Reaction sintering and microstructural evolution in metakaolin- metastable alumina composites Chantale Njiomou Djangang • Arlin Bruno Tchamba • Elie Kamseu • Uphie Chinje Melo • Antoine Elimbi • Anna Maria Ferrari • Cristina Leonelli Received: 12 July 2013 / Accepted: 5 June 2014 / Published online: 8 July 2014 Ó Akade ´miai Kiado ´, Budapest, Hungary 2014 Abstract Fine needles of mullite grains were obtained successfully in a compact and low porous matrix using solid state sintering. We treated high-grade kaolin and sand-rich kaolin at 750 °C to amorphous metakaolins, and bauxite at 1,000 °C to metastable alumina. By designing a stochiometric composition of mullite, each amorphous metakaolin was added to metastable alumina. Fine grains of mullite with almost complete crystallization were obtained from 1,350 °C in a case of amorphous metakaolin from high-grade kaolin and at 1,550 °C in the other case where amorphous metakaolin is from sand-rich kaolin. The difference in the temperatures of mullitization was linked to the late dissolution of silica from the cristobalite and quartz phases which were still present in the sand-rich metakaolin sample at 1,350 °C. The use of metastable alumina and metakaolin instead of kaolin to design the mullite matrix allows the increase in number of mullite nucleation sites. This results to high densification and crystallization, fine grain size, and high mechanical prop- erties of the final matrix. Keywords Densification  Metastable  Microstructure  Mullite  Solid state sintering Introduction Excellent high-temperature strength and hardness, creep resistance, good chemical and thermal stability, low ther- mal expansion coefficient, dielectric properties,and so on are the reasons of attraction that mullite is subjected. These exceptional properties are linked to the nucleation, grain growth, and densification behavior during sintering of mullite precursors: classically, mullite is produced through thermal transformation of kaolin at high temperature [1, 2]. Many alternative techniques have been proposed for the traditional solid state transformation of kaolin to mullite [3–8]. The interdiffusion rates of Si 4? and Al 3? within mullite lattice are relatively slow making the kinetics of mullite formation strongly linked on the precursor mixing [1]. The mullitization temperature for the solid state reac- tion using kaolin can be higher than 1,650 °C. Coating amorphous SiO 2 onto the surface of c-Al 2 O 3 particles, mullite can form at a temperature lower than 1,300 °C[9]. The technique includes: viscous flow of the amorphous silica coating on the particles, avoidance of mullite for- mation until the higher temperatures, and increased threshold concentration for the development of percolation networks [10]. The objectives of the alternative techniques have been directed toward preparation of mullite which has fine particle size to reduce diffusion distances and increase the driving force for sintering, enhance the densification using homogeneous powders compacts in which particles are packed to high relative density in which large pores and powder agglomerates are absent. These techniques require efforts to improve the green microstructure since with optimum processing mullite powder compacts can be pressure-less sintered to high relative density (C98 %) and fine average grain size ( \ 1–2 lm) at temperatures of *1,550 °C[11–15]. C. N. Djangang (&)  A. B. Tchamba  U. C. Melo  A. Elimbi Laboratory of Applied Inorganic Chemistry, University of Yaounde I, P.O. Box 812, Yaounde ´, Cameroon e-mail: djangangc@yahoo.fr A. B. Tchamba  E. Kamseu  U. C. Melo Laboratory of Materials, MIPROMALO, P.O. Box 2396, Yaounde ´, Cameroon E. Kamseu  A. M. Ferrari  C. Leonelli Department of Engineering ‘‘Enzo Ferrari’’, University of Modena and Reggio Emilia, Via Vignolese 905/A, 41125 Modena, Italy 123 J Therm Anal Calorim (2014) 117:1035–1045 DOI 10.1007/s10973-014-3937-6