This paper introduces a theoretical model for a Loop Heat Pipe (LHP) utilizing a coherent porous silicon (CPS) wick. The paper investigates the effects of different parameters on the performance of the LHP such as evaporator temperature, condenser temperature, total mass charge, wick thickness, porosity, and pore size. A LHP is a two-phase device with extremely high effective thermal conductivity that uses capillary forces developed inside its wicked evaporator to pump a working fluid through a closed loop. The loop heat pipe is developed to efficiently transport heat that is generated by a highly localized concentrated heat source and then to discharge this heat to a convenient sink. This device is urgently needed to cool electronic components, especially in space applications. The LHP has been modeled utilizing the conservation equations and thermodynamic cycle. The loop heat pipe cycle is presented on a T-s diagram. A direct relation is developed between the ratio of heat going for evaporation as well as heat leaking to the compensation chamber.

This paper discusses the theory, modeling, design, fabrication and preliminary test results of the MEMS loop heat pipe being developed at the Center for Microelectronic Sensors and MEMS at the University of Cincinnati. The emphasis is placed upon the silicon micro wick and its production through a novel technique known as Coherent Porous Silicon (CPS) Technology.

A MEMS bubble pump utilizing the explosive propulsion of vapor as a driving force to circulate a working fluid was considered and constructed to demonstrate its feasibility as a MEMS pump-thermal control device. This device can dissipate waste heat without any external pump or mechanical moving parts through the utilization of a porous silicon wick, which acts as a one-way valve. With respect to thermal control, it demonstrated a heat dissipation capability of approximately 18.5 W cm −2 in a closed loop configuration. As a MEMS pump, it exhibited excellent potential and demonstrated a pumping capacity up to 2.04 ml min −1 .

Work towards the development of an innovative, potentially high power density, MEMS loop heat pipe is in progress at the Center for Microelectronic Sensors and M E M S at the University of Cincinnati. The design of the loop heat pipe is based upon the very unique coherent porous silicon technology, a technique in which vast arrays of micrometer-sized through-holes are photo-electrochemically etched into a silicon wafer perpendicular to the (100) surface. The initial mathematical model, the design, fabrication and characterization of the device in the open loop configuration were previously reported at this conference, STAIF 2002. This paper begins with a very brief explanation of the device and its theory of operation. The design of the device components and their production utilizing the various techniques of microelectronic and microelectromechanical fabrication are presented. The modifications made to the photon-induced, electrochemical etch process, which significantly increase the etch rate of the pores, are explained. Attention is given to the mathematical model of the planar, MEMS, loop heat pipe with respect to the generation of the dimensions of the components through a summary of the recent advances. The emphasis of this paper is upon the design, construction and the characterization of the evacuated closed loop test cell structure. GENERAL DESCRIPTION OF THE MEMS LOOP HEAT PIPE FIGURE 1. This is a schematic representation of the MEMS loop heat pipe and its components.

The previous papers presented at STAIF 2002 and STAIF 2003 discussed the design, fabrication and characterization of the evaporator section and the initial test cell of a planar MEMS loop heat pipe based upon coherent porous silicon or "CPS" technology. The potentially revolutionary advantage of CPS technology is that it is planar and allows for pores or capillaries of absolutely uniform diameter. Coherent porous silicon can be mass-produced by various MEMS fabrication techniques. The preliminary experiments made with the original test structure exhibited the desired temperature and pressure differences, but these differences were extremely small and oscillatory. This paper describes modifications made to the initial test cell design, which were intended to improve its evacuated, closed loop performance. Included among these changes were the redesign of the compensation chamber and condenser, an increase in the porosity of the coherent porous silicon wick, the fabrication of silicon top "hot" plates with an increased depth of the vapor reservoir and the integration of metal resistive heater elements onto the backside of the top plates to simulate the input heat. Some changes were made in the test sequence to produce more discernable differences in temperatures and pressures. The most recent results of the tests made with the modified system will be presented.