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The reduction behavior of the surface oxide on Cu particles under a reducing gas atmosphere was investigated for a pressureless sinter joining. We conducted x-ray thermodiffraction analysis and simultaneous thermogravimetry, differential thermal analysis, and mass spectroscopy (TG–DTA–MS) under a reducing atmosphere to investigate the reduction and subsequent sintering behaviors of copper particles at different oxygen concentrations. The shear strength of the pressureless sinter joint decreased with increasing oxygen concentration. The thermodiffraction results revealed that the reduction onset of Cu2O started at the same temperature (220°C), whereas the reaction markedly persisted at higher oxygen concentrations. Cu sintering progressed significantly after the reduction due to generation of Cu nanoparticles. The TG–DTA–MS results indicated that H2O formation temperature associated with the reduction depends on the oxygen concentration, consistent with the thermodiffraction results. The surface oxides were found to play an important role in pressureless sinter joining via nanoparticle formation, while the presence of a large amount of oxide delayed the reduction and subsequent sintering.

Pressureless sinter joining of bare Cu substrates using submicron Cu particles was successfully achieved at 250–300 °C in N2–3%H2 forming gas atmosphere via the in-situ generation of Cu nanoparticles reduced from a thin oxide layer present on the surface of Cu particles. The joining strength of the Cu joints at 300 °C reached 26.2 MPa in the forming gas atmosphere, while it was 7.4 MPa under N2 gas atmosphere. Thermogravimetry–differential thermal analysis and X-ray diffraction showed that the natural oxide layer (Cu2O) started to reduce to Cu at approximately 220 °C only under the N2–H2 gas atmosphere. Cu sintering progressed rapidly above the reduction temperature, and the joining strength improved with increasing temperature. Transmission electron microscopy demonstrated the generation of Cu nanoparticles around the submicron Cu particles and the subsequent promotion of sintering behavior. These results suggest that reduction of the initially present oxide layer on submicron Cu particles is crucial for pressureless sinter joining. Based on the reduction and sintering behavior of submicron Cu particles, pressureless sinter joining was realized at 250 °C by increasing the holding time, and the joining strength was > 18 MPa.

Energy storage using liquid organic hydrogen carrier (LOHC) is a long-term method to store renewable energy with high hydrogen energy density. This study investigated a simple and low-cost system to produce methylcyclohexane (MCH) from toluene and hydrogen using fluctuating electric power, and developed its control method. In the current system, hydrogen generated by an alkaline water electrolyzer was directly supplied to hydrogenation reactors, where hydrogen purification equipment such as PSA and TSA is not installed to decrease costs. Hydrogen buffer tanks and compressors are not equipped. In order to enable MCH production using fluctuating electricity, a feed-forward toluene supply control method was developed and introduced to the system. The electrolyzer was operated under triangular waves and power generation patterns of photovoltaic cells and produced hydrogen with fluctuating flow rates up to 7.5 Nm3/h. Consequently, relatively high purity of MCH (more than 90% of MCH mole fraction) was successfully produced. Therefore, the simplified system has enough potential to produce MCH using fluctuating renewable electricity.

Thermal stability and characteristic properties of the microstructure of sintered Ag layers were examined with morphological, crystallographic and mechanical analyses. The sintered Ag layer was prepared by a pressureless sintering procedure with an Ag paste including Ag nanoparticles having a diameter distribution of 50–300 nm. The microstructure of the sintered Ag had a porosity of 8% and was morphologically and crystallographically dense. Cross-sectional SEM images showed that pores having a diameter of 0.2–1.0 μm were homogenously located in an Ag matrix. The dense microstructure remained intact after storage at 250 °C and for 1000 h. Shear strength of the sintered Ag layers was over 40 MPa and elastic modulus of the sintered Ag layer was 14 GPa before and after the storage tests at 250 °C and for 1000 h.

We conduct high-resolution microscale tensile testing of galvannealed steel/Al–Mg–Si alloy dissimilar joints and find that the interfacial reaction layer is sufficiently strong to induce microscale aluminum fractures. The interfacial reaction layer with the 5-µm-thickness generated by friction stir spot welding contains Fe–Al intermetallic compounds (IMCs) with grain diameters of 10 nm to 2 µm, Zn–Mg compounds present around the Fe–Al IMC, and microscale defects within the layer. The microtensile test demonstrates the slip fracture of aluminum regardless of defects present within the reaction layer. The maximum stress withstood by the specimen is 309 MPa.

Micron-scale Ag2O paste facilitates direct sintering bonding to silicon and silicon ceramic substrates because of in-situ formation of Ag during the decomposition of Ag2O. However, it remains difficult to obtain a high-strength joint at low bonding temperatures using the conventional process (even at 300 °C) despite their good performance at temperatures above the thermal decomposition temperature. In the present study, the dominant factors for low-temperature direct sintering bonding of Ag to Si and SiC substrates using Ag2O decomposition were investigated by controlling the preheating temperature and evaluating the bondability. The influence of preheating temperature was evaluated using thermal analysis and the assessment of joint strength. Lower preheating temperatures led to higher joint strength at lower bonding temperatures. It was observed that Ag nanoparticles were generated during the preheating process at high temperatures, indicated from the occurrence of the exothermic Ag2O decomposition reaction. However, the initial formation of Ag nanoparticles during the preheating process did not contribute to the sintering process. We propose that lowering the preheating temperature plays an important role in precisely controlling the amount of organic solvent required for the decomposition reaction of Ag2O. Finally, we confirmed that the low-temperature preheating process realized a sound joint for the Ag sintering bonding to SiC, even at 300 °C.

