Dual structural transition in small nanoparticles of Cu-Au alloy

Cu-Au alloy nanoparticles are known to be widely used in the catalysis of various chemical reactions as it was experimentally defined that in many cases the partial substitution of copper with gold increases catalytic activity. However, providing the reaction capacity of alloy nanoparticles the surface electronic structure strongly depends on their atomic ordering. Therefore, to theoretically determine catalytic properties, one needs to use a most real structural model complying with Cu-Au nanoparticles under various external influences. So, thermal stability limits were studied for the initial L1₂ phase in Cu₃Au nanoalloy clusters up to 8.0 nm and Cu-Au clusters up to 3.0 nm at various degrees of Au atom concentration, with molecular dynamics method using a modified tight-binding TB-SMA potential. Dual structural transition L1₂?→?FCC and further FCC?→?Ih is shown to be possible under the thermal factor in Cu₃Au and Cu-Au clusters with the diameter up to 3.0 nm. The temperature of the structural transition FCC?→?Ih is established to decrease for small particles of Cu-Au alloy under the increase of Au atom concentration. For clusters with this structural transition, the melting point is found to be a linear increasing function of concentration, and for clusters without FCC?→?Ih structural transition, the melting point is a linear decreasing function of Au content. Thus, the article shows that doping Cu nanoclusters with Au atoms allows to control the forming structure as well as the melting point.     

Thermodynamic model for the size-dependent melting of prism-shaped nanoparticles

We report on the size-dependent melting of prism-shaped nanoparticles based on thermodynamic model and applied to understand the melting of prism-shaped indium nanoparticles. It is shown here that the bulk melting temperature cannot be extrapolated from the nanoscale and the extrapolated value will always be lower than the bulk melting temperature as has been observed experimentally.     

Effect of pressure on melting and solidification of metal nanoparticles

A thermodynamic model was developed to clarify the dependence of melting temperature on hydrostatic pressure in the nanoscopic scale. It is based on the classic Clausius-Clapeyron relation and the size dependence of the melting entropy. The melting of nanoparticles in matrix with coherent and incoherent boundaries was also under consideration. It was shown that external hydrostatic pressure leads to the appearance of extrema of the melting temperature that was considered as a function of the characteristic size of nanoparticles.     

Size-dependent melting of nanoparticles: Hundred years of thermodynamic model

Thermodynamic model first published in 1909, is being used extensively to understand the size-dependent melting of nanoparticles. Pawlow deduced an expression for the size-dependent melting temperature of small particles based on the thermodynamic model which was then modified and applied to different nanostructures such as nanowires, prism-shaped nanoparticles, etc. The model has also been modified to understand the melting of supported nanoparticles and superheating of embedded nanoparticles. In this article, we have reviewed the melting behaviour of nanostructures reported in the literature since 1909. This article is dedicated to Indian Institute of Science which is also celebrating its centenary this year.     

Zn-Al二元合金熔化过程的低频内耗研究

利用强迫振动扭摆方法对ZnAl二元合金熔化过程的低频内耗进行了研究.结果表明,ZnAl二元合金熔化过程的内耗峰与其固态相变内耗峰的特征有较大差异.结合该合金熔化过程的微观结构变化,初步分析了内耗峰的形成机理. 关键词： Zn-Al合金 低频内耗 熔化     

Melting point trends in intermetallic alloys

We have examined the melting points of approximately 500 intermetallic binary alloys. We attempted to correlate the melting point behavior of the binary (1:1) alloys with a number of elemental variables including electron number, atomic size, orbital radii, electronegativity, etc. We find that a “Vegards's Law” of melting points works very well for predicting the melting points of binary transition metal alloys, i.e. the melting point of the alloy correlates with the linear average of the elementary melting points. However, this “law” works only moderately well for alloys involving simple metals. In addition, we find that transition metal alloys tend to have melting points below the averaged elemental melting points. This finding is in sharp contrast to simple metal alloys where the opposite trend is observed and it is indicative of fundamental differences between transition metal and simple metal binding. Finally, we have attempted to correlate deviations from a Vegard's law of melting with elemental variables. We found no strong correlation with elemental variables (or the heats of formation of the alloy in question) with the possible exception being a correlation with elemental volume changes upon alloying. The consequences of this correlation upon alloy design and metallic alloy formation are briefly discussed.     

