Atomic Diffusion Processes Research Articles

Effects of the Ni electrodeposit on microstructure evolution and electrical resistance of the P-type Bi2Te3 solder joint

Abstract A systematic investigation was carried out to evaluate the effect of Ni electrodeposit on the microstructure evolution and electrical resistance of the P-type Bi2Te3-based thermoelectric (TE) joint. In this study, two systems (SAC305/(Bi,Sb)2Te3 and SAC305/Ni/(Bi,Sb)2Te3) were set up. The intermetallic compounds (IMCs) were mainly detected by using scanning electron microscope (SEM), X-ray diffractometer (XRD) and transmission electron microscope (TEM). A model of overlapping joint was set up for electrical resistance testing. Results indicated that the SnTe phase was the major and thermodynamically stable product in the SAC305/(Bi,Sb)2Te3 system, and its porous structure determined the presence of micro-tunnels, which facilitated the diffusion process of Sn and Te atoms, therefore the SnTe layer continuously grew in the whole reflowing duration. In this system, the time exponent was analyzed to be 1.2741, indicating that the growth of the IMC layer followed the linear law, and was reaction-controlled. In the SAC305/Ni/(Bi,Sb)2Te3 system, it was remarkable that the Ni barrier inhibited the rapid growth of the IMC layer, and the dominated product was detected to be (Ni,Cu)3Sn4 whose dense distribution could effectively suppress the diffusion of Sn atoms. The time exponent in this system was 0.5010, indicating that the growth of the Ni–Cu–Sn ternary IMC followed the parabolic laws and was mainly volume diffusion-controlled. After reflowing treatment, the electrical resistance remained at 2.203 mΩ for NiTL (the TE leg with the Ni electrodeposit) system while that of TL (the TE leg without the Ni electrodeposit) system rapidly increased to a higher value about 4.879 mΩ. Owing to the lower growth rate of the IMC layer in NiTL system, it could reasonably conclude that the Ni electrodeposit was of great importance in depressing the electrical resistance.

Combustion Behavior and Mechanism of Ti14 Titanium Alloy.

The combustion behavior and mechanism of Ti14 titanium alloy are studied by promoted ignition combustion tests at different oxygen pressures in this paper. The burning velocity increases at higher oxygen pressures and also increases with longer burning times instead of a constant at the same pressure. The Cu atoms are found enriched in two zones—i.e., the heat affected zone and melting zone during the combustion process—which can prevent the diffusion process of oxygen atoms. The different combustion behavior of Ti14 and Ti-Cr-V alloys is basically controlled by the characteristics of phase structures and chemical reactions.

Effects of oxides produced during welding on oxidation behavior of the 304L welded joint in simulated primary circuit solution of PWR

In order to study the effect of welding oxide film on high temperature water oxidation, welded joint samples with welding oxide film were immersed in simulated primary loop solution of pressurized water reactor for 90 days. Both the type and thickness of the welding oxide film affect the atomic diffusion process of the high temperature water oxidation reaction, thus affecting the oxide type and oxidation rate. The chromium oxide in the chromium-rich welding oxide film was basically insoluble in high temperature water. The matrix metal atoms dissolved after passing through the film. The dissolved ions participated in the oxidation reaction and precipitate to form a new oxide film. The iron oxide in the chromium-poor welding oxide film and metal atoms in the matrix partially dissolved in high temperature water. The dissolved ions participated in the oxidation reaction and precipitated to form a new oxide film as well. The experimental results showed that the diffusion range of the dissolution process in the dissolution-reaction-precipitation mechanism was confined to the micron range.