We successfully used CoCrW alloy as the feedstock for directly shaping wear-resistant products by additive manufacturing. We present a guideline for forming substantive, high-density, crack-free CoCrW alloys via laser metal deposition (LMD) and evaluated the relationship between the microstructure and mechanical properties. CoCrW alloy with a scale of several tens of cubic centimeters has not been previously reported because the brittle material undergoes cracking due to residual stresses. Densification and crack suppression were achieved by controlling the oxygen concentration in an Ar atmosphere and preheating the base plate to 400 °C. The tensile strength (MPa) of the LMDed CoCrW alloy was 1492 ± 141 along the scan direction, 1464 ± 234 along the overlap direction, and 1359 ± 72 along the build direction. The overlap direction had almost the same strength as the scan direction. In the microtensile test for the overlap direction, the fracture ratio in the overlap region was only 17.4%, and it was found that softening in this region did not contribute to the decrease in strength.

In this study, the synergistic effects of a hybrid surface treatment involving hydrochloric acid (HCl) immersion and silanization on friction stir spot welding (FSSW) of aluminum and carbon fiber reinforced thermoplastic (CFRTP) were investigated. Maximum tensile shear strength of 10.2 kN and maximum cross-tension strength of 1.92 kN were achieved. An unloading test and a miniature tensile test revealed that the fracture behavior changed from interfacial to CFRTP fracture due to increased interfacial strength with the hybrid treatment and utilization of the CFRTP strength enhanced the joint strength. The nanostructures at the interface with HCl immersion treatment indicated that the interfacial properties were improved mechanically in addition to the chemical enhancement by silanization. Owing to the hybrid treatment, aluminum and CFRTP were efficiently joined by the adhesion behavior of spreading melted plastic resin during the FSSW process.

Joining of 5083 and 6063 aluminum alloys to 304 stainless steel was successfully achieved by high-frequency linear friction welding working at 245 Hz with varied friction pressure and time, which affect the heat input. Increasing the friction pressure and friction time resulted in the softening of aluminum alloys, which facilitated plastic flow and subsequent squeezing behavior. The morphology of the intermetallic compound layer, dependent on the plastic flow, determined the performance of 5083 aluminum alloy/steel joints. Excessive friction pressure and time led to inefficient welding in 6063 aluminum alloy/steel joints because the softening and squeezing behavior of the heat-treatable alloy was susceptible to the heat input. Highly efficient joints were obtained in a short duration, i.e., 1 s, through the formation of a thin intermetallic compound layer at the interface below 500 and 50 nm for 5083 and 6063 aluminum alloy/steel joints by controlling the appropriate friction pressure. High-frequency linear friction welding was demonstrated to be effective for the sound joining of various materials by controlling the friction pressure.

Thin metallic Cu and Ni meshes were successfully embedded within SAC305 solder joints. Defects (residues) within the joint were removed by a vacuum step in the bonding sequence. The metallic mesh inlets were metallurgically bonded by the formation of scallop-like Cu6Sn5 and faceted Ni3Sn4 intermetallic compounds (IMCs) at the Cu mesh-SAC305 solder (CMS) and Ni mesh-SAC305 solder (NMS) interfaces, respectively. After process optimization, the averaged shear strengths were found to be 44.1 (reference), 44.7 (+1%), and 51.4 MPa (+16%) for SAC305, NMS, and CMS composite joints, respectively. The solder-substrate-mesh interaction resulted in a characteristic fracture pattern, with the cracking path partly within the solder and partly at the solder-mesh interface. Finite element (FE) simulations suggested a mesh-induced stiffening effect of the solder joint by 5%. The additional reinforcement (11%) introduced by the Cu mesh inlets was attributed to a locally enhanced fracture resistance. Keywords: Solder-mesh composite joints, Microstructure, Shear strength, Finite element modeling, Fracture

We demonstrate hyperspectral imaging by visible-wavelength two-photon excitation microscopy using line illumination and slit-confocal detection. A femtosecond pulsed laser light at 530 nm was used for the simultaneous excitation of fluorescent proteins with different emission wavelengths. The use of line illumination enabled efficient detection of hyperspectral images and achieved simultaneous detection of three fluorescence spectra in the observation of living HeLa cells with an exposure time of 1 ms per line, which is equivalent to about 2 µs per pixel in point scanning, with 160 data points per spectrum. On combining linear spectral unmixing techniques, localization of fluorescent probes in the cells was achieved. A theoretical investigation of the imaging property revealed high-depth discrimination property attained through the combination of nonlinear excitation and slit detection.

