纳米铜粒子的热稳定性研究

利用惰性气体蒸发法制备了平均粒径为40～50nm的纳米铜粒子。采用示差扫描量热法(DSC)对纳米铜粒子的热性能进行了研究。纳米铜粒子在477℃开始熔化，但不同的升温速度有着不同的熔化过程。升温速率低于30℃／min时，纳米铜粒子有足够的时间长大因而观察不到明显的熔点降低。升温速率在30℃／min时，纳米铜粒子快速吸热熔化，在490℃出现明显的熔点，且绝大部分纳米粒子在此温度下熔化。     

高应变率下温度对单晶铜拉伸微观变形的影响机理

目的 以单晶铜为研究对象,探究5×109 s^-1高应变率下温度对单晶铜的应力及微观变形的影响,为设计、制备高性能单晶铜导线提供理论依据。方法 运用分子动力学模拟技术,构建尺寸为10.8 nm×10.8 nm×10.8 nm的单晶铜模型,在应变率为5×109 s^-1,温度为100～1 100 K范围内对单晶铜进行x、y、z三轴拉伸,模拟其应力应变、位错密度、晶体结构转变规律,对晶体的有序性和孔洞体积分数的微观结构变化进行研究。结果 随着温度的升高,单晶铜的屈服强度降低,在温度为1 100 K时单晶铜的屈服强度比100 K时降低了约55%,与屈服强度相对应的应变数值会提前约5%。得到了100～1 100 K温度范围内应力-应变曲线,该曲线包括3个阶段,即弹性变形阶段、塑性变形阶段和应力下降阶段。对应力变化的原因进行分析,当应力达到屈服点后,单晶铜内部出现孔洞形核,孔洞快速长大并合并;在变形的同时,晶格结构发生转变,在1 100 K温度时FCC结构全部转变为Other结构;利用径向分布函数对晶格有序性进行分析,发现在高应变下会产生非晶结构。结论 随...     

模具钢类多元合金原子间势能模型的构建

分子动力学是研究超精密加工机理的一种理想方法,但其研究的对象较为单一,其中多数为无缺陷的单晶材料（主要是铜、铝、硅等）．由于受势函数的限制,目前还没有对模具钢类多元合金体系的分子动力学仿真研究．针对势函数问题,提出了一种多势能函数耦合叠加的方法,分别应用Morse势能函数、Tersoff势能函数和F—S多体势势能函数描述金属与非金属原子之间、非金属原子之间和金属原子之间的作用关系．通过势能函数的耦合叠加构建了模具铜类多元舍金原子间的势能模型．应用了一种简单的模具钢类多元合金体系的构型方法,在纯铁结构基础上,通过置换原子和添加间隙原子,构建了一种含铬镍的假想合金的结构模型．通过构建的模具铜类多元合金原子间势能函数模型进行计算,得到假想合金的体弹性模量约为101．22GPa,与实际铬镍舍金体弹性模量值比较接近．     

阳极弧等离子体制备纳米镍粉的微结构研究

利用X射线衍射(XRD)、透射电子显微镜(TEM)和相应选区电子衍射(SAED)等测试手段,对阳极弧放电等离子体技术制备的不同晶粒尺寸的纳米镍粉体的晶体结构、晶格参数、形貌、粒度进行性能表征。实验结果表明:阳极弧等离子体法制备的纳米镍粉体的晶格常数均大于完整单晶镍的晶格常数,晶格畸变表现为晶格膨胀。18 nm~52 nm之间纳米粉的晶格常数随着晶粒尺寸的减小而增大,晶格畸变量与晶粒尺寸的倒数成正比。     

单晶高温合金雀斑研究进展

为满足先进航空发动机发展需求,航空发动机涡轮叶片的结构日趋复杂,并且作为涡轮叶片首选材料的单晶高温合金中高熔点合金元素含量不断增加,由此导致单晶高温合金涡轮叶片制备过程中结晶缺陷形成倾向增大,直接影响单晶涡轮叶片的质量.本文以单晶高温合金定向凝固过程中出现的一种晶体缺陷——雀斑为讨论对象,综述了近年来雀斑形成机制、判据模型及其控制方法相关研究工作,分析了合金成分、叶片结构、定向凝固工艺和结晶取向对雀斑形成机制的影响,指出考虑不同合金体系中的合金元素与定向凝固过程的参数对雀斑形成的影响,进一步研究复杂结构单晶涡轮叶片雀斑形成规律,建立雀斑预测与控制的有效方法是未来的研究方向.     

