Modeling the thermodynamic properties of bimetallic nanosolids

A model has been developed to account for size, shape, surface segregation, composition and dimension dependent cohesive energy of bimetallic nanosolids, and further been extended to predict the size dependent thermodynamic properties, such as melting temperature, Curie temperatures, ordering temperature and phase diagram. The cohesive energy, melting temperature, Curie temperatures and ordering temperature of bimetallic nanosolids decrease with decreasing the particle size. The depression is dramatic in the lower range of size, while it becomes smoothly in large size. For nano phase diagram, the solidus and liquidus curves drop and the two-phase zones become small, as the size of the nanosolids decreases. The two-phase zones of the nano phase are always lower than the regions indicated in the bulk Ag-Pd alloy phase diagram, and they may deteriorate into a curve at a critical size. It is also found that the thermodynamic properties of nanosolids not only depend on the compositions, the atomic diameter and the cohesive energy of each component, but also depend on the size and the shape. The model predictions are consistent with the corresponding simulation, semi-empirical model and experimental data.     

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。     

Ag-Cu双金属团簇中Ag原子偏析行为的 分子动力学研究

采用了恒温分子动力学方法系统模拟研究了不同尺寸不同组分的Ag-Cu双金属团簇的退火过程。分析低温退火结构可得团簇中Ag原子的偏析行为：在Ag原子所占比率较少时,Ag原子全部占据在团簇表面;随着Ag原子数的增多,直到Ag和Cu的比率接近时,绝大多数Ag原子仍占据在团簇表面;这与实验观测Ag-Cu混合团簇中Ag原子的偏析行为完全一致。通过细致研究Ag-Cu双金属团簇中Ag原子的偏析行为与团簇尺寸、组分和温度的关系发现：当Ag原子所占比重明显少于Cu原子时,在各不同尺寸下,Ag原子偏析温度点均高于熔点,即在熔点以上一定温度范围内仍会出现Ag原子的偏析现象;当Ag原子所占比重明显多于Cu原子时,在各不同尺寸下,体系偏析温度点均低于团簇熔点;对于较大尺寸团簇,在Ag原子所占比重接近于Cu原子时,体系偏析温度点与团簇熔点相同。     

Surface composition and oxygen chemisorption on Ag-Pd alloys

Ag-Pd alloys prepared by room temperature coprecipitation from solutions of the corresponding nitrates have been subjected to surface analysis by Auger electron spectroscopy. Previous work on the adsorption of O₂ on the same alloys showed that except for very high Pd concentration the adsorption heat was independent of alloy composition. On untreated and Ar ion sputtered alloys Pd enrichment in the surface layers was found, while on thermally annealled alloys (<0.1 Ton or 1 atm of H₂ or O₂), surface segregation of Ag took place. This is the expected result from surface tension and/or atomic size considerations. The relation of these findings with the earlier data on the chemisorption heat of oxygen is discussed. The implications of these conclusions upon present models of metallic alloys is pointed out.     

温度对金属纳米线势能分布的影响

采用三维分子动力学模拟方法,以面心立方金属银为研究对象,基于Finnis-Sinclair型嵌入原子法(EAM)多体势,模拟研究了纳米线势能分布特征在常温下及其在不同温度直到熔化过程中的变化,给出了常温及不同温度银纳米线势能分布比例和势能分布函数.结果表明:常温下,纳米线高势能原子比例随纳米线横截面尺寸的减小而增大,势能分布函数曲线各峰位几乎与纳米线横截面尺寸无关;纳米线熔化前的势能分布函数曲线具有多个波峰,随着温度增加,峰数减少且峰位右移;熔化后,多峰特征消失,只有一个宽化的峰.     

Melting Behaviour of Core-Shell Structured Ag-Rh Bimetallic Clusters

The melting behaviour of four typical core-shell structured 309-atom Ag-Rh bimetallic clusters, with decahedral and icosahedral geometric configurations, is investigated by using molecular dynamics simulation, based on the Sutton-Chen potential. The initial atomic configurations are obtained from semi-grand canonical ensemble Monte Carlo simulations. It is found that the melting point temperature Tm increases with the mole fraction of Rh in the bimetallic clusters, and Tm of Ag-Rh icosahedral clusters is higher than those of Ag-Rh decahedral clusters with the same Rh mole fraction. It is also found that the Ag atoms lie on the surface of Ag-Rh bimetallic clusters even after melting.     

