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The phase formation of Nb 2 AlC was studied by combinatorial thin film synthesis and ab initio calculations. Thin films with lateral chemical composition gradients were synthesized by DC magnetron sputtering at substrate temperatures of 710–870 °C. The lowest formation temperature for Nb 2 AlC is between 710 and 750 °C. A predominantly single phase Nb 2 AlC region where 99% of the X-ray diffraction intensity originate from Nb 2 AlC was identified. Furthermore, selected area electron diffraction analysis reveals the local formation of single phase Nb 2 AlC. The limited Al solubility in Nb 2 AlC compared with Cr 2 AlC can be understood by comparing the defect formation energy of Al substituting Nb and Cr in Nb 2 AlC and Cr 2 AlC, respectively. This methodology may serve as indicator for the magnitude of the A-element homogeneity range in M n +1 AX n phases. The structural and elastic properties of Nb 2 AlC determined experimentally are in very good agreement with the ab initio calculated data. [ABSTRACT FROM AUTHOR]

The surface oxidation of TiAlN High Power Pulsed Magnetron Sputtering (HPPMS) hard coatings was investigated using a combined electrochemical and electron spectroscopic approach. The reversible and irreversible surface redox reactions as well as the resulting surface electronic properties were investigated by means of Cyclic Voltammetry (CV) and Electrochemical Impedance Analysis (EIS). X-ray Photoelectron Spectroscopy (XPS) of the TiAlN near-surface region as oxidised in air after deposition revealed a duplex surface film consisting of oxynitridic TiAl(O,N) covered by a shallow Al rich oxide layer. Complementary EIS revealed metallic conductivity of this TiAl(O,N) phase. A detailed microscopic model of the electrochemical oxidation process highlighting the importance of O interstitials for the oxidation process is presented. The electrochemical oxidation induced by CV resulted into a transition from metallic to n-type semiconducting properties of the surface oxide film. As disclosed by XPS and XPS sputter profiling, a complex multi-layered film was formed comprising segregated a-(TiO 2 ) x (Al(OH) 3 ) y located on top of a more ordered electrochemically inactive oxide film. Further, an intermediary nitrogen doped but segregated a-(TiO 2 ) x (Al(OH) 3 ) y :N phase was also observed. The results are of high importance for understanding the electrochemical oxidation and corrosion resistance of these hard coatings. [ABSTRACT FROM AUTHOR]

A method to estimate the activation energy for surface diffusion during magnetron sputtering based on experimental metastable phase formation data has been developed. The so obtained activation energies are consistent with diffusion activation barriers determined by ab initio calculations. The here adopted comparative experimental and theoretical research strategy enables the efficient acquisition of activation energy data and constitutes an improved description of surface diffusion processes of metastable materials during magnetron sputtering. [ABSTRACT FROM AUTHOR]

Abstract: Oxygen incorporation in the MAX phases Ti2AlC, V2AlC and Cr2AlC was studied by ab initio calculations. Comparing the calculated energies for oxygen incorporation indicates that oxygen substitutes for carbon in Ti2AlC and V2AlC, but is incorporated interstitially in the Al layer of Cr2AlC, even for carbon-deficient Cr2AlC. To evaluate these predictions, combinatorial d.c. sputtering was used to deposit thin films with different oxygen concentrations. Two phase regions of Cr2AlC and Cr2Al were investigated in order to study oxygen incorporation in carbon-deficient Cr2AlC. X-ray strain analysis data indicate that the a and c lattice parameters increase with oxygen content. These trends are in good agreement with the change in lattice parameters predicted by ab initio calculations and therefore corroborate the notion of interstitial oxygen incorporation in Cr2AlC. A metastable solubility limit for oxygen of 3.5at.% was determined experimentally. This is the first report on interstitial oxygen in a MAX phase and may be of relevance during the initial stages of oxidation. [Copyright &y& Elsevier]

Abstract: X-ray amorphous V2AlC and hexagonal (V,Al)2Cx thin films were synthesized by magnetron sputtering from a compound target with the composition of V2AlC. The crystallization kinetics was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). During continuous heating up to 1200°C, one exothermal peak is observed between 565 and 675°C. XRD data suggest that the DSC peak is associated with the formation of V2AlC. The activation energy of crystallization of V2AlC is ~308kJ/mol based on the Kissinger approach. This value is close to the 287kJ/mol activation energy obtained here for the transformation of magnetron sputtered (V,Al)2Cx thin films to V2AlC. The here reported phase formation temperature of V2AlC is about 800°C lower than during hot pressing of elemental powders. [Copyright &y& Elsevier]

