Interfacial Phase Formation Research Articles

Prediction of interfacial phase formation and mechanical properties of Ti6Al4V–Ti43Al9V laminate composites

Ti6Al4V–Ti43Al9V laminate composites were successfully fabricated by hot-pack rolling using as-forged Ti43Al9V alloy and as-rolled Ti6Al4V alloy. The microstructure evolution of composites was investigated by electron probe microanalysis (EPMA) equipped with energy dispersive X-ray spectrometry (EDS) and the electron back scattered diffraction (EBSD). In order to better control the properties of laminate composites, two models are established for better control the thickness and phase formation of interfacial region, respectively. The mechanical properties of composites show a negative correlation with rolling temperature. The composite fabricated at 1125 °C exhibits the highest tensile strength 805 MPa and fracture toughness 49.2 MPa•m1/2 which are much higher than Ti43Al9V alloy and other Ti–Al based laminate composites. The fracture features reveal that the increase of tensile strength is attributed to high density of dislocations and sub-structures stored in TiAl alloy and the crack propagation features show that the enhancement of toughness is associated with the aggravation of misorientation of α2 phase and the increase of the amount of γ/B2 boundaries.

Atomic-resolution interfacial structures and diffusion kinetics in Gd/Bi0.5Sb1.5Te3 magnetocaloric/thermoelectric composites

The demand of a full solid-state cooling technology based on magnetocaloric and thermoelectric effects has led to a growing interest in screening candidate materials with high-efficiency cooling performance, which also stimulates the exploration of magnetocaloric/thermoelectric hybrid cooling materials. A series of Gd/Bi0.5Sb1.5Te3 composites was fabricated in order to develop the hybrid cooling technology. The chemical composition, phase structure and diffusion kinetics across the reaction layers in Gd/Bi0.5Sb1.5Te3 composites were analyzed at different reaction temperatures. Micro-area elemental analysis indicates that the formation of interfacial phases is dominated by the diffusion of Gd and Te while the diffusion of Bi and Sb is impeded. The interfacial phases, including GdTe2, GdTe3, and intermediate phases GdTex, are identified by atomic-resolution electron microscopy. The concentration modulation of Gd and Te is adapted by altering the stacking of the Te square-net sheets and the corrugated GdTe sheets. Boltzmann-Matano analysis was applied to reveal the diffusion kinetics of Gd and Te in the interfacial layers. The diffusion coefficients of Te in GdTe2 and GdTe3 are much higher than that of Gd while in GdTe the situation is reversed. This study provides a clear picture to understand the interfacial phase structures down to an atomic scale as well as the interfacial diffusion kinetics in Gd/Bi0.5Sb1.5Te3 hybrid cooling materials.

Van der Waals interfacial reconstruction in monolayer transition-metal dichalcogenides and gold heterojunctions

The structures and properties of van der Waals (vdW) heterojunctions between semiconducting two-dimensional transition-metal dichalcogenides (2D TMDs) and conductive metals, such as gold, significantly influence the performances of 2D-TMD based electronic devices. Chemical vapor deposition is one of the most promising approaches for large-scale synthesis and fabrication of 2D TMD electronics with naturally formed TMD/metal vdW interfaces. However, the structure and chemistry of the vdW interfaces are less known. Here we report the interfacial reconstruction between TMD monolayers and gold substrates. The participation of sulfur leads to the reconstruction of Au {001} surface with the formation of a metastable Au4S4 interfacial phase which is stabilized by the top MoS2 and WS2 monolayers. Moreover, the enhanced vdW interaction between the reconstructed Au4S4 interfacial phase and TMD monolayers results in the transition from n-type TMD-Au Schottky contact to p-type one with reduced energy barrier height.

Investigation of Interfacial Phase Formation During Corrosion of Stainless Steel

Abstract: The paper shows an adaptation of thermoanalytical techniques such as Differential Scanning Calorimetry (DSC) for the investigation of interfacial phase formation during corrosion of stainless steel 316L (FeCr18Ni10Mo3)- and 16-7-3 (FeCr16Mn7Ni3)-based composites with TiO2 and ZrO2 in molten aluminium alloy (AlSi7Mg0.3). The DSC was able to detect the formation of corrosion phases on cooling after two hours of retention at 850 °C. The method allows the recognition of the solidification temperature of new corrosion phases (binary η-AlFe and ternary τ3, τ5, τ5, τ11-AlFeSi) as well as the assessment of the conversion rate of both materials. Scanning Electron Microscopy (SEM) was used for the analysis of the corrosion products.

