Inhomogeneous Composition Distribution Research Articles

Numerical and experimental investigation on the heat transfer and mass transport in LPBF in-situ alloying of Al/Cu alloy

PurposeLaser powder bed fusion (LPBF) in-situ alloying is a recently developed technology that provides a facile approach to optimizing the microstructural and compositional characteristics of the components for high performance goals. However, the complex mass and heat transfer behavior of the molten pool results in an inhomogeneous composition distribution within the samples fabricated by LPBF in-situ alloying. The study aims to investigate the heat and mass transfer behavior of an in-situ alloyed molten pool by developing a three-dimensional transient thermal-flow model that couples the metallurgical behavior of the alloy, thereby revealing the formation mechanism of composition inhomogeneity.Design/methodology/approachA multispecies multiphase computational fluid dynamic model was developed with thermodynamic factors derived from the phase diagram of the selected alloy system. The characteristics of the Al/Cu powder bed in-situ alloying process were investigated as a benchmark. The metallurgical behaviors including powder melting, thermal-flow, element transfer and solidification were investigated.FindingsThe Peclet number indicates that the mass transfer in the molten pool is dominated by convection. The large variation in material properties and temperature results in the presence of partially melted Cu-powder and pre-solidified particles in the molten pool, which further hinder the convection mixing. The study of simulation and experiment indicates that optimizing the laser energy input is beneficial for element homogenization. The effective time and driving force of the convection stirring can be improved by increasing the volume energy density.Originality/valueThis study provides an in-depth understanding of the formation mechanism of composition inhomogeneity in alloy fabricated by LPBF in-situ alloying.

Characterization of phase separation on AlGaN surfaces by <i>in-situ</i> photoluminescence spectroscopy and high spatially resolved surface potential images

AlGaN is a key material for deep ultraviolet optoelectronic and electronic devices. With the increase of the Al composition ratio, the phase separation on the surface, caused by small-scale compositional fluctuations, is prone to affecting the performance of the device. In order to explore the mechanism of the phase separation on a nanoscale, the AlGaN wafers with different quantities of Al compositions are investigated by the confocal photoluminescence spectroscopy and the single-pass Kelvin force probe microscopy. The composition ratios of Al for the three samples are about 0.3, 0.5, and 0.7, respectively. The single-pass Kelvin force probe microscopy based on dual-frequency phase-locking is used to obtain high spatially resolved (about 10 nm) surface potential images. In the area where the phase separation phenomenon is obvious in the photoluminescence spectrum, the sharp change of the surface potential can be observed at the irregular steps and the edges of the surface pits. The potential changes can be ascribed to the inhomogeneous composition distribution. In the area where the topography turns into step flow, the surface pits shrink and merge. No obvious surface potential domain boundaries appear at the steps nor on the edges of the surface pits. Meanwhile, the phase separation phenomenon in the photoluminescence spectrum almost disappears. Our experiments show that the steps and the edges of the surface pits on AlGaN surfaces are main reasons for small-scale compositional fluctuations and the phase separation in the spectrum. Combining with <i>in-situ</i> confocal photoluminescence spectra, high spatially resolved surface potential image by single-pass Kelvin force probe microscopy is an effective method to characterize the phase separation on AlGaN surface on a nanoscale.

Structure and properties of thin PZT films with inhomogeneous composition distribution

The paper presents the results of studies of bilayer thin ferroelectric PZT films with a lead content variation on the films thickness, formed by the method of radio-frequency magnetron deposition at different pressures of the gas mixture. It was shown that, depending on the sequence of deposition of the layers, both the structure and the dielectric parameters of the samples change.

