Sample calculations of flash temperatures at a silver-graphite electric contact sliding on copper

Explicit equations are presented for calculating the flash temperatures at plastically deformed circular contact spots due to friction and/or Joule heat. Numerical examples calculated for a metal-graphite composite in a very wide range of speeds and loads suggest that intuitive estimates of the flash temperatures tend to be too high. On the basis of the figures it should be possible to deduce approximate values for the flash temperature for a great variety of sliding couples and sliding conditions.     

Verhalten von unbestrahltem ZrNb 1-Rohr unter tangentialer Zugdehnung in jodhaltiger Atmosphäre

The effect of iodine on the mechanical properties of unirradiated recrystallized ZrNb 1 tubing at 350°C was studied by means of the ring tension test. At strain rates < 10⁻³/s iodine leads to a substantial decrease in the totale elongation and the reduction of area, but does neither change the 0.2% yield stress, the ultimate tensile stress nor the uniform elongation. The results indicate that iodine induced crack formation in unirradiated recrystallized ZrNb 1 occurs only in the necking region after marked plastic deformation.     

Role of zirconia phase transformation,interface processes,and Ta2O5 doping on the blistering phenomenon of molten glass in contact with zirconia-based refractories

Bubble formation and removal within the molten glass is an important issue in glass industry. Various sources of bubbles have been identified in glass manufacturing: decomposition of the glass components, air trapping, oxidation/reduction reactions, precipitation resulting from insufficient refining, etc. It has been demonstrated in a previous paper that the blistering phenomenon at the interface between a molten glass and a zirconia-based refractory can be ascribed to the oxygen semipermeability through the zirconia phase. The objective of this study is to clarify the role of temperature on the blistering process, and especially, below and above the phase transition temperature of zirconia (monoclinic/tetragonal transformation) and to evaluate the role of zirconia doping on the blistering level. The influence of the kinetics of the surface processes at the glass/refractory interface is emphasized. Quantitative measurement of the slight blistering ascribed to the so-called “redox shock” is also given.     

Oxidation of graphene-modified HfB2-SiC ceramics by supersonic dissociated air flow

The oxidation resistance of ultra-high-temperature ceramic material (HfB₂-30 vol%SiC)-2 vol%rGO (rGO: reduced graphene oxide) under long-term exposure (2000s) to a supersonic air flow has been studied. The ceramics were obtained by reactive hot pressing of HfB₂-(SiO₂-C)-rGO composite powder at a temperature of 1800°C (pressure 30 MPa, holding time 15 min, Ar). The surface temperature of graphene-modified ceramics under the influence of heating by high-enthalpy air flow (heat flow q reached 779 W·cm^–2) did not exceed 1700°C, which is 650–700°C less than for the HfB₂-30 vol%SiC baseline ceramics. This may be related to an increase in the efficiency of heat transfer from the sample to the water-cooled module, due to the higher thermal conductivity of the rGO-containing material. Thereby, a decrease in the material degradation degree has been noted, i.e. decrease in the recession rate and decrease in the total thickness of the oxidised ceramic layer by tenth. The peculiarities of the oxidised surface and near-surface region microstructure upon aerodynamic heating of the graphene-modified ceramic material, have been shown.     

Insight into t-&gt;m transition of MW treated 3Y-PSZ ceramics by grazing incidence X-ray diffraction

The influence of a microwave hybrid heat treatment (MHH) on the surface and in-depth mineralogical transformation of pre-sintered 3Y-PSZ was investigated. 3Y-PSZ samples were prepared by slip casting and sintered by conventional firing (1270 °C). Then, different MHH treatments from 5 to 15 min. at 1200 °C were applied to obtain a fully stabilized 3Y-TZP. The monoclinic fraction depth profiles in the first micrometres (up to 5) of thickness were investigated by means of the grazing incident X-ray diffraction technique (GIXRD). A good sintering degree with practically nil closed porosity and grain growth was achieved after MHH of 15 min. MHH increases the tetragonal phase content both in the surface and in-depth, reducing completely the monoclinic phase shell typically found after conventional sintering. A new parabolic model is proposed for the convoluted monoclinic fraction depth profile, which through the value of its horizontal asymptote allows the determination of the monoclinic shell thickness.     

