Monique Lallemand

Fabrication and experimental investigation of silicon micro heat pipes for cooling electronics

An experimental investigation was conducted to determine the thermal behaviour of micro heat pipe (MHP) arrays micromachined in silicon wafers. Two types of MHP arrays were tested, one with triangular channels, 230 μm wide, 170 μm deep, and the other with triangular channels, 500 μm wide, 340 μm deep, coupled with arteries. Both types of arrays were fabricated using an anisotropic etching process. Once fabricated, a plain Si wafer was used to seal the pipe array hermetically. Two working fluids were tested, ethanol and methanol. A polysilicon heater was used to supply the heat input, and cooling water flowing through the condenser was used as a heat sink. Fill charges from 0% up to 66% were tested. The axial temperature variation along the length of the pipe was measured using T-type thermocouples connected to a data acquisition system. The effective thermal conductivity was evaluated using the experimental temperature profiles and 3D thermal modelling. The results show a maximum improvement of 300% in effective thermal conductivity at high heat flux, which demonstrates enhanced heat transfer in a prototype with liquid arteries.

The performance of micro heat pipes measured by integrated sensors

This study deals with silicon micro heat pipe (MHP) arrays which are powerful thermal devices for the cooling of microelectronic components. MHP arrays, consisting of non-circular hermetically sealed channels, are passive cooling systems. Their working principle is based on two-phase heat transfer. We report in this work on the fabrication and the characterization of all-silicon MHP arrays using integrated temperature sensor arrays. The MHP arrays consist in series of several tens of triangular shaped channels hermetically sealed using Si–Si direct bonding. We first show how we proceeded to integrate sensor arrays consisting of polycrystalline silicon thermistors in the all-silicon MHP arrays using simple and reliable microfabrication steps. Then, experimental thermal performances obtained using methanol as the working fluid are presented when varying the liquid charge, the heat sink temperature and the input power.

Hydrodynamic and Thermal Study of a Water-Filled Micro- Heat-Pipe Array

A detailed mathematical model for predicting the heat transport capability and the temperature distribution along the axial direction of a 55-micro-heat-pipe array, filled with a polar fluid (water), has been developed. This steady-state model combines hydrodynamic flow equations with heat transfer equations in both the condensing and evaporating thin films. The velocity, pressure, and temperature distributions in the vapor and liquid phases are calculated. Various boundary conditions fixed to the micro-heat-pipe evaporator and condenser have been simulated to study the thermal performance of the micro-heat-pipe array below and above the capillary limit. The effect of the dryout or flooding phenomena on the micro-heat-pipe performance, according to boundary conditions and fluid fill charge, can also be predicted.

Influence of the Fluid on the Experimental Performances of Triangular Silicon Microheat Pipes

An experimental study was conducted to determine the performances of silicon microheat pipes (MHPs) filled with different fluids. The MHP arrays are made of 27 triangular microchannels chemically etched in a silicon wafer. This wafer is closed by molecular bonding of a bulk oxidized silicon wafer. The results show that the use of MHPs with low liquid charges can increase the effective thermal conductivity of the array. The best improvement of the effective thermal conductivity of the MHP array is equal to 41% for methanol andpentane, 12% for ethanol and nil for water and FC72.

Nucleate Pool Boiling of Binary Zeotropic Mixtures

This article deals with an experimental study of pool boiling of acetone/water and propane/isobutane zeotropic mixtures. Water and acetone form non-ideal mixtures characterized by a very wide phase equilibrium diagram. Propane and isobutane, which are alkanes with very close thermophysical properties, form nearly ideal mixtures. The whole nucleate pool boiling curves of these mixtures have been performed for decreasing heat fluxes. The experimental heat transfer coefficients have been compared to the literature models. A reference heat transfer coefficient that is different from the usual ideal heat transfer coefficient is proposed to determine the heat transfer coefficient of mixtures for the whole nucleate pool boiling curve. That is not the case of the ideal heat transfer coefficient, which cannot be calculated for heat fluxes higher than the lower critical heat flux of the two components.