Achieving Both High Ionic Conductivity and High Interfacial Stability with the Li2+xC1-xBxO3 Solid-State Electrolyte: Design from Theoretical Calculations.

A crystalline solid electrolyte interphase Li2CO3 material with a large band gap shows promise toward next-generation all-solid-state lithium batteries (ASSLBs). However, the inferior ionic diffusivity restricts such structures to a real battery setup. Herein, based on density functional theory calculation and Python materials genomics, we theoretically develop the chemistry and local structural motifs to build a mixed boron-carbon framework Li2+xC1-xBxO3 (LCBO). We examine the electrochemical and chemical stabilities of LCBO-electrode interfaces by analyzing the thermodynamics of formation of interfacial phases. Interestingly, the LCBO material is automatically protected from further decomposition through the self-generated resistive interphase (Li2CO3 and Li3BO3), which gives a wide range of operating potential. LCBO shows high interfacial stability with LiCoO2, LiMnO2, and LiMn2O4. More importantly, the theoretical Li-ion migration barrier of LCBO (x = 0.375) is approximately 0.23 ± 0.02 eV through a cooperative migration mechanism. Therefore, the LCBO material combines high Li-ion diffusivity with good interfacial stability, which makes it a promising solid-state electrolyte material for ASSLBs.

Enhancing interface bonding strength of Ti-ZrO2 joints using graphene as brazing materials

Ti-ZrO2 joints with a graphene sponge interlayer were prepared and the interface bonding was characterized by three-point flexural strength. Compared with direct Ti-ZrO2 jointing showing a formation of monoclinic TiO2 and ZrTiO4 at interface and a negligible bonding, interfacial strength of Ti/graphene sponge/ZrO2 joints has been significantly increased up to 48.5 ± 7.0 MPa without any new interfacial phase formation. The enhanced interface bonding can be ascribed to the preferred interstitial diffusion of carbon atoms originated from the decomposition of graphene sponge interlayer at high temperature. The carbon diffusion reduces the available interstitials and vacancies for Ti, O and Zr diffusions, which retards the interdiffusion between titanium and zirconia, achieving a preferred diffusion bonding to enhance the strength of Ti-ZrO2 joints.

Origin of High Interfacial Resistances in Solid‐State Batteries: Interdiffusion and Amorphous Film Formation in Li0.33La0.57TiO3/LiMn2O4 Half Cells

AbstractThe large interfacial resistance between electrolyte and electrodes poses a significant roadblock for the application of all‐solid‐state batteries. The formation of interfacial phases (interphases) has been identified as one of the most significant sources for such high resistance. Therefore, studying the mechanism of interphase formation, along with investigating its effect on ionic conductivity, could lead to the discovery of avenues towards designing high‐performance all‐solid‐state batteries. In this work, we studied the interphase formation in the perovskite electrolyte Li0.33La0.57TiO3 (LLTO) and spinel cathode LiMn2O4 (LMO) pair by co‐sintering experiments via spark plasma sintering (SPS), as well as conventional sintering. Although the processing method has an influence on the electrode/electrolyte contact, the formation of an interphase could not be avoided. At the LLTO/ LMO interface, we observed both an interphase formed by interdiffusion, as well as a complexion‐like amorphous layer. We directly characterized the complexion layer morphology by using HRTEM. Analytical TEM and SEM were used to reveal the elemental composition of the interphase and the interdiffusion layer. Furthermore, we used impedance spectroscopy to measure the electrical properties of the LLTO/LMO interphase and identified the interfacial resistance from the interdiffusion induced interphase to be larger than the individual phases by a factor of 40, whereas the amorphous layer was not visible in the impedance.

The formation mechanism of a novel interfacial phase with high thermal stability in a Mg-Gd-Y-Ag-Zr alloy

Due to their unique precipitation behavior, magnesium-rare earth (Mg-RE) alloys exhibit excellent mechanical properties and decent thermal stability. In this work, a Mg-Gd-Y-Ag-Zr alloy was employed to investigate the segregation and interfacial phase formation at grain boundaries after plastic deformation and heat treatment. The interfacial phase was unequivocally investigated by aberration-corrected high-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) from three different crystal directions and modeling, which reveals a hitherto unknown crystal structure (monoclinic: β = 139.1°, a = 1.20 nm, b = 1.04 nm and c = 1.59 nm). Its orientation relationship with the Mg matrix is: [101]//[112¯0]α, [302]//[101¯0]α and (010)//(0001)α. Different from the precipitates in matrix, the size of the interfacial phase was not sensitive to annealing temperature between 250 °C and 400 °C. Transformation of twin boundaries to coaxial grain boundaries via multiple twinning led to the generation of many high strain sites along the boundaries, which promoted the formation of the interfacial phase. The interfacial phase was stable up to 400 °C, which was about 100 °C higher than the dissolution temperature of β′ and γ" precipitates.

