Formation of grain boundary second phase in BaTiO₃ polycrystal under a high DC electric field at elevated temperatures

Abstract

The microstructure of the damaged area created during a flash sintering event in BaTiO 3 was examined by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectrometry (EELS) measurement. A DC electric field of 133 V/cm was applied to a fully-densified BaTiO 3 body prepared by conventional sintering, and the specimen temperature was elevated at a constant heating rate. The flash sintering event, at which the electric current in the specimen abruptly increases at the threshold field and temperature, took place at 890°C. After the flash event, tunnellike physical damage was observed in the direction of the field through the specimen. Formation of grain boundary second phase layers around the damaged area was confirmed by HRTEM observations. The grain boundary second layer was a crystalline phase with a low Ba/Ti atomic ratio of less than one. The temperature in some of the grain boundaries must increase by Joule heating during the flash event, thus the chemical composition in the vicinity of the grain boundaries changed due to vaporization of the Ba cations, resulting in the formation of the grain boundary second phase layers.

Flash sintering of ceramic materials

“…However, at between 0.6m for 250V/cm and 0.3-0.4m for 500V/cm the grain size in the flash sintered samples was significantly smaller than for conventional sintering (15m after 1h) [109]. M'Peko et al [109] also demonstrated that raising the current density limit too high can result in significant inhomogeneities in the microstructure, which is confirmed in a later study by Yoshida et al [35] who examined damage in barium titanate polycrystalline ceramics exposed to electric fields of 133V/cm. Layers of second phase material with reduced barium content were found at the grain boundaries, suggesting that electric conduction through the grain boundaries may trigger the flash sintering event in barium titanate.…”

“…Ceramic materials moulded into bars [35] and dogbones [20] with rectangular cross-sections and pellets of various diameter to height ratios [36][37][38] have all been flash sintered ( Figure 4). Flash sintered samples generally have small dimensions [28].…”

“…Three ferroelectric materials have so far been shown to undergo flash sintering; strontium titanate [108], barium titanate [35,109,110] and potassium niobate [111].…”

On thermal runaway and local endothermic/exothermic reactions during flash sintering of ceramic nanoparticles

“…First, the flash experiments are controlled by limiting the current density immediate to the flash event. If this limitation is removed, the excess Joule heat may locally melt the compact [35], or fully melt the glass (see supplement in Ref. [4]).…”

“…The lack of liquid residue in many flash sintered microstructures may be due to the limited extent of the softened/melted surface (i.e., a few atomic layers at the particle surfaces) as well as due to its metastable and transient nature. Nevertheless, when formation of high-temperature liquid led to selective evaporation of some elements, liquidlike phases were observed at the particle surfaces and grain boundaries [35,36]. Nevertheless, local melting was observed at the pore surfaces during flash spark plasma sintering (FSPS) of thermoelectric Sb-doped magnesium-tin silicide [37].…”