贵金属Au冷却过程中结构及能量变化的分子动力学计算机模拟

通过分子动力学方法,研究了不同冷速下贵金属Au在温度2000～300K的冷却过程中微观结构的变化特点。结果发现,冷却速度对Au的微观结构产生重要影响。采用偶关联函数和键对分析技术对原子局域团簇结构进行分析,并考察了冷却过程中原子势能随温度的变化,比较了Au的微观结构转变与能量变化的对应关系,从能量转化的角度对冷却过程中Au的结构变化进行了说明。     

耐蚀合金Au3 Cu高温冷却过程中能量及结构转变的分子动力学模拟

用分子动力学模拟方法对液态Au3Cu冷却过程进行了研究,考察了不同冷却速度下Au3Cu结构变化特点,原子间相互作用势采用F-S多体势,结构分析采用键取向序和对分析技术.计算结果表明,冷却速度对液态Au3Cu能量及结构转变有重要影响,给出了不同冷却速度下液态Au3Cu结构转变的微观信息.     

Calibration of a quenching and deformation differential dilatometer upon heating and cooling: Thermal expansion of Fe and Fe-Ni alloys

Dilatometry is a technique for precise measurement of thermal dilatation of materials during heating or cooling. A procedure has been presented for calibration of a differential dilatometer operating with electromagnetic heating for metallic specimens both upon heating and cooling as well as under uniaxial compressive and tensile loading. The dilation signal has been calibrated for both heating and cooling and for uniaxial loading (compressive and tensile) using platinum or iron reference specimens, for which recommended dilational data are available. The ferro- to paramagnetic transition (characterised by the Curie temperature) of pure iron or iron-based alloys has been adopted to calibrate the temperature in the dilatometric measurement under different loading modes during heating and cooling. On this basis calibrated data for the thermal expansion coefficients of Fe and Fe-Ni alloys have been obtained.     

Impacts of moving wall and heat-generating element on heat transfer and entropy generation of Al2O3/H2O nanofluid

Development of various electronic devices demands to create effective cooling complexes. The present paper deals with computational analysis of mixed convection cooling of heat-conducting and heat-generating element located inside an alumina–water nanofluid enclosure with upper moving wall. Usage of upper moving wall, nanofluid and cooling vertical walls allows to create the effective cooling process. Analysis has been performed numerically using the Oberbeck–Boussinesq equations. The effects of nanoparticles concentration, heat source location and upper wall velocity on flow structures, heat exchange and entropy generation have been investigated. It has been ascertained that effective cooling of the heated element occurs for high Reynolds number and central position of the heat-generating element.

     

A calorimetric study of phase evolution in a WE43 Mg alloy

A DSC investigation has been performed on a Mg–RE–Y–Zr (RE=rare earth) technical alloy WE43. Hardness trend during isothermal treatments has been correlated to the calorimetric traces evolution and to the forming β phases with its precursors. Oversaturation of solute elements occurs at temperatures higher than 150±C, on cooling at room temperature after the anneals. Activation energies, found under non-isothermal conditions on artificially aged samples, suggest a slow transformation velocity, while the hardness response is relatively fast.     

Cooling rate effect on the phase transitions in a polymer liquid crystal: DSC and real-time MAXS and WAXD experiments

The importance of the cooling rate for the structural transformations in a main-chain poly(hexamethylene-4,4′-bibenzoate) has been presented. Detailed analysis of the phase transitions, main structural parameters and their temperature changes has been performed by differential scanning calorimetry, real-time middle-angle X-ray scattering and wide-angle X-ray diffraction methods. The thermodynamic nature of the initial transformation into a smectic A phase has been discussed. The material in the smectic state is supposed to be organized in smectic domains. The crystallization from the smectic phase depends strongly on the kinetics. The crystallization inside the smectic domains results into different final structures determined by the cooling rate applied. At the highest cooling rates, only one crystalline form has been observed. Different possible modifications have been discussed for the case: either a γ-polymorphic form or still some mesophase of high order, as a frozen metastable state. There is a possibility that the phase might be also identified as a condis crystal. At decreasing cooling rates, a new crystalline form, named α∗, appears together with the first one. Lowering the cooling rate, the volume fraction of the α∗-polymorph gradually increases, at the expenses of the first form. The interesting feature of the new observed α∗-polymorph is that it has some similarities with α- and δ-phases of the same material. Contrary to the previous observations, no γ?α^∗ transformation has been observed neither during the course of single crystallization nor during the subsequent heating. A model describing the gradual transformation of the material during its temperature treatment has been proposed.     

