Urged by the increasing power and packing densities of integrated circuits and electronic devices, efficient dissipation of excess heat from hot spot to heat sink through thermal interface materials ...(TIMs) is a growing demand to maintain system reliability and performance. In recent years, graphene‐based TIMs received considerable interest due to the ultrahigh intrinsic thermal conductivity of graphene. However, the cooling efficiency of such TIMs is still limited by some technical difficulties, such as production‐induced defects of graphene, poor alignment of graphene in the matrix, and strong phonon scattering at graphene/graphene or graphene/matrix interfaces. In this study, a 120 µm‐thick freestanding film composed of vertically aligned, covalently bonded graphene nanowalls (GNWs) is grown by mesoplasma chemical vapor deposition. After filling GNWs with silicone, the fabricated adhesive TIMs exhibit a high through‐plane thermal conductivity of 20.4 W m−1 K−1 at a low graphene loading of 5.6 wt%. In the TIM performance test, the cooling efficiency of GNW‐based TIMs is ≈1.5 times higher than that of state‐of‐the‐art commercial TIMs. The TIMs achieve the desired balance between high through‐plane thermal conductivity and small bond line thickness, providing superior cooling performance for suppressing the degradation of luminous properties of high‐power light‐emitting diode chips.
Graphene nanowalls, composed of high‐quality, vertically aligned, and covalently bonded graphene frameworks, exhibit excellent ability to improve the thermal conductivity of polymer‐based thermal interface materials. The resulting composites show a through‐plane thermal conductivity of 20.4 W m−1 K−1 at a filler content of 5.6 wt%, resulting in ≈1.5 times higher cooling efficiency compared to that of a commercial thermal pad.
Modelling of soil solid thermal conductivity He, Hailong; Li, Min; Dyck, Miles ...
International communications in heat and mass transfer,
July 2020, 2020-07-00, Letnik:
116
Journal Article
Recenzirano
Soil solid thermal conductivity (λs) is critical to model effective soil thermal conductivity (λeff) that is required for engineering design and estimate of soil surface energy flux and soil ...temperature. However, it is impossible to measure λs because soil is a porous medium and there is no way to compact the soil to a continuous solid state without any pore spaces. The indirect estimation of λs requires saturation of the soils or a complete soil mineralogical information or mineral component such as quartz that has much greater thermal conductivity. Therefore, many approximation approaches of various complexities to predict λs have been proposed. However, few studies have been conducted to assess these models. An extensive review were conducted and returned 20 models to calculate λs. These models were categorized and their performances were assessed with a compiled dataset consisting of 65 soils from five studies. The results showed that the Johansen approach can give satisfactory λs given that quartz content is available (RMSE = 0.60 W m−1 °C−1, NSE = 0.91) and the Tarnawski et al. model suitable for Canadian soils (RMSE = 0.55 W m−1 °C−1, NSE = 0.92). The Côté and Konrad approach that inversely model λs based on the geometric mean model and measured soil thermal conductivity at full saturation (λsat) give accurate λs (RMSE = 0.22 W m−1 °C−1, NSE = 0.99), but cannot be applied to soils without λsat measurement. The other approaches that take use of soil thermal conductivity at dryness (λdry) give unsatisfactory λs. Therefore, a new three-point method (three measurements between λdry and λsat) based on the He et al. model was proposed to predict λs. The results showed this approach provides a reliable method to estimate λs (RMSE = 0.17 W m−1 °C−1, NSE = 0.99) at various textures and water contents without knowledge on mineralogical information.
•20 models to estimate solid thermal conductivity (λs) were evaluated•λs can be accurately estimated with the inversing geometric mean model at saturation (one point method)•A new three-point method based on the He et al. 2017 model were proposed to accurately model λs
Electrostatic flocking is applied to create an array of aligned carbon fibers from which an elastomeric thermal interface material (TIM) can be fabricated with a high through‐plane thermal ...conductivity of 23.3 W/mK. A high thermal conductivity can be achieved with a significantly low filler level (13.2 wt%). As a result, this material retains the intrinsic properties of the matrix, i.e., elastomeric behavior.
