Tao Tong

Dense Vertically Aligned Multiwalled Carbon Nanotube Arrays as Thermal Interface Materials

Carbon nanotube (CNT) arrays are being considered as thermal interface materials (TIMs). Using a phase sensitive transient thermo-reflectance technique, we measure the thermal conductance of the two interfaces on each side of a vertically aligned CNT array as well as the CNT array itself. We show that the physically bonded interface by van der Waals adhesion has a conductance ~105W/m2K and is the dominant resistance. We also demonstrate that by bonding the free-end CNT tips to a target surface with the help of a thin layer of indium weld, the conductance can be increased to ~106W/m2K making it attractive as a TIM

Interfacial energy and strength of multiwalled-carbon-nanotube-based dry adhesive

Vertically aligned multiwalled carbon nanotube (MWCNT) arrays can mimic the hairs on a gecko’s foot and act as a dry adhesive. We demonstrate the van der Waals interactions originated dry adhesion between MWCNT array surfaces and various target surfaces over millimeter-sized contact areas. The adhesive strengths were measured over 10N∕cm2 in the normal direction and about 8N∕cm2 in the shear direction with glass surface. The adhesion strength over repeated cycles is limited by the relatively poor adhesion of MWCNTs to their growth substrate, which was improved significantly by adding molybdenum to the catalyst underlayer. We also measured the interfacial work of adhesion as a fundamental adhesion property at the interface. Our measured values of a few tens of mJ∕m2, which falls in the range of typical van der Waals interactions energies, provide a direct proof of the van der Waals dry adhesion mechanism. Furthermore, in contrast to other dry adhesives, we show that MWCNT adhesives are electrically and the...

Reexamining the 3-omega technique for thin film thermal characterization

The 3-omega method is widely used to measure thermal properties of thin films and interfaces. Generally, one-dimensional heat conduction across the film is assumed and the film capacitance is neglected. The change in the in-phase (real part) temperature response for the film-on-substrate case relative to the substrate-only case is, therefore, attributed to the sum of the bulk thermal resistance of the film and the thermal boundary resistance between the film and the substrate. Based on a rigorous and intuitive mathematical derivation, it is shown that this approach represents a limiting case, and that its use can cause significant errors in rather realistic situations when the underlying assumptions are not met. This article quantifies the error by introducing a new parameter called the ratio function R, which modifies the film thermal resistance and mathematically shows that it depends only on three dimensionless parameters that combine thermal properties and geometries of the film and the heated linewid...

Optical Measurement of Thermal Conductivity Using Fiber Aligned Frequency Domain Thermoreflectance

Fiber aligned frequency domain thermoreflectance (FAFDTR) is a simple noncontact optical technique for accurately measuring the thermal conductivity of thin films and bulk samples for a wide range of materials, including electrically conducting samples. FAFDTR is a single-sided measurement that requires minimal sample preparation and no microfabrication. Like existing thermoreflectance techniques, a modulated pump laser heats the sample surface, and a probe laser monitors the resultant thermal wave via the temperature dependent reflectance of the surface. Via the use of inexpensive fiber coupled diode lasers and common mode rejection, FAFDTR addresses three challenges of existing optical methods: complexity in setup, uncertainty in pump-probe alignment, and noise in the probe laser. FAFDTR was validated for thermal conductivities spanning three orders of magnitude (0.1-100 W/m K), and thin film thermal conductances greater than 10 W/m(2) K. Uncertainties of 10-15% were typical, and were dominated by uncertainties in the laser spot size. A parametric study of sensitivity for thin film samples shows that high thermal conductivity contrast between film and substrate is essential for making accurate measurements. DOI: 10.1115/1.4003545

An acoustic and dimensional mismatch model for thermal boundary conductance between a vertical mesoscopic nanowire/nanotube and a bulk substrate

A theoretical model to calculate the thermal boundary conductance (Kapitza conductance) or, alternatively, thermal boundary resistance (Kapitza resistance) between a vertically grown mesoscopic nanowire/nanotube and a bulk substrate is presented. The thermal boundary resistance at the interface between the mesoscopic geometry and a three-dimensional substrate is primarily due to two reasons: (1) dimensional mismatch in the phonon density of states and (2) mismatch in the acoustic properties. Our model based on the solution of the elastic wave equation in the substrate and the mesoscopic geometry incorporates both these effects.

