793 research outputs found
Thermophysical Properties of Fe-Si and Cu-Pb Melts and Their Effects on Solidification Related Processes
No full textAmong thermophysical properties, the surface/interfacial tension, viscosity, and density/molar volume of liquid alloys are the key properties for the modelling of microstructural evolution during solidification. Therefore, only reliable input data can yield accurate predictions preventing the error propagation in numerical simulations of solidification related processes. To this aim, the thermophysical properties of the Fe-Si and Cu-Pb systems were analysed and the connections with the peculiarities of their mixing behaviours are highlighted. Due to experimental difficulties related to reactivity of metallic melts at high temperatures, the measured data are often unreliable or even lacking. The application of containerless processing techniques either leads to a significant improvement of the accuracy or makes the measurement possible at all. On the other side, accurate model predicted property values could be used to compensate for the missing data; otherwise, the experimental data are useful for the validation of theoretical models. The choice of models is particularly important for the surface, transport, and structural properties of liquid alloys representing the two limiting cases of mixing, i.e., ordered and phase separating alloy systems
Chemical ordering in magic-size Ag-Pd nanoparticles
No full textChemical ordering in magic-size Ag-Pd nanoalloys is studied by means of global optimization searches within an atomistic potential developed on the basis of density functional theory calculations. Ag-rich, intermediate and Pd-rich compositions are considered for fcc truncated octahedral, icosahedral and decahedral geometric structures. Besides a surface enrichment in Ag, we find a significant subsurface enrichment in Pd, which persists to quite high temperatures as verified by Monte Carlo simulations. This subsurface Pd enrichment is stronger in nanoparticles than in bulk systems and is rationalized in terms of the energetics of the inclusion of a single Pd impurity in an Ag host nanoparticle. Our results can be relevant to the understanding of the catalytic activity of Ag-Pd nanoparticles in those reactions in which subsurface sites play a role. This journal is © the Owner Societies 2014
Design of Composites by Infiltration Process: A Case Study of Liquid Ir-Si Alloy/SiC Systems
No full textThe design of processing routes involving the presence of the liquid phase is mainly associated with the knowledge of its surface and transport properties. Despite this need, due to experimental difficulties related to high temperature measurements of metallic melts, for many alloy systems neither thermodynamic nor thermophysical properties data are available. A good example of a system lacking these datasets is the Ir-Si system, although over the last fifty years, the structures and properties of its solid phases have been widely investigated. To compensate the missing data, the Gibbs free energy of mixing of the Ir-Si liquid phase was calculated combining the model predicted values for the enthalpy and entropy of mixing using Miedema's model and the free volume theory, respectively. Subsequently, in the framework of statistical mechanics and thermodynamics, the surface properties were calculated using the quasi-chemical approximation (QCA) for the regular solution, while to obtain the viscosity, the Moelwyn-Hughes (MH) and Terzieff models were applied. Subsequently, the predicted values of the abovementioned thermophysical properties were used to model the non-reactive infiltration isotherm of Ir-Si (eutectic)/SiC system
Multiscale approach for studying melting transitions in CuPt nanoparticles
No full textA multiscale approach, based on the combination of CALPHAD and molecular dynamics (MD) simulation, is applied in order to understand the melting transition taking place in CuPt nanoalloys. We found that in systems containing up to 1000 atoms, the morphology adopted by the nanoparticles causes the icosahedral CuPt to melt at temperatures 100 K below that of the other morphologies, if the chemical composition contains less than 30% of Pt. We show that the solid-to-liquid transition in CuPt nanoparticles of a radius equal to or greater than 3 nm could be studied using classical tools
Au-Ge based alloys for novel High-T Lead Free Solder materials - Fundamentals and applications(
