolution of the specimens and their mechanical properties are
investigated. Primary mullite appears at a temperature around 1200C. The alumina particles are inert to the formation of primary
mullite. Alumina starts to react with the silica in glassy phase to form secondary mullite above 1300C. The formation of secondary
mullite decreases the amount of glassy phase. Furthermore, the addition of alumina reduces the size of mullite grains and their aspect ratio. The strength and toughness of the resulting mullite increase with the increase of alumina content; however, the mechanical properties of the mullite and mullite/alumina composites are lower than those of alumina for their relatively low den-sity. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Al2O3; Kaolinite; Mechanical properties; Microstructure-®nal; Mullite; Reaction sintering
1. Introduction
Kaolinite (2SiO2.Al2O3.2H2O), an
aluminosilicate-based ceramic, is widely used in ceramic industries for many years. The stoichiometric 3:2 mullite (3Al2O3.
2SiO2) is a thermodynamically stable phase in the SiO2±
Al2O3 system.1 The mullite is thus formed after ®ring
kaolinite at elevated temperature.2,3 Nevertheless, the
amount of SiO2in kaolinite is much higher than that in
mullite; the excess SiO2together with the impurities in
kaolinite forms a glassy phase and cristobalite to accompany the formation of mullite at a temperature higher than 1000C.4 The cristobalite may also
trans-form to a glassy phase as kaolinite is ®red above 1500C.5 The strength of glass is usually low at room
temperature; furthermore, the glass is softened at ele-vated temperature. The presence of a large amount of glass phase is thus detrimental to the mechanical prop-erties of the mullite prepared from kaolinite.
The amount of the SiO2 in the glassy phase can be
consumed by adding Al2O3. Furthermore, the reaction
product of the SiO2in glass and Al2O3is also a mullite
phase. The addition of Al2O3can therefore reduce the
amount of glass phase and increase the amount of mullite. In the present study, the interactions between kaolinite and alumina at elevated temperature are investigated. Furthermore, the microstructural evolution of the mullite prepared by the reaction between kaolinite and alumina during sintering is also studied.
2. Experimental
A kaolin (AKIMA 35, Akima Co., Malaysia) powder and various amounts of alumina (AES-11, Sumitomo Chem., Japan, 99.8% Al2O3) were milled together in
ethyl alcohol with a turbo-mixer. The composition of the kaolin powder was determined with the induced coupled plasma emission spectroscopy (ICP). The grinding media was zirconia balls and milling time was 4 hours. After drying, the dried lumps were crushed and passed through the plastic sieve with an aperture size of 149 mm. The particle size and its distribution were determined with a laser particle-size analyzer (LS 230, Coulter Co., USA). The powder compacts were pre-pared by the die-pressing technique. The pressure applied was 27 MPa. The ®ring was carried out at a
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temperature varied from 1000 to 1600C for 1 h. The
heating rate and cooling rate were 5C/min.
The reaction as following was taken place during the sintering of the powder mixtures of kaolinite and alumina,
2SiO2Al2O32H2O 2Al2O3
! 3Al2O32SiO2 2H2O 1
The phase identi®cation was performed by X-ray dif-fractometry (Philips PW1710, Philips Co., Netherlands) with CuK radiation. In order to determine the amount
of mullite and alumina phases in the sintered specimens, a high-purity mullite was used to mix with the alumina powder. The mullite/alumina powder mixtures with the ratio of 1/4, 2/3, 3/2 and 4/1 were prepared with the same procedures as those for the kaolin/alumina pow-der mixtures. A slow scanning speed, 0.005 2/s, was
used to determine the integral intensity of the (120) plane of mullite and of the (012) plane of alumina within the 2 ranged from 25 to 27. Three runs were
conducted to determine the reproducibility of the XRD analysis. The calibration curve was then established by correlating the intensity ratio to mullite/alumina ratio. The ratio of mullite and residual alumina in the sintered kaolinite/alumina specimens could thus be determined. Since the amount of SiO2 and Al2O3 in the starting
materials was known; mullite, alumina and glass phase were the major phases in the ®red specimens. The amount of mullite, residual alumina and glassy phase in the sintered specimens could therefore be determined after the mullite/alumina ratio was measured. The ®nal density was determined by Archimedes' method. By using the theoretical density of 3.16 g cmÿ3 for mullite,
3.98 g cmÿ3 for alumina and assuming 2.5 g cmÿ3 for
glass, the relative density of the ®red specimens could then be estimated. Since the composition of stoichio-metric 3:2 mullite formed at elevated temperature varied in a rather small range1and the density of Al
2O3±SiO2±
K2O glass was very close to 2.5 g cmÿ3,6the error that
may thus be caused by using the above values to calcu-late the relative density should be relatively minor.
