Objective

1. System-Level Approach for Multi-Phase Nanotechnology-Enhanced Cooling of High-Power Microelectronic Systems

2. Objective Our objective is to develop a robust, low-temperature thermal management approach for distributed, large-scale, high-power electronic systems using improved heat transfer technologies and system modeling/control concepts to reduce the total system thermal resistance so that chip and device temperatures are maintained below 65°C.

3. Overall Challenge Problem: Operation of a large-scale, distributed electrical/electronic system including devices with ultra-high heat fluxes that require low surface temperatures. 65°C

4. State-of-the-art and Our Goals Estimates of Conductances in State-of-the-Art Systems and Future Systems

5. A Multidisciplinary Approach to a Multiscale Problem: Chip to Ship Scale: Nanoscale Energy Transport and Conversion: Length: 10-9 nm Electrical Engineering, Material Science, Solid-State Physics, Statistical Mechanics Thermal Management of Electronics: 10-6 μm Heat Conduction, Material Science, Microfluidics, Mechanics, Fatigue 10-3 mm Macroscopic Energy Transport and Conversion: Convective Heat Transfer, Fluid Dynamics, Thermodynamics 100 m System Scaling and Optimization: Electrical Engineering, Control Systems 103 km

6. Primary Thrusts Thrust 1 – Solid/Solid Interfaces Understanding the origins (interfacial chemistry, structure, and electron/phonon transport) of solid-solid interfacial thermal resistance, will lead to the development of a new thermal interface material with a target conductance of G~5 MW/m2-K. Thrust 2 – Solid/Liquid Interfaces Studying flow boiling and condensation at the micro-scale through novel noninvasive thermometry tools will guide the enhancement of convective heat transfer using advanced micro-domain configurations, engineered surface topography, and changes in chemical composition for control of interfacial physics. Thrust 3 – System Design and Optimization Sensor feedback and modern control theory will be used to develop design and control methodology for the system and devices and the results of Thrusts 1 and 2 will be integrated in a modular and scalable testbed.

7. Integration

8. Thrust 1: Solid/Solid Interfaces • Primary Contributors • UVa: Pam Norris • ASU: Ravi Prasher • UC Berkeley: Costas Grigoropoulos • Thrust Statement/Objectives • Increase the long-term stable conductance at solid/solid interfaces • Understand the origins (interfacial chemistry, structure, and electron/phonon transport) of solid-solid interfacial thermal resistance • Use this knowledge to help develop new thermal interface materials

9. Thrust 1: Accomplishments for 1st Yearas of June 2008 • Theoretical models developed for thermal transport in mesoscopicnanowires/nanotubes (thermal conductivity, thermal boundary resistance, specific heat). • Measured elastic modulus of vertically aligned CNT arrays under compression, and studied mechanism. • Studied the growth mechanism of vertically aligned CNT arrays. • Fabricated TIM with CNT array with G ~ 1 MW/m2-K through In bonding technique. • Compared the interface thermal conductance of glass-CNT-Si sandwich structures with/without In bonding, and with Ti wetting layer as well. • Investigated the adhesion problem between In film and Ti film. • Investigated the possibility of enhancing contact between In and Ti by adding Au bonding layer. • Examined effects of atomic mixing at Cr/Si interface on G – developed new model to take into account multiple scattering events from disorder. • Studied phonon scattering processes at interfaces – developed model to account for inelastic scattering and separated role of elastic and inelastic scattering in G. • Experimentally investigated electron-phonon equilibration in thin Au films subject to short pulsed heat in the presence of the film/substrate interface. • Determined that interfacial scattering can affect electron-phonon equilibration time – developed new model based on ballistic electron transport and ballistic-diffusive approximation to the Boltzmann Transport Equation.

10. Thrust 1: Accomplishments for 2nd Yearas of June 2009 • Characterized the packing density of vertically aligned CNT array using high resolution SEM. • Investigated the effect of hydrogen pretreatment on the CNT array density. • Comprehensively studied the sensitivity of transient opto-thermoreflectance technique on ultra-small thermal interface resistance measurement. • Improved the experimental technique to allow high resolution thermal measurement with improved sensitivity. • Investigated the effect of variation of packing densities on G. • Investigated the influence of interstitial metallic layers on G at metal-nonmetal interfaces. • Emulated the behavior of metal-CNT contact by making thermal measurements on metal-graphite samples. • Identified most probable material systems for ultimate TIM package assembly. • Preliminary development of the multidimensional diffuse mismatch model. • Refined UVA TTR setup to allow for measurement on more diffuse surfaces. • Fabricated CNT-array-based TIM with G ~ 1.5 MW/m2-K.

11. Thrust 1: Accomplishments for 3rdYearas of August 2010 • Developed a theoretical model for thermal transport between isotropic films and anisotropic substrates considering contributions of inelastic scattering. • Measured boundary conductance between Au films and sp2 carbon structures, investigating the role of interface structure and chemistry. • Investigated the role of subconduction band excitations on thermal conductance at metal-metal interfaces. • Reviewed the assumptions made in modeling diffusive transport in nanostructures and their application to specific systems. • Increased the CNT array packing density to 11% volume fraction. • Fabricated CNT arrays of various lengths  (~250, 200, 150, 100, and 50 μm) by decreasing growth duration of water-assisted synthesis from 3 minutes to 15 seconds, and with nearly fixed CNT array volume fraction of 3%. • Measured thermal interface (CNT – bonding surface) conductance of high density (11%) sample to be over 5 MW/m2-K.

