Structural characterization of polypyrrole in the solid state by high resolution 15N NMR spectroscopy [II]

The solid-state ¹⁵N CP/MAS NMR spectra and ¹⁵N spin-lattice relaxation times (T₁) of doped and dedoped ¹⁵N-labeled polypyrroles prepared by electrochemical polymerization, have been measured by means of high-resolution solid-state ¹⁵N NMR. The ¹⁵N signal of polypyrrole consists of four peaks decomposed by line shape analysis. The four peaks obtained have been assigned to the various structures of polypyrrole. Further, the half-width of the ¹⁵N NMR spectra of polypyrroles is discussed as related to the electrical conductivity. © 1995 John Wiley & Sons, Inc.     

Electrical characterisation of Cu-Diffusedn-GaAs epitaxial layers using optical deep level transient spectroscopy

Undopedn ^--GaAs epitaxial layers were grown by OMVPE onn ⁺ (1 × 10¹⁸ cm³) and semi-insulating (SI) GaAs substrates. The as-grown epitaxial layers grown onn ⁺ substrates contained several deep level defects whereas those grown on SI substrates were, apart from the EL2, virtually “defect free”. Upon Cu diffusion, deep levels which may reduce hole and electron diffusion lengths and lifetimes, were formed. Optical deep level transient spectrocopy (ODLTS) has been used to identify such levels atE _v + 0,41 eV andE _c-0,31 eV respectively. The EO1 (EL2) trap concentration reduced after Cu had been diffused into the epitaxial layers. The magnitude of this reduction was approximately equal to the concentration of the trap found atE _c - 0,31 eV which suggests that the two may be related. Activation energies and capture cross-section values are presented for the deep levels detected in these epitaxial layers.     

Compositional, structural, optical and electrical characterization of CulnTe2 grown by the tellurization of stoichiometric Cu and in in the liquid phase

A new method to grow single crystals with stoichiometry close to 1:1:2 of CuInTe₂, an important member of the Cu-III-VI₂ family of semiconductors is described. This consists of tellurization in the liquid phase of stoichiometric Cu and In and later solidification under directional freezing. It is found from energy dispersive spectroscopy that the ingots obtained with the evaporation temperature (T_t) of Te at 590 and 635°C were very close to the ideal stoichiometry 1:1:2. Samples of all ingots obtained with T_t between 530 and 690°C, as studied by x-ray and differential thermal analysis, were of single phase having chalcopyrite structure and p-type conductivity. Optical and electrical characterization of different ingots were made. The acceptor ionization energy E_A and the density of states effective mass m _p ^* of the holes are estimated from the temperature dependence of the hole concentration p in samples of different ingots. m _p ^* = (0.78 ± 0.02) m_e agrees quite well with the reported value. From a combined analysis of the variation of E_A with p^1/3 and the knowledge of molecularity and valence stoichiometry of each sample, it is established that the shallow acceptor level observed in different samples with E_A (0) ⋍ 30 meV is due to V_In.     

Enhancement of electrical properties due to Cr substitution in Co-ferrite nanoparticles synthesized by two chemical techniques

Nanocrystalline cobalt ferrites with nominal composition CoCr_xFe_2−xO₄ ranging from x=0.0 to 0.5 with step increment of 0.25 were prepared by sol–gel auto combustion and chemical co-precipitation techniques. A comparative study of structural, electrical and magnetic properties of these ferrites has been measured using different characterization techniques. Structural and micro-structural studies were measured using X-ray diffraction, Fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy and atomic force microscopy. Crystallite sizes of the series are within the range of 12–29±2 nm. Lattice parameters decrease by increasing Cr³⁺ concentration. FTIR confirms the presence of two lattice absorption bands. DC electrical resistivity increases to a value of ∼10¹⁰ Ω-cm with increase in Cr³⁺ concentration, but the most significant increase is in samples prepared by sol–gel combustion. Dielectric properties have been measured as a function of frequency at room temperature. Dielectric loss decreases to 0.1037 and 0.0108 at 5 MHz for chemical co-precipitation and sol–gel combustion, respectively. Impedance measurements further helped in analyzing the electrical properties and to separate the grain and grain boundary resistance effects using a complex impedance analysis. Magnetic parameters were studied using a vibrating sample magnetometer in the applied field of 10 kOe. The saturation magnetization decreased from 63 to 10.8 emu/gm with increase in Cr³⁺ concentration.     

