Advanced Materials Research Vols. 941-944

Abstract: Based on the finite element analysis method, the relationship between two systems of crack (radial crack and lateral crack) and pop-in phenomenon were compared during the instrumented indentation process in ceramic materials. In order to give a reasonable explanation for mechanism analysis of pop-in phenomenon in the process of ceramic materials instrumented indentation, this paper proposed the viewpoint that pop-in phenomenon is caused by the lateral crack in the process of ceramic materials instrumented indentation. This work provided a theoretical basis for the study of instrumented indentation methodologies based on the pop-in phenomenon to determining mechanical properties of ceramic materials.

Abstract: The photometric, electrical, thermal and life features of LEDs are highly dependent on each other. All these factors should be considered together in order to optimize the operating point of LEDs. A thermal life theory about LEDs is developed. This theory shows the inner inks among photometric thermal and 1ife features of a LED. A life prediction model of LEDs and the relationship between output luminous flux and lifetime was found out by this theory. The life of LEDs can be predicted and find the proper operating point, at which the LED will generate the maximum amount of luminous flux in its total life cycle.

Abstract: Femtosecond laser with ultrashort pulse (≤35fs) is used to investigate the damage threshold and light-to-electric conversion efficiency of gallium arsenide solar cells. Different from nanosecond or picosecond lasers, the damage threshold of the solar cell illuminated by femtosecond laser is slightly higher than that under the illumination of continuous-waves laser with the same wavelength. Because the thermal non-equilibrium effect becomes obvious, heat conduction phenomenon no longer obeys the law of Fourier, when the heating pulse time of femtosecond laser is equal to or shorter than the electron-phonon coupling time. The photovoltaic properties of gallium arsenide solar cells under femtosecond pulse laser illumination are measured. The device shows better short circuit current and the light-to-electric conversion efficiency under femtosecond pulse laser than that under continuous-waves laser of the same wavelength.

Abstract: Study on contact resistance is a fundamental content of electrical contact field. The causes of contact resistance are complex, and a reasonable theoretical model poses significant impacts on the study and practical application of contact resistance. This paper describes the generating mechanism and components of contact resistance, highlights the summary on the development of the model, and gives the calculation method of contact resistance in practical engineering application.

Abstract: In order to predict the life of white LED in a short time, three groups constant stress accelerated life tests were conducted by increasing the working current. Life information of LEDs was obtained by using bilinear regression method. The numerical results indicated that the scheme of the accelerated life tests was correct and feasible. The estimation of the LED life was accurate by acceleration parameters．

Abstract: The electronic of S-doped and N-doped anatase TiO₂(001) surfaces were investigated using the density functional theory (DFT). Substitutional and interstitial configurations of N and S atoms doped in anatase TiO₂ (001) surface and sub-surface at different sites were considered and all the formation energies were obtained. The perfect strutures of atoms doping were obstained by formation energies comparisions. The electronic structures of the results indicated that the elemental doping has much more efficient and stable photocatalyst than the pristine one, which narrowed the band gap of TiO₂ and improved its visible-light response activity.

Abstract: Spinel LiMn₂O₄ was prepared by a molten-salt combustion synthesis using eutectic acetate salts as starting materials without any additional molten-salt at 400°C, 500°C, 600°C and 700°C for 3h. The experimental results show that the main phase of the produts is spinel LiMn₂O₄, and the impurities are Mn₂O₃ or Mn₃O₄. It has been found that elevated temperature was easy to generate Mn₃O₄, and low temperature was easy to generate Mn₂O₃. The product prepared at 600 °C is single phase LiMn₂O₄ and has good crystallinity. With increasing combustion reaction temperature, the particle sizes of the products were decreased. The product prepared at 600 °C has the highest initial specific capacity of 116.5 mAh•g^-1 at 0.2C, the capacity retention was only 77.2% after 50 cycles.

Abstract: LiMn₂O₄ was synthesized by flameless solution combustion at 600°C for 3 hours (h). The influence of HNO₃ on the morphologies, crystal structure and electrochemical performances of the material was investigated. The results show that the main phase of all synthesized products is LiMn₂O₄, and the impurities are Mn₂O₃ or Mn₃O₄ depending on the concentration of HNO₃ (C_HNO3). While C_HNO3=0 and 15 mol L^-1, the impurity was Mn₂O₃, when the concentrations of HNO₃ from 3 to 9 mol L^-1, the impurity was Mn₃O₄; at C_HNO3=12 mol L^-1, the synthesized product was single phase; C_HNO3≤12 mol L^-1, with the increase of C_HNO3, particles size grew from 70-130 nm to 140-500 nm, however, C_HNO3 is up to 15 mol L^-1, particles become small (70-140 nm); the single phase of LiMn₂O₄ obtained the maximum first discharge capacity (119.7 mAh g^-1) at C_HNO3=12 mol L^-1, but its retention rate was undesirable, while C_HNO3=15 mol L^-1, the cycling performance of the product was the optimum with first discharge capacity of 118.5 mAh g^-1 and capacity retention of 90.9 % after 40 cycles at 0.2 C.

Abstract: The carrier injection in high gain semi-insulating GaAs photoconductive semiconductor switches (PCSS) is studied. A great quantity of carrier generation in the insulating region of the GaAs PCSS depends upon photo-ionization and impact ionization. The impact ionization avalanche carrier generation exists in the electron avalanche domains (EAD). The EAD is closely related to the growing domains associated with carrier injection. Carriers are injected either at the contacts (Cathode and Anode) or at the tip of the current filaments which works as a “contact reaching further into the gap”. Carrier injection plays an important role for the EAD formation.

Abstract: In the most of its engineering applications, ferroelectric materials are often subjected to combined loadings in the electric and mechanical fields. To simulate the influence of the biased stresses on the hysteretic dynamics of the materials, a macroscopic differential model is proposed to model the hysteresis loops and butterfly-shaped behaviors caused by the polarization orientation switching with biased stresses. A group of numerical simulations are presented, and the comparison of theoretical results with its experimental counterparts is also presented.