12. Thrust 2: Fluid/Solid Interfaces • Primary Contributors • RPI: Michael Jensen, YoavPeles • UIUC: David Cahill, Steve Granick • Thrust Statement/Objectives • Reduce convective thermal resistances in heat sinks and heat exchangers • Understand flow boiling and condensation at the micro-scale • Enhance heat transfer through modifications of surface chemistry, surface topography, and micro-domain configurations

13. Thrust 2: Accomplishments for 1st Yearas of June 2008 • 95%+ complete on construction of three flow loops (boiling, condensation, microjet). • 99% complete on test sections for all three loops. • Calibrations of components in three loops done except for test sections; data acquisition and reduction programs complete. • Developed a program to optimize different microdomain geometries (microchannels, pin fins, etc.) that takes into account heat transfer and pressure drop characteristics; this will be used for guidance on developing enhanced boiling and condensing geometries. • Completed construction and testing of an IR (1.55 micron) femtosecond pump-probe measurement system capable of probing through the back side of Si wafers with low doping levels. • Completed first-generation construction of homebuilt apparatus for surface plasmon resonance imaging of bubble nucleation events.

14. Thrust 2: Accomplishments for 2nd Yearas of June 2009 • Obtained data for critical heat flux and boiling heat transfer data for R134a in microtubes. • Obtained data for single-phase heat transfer with single micro-jet and with micro-jet arrays. • Designed new experiment for multi-jet array impinging on enhanced surface. • Performed optimization study for single-phase flows with various enhanced heat transfer techniques in micro-domains. • Invented two-photon thermometry for ultra-high space and time resolution of thermal transport. • Invented surface plasmon thermometry for imaging local nucleation events. • Invented pump-probe ellipsometry for molecular understanding of heat transfer from solid to liquid on the picosecond time scale.

15. Thrust 2: Accomplishments for 3rdYearas of August 2010 • Completed a detailed experimental study of heat transfer coefficients and CHF condition for flow boiling of R134a in circular microtubes. • Completed a parametric study of area-averaged heat transfer coefficient of a microjet array varying area ratio and using air and water. • Converted the experimental microjet array apparatus to a closed, pressurized, refrigerant system charged with R134a, and conducted flow boiling experiments with R134a. • Designed and constructed test sections to measure condensation heat flux, and, using these, completed condensation heat transfer experiments on square mini-channels. • Designed micro devices for studying single-phase and flow boiling heat transfer of jet impingement on micro pin-fin structures; microfabricated the micro devices using MEMS microfabrication technologies. • Conducted heat transfer and pressure drop experiments using water as coolant for jet impingement on micro pin fins. • Discovered that time-averaged heat transfer during water droplet impacts can reach 500 W/cm2. • Observed thermal accommodation between a solid and a condensing vapor for the first time. • Directly visualized heat transfer in the nucleation of single bubbles for the first time.

16. Thrust 3: System Design and Optimization • Primary Contributors • RPI: J. Wen, M. Jensen, Y. Peles • ASU: P. Phelan, R. Prasher • Thrust Statement/Objectives • Integrate results of Thrusts 1 and 2 into a modular and scalable refrigeration system • Demonstration of 1 kW/cm2 on single module • Demonstration of scalability to multi-module systems • Develop design & control methodology for system and devices using sensor feedback and modern control theory

17. Thrust 3: Accomplishments for 1st Yearas of June 2008 • Development of steady-state vapor compression cycle modeling and optimization tool (in MATLAB) including CHF consideration. Extensibility to multiple evaporators, multiple loops, pump cycle. • Establishment of the two-loop architecture for system design. • Initial design and setup of a fully instrumented vapor compression cycle testbed capable of operating at a wide range of conditions. • Initial design and equipment purchase for the high-heat-flux, single-module testbed. • Initial development of dynamic modeling.

18. Thrust 3: Accomplishments for 2nd Yearas of June 2009 • Developed optimization module for steady-state system design at multiple operating conditions for a single vapor compression cycle. • Developed initial steady-state design tool for two-loop system analysis. • Completed the first phase RPI test facility beds and started model identification for expansion valve and compressor. • Nearly (99%) completed construction of ASU test facility. • Obtained initial transient stability result using singular perturbation. • Developed Ledinegg instability model based on data from Intel. • Developed preliminary pressure drop instability model based on data from Intel.

19. Thrust 3: Accomplishments for 3rdYearas of August 2010 Macro-scale Systems • Validated steady-state vapor compression cycle models (compressor, valve, heat exchangers) with experimental test facility at RPI. • Developed steady-state design and optimization module for two-loop cooling system at multiple operating conditions. • Completed dynamic identification for empirical modeling and controller design for single evaporator case. • Completed evaluation of linear controller design using expansion valve opening to control evaporator wall temperature. • Investigated the effects of transient heat load on two-phase flow instabilities in electronics cooling systems. Micro-scale Systems • Studied micro-thermal-fluid transients, developed new HTC correlation, and combined extremum-seeking and thermal-fluid stabilizing controller. • Developed pressure drop flow oscillation model and active instability control strategies for microchannel boiling system. • Proposed non-identical structures for parallel-channel flow instability control.