Hydrothermal Synthesis Optimization of High-Aspect Ratio α-Al2O3 Microfibers for Thermally Conductive Soft Composites

Ceramic substrates are commonly used in electronic techniques. These substrates have to meet specific requirements for applications in microelectronics. Beside a moderate mechanical strength, a high thermal conductivity and a good insulating these materials should show a low dielectric permittivity and small dielectric loss. Glass-ceramics based on cordierite offer a low thermal expansion and a good thermal shock resistance. Due to their low dielectric permittivity they have a high potential for use as substrate-materials for highly integrated microwave circuits. The low thermal conductivity can be increased by generating thermal conducting paths using a flexible process. Thus an enhancement of applications can be obtained. In the present study a laserinduced surface-modification process was examined to fabricate thermal and electrical conducting paths. The ceramic substrates were produced by sintering of powders with cordierite composition. Using a CO 2 -laser the cordierite-ceramic was remelted and different additives (TiC, WC, W) with higher thermal and electrical conductivity were embedded. In order to reduce thermoshock the substrate which was precoated with the additive material must be preheated. To achieve this a vacuum-furnace was developed which allowed an in-situ laser treatment. The composite structure which developed after solidification of the ceramic melt was characterised by microstructural examinations and determination of thermal and electrical conductivity. The result of the characterisation has been used to find optimised process-parameters. The result of the laserinduced modification process depended strongly on the selected additive and the process parameters. Paths fabricated with TiC introduced into the melt showed a significant increase of thermal and electrical conductivity over small dimensions. Because of the existing inhomogeneities of the microstructure (cracks, pores) this effect was not reproducible over larger distances. The areas modified with WC showed also an increase of thermal and electrical conductivity. However their adhesion strength to the substrate was limited due to the fact that the WC-particles were not embedded into the substrate but stayed top of it as a surface layer. The additive W showed a very good compatibility with the cordierite-substrate. Using optimised process-parameters e.g. an adjusted level of preheating temperature and power density conducting paths were fabricated which offered a very good adhesion to the substrate and showed a microstructure with homogeneously distributed W-particles inside the ceramic matrix. Follow this procedure any planar structures could be generated on the cordierit-ceramic, which had a ten times higher thermal conductivity than the substrate and an electrical resistivity of 10 - 5 -10 - 6 Ωm.

Effective heat transfer between the working fluid and subterranean rocks is essential for producing green and low-carbon geothermal energy. As the primary thermal conductive medium, cement has low thermal conductivity, leading to high thermal resistance and significantly reducing geothermal wells’ efficiency. Therefore, high thermal conductivity cement has emerged as a widely anticipated new research area. The purpose of this research is to address the substantial harm of traditional carbon-based thermal conductive fillers to cement. A novel expanded graphite (EG)/epoxy resin (EP) composite additive (MEG) was designed to increase cement’s thermal conductivity while preserving its mechanical strength and pumpability. Firstly, the physicochemical properties of MEG were revealed by FT-IR, UV-Vis, SEM, and TGA. Then, the applicability of MEG cement in adverse geological environments (high-temperature 60–100 ℃, high-mineralization 5–36% NaCl) was evaluated through simulated maintenance experiments. Finally, the hydration products and pore structure of MEG-cement were analyzed by XRD/FT-IR and SEM/MIP, revealing the thermal conductivity enhancing mechanism. The results showed that: 1) MEG uses ZDMA as a bridge to promote the ring opening and curing of EP, and is formed by strong cation -π interaction with EG. 2)After curing at 60–100 ℃, MEG-cement exhibits a significant increase (46.6-182.1%) in thermal conductivity within the optimal dosage range of 5–10%, fully meeting the requirements for compressive strength (10.4–21.7 MPa) and fluidity (19.3–21.2 cm) of cementing. In addition, MEG-cement maintained stable density and significant high thermal conductivity advantage in high-mineralization environments (5–36% NaCl), with an increase in thermal conductivity of 23.8- 54.1%. 3) The mechanism of MEG promoting heat transfer in cement is summarized as the enhancement of the hydration process and the production of C-S-H gels. C-S-H gels filled the gel pores and transition pores in the cement skeleton and formed a dense, high thermal conductivity network, which shortens the heat transfer path and thus greatly improves the thermal conductivity of cement. In summary, this study has successfully developed a MEG geothermal cement with independent intellectual property rights that provides reliable technical support for the efficient development of geothermal resources and has important engineering application value.