Modeling of a free-burning,high-intensity arc at elevated pressures

A previously established model has been applied to a free-burning high-intensity argon arc at elevated pressures for simulating the situations experienced, for example, in plasma processing or in underwater welding. With the calculated thermodyanmic and transport properties and appropriate boundary conditions, solutions of the entire arc are obtained with exception of the electrode sheath regions. The results show that the arc contracts as the pressure increases. As a consequence of this contraction, the current density, the enthalpy flux, and the voltage drop increase also while the velocity of the induced cathode jet decreases.     

A temperature correction procedure for temperature inhomogeneity in dilatometer specimens

Dilatometry is a thermo-analytical technique used to measure the expansion or shrinkage of materials during heating or cooling, whether or not in association with a phase transformation. A temperature correction procedure has been developed to correct for the temperature inhomogeneity that exists in an inductively heated specimen during the heating/cooling process and to represent the dilation as a function of a homogeneous temperature. As an example, taking an Fe-5.91 at.% Ni specimen and subjecting it to two different cooling rates, the temperature correction has been performed for the temperature range where the austenite to ferrite phase transformation takes place as well as for the pure austenite and ferrite phases close to the temperature range of the transformation.     

金属Cu液固转变及晶体生长的分子动力学模拟

采用分子动力学模拟研究了液态Ｃｕ在不同冷却速度下的凝固特点,模拟采用ＥＡＭ作用势,计算了不同温度,不同冷却速度下Ｃｕ的偶相关函数,结果表明ＥＡＭ作用势能很好地描述液态Ｃｕ的结构特征,当冷却速度较快时,液Ｃｕ形成非晶;当冷却速度较慢时,液Ｃｕ形成晶体,分析了不同冷却速度下体系的相变热力学及相变动力学过程,最后采用液固两层构型法,描述了Ｃｕ晶体的生长过程。     

A Coupled Chemical Kinetic and Nucleation Model of Fume Formation in Metal–Inert-Gas/Metal–Active-Gas Welding

A computational model of the formation of welding fume in arc plasmas, under conditions occurring in metal–inert-gas (MIG) and metal–active-gas (MAG) welding, is presented. The model couples the chemical kinetics occurring in high-temperature mixtures of iron vapour, oxygen and argon with a moment model of the nucleation and growth by condensation of iron and iron oxide nanoparticles. Results are presented for different iron vapour concentrations, oxygen-to-argon ratios, and quench rates. It is found that the presence of oxygen has important effects on the gas-phase chemistry and the properties of the nanoparticles. FeO nanoparticles are preferentially nucleated, and have smaller diameter than the Fe nanoparticles that are produced in the absence of oxygen. The final composition of the nanoparticles depends on the relative concentrations of iron and oxygen in the plasma. A three-dimensional arc model that includes vaporization of the wire electrode is used to predict temperature, velocity and iron vapour mass fraction distributions in typical MIG and MAG welding conditions. Calculations of nanoparticle formation and growth along streamlines confirm the importance of oxygen in determining the fume particle properties.     

Reversible and irreversible variation of the superstructure of highly oriented low-density polyethylene during heat treatment

The small-angle x-ray scattering (SAXS) intensity of highly-oriented, low-density polyethylene (LDPE) with fixed draw ratio has been investigated during several heating and cooling cycles. Using a three-dimensional, monoclinic, paracrystalline superlattice to describe the superstructure of the sample, it has been possible to calculate the SAXS patterns completely. A very large irreversible variation of the superstructure during the first heating cycle, and a smaller reversible variation of the average size and distance of the crystallites during subsequent temperature cycles, could be obtained. These results can be explained using the thermodynamic theory of crystallization of polymer multicomponent systems of Kilian.     