Optimization of Metal‐Organic Chemical Vapor Deposition Regrown n‐GaN

GaN devices for high‐frequency and high‐power applications often need n‐doped GaN layers on top of their structures. Such layers can be either grown in an epitaxial reactor or formed by implantation or annealing of Si‐containing layers (e.g., a SiO2 mask). These processes are typically performed at high temperatures, which generate the undesired effect of atom diffusion between the different epitaxial layers; consequently, the electrical performance of the final device will be hampered. Herein, an optimized epitaxial growth process of n‐GaN layers is developed with the focus on minimizing the atom diffusion process, while preserving a high material quality and excellent electrical characteristics, such as very low contact resistance for n‐GaN ohmic contacts or high electron mobility in GaN npin structures. A low growth temperature process combined with improved growth conditions to minimize the incorporation of impurities is successfully optimized and demonstrated on different epitaxial reactors.

Dynamic observation on the functional metal oxide conversion behaviors in Fe3O4/ZnO heterostructures

We directly observed the atomic diffusion process of bilayered Fe/ZnO transforming into a Fe3O4/ZnO heterostructure. Significantly, we suggested a unique two-step route in which Fe diffused along [0001]ZnO via boundaries of ZnO and then reacted along [1¯21¯0]ZnO. The specific orientation relationship of [0001]ZnO//[111¯]Fe3O4 could be precisely controlled. In addition, the formation of a ZnO/Fe3O4 heterostructure can be tuned in terms of reaction depth by controlling the annealing time. This study provided a novel nanofabricated method to produce oriented metal oxide heterostructures while visualizing the atomic mechanism of metal oxide formation, which allows for further investigation of promising devices.

Multi-modal and multi-scale non-local means method to analyze spectroscopic datasets

A multi-modal and multi-scale non-local means (M3S-NLM) method is proposed to extract atomically resolved spectroscopic maps from low signal-to-noise (SNR) datasets recorded with a transmission electron microscope. This method improves upon previously tested denoising techniques as it takes into account the correlation between the dark-field signal recorded simultaneously with the spectroscopic dataset without compromising on the spatial resolution. The M3S-NLM method was applied to electron energy dispersive X-ray and electron-energy-loss spectroscopy (EELS) datasets. We illustrate the retrieval of the atomic scale diffusion process in an Al1-xInxN alloy grown on GaN and the surface oxidation state of perovskite nanocatalysts. The improved SNR of the EELS dataset also allows the retrieval of atomically resolved oxidation maps considering the fine structure absorption edge of LaMnO3 nanoparticles.

High-performance integrated additive manufacturing with laser shock peening –induced microstructural evolution and improvement in mechanical properties of Ti6Al4V alloy components

High-performance integrated additive manufacturing with laser shock peening (LSP), is an innovative selective laser melting (SLM) method to improve mechanical properties, and refine microstructure in the surface layer of metallic components. Phase, residual stress distribution, surface micro-hardness, tensile properties and microstructural evolution of SLMed and SLM-LSPed specimens in horizontal and vertical directions were examined. In particular, typical microstructural features in the surface layer were characterized by transmission electron microscopy (TEM) observations. Results indicated that surface micro-hardness subjected to massive LSP treatment had significantly improved, tensile residual stress was transformed into compressive residual stress by LSP-induced plastic deformation, and both SLMed specimens in two directions exhibited a good combination of the ultimate tensile strength (UTS) and ductility. Meanwhile, high-density dislocations and a large number of mechanical twins were generated in the coarse α′ martensites by laser shock wave (LSW), and gradually evolved into refined α′ martensites. Furthermore, according to the included angle between LSW and the deposited plane, two kinds of LSW-induced atomic diffusion processes at the interfaces between both adjacent deposited layers were presented, and the influence mechanisms of the included angle between LSW and the deposited plane on tensile properties of both SLM-LSPed specimens were revealed. The hybrid additive manufacturing technology combined SLM with LSP realizes the high-efficiency and high-quality integrated manufacturing of the formed metallic components for practical applications.

