# Synthesis of Al203-SiC from kyanite precursor

Ali Amroune, Gilbert Fantozzi

► To cite this version:

Ali Amroune, Gilbert Fantozzi. Synthesis of Al203-SiC from kyanite precursor. Journal of Materials Research, 2001, 16 (6), pp.1609-1613. 10.1557/JMR.2001.0223. hal-00475367

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# Synthesis of Al₂O₃–SiC from kyanite precursor

A. Amroune and G. Fantozzi

Ceramics and Composites Research Group, GEMPPM Bat Blaise Pascal, INSA de Lyon, 69621 Villeurbanne Cedex, France

Carbothermal reduction of kyanite, a natural aluminosilicate with high alumina content (Al₂O₃ · SiO₂), was used as a way to synthesize SiC–Al₂O₃ powder. Carbon black was mixed with the mineral precursor in a molar ratio of (C/SiO₂) = 5.5. The Carbothermal reaction sequence was studied in the temperature range 1260–1550 °C. Completion of the reaction at 1550 °C gave β-SiC-whiskers and α-Al₂O₃ particles as final products. From observations, the impurities contained in the mineral precursor behave as catalysts for the vapor–liquid–solid whisker growth mechanism. When the specific surface area of the starting carbon was increased from 330 to 996 m²/g, the carbothermal reaction rate increased but the morphology of the SiC-whiskers became very irregular. This study aimed to identify the main reaction conditions for obtaining a favorable morphology of the synthesized powder for elaborating Al₂O₃–SiC-whisker composite materials by using low cost starting materials and a relatively simple *in situ* synthesis route in comparison to the conventional way of separately preparing the phases SiC (whiskers) and Al₂O₃ before elaborating composites.

## I. INTRODUCTION

Due to their highly ordered structure, chemical stability, and high strength approaching the theoretical value, ceramic whiskers have been attracting interest for use as reinforcements in ceramic matrices for thermostructural applications. Among these, silicon carbide whiskers (SiCw) have been extensively studied as reinforcing materials in ceramic matrices.¹⁻⁴ Three main methods have been utilized to synthesize SiCw: (i) carbothermal reduction of silica in the presence of metal catalysts including rice-hulls as starting material,⁵⁻¹¹ (ii) thermal decomposition of organic silicon compounds,¹²,¹³ and (iii) reaction between halides in presence of hydrogen.¹⁴,¹⁵ Because of its simplicity, the first method is the most utilized. However, the toxicity (mainly by inhalation) related to whisker handling and the high cost of their production have limited their application. To reduce these disadvantages, an *in situ* SiCw synthesis approach has been attempted. Partial carbothermal reduction of Si₃N₄ has been utilized¹⁶,¹⁷ to obtain Si₃N₄/SiCw powders. Synthetic or natural aluminosilicates nAl₂O₃ · mSiO₂ may give Al₂O₃ and SiCw as final products by carbothermal reduction at high temperatures in an inert atmosphere.¹⁸,¹⁹ It has been reported¹⁸,¹⁹ that the fracture toughness KIC values of Al₂O₃–SiCw composites prepared by hot pressing from *in situ* synthesized powders range between 5 and 7.6 MPa m¹/². Similar composites prepared by conventional physical mixing of the components and sintering generally present a fracture toughness

ranging between 6 and 9.5 MPa m¹/². These values indicate the possibility of obtaining *in situ* composites having mechanical properties comparable to those of similar conventional composites prepared from separately synthesized SiCw and Al₂O₃. In this study, carbothermal reduction of kyanite, a large availability aluminosilicate mineral with high alumina content, was used as a way to *in situ* obtain β-SiC whisker and α-Al₂O₃ particles. This route would simplify the conventional costly way to prepare powder for elaborating Al₂O₃–SiCw composites. Some reaction conditions were studied to synthesize a powder with a favorable morphology for elaborating such composites.