矢量控制变频调速式连铸辊埋弧堆焊驱动装置的研发

为使不同辊径连铸辊在施行埋弧堆焊时的焊接线速度与优化的焊接参数库相匹配,基于矢量控制技术研发了变频调速式连铸辊驱动装置,该装置具有控制系统调速范围宽、加减速性能好、速度精度高的特点,为使用于不同区段、不同辊径的连铸辊埋弧堆焊提供了装备保障。     

PrBaCo2-xTaxO5+δ based composite materials as cathodes for proton-conducting solid oxide fuel cells with high CO2 resistance

PrBaCo₂O_5+δ (PBC) with high catalytic activity is identified as a prospective cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). However, its poor chemical stability hinders its application. To address this problem, a Ta-doping strategy was presented in this study. The cathode with Ta-doping PBC was applied in proton conducting SOFCs. And the influence of Ta-doping on the crystal structure, electrochemical performance, structure stability and electrical conductivity of PBC was investigated. The resistance to CO₂ of PBC at elevated temperature is significantly improved with Ta-doping. The electrochemical performance measurements indicated that a low Ta-doping concentration did not change the performance of the cells obviously, while large Ta-doping concentration could lower the fuel cell performance.     

基于统计最优的近场声全息理论与仿真实验

对统计最优平面近场声全息（SOPNAH）技术进行研究,提出了一种吉洪诺夫（Tikhonov）正则化方法并选择最优正则化参数λ。通过仿真实验验证了结合吉洪诺夫正则化方法的SOPNAH能够对噪声源进行准确定位及实现声场重建,并且重建的精度比传统的方法更高。     

Non-interactive multisignatures in the plain public-key model with efficient verification

Multisignatures extend standard digital signatures to allow an ad hoc set of users to jointly sign a message. Multisignature schemes are often evaluated from the following perspectives: (1) the cryptographic assumptions underlying the schemes; (2) the operational assumptions about the bootstrapping of the schemes in practice; (3) the number of communication rounds for signing a message; (4) the time complexity for signing a message; (5) the amount of communication for signing a message; (6) the time complexity for verifying a multisignature; (7) the length of the resulting multisignatures. Existing multisignature schemes achieve various trade-offs among these measures, but none of them can achieve simultaneously the desired properties with respect to all (or even most) of these measures. In this paper, we present a novel multisignature scheme that offers desired properties with respect to the above (1)–(7) simultaneously, except that it uses random oracles (which however are often required in order to design practical schemes). In particular, our scheme is featured by its weak operational (i.e., plain public-key) model, non-interactive signing, and efficient verification.     

Three-dimensional modeling of arc plasma and metal transfer in gas metal arc welding

An integrated comprehensive 3D model has been developed to study the transport phenomena in gas metal arc welding (GMAW). This includes the arc plasma, droplet generation, transfer and impingement onto the weld pool, and weld pool dynamics. The continuum formulation is used for the conservation equations of mass, momentum, and energy in the metal zone. The free surface is tracked using the volume-of-fluid (VOF) technique. The 3D plasma arc model is solved for the electric and magnetic fields in the entire domain. The interaction and coupling between the metal zone and the plasma zone is considered. The distributions of velocity, pressure, temperature, and free surface for the metal zone and the velocity, pressure, and temperature for the plasma zone are all calculated as a function of time. The numerical results show the time-dependant distributions of arc pressure, current density, and heat transfer at the workpiece surface are different from presumed Gaussian distributions in previous models. It is also observed that these distributions for a moving arc are non-axisymmetric and the peaks shift to the arc moving direction.