Visible-wavelength two-photon excitation (v2PE) is a powerful technique for simultaneous multicolor fluorescence imaging via simultaneous excitation of fluorescent proteins (FPs) with different emission wavelengths. We implemented v2PE into a slit-scanning confocal microscope in order to realize faster simultaneous multicolor fluorescence imaging with utilizing the capability of spectral detection. We demonstrated simultaneous multicolor imaging of living HeLa cells with expressing three types of FPs with different emission wavelengths localized at different intracellular structures. Linear un-mixing of hyperspectral images successfully separated the distribution of multiple FPs expressed in the sample.

The relationship between the intermetallic compound (IMC) layer thickness including the deviation and joint strength was evaluated for the dissimilar joints between aluminum alloys and austenitic stainless steel. To evaluate the interface with various IMC layer thicknesses, the joints by friction stir welding were solution-heat-treated and artificially aged. The transitions of joint strength and fracture position were characterized for the IMC layer thickness ranging from 0 to 2.5 µm. Further, we demonstrated that the fracture position can be determined by the percentage of the joining area via the IMC layer thickness below a threshold value, which was 0.4 µm.

Bonding between AlN and metals conventionally requires a surface modification process at high temperature such as metallization. The AlN-to-metal direct bonding process by sintering of Ag nanoparticles derived from in situ reduction of Ag2O microparticles mixed with diethylene glycol is examined. Bonding was conducted at 300–500 °C after a preheating process at 100 °C, and the shear strength exceeded 20 MPa for the joint bonded at 500 °C. The role of preheating in the direct bonding of AlN and Ag was identified. The Ag nanoparticles generated during preheating at 100 °C cover the AlN surface, and they are converted into a thin Ag film. The thin film promotes the formation of a bonding layer, owing to the effective adhesion of the sintered Ag to the film, and contributes to the bonding of Ag and AlN. No interfacial reaction layer is observed. The same bonding process can be applied to other ceramics with poor wettability.

High-frequency linear friction welding (HFLFW) with a 250 Hz oscillation frequency was applied on similar joining of AA5052 and AA6063, i.e., work- and precipitation-strengthened Al alloys, with 11 × 11 mm2 friction surface. Sound joints were achieved under a 30 MPa friction pressure and with processing times longer than 1 s. The thermomechanically affected zone (TMAZ) of AA6063 joints suffered from softening at a distance of 2–3 mm from the joined interface in contrast to the TMAZ of AA5052 joints. The pressure reduction to 7 MPa narrowed down the thickness of the softened TMAZ to 1 mm from the interface. A thermal finite element (FE) model was calibrated against in situ temperature measurements. Significant temperature differences of up to 300 °C at the welding interface were assigned to the process parameters as well as to the different thermal and mechanical properties of the Al alloys. Linear hardening and softening effects in AA5052 and AA6063, respectively, were quantified in hardness vs. temperature diagrams, and a pressure-hardening phenomenon was identified. A user subroutine was fed with these correlations, which visualized the hardness distribution over the near-interface volume for a low- and a high-pressure process.

This study introduces a novel concept that uses composite structures of thin metallic meshes and Sn3.0Ag0.5Cu (SAC305) solder alloys to interconnect semiconductor chips to DBC substrates. The feasibility was proven by bonding Cu-to-Cu substrates. The averaged shear strength was measured as 44.1, 44.7 and 51.4 MPa for samples bonded with SAC305 (reference), Ni mesh/SAC305 and Cu mesh/SAC305 composites, respectively. The solder-mesh joints revealed a specific fracture behavior where crack propagation occurred partly within the solder and partly at the solder-mesh interface. Microstructural analyses confirmed that the metallic meshes were bonded by the formation of intermetallic compounds (IMC) while almost no (larger) defects were found. The solder-mesh concept was subsequently applied on Si-to-DBC assemblies. A very good resistance of Cu mesh/SAC305 composite joints against cyclic temperature between 80 and 200 °C was observed when the bonded area only reduced by 4.2 % after 8000 cycles. Thermal finite element (FE) simulations indicated that in particular Cu mesh/SAC305 composites can significantly reduce the thermal resistance of the interconnections which is equivalent to a better heat dissipation through the bonding layer. Thus, solder-mesh composite joints seem to be an attractive solution for high-temperature applications up to 200 °C.

