Papers by Keyword: Temperature Dependence

Abstract: Metal forming of magnesium alloys often performed at elevated temperature, because magnesium alloys exhibit peculiar stress-strain relation and inferior ductility compared to conventional metals at room temperature. In the present study, deformation behavior and formability of cast and extruded AZ31 magnesium alloys under uniaxial and biaxial compressions at room temperature and at elevated temperatures were investigated. The results revealed that the compressive stress-strain relation of AZ31 magnesium alloy changed not only with the initial texture but also with the deformation temperature. The temperature dependency of flow stress of the cast alloy was smaller than that of the extruded alloy probably because of less influence of pre-deformation. In addition, the influence of compressive deformation pattern upon flow stress of the extruded alloy remained even at elevated temperature to 523 K. The temperature dependency of compressive fracture was also discussed and it was found that the equi-biaxial condition improved the compressive formability at elevated temperatures.

Abstract: The viscoelastic properties of glass under different temperature are essential for the high-precision thermo-plastic-forming of glass. But it is exceptionally difficult to establish a quantitative relation between the thermal history and the viscoelasticity owing to the lack of constitutive model of glassy materials’ relaxation. The present work investigates the validity of Young’s modulus measurement in impulse excitation technology and then the viscosity predicted by Kelvin and Maxwell model. It is demonstrated that the classical Kelvin model, leads to the seemingly unphysical result that viscosity increases with temperature since the experimental loss rate of damped vibration increases with temperature. Although Maxwell model can be employed to explain the positive temperature dependence of loss rate, the magnitude is even smaller than the viscosity at glass transition temperature and is therefore also unreasonable. The further theoretical work suggests the intermediate zone of Kelvin and Maxwell model.

Abstract: Dynamic and static characteristics of SiC power MOSFETs at high temperature up to 380°C were investigated. Investigated devices have exhibited a behavior as a normally-off MOSFET even at such high temperature as 380°C. Temperature dependence of the MOSFET characteristics are reported in this paper, such as threshold voltage (V_TH), on-resistance, internal gate resistance, and turn-on and turn-off losses (E_ON, E_OFF). E_ON decreases and E_OFF increases with increased temperature. Temperature dependence of switching losses is affected by transfer time of V_DS, which was mainly determined from V_TH.

Abstract: This paper proposes a method of characterizing silicon carbide Schottky diodes with inhomogeneous contacts in temperature sensing applications. Using the energy activation technique, temperature intervals where the effective barrier height is constant are determined. Unlike the conventional barrier which increases with temperature for inhomogeneous diodes, the effective barrier has physical meaning and can be used for sensor performance evaluation. The utility of effective barrier analysis is confirmed on fabricated Ni/4H-SiC Schottky diodes with different annealing conditions and different degrees of barrier non-uniformity. The good agreement between calculated and experimental data proves the suitable behavior of inhomogeneous diodes as sensors for different temperature ranges.

Abstract: The temperature and concentration dependences of kinematic viscosity of alloys of Cu-Ag system with Ag content to 99 mass % in liquid state have been studied. It has been found that heating above certain critical temperatures leads to transform the melt to equilibrium state. The kinematic viscosity of the Cu-Ag melts are extremely depend from silver content in the alloy. The new regime of amorphous ribbon production is selected. Proposed regime leads to increasing of microhardness and weld strength on 10 % and corrosion resistivity on 15 %.

Abstract: We present a detailed study on near-infrared (NIR) reflection spectra of Cu nanowire arrays (NWAs) which are embedded in porous anodic alumina oxide templates and with pore diameters from 35 nm to 80 nm. We find that the NIR reflection of these samples is out of the frequency regime for surface-plasmon resonance induced by intra-and inter-band excitations. However, the intensity of the NIR reflection of Cu NWAs depends strongly on sample parameters and temperature. The measurements are carried out at temperatures setting to be 4 K, 77 K, 200 K, and at room temperature. The optical response of the Cu NWAs in NIR bandwidth is attributed to localized surface-plasmon oscillations and the NIR reflectance increases with temperature up to room-temperature. The physical mechanisms behind these interesting findings are discussed.

Abstract: Single phase LPS powders with nanosize particles were synthesized by a sol-gel process. Temperature and calcination temperature dependence of luminescence of the synthetic LPS:Ce powders were investigated under vacuum ultraviolet (VUV)-UV excitation. Luminescence intensity of LPS:Ce powders increase with calcination temperature, but it reaches the maximum at 1250 °C. Decay time of the LPS:Ce powders also slightly increasing from 27.1 to 30.5 ns as calcining temperature elevating. Both of them attribute to improved crystallization of LPS:Ce powders. Total luminescence intensity of LPS:Ce powder calcined at 1350 °C for 2 h has no noteworthy variation as the temperature rising in temperature 30-260 K range, this luminescence stability suggests that LPS:Ce would also be a good scintillator for high energy radiation detection in low temperature condition with temperature variation.

Abstract: In this paper we report on the effect of temperature and injection level on the effective lifetime of non-equilibrium charge carriers in p-base of 4H-SiC PiN diodes. Studies were carried out on 1kV drift step recovery diodes (DSRDs) with p⁺-p^--n⁺. The lifetime of non-equilibrium charge carriers in 4H-SiC p⁺-p^--n⁺ structures increases by an average of 6 times from 250ns to 1.4μs with the increase of the samples temperature from 300K to 673K. However, increase of the injection level in the drift region from 2.3·10¹⁶cm^-3 to 5.9·10¹⁶cm^-3 does not affect the lifetime indicating that Shockley-Read-Hall recombination processes are dominating.

Abstract: Shape memory alloys (SMAs) are a fascinating group of metals that have two remarkable properties, the shape memory effect and superelasticity. The TiPt structure with the B2 phase has been reported to undergo a reversible displacive transformation to B19 martensite at about 1200K. However, this system could serve in principle as the basis of high-temperature shape memory alloys. Molecular dynamics study of martensitic transformation in platinum titanium alloys was performed to investigate the effect of temperature dependence on B2 and B19 structures at 50 at.%Pt. The NPT ensemble was used to determine the properties of these systems and we found good comparisons with recent experimental work. The temperature dependence of TiPt shows potential martensitic change when B19 is heated to extreme high temperatures of 273K up to 1573K.

Abstract: Firstly, this paper reminds the reader of some basic facts about the glassy state, then of the various ways to produce amorphous metals with particular emphasis on the route of vitrification from the melt. Vitrification of an undercooled melt is the most important route from the viewpoint of the application of metallic glasses. We compare diffusion in some metallic glasses with related crystalline metals. Glassy metals, also called metallic glasses, comprise conventional [1] and bulk metallic glasses [2,3]. We remind the reader of the major experimental techniques for diffusion studies in metallic glasses. The paper then reviews our current understanding of diffusion in glassy metals (see also [4,5,6]), including conventional as well as bulk metallic glasses and undercooled melts. We cover the temperature dependence of diffusion in metallic glasses and discuss the spectrum of activation parameters of glassy metals and its difference to the corresponding one of crystalline metals. We mention the pressure dependence and the isotope effect and we discuss tracer diffusion and viscosity diffusion for a bulk metallic glass and its undercooled melt. Finally we mention computer simulations of atomic jump processes. The diffusion mechanism in metallic glasses differs from that in crystalline metals and involves thermally activated, highly collective (chain-like or caterpillar-like) diffusion jumps. Finally, we mention diffusion along shearbands in a plastically deformed glassy metal.