Hydrogen and humidity sensing characteristics of Nafion,Nafion/graphene,and Nafion/carbon nanotube resistivity sensors

Journal of Nanoparticle Research - The melting of micro-/nanoparticles in an alloy melt is investigated by using the asymptotic method. The asymptotic solution of the dynamic model for...     

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.     

Molecular dynamics simulations on the melting of gold nanoparticles

Molecular dynamics is employed to study the melting of bulk gold and gold nanoparticles. PCFF, Sutton-Chen and COMPASS force fields are adopted to study the melting point of bulk gold and we find out that the Sutton-Chen force field is the most accurate model in predicting the melting point of bulk gold. Consequently, the Sutton-Chen force field is applied to study the melting points of spherical gold nanoparticles with different diameters. Variations of diffusion coefficient, potential energy and translational order parameter with temperature are analyzed. The simulated melting points of gold nanoparticles are between 615～1115 K, which are much lower than that of bulk gold (1336 K). As the diameter of gold nanoparticle drops, the melting point also descends. The melting mechanism is also analyzed for gold nanoparticles.     

The effect of matrix on melting and solidification behaviours of embedded Pb-Sn alloy nanoparticles

The present investigation is aimed at understanding the effect of a matrix on the phase transformation of biphasic embedded Pb–Sn alloy nanoparticles. The melting and solidification behaviours of eutectic (Pb_26.1Sn_73.9) nanoparticles embedded in icosahedral (IQC) as well as decagonal quasicrystalline (DQC) matrix have been studied. Electron microscopic observations reveal that the major portion of the alloy nanoparticle consists of body-centred tetragonal β-(Sn) with face-centred cubic (Pb) constituting the cap. (Pb) bears specific orientation relationships (OR) with the surrounding IQC matrix, whereas β-(Sn) does not have any specific OR. For alloy particles embedded in the DQC matrix, both (Pb) and β-(Sn) bear specific OR. In case of IQC matrix, differential scanning calorimetric measurements reveal sharp melting but diffuse solidification peaks for the embedded nanoparticles. On the other hand, sharp melting and solidification peaks are observed for the nanoparticles embedded in the DQC matrix. The IQC and DQC are heat-treated at different temperatures to observe the effect of the matrix on the phase transformation of the alloy nanoparticles. The formation of well- developed facets in the nano-particles and defects in the matrix have been found to play a crucial role in determining the phase transformation of the alloy nanoparticles in the heat-treated samples. The experimental observations are rationalized using available literature.     

Synthesis and characterization of polymer encapsulated Cu–Ni magnetic nanoparticles for hyperthermia applications

Copper nickel alloy nanoparticles were synthesized by polyol reduction method and by physical melting process. The particles were further coated with a biodegradable polymer, polyethylene glycol. The particles have a curie temperature in the range of 43–46 °C and are designed to be used for hyperthermia applications. Morphology of these encapsulated particles was determined by electron microscopy. The curie temperature for alloy particles and encapsulated particles was also measured.     

Effects in the interphase boundary with contact melting in three-component systems

The crystallized, contact melting zone of binary eutectic alloys with a third eutectic component is investigated by an x-ray micrography method. Two layers are formed in the zone; one layer is liquid and the other, adjacent to the alloy, is solid-liquid. The dependence of the thickness of the layers on temperature and contact time is studied.     

Superheating and solid-liquid phase coexistence in nanoparticles with nonmelting surfaces

We present a phenomenological model of melting in nanoparticles with facets that are only partially wet by their liquid phase. We show that in this model, as the solid nanoparticle seeks to avoid coexistence with the liquid, the microcanonical melting temperature can exceed the bulk melting point and that the onset of coexistence is a first-order transition. We show that these results are consistent with molecular dynamics simulations of aluminum nanoparticles which remain solid above the bulk melting temperature.     

