Photoacoustic characterization and optimization of carbon nanotube array thermal interfaces

Baratunde A Cola, Purdue University

Abstract

Because of their high thermal conductivity and excellent mechanical properties, carbon nanotube (CNT) arrays have emerged as a promising class of materials to enhance thermal interface conductance. However, traditional measurement techniques suffer larger errors when used to characterize the thermal performance of such high conductance structures, and little is known about the effects of CNT array morphology on thermal performance. Additionally, integrating CNT arrays with technically relevant substrates and functional devices and modeling thermal performance remain as significant challenges. In this dissertation, a photoacoustic (PA) technique was developed to measure precisely the resistances of various CNT array thermal interfaces directly synthesized on substrates of technical interest such as Si, SiC, diamond, and Cu. The PA technique was used to measure the total thermal resistances of interfaces with CNT arrays grown on one and both sides of the interface. The two-sided interface achieved a very low resistance of 4 mm2˙K/W at moderate pressures. The total resistances of other interfaces that consist of CNT arrays synthesized on the surfaces of insertable metal foils were measured using the PA technique, and structures with arrays synthesized on both sides of Cu foil achieved values as low as 8 mm2˙K/W. The local CNT-substrate resistances and the intrinsic resistance of the CNT arrays in the one-sided, two-sided, and CNT/foil configurations were measured with the PA technique. The CNT-substrate resistances were observed to dominate the total resistances of each structure, and, for the one-side configuration, the resistances at the free CNT ends were measured to be an order of magnitude higher that the resistances between CNTs and their growth substrate. To reduce resistance at this thermal bottleneck, a wicking technique was developed to add consistent amounts of paraffin wax to the free ends of CNT arrays, and resistance values as low as 2 mm2˙K/W were achieved. A templated catalyst structure and microwave plasma chemical vapor deposition (MPCVD) was used to grow CNT arrays from 500 to 800°C with systematic control over defect density, CNT number density, and CNT diameter. Reduced CNT diameter and number density were revealed to produce higher thermal resistances because of less interface contact and reduced phonon transmission at small diameter CNT contacts, and increased defect density was revealed to produce lower thermal resistances because of enhanced array compliance. The performances of CNT/foil and wax/CNT/foil interface materials were tested for more than 1000 thermo-mechanical cycles in a burn-in process employed by the Intel Corporation. The thermal resistances of the CNT/foil interfaces exhibited excellent durability and resilience over the test range and for a large sample set. A model was developed to predict the thermal contact resistance of CNT array interfaces with CNT arrays synthesized directly on substrate surfaces. An analytical model for contact mechanics was first developed in conjunction with prior data from load-displacement experiments to predict the real contact area established in CNT array interfaces as a function of applied pressure. The contact mechanics model was integrated with a detailed thermal model that treats the multitude of individual CNT-substrate contacts as parallel resistors and considers the effects on phonon transport in the confined geometries that exist at such contacts. The influence of CNT array properties, e.g. diameter and density, are explicitly incorporated into the thermal model, which agrees well with experimental measurements of thermal resistances as a function of pressure for different types of interfaces. The model reveals that: (1) ballistic thermal resistance dominates at the CNT array interface; (2) the overall performance of CNT array interfaces is most strongly influenced by the thermal resistance at the contacts between free CNT ends and the opposing substrate surface (one-sided interface) or the opposing CNT array (two-sided interface); and (3) dense arrays with high mechanical compliance reduce the thermal contact resistance of CNT array interfaces by increasing the real contact area in the interface.

Degree

Ph.D.

Advisors

Xu, Purdue University.

Subject Area

Electrical engineering|Mechanical engineering|Materials science

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