Papers by Author: X.P. Wang

Abstract: The relaxation mechanism of lithium ions in Li₅La₃Bi₂O₁₂ electrolyte was investigated by internal friction (IF) method. A prominent relaxation-type IF peak was observed. From the shift of peak position with frequency, the activation energy of E=1.0-1.1 eV and the pre-exponential factor of relaxation time in the order of τ₀ =10^-25 ~10^-18 s were obtained if one assumes a distributed Debye relaxation process. These values of relaxation parameters strongly suggest the existence of interaction between the relaxation species (here lithium ions or vacancies). Basing on the coupling model, the values of E and τ₀ were determined as 0.5-0.6 eV and 10^-17 ~ 10^-15 s, which still deviated from the typical values for point defect relaxation. A new phenomenological model was used to describe the relaxation process, and the values of E and τ₀ were determined as 0.46 eV and 3.6×10^-14 s, which is in agreement with the typical values for point defect relaxation.

Abstract: The relaxation and phase transition behaviors of rare-earth ion substituted fast oxide-ion conductors (La_1-xRe_x )₂Mo₂O₉ (Re=Nd, Gd) were investigated by internal friction (IF) measurement in the temperature range 300 K - 950 K. Three different IF peaks (labeled as P_L, P_H, and P_G, respectively) were observed in the rare-earth ion doped La₂Mo₂O₉ samples. Peak P_L corresponds to short diffusion processes of oxygen ions among different oxygen vacancy sites. Peak P_H is associated with the static/dynamic disorder transition in oxygen ion distribution. Peak P_G is a newly discovered peak embodying phase transition-like characteristics and is suggested to be related to order-disorder transition associated with the rearrangement of La/ Re sub-lattice.

Abstract: In this paper, we review the damping mechanisms in oxide materials, such as the short-range jump of oxygen vacancies and cation vacancies, movement of domain walls, and grain boundary sliding. Some examples in doped ZrO2, La2CuO4+δ, La2Mo2O9 and other oxide materials are briefly discussed, in which the damping capacity can reach as high as 30%. These oxides could be possibly applied as high damping materials either in the form of bulk components, or as additives in composites, or as hard damping coatings. In the last two potential applications, the high hardness and strength as well as high damping capacity of the oxides are simultaneously exploited, which cannot be realized by the usual high-damping metals and alloys.

Abstract: The novel oxide-ion conductors (La1-xAx)2Mo2O9-δ (A = Ca, Bi, K, x = 0 ~ 0.075) are investigated in this paper by internal friction and dielectric relaxation techniques. Two relaxation peaks associated with the short-distance diffusion of oxygen vacancies were observed, indicating that there are at least two relaxation processes for diffusion of oxygen vacancies. Doping at La site with different elements shifts both relaxation peaks toward higher temperature and increases the activation energy of oxygen vacancy diffusion. In the case of internal friction, the height of the higher-temperature peak (dominant component) decreases with increasing doping content. In the case of dielectric relaxation, however, the variation of the peak heights as a function of doping content exhibits a maximum around 2.5 % K and 5 % Bi. After properly doping, the conductivity at low temperature of doped La2Mo2O9 increases by different degrees, and a peak of the conductivity at 500° C is observed in the doping content where the highest dielectric relaxation peak appears. Based on the experimental results and the crystalline structure, the mechanism of oxygen vacancy diffusion in (La1-xAx)2Mo2O9-δ samples is discussed.