Whole Range Of Alloy Compositions Research Articles

Determination of the Dynamic Viscosity of Liquid Alloys from the Phase Diagram

That viscosity and fluidity are equilibrium processes may be established on the basis of the Boltzmann distribution in terms of randomized particles. Specifically, the virtual presence of three classes of particles— with crystal mobility, liquid mobility, and vapor mobility—is assumed. Accordingly, the viscosity and fluidity of solutions—in particular, molten metallic alloys—may be considered in terms of the equilibrium contributions of each component to the total viscosity and fluidity, despite the kinetic interpretation of the usual expressions for these liquid properties. A linear additive equation for the viscosity is only possible for perfect solutions; for present purposes, that corresponds to alloys with unlimited mutual solubility of the components. Alloys with eutectics, chemical compounds, and other singularities in the phase diagram are characterized by viscosity equations that reproduce the shape of the liquidus curve over the whole range of alloy composition at different temperatures. Greater smoothness and closeness of the curves is observed at higher temperatures. These aspects of the temperature dependence of the viscosity may be explained on the basis of randomized particles and the visual cluster model of viscosity, with calculation of the proportions of the clusters that determine the alloy viscosity. Such viscosity is determined from a formula in which the thermal barrier to randomization is the thermal energy RTcr at the liquidus temperature, which is characterized by the temperature of melt crystallization Tcr, analogously to the melting point of the pure materials. On that basis, we propose a method of calculating the alloy viscosity from the phase diagram. From the temperature dependences of the viscosity of the pure components, the alloy viscosity may be determined from the proportion of clusters at any temperature above the liquidus curve and the viscosities of the pure components, taking account of the mole fraction of each component. The result is a trifactorial model of the viscosity of a liquid alloy. In this model, the variable is the thermal barrier RTcr to randomization, which determines the proportion of clusters both for pure materials (at RTcr = RTm) and for alloys. Overall, this model corresponds to the virtual cluster theory of viscosity and is consistent with the concept of randomized particles.

THERMODYNAMICS OF OXYGEN SOLUTIONS IN THE Fe – Ni, Fe – Co AND Ni – Co MELTS

Thermodynamic analysis of oxygen solutions in the Fe – Ni – O, Fe – Co – O, Co – Ni – O melts was determined. Composition of the oxide phase and the value of the equilibrium oxygen concentration in the melt of those systems were fi rst time obtained in a whole range of alloy compositions. In Fe – Ni – O and Fe – Co – O systems with increasing content of nickel and cobalt in the melts the oxide phase contains mainly FeO in a suffi ciently wide range of content of nickel and cobalt. Only when the mole fraction of nickel is close to unity and the mole fraction of cobalt is more than 0.8, content of NiO and CoO increases sharply. Oxide phase of Co – Ni – O system contains CoO and NiO in the entire range of alloy compositions. In the Fe – Ni and Fe – Co systems nickel and cobalt additives in the melt lead to decreased solubility of oxygen due to the attenuation, both nickel and cobalt, oxygen bonds in the melt and thus increases its activity. Further, by increasing the content of nickel and cobalt, the oxygen concentration in the melt increases slowly at fi rst and then quite rapidly. In Co – Ni system nickel additives to cobalt lead to increasing solubility of oxygen in the whole range of alloy compositions due to a substantially greater solubility of oxygen in nickel than in cobalt.