No full textLow melting Au based alloys offer a combination of high corrosion and creep resistance as well as good electrical and thermal conductivity, and they support the capability for fluxless soldering. Despite their high price, Au alloys are of special interest for high temperature solder applications where these characteristics are essential, e.g. special MEMS devices, sensors exposed to aggressive media, biomedical applications or in space technology. The alloying system Au-Ge is characterized by a deep eutectic point with a melting temperature of 360°C, but information on the use of eutectic Au-Ge as solder material is very limited. In this work, the results of a group project within the European COST Action MP0602 "High Temperature Lead Free Solders" on the feasibility of Au-Ge based alloys as high-temperature lead free solder will be summarized. Beside the thermodynamic assessment of a number of important alloying systems (Cu-Ge, Ge-Ni, Au-Ge-Sb, Au-Ge-Si, Au-Ge-Sn) wetting and soldering tests using eutectic Au-Ge alloy and Cu and Ni substrates were performed. Sound joints with a high strength could be produced
Experimental Thermodynamics and Surface Properties of Ag-Cu-Ge Solder/Braze Alloys
No full textAg-based alloys have industrial importance in relation to their use as high-temperature solders in jewellery or braze alloys for thermoelectric modules. Good wetting properties and a tarnish-resistance of Ag-Ge and Ag-Cu-Ge alloys together with appropriate mechanical properties make them good candidates for bonding sterling silver (Ag-7.5Cu, in wt.%). The melting temperature and the heat of melting of Ag-Cu, Ag-Ge and Ag-Cu-Ge eutectic alloys have been measured by differential scanning calorimetry. From a technological point of view, particular attention should be paid to the surface tension, a key property of the joining processes. The aim of this study is to correlate the thermodynamic properties of the Ag-Cu-Ge system and its subsystems with their surface properties and to compare the model predicted property values to the data available in the literature
Micro-bioreactor arrays for controlling cellular environments: Design principles for human embryonic stem cell applications
No full textWe discuss the utilization of micro-bioreactor arrays for controlling cellular environments in studies of factors that regulate the differentiation of human embryonic stem cells. To this end, we have designed a simple and practical system that couples a microfluidic platform with an array of micro-bioreactors, and has the size of a microscope slide [E. Figallo, C. Cannizzaro, S. Gerecht, J.A. Burdick, R. Langer, N. Elvassore, G. Vunjak-Novakovic, Lab Chip 7 (2007) 710-719]. The system allows quantitative studies of cells cultured in monolayers or encapsulated in three-dimensional hydrogels. We review the operating requirements for studies of human embryonic stem cells (hESCs) under steady-state and dynamic conditions, and the related control of the mass transport and hydrodynamic shear. We describe the design and fabrication of the individual bioreactor components, and the criteria for selecting the bioreactor configuration and operating parameters, based on the analysis of the characteristic times and scales of reaction, convection and diffusion. To illustrate the utility of the bioreactor, we present a "case study" of hESC cultivation with detailed experimental methods and representative biological readouts. (C) 2008 Elsevier Inc. All rights reserved
Interfacial reactions in the Sb-Sn/(Cu, Ni) systems: Wetting experiments
No full textInterfacial reactions in the SbeSn/Cu and SbeSn/Ni systems have been investigated by means of wetting experiments. The wetting behaviour of two lead-free alloys, namely, Sb2.5Sn97.5 and Sb14.5Sn85.5 (at.%), in contact with Cu and Ni-substrates has been studied in view of possible applications as high-temperature solders in the electronics industry. The contact angle measurements on Cu and Ni plates were performed by using a sessile drop apparatus. The solder/substrate interface was characterised by the SEM-EDS
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Diffusion-driven instabilities in the BT-GN oscillatory carbonylation reaction network
No full text\ua9 2024 Author(s).This study explores the role of diffusion in creating instabilities in the Bruk Temkin-Gorodsky Novakovic (BT-GN) oscillatory carbonylation reaction network. Stoichiometric network analysis and numerical methods revealed the presence of two destabilizing feedback cycles responsible for these instabilities. Analysis of a spatially uniform system showed that the saddle-node bifurcation can be simulated within the reaction network. The introduction of diffusion results in two types of instabilities: one occurs when a spatially uniform system is already unstable, leading to a reaction-diffusion front; and another involves diffusion-driven instabilities where introducing diffusion destabilizes a stable spatially uniform system. Slower PdI2 diffusion plays a key role in inducing these instabilities. Equations describing conditions for the emergence of the instabilities in both cases were derived
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