The subject of thermal transport at the mesoscopic scale and in low-dimensional systems is interesting for both fundamental research and practical applications. As the first example of truly two-dimensional materials, graphene has exceptionally high thermal conductivity, and thus provides an ideal platform for the research. Here we review recent studies on thermal and thermoelectric properties of graphene, with an emphasis on experimental progresses. A general physical picture based on the Landauer transport formalism is introduced to understand underlying mechanisms. We show that the superior thermal conductivity of graphene is contributed not only by large ballistic thermal conductance but also by very long phonon mean free path (MFP). The long phonon MFP, explained by the low-dimensional nature and high sample purity of graphene, results in important isotope effects and size effects on thermal conduction. In terms of various scattering mechanisms in graphene, several approaches are suggested to control thermal conductivity. Among them, introducing rough boundaries and weakly-coupled interfaces are promising ways to suppress thermal conduction effectively. We also discuss the Seebeck effect of graphene. Graphene itself might not be a good thermoelectric material. However, the concepts developed by graphene research might be applied to improve thermoelectric performance of other materials.

Thermal and Thermoelectric Properties of Graphene

This paper deals with a review of the main research aspects concerning the formulation of the parameters indicating the characteristics and performance of thermoelectric cooling devices, with particular reference to a number of recent publications. The specific aspects addressed include some practical considerations referring to the thermoelectric figure of merit, the characterization of the cooling capacity, and the assessment of the coefficient of performance ( COP ). The contribution of this paper starts by categorizing the topics addressed by recent review papers, showing that these reviews generally had a wide focus and provided little specific details on thermoelectric cooling parameters and performance. Then, the dimensionless thermoelectric figure of merit is addressed by focusing on its conventional and modified definitions and indicating the values obtained for different thermoelectric cooling materials. Furthermore, the expressions of the cooling capacity for single-stage and multi-stage thermoelectric coolers are reviewed. Concerning the COP , its dedicated expressions are constructed starting from the classical formulation and introducing additional factors or modifications in order to take into account the Thomson effect, the dependence on temperature of the thermoelectric materials, and the effects of the electrical contact resistance, thermal resistance, thermoelement length and current. Finally, on the basis of the indications taken from the literature, further considerations are included on the COP values found in thermoelectric cooling applications, as well as on how to obtain COP improvements.

A review on thermoelectric cooling parameters and performance

Thermal and thermoelectric properties of graphene.

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Electrons And Phonons The Theory Of Transport Phenomena In Solids

Carbon nano tube (CNT)/graphene filled organic composites have great potential for making cheaper thermoelectric materials towards their gear up for applications in energy harvesting due to their low cost, low density, facile routes of preparations, versatile processability and low thermal conductivity. These properties make them superior compared to previously reported hybrid alloys. Now a day׳s CNT and graphene are most frequently used nanofillers due to their unique shape and characteristics such as superconductivity, light weight, high stiffness and axial strength. Moreover, to achieve superior dispersion and properties of composites, various functionalizations on the graphene and CNT have been accomplished by various research groups. Foremost objective of this paper is to highlight the recent research advances on CNT and graphene filled thermoelectric (TE) materials for the replacement of inorganic semiconductors. Fundamentally, for polymer composite based thermoplastic (TP) materials, a thermally nonconductive but electrically connected nano-structured network should be established.

Recent advances in CNT/graphene based thermoelectric polymer nanocomposite: A proficient move towards waste energy harvesting

