Pressure dependence of energy gap of III–V and II–VI ternary semiconductors

A general expression for the pressure dependence of the energy gap of a series of group III–V and group II–VI ternary semiconductors have been derived based on Van Vechten’s dielectric theory. The results obtained are in good accord with the available experimental data. The trends in the variation of the pressure dependence of the energy gap with the nearest neighbor distance and Phillips ionicity are explored qualitatively.     

The Tl–I phase diagram revisited and the thermodynamic properties of thallium iodides

The Tl–I system has been studied using differential thermal analysis, X-ray diffraction, and emf measurements on TlI concentration cells. A more accurate Tl–I phase diagram is presented, according to which the compounds existing in the Tl–I system are TlI, Tl₂I₃, and TlI₃. Thallium monoiodide melts congruently at 715 K and undergoes a polymorphic transformation at 440 K. The other iodides melt peritectically at 535 and 404 K, respectively. In contrast to what was reported previously, no compound of composition Tl₃I₄ has been obtained. Using experimental emf data, we evaluated relative partial molar thermodynamic functions of the TlI in alloys of the TlI–I system and the standard Gibbs free energy, enthalpy of formation, and standard entropies of TlI₃ (?ΔG ₂₉₈ ⁰ = 142.79 ± 0.73 kJ/mol, ?ΔH ₂₉₈ ⁰ = 135.37 ± 2.85 kJ/mol, and S ₂₉₈ ⁰ = 263.3 ± 7.4 J/(mol K)) and Tl₂I₃ (271.39 ± 1.47, 262.40 ± 5.34, and 322.8 ± 13.2).     

Heat capacity and thermodynamic properties of GaFeO3 in the range 330–900 K

The heat capacity of multiferroic GaFeO₃ has been measured in a wide temperature range and the results have been used to evaluate its thermodynamic functions (enthalpy increment and entropy change).     

Relationship between the structure and mechanical properties in ß III titanium alloy

The ageing reactions that take place during the heat-treatment of solution-treated III titanium (11.5 wt% Mo, 6 wt% Zr, 4.5 wt% Sn, balance Ti) were followed by detailed structure characterization using electron microscopy. The variations in mechanical properties with heat treatment were also followed systematically. The electron microscopy investigations indicated that the omega phase formed on quenching. The size and volume fraction of the omega phase increased on subsequent ageing,, and phases were found to co-exist at ageing temperatures between 800 and 900° F (427 and 482° C) for short ageing times. From the observations of interfacial dislocations at the/ interface and the precipitation of fine alpha near the omega particles, with a morphology that is characteristic of the prior morphology, it is suggested that the-phase forms directly from the omega phase. The observed increase in yield strength over the solution-treated condition, due to the precipitation of phase, was found to agree well with that predicted by the Orowan hardening mechanism. Since the precipitation of fine ellipsoidal-phase was found to increase the yield strength of the alloy with reasonable ductility, it is suggested that the optimum heat treatment to produce high strength with good ductility in III titanium is to age at 900° F (482° C) for 10 to 25 h.     

Density function theory investigation on the thermodynamic properties of the Li–N–H system

The electronic structures and formation enthalpies of compounds in the Li–N–H system have been studied by using the density functional theory. In order to evaluate the competition between each reaction in the system, the chemical potential phase diagrams of compounds in the Li–N–H system have been computed and discussed. Our calculations show that for LiNH₂, Li⁺ combines with [NH₂]^- by an ionic bond. For Li₂NH, the N–H bond displays covalent characteristics. The calculated formation enthalpy of compounds in the Li–N–H system is in agreement with previous results, the LiNH₂ is −212.27 kJ mol⁻¹, LiH is −91.66 kJ mol⁻¹, Li₂NH is −243.14 kJ mol⁻¹, Li₄NH is −309.72 kJ mol⁻¹, Li₃N is −189.11 kJ mol⁻¹, and NH₃ is −102.27 kJ mol⁻¹, respectively. Using the chemical potential phase diagrams, six reversible reactions are discussed. It is found that Li₄NH takes part in the three reversible reactions and some NH₃ formed in the system react with other compounds in the Li–N–H system. These reversible reactions are confirmed by the proposed mechanism from experiments.     

