Lanthanum Silicate Oxyapatite Research Articles

Microstructure of polycrystalline lanthanum silicate oxyapatite formed by solid–gas reactive diffusion

Microstructure of polycrystalline lanthanum silicate oxyapatite formed by solid–gas reactive diffusion

Optimized preparation of <i>c</i>-axis-aligned polycrystal of doped lanthanum silicate oxyapatite and evaluation of its electrical properties

Optimized preparation of <i>c</i>-axis-aligned polycrystal of doped lanthanum silicate oxyapatite and evaluation of its electrical properties

Preparation of grain-aligned polycrystal of doped lanthanum silicate oxyapatite by templated grain growth method and evaluation of its structural and electrical properties

The disk-shaped sintered compacts consisting mainly of c-axis-aligned lanthanum silicate oxyapatite (LSO) polycrystals doped with K2O and Al2O3 were prepared by templated grain growth method. The tabular single crystals of K2O- and F-doped LSO were used as template particles, and the Al2O3-doped LSO as matrix powder. The chemical formula derived from the average chemical composition of the constituent LSO crystal grains was (La9.60K0.16□0.24)Σ=10(Si5.48Al0.38□0.14)Σ=6O26, where □ denotes vacancies in La and/or Si sites. One of the doped LSO grains was examined by single-crystal X-ray diffraction to confirm that the crystal structure was isomorphous to those previously reported. Other coexisting phases were 0.16 mol% LaAlO3 and 0.34 mol% SiO2-rich interstitial material (probably in a liquid state at high temperature). With increasing temperature from 773 to 1073 K, the total oxide-ion conductivity (σtotal) along the grain-alignment direction steadily increased from 1.31 × 10−4 to 3.21 × 10−3 S cm−1, each value of which was, at the same temperature, more than 7.7 times larger than that of the randomly grain-oriented polycrystal with the same bulk chemical composition. The larger σtotal-value of the former polycrystal would be due to the significantly higher oxide-ion conductivity along the c-axis direction of the doped LSO. A solid oxide fuel cell (SOFC) using the textured polycrystal as the electrolyte was constructed and tested for power generation. The maximum power density was satisfactorily high at 873 K, indicating that the c-axis-aligned polycrystals of doped LSO would be potentially applicable as electrolytes for the SOFCs operating at medium to low temperatures.

Diffusion mechanisms between La2SiO5 and SiO2 during formation of textured lanthanum silicate oxyapatite crystals

Lanthanum silicate oxyapatite (LSO) is a promising alternative electrolyte for solid oxide fuel cell (SOFC) applications. They exhibit anisotropic oxide-ionic conduction; therefore, a preferential crystal orientation along the c-axis is necessary to achieve optimal conductivity. This study is performed to understand the reaction mechanisms involved in the formation of an LSO layer at the interface of the La2SiO5/SiO2 diffusion pair during reactive sintering at high temperatures. Different La2SiO5/SiO2 bilayers were fabricated in sandwich-type structures using silica-type quartz or La2SiO5 supports obtained via electrophoresis and uniaxial pressing, respectively. These supports were pre-sintered and then coated with a suspension of the opposite component. The diffusion pairs were isothermally heated at 1500 °C at different holding times (10, 20, and 40 h) and at 1600 °C for 10 h. Their surfaces and cross-sections were investigated via X-ray diffraction, scanning electron microscopy (SEM) observations, energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy.Experimental results show that two different driving forces were involved in the nucleation and growth of apatite crystals. The first is the chemical potential gradient in lanthanum between the components of the layers, and the second is the electric field created to preserve the electroneutrality of the system. The predominant diffusing species in this bilayer system were La+III and O-II, and their diffusion is enhanced by the formation of a glassy phase on the silica grains through the transformation of silica-type quartz into cristobalite. The formation of a La2Si2O7 intermediate phase was detected, which is vital to the interdiffusion of species and the nucleation and growth of oriented apatite crystals. The porosity gradient observed in the cross-section after reactive diffusion suggests the possibility of achieving an SOFC half-cell device via this process.