Mn site substitution of La0.67Ca0.33MnO3 with closed shell ions: Effect on magnetic transition temperature

Mn site is substituted with closed shell ions (Al, Ga, Ti, Zr and a certain combination of Zr and Al) and also with Fe and Ru ions carrying the magnetic moment (S=5/2 and 2 respectively) at a fixed concentration of 5 at %. Substitution did not change either the crystal symmetry or the oxygen stoichiometry. All substituents were found to suppress both the metal-insulator and ferromagnetic transition temperatures (T _p(ρ) and T _C, respectively) to varied extents. Two main contributions identified for the suppression are the lattice disorder arising due to difference in the ionic radii between the substituent (r _M) and the Mn³⁺ ion (r _Mn ³⁺) and in the case of the substituents carrying a magnetic moment, the type of magnetic coupling between the substituent and that of the neighboring Mn ion.     

基于神经网络的电火花线切割工艺目标建模

利用BP神经网络技术建立了电火花线切割加工工况参数与工艺目标间的预测模型.以脉冲宽度、脉冲间隙、峰值电流、间隙电压及工件厚度等工况参数为网络输入,加工效率和表面粗糙度等工艺目标为网络输出,通过用样本数据对网络的训练,实现了对工艺目标的预测.试验结果表明:所建预测模型能较好地反映线切割机床的工艺规律,实现对指定切割条件下加工效率和表面粗糙度的预测,最大预测误差小于10%.     

Piezoelectric resonators with mechanical damping and resistance in current conduction

A novel design method for high Q piezoelectric resonators was presented and proposed using the 3-D equations of linear piezoelectricity with quasi-electrostatic approximation which include losses attributed to mechanical damping in solid and resistance in current conduction. There is currently no finite element sofware for estimating the Q of a resonator without apriori assumptions of the resonator impedance or damping. There is a necessity for better and more realistic modeling of resonators and filters due to miniaturization and the rapid advances in frequency ranges in telecommunication. We presented new three-dimensional finite element models of quartz and barium titanate resonators with mechanical damping and resistance in current conduction. Lee, Liu and Ballato’s 3-D equations of linear piezoelectricity with quasi-electrostatic approximation which include losses attributed to mechanical damping in solid and resistance in current conduction were formulated in a weak form and implemented in COMSOL. The resulting finite element model could predict the Q and other electrical parameters for any piezoelectric resonator without apriori assumptions of damping or resistance. Forced and free vibration analyses were performed and the results for the Q and other electrical parameters were obtained. Comparisons of the Q and other electrical parameters obtained from the free vibration analysis with their corresponding values from the forced vibration analysis were found to be in excellent agreement. Hence, the frequency spectra obtained from the free vibration analysis could be used for designing high Q resonators. Results for quartz thickness shear AT-cut and SC-cut resonators and thickness stretch poled barium titanate resonators were presented. An unexpected benefit of the model was the prediction of resonator Q with energy losses via the mounting supports.     

Effect of annealing prior to cold rolling on magnetic and mechanical properties of low carbon non-oriented electrical steels

The effects of annealing prior to cold rolling on the microstructure, magnetic and mechanical properties of low-C grain non-oriented (GNO) electrical steels have been investigated. The grain structure of hot-rolled electrical steel strips is modified by annealing at temperatures between 700 and 1050 °C. Annealing at temperatures less than the ferrite to austenite+ferrite transformation temperature on heating (Ac₁) causes a marginal effect on the grain size. However, annealing in the intercritical region at temperatures between Ac₁ and Ac₃ (the ferrite+austenite to austenite transformation temperature on heating) causes rapid decarburization and development of large columnar ferrite grains free of carbide particles. This microstructure leads, after cold rolling and a fast annealing treatment, to carbide free, large ferrite grain microstructures with magnetic and mechanical properties superior to those observed typically in the same steel in the industrially fully processed condition. These results are attributed to the increment in grain size and to the {1 0 0} fiber texture developed during the final annealing at temperatures up to 850 °C. Annealing at higher temperatures, T>Ac₃, results in a strong {1 1 1} fiber texture and an increase of the quantity of second phase particles present in the microstructure, which lead to a negative effect on the final properties. The results suggest that annealing prior to cold rolling offers an attractive alternative processing route for the manufacture of fully processed low C GNO electrical steels strips.     

Deflagration-to-detonation transition process for spherical aluminum dust/epoxypropane mist/air mixtures in a large-scale experimental tube

The deflagration-to-detonation transitions (DDTs) for clouds of spherical aluminum dust (SAD) mixed with air or epoxypropane mist (EPM) and air were investigated in a 29.6-m-long experimental tube of 199 mm in diameter. The clouds formed through the injection of SAD and SAD/liquid epoxypropane samples into the experimental tube. Explosions of the SAD/air mixture were initiated using a 7-m-long EPM/air cloud explosion ignited by a 40-J electric spark. Explosions in SAD/EPM/air clouds were initiated using a 1...