Exploring the increasing behavior of thermal conductivity for high-entropy zirconates at high temperatures

Polymers are widely used in applications due to their diverse and controllable properties in many physical domains. However, polymers have not historically been used in applications for which a high thermal conductivity is required as bulk polymers are typically thermal insulators. However, research in recent decades on a handful of highly oriented or semi-crystalline polymers has shown the potential for dramatically increased uniaxial thermal conductivity by factors exceeding 100. This dramatic increase in thermal conductivity is because heat is conducted by atomic vibrations along the covalently bonded polymer backbone rather than across chains by weak van der Waals bonds as in unoriented polymers. While it is known that polymers can be processed to yield these properties, much remains unknown about the microscopic transport properties of atomic vibrations in these materials and the true upper limits to thermal conductivity. In this thesis, we address these knowledge gaps by using a combination of simulations and experiments to investigate thermal conduction in semi-crystalline and crystalline polymers. First, we present molecular dynamics simulations of a perfect polymer crystal, polynorbornene. While polymer crystals studied typically exhibit substantially enhanced thermal conductivities above those of the amorphous form, polynorbornene exhibits a glass-like thermal conductivity of less than 1 Wm-1K-1 even as a perfect crystal. This unusual behavior occurs despite the polymer satisfying many of the conventional criteria for high thermal conductivity. Using our simulations, we show that the origin of this unusual behavior is excessively anharmonic bonds and a complex unit cell. Second, we move to experimental studies of thermal transport in polymers. A key requirement to perform materials science is a method to routinely and easily characterize the property of interest in diverse samples. For polymers, this property is typically the in-plane thermal conductivity. This property turns out to be surprisingly difficult to measure using conventional thermal characterization methods. In this work, we adapt transient grating spectroscopy (TG), a well-known method in the chemistry community, to perform in-plane thermal conductivity measurements of polymer films. TG can resolve the in-plane thermal anisotropy of a sample without any physical contact and at tunable length scales, a substantial advance in capability over all prior characterization methods. We extend the application of TG to probe sub-µm length scales, and we successfully apply the technique to numerous poor quality polymer samples as well as thin films. Finally, we exploit the capability of TG to probe thermal conduction over sub-µm length scales to provide the first experimentally resolved microscopic transport properties of atomic vibrations in semi-crystalline polyethylene (PE). Despite the intense interest over decades in PE due to its high intrinsic thermal conductivity, no experimental measurement has yet been able to directly probe the heat-carrying phonons, leading to many questions about the relevant scattering mechanisms and absolute upper limits of thermal conductivity in real samples. Using TG, we present the first observation of quasi-ballistic thermal transport at sub-µm length scales, from which we obtain the phonon mean free path spectra of a semi-crystalline PE sample. Further, we pair these results with Small-Angle X-ray Scattering measurements to show that thermal phonons propagate ballistically within and across nanocrystalline domains, contrary to the conventional viewpoint. These results provide an unprecedented microscopic view of thermal transport in polymer crystals that was previously experimentally inaccessible.

SiCw is expected to be candidate material for Accident Tolerant Fuel (ATF). Nevertheless, the distribution, morphology and size of SiCw have impacts on the thermal conductivity of composite fuels. An analysis model for UO2-SiCw was established using finite element method (FEM), and the effects of whisker orientation, size, volume fraction and aspect ratio on thermal conductivity were analyzed. The generation algorithm of SiCw was improved, and the results of thermal conductivity calculations showed that the improved model is closer to the experimental value. The effect of whisker orientation on thermal conductivity can be calculated by the cosine law of zenith angle and azimuth angle, and deriving a model for calculating thermal conductivity with specific whisker orientation. With the decrease of whisker size, the influence of Kapitza thermal resistance is significant. The formula used to calculate the whisker threshold diameter was deduced, and the results are in good agreement with the FEM results. With the increase of volume fraction of whiskers, the thermal conductivity increases linearly. For morphology, whiskers with high aspect ratio can achieve higher thermal conductivity. There is a value for the ratio of whisker length to matrix size, above which increasing whisker length will slow the increase in thermal conductivity (0.3 for whiskers along the direction of thermal conduction). The thermal conductivity of UO2-SiCw was optimized using the above conclusions, and the radial temperature distribution of pellet was analyzed. The results show that taking full advantage of the anisotropy of whisker heat transfer can significantly improve the thermal conductivity of composites and improve the operation condition of the pellet. The results can provide a good theoretical basis for designing whisker-doped composites with high thermal conductivity.

