Temperature Dependence of Local Structure of Rb3C60 Superconductor

EXAFS studies on Rb₃C₆₀ are made as a function of temperature from 6K to 300 K. It is found that the distance r_Rb-c and the second, third and fourth cumulant terms of Debye-Waller factors, σ(2), σ_c(3) and σ_c(4), show the anomalous behavior near T_c.     

Flux noise and flux creep in YBCO thin films

The authors used a DC SQUID (superconducting quantum interference device) to measure the low-frequency magnetic flux noise produced by thin-film rings of YBa₂Cu₃O_7-δ (YBCO) with various microstructures. Below the transition temperature T _c of the YBCO, the spectral density of the noise scales as 1/f (f is the frequency) from 1 Hz to 1 kHz. This noise generally increases with temperature and vanishes abruptly at T_c . Improvements in crystalline microstructure greatly reduce the magnitude of the noise, which was lowest for a highly orientated sample with its c-axis perpendicular to the substrate. Making a radial cut to interrupt current paths around the sample ring does not significantly affect the magnitude of the noise, demonstrating that the noise arises from a local mechanism such as the thermally activated hopping of flux bundles. Flux creep was observed in one sample cooled in a magnetic field of 1 mT, and the creep rate exhibited a sharp maximum near 80 K. It is concluded that SQUIDs and flux transformers of YBCO must be fabricated from highly orientated films to obtain low noise at low frequencies     

The elastically equivalent softening zone size for an elastic-softening material: II. A simple piece-wise softening law

The paper determines the elastically equivalent softening zone size R_E for an elastic-softening material when there is a semi-infinite crack in a remotely loaded infinite solid; the parameter R_E plays a central role in size effect expressions that are used to correlate the maximum loads that can be sustained by solids having different dimensions. The stress (p) versus displacement (v) softening law considered is that for which P = p_c for oc and P = ; qp_c = p_* for λδ_c< v <δ_c. Particular attention is focussed on the case where the parameters λ and q are both small. Such a softening law simulates the softening behaviour of plain concrete, where there is an initially rapid softening to a low stress value at a small displacement, followed by a long tail that is associated with a low stress. The paper shows that the behaviour of a material with such a softening law can be conveniently analysed by assuming that there is a non-zero stress intensity (K_IC) at the crack tip followed by a constant stress p_* within the softening zone; , where E_o is the reduced modulus and G_{I = λpcδc} is the contribution to the specific fracture energy arising from the initially rapid softening region. Analysis of a specific model demonstrates the viability of this approach by showing that there is consistency with the proposed size effect expressions based on R_E.     

Moving singularity creep crack growth analysis with the (ΔT)c and C integrals

The physical meaning of (ΔT)_c and its applicability to creep crack growth are reviewed. Numerical evaluation of(ΔT)_c and C^* is discussed with results being given for compact specimen and strip geometries. A moving crack-tip singularity, creep crack growth simulation procedure is described and demonstrated. The results of several crack growth simulation analyses indicate that creep crack growth in 304 stainless steel occurs under essentially steady-state conditions. Based on this result, a simple methodology for predicting creep crack growth behavior is summarized.     

Experimental study of Hopkinson effect in single domain CoFe2O4 particles

Nanosize particles of CoFe₂O₄ have been synthesized by the citrate precursor technique. Considerably higher coercive force (1.68 kOe) than that obtained by the conventional technique (1.00 kOe) is associated with the nanostructure of CoFe₂ O₄. These nanosize ferrimagnetic CoFe₂O₄ particles exhibit chainlike clusters indicating strong interparticle interactions and reduced magnetic moment, which is attributed to anisotropy and canted spin structure at the surface of the particle. The magnetization shows a peak just below the Curie temperature T_c during heating in the presence of a small magnetic field (the Hopkinson effect), On the other hand, the magnetization increases monotonically when the system is cooled from T _c. This peak is associated with the single domain behavior of nanocrystalline CoFe₂O₄ particles and explained within the mathematical formalism given by Stoner and Wohlfarth in conjunction with other explanations of Hopkinson effect     

