Structural and optical properties of GaN films grown on GaAs substrates by molecular beam epitaxy

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. X-ray diffraction and transmission electron microscopy indicate that this method promotes prismatic growth of c-oriented α-GaN. Photoluminescence studies show that the emission from cubic β-GaN inclusions dominates the spectrum.     

Fine yellow α-SiAlON:Eu phosphors for white LEDs prepared by the gas-reduction–nitridation method

Yellow-emitting α-SiAlON:Eu²⁺ phosphors were synthesized by the gas reduction and nitridation of a homogeneous oxide precursor in a CaO–Al₂O₃–SiO₂–Eu₂O₃ system at 1400–1450 °C using an NH₃–CH₄ mixture gas as a reduction–nitridation agent. The precursor was prepared by a sol–gel process using a low-cost nitrate, tetraethyl orthosilicate and citric acid as the starting materials. The effects of reaction parameters such as heating rate, temperature, holding time and CH₄ content on the composition, microstructure and photoluminescence of the prepared powders were investigated. Nearly single-phase α-SiAlON was successfully synthesized by the one-step gas reduction and nitridation without the need for post-annealing at a higher temperature. The prepared powders consisted of relatively well-dispersed and uniform crystals with a hexagonal shape. The photoluminescence spectra of Eu-doped Ca-α-SiAlON phosphors excited by near-ultraviolet or blue light showed a broad, yellow emission band at 500–700 nm, which agrees well with that obtained from phosphors prepared by the conventional solid-state reaction.     

Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.     

Synthesis of single nanocrystal phase γ′-Fe4N on NaCl substrate by DC magnetron sputtering

The single-phase γ′-Fe₄N nanocrystal magnetic films with grain size of d = 40–60 nm were synthesized on single crystal NaCl (1 0 0) substrate by DC magnetron sputtering at 150 °C. The structure, morphology of the single-phase γ′-Fe₄N films were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and the magnetic properties of samples prepared at different substrate temperatures were investigated by superconducting quantum interference device (SQUID). It is shown that substrate temperature has a significant influence on the crystalline structure and magnetic properties for Fe–N films. As substrate temperature was increased, the saturation magnetization for the deposited films increased, but the coercivity reduced.     

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy was studied. The results show that there are obvious yield points on true stress–true strain curves of annealing structures, then a stress drop occurs. The curves show linear work-softening after yielding at annealing temperature of 720–780 °C and linear work-hardening at annealing temperature of 800–840 °C. Elastic energy stored in the α-phase is dramatically released after plastic deformation of the β-phase, which leads to the stress drop.     

Kinetics of direct nitridation of pelletized silicon grains in a fluidized bed: experiment, mechanism and modelling

A fluidized-bed nitridation of pelletized silicon grains having a wide size distribution was carried out in the temperature range 1200–1300°C under conditions free of external heat and mass transfer effects. N2(30%–90%)–H2(5%–50%)–Ar (balance) mixtures were used as the nitriding gas at atmospheric pressure. Both the yield of -Si3N4 and the final overall conversion of silicon are affected by temperature and nitrogen gas concentration in a nitriding atmosphere, but hydrogen gas has a minor effect on either of these. After accounting for some of the structural changes that occur during nitridation, a simple model was derived. The model has shown that the pseudo-asymptotic exponential conversion trend in the second nitridation stage could be explained by various reaction mechanisms, adjusted for properties of the size distribution of silicon grains and the experimentally observed spalling of the product scale from the silicon surface. In the investigated range of experimental conditions, nitridation could be considered as having an apparent activation energy of Eapp340 kJ mol-1. © 1998 Chapman & Hall     

Effect of Cu and Sn on liquid–liquid interfacial energy in ternary and quaternary Al–Bi-based monotectic alloys

The influence of Cu and Sn on the interfacial energy between Al-rich and Bi-rich liquids has been studied. The liquid–liquid interfacial tension is increased when Cu is added to the Al_34.5Bi_65.5 (numbers indicate wt.%) binary and it is decreased when Sn is added. Simultaneous addition of Cu and Sn in equal quantity to the binary Al–Bi alloy results in a decrease of the interfacial tension at low temperature and in its increase at high temperature. Temperature dependences of the interfacial tension in the alloys studied are well described by the function σ_αβ = σ₀ (1 − T/T_C)^μ with a constant σ₀ and the critical-point exponent μ = 1.3.     

Effect of Mg17Al12 precipitates on the microstructural changes and mechanical properties of hot compressed AZ91 magnesium alloy

During hot compression, Mg₁₇Al₁₂ (β) precipitates show strong influence on the microstructural changes of 415 °C-24 h homogenized AZ91 alloy. When compressed at 300 °C and 350 °C, dynamic recrystallization (DRX) only occurs near grain boundaries with discontinuous β precipitate pinning at the newly DRXed grain boundaries. With increasing compression temperature and decreasing strain rate, the β-precipitating region expands; however, the amount of pinning precipitates decreases, resulting in increases in the DRX ratio and average DRXed grain size. With a compression ratio of only 50%, the specimen compressed at 350 °C and a strain rate of 0.2 s⁻¹ (designated 350 °C-0.2 s⁻¹ compressed specimen) shows an ultimate tensile strength (UTS) of 334 MPa, a 0.2% proof stress (PS) of 195 MPa and an enough elongation of 17.9%. After a subsequent aging treatment at 180 °C, due to the large number of β precipitates, the strength of the compressed specimens are further improved, and the specimen peak aged after compression at 400 °C and 0.2 s⁻¹ shows UTS of 364 MPa and PS of 248 MPa with a moderate elongation of 7.7%.     

