Jiří Sopoušek

Ag-Cu colloid synthesis: bimetallic nanoparticle characterisation and thermal treatment

The Ag-Cu bimetallic colloidal nanoparticles (NPs) were prepared by solvothermal synthesis from metalloorganic precursors in a mixture of organic solvents. The nanoparticles were characterized by dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). The properties of metallic core and organic shell of the nanoparticles were studied by direct inlet probe mass spectrometry (DIP/MS), Knudsen effusion mass spectrometry (KEMS), double-pulse laser-induced breakdown spectroscopy (DPLIBS), and differential scanning calorimetry (DSC). The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used for particle characterization before and after thermal analysis. The experiment yielded results that were for AgCu nanoparticles for the first time. The detected liquidus temperature has been compared with the prediction obtained from calculation of the phase diagram of Ag-Cu nanoalloy. The experimental results show that of near-eutectic composition AgCu nanoparticles possess the fcc crystal lattice. Surprisingly, spinodal decomposition was not observed inside the AgCu nanoparticles at temperatures up to 230°C. The depression of the eutectic AgCu melting point was calculated but not observed. The eutectic AgCu microparticles are formed before melting.

Simulation of dissimilar weld joints of steel P91

Abstract Results of computer simulations of long term service exposure for weldments of the CSN 15 128/P91 and SK3STC/P91 steels are presented and compared with corresponding results of phase and composition experiments. The welded material P91 (EU designation: X10CrMoVNb 9–1) represents progressive chromium steel alloyed with molybdenum, vanadium, carbon, and nitrogen. The CSN 15 128 (13CrMoV 2–5) material is low alloy Cr–Mo–V steel. The SK3STC alloy (12CrMo 10–10) represents the consumable electrode material. The stability of the weldment microstructure is investigated at elevated temperatures (500–700° C). The simulation method is based on the Calphad approach complemented with the theory of multicomponent bulk diffusion, local conditions of phase equilibrium, and the assumption that diffusion is the process that controls the rate of phase transformation. Significant phase profiles, concentration profiles, and phase transformation processes in the diffusion affected zone are simulated, investigated, and compared with experimental results. The potentially deleterious carbon depleted region inside each weld joint is discussed. The method described can be used to predict microstructure instability in weld joints.

Phase changes in superaustenitic steels after long-term annealing

A structural study was performed on the austenitic steels Avesta 254 SMO and Avesta 654 SMO after annealing at 700 C for 500, 3188 and 6170 h. Both Avesta steels initially show an unexpectedly large amount of the Laves phase, followed by a relative slow development of the Sigma phase with equilibrium apparently not yet reached after 3188 h. Thermodynamic calculations confirm that the driving forces for alternative precipitates are very similar thus making it easy to form metastable precipitates that only change very slowly to the equilibrium state. TTT calculations also comnfirm that the Laves phase precipitates earlier than the Sigma phase as the temperature is lowered.

Shear Strength of Copper Joints Prepared by Low Temperature Sintering of Silver Nanoparticles

In this work, mechanical properties of Cu-to-Cu joint samples prepared by low temperature sintering of Ag nanoparticle paste have been investigated. The silver nanopaste was prepared by a controlled thermal decomposition of an organometallic precursor. The as-synthesized Ag particles were spherical, with an average diameter of 8.5 nm. The Cu-to-Cu joint samples were made by placing a small amount of Ag nanopaste between two polished Cu plates and sintering at 150°C, 200°C, 220°C and 350°C in air. A normal load was applied to aid sintering. Mechanical properties were measured by imposing a uniform stress across the sample bond area and measuring the corresponding strain. The application of external load was found to have a positive effect on the material’s mechanical properties. Furthermore, interestingly high values of shear strength were observed.

More sophisticated thermodynamic designs of welds between dissimilar steels

Abstract In the present paper, steels that are mostly used in technical practice as base materials, filler metals and consumable electrodes in the design of dissimilar weld joints are studied. These steels differ mainly in the content of Cr, Mo, V, W and N. It is explained and emphasised in the present paper that the effect of thermodynamics, diffusion and phase transformations on the stability of dissimilar weld joints that are exposed to temperatures of over ∼500°C cannot be underrated during operation. For all the materials under study the carbon activity at 600°C is given as calculated by the CALPHAD method, and a correlation is shown to exist between the carbon activity and the total chromium content. This correlation can be used as a most general alternative in the selection of filler metals or consumable electrodes in the design of dissimilar weld joints. The primary goal of the present paper is, however, to present a more sophisticated alternative based on a mutual comparison of the temperature dependence of the carbon activities of the base alloys, filler metals and consumable electrodes under consideration. This alternative is suitable for routine engineering purposes. Attention is also drawn to the currently most advanced alternative, which enables the prediction of the redistribution of elements and phases after prolonged exposure.

