Interdiffusion Of Atoms Research Articles

Microstructural evolution and mechanical properties of diffusion bonding WC-Co cemented carbide to steel using Co and composite Ni/Co interlayers

This study focuses on diffusion bonding of WC-Co cemented carbide to a steel using Co and composite Ni/Co interlayers. The typical microstructure and microstructural evolution with variable bonding temperatures were investigated, and mechanical properties of the joint were evaluated. The results illustrated that increasing temperature promoted interdiffusion of interfacial atoms, leading to the elimination of interfacial voids and the enhancement of Fe-Co-Cr interdiffusion zone on steel side. As the temperature increased, so did the thickness of Co6W6C formed near the WC-Co substrate (using Co interlayer). On the other hand, the Co6W6C was inhibited due to the addition of Ni foil, and replaced by the WC and (Ni, Co) solid solution resulting in satisfactory shear strength of the diffusion-bonded joint, where referring to using composite Ni/Co interlayer. The maximum shear strength of 418 MPa was achieved when the joint was diffusion-bonded at 1100 °C for 60 min using composite Ni/Co interlayer. In addition, ductile fracture occurred in the joint indicating propagation of cracks in the Ni-Co interdiffusion zone, and passing through the WC-Co substrate. Added up, this study provided experimental support for reliable joining cemented carbides to Fe- or Co-based alloys.

Synthesis of bi-functional Ni/Co phosphate nanocomposites for Peroxymonosulphate activation and supercapacitor electrode

The hierarchical Ni3(PO4)2/Co3(PO4)2 nanocomposites were fabricated by one step hydrothermal technique via immersing Ni2+ and Co2+ with PO43-. Varing the cations, e.g., Na2HPO4, K2HPO4 and (NH4)2HPO4, the morphological changes of leaf, rod and platelet morphology of prepared nanocomposites were observed due to the Kirkendall effect modifying the nanomaterial structure via interdiffusion of transition metal atoms with phosphate ions source. The developed bimental phosphates displayed enhanced activity in dye degradation and supercapacitive performance. The Ni3(PO4)2/Co3(PO4)2 composite as an effective catalyst was applied for degradation of Rhodamine B by PMS activation. The maximum dye degradation efficiency was about 97% after 40 min. The prepared composite effectively produced sulfate radicals even after fifth cycle. In addition, the cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance analysis were investigated in 1 M Na2SO4 neutral electrolyte. The electrochemical investigations clearly indicated the Ni3(PO4)2/Co3(PO4)2 composites enhanced super capacitive performance with good stability nature. Hence, the bi-functional activities of Ni3(PO4)2/Co3(PO4)2 have the potential candidate to the water purification and energy storage applications.

Ultrafast and robust joining of 3YSZ and GH3128 superalloy using Cu interlayer under an electric field

3 mol% yttria-stabilized zirconia (3YSZ) and a nickel-based superalloy (GH3128) were robustly joined in seconds using a Cu interlayer at 1000 °C under the application of an electric field. The shear strength of the joints reached 190 ± 6 MPa within 2 s of energization and increased to 250 ± 15 MPa with the extension of the energization time to 30 s. The electric field-induced electrochemical reactions greatly accelerated atomic interdiffusion, generated Cu−Zr eutectic liquid at the interface and enabled the interfacial gap to be rapidly filled due to the good wettability of 3YSZ by the melt, thus yielding the strong joints in seconds. The key joining parameters (current density, time and temperature) were investigated and the correlations between interfacial structural evolution and fracture characteristics were addressed. This work provides a reliable method for ultrafast and robust joining of zirconia to metals.

