Optimized synthesis of mullite ceramics in thermal plasma

   Mullite has low thermal expansion coefficient, thermal conductivity and excellentresistance to thermal shock, and plays therefore an important role in the synthesis of modern ceramic materials. The paper presents the experimental results of mullite ceramics synthesis using thermal plasma. The ceramic samples are obtained from natural raw materials such as boehmite and silica sand in the stoichiometric Al₂O₃/SiO₂ ratio of 1.86, 3 and 5.67. It is shown that the optimal operation mode of the arc plasma torch in the synthesis of mullite-containing samples from an 8 g charge weighing includes 80 A current, 100 V voltage, and 60 s exposure time. With such technological parameters, the melting product is a hemisphere with diameter 23 mm and thickness 11 mm, which indicates to a complete transition of the initial components to the molten state. The synthesized ceramic samples are characterized by the crystalline phase of mullite 3Al₂O₃ ·2SiO₂. At the same time, at the Al₂O₃/SiO₂ ratio of 1.86, cryptocrystalline diffraction maxima are observed at 2θ = 53–54° and 69–76° due to the increased content of SiO 2 . According to morphological studies performed for the Al 2O₃/SiO₂ ratio of 3, which is the traditional stoichiometric ratio for the mullite phase synthesis, the matrix of the synthesized sample is represented by the amorphous phase penetrated by filamentary single crystals of mullite with the average length of 3.3 μm and 0.86 μm thickness. It is found that the transition layer between these phases has a glass-ceramic structure, which is consistent with the results of the X-ray phase analysis.

1. Costa Oliveira F. A., Livramento V., Delmas F. Novel mullite-based ceramics manufactured from inorganic wastes: I. Densification behaviour. Journal of Mechanical Working Technology. 2008. V. 196. No. 1–3. Pp. 101–108.

2. Zhou Y. D. J., Lin B., Wang Y. et al. Reaction-sintered porous mineral-based mullite ceramic membrane supports made from recycled materials. Journal of Hazardous Materials. 2009. V. 172. No. 1. Pp. 180–186.

3. Bagchi B., Das S., Bhattacharya A., et al. Nanocrystalline mullite synthesis at a low temperature. Journal of the American Ceramic Society. 2009. V. 92. No. 3. Pp. 748–751.

4. Kim, H. S. Nicholson P. S. Use of mixed-rare-earth oxide in the preparation of reaction-bonded mullite at temperature < 1700 °C. Journal of the American Ceramic Society. 2002. V. 85. No. 7. Pp. 1730–1734.

5. Aryal S., Rulis P., Ching W. Y. Mechanical properties and electronic structure of mullite phases using first-principles modeling. Journal of the American Ceramic Society. 2012, V. 95. Pp. 2075–2088.

6. Liu M. Z., Zhu Z. W., Zhang Z., Chu Y. C., Yuan B., Wei Z. L. Development of highly porous mullite whisker ceramic membranes for oil-in-water separation and resource utilization of coal gangue. Separation and Purification Technology. 2020. V. 237. Pp. 1–10.

7. Volokitin G. G., Skripnikova N. K., Volokitin O. G., Shekhovtsov V. V., Khaysundinov A. I. Elektrodugovyye i elektroplazmennyye ustroystva dlya pererabotki silikatsoderzhashchikh otkhodov [Electric arc and plasma devices for recycling silicate-containing waste]. Izvestiya vysshikh uchebnykh zavedeniy. Fizika. 2014. V. 57. No. 3–3. Pp. 109–113. (rus)

8. Shekhovtsov V. V., Volokitin G. G., Skripnikova N. K., Volokitin O. G., Chibirkov V. K. Ustroystvo dlya polucheniya mikrosfer i mikrosharikov iz oksidnykh materialov [Device for producing microspheres from oxide materials]. Invention Patent Russ. Fed. N 2664287. 2018. (rus)

9. Volokitin O. G., Shekhovtsov V. V. Protsessy polucheniya silikatnykh rasplavov i materialov na ikh osnove v nizkotemperaturnoy plazme [Silicate melts and materials produced therefrom using low-temperature plasma]. Vestnik of Tomsk State University of Architecture and Building. 2017. No. 1 (60). Pp. 144–148. (rus)

10. Gafarov, R. Ye., Shekhovtsov V. V. Polucheniye mullita iz kaolinovoy gliny s pomoshch'yu energii plazmy [Mullite sunthesis from kaolin clay using plasma energy]. In: Perspektivy razvitiya fundamental'nykh nauk: Sbornik nauchnykh trudov XVIII Mezhdunarodnoy konferentsii studentov, aspirantov i molodykh uchenykh (Proc. 18th Int. Sci. Conf. of Students and Young Scientists ‘Prospects of Fundamental Sciences Development’). 2021. Pp. 71–73. (rus)

11. Volokitin O. G., Vereshchagin V. I., Volokitin G. G., Skripnikova N. K., Shekhovtsov V. V. Polucheniye silikatnykh rasplavov s vysokim silikatnym modulem iz kvartspolevoshpatsoderzhashchego syr'ya po plazmennoy tekhnologii [Production of silicate melts with high silicate modulus from quartz-feldspar-containing raw materials using plasma treatment]. Izvestiya vysshikh uchebnykh zavedeniy. Seriya: Khimiya i khimicheskaya tekhnologiya. 2014. V. 57. No. 1. Pp. 73–77. (rus)