Interfacial reactions to form high-barrier-height ITO-based Schottky contacts on p-type GaN using a Ti interlayer

We report the development of high-barrier-height and transparent Ti/ITO Schottky contacts on p-GaN for optoelectronic and transparent electronic devices. The Schottky barrier heights (SBHs) and ideality factors estimated using the current–voltage characteristics were in the ranges of 0.36–0.39 eV and 1.74–2.07, respectively, depending on the annealing temperature. However, the barrier inhomogeneity and modified Richardson plot methods yielded much larger SBHs in the range of 0.82–1.18 eV. At 560 nm, the transmittance of the Ti/ITO samples was in the range of 47.4–89.9%. The Ga 2p core levels obtained from the X-ray photoemission spectroscopy (XPS) of the interface regions of the ITO/Ti/GaN samples shifted toward higher or lower energies, depending on the annealing temperature. The normalized N/Ga atomic ratio showed that N and Ga vacancies were formed at the p-GaN surface region at 300 and 500 °C, respectively. The XPS Ti 2p, N 1s, and O 1s core level results showed the formation of interfacial TiN and TiO2 phases at 300 and 500 °C, respectively. The elemental mapping results obtained using scanning transmission electron microscopy (STEM) demonstrated the outdiffusion of Ga atoms in the sample annealed at 500 °C. On the basis of XPS and STEM results, the dependence of the SBHs on the annealing temperature is described and discussed.

Wettability of SiC and graphite by Co–Ta alloys: evaluation of the reactivity supported by thermodynamic calculations

Within the context of the design of high-temperature brazing process for C and SiC-based composites, a basic study is presented here about the wetting and interfacial reactivity of Co–Ta alloys in contact with graphite or SiC. Wetting results are presented for the first time showing that Co–Ta alloys wet C or SiC fairly or excellently depending on the relative amount of Ta. The final interfacial microchemistry and microstructures are the result of the interplay between the typical interfacial phenomena of the pure elements. Specifically, depending on the alloy composition, dissolution of the ceramic phase by Co or formation of a continuous interfacial layer of TaC that prevents dissolution prevails. The discussion about the interfacial reactivity between liquid Co–Ta alloys and graphite or SiC, as well as the interpretation of solidification phenomena and the formation of interfacial phases, is supported by making reference to new multi-component phase diagrams calculated by CALPHAD method.

Interfacial reaction and wetting behavior of active Ag-Cu filler alloys on surface of cBN grits and its influence on failure patterns of brazed joint

Cubic boron nitride (cBN) abrasive grits are often brazed on steel to produce monolayer grinding wheels. Establishing a strong cBN brazement is a challenge. In the current work, performance of three different active Ag-Cu alloys have been investigated in-depth considering microstructure of alloys, interfacial reaction phase formation and wetting capability of the reaction product towards the alloy. In the current study, all three alloys contain Ti as active element but with different wt%. One of alloys was so chosen that Indium (In) was present as an additional alloying element, which is responsible for its low liquidus temperature, high Young's modulus and good ductility. Segregation of Ti towards the interface was critically examined. To evaluate joint strength, a miniature twin-ring grinding wheel was produced with these alloys. Interestingly, the alloy containing higher Ti, which required higher brazing temp, resulted in intense reaction but with inferior joint strength. On the other hand, the very presence of indium in Ag-Cu alloy, containing low wt% of Ti not only lowered the brazing temperature but also led to effective wetting and higher joint strength. Failure pattern indicates the prevalence of high residual stress at the bond level for the alloy containing the highest percentage of titanium.