Thermodynamics of oxygen solutions in Fe-Ni, Fe-Co, and Co-Ni melts

By thermodynamic analysis of oxygen solutions in Fe-Ni-O, Fe-Co-O, and Co-Ni-O melts, the composition of the oxide phase is established for the first time. In addition, the equilibrium oxygen concentrations in these melts are determined over the whole range of alloy compositions. In Fe-Ni-O and Fe-Co-O melts, the oxide phase mainly contains FeO over a relatively broad of alloy compositions. Sharp increase in NiO content is only observed when the molar fraction of nickel exceeds one; sharp increase in CoO content is only observed when the molar fraction of cobalt exceeds 0.8. In the Co-Ni-O system, the oxide phase contains both CoO and NiO over the whole range of alloy compositions. In the Fe-Ni system, adding nickel to the melt reduces the solubility of oxygen as a result of weakening of the oxygen bonds in the melt by nickel and consequent increase in oxygen’s activity. With further increase in nickel content in the melt, the oxygen content rises at first slowly and then very sharply. In the Fe-Co system, analogously, adding cobalt to the melt reduces the solubility of oxygen as a result of weakening of the oxygen bonds in the melt and consequent increase in oxygen’s activity. With further increase in cobalt content, the oxygen content rises at first slowly and then relatively rapidly. In the Co-Ni system, adding nickel to cobalt increases the solubility of oxygen over the whole range of alloy compositions, on account of the significantly greater solubility of oxygen in nickel than in cobalt.

Band gap bowing and band offsets in relaxed and strained Si1−xGex alloys by employing a new nonlinear interpolation scheme

We present nonlocal empirical pseudopotential calculations for SiGe alloys employing a novel nonlinear interpolation scheme. Our interpolation scheme is able to correctly model for the first time the band gap bowing observed in relaxed SiGe alloys. The valence-band-edge and conduction-band-edge energies in relaxed Si1−xGex for arbitrary x, which are difficult to obtain by experimental techniques, have been evaluated using pseudopotential calculations. We have also calculated the band energies of pseudomorphic [100]-strained Si1−xGex alloys grown over unstrained Si1−yGey substrates. The energy gaps, valence and conduction band offsets, effective masses, and strain induced splittings in pseudomorphic SiGe layers are calculated for the whole range of alloy compositions x and y.

A multi-color quantum well photodetector for mid- and long-wavelength infrared detection

The authors report a two-color quantum well infrared photodetector at room temperature operating in the mid- and long-wavelength infrared detection. To this purpose, the band alignment is tailored and electronic properties are investigated for the proposed structure based on Ga1−xInxAsySb1−y/GaSb and AlxGa1−xAsySb1−y/GaSb. As accurate knowledge of band offsets is required in device modeling, we have proceeded to theoretical investigations of the band offsets for pseudo-morphically strained and lattice-matched Ga1−xInxAsySb1−y/GaSb and AlxGa1−xAsySb1−y/GaSb heterointerfaces in the whole range of alloy compositions 0 ≤ x, y ≤ 1. The carrier effective masses are deduced from the laws extracted from the k.p strain Hamiltonian laws. For the modeled heterostructure, the dark current of about 10−1 A cm−2 at ambient temperature shows the high performance of this multi-color infrared photodetector around 5 and 12.5 µm wavelengths.

Structural and magnetic ground-state properties ofγ-FeMn alloys fromab initiocalculations

The magnetic properties of fcc-FeMn alloys, especially at the Fe${}_{0.5}$Mn${}_{0.5}$ composition, have been the subject of intense experimental and theoretical investigations for several decades. We carry out an ab initio theoretical study of this system, including simultaneous optimization of structural and magnetic properties, and find that the ground state is the locally relaxed noncollinear $3Q$ antiferromagnetic structure. We also show that the two most frequently used parameterizations of the generalized gradient approximation not only fail to reproduce the equilibrium lattice constant of FeMn alloys, and consequently the magnetic properties, but also internally yield qualitatively different results. For practical studies of these alloys, which currently attract great attention, we propose a set of approximations, which is internally consistent, and brings the equilibrium lattice constant and magnetic properties in good agreement with the experiment in the whole range of alloy compositions.

A study of the growth and CO electrooxidation behaviour of PtRh alloys on Pt{1 0 0} single crystals

The formation of quasi-crystalline PtRh two dimensional films supported on Pt{1 0 0} is described. For the first time, the voltammetry of PtRh{1 0 0} single crystal alloys covering the whole range of alloy composition is reported. Synthesis follows a similar procedure to that described previously for the formation of PtPd alloys supported on Pt{ h k l} but with some important provisos concerning the final annealing step. CO electrooxidation was used as a probe reaction and for certain PtRh{1 0 0} surface alloys, unusually high electrocatalytic activity was observed relative to monometallic Pt and Rh electrodes. The flame annealing of a “PtRhPt sandwich” precursor structure was found to be the best method of forming the PtRh alloy surfaces. For PtRh films annealed under nitrogen, significant phase separation was observed in agreement with previous surface science studies of PtRh adlayers on Pt{1 0 0} annealed in the absence of oxygen. In addition, an excess of Rh in the “sandwich structure” from which the alloy was formed tended to preclude good alloy formation. It is suggested that the protocols for thin film formation described may prove useful in many other important electrocatalytic systems.

