Natural vibration of pre-twisted shear deformable beam systems subject to multiple kinds of initial stresses

Free vibration and buckling of pre-twisted beams exhibit interesting coupling phenomena between compression, moments and torque and have been the subject of extensive research due to their importance as models of wind turbines and helicopter rotor blades. The paper investigates the influence of multiple kinds of initial stresses due to compression, shears, moments and torque on the natural vibration of pre-twisted straight beam based on the Timoshenko theory. The derivation begins with the three-dimensional Green strain tensor. The nonlinear part of the strain tensor is expressed as a product of displacement gradient to derive the strain energy due to initial stresses. The Frenet formulae in differential geometry are employed to treat the pre-twist. The strain energy due to elasticity and the linear kinetic energy are obtained in classical sense. From the variational principle, the governing equations and the associated natural boundary conditions are derived. It is noted that the first mode increases together with the pre-twisted angle but the second decreases seeming to close the first two modes together for natural frequencies and compressions. The gaps close monotonically as the angle of twist increases for natural frequencies and buckling compressions. However, unlike natural frequencies and compressions, the closeness is not monotonic for buckling shears, moments and torques.     

Creep behaviour study of virgin and service exposed 5Cr–0.5Mo steel using magnetic Barkhausen emissions technique

Creep damage behaviour of water quenched 5Cr–0.5Mo steel has been studied using magnetic Barkhausen emissions (MBE) technique. The results were compared with the materials having same composition but used in service for 15 years to demonstrate the potentiality of the magnetic technique for in-situ evaluation of extent of creep damage of components. The rms voltage of magnetic Barkhausen signal for the virgin sample decreased at the initial stage of the expended creep life where new carbides are formed. As soon as the growth of the carbides took place at the expense of the smaller ones, MBE voltage started increasing due to the decrease of pinning density. However, in case of 15 years of service exposed sample, growth of carbides already took place and hence MBE voltage increased even during the initial stage of laboratory creep testing. As soon as the void started forming in the samples (both for virgin and service exposed one), the rate of increase of MBE voltage started decreasing. The formations of such cavities were observed through SEM micrograph analysis.     

The effect of hydrogenation/dehydrogenation cycles on palladium physical properties

A series of hydrogenation/dehydrogenation cycles have been performed on palladium wire samples, stressed by a constant mechanical tension, in order to investigate the changes in electrical and mechanical properties. A large increase of palladium electrical resistivity has been reported due to the combined effects of the production of defects linked to hydrogen insertion into the host lattice and the stress applied to the sample. An increase of the palladium sample strain due to hydrogenation/dehydrogenation cycles in α→β→α phase transitions is observed compared to the sample subjected to mechanical tension only. The loss of initial metallurgical properties of the sample occurs already after the first hydrogen cycle, i.e. a displacement from the initial metallic behavior (increase of the resistivity and decrease of thermal coefficient of resistivity) to a worse one occurs already after the first hydrogen cycle. A linear correlation between palladium resistivity and strain, according to Matthiessen's rule, has been found.     

Modification of the CVD-graphene resistivity by post-processing sample annealing

In this paper, we present the results of an additional annealing effect on the temperature dependences of the resistivity for CVD-graphene samples of a large area. We found that an annealing in a Ar/H₂ mixture at different temperatures modifies both the value of the resistivity and the slope of its temperature dependence. The annealing effect on the resultant sample quality depends on the type of the ρ(T) dependence for the initial sample. For samples with a metallic-like ρ(T) dependence, a low-temperature annealing (at T = 250 °C) results in a slight decrease in the resistivity value and an increase of the ρ(T) curve slope. Increasing the annealing temperature up to T = 400 °C leads to a stronger increase in the ρ(T) curve slope but to an increase in the resistivity value. For samples with a semiconductor-like ρ(T) dependence, increasing the annealing temperature up to T = 750 °C results in a gradual suppression of the activation character of the resistivity behavior at low temperatures. The additional annealing is concluded to be accompanied by two processes: a cleaning of the graphene surface from adsorbed contaminations and an additional defect formation in the graphene structure. A relative role of these processes in dependence on the annealing temperature and the type of the ρ(T) dependence for the initial sample is discussed.     