Interfacial phase formation of Al-Cu bimetal by solid-liquid casting method

The solid-liquid method was used to prepare the continuous casting of copper cladding aluminium by liquid aluminum alloy and solid copper, and the interfacial phase formation of Al-Cu bimetal at different pouring temperatures (700, 750, 800 °C) was investigated by means of metallograph, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) methods. The results showed that the pouring temperature of aluminum melt had an important influence on the element diffusion of Cu from the solid Cu to Al alloy melt and the reactions between Al and Cu, as well as the morphology of the Al-Cu interface. When the pouring temperature was 800 °C, there were abundant Al-Cu intermetallic compounds (IMCs) near the interface. However, a lower pouring temperature (700 °C) resulted in the formation of cavities which was detrimental to the bonding and mechanical properties. Under the conditions in this study, the good metallurgical bonding of Al-Cu was achieved at a pouring temperature of 750 °C.

Effect of Indium Additions on the Formation of Interfacial Intermetallic Phases and the Wettability at Sn-Zn-In/Cu Interfaces

The wettability of copper substrates by Sn-Zn eutectic solder alloy doped with 0, 0.5, 1, and 1.5 at.% of indium was studied using the sessile drop method, with flux, in air, at 250°C and reflow time of 3, 8, 15, 30, and 60 min. Wetting tests were performed at 230, 250, 280, 320, and 370°C for an alloy containing 1.5 at.% of indium, in order to determine activation energy of diffusion. Solidified solder/substrate couples were studied using scanning electron microscopy (SEM), the intermetallic phases from Cu-Zn system which formed at the solder/substrate interface were identified, and their growth kinetics was investigated. The ε-CuZn4 was formed first, as a product of the reaction between liquid solder and the Cu substrate, whereas γ-Cu5Zn8 was formed as a product of the reaction between ε-CuZn4 and the Cu substrate. With increasing wetting time, the thickness of ε-CuZn4 increases, while the thickness of ε-CuZn4 does not change over time for indium-doped solders and gradually disappears over time for Sn-Zn eutectic solder.

On the thermal conductivity of bimodal SiC/A356 composites fabricated via powder metallurgy route

ABSTRACTThe present study investigates the thermal conductivity of bimodal SiC particulate distribution in aluminum matrix composites fabricated via powder metallurgy route. The effects of the SiCp reinforcement size distribution and processing parameters such as sintering time and temperature on the thermal conductivity have been examined. The Box–Behnken experimental array was employed to identify the effects of selected variables on the thermal conductivity of the composite. A reasonable augmentation in the thermal conductivity was observed with an increase in sintering time and %volume fraction of fine SiC particulates. It has been demonstrated that the matrix doped with fine SiC particulates (37 µm) occupied interstitial positions and formed continuous SiC–matrix network resulting in minimizing the micropores that contributed for good thermal conductivity, that is, 235 W/mK. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were conducted to evaluate the microstructure architecture and interfacial phase formation.

Super‐toughened poly(l‐lactic acid) fabricated via reactive blending and interfacial compatibilization

AbstractSuper‐toughened poly(l‐lactic acid) (PLLA) was prepared by reactive blending of PLLA with poly(ϵ‐caprolactone) (PCL), glycerol and 4,4′‐methylenediphenyl diisocyanate. The reactive interfacial compatibility between PLLA and the formed crosslinked polyurethane (CPU) in the PLLA matrix was studied in detail. The morphology and the toughness of the blends can be tuned by changing the CPU content. The results indicate that the impact strength of PLLA shows a tendency to higher values with the increasing PCL content up to 20 wt%. The notched impact strength of the blend with 20 wt% PCL increases to 55.01 kJ m−2, which is 24.9 times higher than that of neat PLLA. The elongation at break is also increased from 5% to 139.4%, indicating the brittle − ductile transition. The increased interfacial binding strength through the reactive interfacial compatibility and the formation of a CPU network in the PLLA matrix account for the improved toughness of PLLA/CPU blends. Dynamic mechanical analysis results indicate that the compatibility between PLLA and CPU is improved with increasing CPU content resulting in the formation of more interfacial phase. In addition, rheological property measurements indicate that the improvement in storage modulus and complex viscosity is ascribed to the formation of a CPU network in the PLLA matrix. © 2016 Society of Chemical Industry

First principles calculations of point defect diffusion in CdS buffer layers: Implications for Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4-based thin-film photovoltaics

