1. In-situ micro-deformation & texture analyses of crystals or nano fibers
In general, a lot of structural or functional materials are composed of poly crystals (poly grains). When poly crystals are loaded in use, their deformations are very complex due to existence of grain boundaries. In addition, most deformations are not simple tension or compression but complex deformation modes. Therefore, in order to enhance strength and toughness of the materials, it is very significant to understand their deformation behaviors in various deformation modes.
Single or poly crystals of metallic materials such as aluminum, magnesium, and titanium are elongated using the in-situ micro-deformation devices (uniaxial tension, biaxial tension, plain strain tension, and shear deformation) which are equipped in a SEM/EBSD system. Microstructural evolution of the materials are observed using a SEM, and texture evolutions during the deformations are analyzed using a EBSD or XRD system.
2. Development of Al alloy with high formability and low planar anisotropy
Steel exhibits high r-value ( > 1.8 ) and low Dr-value ( < 0.4 ), resulting in high formability and low planar anisotropy. On the other hand, Al and Cu tend to have low r-value ( < 0.8 ) and high Dr-value ( ~ 0.6 ), i.e., low formability and high planar anisotropy. In order to enhance the formability and reduce the planar anisotropy of FCC metals such as Al and Cu, the presence of the shear textures, such as {111}< 110 > and {111}< 112 >, is indispensable. Such shear textures in FCC metals can be introduced by the shear deformation. However, most conventional metal forming processes including rolling were not successful for introducing the shear deformation along a direction parallel to the surface of the work piece. To date, extensive research works have been carried on shear deformation using ECAP (Equal Channel Angular Pressing), DCAP (Dissmilar Channel Angular Pressing), and asymmetric rolling. Most of these works have been focused on grain refining in an attempt to produce ultra fine-grained bulk metals. Considering that a significant amount of the shear deformation can be introduced into metals through ECAP, this process can be used as a technique for promoting shear textures in Al and Cu alloys.
In our group, we try to enhance the formability of Al alloys by controlling their texture using asymmetric rolling at elevated temperatures, where effects of dynamic recrystallization and pre-precipitation on shear deformation have been investigated.
3. Development of non-rusting Cu-Sn alloys with high formability
Cu-Sn alloy is well known as bronze, which have been used for cultural heritage, bowls, or musical instruments. In spite of high cost of the Cu-Sn alloys, the demand for the Cu-Sn alloys is increasing due to their cultural value, thermal insulation properties, and excellent antibacterial properties. However, the Cu-Sn alloys fabricated by casting have low mechanical strength and low elongation, which is due to presence of brittle δ phase. Their mechanical properties can be improved by microstructure controlling via various plastic deformations and heat treatments.
In our group, microstructure of Cu-Sn alloy was controlled by various heat treatment processes. We changed the cooling temperature and cooling method (water quenching and air cooling), and analyzed the changes of microstructures and mechanical properties. In addition, tempering was carried out after quenching for softening of martensite phases, and tempering temperature and time were varied. In addition, we are trying to find out key factor controlling the elongation of brittle Cu-Sn alloys.
4. Magnetic shape memory alloys for bio actuators
Shape memory alloys can recover a large thermoelastic strain by martensitic transformation from martensite to austenite. Shape memory alloy thin films have attracted considerable attention because of their rapid actuation response due to rapid heat dissipation. Even faster actuation response can be obtained from magnetic shape memory alloys (MSMA). This enhanced actuation results from the change in the driving mode for phase transformation from temperature to magnetic field. The shape memory effect induced by temperature requires a long time for heat dissipation, but the shape memory effect induced by a magnetic field does not increase the temperature so no heat dissipation is needed. The MSMA can be used for bio-actuators such as artificial muscle, micro robot propulsion devices, bio-switches etc.
In our group, a 7x7 array of cantilever beams was fabricated using bulk Si micromachining processes. Magnetic shape memory alloys such as Fe-Pd and NiMnGa thin films were deposited on the cantilevers by co-sputtering individual elemental targets. Lateral composition gradients in the coating were generated by varying the power and by changing the inclination of the individual magnetrons in the deposition system. The curvature, i.e., residual stress of the cantilever beams was measured by scanning using an array of parallel laser beams along the cantilever beams as a function of temperature or magnetic field.
5. Magnetic resonance (MR) image distortion reduction of shape memory alloy medical stents
Medical stent, which is a tube or other device placed in the body to create a passage between two hollow spaces, or catheter is made of NiTi shape memory alloy. In order to observe of the stents in the body, magnetic resonance image (MRI) is used. However, the distortion of the MR image occurs due to difference in magnetic susceptibility between human body and NiTi alloy, and becomes worse with magnetic field.
In our group, in order to decrease the MR image distortion, carbon materials such as graphene, carbon nano tube (CNT), or graphene oxide (GO) having a magnetic property opposite to that of the NiTi alloy was coated on the surface of the NiTi stents by spray coating, transferring, or direct growing. Magnetic susceptibility is measured using PPMS or SQUID and MR image distortion is evaluated using MRI system.
