Qianfeng Fang

Dielectric relaxation studies of Bi-doping effects on the oxygen-ion diffusion in La2−xBixMo2O9 oxide-ion conductors

Two dielectric relaxation peaks associated with oxygen-ion diffusion in the oxide-ion conductors La2−xBixMo2O9 (x=0.05, 0.1, and 0.15) have been studied. It is found that the activation energies of the two peaks increase with increasing Bi-doping concentration, which results from the blocking effects of the lone-pair electrons of Bi3+ ions. From the different effects of Bi doping on the two peaks, the diffusion paths of oxygen ions corresponding to each peak are confirmed. Significantly, it is revealed that Bi doping could enhance the ionic conductivity of La2Mo2O9 at lower temperatures.

Dielectric properties of Pb(Zr20Ti80)O3/Pb(Zr80Ti20)O3 multilayered thin films prepared by rf magnetron sputtering

A series of Pb(Zr,Ti)O3 (PZT) films with tetragonal/rhombohedral multilayered structures has been grown on Pt/TiO2/SiO2/Si substrates by rf magnetron sputtering at a relative low temperature. All the films comprise 12 periodicities of Pb(Zr20Ti80)O3/Pb(Zr80Ti20)O3 in constant thickness of 40 nm, but the layer thicknesses of tetragonal phase (dT) and rhombohedral phase (dR) in one periodicity are varied. The electric properties of the films are investigated as a function of dT/dR from 10/30 to 35/5. An enhanced dielectric property is observed in the multilayered films. Especially, a optimal value of dT/dR=30/10 is obtained, where the dielectric constant reaches maximum value of 469 at 100 kHz with a loss tangent of 0.037, and the dielectric constant is about five times that of the single tetragonal phase PZT film formed under the identical condition. Moreover, the polarization also increases in the multilayered films, and remarkably, the film of 30/10 exhibits larger remanent polarization, lower coercive v...

Design, Fabrication, and Properties of High Damping Metal Matrix Composites—A Review

Nowadays it is commonly considered that high damping materials which have both the good mechanical properties as structural materials and the high damping capacity for vibration damping are the most direct vibration damping solution. In metals and alloys however, exhibiting simultaneously high damping capacity and good mechanical properties has been noted to be normally incompatible because the microscopic mechanisms responsible for internal friction (namely damping capacity) are dependent upon the parameters that control mechanical strength. To achieve a compromise, one of the most important methods is to develop two-phase composites, in which each phase plays a specific role: damping or mechanical strength. In this review, we have summarized the development of the design concept of high damping composite materials and the investigation of their fabrication and properties, including mechanical and damping properties, and suggested a new design concept of high damping composite materials where the hard ceramic additives exhibit high damping capacity at room temperature owing to the stress-induced reorientation of high density point defects in the ceramic phases and the high damping capacity of the composite comes mainly from the ceramic phases.

Dielectric relaxation studies on the submicron crystalline La2Mo2O9 oxide-ion conductors

Abstract Submicron crystalline La 2 Mo 2 O 9 bulk samples with grain size of about 0.3 μm were successfully synthesized by sol–gel method and sintering process assisted by phase transformation. In the dielectric measurement, two prominent relaxation peaks with almost the same height (P d1 at lower-temperature and P d2 at higher-temperature) are observed in temperature spectrum as well as in frequency spectrum, which are associated with the short-distance diffusion of oxygen vacancies. The activation energies and relaxation times at infinite temperature of these two peaks are deduced as (1.22 eV, 3.3×10 −16 s) and (1.35 eV, 5.9×10 −16 s), respectively. The DC conductivity of the submicron crystalline samples, which is about 0.02 S/cm when extrapolated to 800 °C, is much smaller than that of the macrocrystalline samples. The reciprocal of the peak height for P d1 peak is linearly proportional to temperature, while that for P d2 peak is temperature independent. The reason for the co-appearance and the possible relaxation dynamics of the two peaks are discussed.

Phase transition process in oxide-ion conductor β-La2Mo2−xWxO9 assessed by internal friction method

The oxygen ion diffusion and phase transition in La2Mo2−xWxO9 (x=0, 0.25, 0.75, 1.0, and 1.4) have been investigated by the internal friction method. In addition to the low-temperature relaxation peak associated with oxygen ion diffusion, an internal friction peak of phase transition type is observed around 350°C in all tungsten substituted La2Mo2O9 compounds. Based on the behavior of this peak and the ionic conduction properties, the mechanism of this peak is suggested to be associated with a transition from static disordered state to dynamic disordered state of oxygen ion distribution in anion sublattice that most probably results in a transition of the ionic conduction from the Arrhenius type to the Vogel-Tamman-Fulcher type.

