Development of Microvalves for High-Performance Cooling of Microelectronics (Ontwikkeling van microkleppen voor hoogperformante koeling van microelektronica)

Driven by miniaturization and the continuing increase in clock speed and data transfer, power dissipation in many electronic components has exceeded the limits of conventional cooling methodologies, such as fan-blown air cooling. Given the expected increase in power density, microprocessor designers have identified cooling as one of the major challenges for the next decade. Similar needs arise from the further development of other electronic devices such as Light Emitting Diodes (LED), where future generations of high light-output multichip LED modules are being developed for general and automotive illumination applications as well as for power electronics. Hence, there is a tremendous need for innovative cooling technologies. One of the most important factors in reducing the lifetime of micro-electronic components is the high operation temperature and thermal cycling. Liquid cooling has been identified as one of the most promising solutions in reducing the operation temperature of high power electronics. Liquid cooling is often not optimized, leading to excess of cooling and large temperature variations over the IC. If the coolant is allowed to boil in the cooling channels, the heat transfer coefficient is increased with one order of magnitude. The problem of phase transition regime is the boiling instability.A solution of these problems is addressed by implementation of valves in the cooling system. For single-phase systems they have the role of maintaining a more uniform temperature over the IC and to reduce the pumping power. For two-phase systems the valve purpose is to maintain the most optimum boiling conditions.A comparative study of different actuation principles was performed to identify the best actuation technology applied for chip cooling valves which meet the cooling requirements targeted in this PhD project. This classifies the valve size between the micro and macro-scale, a field which has been mainly overlooked by prior research and potentially promising applications have not been yet explored. It is for the first time that valves are especially designed for chip cooling applications. The principal requirements for the mechanical valves are the following: no leakage to the environment, low power consumption, continuous and proportional flow control, low cost, fast response time, small size and high reliability.The most promising actuation technology found for single-phase flow is thermopneumatic actuation. Based on this principle, a valve has been designed manufactured and tested on a cooling system. The valve actuation is performed by the thermal expansion of a liquid (actuation fluid) which, at the same time, actuates the valve and provides feed-back sensing. The thermopneumatic valve shows a novel actuation principle that combines the advantages of zero power consumption, small size compared with the high flow rate, and low manufacturing costs. This valve has the major advantage that it can be used together with any heat sink used for microelectronic cooling. Its small size and its independence from external energy make it suited for portable devices. No electrical connection, no wires and no electronic control are needed, increasing its simplicity and reliability. A maximum flow rate of 38 kg/h passes through the valve for a heat load up to 133 W. The valve is able to reduce the pumping power by up to 60 % and has the capability to reduce the temperature variation over the IC with up to 24 %.The most suitable actuation principle found for two-phase cooling is electro- magnetic actuation for large bandwidth, large forces, relatively high strokes and low manufacturing cost. The electromagnetic valve uses the reluctance actuation principle. It makes use of a pilot valve to reduce the size and the required actuation force. The emphasis is placed on a compact planar design to fit in planar devices. A pressure balance design allows operation at high system pressures. Because the pressure balance is used to drive the valve, an analytical and numerical model has been developed to predict the pressure distribution over the valve. The pilot valve has also been modelled as well and linearised using the results obtained from the model. The results of the model have been successfully validated by measurements. The valve is capable to control a flow rate up to 15 kg/h. This flow rate is sufficient to remove a heat input of 133 W equivalent with a heat flux of 500 W/cm2 at a temperature difference of 20 deg. C. Its equilibrated design allows a pressure drop control of up to 100 kPa at maximum system pressures of 600 kPa. Compared with the latest research published in literature and the latest developments in industry, this electromagnetic valve classifies on top of the list when it comes to the following combined properties in a