A thermo dynamical model for the shape and size effect on melting of boron carbide nanoparticles

The size and shape dependence of the melting temperature of Boron Carbide (B4C) nanoparticles has been investigated with a numerical thermo dynamical approach. The problem considered in this paper is the inward melting of nanoparticles with spherical and cylindrical geometry. The cylindrical Boron Carbide (B4C) nanoparticles, whose melting point has been reported to decrease with decreasing particle radius, become larger than spherical shaped nanoparticle. Comparative investigation of the size dependence of the melting temperature with respect to the two shapes is also been done. The melting temperature obtained in the present study is approximately a dealing function of radius, in a good agreement with prediction of thermo dynamical model.     

Ti-5Al-2.5Sn合金ISM熔炼工艺参数优化设计

运用直接差分方法,建立了感应凝壳熔炼温度场计算模型,并在此基础上模拟计算了熔炼Ｔｉ－５Ａｌ－２．５Ｓｎ合金时坩埚内温度变化。提出了合理的功率进给方式。研究了炉料和熔炼功率对熔体最终温度和出液量的影响,优化了熔炼工艺参数,为实际熔炼提供了理论指导。     

Three-dimensional finite element analysis of temperatures and stresses in wide-band laser surface melting processing

During laser surface melting of steel components, obtaining the desired distribution of microstructure and residual stresses with minimum distortion is essential for production goals and reliable service performance. In this study, a three-dimensional finite element based model, which is integrated into commercial finite element analysis (FEA) software SYSWELD by means of user subroutines, has been developed to simulate the wide-band laser surface melting (LSM) processing and predict temperature history and stress field with different laser scanning speed. In the proposed computational procedure, thermal, metallurgical transformation and mechanical aspects were taken into account, and the heat transfer analysis, the temperature dependent on material properties and a coupled transient thermo-mechanical analysis were used. Effects of laser scanning speed on melting temperature field and residual stress were investigated. The simulation results show that laser scanning speed changes have significant effects on melting residual stress. For experimental verification, laser surface melting of thin plate 42CrMo4 steel was achieved by a 5 kW continuous wave CO₂ laser with laser scanning speed from 10 m/s to 30 m/s. The computational results are in good agreement with experimental measurements.     

Temperature control of TiAl melt during induction skull melting

Abstract

The properties of intermetallic compounds are sensitive to alloy composition and interstitial element content determined by the melting process. Induction skull melting is one of the best methods for melting reactive alloys. For induction heating under vacuum, melt temperature control is problematic. A numerical model for simulating temperature field during induction skull melting has been developed using the direct finite difference method. Factors such as water cooling boundaries, electromagnetic stirring meniscus, and power distribution in the charge were analysed. The chargecrucible interface was taken as a radiation boundary before melting and as a combined radiation and conduction boundary after melting. The free surface was taken as a radiation boundary. The relationship between the height of electromagnetic stirring meniscus and charge weight and melting power was reduced based on the conservation of mass. Based on the conservation of energy, the distribution density of induction current was ascertained. With the program, the relationship between melting power, charge weight, and melt temperature was established. During induction skull melting of gamma-TiAl based alloys, the melt temperature was measured carefully. The theoretical and experimental results were found to be in agreement.     

Texturing by cooling a metallic melt in a magnetic field

Abstract

Processing in a magnetic field leads to the texturing of materials along an easy-magnetization axis when a minimum anisotropy energy exists at the processing temperature; the magnetic field can be applied to a particle assembly embedded into a liquid, or to a solid at a high diffusion temperature close to the melting temperature or between the liquidus and the solidus temperatures in a region of partial melting. It has been shown in many experiments that texturing is easy to achieve in congruent and noncongruent compounds by applying the field above the melting temperature T_m or above the liquidus temperature of alloys. Texturing from a melt is successful when the overheating temperature is just a few degrees above T_m and fails when the processing time above T_m is too long or when the overheating temperature is too high; these observations indicate the presence of unmelted crystals above T_m with a size depending on these two variables that act as growth nuclei. A recent model that predicts the existence of unmelted crystals above the melting temperature is used to calculate their radius in a bismuth melt.     

Size,composition and shape dependent melting temperature for AuPt bimetallic nanoparticles

Abstract

According to the shape dependent melting temperature model for nanoparticles, an analytical model has been developed to describe the size, composition and shape dependent melting temperature. Results show that the size dependent melting temperature of bimetallic particles has a similar trend to that of the monometallic nanoparticles. The effect of composition on the melting temperature depends on the melting point of the component elements. The shape effect can be estimated by the surface/volume ratio of the bimetallic nanoparticles. The accuracy of our model is confirmed by recent molecule dynamic simulations.     