Ultraviolet-C Photodetector Fabricated Using Si-Doped n-AlGaN Nanorods Grown by MOCVD

Aluminum gallium nitride (AlxGa1–xN) alloy films and nanostructures have attracted extensive research attention for ultraviolet (UV) and deep ultraviolet optoelectronic applications. However, the morphology-controlled growth of high-quality AlxGa1–xN quasi one-dimensional nanostructures has been limited by the complex multicomponent phase diagram and inhomogeneous composition distribution. Here, we demonstrated the growth of Si-doped n-type compositionally uniform Al0.45Ga0.55N nanorods employing a metal organic chemical vapor deposition (MOCVD) technique for the application in UV-C photodetectors. A two-step growth process, namely, growth of undoped GaN seeds and subsequent growth of n-AlGaN nanorods over GaN seeds, has been developed. Various characterization techniques have been used to study the crystalline quality, orientation, and optical properties of the realized nanorods. Field emission scanning electron microscopy revealed a uniform distribution of vertically aligned n-AlGaN nanorods over the Ga...

Inhomogeneous composition distribution in monolayer transition metal dichalcogenide alloys

Alloying with various compositions is an efficient method to tailor the optoelectronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). However, the composition distribution in the monolayer of TMDs alloys lacks detailed investigation. Here, by exploring scanning Auger electron spectroscopy, we investigate the composition distribution in MoS2(1−x)Se2x monolayers with high spatial resolution. Our results demonstrate that inhomogeneous composition distribution exists not only among different nanosheets on a substrate but also within individual nanosheets. Our study would be helpful to develop new methods for controllable synthesis of TMDs alloys and other 2D materials.

Dominant effect of particle size on the CeO2 preferential evaporation during plasma spraying of La2Ce2O7

The inhomogeneous composition distribution of atmospheric plasma sprayed La2Ce2O7 limits its operation in thermal barrier coatings. Two types of La2Ce2O7 powders were used to investigate the effect of the powder size and structure on the preferential evaporation of Ce during plasma spraying. The results indicate that powder particle size is dominant for the Ce evaporation loss. Such effect is significantly intensified with the decrease of particle size. When the particle size is larger than 30μm, the Ce loss is less or can be neglected. Two mechanisms including the element diffusion mechanism and convection mass transfer mechanism are proposed to well explain the evaporation rule. Moreover, the loss of Ce is higher using the agglomerate-sintered powder due to its higher thermal conductivity than the as-agglomerated powder. Correspondingly, the CeO2 loss can be neglected in the coatings as the agglomerated powders with a size <30μm were used.

Injection molding of ultra-fine Si3N4 powder for gas-pressure sintering

The ceramic injection molding technique was used in the gas-pressure sintering of ultra-fine Si3N4 powder. The feedstock’s flowability, debinding rate, defect evolution, and microstructural evolution during production were explored. The results show that the solid volume loading of less than 50vol% and the surfactant mass fraction of 6wt% result in a perfect flowability of feedstock; this feedstock is suitable for injection molding. When the debinding time is 8 h at 40°C, approximately 50% of the wax can be solvent debinded. Defects detected during the preparation are traced to improper injection parameters, mold design, debinding parameters, residual stress, or inhomogeneous composition distribution in the green body. The bulk density, Vickers hardness, and fracture toughness of the gas-pressure-sintered Si3N4 ceramic reach 3.2 g/cm3, 16.5 GPa, and 7.2 MPa·m1/2, respectively.

Electrophysical and photoelectrical properties of MIS structures based on MBE grown heteroepitaxial HgCdTe MIS structures with inhomogeneous composition distribution

The electrophysical and photoelectrical properties of MIS structures are studied using MBE grown heteroepitaxial HgCdTe with inhomogeneous composition distribution. It is shown that surface varib- and layers with increased surface CdTe composition in n-Hg1 − x Cd x Te-based MIS structures (x = 0.21–0.23) strongly affect the dependences of capacity and photo-emf on the voltage bias and frequency. Parameters of n-Hg0.7Cd0.3Te-based MIS structures with periodic barrier-type regions of increased CdTe composition are studied, and their effect is shown on the parameters of MIS structures when they are located near the dielectric-semiconductor interface. Electrical properties of n-Hg1 − x Cd x Te-based MIS structures (x = 0.62–0.73) with potential-well regions of decreased CdTe composition in the surface region are studied.