single package: high speed proportional control, compact dimensions, high flow rates at low pressure drop, high reliability and low manufacturing cost.This research demonstrates the need for valves and the benefits of using valves in high performance cooling systems.Abstract v List of Abbreviations vii List of Symbols ix Contents xix List of Figures xxv List of Tables xxxv 1 Introduction 1 1.1 Liquid cooling: why? 2 1.1.1 Benefits and drawbacks of liquid cooling 9 1.2 Research objectives and contributions 11 1.2.1 Goals of this research 11 1.3 Outline of the thesis 12 1.3.1 Contributions 13 2 State of the art in existing cooling solutions 15 2.1 Mature cooling solutions 15 2.1.1 Air cooling: heat sinks and fans 15 2.1.2 Liquid cooling: evaporators and heat-pipes 16 2.2 Experimental cooling solutions 18 2.2.1 Increasing cooling performance with higher thermal conductivity 18 2.2.2 Increasing cooling performance with higher heat transfer 21 2.3 Single-phase cooling 26 2.4 Two-phase cooling 29 3 Selection of the actuation technology for chip cooling valves 35 3.1 Design considerations 35 3.2 Considered actuation principles 37 3.2.1 Capillary and diffuser based actuation 39 3.2.2 Actuation based on rheologic fluids 39 3.2.3 Actuation based on ferrofluids 40 3.2.4 Actuation by phase change fluids 41 3.2.5 Actuation by paraffin materials 42 3.2.6 Bimetallic actuation 43 3.2.7 Shape memory alloy actuation 50 3.2.8 Thermopneumatic actuation 58 3.2.9 Electrostatic actuation 61 3.2.10 Piezoelectric actuation 65 3.2.11 Electromagnetic actuation 73 3.3 Back to back comparison 80 3.4 Heat sink dimensioning for micro-electronic cooling 82 3.4.1 Heat sink channel aspect ratio optimization 83 3.4.2 Heat sink wall thickness optimization 85 3.4.3 Experimental results 87 4 Thermopneumatic valve for single-phase cooling 93 4.1 Introduction 93 4.2 Design and operating principle 96 4.2.1 Valve operating principle 96 4.2.2 Detailed valve design 97 4.2.3 Beam geometry optimization 98 4.2.4 Thermopneumatic reservoir 100 4.3 Fabrication technology 101 4.3.1 Heat sink 101 4.3.2 Valve body 102 4.3.3 Actuation beam 103 4.3.4 Silicon tubing 103 4.3.5 Transparent cover with silicone sealing membrane 105 4.4 Measurement setup 107 4.5 Measurements 109 4.5.1 Tube characterization 109 4.5.2 Beam actuation capability 111 4.5.3 Beam pressure drop sensitivity 113 4.5.4 Valve flow rate regulation as a function of temperature 113 4.5.5 Dynamic response 115 4.5.6 Pressure drop regulation as function of deflection 117 4.5.7 Cooling performance 118 4.5.8 Power saving performance 119 4.6 Conclusion 120 5 Electromagnetic valve for two-phase cooling 123 5.1 Introduction 123 5.2 Design and operating principle 124 5.2.1 Valve operating principle 125 5.2.2 Detailed valve design 125 5.2.3 Membrane deflection modelling 128 5.2.4 Flow path modelling 132 5.2.5 Determination of the response time 148 5.2.6 Electromagnetic actuator modelling 151 5.3 Fabrication technology 162 5.3.1 Iron core and coiling 162 5.3.2 Valve body 164 5.3.3 Plunger 165 5.3.4 Spring holder 167 5.3.5 Flexible membrane and membrane holder 168 5.3.6 Transparent cover 170 5.4 Measurement setup 171 5.5 Measurements and results 172 5.5.1 Flow-pressure drop characteristic 172 5.5.2 Closed-loop control strategy 173 5.5.3 Closed-loop plunger position control 174 5.5.4 Total valve response time 175 5.5.5 Pressure drop influence over the valve 177 5.5.6 Closed-loop pressure drop control 179 5.5.7 Closed-loop flow control 181 5.5.8 Closed-loop temperature control 183 5.5.9 Coil temperature during operation 186 5.6 Conclusion 187 6 General conclusions and recommendations for future work 189 6.1 General conclusions 189 6.2 Suggestions for future work 192 6.2.1 Suggestions addressed to the valve for single-phase cooling 193 6.2.2 Suggestions addressed to the valve for two-phase cooling 194 A Design, fabrication and testing of a heater 195 A.1 Introduction 195 A.2 Design of the heat spreader 196 A.3 Design of the isolation 198 A.4 Fabrication and assembly 202 A.5 Measurements 206 A.5.1 Theoretical and measured maximum power 206 A.5.2 Response time 209 A.5.3 Flow rate influence on heater temperature 210 A.5.4 Temperature gradient over the heater 211 A.5.5 Heater efficiency in transmitting power to the coolant 211 A.6 Conclusion 213 B Demonstrator for single and two-phase cooling 215 B.1 A single-phase cooling system 215 B.2 A two-phase cooling system 218 C An experimental two-phase cooling system 223 C.1 Design and construction 223 C.2 Preliminary results 228 C.2.1 Heating surface temperature and temperature gradient during two-phase transition 228 C.2.2 Coolant temperature increase and power absorption during two-phase transition 230 C.2.3 Pressure variation during two-phase transition 231 C.2.4 Electromagnetic valve response time in two-phase system 233 D Oxidation experiments on aluminium 7075 235 Bibliography 237 Curriculum Vitae 259 List of publications 261nrpages: 304status: publishe