Low-energy high-current electron beam heating of target with second-phase microinclusions

The temperature field generated by microsecond pulsed low-energy high-current electron beam (LEHCEB) in the surface layer of a stainless-steel target containing second-phase (manganese sulfide, MnS) microinclusions has been numerically simulated. The results of calculations show that the temperature is nonuniformly distributed over the target surface. By the end of the LEHCEB pulse, the temperature in the regions of MnS inclusions significantly exceeds that of the steel matrix. This nonuniformity is related to (i) markedly greater thermal conductivity of steel compared to that of MnS and (ii) the pulsed character of the electron-beam-induced heating of the target surface. It is also established that LEHCEB-induced melting begins at the inclusion-steel interface and then involves the inclusion and spreads over the entire irradiated surface. The dependence of the characteristics of the irradiation-induced temperature field on the parameters of the pulsed electron beam has been studied.     

On the temperature of melting of nanoparticles and nanocrystalline substances

A new expression for the temperature of melting of a nanoparticle, which agrees with the well-known result of the Debye model, has been obtained within the framework of a rather strict thermodynamic approach. It is shown that the characteristic Debye temperature and the melting temperature can depend in a critical manner on the fractal structure of the nanocrystalline solid.     

Temperature distribution in a sample with second-phase microinclusions during irradiation by a low-energy high-current pulsed electron beam

Using the methods of numerical integration, a temperature field has been calculated that arose in the surface layer of titanium nickelide target with NiTi₂ intermetallic inclusions during irradiation by a lowenergy high-current electron beam with a duration of the order of a microsecond. The calculated temperature field has been compared with that obtained previously for a target of stainless steel 316L containing MnS inclusions. It has been found that, as in the case of stainless steel, the regions of inclusions are overheated. However, the temperature increase for NiTi₂ (12 K) is significantly lower than in the case of stainless steel 316L (283 K). The dynamics of melting of these systems are also considerably different.     

Melting of Krypton Layers Adsorbed in Cylindrical Pores

The mechanism of melting and its correlation to adsorption on cylindrical surfaces of silica-like cylindrical nanopores has been studied for the first time. The size of the pores has been defined by its diameter (4 nm in this work). As an example, we have analyzed krypton atoms as adsorbed system. We have shown, that the layer adsorbed on the surface of the pore has much lower melting temperature (T = 77.5 K) than both the filled pore (T = 90 K) and the bulk system (T = 118 K). The mechanism of melting in the layer is a continuous transition and depends on the vacancy formation. The layering transition changes its character with temperature from a sharp transition below the melting temperature into a continuous one above it.     

固相缩聚与PET的熔融性能

应用差示扫描量热法（ＤＳＣ）和广角Ｘ射结衍射（ＷＡＸＤ）研究了低温干燥高温固相缩聚ＰＥＴ的熔融行为和结晶结构。根据ＤＳＣ结果分析了固相缩聚反应温度和时间对熔融双峰的影响,得到熔点与固相缩聚反应温度有红性关系,熔点和结晶度与固相缩聚反应时间的对数呈线性关系。利用ＷＡＸＤ研究了固相缩聚反应温度和时间对表观晶粒尺寸的影响。结果说明低温结晶时形成的不完善晶体在固相缩聚过程中发生部分熔融、重结晶,结晶结构趋     

Effect of twin-boundaries on melting of aluminum

Molecular dynamics simulation technique has been applied to investigate melting temperature of aluminum. Semi-empirical potentials, based on the embedded atom method have been employed to calculate lattice parameter, energy per atom and mean square displacements. Melting temperature is found to compare well with the experimental results. Computer simulation studies of some low index (111), (113) and (112) twin boundaries at various temperatures and their effect on the melting temperature are also carried out. It is observed in this study that in the presence of twin boundaries, aluminum melts at lower temperatures, as compared to normal melting point.     

Crystal Growth and Melting of Nuclear-Ordered Solid 3He

We measured crystal growth and melting rate of nuclear-ordered solid ³ He in a rectangular sample cell. The melting rate was much faster than the growth rate at all temperatures in the low field phase (U2D2) and strongly depended on temperature. The melting occurred on a rough surface and the dissipation mechanism of the melting was compared with the surface magnon scattering mechanism. The crystal growth rate did not depend on temperature and was compared with the spiral growth model associated with screw-dislocations. We found that a large chemical potential difference started to develop when the crystal grew large enough so that the sample became almost single domain at the upper part of the sample cell. We attributed this saturation to the pinning of the screw dislocation on the sample wall. This chemical potential difference decreased rapidly when the crystal was in the high field phase and the crystal grew as it did before the saturation. The crystal continued to grow even after the crystal returned to the U2D2 phase from the high field phase.     

