Encyclopedia of Thermal Packaging

The following sections are included:

• Foreword to the Encyclopedia of Thermal Packaging

• Preface

• List of Symbols

• Contents

Thermoelectric cooling is the direct conversion of electrical voltage to temperature difference, in which electrons or holes act as the working fluids to carry energy from heat source to sink under an external electric field. This chapter discusses thermoelectric effects and transport properties on both macroscopic and microscopic levels. It starts with an introduction to thermoelectrics, which defines three basic thermoelectric effects: the Seebeck effect, Peltier effect and Thomson effect, and their corresponding coefficients. The Peltier effect is the principle at work behind thermoelectric cooling. It is followed by the analysis of cooling performance and figure of merit ZT of single- and multi-stage thermoelectric modules. Thermoelectric transport properties in bulk materials, including the Seebeck coefficient, electrical resistivity, electron and phonon thermal conductivities, have been derived using the Boltzmann transport equations. Nanostructured materials provide additional parameter space that can be engineered to steer the thermoelectric properties for an enhanced figure of merit ZT. Electron and phonon thermoelectric transport in nanostructures have also been discussed in the last part of this chapter.

Realizing the widespread use of thermoelectric technology in electronic cooling is contingent on development of materials with high figure-of-merit Z. This chapter provides a review of thermoelectric materials and modules most relevant to electronic cooling. It starts with discussion of challenges facing the development of thermoelectric materials, which is due to the conflicting combination of material properties that are required. Then it is followed by the discussion of physical properties and thermoelectric transport properties of several materials, such as crystalline silicon, Bi₂Te₃-based alloys and Bi-Sb alloys. Bi₂Te₃-based alloys are the best commercially-available thermoelectric materials for cooling applications. Nanostructured materials, such as Bi₂Te₃-based superlattices and nanocomposites, are found to have ZT values higher than their bulk forms. In addition to aforementioned semiconductors, thermoelectric properties of metallic materials are also discussed as these materials can serve as one branch of a thermoelectric thermocouple or interconnect. The last part of this chapter provides a general overview of thermoelectric modules for electronic cooling. This chapter is not intended to serve as a complete description of all thermoelectric materials available for electronic cooling. The selection of the coverage was influenced by the research focus of the authors and reflects their assessment of the field.

Knowledge of thermoelectric material properties (i.e., electrical resistivity, thermal conductivity, Seebeck coefficient, and figure-of-merit) is critical for the application of thermoelectrics in electronic cooling. The accurate measurement and characterization of these properties in bulk and thin-film materials can pose many challenges, and extensive efforts have been undertaken since 1950s. An overview of the typical measurement techniques used to determine thermoelectric properties is provided in this chapter. It starts with an introduction of electrical resistivity measurement, which includes ohm's law method, van der Pauw's method, and modified transmission line method. The Seebeck coefficient is determined by the ratio of the open-circuit voltage to the associated temperature difference along the sample, but its measurement in the cross-plane direction of thin films is extremely challenging. Thermal conductivity measurement is difficult to make with relatively high accuracy due to various heat loss mechanisms. The techniques presented include steady-state method, laser flash method and 3ω method. The last part of this chapter is devoted to Harman's technique that can be used to determine the effective figure-of-merit Z of thermoelectric materials and modules. This chapter also provides an extensive reference list for more discussion of the measurement concepts and techniques.

Significant progress has been made over the past 20 years in research and development of thin-film thermoelectric materials and devices that can be integrated into electronic systems for thermal management applications. This chapter provides an overview of state-of-the-art thin film thermoelectric materials and devices for electronics cooling and addresses some principles and potential applications of thin-film TECs for on-chip hot spot cooling and isothermalization of high power electronics chips that are currently being developed.

Self-cooling capability in silicon, germanium, and silicon carbide substrates by utilization of the substrates themselves as thermoelectric elements and development of thermoelectric circuit within them, can offer great promise in removing local high heat flux hot spot on these chips. In this chapter, the principle, development, and application of self-cooling concept for thermal management of on-chip hot spot is addressed and discussed in details. The emphasis is placed on the numerical analysis of on-chip self-cooling for hot-spot thermal management in silicon, germanium and silicon carbide substrates.

A thermally-conductive mini-contact pad, sandwiched between a thermoelectric cooler and a microprocessor chip, can be utilized to concentrate the Peltier cooling power onto a small spot of the chip to achieve high heat flux localized cooling and enhance thermoelectric cooling performance. In this chapter the principle and proof-of-concept of mini-contact enhanced thermoelectric cooling for on-chip hot spot thermal management is discussed and various geometric and parasitic effects on hot spot cooling are addressed.

Pulsed thermoelectric cooling is interesting in thermal management because of its enhanced, strong transient heat pumping capacity compared to the steady-state maximum. This chapter describes the theories and practices of pulsed thermoelectric cooling with an attention focused on the influence of geometric parameters, operating conditions, and parametric effects on pulsed cooling performance and its potential application for hot spot thermal management. The presented transient analysis can also be extended to response of thermoelectric cooling during start-up and shut-down or for time-dependent thermal loads.

The following sections are included:

• Author Index

• Subject Index

• About the Authors

• About the Editor-in-Chief