## II. EXPERIMENTAL

Table I lists chemical compositions of the utilized kyanite (Virginia, kyanite supplied by SOGEMET, Paris, France) as aluminosilicate precursor and indicates the content of impurities in the mineral, primarily iron and titanium oxides. X-ray analysis showed the good crystallinity of this mineral. Scanning electron microscopy (SEM) observation of the powder reveals that the particle size ranges between 5 and 53 µm. As carbon sources, two types of carbon black have been utilized (supplied by DEGUSSA, Paris, France) with specific surface areas of 330 and 996 m²/g. XRD analysis of these two types of carbon reveals their amorphous states. Kyanite powder was mixed (in ethylic alcohol) with carbon black in a
<table><caption>TABLE I. Chemical composition of kyanite</caption><thead><tr><th>Oxydes</th><th>wt %</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>37.64–43.70</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>54.00–60.06</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>0.40–1.16</td></tr><tr><td>TiO<sub>2</sub></td><td>0.52–1.65</td></tr><tr><td>CaO</td><td>0.03</td></tr><tr><td>K<sub>2</sub>O</td><td>0.42</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.42</td></tr><tr><td>Weight loss</td><td>0.21</td></tr></tbody></table>

molar ratio of C/SiO₂ ≈ 5.5 by ball milling. The slurries were then dried and the powder mixtures were sieved to less 500 µm.

A vertical tubular furnace (Pyrox, Paris, France) at 1260–1550 °C was used for carbothermal reduction of kyanite. A bed of the mixed starting powder (10 mm in height and 50 mm in diameter) was prepared in a graphite crucible and heat treated for 2 h at each reaction temperature under argon. The heating and cooling rates were both 300 °C / h. After each heat treatment, unreacted carbon was removed by oxidation in air at 625 °C for 5 h before characterization.

X-ray diffraction (XRD; Cu Kα radiation with a graphite monochromator) was used for phase analysis. XRD quantitative analysis was performed on the synthesized powders, too. Mixtures of Al₂O₃ and SiC-whiskers in different proportions have been prepared and a calibration curve established. The morphology was investigated by SEM (JEOL 840, Tokyo, Japan) with an energy dispersive x-ray spectroscopy (EDS) system (EDAX TRACOR, Paris, France).

### III. RESULTS AND DISCUSSION

#### A. Phase evolution

XRD phase analysis (Fig. 1) shows that the carbothermal reaction goes through many steps leading to β-SiC and α-Al₂O₃ as final products. Mullite and a small amount of an amorphous phase were detected as intermediate solid phases. The formation of β-SiC and α-Al₂O₃ began at about 1320 and 1370 °C, respectively. Figure 1 shows that the rate of reduction of kyanite started very slowly at 1320 °C and increased with increasing temperature.

On the basis of XRD analysis the following reaction scheme is proposed. The thermal decomposition of kyanite to mullite and amorphous silica occurs below 1370 °C.

$$3(\underset{\text{kyanite}}{\mathrm{Al_2O_3}} \cdot \mathrm{SiO_2}) \xrightarrow{<1370 \,^{\circ}\mathrm{C}} \underset{\text{kyanite}}{3\mathrm{Al_2O_3}} \cdot 2\mathrm{SiO_2} + \mathrm{SiO_2}, \quad (1)$$

The silica produced by this decomposition is reduced to SiC above 1320 °C.

$$\mathrm{SiO_2} + 3\mathrm{C} \xrightarrow{>1320 \,^{\circ}\mathrm{C}} \mathrm{SiC} + 2\mathrm{CO}, \quad (2)$$

<figure><img src="image_2.png" /><figcaption>FIG. 1. X-ray diffraction patterns on heat treated powders from kyanite-carbon system in the temperature range 1260–1550 °C under argon. (A) α-Al₂O₃; (B) β-SiC; (K) kyanite; (M) mullite.</figcaption></figure>

For the last reaction, several mechanisms have been proposed, all of which involve two important steps⁵,⁷,⁸,²⁰,²¹ The first step is related to the formation of silicon monoxide by the reaction:

$$\mathrm{SiO_2} + \mathrm{C} \rightarrow \mathrm{SiO} + \mathrm{CO} \quad . \quad (2a)$$

This reaction is followed by

$$\mathrm{SiO} + 2\mathrm{C} \rightarrow \mathrm{SiC} + \mathrm{CO} \quad . \quad (2b)$$

Combination of reactions (2a) and (2b) gives reaction (2).