Investigation of Calcium Carbonate Scaling Inhibition and Scale Morphology by AFM

The calcium carbonate scale inhibition by two inhibitors, polyacrylic acid (PAA) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), has been studied in two heat transfer systems: recirculating cooling water and pool boiling systems. It is found that PBTCA has a better inhibition effect than PAA under identical conditions. The inhibition effect increases with increasing fluid velocity for the cooling water system, whereas in the presence of inhibitors, the fluid velocity has less effect on the scaling behavior. When the initial surface temperature increases, the inhibition efficiency decreases. In the presence of inhibitors, the scaling behavior is insensitive to the change of surface temperature. The relationship between the inhibition effect and the fractal dimension has also been investigated. The results show that the fractal dimension is higher in the presence of inhibitors. The better the inhibition effect, the higher the fractal dimension. XRD and FTIR analyses demonstrate that for the CaCO₃ formed in the pool boiling system, the content of vaterite increases with the increase of inhibition effects. The metastable crystal forms of vaterite and aragonite are stabilized kinetically in the presence of inhibitors. The step morphology has been observed by atomic force microscopy. It is shown that the step space on the CaCO₃ surface increases in the presence of inhibitors. Moreover, with the increase in inhibition effect, both the step space and the fractal dimension increase. Step bunching is also found and discussed in this paper.     

The effect of polyolefin's molecule and supermolecule structures on some industrial processes

In this study, the raw materials and their structural properties were analyzed in synthetic polymer pipe production by the following events: 1) The effect of extruder processes to dynamic-mechanical properties of polyethylene (PE) and polypropylene (PP) pipes in pipe production; 2) The effect of the fiber direction on dynamic-mechanical properties and extruder materials; 3) The heat process on welding during the pipe production and the effect of polymer structure on welding duration; 4) Physical properties and structural changes of pigmented PE and PP raw materials. 5) The relationship between dynamic-mechanical properties of polymers with thermodynamic and kinetic concepts and effect of temperature. The results (that were) derived after several measurements are as follows: 1) The best stability properties were observed on the pipe samples which have same fibre direction. In samples with opposite and vertical fiber directions the stability properties are 30% lower. 2) The maximum dynamic-mechanical properties of the pipe samples were observed when the extruder temp. Is 463 k. These properties were lower in the pipe compared to the samples taken from the inner and outer part of the extruder. For the sample during this time the concentration of the oxygen groups are at the lowest and amorphous regions are at the highest level. These results prove that the temperature, the pressure and the cooling rate of the material taken from the extruder have too much effect on polymer structure. 3) Welding duration is investigated without changing the fibrillar structure. The best welding conditions have been obtained in case of deformation graphs with minimum areas. The results can be explained with the difference in polymer structures at different temperatures during pipe manufacturing process. 4) The σ, τ, γ values differ greatly in the samples taken from different companies (∼ 50%). The analyses of IR spectra and microphotographs show that this difference depends on the differences of the polymer structures and the initial data.     

Na-beam cooling by a mode-locked laser

Controlled mode-locked laser cooling of sodium atoms has been demonstrated for the first time. The broadband laser spectrum covers most of the Doppler-broadened atomic resonance spectrum. Atoms of a broad velocity range are thus decelerated simultaneously. In order to stop this process at a defined atomic velocity a two-mode laser beam tuned to resonance with both sodium groundstate levels is copropagating with the atomic beam and counterpropagating to the mode-locked laser beam. In this optical molasses sodium atoms ofv=20 m/s have been piled up to densities of about 2·10⁶ cm^?3. The density dependence on variable laser parameters has been investigated systematically.     

Emission of gases and degradation volatiles from polymeric wood constituents in friction welding of wood dowels

Analysis of volatile compounds and gases emitted as smoke at the welding interface during rotational wood dowel welding of a hardwood (beech) and of a softwood (Norway spruce) has shown that the compounds in such a smoke are water vapour, CO₂, degradation compounds from wood polymeric carbohydrates and from amorphous lignin, as well as some volatile terpenes, these latter only for the softwood used, Norway spruce. The main carbohydrates contributing to the volatile compounds are xylans for beech and glucomannans for spruce. Numerous compounds, in very small proportions derived from the degradation and rearrangement reactions of lignin, have also been identified. The proportion of CO₂ emitted is very low, and neither CO nor methane is emitted due to the relatively low temperature of dowel welding. Experiments at temperature slightly higher than that of dowel welding but prolonged in time have shown that the main component of the smoke produced during welding appears to be water vapour.     