Microelectronics
The landscape represents a GeTe‐C nanocomposite, a heterogeneous medium made of materials with different elastic properties, as symbolized by mountains and valleys. A travelling ...phonon is scattered at the interfaces and strongly attenuated. This strong scattering, like in glasses, modifies phonon properties and leads to a low and weakly temperature dependent thermal conductivity. More in article number 2310209, V. M. Giordano and co‐workers.
The temperature dependent lattice thermal conductivity (κph) of MAX phases, Mn+1AXn are calculated using the Debye theory as outlined by Slack. At high temperature the formula derived by Slack is a ...reasonable approximation to estimate the lattice thermal conductivity. The calculation used the large data base of elastic coefficients of stable MAX phases established recently. It is found that MAX phases with “A”=Al have higher κph at 1300K, and the majority of MAX carbides have higher κph than MAX nitrides. We have also calculated the minimum thermal conductivities of these MAX phases using the empirical formula suggested by Clarke. It is shown that the minimal lattice thermal conductivities of MAX carbides and nitrides are closer to each other in the 211 phases than in higher n phases. The calculated κph for 8 MAX phases at 1300K are in reasonable agreement with experimental data, especially in Ti2AlC, Nb4AlC3, Ta4AlC3, Nb2AlC and Nb2SnC phases.
Polymer‐based thermal management materials (TIMs) show great potentials as TIMs due to their excellent properties, such as high insulation, easy processing, and good flexibility. However, the limited ...thermal conductivity seriously hinders their practical applications in high heat generation devices. Herein, highly transparent, insulating, and super‐flexible cellulose reinforced polyvinyl alcohol/nylon12 modified hexagonal boron nitride nanosheet (PVA/(CNC/PA‐BNNS)) films with quasi‐isotropic thermal conductivity are successfully fabricated through a vacuum filtration and subsequent self‐assembly process. A special structure composed of horizontal stacked hexagonal boron nitride nanosheets (h‐BNNSs) connected by their warping edges in longitudinal direction, which is strengthened by cellulose nanocrystals, is formed in PVA matrix during self‐assembly process. This special structure makes the PVA/(CNC/PA‐BNNS) films show excellent thermal conductivity with an in‐plane thermal conductivity of 14.21 W m−1 K−1 and a through‐plane thermal conductivity of 7.29 W m−1 K−1. Additionally, the thermal conductive anisotropic constants of the as‐obtained PVA/(CNC/PA‐BNNS) films are in the range of 1 to 4 when the h‐BNNS contents change from 0 to 60 wt%, exhibiting quasi‐isotropic thermal conductivity. More importantly, the PVA/(CNC/PA‐BNNS) films exhibit excellent transparency, super flexibility, outstanding mechanical strength, and electric insulation, making them very promising as TIMs for highly efficient heat dissipation of diverse electronic devices.
Quasi‐isotropically thermal conductive cellulose reinforced polyvinyl alcohol/nylon12 modified hexagonal boron nitride nanosheet films with high transparency, good electrical insulation, and super‐flexibility are successfully fabricated through a vacuum filtration and subsequent self‐assembly process. These films are very promising as thermal interface management materials for highly efficient heat dissipation of diverse electronic devices.
Abstract
The characteristics of metals are very important to know in order to get optimal utilisation. In this research, a system for measuring the heat propagation of various types of metals is ...developed. Measurements will be made using a number of LM35 sensors placed scattered on the metal to be tested. In this research, Arduino is used to read the data through ADC which then sends the data to the computer. The data is then processed simultaneously for several metal samples so that the heat propagation of each can be compared. The fixed parameters used in this research are the area of each plate and also the distance between the plates. Finally, the success of the method to measure the heat propagation of different metals is reported. Further results can be developed to identify different types of metals and their purity.
