Xinghua Zheng

Length-dependent thermal conductivity of an individual single-wall carbon nanotube

The thermal conductivity of single-wall carbon nanotubes (SWCNTs) is predicted to increase with length, but this has never been proved experimentally because of limitations in previous measurement methods. Here, the authors report the measurement of the length-dependent thermal conductivities of individual SWCNTs on a Si substrate using a four-pad 3 omega method. An increase in thermal conductivity with length was observed at room temperature, which is consistent with a theoretical prediction that considers higher order three-phonon processes. When SWCNTs are longer than the phonon mean path, they showed dissipative thermal transport. The observed increase of thermal conductivity with length makes SWCNTs ideal for thermal management.(c) 2007 American Institute of Physics.

The freestanding sensor-based 3ω technique for measuring thermal conductivity of solids: Principle and examination

In recent two decades, the 3ω technique has been proven to be valuable for characterizing thermophysical properties of materials from nanoscale to bulk, but some inherent deficiencies in this technique such as laborious and repeated four-pad micro strip heater/sensor deposition process and flimsiness of the micro heater/sensor limit its practical applications. Here, the authors report a novel 3ω technique, based on a freestanding sensor replacing the conventional 3ω heater/sensor adjacent to the specimen surface. A zigzag temperature response curve of the new sensor instead of the classical straight line was observed and used to extract the specimen thermal conductivity. Experimental results which excellently agree with calculation values show that the new technique is of great application value to thermal properties characterization of amorphous bulks and hundreds of microns thick wafers.

Remarkably enhanced thermal transport based on a flexible horizontally-aligned carbon nanotube array film.

It has been more than a decade since the thermal conductivity of vertically aligned carbon nanotube (VACNT) arrays was reported possible to exceed that of the best thermal greases or phase change materials by an order of magnitude. Despite tremendous prospects as a thermal interface material (TIM), results were discouraging for practical applications. The primary reason is the large thermal contact resistance between the CNT tips and the heat sink. Here we report a simultaneous sevenfold increase in in-plane thermal conductivity and a fourfold reduction in the thermal contact resistance at the flexible CNT-SiO2 coated heat sink interface by coupling the CNTs with orderly physical overlapping along the horizontal direction through an engineering approach (shear pressing). The removal of empty space rapidly increases the density of transport channels, and the replacement of the fine CNT tips with their cylindrical surface insures intimate contact at CNT-SiO2 interface. Our results suggest horizontally aligned CNT arrays exhibit remarkably enhanced in-plane thermal conductivity and reduced out-of-plane thermal conductivity and thermal contact resistance. This novel structure makes CNT film promising for applications in chip-level heat dissipation. Besides TIM, it also provides for a solution to anisotropic heat spreader which is significant for eliminating hot spots.

Robust and superhydrophobic thiourethane bridged polysilsesquioxane aerogels as potential thermal insulation materials

Aerogels with low density and high porosity exhibit outstanding performances as thermal or acoustic insulators. However, most aerogels are mechanically brittle and need further chemical or physical surface treatments to obtain elasticity and superhydrophobicity. Structurally new types of polysilsesquioxane (PSQ) aerogels containing thiourethane as their bridging groups are now reported. These novel PSQ aerogels were prepared from thiourethane bridged silsesquioxane through a simple sol–gel process and vacuum-drying method. The PSQ aerogels exhibit inherent superhydrophobicity, excellent mechanical toughness and good thermal insulation properties. Moreover, the morphology and mechanical performance of these aerogels can be tailored by changing the rigidity of the bridging group.

Note: Non-destructive measurement of thermal effusivity of a solid and liquid using a freestanding serpentine sensor-based 3ω technique

A non-destructive thermal effusivity characterization method described as a freestanding serpentine sensor-based 3ω technique was reported. This freestanding serpentine sensor was fabricated by the mature flexible printed circuit production technique. Expression for the temperature response of the freestanding serpentine sensor with respect to the thermal effusivity of the test sample was presented. The technique was further verified by measuring four kinds of standard samples at room temperature. Experimental results which well agree with reference values demonstrate the new technique is of great application value to thermal effusivity characterization of solids, liquids, and structures to which the conventional 3ω technique is not applicable, e.g., solids with porous surfaces.