化学链制氧技术中铜-锆氧载体的动力学分析

化学链空气分离制氧是利用氧载体在脱氧反应器中脱氧-在吸氧反应器中吸氧、再生实现连续或间断制氧的新颖技术。本研究通过机械混合法制备铜-锆氧载体, 并对制备材料的物相组成、表面形貌及比表面积进行表征, 在STA409PC热重分析仪上采用程序升温法作相关机理探索, 分析气体流量、样品质量、升温速率、惰性载体添加比例对铜-锆氧载体脱氧和吸氧性能的影响。采用等转化率法求解活化能, 采用主曲线法确定动力学模型机理函数。 结果表明, 所制备的铜-锆氧载体物相稳定, 没有烧结现象的发生, 随ZrO₂添加比例的增大, 制备氧载体的比表面积逐渐增大; 当气体流量大于30 mL/min, 样品质量小于10 mg时, 氧载体的转化速率已不受样品传热和传质等内外扩散的影响; 随升温速率的增大及惰性载体添加比例的减小, 氧载体反应起始温度、最大反应速率出现温度及转化完全温度均向高温移动。等转化率法计算得到不同转化率下氧载体的活化能基本相同, 且不同惰性载体添加比例下活化能数值也相差不大; 氧载体脱氧和氧化反应都可用成核和核增长机理模型描述, 但模型中两种反应的级数不同, 前者为3, 后者为1.5。     

砷化镓单晶中微晶和非晶的形成机制

利用高分辨电子显微镜对0.0049N和0.049N荷载Vickers压痕锈发砷化镓单晶的相转变进行了观察和研究,结果表明,在大小压痕作用下分别发生了单晶向和微晶的转变,微晶的结构由小于10nm,取向各异的纳米晶和非晶组成,在完全非晶化的结构中存在少量由几个原子组成的原子簇,在非晶和晶体的交界区能观察到许多晶体缺陷以及沿这些缺陷产生的晶格扭曲和非晶相岛,对这种非晶化现象提出了两种可能的诱发机制,高压力诱导非晶化和剪切诱导非晶化。     

分子动力学模拟Au-Pd和Ag-Pt合金的热学和力学性质

利用Finnis-Sinclair势,对金属Au、Pd、Ag、Pt和合金Au3Pd、AuPd3、Ag3Pt、AgPt3的热学和力学性质进行了分子动力学模拟.首次计算了不同温度下合金的晶格常数、结合能和弹性常数,并预测了它们的熔点.通过比较发现,Au3Pd、AuPd3和Ag3Pt这3种合金的晶格常数、结合能和弹性常数介于其组分金属之间,而AgPt3的剪切模量和熔点高于其组分hg和Pt.     

晶格常数的变化对钛酸铝热稳定性的影响

深入研究了晶格常数的变化和钛酸铝热稳定性的关系．通过仔细分析钛酸铝的晶格常数随温度变化的特点,发现钛酸铝的晶格常数Ｃ随温度升高而降低这一反常现象．提出了钛酸铝的稳定性与其晶格常数Ｃ的大小密切相关的论点．在钛酸铝中引入多种添加剂以改变其晶格常数．结果发现晶格常数Ｃ的大小反映了钛酸铝的稳定性．晶格常数Ｃ越大,钛酸铝就越稳定.钛酸铝稳定性提高的原因是：晶格常数ｃ对应于钛酸铝晶体结构中畸变的［ＭｅＯ₆］八面体的高度,Ｃ值增大导致八面体的畸变程度降低,结果就使得钛酸铝更稳定．还研究了不同烧成工艺对钛酸铝的晶格常数和稳定性的影响,结果发现：随着烧成温度的提高和保温时间的延长,钛酸铝的晶格常数Ｃ增大,其稳定性也相应提高．     

单晶镍基合金中γ,γ′两相的晶格常数及影响因素

通过对单晶镍基合金的组织形貌观察和XRD谱线分析,研究了元素铼对单晶合金晶格常数及错配度的影响。结果表明:2%Re镍基单晶合金经完全热处理后,其组织结构是由尺寸约为350～400nm的立方γ′相以共格方式嵌镶在γ基体相所组成;在γ,γ′两相镍基合金的XRD谱线中,衍射峰不对称的原因是由于γ,γ′两相衍射峰叠加所致。对不同合金在不同条件下XRD分离谱线的的分析表明,随元素Re含量增加,合金中γ,γ′两相在室温的晶格常数略有增大,而晶格错配度的绝对值减小。随温度提高,合金中γ,γ′两相的晶格常数及晶格错配度的绝对值增大,并进一步测算出合金中γ,γ′两相在不同温度区间的热膨胀系数。     