We investigate point defects in CdS buffer layers that may arise from intermixing with Cu(In,Ga)Se2 (CIGSe) or Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers in thin-film photovoltaics (PV). Using hybrid functional calculations, we characterize the migration barriers of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities and assess the activation energies necessary for their diffusion into the bulk of the buffer. We find that Cu, In, and Ga are the most mobile defects in CIGS-derived impurities, with diffusion expected to proceed into the buffer via interstitial-hopping and cadmium vacancy-assisted mechanisms at temperatures ∼400 °C. Cu is predicted to strongly favor migration paths within the basal plane of the wurtzite CdS lattice, which may facilitate defect clustering and ultimately the formation of Cu-rich interfacial phases as observed by energy dispersive x-ray spectroscopic elemental maps in real PV devices. Se, Zn, and Sn defects are found to exhibit much larger activation energies and are not expected to diffuse within the CdS bulk at temperatures compatible with typical PV processing temperatures. Lastly, we find that Na interstitials are expected to exhibit slightly lower activation energies than K interstitials despite having a larger migration barrier. Still, we find both alkali species are expected to diffuse via an interstitially mediated mechanism at slightly higher temperatures than enable In, Ga, and Cu diffusion in the bulk. Our results indicate that processing temperatures in excess of ∼400 °C will lead to more interfacial intermixing with CdS buffer layers in CIGSe devices, and less so for CZTSSe absorbers where only Cu is expected to significantly diffuse into the buffer.

Thin Film Oxy-Apatite Anodes for Solid Oxide Fuel Cells

Thin film ZnO-doped Ce4.67(SiO4)3O (ZCS) oxy-apatite was synthesized via room temperature sputtering of binary oxide precursor components followed by low oxygen partial pressure (p(O2) ∼10−20 atm) annealing at temperatures not exceeding 1100 K to enable interfacial reactions and phase formation. During the synthesis, the emergence of apatite phase is identified by in-situ electrical conductivity measurements and this is correlated with ex-situ investigations including X-ray diffraction and X-ray photoelectron spectroscopy. The synthesized ZCS apatite films show good mixed conductivity. Solid oxide fuel cells (SOFC) implementing Ni–apatite co-deposited composite electrodes are demonstrated for the first time with a peak power density of ∼105 mW cm−2 at 823 K which presents feasibility of thin film solid state reactions as a means of synthesis of challenging metastable phases and their explorations in energy conversion.

Interface Stability in Solid-State Batteries

Development of high conductivity solid-state electrolytes for lithium ion batteries has proceeded rapidly in recent years, but incorporating these new materials into high-performing batteries has proven difficult. Interfacial resistance is now the limiting factor in many systems, but the exact mechanisms of this resistance have not been fully explained - in part because experimental evaluation of the interface can be very difficult. In this work, we develop a computational methodology to examine the thermodynamics of formation of resistive interfacial phases. The predicted interfacial phase formation is well correlated with experimental interfacial observations and battery performance. We calculate that thiophosphate electrolytes have especially high reactivity with high voltage cathodes and a narrow electrochemical stability window. We also find that a number of known electrolytes are not inherently stable but react in situ with the electrode to form passivating but ionically conducting barrier layers. A...

WC-Co Particles Reinforced Aluminum Matrix by Conventional and Microwave Sintering

Metal matrix composites of Al-WC (10, 15 and 20 wt%) were prepared by microwave (without holding) and conventional sintering (held for 1 hour) processes at various temperatures between 650-950 °C. The results indicated that the highest density of conventional and microwave sintering corresponds to 97.4 ± 1.2% and 98.6 ± 0.8 of theoretical density, respectively; The highest bending strength of conventional and microwave sintered samples were 223 ± 12 and 256 ± 12 MPa, respectively. XRD patterns showed the decomposition of WC particles and formation of Al5W and Al12W interfacial reaction product phases in both processes. SEM studies showed that WC reinforcement particles were more likely to be agglomerated in microwave compared to conventional sintering process.

Intermetallic formation and interdiffusion in diffusion couples made of uranium and single crystal iron

We studied the interfacial phase formation and diffusion kinetics in uranium–iron diffusion couples. A comparison was made between polycrystalline uranium (U) bonded with polycrystalline iron (FeP) and polycrystalline uranium bonded with single crystalline Fe (FeSC). After thermal annealing at 575 °C, 600 °C, 625 °C and 650 °C, respectively, diffusion and microstructures at the interface were characterized by scanning electron microscopy and transmission electron miscopy. The presence of grain boundaries in iron has a significant influence on interface reactions. In comparison with U–FeP system, interdiffusion coefficients of the U–FeSC system are significantly lower and were governed by much higher activation energies. Integrated interdiffusion coefficients and intrinsic diffusion coefficients were obtained. The intrinsic diffusion coefficients show faster diffusion of iron atoms in both U6Fe and UFe2 intermetallic phases than uranium.