Mullite (3Al₂O₃·2SiO₂) ceramics obtained by reaction sintering of rice husk ash and alumina, phase evolution, sintering and microstructure

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

Figures & data

Fig. 1 XRD pattern of the rice husk ash (RHA).

Table 1 Chemical composition of the as received rice husk ash.

Oxide	(% wt.)
SiO2	94.72
SO3	0.17
CO2	0.23
P2O5	0.51
CaO	0.21
MgO	0.18
Na2O	0.05
K2O	0.46
Al2O3	<0.05
Fe2O3	0.05
Cl^−	0.02
C	3.39
Melting point (°C)	>1200
Calorific power (cal/g)	351
Specific surface (BET: m^2/g)	11.22

Table 2 Particle size distribution of the studied RHA.

Sieve number (ASTM)	Aperture (mm)	Retained (% wt.)
18	1.00	0.78
30	0.595	5.61
50	0.297	42.05
80	0.177	24.32
100	0.149	5.43
120	0.125	6.81
150	0.105	2.33
≤150		12.62

Fig. 2 Digital image of the as received rice husk ash (RHA), grid size is 5 mm.

Fig. 3 SEM images of the as received rice husk ash (RHA) with a 150× magnification.

Fig. 4 Grain size distribution of the stoichiometric milled alumina–RHA mixtures, evaluated by laser scattering.

Fig. 5 Differential thermal analysis (DTA), thermogravimetric analysis (TG), and thermogravimetric derivative (DTG) of the as received rice husk ash (RHA).

Fig. 6 Sintering parameters: density, porosity and shrinkage.

Fig. 7 XRD pattern of the studied materials (M13–M16).

Fig. 8 Phase content of the developed ceramics as a function of the sintering temperature, evaluated by XRD-Rietveld.

Table 3 Aluminum and silicon oxide percentile weight proportions of the M15 material.

	Theoretical (% wt.)	Global (% wt.)	Grain (point A in Fig. 10) (% wt.)	Border (point B in Fig. 10) (% wt.)
Al2O3	71.8	73.7	72.8	56.7
SiO2	28.2	26.3	27.2	43.3

Fig. 9 SEM image of the M15 and M16 materials (200×, 1000×).

Fig. 10 SEM image of the M15 material. Points A and B correspond with the punctual EDX analysis reported in Table 3.