Thermal Management of Electronics, Volume II

Phase Change Material-Based Composite Heat Sinks—An Experimental Approach

Rajesh Baby, C. Balaji

• Publisher: Momentum Press
• Year: 2019
• Print ISBN: 9781949449419
• eBook ISBN: 9781949449426
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Phase change material (PCM)-based composite heat sinks have attracted great interest in recent decades, especially in the context of thermal management of portable electronic devices such as mobile phones, digital cameras, personal digital assistants, and notebooks.

In this monograph, a detailed analysis of plate fin heat sinks and plate fin heat sink matrix is presented, based on in-house experiments. Performance benchmarks are articulated and presented for these heat sinks. The state of the art in the development of PCM-based heat sinks and the challenges are outlined, and directions on future development are provided. It is our sincere hope and trust that this book will not only be informative but also awaken curiosity and inspire thermal management solution seekers to delve deep into the ocean of research in PCM-based heat sinks and discover their own pearls and diamonds.

CHAPTER 1

INTRODUCTION

1.1 BACKGROUND

The world has been witnessing a hectic growth in electronics. Added to this is the rapid miniaturization of the electronics and the ever-increasing heat flux densities. Such developments have opened new vistas and also challenges to researchers working on thermal management solutions. Heat is an inevitable byproduct of every electronic device, and the lack of proper heat dissipation mechanisms causes heat buildup that hampers device performance and reliability. The reliability of any electronic device or equipment is a direct measure of the frequency of failure as a function of time. Many factors affect the reliability of electronics: temperature, humidity, vibration, shock, and so on. Both routine, everyday Applications like laptops, mobile phones, and digital cameras and critical advanced electronics used in spacecraft need thermal management solutions that are reliable, safe, and energy-and cost-efficient. Cost is a critical concern mostly in consumer electronics, and it is not a critical factor in the designing of cooling technologies or thermal management solutions for the kind of advanced electronics used in space crafts. Growing flux densities, harsher environments, miniaturization coupled with ever-increasing performance expectations continue to challenge the thermal engineer.

The aforementioned growth in electronics demands innovative solutions in thermal management. The absolute power levels in microelectronic devices range from a few tens to a few hundreds of Watts. Even so, heat fluxes can be significant and can vary from 5 W/cm² for a printed wiring board to 2,000 W/cm² for a semiconductor laser. Providing cooling solutions for high heat flux requires considerable novelty. The cost also needs to be controlled. A bird’s-eye view of the history of the development of thermal management solutions is presented in Figure 1.1. The exponential curve shows the rapid increase in the heat flux and the changing cooling technologies. New cooling solutions are being developed in the area of multiphase heat transfer technologies among other alternatives to tackle emerging challenges in heat dissipation of complex, advanced electronic equipment of the future.

Figure 1.1. Thermal management scenario

Source: IBM, USA.

1.2 POSSIBLE COOLING STRATEGIES

Thermal management solutions can be broadly classified as follows: (a) active cooling techniques and (b) passive cooling techniques. The term active cooling suggests that the cooling process is assisted with devices such as a pump or a blower or a compressor. So, it is apparent that active cooling techniques can help handle high fluxes. The possibility bringing the temperatures below the ambient level will no longer be a constraint with new technologies being developed in the field of thermal management. Jet impingement, a method that uses air or liquid, and forced convection, spray cooling, and refrigeration systems are some of the most effective cooling solutions. The hitch here is that the use of these technologies leads to higher operating costs and increased size and weight of the device. Passive cooling subsystems do not require external power. Important methods in this category of cooling mechanisms include natural convection, use of phase change materials (PCMs), and thermosiphons. Passive cooling techniques are generally used in devices with lower heat flux densities. Some of the commonly used active and passive cooling techniques are discussed in the following sections.

1.2.1 ACTIVE COOLING

Forced convection with air: Here, the electronic devices are cooled by circulating air with a fan. The heat transfer coefficients associated with air are very low. In view of this, in order to reduce high heat fluxes, the air velocity required can be very high and in turn demand higher pumping power. The noise level will also increase with increasing fan speed. In addition, fans increase the possibility of mechanical failure. In consideration of the above, applications such as forced convection with air are rather limited in practical use.

Forced convection with liquid: For cooling high heat flux in electronic components, such as a computer microprocessor or a switch-mode power supply (SMPS), liquid cooling is a good choice. Liquid cooling involves circulating a coolant (water or refrigerant–water mixtures) through a cold plate. This plate absorbs the heat from heat-generating electronic components and then transfer that heat with a liquid-to-air heat exchanger or liquid-to-liquid heat exchanger to the ambient space. This is a cyclic process and the cooled liquid becomes ready once again to absorb the heat.