Influence of crystal plane on the welding quality of YBCO bulk superconductor

The microstructure of welds parallel to the {1 0 0} and the {1 1 0} crystal planes of the single-grain YBa₂Cu₃O₇/Y₂BaCuO₅ (Y123/Y211) bulk superconductors welded by Ag foil has been studied. It is shown that a zone free of small Y211 particles is formed along the both sides of the welds. Formation of this zone is explained by higher Y content in the melt produced during heating and leaking out of the weld. The different morphology of the rest solidified liquid in the welds, which has a tendency to form films in the {1 0 0} welds and granules in the {1 1 0} welds, is associated with different surface energies of welded surfaces. Cracking suppression of the {1 1 0} welds was observed and is ascribed to higher fracture toughness for this plane. The quality of the weld is confirmed by the trapped field measurements.     

Microstructures and properties of biomedical TiNbZrFe β-titanium alloy under aging conditions

A metastable β-titanium alloy Ti–28Nb–13Zr–0.5Fe (TNZF alloy for short) was designed for implant biomedical application. The forged specimens were solute-treated at 850 °C followed by water quenching and then aged at 350 °C, 450 °C, and 550 °C for 2–6 h in order to evaluate the effect of phase transformation during ageing on the biomechanical compatibility of the alloy. The quenched microstructure consists of lath α″ martensite and β phase. A large quantities of shuttle-like ω phase precipitate at 350 °C, leading to the drastic increase of strength and elastic modulus and the decrease of plasticity. Ageing at 450 °C for 4 h, small amount of elliptic ω phase and dot α phase precipitate from β matrix. With increasing ageing time α precipitations begin to coarsen and precipitation free zones (PFZs) form around prior β grain boundaries. Needle-like α phase precipitates on grain boundaries and intra-grains when aged at 550 °C. Both PFZs and grain boundary α precipitates are prone to bring about the intergranular fracture and thus have adverse effects on the tensile strength and fracture plasticity. The quenched microstructure has good combination properties of high strength, high plasticity and low elastic modulus.     

Thermomechanical behavior of single crystalline tantalum in the static and dynamic regime

This paper reports the thermomechanical behavior of single crystalline tantalum (Ta) in the [1 0 0] and [1 1 0] orientations. Mechanical testing was carried out at low and high strain rates using a Kolsky bar together with the simultaneous recording of the specimen’s temperature by means of an infrared detector. The results show a marked difference in terms of flow curve and strain hardening between the two orientations, irrespective of the strain-rate. Similarly, the thermomechanical behavior, namely the efficiency of the thermomechanical conversion at high strain-rates (β_int), is observed to be different for each orientation. A comparison of the present results with those obtained for pure polycrystalline Ta (Rittel et al., 2007) reveals some similarity of flow curves with the [1 0 0] orientation. By contrast, the [1 1 0] orientation is observed to possess β_int characteristics that are similar to those of the polycrystalline material. These results are presented and discussed, thus completing the overall experimental characterization of this material in order to enable the simulation and validation of the dynamic behavior of polycrystalline Ta on the basis of its single-crystal characteristics.     

High temperature deformation behavior of a near alpha Ti600 titanium alloy

The high temperature deformation behavior of a near alpha Ti600 titanium alloy was investigated with isothermal compression tests at temperatures ranging from 800 to 1000 °C and strain rates ranging from 0.001 to 10.0 s⁻¹. The apparent activation energy of deformation was calculated to be 620.0 kJ mol⁻¹, and constitutive equation that described the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of Ti600 titanium alloy in the α + β phase region. The processing map was calculated to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. High efficiency values of power dissipation over 55% obtained under the conditions of strain rate lower than 0.01 s⁻¹ and temperature about 920 °C was identified to represent superplastic deformation in this region. Plasticity instability was expected in the regime of strain rate higher than 1 s⁻¹ and the entire temperature range investigated.     

Influence of the nitridation parameters on the stoichiometry, structure and specific surface area of zirconium phosphate oxynitride catalysts

Oxygen can be exchanged by nitrogen atom in high surface specific and amorphous zirconium phosphate solids thank to nitridation process. Various nitridation parameters influence the O/N substitution. A minimum temperature of 550°C is required for efficient nitridation whereas temperature higher than 850 and 1300°C induces reduction of phosphorus and zirconium atoms respectively. In intermediate temperature range, O/N exchange increases with temperature as well as with time of nitridation. In these conditions, the ZrPON solids obtained are X-ray amorphous and a slight decrease of precursor specific surface area is observed (110–210 m² g^-1).     