Influence of ionic radii on the conduction mechanism in lanthanum silicate oxyapatite

Lanthanum Silicate oxyapatite has attracted researchers due to its interesting ion channels and conduction pathways. In the present work, the average ionic radii of La site are altered via substitution of La3+ with Ba2+ and Ca2+ to form (La1-xAx)9.67(SiO4)6O2+δ (x = 0.0, 0.05, 0.10 and 0.15) where A = Ca and Ba. Further, the ionic radii of substituents will alter the average area of critical triangle (saddle point). Therefore, it is expected to affect the ion mobility. The single-phase Ca and Ba substituted samples are observed and Rietveld refinement suggests no phase change with the ionic radii. The elemental content obtained from Rietveld refinement, Energy dispersion spectra, X-ray photoelectron spectroscopy and thermogravimetric analysis suggests La deficiency in Ca substituted sample and O deficiency in Ba substituted sample. Temperature and frequency dependent ac conductivity in the temperature range (548 K–973 K) suggests that the conductivity is higher at x = 0.05 than x = 0.1 in Ba substituted sample. On a comparative note, the conductivity is higher in Ca substituted sample than that of the Ba substituted sample for x = 0.1.

Synthesis of transition metal doped lanthanum silicate oxyapatites by a facile co-precipitation method and their evaluation as solid oxide fuel cell electrolytes.

Transition metal doped apatite La10Si6-x Co x O27-δ (x = 0.0; 0.2; 0.8) and La10Si5.2Co0.4Ni0.4O27-δ are synthesized by co-precipitation method followed by sintering. The precursor precipitates and apatite products are characterized by XRD, FTIR, TGA/DTA, Raman Spectroscopy, SEM-EDX and electrochemical impedance spectroscopy. The presence of apatite phase with hexagonal structure is confirmed through the XRD results. The conductivity measurements of the samples sintered at 1000 °C show that the ionic conductivity increases with increasing content of Co2+ doping into apatite that is further increased by co-doping of Ni2+. The Co doped apatite (La10Si5.2Co0.8O27-δ ) exhibited conductivity of 1.46 × 10-3 S cm-1 while Co-Ni co-doped sample (La10Si5.2Co0.4Ni0.4O27-δ ) exhibited highest conductivity of 1.48 × 10-3 S cm-1. The maximum power density achieved is also for Co, Ni co-doped sample i.e., 0.65 W cm-2 at 600 °C. The results represented show that Co and Ni enhances the SOFC performance of apatite and makes it potential electrolyte candidate for solid oxide fuel cell application.

Overlapping large polaron tunnelling in lanthanum silicate oxyapatite

Amongst the various fast ion conductors, lanthanum excess lanthanum silicate oxyapatite (La(SiO4)6O ) has shown higher oxide ion conductivity with lower activation energy. On the other hand, the activation energy increases with La vacancies (La at 4f site). In the present work, La site is altered with Ca to form (LaCa x )9.67(SiO4)6O ( and 0.15) with minimum oxygen non-stoichiometry and studied the hopping/tunnelling mechanism with the Ca substitution. The elemental content obtained from Rietveld refinement of the x-ray diffractograms suggests La deficiency with minimum oxygen deficiency. Further, XPS and TGA studies confirm the formation of La deficient samples. Temperature and frequency dependent ac conductivity in the temperature range (548–973 K) suggests that the conduction takes place via overlapping large polaron tunnelling. Further, the tunnelling distance and polaron radii as a function of temperature and frequency are observed to be altered with Ca and affecting the ion conducting channel through the elongation of La(6 h) triangles. Our study suggests the phononic contribution play a pivotal role in ionic transport.

