Andrew J. McNamara

High-Quality Vertically Aligned Carbon Nanotubes for Applications as Thermal Interface Materials

Vertically aligned carbon nanotube (VACNT) array is an ideal form for heat dissipation in electronic packaging, due to its high-intrinsic thermal conductivity and robust mechanical properties. In this paper, we report the growth of high-quality VACNTs for the applications as thermal interface materials (TIMs). The high-quality VACNTs were grown and confirmed by the characterizations of Raman and thermogravimetric analyses. Metalized VACNT array was transferred and bonded to a metalized silicon or copper substrate. The VACNT-based TIM structure (Si-Ti/Ni/Au-In-Ti/Ni/Au-VACNT-Ti/Ni/Au-In-Ti/Ni/Au-Cu) was then successfully made after bonding to another substrate (copper or silicon). The total boundary resistance between the VACNT array and the surrounding materials was measured by an infrared thermal imaging method. Compared with the TIM sample made from carbon nanotubes grown in our laboratory chemical vapor deposition (CVD), the thermal boundary resistance of the TIM sample made from CNTs in the black magic CVD was greatly reduced from 11.6±0.5 to 3.4±0.1 mm2 KW-1. Overall, these high quality, and bonded VACNT arrays demonstrate properties promising for next-generation TIM applications.

Infrared Imaging Microscope as an Effective Tool for Measuring Thermal Resistance of Emerging Interface Materials

The reduction of interfacial resistance continues to be a significant challenge in thermal management of semiconductor and other microscale devices. Current state-of-the-art thermal interface materials (TIMs) have resistances in the range of 5–10 mm2 ·K/W. At these values, particularly for the emerging highly nonhomogeneous materials, standard measurement techniques often fail to provide accurate results. This paper describes the use of infrared microscopy for measuring the total thermal resistance across multiple interfaces. The method is capable of measuring samples of wide ranging resistances with thicknesses ranging from 50–250 μm. This steady-state technique has several advantages over other methods, including the elimination of the need for intrusive temperature monitoring devices like thermocouples at the area of interest and the need for a priori knowledge of the specific heat and density of the materials of interest, as in the transient techniques for determining thermal resistances. Results for three different commercially available TIM and uncertainty analysis are presented.Copyright

Two-phase flow and heat transfer in pin-fin enhanced micro-gaps

Thermal management of integrated circuits (IC) has emerged as one of the key challenges for continued performance enhancement of modern microprocessors. Cooling schemes utilizing two-phase, microfluidic technologies are some of the more promising modular thermal management solutions for next generation devices. In this study, the flow and heat transfer in pin-fin enhanced micro-gaps are experimentally investigated. It has been known that pin-fin structures inside micro-gaps can increase convective heat transfer coefficients in single phase flow conditions. However, two-phase microfluidic cooling is becoming an increasingly popular method in thermal control of electronics, and this cooling strategy has not been well characterized for pin-fin enhanced micro-gaps. Pin-fin, micro-gap structures studied had a pin diameter, height and pitch of 150μm, 200μm and 225μm, respectively, providing an aspect ratio of 1.33. Both the overall micro-gap width and length are 1cm. The working fluid used was R245fa. The structure contained a transparent cover which allowed for visualization of flow through the micro-gap. A high speed camera allowed for image capture and characterization of various two-phase flow regimes. The thermal performances of the heat sink were experimentally evaluated using pressure drop and temperature measurements.

Single/few-layer boron nitride-based nanocomposites for high thermal conductivity underfills

Thermal management in 3D packaging plays an important role in the device performance and reliability. The development of thermally conductive underfills is highly crucial, but still challenging. In this work, single/few-layer boron nitride (BN) was exfoliated from bulk h-BN flakes and was incorporated into epoxy resin via a solvent transfer method. [1] The structure of exfoliated BN was characterized by varieties of techniques, including scanning electron microscopy, transmission electron microscopy, electron diffraction, Raman microscopy, and UV-vis microscopy. The single/few layer boron nitride/epoxy composite was characterized by thermomechanical analysis and thermogravimetric analysis. The thermal conductivity of exfoliated BN was measured by an infrared thermal imaging method. A significant enhancement of thermal conductivity (220%) is observed at a low filler loading of 5 wt%, indicating that the single/few-layer BN is a promising filler for the development of novel underfill for 3D packaging.

Quantum Size Effect on the Lattice Specific Heat of Nanostructures

For structures consisting of a few atomic layers (in one, two, or three dimensions), discretized modes of lattice vibration must be considered and their contributions need to be evaluated by summation (in the confined directions) rather than integration over the wavevector components as done for bulk materials in the Debye theory. Existing studies on the specific heat of nanostructures have left a question as to whether the lattice specific heat of nanostructures will be suppressed or enhanced over the corresponding bulk value at cryogenic temperatures. This study presents explicit analytical formulations of the lattice specific heat of nanostructures under the assumption of linear dispersion. The specific heat at sufficiently low temperatures is dominated by the lowest-energy vibration modes. Subsequently, the planar modes (for which one of the wavevector components becomes zero) or axial modes (for which two of the wavevector components become zero) are responsible to the T2 or T dependence of specific heat for thin films or nanowires, respectively, resulting in a specific-heat enhancement over the bulk materials (whose specific heat depends on T3 at low temperatures). As the temperature is reduced, however, the specific heat decays faster for cubic nanoparticles than for bulk materials, because the contribution of the lowest vibration modes in nanoparticles is proportional to (1/T2) exp(−1/T) at extremely low temperatures. The effects of the aspect ratio and shape on the specific heat of nanoparticles as well as boundary conditions are also discussed. This work clarifies the trends in the lattice specific heat of nanostructures at low temperatures.