Above 1370 °C (after consumption of silica), the mullite is reduced. This reduction forms α-alumina (corundum).

$$3\underset{\text{mullite}}{\mathrm{Al_2O_3}} \cdot 2\mathrm{SiO_2} + 6\mathrm{C} \xrightarrow{>1370 \,^{\circ}\mathrm{C}} \underset{\text{mullite}}{3\mathrm{Al_2O_3}} + 2\mathrm{SiC} + 4\mathrm{CO} \quad . \quad (3)$$

The combination of reactions (2) and (3) gives

$$3\mathrm{Al_2O_3} \cdot 2\mathrm{SiO_2} + \mathrm{SiO_2} + 9\mathrm{C} \rightarrow \underset{\text{mullite}}{3\mathrm{Al_2O_3}} + 3\mathrm{SiC} + 6\mathrm{CO} \quad .$$

The reaction in the system is complete at 1550 °C.

#### B. Morphology of the α-Al₂O₃/β-SiC powder

SEM observations of the powder synthesized at 1550 °C revealed that whiskers coexist with spherical particles. The synthesized powder has a woolly aspect.
<figure><img src="image_3.png" /><figcaption>FIG. 2. SEM micrographs showing difference in morphology between reacted powders from upper and lower layer of the sample. Whiskers from (a) the upper layer have relatively higher aspect ratio than from (b) the lower one.</figcaption></figure>

EDS analysis, with the help of the above results, indicates that the whiskers and the particles are respectively composed of SiC and Al₂O₃. Differences in morphology were observed between reacted powders from the upper layer (width of approximately 1 µm) of the sample and the lower one [Figs. 2(a) and 2(b)]. Whiskers from the upper layer have a higher aspect ratio. This is likely due to the lower supersaturation degree of silicon and carbon monoxides at the upper layer. A higher supersaturation degree is not favorable to SiC whisker growth.⁹ The mean dimensions of SiC whiskers and Al₂O₃ particles in different parts of the reaction zone are presented in Table II.

<figure><img src="image_4.png" /><figcaption>FIG. 3. SEM micrograph showing a globule at the tip of a whisker. The globule is a solidified droplet which forms at high temperatures during the VLS growth mechanism of the SiC-whisker.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>FIG. 4. XRD patterns of heat treated kyanite-carbon powder at 1500 °C. The reaction for the formation of SiC and Al₂O₃ is completed with the higher specific surface area carbon. (A) α-Al₂O₃; (B) β-SiC; (M) mullite.</figcaption></figure>

Globules were observed at the tip of some whiskers (Fig. 3) and EDS analysis reveals that these globules are iron-rich regions with the presence of silicon. As in the case of separate synthesis of SiC whiskers, these globules are solidified droplets which form at high temperatures during the vapor-liquid-solid (VLS)⁶,⁷ growth mechanism of SiC whiskers. The impurities, such as transition metals, constitute energetically favourable sites

<table><caption>TABLE II. Dimensions (µm) of the synthesised SiC-whiskers and Al₂O₃-particles at 1550°C under argon atmosphere.</caption><thead><tr><th colspan="4">Synthesised powder from the lower layer of the sample</th><th colspan="4">Synthesised powder from the upper layer of the sample</th></tr><tr><th colspan="2">SiC whiskers</th><th colspan="2">Al<sub>2</sub>O<sub>3</sub> particles</th><th colspan="2">SiC whiskers</th><th colspan="2">Al<sub>2</sub>O<sub>3</sub> particles</th></tr><tr><th>Length</th><th>Diameter</th><th>Size</th><th>Size</th><th>Length</th><th>Diameter</th><th>Size</th><th>Size</th></tr></thead><tbody><tr><td>10</td><td>0.5</td><td>6</td><td>6</td><td>20</td><td>0.6</td><td>4</td><td>4</td></tr></tbody></table>
for SiC nucleation. The presence of whiskers having a conical tip instead of balls indicates that there was another mechanism of crystal growth. Although the nature of this mechanism is uncertain, vapor-solid (VS) growth mechanism of whiskers has been proposed,²² and carbon black may constitute nucleation sites for SiC.²³ In the VS mechanism, SiC crystallisation occurs directly from the vapor phases SiO and CO formed at high temperatures. The tip growth is postulated to occur on an axial screw dislocation, but such dislocations are rarely seen.²⁴