Effects of coherency stress and vacancy sources/sinks on interdiffusion across coherent multilayer interfaces – Part I: Theory

A phase field model corresponding to vacancy-mediated interdiffusion in coherent multilayers with completely miscible constituents is developed to explore the effects of several factors on interdiffusion across coherent multilayer interfaces, such as: (1) the dependence of diffusion potentials and mobilities on coherency stress; (2) the dependence of diffusion potentials and mobilities on composition; (3) the elastic constant inhomogeneity resulting from a inhomogeneous composition distribution; and (4) the properties of vacancy sources/sinks. The Gibbs free energy of the system consists of chemical and elastic energies. The gradient energy is neglected as the multilayers under consideration can be chemically well approximated by an ideal substitutional solution model. Elastic energy is a function of the stress-free strain and inhomogeneous elastic moduli distributions, while the stress is solved by anisotropic phase field microelasticity theory. The diffusion potentials are obtained straightforwardly as functional derivatives of the free energy with respect to composition and are in keeping with previous derivations that involved many mathematical manipulations or quite advanced theories. The diffusion mobilities are affected by the stress through modification of the vacancy formation and migration energies. Two limiting cases of vacancy sources/sinks are taken into account: ideal vacancy sources/sinks are uniformly and densely distributed, or not present at all, so the vacancy concentration is in equilibrium all the times, as determined by the local stress and composition in the former case, but deviates from the equilibrium concentration in the latter. The model can be conveniently extended to consider the non-ideal activity of vacancy sources/sinks by introducing a general kinetic relation for the vacancy creation rate.

Preparation and Characterization of Ba8Ga16Ge30/Sr8Ga16Ge30 Core−Shell Single Crystals

Ba- and Sr-filled type I Ge clathrate single crystals were synthesized by the Ga flux method, using high purity elemental Ba, Sr, Ga, and Ge as starting materials. Scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and Raman spectroscopy were used to characterize the cross-section of a single crystal. The results indicate an inhomogeneous composition distribution of the crystal: the core of the crystal is composed of Ba 8 Ga 16 Ge 30 and the shell is composed of Sr 8 Ga 16 -Ge 30 with a transition layer in between. This inhomogeneous composition of the crystal is confirmed by a locally resolving Seebeck coefficient measurement and thermal analysis by differential scanning calorimeter.

Effect of WO 3 doping on dielectric temperature characteristics of lead complex perovskite ferroelectric ceramics

The effect of WO 3 doping on the dielectric temperature characteristics and the microstructure of a ternary ceramic system were investigated. Both the increase of WO 3 doping level and the increase of sintering temperature led to a decrease in dielectric constant and the flatting of dielectric curves. The presence of a liquid phase and the pyrochlore phase cause this decrease. Also, microstructural and elemental analyses of the ceramics revealed the structure within one grain consists of three parts: the core, the shell and in-between one. This inhomogeneous composition distribution contributes to flat dielectric temperature dependence.

Microstructural characteristics of microwave-sintered semi-conductive Pb0.6Sr0.4Tio3ceramics

Microstructures of microwave-sintered semi-conductive Pb0.6Sr0.4TiO3 ceramics were investigated using electron microscopy and energy dispersive spectroscopy. The results imply that different atomic species may interact differently with microwave and induce various heating and reaction behavior at different regions. Consequently, inhomogeneous composition distribution occurs and complicated domain structures form when cooling through the Curie temperature. Moreover, core-shell structures, which may induce peculiar electric properties, were constantly observed. The shell regions contain higher Pb-concentration, which may be attributed to the re-precipitation of the supersaturated liquid phase to form Pb-rich perovskite structure during cooling.