Crack growth resistance characterized by the strain energy density function

Stable ductile fracture of a typical metal alloy is found to be governed by the condition dS/da = const., i.e. the rate change of the strain energy density S with crack length 2a (or a) remained constant. Since fracture and/or yielding are load rate dependent, the incremental theory of plasticity is employed for analyzing crack growth where unloading in the material near the crack can take place. Attention is focused on the energy per unit volume, dW/dV, stored along the prospective path of crack growth. The nearest neighbor continuum element must necessarily be at a finite distance r from the crack front. This leads to the general relation dW/dV = S/R. The critical value (dW/dV)_c representing the area under the uniaxial true stress and strain curve is assumed to correspond with failure of material elements. If yielding and unloading occurred locally, a certain amount of irrecoverable energy will not be available for dissipation during macrocracking. Hence, the threshold energy density must be modified to read as (dW/dV)_c^* < (dW/dV)_c. The quantity (dW/dV)_c may be regarded as the crack growth resistance whose magnitude decreases with increasing distance from the crack tip at which point yielding is most intensified.

The results are displayed graphically and shown that the condition dS/da = const. provides a rational means of collating and interpreting ductile fracture data.     

A new diagram for evaluating the relationship between convective and evaporative heat transfer during the cooling of carcasses

During the cooling of a carcass, both its heat losses by convection and by evaporation are governed by the convective heat transfer coefficient c. While both phenomena occur at the same temperature difference δ_w between the surface of the carcass and the cold air, the quantity of heat flux of the former part becomes _cδ_w, and the latter part becomes the product of _c multiplied by an equivalent temperature difference δ_D which is a function of the vapour pressure differences between the wet surface and the cold air. The ratio δ_D/δ_w can be established from the diagram and the heat loss by convection and by evaporation can be calculated for any given value of _c. Consequently, the anticipated theoretical weight loss of the carcass can be calculated.     

The elastically equivalent softening zone size for an elastic-softening material: I. Power law softening behaviour

The paper is concerned with the determination of the elastically equivalent softening zone size (R_E) associated with a semi-infinite crack in a remotely loaded infinite solid, with the material being of the elastic-softening variety. R_E plays a prominent role in size effect expressions that are used to correlate the failure loads for solids having different dimensions. R_E is determined for a range of softening behaviours, characterised by a power law variation, and a comparison is made with the value R_A: the actual softening zone size. The ratio R_E/R_A increases from a value 0.333 for the case where the stress is constant with the zone, to a value of unity for the other limiting case where G_F/P_cδ_c→0;G_F is the specific fracture energy, while p_c and δ_c, are respectively the maximum stress and displacement within the softening zone.     

CdGeAs2晶体的热膨胀行为研究

采用WinTA 100热膨胀仪研究了四方黄铜矿CdGeAs₂晶体在320~620 K温度范围内的热膨胀行为, 探索了CdGeAs₂晶体热膨胀各向异性的物理机制。测定晶体a轴和c轴方向的热膨胀系数α_a和α_c发现, α_a>>α_c>0, 表现出强烈的各向异性热膨胀特性。利用最小二乘法, 拟合出CdGeAs₂晶体的晶格常数(a, c)与温度(T)的函数关系式, 与文献报道值吻合。分别计算出不同温度下的四方畸变因子δ=2-c/a, Cd-As 键长(l_Cd-As)和 Ge-As 键长(l_Ge-As)以及相应的热膨胀系数α_Cd-As和α_Ge-As。结果表明, a、c、δ、l_Cd-As、l_Ge-As和α_Cd-As均随着温度的升高而增大, c/a和α_Ge-As则随着温度的升高而减小。当T=360 K时,α_Cd-As是α_Ge-As的6.36倍, 是造成CdGeAs₂晶体强烈热膨胀各向异性的主要原因。     

A finite element analysis of quasistatic crack growth in a pressure sensitive constrained ductile layer

Polymeric adhesive layers are employed for bonding two components in a wide variety of technological applications. It has been observed that, unlike in metals, the yield behavior of polymers is affected by the state of hydrostatic stress. In this work, the effect of pressure sensitivity of yielding and layer thickness on quasistatic interfacial crack growth in a ductile adhesive layer is investigated. To this end, finite deformation, finite element analyses of a cracked sandwiched layer are carried out under plane strain, small-scale yielding conditions for a wide range of mode mixities. The Drucker–Prager constitutive equations are employed to represent the behavior of the layer. Crack propagation is simulated through a cohesive zone model, in which the interface is assumed to follow a prescribed traction–separation law. The results show that for a given mode mixity, the steady state fracture toughness |K|_ss is enhanced as the degree of pressure sensitivity increases. Further, for a given level of pressure sensitivity, |K|_ss increases steeply as mode II loading is approached.     

