| Oxyapatite materials, particularly the rare earth silicate and germanate, are attracting great interest as a new class of oxygen anion conductor. At intermediate temperature the conductivity of oxyapatite is very high, so it appreciated to be utilized as the electrolyte, for example, its conductivities is higher than 10-3 S·cm-1 at 500℃This electrolyte can reduce the operating temperature and the cost of SOFC construction.According to the difference on the ratio of oxyapatite, it is classified into four types:stoichiometric, cation-vacancy, oxygen-vacancy and oxygen-excess. The studies indicate the conductivity of cation-vacancy OA and oxygen-excess OA are higher than others, so the studies of oxyapatite focus on the doping of these two types of apatite. It assumes the vacancies of cation-vacancy OA occur on 4f site, not referring to 2a site oxygen, but the vacancies on 4f site result in local charge imbalance, so channel oxygen 0(4) on 2a site gets into interstitial site,that is 12i site, to attain the charge balance, so part of channel oxygen 0(4) becomes interstitial oxygen 0(5) and produce vacancy on 2a site,thus 0(4) and 0(5) together act as charge carriers, significantly increasing the conductivities of cation-vacancy OA. The high oxide ion conduction of oxygen-excess attribute to excessive oxygen, which gets into the interstitial site (12i) by the side of channel oxygen 0(4) and results in local charge imbalance, thus the coulomb force between 0(4) on 2a site and La atom on 6h site is reduced,the structure becomes looser, so the migration of oxygen anion becomes easier along c axis in channels, which should be responsible for the high oxide ion conduction.We synthesized cation-vacancy oxyapatite by sol-gel method, the study focus on that Cr, Fe and Al are doped into the Si site of La9.33Si6O26, obtaining a series of sample La9.33+x/3Si6-xFexO26(x=0.5,1.0,1.5 and 2.0),La9.33+x/3Si6-xCrxO26(x=0.5,1.0,1.5 and 2.0) and La9.33+x/3Si6-xAlxO26(x=0.5,1.0,1.5 and 2.0). The samples were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Infrared spectrum and TG-DTA, which prove the sample is apatite. We investigated the impact of doping on the conductivities, and we found the conductivity increases with doping content up to a maximum value for x=1.5 in Cr, Fe and Al doping. When the value of doping is 1.5, the conductivity of Fe-doped, Cr-doped and Al-doped is respectively 6.04×10-3 S·cm-1,5.59×10-4 S·cm-1 and 3.03×10-3 S·cm-1. The conductivities of Si-doped cation-vacancy oxyapatite are mainly determined by two factors:the number of cation vacancy and the amount of doping. Taking two factors into account, the conductivities of sample reach a maximum value for x=1.5.We also synthesized oxygen-excess oxyapatite by sol-gel method, a series of B-dopes and Mg-doped lanthanum silicate apatite-type phases, La10Si5.5B0.5O26.75,La10Si5BO26.5andLa10Si5.25Mg0.75O26.25 had been obtained. The samples were studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Infrared spectrum and TG-DTA. This work aimed to synthesis Si-doped oxygen-excess oxyapatite having higher crystallinity and purity through sol-gel method.
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