Molecular dynamics simulation of atomic diffusion in friction stir spot welded Al to Cu joints

Dissimilar metals joining, especially Aluminum (Al) to copper (Cu), have gained importance in batteries for electric vehicles. Although friction stir spot welding (FSSW) has recently been used for welding dissimilar materials, progress has been very slow toward understanding the effect of temperature on diffusion condition between the two materials with the same FCC crystal structure. The thermo-mechanical modeling has been used to define the trajectory of Al and Cu particles at the weld interface, but it had a limitation to quantified the diffusion coefficient. Hence, the molecular dynamics (MD) study has been used to investigate the atomic interdiffusion of Al and Cu. The transmission electron microscopy results are used to validate the MD simulation outcome to understand the formation of dislocations and intermetallic compounds. The MD results implicated the formation of γ-phase (BCC), i.e., Al4Cu9 IMC toward the Cu side. Further, the In-situ investigation of non-FCC phase formation at FSSW condition has also been studied.

Interface design and the strengthening-ductility behavior of tetra-needle-like ZnO whisker reinforced Sn1.0Ag0.5Cu composite solders prepared with ultrasonic agitation

To develop a lead-free solder with high strength and ductility, a novel interface was designed between the tetra-needle-like ZnO whisker (T-ZnOw) and Sn1.0Ag0.5Cu solder matrix. First, surface modification of T-ZnOw was conducted using a pyrolysis method based on self-assembly to deposit NiO nanoparticles densely and evenly on the whisker surface. The interfacial relationship between T-ZnOw (100) and NiO (101) planes was semi-coherent with a misfit of 0.145. In-situ transmission electron microscopy was used to explore the surface modification mechanism. Next, a composite solder of 0.3mass% NiO/T-ZnOw reinforced Sn1.0Ag0.5Cu was prepared with ultrasonic agitation, and the introduced reinforcement was uniformly distributed in the solder matrix. A thin NiO transition layer existed between the reinforcement and matrix, indicating the formation of the Sn1.0Ag0.5Cu/NiO/T-ZnOw sandwich structure interface. No gaps, pores, or new phases were formed at the interface, and atomic interdiffusion occurred at the interface to form effective bonding. The composite solder shows improved wettability compared with plain solder. The ultimate tensile strength (UTS) and elongation of the composite solder were 32.5 MPa and 28.1%, respectively, being 19.0% and 35.1% higher than those of the plain Sn1.0Ag0.5Cu solder. The strengthening-ductility mechanisms primarily included load transfer, grain refinement, and misfit dislocation effects.

Studies of Cu-Sn interdiffusion coefficients in Cu3Sn and Cu6Sn5 based on the growth kinetics

In this work, we set up a kinetic model of Cu3Sn (ε) and Cu6Sn5 (η) growth based on the reaction/diffusion theory. The kinetic equations of the ε and η layers growth were established according to the principle that the growth was controlled by the interdiffusion of Cu and Sn atoms. We derived the kinetic equations with and without consideration of the volume change caused by phase transitions, where the Cu-Sn interdiffusion coefficients ((D˜ε, D˜η) were all included as dominant factors. On this basis, the interdiffusion coefficients at 130 °C ~ 200 °C were determined under the two conditions, and results showed that the volume change affected the accuracy of the calculation of D˜ε and D˜η to some extent.

Effects of substrate temperature and intermediate layer on adhesion, structural and mechanical properties of coaxial arc plasma deposition grown nanodiamond composite films on Si substrates

Low-temperature fabrication of nanodiamond films on untreated silicon substrates, which are preferred substrates and widely employed in industry, can extend nanodiamond film applications, for instance in biosensing and miniaturization technologies. For its realization, the weak adhesion of nanodiamond films on silicon substrates due to insufficient atomic interdiffusion at the interfaces is a serious issue that needs to be overcome. In the present study, coaxial arc plasma deposition (CAPD) grown nanodiamond films were fabricated at low temperatures on silicon substrates. The effects of the substrate temperature, which is an important parameter in the CAPD nanodiamond film growth process, were structurally and physically investigated. It was found that the films softened with increasing substrate temperature and atomic interdiffusions at interfaces between the films and substrates were activated, which resulted in the formation of SiC at the interfaces. On the other hand, the films fabricated directly on silicon at room temperature were exfoliated due to the poor adhesion and high residual stress. Consequently, intermediate layers that were fabricated at elevated substrate temperatures were employed to improve the adhesion strength of nanodiamond films on silicon substrates. This way, the atomic interdiffusion at the interfaces could be promoted and the residual stress in the films was relieved. In the second stage, the films were fabricated on the intermediate layers at different temperatures. It has shown that the top layer films fabricated at low temperature gained distinctive mechanical and structural properties. By applying this strategy, several micrometres of hard nanodiamond films can be fabricated on Si substrates without easily being peeled off.