Abstract: Amorphous Cr2AlC thin films were produced by room temperature magnetron sputtering on NaCl substrates with subsequent dissolution of the NaCl. The crystallization kinetics of Cr2AlC was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Two exothermal reactions are observed during DSC up to 1200°C. Comparing lattice parameters obtained from XRD and ab initio calculations it is suggested that the first reaction is associated with the formation of hexagonal (Cr,Al)2Cx, while after the second reaction Cr2AlC is formed. The activation energy for the phase transformations are 426 and 762kJ/mol, respectively. [Copyright &y& Elsevier]

An extension of the rule of mixture (ROM) to the sub unit-cell level is reported. This approach—which was tested for MAX and M n Al4C3+ n phases—provides a reliable estimate of the bulk moduli of single phases with layered crystal structure based on the calculated moduli of the constituents. Based on the ROM, density functional theory combinatorics is developed, reducing the calculation time for elastic properties by some orders of magnitude. This is a significant step towards knowledge-based materials design since elastic properties can be predicted based on an efficient high-throughput methodology. [Copyright &y& Elsevier]

Decomposition of Cr 2 AlC deposited onto a Zr substrate and vacuum-annealed is observed at 800 °C as Al diffuses from the MAX phase into the Zr substrate. A double layer of ZrN and AlN has been predicted by CALPHAD calculations to act as diffusion barrier between the Zr substrate and Cr 2 AlC. Experimental thermal stability investigations corroborate this prediction by confirming that the proposed double layer diffusion barrier coatings suppress the decomposition of Cr 2 AlC for one hour at temperatures of up to 1000 °C. [ABSTRACT FROM AUTHOR]

Highlights: [•] The chemisorption of oxygen on TiAlN was studied at low oxygen partial pressures of 1×10−3 mPa. [•] The temporal evolution of the surface chemistry was investigated by means of time resolved XPS. [•] The process of oxygen chemisorption on TiAlN was simulated by ab initio molecular dynamics. [•] The experimental and theoretical data imply that Ti and not Al determines the oxygen chemisorption process. [ABSTRACT FROM AUTHOR]

Abstract: The energetics of point defects and twins in MC x (x <1, M =Ti, V, Cr, space group ) was studied using density functional theory. Our goal is to contribute towards understanding the underlying atomic mechanisms enabling the Al intercalation into MC x . As the valence electron concentration is increased by substituting Ti in TiC x with V and further with Cr, the energy of formation for C vacancies is decreased. This may be understood based on the electronic structure. Upon increasing the valence electron concentration, the bonding becomes less ionic and more covalent. In covalent crystals, directional bonding may be rearranged and local relaxation is observed upon vacancy creation, while this gives rise to repulsive Coulomb forces in ionic crystals, and hence the energy of formation is expected to decrease as the valence electron concentration of M is increased. The difference between the energy of formation for an Al substitution at a C site and a C vacancy, the migration energy for Al, the point defect ordering energy, and the twin boundary energy may be overcome, for instance, during vapor phase condensation. These results may be of general relevance for the formation of MAX phases (space group P63/mmc) at low temperatures. [Copyright &y& Elsevier]

Through the judicious use of "virtual elements" that have zero atomic mass but that are included in the materials balances, it is possible to apply a variety of constraints to chemical equilibrium calculations without the necessity of writing dedicated software for each individual application. Several examples are presented, including the suppression of decomposition of metastable molecules and ions or redox reactions in aqueous solutions, the suppression of internal equilibria in molten salts and ceramics, the calculation of the surface tension of solutions, following the course of reactions with time, paraequilibrium calculations and limiting the extent of a reaction. • Constrained chemical equilibrium calculations through use of virtual elements. • Suppression of internal sub-lattice equilibria in chemical equilibrium calculations. • Suppression of redox equilibria in chemical equilibrium calculations. • Applying equilibrium constraints with FactSage software. [ABSTRACT FROM AUTHOR]

The effect of Si additions on the oxidation behavior of Cr 2 AlC based coatings is investigated. Oxidation experiment was performed at 1120 °C in air for 4 h for Cr 2 AlC and Cr 2 Al 1 − x Si x C (0 < x ≤ 0.06) coatings. The crystal structure, microstructure and chemical composition of the as-deposited as well as oxidized coatings have been investigated. Alloying Cr 2 AlC with up to 0.7 at.% Si causes an increase in Al 2 O 3 scale thickness by up to 40 ± 17%. Electron microscopy and atom probe tomography data support the notion that the here reported Si concentration induced 40% increase in Al 2 O 3 layer thickness (during oxidation at 1120 °C for 4 h) is enabled by the Si concentration induced, and hence concomitant, increase in nucleation density of Al 2 O 3 . [ABSTRACT FROM AUTHOR]