Effect of a metallic coating on the thermal conductivity of carbon nanofiber–dielectric matrix composites

Theoretical models are proposed for predicting the longitudinal and transverse thermal conductivities of wood-polymer composites. The predictions of the models are in good agreement with the measured thermal conductivities of red maple boards impregnated with either polystyrene, polymethyl methacrylate or polyfurfuryl alcohol. The density, heat capacity, transverse thermal conductivity and longitudinal thermal conductivity of the red maple boards were 589 kg/m3,1290 J/kg K, 0.155 W/mK and 0.358 W/mK, respectively. Polymer impregnation moderately altered the thermophysical properties of the boards. The increase in density of the boards ranged from 60% to 79%, the increase in transverse thermal conductivity ranged from 12% to 33%, the increase in longitudinal thermal conductivity ranged from 3% to 13% and the decrease in heat capacity ranged from 3% to 11%. Polystyrene provided the largest increase in density whereas polymethyl methacrylate yielded the greatest increase in thermal conductivity and the largest decrease in heat capacity. Treatment with polyfurfuryl alcohol caused the samples to swell and resulted in the lowest increases in thermal conductivity and density. On average the thermal diffusivity of the composites was 26% smaller than that of the parent wood.

Thermal conductivity and mechanical properties of thermally conductive composites based on multifunctional epoxyorganosiloxanes and hexagonal boron nitride

Aluminum nitride, with its high thermal conductivity and insulating properties, is a promising candidate as a thermal dissipation material in optoelectronics and high-power logic devices. In this work, we have shown that the thermal conductivity and electrical resistivity of AlN ceramics are primarily governed by ionic defects created by oxygen dissolved in AlN grains, which are directly probed using 27Al NMR spectroscopy. We find that a 4-coordinated AlN3O defect (ON) in the AlN lattice is changed to intermediate AlNO3, and further to 6-coordinated AlO6 with decreasing oxygen concentration. As the aluminum vacancy (VAl) defect, which is detrimental to thermal conductivity, is removed, the overall thermal conductivity is improved from 120 to 160 W/mK because of the relatively minor effect of the AlO6 defect on thermal conductivity. With the same total oxygen content, as the AlN3O defect concentration decreases, thermal conductivity increases. The electrical resistivity of our AlN ceramics also increases with the removal of oxygen because the major ionic carrier is VAl. Our results show that to enhance the thermal conductivity and electrical resistivity of AlN ceramics, the dissolved oxygen in AlN grains should be removed first. This understanding of the local structure of Al-related defects enables us to design new thermal dissipation materials.

Coating polyrhodanine onto boron nitride nanosheets for thermally conductive elastomer composites

The interest in polymers with high thermal conductivity increased much because of their inherent properties such as low density, low cost, flexibility, and good chemical resistance. However, it is challenging to engineer plastics with good heat transfer characteristics, processability, and required strength. Improving the degree of the chain alignment and forming a continuous thermal conduction network is expected to enhance thermal conductivity. This research aimed to develop polymers with a high thermal conductivity that can be interesting for several applications. Two polymers, namely poly(benzofuran-co-arylacetic acid) and poly(tartronic-co-glycolic acid), with high thermal conductivity containing microscopically ordered structures were prepared by performing enzyme-catalyzed (Novozyme-435) polymerization of the corresponding α-hydroxy acids 4-hydroxymandelic acid and tartronic acid, respectively. A comparison between the polymer’s structure and heat transfer obtained by mere thermal polymerization before and enzyme-catalyzed polymerization will now be discussed, revealing a dramatic increase in thermal conductivity in the latter case. The polymer structures were investigated by FTIR spectroscopy, nuclear magnetic resonance (NMR) spectroscopy in liquid- and solid-state (ss-NMR), and powder X-ray diffraction. The thermal conductivity and diffusivity were measured using the transient plane source technique.