The DX Center in GaAsP Alloys

Using Deep Level Transient Spectroscopy (DLTS), we have investigated the properties of the DX center in GaAsP for the whole range of alloy composition x. We have determined the variation of the defect characteristics (thermal ionization energy E i , barrier for electron capture B, and energy level location E DX ) versus x. From the relationship that exists between E i , B and E DX , and between B and Δ, the energy difference between the L band and the bottom of the conduction band, we deduce that electron emission and capture occur from and to the DX center via the L band in the same fashion as in GaAlAs alloys. A good fit of the variation of E i and B versus x is obtained in a model where the DX level is a donor state associated with the L band which is 200 meV deep, like in GaAlAs, as a result of intervalley mixing.

Electronic-band parameters in strained Si1-xGex alloys on Si1-yGey substrates.

A systematic theoretical study of the electronic properties of pseudomorphic (100)-strained ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys grown on unstrained ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Ge}}_{\mathit{y}}$ substrates is presented. Based on nonlocal empirical pseudopotential calculations with spin-orbit interactions, realistic estimates of the conduction- and valence-band-edge energies, higher-energy-band minima, effective masses, deformation potentials, and heterostructure band offsets for the whole range of alloy compositions x and y and strain are presented. The theory predicts that the band edges of weakly stressed Ge fall within the wider gap of the ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Ge}}_{\mathit{y}}$ substrate for 0.7y1 (type-I alignment), in contrast to any Si-rich combination of active layer and substrate.

Raman Spectral Behavior of In1-xGaxP (0

We report on a Raman study of In1-xGaxP over the whole range of alloy compositions, x. The additional peak, which appeared for x\\lesssim0.98 and about which various interpretations have been proposed, is found to be constructed by two unresolved modes. The Raman spectra indicate that this alloy system has a modified two-mode behavior. The spectral half width of a TO mode broadened in the middle of the alloy composition. This is partly due to the strong two-acoustic-phonons scattering process.

Fe - Co - Ni thin films

Binary NiFe, NiCo, and FeCo films and ternary NiFeCo films have been investigated over a large range of alloy composition. Measurements were carried out for the uniaxial anisotropy K u , the structure constant S , and the wall coercive force H w . It is qualitatively shown that the uniaxial anisotropy is mainly caused by directional ordering of unequal pairs of atoms. The model of directional pair ordering of equal pairs, as usually discussed in the literature, appears incorrect. The strongest contribution comes from FeCo pairs, the next from NiCo pairs, while the influence of NiFe pairs seems to be small. Investigation of well prepared ternary films will allow the determination of the six anisotropy constants which are related to the six possibilities of pairing three different sorts of atoms. The dependence of the uniaxial anisotropy as given by the experiment is shown by a plot in the ternary alloy diagram. The relation for the coercive force H_{w} \propto K_{u}^{1/4}D^{1/2}M_{w}^{-1} S was verified over the whole range of alloy composition. A plot of H w in the ternary alloy diagram is also given.

Catalytic hydrogenolysis and dehydrogenation over copper-nickel alloys

The hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene were investigated over a series of copper-nickel alloys. The alloys were characterized by adsorption, X-ray diffraction, and magnetic measurements. The effect of adding copper to nickel was very different for the two reactions investigated. In the case of ethane hydrogenolysis, the catalytic activity decreased markedly and continuously with addition of copper to nickel over the whole range of alloy composition, although much of the activity decline was observed on addition of the first few percent of copper. With cyclohexane dehydrogenation, however, the catalytic activity increased initially with addition of small amounts of copper to nickel, and then remained insensitive to alloy composition over a wide range, finally decreasing sharply at compositions approaching pure copper. The effects of alloying are clearly very specific to the nature of the reaction.