Negative thermal expansion in porous glass-ceramics based on Mg2Al2B2Si5O18 prepared from Saudi raw materials

Abstract

A porous glass-ceramic enjoyed very low and negative thermal expansion was prepared. A glass of the base molar composition, 2 MgO·2 Al2O3·2 B2O3·5 SiO2 with the addition of 1.61 ma% TiO2, was melted from kaolin, magnesite, silica sand and boric acid. The composition is based on cordierite where 50 % Al2O3 were replaced by an equimolar concentration of B2O3. The glass was crashed and powdered, subsequently plastified, uniaxially pressed and finally sintered at temperatures in the range from 1100 to 1300 °C. X-ray diffraction of sintered samples gave evidence of a cordierite like crystalline phase. Scanning electron microscopy and electron dispersive microanalysis of samples prepared at 1300 °C showed crystals with hexagonal shape doped by TiO2, B2O3 and Fe2O3 embedded in a glassy matrix. With increasing crystallization temperature, the porosity of the glass-ceramics increased from 8.99 % (at 1200 °C for 3 min) to 56.42 % (at 1300 °C for 3 min). This led to a decrease of the specimen density from 1.9880 to 1.1278 g/cm3 whereas the skeletal density increased from 2.1303 g/cm3 to 2.5878 g/cm3. The microhardnesses were in the 5.60 and 6.00 GPa range. The coefficient of thermal expansion was between −3.26 and −2.27×10−6 K−1 from room temperature up to 300 oC and −0.45 and 1.89×10−6 K−1 from room temperature up to 500 °C. For the first time, to the best of our knowledge, a porous cordierite glass-ceramic with negative thermal expansion is reported. The partial replacement of Al2O3 by B2O3 enables to apply lower melting temperature which is advantageous for the production process.