立方铂纳米粒子的形状变化与熔化特性的分子动力学研究

采用分子动力学方法结合嵌入原子多体势,对立方铂纳米粒子的热稳定性进行了模拟研究.计算结果表明,立方纳米粒子在升温过程中首先转变为由{111}和{100}面所构成的十四面体,然后再转变为球形,最后熔化为液态.通过计算立方铂钠米粒子的统计半径,发现形状转变温度在1250 K左右.尽管形状不同,立方纳米粒子和球形纳米粒子的熔点是相同的. 关键词： 纳米粒子 热稳定性 分子动力学     

纳米多晶铁的冲击相变研究

利用分子动力学方法研究了不同晶粒度的纳米多晶铁在冲击压缩下的结构相变过程,模拟结果表明:纳米多晶铁的冲击结构相变(由体心立方(bcc)结构 α 相到六角密排(hcp)结构 ε 相)发生的临界冲击应力在15 GPa左右.纳米多晶铁在经过弹性压缩变形后,晶界导致的塑性变形开始发生,然后大多数相变从晶界成核并最终发展为大规模相变.不同变形过程在应力和粒子速度剖面上能得到清晰的体现,并通过微观原子结构分析分辨.冲击压缩后的微观结构以晶界原子和以fcc结构原子充当孪晶界的hcp原子为主.晶粒度明显影响晶界变形及相变 关键词： 冲击相变 纳米多晶铁 冲击波 分子动力学     

单晶铁沿[101]晶向冲击过程中面心立方相的形成机制

铁的冲击相变过程是科研工作者们关注的热点领域之一.铁沿[100]晶向冲击时会发生体心立方相到密排六方相的转变;而沿[101]晶向冲击时,相变产物除了密排六方相之外还出现一定量的面心立方相.人们已经明确了体心立方到密排六方相的转变机制,然而对于面心立方相的形成机制问题至今还在探索.本文通过分子动力学方法模拟了体心立方单晶铁沿[101]晶向的冲击过程,模拟结果显示体心立方相将转变为高压密排结构(密排六方相和面心立方相);并分析了面心立方相的形成机制:在冲击过程中,单晶铁沿[101]和101]晶向突然收缩,同时沿[010]晶向突然扩张,从而导致体心立方到面心立方相的转变.此外,本文进一步研究了不同应力状态下单晶铁的相变机制,发现沿[101]晶向单轴压缩以及沿[101]和[101]晶向双轴压缩时铁将发生体心立方到面心立方相的转变;而沿[101]和[010]晶向双轴以及三轴压缩时将会发生体心立方到密排六方相的转变.最后进一步计算了三个相的吉布斯自由能随压力的变化,并对冲击模拟结果进行了能量分析,给出了沿[101]晶向冲击条件下高压密排相产生的原因.     

铂纳米晶在升温过程中结构演化与熔化特征的原子级模拟研究

采用分子动力学方法结合量子修正Sutton-Chen型多体力场,对由{100}面和{111}面构成的十四面体Pt纳米晶在升温过程中的热稳定性和熔化机制进行了计算机模拟研究,并引入统计半径和Lindemann指数来分析它的结构和形状演化过程. 结果表明：该纳米晶在1500 K时形状开始发生变化,并在1700 K时转变为球形. 铂纳米晶粒在1500 K时开始出现表面预熔,在1650 K时表面完全熔化并开始向内部传播,最终在1730 K时整体熔化为液态粒子. 表面预熔的出现对形状转变的发生是有利的. 关键词： 纳米晶 结构 熔化 分子动力学     

Pt-Au核-壳结构纳米粒子热稳定性的分子动力学研究

采用分子动力学方法结合嵌入原子势, 对Pt-Au核-壳纳米粒子的热稳定性进行了研究. 计算结果表明: Pt-Au纳米粒子的熔点明显高于Au纳米粒子而低于Pt纳米粒子. 通过计算Lindemann指数发现: 壳层中的Au首先熔化, 然后逐渐向内部扩展, 最终导致核中的Pt完全熔化; 熔化所经历的温度区间明显宽于单质纳米粒子, 而且该熔化过程呈现典型的两阶段熔化特征; 在两次熔化之间, 存在着固(核)液(壳)共存的结构. 关键词： 纳米粒子 熔化 分子动力学     