The ¯exural strength of the rectangular specimens was determined by the three-point bending technique at ambient, room-temperature condition. The lower span for the ¯exural testing ®xture was 30 mm. The loading rate was 0.5 mm/min. The fracture toughness was determined by the three-point single-edge-notched-beam (SENB) technique. The notch was generated by cutting with a diamond saw. The width of the notch was around 0.45 mm. The microstructure was observed by scanning electron microscopy (SEM). To reveal the grain boundaries of mullite, a concentrated hydro¯uoric acid was used as etching solution. The mean size of mullite grains was determined with the line intercept technique. The line intercept was taken randomly from several SEM micrographs for each composition.
Further-more, more than 800 grains were counted. A statistical procedure7 to determine the aspect ratio, the length/
width ratio, of elongated Si3N4 grains in the in-situ
reinforced silicon nitride was adopted in the present study to evaluate the aspect ratio of mullite grains. 3. Results and discussion
The mean size of kaolinite and alumina particles is 1.7 and 0.3 mm, respectively. The morphology of the kaoli-nite particles and of a powder mixture of kaolikaoli-nite and alumina is shown in Fig. 1. Fig. 1(b) shows that ®ne alumina particles are mixed uniformly with the coarse kaolinite particles after milling and drying. Fig. 2 shows the XRD patterns of the powder mixture of kaolinite and alumina. A minor phase, quartz, is detected in the starting kaolin powder. The XRD patterns of the kao-lin/42 wt.% alumina powder compact at elevated tem-perature are also shown in Fig. 2. Mullite is ®rst noticeable in the specimen ®red at the temperature of 1200C. At the temperature, the amount of Al2O3is not
reduced, indicating that alumina remains inert chemi-cally until 1200C. This ®nding is the same as the result
Fig. 1. Morphology of (a) kaolin powder and (b) kaolin+52 wt.% alumina powder mixtures.
reported by Liu et al.8As the temperature is raised to
1400C, the amount of alumina is decreased
con-siderably, the amount of mullite increased instead. The mullite formed at 1200C is termed as primary
mul-lite;8,9 the mullite formed above 1300C as secondary
mullite. The formation of secondary mullite is mainly contributed by a solution and precipitation mechanism through the glassy phase.9The formation of secondary
mullite consumes the SiO2 in the glassy phase. The
amount of glassy phase is thus decreased.
The ICP analysis indicates that the composition of the kaolin used in the present study is 48.6 wt.% SiO2, 35.7
wt.% Al2O3, 1.2 wt.% K2O, 0.9 wt.% Fe2O3, 0.4 wt.%
TiO2, 0.2 wt.% MgO, 0.1 wt.% CaO, 0.1 wt.% BaO
and 12.6 wt.% ignition loss. Assuming that the SiO2in
the kaolinite is reacted completely with Al2O3to form
mullite as the reaction shown in Eq. (1); since there is 48.6 wt.% SiO2in the starting kaolin powder, an excess
amount of 41.7 wt.% alumina is needed to react all SiO2
in the kaolin powder to form mullite. Therefore, there is no Al2O3 found in the kaolin/32 wt.% alumina and
kaolin/42 wt.% alumina powder compacts after ®ring at high temperature (Fig. 2). However, residual alumina can be detected in the specimens containing more than 52 wt.% Al2O3. Therefore, the specimens containing 52,
62 and 72 wt.% Al2O3 are virtually mullite/alumina
composites after sintering at a temperature above 1400C.
