• K T Jacob

Articles written in Bulletin of Materials Science

• Compatibility of RuO2 electrodes with PZT ceramics

Because of its high electrical conductivity and good diffusion barrier properties ruthenium dioxide (RuO2) is a good electrode material for use with ferroelectric lead zirconate–titanate (PZT) solid solutions. Under certain conditions, RuO2 can react with PZT to form lead ruthenate (Pb2Ru2O6.5) during processing at elevated temperatures resulting in lead depletion from PZT. The standard Gibbs energies of formation of RuO2 and Pb2Ru2O6.5 and activities of components of the PZT solid solution have been determined recently. Using this data along with older thermodynamic information on PbZrO3 and PbTiO3, the stability domain of Pb2Ru2O6.5 is computed as a function of PZT composition, temperature and oxygen partial pressure in the gas phase. The results show PbZrO3-rich compositions are more prone to react with RuO2 at all temperatures. Increasing temperature and decreasing oxygen partial pressure suppress the reaction. Graphically displayed are the reaction zones as a function of oxygen partial pressure and PZT composition at temperatures 973, 1173 and 1373 K.

• Phase diagram of the system Ca–Ti–O at 1200 K

Phase relations in the system Ca–Ti–O have been established by equilibration of several samples at 1200 K for prolonged periods and identification of phases in quenched samples by optical and scanning electron microscopy, XRD and EDS. Samples representing 20 compositions in the ternary system were analyzed. There was negligible solid solubility of Ca in the phases along the binary Ti–O, and of Ti in CaO. Four ternary oxides were identified: CaTiO3, Ca4Ti3O10 and Ca3Ti2O7 containing tetravalent titanium, and CaTi2O4 containing trivalent titanium. Tie-lines link calcium titanite (CaTi2O4) with the three calcium titanates (CaTiO3, Ca4Ti3O10 and Ca3Ti2O7), CaO, oxygen excess TiO1+𝛿 and stoichiometric TiO. Tie-lines connect CaTiO3 with TiO2–𝑥, Magneli phases Ti$_{n}$O$_{2n–1}$ (28 ≥ 𝑛 ≥ 4), Ti3O5, Ti2O3 and TiO1+𝛿. CaO was found to coexist with TiO, and Ti–O solid solutions 𝛼 and 𝛽. The phase diagram is useful for understanding the mechanisms and kinetics of direct calciothermic reduction of TiO2 to metal and electrochemical reduction of TiO2 using graphite anode and molten CaCl2 electrolyte.

• Thermal expansion of doped lanthanum gallates

Thermal expansion of several compositions of Sr and Mg-doped LaGaO3 including an 𝐴-site deficient composition (La0.9Sr0.1)0.98(Ga0.8Mg0.2)O2.821 were measured in the temperature range from 298 to 1273 K. The effect of doping on thermal expansion was studied by varying the composition at one site of the perovskite structure (either 𝐴 or 𝐵), while keeping the composition at the other site invariant. Thermal expansion varied nonlinearly with temperature and exhibited an inflexion between 550 and 620 K, probably related to the change in crystal structure from orthorhombic to rhombohedral. The dependence of average thermal expansion coefficient (𝛼av) on the dopant concentration on either 𝐴 or 𝐵 site of the perovskite structure was found to be linear, when the composition at the other site was kept constant. Mg doping on the 𝐵-site had a greater effect on the average thermal expansion coefficient than Sr doping on the 𝐴-site. Cation deficiency at the 𝐴-site decreases thermal expansion when compositions at both sites are held constant.

• Resolution of conflicting views on thermodynamics of glass transition: A unified model

Classical description of thermodynamic properties during glass transition has been questioned by the entropy-loss model. The uncompensated loss of entropy at the glass transition temperature and zero residual entropy is at the heart of the controversy. Both the models are critically reviewed. A unified model is presented which incorporates features of both entropy loss and residual entropy. It implies two different types of contributions to the entropy of the supercooled liquid, one of which vanishes at the transition and the other which contributes to residual entropy. Entropy gain during spontaneous relaxation of glass, and the nature of heat capacity ‘hysteresis’ during cooling and heating through the glass transition range support the proposed model. Experiments are outlined for differentiating between the models.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019