Thermal Conductivity of Polymer-Based Composites: Fundamentals and Applications

Thermophysical Properties Research Literature Retrieval Guide Edited by Y. S. Touloukian, J. K. Gerritsen and N. Y. Moore Second edition, revised and expanded. Book 1: Pp. xxi + 819. Book 2: Pp.621. Book 3: Pp. ix + 1315. (New York: Plenum Press, 1967.) n.p.

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Heat and Mass Transfer

The dependence of the strength of the electron-phonon coupling and the electron heat capacity on the electron temperature is investigated for eight representative metals, Al, Cu, Ag, Au, Ni, Pt, W, and Ti, for the conditions of strong electron-phonon nonequilibrium. These conditions are characteristic of metal targets subjected to energetic ion bombardment or short-pulse laser irradiation. Computational analysis based on first-principles electronic structure calculations of the electron density of states predicts large deviations (up to an order of magnitude) from the commonly used approximations of linear temperature dependence of the electron heat capacity and a constant electron-phonon coupling. These thermophysical properties are found to be very sensitive to details of the electronic structure of the material. The strength of the electron-phonon coupling can either increase (Al, Au, Ag, Cu, and W), decrease (Ni and Pt), or exhibit nonmonotonic changes (Ti) with increasing electron temperature. The electron heat capacity can exhibit either positive (Au, Ag, Cu, and W) or negative (Ni and Pt) deviations from the linear temperature dependence. The large variations of the thermophysical properties, revealed in this work for the range of electron temperatures typically realized in femtosecond laser material processing applications, have important implications for quantitative computational analysis of ultrafast processes associated with laser interaction with metals.

Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium

Advanced models of fuel droplet heating and evaporation

Low efficiencies and costly electrode materials have limited harvesting of thermal energy as electrical energy using thermo- electrochemical cells (or "thermocells"). We demonstrate thermocells, in practical configurations (from coin cells to cells that can be wrapped around exhaust pipes), that harvest low-grade thermal energy using relatively inexpensive carbon multiwalled nanotube (MWNT) electrodes. These electrodes provide high electrochemically accessible surface areas and fast redox-mediated electron transfer, which significantly enhances thermocell current generation capacity and overall efficiency. Thermocell efficiency is further improved by directly synthesizing MWNTs as vertical forests that reduce electrical and thermal resistance at electrode/substrate junctions. The efficiency of thermocells with MWNT electrodes is shown to be as high as 1.4% of Carnot efficiency, which is 3-fold higher than for previously demonstrated thermocells. With the cost of MWNTs decreasing, MWNT-based thermocells may become commercially viable for harvesting low-grade thermal energy.

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Harvesting Waste Thermal Energy Using a Carbon-Nanotube-Based Thermo-Electrochemical Cell

This work describes an experimental study of thermal conductance across multiwalled carbon nanotube (CNT) array interfaces, one sided (Si-CNT-Ag) and two sided (Si-CNT-CNT-Cu), using a photoacoustic technique (PA). Well-anchored, dense, and vertically oriented multiwalled CNT arrays have been directly synthesized on Si wafers and pure Cu sheets using plasma-enhanced chemical vapor deposition. With the PA technique, the small interface resistances of the highly conductive CNT interfaces can be measured with accuracy and precision. In addition, the PA technique can resolve the one-sided CNT interface component resistances (Si-CNT and CNT-Ag) and the two-sided CNT interface component resistances (Si-CNT, CNT-CNT, and CNT-Cu) and can estimate the thermal diffusivity of the CNT layers. The thermal contact resistances of the one- and two-sided CNT interfaces measured using the PA technique are 15.8±0.9 and 4.0±0.4mm2K∕W, respectively, at moderate pressure. These results compare favorably with those obtained using a steady state, one-dimensional reference bar method; however, the uncertainty range is much narrower. The one-sided CNT thermal interface resistance is dominated by the resistance between the free CNT array tips and their opposing substrate (CNT-Ag), which is measured to be 14.0±0.9mm2K∕W. The two-sided CNT thermal interface resistance is dominated by the resistance between the free tips of the mating CNT arrays (CNT-CNT), which is estimated to be 2.1±0.4mm2K∕W.