Dissolving, trapping and detrapping mechanisms of hydrogen in bcc and fcc transition metals

First-principles calculations are performed to investigate the dissolving, trapping and detrapping of H in six bcc (V, Nb, Ta, Cr, Mo, W) and six fcc (Ni, Pd, Pt, Cu, Ag, Au) metals. We find that the zero-point vibrations do not change the site-preference order of H at interstitial sites in these metals except Pt. One vacancy could trap a maximum of 4 H atoms in Au and Pt, 6 H atoms in V, Nb, Ta, Cr, Ni, Pd, Cu and Ag, and 12 H atoms in Mo and W. The zero-point vibrations never change the maximum number of H atoms trapped in a single vacancy in these metals. By calculating the formation energy of vacancy-H (Vac-Hn) complex, the superabundant vacancy in V, Nb, Ta, Pd and Ni is demonstrated to be much more easily formed than in the other metals, which has been found in many metals including Pd, Ni and Nb experimentally. Besides, we find that it is most energetically favorable to form Vac-H1 complex in Pt, Cu, Ag and Au, Vac-H4 in Cr, Mo and W, and Vac-H6 in V, Nb, Ta, Pd and Ni. At last, we examine the detr...

Damping mechanism in the novel La2Mo2O9-based oxide-ion conductors

Abstract In this paper, the microscopic diffusion mechanism of oxygen vacancies in the oxygen-ion conductors La 2− x A x Mo 2 O 9 with A=Bi, K and x =0–0.15 are studied by the low frequency internal friction measurements. An internal friction peak associated with the phase transition around 833 K and two relaxation peaks associated with the short-distance diffusion of oxygen vacancies were observed in all samples. With increasing K and Bi doping contents, the activation energies of both relaxation peaks increase; the high-temperature peak decreases in height while the other relaxation peak increases. The phase transition can be completely suppressed by 10% K or 15% Bi doping. It is found that the effect of K-doping is stronger than that of Bi-doping. Combining with the analysis of the crystal structure of La 2 Mo 2 O 9 , the microscopic mechanism of oxygen vacancy diffusion for the two relaxation peaks are suggested.

Tribological properties of transition metal di-chalcogenide based lubricant coatings

Transition metal di-chalcogenides MX2 (X = S, Se, Te; and M= W, Mo, Nb, Ta) are one kind of solid lubricant materials that have been widely used in industry. The lubricant properties of such lubricant coatings are dependent not only on microstructure, orientation, morphology, and composition of the coatings, but also on the substrate, the interface between substrate and lubricant coatings, and the specific application environment. In this review, the effects of parameters on tribological properties of such kind of lubricant coatings were summarized. By comparing advantages and disadvantages of those coatings, the special treatments such as doping, structural modulation and post-treatment were suggested, aiming to improve the tribological performance under severe test conditions (e.g. high temperature, oxidizing atmosphere or humid condition).

Influence of service temperature on tribological characteristics of self-lubricant coatings: A review

Self-lubricating coatings have been widely used to reduce friction in moving machine assemblies. However, the tribological performance of these coatings is strongly dependent on the service temperature. In this paper, an extensive review pertaining to the influence of operating service temperature on tribological performance of self-lubricating coatings has been carried out. Based on the effective lubricating temperature range, the self-lubricating coatings developed in the past have been divided into three groups: low temperature lubricant coating (from −200°C to room temperature), moderate temperature lubricant coating (from room temperature to 500°C) and high temperature lubricant coating (> 500°C). Ideas concerning possible ways to extend the operating temperature range of self-lubricating coatings have been presented as follows: hybridized tribological coating, adaptive tribological coatings, and diffusion rate limited solid lubricant coating. In addition, a new self-lubricating coating formulation for potential application at a wide operating temperature range has been proposed.

Core structure and mobility of an edge dislocation in aluminum

Abstract The core structure of an edge dislocation in aluminum is studied by molecular dynamics simulation with the glue potential. The dislocation splits into two partials. The separation distance between the two partials is about 9 A. The half width of the two partial dislocations is deduced to be 6.5 A by fitting the Burgers vector density to an arctangent function, giving a half width of the whole dislocation of 12 A. Dislocation mobility is studied by applying a shear stress on the crystal and observing the corresponding shift of the Burgers vector density. By considering the minor force acting on the dislocation, a Peierls stress in the order of 10 −4 μ for the motion of the whole dislocation in aluminum is obtained.