Electrical properties of lithium–lanthanum silicate oxyapatites

Lithium–lanthanum silicate oxyapatite exhibits high lithium-ion conductivity at temperatures above 400 °C. To further enhance its ionic conductivity, the relationships among the microstructure, bulk conductivity, and grain boundary conductivity were investigated for Li-rich compositions. LixLa10−xSi6O27−x (x = 1–8), Li5La5Si5O20, Li5La5Si4.5O19, Li6La4Si4.5O18, and Li7La3Si4O16 were synthesized, and their ionic conductivities were determined. Using X-ray diffraction, Li2La8Si6O25, Li5La5Si5O20, Li5La5Si4.5O19, Li6La4Si4.5O18, and Li7La3Si4O16 were analyzed to be close to the apatite phase with few impurities. Scanning electron microscopy/energy-dispersive X-ray spectroscopy and nuclear magnetic resonance analyses revealed that the lithium–silicate glass phase increases with respect to the apatite phase in the following order: Li2La8Si6O25 < Li5La5Si5O20 < Li5La5Si4.5O19 < Li6La4Si4.5O18 < Li7La3Si4O16. Further, as the La/Si ratio of the apatite phase increases, the chemical composition changes from Li1La9Si6O26 to Li3La7Si6O24. Both the bulk (apatite phase) and the grain boundary (glass phase) resistivities at 400 °C decreased in the order Li5La5Si5O20 > Li5La5Si4.5O19 > Li6La4Si4.5O18 > Li7La3Si4O16. The apparent activation energies for the bulk ion conductivity of Li5La5Si5O20, Li5La5Si4.5O19, Li6La4Si4.5O18, and Li7La3Si4O16 were 57.3, 47.6, 43.0, and 40.4 kJ mol−1, respectively. The total conductivities (bulk + grain boundary conductivities) of Li7La3Si4O16 (8.1 × 10−3 S cm−1 at 400 °C) was approximately four times higher than that of Li1.08LaSiO4.04 (2.2 × 10−3 S cm−1 at 400 °C), which has been previously reported to have the highest ionic conductivity.

Influence of Ionic Radii on the Conduction Mechanism in Lanthanum Silicate Oxyapatite

Influence of Ionic Radii on the Conduction Mechanism in Lanthanum Silicate Oxyapatite

Production of crystal-oriented lanthanum silicate oxyapatite ceramics with anisotropic electrical conductivity and thermal expansion

Lanthanum silicate oxyapatite (LSO) has a very high oxide-ion conductivity along the c-axis of its hexagonal crystal. The effect of the crystal orientation in LSO ceramics on the anisotropy of their electrical and mechanical properties has not been studied systematically because centimeter-scale ceramics with different degrees of crystal orientation are difficult to produce. We produced crystal-oriented LSO ceramics by slip casting under a strong magnetic field and subsequent sintering. Using LSO ceramics with different degrees of crystal orientation, the electrical conductivity (σ) and linear thermal expansion coefficient (α) were measured. σ parallel to the c-axis (σ||c) increased as the degree of crystal orientation increased, whereas σ perpendicular to the c-axis (σ⊥c) was relatively insensitive to this parameter. The σ||c and σ⊥c values of our samples were almost the same as those of LSO single crystals. α also exhibited anisotropy originating from the asymmetry of the crystal structure.

Development of Thin Film Production Process for Lanthanum Silicate Oxyapatite Electrolyte By Tape Casting Method

