Study On Preparation Process, Structure And Properties Of Ca- And Mg-Doped Lanthanum Chromites

Doped lanthanum chromites powders were synthesized by sol-gel combustion method and the key process parameters were analyzed and optimized systematically. The powders of Ca- and Mg-substituted lanthanum chromites were prepared with the optimized process parameters, and then pressed into sintered bodies. The influence of Mg-doping on sintering c haracteristic and e lectrical property of La_0.9Ca_0.1iCrO₃ was investigated. The crystalline structure of La_0.9Ca_0.1Cr_1-yMg_yO₃ (0≤y≤0.4) solid solution powders was refined by Rietveld method (Fullprof program).The powders of La_1-XCa_xCrO₃ were synthesized by sol-gel combustion method using citric acid-nitrate system. Aiming at the maximal BET surface area, the value of six key process parameters (the amount of citric acid, the amount of ethylene glycol, pH of sol, ageing time of sol, calcination temperature of powders and doped dose of calcium) was optimized by orthogonal experiment method with the orthogonaltable L₂₅ (5⁶). It was found that the sequence of primary and secondary factors is thecalcination temperature, the amount of citric acid, pH of sol, the amount of ethylene glycol, the ageing time of sol and the dose of calcium. The optimized parameters of the mol ratio of metal cations and citric acid, the mol ratio of citric acid and ethylene glycol, the pH of sol, the ageing time of sol, the calcination temperature of powders and doped dose of calcium are 1:1.2, 1:5, 8.0, 2d, 500°C and 20mol% respectively. The BET surface area of powders prepared by the optimized technology was 18.2m²/g. It was shown by X-ray diffractometer (XRD) that both the combustion decomposition of organic gel and the formation of Ca-doped lanthanum chromites powders with perovskite structure can be completed at the same time. The micrographies of scanning electron microscope (SEM) and high resolution electron microscope (HREM) revealed that the spherical nano-particles are transformed by combustion powders with the state like gel during the calcination process.According to the optimized process parameters, the nanopowders of nominal composition La_0.9Ca_0.1Cr_1-yMg_y03(0≤y≤0.8) were synthesized. XRD results of powders demonstrated that the samples with 0≤y≤0.5 take on single phase with orthorhombic perovskite struture. It was also obtained that Cr-sites are substituted by doped element Mg, which induces the expansion of crystal cells. The sintering experiments of powders revealed that the sinterability of La_0.9Ca_0.1Cr_1-yMg_yO₃ (0≤y≤0.3) is improved by Mg-doping. The unknown second phase precipitated from La_0.9Ca_0.1Cr_1-yMg_yO₃(0.4≤y≤0.5) which are the supersaturated solid solution of element Mg when samples were sintered at elevated temperature. Further study showed that the precipitation of unknown phase has an adverse effect on sintering. Lao.9Cao.1Cro.8Mgo.2O3 sintered bodies with the maximum apparent density of 97.5% of theory density were obtained when nanopowders were uniaxially pressed under 400MPa at room temperature and sintered at 1500°C for 4h.Electrical resistivity of sintering pellets was measured by four point resistivtity test system. The results demonstrated that the electrical resistivity of La_0.9Ca_0.1Cr_1-yMg_yO₃ increases with the content of Mg-doping. For y≤0.3, the reason of the electrical resistivity increasing is that Mg-doping reduces the number of Cr-ions which give rise to habridization state with O-anions, and Mg-O ionic bonds formed by Mg-doping hinder the motion of electron vacancies. While in the case of y≥0.4, the electrical resistivity increases suddenly. The room-temperature electrical resistivity of La_0.9Ca_0.1Cr_1-yMg_yO₃(0≤y≤0.3) decreases with the calcination temperature increasing.The results of crystal structure refinement demonstrated that the B-sites of ABO₃ perovskite compound La_0.9Ca_0.1CrO₃ are substituted by element Mg. With y increasing, the lattice parameters and crystal cells volume increase, and the difference of bond angle between Cr(Mg)-O1-Cr(Mg) and Cr(Mg)-O2-Cr(Mg) increases gradully.