Pavel Brož

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.

Microstructural and phase equilibria study in the Ni-Al-Cr-W system at 1173 and 1273 K

Abstract Several alloys of the Ni–Al–Cr–W quaternary system were prepared and their microstructure was studied quantitatively after long-term annealing at 1173 and 1273 K using scanning and transmission electron microscopy with energy-dispersive X-ray analysis. Thermodynamic modelling of the system was done by means of the Thermo-Calc software package. The thermodynamic parameters for the calculations were taken from commercial database for Ni-base superalloys. Two sections of quaternary phase diagrams at 1173 and 1273 K with constant Ni level of 71 and 76 at.% were discussed. Based on the comparison of our new experimental data with the results of thermodynamic modelling, a necessity of adjustment of some thermodynamic parameters was stated. Two of the interaction parameters describing the Ni–Al–W ternary subsystem were adjusted and better agreement of calculations with experiments was attained.

Theoretical and experimental study of the γ and γ' equilibrium in Ni-based superalloys

Abstract Ni-based superalloys with aluminium belong to systems with order–disorder transition for the γ and γ′ structures. Because of the complexity of these alloys, quaternary systems can be taken as suitable and sufficiently representative model systems for a theoretical study of their phase equilibria. In recent years several assessments including various model descriptions of ordered phases and ordering based on the CALPHAD formalism have been made with the aim to describe phase diagrams of relevant binary, ternary and some quaternary systems. For our study, the Ni–Al–Cr–Co system was chosen and phase boundaries of γ and γ′ phase equilibrium were calculated based on the data for lower-order systems. The calculations were performed by means of the software ‘Thermo-Calc’. Values obtained were compared with experimentally determined phase information for five alloys with constant Ni content (70 at.%) after long-term annealing at 900°C. The experiments were realized by means of energy-dispercive microanalysis in scanning transmission electron microscopy. For better agreement of theory with experiment, an thermodynamic optimization was performed.

Synthesis, characterization and biological activity of two nickel(II) complexes with 6-(2-chlorobenzylamino)purine

Compounds of composition [Ni(L)(H2O)2Cl] and [Ni(L)(H2O)(NO3)] · EtOH [HL = 6-(2-chlorobenzylamino)purine] have been synthesized and characterized by elemental analyses, i.r. and electronic spectra, magnetic measurements and mass spectroscopy as tetrahedral nickel(II) complexes. The geometry of both complexes has been optimized using molecular mechanics modelling. Although the monoanionic ligand L is potentially bidentate, we assume that the coordination to nickel is via the N9 atom only. Cytokinin and anti-cancer activities of the complexes were also tested in an Amaranthus cytokinin bioassay and in an in vitro MTT-based cytotoxicity assay, respectively. In human T-lymphoblastic leukemia cell line CCRF-CEM both complexes showed potent cytotoxic activity.

The system Ce–Zn–Si for <33.3 at.% Ce: phase relations, crystal structures and physical properties

