Glass–ceramics with zero thermal expansion in the system BaO/Al2O3/B2O3

A glass with the stoichiometry BaO · Al2O3 · B2O3 was melted from the raw materials. Three types of samples were prepared: (i) the as melted glass was thermally treated, (ii) the glass was powdered, pressed and thermally treated to obtain sinter glass–ceramics and (iii) the crushed glass was mixed with a platinum salt solution, remelted, cooled and then thermally treated. All three samples contained BaO · Al2O3 · B2O3 as crystalline phase and showed negative thermal expansion coefficients. However, those of as melted and crystallized sample was most and that of the sintered sample least negative. High temperature X-ray powder diffraction was performed in order to study the thermal expansion in different crystallographic directions. It is shown that thermal expansion is strongly negative in the crystallographic c-axis while it is positive with respect to the a-axis.