Papers by Author: Qian Feng Fang

Abstract: The proton/lithium exchange property of the garnet-related lithium-ion conductors Li₅La₃Ta₂O₁₂ is shown to occur at room temperature under ambient air. The internal friction, TGA analysis, and IR spectroscopy techniques are used to investigate the reaction mechanism. XRPD analysis demonstrates the topotactic character of the exchange reaction. The water gas in ambient air is adsorbed on the grain surface and then to exchange proton for lithium ion into the garnet structure,(Li_5-xH_x)La₃Ta₂O₁₂. The H⁺/Li⁺ exchanging processes are reversible. When the measured temperature is higher over 573K, the internal friction peak gradually shifts toward lower temperature and the like garnet-like phase is recovered.

Abstract: In this paper the hydroxyapatite fibers reinforced chitosan nanocomposites with high hydroxyapatite dosage (70~90 wt%) were synthesized by in-situ hybridization. The semi-permeable membrane was used to control the process of hybridization and morphology of hydroxyapatite. The compositional and morphological properties of nanocomposites were investigated by FTIR spectroscopy, X-ray diffraction, and transmission electron microscopy. The results showed that the hydroxyapatite were carbonated nanometer crystalline fibers with high aspect ratio (about 25) and dispersed uniformly in the nanocomposites. The high-resolution image indicated that the growth of nano-hydroxyapatite crystallites in the chitosan matrix preferred in the c-axis. The mechanical properties of these nanocomposites were enhanced dramatically and the compressive strength increases almost to 170MPa when the hydroxyapatite content is 70 wt%. The in vitro tests indicated that the composites have high bioactivity and degradation. These properties illustrated the potential application of this kind of nanocomposites for bone tissue engineering.

Abstract: The diffusion processes of lithium ions in La_2/3-xLi_3xTiO₃ (x=0.06, 0.11) compounds were investigated by dielectric relaxation method. Prominent relaxation dielectric loss peaks, peaks P1 and P2 in La_0.56Li_0.33TiO₃ and peaks P3 and P4 in La_0.61Li_0.18 TiO₃, were observed. From the shift of peak position with frequency, the activation energy of 0.36~0.42 eV and the pre-exponential factor of relaxation time in the order of 10^-14 ~ 10^-13 s were obtained if one assumes Debye relaxation processes. The activation energies of lithium ion diffusion in La_2/3-xLi_3xTiO₃ compounds and the characteristic vibration frequency for the ionic hopping motion are higher than those measured by ⁷Li nuclear magnetic resonance (NMR) spectroscopy and that of the typical phonon frequency (about 10¹³Hz). 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 decoupled relaxation parameters of the dielectric relaxation peaks is: P₁ (0.14 eV, 2×10^-13 s), P₂ (0.25 eV, 1.8×10^-13 s), P₃ (0.17 eV, 4×10^-13 s) and P₄ (0.3 eV, 2.7×10^-13 s). These decoupled parameters are very close to the NMR measurement results and the reciprocal of the typical phonon frequency. Judging from the relaxation parameter of the peaks and combining with the crystallographic characterization, it is suggested that the P₁ (P₃) and P₂ (P₄) peaks are associated with the lithium ion diffusion in the ab planes and between the adjacent ab planes, respectively.

Abstract: In recent years, we have investigated the grain boundary (GB) internal friction in high-purity Al bicrystals with a wide variety of misorientations of tilt and twist GBs. The main results are reviewed and divided into the following sections: (1) the origin of the GB internal friction peak; (2) the individual behaviors of different types of GBs; and (3) the coupling effect and compensation effect involved in GB relaxation.

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: The effect of ternary alloying elements on the oxygen Snoek-type relaxation in the Ti-24Nb-2X-1.7O alloys (X = Al, Sn, Cr, Mn, Fe) was investigated. The dipole shape factor (δλ) of the Snoek-type relaxation was figured out for each ternary alloy based on the measured damping peak with the variable temperature. The value of δλ in the Ti-Nb-Al alloy was the highest among the present ternary alloys. It was found that δλ increased with the decreasing lattice constant as well as the decreasing valence electron number per atom (e/a) and came to a maximum value when the e/a value was around 4.24, which defined the β phase boundary. Therefore, decreasing the lattice constants and the e/a value as largely as possible with alloying elements in the β-Ti alloys is one of the feasible ways to increase δλ and to design the high damping Ti alloys.

Abstract: High-quality 5% W- and Gd-doped La2Mo2O9 nanocrystalline films were successfully synthesized on poly-alumina substrates via a PVP-assisted sol-gel method with inorganic salts as precursors, combined with the rapid thermal annealing and novel three stages annealing procedures. Dense and uniform La2Mo2O9-based films with 90 nm in grain size and 1 μm in thickness were obtained by only twice spin coatings and sintering at 600 oC. The electrical conductivity of the films at 600 oC reaches 0.028 and 0.025 S/cm for W- and Gd-doped samples, respectively.

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.