Ti纳米粒子熔化与凝结的原子尺度模拟

本文采用基于嵌入原子势的分子动力学方法模拟研究了不同尺寸Ti纳米粒子在熔化与凝结过程中的原子堆积结构变化.温度变化过程中对Ti纳米粒子中原子平均能量、对分布函数、键对和比热容的计算结果表明,粒子尺寸和温度变化方式对粒子的结构转变具有重要影响.小尺寸Ti粒子更易于形成二十面体构型.随着Ti纳米粒子粒径的增大,室温下粒子趋于保持初始密排六方的堆积结构.升温过程中,大粒径的纳米粒子内出现HCP向BCC的部分结构转变,导致HCP和BCC结构共存现象.大粒径粒子的熔化与体相材料相似,具有一个熔化温度.熔融粒子降温时,纳米粒子内部原子发生熔融态→BCC→HCP堆积结构的转变,且凝结温度较熔化温度滞后.该原子尺度的模拟提供了可用经典理论估算Ti粒子熔化所需能量的临界尺寸.     

铂纳米粒子热稳定性的原子级模拟研究

本文采用分子动力学结合嵌入原子多体势,模拟了铂纳米粒子在升温过程中的热稳定性和熔化机制,并利用共近邻分析方法分析了它的微结构演化过程。模拟的结果表明：铂纳米粒子的熔点明显低于体材料的熔点;由于表面层原子的结合力较弱,在升温过程中表面会首先出现预熔;纳米粒子的熔化是从表面层开始的,并随着温度的升高,熔化的表面层会逐渐向内部扩展,最终导致纳米粒子整体转变为液态结构;当温度低于表面预熔温度时,纳米粒子保持良好的晶态结构。     

Au-Pd共晶纳米粒子熔化行为的分子动力学研究

采用分子动力学方法结合嵌入原子势, 对Au-Pd共晶纳米粒子的热稳定性进行了模拟研究. 计算结果表明: Au-Pd纳米粒子的熔点明显高于Au单质纳米粒子而低于Pd纳米粒子. 通过计算Lindemann指数发现Au-Pd共晶纳米粒子中的Au原子首先熔化, 然后带动Pd原子的熔化; 熔化所经历的温度区间明显要宽于单质纳米粒子.     

Ag309团簇升温过程中结构转变的分子动力学模拟研究

采用分子动力学模拟研究了具有面心立方(fcc)晶格结构的截断八面体Ag309团簇升温过程中结构演变.对团簇的能量曲线变化、快照图演变和键对分析表明:无缺陷截断八面体Ag309团簇在410 K时转变为二十面体,在840 K时熔化;不同缺陷诱导二十面体结构转变温度异常变化,沿晶面滑移缺陷使二十面体转变温度升高,沿晶面旋转缺陷使二十面体结构转变温度降低;不同缺陷对团簇键型和势能产生的影响是使二十面体结构转变温度异常变化的主要诱导因素.这种通过缺陷控制团簇结构转变的研究为新型纳米结构的可控制备提供理论基础.     

碳纳米管表面金纳米颗粒的形成与结构转变

利用分子动力学模拟研究了室温下金纳米颗粒在碳纳米管表面的结构和作用能.研究结果表明,金纳米颗粒随着尺寸的增大会发生不同于孤立状态下的结构转变.当原子数小于130时,颗粒属于无序结构;当原子数大于140时,呈现面心立方晶体结构.小金纳米颗粒和碳纳米管结合紧密,相互作用能正比于面对碳纳米管的颗粒表面面积. 关键词： 金纳米颗粒 碳纳米管 分子动力学模拟     

Realizing high magnetic moments in fcc Fe nanoparticles through atomic structure stretch

We describe the realization of a high moment state in fcc Fe nanoparticles through a controlled change in their atomic structure. Embedding Fe nanoparticles in a Cu(1-x)Au(x) matrix causes their atomic structure to switch from bcc to fcc. Extended x-ray absorption fine structure (EXAFS) measurements show that the structure in both the matrix and the Fe nanoparticles expands as the amount of Au in the matrix is increased, with the data indicating a tetragonal stretch in the Fe nanoparticles. The samples were prepared directly from the gas phase by co-deposition, using a gas aggregation source and MBE-type sources respectively for the nanoparticle and matrix materials. The structure change in the Fe nanoparticles is accompanied by a sharp increase in atomic magnetic moment, ultimately to values of ~2.5 ± 0.3 μ(B)/atom .     