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Photoacoustic characterization of carbon nanotube array thermal interfaces

The mechanisms of decomposition of a metal (nickel) during femtosecond laser ablation are studied using molecular dynamics simulations. It is found that phase explosion is responsible for gas bubble generation and the subsequent material removal at lower laser fluences. The phase explosion process occurs as combined results of heating, thermal expansion, and the propagation of tensile stress wave induced by the laser pulse. When the laser fluence is higher, it is revealed that critical point phase separation plays an important role in material removal.

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Mechanisms of decomposition of metal during femtosecond laser ablation

In this work, Molecular Dynamics (MD) simulation is employed to investigate femtosecond laser ablation of copper, with an emphasis on the understanding of the mechanism of phase change during laser ablation. Laser induced heat transfer, melting, surface evaporation, and material ablation are studied. Theoretically, it has been suggested that under intense femtosecond laser irradiation, the material undergoes a volumetric phase change process; its maximum temperature can be close to or even above the thermodynamic critical point. The MD simulations allow us to determine the transient temperature history of the irradiated material and to reveal the exact phase change process, which explains the mechanisms of femtosecond laser ablation. A finite difference calculation is also performed, which is used to compare results of heating and melting prior to a significant amount of material being ablated.

Molecular dynamics study of phase change mechanisms during femtosecond laser ablation

The critical point of nickel and the phase diagram near the critical point are numerically evaluated using molecular dynamics (MD) computations. Thermodynamic states on the phase diagram are calculated for a homogeneous material at equilibrium states. Isothermal lines on p–v diagrams are constructed at temperatures below and above the critical temperature, and the liquid-gas coexistence lines and regimes are obtained. The critical point of nickel is obtained as T
 c
 =  9460± 20 K, ρ
 c
 =  2560± 100 kg· m−3, and p
 c
 =  1.08± 0.01 GPa. The method used in this work can be used to estimate thermodynamic properties of other materials at high temperature/pressure.

https://engineering.purdue.edu/NanoLab/PDF/2007%20Molecular%20Dynamics%20Calculation%20of%20Critical%20Point%20of%20Nickel.pdf

Molecular dynamics calculation of critical point of nickel

This work describes an experimental study of the cross-plane thermal conductance of plasma-enhanced chemical vapor deposited (PECVD) diamond films grown as a result of bias-enhanced nucleation (BEN). The diamond films are grown on silicon wafers using a two-step process in which a nucleation layer of amorphous or diamond like (DLC) carbon is first deposited on the silicon under the influence of a voltage bias. Then, conditions are adjusted to allow for polycrystalline diamond (PD) growth. The nucleation layer is essential for seeding diamond growth on smooth substrates and for optimizing PD properties such as grain size, orientation, transparency, adhesion, and roughness. A photoacoustic (PA) technique is employed to measure the thermal conductivities of and the thermal interface resistances between the layers in the diamond film structure. The influence of nucleation layers that are 70, 240, 400, and 650 nm thick on the thermal conductance of the diamond film structure is characterized. The thermal conductivity of the nucleation layer exhibits a thickness dependence for relatively thin layers. For each sample, the thermal conductivity of the PD is higher than 500 Wldrm-1K-1 (measurement sensitivity limit). A resistive network for the diamond film structure is developed. The resistance at the silicon/nucleation interface is less than 10-9m2ldrKldrW-1 (measurement sensitivity limit), which is of the order of theoretical predictions. The minimum diamond film structure resistance occurs when the nucleation layer is thinnest. When the nucleation layer is sufficiently thick, it begins to exhibit bulk behavior, and the resistance at the nucleation/PD interface dominates the thermal resistance of the diamond film structure.

/pdf/influence-of-bias-enhanced-nucleation-on-thermal-conductance-etkc106svc.pdf

Changrui Cheng

Papers

Photoacoustic characterization of carbon nanotube array thermal interfaces

Mechanisms of decomposition of metal during femtosecond laser ablation

Molecular dynamics study of phase change mechanisms during femtosecond laser ablation

Molecular dynamics calculation of critical point of nickel

Influence of Bias-Enhanced Nucleation on Thermal Conductance Through Chemical Vapor Deposited Diamond Films