Lanthanum silicate oxyapatite (LSO) has attracted attentions for applications as solid electrolyte of solid oxide fuel cells (SOFCs) and chemical sensors due to its high oxide ion conductivity along to the c-axis [1]. For applications as solid electrolyte of SOFC and chemical sensors, thin and dense electrolyte is preferable because of lower resistance of the electrolyte. Among thin film production processes, tape casting method has an advantage to produce large area thin film ceramics with low cost. In this research, we developed the tape casting process for LSO thin film electrolyte production. The conditions for preparation of slurry and sintering temperature were optimized in order to produce a dense thin film electrolyte without cracks.LSO was synthesized from La(OH)3 and SiO2 raw powders using solid state reaction method [2]. After the synthesis, the LSO was grinded by planetary ball mill in ethanol with polyethyleneimine as dispersant. Then, methyl ethyl ketone (MEK) was added in the slurry. It was known that mixture of ethanol and MEK at volume ratio of 4 : 6 becomes an azeotropic state [3]. Solid content of LSO powder in the prepared slurries was 5, 10, and 15 vol.%. A binder was added to the slurry at 5, 10, and 15 wt.%. Also, 0, 5, 10, and 15 wt.% of plasticizers were added to improve flexibility without crack formation in a green tape. After stirring the slurry, residual gas was removed in vacuum desiccator. Green film was cast at 1 mm thickness on PET (polyethyleneterephthalate) film using metal applicator. The cast film was dried in air at room tempareture. Frist, we focused on finding concentration in which the cast green tape can be peeled off from PET film without macroscopic crack. The films were heated at 773 K to remove organic additives. Optimized concentration of slurry was determined by the results that macroscopic crack and warp were not formed after the heating. This heated film produced from the optimized slurry was represented by preheated film. Final sintering tempreture was 1873 K. Warp was observed by the sintering even use of the preheated film when the film was set at free space. In order to avoid the warp, the preheated film was sandwitched by LSO ceramic disks during sintering. The disks were produced by conventional sintering using LSO powder.A sintered thin film was obtained under the conditions ; solid content at 15 vol.% ; binder concentration at 5 wt.% ; plasticizer concentration at 0 wt.% at this stage. The prepared electrolyte thin film was characterized by scanning electron microscope (SEM) to observe microstructure and film thickness. Electrical conductivity was measured by impedance spectroscopy. In addition, we produced a SOFC single cell by obtained film and platinum electrode. The results of SOFC cell performance test from 1273 K to 1073 K are shown in the figure. Thickness of the LSO thin film was 125 µm. From the cell performance test, mechanical gas leak was negligible because open circuit voltage at 1273 K was 1.04 V.Due to the use of platinum electrode, cell performance was still low at 13.1 mW cm-2 at 1273 K. To improve the cell performance, more suitable electrode materials for SOFC cell is necessary. Moreover, alignment of crystal orientation is effective to improve the cell performance by decreasing of electrolyte resistance.Reference[1] S. Nakayama, et al, J. Mater. Chem. 5 (1995), 1801-1805[2] K. Kobayashi, et al., J. Ceram. Soc. Japan. 123 (2015), 274-279[3] S. Beaudet Savignat et al., J. Eur. Ceram. Soc. 27 (2007), 673-678 Figure 1

Templated grain growth of textured lanthanum silicate oxyapatite ceramics

Templated grain growth of textured lanthanum silicate oxyapatite ceramics

Lanthanum silicate-based layered electrolyte for intermediate-temperature fuel cell application

An oxide ion (O2−) conducting membrane cell, based on lanthanum silicate oxyapatite (La9.33+xSi6O26+1.5x, LSO), exhibiting low ohmic and polarization resistances in the intermediate-temperature region (~873 K) was developed. As a solid electrolyte, highly conductive Mg-doped LSO, La9·8(Si5·7Mg0.3)O26.4 (MDLS), modified with another kind of non-conductive lanthanum silicate, La2SiO5 was employed. Two kinds of silicate layers were successively spin-coated on a Ni-MDLS porous anode support, followed by thermal treatment aimed at improving ionic conductivity as well as densification of MDLS through solid state reactive diffusion. In addition, the Gd-doped CeO2, (Ce0.9Gd0.1)O1.95 (GDC) electrolyte layer, which plays an important role to prevent a reaction between the electrolyte and cathode materials, was spin-coated on the modified MDLS electrolyte. Finally, the cathode layer of porous (La,Sr)(Co,Fe)O3-δ was screen printed on the electrolyte layers. The resulting cell obtained from this study showed good fuel cell performance with a maximum power density of 94 mW cm−2 at 873 K, when operated with argon-diluted hydrogen and pure oxygen gasses.

Morphology and oxide-ion conductivity of flux grown single crystals of BaO-doped lanthanum silicate oxyapatite

Well-faceted and chemically homogeneous single crystals of BaO-doped lanthanum silicate oxyapatite were grown by a BaCl2 flux method. They were characterized by optical microscopy, scanning electron microscopy, electron probe microanalysis (EPMA), X-ray diffractometry (XRD), and electrochemical impedance spectroscopy. The developed faces were identified for the two types of single crystals with different habit. The face indices were {100}, {101}, {110}, and {210} for the prismatic crystals, and {100}, {101}, {111}, and {001} for the isometric ones. With prismatic crystals, the general formula was derived from the numbers of cations determined by EPMA and structural data refined by XRD to be (La10−xBax)Σ=10(Si5.5+0.25x□0.5−0.25x)Σ=6O26 (1.78 ≤ x ≤ 1.81), where □ denotes a vacancy in Si site. We determined the oxide-ion conductivity (σ) of the prismatic single crystal of (La8.22Ba1.78)(Si5.94□0.06)O26 (x = 1.78) along the elongation direction, which is parallel to the c-axis. The σ-value steadily increased from 7.69 × 10−3 to 5.30 × 10−2 S cm−1 with increasing temperature from 823 to 1073 K. When compared at the same temperature, the σ-values were >230 times higher than those of the randomly grain-oriented polycrystal with the same chemical composition, the latter of which ranged from 1.74 × 10−5 S cm−1 at 823 K to 2.24 × 10−4 S cm−1 at 1073 K. The remarkable difference in conductivity between the single crystal and randomly grain-oriented polycrystal was found to be principally induced by the marked conduction anisotropy in the crystal structure.