We developed High-Frequency Linear Friction Welding (HFLFW) operating at 250 Hz, which is larger than conventional LFW methods, to realize welding aluminum alloys. In order to establish the fundamental HFLFW process for the welding of Al alloys, the relationship between the mechanical properties and microstructure of the HFLFW joint was investigated for both 6063 and 5052 Al alloys, which have different strengthening mechanisms. Sound welds were achieved for both 6063 and 5052 Al alloys. The change in the shape of flash with increasing process time indicated that the material flow in the joint was largely affected by the material properties such as proof stress at high temperature and thermal conductivity. Further, the hardness increased and refined grains were observed at the interface of the 6063 Al alloy joints welded in the low-heat-input condition, which led to a higher joint strength at a shorter process time. The size of the refined grains was about 100 nm, as observed by transmission electron microscopy. Our findings reveal that a high frequency is effective and HFLFW is a promising process for the welding of precipitation-hardening aluminum alloys with a high joint strength owing to the less heat effect during the process. Keywords: Linear friction welding, Aluminum alloy, Precipitation hardening, Frequency, Microstructure, Mechanical properties

A bonding method utilizing redox reactions of metallic oxide microparticles achieves metal-to-metal bonding in air, which can be alternative to lead-rich high-melting point solder. However, it is known that the degree of the reduction of metallic oxide microparticles have an influence on the joint strength using this bonding method. In this paper, the reduction behavior of CuO paste and its effect on Cu-to-Cu joints were investigated through simultaneous microstructure-related x-ray diffraction and differential scanning calorimetry measurements. The CuO microparticles in the paste were gradually reduced to submicron Cu2O particles at 210–250°C. Subsequently, Cu nanoparticles were generated instantaneously at 300–315°C. There was a marked difference in the strengths of the joints formed at 300°C and 350°C. Thus, the Cu nanoparticles play a critical role in sintering-based bonding using CuO paste. Furthermore, once the Cu nanoparticles have formed, the joint strength increases with higher bonding temperature (from 350°C to 500°C) and pressure (5–15 MPa), which can exceed the strength of Pb-5Sn solder at higher temperature and pressure.

Metallization-free Ag sinter bonding to Si and SiO2 substrates was achieved via the thermal decomposition of Ag oxalate. Ag oxalate generated atomic-scale Ag through decomposition at approximately 200 °C and transformed into sintered Ag. Ag derived from Ag oxalate successfully bonded to Si substrates through close contact formation with Si surface oxide. Similarly, amorphous and α-quartz SiO2 substrates were directly bonded with Ag. These results indicate that the atomic-scale Ag generated on SiO2 contributed to interface formation over a large area. Moreover, the joints of the bare Si chip and metal substrate showed a maximum shear strength of 29 MPa at a bonding temperature of 250 °C and a bonding pressure of 5 MPa. The in-situ generation of Ag through the decomposition of Ag precursors can facilitate the direct bonding of electronic materials, making the proposed method a promising one for electronics applications.

Herein, we investigated the fracture behavior of thermally aged Ag–Cu composite sinter joints through nano-X-ray computed tomography (nano-CT) combined with microscale tensile testing. Nano-CT results showed that the joint comprised Ag/oxidized Cu flake sintered layer and Cu oxide, which was formed on the Cu substrate; further, pores were present at the interface between the substrate and Cu oxide layer. The tensile test showed that the stress increased with plastic deformation up to a maximum of 308 MPa, before a subsequent steep decrease due to crack initiation inside the sintered layer. The nano-CT and in situ observation results suggest that the microscale joint fractured through the interface between the oxidized Cu flake and Ag. Results obtained herein suggest that microscale tensile testing coupled with nano-CT is an effective approach to describe the intrinsic fracture behavior of joints.

Metal-to-ceramics direct sintering was carried out with Al2O3 and Cu / 3 μm Ni / 1 μm Au substrates. The bonding paste consisted of micron-sized Ag2O particles and a reducing solvent that provokes Ag2O-to-Ag reduction during processing accompanied by the formation of Ag nano particles. Five different sets of process parameters resulted in different joint microstructure and strength. The experimental data was used to calibrate an elasto-visco-plastic finite element model of the sintered assembly which yielded a quantitative damage function and criterion to predict the strength of direct-sintered joints. The developed ductile damage formulation introduced a parameter ξ, i.e. the product of equivalent creep strain and stress triaxiality, that controls the tolerable plastic strain at fracture. An extended numerical parameter study subsequently revealed the complex interaction between the joint strength and microstructural joint features. Thinner joints were found to provide a slightly higher strength while the amount of sinter material and costs is significantly reduced. Moreover, it is recommended to aim at a higher level of densification at the edges and corners of sintered joints since these areas apparently contribute more to the overall mechanical strength. The developed concept is capable of tailoring the microstructure of direct-sintered joints according to the requirements or vice versa.

In the present study, low temperature bonding of gold-to-alumina was investigated by the in situ generation of silver via a redox reaction between silver oxide microparticles and diethylene glycol. The shear strength of gold-to-alumina joints with a silver layer bonded at 400 °C was comparable to that of conventional methods that require a high bonding temperature. Microstructural observations revealed that the adhesion of silver nanoparticles, which are formed by redox reaction, on an alumina surface improves the strength leading to the fracture of alumina. High-resolution transmission electron microscopy and energy dispersive spectroscopy analysis showed the direct bonding between silver and alumina at the interface without diffusion or formation of a reaction layer. Additionally, x-ray photoelectron spectroscopy analysis at the interface showed that the bonding of silver and alumina could be attributed to the silver-oxygen bond, which revealed that silver ions generated during the redox reaction could contribute to the bond formation. Further, molecular dynamics simulation confirmed that the presence of silver ions promotes the formation of an initial silver layer, which plays a role in bonding silver nanoparticles to an alumina surface. As mentioned above, it was found that a bonding method utilizing the reduction of silver oxide by reducing agent involves a significant, unique process that realizes metal-to-alumina bonding at a low temperature.