New beta(fcc)-cobalt to 210 GPa

We report a new phase transition in cobalt from the magnetic varepsilon(hcp) to a beta(fcc) phase, likely nonmagnetic, at 105 GPa. It occurs martensitically in an extended pressure region between 105 and 150 GPa without any apparent volume change. The fcc phase of Co is in systematic accordance with the 4d and 5d counterparts. The pressure-volume isotherm of beta-Co resembles those of alpha(fcc)-Ni and varepsilon(hcp)-Fe within 1%. The phase diagram of cobalt suggests that the fcc stability increases with increasing occupancy of d-band electrons from Fe to Co to Ni.     

Crystal structure and compressibility of FePt nanoparticles under high pressures and high temperatures

FePt nanoparticles with an average grain size of 4 nm and equiatomic composition of Fe and Pt was studied under high pressures in a diamond anvil cell to investigate its structural stability and compressibility under high compression. The ambient pressure disordered face-centered-cubic (fcc) phase was found to be stable to the highest pressure of 61 GPa (compression of 15%) at room temperature. The compression of Fe₅₀Pt₅₀ nanoparticles is closer to the compression curve for pure Pt and shows lower compressibility than what would be expected for a bulk Fe₅₀Pt₅₀ alloy. The nanoparticle character of Fe₅₀Pt₅₀ sample is maintained to the highest pressure without any observable grain coarsening effects at ambient temperature. Laser heating of disordered fcc phase at 32 GPa to a temperature of 2000 K resulted in a phase transformation to a microcrystalline phase with the distorted fcc structure.     

Structural and Mössbauer studies of aerosol FeCu nanoparticles in a wide composition range

Structure and magnetic state of aerosol FeCu nanoparticles of 10–30 nm size with Cu content of 0.6–92.1 at.% have been examined by X-ray diffraction and Mössbauer spectroscopy. The FeCu particles have been shown to consist of an iron core surrounded by a copper and Fe oxide shell. With increasing Cu content the iron core having a bcc structure is reduced down to its complete disappearance followed by vanishing ferromagnetism of the particles. Within the copper content from 4.9 to 74.3 at.% the bcc and fcc phases coexist, with the fcc phase having a lattice constant close to that of pure copper and the bcc lattice constant being slightly higher than that for pure Fe due to embedding Cu atoms into the Fe lattice. At Fe-rich FeCu samples a presence of two-spin (ferromagnetic and paramagnetic) components of the fcc Fe is also observed. In the case of a thin copper shell there is only the ferromagnetic fcc Fe, whereas with further thickening of the shell both spin states of the fcc Fe appear existing up to a 20% Cu content. For FeCu samples with a higher Cu content they disappear due to oxidation of the copper grains. The Cu-rich samples with Cu content higher 80 at.% have a fcc structure, with the lattice constant being slightly higher than that of copper and they are paramagnetic. A slight increase of the lattice constant is due to the penetration of small iron aggregations into the Cu grains. In contact with air, the FeCu particles become covered with Fe₃O₄ and Cu₂O. Their long-term exposure to ambient conditions leads to further oxidation process of Cu₂O to CuO.     

Structural and magnetic phase formation in nanophase brass—iron electron compounds

Starting with Cu0.65Zn0.35 with an e/a ratio of 1.35 we studied the phase formation in nanophase (Cu_0.65Zn_0.35)_1?x Fe _x alloys in the concentration range 0.1 ≤x ≤0.7 to see the effect of altering the electron concentration. The evolution of bcc phase from the fcc phase as a function of Fe concentration was investigated by Mössbauer spectroscopy and X-ray diffraction. The grain size, lattice parameters, and average hyperfine magnetic field distributions were estimated for the nanophase alloys. The fcc phase was observed to persist up to 40 atomic per cent Fe substitutions, a mixed (fcc + bcc) phase region up to 70 atomic per cent Fe and bcc phase beyond 70 atomic per cent Fe. The magnetic state of the alloys changed from nonmagnetic forx ≤0.3 to magnetically ordered state at room temperature forx ≤0.33, which lies in the fcc phase region. The fcc phase alloys of Fe with non-magnetic metals have very low magnetic transition temperatures. However, in this system the room temperature state is unusually magnetic     

