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.