Al2O3-to-Cu/3 μm Ni/1 μm Au assemblies were direct-sintered with a paste consisting of micron-sized Ag2O particles and a reducing solvent that provokes Ag2O-to-Ag reduction during processing accompanied by the formation of Ag nanoparticles. The variation of process temperature, pressure and time resulted in different porosity and shear strength. This experimental data set was used to calibrate a finite element model of the assembly considering the elasto-visco-plastic joint properties and the joint porosity. Local analyses within the sintered Ag layer yielded a damage function D f and a damage criterion that could assess the joint strength as a function of geometry, microstructure and process conditions. The developed damage model introduced a parameter ζ, i.e. the product of equivalent creep strain e cr,eq and stress triaxiality η, that balanced the tolerable equivalent plastic strain e pl,eq at fracture. A Fortran subroutine (UVARM) visualized the damage distribution over the sintered layer. The predominant crack initiation and propagation were identified in good agreement with the experimental fracture behavior. A numerical parameter study subsequently revealed the complex interaction between the ceramic–metal interfacial porosity, the total bulk porosity and the shear strength.

We propose a room-temperature multiple-reaction bonding method for aluminum, by combining the thermite reaction of Al-CuO powder and self-propagating formation reaction of Ti-B powder with no external heat source. The joint microstructure consists in various composites (Al2O3, TiB2 and Al-Cu intermetallic), and their distribution affects the joint bonding properties. The multiple reaction leads to a compact joint layer with a very low porosity, in contrast to joints formed only by a formation reaction. This observation is explained by the inflow of molten aluminum metal into the joint layer during the joint formation, a process which is central in the formation of a sound joint. Furthermore, bonding between different composites in the joint is achieved by lattice defects at their interface. Analysis of shear tests show that fracture occurs in regions at the metal base/joint interface where Al2O3 particles were segregated. It is concluded that appropriate reactant design plays a significant role in controlling the microstructure, and thus, the bonding properties of the joint. Keywords: Self-propagating high-temperature synthesis, Room-temperature bonding, Composite structure, Aluminum alloys, Interface structure

Solder joints between leads and printed circuit boards in thin small outline packages were produced with conventional Sn1.0Ag0.7Cu (SAC107) and Sn3.0Ag0.7Cu (SAC305) solders as well as various solder alloys with gradually increasing amounts of Bi (up to 3.0 wt.%) and In (up to 1.0 wt.%) within the SAC107 base solder. The reliability of soldered leads in temperature cycle (TC) tests improved most with solder alloys containing both Bi (1.6 wt.%) and In (0.5 wt.%). Microindentation and electron probe microanalysis mappings revealed that the effect originates from a combination of solution and precipitation strengthening of the initial SAC alloy. The distribution of inelastic strain accumulation (ISA), as a measure for degradation, was determined in the solder joints by finite element calculations. It was shown that defects in the solder proximal to the lead ( 60–75 μm), which was underpinned by similar cracking characteristics along the lead–solder interface. The ISA was confirmed to be lower in SAC+Bi/In alloys owing to their enhanced elasto-plastic properties. Moreover, the addition of a thin Cu coating on the leads could improve the joint reliability, as suggested by the calculation of the ISA and the acceleration factor.

A bonding process utilizing self-heating due to exothermic heat from the combustion synthesis of TiB2 was investigated for A5056 aluminum alloys. Aluminum particles were added to a Ti–B mixed powder to not only reduce the porosity, but also to produce composites between TiB2 and aluminum in the bonding layer. The bonding process using exothermic rather than external heat was successfully achieved by controlling aluminum addition, which significantly improved the joint tensile strength. The fracture surfaces showed the presence of aluminum oxide, while joint cross-sections showed that the bonding material and added aluminum filled pores of synthesized TiB2, which contributed to the decrease in the joint porosity. Both porosity and aluminum oxide content, dependent on aluminum addition, were correlated with the joint strength. Therefore, appropriate control of aluminum addition can be used to produce sound joints in self-heating bonding at lower temperatures. Keywords: Aluminum alloy, Combustion synthesis, Self-heating bonding, Aluminum addition, Tensile strength

We have proposed a low-temperature bonding technology utilizing the sintering of Cu particles with transient liquid-phase of Sn-based solder, called transient liquid-phase sintering (TLPS), as a die-attach solution for high-temperature power modules. A copper-intermetallic compound-resin (Cu-IMC-resin) microstructure, which consists of Cu particles connected with Cu&ndash, Sn intermetallic compounds (IMCs) partially filled with polyimide resin, is obtained by the pressureless TLPS process at 250 &deg, C for 1 min using a novel Cu-solder-resin composite as the bonding material in a nitrogen atmosphere. Macro- and micro-deformation properties of the unique microstructure of the TLPS Cu-IMC-resin are evaluated by finite element analysis using a three-dimensional image reconstruction model. The macroscopic computational uniaxial tensile tests of the Cu-IMC-resin model reveal that the utilization of the IMCs and the addition of the easily-deformable resin facilitates the temperature-stability and low-stiffness of the mechanical properties. The microstructure exhibits a significantly low homogenized Young&rsquo, s modulus (11 GPa). Microscopic investigations show that the local stresses are broadly distributed on the IMC regions under uniaxial macroscopic tensile displacement, indicating highly reliable performance of the joint within a specific macroscopic strain condition. Numerical and experimental investigations demonstrate the excellent thermal cyclic reliability of die-attached joints between silicon carbide chips and directly bonded copper substrate.

