Papers by Author: Dan Li

Abstract: The influence of barium doping on the oxygen-ion diffusion and phase transition in the La₂Mo_2-yW_yO_9-δ (y=0, 1.0) oxide-ion conductors has been systematically investigated via dielectric techniques. In the Ba-doped La₂Mo₂O₉ samples there are only two dielectric relaxation peaks P_d1 and P_d2, which are associated with the short-distance diffusion of oxygen vacancies. But in the Ba-doped La₂MoWO₉ members, three peaks are detected, including peak P_d1, P_d2, and peak P_h. The last is associated with the phase transition process from the static disordered state to the dynamic disordered state of oxygen ion/vacancy distribution.

Abstract: The compounds of Fe substitution of Al in nanotructured Sr₂Fe_1-x Al _xMoO₆ (0≤x≤0.2) double perovskite have been prepared by sol-gel method. The x-ray diffraction patterns of the samples show that the samples are in nanometer range. The Curie temperature and thesaturation magnetization of the samples decrease with the increasing of Al substitution. And the magnetoresistance of the samples increase with the increasing of x.

Abstract: The reducibility of oxide-ion conductors La_2-xBa_xMo_2-yW_yO_9-δ(x = 0, 0.06; y = 0, 0.1) were studied in the reducing atmosphere of 5%H2 + Ar by means of impedance spectroscopy measurement.. All substitutions can enhance the conductivity and improve the reducibility in the temperature range from 548 to 923 K. The double substitution of Ba and W has a better stabilizing effect than the single tungsten substitution.

Abstract: The reducibility of oxide-ion conductors La₂Mo_2-Yw_yo₉(y=0, 0.1) were studied in the reducing atmosphere of 5%H2+Ar by means of impedance spectroscopy measurement. 5% W-doped La₂Mo_2-Yw_yo₉ can enhance the conductivity and improve the reducibility in the temperature range from 275oC to 650oC.

Abstract: The compounds of Fe substitution of Cr in nanotructured Sr2Fe1-xCrxMoO6 (0≤x≤0.2) double perovskite have been prepared by sol-gel method. And the x-ray diffraction patterns of the samples show that the samples are in nanometer range. All the samples show a paramagnetic to ferromagnetic transition with temperature, and the Curie temprature increase with the increasing of x. The resistivity of the samples decrease with the increasing of temprature, and increase with the increasing of x.

Abstract: (Sr₂FeMoO₆)_1-x/(CeO₂)_x composites with nanosized crystallite were prepared by Sr₂FeMoO₆ obtained from the sol-gel method and nanosized CeO₂ powders. The magnetoresistance of composites increase with the CeO₂ increasing. And the MR ratio at 10 K is 1.23%, and 2.48% for the x = 0.05 and 0.2 samples with H = 1 kOe, which are 1.25 times and 2.53 times as large as that for pure Sr₂FeMoO₆, respectively. The enhanced magnetoresistance was attributed to the spin-dependent tunneling at the interface of grain boundaries.

Abstract: In this paper, the ceramic samples of La₂Mo₂O₉ and La₂Mo_2-xGa_xO _9-δ(x=0.1,0.2)were prepared by conventional solid-state reaction method. The reducibility of pure La2Mo2O9 and La₂Mo_2-xGa_xO _9-δ(x=0.1,0.2) samples were studied in the reducing atmosphere of 5%H2+Ar by means of impedance spectroscopy measurement. For the La₂Mo_2-xGa_xO_9-δ(x=0.1,0.2) the difference between the conductivity in the reducing atmosphere and the one in air is small, indicating that partial substitution of Ga for Mo is effective to improve the reducibility of La₂Mo_2-xGa_xO_9-δ(x=0.1,0.2).

Abstract: The influence of aluminum doping on the ionic conductivities and phase transition in the La2Mo2–xAlxO9-δ(x = 0.1, 0.2, 0.3) oxide-ion conductors has been investigated by XRD, DSC and dielectric relaxation measurements. The results indicate that the solubility of Al3+ in La2Mo2O9 sample is about 10mol%, Al3+ can not suppress the α/β phase transition in the La2Mo2O9, and the aluminum substituted La2Mo2O9 has a higher conductivity in whole temperature than that of pure La2Mo2O9.

Abstract: The effects of tungsten doping on the oxygen-ion diffusion and the phase transition in La2Mo2–xWxO9 samples (x = 0.5, 0.75, 1.0, 1.2) were studied using internal friction method. The results show that two peaks were detected in the internal friction-temperature spectra in the La2Mo2-xWxO9 samples, the low-temperature relaxation peak is associated with oxygen ion diffusion, and the high-temperature peak is associated with a phase transition from the static disordered state to the dynamic disordered state of oxygen ion/vacancy distribution.

Abstract: The effects of tungsten doping in La₂Mo_2–yW_yO₉ (y =0, 0.25, 0.5, 1.0, 1.4) samples were studied using dielectric relaxation measurements. The results indicate that the solubility of W⁶⁺ in La₂Mo₂O₉ sample is about 25mol%, W⁶⁺ can suppress the α/β phase transition in the La₂Mo₂O₉. Additional to the low-temperature relaxation peak P_d associated with oxygen ion diffusion, a new dielectric loss peak P_h associated with a phase transition from the static disordered state to the dynamic disordered state of oxygen ion distribution was observed around 740 K. The conductivity of La₂Mo_1.9W_0.1O₉ sample is higher than that of the pure La₂Mo₂O₉, and then decrease with the increasing of the tungsten doping in La₂Mo_2–yW_yO₉ (y = 0.5, 1.0) samples in the whole measurement temperature.