The calibration curve to correlate the XRD intensities to mullite/alumina ratio is shown in Fig. 3. The results between the three tests for each composition are very close, indicating the reproducibility is high. The ratio of mullite/(mullite+alumina) is shown as a function of sintering temperature in Fig. 4. Fig. 5 shows the amount of residual alumina after reacting with kaolinite as a
Fig. 2. XRD patterns of the kaolinite+42 wt.% alumina powder compacts at elevated temperatures.
Fig. 3. Calibration curve for the determination of mullite in the ®red specimens.
Fig. 4. Ratio of mullite to (mullite+alumina) in the sintered speci-mens as a function of sintering temperature.
Fig. 5. Amount of residual alumina in the sintered specimens as a function of sintering temperature.
function of sintering temperature. The mullite formed at 1200C is attributed to the formation of primary mullite.
The amount of alumina remains approximately the same at the temperature of 1200C, suggesting that alumina is
inert to the formation of primary mullite. In the tem-perature ranges from 1300 to 1500C, the amount of
mullite increases rapidly, the amount of alumina decreases rapidly instead. The increase in mullite amount in this temperature range is contributed by the formation of secondary mullite through the solution of alumina particles and precipitation of mullite grains.9
The amount of mullite is not increased any further above 1500C. Fig. 6 shows the amount of glassy phase
as a function of sintering temperature. The amount of glass phase in the kaolinite specimen is also shown in the ®gure for comparison. The amount of glass phase is reduced as alumina is added into kaolinite. The amount of glass phase decreases rapidly between 1300 and 1500C. The glass in the specimen containing 72 wt.%
Al2O3 is consumed nearly complete. However, due to
the existence of impurities in the starting materials, glass phase will always present at the grain boundaries. From Figs. 5 and 6, the alumina and glass are consumed rapidly to accompany the formation of secondary mul-lite in the temperature region from 1300 and 1500C.
The absolute density of the (kaolin+alumina) powder compacts after sintering is shown as a function of sin-tering temperature in Fig. 7. The density of the kaolin powder compact and alumina powder compact is also shown in the ®gure for comparison. The relative density of the powder compacts is shown as a function of mina content in Fig. 8. The relative density of the alu-mina-containing specimens is higher than 90% as the specimens are sintered at 1600C. In the temperature
ranges between 1300 and 1500C, the densi®cation rate
is relatively slow (Figs. 7 and 8). Secondary mullite is formed within this temperature region, suggesting that
the reaction between the glass phase and alumina to form mullite is detrimental to the densi®cation.
The density of kaolin powder compact is decreased above 1500C. The Fe3O4 is formed at a temperature
above 1500C.10The formation of Fe3O4from Fe2O3can
generate oxygen as
3Fe2O3 ! 2Fe3O4 1=2O2 2
large holes are formed consequently in the sintered compact.5 The density of the kaolin specimen thus
decreases considerably above 1500C. In the
alumina-containing specimens, the amount of iron oxide is diluted due to the addition of alumina. Furthermore, iron oxide can dissolve into alumina at elevated temperature (the solubility of Fe2O3 in Al2O3 is higher than 15 wt.%
above 1400C),11the decrease of density in the
alumina-containing specimens above 1500C is thus not
observed.
Fig. 6. Amount of glassy phase in the sintered specimens as a function of sintering temperature.
Fig. 7. Absolute density of the sintered specimens as a function of sintering temperature.
Fig. 8. Relative density of the sintered specimens as a function of alumina content.
