Microstructure And Electrical Properties Of Oxy-apatite Lanthanum Silicate Doped With Cations

Solid oxide fuel cells （SOFCs） is a high-efficiency and environmentallyfriendly all-solid energy conversion device, in which the electrolyte is the mostimportant component.8mol.%Y₂O₃-stabilized ZrO₂（8YSZ） is widely used as solidelectrolyte material of SOFCs due to its high oxide-ion conductivity. Nevertheless,8YSZ needs a high operating temperature, which leads to the decrease in thelong-term stability and the limitations in the selection of materials. Therefore, novelelectrolyte materials with good electrical properties, high chemical stability andintermediate operating temperatures of873-1073K are desirable. In consideration ofthe improved electrical conductivity, low operation temperature and relativelysuitable thermal expansion coefficient with cathode and anode materials, oxy-apatitelanthanum silicates become the most potential candidate electrolytes of solid oxidefuel cells.Oxy-apatite La₁₀Si₆O₂₇lanthanum silicate and its doped material systems ofLa_10-xA_xSi₆O₂₇(A=Nd³⁺, Gd³⁺, Yb³⁺) and La₁₀Si_6-xB_xO（27±δ）(B=In³⁺, Nb⁵⁺, W⁶⁺) havebeen successfully prepared by low-temperature reaction synthesis and hightemperature densification process. The microstructure was characterized by X-raydiffraction, scanning electron microscopy, Raman spectroscopy and transmissionelectron microscopy. The electrical conductivity of doped oxy-apatite La₁₀Si₆O₂₇atdifferent measurement temperatures was investigated by AC impedancespectroscopy, as well as the electrical conductivity under different oxygen partialpressures conditions, to evaluate their conduction mechanisms. In addition, thethermal expansion coefficients of doped oxy-apatite La₁₀Si₆O₂₇were also studied toobtain a better understanding on the relationship between the thermal expansionmechanisms and crystal structure.Oxy-apatite La₁₀Si₆O₂₇ceramics prepared by low-temperature reactionsynthesis and high temperature densification process generally contain a smallamount of second phase La₂SiO₅. In this work, the densification process has beensignificantly improved by tailoring the technological parameters, which areoptimized as T_s=1923K, t_s=10h, T_c=1623K and t_c=10h. At1073K， the totalconductivity of La₁₀Si₆O₂₇ceramics obtained under the optimium sintering conditions reaches1.28×10^-2S·cm^-1. The content of second phase La₂SiO₅isinversely proportional to the total conductivity of La₁₀Si₆O₂₇ceramics, which is thedominated factor influencing the total conductivity. However, the bulk density isdirectly proportional to the total conductivity of La₁₀Si₆O₂₇ceramics.In order to investigate the influence of oxy-apatite La₁₀Si₆O₂₇ceramics dopedwith different ionic radii on electrical conductivity, three kinds of rare-earth cationsof Nd³⁺, Gd³⁺, Yb³⁺that have the same ionic valency but different ionic radii werechosen to dope at the La site. The second phase La₂SiO₅in the La_10-xA_xSi₆O₂₇(A=Nd³⁺, Gd³⁺, Yb³⁺) ceramics is effectively restrained or even eliminated afterdoping with rare-earth elements. In addition, the diffraction peaks of correspondingcrystal planes shift to the high angle side, and the lattice constants and unit cellvolumes decrease with increasing the contents of Nd³⁺, Gd³⁺, Yb³⁺cations.It is effective in promoting the densification process, and in reducing thecontent of second phase La₂SiO₅in La₁₀Si_6-xB_xO（27±δ）(B=In³⁺, Nb⁵⁺, W⁶⁺) ceramicsby doping strategy with cations. However, when the doping amount of Nb⁵⁺and W⁶⁺ion is excessive, a small amount of La₃NbO₇and La₆W₂O₁₅phases will precipitateat the grain boundaries, respectively. The microstructures of La₁₀Si₆O₂₇and theprecipitates in La₁₀Si_6-xB_xO（27±δ）(B=Nb⁵⁺, W⁶⁺) ceramics were characterized byRaman spectra and TEM. The secondary phase is found to be the weberite-typeLa₃NbO₇in La₁₀Si_6-xNb_xO（27+δ）ceramics and the orthorhombic La₆W₂O₁₅inLa₁₀Si_6-xW_xO（27+δ）ceramics, respectively.Doped La_10-xA_xSi₆O₂₇(A=Nd³⁺, Gd³⁺, Yb³⁺) ceramics in this study are all pureoxide-ionic conductors, with a conductivity slightly lower than that of undopedLa₁₀Si₆O₂₇ceramics. However, when the doping content increases from x=0.2tox=1.0, there is a maximum value σmaxof conductivity for La_10-xA_xSi₆O₂₇ceramics,which gradually shifts with increasing the doping contents of rare earth cations. Thatis, the σmaxis located at x=0.4when doped with Nd³⁺, at x=0.6when doped withGd³⁺, and at x=0.8when doped with Yb³⁺, respectively. The conductivity is closelyrelated to its crystal structure. The smaller cations of Nd³⁺, Gd³⁺and Yb³⁺replacepartially the larger cation of La³⁺at the6h site, and will expand the space of C2,which is in favor of freedom oxygen O（4） and interstitial oxygen O（5） to shift.However, the decrease in polarization ratio would leads to the increase in theaffinity of the6h-site cation for O（4） and O（5）, which is disadvantageous to the shift of O（4） and O（5）. These two factors are competing to result in the finalconsequence.In order to investigate the influence of oxy-apatite La₁₀Si₆O₂₇ceramics dopedwith different ionic valences on electrical conductivity, In³⁺, Nb⁵⁺and W⁶⁺cationsare chosen to dope at the Si site to obtain La₁₀Si_6-xB_xO（27±δ）ceramics with a highionic conductivity. At1073K, the measured total conductivities ofLa10Si5.8In0.2O26.90, La10Si5.9Nb0.1O27.05and La10Si5.9W0.1O27.1ceramics are3.14×10^-2S·cm^-1,1.74×10^-2S·cm^-1and4.45×10^-2S·cm^-1, respectively. All theLa₁₀Si_6-xB_xO（27±δ）(B=In³⁺, Nb⁵⁺, W⁶⁺) ceramics are pure oxide-ionic conductors.Both the oxygen vacancies and interstitial oxygen anions conduct together whendoping with In³⁺ions, however, the interstitial oxygen anions play an important rolewhen doping with Nb⁵⁺and W⁶⁺ions. The thermal expansion coefficient ofLa₁₀Si_6-xB_xO（27±δ）(B=In³⁺, Nb⁵⁺, W⁶⁺) ceramics is directly proportional to the ionicradius of B cation, however, it is not closely related to the doping content of Bcation.