Static coarsening behavior of a pre-deformed Ti2AlNb-based alloy during heat treatment

Microstructure evolution, static coarsening mechanism and coarsening kinetics of a pre-deformed Ti2AlNb-based alloy during aging at 945 °C and 970 °C were investigated in this study. The corresponding microstructures were characterized by scanning electron microscopy and transmission electron microscopy. The results show that the pre-deformed alloy exhibited different morphologies aged at 945 °C and 970 °C. When the alloy was aged at 970 °C, the coarsening mechanism of the O precipitates was mainly controlled by the Ostwald ripening mechanism at first aging stage; after that, it was controlled by the boundary splitting mechanism and termination migration mechanism. Due to the difference in interface energy between the terminals and the long-axis direction of the O lath, diffusion process of solute atoms leads to the coarsening and fragmenting of O lath. The dimension of the O precipitates in the studied alloy continued to grow with the prolonged time during aging at 945 °C or 970 °C while the coarsening rate decreased. The static coarsening kinetic of the studied alloy was established in term of the modified Lifshitz-Slyozov-Wagner (LSW) theory. Static coarsening behavior of the pre-deformed alloy was mainly controlled by bulk diffusion at 945 °C and interfacial diffusion at 970 °C, respectively.

Direct Observation of Yolk-Shell Transforming to Gold Single Atoms and Clusters with Superior Oxygen Evolution Reaction Efficiency.

Noble metal particles and atoms tend not to react with other media even at elevated temperatures due to their inert nature. Herein, we report surprising findings of Au diffusion and dissolution in a Ni2P matrix, forming single Au atoms and tiny clusters. The dynamic atomic diffusion process was directly probed by in situ heating scanning transmission electron microscopy (STEM). The Au yolks can diffuse and dissolve completely into the Ni2P shell at temperatures of 350 °C or higher, resulting in an inward volume expansion of the Ni2P shell and formation of single Au atoms and tiny Au clusters in Ni2P. The resulting structure exhibited outstandingoxygen evolution reaction (OER) efficiency that is 16-fold that of commercial IrO2. The in situ STEM finding explains its drastically improved catalytic performance after annealing, confirming Au single atoms and clusters on a Ni2P surface as active OER sites. Our finding emphasizes the role of dynamic structural changes during simple heat treatment to enhance catalysis efficiency.

MnBi hard magnetic films optimised through the correlation between resistivity, morphology and magnetic properties

Abstract Hard magnetic MnBi films were fabricated using pulsed laser deposition (PLD). The films consisted of Mn/Bi bilayers and the thickness of each sample constant was 185 nm. The number of bilayers (5 to 80 bilayers) and consequently, the thickness of the Mn and Bi layers changed among the different samples, although the proportion of Mn and Bi remained constant in each bilayer. The structural and morphological changes in the Mn/Bi multilayered films were produced by thermal treatments and were monitored by measuring the change in their resistance as a function of the temperature. These measurements, along with the magnetic measurements, allowed us to determine the optimum heat treatments that induced the formation of the low temperature (LT) α-MnBi phase. A maximum value of both the magnetisation (460 emu/cm3) and the maximum BH product (46 kJ/m3) was observed in the sample with 40 bilayers, showing, in addition, a magnetic anisotropy, wherein the easy magnetisation direction was predominantly perpendicular to the plane of the sample. A detailed explanation of these changes and of the atomic diffusion processes present for such optimisation has been achieved.

Creep behaviour and microstructural evolution of AlxCrMnFeCoNi high-entropy alloys

ABSTRACTThe creep deformation behaviour of thermo-mechanically treated AlxCrMnFeCoNi high-entropy alloys was investigated in the high-temperature range of 600–700°C. Compared with the Al-free CrMnFeCoNi alloy, the AlxCrMnFeCoNi alloys showed higher creep resistance under the same creep conditions due to strengthening contribution from elemental Al. The Al0.4CrMnFeCoNi alloy exhibited a stress-dependent transition from the low-stress region to high-stress region. The Al0.6CrMnFeCoNi alloy showed decreased creep resistance compared to the Al0.4CrMnFeCoNi alloy, which was ascribed to the higher activation volume and stacking-fault energy in the former alloy. Specifically, the intragranular formation of Cr-rich σ precipitation was widely found in the matrix grain among testing conditions, indicating a striking atomic diffusion process due to the limited entropic stabilisation in Al-containing CrMnFeCoNi alloys.