ATS／PEEK共混物的结晶熔融行为

用ＤＳＣ研究了ＰＥＥＫ和ＡＴＳ／ＰＥＥＫ共混物的结晶熔融行为。ＰＥＥＫ的结晶熔融行为取决于其物理状态、熔融、退火结晶和固相处理条件。ＡＴＳ的加入使ＰＰＳ的冷结晶温度Ｔ∝提高，Ｔｃ和Ｔｍ降低。固相处理引起ＰＥＥＫ无定形区热氧化交联，淬火样品退火结晶后出现双熔融峰，随退火温度提高，高温峰Ｔｍ２基本不变，低温峰Ｔｍ１移向高温，最后与Ｔｍ２重迭成单峰。ＡＴＳ对ＰＥＥＫ的Ｔｍ１影响不大，但使其Ｔｍ２降低。     

Kapton Capacitance Thermometry at Low Temperatures and in High Magnetic Fields

A sensitive Kapton foil capacitance sensor, with size of 9.5 mm×4.5 mm, has been developed and used as a thermometer at ultra-low temperatures down to 1.2 mK and in high magnetic fields. There is no visible magnetic field dependence up to 15 T. The sensor was calibrated with ³He melting pressure thermometer (MPT) and vibrating wire (VW) viscometer. With the silver powder sintered heat exchanger sandwich-like design, the thermal relaxation time is as short as a few minutes at the base temperature. The low temperature (below 1.2 K) reproducibility has been tested and is satisfied within experimental errors.     

The Dependence of the Melting Temperature of Cobalt–Carbon Eutectic on the Morphology of its Microstructure

Fixed points provide a reliable way to realize and verify temperature scales. High-temperature fixed points are being developed based upon alloys, since single-phase materials are impractical to use above the copper freezing point. In particular, eutectic alloys have been shown to be sufficiently reproducible to warrant consideration as a way to significantly improve high-temperature metrology. However, eutectic alloys have certain characteristics requiring that they are used differently from the current ITS-90 fixed points. As their freezing temperature depends on the freezing rate, the melting temperature is preferred, though it has been shown that for some alloys, notably iron–carbon and cobalt–carbon, the apparent melting temperature can depend on the rate of the preceding freeze. This behavior will need to be explained and quantified if such fixed points are to be acceptable. The effect of varying the freezing rate on subsequent melting has been investigated for cobalt–carbon eutectic fixed points. The apparent melting temperature varies by up to 50 mK. Measurements were made of two different fixed-point blackbodies with very similar results. Optical microscopy of samples produced at different freeze rates shows a change in scale of the microstructure. Electron back-scatter diffraction (EBSD) shows evidence of high levels of residual strain in rapidly frozen samples. The effect of annealing on the melting behavior and microstructure has also been investigated. It is suggested that disordered phase boundaries and residual strain lead to changes in the melting behavior as nonequilibrium conditions may lead to a significant level of pre-melting. Whether this actually changes the liquidus temperature, or whether the melting temperature variation is due to the way the melting point is defined, is also discussed. The variation requires consideration if the highest accuracy is to be achieved, and will be a contributory factor to any uncertainty budget.     

低分子量聚丙烯的制备及平衡熔点的测定

采用不同温度下溶剂抽提和熔融态下温度梯度萃取两种方法对聚丙烯进行了分级。后一种分级方法可获得相同等规度，分子量分布窄的聚丙烯级份。测得低分子量聚丙烯（Ｍｎ＝２４００，Ｍｗ／Ｍｎ＝２．０）的平衡熔点为３９８．７Ｋ。分析该关品的熔融行为，发现晶体内的晶片厚度有一分布，其随结晶温度的升高而变窄。按照Ｈｏｆｆｍａｎ－Ｇｉｂｂｓ方程，计算出片晶的平均厚度为３．５ｍｍ。     

固相缩聚与PET的熔融性能

应用差示扫描量热法（ＤＳＣ）和广角Ｘ射线衍射（ＷＡＸＤ）研究了低温干燥高温固相缩聚ＰＥＴ的熔融行为和结晶结构。根据ＤＳＣ结果分析了固相缩聚反应温度和时间对熔融双峰的影响,得到熔点与固相缩聚反应温度有线性关系,熔点和结晶度与相缩聚反应时间的对数呈线性关系。利用ＷＡＸＤ研究了固相缩聚反应温度和时间对表观晶粒尺寸的影响。结果说明低温结晶时形成的不完善晶体在固体缩聚过程中发生部分熔融、重结晶,结晶结构趋于     

Melting, Oxidation, Evaporation of Particle in-Flight in Plasma Spray Processes

A comprehensive numerical model has been developed to investigate the transient behaviors of particle characteristics, such as temperature, melting, evaporation, and oxidation status during argon-hydrogen DC plasma spraying.This model includes heat, momen     

Melting entropy and enthalpy of metallic nanoparticles

The lattice-type-sensitive model has been developed to predict the size-dependent depression of melting entropy and enthalpy of nanoparticles. The size-dependency of melting entropy and enthalpy of nanoparticles has been obtained based on the relation between cohesive energy and melting point of nanoparticles obtained in our previous work. In this model the effects of particle size, lattice and surface packing factors, and the coordination numbers of the lattice and surface crystalline planes are considered. The presented equations of melting entropy and enthalpy have been corroborated by the experimental data of In and molecular dynamic (MD) simulation results of Cu nanoparticles. The model confirms that the size-dependency of the entropy and enthalpy of melting for nanoparticles is quite dependent on their lattice structure.