Two-photon excitation microscopy is one of the key techniques used to observe three-dimensional (3-D) structures in biological samples. We utilized a visible-wavelength laser beam for two-photon excitation in a multifocus confocal scanning system to improve the spatial resolution and image contrast in 3-D live-cell imaging. Experimental and numerical analyses revealed that the axial resolution has improved for a wide range of pinhole sizes used for confocal detection. We observed the 3-D movements of the Golgi bodies in living HeLa cells with an imaging speed of 2 s per volume. We also confirmed that the time-lapse observation up to 8 min did not cause significant cell damage in two-photon excitation experiments using wavelengths in the visible light range. These results demonstrate that multifocus, two-photon excitation microscopy with the use of a visible wavelength can constitute a simple technique for 3-D visualization of living cells with high spatial resolution and image contrast.

Thermal fatigue generally occurs in ultrasonically bonded copper joints in electronic devices as the bonding substrate is composed of plural materials, leading to differences in the coefficient of thermal expansion. In this study, we found that the thermal fatigue resistance of the ultrasonically bonded copper joints was influenced by the grain size and hardness of the bonding substrate through the evaluation of the thermal fatigue properties. Copper alloys C1020 and C1940 were used as substrate materials to investigate the influence of the initial properties of the bonding material on the thermal fatigue resistance. We evaluated the crack propagation due to thermal fatigue via thermal cycle tests. Microstructural observations of the region fractured because of thermal fatigue revealed that cracks resulting from thermal fatigue did not progress in the fine grain region formed at the bonded interface. It was inferred that grain boundaries were an obstacle to crack propagation. C1940 has higher hardness and finer grains than C1020, and showed a lower decreasing rate of the peel strength and bonding area after the thermal cycling test than C1020 joints. Thus, a hard copper material with fine grains is effective in suppressing thermal fatigue fracture of ultrasonically bonded copper joints.

The evolution of the transient liquid-phase sintered (TLPS) Cu&ndash, Sn skeleton microstructure during thermal aging was evaluated to clarify the thermal reliability for die-attach applications. The Cu&ndash, Sn skeleton microstructure, which consists of Cu particles connected with Cu&ndash, Sn intermetallic compounds partially filled with polyimide resin, was obtained by the pressure-less TLP sintering process at 250 &deg, C for 1 min using a novel Cu-solder-resin composite as a bonding material in a nitrogen atmosphere. Experimental results indicate that the TLPS joints were mainly composed of Cu, Cu6Sn5, and Cu3Sn in the as-bonded state, where submicron voids were observed at the interface between Cu3Sn and Cu particles. After thermal aging at 150, 175, and 200 &deg, C for 1000 h, the Cu6Sn5 phase fully transformed into Cu3Sn except at the chip-side interface, where the number of the submicron voids appeared to increase. The averaged shear strengths were found to be 22.1 (reference), 22.8 (+3%), 24.0 (+9%), and 19.0 MPa (&minus, 14%) for the as-bonded state and specimens aged at 150, 175, and 200 &deg, C for 1000 h, respectively. The TLPS joints maintained a shear strength over 19 MPa after thermal aging at 200 &deg, C for 1000 h because of both the positive and negative impacts of the thermal aging, which include the transformation of Cu6Sn5 into Cu3Sn and the formation of submicron voids at the interface, respectively. These results indicate an excellent thermal reliability of the TLPS Cu&ndash, Sn skeleton microstructure.

One of the mechanisms for the dissimilar materials joining is the formation of interfacial reaction layers. However, this reaction is disrupted owing to the presence of oxide layers and voids, leading to lower strength, and the elimination of these factors is essential. Thus far, studies on linear fraction welding (LFW) with regard to the relationship between the joint strength and the microstructure of the base alloy and interface are lacking. Therefore, herein, the influence of microstructural changes in base alloys and interfacial structures on the joint tensile strength was investigated for the dissimilar joint between the stainless steel and an A5083 alloy after high-frequency linear friction welding at 245 Hz. After the initial joining process up to 1 s, the joint strength decreased as the friction increased. The crystal orientation analyses using electron backscatter diffraction demonstrated a minimal change in the microstructure of the A5083 alloy, particularly in the grain size in the geometric dynamic recrystallization region. This variation in the microstructure was minimal, regardless of the friction time, which was consistent with the result of the hardness distributions. The observation of microstructure using transmission electron microscopy revealed the formation of metastable Al-Fe intermetallic compounds and Al-Mg-Cr compounds derived from the reaction between the oxide layer and alloying elements in the A5083 after experiencing friction for a short time. Conversely, we confirmed the formation of a fragile boundary within the interfacial reaction layer after the exposure to friction for an extended period due to differences in the formation process of the layer: mechanical mixing or interdiffusion. Hence, it was proposed that mechanical removal as well as the interfacial reaction between the oxide layer and the Mg contained in the aluminum alloy function in removing the surface oxide layer, which plays a significant role in designing the interfacial structure and joint properties.