Effect of Zn substitution for Co on the transport properties of Y BaCo4O7

Electrical resistivity and Seebeck coefficients of Y BaCo_4−xZn_xO₇ (x=0.0,0.5,1.0,2.0) were investigated in the temperature range 350-1000 K. It was found that the electrical resistivity and activation energy increase with increasing Zn concentration, while Seebeck coefficients do not increase but decrease when electrical resistivity increases. We explained the increase of electrical resistivity and the drop of Seebeck coefficients for Zn-substituted samples by the decrease of carrier mobility, rather than of carrier concentration. The effect of oxygen absorption and desorption on the electrical resistivity and Seebeck coefficients was also investigated. An abrupt change of transport properties happens at about 650 K for x=0.0 and 0.5 samples measured in oxygen. For x=1.0 and 2.0 samples, however, such change disappears and the transport behavior in oxygen is almost same as that in nitrogen due to the significant suppression of oxygen diffusion caused by the higher Zn concentration in these samples.     

Electron transport properties of Co71−xFexCr7Si8B14 (x=0, 2, 3.2, 4, 6,8 and 12 at%) amorphous alloys during devitrification

The investigation addresses the electron transport properties of Co_71−xFe_xCr₇Si₈B₁₄ (x=0, 2, 3.2, 4, 6, 8 and 12 at%) amorphous alloys. The variation in electrical resistivity of as-cast amorphous materials with thermal scanning from room temperature to 1000 K was measured. The CoFe-based alloys revealed an initial decrease in temperature coefficient of resistivity (TCR), a characteristic of spin-wave phenomena in glassy metallic systems. This behaviour in the present alloys was in a sharp contrast to the Co-based amorphous materials that indicate the drop in resistivity much below room temperature. In the studied alloys, the variation in initial TCR values and the full-width at half-maxima determined from X-ray diffraction of as-quenched materials exhibited a similar trend with increasing Fe content, indicating the compositional effect of near neighbouring atoms. After the initial decrease in resistivity, all the alloys indicated a subsequent increase at T_min. The Curie temperature (T_C), which was measured from thermal variation of ac susceptibility showed non-monotonic change with Fe content. In the temperature range between T_min and T_C the relative scattering by electron-magnon and electron-phonon resulted in the non-monotonic change in Curie temperature. At crystallization onset (T_X1) all the alloys except there with X=6, showed a sharp decrease in electrical resistivity which was attributed to ordering phenomena. In contrast to this resistivity decrease, X=6 alloy exhibited a drastic increase in resistivity around T_X1 observed during amorphous to nanocrystalline transformation. Such nanocrystalline state was observed by Transmission electron microscopy.     

Dc electrical conductivity of Ge2S3Agx glasses at high hydrostatic pressure

The paper deals with the influence of hydrostatic pressure on d.c. electrical conductivity in Ge₂S₃Ag _x glasses forx10%. The initial material exhibits high resistivity and the presence of Ag impurity yields strong increase in electrical conductivity. The experimental results suggest that there is a non-linear decrease of electrical resistivity at pressure ranging from 0·1 to 10³ MPa. The pressure coefficient of resistivity is a function ofx. All measurements were performed on bulk samples using graphite contacts. The experimental results are interpreted by means of ionic conductivity.     

Magnetic properties and DC electrical resistivity studies on cadmium substituted nickel-zinc ferrite system

Polycrystalline Ni_0.65−xCd_xZn_0.35Fe₂O₄ ferrites with x varying from 0.00 to 0.20 in steps of 0.04 have been prepared by conventional ceramic route. Calcination and sintering of samples were performed at 950 and 1250 °C for 4 and 2 h, respectively. The prepared samples were characterized by powder X-ray diffraction. The observed modifications in structure and increase in lattice constant are attributed to the difference in ionic radius of substituted Cd²⁺ ion and displaced Ni²⁺ ion. The room temperature specific saturation magnetization and Curie temperature are observed to decrease continuously with decrease in cadmium content and are attributed to the decline of A-B exchange interaction. The monotonic increase in initial permeability and decrease in magnetic loss are observed with cadmium concentration. An increase in dc electrical resistivity is observed up to x=0.12 of cadmium followed by a continuous decrease. The variation of electrical resistivity with temperature was measured in the temperature range of RT-140 °C and the corresponding activation energies for conduction obtained from the log ρ vs 1/T graphs.     