Surface segregation of the metal impurity to the (100) surface of fcc metals

The surface segregation energies for a single metal impurity to the (100) surface of nine fcc metals (Cu, Ag, Au, Ni, Pd, Pt, Rh, Al and Ir) have been calculated using the MAEAM and molecular dynamics (MD) simulation. The results show that the effect of the surface is down to the fourth-layer and an oscillatory or monotonic damping (|E ₁| > |E ₂| > |E ₃| > |E ₄|) phenomenon in segregation energy has been obtained. The absolute value of the segregation energy E ₁ for a single impurity in the first atomic layer is much higher than that in the nether layers. Thus, whether the surface segregation will work or not is mainly determined by E ₁ which is in good relation to the differences in surface energy between the impurity and host crystals ΔQ = Q _imp − Q _hos. So we conclude that an impurity with lower surface energy will segregate to the surface of the host with higher surface energy.      

Empirical correlation between melting temperature and cohesive energy of binary Laves phases

A list of 143 binary Laves phases with their melting temperature and melting type is collected, and used to study a correlation between melting temperature and cohesive energy. It is found that the melting temperature of Laves phases is roughly proportional to its cohesive energy calculated by Miedema's empirical model from their intrinsic atomic properties. The average predicted error of melting temperature of compounds is as low as 8.0%. This empirical rule is consistent with the result of the universal binding energy theory of solids.     

银纳米团簇负热容现象的分子动力学模拟

采用分子动力学方法和嵌入原子法(EAM)多体势函数,模拟研究了银纳米团簇在不同温度直到熔化过程中的结构变化,得到了体系能量和热容量随温度的变化关系.结果显示:银纳米团簇在临近熔点附近出现了负热容现象.研究了弛豫后银纳米团簇的稳态结构变化及其在不同时刻结构的演变过程.结果表明:产生负热容现象的主要原因是纳米团簇在熔点附近,结构发生了巨大的变化,形成由{111}和{100}面围成的结构十分稳定和能量更低的多面体.     

Reversed surface segregation in palladium-silver alloys due to hydrogen adsorption

It is well known that silver segregates to the surface of pure and ideal Pd–Ag alloy surfaces. By first-principles band-structure calculations it is shown in this paper how this may be changed when hydrogen is adsorbed on a Pd–Ag(1 1 1) surface. Due to hydrogen binding more strongly to palladium than to silver, there is a clear energy gain from a reversal of the surface segregation. Hydrogen-induced segregation may provide a fundamental explanation for the hydrogen or reducing treatments that are required to activate hydrogen-selective membrane or catalyst performance.     

The physical process of melting and freezing

It is suggested that at the melting temperature the wavelength of the average thermal phonon vibration is equal to or is a harmonic of the distance separating the atomic layers in the crystal. This resonance between the phonon and lattice vibrations equals out the energy of the vibrating atoms in the surface layer. If this “uniform” energy is higher than the energy corresponding to the metastable transition state then all the surface atoms lose its position stability. In order to make the jump to the next potential well energy is required to overcome on the viscous resistance of the liquid. If this energy, latent heat of fusion, is supplied then the atomic/molecular sheet or platelets from the surface are detached and melting occurs. The proposed model is consistent with all of the characteristic features of melting and freezing. Equations calculating the average phonon wavelength and the corresponding lattice distance at the melting temperature are derived from fundamental thermodynamic relationships. The required thermodynamic parameters are determined from experiments of the nine selected highly symmetrical solids. The calculated wavelengths of the phonon vibration are equal to or is a harmonic of the $d$-spacing of the atomic/molecular layers in agreement with theory.     

小Cu团簇熔化过程中的预熔化行为

用分子动力学方法研究了不同大小的Cu原子团簇(Cu13,Cu38,Cu55)的熔化行为.发现对于小的团簇(Cu13),在接近熔化温度时,其内部存在着一种特殊的预熔化行为:内部原子的相对扭转,这种扭转使得Cu13团簇在保持基态结构不变的同时,其内部的原子又具有一定的流动性,对于较大的团簇(Cu38,Cu55)则不存在这种现象.     