高压下Fe从bcc到hcp结构相变机理的第一性原理计算

采用基于密度泛函理论的第一性原理方法, 分别研究了压力作用下Fe从体心立方 (bcc,α 相) 结构到六角密排(hcp, ε相) 结构相变的两种不同的相变机理: 相变过程中出现亚稳定的面心立方(fcc) 结构(bcc-fcc-hcp) , 以及相变过程中没有出现亚稳定的fcc结构(bcc-hcp) . 计算结果表明: 静水压力条件下, 相变过程中并不会产生亚稳定的fcc结构, 这与最近的原位XRD实验测量结果相一致. 随着压力的增加, fcc-hcp的相变势垒逐渐增加, 压力趋向于阻止Fe从fcc结构到hcp结构的相变. 计算得到了相变过程中原子磁性和结构的详细信息, 分析表明相变过程中涉及复杂的磁性转变, 并且讨论了原子磁性对结构转变影响的物理机理. 此外, 对分子动力学模拟中产生亚稳定的fcc结构的原因也进行了讨论. 关键词： 相变机理 静水压力 第一性原理 铁     

The investigation of multiply twinned L10-type FePt nanoparticles by transmission electron microscopy

Thin films of FePt nanoparticles were prepared by co-deposition of Fe and Pt on to amorphous C films kept at 350°C. As-prepared films were composed of disordered Fe–Pt nanoparticles with a fcc structure, where twinned and multiply twinned Fe–Pt nanoparticles could be identified by transmission electron microscopy (TEM) and electron diffraction. Atomic ordering from fcc to L1₀ structure was followed by in-situ TEM observation during heating up to 750°C. Multiply twinned (fivefold) nanoparticles of the L1₀ FePt were observed for the first time by high-resolution TEM observation. In these nanoparticles the crystallographic c axes of L1₀ structure is oriented parallel to the film plane in each segment. The stability of the fivefold FePt nanoparticles is briefly discussed.     

Structural transitions in small nickel clusters

The processes of a thermal impact on Ni nanoclusters with a radius of up to 0.8 nm have been studied by means of molecular dynamics with the use of a tight-binding potential. The simulation indicates that the structural transition from the initial fcc phase to the icosahedral modification occurs under the influence of temperature. The transition temperature is shifted towards the cluster melting temperature with an increase in the cluster size. A similar behavior has been observed for copper and gold nanoparticles. A conclusion has been drawn that 200–250 atoms is presumably the limiting size of a metallic cluster, below which the initial fcc modification cannot be kept under realistic industrial conditions. The adequacy of the results is checked in the computer experiments with Lennard-Jones nanoparticles. The results for the Lennard-Jones and metallic nanoparticles have been shown to agree with each other.     

Ball milling of Fe40Ni40P20-xSix (x = 6, 10 and 14): production and characterization

Progress in the ball milling amorphization of elemental powders with the overall composition Fe₄₀Ni₄₀P_{20 ? x}Si_x (X = 6, 10 and 14) and thermally induced crystallization of obtained alloys were characterized by differential scanning calorimetry, X-ray diffraction and transmission Mössbauer spectroscopy (TMS). Diffusion of Si into Fe and Ni alloys promotes the formation of the amorphous phase, via previous formation of (Fe, Ni) phosphides. After milling for 32–64 h, most of the powders are amorphous but bcc Fe(Si) crystallites remain (about 5% in volume). TMS results indicate that homogenization of the amorphous phase occurs by interdiffusion of Ni and Fe in Fe(Si,P)-rich and Ni(Si,P)-rich zones respectively. Annealing induces structural relaxation of stresses induced by milling, growth of bcc Fe(Si) crystallites, precipitation of bcc Fe(Si) and fcc Ni–Fe, and minor phases of Ni-rich silicides and (Fe, Ni) phosphides. The main ferromagnetic phase is bcc Fe(Si) for Fe₄₀Ni₄₀P₁₀Si₁₀ powders obtained after milling for 32 h. However, it is fcc Fe–Ni for the same alloy after milling for 64 h. In the later powders, as well as for alloys with x = 6 and 14 milled for 32 h, the fcc Fe–Ni shows the Invar magnetic collapse.     

STUDY OF IRON OVERLAYER ON SULPHUR PASSIVATED GaAs(100) BY SYNCHROTRON RADIATION PHOTOEMISSION

We have studied the interface electronic structures and the chemical reaction of the Fe overlayer deposited on S-passivated GaAs(100). The chemical bond and electronic structure are different from Fe/GaAs, and the reaction between As and Fe is weakened by S atoms. This is beneficial to the magnetism in the interface. In the first stage of deposition, Fe clusters is form near S atoms due to the large electronegativity of S. The S atoms remain at the interface with Fe coverage. Magnetic ordering feature is found at a coverage higher than 0.6 nm. According to the large exchange splitting in valence band spectra, we suggest that Fe phase transition from bcc to fcc occurs with increasing coverage.