The strength of a dissimilar joint between aluminum and steel is generally dependent on the thickness of the intermetallic compound (IMC) layer. Mechanical factors such as the residual stress and stress field under loading also likely contribute to the fracture behavior of the joint. However, a clear understanding of the effects of the interfacial structure (i.e., the IMC layer thickness) and mechanical factors on joint strength is lacking in the existing literature. The purpose of this study was to elucidate the influence of both the IMC layer thickness and the difference in the strengths of dissimilar metals on the interfacial strength of aluminum/steel joints through experimental evaluations and finite element (FE) analyses. The relationship between the strength of a friction-stir-welded joint of 6061 aluminum alloy (A6061) and 780-MPa grade high-tensile-strength steel (HT780) and the IMC layer thickness was investigated by controlling the IMC growth promoted by the annealing process. The results showed that the joint strength decreased with increasing IMC layer thickness. Fracture mainly occurred at the joint interface for a thin IMC layer of approximately 100 nm, whereas the joint between A6061 and 304 stainless steel, with a lower proof stress than HT780, showed a higher joint strength that resulted in fracture in the A6061 alloy. The FE analysis revealed that a high maximum principal stress is applied near the interface due to the difference in the degree of deformation between steel and aluminum, a behavior that is likely to appear in any joint consisting of materials with considerably different strengths.

Dissimilar joints generally involve the formation of a non-uniform interfacial reaction layer, which affects the joint strength significantly. However, a clear understanding of the effect of interfacial local strength on the macroscopic fracture behavior of the dissimilar joints is lacking in the existing literature. Here, we demonstrate the influence of interfacial reaction layer on the fracture behavior of the friction stir spot welded dissimilar joints between galvanized steel and aluminum alloy through the microscale evaluation of interfacial strength in addition to the conventional macroscale evaluations. Macroscale evaluations showed that the rim region of the joint, which includes a thin reaction layer comprising of both intermetallic compound (IMC) with several hundred nm thickness and Zn-concentrated area partly including voids derived from the discharge of galvanized layer, primarily resists the fracture in tensile shear and cross tension tests. Microscale cantilever beam bending tests showed that the interfacial strength increases with the decrease of reaction layer thickness except in the case of voids. Our study highlights the significance of rim region in spot welding, which has been indicated only by macroscale evaluations.

Silicon-based materials are widely promising electronic components by the combination with metals in power electronics field. However, bonding metal and silicon-based materials generally requires specific surface modification due to their different chemical bonds. Here, we demonstrate a process for directly bonding metals to silicon-based materials that does not require surface treatment, based on the in situ decomposition of Ag2O paste, forming Ag nanoparticles (AgNPs). We demonstrate sound joints of Ag/silicon-based materials at 300–500 °C with the formation of a silicon oxide interlayer containing AgNPs. We propose that Ag in the interlayer attracted other Ag particles to the interface, playing a unique role in this direct bonding process. This process is suitable for various bonding applications in electronics, as well the fabrication of conducting paths for photovoltaic and other applications.

Super-resolution imaging techniques based on single molecule localization microscopy (SMLM) broke the diffraction limit of optical microscopy in living samples with the aid of photoswitchable fluorescent probes and intricate microscopy systems. Here, we developed a fluorescent protein, SPOON, which can be switched off by excitation light illumination and switched on by thermally induced dehydration, resulting in an apparent spontaneous blinking behavior. This unique property of SPOON provides a simple SMLM-based super-resolution imaging platform which requires only a single 488 nm laser.

Sano T., Eimura T., Kashiwabara R., et al. Femtosecond laser peening of 2024 aluminum alloy without a sacrificial overlay under atmospheric conditions. Journal of Laser Applications, 29, 1, 012005. https://doi.org/10.2351/1.4967013., The authors have successfully performed femtosecond laser peening on a 2024 aluminum alloy without any sacrificial overlays. Laser pulses were directly irradiated to the surface of specimens in the air without water film as a plasma confinement medium during the peening treatment. The fatigue life was improved as much as 38 times in comparison with base material at a stress amplitude of 195 MPa. The fatigue strength of the peened specimen after 2 × 106 cycles was 58 MPa larger than that of the base material. The femtosecond laser peening process has a great potential to be applied in various fields where conventional peening methods cannot be used, as this process can be performed under ambient conditions without the use of a plasma confinement medium such as water or transparent materials.

