Heat conduction in fine scale mixtures with interfacial contact resistance

Heat conduction in a fine scale mixture of two conductors is examined in the presence of a contact resistance between phases. The problem is studied rigorously in the context of periodic homogenization. Unlike the case of perfect heat transmission between phases, the temperature gradients converge weakly as Radon measures. The strict ellipticity of the homogenized transport equation depends upon the geometry of the interface. The effective conductivity associated with the overall heat dissipation rate inside a composite cube is considered. It is shown that this property exhibits a size effect under rescaling

Thermal Conductivity of Carbon Nanotubes and their Polymer Nanocomposites: A Review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT. This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composite

Mixed-conducting LSC/CGO and Ag/CGO composites for passive oxygen separation membranes

Dense ceramic oxygen separation membranes can pass oxygen perm-selectively at elevated temperature and have potential for improving the performance and reducing the cost of several industrial processes: such as the conversion of natural gas to syngas, or to separate oxygen from air for oxy-fuel combustion in electricity generation (to reduce NOx emissions and facilitate CO2 sequestration). These pressure-driven solid state membranes are based on fast oxygen-ion conducting ceramics, but also need a compensating flow of electrons. Dual-phase composites are attractive since they provide an extra degree of freedom, compared with single phase membranes, for optimising the overall membrane performance. In this study, composites containing gadolinia doped ceria (CGO, Ce0.9Gd0.1O2- ) and either strontium-doped lanthanum cobaltite (LSC, La0.9Sr0.1CoO3- or La0.6Sr0.4CoO3- ) or silver (Ag) were investigated for possible application as oxygen separation membranes in oxy-fuel combustion system. These should combine the high oxygen ion conductivity of CGO with the high electronic conductivity and fast oxygen surface exchange of LSC or silver. Dense mixed-conducting composite materials of LSC/CGO (prepared by powder mixing and sintering) and Ag/CGO composites (prepared by silver plus copper oxide infiltration method) showed high relative density (above 95%), low background gas leakage and also good electrical conduction. The percolation threshold of the electronic conducting component was determined to be approximately 20 vol.% for both LSC compositions and 14 vol.% for Ag. Isotopic exchange and depth profiling by secondary ion mass spectrometry was used to investigated the oxygen tracer diffusion (D*) and surface exchange coefficient (k*) of the composites. Composites just above the electronic percolation threshold exhibited high solid state oxygen diffusivity, fast surface exchange activity moderate thermal expansion and sufficient mechanical strength thus combining the benefits of their constituent materials. The preliminary work on oxygen permeation measurement showed that the reasonable magnitude of oxygen fluxes is possible to be achieved. This indicates that the composites of LSC/CGO and Ag/CGO are promising for further development as passive oxygen separation membranes

Thermal Conductivity of Sand-Silt Mixtures

Heat flow controls the design and operation of a wide range of engineered geosystems. This study uses transient thermal probe measurements to determine the evolution of the thermal conductivity of air-dry and water-saturated sand–silt mixtures as a function of effective stress. Results confirm that the thermal conductivity of soils varies with state of stress, dry mass density, mineralogy, and pore fluid properties and highlight the effect of thermal contact resistance on the thermal conductivity of granular materials. Thermal conductivity follows a linear relationship with the logarithm of effective stress as a consequence of fabric compaction, increased coordination number, contact deformation, and reduced thermal contact resistance. The bulk thermal conductivity of water-saturated soils is more than seven times that of air-dry soils for the same fines content (FC) and effective stress. Pore-filling fines contribute conduction paths and interparticle coordination; the peak in thermal conductivity takes place at FC ≈ 0.4; this mixture range corresponds to the transition from fines-controlled to coarse-controlled mechanical response (i.e., both fines and coarse grains are load bearing), in agreement with the revised soil classification system