Electrical and humidity sensing properties of lead(II) tungstate–tungsten(VI) oxide and zinc(II) tungstate–tungsten(VI) oxide composites

Lead(II) tungstate and zinc(II) tungstate were prepared by a solution route and sintered at 973 K in the form of cylindrical discs. Experimental results on PbWO₄ (PW) and WO₃ (WO) composites for humidity sensing are described. Sintered polycrystalline discs of PbWO₄ (PWWO-10), WO₃ (PWWO-01), ZnWO₄ (ZWWO-10) and composites of PW or ZW and WO in the mole ratios 8:2, 6:4, 4:6, 2:8 designated as PWWO and ZWWO-82, 64, 46 and 28, respectively and doped with 2 mol% of Li⁺ were studied. The composites were subjected to dc conductance measurements over the temperature range 373–673 K in air atmosphere from which activation energies were determined. The activation energy values for dc conductance were found to be in the range of 1.09–1.30 eV. The composites were identified by powder XRD data. The scanning electron microscopy (SEM) studies were carried out to study the surface and pores structure of the sensor materials. The composites were subjected to dc resistance measurements as a function of relative humidity in the range of 5–98% RH, achieved by different water vapor buffers thermostated at room temperature. The sensitivity factor (S_f=R_5%/R_98%) measured at 298 K revealed that PWWO-28 and ZWWO-46 composites have the highest humidity sensitivity factor of 17 615±3000 and 2666±550, respectively. The response and recovery time for these humidity sensing composites were good.     

Size quantization effects in optical and electrical properties of II–VI semiconductor films in nanocrystalline form

The electrical properties of CdTe and optical properties of ZnS in nanocrystalline thin film form are studied with a view to have a clearer understanding of the optical processes and the carrier transport mechanisms in nanocrystalline II–VI semiconductors, in general. Nanocrystalline ZnS and CdTe films were deposited by magnetron sputtering of respective targets in argon plasma. The optical absorption data of nanocrystalline ZnS films (thickness 10–40 nm) could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites. The conductivity of CdTe (grain size within 4–4·7 nm) showed (T ₀/T) ^p dependence withp ∼ 0·5 indicating the presence of a Coulomb gap near the Fermi level. The width of the Coulomb gap varied within 0·02–0·04 eV depending on the deposition condition. The existing theoretical models were used for estimating hopping energy (0·02–0·04 eV) and hopping distance (2·8–5·1 nm) in nano CdTe films.     

Correlations between aging heat treatment, ω phase precipitation and mechanical properties of a cast Ti–Nb alloy

Ti–Nb alloys were arc melted in a water-cooled copper hearth in an inert atmosphere. After preparation, the samples were centrifugally cast in copper molds, and rapidly cooled, resulting in a martensitic microstructure. They were then aged at different temperatures. The microstructural characterization of this material suggested that martensite decomposition occurred, leading to precipitation of α, β and ω phases. Aging at higher temperatures led to ω phase decay. Mechanical characterization indicated that the heat treatment enhanced the strength and ductility of the alloys. Correlations between heat treatment, ω precipitation and mechanical behavior are discussed.     

Magnetic properties of Ni(II)–Mn(III) LDHs

The synthesis of Ni_1−xMn_x(OH)₂(CO₃)_x/2·nH₂O Layered Double Hydroxides (LDHs) for x = 0.2, 0.25 and 0.33, their characterisation by electron microscopy, X-ray diffraction and their magnetic properties are reported in this study. When x increases, the crystallinity of the nanoparticles is improved. The low temperature magnetic behaviour of these compounds is characteristic of the competition between in plane ferromagnetic and interlayer antiferromagnetic interactions. The ferromagnetism is due to in plane Ni cations interaction and decreases when manganese content increases (Tc decreases from 26 to 15 K when x increases from 0.2 to 0.33). It was found that the substitution of Ni by Mn ions favours the in plane antiferromagnetic order. This study demonstrates that magnetic interactions occur in LDH with non magnetic interlayer anions.     

The relationship between the electrical and mechanical properties of polymer–nanotube nanocomposites and their microstructure

A range of polymer–nanotube nanocomposites were produced using different processing routes. Both polymer-grafted and as-grown nanotubes were used and latex and polystyrene matrices investigated. The microstructures of the nanocomposites were studied, mainly by electron microscopy, in terms of the dispersion state of the nanotubes and the polymer–nanotube interface. The mechanical and electrical properties of the composites were also measured. The relationship between the microstructures observed and the resulting physical properties are discussed. It is found that composites with apparently similar microstructures can exhibit similar mechanical properties but very different electrical behaviours. Moreover, the nanocomposites produced using polymer-grafted nanotubes exhibit a clear improvement of the stress at large deformation. Thus, from our results, it appears that the mechanical and electrical properties do not necessarily depend on the same microstructural parameters. However it is still a challenge to simultaneously improve both physical properties.     

Transition from a disordered to a crystalline state in II–VI and III–V films

The initial stages of HgCdTe growth on Al₂O₃, GaAs, CdTe, and KCl substrates have been studied by electron diffraction. HgCdTe films were produced by pulsed laser deposition and isothermal vapor phase epitaxy. InGaAs films were grown by isothermal chloride epitaxy on GaAs substrates. In the initial stages of the growth process, we observed a transition from an amorphous to a textured polycrystalline phase and then to a mosaic single-crystal structure. We have calculated the critical size of crystalline grains below which amorphization occurs in II-VI and III-V compounds. The critical grain size agrees with the grain size of the disordered (amorphous) phase that forms in the initial stage of epitaxy. We have determined some characteristics of the heterostructures: critical film thickness below which pseudomorphic growth is possible without misfit dislocation generation, elastic stress in the epitaxial system, surface density of dangling bonds at dislocations, and the critical island radius above which no interfacial misfit dislocations are generated.     