Growth and structural investigations of epitaxial hexagonal YMnO3 thin films deposited on wurtzite GaN(001) substrates

Epitaxial hexagonal YMnO₃ (h-YMnO₃) films having sharp (00l) X-ray diffraction peaks were grown above 700 °C in 5 mTorr O₂ via pulsed laser deposition both on as-received wurtzite GaN/AlN/6H-SiC(001) (w-GaN) substrates as well as on w-GaN surfaces that were etched in 50% HF solution. High-resolution transmission electron microscopy revealed an interfacial layer between film and the unetched substrate; this layer was absent in those samples wherein an etched substrate was used. However, the substrate treatment did not affect the epitaxial arrangement between the h-YMnO₃ film and w-GaN substrate. The epitaxial relationships of the h-YMnO₃ films with the w-GaN(001) substrate was determined via X-ray diffraction to be (001)_YMnO3 ‖ (001)_GaN : [11¯0]_YMnO3 ‖ [110]_GaN; in other words, the basal planes of the film and the substrate are aligned parallel to one another, as are the most densely packed directions in planes of the film and the substrate. Interestingly, this arrangement has a larger lattice mismatch than if the principal axes of the unit cells were aligned.     

Polymorphism of Bismuth Sesquioxide. I. Pure Bi2O3

Stability relationships of the four polymorphs of bismuth oxide have been determined by means of DTA and high-temperature x-ray studies. The stable low-temperature monoclinic form transforms to the stable cubic form at 730 ±5 °C, which then melts at 825 ± 5 °C. By controlled cooling, the metastable tetragonal phase and/or the metastable body-centered cubic (b.c.c.) phase appear at about 645 °C. Whereas b.c.c. can be preserved to room temperature, tetragonal will transform to monoclinic between 550 and 500 °C. Tetragonal Bi₂O₃, however, is easily prepared by decomposing bismutite (Bi₂O₃·CO₂) at 400 °C for several hours. The greatest transition expansion occurs at the monoclinic to cubic inversion, and cubic Bi₂O₃ shows the greatest coefficient of volume expansion. With exposure to air, Bi₂O₃ carbonates and partially transforms to bismutite and an unknown phase.     

Microwave dielectric properties and sintering behavior of nano-scaled (α + θ)-Al2O3 ceramics

The dielectric properties at microwave frequencies and the microstructures of nano (α + θ)-Al₂O₃ ceramics were investigated. Using the high-purity nano (α + θ)-Al₂O₃ powders can effectively increase the value of the quality factor and lower the sintering temperature of the ceramic samples. Grain growth can be limited with θ-phase Al₂O₃ addition and high-density alumina ceramics can be obtained with smaller grain size comparing to pure α-Al₂O₃. Relative density of sintered samples can be as high as 99.49% at 1400 °C for 8 h. The unloaded quality factors Q × f are strongly dependent on the sintering time. Further improvement of the Q × f value can be achieved by extending the sintering time to 8 h. A dielectric constant (_r) of 10, a high Q × f value of 634,000 GHz (measured at 14 GHz) and a temperature coefficient of resonant frequency (τ_f) of −39.88 ppm/°C were obtained for specimen sintered at 1400 °C for 8 h. Sintered ceramic samples were also characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

The influence of dynamic strain aging on the low cycle fatigue behavior of near alpha titanium alloy IMI 834

Total strain controlled low cycle fatigue tests on IMI 834 have been conducted in air in the temperature range between 375 and 500 °C at a temperature interval of 25 °C at the nominal strain rate of 6.67 × 10⁻⁴ s⁻¹. The observed maximum peak stress ratio, minimum half-life plastic strain range and lower fatigue life at 425 °C indicates the occurrence of dynamic strain aging (DSA). Pronounced deformation bands, increased dislocation density and non-uniform dispersion of dislocations inside primary α grains observed by the study of transmission electron microscopy supports the occurrence of dynamic strain aging. Initial cyclic softening was attributed to shearing of Ti₃Al precipitates as revealed by TEM evidences.     

Preparation and phase transformation behavior of χ-alumina via solvothermal synthesis

Solvothermal reaction of aluminum isopropoxide (AIP) in mineral oil at 250–300 °C over 2 h duration provides χ-alumina powder, which transforms directly to α-alumina after calcination at high temperature. The mechanism of the crystallization process appears to be the initial formation of a spherical complex which subsequently decomposes further to precipitate a solid. This mechanism is suggested by XRD, IR, TG/DTA, SEM and TEM characterization of the powder formed. χ-Alumina attains a critical crystallite size around 15 nm through accretion on calcination and then transforms directly to α-alumina through nucleation and growth process. Direct α-phase transformation of χ-alumina powders rather than passage through κ-alumina can be explained by the absence of the cation contamination and the higher crystallinity of χ-alumina in the AIP decomposition process.     

Allotropic phase transformation of pure zirconium by high-pressure torsion

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1–40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above 4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.