The efficient usage of thermoelectric (TE) devices for microelectronics cooling application requires investigation and remedy of various obstacles such as integration of these devices with electronic package, parasitic contact resistances, and utilization of appropriate current pulses. We develop a computational model to investigate the effect of steady state and transient mode of operation of ultrathin thermoelectric cooler (TEC) devices on hot-spot cooling considering the effect of crucial thermal and electrical contact resistances. Our analysis shows that the transient pulses can be very effective in reducing the hot-spot temperature by 6-7°C in addition to the cooling achieved by the steady state current through the TEC device. We correlate the important characteristics of the transient temperature behavior of hot-spot under the TEC operation such as the maximum temperature drop (ΔTmax), time taken to achieve ΔTmax and temperature overshoot after turning off pulse current with the electrical and thermal contact resistances and Seebeck coefficient of the TE material. It has been observed that thermal and electrical contact resistances play a very crucial role in the performance of TEC devices as high values of these resistances can significantly diminish the effect of Peltier cooling during steady state operation. The effect of these parasitic resistances is even higher for the transient cooling of hot-spots by the pulsed current through the TEC device. High Seebeck coefficient of TE materials is desirable as it increases the figure of merit of TE devices. However, cooling capabilities of heat sink may become bottleneck to realize the benefits of very high Seebeck coefficient as the back heat flow from the hot side to cold side of TEC device diminishes the degree of cooling achieved by these ultrathin TECs.

Ultrathin Thermoelectric Devices for On-Chip Peltier Cooling

Site-specific on-demand cooling of hot spots in microprocessors can reduce peak temperature and achieve a more uniform thermal profile on chip, thereby improve chip performance and increase the processor’s life time An array of thermoelectric coolers (TECs) integrated inside an electronic package has the potential to provide such efficient cooling of hot spots on chip This paper analyzes the potential of using multiple TECs for hot spot cooling to obtain favorable thermal profile on chip in an energy efficient way Our computational analysis of an electronic package with multiple TECs shows a strong conductive coupling among active TECs during steady-state operation Transient operation of TECs is capable of driving cold-side temperatures below steady-state values Our analysis on TEC arrays using current pulses shows that the effect of TEC coupling on transient cooling is weak Various pulse profiles have been studied to illustrate the effect of shape of current pulse on the operation of TECs considering crucial parameters such as total energy consumed in TECs peak temperature on the chip, temperature overshoot at the hot spot and settling time during pulsed cooling of hot spots The square root pulse profile is found to be the most effective with maximum cooling and at half the energy expenditure in comparison to a constant current pulse We analyze the operation of multiple TECs for cooling spatiotemporally varying hot spots The analysis shows that the transient cooling using high amplitude current pulses is beneficial for short term infrequent hot spots, but high amplitude current pulse cannot be used for very frequent or long lasting hot spots

Array of Thermoelectric Coolers for On-Chip Thermal Management

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Array of Thermoelectric Coolers for On-Chip Thermal

The high field properties of carbon nanotube (CNT) network thin film transistors (CN-TFTs) are important for their practical operation, and for understanding their reliability. Using a combination of experimental and computational techniques we show how the channel geometry (length LC and width WC) and network morphology (average CNT length Lt and alignment angle distribution ) affect heat dissipation and high field breakdown in such devices. The results suggest that when WC Lt, the breakdown voltage remains independent of WC but varies linearly with LC. The breakdown power varies almost linearly with both WC and LC when WC Lt. We also find that the breakdown power is more susceptible to the variability in the network morphology compared to the breakdown voltage. The analysis offers new insight into the tunable heat dissipation and thermal reliability of CN-TFTs, which can be significantly improved through optimization of the network morphology and device geometry.

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High field breakdown characteristics of carbon nanotube thin film transistors

estimate the average CNT-SiO2 TBC as g � 0.16Wm � 1 K � 1 and the TBC at CNT junctions as GC � 2.4 pWK � 1 . We find the peak power dissipation in CN-TFTs is more strongly correlated to the TBC of the CNT-substrate interface than to the TBC at CNT junctions. Molecular dynamics simulations of crossed CNT junctions also reveal that the top CNT is buckled over � 30nm lengths, losing direct contact with the substrate and creating highly localized hot-spots. Our results provide new insights into CNT network properties which can be engineered to enhance performance of CN-TFTs for macro and flexible electronics applications. V C 2012 American

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Man Prakash Gupta

Papers

Ultrathin Thermoelectric Devices for On-Chip Peltier Cooling

Array of Thermoelectric Coolers for On-Chip Thermal Management

Array of Thermoelectric Coolers for On-Chip Thermal

High field breakdown characteristics of carbon nanotube thin film transistors

Impact of thermal boundary conductances on power dissipation and electrical breakdown of carbon nanotube network transistors