Facile synthesis of Cu10Sn3 nanoparticles and their sintering behavior for power device packaging

Abstract Low temperature nanojoining and high temperature service in the microelectronic packaging technology has received much focused attention. Here, we suggest a facile thermal decomposition method for preparation of Cu10Sn3 nanoparticles (NPs) with high bulk melting point (640 ​°C) by using common organic salts, oleylamine (OLA), and oleic acid (OA). The uniform and dense sintered Cu/Cu10Sn3/Cu joints can obtain at 300 ​°C, 10 ​MPa, 20 ​min, which presents the average shear strength of 25.1 ​MPa. In addition, the interdiffusion of Sn and Cu atoms at interface between the sintered Cu10Sn3 layer and Cu makes the interface a metallurgical bonding. This sintered Cu/Cu10Sn3/Cu joints can provide a new interconnect material and technology for high-power device package.

Alloying/dealloying mechanisms, microstructural modulation and mechanical properties of nanoporous silver via a liquid metal-assisted alloying/dealloying strategy

Nanoporous metals show great potentials in various applications including catalysis, sensing, actuation and supercapacitors. The liquid Ga-assisted alloying/dealloying strategy is a feasible and scalable way to fabricate substrate-supported nanoporous metals. However, the influence of intrinsic alloying mode and mechanism on the formation and modulation of nanoporous structure has not been thoroughly explored before. In this work, after painting liquid Ga on Ag foil, both the bulk-like Ag3Ga (in the Ag-rich zone) and wire-like Ag3Ga (in the liquid Ga-rich zone) form owing to the interdiffusion of Ag and Ga atoms. Correspondingly, the bimodal wire-like and unimodal bulk-like nanoporous Ag is obtained because of the structure inheritance during dealloying. In addition, the thickness of alloy layer (nanoporous layer) versus mass loading of liquid Ga follows a good linear relationship. The in-situ X-ray diffraction of dealloying from Ag3Ga to nanoporous Ag illustrates no other intermetallic phase appears and the dealloying process can be described by the dissolution-surface diffusion model. More importantly, the substrate-supported nanoporous Ag exhibits excellent mechanical properties which are of great importance to the future applications of nanoporous metals.

Atomic layer deposition of vanadium oxides using vanadyl acetylacetonate as the precursor

Thermochromic VO2 thin films are fabricated by atomic layer deposition with vanadyl acetylacetonate as the precursor. The crystallization process has been performed by post-annealing process, i.e. rapid thermal annealing. The effect of annealing temperature on thermochromic properties is investigated. Though the transmittance increases with increasing annealing temperature, a moderate temperature is suggested by the consideration of atomic inter-diffusion between TiO2 and VO2 layers. It is found that an oxygen-deficient phase of V2O3 appears at low annealing temperatures, while an oxygen-ambient phase of V2O5 presents at high temperatures. Additionally, a ternary compound, Ti3V6O17 is dominant beyond 700°C. The crystalline behavior of monoclinic phase of VO2 and Ti3V6O17 observed from the X-ray diffraction pattern is further examined by high resolution transmission electron microscope. The depth profiling in binding energies of V 2p, O 1s and Ti 2p orbits are compared.