• Thermodynamic modeling of an entire reciprocal system using a single subregular model for the liquid phase. • Thermodynamic modeling of the liquid solution spanning from water to liquid acid and base, and liquid salt as a single phase. • Excellent representations of the phase diagrams and properties of the liquid phase in all binary sub-systems. Over the last few decades, thermochemical databases for aqueous solutions and for high-temperature salt solutions have been developed separately from each other. This is largely due to the fact that the models used for the liquid phases are inconsistent. However, there is interest in highly concentrated aqueous solutions, as well as in the hydrolysis of salt melts, closing the gap between the two communities. Here, the liquid phase was consistently modeled, covering the complete composition space between H 2 O, HCl, KOH, and KCl at temperatures between 200 K and 1,000 K. Thermodynamic modeling of phase equilibria in the KCl–K 2 O–HCl–H 2 O reciprocal system was carried out using an associate solution model for the liquid phase, spanning from water to pure liquid acid and base, and liquid salt. Using the Calphad technique, the H 2 O–KCl, H 2 O–HCl, H 2 O–K 2 O, and KOH–KCl binary systems were critically assessed. While the model does not cover classical aqueous thermochemical properties (pH and osmotic coefficient), an excellent representation of phase diagrams and heat of reactions was achieved. [ABSTRACT FROM AUTHOR]

Vanadium oxides were added to the existing GTOx database because of its important role in the ferro-titanium metallurgy as well as in petroleum coke gasification. The CaO–FeO–Fe 2 O 3 –MgO–SiO 2 system is a part of GTOx database and is chosen because of the existence of experimentally determined vanadium distributions between vanadium ferroalloys and metallurgical slags. The CaO–FeO–Fe 2 O 3 –MgO–SiO 2 –V 2 O 3 –V 2 O 5 system including all binary and ternary sub-systems was thermodynamically assessed using all available experimental data. Vanadium was introduced into the thermodynamic description of solid solution phases such as MeO, Spinel, Corundum and Ca 2 SiO 4 -α using available experimental information. Particular attention was given to the phase Spinel which forms the wide completely miscible solid solution Fe 3 O 4 –FeV 2 O 4 –MgV 2 O 4 –MgFe 2 O 4. The thermodynamic description of the metallic vanadium containing systems was taken from the SGTE alloy database. With the knowledge about the complex behavior of vanadium in slags as well as in liquid metal alloys it is possible to calculate the vanadium distribution ratios (V)/[V] between iron melts and slags in a wide range of temperatures and compositions, corroborating the validity of the model under reducing conditions. The additionally developed viscosity model shows good agreement between the calculated and experimental values in vanadium-containing slags. [ABSTRACT FROM AUTHOR]

• Zinc condensation behavior is investigated under gasification conditions. • Influence of the (HCl/H 2 S) gas mixtures on zinc condensation behavior is studied. • Scheil-Gulliver cooling model provides a good reference for condensation analysis. • The mechanisms for zinc condensation processes in gas mixtures are proposed. • Potential applications in IGCC power plants and future directions are discussed. Fuels used in combustion and gasification, such as coal, biomass and wastes, yield large amounts of trace elements, which can cause both technological and environmental concerns. This work provides an in-depth insight into the condensation behavior of the trace element zinc under gasification-like conditions in atmospheres containing the HCl and H 2 S trace gases. A lab-scale quartz reactor with a multi-stage cooling zone was used to determine the condensation content and species distribution of the zinc deposition in different gasification atmospheres. The Scheil-Gulliver cooling model was used to simulate the zinc condensation process, since it provides a good reference to analyze the degree of supercooling during the condensation process. Competition of the gaseous species HCl and H 2 S with respect to the ZnO condensation behavior has been observed. HCl leads to significant supercooling of the ZnO condensation. It is shown that this can be compensated by ZnS acting as nucleation sites for ZnO if significant amounts of H 2 S are present. It is further shown that there is a significant bypass effect, i.e. even after nucleation has started there is a significant amount of Zn remaining in the gas phase which significantly extends the condensation regime to lower temperatures. To visualize both effects, a H 2 S-temperature-transition diagram is proposed. The potential applications including the prevention of problematic depositions (slagging and fouling) and sorbent selection as well as design for removal of trace element zinc from the syngas in IGCC power plants are proposed and discussed in the light of developing clean power technologies. [ABSTRACT FROM AUTHOR]