Multi-scale modelling of thermal shock damage in refractory materials

Refractories are high-temperature resistant materials used extensively in many engineering structures and assemblies in a wide spectrum of applications ranging from metallurgical furnace linings to thermal barrier coatings. Such structures are often exposed to severe thermal loading conditions in the form of rapid temperature changes (thermal shock) and/or temperature cycles. The understanding and modelling of the failure processes are definitely necessary to achieve reliable life-time predictions of the existing structures and to develop design rules for improvement. Due to their high temperature resistance, alumina based refractory ceramics with a porous granular microstructure being far from homogenous are commonly used in the applications as mentioned above. In such heterogeneous material systems, local thermal expansion (CTE) mismatches, non-uniformities and anisotropy of the different constituents naturally lead to the appearance of internal stresses which are essentially the driving mechanisms for micro-cracking and damage. Under highly transient external thermal loading conditions, the resulting heterogeneous temperature distribution may lead to a complicated mechanical response along with a nonuniform mechanical and physical property degradation accompanied by irreversible geometry changes. The altered distribution of the mechanical properties dictates the macroscopic response when the external loading is further varied. Therefore, a strong coupling between the evolving microstructure and the macroscopic response arises. Moreover, microstructural configurational changes may trigger a significant interaction between the mechanical and thermal fields, for instance due to a reduced heat transport across a damaged interface. Therefore, an approach taking into account these mechanisms sufficiently well would render a versatile tool to improve the understanding of the influence of mechanical and thermal properties at the constituent level and their mutual interaction from a microstructural perspective. In this thesis, a concurrent multi-scale framework for the thermo-mechanical analysis of heterogeneous materials is proposed, with a particular focus on coarse grained refractory ceramics. The framework is essentially based on a rigorous extension of the well established FE2 computational homogenization technique, where the local macroscopic response is determined through the solution of a boundary value problem defined on a representative volume of the underlying microstructure. At first, the computational homogenization ideas are explored in the context of pure heat conduction processes in heterogeneous solids. Subsequently, the framework for coupled thermo-mechanical analyses is constructed by combining the first order mechanical homogenization with the dual procedure developed for heat conduction, within an operator-split (or staggered) solution algorithm which is composed of incrementally uncoupled nested (FE2) solution blocks for thermal and mechanical equilibrium subproblems. For predictive computations, the mechanical and thermophysical properties of individual phases and interfaces at the microstructural level are required, which is a distinctive characteristic of such a multi-scale approach. Due to the lack of material data, particularly for interfaces, direct numerical simulations (DNS) are exploited to identify the parameters inversely by using a limited set of molten aluminium thermal shock test results. On the basis of a microstructure composed of mutually noncontacting large grains embedded in a homogeneous matrix reflecting a compound of very fine grains, molten aluminium thermal shock tests are reproduced in full detail under realistic boundary conditions and a computational procedure is developed to determine the damage distribution along the specimen which is compared to experimental results. The failure mechanisms at the matrix-grain interface level are resolved by introducing thermo–mechanical cohesive zone elements not only capable of accounting for the mechanical decohesion but also including the reduced heat transport through themechanically damaged interfaces. Fine scalemicro-cracks within the matrix are smeared out by using a well-established continuum damage mechanics formulation which is free of any pathological localization and mesh sensitivity problems. Direct numerical simulation of thermal shock tests has also served for the investigation of short range effects (due to the local CTE mismatch) and long range effects (elastic fields accompanying the temperature gradient) on the resulting thermo-mechanical damage profile, through variations of different microstructural material parameters. In the last part of the thesis, predictive capabilities of the developed analysis framework are assessed by means of the two–scale analysis of a real size ladle refractory lining, based on the microstructural parameters identified through direct numerical simulations

Analytical and numerical assessment of the effect of highly conductive inclusions distribution on the thermal conductivity of particulate composites

Highly conductive composites have found applications in thermal management, and the effective thermal conductivity plays a vital role in understanding the thermo-mechanical behavior of advanced composites. Experimental studies show that when highly conductive inclusions embedded in a polymeric matrix the particle forms conductive chain that drastically increase the effective thermal conductivity of two-phase particulate composites. In this study, we introduce a random network three dimensional (3D) percolation model which closely represent the experimentally observed scenario of the formation of the conductive chain by spherical particles. The prediction of the effective thermal conductivity obtained from percolation models is compared with the conventional micromechanical models of particulate composites having the cubical arrangement, the hexagonal arrangement and the random distribution of the spheres. In addition to that, the capabilities of predicting the effective thermal conductivity of a composite by different analytical models, micromechanical models, and, numerical models are also discussed and compared with the experimental data available in the literature. The results showed that random network percolation models give reasonable estimates of the effective thermal conductivity of the highly conductive particulate composites only in some cases. It is found that the developed percolation models perfectly represent the case of conduction through a composite containing randomly suspended interacting spheres and yield effective thermal conductivity results close to Jeffery's model. It is concluded that a more refined random network percolation model with the directional conductive chain of spheres should be developed to predict the effective thermal conductivity of advanced composites containing highly conductive inclusions

Recent developments on fractal-based approaches to nanofluids and nanoparticle aggregation

