Glasses and crystallised glasses in the BaO-B_2O_3-TiO_2 system

摘要

Several compositions in the BaO-B_2O_3-TiO_2 system have been studied, the most important being BaTi_4O_9 with an added amount of (mol percent) 5-50 B_2O_3 and BaTiO_3 together with 25-50 B_2O_3. The melting temperatures were between 1100 and 1450 deg C depending on the amount of B_2O_3. For the compositions with less than 40 mol percent TiO_2 and more than 20 mol percent B_2O_3 the materials obtained were vitreous. If the amount of TiO_2 was greater than 50 mol percent they were crystalline even after ultrarapid quenching. The main physical properties were determined for both the vitreous and crystallised materials. The dielectric constant, e_r and the tangent of dielectric loss, tand, have been measured at different frequencies for uncoated samples and those coated with silver. One of the most important characteristics for these samples was the stability of tand data over time and in the temperature range 0-120 deg C. For structural analysis, x-ray diffractometry, differential thermal analysis (DTA), optical and electron microscopy have been used on samples crystallised by controlled thermal treatments. X-ray analysis of the vitreous samples showed no crystalline compounds, while DTA data characterise the crystallisation tendency of these samples in the temperature range 600-800 deg C. Structural investigations for the crystallised samples indicate the presence of the compounds BaTi_4O_9 and Ba_2Ti_9O_20 together with retile, TiO_2. These crystallised samples obtained from the glass matrix were homogenous, the microstructure being without pores. In the samples with less then 40 mol percent TiO_2 and more then 20 mol percent B_2O_3 a second ternary compound, Ba_2Ti_2B_2O_9, was identified.