Bulk nanostructured materials from severe plastic deformation

Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

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Nanoscale thermal transport

Nanostructured materials: basic concepts and microstructure☆

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

Thermal Conductivity of Polymer-Based Composites: Fundamentals and Applications

A methodology is introduced for predicting the effective thermal conductivity of arbitrary particulate composites with interfacial thermal resistance in terms of an effective medium approach combined with the essential concept of Kapitza thermal contact resistance. Results of the present model are compared to existing models and available experimental results. The proposed approach rediscovers the existing theoretical results for simple limiting cases. The comparisons between the predicted and experimental results of particulate diamond reinforced ZnS matrix and cordierite matrix composites and the particulate SiC reinforced Al matrix composite show good agreement. Numerical calculations of these different sets of composites show very interesting predictions concerning the effects of the particle shape and size and the interfacial thermal resistance.

Effective thermal conductivity of particulate composites with interfacial thermal resistance

It is well known that the Fourier analysis of X-ray diffraction peak profiles (as implemented by Warren and Averbach) can accurately determine the areaweighted average grain size of a fine-grained sample. Less well known is the fact that this method simultaneously yields a volume-weighted average grain size. Under certain circumstances, knowledge of these two weighted average grain sizes is sufficient to permit reliable estimation of the grain-size distribution, even when the distribution cannot be calculated directly from the Fourier coefficients, as is usually the case. We demonstrate this for a nanocrystalline Pd sample prepared by inert-gas condensation; average grain sizes and the grain-size distribution are estimated by X-ray diffraction profile analysis and compared with the same quantities measured directly by transmission electron microscopy (TEM). Very good agreement between the resulting average grain sizes is achieved only after compensating for the fact that diffraction profile analy...

Estimating grain-size distributions in nanocrystalline materials from X-ray diffraction profile analysis

Modelling the influence of grain-size-dependent solute drag on the kinetics of grain growth in nanocrystalline materials

Determining the Kapitza resistance and the thermal conductivity of polycrystals: A simple model

Undoped and Bi-doped nanocrystallineZnO was prepared by the inert gas condensation method. Samples of different grain sizes were obtained by annealing treatments at various temperatures. The dc and acelectrical conductivity of undoped and Bi-doped nanocrystallineZnO was investigated as a function of grain size. In dcmeasurements,nanocrystallineZnO of small grain size ( 40 nm) did not exhibit time dependent conduction but exhibited ohmic behavior. This difference in dc conductivity is discussed in terms of the grain size dependence of the average trap density. A small amount of Bi dopant had no measurableeffect on the electrical behavior in the small grain size range but resulted in higher specimen resistance in the large grain size range. acelectrical properties were characterized by impedance spectroscopy. The impedance spectra of undoped and Bi-doped samples of small grain sizes were rather similar and exhibited two semicircles. In contrast, the impedance spectra of samples with larger grain sizes were single, depressed circles with much higher impedances for the Bi-doped sample, in agreement with the dcmeasurements. A consistent assignment of the contributions due to bulk conductivity, porosity, and grain boundary effects to the overall impedance is presented.

Grain size-dependent electrical properties of nanocrystalline ZnO

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