Melting Temperature Of Particles Research Articles

Structure and properties of additive products manufactured from electroerosion powders

The paper considers the possibility of using electroerosion metal powders based on the Co-Cr alloy with different properties in additive technologies. Use of these powders opens up great opportunities for improving the technical and economic characteristics of products, and will contribute to a significant increase in their reliability, durability, weight reduction, reduction of manufacture and operation costs. The economic efficiency of the use of electroerosion cobaltchromium powders is due to the use of waste and low energy-intensive technology for their production. The aim of this work is studying the technology of powder fusion obtained by electroerosion dispersion of cobaltchromium alloy waste in alcohol, on porosity, microhardness and roughness of additive products. The process of electrodispersion, i.e. grinding of the KKhMS "Cellite" alloy (63 % Co, 27 % Cr, 5 % Mo, 2 % Ni, 2 % Fe) was carried out on an original patented installation. After dispersion of cobalt-chromium alloy waste, its destruction occurred as a result of local exposure to short-term electrical discharges between the electrodes located in the working fluid, with formation of powder particles. Butyl alcohol was used as the working fluid. The fusion of electroerosion cobalt-chromium powders was carried out by plasma on the original installation for layer-by-layer deposition, which allows varying the technological parameters of the process: the nozzle transition speed (mm/min), the distance between the nozzle and the construction zone (mm), and the fusion temperature (°C). Based on a set of experimental studies, it was found that the melting temperature of particles of electroerosion cobaltchromium powder practically does not effect on varying in the elemental and phase composition of samples; at the same time, with an increase of the melting temperature, the pores size and their number decrease, as well as an increase of microhardness with decrease of the height of irregularities and roughness in general are observed. The research was conducted under financial support of the Russian scientific fund. The project number 17-79-20336P.

Melting of micro/nanoparticles considering anisotropy of surface energy

The effect of surface energy on the melting of micro/nanoparticles is studied using the asymptotic method. The asymptotic solution of the dynamic model for micro/nanoparticle melting reveals the dependence of the particle melting temperature on the particle size and the anisotropy of surface energy. Specifically, as the particle radius decreases, the isotropic surface energy reduces the melting temperature and accelerates the interface melting of the particle. Along certain crystal orientations, the anisotropy of surface energy enhances the melting temperature of the micro/nanoparticles, whereas depresses the melting temperature of the micro/nanoparticle along other crystal orientations. The anisotropy of surface energy enhances the melting speed of the micro/nanoparticles along certain crystal orientations, whereas reduces the melting speed of the micro/nanoparticles along other crystal orientations. The result of the asymptotic solution is in good agreement with the experimental data.

Morphological changes and ash fusibility of coal, rice straw and their mixture during CO2 gasification

The gasification behavior of single particle of coal, rice straw, and coal-rice straw mixture were investigated by in-situ heating stage microscope, X-ray diffraction, and scanning electron microscopy. The gasification process of coal particle can be divided into three stages consisting of swelling stage, shrinkage stage, and ash fusion stage, while the swelling stage is not observed for the gasification of rice straw particle. The presence of rice straw particle facilitates the shrinkage of coal particle during the co-gasification process due to the migration and catalytic effect of alkaline components which are abundant in biomass. Further, it was found that the average melting temperature of particles is close to the deformation temperature (DT) of ash sample, implying that the DT may reflect the melting properties of particles in a real gasifier. Finally, the ash fusion mechanisms were explored. It was identified that the formation of high-melting points mullite, quartz and anorthite are responsible for the high ash fusion temperatures of coal (with flow temperature over 1500 °C). The addition of 20 wt% rice straw (with flow temperature of 1133 °C) to coal improves the formation of lower-melting points minerals containing alkaline and alkaline-earth elements, and sulphur in the mixture that facilitates the ash fusion process and induces the slag formation. Thus, the flow temperature of the mixture decreases to 1369 °C. The ash fusion behavior of coal follows the “softening-melting” mechanism, while this behavior of rice straw shows a typical “melting-dissolution” mechanism. The ash fusion behavior of coal-rice straw mixture reveals both mechanisms; however, the “melting-dissolution” mechanism seems to be more characteristic for this mixture.

Optimization of Polyethylene Grade Transitions in Fluidized Bed Reactors with Constraints on the Polymer Sticking Temperature

In gas-phase polyolefin processes, it is important to evaluate the melting temperature of particles since exceeding this temperature may cause particle sticking and aggregation. In this work, a mod...

