Synthesis And Conductivity Of Oxy-apatite Solid Electrolyte

Recently the oxygen-conducting electrolytes with apatite-type have attracted special attention, especially the electrolytes which are oxy of the rare earth silicates. Because of the high oxide ion conductivity and low activation energy under the intermediate temperature and wide oxygen partial pressure, the oxygen-conducting electrolyte is expected to become a new type of intermediate temperature SOFC electrolyte materials. They have a general chemical formula Ln_10-x(SiO₄)₆O_2±y where Ln is rare earth or alkaline earth metals. It seems that several locations can be replaced from the structure. However, there are many different views and interpretations about the oxygen ion conduction mechanism and the inherent physical nature of the understanding of doping currently. So we research special structure and conduction mechanism by systemic doping of lanthanide silicides apatite which has a series of substituent in this paper.Firstly we overviewed the research and development of the apatite materials. The most simple compound La9.33SiO6O26 was selected as the research object. We synthesized La9.33SiO6O26 powder by solid-phase methods, sol-gel method and molten salt methods, and sintered them at different temperatures. The results show: (1) three kinds of synthetic hydroxyapatite powders at the temperature of 1400°C, 900°C, 900°C; (2) The molten salt method is a new way. The powders synthesized by molten salt method are about 250nm in diameter. There is no agglomerated and they have highest activity comparing with others. While the sintering temperature achieved 1600°C, the ceramics are most dense. Moreover the electrolytes made by molten salt methods have a relatively higher conductivity than other two kinds of methods; (3) They respectively have conductivity of 6.48×10^-4S·cm^-1, 3.09×10^-3S·cm^-1, and 4.7×10^-3S·cm^-1 in the 700°C while were sintered in 1600°C.Secondly, we studied the substituents of Ln location of oxy of the rare earth silicide. The chemical general formula became La_9.33-2x/3A_x(SiO₄)₆O₂ after we doped A to partly replace La (A = Ba,Sr,Ca,Mg;x = 0,0.5,1.0,1.5,2.0). We sintered the powers in 1450°C which synthesized by sol-gel method and studied the impact of the conductivity of the material with different elements doping or different content doping. The results show: (1) for the same kind of element doping, irrespective of their ionic radius, the conductivity was reducing while the doping content is increasing. So we can deduce that the impact to conductivity caused by Ln cations vacancies is larger than the augment of oxygen ions that went into the gap because of Ln cations doping. Suitable Ln cations vacancies is more benefit of the raising of the electrical conductivity of materials; (2) But when we doped with the valence of alkaline-earth metal, the conductivity had a big deal with the doping element ionic radius. While the ionic radius of A is larger than La, and there are suitable Ln cation vacancies, the conductivity increased. On the contrary, the conductivity reduced regardless of the number of Ln cation vacancies. Taken together, the changes of the conductivity of La_9.33SiO₆O₂₆ with dopes is simultaneously affected by the number of cation vacancies and the lattice parameters variety after be doped, but the effects of the cation vacancies are much greater.Then we synthesized the electrolyte samples with replacing Ga, or Al, or B in Si-bit with different components. We can use the chemical general formula La_9.33+x/3(SiO₄)_6-x(AO₄)_xO₂(A = Ga,Al,B;x = 0,0.5,1.0,1.5,2.0)to describe the compounds. After conductivity measurements, the results show: (1) an appropriate amount of doping can more increase its conductivity when the ionic radius of doping element is large than Si, as Ga Al; (2) An appropriate amount of doping can slightly increase its conductivity when the ionic radius of doping element is less than Si, as B; (3) Generally speaking, La_9.33+x/3(MO₄)₆O₂ apatite materials, in the M-bit adoption of appropriate low-valence, large ionic radius elements will increase conductivity a lot, while dope with low-valence but small ionic radius elements only slightly increase conductivity.We synthesized the La_9.33+x/3(Si_0.5Ge_0.5O₄)_6-x(GaO₄)_xO₂(x=0,1.0,1.5,2.0)power and La_9.67(Si_0.5Ge_0.5O₄)₅(AO₄) O₂(A = Al,B)power by solid-phase methods and sintered them became to electrolyte. The experimental results indicate: (1) the sample with doping appropriate amount of Ge, Ga at the same time have a higher conductivity than the sample with doping single Ge, or Ga. (2) While doped Ge, it introduced B which has a smaller ionic radius than Si at the same time, such as the appropriate amount of doping can slightly increase the conductivity. (3) But if introduced elements have larger ionic radius than Si, as Ga, Al, the conductivity will increase a lot.Through comprehensive studies, results indicate: (1) as the oxy apatite Ln_10-x(SiO₄)₆O_2±y materials, in the Ln-site, doping elements with larger ionic radius of proper amount can increase the electrolyte conductivity while doping elements with smaller ionic radius can reduce the conductivity. (2) In the M-site, appropriate adoption of low-valence, small ionic radius elements only slightly increase the electrolyte conductivity while adoption of low-valence but large ionic radius elements will increase the electrolyte conductivity a lot. And if we dope Ge and other elements at the same time, it always increases the conductivity.