Since the early days of phase diagram calculation, the CALPHAD community has accepted a method of data assessment that is based on using Gibbs energy expressions for stoichiometric compounds that ...consist of a part that represents the contributions of the reference components, the so-called Neumann–Kopp sum, and a part that represents the Gibbs energy of formation. Usually, the latter consists only of a simple A + B *
T
term, which implies on the one hand that enthalpy and entropy of formation of a compound are temperature independent, and on the other, that the heat capacity of the respective compound is given by the Neumann–Kopp sum of the
C
p
s of the constituents. In most cases this method yields acceptable results, also for the heat capacity. However, if certain elementary components are involved, this is not so. The paper highlights the problem outlining a remedial treatment that can be applied to problematic heat capacity functions present in pre-existing assessments, and thus giving a suggestion of how to prevent the problem in the future.
Abstract
The atomic distribution in amorphous FeZr alloys is found to be close to random, nevertheless, the composition can not be viewed as being homogenous at the nm-scale. The spatial variation of ...the local composition is identified as the root of the unusual magnetic properties in amorphous
$$\hbox {Fe}_{1-x}\hbox {Zr}_{x}$$
Fe
1
-
x
Zr
x
alloys. The findings are discussed and generalised with respect to the physical properties of amorphous and crystalline materials.
•Zinc condensation behavior is investigated under gasification conditions.•Influence of the (HCl/H2S) 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 H2S 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 H2S 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 H2S 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 H2S-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.
The temperature-dependent composition and phase formation during the physical vapor deposition (PVD) of Mg-Ca thin films is modeled using a CALPHAD-based approach. Considering the Mg and Ca ...sublimation fluxes calculated based on the vapor pressure obtained by employing thermochemical equilibrium calculations, the experimentally observed synthesis-temperature trends in the thin-film composition and phase formation were reproduced. The model is a significant step towards understanding how synthesis parameters control composition and, therefore, phase formation in the PVD of metals with high vapor pressures.
A method to model the metastable phase formation in the Cu-W system based on the critical surface diffusion distance has been developed. The driver for the formation of a second phase is the critical ...diffusion distance which is dependent on the solubility of W in Cu and on the solubility of Cu in W. Based on comparative theoretical and experimental data, we can describe the relationship between the solubilities and the critical diffusion distances in order to model the metastable phase formation. Metastable phase formation diagrams for Cu-W and Cu-V thin films are predicted and validated by combinatorial magnetron sputtering experiments. The correlative experimental and theoretical research strategy adopted here enables us to efficiently describe the relationship between the solubilities and the critical diffusion distances in order to model the metastable phase formation during magnetron sputtering.
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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.
Decomposition of Cr2AlC 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 Cr2AlC. Experimental thermal stability investigations corroborate this prediction by confirming that the proposed double layer diffusion barrier coatings suppress the decomposition of Cr2AlC for one hour at temperatures of up to 1000 °C.
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–Fe2O3–MgO–SiO2 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–Fe2O3–MgO–SiO2–V2O3–V2O5 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 Ca2SiO4-α using available experimental information. Particular attention was given to the phase Spinel which forms the wide completely miscible solid solution Fe3O4–FeV2O4–MgV2O4–MgFe2O4. 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.
The phase stability of Al-containing cubic transition metal (TM) nitrides, where Al substitutes for TM (i.e. TM
1−
x
Al
x
N), is studied as a function of the TM valence electron concentration (VEC). ...X-ray diffraction and thermal analyses data of magnetron sputtered Ti
1−
x
Al
x
N, V
1−
x
Al
x
N and Cr
1−
x
Al
x
N films indicate increasing phase stability of cubic TM
1−
x
Al
x
N at larger Al contents and higher temperatures with increasing TM VEC. These experimental findings can be understood based on first principle investigations of ternary cubic TM
1−
x
Al
x
N with TM = Sc, Ti, V, Cr, Y, Zr and Nb where the TM VEC and the lattice strain are systematically varied.
However, our experimental data indicate that, in addition to the decomposition energetics (cubic TM
1−
x
Al
x
N → cubic TMN + hexagonal AlN), future stability models have to include nitrogen release as one of the mechanisms that critically determine the overall phase stability of TM
1−
x
Al
x
N.
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-(TiO2)x(Al(OH)3)y located on top of a more ordered electrochemically inactive oxide film. Further, an intermediary nitrogen doped but segregated a-(TiO2)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.
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