Phase equilibria of the system Ce–Zn–Si have been determined for the isothermal section at 600 °C for <33.3 at.% Ce by XRPD and EPMA. This partial section is characterized by the formation of five ternary compounds with homogeneity regions at constant Ce-content and partial substitution of Zn/Si: τ1-Ce7Zn21(Zn1−xSix)2 (unique type; 0.45 ≤ x ≤ 0.99), τ2-Ce(Si1−xZnx)2 (AlB2-type; 0.36 ≤ x ≤ 0.73), τ5-CeZn(Zn1−xSix)2 (CeNiSi2-type; 0.68 ≤ x ≤ 0.76), τ6-CeZn2(Si1−xZnx)2 (ThCr2Si2-type; 0.25 ≤ x ≤ 0.30) and τ7-Ce37Zn48Si15 (structure unknown). Whereas τ1, τ2 and τ5 are stable at 600 and 800 °C, the phases τ6, τ7 are unstable at 800 °C. Atom site distribution in the crystal structures of τ5, τ6 and {La,Ce}7Zn21(Zn1−xGex)2 have been elucidated from X-ray intensity refinements on single crystals. The small amounts of the stabilizing tetrel element in {La,Ce}7Zn21[Zn1−xSi(Ge)x]2 suggest a hypothetical binary phase “{La,Ce}7Zn23”. The stabilizing effect of Ge in Ce7Zn23−xGex has been elucidated from density functional theory (DFT) calculations discussing the electronic structure in terms of the density of states (DOS) and defining enthalpies of formation for Ce7Zn23−xGex (x = 0, 0.5, 2) as well as for several neighbouring binary Ce–Zn phases. A Schultz–Scheil diagram for the solidification behaviour in the (Zn,Si)-rich part of the diagram was constructed from DTA measurements in closed silica crucibles along with partial isothermal sections determined in the temperature range from 400 to 900 °C. The phases τ5 and τ6 both form in degenerate ternary peritectic reactions: L + CeSi2,β-Ce2Zn17 ⇔ τ5 at 865 ± 5 °C and L + τ5,CeZn11 ⇔ τ6 at 695 ± 5 °C, respectively. Magnetic susceptibility, specific heat and resistivity measurements of τ5-CeZn(Zn1−xSix)2 revealed Kondo lattice behavior with ferromagnetic ordering below TC = 4.4 K, whereas susceptibility and specific heat studies of τ6-CeZn2(Zn0.28Si0.72)2 revealed Curie–Weiss paramagnetic behaviour down to 3 K. The effective paramagnetic moments of Ce obtained from Curie–Weiss fits of τ5 (2.50μB) and τ6 (2.34μB) reveal a ground state close to trivalent Ce.

Experimental study and thermodynamic assessment of the Ni-Al-Cr-Mo system at 1173 K

The Ni-Al-Cr-Mo system was studied experimentally and theoretically at 1173 K. Reasonable agreement between theory and experiment was stated.

Saturation effects occurring in oxidation of Fe, (Fe–C) and (Fe–3%Cr) alloys

Abstract The following oxidation experiments and their analysis are reported: (i) Time dependence of the oxidation of Fe-wires in ambient air at 1100°C; (ii) Time dependence of the oxidation of (Fe–C) alloy in purified O 2 at 500°C; and (iii) Time dependence of the oxidation of (Fe–Cr) alloy in dry O 2 at 800°C. All the measured time dependences reveal saturation effect. A new model for the description and evaluation of experimental data is proposed. It expresses the fact that the high-temperature oxidation of thin wires and strips follows an exponential law.

Mechanical and electrical properties of ternary Ag-Bi-Ga system at 250 °C

This paper presents a comparative review of the experimental and thermodynamic assessment of a ternary Ag-Bi-Ga system. An isothermal section at 250 °C was calculated using optimized thermodynamic data for the constitutive binaries. Microstructures and phase compositions of studied alloys were analyzed by scanning electron microscopy in combination with energy dispersive spectrometry and X-ray powder diffraction technique. The obtained experimental results were found to support the predicted phase equilibria rather well. The hardness of alloys from three vertical sections (Bi-AgGa, Ag-BiGa, and Ga-AgBi) was determined using Brinell hardness test while the hardness of the individual identified phases was determined using Vickers microhardness test. Additional electrical conductivity measurements were carried out on the same alloy samples. Based on the experimentally obtained results iso-lines of Brinell hardness and electrical conductivity for the entire compositional range were calculated.

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.

Determination of Polychlorinated Biphenyls by Means of Electrochemical Methods

Determination of polychlorinated biphenyls (PCBs) in a technical mixture — DELOR 103 — by means of several electroanalytical methods as polarography (DC), alternating polarography (AC), fast differential pulse voltammetry (FDPV) and cyclic voltammetry (CV) in nonaqueous media is described. Direct electroreductive PCB determination in aqueous media is not possible in the available potential range but it is possible by a tensammetric determination using adsorptive properties of PCB in buffered neutral water-methanol solutions. The possibilities of adsorptive electrochemical determination of PCBs and the effects of experimental conditions have been studied by adsorptive cyclic voltammetry (AdCV) and adsorptive alternating voltammetry (AdAV).