Effect of Boron Substitution on Oxide-Ion Conduction in c-Axis-Oriented Apatite-Type Lanthanum Silicate

Abstract

Apatite-type lanthanum silicate (LSO) is a material with high oxide-ion conductivity in the low- and intermediate-temperature range (573–873 K) and is, therefore, a promising solid electrolyte for low-temperature applications such as solid oxide fuel cells and oxygen sensors. Herein, the effect of B substitution at the Si site in a c-axis-oriented apatite-type lanthanum silicate (La9.7Si5.3B0.7O26.2, c-LSBO) polycrystal on oxide-ion conduction is investigated. A highly c-axis-oriented LSBO polycrystal is fabricated by a vapor–solid reaction in which a dense La2SiO5 disk is heated in B2O3 vapor at ≥1673 K. The oxide-ion conductivity of c-LSBO reaches 16 mS cm–1 at 678 K with an activation energy of 0.4 eV. The obtained oxide-ion conductivity of c-LSBO is approximately 190 times higher than that of yttria-stabilized zirconia and 5.8 times higher than that of the polycrystalline c-axis-oriented nondoped lanthanum silicate. Based on 11B nuclear magnetic resonance measurements, B is located at the SiO4 site as BO4, suggesting the formation of an oxygen vacancy at the O4 site located along the c-axis due to charge compensation. In addition, molecular dynamics simulations indicate that the oxide-ion diffusion coefficient of the B-doped LSO is higher than that of the nondoped LSO. The high oxide-ion conductivity of c-LSBO is likely attributable to the formation of an oxygen vacancy at the O4 site by B doping, which has a lower valency than Si. Therefore, c-LSBO is a promising candidate as a solid electrolyte in electrochemical devices operating at low and moderately high temperatures.

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.…”

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

“…Recently, we successfully synthesized a highly c -axis-oriented ( f 00 l = 0.99) boron-substituted apatite-type lanthanum silicate ( c -LSBO) through a vapor–solid reaction using B 2 O 3 as the vapor precursor . Through this method, we achieved the highest degree of c -axis orientation ever reported for LSO synthesized by vapor-phase methods.…”

High-purity synthesis of La2SiO5 by solid-state reaction between La2O3 and different characteristics of SiO2

“…Moreover, Fukuda et al [8][9][10] have used this lanthanum silicate to obtain a highly c-axis oriented apatite-type lanthanum silicate by reactive diffusion between sandwich-type layers of La 2 SiO 5 and La 2 Si 2 O 7. Likewise, other sandwichtype layer combinations such as La 2 SiO 5 /SiO 2 [11] and La 2 SiO 5 /B 2 O 3 vapors [12] have been proposed with successful results in apatite orientation . Generally, the La 2 SiO 5 compound must be of high purity for this type of applications, but few studies documented adequate requirements to synthesize lanthanum monosilicate in the literature.…”

“…Christensen et al [13] used amorphous silica known as kieselguhr or diatomite, which has a high amount of impurities such as Al 2 O 3 , MgO, and Fe 2 O 3 [19] . Usually, the studies related to the synthesis of rare earth silicates have in common the use of high-purity silica powders (>99.0%) [4,5,8,12,13,15,16,[20][21][22]. Due to the low reactions between La 2 O 3 and SiO 2 , the synthesis by solid-state reaction of this type of materials broadly requires high temperatures (1400-1700°C), long sintering time (20-200 h), and alternated grinding cycles [13,[23][24][25][26][27][28][29][30].…”