This project was supported by the National Natural Science Foundation of China (Nos. 41572116, 51576114, ​41630317), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG160602) and the Natural Science Foundation of Fujian Province of China (No. 2016J01254). The authors of the figures that used in presented review are also highly appreciated.Peer reviewedPostprin

Carbon nanotubes for thermal interface materials in microelectronic packaging

As the integration scale of transistors/devices in a chip/system keeps increasing, effective cooling has become more and more important in microelectronics. To address the thermal dissipation issue, one important solution is to develop thermal interface materials with higher performance. Carbon nanotubes, given their high intrinsic thermal and mechanical properties, and their high thermal and chemical stabilities, have received extensive attention from both academia and industry as a candidate for high-performance thermal interface materials. The thesis is devoted to addressing some challenges related to the potential application of carbon nanotubes as thermal interface materials in microelectronics. These challenges include: 1) controlled synthesis of vertically aligned carbon nanotubes on various bulk substrates via chemical vapor deposition and the fundamental understanding involved; 2) development of a scalable annealing process to improve the intrinsic properties of synthesized carbon nanotubes; 3) development of a state-of-art assembling process to effectively implement high-quality vertically aligned carbon nanotubes into a flip-chip assembly; 4) a reliable thermal measurement of intrinsic thermal transport property of vertically aligned carbon nanotube films; 5) improvement of interfacial thermal transport between carbon nanotubes and other materials. The major achievements are summarized. 1. Based on the fundamental understanding of catalytic chemical vapor deposition processes and the growth mechanism of carbon nanotube, fast synthesis of high-quality vertically aligned carbon nanotubes on various bulk substrates (e.g., copper, quartz, silicon, aluminum oxide, etc.) has been successfully achieved. The synthesis of vertically aligned carbon nanotubes on the bulk copper substrate by the thermal chemical vapor deposition process has set a world record. In order to functionalize the synthesized carbon nanotubes while maintaining their good vertical alignment, an in situ functionalization process has for the first time been demonstrated. The in situ functionalization renders the vertically aligned carbon nanotubes a proper chemical reactivity for forming chemical bonding with other substrate materials such as gold and silicon. 2. An ultrafast microwave annealing process has been developed to reduce the defect density in vertically aligned carbon nanotubes. Raman and thermogravimetric analyses have shown a distinct defect reduction in the CNTs annealed in microwave for 3 min. Fibers spun from the as-annealed CNTs, in comparison with those from the pristine CNTs, show increases of ~35% and ~65%, respectively, in tensile strength (~0.8 GPa) and modulus (~90 GPa) during tensile testing; an ~20% improvement in electrical conductivity (~80000 S m⁻¹) was also reported. The mechanism of the microwave response of CNTs was discussed. Such an microwave annealing process has been extended to the preparation of reduced graphene oxide. 3. Based on the fundamental understanding of interfacial thermal transport and surface chemistry of metals and carbon nanotubes, two major transfer/assembling processes have been developed: molecular bonding and metal bonding. Effective improvement of the interfacial thermal transport has been achieved by the interfacial bonding. 4. The thermal diffusivity of vertically aligned carbon nanotube (VACNT, multi-walled) films was measured by a laser flash technique, and shown to be ~30 mm² s⁻¹ along the tube-alignment direction. The calculated thermal conductivities of the VACNT film and the individual CNTs are ~27 and ~540 W m⁻¹ K⁻¹, respectively. The technique was verified to be reliable although a proper sampling procedure is critical. A systematic parametric study of the effects of defects, buckling, tip-to-tip contacts, packing density, and tube-tube interaction on the thermal diffusivity was carried out. Defects and buckling decreased the thermal diffusivity dramatically. An increased packing density was beneficial in increasing the collective thermal conductivity of the VACNT film; however, the increased tube-tube interaction in dense VACNT films decreased the thermal conductivity of the individual CNTs. The tip-to-tip contact resistance was shown to be ~1×10⁻⁷ m² K W⁻¹. The study will shed light on the potential application of VACNTs as thermal interface materials in microelectronic packaging. 5. A combined process of in situ functionalization and microwave curing has been developed to effective enhance the interface between carbon nanotubes and the epoxy matrix. Effective medium theory has been used to analyze the interfacial thermal resistance between carbon nanotubes and polymer matrix, and that between graphite nanoplatlets and polymer matrix.PhDCommittee Chair: Wong, C. P.; Committee Member: Graham, Samuel; Committee Member: Hess, Dennis; Committee Member: Jacob, Karl; Committee Member: Wang, Z. L.; Committee Member: Yao, Don