Contributing to solve the heat concentration problem in power
semiconductors
Developing a boiling immersion cooling technology using the promotion of boiling by lotus metals

Date:
October 12, 2021
Source:
Japan Science and Technology Agency
Summary:
Towards solving the heat concentration problem in power
semiconductors, researchers have developed a highly efficient
boiling immersion cooler using lotus metals. LTS succeeded in
increasing the cooling performance from about 200 W/cm2 of the
conventional cooler to 530 W/cm2 or more by using the boiling
promotion technology using lotus metals. This technology is also
considered as a highly efficient cooling technology for CPUs for
conventional workstations and large-scale servers.



FULL STORY ==========================================================================
In high-performance CPUs used in large servers and power semiconductors
used in inverters for hybrid electric vehicles (HEVs), as the
integration density rises and the higher the power consumption becomes,
the semiconductor package is also becoming smaller. Therefore, the power consumption per area of the semiconductor increases. As a result, the
heat generation density increases, and the current situation is that
the heat removal limit from the device is approaching.


==========================================================================
JST commissioned the company-led phase NexTEP-B type development
project of Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP*) "high-performance in-vehicle cooler with spontaneous cooling promotion mechanism" to Lotus Thermal Solutions Co.,
Ltd. to proceed the practical development based on the research results
of Professor Kazuhisa Yuki et al. of Sanyo-Onoda City University.

In the research by Professor Yuki et al., they realized the structure
hard for occurring the film boiling by engraving about 1 mm wide grooves
at a regular interval on a heat conductor, such a copper, which contacts
with a heating element, and combining it with a lotus metal. Lotus Thermal Solution has established a method that determines the appropriate groove cross-sectional areas and pore diameters according to the refrigerant,
and developed a highly efficient boiling immersion cooler (1) using lotus metals (2). Silicon carbide (SiC), which is expected as a next-generation
power semiconductor, has a heat generation density of 300 to 500 watts
per square centimeter (W/cm2). Thus, to use SiC in devices, a cooler with
a critical heat flux (CHF)(3) larger than this heat generation density
is required. In this development, we succeeded in increasing CHF from
about 200 W/cm2 of the conventional cooler to 530 W/cm2 or more by using
the boiling promotion technology using lotus metals.

The boiling immersion cooler prototyped in this development has
the capability to cool the inverter with Si semiconductors and
SiC semiconductors and is expected as a technology to solve the heat concentration problem of in-vehicle power semiconductors with increasingly
high heat generation density.

Furthermore, this technology is considered as a highly efficient cooling technology for CPUs for conventional workstations and large-scale servers.

(1) Boiling immersion cooling This is a method that a liquid refrigerant
is boiled with the heat of a heat source and then cooled. The conventional cooling method uses the temperature difference for heat transfer from
the heat source to the refrigerant such as water or air, and it cools
by natural convection or forced convection. However boiling cooling can
utilize latent heat of evaporation (heat of vaporization) when vaporizing; therefore, it is said to have cooling capacity several times that of
the conventional method.

(2) Lotus metal This is a lotus root-like porous metal in which
many elongated pores are arranged in the same direction. It has
cooling characteristics owing to a refrigerant flowing through the
pores. When a molten metal containing hydrogen is solidified, pores are
formed by hydrogen that cannot completely be dissolved in the molten
metal. Utilizing this phenomenon, lotus metals can be produced at a
low cost.

(3) Critical Heat Flux CHF When the heat load increases in boiling
immersion cooling, the nucleate boiling with a good heat transfer
efficiency cannot be maintained at certain point, and suddenly
transitions to film boiling where the heating surface is covered with
a vapor film. The heat flux (heat flow per unit area, unit [W/cm2])
at the transition point is called the critical heat flux.

*A-STEP is a technology transfer support program whose aim is to put the research results by public research institutes into practical applications
as important technology in the national economy, and thus to give some
of their profit back to society.

========================================================================== Story Source: Materials provided by
Japan_Science_and_Technology_Agency. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Risako Kibushi, Kazuhisa Yuki, Noriyuki Unno, Tetsuro Ogushi,
Masaaki
Murakami, Tomiyuki Numata, Takuya Ide, Hikaru Nomura. Enhancement
of the critical heat flux of saturated pool boiling by the
breathing phenomenon induced by lotus copper in combination
with a grooved heat transfer surface. International
Journal of Heat and Mass Transfer, 2021; 179: 121663 DOI:
10.1016/j.ijheatmasstransfer.2021.121663 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012095032.htm

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