Modeling of thermoelectric heat transfer in shape memory alloy actuators: Transient and multiple cycle solutions

In this paper we study the transient and multiple cycle solutions of one-dimensional symmetric thermoelectric shape memory alloy (SMA) actuators. From the model proposed by Bhattacharya et al. [1], an approximately equivalent simpler model governed by an integro-differential equation is derived first, to describe the temperature distribution in SMA. Then a numerical algorithm is proposed to solve the integro-differential equation. It is found that for the transient cooling problem with constant electric current density of magnitude |J| there is a critical value J₀ of |J| such that when |J|>J₀, the temperature in SMA may not be always decreasing. The heat transfer problem and the resulting phase transition of SMA, induced by a piecewise constant electric current source, are also studied. It is found that there exist restrictions on the maximum and minimum values of the current density J ff a repeated complete phase transition to take place in SMA. Explicit expressions for the critical values of J are derived and their physical implications are discussed.     

Enhanced piezoelectric properties of potassium niobate single crystals with fine engineered domain configurations

Potassium niobate (KNbO₃) single crystals were grown by a top seed solution growth (TSSG) method. At first, the electric field was applied along [0 0 1]_c (cubic notification system) direction of KNbO₃ crystals to induce the engineered domain configurations into KNbO₃ crystals. Prior to domain engineering, the piezoelectric properties of [0 0 1]_c oriented KNbO₃ single-domain crystals were measured. These measured values were completely consistent with the calculated apparent d₃₁ and d₃₂. Finally, the engineered domain configurations were induced into KNbO₃ crystals. As a result, piezoelectric properties increased with decreasing domain sizes of the engineered domain configuration.     

Predicted modifications in the direct and indirect gaps of Ge

The charge density of Ge was studied at various k-points and for various bands, by the ab initio pseudopotential method, using additionally the interstitial sites. The lowest X_c conduction-band points were found to be unique in having a high charge density in the interstitial site. It has been therefore predicted and verified that the X_c points move up in energy relative to the Γ_c point when closed-shell atoms (like H) are substituted at the interstitial sites. The calculations also indicate the change of the band-gap for HGeH.     

Anomalous behavior of electron temperature in silane glow discharge plasmas

The electron temperature measured by an optical emission intensity ratio of Si* to SiH* in a silane (SiH₄) glow-discharge plasma shows an anomalous behavior against film preparation conditions such as gas pressure and substrate temperature. When increasing the gas pressure, the electron temperature decreases first, takes a minimum value at a certain pressure range and then it increases. The electron temperature decreases with increasing substrate temperature, which is quite the opposite trend to a conventional non-reactive hydrogen plasma. These anomalous behaviors of electron temperature in silane plasmas have been explained in terms of feed-back phenomenon in the plasma, starting from an electron-attachment event to higher silane molecules produced in the plasma, causing an increase of electron temperature due to an increase of electron-loss rate, followed by an enhanced production of higher silane molecules. It has also been suggested that the responsible higher silane molecules for the above mentioned feedback phenomenon is penta-silane, Si₅H₁₂.     

L∞范数拟合正则化方法在飞行器动态载荷识别中的应用

基于变限积分理论,构造了响应函数最小二乘意义下的加权变限积分。通过适当次数的积分滑动平均,有效过滤测量噪声中的高频噪声。针对测量响应中残留的低频噪声,使用L_∞范数拟合正则化方法识别载荷,提出了一种选取L_∞范数拟合正则化方法最优正则化参数的单调性检验方法。数值仿真及试验验证说明单调性检验方法可以有效确定L_∞范数拟合正则化方法的最优正则化参数,得到比传统L_2范数正则化方法拟合性质更好精度更高的识别载荷;针对遥测数据中噪声特点,使用模拟遥测数据利用L_∞范数拟合正则化方法对冲击载荷进行了有效识别。     

Dimensionally scaled penetration experiments to extract projectile sizes from space exposed surfaces

Impact experiments were conducted which employed soda-lime glass projectiles (50, 150, 1000 and 3175 μm in diameter; D_p) and aluminum (1100 series) and Teflon (FEP) targets of variable thickness (T; ranging from thick infinite halfspace targets [D_p/T < 0.1] to foil thicknesseof a few microns [D_p/T > 100]). The objectives of these impact experiments were determine, at constant impact velocity, the relationships between the diameter of the resulting penetration hole (D_h), the foil thickness (T) and the projectile size (D_p). We found that D_h, and other morphologic features such as rim structures in aluminum or spall phenomena in Teflon exhibit a systematic relationship to the target thickness. This relationship is described by polynomial fits which permit unique solutions for unknown projectile sizes (D_p) from the measurement of T and D_h on space-exposed surfaces.     