Study on the melting of interstitial alloys FeH and FeC with BCC structure under pressure

From the model of interstitial alloy AB with BCC structure and the condition of absolute stability for crystalline state we derive analytic expression for the temperature of absolute stability for crystalline state, the melting temperature and the equation of melting curve of this alloy by the way of applying the statistical moment method. The obtained results allow us to determine the melting temperature of alloy AB at zero pressure and under pressure. In limit cases, we obtain the melting theory of A main metal with BCC structure. The theoretical results are numerically applied for alloys FeH and FeC with using different potentials.     

A study of melting of various types of Pt–Pd nanoparticles

The melting processes of various Pt–Pd nanoparticles (binary alloy, core–shell, D ≤ 4.0 nm) with different percent platinum atom content are investigated via the molecular dynamics using the embedded atom method potential in order to establish the thermal stability of simulated particle structure. In accordance with the data obtained, the most thermally stable are Pt–Pd nanoalloys with a diameter above 2.0 nm and core–shell Pd@Pt particles. As is shown, heating of binary Pt–Pd cluster alloys with the particle diameters less than 2.0 nm may cause the transition to pentagonal symmetry structures and core–shell-like complex formations.

     

The melting mechanism in binary Pd0.25Ni0.75 nanoparticles: molecular dynamics simulations

The melting mechanism for Pd_0.25Ni_0.75 alloy nanoparticles (NPs) was investigated using molecular dynamics (MD) simulations with quantum Sutton-Chen many-body potentials. NPs of six different sizes ranging from 682 to 22,242 atoms were studied to observe the effect of size on the melting point. The melting temperatures of the NPs were estimated by following the changes in both the thermodynamic and structural quantities such as the total energy, heat capacity and Lindemann index. We also used a thermodynamics model to better estimate the melting point and to check the accuracy of MD simulations. We observed that the melting points of the NPs decreased as their sizes decreased. Although the MD simulations for the bulk system yielded higher melting temperatures because of the lack of a seed for the liquid phase, the melting temperatures determined for both the bulk material and the NPs are in good agreement with those predicted from the thermodynamics model. The melting mechanism proceeds in two steps: firstly, a liquid-like shell is formed in the outer regions of the NP with increasing temperature. The thickness of the liquid-like shell increases with increasing temperature until the shell reaches a critical thickness. Then, the entire Pd–Ni NP including core-related solid-like regions melts at once.     

Molecular dynamics study of the role of material properties on nanoparticles formed by rapid expansion of a heated target

Rapid expansion of a heated target and its decomposition into fragments is investigated by using molecular dynamics simulations. Particular attention is focused on the void formation and nucleation that governs the target disintegration. The cluster formation process is investigated as a function of material properties (initial temperature, interaction potential and composition). Calculation results demonstrate the influence of these properties on void nucleation and growth and on the characteristic parameters of nanoparticles to be formed. In particular, larger initial temperature and expansion rate lead to the formation of smaller fragments. These effects are found to be similar for three different materials (silicon, nickel and metal alloy). In addition, the stoichiometrical cluster composition obtained in the expansion of a binary alloy is found to be fairly well preserved. The calculation results can be used for the interpretation of the experimental findings showing the formation of nanoparticles by short and ultra-short pulse laser ablation of both simple and more complex materials.     

Disorder effect on heat capacity,self-diffusion coefficient,and choosing best potential model for melting temperature,in gold–copper bimetallic nanocluster with 55 atoms

Molecular dynamics simulation has been implemented for doping effect on melting temperature, heat capacity, self-diffusion coefficient of gold–copper bimetallic nanostructure with 55 total gold and copper atom numbers and its bulk alloy. Trend of melting temperature for gold–copper bimetallic nanocluster is not same as melting temperature copper–gold bulk alloy. Molecular dynamics simulation of our result regarding bulk melting temperature is consistence with available experimental data. Molecular dynamics simulation shows that melting temperature of gold–copper bimetallic nanocluster increases with copper atom fraction. Semi-empirical potential model and quantum Sutton–Chen potential models do not change melting temperature trend with copper doping of gold–copper bimetallic nanocluster. Self-diffusion coefficient of copper atom is greater than gold atom in gold–copper bimetallic nanocluster. Semi-empirical potential within the tight-binding second moment approximation as new application potential model for melting temperature of gold–copper bulk structure shows better result in comparison with EAM, Sutton–Chen potential, and quantum Sutton–Chen potential models.     

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.