Thermodynamics of laser plasma removal of micro and nano-particles

Micro-impurity pollution is always one of the key factors affecting the quality and service life of precision devices. Micro-nano impurity particles are difficult to remove by traditional cleaning methods (ultrasonic cleaning, etc.) and low removal efficiency by laser cleaning methods (dry laser cleaning, etc.). The laser plasma shock wave has high pressure and high temperature characteristics, which can remove nano-scaled impurity particles, and has great potential applications. In this work, we mainly study the thermodynamic effect of the laser plasma in the process of removing micro- and nano-particles. In this work, the Al particles on the Si substrate are removed by laser plasma shock wave, and the transformation of the particle state is discussed through the changes of the experimental sample morphology after different pulse effects. The experimental results show as follows With the increase of the pulse number, the micro- and nano-particle residues gradually decrease. At the same time, on the surface of the sample after these particles are removed, it can be found that large particles break up into small particles, and some of the particles will change into smooth spheres when their temperatures reach the melting point. These phenomena are the result of the interaction of the thermodynamic effect of the plasma. In order to study the transformation process of particle state, based on the plasma shock wave propagation theory, the evolution law of pressure characteristic and temperature characteristic of shock wave are obtained. From the evolution law, it can be seen that with the increase of shock wave radius, the pressure and temperature gradually decrease. When the shock wave propagates to the surface of a sample, it can reach the compression threshold and correspondingly the surface temperature arrives at melting temperature of particles, which are consistent with the experimental results. By using the finite element simulation method, the pressure and temperature of laser plasma shock wave acting on particles are studied. The stress distribution and temperature distribution in particles varying with time are obtained. The analysis results are consistent with the experimental results, and therefore the thermodynamic mechanism of plasma on particles is obtained.

Solid-state wetting at the nanoscale: molecular dynamics and surface diffusion approach

The authors simulated spreading of solid copper (Cu) and gold (Au) nanoparticles (5–7 nm size) on the (100) face of the same metal by using molecular dynamics. Then, the results obtained for the copper (nanoparticle)/copper (substrate) system were qualitatively compared to some available experimental data on the spreading of copper microparticles (5–20 μm) on the surface of polycrystalline copper. Before simulating the nanoparticle spreading, the size dependence of the melting temperature was investigated for copper and gold nanoparticles. When temperatures were by 25–35 K lower than the copper particle melting temperature, the observed value 40° of the equilibrium contact angle qualitatively agreed with the range 20–25° that the authors evaluated using an experimental snapshot of a copper microparticle on the copper polycrystalline substrate at temperature by 33 K lower than the bulk copper melting point. However, the characteristic spreading times 0·5–10 ns found for nanoparticles in computer experiments differ from that for copper microparticles (3 h ≈ 105 s) by many orders of magnitude. The characteristic times of spreading for copper and gold nanoparticles were also estimated by using the capillary-induced surface diffusion concept, the similarity theory and an available experimental value of the spreading duration for solid copper microparticles. The theoretical estimations in question satisfactorily agree with the authors’ molecular dynamics results.

Size- and Composition-Dependent Structural Stability of Core–Shell and Alloy Pd–Pt and Au–Ag Nanoparticles

The structural stability and thermodynamic properties of core–shell nanoparticles (NPs) are very important for their applications. In this work, the bond energy (BE) model, originally for cohesive energy of pure NPs, has been generalized to core–shell nanosolids. The core–shell NPs can be divided into three regions: the core, the interface, and the shell. By summation of the atomic contributions of the three regions, the relation for the cohesive energy of core–shell NPs can be derived. Using this model, we discuss the cohesive energy and the phase stability of Pd–Pt and Au–Ag core–shell and alloy NPs. For 6.2 nm PtPd bimetallic NPs, the cohesive energy exhibits a linear relationship with the composition of Pd. Compared with Pt@Pd (core @shell) and PdPt nanoalloy NPs, the Pd@Pt NPs are most unstable. There exists critical composition for the stability of Pt@Pd NPs and PdPt nanoalloy, i.e., 6.2-nm PdPt nanoalloy is more stable than Pt@Pd NPs when the compositing of Pd exceeds 62%. The critical size increases with the increase of the composition of Pt. For Au–Ag bimetallic nanoparticles, the Ag@Au NPs are most unstable compared with Au@Ag and AuAg nanoalloy NPs. There exists critical composition for the stability of Au@Ag and AuAg nanoalloy NPs. The critical size increases with increasing of the composition of Au. We also use the generalized BE model to predict the melting of core shell NPs. For Pt@Pd NPs, the melting temperature of the Pt core increases with the increase of core size. Furthermore, the predicted surface melting temperature of Pt@Pd NPs is 1246 K, while the melting temperature of the Pd shell in Pt@Pd NPs lies in the region between the surface melting temperature and the particle melting temperature of Pd NPs.

