Shakti Singh Chauhan

On the charging and thermal characterization of a micro/nano structured thermal ground plane

As power densities in electronic devices have increased dramatically over the last decade, advanced thermal management solutions are required. A significant part of the thermal resistance budget is commonly taken up by the heat spreader, which serves to reduce the input heat flux and connect to an increased area for heat removal. Thermal ground planes are devices that address this issue by utilizing two-phase heat transfer achieving higher effective thermal conductivities than conventional solid heat spreaders. This study describes the need for and design of a charging station to accurately dispense the working fluid and a thermal characterization experiment to characterize performance. The design study includes detailed analysis of accuracy and validation of the setup.

Charging Station of a Planar Miniature Heat Pipe Thermal Ground Plane

Thermal ground planes (TGPs) are planar, thin (thickness of 3 mm or less) heat pipes which use two-phase heat transfer. The objective is to utilize TGPs as thermal spreaders in several microelectronic cooling applications. TGPs are innovative high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. Moreover, being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and can operate in high adverse gravitational fields due to nanoporous wicks. A key factor in the design of the TGP is evacuation prior to filling and introduction of the proper amount of working fluid (water) into the device. The major challenge of this work is to fill heat pipes with a total liquid volume of less than 1 ml, without being able to see into the device. The current filling station is an improvement over the current state of the art as it allows for accurate filling of microliter sized volumes. Tests were performed to validate performance of the system and to verify that little to no noncondensable gasses were introduced to the system. Careful calibration of the amount of liquid introduced is important. Therefore, calibration of the burettes utilized for a liquidfill range of 0.01 ml to 100 ml was important. The magnitude of the pressure inside the TGP device is also an important factor. Charging station validation demonstrated the capability of charging TGPs with accuracy of ±1.64 μl. Calibration curves for the burettes and error characterization curves for a range of liquid charging volumes will be presented and discussed in this paper.

Thermo-Fluid Model for High Thermal Conductivity Thermal Ground Planes

The thermal ground plane (TGP) is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the TGP, including the effects of the presence of non-condensable gases (NCGs). Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the TGP decreases when the substrate and/or wick are thicker and/or with the presence of NCGs. Moreover, it was demonstrated that the thermo-fluid model may be utilized to optimize the performance of the TGP by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the TGP. The wick porosity plays an important effect on maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat transport capability of the TGP. Axial effective thermal conductivity is mostly unaffected by the gravitational field. The maximum length of the TGP before reaching the capillary limit is inversely proportional to input power.© 2012 ASME

Development and Experimental Validation of a Micro/Nano Thermal Ground Plane

Heat pipes are commonly used in electronics cooling applications to spread heat from a concentrated heat source to a larger heat sink. Heat pipes work on the principles of two-phase heat transfer by evaporation and condensation of a working fluid. The amount of heat that can be transported is limited by the capillary and hydrostatic forces in the wicking structure of the device. Thermal ground planes are two-dimensional high conductivity heat pipes that can serve as thermal ground to which heat can be rejected by a multitude of heat sources. As hydrostatic forces are dependent on gravity, it is commonly known that heat pipe and thermal ground plane performance is orientation dependent. The effect of variation of gravity force on performance is discussed and the development of a miniaturized thermal ground plane for high g operation is described. In addition, experimental results are presented from zero to −10g acceleration. The study shows and discusses that minimal orientation or g-force dependence can be achieved if pore dimensions in the wicking structure can be designed at micro/nano-scale dimensions.Copyright

Thermal-Fluid Modeling For High Thermal Conductivity Heat Pipe Thermal Ground Planes

The thermal ground plane is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the thermal ground plane, including the effects of the presence of noncondensable gases. Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the thermal ground plane decreases when the substrate and/or wick are thicker and/or with the presence of noncondensable gases. Moreover, it was demonstrated that the thermal-fluid model may be used to optimize the performance of the thermal ground plane by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the thermal ground plane. The wick porosity plays a significant role in maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat ...

Charging Station of a Planar Miniature Thermal Ground Plane

Thermal ground planes (TGPs) are flat thin (less than 1 mm thick) heat pipes that can be used as a thermal spreader in a variety of microelectronic cooling applications. Like conventional heat pipes, TGP’s utilize two-phase cooling. Major advantages, include the ability to integrate directly with the microelectronic substrate for a wide range of applications; and the ability to operate in an adverse gravity environment of up to 20g. Other advantages include a very high thermal conductivity, reliability, no moving parts, electrodes, or need for external power. A key factor in the design of the TGP is evacuation prior to filling and introduction of the proper amount of working fluid (water) into the device. The major challenge of this work is to fill heat pipes with a total liquid volume of less than 1 ml, without being able to see into the device. The current filling station is an improvement over the current state of the art as it allows for accurate filling of micro liter sized volumes. Tests were performed to validate performance of the system and to verify that little to no non-condensable gasses were introduced to the system. Careful calibration of the amount of liquid introduced is essential. Therefore, calibration of the burettes utilized for a liquid fill range of 0.1 ml to 100ml was important. The magnitude of the pressure inside the TGP envelope is also an important factor. Calibration curves for the burettes and error characterization curves for a range of liquid charging volumes will be presented and discussed.Copyright

Thermal Modeling and Experimental Validation for High Thermal Conductivity Heat Pipe Thermal Ground Planes

Thermal ground planes (TGPs) are flat, thin (external thickness of 2 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. In addition to TGPs and flat heat pipes, some investigators refer to similar devices as vapor chambers. TGPs are novel high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. In addition to being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and (because of nanoporous wicks) can operate in high adverse gravitational fields. A three-dimensional (3D) finite element model is used to predict the thermal performance of the TGP. The 3D thermal model predicts the temperature field in the TGP, the effective axial thermal conductivity, and the evaporation and the condensation rates. A key feature of this model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor–liquid saturated wick interface. Wick samples for a TGP are tested in an experimental setup to measure the interfacial heat transfer coefficient. Then the experimental heat transfer coefficient data are used for the interfacial energy balance. Another key feature of this model is that it demonstrates that for the Jakob numbers of interest, the thermal and flow fields can be decoupled except at the vapor–liquid saturated wick interface. This model can be used to predict the performance of a TGP for different geometries and implementation structures. This paper will describe the model and how it incorporates empirical interfacial heat transfer coefficient data. It will then show theoretical predictions for the thermal performance of TGPs, and compare with experimental results.

Thermal Modeling for High Thermal Conductivity Thermal Ground Planes

Thermal ground planes (TGPs) are flat, thin (external thickness of 2 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. TGPs are novel high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. In addition to being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and (because of nano-porous wicks) can operate in high adverse gravitational fields.A three-dimensional (3D) finite element model is used to predict the thermal performance of the TGP. The 3D thermal model predicts the temperature field in the TGP, the effective axial thermal conductivity, and the evaporation and the condensation rates. A key feature of this model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor-liquid saturated wick interface. Wick samples for a TGP are tested in an experimental setup to measure the interfacial heat transfer coefficient. Then the experimental heat transfer coefficient data are used for the interfacial energy balance. Another key feature of this model is that it demonstrates that for the Jakob numbers of interest, the thermal and flow fields can be decoupled except at the vapor-liquid saturated wick interface. This model can be used to predict the performance of a TGP for different geometries and implementation structures. This paper will describe the model and how it incorporates empirical interfacial heat transfer coefficient data. It will then show theoretical predictions for the thermal performance of TGP’s, and compare with experimental results.Copyright