BaTi0.975Sn0.025O3/BaTi0.85Sn0.15O3 functionally graded materials: master sintering curves and electrical properties

Abstract

The most important aim in the design and processing of functionally graded materials (FGMs) is to obtain high-quality microstructure, and to produce devices free from any form of deformation. During sintering of BTS2.5/BTS15 FGMs, barium tin-titanate powders with different tin content (BaTi0.975Sn0.025O3 and BaTi0.85Sn0.15O3, denoted as BTS2.5 and BTS15, respectively) show different shrinkage rates and different extents of shrinkage, as well as different final density. This phenomenon can lead to excessive shape distortion, warping, delamination, crack development and microstructural damage in the FGMs. Therefore, to achieve high-quality FGM it is desirable to predict the sintering process for each layer in FGM and to design sintering strategies. The master sintering curve (MSC) model is suitable approach to make quantitative predictions of the densification behavior, and to design sintering procedure to prepare high-quality ceramics. Here, BTS2.5/BTS15 FGMs are prepared by the powder stacking method and uniaxially-pressing process, followed by sintering. The MSCs were constructed, for BTS2.5 and BTS15 graded layers in FGMs, using shrinkage data obtained by a heating microscope during non-isothermal sintering up to 1420 ◦C with heating rates of, 2, 5, 10 and 20 ◦/min. The effective activation energies, determined using the concept of MSC, were 359.5 and 340.5 kJ/mol for graded layers BTS2.5 and BTS15, respectively. A small difference of the effective activation energies of chosen powders allowed preparation of high-quality FGMs, without delamination, distortion or other forms of defects. Electrical characteristics of such BTS2.5/BTS15 FGMs prepared by non-isothermal sintering were determined by an ac impedance spectroscopy (IS). The IS results were used to distinguish the grain-interior and grain boundary resistivity of the ceramics, furthermore, to calculate activation energies for ionic conductivity. The activation energy as deduced from grain-interior conductivity is defined by chemical composition and does not depend on microstructure, while activation energy from grain boundary conductivity is influenced by microstructural development (density and average grain size), and vary for differently nonisothermally treated BTS2.5/BTS15 FGMs. No point dissipation is observed by IS, confirming that no insulator interfaces (cracks and/or delamination) between graded layers exist. We found that a smart choice of the heating rate during sintering is important for the fabrication of FGMs with desired electrical properties.