6. Copper/Polymer composite film substrate for flexible electronic devices
As the market for wearable electronics has continued to grow, interest in light weight, low cost, and high durability flexible circuits has increased. In general, conventional flexible circuits are fabricated by patterning, using the deposition of conducting metallic films such as Cu on polymer substrates and lithography. This patterning process is complicated and expensive due to the number of processing steps, which include surface modification of the polymer, Cu coating, photoresist (PR) coating, exposure, developing, and etching. A large amount of harmful chemicals are also used.
Our group fabricates the Cu circuit on a polyethylene terephthalate (PET) substrate without the conventional etching process, using selective surface modification by plasma etching and electroless plating. Effects of plasma treatment time on the adhesion strength between Cu coating and PET substrate and fatigue strength of the Cu/PET flexible substrate are studied. In addition, we proposed a new simple method for Cu patterning on PET substrates without the pretreatment steps (sensitization and activation) needed for Cu electroless plating as well as the etching process needed for Cu circuit patterning. Instead of six steps (plasma surface modification of polymer, Cu coating, PR coating, exposure, developing, and etching), the proposed method requires only two steps (plasma surface modification and Cu coating). The proposed method makes it possible that the two processes (plasma surface modification and catalyst patterning) occur simultaneously. A patterned mask coated with Ag, used as a catalyst material for the Cu electroless plating, was placed on a PET substrate during oxygen plasma treatment. The oxygen plasma treatment promotes the formation of nanostructures coated with the co-deposited Ag nanoparticles on PET, which is capable of the direct formation of a flexible Cu circuit with strong adhesion.
7. Development of high specific strength and high elastic modulus composites for automobiles (matrix: Al, reinforcement: CNF, CNT, CF)
Nano carbon materials such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are widely used as reinforcement materials of composites because of their high elastic moduli, high mechanical strengths, and outstanding electrical conductivities, which can improve the mechanical and physical properties of automobiles, aircrafts, and fuel cells. Especially, in the automobile industry, to reduce the CO2 emission and improve the combustion efficiency of fuel gas, lightweight components have become essential. In this respect, aluminum (Al) alloys with high specific strength has received much attention. Therefore, Al matrix composites reinforced with light nano carbon materials having outstanding mechanical properties have come into more prominence.
In general, two processes, the powder metallurgy process and the liquid process such as melt stirring, are available for the fabrication of metal matrix composites. The powder metallurgy process can yield well dispersed CNF reinforced metal matrix composites, while it cannot be easily applied to large scale products and is more costly than the liquid process. In contrast, the liquid process is simple and cheap, and can fabricate large scale composite billets. However, in spite of such merits of the liquid process, to fabricate a sound CNF/Al composite, difficulties such as the poor wetting of molten Al on CNF, entanglement of the CNF particles due to Van der Waals interaction, and floating of CNFs (ρ= ~2.0) on the surface of the Al melt (ρ=2.4) due to the differences in specific gravity have to be resolved.
In our group, Al matrix nanocomposites reinforced by CNFs were fabricated by the melt stirring method. To overcome the entanglement of the CNFs and disperse them uniformly in the Al matrix, we improved the wetting of molten Al on the CNFs and decreased the floating of the CNFs on the surface of the Al melt by electroless Cu coating on the surface of the CNFs.
8. Development of high heat conductivity and low thermal expansion coefficient composites for LED and power semiconductor devices ( matrix: Al or Cu, reinforcement: CNF, CNT, CF, graphene, SiC, AlN)
As microelectronic devices continue to evolve toward further miniaturization and higher component density, the power that the compact devices consume has been significantly increasing. Since most of that power is converted to heat, efficient dissipation of the heat generated in the compact devices during operation has become an important issue, which affects their reliability and lifetime. Heat dissipation can be achieved by employing a heat sink made of thermally conductive materials. Metals such as aluminum and copper have been widely used as heat management materials in many electronic systems. However, typically there is a large difference in CTE between the ceramic substrate of the devices and the metallic heat sink, and this mismatch results in thermal stress which can lead to the thermal failure of solder joints or the ceramic substrate. To address these problems, it is necessary to design novel heat dissipation materials which have a high thermal conductivity, comparable to that of copper, and a low CTE, comparable to the CTE of ceramics.
In recent studies, copper composite materials reinforced with carbonaceous nanomaterials such as carbon nanofibers (CNFs), carbon nanotubes (CNTs), and graphene have appeared to be promising candidates as heat management materials, due to the high thermal conductivities and very low CTEs of the carbonaceous nanomaterials. However, the addition of carbonaceous nanomaterials also causes a significant decrease in the thermal conductivity of the composites, which is largely due to the difficulty of achieving homogeneous dispersion of the nanomaterials. Therefore, developing C/Cu composites which have a low CTE without degrading their thermal conductivity has been a very important goal.
In our group, each CNF particle is coated with Cu using electroless plating to enhance the dispersion of the CNFs. The CNF/Cu composite powders are solidified using hot pressing or spark plasma sintering (SPS).