Diffraction-limited phonon thermal conductance of nanoconstrictions

Thermal transport across nanosized constrictions is calculated considering wave effects. It is shown that Rayleigh-type phonon diffraction reduces thermal transport across nanosized constrictions at low temperatures. We show that for aT∕vDebye<0.01×10−9Ks, where a is the radius of the constriction, T the temperature, and vDebye the Debye velocity of the solid material, diffraction effects are important.

Indium Assisted Multiwalled Carbon Nanotube Array Thermal Interface Materials

The state-of-the-art thermal interface materials (TIMs) for electronic cooling is briefly reviewed with an emphasis on the emerging trend of employing carbon nanotubes (CNTs) for enhanced thermal conduction across interfaces. Previous studies on CNT TIMs, however, indicated that despite the high thermal conductivities of CNTs themselves, heat conduction is limited by the direct contact interface between CNTs and the target surface. This has greatly limited the practical application of CNT TIMs. In this study, we thermally welded a thin indium layer between the CNT surface and the target surface, and observed an order of magnitude increase in thermal conductance ~106 W/m2 middot K across the interface compared with direct contact interface. With vacuum and cryogenic temperature compatibility, indium assisted CNT TIMs may find their future applications in areas from electronic chip cooling, cryogenic pumps, to thermal management in spacecrafts

Multiwalled Carbon Nanotube/nanofiber Arrays as Conductive and Dry Adhesive Interface Materials

We demonstrate the possibility of making conductive and dry adhesive interfaces between multiwalled carbon nano-tube (MWNT) and nanofiber (MWNF) arrays grown by chemical vapor deposition with transition-metal as catalyst on silicon substrates. The maximum observed adhesion force between MWNT and MWNF surfaces is 3.5 mN for an apparent contact area of 2 mm by 4 mm. The minimum contact resistance measured at the same time is ∼20 Ω. Contact resistances of MWNT-MWNT and MWNT-gold interfaces were also measured as pressure forces around several milli-Newton were applied at the interface. The resulting minimum contact resistances are on the same order but with considerable variation from sample to sample. For MWNT-MWNT contacts, a minimum contact resistance of ∼ 1 Ω is observed for a contact area of 2 mm by 1 mm. The relatively high contact resistances, considering the area density of the nanotubes, might be explained by the high cross-tube resistances at the contact interfaces and limited inter-penetration of the nanotube arrays.Copyright

Vertically Aligned Multi-Walled Carbon Nanotube Arrays as Thermal Interface Materials and Measurement Technique

State-of-the-art thermal interface materials are briefly reviewed with an emphasis on the emerging trend of using carbon nanotubes to increase interface thermal performance. Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were grown and applied as thermal interfacial enhancing materials. It is expected that the highly thermally conductive channels directly bridging the mating surfaces would significantly enhance the interface thermal conductance. We extended the all-optical pump and probe phase sensitive transient thermo-reflectance (PSTTR) method and used it to measure the interfacial properties of a three-layer sample of a vertically aligned MWCNT array grown on silicon (Si) substrate dry adhered to a glass plate. The dominant thermal resistance is identified as the dry adhered MWCNT-glass interface with a thermal conductance of ~5.9 × 104 W/m2 ·K, compared with MWCNT-Si interface of almost two orders of magnitude higher. Tentative explanations on the difference in the two interfaces and ways for future improvements are provided. The PSTTR measurement principle and issues are also discussed in the context.Copyright

Micro-thermal-fluid transient analysis and active control for two-phase microelectronics cooling

Because of increasing power densities, microchannel systems are being explored for two-phase cooling of ultra high power electronic devices. Flow instability is a potential problem in any two-phase microchannel cooling system especially for transient applications. With various two-phase flow stabilities possible in a microscale boiling system, the overall cooling performance deteriorates significantly. For better dynamic thermal management of microelectronic systems, a family of oscillatory flow boiling heat transfer correlations and active stabilizing flow control methods have been developed.