A review of experimental/computational studies to enhance the thermal conductivity of phase change materials (PCM) that were conducted over many decades is presented. Thermal management of ...electronics for aeronautics and space exploration appears to be the original intended application, with later extension to storage of thermal energy for solar thermal applications. The present review will focus on studies that concern with positioning of fixed, stationary high conductivity inserts/structures. Copper, aluminum, nickel, stainless steel and carbon fiber in various forms (fins, honeycomb, wool, brush, etc.) were generally utilized as the materials of the thermal conductivity promoters. The reviewed research studies covered a variety of PCM, operating conditions, heat exchange and thermal energy storage arrangements. The energy storage vessels included isolated thermal storage units (rectangular boxes, cylindrical and annular tubes and spheres) and containers that transferred heat to a moving fluid medium passing through it. A few studies have focused on the marked role of flow regimes that are formed due to the presence of thermally unstable fluid layers that in turn give rise to greater convective mixing and thus expedited melting of PCM. In general, it can be stated that due to utilization of fixed high conductivity inserts/structures, the conducting pathways linking the hot and cold ends must be minimized.
Excellent through‐plane thermally conductive composites are highly demanded for efficient heat dissipation. Giant sheets have large crystalline domain and significantly reduce interface phonon ...scattering, making them promising to build highly thermally conductive composites. However, realizing vertical orientation of giant sheets remains challenging due to their enormous mass and huge hydrodynamic drag force. Here, we achieve highly vertically ordered liquid crystals of giant graphite oxide (more than 100 µm in lateral dimension) by microwire shearing, which endows the composite with a recorded through‐plane thermal conductivity of 94 W m−1 K−1. Microscale shearing fields induced by vertical motion of microwires conquer huge hydrodynamic energy barrier and vertically reorient giant sheets. The resulting liquid crystals exhibit extremely retarded relaxation and impart large‐scale vertical array with bidirectional ordering degree as high as 0.82. The graphite array‐based composites demonstrate an ultrahigh thermal enhancement efficiency of over 35 times per unit volume. Furthermore, the composites improve cooling efficiency by 93% for thermal management tests compared to commercial thermal interface materials. This work offers a novel methodology to precisely manipulate the orientation of giant particles and promote large‐scale fabrication of vertical array with advanced functionalities.
Highly vertically ordered liquid crystals of giant graphite oxide (more than 100 µm in lateral dimension) is realized by microwire shearing, which endows the composite with a record high through‐plane thermal conductivity of 94 W m−1 K−1, 15% higher than that of pure Indium foil (81.8 W m−1 K−1).
Effective soil thermal conductivity (λ
eff) is a critical parameter for environmental and earth science as well as engineering applications. Models to predict λ
eff are required in diverse global and ...community land surface schemes as well as climate models to investigate coupled water and heat transport in soils and heat exchange at the earth surface. Among the many soil thermal conductivity models, models based on the normalized concept are most often developed and utilized for estimating λ
eff. However, at present no systematic study has been performed to investigate the origin and evolution of the normalized thermal conductivity models, nor to evaluate their performance with large datasets. The objectives of this study were to: (a) review the development and evolution of the normalized thermal conductivity models, and (b) assess their performance with datasets consisting of soils with a full range of water saturation and a wide range of soil textures and bulk densities. A total of 38 normalized thermal conductivity models were critically reviewed and their relationships were clearly outlined. Their performance was evaluated by five categories according to model characteristics with a compiled dataset consisting of 71 soils and 669 tests collected from nine studies. Our analysis demonstrated key roles of the quartz content, solid thermal conductivity and choice of the Kersten functions in the model applicability and accuracy of estimating λ
eff. The results showed that the Y2018, CK2005, CK2006, J1975, L2007 and T2009 models have the best performance among the models without fitting parameters, but further improvements are required to apply them universally. Although the models of H2017, LD2015, M2006 and K2007 are the best performing models with fitting parameters, approaches to calculate these parameters are required so they can be easily applied. Future studies on parametrization of currently well‐performing models for wider and more accurate application, development of a soil thermal conductivity database for model evaluation and calibration purposes, and connecting soil thermal conductivity models to hydraulic properties are recommended.
Highlights
The history and evolution of normalized thermal conductivity models and the potential Kersten (K
e) functions are collated and synthesized.
A total of 38 models were reviewed and their performance was evaluated with a total of 71 soils and 669 tests from nine studies.
The Y2018, CK2005, CK2006, J1975, L2007 and T2009 are the best ranked models without fitting parameters.
The models of H2017, LD2015, M2006 and K2007 are the best ranked models with fitting parameter.
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