Thin cooling device targets compact electronic devices

Smartphones, tablets and other similar mobile devices are increasingly multi-functional and fast. These spec improvements, however, have increased heat generated from internal components, and the overheating of localised parts in devices has become problematic.

To solve this problem, Fujitsu Laboratories Ltd has developed the world's first loop heat pipe less than 1mm thick, which is designed to cool down small, thin electronic devices. This new tool was developed using technologies for stacking metal sheets. It is capable of transferring approximately five times more heat than current thin heat pipes.

This technology will make it possible for CPUs and other heat-generating components to run cooler and to avoid concentrated hot-spots inside devices.

Loop heat pipe

A loop heat pipe is a heat-transfer device that consists of an evaporator that absorbs heat from the heat source and a condenser that dissipates that heat away, with the two components connected by pipes into a loop (Figure 1). A working fluid is encapsulated inside this closed loop as a coolant. The heat from the heat source evaporates the coolant, and the energy that goes into evaporating the coolant is taken away from the heat source, lowering its temperature. It is based off of the same principle used when sprinkling water on pavement to reduce heat.

Figure 1: Configuration of a loop heat pipe

The thin loop heat pipe that Fujitsu has developed can be installed on a heat-generating component, such as a CPU, in an electronic device so that the heat generated by the component is carried to a relatively cool spot inside the device to diffuse the heat (Figure 2).

Figure 2: New thermal management concept for a smartphone equipped with a loop heat pipe

Features of the technology are as follows:

1. Structural design for high-efficiency heat transfer

Heat transfer using a loop heat pipe is based on the same sort of capillary action that absorbs water in fibres, sponges and plants. The evaporator contains a porous structure, with the numerous holes driving the fluid with capillary action. To achieve this action, Fujitsu stacked together copper sheets to develop a structure containing minuscule pores. The pattern of holes etched into the sheets is such that the holes of each layer are slightly offset from the adjacent layers. When these sheets are stacked, capillary action is created that causes the fluid to circulate.

Figure 3: Prototype model of a thin loop heat pipe

In addition, by separating the vapour phase and liquid phase, there are two flows of the working fluid within the stacked structure, which enables efficient heat transfer. Furthermore, because the liquid line that returns fluid to the evaporator operates on capillary action, enabling it to stably transfer heat regardless of the device's orientation, this technology can now be applied to mobile devices.

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