Spray cooling: Spray cooling involves dispersing, usually fine, droplets onto a heated surface. Here, large amounts of heat can be removed as the droplets evaporate because of the latent heat of evaporation. Higher heat transfer rates can be obtained from boiling with sprays, for example, from a pool. Spray cooling can be carried over large surfaces, and the impact of droplets is usually too small to cause erosion. However, filters, pumps, and auxiliary equipment add to cost and increase weight.

Jet impingement cooling: Here, a cold jet of fluid impinges on the target surface. The impinging jet usually has a high velocity jet with a nozzle that emanates from a hole or slot or an array of holes/slots. The impingement results in very high heat transfer rates. Other cooling techniques like Peltier cooling or thermoelectric cooling have also been developed and used.

1.2.2 PASSIVE COOLING

Natural convection: Natural convection is a type of heat transfer mechanism that occurs due to density differences in the fluid caused by temperature or concentration gradients. The heated fluid becomes less dense and rises up. The surrounding fluid rushes into the replace the heated fluid. Standalone devices such as modems and small computers with an array of printed circuit boards mounted within an enclosure are suitable for this method. Use of natural convection is limited to handling only small heat fluxes. Even so, it is noiseless and has high reliability.

Thermosiphons: Thermosiphons operate much in the same way as a heat pipe does. The key difference, though, is that no capillary structure is present as may be seen in heat pipes. In view of this, the evaporator needs to be located vertically below the condenser, ensuring that the condensate returns to the evaporator aided by gravity. Thermosiphons are used in heat pumps, water heaters, and boilers. Although thermosiphons have limitations like orientation and capacity, they are often an ideal alternative to using mechanical pumps.

Heat pipes: A heat pipe essentially consists of an evaporator and a condenser separated by an adiabatic section. The working fluid is converted into vapor and the heat is absorbed by the evaporator. The vapor dissipates heat at the condenser and becomes liquid. A capillary wick is then used to bring the liquid condensate back to the evaporator. The thermal performance of the heat pipe is a strong function of the material of wick, working fluid, the size and shape of the wick, and is a weak function of orientation.

PCM-based cooling: PCMs are highly effective heat storage materials that undergo a phase change at a certain temperature known as the phase change temperature. Here, the material latent heat is absorbed at the place where the heat buildup of electronic components is dissipated. When the latent heat is released, the PCM gets solidified once again.

Melting and solidification are nearly isothermal processes. Thermal management using PCM-based heat sinks have been widely used for various applications, such as spacecraft and avionics thermal control, personal digital assistants, iPods, mobile phones, digital cameras, and notebooks. They can also be used in situations where the heat dissipation is intermittent or periodic as is the case with most of these devices.

1.3 ADVANTAGES OF PASSIVE COOLING TECHNIQUES OVER ACTIVE COOLING METHODS

Passive cooling technologies offer many advantages over active cooling methods. They are useful in situations where space is premium and active cooling devices cannot be utilized. No electricity is required to run them and thus they need only low maintenance. Passive systems are quite efficient and cost-effective in certain applications like avionics.

The ever-increasing demands for better performance of electronic equipment have propelled the development of new and innovative thermal management solutions, for example, PCM-based cooling. As the latent heat of fusion of PCMs is relatively high, only a small amount of PCM is required to handle the heat storage requirements for a plethora of applications. The types of PCMs and their salient features are discussed in the next section.

1.4 PHASE CHANGE MATERIALS

PCMs can be broadly classified as follows: (a) organic, (b) inorganic, and (c) eutectic (see Figure 1.2).

Figure 1.2. Classification of PCMs

Paraffin and non-paraffin compounds come under the category of organic PCMs. Organic materials are used in congruent melting, which means that melting and freezing, even if they occur repeatedly, no phase segregation is caused. Paraffins can be used for managing a large range of temperatures and are safe, reliable, less expensive, and noncorrosive. The stability of the paraffin compounds below 500°C makes them suitable ...

Table of contents

• Cover
• Halftitle Page
• Title Page
• Copyright Page
• Contents
• List of Figures
• List of Tables
• Abbreviations
• Notations
• Preface
• Acknowledgments
• Chapter 1 Introduction
• Chapter 2 Review of Literature
• Chapter 3 Characterization of Pcm and Tce
• Chapter 4 Experimental Setup and Methodology
• Chapter 5 Thermal Performance and Optimization of Pin Fin Heat Sinks
• Chapter 6 Performance Studies on Metal Foam-filled Pcm-based Heat Sinks
• Chapter 7 Deductions and Suggestions for Future Work
• Appendix A Schematic Diagrams of Various Heat Sinks Used in the Present Study
• References
• List of Publications Used for the Book
• About the Authors
• Index