Phonon and thermodynamic properties of Al–Mn compounds: A first-principles study

Based on first-principles calculations within the projector augmented wave method and the generalized gradient approximation, the structural, thermodynamic as well as phonon properties of Al–Mn compounds have been investigated. The compounds studied include Al₁₂Mn, Al₆Mn, Al₈Mn₅, Al₁₀Mn₃, Al₁₁Mn₄ (low temperature phase), γ-AlMn, ε-AlMn, and τ-AlMn. Besides phonon calculations, Debye model is also used to evaluate the thermodynamic properties at elevated temperatures, which are compared with available data from experiments and thermodynamic modeling. A good agreement is found between first-principles calculations and experimental values or CALPHAD-type calculations. For Al–Mn compounds with lower Mn content, it is observed that (i) the equilibrium volume decreases roughly linearly while the bulk modulus increases roughly linearly with increasing Mn content, (ii) the bonding strength follows the trend of Al–Al > Al–Mn (and Mn–Mn) at a fixed bond length, and (iii) roughly the higher the Mn content of Al–Mn compounds, the smaller the vibrational contribution to entropy, and in turn, to Gibbs energy. The demonstrated methodology herein, as well as the predicted thermodynamic properties, provides helpful insights into the stability of phases and thermodynamic modeling, especially for system where the experimental information is lacking or less reliable.     

Modulation spectroscopy of solid solutions between II—VI compounds

The electroreflectance spectra and microhardness of Hg_0.8Cd_0.2Te, Hg_0.84Mn_0.16Te, and Hg_0.81Cd_0.18Mn_0.01Te crystals, whose surfaces were mechanically polished, chemically etched, and exposed to argon plasma, were studied. The thickness of the polishing damage layer was found to correlate with microhardness. The penetration depth of Ar⁺ ions in Hg_0.81Cd_0.18Mn_0.01Te was determined     

Influence of electron bombardment on recombination and attachment in II–VI—IV–VI film photoconductors

An investigation is made of the action of electron bombardment on wide-gap photosensitive films of II–VI compounds with submicron IV–VI inclusions which form bounded solid solutions with these compounds. It is shown that the radiation resistance of II–VI compounds is enhanced by adding IV–VI compounds. It is observed that the radiation acts differently on shallow and deep levels in the wide-gap semiconductor. An explanation is proposed for the experimental data using a heterophase semiconductor model. Pis’ma Zh. Tekh. Fiz. 25, 66–72 (February 12, 1999)     

Experimental investigation and thermodynamic assessment of the Hf–Ge system

The Hf–Ge system has been studied via experiment, first-principles calculations and thermodynamic modeling. 15 annealed Hf–Ge alloys were analyzed by means of X-ray diffraction, scanning electron microscope with energy dispersive X-ray analysis, differential thermal analysis and high temperature reaction calorimetry. The major experimental results are as follows: (I) the heat contents of the Hf₃Ge and HfGe₂ phases were measured from 400 to 1050 °C; (II) the enthalpy of formation at 298 K for the HfGe₂ phase (?67 ± 4 kJ/mol atoms) was measured; (III) the invariant reaction temperature of Liquid ? (Ge) + HfGe₂ was determined to be at 932.6 ± 2.3 °C; and (IV) the previously reported phases HfGe and Hf₃Ge₂ were not found in the present experiment. In addition, the enthalpies of formation at 0 K for the Hf₃Ge, Hf₂Ge, Hf₅Ge₃, Hf₅Ge₄, Hf₃Ge₂, and HfGe₂ phases were computed from first-principles calculations in order to provide the necessary energy levels for the thermodynamic properties of the binary compounds. Based on the present experimental results and first-principles calculations as well as the reliable literature data, the Hf–Ge system was modeled by using a CALPHAD approach.     

Experimental investigation and thermodynamic description of the Mg–Y–Zr system

The Mg–Y–Zr system was studied via experimental investigation and thermodynamic modeling. Four diffusion couples and four key alloys of the Mg–Y–Zr system at 500 °C were prepared. The phase relations of the Mg–Y–Zr system were investigated by means of X-ray diffraction, scanning electron microscopy, and electron probe microanalysis. No ternary compound was found at 500 °C. The solubility of (αZr) in the Mg–Y intermetallics, i.e., Mg₂₄Y₅, Mg₂Y and MgY, was determined to be negligible. The differential scanning calorimetry measurement was performed on the Mg–Y–Zr alloys to obtain the phase transition temperature. The present thermodynamic calculations of the Mg–Y–Zr system matched well with the experimental data. The presently established Mg–Y–Zr phase diagram can offer a better understanding of the recent processing technique of creep-resistant magnesium alloys.