Friction self-piercing riveting (F-SPR) of aluminum alloy to magnesium alloy using a flat die

Friction self-piercing riveting (F-SPR) process based on a pip die has been invented to solve the cracking problems in riveting high-strength and low-ductility light metals, such as magnesium alloys, cast aluminum, and 7 series aluminum alloys. In this paper, in order to solve quality issues caused by the misalignment between rivet and pip-die in F-SPR, a flat-die based F-SPR process was proposed and employed to join 1.27 mm-thick AA6061-T6 to 3 mm-thick AZ31B. The results indicate that a 1.0 mm die distance is effective to avoid rivet upset and insufficient flaring. As the feed rate increases, the heat input in the whole process decreases, resulting in a larger riveting force, which in turn increases both the bottom thickness and interlock amount. Besides, solid-state bonding, including Al-Mg intermetallic compounds (IMCs), Al-Mg mechanical mixture, and Al-Fe atom interdiffusion was observed at the joint interfaces. The upper Al layer was softened, but the lower Mg layer was hardened, and both sheets exhibited a narrowed affected region with the increase of feed rate, while the rivet hardness shows no obvious change. Three fracture modes appeared accompanying the variations in lap-shear strength and energy absorption as the feed rate increased from 2 mm/s to 8 mm/s. Finally, the F-SPR process using a flat die was compared to those using a pip die and a flat bottom die to show the advantage of flat die on coping with the misalignment problem.

Pressure-assisted direct joining of Ti3AlC2 ceramics in air

We introduced a novel strategy for direct joining Ti3AlC2 ceramics in air without the need for any interlayer and vacuum/ inert gas protection. This was achieved by applying constant external pressure to manipulate the amount of available oxygen for oxidation at the Ti3AlC2―Ti3AlC2 interface. The shear strength of the joints bonded under a pressure of 10 MPa was approximately 3 times higher than that without pressure. The joining mechanisms were addressed based on the atomic interdiffusion and preferential oxidation of Al in lamellar Ti3AlC2 grains.

Effect of deposition substrates on surface topography, interface roughness and phase transformation of the Al/Ni multilayers

The Al/Ni multilayers with modulation periods of 50, 250, and 500 nm were deposited on AlN, AlNi3, Si, and Al2O3, respectively. Surface topography, interface roughness and phase transformation show strong dependence on the substrates. The substrate roughness can much increase the interface roughness, and this effect can accumulate for the later layers. The severely roughened surface of the AlN substrate causes much rougher interfaces, poorer surface flatness, and hence low compactness of the as-deposited multilayers. Phase transformation varies with the modulation period below 673 K. The stable AlNi preferentially formed in the 500 nm period multilayers instead of in smaller period samples at 773 K, which was related to the interface roughness. The roughened interface facilitates atomic interdiffusion to meet the nucleation requirement at the initial stage. The pre-formed intermetallic phase layers would hinder the diffusion of Ni and finally slower the formation of AlNi when annealing at 773 K. Substrates could also affect the intermixing region at interfaces. Larger hardness combined with smaller thermal conductivity coefficient of Al2O3 gives rise to clearer and sharper interfaces and finally leads to slower phase transformation in 500 nm period multilayers. This result suggests that substrates may provide advantages to tailor the performance of multilayers.

Elucidating the surface compositions of Pd@PtnL core-shell nanocrystals through catalytic reactions and spectroscopy probes.

The catalytic behaviors or properties of bimetallic catalysts are highly dependent on the surface composition, but it has been a grand challenge to acquire such information. In this work, we employ Pd@PtnL core-shell nanocrystals with an octahedral shape and tunable Pt shell thickness as a model system to elucidate their surface compositions using catalytic reactions based upon the selective hydrogenation of butadiene and acetylene. Our results indicate that the surface of the core-shell nanocrystals changed from Pt-rich to Pd-rich when they were subjected to calcination under oxygen, a critical step involved in the preparation of many industrial catalysts. The inside-out migration can be attributed to both atomic interdiffusion and the oxidation of Pd atoms during the calcination process. The changes in surface composition were further confirmed using infrared and X-ray photoelectron spectroscopy. This work offers insightful guidance for the development and optimization of bimetallic catalysts toward various reactions.

