Exploiting the growth rate dependency of intermetallic compounds (IMCs) on the substrate composition, the possibility of controlling porosity during transient liquid phase (TLP) bonding via a composition gradient is investigated. A Cu substrate with a variable Ni concentration was prepared through selective electroplating and subsequent heat treatment. When this substrate reacted with a molten Sn-rich alloy, there was a non-uniform growth of the Cu₆Sn₅ IMC. It is proposed that by tailoring the direction of the composition gradient in the substrate, the IMC growth rate can be controlled in such a way as to progressively move the solid-liquid front to redistribute any porosity away from the central region of the joint and thereby improve reliability.

The real-time monitoring of electromigration in ball-grid-array solder joints is limited to measuring the electrical resistance increase of the solder joints. Tracking the electromigration induced microstructural changes in solder balls requires cross sectioning which is a destructive technique. A novel planar solder geometry was invented and described here that allows real-time, non-destructive monitoring of microstructural changes and the rate of elemental segregation at the anode while simultaneously tracking the extent of electromigration by electrical resistance means. Electromigration in planar geometry tin-bismuth eutectic solder was studied by two means, (a) by the rate of Bi segregation at the anode and (b) by the rate of increase of electrical resistance of the solder, as a function of joint length, solder temperature and electrical current density. At low temperature and low electrical current density there was an extended initial period during which the joint resistance decreased before it increased. At higher temperatures and electrical current densities this initial period of decreasing resistance became less pronounced and at much higher temperature and current density stressing it became non-existent. The rate of bismuth segregation at the anode was somewhat proportional to the solder joint length indicating a probable Blech back-stress effect. Electromigration results from the rate of Bi segregation and the rate of increase of solder joint resistance were summarized using Arrhenius plots. The two plots gave similar electromigration activation energies of 0.7 eV from the electrical measurements and 0.75 eV from the Bi segregation measurements. The Arrhenius plot based on resistance rate increase was also used to predict the electromigration life of Sn-Bi solder joints under typical application conditions.

With the increasing popularity of hybrid vehicles (HVs) and electric vehicles (EVs), the time required for their mass scrapping is also increasing. Therefore, attempts have been made to use storage batteries acquired from scrapped vehicles as fixed storage batteries; however, these batteries have not been widely used because of their lack of versatility and poor cost-effectiveness. In this study, we developed a versatile control program and hardware to use all vehicle-mounted storage batteries as stationary storage batteries regardless of the vehicle type, using the entire vehicle-mounted storage battery and parts. As a result, we realized batteries with a more stable system whose selling price was lower than that of new batteries. Moreover, attempts are in progress to develop a high-performance energy storage system that is not possible using fixed batteries by developing a system that integrates high-power (e.g., in HVs) and standard-power (e.g., in EVs) storage batteries. To use vehicle-mounted storage batteries from HVs and other vehicles as fixed storage batteries, we analyzed their charge/discharge control signal data to construct a basic algorithm for the control program, and we further succeeded in developing an original hardware design for incorporating the storage batteries into the system.

In this study, we investigated the heat transfer performance of subcooled flow boiling using a vortex & boiling bubble generator in a narrow channel for cooling automotive SiC inverters. The generator had continuous, perpendicular, but hydraulically nonsmooth vertical fins for effective flow management. Heat transfer experiments and computational fluid dynamics (CFD) simulations were performed to investigate the effects of the generator on boiling heat transfer and critical heat flux. The boiling curve verified that the boiling heat transfer performance increased by using the generator and that the maximum heat flux achieved 14.4 MW/m² under highly subcooled conditions. Furthermore, visualization experiments demonstrated that boiling bubbles were generated from the bottom corners of the fin channel, detached, and repeatedly condensed. The CFD simulation clarifies that a high-velocity flow in a narrow channel produces a strong vortex in the fin channel and the superheated liquid near the bottom corners of the fin channel. These results indicate that the proposed vertical fins in the narrow channel function as a vortex & boiling bubble generator and can improve the boiling heat transfer as well as enhance the critical heat flux.

In response to the demand for intraoral sensing applications, we have developed a wireless charging scheme for rechargeable batteries that utilizes a 2.45-GHz quasi-electrostatic field power transfer. Our comparison of a coupler capacitor and a dipole antenna for power transmitting showed that the dipole antenna using a quasi-electrostatic field is more suitable for intraoral sensing. We chose a sleeve antenna as the power transfer receiver. Several rectifier circuits were tested, and a quadruple-voltage rectifier circuit was deemed most suitable for this system. The received power was over 3.5 mW for a transmission power of 50 mW using wireless power transfers. We also developed an antenna design technique to improve the charging efficiency by placing the charging dock inside the metal can. Experimental results showed that charging efficiency when the lid of the can was closed was 30% higher than when it was open. Our proposed wireless power transfer scheme makes it possible to charge a battery with the capacity of 2 mAh for 9 hours and continuously measure the intraoral temperature for 6 hours.

This study investigated the heat transfer performance of subcooled flow boiling using chevron fins in a narrow channel to cool automotive SiC inverters. The chevron fins were inclined at an angle to the flow direction, but were not hydraulically smooth. These fins generate a strong swirl flow and improve the transport of the heated liquid along the fin channel, which enhances the heat transfer performance over the entire heat transfer surface. We call this the "Hot Coolant Refreshing Method." The effectiveness of the proposed method on the boiling heat transfer and critical heat flux was evaluated using heat transfer experiments and computational fluid dynamics (CFD) simulations. The chevron fin successfully removed more heat flux than the vertical V-shaped fin. Additionally, visualization experiments indicated that small boiling bubbles were transported along the chevron fin direction. The CFD simulation also clarified that the coolant inside the fin channel flows so as to swirl under the influence of the flow in the narrow channel. The "Hot Coolant Refreshing Method" with chevron fins in the narrow channel can improve the boiling heat transfer and enhance the critical heat flux.

In order to cool a high heat density of a SiC-based inverter installed in electric vehicles, this study evaluates the influence of scale inhibitor EDTA (Ethylenediaminetetraacetic acid) on heat transfer performance of subcooled flow boiling of Long-life-coolant (LLC) in a narrow channel. The boiling surface is smooth and flat, and the inlet temperature of the LLC solution at 30 vol% is 60°C (the liquid subcooling is approximately 40 K). The average flow velocities of the coolants in the narrow channel are 1 m/s and 5 m/s. The heat transfer tests verify that the utilization of EDTA prevents the cooling performance of the LLC solution from deteriorating under the flow boiling conditions. This is conceivable because EDTA not only suppresses the scale deposition but also dissolves the scale on the boiling surface. For instance, the heat flux in the case of the LLC solution with EDTA at the flow velocity of 5 m/s increases by 3 MW/m² at the wall superheat of 50 K, compared to the case without EDTA. Inhibitor EDTA is certainly useful and effective in preventing the cooling performance from deteriorating under flow boiling conditions.