<figure><img src="image_6.png" /><figcaption>FIG. 5. Evolution of the most intense peak (111) of β-SiC formed from carbothermal reduction of kyanite in the temperature range 1370–1550 °C. With the higher value of specific surface area of carbon black the conversion to SiC is accelerated.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>FIG. 6. SEM micrograph showing unfavorable morphology of the synthesized Al₂O₃/SiC powder by using a too-high specific surface area of carbon during carbothermal reduction of kyanite.</figcaption></figure>

XRD quantitative analysis of the phases present in the powder after completion of the synthesis reaction gave approximately 28 wt% SiC in Al₂O₃/SiC powder. Optimal strength values for densified Al₂O₃/SiCw composites prepared by conventional mixing of alumina and SiC whiskers are obtained with 25 wt% SiC.²⁵ This value may be achieved by adding alumina after synthesis. Adding alumina before synthesis to the raw materials does not contribute to a good morphology of the whiskers.¹⁹

Some too-long, bent, or twisted whiskers and agglomerates are also encountered in the synthesized powders. These characteristics are also observed in the case of separate synthesized SiC whiskers.²⁶,²⁷ This needs further operation of milling-deagglomeration (for example by ball milling) before sintering the powder to form composite materials with good dispersion of the phases.²⁸

### C. Effect of carbon specific surface area

Figure 4 shows XRD patterns of heat-treated mixtures of kyanite and carbon black at the same temperature 1500 °C with two different values of Brunauer–Emmett–Teller (BET) specific surface area of carbon: 330 and 996 m²/g. Higher specific surface area of carbon lead to completion of the carbothermal reduction reaction, whereas a significant amount of mullite is detected in the sample with lower specific surface area at the same temperature of 1500 °C (the reaction in this latter case is completed between 1500 and 1550 °C).

Figure 5 shows the evolution of the most intense peak (111) of β-SiC formed from carbothermal reduction of kyanite in the temperature range 1370–1550 °C. A higher specific surface area of carbon black leads to an increase in the conversion rate to SiC. Quantitative analysis shows that the amount of SiC phase in Al₂O₃/SiC powder (about 28 wt%) does not change with increasing the temperature from 1500 to 1550 °C in the case of the higher surface area. A similar effect of specific surface area of carbon has been observed in the case of the carbonitriding of silica.²⁹,³⁰

Higher specific surface area of carbon does not influence favorably the morphology of the synthesized powder. The SEM micrograph (Fig. 6) shows that the synthesized powder is mainly composed of agglomerates, and the fibrous phase SiC is not morphologically regular. Whiskers are bent, twisted, and very thin. This morphology is not favorable for Al₂O₃–SiCw composites, since these characteristics lead to large flaws, porosity, and inhomogeneous microstructure in the final composite. A carbon black with a relatively high surface area would lead to a large number of sites for the nucleation and growth of SiC during synthesis. These conditions favor irregular morphology of the whiskers.
## IV. CONCLUSION

Carbothermal reduction of kyanite leads to *in situ* $Al_2O_3$ particles and SiC-whisker formation with 28 wt% SiC. The completion of the reaction is obtained at 1550 °C when using a carbon with a specific surface area 330 m²/g and a heating time of 2 h. With a higher value of 996 m²/g, the completion of the reaction occurred at 1500 °C. During carbothermal reduction of kyanite, this mineral decomposes to free silica and mullite as intermediate phases. The reduction of mullite occurs at higher temperature than that of free silica. Mullite reduction does not occur before full consumption of the free silica. The aspect ratio of the synthesized SiC in the upper layer of the samples is relatively higher than that in the lower one. Both VLS and VS crystal growth mechanisms for SiCw are suggested. The morphology of the synthesized powder is not favorable with a too-high specific surface area of carbon. The principal advantages of this *in situ* route are reducing the health hazard related to the handling of whiskers, the use of raw materials with large availability such as kyanite, and the simplification of the process of $Al_2O_3$/SiCw composites. The principal disadvantage of this method is the difficulty of morphological control of the phases because they are not synthesized separately.

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