Состав и морфология поверхности Si(111) с поверхностной пленкой SiO-=SUB=-2-=/SUB=- разной толщины

In this work, the composition, morphology, and electronic structure of SiO2 nanofilms of various thicknesses, created by thermal oxidation on the Si(111) surface, have been studied. It is shown that up to a thickness of 30–40 Å, the film has an island character. At d ≥ 60 Å, a homogeneous continuously film of SiO2 is formed and the stoichiometric surface roughness of which does not exceed 1.5 - 2 nm. Regardless of the film thickness of the SiO2 appreciable interdiffusion of atoms at the interface SiO2-Si not observed. The regularities of the change in the composition, the degree of surface coverage, and the energy of plasma oscillations with a change in the thickness of the SiO2/Si(111) films in the range from 20 to 120 Å have been determined.

Evolution of interfacial structure of the joints between a tungsten-copper composite and austenitic stainless steel

Joining of CuW70 composite and 0Cr18Ni9 stainless steel was successfully achieved at temperatures 1150 °C with durations from 15 to 90 min after adding a pure copper intermediate layer. Microstructure, phase composition and elemental distribution at the interfacial region were studied. The optimum joining interface with a dense microstructure was obtained at a joining temperature of 1150 °C with a duration of 60 min. Interdiffusion of atoms occurred at the interface between the tungsten-copper composite and stainless steel, resulting in formation of various compounds such as W0.6Cu0.4, Cu3.8Ni and Fe0.946Ni0.054. An iron chromium-rich layer was formed at the interface near the tungsten-copper composite side, and an island structure containing Cr, Cu, and Ni was formed near the stainless steel side.

Properties of lithium phosphorus oxynitride (LiPON) solid electrolyte - Li anode interfaces

In recent years, all-solid-state thin-film batteries have been used to power low-energy devices such as microchips, smart cards, microelectromechanical systems, wireless sensors, and implantable medical devices. All-solid-state thin-film batteries have become an important research direction of rechargeable solid-state batteries (SSBs). However, the solid-solid interface between electrodes and electrolytes seriously affects the further improvement of battery performance, which has attracted extensive attention. Lithium phosphorus oxynitride (LiPON) was found to be a useful inorganic electrolyte in lithium batteries because of its favorable electrochemical properties. For instance, LiPON has good electrical and chemical stability, negligible electronic conductivity and excellent cyclability as well as ease of integration with thin film battery with an electrochemical stability window. The LiPON can present two states, <i>i.e.</i> amorphous state and crystalline state. Here, we adopt <i>ab initio</i> molecular dynamics to study amorphous-LiPON/Li(100) interface and crystalline-Li<sub>2</sub>PO<sub>2</sub>N(100)/Li(100) interface. Our results demonstrate that the atomic inter-diffusion occurs in the interfacial region, forming a thin interfacial layer, and the ionic conductivity is increased after the interface layer has formed. Meanwhile, comparing with the Lipon bulk phase structure, the proportion of Li[O<sub>2</sub>N<sub>2</sub>], Li[O<sub>3</sub>N], and Li[O<sub>4</sub>] tetrahedral local structure in the interface layer with Li atom as the center decrease obviously, and the average coordination number of Li-O, Li-N, P-O, and P-N in the interfacial layers also decrease in the LiPON/Li interface. Due to the change of structure and coordination number at the interface, the ionic bonds between Li and O, N are weaker, which explains the increase of ionic conductivity at the LiPON/Li interface. Previous experiments showed that element interdiffusion occurs at the LiPON/Li interface and the interface layer is formed, and found that the decrease in impedance of the interface layer can confirm that the ionic conductivity of the interface layer indeed increases. In addition, the tetrahedral structure of the interface layer will be decomposed into other smaller structures. Our computational results are consistent with the previous experimental results, which indicates the rationality and reliability of our conclusion. This feature plays a positive role in promoting the performance of LiPON electrolytes in practical battery applications.