Spin-glass and spin-fluctuation in Mo-doped Ca3Co4O9 system

Structural, magnetic, resistivity and thermal transport measurements have been performed to study the Mo-doping effect on a layered cobaltite Ca₃Co_4−xMo_xO₉(0≤x≤0.4) system. The results indicate that the low-temperature magnetic behavior of the system changes from a ferrimagnetic state to a spin-glass-like state upon Mo doping, which is due to the decrease in the average valence of Co ions. Moreover, all the Mo-doped samples have a higher resistivity and larger thermopower S compared with the Mo-free sample. The variation in the resistivity and thermopower between the Mo-doped and the Mo-free samples is dominated by the change in the carrier concentration of the samples. In the Mo-doped samples with x≥0.1, both the resistivity and thermopower decrease gradually with increasing Mo-doping level, which is suggested to mainly originate from the variation in the carrier mobility of the samples. In addition, an obvious thermopower upturn is observed in the S(T) curve of all the Mo-doped samples, which can be explained by the enhancement of spin-fluctuation induced by Mo-doping.     

Characteristics of heat treated polyparaphenylene for lithium-ion secondary batteries

In order to clarify the mechanism of Li-ions insertion/deinsertion into/from polyparaphenylene (PPP)-based carbons, the PPP carbonization has been analyzed. The weight loss of the PPP carbonized samples increased at a faster rate in the temperature range 600–680 °C. Hydrocarbons such as benzene and biphenyl were mainly detected by gas chromotography–mass spectrometry at a temperature of 710 °C during thermo-gravimetric analysis of PPP. The microtexture and the structure of heat treated PPP samples were characterized by high resolution transmission electron microscopy and investigated by image processing. The resistivity measurements of heat treated PPP showed a drastic decrease in the 680–730 °C HTT range. We related the microtexture evolution, the hydrocarbons release and the resistivity decrease of PPP carbonized samples to Li-ion battery capacity.     

纳米NiAl-Ni和Ni3Al-Ni合金的低温电阻特性

为了深入理解纳米Al-Ni合金低温下的电子输运过程，使用自主研发的电磁感应加热-自悬浮定向流法制备出Al，Ni和Al-Ni纳米合金粉末，并采用真空热压设备将纳米粉末压制成纳米晶块体，利用自主搭建的低温热电测量系统研究了Al-Ni纳米合金的电阻率随温度(8~300 K)的变化规律。研究结果表明:Al-Ni纳米合金由于形成有序晶相而仍然与Al，Ni纳米晶一样，电阻率随温度的降低而降低。纳米Ni₃Al-Ni和NiAl-Ni在居里温度点附近出现了电阻率随温度变化的极大值点，因为单质Ni的影响，Ni₃Al-Ni的居里温度比粗晶Ni₃Al提高了20 K。由于磁子-电子散射作用和声子-电子散射作用，纳米Ni₃Al-Ni，NiAl-Ni和Ni的电阻率在低温下(8~40 K)与温度呈T2和T4关系。     

Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering in Ar–O2 mixture

The structure, composition, and temperature coefficient of resistance of tantalum films sputtered in Ar–O₂ mixture were studied as a function of deposition parameters and substrates temperature. As the sputtering power increased from 25 to 100 W, the samples deposited at 300 °C only consisted of the β phase, the preferred-growth orientation of films changed from (2 0 0) to (2 0 2) and the temperature coefficient of resistance reduced from −289.8 to −116.7 ppm/°C. The decrease of the oxygen and other impurity in the films was observed as the increase of the sputtering power. In addition, the O/Ta ratio decrease and grain size reduction in the films related to a change of electrical resistivity were observed at substrate temperatures in the range of 300–500 °C. These results suggested that the electrical properties were due to the oxygen and other impurity content and grain size in the films rather than to growth orientation. At 650 °C, the deposited films contained both partial stable body-centered-cubic α phase with low resistivity and tetragonal β phase of Ta. The presence of α phase of Ta causes a sharp decrease of the electrical resistivity and a significant change in the microstructure of the samples.     