A comparative study on microstructure and properties of Ti(C,N)-based cermets with the various Cr doping methods

In this work, the effects of chromium doping methods on the microstructures and properties of Ti(C,N)-based cermets were investigated systematically. The results showed that the existence state of Cr element in the cermet was influenced by the different doping methods. The additive pure Cr powders might cause the appearance of Cr particles and solutions in the binder phase of cermets. However, with the addition of NiCr powders, the Cr element might distribute in the binder phase of cermet. The Cr3C2 addition could make Cr element disperse in the rim and binder phase. Moreover, the Cr atoms dissolved into binder phase might have the solid solution strengthening effect on the binder phase, which increased the hardness of cermets. The Cr element in binder phase could improve the oxidation resistance of cermets owing to the existence of chromium oxide layer in the internal oxidation region. Furthermore, the diffusion process of Mo atoms to the surface during oxidation was prevented by the dissolved Cr element in the binder phase, which led to the increased bonding strength between oxidation layer and unoxidized substrate. In addition, a larger amount NiCr powder addition had the excellent reinforcing effect on the corrosion resistance of cermets due to the appearance of stable chromium oxide film on the binder phase.

Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors

Gasochromic based optical hydrogen sensors have attracted much attention as normal temperature sensors. The behaviors of the hydrogen diffusion largely affect the reaction process and the sensitivity. However, few researches focused on the influence of hydrogen diffusion in gasochromic films. Here, we report a method to pre-split the H2 molecule and separate the hydrogen molecule and hydrogen atom diffusion process by deposition of porous PdCl2/SiO2 catalyst films on WO3. The response time of WO3 at 844 Pa H2 partial pressure reduced from 800 s to 200 s. The stability of WO3 film was largely improved to be more than 160 cycling lifetime. A validation experiment was designed and proved the diffusion of the hydrogen atom and the two-step model of H2 reaction on WO3 films, on the basis of which the diffusion coefficient of the hydrogen atom has been estimated. Moreover, the PdCl2 doping SiO2 catalyst exhibits substrate versatility, which can also coat on the nano-structured WO3.

The core structure of 60° mixed basal dislocation in alumina (α-Al2O3) introduced by in situ TEM nanoindentation

The dislocation associated with the (0001)1/3<112¯0> basal slip in alumina plays an important role on its plastic deformation at elevated temperatures. A 60° mixed basal dislocation was preferentially introduced by in situ transmission electron microscopy (TEM) nanoindentation, and characterized by atomic-resolution scanning TEM (STEM). It was revealed that the 60° mixed basal dislocation dissociates into two partial dislocations via self-climb with a stacking fault on the 11¯00 plane. The dissociation mechanisms of the basal dislocation have been discussed in terms of its core structure and atomic diffusion process.

Distribution of Elements Inside Stars

The chemical composition measured in stellar atmospheres is not necessarily the same as in deeper layers (outside the core). Indeed, for a significant fraction of main-sequence G to B types stars the discrepancies between superficial and internal abundances go from a few percent (for the coldest of these stars) to huge factors (for hot chemically peculiar stars). This is due to atomic diffusion process, which may produces elements segregation at some stages of the stellar evolution.

The mic-mac connection inside stars: the interdependence of atomic diffusion and hydrodynamic instabilities

The interdependence of microscopic (atomic) and macroscopic (hydrodynamic) processes inside stars and their consequences for stellar structure and evolution were recognized by Jean-Paul Zahn several decades ago. He was a pioneer in that respect, discussing the importance of the macroscopic motions related to stellar rotation, in competition with the chemical stratification induced by gravitational settling and radiative accelerations. This has been much developed in recent years, in connection with the improvement of observational data, including asteroseismology. Morover, it has been recently discovered that the microscopic atomic diffusion processes can lead to macroscopic results which may infuence in a non negligible way the internal stellar structure, independently of the abundances observed at the surface.

