Effect of LiF addition on sintering behavior and dielectric breakdown mechanism of MgO-based microwave dielectric ceramics

Glass-free MgO-based microwave dielectric ceramics(1-x)wt%(0.98 MgO-0.02 Al2O3)-x wt%LiF(x=0.5,1,2,3,4)were synthesized where LiF and Al2O3 were utilized as sintering additive and reinforcement phase in MgO matrix respectively.It was found that ion substitution is apt to occur between LiF and MgO,leading to the formation of oxygen vacancies and MgF2.Nevertheless,different from ordinary liquidphase sintering,morphologies of ceramics were distinctly altered with grains changing from polyhedron to sphere-like shape and densities underwent obvious decrease when excessive amount of LiF was introduced and we call it excessive liquid-phase sintering.Grain boundary weakening caused by this circumstance would exert an adverse effect on physical properties.Moreover,LiF addition dramatically reduced dielectric breakdown strength and altered the dielectric breakdown behavior of MgO-based ceramics,which is dominated by electrical breakdown mechanism.Combination of good properties were achieved in 1 wt%LiF modified MgO-based ceramics sintered at 950℃ which exhibited superior microwave properties(εr=9.56,tanδ=9.2×10^(-5),Qf=124,600 GHz),high flexural strength(184.5 MPa),high thermal conductivity(21.3 W/(m,K)),high coefficient of thermal expansion(-12 ppm/℃)and moderate electrical properties(Eb=35.9 kV/mm,r=4.9×10^(12) U cm).

基于PIC-MCC法的真空灭弧室电弧放电机理(英文)

With the raise of voltage level in electric power grid,the phenomena of high voltage gas insulation has received extensive attention from all over the world.The research on the breakdown mechanism of vacuum which is the main insulation gas in high voltage level is one of the most important issues.It is also important to the study of vacuum arc in vacuum switch.But for the limitations of available method used in analyzing the breakdown mechanism of vacuum,the main research on vacuum breakdown is macroscopic experiment.The experiments are greatly influenced by environmental factors and high vacuum degree is difficult to be ensured.So the data from the experiments are dispersive and the complex physical change in vacuum breakdown can not be revealed.The purpose of this work is to analyze the mechanism of vacuum breakdown quantitatively by microscopic numerical simulation.The particle in cell and Monte Carlo methods are used here to solve microscopic dynamic equation of gas.Based on the field emission theory in vacuum,electrons produced by the cathode and ions produced by the collision between electron and metal vapor molecule are the objects of this study.The motions of microscopic particles which are at the functions of the applied and self-consistent electric filed are traced in time and two space dimensions.Mont Carlo method is used here to cope with the collisions between electrons and metal vapor molecules.The cross sections of the collision which is related with the energy are all from the experiments.The secondary electron emission,exciting,elastic and ionizing collisions between electrons and metal vapor molecules have been considered in this paper.By the simulation,the number densities of electron and ion are acquired and the microscopic dynamic electric field produced by space charge is also calculated. The effect of vacuum degree on discharge voltage is also discussed here.According to the simulation data,we draw the conclusion that the main reason for vacuum arc formation is metal vapor ionization and large amount of metal gas is from high energy electrons' collision with the anode.

Fast pore etching on high resistivity n-type silicon via photoelectrochemistry

In this paper, five factors, namely the HF （hydrofluoric acid） concentration, field strength, illumination intensity as well as the oxidizing-power and conductivity of electrolytes were found to strongly affect the fast pore etching. The oxidizing power of aqueous HF electrolyte of different concentrations was especially measured and analysed. A positive correlation between optimal bias and HF concentration was generally observed and the relationship was semiquantitatively interpreted. Pore density notably increased with enhanced HF-concentration or bias even on patterned substrates where 2D （two-dimensional） nuclei were densely pre-textured. The etch rate can reach 400μm/h and the aspect ratio of pores can be readily driven up to 250.

Spatiotemporal characteristics of nanosecond pulsed discharge in an extremely asymmetric electric field at atmospheric pressure

In this paper,high resolution temporal-spatial diagnostics are employed to research the optical characteristics of nanosecond pulsed dielectric barrier discharge in needle-plate electrode configuration.Temporal-spatial distributions of discharge images,the emission intensities of optical emission spectra,and plasma vibrational and rotational temperatures are investigated.By analyzing the evolution of vibrational and rotational temperatures in space and time dimensions,the energy distribution and energy transfer process in plasma are also discussed.It is found that a diffuse structure with high density plasma concentrated in the region near the needle tip can be presented in nanosecond pulsed discharge,and an obvious energy transfer from electronic energy to vibration energy can be observed in each discharge pulse.

Breakdown Behavior of Eutectic Carbide in High Speed Steel During Hot Compression

The evolution of euteelie carbide in as CaSl M2 high speed steeL was invesligated with hot compression test and metallographic examination. Initial rodlike or irregular eutectic carbides were broken into smaller particles during hot deformation by thermomechanical disintegration, while diffusion controlled phase transformation was not remarkable. Combining with numerical simulation, the relationship between breakdown ratio of carbide network and deformation parameters was obtained. Strain was the most important driving force to shatter euteclic carbides and disperse products. Furthermore, critical strain values were obtained, beyond which carbide network disappeared, and fractured carbides kept a stable profile and they were deformed with matrix coordinately. A higher temperature or lower strain rate resulted in a lower crilical strain.