X-ray diffractometric determination of mullite in silica-alumina ceramics using multiple analytical lines

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Abstract
A powder diffraction method has been applied to the determination of mullite in several porcelain and pottery wares. Two calibration methods, namely, the external standard and diffraction-absorption methods, were examined. Calibration mixtures for use in the external standard method were prepared by mixing a standard mullite powder extracted from a heat-resistant ceramic material by treatment of the latter with HF at 20 °C to remove the matrix. The matrix for this method was SiO2-Al2O3 glass powder, prepared by calcination of SiO2-Al2O3 gel at 900 °C. The SiO2-Al2O3 gel was prepared by hydrolysis of a mixture containing silicon ethoxide and aluminium isopropoxide in an 8:2 molar ratio. The intensity of mullite was calculated from the sum of the diffraction intensities of the (110), (220), (111) and (121) lines, in order to eliminate the preferred orientation of the (110) plane. A linear calibration graph was obtained over the range 0–100%m/m of mullite using the external standard method. The analytical results obtained by the two calibration methods showed good agreement. The values obtained by gravimetric analysis were lower than those given by the X-ray method owing to slight dissolution of the mullite in the hydrofluoric acid. The determination limit was 0.22%m/m of mullite, which corresponds to three times the standard deviation of the sum of the intensities of the four lines. The relative standard deviation for 57.9%m/m of mullite was 0.66% for the external standard method and 1.3% for the diffraction-absorption method.

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