Crystal Growth Mechanism of Highly c-Axis-Oriented Apatite-Type Lanthanum Borosilicate Using B₂O₃ Vapor

Abstract

Apatite-type lanthanum silicate (LSO) exhibits high oxide-ion conductivity and has recently garnered attention as a potential solid electrolyte for high-temperature solid oxide fuel cells and oxygen sensors that operate in the low- and intermediate-temperature ranges (300–500 °C). LSO exhibits anisotropic oxide-ion conduction along with high c-axis-oriented oxide-ion conductivity. To obtain solid electrolytes with high oxide-ion conductivity, a technique for growing crystals oriented along the c-axis is required. For mass production and upscaling, we have thus far focused on the vapor-phase synthesis of c-axis-oriented apatite-type LSO and successfully grew polycrystals of highly c-axis-oriented boron-substituted apatite-type lanthanum silicate (c-LSBO) using B2O3 vapor. Here, we investigated the mechanism of c-LSBO crystal growth to determine why the utilization of B2O3 vapor resulted in such a strong c-axis crystal orientation. The synthesis of c-LSBO by the B2O3 vapor-phase method results in crystal growth accompanied by the diffusion of B2O3 supplied from another new compound that formed on the surface of the La2SiO5 disk, LaBO3. In addition, c-LSBO crystals are formed not only by vapor–solid reactions but also by solid–solid and liquid–solid reactions. The increase in the c-axis orientation degree might be due to the increase in the amount of the liquid-phase interface.

DC-Voltage-Induced High Oxygen Permeation through a Lanthanum Silicate Electrolyte with a Cerium Oxide Thin Film

“…The c-LSBO solid electrolyte was prepared by reacting sintered La2SiO5 with B2O3 vapor at 1570 °C for 40 h in an electric vertical double furnace. 30,31 The obtained c-LSBO was polished to a thickness of 350 µm. Then, the SDC intermediate layer (thickness: 200, 300, and 400 nm) was deposited on the cathode side surface of c-LSBO by radio frequency sputtering using a Ce0.8Sm0.2O2-δ sputtering target and annealed at 1400 °C for 1 h in air.…”

“…29 We synthesized a La9.66Si5.3B0.7O26.14 (c-LSBO) solid electrolyte with crystal orientation along the c-axis and reported a high σo (0.016 S cm −1 at 400 °C). 30,31 Additionally, by employing c-LSBO in a novel oxygen separation device and gas sensor, our group demonstrated lowto-intermediate temperature operability. 32,33 In terms of the oxygen pumping properties, cell resistance decreased by more than 80% at a DC voltage of 2.0 V upon the inclusion of an intermediate Sm0.2Ce0.8O2 (SDC) thin film layer between the c-LSBO electrolyte and the Pt electrode, compared with that of the cell without the SDC.…”