Temperature calibration of heat flux DSC's on cooling

The temperature calibration of a TA Instruments 3200-2920 DSC has been performed on cooling using the isotropic → nematic, isotropic → cholesteric and other liquid crystal → liquid crystal transitions of thermally stable, high purity liquid crystals. The thermal stability of these liquid crystals has been verified by measuring the temperature of the mentioned transitions during cyclic heating and cooling experiments. Correspondence has been established between the real and indicated temperature during cooling for all combinations of heating and cooling rates of practical interest: correction values were determined to the indicated temperature in order to obtain the real temperature on cooling. These correction values were calculated as the average from the temperatures of four or five different liquid crystal transitions for each heating-cooling rate combination. The accuracy of the temperature calibration on cooling is ca. 0.2?C for heating and cooling rates up to 20?C min^?1.     

A study of the crystal structure of α-glycylglycine in the temperature range 100–295 K

Changes in the structure of α-modification of glycylglycine during cooling down to 100 K have been studied; the results were compared with those obtained earlier for glycine polymorphs. A relative volumetric contraction of the structure during cooling is found to be greater for glycylglycine than for all other glycine polymorphs. The most rigid direction, along which linear dimensions increase during cooling, whereas overall volumetric contraction of the structure takes place, correlates with the direction of the chains in which zwitterions of glycylglycine are linked by H-bonds “head-to-tail.” The maximum rigidity of the structure in the direction of the “ head-to-tail” chains was also observed in all glycine polymorphs. The behavior of H-bonds in the crystals of glycylglycine during cooling is examined. The role of conformational flexibility of the molecule in the anisotropy of structural deformation during cooling is discussed.     

Ni3Al合金液态与非晶中的原子团簇

采用常温常压分子动力学模拟技术，模拟了液态Ni3Al中原子团簇在快速凝固条件下的演变过程，模型采用的是TB(tight binding)作用势.用偶分布函数、键对和多面体等结构参数来描述快速凝固条件下团簇种类和数量的变化，并将团簇结构可视化.在2 000 K下，液态Ni3Al中团簇数量较少，且都是由缺陷二十面体构成；在4×1013 K•s－1的冷速下，团簇的数量随温度的降低不断增加，且出现完整二十面体团簇，体系最终形成了由二十面体和缺陷二十面体团簇网络所组成的非晶结构.     

Phase separation by continuous quenching: similarities between cooling experiments in polymer blends and reaction-induced phase separation in modified thermosets

Small angle light scattering (SALS) has been applied to study the phase separation kinetics in a binary polymer mixture of poly(ethyl methyl siloxane) (PEMS) and poly(dimethyl siloxane) (PDMS). The phase separation was induced by cooling an initially homogeneous mixture with well defined cooling rates. The results have been compared to time resolved SALS and microscopy in the course of reaction-induced phase separation in mixtures of an epoxy resin and polysulfone (PSU). For the critical PEMS/PDMS mixture with an upper critical point it was found in a continuous quenching experiment that the time evolution of the scattered light intensity I(q,t) scales with the cooling rate. The similarity to the scaling behavior of I(q,t) in isothermal experiments after fast quenches (scaled by the quench depth) is discussed. A secondary phase separation was found and has been explained by the competition between the growth of the two phase structure during cooling and the mutual diffusion without the assumption of gelation or vitrification. For the epoxy/PSU mixture with 15% PSU, after the appearance of a bicontinuous structure a secondary phase separation was observed. Mixtures with higher PSU-contents formed epoxy-rich droplets in the PSU-rich matrix by nucleation and growth mechanism. The frustration of the structure growth can be explained by approaching vitrification of one or both phases. The similarity between continuous cooling experiments in blends and the reaction-induced phase separation have been discussed in the generalized χN vs. composition phase diagram (N: degree of polymerization, χ: Flory-Huggms interaction parameter).