Microstructural and mechanical evaluation of a dissimilar joining between SAE 1020 and AISI 304 steel obtained via ultra-high-frequency-pulsed GTAW

Two welding process parameters (the amplitude and frequency of the pulsed current) were varied in order to investigate their influence on the microstructure of the weld region and the mechanical properties of the joints. This investigation and the subsequent discussion were made based on the results of metallographies, tensile tests, and Vickers microhardness tests. The introduction of pulsed current at ultra-high frequencies resulted in a significant reduction in the size of the heat-affected zone (HAZ) for both materials: up to 54% for carbon steel and up to 73% for stainless steel. An increase in the grain size in the carbon steel HAZ was also noted. There are indications that the pulsed current at ultra-high frequencies has accelerated the atomic diffusion process of alloying elements from the fusion zone (FZ) to the carbon steel HAZ. Martensitic grains were observed in the FZ and the microhardness test verified this finding, once the microhardness values measured were around 400 HV0.3. The tensile strength was around 450 MPa and the rupture occurred on the carbon steel side, for all samples.

Fabrication and electrochemical performance of La 0 : 595 V 0 : 005 Sr 0 : 4 MnO 3

This study reports the effect of vanadium on the crystal structure and electrical and electrochemical properties of La$_{0.6-x}$V$_{x}$Sr$_{0.4}$MnO$_{3-\delta \, }(x =$ 0.005--0.1) perovskite-based cathode materials in solid oxide fuel cells. Crystal structure, morphology, and porosity of prepared cathode materials are characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. Using XPS analysis it was established that the La$^{3+}$ cation is exchanged by the V$^{4+/5+}$ cation in the perovskite structure. This ion replacement improves the ionic conductivity and catalytic activity for oxygen reduction reaction in the perovskite structure, presumably due to the smaller size of the V$^{4+/5+}$ cation than the La$^{3+}$ ion. Oxygen partial pressure-related polarization experiments suggest that the adsorption-desorption process and the reactions controlled by the atomic oxygen diffusion process followed by charge transfer are the cathode reaction rate-limiting steps.

First-Principles Investigation of Atomic Hydrogen Adsorption and Diffusion on/into Mo-doped Nb (100) Surface

To investigate Mo doping effects on the hydrogen permeation performance of Nb membranes, we study the most likely process of atomic hydrogen adsorption and diffusion on/into Mo-doped Nb (100) surface/subsurface (in the Nb12Mo4 case) via first-principles calculations. Our results reveal that the (100) surface is the most stable Mo-doped Nb surface with the smallest surface energy (2.75 J/m2). Hollow sites (HSs) in the Mo-doped Nb (100) surface are H-adsorption-favorable mainly due to their large adsorption energy (−4.27 eV), and the H-diffusion path should preferentially be HS→TIS (tetrahedral interstitial site) over HS→OIS (octahedral interstitial site) because of the correspondingly lower H-diffusion energy barrier. With respect to a pure Nb (100) surface, the Mo-doped Nb (100) surface has a smaller energy barrier along the HS→TIS pathway (0.31 eV).

The influence of short-range structures on atomic caging in glass-forming Cu-Zr-Al melts

Atomic caging is an important dynamic process that influences liquid vitrification. In this article, we have studied bulk glass-forming copper-zirconium-aluminum (Cu-Zr-Al) systems using quasielastic neutron scattering and molecular dynamics simulation in order to understand the influence of short-range structures on atomic caging. We found that, in Cu-Zr-Al melts, the long range atomic transport process occurs via a jump diffusion process, and temperature dependence of the atomic diffusion process is non-Arrhenius. Furthermore, the Cu diffusion coefficient at a given temperature decreases with Al addition. Inherent structures obtained from molecular dynamic simulation trajectories show that the dominant short range structures above the melting temperature of these melts are 〈0,3,6,4〉 and 〈0,2,8,2〉 and the percentage of these structure increases with decreasing temperature. The residence time of the atoms in the cage was found to be directly correlated with the number of dominant short-range structures in these melts. Our results show that transient local short-range structures have a strong influence on atomic caging in the glass-forming metallic melts.