A.J. Robinson

A high-precision apparatus for the characterization of thermal interface materials.

An apparatus has been designed and constructed to characterize thermal interface materials with unprecedented precision and sensitivity. The design of the apparatus is based upon a popular implementation of ASTM D5470 where well-characterized meter bars are used to extrapolate surface temperatures and measure heat flux through the sample under test. Measurements of thermal resistance, effective thermal conductivity, and electrical resistance can be made simultaneously as functions of pressure or sample thickness. This apparatus is unique in that it takes advantage of small, well-calibrated thermistors for precise temperature measurements (+/-0.001 K) and incorporates simultaneous measurement of electrical resistance of the sample. By employing precision thermometry, low heater powers and minimal temperature gradients are maintained through the meter bars, thereby reducing uncertainties due to heat leakage and changes in meter-bar thermal conductivity. Careful implementation of instrumentation to measure thickness and force also contributes to a low overall uncertainty. Finally, a robust error analysis provides uncertainties for all measured and calculated quantities. Baseline tests were performed to demonstrate the sensitivity and precision of the apparatus by measuring the contact resistance of the meter bars in contact with each other as representative low specific thermal resistance cases. A minimum specific thermal resistance of 4.68x10(-6) m(2) K/W was measured with an uncertainty of 2.7% using a heat transfer rate of 16.8 W. Additionally, example measurements performed on a commercially available graphite thermal interface material demonstrate the relationship between thermal and electrical contact resistance. These measurements further demonstrate repeatability in measured effective thermal conductivity of approximately 1%.

A Thermal–Hydraulic Comparison of Liquid Microchannel and Impinging Liquid Jet Array Heat Sinks for High-Power Electronics Cooling

In this paper, two single-phase liquid cooling strategies for electronics thermal management are compared and contrasted; impinging jet arrays and laminar flow in microchannels. The comparison is posed for a situation in which an electronic device must dissipate 250 W/cm2 while being maintained at a temperature of 85degC. The calculations indicate that both the impinging jet and microchannel heat sinks can provide the necessary cooling with less than 0.1 W of pumping power. Microchannels achieve this heat transfer target with such low pumping power by the relatively high pressure drop being offset by a low volumetric flow rate. In contrast, impinging jet heat sinks require a lower pressure drop and higher volumetric flow rate. From a practical point of view, lower operating pressure and larger mass flow rates are desirable characteristics, since they will be less prone to leakage and will provide better temperature uniformity across the heated component.

Bubble growth in a uniform and spatially distributed temperature field

Abstract A theory has been developed which has been shown to predict experimental bubble growth data for both spherical growth in an unbounded liquid and hemispherical growth at a heated plane surface in microgravity. The theory is able to accommodate both spatial and temporal variations in the temperature and velocity fields in the liquid surrounding the bubble as it grows. Utilising the present theory, the complicated thermal and hydrodynamic interactions between the vapour, liquid and solid have been manifested for a single isolated bubble growing on a heated plane surface from inception.

An Experimental Study of Small-Diameter Wickless Heat Pipes Operating in the Temperature Range 200°C to 450°C

An experimental investigation is reported for medium-temperature, wickless, small-diameter heat pipes charged with environmentally sound and commercially available working fluids. The wickless heat pipes (thermosyphons) studied have many applications in heat recovery systems since their operational temperature range is between 200°C and 450°C. The heat pipes investigated had an internal diameter of 6 mm and a length of 209 mm. The lengths of evaporator and condenser sections were 50 mm and 100 mm, respectively. The working fluids tested were diphenyl based: Therminol VP1 and Dowtherm A. High-grade stainless steel was chosen as the shell material for the heat pipes to provide chemical compatibility between heat pipe casing material and working fluids at elevated temperatures. Thermal resistances of less than 0.4 K/W have been achieved at working temperatures of up to 420°C with an effective thermal conductivity of 20 kW/m-K, which corresponds to an axial heat flux of 2.5 MW/m2. Even for such small-diameter heat pipes, the experimental data for the evaporator showed good agreement with Rohsenows pool boiling correlation.

Analysis of quasi-static vapour bubble shape during growth and departure

In an effort to better understand the physical mechanisms responsible for pool boiling heat transfer, a numerical solution to the capillary equation is used to describe bubble shape evolution. Indeed, any analysis of thermal transport due to nucleate pool boiling requires bubble frequency and volume predictions, which are intimately linked to bubble shape. To this end, a numerical treatment of the capillary equation is benchmarked to profiles measured from captured images of vapour bubble formations. The bubble growth is quasi-static in a quiescent liquid with a triple contact line fixed to the perimeter of a needle orifice. This investigation provides insight into the dependence the bubble shape evolution has on the physical mechanisms quantified in the Bond number with characteristic length equal to the cavity radius.

Development of a high-accuracy thermal interface material tester

An experimental apparatus has been designed and constructed to accurately measure the next generation of high-performance thermal interface materials with unprecedented precision and accuracy. The apparatus is based on a common implementation of ASTM D5470 using meter bars. However, the apparatus in the present study is unique in that it utilizes small thermistors to make precise thermal measurements (plusmn0.003 K). These measurements are used to calculate the thermal impedance at the interface of two conducting bodies while keeping input power at a minimum. Furthermore, a robust and conservative uncertainty analysis is employed to calculate how the measured uncertainties contribute to the calculated quantities of thermal impedance and effective thermal conductivity. Baseline tests are performed to demonstrate the sensitivity and uncertainty of the apparatus by measuring the contact resistance of the meter bars in contact with each other as a representative low-thermal- impedance case. A contact thermal impedance as low as 2.81E-5 m2ldrK/W was measured with a calculated absolute uncertainty of approximately 2%. The effective thermal conductivity of a gap pad was also measured to further validate the apparatus.