Anisotropic Electric Conductivity and Battery Performance in &lt;i&gt;C&lt;/i&gt;-axis Oriented Lanthanum Silicate Oxyapatite Prepared by Slip Casting in a Strong Magnetic Field

Anisotropic Electric Conductivity and Battery Performance in &lt;i&gt;C&lt;/i&gt;-axis Oriented Lanthanum Silicate Oxyapatite Prepared by Slip Casting in a Strong Magnetic Field

Electrical Properties of Oxyapatite-Type Solid Electrolyte and Its Application to Solid Oxide Fuel Cell

Lanthanum silicate oxyapatite (LSO) attracts attention as an alternative electrolyte material for the intermediate-temperature solid oxide fuel cells (SOFCs). However, controlling factors of its conductivity and activation energy values have not been well identified. In this study, its conducting properties were reinvestigated in detail aiming at obtaining highly conductive LSO specimen. In addition, an anode-supported electrochemical cell using LSO-based electrolyte was developed and operated at 873 K.

Flux growth of doped lanthanum silicate oxyapatite crystals with hexagonal tabular morphology

Flux growth of doped lanthanum silicate oxyapatite crystals with hexagonal tabular morphology

Influence of the porosity caused by incomplete sintering on the mechanical behaviour of lanthanum silicate oxyapatite

Lanthanum silicate oxyapatite (LSO) is an encouraging material for its use as an electrolyte in intermediate temperature solid oxide fuel cells. There is a vacancy in knowledge respecting to LSO's mechanical properties, and thus they are the scope of research of the present work.Isostatically pressed bars were sintered at temperatures from 1200 °C to 1500 °C. The dependence of Young's modulus with porosity was fitted using three different modeling curves, and extrapolated for a fully dense material. The fitting of these equations give an E0 between 144 GPa and 169 GPa. This is the first time that LSO's Young's modulus is reported.Flexural strength,Vickers hardness, fracture toughness (KIC) from microindentation cracks and nanohardness were also measured. Scanning electron micrography was performed on both a sub-sintered and the most sintered sample, and the microstructure and fracture mechanism were analyzed.In general, the mechanical behaviour is similar to other intermediate temperature solid oxide electrolytes. The fracture mechanism of the most sintered sample makes it well-suited for future SOFC applications.

Dense lanthanum silicate oxyapatite ceramics obtained by uniaxial pressing and slip casting

Lanthanum silicate oxyapatite (LSO) is a promising ion conductive ceramic material, which has higher oxygen ion conductivity at intermediate temperatures (600-800?C) compared to yttria-stabilized zirconia. Its mechanical properties, though important for any of its applications, have been scarcely reported. In this study, we compare apparent densification, open porosity and Vickers hardness of samples conformed by uniaxial pressing and slip casting and fired up to 1600?C. Colloidal processing was optimized for slip casting in order to get high green densities. At sintering temperatures higher than 1400?C, both processing routes yielded comparable densities, although uniaxially pressed samples show slightly better mechanical properties, evidencing that slip cast ones already underwent a grain growth process.

Anisotropic Electronic Conductivity and Battery Performance in C-axis Oriented Lanthanum Silicate Oxyapatite Prepared by Slip Casting in a Strong Magnetic Field

Anisotropic Electronic Conductivity and Battery Performance in C-axis Oriented Lanthanum Silicate Oxyapatite Prepared by Slip Casting in a Strong Magnetic Field