Heat pipe is a sealed vessel as a thermal conductance device. Working fluid is charged in heat pipe. The phase of working fluid at evaporator section (heat source) is changed from liquid to vapor and contrarily changed at condenser section and cooled. Cooled working fluid is returned to from condenser to evaporator by capillary action within wick structure. It dissipates energy from heat source by the latent heat of evaporation in a nearly isothermal operation. Working fluid is circulated inside heat pipe accompanying with the phase change at both evaporator and condenser. So, also called two-phase convection device. The basic elements of heat pipe are shown Fig. 1. A number of heat pipes have been developed to cool numerous applications, such as microelectronics chip, space, reactor, engine, etc.
< Fig. 1 Heat pipe scheme >
As integrated circuits become faster and more densely packed with transistors, the power density increases and the heat generated as a by-product becomes more severe. Conventional methods of cooling are not an ideal way to overcome the heat problem so that small scale and high performance cooling devices are needed. A simple solution would be using micro heat pipe as integrated part in the silicon substrate of the processors. Currently, micro heat pipe are being fabricated using micro machining technology (MEMS) and tested to verify the operation of the micro heat pipe as a thermal heat spreader.
Definition of micro heat pipe(MHP)
The theory of micro heat pipe was introduced by Cotter in 1984. He defined a micro heat pipe as "so small that the mean curvature of the liquid-vapor interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel".
A MHP is a small-scale device with a hydraulic diameter on the order of 100 зн and a length of several centimeters. It differs from a conventional heat pipe in that it is much smaller, 5 зн to 500 зн in hydraulic diameter. In general, it does not contain a wick structure to assist the return of the condensate to the evaporator section. It rather uses capillary forces generated in the sharp edges of the pipe's cross section.
< Fig. 2 Schemetical Micro Heat Pipe