High thermal conductivity materials show promise for thermal mitigation and heat removal in devices. However, shrinking the length scales of these materials often leads to significant reductions in thermal conductivities, thus invalidating their applicability to functional devices. In this work, we report on high in-plane thermal conductivities of 3.05, 3.75, and 6 μm thick aluminum nitride (AlN) films measured via steady-state thermoreflectance. At room temperature, the AlN films possess an in-plane thermal conductivity of ∼260 ± 40 W m-1 K-1, one of the highest reported to date for any thin film material of equivalent thickness. At low temperatures, the in-plane thermal conductivities of the AlN films surpass even those of diamond thin films. Phonon-phonon scattering drives the in-plane thermal transport of these AlN thin films, leading to an increase in thermal conductivity as temperature decreases. This is opposite of what is observed in traditional high thermal conductivity thin films, where boundaries and defects that arise from film growth cause a thermal conductivity reduction with decreasing temperature. This study provides insight into the interplay among boundary, defect, and phonon-phonon scattering that drives the high in-plane thermal conductivity of the AlN thin films and demonstrates that these AlN films are promising materials for heat spreaders in electronic devices.

Molecular dynamics study on the mechanism of nanofluid coolant's thermal conductivity improvement

High thermal conductive polyvinyl alcohol composites with hexagonal boron nitride microplatelets as fillers

<p indent="0mm">In the 5G era, the effective thermal management has become more demanding due to the ever-rising integration of electronic devices. High thermal conductivity materials play a crucial role in the field of thermal management, for example, thermal interface materials (TIMs) are used to fill the gap between the electronic chip and the heat sink to improve thermal transfer. In recent years, polymers have become a popular choice for thermal conductive materials because of their light, economical and excellent insulation and processability. To improve the thermal conductivity of materials, inorganic fillers with high thermal conductivity are generally composited with polymers. With the merits of high thermal conductivity, desirable chemical stability, carbon nanotubes (CNTs) are considered to have broad application potential in thermal conductive composites. Simple composition methods failed to increase thermal conductivity of composite materials to expected levels due to the large interfacial thermal resistance between CNTs and polymers and the disorderly distribution of CNTs in polymers. Therefore, reasonable design of polymer composites filled with CNTs is the key to achieving high thermal conductivity. This review mainly introduces the application of CNTs in thermal conductive polymer composites. Based on the existing theoretical research on thermal conductivity of composites and the application of molecular dynamics, more feasible strategies for improving the thermal conductivity of polymer composites filled with CNTs have been proposed. The approaches that improve the thermal conductivity of composites are mainly introduced from three aspects. (1) The intrinsic thermal conductivity of CNTs is an important factor affecting the thermal conductivity of polymer-based composites. CNTs and their macroscopic bulk materials both have excellent thermal conductivity. However, the thermal conductivity test results of the macroscopic materials of CNTs (such as CNTs fibers, arrays, and films) showed that the thermal conductivity of the macroscopic materials of CNTs was much smaller than that of single CNTs due to impurities, defects and inter-tube contact thermal resistance. Studies have shown that purification of CNTs and reduction of intertubular contact thermal resistance can improve the intrinsic thermal conductivity of CNTs. (2) From a microscopic perspective, phonons, the quantized energy of lattice vibration, are the main mechanism of heat conduction in most carbon fillers and polymers. Phonon scattering occurs in the process of phonon transfer, including the scattering between phonons and the scattering at the interface caused by defects and impurities, resulting in thermal resistance. The bonding strength of fillers and polymer interfaces are the crucial factors affecting the transmission of phonons. Hence, the thermal conductivity of composites could be effectively enhanced by surface treatment of CNTs, including covalent functionalization and non-covalent functionalization. Studies have shown that the functionalization can enhance the interfacial interaction between CNTs and polymers, while improving the dispersion of CNTs in polymers. (3) According to the thermal conduction network theory, the key to improving the thermal conductivity is whether the fillers can form a large number of continuous thermal conduction paths in the polymers and maintain a stable existence. However, high CNTs content usually affects the comprehensive properties of composites. To solve this problem, the arrangement and distribution of CNTs in the polymer should be improved to construct more heat conduction pathways, which can achieve high thermal conductivity at a low filling content. Here we introduce some effective methods, including the synergy effect, field orientation and the construction of 3D network structures. In this review, the characteristics and improvement effects of different technical approaches are summarized, which provides a reference for the research and application of CNT-filled polymer-based composites with high thermal conductivity. Finally, the future development prospects of carbon nanomaterial-filled polymer composites are discussed from perspectives of theoretical research, experimental design and engineering application.