The role of FeCo and FeCoCr nanowires array on the formation of single phase bi-magnets

Nanowire (NWs) arrays of FeCo and FeCoCr were prepared by AC pulsed electrodeposition embedded into an alumina template. They were then employed as an underlayer to provide the dot array of cobalt ferrite thin films. Highly ordered arrays of NWs with the diameter of 30 nm and length of 10 µm were fabricated and then annealed in various environments. The cobalt ferrite thin films were deposited by the sputtering approach with the almost constant thickness of 70 nm as the hard magnetic phase. The combination of FeCo and cobalt ferrite could provide an exchange spring magnet with the magnetic coupling feature. X-ray diffraction (XRD) patterns of the NW arrays indicated the formation of the body centered cubic (bcc) FeCo phase with [110] texture. Field emission electron microscopy (FESM) equipped by EDS also characterized the morphological features and elemental composition. Magnetization and coercivity of NWs were examined using the vibrating sample magnetometer. The maximum coercivity of NWs array is 2950 Oe which is related to H2 annealing FeCo NWs array. The magnetization reversal process of the as-deposited and annealed FeCo and FeCoCr NWs arrays was performed by angular dependence coercivity and squareness. Further, FORC diagrams indicated an interacting single domain behavior for as-deposited FeCo NWs and a weakly-interacting single domain state for the annealed samples. The morphological and structural features of cobalt ferrite were also studied by XRD, FE-SEM and AFM techniques. It was found that the cobalt ferrite dot array with a uniform chemical composition and structure was formed. The inter-diffusion of atoms between underlayer and thin films was confirmed by EDS. The maximum coercivity of the nanocompsite was 5800 Oe that is obtained for FeCoCr underlayer with argon annealing. The VSM analysis showed that sufficient exchange coupling was formed by using the FeCoCr underlayer in hydrogen annealing, while argon annealing provided a kink in the hysteresis loops. The obtained nanocomposites could be, therefore, considered as the potential candidate for use in the exchange spring magnet.

Fusion bonding of copper and silicon at -70 °C by electrochemistry

Wafer bonding processing typically employs thermal energy to fuse two surfaces by stimulating atomic interdiffusion at high temperatures. However, we found that the fusion bonding of copper and silicon can occur at an extremely low temperature in cryo-electrochemical processing cooled by dry-ice (-20 °C) or even by liquid nitrogen (-70 °C). The results demonstrate that electrical energy can replace the thermal energy that must be used in semiconductor processes. The bonding phenomenon reproducibly occurred, and even the copper surface was not favorable for spontaneous bonding. Notably, the bonding strength of Cu/Si was very high. Even after forcibly inserting a razor at the bonding interface, a copper layer was split from the Cu host substrate to enable a transfer onto silicon. The secondary-ion mass spectrometry (SIMS) analysis revealed that the bonding was caused by nanoscale interdiffusion between surface copper and silicon atoms. We propose a possible mechanism where holes are driven into the bonding interface of Cu/Si under bias, positively charge the Cu atoms and form cations (i.e., surface activation). The electric field continuously drives Cu cations to bond with the dangling bonds on the mating silicon to form Si-Cu bonds. The Cu cations that arrive later can pass over the bonding interface and quickly diffuse into the silicon interstitials. This study of fusion bonding at -70 °C by electrochemistry-assisted interdiffusion instead of thermal energy has profound implications for wafer bonding processes and applications.

Single-pass laser welding of TC4 Ti alloy to 304 SS with V interlayer and V/Cu bilayer

Two novel single-pass laser welding methods were performed to join TC4 Ti alloy and 304 SS using V interlayer and V/Cu bilayer. In order to avoid the formation of brittle Ti-based IMCs, two hybrid joints based on different welding mechanisms were created. The joint using the V interlayer with a fusion weld and a diffusion weld was obtained. The fusion weld was formed on Ti alloy, diffusion weld was formed at the Ti alloy-SS interface. The joint using the V/Cu bilayer with a fusion weld, a diffusion weld, a braze weld was obtained. The fusion weld was formed on Ti alloy, V and Ti alloy were joined by atomic interdiffusion, SS and Cu were joined by brazing. The tensile strength and quality of joints was evaluated.