Activation energy and hall-coefficient measurements in lightly dopedn-InP in the thermal freeze-out region

Summary The direct-current resistivity, β, and Hall coefficient,R _H, of lightly dopedn-type InP samples were measured at temperatures (T) down to 12K and magnetic fields up to 4.8 kG. A sharp exponential increase in β, asT was decreased, was observed for temperatures below 80 K. The Hall coefficient showed a similar trend,i.e. R _H increased sharply asT was reduced below 80 K. This is attributed to the freeze-out of conduction electrons onto their donor sites. The donor activation energy,E _d, calculated from the temperature dependence of the resistivity, was less than the theoretical prediction. An enhanced dielectric constant would be a possible candidate for such behaviour. The initial decrease in β (asT is reduced) recorded in the higher-temperature region is due to impurity scattering probably combined with lattice scattering. The authors of this paper have agreed to not receive the proofs for correction.     

Effect of MgO additive on the high-frequency properties of Z-type hexaferrites

MgO was introduced into low-temperature sintered Z-type hexaferrites in order to improve their high-frequency electromagnetic properties. In the doped samples the major Z-type phase coexists with a small amount of W-type magnetoplumbite phase. The addition of MgO causes a decrease of the average grain size and an increase of the magnetocrystalline anisotropy (K₁) and the saturation magnetization (M_s) with the increment of K₁ being larger than that of M_s. These factors result in a reduce of the initial permeability. Also, the samples with MgO additive exhibit higher Q-factor and dc resistivity. Furthermore, the introduction of MgO can decrease the dielectric constant and improve the dielectric loss tangent of the samples by reducing the electronic transition in octahedral site (B-site) between Fe²⁺ and Fe³⁺ ions.     

超镁铁质岩、榴辉岩和辉长岩中主要矿物的高压电阻率研究

 为了研究3~20 GPa压力范围内地幔物质的电阻率随压力变化的情况，选择了辉长岩、榴辉岩和二辉橄榄岩中的主要矿物：单斜辉石、拉长石、绿辉石、石榴石和顽辉石作研究对象，在DAC装置中测定其在不同压力下的电阻率。结果表明，在3~8 GPa压力区间，矿物的电阻率随压力增高而明显增大。超过8 GPa，电阻率受压力的影响减小，而辉石的电阻率随压力增高而不同程度地减小，在14~20 GPa压力范围内，顽辉石的电阻率再次随压力增高而明显增大，对其它矿物的影响不明显。在约400 km深处的地幔中出现的电导率突然上升，恰好发生在辉石电阻率与压力成负相关的压力范围内（12 GPa左右），这是否是一种巧合，将在以后研究。     

Effect of 100 MeV O7+ ion beam irradiation on structural,optical and electronic properties of SnO2 thin films

In this paper, we present the impact of swift heavy ion beam irradiation on the structural, optical and electronic properties of SnO₂ thin films. Thin films were deposited using the pulsed laser deposition technique on Al₂O₃ substrates. Atomic force microscopy, X-ray diffraction, UV–visible absorption and temperature-dependent resistivity measurements were performed to explore the morphological, structural, optical and electronic properties of the as-deposited and irradiated samples. The peak intensity of the (200) peak was found to decrease monotonously with increasing irradiation fluence. The band gap energy of the 1×10¹¹ ion/cm² irradiated sample was found to increase. The electrical resistivity of the samples showed a continuous increase with the irradiation fluence.     