Development of EHD ion-drag micropump for microscale electronics cooling

In this investigation, the numerical simulation of electrohydrodynamic (EHD) ion-drag micropumps with micropillar electrode geometries have been performed. The effect of micropillar height and electrode spacing on the performance of the micropumps was investigated. The performance of the EHD micropump improved with increased applied voltage and decreased electrode spacing. The optimum micropillar height for the micropump with electrode spacing of 40 mum and channel height of 100 mum at 200 V was 40 mum, where a maximum mass flow rate of 0.18g/min was predicted. Compared to that of planar electrodes, the 3D micropillar electrode geometry enhanced the overall performance of the EHD micropumps.

Numerical Investigation of Bubble‐induced Marangoni Convection

The liquid motion induced by surface tension variation, termed the thermocapillary or Marangoni effect, and its contribution to boiling heat transfer has long been a very controversial issue. In the past this convection was not the subject of much attention because, under terrestrial conditions, it is superimposed by the strong buoyancy convection, which makes it difficult to obtain quantitative experimental results. The scenario under consideration in this paper may be applicable to the analysis of boiling heat transfer, specifically the bubble waiting period and, possibly, the bubble growth period. To elucidate the influence of Marangoni convection on local heat transfer, this work numerically investigates the presence of a hemispherical bubble of constant radius, Rb= 1.0 mm, situated on a heated wall immersed in a liquid silicone oil (Pr= 82.5) layer of constant depth H= 5.0 mm. A comprehensive description of the flow driven by surface tension gradients along the liquid–vapor interface required the solution of the nonlinear equations of free‐surface hydrodynamics. For this problem, the procedure involved solution of the coupled equations of fluid mechanics and heat transfer using the finite‐difference numerical technique. Simulations were carried out under zero‐gravity conditions for temperatures of 50, 40, 30, 20, 10, and 1 K, corresponding to Marangoni numbers of 915, 732, 550, 366, 183, and 18.3, respectively. The predicted thermal and flow fields have been used to describe the enhancement of the heat transfer as a result of thermocapillary convection around a stationary bubble maintained on a heated surface. It was found that the heat transfer enhancement, as quantified by both the radius of enhancement and the ratio of Marangoni heat transfer to that of pure molecular diffusion, increases asymptotically with increasing Marangoni number. For the range of Marangoni numbers tested, a 1.18‐fold improvement in the heat transfer was predicted within the region of Rb≤r≤ 7Rb.

EHD Augmented Convective Boiling: Flow Regimes and Enhanced Heat Transfer

This work investigates the influence of electrohydrodynamics (EHD) on the flow and heat transfer during convective boiling of HFE7000. A unique tube-and-shell heat exchanger has been constructed with heated water flowing on the shell side and a saturated mixture of refrigerant flowing within the tube side. The heat exchanger is novel in that it allows full visual access to the flow in the inner tube while being both thermally and electrically conductive. This permits observation of the two-phase flow regimes, which is not possible with metallic test sections. In this work the influence of EHD on the flow regimes and subsequent overall heat transfer is investigated for fixed inlet refrigerant mass flux of 100 kg/m2-s, inlet quality of 3%, and wall superheat of approximately 11.5°C. For these conditions the applied voltage across a concentric inner electrode and the outer wall of the tube was varied between 0 kV and 10 kV at 60 Hz AC. In particular, this work focuses on quantifying the level of overall enhancement that is achievable with EHD for this heat exchanger. This is done in the context of the additional heat extracted by the working fluid in the heat exchanger compared with the field-free case and the additional power penalties required to do so. Heat transfer enhancements of up to 1.8 -fold were realized in this heat exchanger. Even so, there were hydraulic power increases as well as electrical power required to achieve the heat transfer enhancement. It was found that the electrical power was the dominant penalty and that an overall enhancement of 40 times more heat power extracted than input required was achieved. Finally, a proportional–integral–derivative (PID) control system has been utilized in conjunction with a high-voltage amplifier in order to accurately control the heat transfer rate of the heat exchanger. To our knowledge this is the first solid-state control system of this type for a two-phase heat exchanger.

Present and future thermal interface materials for electronic devices

ABSTRACT Packaging electronic devices is a growing challenge as device performance and power levels escalate. As device feature sizes decrease, ensuring reliable operation becomes a challenge. Ensuring effective heat transfer from an integrated circuit and its heat spreader to a heat sink is a vital step in meeting this challenge. The projected power density and junction-to-ambient thermal resistance for high-performance chips at the 14 nm generation are >100 Wcm−2 and <0.2 °CW−1, respectively. The main bottleneck in reducing the net thermal resistance are the thermal resistances of the thermal interface material (TIM). This review evaluates the current state of the art of TIMs. Here, the theory of thermal surface interaction will be addressed and the practicalities of the measurement techniques and the reliability of TIMs will be discussed. Furthermore, the next generation of TIMs will be discussed in terms of potential thermal solutions in the realisation of Internet of Things.