Defect structure and the phase diagram of uranium dioxide

Positions of the phase boundaries of pure uranium dioxide UO_{2 ? x} are calculated under the assumption that damage (melting) of the crystal lattice is caused by filling it with defects. The conditions of complete filling of the crystal lattice with individual defects are analyzed. The equilibrium reaction rate constants are determined for the high-temperature forms of interaction of the crystal with oxygen of ambient gaseous medium. The position of the monotectic point is found for x > 0. The melting point parameters are determined for stoichiometric uranium dioxide and uranium dioxide with the maximum dense packing of the defects. A retrograde behavior of the solidus curve in the region x > 0 and anisotropy of properties near the point of congruent melting are predicted. The complex character of the behavior of the boundaries of U₄O₉ and U₃O₈ phases is explained. The reason of discrepancy between the results of two different measurements of solidus temperature in the region x < 0 is explained qualitatively, and the possibility of the existence of solid uranium dioxide up to 3400 K under certain conditions is predicted.     

Application of the embedded-atom method to liquid copper

A procedure for calculating the embedded-atom method (EAM) potential with the use of diffraction data for the metal near its melting point has been applied to copper at temperatures from 1423 to 7400 K. In optimizing the parameters of the EAM potential, we used the pair correlation functions of copper at 1423 and 1873 K, the thermodynamic properties of liquid copper under ordinary conditions, and flash heating and shock compression results. Molecular dynamics simulation with the EAM potential adequately represents the thermodynamic properties and structural characteristics of liquid copper up to 1873 K. The simulated 1423-K bulk modulus is close to the experimentally determined one. At low pressures, the self-diffusion coefficient rises as a power law function of temperature with an exponent close to 2.10. The simulated melting point of copper, 1384 ± 3 K, is close to the actual one. Simulations were performed at temperatures of up to 7400 K and densities a factor of 1.6 higher than the normal one. The melting point was evaluated at pressures of up to 50 GPa. The EAM potential obtained is suitable for the liquid phase but fails to accurately describe properties of crystalline copper.     

Homogeneous and heterogeneous melting behavior of bulk and nanometer-sized Cu systems: a numerical study

Molecular dynamics simulations have been used to investigate the solid–liquid transition of different Cu systems. These consisted of surface-free crystalline bulks and semi-crystals terminating with a free surface as well as of particles and wires with different shape and size in the mesoscale regime. The characteristic melting points of the various systems were attained by gradual heating starting from 300 K. Apart from surface-free bulk systems, where the phase transition at the limit of superheating is homogeneous, melting displays heterogeneous character. This is due to the existence of surface layers with structural and energetic properties different from the ones of bulk-like interior. Simulations point out a significant depression of both the melting point and latent heat of fusion for nanometer-sized systems respect to semi-crystals. Below the characteristic melting point, free surfaces are involved in pre-melting processes determining the formation of a solid–liquid interface. The onset of melting is related to the formation of a critical amount of lattice defects and this provides a common basis for the rationalization of homogeneous and heterogeneous melting processes despite their intrinsic differences.     

移动加热器法晶体生长工艺的研究进展

移动加热器法被认为是一种切实可行的生长大尺寸、高质量单晶体的方法,它结合了液相外延和区熔提纯的优点.本文阐述了移动加热器法晶体生长的基本原理、优缺点,综述了相关加热方式、晶体生长过程中的质量输运和热交换,并探讨了工艺条件(如重力场、磁场、强迫对流等和晶体生长速率)对移动加热器法生长晶体过程与晶体质量的影响,最后对移动加热器法的发展趋势进行了展望.     

Investigation of the lattice expansion for Ni nanoparticles

Nickel nanoparticles with various grain sizes were successfully prepared by anodic arc discharge plasma method. The samples prepared by this method were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and the corresponding selected-area electron diffraction (SAED) to determine the crystal structure, lattice parameter, morphology and particle size. The experiment results indicate that the crystal structure of the samples is FCC structure as same as the bulk materials. The lattice parameters for Ni nanoparticles are always larger than the equilibrium values of the perfect single crystal lattice respectively, the lattice parameter increase significantly with the decrease of the grain size, the lattice expansion was found, and the expansion quantity of the lattice parameter is in direct proportion to the reciprocal of grain size. The values of the unit cell volume increase remarkably with the decrease of the grain size. The lattice expansion is the result of the interfacial energy and surface tension mutual attraction of Ni nanoparticles, this phenomenon can be explained according to the thermodynamic theory.     