Effect of intermediate plastic deformation on the high-temperature creep and lifetime of aluminum

The effect of various types of intermediate plastic deformation on the high-temperature creep of polycrystalline aluminum is studied. Intermediate deformation is performed after testing for 0.44 of the time to failure t _f via the single or multiple action of a hydrostatic pressure of 1000 MPa on porosity or via tension or compression at atmospheric pressure. Intermediate deformation is shown to decrease the creep rate, to increase the time to failure, and to increase the grain size. The change in the creep rate is maximal upon the cyclic (in the same test time intervals) action of pressure. A relation between the creep rate and the grain size has been reveled. The detected decrease in the creep rate is assumed to be caused by a decrease in the density of mobile dislocations (due to recrystallization).     

Low-energy(40 keV) proton irradiation of YBa_2Cu_3O_(7-x) thin films:Micro-Raman characterization and electrical transport properties

To investigate the damage profiles of high-fluence low-energy proton irradiation on superconducting materials and related devices, Raman characterization and electrical transport measurement of 40-keV-proton irradiated YBa_2Cu_3O_(7-x)(YBCO) thin films are carried out. From micro-Raman spectroscopy and x-ray diffraction studies, the main component of proton-radiation-induced defects is found to be the partial transition of superconducting orthorhombic phase to the semiconducting tetragonal phase and non-superconducting secondary phase. The results indicate that the defects induced in the conducting CuO_2 planes, such as increased oxygen vacancies and interstitials, can result in an increase in the resistivity but a decrease in the transition temperature TCwith the increase in the fluence of proton irradiation, which is confirmed in the electrical transport measurements. Especially, zero-resistance temperature TC_0 is not observed at a fluence of 10~(15)p/cm~2.Furthermore, the variation of activation energy U_0 can be explained by the plastic-flux creep theory, which indicates that the plastic deformation and entanglement of vortices in a weakly pinned vortex liquid are caused by disorders of point-like defects. Point-like disorders are demonstrated to be the main contribution to the low-energy proton radiation damage in YBCO thin films. These disorders are likely to cause flux creep by thermally assisted flux flow, which may increase noise and reduce the precision of superconducting devices.     

Electrical resistance and 1/f fluctuations in thin titanium films

The thickness of a metallic layer has been determined and the resistivity and spectral density of voltage fluctuations in thin titanium films with initial thicknesses of 5–100 nm, which were obtained by magnetron sputtering and intended for promising elements of the neutron optics, have been investigated. It has been found that a continuous metallic layer necessary for functioning is retained even in thinnest samples, and excess fluctuations of the layer resistance with the 1/f-type spectrum are observed. It has been shown that the method of measuring film resistivity can be used as effective express-method of determining the thickness of metallic nanolayers.     

Structural micromechanics of oriented polymers

To clarify whether the interfibrillar slippage occurs on plastic deformation of oriented polymers, flow creep of ultrahigh molecular weight polyethylene (UHMW PE) samples with various connectedness of microfibrils has been studied in a dead load mode at room temperature. The flow creep rate of melt-crystallized and gel-cast UHMW PE films drawn to various draw ratios, as well as of modified gel-crystallized samples (cross-linked/grafted or washed free of low molecular weight fraction) has been measured with the help of a unique laser interferometric technique (Doppler creep rate meter). The technique allows one to measure creep rates for deformation increments as small as 0.3 μ within an accuracy 1%. The interferometric technique enabled us to observe an extremely high variability of flow creep rate. It was recognized that the creep process accelerates or slows from time to time. A length of a loaded sample increased by multiple consecutive deformation jumps (or steps). The size distribution of the steps appeared to be controlled by the structure of interfibrillar regions. The influence of the latter on the variability of creep rate confirms a hypothesis that suggests a contribution of interfibrillar slippage to plastic deformation of oriented polymers. The observed phenomenon has been attributed to stick-slip motion of microfibrils and their aggregates sliding on each other under the action of applied stress. It was found that the creep rate decreases with increasing interfibrillar interaction.