Study on heat-storage and release characteristics of multi-cavity-structured phase-change microcapsules

Based on the solutions of apparent heat capacity method for phase-change problems, numerical simulations were performed to study the heat-storage and release processes for the multi-cavity-structured phase-change microcapsules in this paper. Moreover, the influence of the cavity structures of the phase-change microcapsules on the heat-storage and release capacity was analyzed. The results show that the rate of heat-storage and release will be accelerated by increasing the number of cavities in microcapsules, and cavity interlayer is the key factor to enhance the heat transfer.

Study on the thermal resistance in secondary particles chain of silica aerogel by molecular dynamics simulation

In this article, molecular dynamics simulation was performed to study the heat transport in secondary particles chain of silica aerogel. The two adjacent particles as the basic heat transport unit were modelled to characterize the heat transfer through the calculation of thermal resistance and vibrational density of states (VDOS). The total thermal resistance of two contact particles was predicted by non-equilibrium molecular dynamics simulations (NEMD). The defects were formed by deleting atoms in the system randomly first and performing heating and quenching process afterwards to achieve the DLCA (diffusive limited cluster-cluster aggregation) process. This kind of treatment showed a very reasonable prediction of thermal conductivity for the silica aerogels compared with the experimental values. The heat transport was great suppressed as the contact length increased or defect concentration increased. The constrain effect of heat transport was much significant when contact length fraction was in the smal...

Stethoscope-type 3ω independent detector for fast measurement of material thermal conductivity

As one of the most basic properties of materials, thermal conductivity is a key parameter to investigate the analysis and design of the particular thermal process, such as the analysis of the thermal/cool energy storage and release process, the thermal/cool energy storage system design, and so on. Also, it is the foundation of energy technology development. In order to overcome the problem that the measurement frequency of a polyimide substrate independent detector is lower and the measurement time is longer, based on the relationship among 3ω detection frequency, thermal wave penetration depth, and thermal properties of the substrate material, this paper developed a stethoscope-type 3ω independent detector based on a sapphire substrate for fast measurement of material thermal conductivity. Nickel, with a high resistance temperature coefficient, was used as the metal detector. The influence of the width of the nickel metal detector and the thickness of the sapphire substrate on the measurement accuracy was analyzed by introducing an effective heat flow ratio. The sapphire substrate independent detector was calibrated by using copper, aluminum, 304 stainless steel, quartz glass, ethylene glycol, and plexiglass with known thermal conductivities. The experiment shows that the measurement frequency of the stethoscope detector can be ten times higher than that of a Kapton film independent detector. Its measurement time is only one tenth of that of the Kapton film independent detector.

3ω slope comparative method for fluid and powder thermal conductivity measurements

By analyzing the relationship among the heat penetration depth, measurement frequency and detector characteristic parameters, a simple and practical 3ω slope comparative method has been proposed. The corresponding measurement system for measuring the thermal properties of fluids and powder materials was established and verified using several specimens with known thermophysical parameters, such as alcohol, distilled water, and air. Compared to the two-dimensional model, the data processing of the method is relatively simple and quick. Due to the elimination of errors introduced by the detector parameter measurement, the measurement accuracy of the method is higher than the conventional one-dimensional model. By using an appropriate frequency range, the new method is time saving and convenient for measuring the thermal conductivity of fluids and powders with low thermal conductivity. Based on the analysis, the effective thermal conductivity of nano-SiO2 powder is accurately determined.

The Effective Thermal Conductivity of Porous Polymethacrylimide Foams

A freestanding sensor-based 3ω method was employed to measure the effective thermal conductivity of porous polymethacrylimide (PMI) foams with different densities at different temperatures. Experimental data showed that within the measuring temperature range, the effective thermal conductivity increased with temperature. Moreover, the formation mechanism of the relationship between the effective thermal conductivity and temperature was analyzed in this paper.