The fracture surfaces of the specimens containing 42 wt.% Al2O3are shown in Fig. 9. In the specimen
sin-tered at 1400C, large holes are observed [Fig. 9(b)].
Such large holes are not found in the green compact and in the specimens sintered below 1300C [Fig. 9(a)].
These large holes are nearly spherical in shape. Fur-thermore, the fracture path around the large holes is very much ¯at, indicating the surface of large holes is
covered with glass phase. The holes are no longer sphe-rical in shape in the specimen sintered at 1600C [Fig.
9(c)]; mullite grains intrude into the large holes. The density of the kaolinite, alumina and mullite is dierent, a shrinkage is accompanied with the reaction between kaolinite and alumina. The formation of mullite is mainly taken place between 1300 and 1500C. Large
holes are thus formed in this temperature region. Glass phase can ¯ow to the surface of holes, the holes are thus covered with glassy phase. More glassy phase is con-sumed by alumina to form mullite at higher tempera-ture; furthermore, mullite grains grow in their size at 1600C. The holes are thus no longer spherical at the
temperature. Nevertheless, the presence of the large holes renders the density of the kaolin-containing spe-cimens lower than that of alumina (Fig. 8).
The microstructures of the specimens sintered at 1600C for 1 h are shown in Fig. 10. The average
inter-cept and the aspect ratio of mullite grains are shown in Fig. 11. As 32 wt.% Al2O3 and 42 wt.% Al2O3 are
added into kaolin, both the average intercept and aspect ratio of mullite decrease with the increase of alumina content. In the specimens containing more than 52
Fig. 9. Micrographs of the fracture surface of kaolinite+42 wt.%
wt.% Al2O3, mullite and alumina are coexisted.
There-fore, the intercept and aspect ratio in Fig. 11 are virtually the values for both mullite and alumina grains. The alumina grains can act as the nuclei for the formation of mullite grains.10 The increase of alumina particles
increases the amount of nuclei and decreases the amount of glass, the size and aspect ratio of mullite grains are thus reduced.
Fig. 12 shows the strength of the specimens sintered at 1600C for 1 h as a function of alumina content. The
strength of the kaolinite specimen sintered at 1600C is
low for its low density. Therefore, the strength of the kaolinite specimen sintered at 1500C is also shown in
the ®gure for comparison. The strength of the (kaoli-n+alumina) specimens is in a rather small region from 170 to 195 MPa, indicating that the size of the critical ¯aws in the kaolin-containing specimens is more or less the same. The strength of the (kaolin+alumina) specimens is
higher than that of kaolin specimen due to the decrease of glassy phase, and lower than the strength of alumina specimen due to the presence of large holes. The tough-ness of the specimens sintered at 1600C is also shown
in Fig. 12. The presence of rounded holes has little in¯uence on toughness. Due to the amount of brittle glass is reduced, the toughness of the specimens thus increases with the increase of alumina content.
4. Conclusions
The present study demonstrates that mullite can be prepared by reaction sintering kaolinite and alumina. The advantage of this process is its economic feasibility. The disadvantage is its relatively high sintering tempera-ture, low density and consequently low strength. The alumina particles are inert to kaolinite until 1200C.
The reaction between alumina and the glass phase to form mullite starts from 1300C. The sintering
tem-perature of the (kaolin+alumina) powder compacts has therefore to be higher than 1300C. Nevertheless, this
disadvantage can be coped with by adding ¯ux such as feldspar into kaolin.12 The reaction between kaolinite
and alumina is accompanied with a shrinkage. The pre-sence of glassy phase facilitates the formation of large holes. Fully dense mullite specimens are thus dicult to prepare by using the process employed in the present study. The strength of the specimens is therefore low. However, the toughness of the specimen increases with the increase of alumina content.
Acknowledgement
The present study was supported by the National Science Council, Republic of China, through contract number NSC88-2216-E002-027.
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