Detection of surface lattice defects using line scan method

The presence of different kinds of surface lattice defects such as missing atom, interstitial atom, line defects, in graphite single crystal have been identified by using scanning tunneling microscope. These defects cause displacement of atoms from their mean position and lattice strain is introduced. By measuring the displacement of atoms from their mean position. lattice strain has been calculated. It is found that among single point defects, vacancies cause maximum lattice strain. Paper presented at the poster session of MRSI AGM V1, Kharagpur, 1995     

Synthesis and Characterization of Tb-incorporated Apatite Nano-scale Powders

Nano-scale Tb-incorporated apatite (nano-Tb-AP) particles with different Tb contents (Tb/(Tb+Ca)) of 0%, 5%, 10% and 20% were synthesized through a simple wet chemical method in this study. The crystal structure, thermal stabilities, chemical groups, crystal morphologies and crystal sizes of the nano-Tb-AP particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM), respectively. It was found that lattice constants, particle sizes, crystalline and thermal stability varied with the doped Tb contents. With the increasing of Tb content, the lattice constants, particle size, length/diameter ratio, crystalline and thermal stability of nano-Tb-AP gradually decrease. Especially, almost all the 20%Tb-AP nano particles had been decomposed at 1200 C while only a few of the decomposed products (β-TCP) were detected in the Tb-free nano apatite powders. This kind of nano-scale Tb-incorporated apatite exhibits an extremely potential clinic application because it integrates both the excellent biological functions of Tb element and apatite in human body.     

Melting behavior and origin of strain in ball-milled nanocrystalline Al powders

Ball-milled aluminum powders have been investigated by differential scanning calorimetry and high resolution X-ray line profile analysis. Ball-milling was carried out for different milling times ranging from 45 min to 32 days. The milling time dependence of the average grain-size and of the density of lattice defects, mainly dislocations, were determined by the modified Williamson-Hall and Warren-Averbach procedures based on the dislocation model of mean square strain. Characteristic grain sizes of ball-milled Al-powders decreases with increasing milling time and simultaneously, the grain-size distribution becomes sharper.Calorimetric measurements indicated the decrease of the melting point, T_m with increasing milling time. The melting point depression was found to be proportional to inverse grain size.The strain accumulated in the powder particles is mainly caused by intergrain dislocations.     

Control of hydroxyapatite crystallinity by mechanical grinding method

Crystallinity of hydroxyapatite reflecting crystal size and crystal elastic strain was controlled by the mechanical grinding (MG) technique using a set of container and balls made of SUS304 stainless steel or agate. Variation in the crystallinity through MG was monitored by the XRD method and represented by the broadening of the diffraction peak. Effect of changes in crystallite size and strain on the crystallinity was also examined using the Hall-plot method.Crystallinity rapidly decreased with milling time. Significant crystallographic diffraction peaks disappeared and a broad diffraction around 2=32° was observed after MG for 72 h. The broadening was dominantly due to an increase in crystal strain in addition to fine crystallite size. Contamination from the container and balls during MG was more suppressed using agate than SUS304 stainless steel.The recovery process of crystallinity during heating between 300 °C and 1200 °C was examined focusing on the decrease in residual elastic strain. Low crystallinity was maintained at annealing temperatures below 800 °C but lattice defects were recovered above 1000 °C.© 2001 Kluwer Academic Publishers     

Application of Microwave Melting for the Recovery of Tin Powder

The present work explores the application of microwave heating for the melting of powdered tin. The morphology and particle size of powdered tin prepared by the centrifugal atomization method were characterized. The tin particles were uniform and spherical in shape, with 90% of the particles in the size range of 38–75 μm. The microwave absorption characteristic of the tin powder was assessed by an estimation of the dielectric properties. Microwave penetration was found to have good volumetric heating on powdered tin. Conduction losses were the main loss mechanisms for powdered tin by microwave heating at temperatures above 150 °C. A 20 kW commercial-scale microwave tin-melting unit was designed, developed, and utilized for production. This unit achieved a heating rate that was at least 10 times higher than those of conventional methods, as well as a far shorter melting duration. The results suggest that microwave heating accelerates the heating rate and shortens the melting time. Tin recovery rate was 97.79%, with a slag ratio of only 1.65% and other losses accounting for less than 0.56%. The unit energy consumption was only 0.17 (kW·h)·kg^–1—far lower than the energy required by conventional melting methods. Thus, the microwave melting process improved heating efficiency and reduced energy consumption.