Ionoluminescence in the presence of ionizing radiation

An increase in the ionoluminescence intensity from a ZnS-CdS:Ag sample by a factor of more than 20 upon additional electronic excitation by UV light has been found. The effect decreases in magnitude with an increase in the energy of bombarding H₂⁺ ions and peaks at an energy below 175 eV. This effect manifests itself in samples having a system of shallow electron traps and is due to the relaxation of highly excited anharmonic atomic vibrations, caused by the ion impact, through the electronic channel. A mechanism is proposed and computed for the phenomenon revealed.     

Effect of element Re on the grain boundary cohesion of α-Fe

The effect of Re segregation on the α-Fe ∑5 [001] （010） grain boundary （GB） is investigated by using a software called DMol and discrete variational method （DVM）. Based on the Rice Wang model, the calculated segregation energy and defect formation energy show that Re is a strong cohesive enhancer. We also calculated the interatomic energy （IE） and bond order （BO） of several atomic pairs to investigate the mechanism of the cohesive effect of Re microscopically and locally. The results show that IEs of atomic pairs formed by those atoms which cross the plane of GB are strengthened due to the segregation of Re, while the BOs of the corresponding pairs are slightly decreased. This discrepancy demonstrates that IE which contains the Hamiltoniaa of interaction between atoms is a good quantity to describe the bonding strength. The analysis suggests that the electronic effect between atomic pair which comes directly from Hamiltonian is the key factor, The charge density is also presented, and the result indicates that the bonding strength between the Fe atoms on the GB is enhanced due to the segregation of Re, which is consistent with the analysis of IE.     

Prediction of nanoparticles’ size-dependent melting temperature using mean coordination number concept

Many models have been developed to predict size-dependent melting temperature of nanoparticles. A new model based on the cluster mean coordination number (MCN) calculations is developed in this work. Results of the model for Al, Au, Pb, Ag, Cu, In, Sn, and Bi were compared with other models and experiments. The comparison indicated that the MCN model is in good agreement with available experimental values. It is also found that the melting temperature is more dependent on particle size as the atomic radius increased.     

激光照射金颗粒在散射效应下的相变传热

采用二维双温度模型结合散射作用所引起颗粒表面光强不均匀分布的结果,对单脉冲激光垂直照射金颗粒的相变传热进行研究,通过将界面能量平衡方程,成核动力学的界面追踪法相耦合来确定固液界面的位置并研究激光参数对烧结过程的影响.结果表明：当激光垂直照射金颗粒时,熔化现象主要发生在颗粒的两极且底部熔化开始时间早,熔化体积也比较小.激光的脉宽越短熔化开始时间越早,熔化体积越大.提高激光的能量密度,颗粒的熔化体积也随之增加.     

He,Au离子辐照AuCu_3致元素表面偏析

采用磁控溅射方法在单晶硅(111)衬底上制备了AuCu_3薄膜,用2 MeV He离子和1 MeV Au离子对薄膜进行辐照,用卢瑟福背散射对He,Au离子辐照前后AuCu_3薄膜近表面的成分变化进行了分析,对不同离子辐照导致的表面元素偏析行为进行了研究.结果表明:当2 MeV He离子辐照时,随着辐照剂量增大,观察到样品近表面Au元素偏析的趋势;当1 MeV Au离子辐照时,随着辐照剂量增大,观察到样品近表面Cu元素偏析的趋势,与He离子辐照相反.通过对He,Au离子在样品中产生的靶原子空位及其分布分析,发现靶原子空位浓度分布的梯度是导致两种不同表面元素偏析趋势的原因,空位扩散是其中的主要机理.     

The Melting Temperature of 99.97 wt.% Al Estimated from Activation Energy for Viscous Flow

The viscous flow phenomenon in molten aluminium and the viscosity, in the vicinity of its melting temperature, was analyzed. When the temperature decreases, both the viscosity and the activation energy for viscous flow increase. This is explained on the basis of the atomic rearrangement due to interatomic interactions prior to the separation of the solid phase. The latter was found to be nonlinear as the temperature is decreased wherein the solid phase is separated from the liquid. The second derivative of the activation energy for viscous flow with respect to temperature was found to show discontinuity in the vicinity of the melting temperature. Thus, the second derivative offers a way of estimating the melting temperature of metals. Our estimations indicate that the break of the second derivative vs. temperature could be observed at 938 K.