Introduction It is extremely important to know the change in the ion concentration in order to understand the dynamics of living cells. Nowadays, considerable attention has been paid to the method which can display ion concentration intuitively. In our laboratory, we have developed several ion image sensors by modifying a CCD-type pH image sensor with plasticized poly(vinyl chloride) (PVC) membranes. Recently, Na+-K+ image sensor [1] was fabricated using a inkjet method. In this paper, we developed Ca2+-K+ image sensor. Ca2+ and K+are very important ions in cells. Cell membranes have each ion channel and each ion pump. Both of ions exist on lower concentration in extracellular fluid than in cells. Materials and methods The half of a CCD-type pH image sensor was modified with Ca2+ selective membrane using a inkjet method. Similarly, the other half was modified with K+ selective membrane. The K+ selective membrane was prepared in our previous paper [1].The inkjet condition of the Ca2+ selective membrane was optimized, and the membrane with Nernstian response was prepared. The K+ selective membrane consisted of 30.0 % of PVC (MW = 82000), 30.0 % of 1,3,5,7,9,11,13,15-octa(propylmethacryl)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane (POSS), 36.0 % of dioctyl sebacate (DOS), 2.7 % of valinomycin, and 1.3 % of potassium tetrakis (4-chlorophenyl)borate (K-TCPB). Ca2+ selective membrane consisted of 30.0 % of PVC, 10.0 % of POSS, 55.0 % of 2-nitrophenyl octyl ether (NPOE), 3.6 % of 10,19-bis[(octadecylcarbamoyl)methoxyacetyl]-1,4,7,13,16-pentaoxa-10,19-diazacycloheneicosane (K23E1) and 1.4 % of sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na-TFPB). Real-time bio-images for PC12 cells (rat pheochromocytoma cell line) and Hela cells (human cervical cancer cell line) were captured by the Ca2+-K+image sensor. Results and Discussion After several days culture of PC12 cells on the collagen sheet, the cells on the Ca2+-K+ image sensor were stimulated by acetylcholine (Ach). As a result, the Ca2+ concentration decreased after the stimulation. On the other hand, there was no significant change in K+ concentration. The previous study [2] reported that the stimulation brought about the decrease in the Ca2+ concentration. This study confirmed that the decrease of Ca2+ wasn’t accompanied by the change in the K+concentration. After a day culture of HeLa cells, the cells were stimulated by Amphotericin B. As a result, the K+ concentration increased after the stimulation. Meanwhile, there was no significant change in Ca2+ concentration. A related result that showed the Amphotericin B-induced decreasing of K+ concentration in HeLa cells due to the K+ efflux was reported by Ohtsuka et al. [3] through the fluorometric imaging. The present study indicated that the change can be recorded only in the K+concentration with a non-invasive and easy method. In conclusion, the Ca2+-K+image sensor demonstrated successfully the real-time monitoring of living cells stimulated. [1]T. Hattori, H. Sato, K. Tokunaga, R. Kato, K. Sawada, Sensors and Materials, 27, (2015) ,1023-1034 [2] H. Taki, T. Sakurai, Y. Masaki, T. Hattori, K. Takahashi, S. Terakawa, M. Ishida, K. Okumura, K. Sawada, 2012 International Symposium on Chemical-Environmental-Biomedhical Technology, P_BE1_09, September 2-5, Tainan, Taiwan (2012) [3]K. Ohtsuka, S. Sato, Y. Sato, K. Sota, S. Ohzawa, T. Matsuda, K. Takemoto , N. Takamune, B. Juskowiak, T. Nagai, S. Takenaka, Chem. Commun., 48, (2012), 4740-4742

We found that a femtosecond laser-driven shock wave induces marked changes in dislocation structure in iron over a fluence range from 1.3 to 8.3 J/cm2. Transmission electron microscopy observations showed a change in dislocation structure from lath structures with twist boundaries to only laths, and an increase in depth where laths begin to appear, with increasing fluence. X-ray diffraction results showed the distribution of crystallite sizes corresponding to the change in dislocation structure. We proposed that the dislocation structure is determined by the laser fluence, through the change in the duration of the shock wave.

Matsuda T., Sano T., Arakawa K., et al., Applied Physics Letters, 105(2), 021902 (2014) https://doi.org/10.1063/1.4890389, We found that multiple shots of femtosecond laser-driven shock pulses changed coarse crystalline iron grains with a size of 140 μm into nanocrystals with a high density of dislocations, which had never been observed in conventional shock processes. We performed metallurgical microstructure observations using transmission electron microscopy (TEM) and hardness measurements using nanoindentation on cross-sections of shocked iron. TEM images showed that grains with sizes from 10 nm through 1 μm exist within 2 μm of the surface, where the dislocation density reached 2 × 10 15m-2. Results of the hardness measurements showed a significant increase in hardness in the nanocrystallized region. We suggest that the formation of a high density of dislocations, which is produced by a single shock, induces local three-dimensional pile-up by the multiple-shocks, which causes grain refinement at the nanoscale. © 2014 AIP Publishing LLC.