Anomalous elastic properties of RF-sputtered amorphous TeO2+x thin film for temperature-stable SAW device applications

The anomalous elastic properties of TeO_2+x thin films deposited by rf diode sputtering on substrates at room temperature have been studied. The deposited films are amorphous, and IR spectroscopy reveals the formation of Te-O bond. X-ray photoelectron spectroscopy confirms the variation in the stoichiometry of TeO_2+x film from x = 0 to 1 with an increase in the oxygen percentage in processing gas composition. The elastic parameters of the films in comparison to the reported values for TeO_2+x single crystal are found to be low. However, the temperature coefficients of elastic parameters of all deposited films exhibit anomalous behavior showing positive values for TC(C₁₁) in the range (32.0 to 600.0) x 10^-40 C^-1 and TC(C¹¹) = (35.0 to 645.5) x 10^-4degC^-1 against the negative values TC(Cn) = -2.7 x 10^-4degC^-1 and TC(C₁₁) = -0.73 x 10^-4degC^-1 reported for TeO₂ single crystal. The variation in the elastic parameters and their temperature coefficients is correlated with the change in the three-dimensional network of Te-O bonding. The anomalous elastic properties of the TeO_2+x films grown in 100% O₂ are useful for potential application in the design of temperature stable surface acoustic wave devices.     

Inference of critical cod from Charpy V test result

Correlation between the Charpy absorbed energy and critical COD is investigated to obtain a useful method for estimating critical COD from Charpy V data. The round bar tension test, Charpy V-notch test and static 3-point bend test with fatigue notched specimen are carried out using mild steel, 785 MPa grade high strength steel and A5083 aluminum alloy. Correlation is found between W'_c/σ_Y and δ_c as well as between EW'_c/σ_Y² and Eδ_c/σ_Y, where W'_c is the Charpy absorbed energy obtained by considering temperature difference between the Charpy transition temperature and COD transition temperature. The symbols σ_Y, δ_c and E are yield strength, critical COD and Young's modulus, respectively. The correlations are established for various kinds of metals and over a wide temperature range including not only upper shelf range but also the transition range.     

Development of Low Power Cryogenic Readout Integrated Circuits Using Fully-Depleted-Silicon-on-Insulator CMOS Technology for Far-Infrared Image Sensors

We are developing low power cryogenic readout integrated circuits (ROICs) for large format far-infrared image sensors using fully-depleted-silicon-on-insulator (FD-SOI) CMOS technology. We have evaluated the characteristics of MOS FETs fabricated by the FD-SOI CMOS technology and have found that both p-ch and n-ch FETs show good static performance below the liquid helium temperature, where n-ch FETs fabricated by conventional bulk-CMOS technology usually suffer from anomalous behaviors such as kink and hysteresis. We have also designed and fabricated an operational amplifier (OP-AMP) and have successfully demonstrated that the OP-AMP works at the liquid helium temperature with an open loop gain of 7000 and a power consumption of 1.3 μW. The noise is dominated by mainly 1/f and has a value of at?1?Hz.     

Methods for direct-reading noise measurements of low-gain two-ports

This paper describes direct-reading measurement methods of noise figure F_m displayed by an automatic noise figure indicator. The F_m readings should be equal to F_x, which are the true noise figure values of the two-port under test, regardless of its available power gain G_Ax, and the noise figure of the measurement receiver F_t. Technical problems of these F_x direct readings are shown. They particularly occur while using the conventional measurement methods. Methods that allow elimination of the influence of the F_r value on F_m readings as well as the influence of both F_r and G_ax on F_m are discussed. An important advantage of using the method with elimination of the F_r value is presented. Formulas are given which allow the estimation of the technical requirements for additional blocks of measurement systems or to count measurement error components, specific for the chosen measurement method