AgCu Bimetallic Nanoparticles under Effect of Low Intensity Ultrasound: The Cell Viability Study In Vitro

The effects of metallic nanoparticles as cytotoxicity or antibacterial activity are widely known. It is also obvious that ultrasound is one of the most widely used therapeutic modalities in medicine. The effect of application of therapeutical ultrasonic field in the presence of metallic nanoparticles AgCu <100 nm modified by phenanthroline or polyvinyl alcohol was examined on human ovarian carcinoma cells A2780. Metallic nanoparticles were characterized by electron microscopy and by measuring of zeta potential. The cell viability was tested by MTT test. The experimental results indicate a significant decrease of cell viability, which was affected by a combined action of ultrasound field and AgCu nanoparticles. The maximum decrease of cells viability was observed for nanoparticles modified by phenanthroline. The effect of metallic nanoparticles on human cell in presence of ultrasound exposure was found—a potential health risk or medical advantage of targeted therapy in the future.

Microstructure of super-austenitic steels after long-term annealing

Abstract Longevity of materials, especially technological parts, is influenced by microstructure development during long-time annealing of such parts during their industrial service. Important relations between the volume fractions of phases and the time of service, especially the precipitation of intermetallics need to be revealed. The present study describes the time dependence of the structure development of two corrosion resistant steels with chromium content of 27 and 32 wt.% and various additions of nickel, molybdenum, manganese, silicon, copper and nitrogen. The microstructure was studied on samples annealed at 700 °C for 500, 3 188 and 6 170 h, revealing the kinetics of the equilibrium process. The homogeneity of samples was checked before heat treatment. The experimental microstructures are compared with the predicted phase composition. The effect of using different databases is also discussed.

Spinodal decomposition of Ag-Cu nanoparticles

Ag-Cu nanoparticles (NPs) of near-eutectic composition were prepared by solvothermal synthesis at 230C from metallo-organic precursors in organic solvent. The nanoparticle size was measured by both dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) method. The size and shape were evaluated by electron microscopy (SEM, TEM and HRTEM), the thermal properties were investigated by differential scanning calorimetry (DSC) and temperature controlled Xray diffraction method (XRD). HRTEM results show that AgCu nanoparticles with face centered cube (fcc) crystal lattice exist as a substitutional solid solution in contrast to bulk AgCu system, which reveals spinodal decomposition under eutectic temperature. The phase diagram respecting size of the AgCu NPs was calculated by the CALPHAD method using surface tension of the bulk Ag-Cu alloy in liquid and solid (fcc) states. The predicted eutectic melting point depression was not experimentally observed when the nanoparticles were melted by DSC. The samples melt at eutectic temperature exactly as bulk AgCu alloy. The microparticles consisting of the eutectic copper and silver rich phases were observed by SEM after melting. A spontaneous separation of copper and silver atoms was observed by temperature controlled X-ray diffraction method (XRD). The temperature treatment of the AgCu NPs showed that a spinodal decomposition and crystallinity changes (size evaluation using Scherrer equation) occur at a narrow range of temperature. The reason for the phenomenon observed can be the metastable state of the nanoparticles but there are also other unknown effects at the nano-scale level that hinder the Cu- and Ag-rich phase separation at low temperatures. The open question is also if the general equilibrium thermodynamics can be used to describe the AgCu NPs system under view.

Carbon and Nitrogen Activities of Materials of Weld Joints

The demand for higher efficiency of energy production in fossil-fired power stations needs the increase of working temperatures up to 625 °C. This limitation is given by creep properties of the materials used for steam turbines and the other parts under long high-temperature exploitation. The most widely used materials for this application remain steels. From thermodynamic points of view, the steels can be thermodynamically treated as Fe-Me-N-C based closed systems. Phase diagrams and temperature dependent carbon and nitrogen activities of the steels can be calculated using CALPHAD approach, which is based on semiempirical thermodynamic model of the multicomponent/multiphase system and integral thermodynamic condition of phase equilibrium. The activities of all elements can be obtained mathematically. In this contribution, the CALPHAD method is applied on an investigation of the heterogeneous welds, for bainitic and martensitic base materials and for electrodes and filler weld alloys. Calculated temperature dependences of carbon and nitrogen activity are very important for a design of dissimilar weld joints. This information is essential, for example, for life-time estimations of the heterogeneous welds under high temperature creep.