Development of Novel Processing Routes for Electrical Ceramics and Glass-Ceramics

The study in this project focuses on using three different processing routes, conventional solid-state sintering, microwave sintering and melt processing, to manufacture electrical ceramics and glass-ceramics in bismuth sodium titanate (BNT) and potassium sodium niobate (KNN) systems. Manufacturing electrical ceramics using a conventional solid-state processing route requires high sintering temperatures (more than 1100oC) for long times (at least 10 hours including heating and cooling), with high energy consumption (20kWh) for electrical furnaces, leading to high cost. Moreover, laboratory-scale preparation of electrical ceramics in the author’s home country, Iraq, is difficult because of equipment shortages and power watts. Therefore, in the present work, a microwave processing route using a standard kitchen microwave oven has been used to produce electrical ceramics, whilst dramatically reducing sintering times (heating between 20-25mins and cooling 2h) and thus energy consumption (0.4kWh). Also, a melt processing route has been used to manufacture electrical glass-ceramics as bulk and fibre samples, thus reducing processing steps and preparing KNN borosilicate glass-ceramic fibres for the first time. Microwave sintering/heating depends on the generation of heating throughout the sample simultaneously. This project has used a combination of direct microwave heating of the sample itself, combined with indirect heating by microwave susceptor disks placed close to the sample. In the present project, microwave susceptor disks composed of 50wt% graphite, 30wt% SiO2, 10wt% Mn2O3, 10wt% Fe3O4 have been used. Both conventional solid-state sintering and microwave processing have been used to prepare four different electrical ceramic compositions: pure bismuth sodium titanate (Bi0.5Na0.5TiO3), pure potassium sodium niobate (K0.5Na0.5NbO3), iron-doped bismuth sodium titanate (Bi0.5Na0.5Ti1-xFexO3-0.5x) and iron-doped potassium sodium niobate (K0.5Na0.5Nb1-xFexO3-x), with high sintering temperatures (900-1100oC) for 2h inside an electrical furnace (conventional sintering) and short heating times (10-25 mins) inside a standard 900W kitchen microwave oven. A melt processing route has also been used, with the aim of preparing bismuth sodium titanate (Bi0.5Na0.5TiO3) and potassium sodium niobate (K0.5Na0.5NbO3) by heat treatment of bismuth sodium titanium silicate, bismuth sodium titanium borate, potassium sodium niobium silicate, potassium sodium niobium borate and potassium sodium niobium borosilicate glasses to form glass-ceramics. In addition, opportunities for preparation of potassium sodium niobium borosilicate glass and glass-ceramic fibres have been studied. A range of structural, thermal and electrical measurements have been carried out, including differential thermal analysis (DTA), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), density, polarisation and strain versus electric field measurement were carried out on the electrical ceramics and glass-ceramics. Pure BNT ceramics have been prepared by solid-state sintering and microwave sintering. In addition, BNT-silicate and BNT-borate glasses have been prepared by melt processing, and subsequent heat treatment with the aim of forming BNT in the resulting glass-ceramics. A Bi0.5Na0.5TiO3 phase with rhombohedral structure has been successfully produced by solid-state sintering and microwave sintering methods, however, it was not possible to produce entirely glassy materials in the silicate and borate systems studied, which instead formed glass + crystalline components upon cooling from molten. Increasing the sintering temperature from 1000oC to 1100oC for pure BNT ceramics prepared by solid-state sintering increased average grain size from ca. 0.4 μm to 2 μm and the relative density increased from approximately 83% of theoretical density to approximately 94% of theoretical density. Also, increasing sintering temperatures from 1050oC to 1100oC enhanced the remnant polarisation from approximately 22 to 40 μC/cm2 and maximum strain increased from about 4.5 to 8%, respectively. Furthermore, increasing the microwave sintering time from 10 to 25 min for pure BNT ceramics prepared by microwave sintering processing, increased average grain size approximately 0.2 to ca 0.9 μm, and the relative density from approximately 70% to approximately 94% of theoretical density. The remnant polarisation (about 42.2μC/cm2) and the maximum strain (8.3%) with a saturated and typical shape for ferroelectric behaviour were obtained at higher microwave sintering times (25 mins). Iron-doped BNT ceramics were produced by solid-state sintering and microwave sintering. A single Bi0.5Na0.5TiO3 phase with cubic structure was successfully produced by both processing methods. Adding iron to the BNT ceramics using solid-state sintering processing increased average grain size with increasing Fe content from 0.2 μm in pure BNT to 5 μm in (Bi0.5Na0.5Ti0.9Fe0.1O2.95) for iron doped BNT ceramics. The relative density was low (69% of theoretical density) for undoped BNT ceramics and high (97% of theoretical density) for 7 mole% Fe-BNT. Pure KNN ceramics have been prepared by solid-state sintering and microwave sintering. In addition, KNN-silicate glasses, KNN-borate glasses, KNN-borosilicate glasses and KNN-borosilicate glass fibres have been prepared by melt processing, and subsequent heat treatment with the aim of forming KNN in the resulting glass-ceramics. A K0.5Na0.5NbO3 phase with orthorhombic structure has been successfully produced by solid-state sintering and microwave sintering processing. Also, a main K0.5Na0.5NbO3 phase with anorthic structure has been presented for KNN-borate glass-ceramics, KNN-borosilicate glass-ceramics and KNN-borosilicate glass-ceramic fibres by melt processing, however, it was not possible to produce entirely glassy materials in the silicate systems studied, which instead formed glass + crystalline components upon cooling from molten. Increasing the sintering temperature from 1000oC to 1100oC for pure KNN ceramics manufactured by solid-state sintering increased average grain size from ca. 0.6 μm to 5 μm and the relative density increased from ca. 69% of theoretical density to approximately 91% of theoretical density. Also, increasing sintering temperatures from 1075oC to 1100oC enhanced maximum strain increased from about 3.6 to 6%, respectively. The remnant polarisation of approximately 33 μC/cm2 with a saturated and ideal P-E hysteresis shape related to the ferroelectric behaviour has obtained at a higher sintering temperature of 1100oC. Additionally, increasing the microwave sintering time from 10 to 25 min for pure KNN ceramics manufactured by microwave sintering processing, increased average grain size approximately 0.2 μm to approximately 3 μm, and the relative density from ca. 68% to ca. 90% of theoretical density. Also, increasing microwave sintering time from 20 to 25 mins improved maximum strain increased from about approximately 5.2 to 7.5%, respectively. The remnant polarisation of approximately 17 μC/cm2 with a saturated and ideal P-E hysteresis shape related to the ferroelectric behaviour has obtained at a higher microwave sintering time of 25 mins. On the other hand, flat surfaces with grain growth, and slimmed P-E hysteresis shapes, indicated the absence of ferroelectric behaviour from the KNN borate glass-ceramics and KNN borosilicate glass-ceramics prepared by melt method. Iron-doped KNN ceramics were prepared by solid-state sintering. A single K0.5Na0.5NbO3 phase was obtained for only one iron-doped sample, K0.5Na0.5Nb0.98Fe0.02O2.98. Adding iron to KNN ceramics increased the relative density from 73% of the theoretical density for undoped KNN ceramics to 86% of the theoretical density for K0.5Na0.5Nb0.9Fe0.1O2.9.