Numerical simulation of nanoparticle melting inside a matrix

A theoretical model is proposed for describing the melting of a metal nanoparticle embedded into a solid matrix. The model is based on a thermodynamic approach that takes into account matrix elasticity. The melting process is described for gold nanoparticles embedded in a solid matrix whose elastic modulus is varied in a wide range. Both spherical and ellipsoidal particles are considered. It is shown that particle melting temperature can be both higher and lower than the melting point of a bulk sample depending on the interaction intensity of the solid and liquid particle surfaces with the matrix. An increase in the shear modulus of the matrix causes a rise in the nanoparticle melting temperature, with the effect of the matrix elasticity becoming noticeable at some critical shear modulus. The conditions are revealed at which only a surface layer of a nanoparticle, the thickness of which depends on the particle radius and temperature, is melted.

Nonhomogeneous surface premelting of Au nanoparticles

A reversible nonhomogeneous surface premelting of Au nanoparticles is demonstratedthrough molecular dynamics simulations. With increasing temperature, liquid-like atomsfirst appear at some vertices and edges of surface facets, then small liquid regions grow and,at temperatures close to the particle melting temperature, most of the remainingsolid-like surface atoms reside on {111} planes which are the most stable againstsurface premelting. The appearance of a contiguous liquid layer (complete surfacepremelting) is size dependent and is not observed in very small nanoparticles.

Nanoparticle coalescence and sintering: molecular dynamics simulation

Molecular Dynamics (MD) simulations are employed to better understand coalescence and sintering processes of gold nanoparticles. During coalescence in the liquid phase, the initial neck growth can be well described by the viscous flow model. With initial temperature right below the single particle melting temperature, the initial neck growth is controlled by viscous flow at first and then, by grain boundary diffusion. At initial temperatures well below melting, the sintering process occurs very rapidly, which may be attributed to a formation of liquid-like neck regions. The sintering of two free Au nanoparticles irradiated by a femtosecond laser pulse is also simulated by combining the two-temperature and MD models. It is shown that by increasing laser input energy, nanoparticles can be melted forming a single larger nanoparticle. The effects of multinanoparticle melting, solidification and sintering are also investigated.

Size effects in phase transitions of semiconductor nanoparticles embedded in glass

Phase transitions of copper halide particles have been investigated by optical means. A decrease of particle melting temperature with decrease of size has been found. It is shown that the observed size dependencies of crystallization and melting temperatures can be described with practically the same value of surface tension in the framework of homogeneous nucleation theory and particle melting model in which the idea about thermally activated formation of a liquid layer on the particle surface is used. It has been found that the lowest temperature at which high temperature modification of CuBr (β-phase) can be stabilized due to extra annealing of samples depends weakly on particle size. Decrease of melting temperature prevents stabilization of β-phase for particles having radii less than 5 nm.

Surface-Induced Melting of Small Particles.

A surface effect on the behavior of solid-liquid phase transition is studied by the Landau theory. We calculate the dependence of melting temperature of a small particleon its surface curvature. We show that taking account of the effect of the curvature is essential in understanding the melting process of small particles, particularly when their radii are in the order of nm. The non-linear relationship between a melting point and a reciprocal radius is derived from a difference between the radius-dependence of the “surface-induced melting temperature (Tst)” and that of the “particle melting temperature (Tend).” The calculated results agree satisfactorily with the recent experimental data. We also show that a surface melting state becomes difficult to observe as the particle radius decreases down to a critical value. This is in contrast to the common belief that surface-induced melting becomes more dominant for smaller particles.

Revisiting the structural instability observed in small particles in the electron microscope

We present experimental results of the structural instability of gold clusters in the electron microscope. Observations of surface melting in a dynamic HREM study of small gold particles (2–5 nm) are provided. We argue that the temperature regime of the particle during observation, remains quite high at about 50–100 K below the particle's melting temperature. At this temperature, a further 15–30 eV would be enough to raise the particle's temperature above the melting point. We conclude that the structural instabilities observed are due to a mechanism based on a recrystallization from a highly excited state.

Size effect on the melting temperature of gold particles

Recently, small particles have been shown to exhibit a melting temperature which depends on the particle size. The various possible experimental methods have been compared and measurements of the melting points of small gold particles have been made using a scanning electron-diffraction technique. This method was applied to particles having diameters down to 20 \AA{}. Consideration of the size distribution over an entire sample makes it necessary to carry out a careful analysis of the experimental results in order to deduce the melting temperature of particles having a well-defined diameter. The experimental results are quantitatively in good agreement with two phenomenological models. The first model describes the equilibrium condition for a system formed by a solid particle, a liquid particle having the same mass, and their saturating vapor phase. The second model assumes the preexistence of a liquid layer surrounding the solid particle and describes the equilibrium of such a system in the presence of the vapor phase. In order to permit a better comparison between both models, a new expression for the thermodynamic equilibrium condition has been derived in the present work. In the case of the first model, the agreement was obtained using only the physical constants of massive gold. In applying the second model, however, one is compelled to assume the existence of a liquid layer having a thickness of about 6 \AA{}.