Influence of high pressure on the relative permittivity of oleic acid

Oleic acid (OA) is a monounsaturated omega-9 fatty acid which undergoes a first-order phase transition when it is influenced by high pressures. The transition results in a change of molecular structure, which was investigated by means of X-ray techniques [J. Przedmojski and R.M. Siegoczyński, X-ray diffraction investigation of oleic acid under high pressure, Phase Transit. 75 (2002), pp. 373–377]. Despite a significant change in the structure, the permittivity of the acid remains barely influenced during and after the phase transition. A rise in relative permittivity has been observed due to the increasing number of molecules per volume unit within a compression cycle. However, no significant fall in the permittivity has been observed as a result of the transition which occurs in the case of edible oils. This may be caused by a strongly dimerised structure and dipole–dipole interaction in the liquid at ambient pressure.     

Influence of thermal stress on the relative permittivity of the AlGaN barrier layer in an AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on AlGaN/GaN heterostructures were fabricated. Some samples were thermally treated in a furnace with N₂ ambience at 600 °C for different times (0.5 h, 4.5 h, 10.5 h, 18 h, 33 h, 48 h, and 72 h), the others were thermally treated for 0.5 h at different temperatures (500 °C, 600 °C, 700 °C, and 800 °C). With the measured current—voltage (I—V) and capacitance—voltage (C—V) curves and by self-consistently solving Schrodinger's and Poisson's equations, we found that the relative permittivity of the AlGaN barrier layer was related to the piezoelectric and the spontaneous polarization of the AlGaN barrier layer. The relative permittivity was in proportion to the strain of the AlGaN barrier layer. The relative permittivity and the strain reduced with the increased thermal stress time until the AlGaN barrier totally relaxed (after 18 h at 600 °C in the current study), and then the relative permittivity was almost a constant with the increased thermal stress time. When the sample was treated at 800 °C for 0.5 h, the relative permittivity was less than the constant due to the huge diffusion of the contact metal atoms. Considering the relation between the relative permittivity of the AlGaN barrier layer and the converse piezoelectric effect, the conclusion can be made that a moderate thermal stress can restrain the converse piezoelectric effect and can improve the stability of AlGaN/GaN heterostructure devices.     

Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AIN/GaN and AIGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated. Based on the measured current-voltage and capacitance-voltage curves, the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrodinger＇s and Poisson＇s equations. It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode, and also much lower than the theoretical values. Moreover, by fitting the measured forward 1-V curves, the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode. As a result, the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer, which is attributed to the more dislocations.     

Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance-voltage curves,the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode,and also much lower than the theoretical values.Moreover,by fitting the measured forward I–V curves,the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode.As a result,the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer,which is attributed to the more dislocations.     

快照的时间反转和逆时偏移法对含有固体分层介质中目标的超声检测

快照的时间反转和逆时偏移方法已实现了液-液分层介质中目标的超声检测和定位,现进一步推广到含有固体分层介质情形。由于在含有固体分层介质中,都存在多种的反射波和散射波,因此提出一种筛选方法,首先从中筛选出散射波以及所属波型,再利用快照的时间反转和逆时偏移方法进行处理。实验分别研究了固-液、液-固和固-固分层介质3种情况。利用快照的时间反转和逆时偏移法进行处理,在空间就形成了一个峰状的分布,其峰顶的位置与实际目标的位置比较吻合,误差在一个波长左右。因而,实现了含有固体分层介质中目标的超声检测和定位。     

一种基于周期轨道统计的混沌信号不可预测性强弱的检测方法

论述了对混沌信号不可预测程度进行检测的基本原理,在此基础上提出了一种对混沌信号不可预测性强弱进行检测的方法.分别对硬件电路和数值仿真产生的混沌时间序列的周期轨道进行了统计分析,寻找热鞍周期轨道(SPO)并对其回归谱和回归频度等特性进行了研究.实验结果表明了该方法用于检测混沌信号不可预测性强弱的有效性. 关键词： 不可预测性 检测 回归 混沌     

基于分布式超声无损检测方法的零件内部缺陷检测

超声检测广泛应用于工业检测,比如超声相控阵检测法和超声A扫应用于零件内部缺陷检测。然而,这些方法可以检测出缺陷位置却很难精确地检测缺陷尺寸,缺陷定量成了急需解决并且很有意义的问题。本文提出了一种分布式超声无损检测方法,将超声探头均匀布置在检测表面,每一个超声探头可以同时发射和接收超声信号,通过对接收到的信号进行处理来重构缺陷轮廓。基于分布式超声无损检测方法,重构零件的人造缺陷并建立相应的声学仿真模型。通过多项式拟合法和聚类法分别处理实验和仿真所获得的数据并重构缺陷轮廓。实验结果和仿真结果显示重构的椭圆形缺陷和正方形缺陷具有一定的精度。结果表明分布式超声无损检测方法有潜在的应用价值和理论意义。     

The study of optimal oxidation time and different temperatures for high quality VO2 thin film based on the sputtering oxidation coupling method

The high quality Vanadium dioxide (VO₂) thin films have been fabricated successfully on sapphire by a simple novel sputtering oxidation coupling (SOC) method. All VO₂ thin film samples exhibit a good metal-insulator transition (MIT) at about 340 K. The optimal oxidation time at different temperatures has been experimentally investigated. We report on the relationship between optimal oxidation time and different temperatures of metal vanadium thin film samples of 101 nm thickness by oxidation in air. It is found that the optimal oxidation time ln(t) as a function of temperature 1/T shows a significant linear relationship among 703 K-783 K, in good agreement with the Wagner's high-temperature oxidation model.