Study On Liquid Metal Cooling Method For Thermal Management Of Computer Chip

With the rapid improvement of computer performance, tremendous heat generation in the chip becomes a major serious concern for the thermal management. As a result, higher performance and reliability are extremely hard to attain. Recently, it was realized that using liquid metal or its alloy with low melting point as the coolant could significantly lower the chip temperature. This new generation heat transfer enhancement method raised many important fundamentals as well as practical issues to be solved. The present thesis was dedicated to present a comprehensive investigation in this area. Progresses achieved were listed as follows:1. Considering that the thermophysical properties of low-melting-point metals or alloys are extremely important however generally not available, this thesis predicted and measured some typical properties needed in designing liquid metal cooling. A theoretical correlation was proposed to calculate the thermal conductivity of binary alloy based on Faber-Ziman theory and Wiedemann-Franz-Lorenz (WFL) law. Further, the thermal conductivities of several typical alloys with low melting point were measured using TCi. The specific heat of those alloy were obtained using DSC.2. Different from conventional liquid, the liquid metal flow has its own characteristic. We discussed the effect of axial heat conduction on the convective heat transfer of liquid flowing in a single tube under isothermal boundary conditions. Using theoretical models for characterizing the liquid metal, we obtained the theoretical solution of the heat transfer equation. It was found that the need to include the axial heat conduction in the analysis is determined by the magnitude of the Peclet number; the larger the Peclet number, the less importance is the effect of axial heat conduction.3. We proposed for the first time the concept of the nano-liquid-metal fluid, aiming to establish an engineering route to make the highest conductive coolant. Using theoretical models for characterizing the nano fluid, the thermal conductivity enhancement of the liquid-metal fluid due to addition of more conductive nano particles was predicted. Further, the effects of particle size, cluster of nano particle, solid-like layer due to adsorption, volume fraction and particle types were evaluated. Having the highest conductivity and surface tension, being electromagnetically drivable, the liquid metal with low melting point is expected to be an idealistic base fluid for making super conductive solution.4. By taking full use of the double merits of the liquid metal, i.e. superior heat transfer performance and electromagnetically drivable ability, we demonstrated for the first time the liquid-cooling concept for the thermal management of computer chip using waste heat to power the thermoelectric generator (TEG) and thus flow of the liquid metal. Such device consumes no external net energy, which warrants it a self-supporting and completely silent liquid cooling module. Experiments on devices driven by one or two stage TEGs indicate that a dramatic temperature drop on the simulating chip has been realized without aid of any fans. The higher heat load, the larger temperature decrease will be caused by the cooling device. This new method is expected to be significant in future thermal management of a desk or notebook computer, where both efficient cooling and extremely low energy consumption are of major concern.5. In an extremely low temperature environment, the liquid metals may subject to solidification due to local freezing, which will cause failure of its role in transferring heat away. To resolve this potential difficulty, we proposed to quickly melt the frozen coolant by implanting in advance a wire heater into the liquid metal. A theoretical model was set up through introduction of moving heat source principle and a closed form analytical solution was obtained using Green's function under the third boundary condition. Meanwhile, proof-of-concept experiments were also performed in a refrigerator environment. Both theoretical prediction and experimental measurements in the sixth chapter show that, the frozen metal can be successfully thawed within several dozens of seconds. This would guarantee a quick start and highly safe running of liquid metal for computer chip cooling.