Vertical Thermal Conductivity Research Articles

Structure and mechanism of directional heat induced asphalt pavement with thermal aggregation effect

Efficiently and actively dissipating accumulated heat within asphalt pavement is crucial for reducing pavement distress and improving subgrade stability. This paper proposed a novel inverted “T” shaped heat induced structure (IT-HIS) that facilitates the rapid transfer of heat from the pavement to the subgrade and shoulders. The IT-HIS consists of two components: a vertical gradient thermal conductivity structure utilizing steel slag, and a horizontal oriented carbon fiber heat induced structure. The heat transfer behavior of the IT-HIS was analyzed using a finite element heat transfer model created in ABAQUS, and the results were validated through indoor irradiation experiments. The findings demonstrated that the IT-HIS enhanced heat flow and promoted secondary aggregation in the vertical direction, while facilitated lateral heat dissipation. Compared to the ordinary group (OG), the heat flux of IT-HIS at depths of 5 cm and 6.5 cm increased by 19.8 % and 64.6 %, respectively. The indoor irradiation experiment results indicated that during the heat absorption period, the IT-HIS reduced temperatures at depths of 0 cm, 2 cm, and 5 cm by 1.5 ℃, 1.7 ℃, and 1.1 ℃, respectively. During the heat dissipation period, the IT-HIS reduced temperatures at depths of 0 cm, 5 cm, and 10 cm by 0.9 ℃, 0.7 ℃, and 1.8 ℃, respectively. The IT-HIS enhanced the cooling effect during daytime and inhibited heat upward during nighttime based on the gradient thermal conductivity structure. The proposal and validation of this structure provide a fundamental approach to forming large-scale high-speed microchannel heat dissipation within asphalt pavements, and is expected to provide experimental and theoretical foundations for the development of directional active cooling technologies.

The preparation and feasibility analysis of magnetic orientation method in the study of thermal insulation composite materials in the field of electronic packaging

AbstractAs an emerging field of current technological development, the heat dissipation performance of electronic packaging materials has become the key to determining product reliability. Therefore, the applied thermal interface materials (TIMs) need to simultaneously meet the performance indicators of thermal conductivity, mechanics, and insulation. Although BN had excellent in‐plane thermal conductivity and insulation performance, it is currently difficult to achieve vertical orientation of powder which leads the layered powder cannot complete a continuous and effective heat transfer path along the Z‐axis direction inside the composite material. In order to solve the above two problems, out‐of‐plane thermal conductive composite was prepared by introducing magnetic oriented method. In order to comprehensively evaluate the thermal conductivity, mechanical and electrical properties, BN@Fe3O4/EP magnetic oriented composites were compared with BN@PDA‐Al2O3/EP composite prepared by mechanical blending method. The vertical thermal conductivity of the magnetic oriented composite material was 2.24 W/(m·K) at 30 vol%, which was 3.39 times that of the blended composite material and 12.4 times that of pure EP. This proves that the magnetic oriented composite materials prepared by the research institute can meet the vertical heat transfer needs in the field of electronic packaging and have high research value in this area.Highlights BN@Fe3O4 core–shell particles were prepared with both thermal conductivity and magnetic controllability. The thermosetting composites were prepared by heated through preliminary gel point and then cured at high temperature. The orientation of powder inside the matrix was constructed, resulted in excellent out‐of‐plane heat transfer performance. Comparisons were made between various properties of magnetic oriented materials and conventional blend materials. The magnetic orientation method is feasible in the field of electronic packaging.

3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and implications for petrophysical properties of potential Mesozoic shale host rocks for nuclear waste storage

AbstractJurassic sedimentary sequences suitable for nuclear waste storage in northern Germany consist of organic-lean claystone and were uplifted to < 100 m depth in the Hils Syncline area (southern Lower Saxony Basin). This Hils Syncline, showcasing a northwestward increase in thermal maturity, facilitates the study of shale petrophysical properties influenced by burial history. This study introduces a 3D-thermally calibrated numerical model of the Hils Syncline area to analyze its geodynamic evolution and maturity variations. It provides new vitrinite reflectance and sonic velocity data for modeling calibration and erosion estimation. The Hils Syncline area has undergone continuous subsidence, interrupted by a Cretaceous uplift documented by an erosional unconformity. During the latest Early Cretaceous, Jurassic rocks underwent maximum burial reaching up to several thousand meters depth and temperatures up to 160 °C in the northwest. The Late Cretaceous inversion caused stronger erosion towards the northwest removing up to 3300 m of sediment compared to about 1300 m in the south, according to vitrinite reflectance-based estimations. Numerical modeling results along the study area indicate decreasing porosity and permeability northwestward with increasing thermal maturity. Porosity and vertical permeability decreased to 5–14% and 2.8 × 10–23 to 1.5 × 10–19 m2 [1 mD = 10−15 m2], respectively, while vertical thermal conductivity increased to 1.30–2.12 (W/m/K). These trends of porosity/permeability and thermal conductivity with burial align with sonic velocity and published experimental porosity data, except for the thermally most mature region (Haddessen). This anomaly is tentatively attributed here to localized overpressure generation in the Posidonia Shale during maximum burial, affecting both the underlying Pliensbachian and overlying Doggerian units. Graphical abstract 3D numerical model of the Hils Syncline and surrounding area revealing that a northwestward increase in maximum burial resulted in higher temperatures and varying maturity levels. While most locations align well with calibration data (i.e. measured vitrinite reflectance and porosity), discrepancies arise in the Haddessen/Bensen area. The mismatch between porosity, vitrinite reflectance, and sonic velocity response indicates local overpressure in the northernmost region mainly during the Cretaceous. It was likely caused by gas generation in the Posidonia Shale affecting nearby Lower and Middle Jurassic units.

Preparation of GNP/SBR thermal conductivity composites with high strength and toughness by synergistic construction of interfacial multiple non‐covalent and covalent bonds

Abstract[AMIm]Cl@GNP/SBR composites were prepared by blending ionic liquid 1‐allyl‐3‐methylimidazolium chloride salt([AMIm]Cl) modified graphene nanoplatelets (GNP) and styrene‐butadiene rubber (SBR). The mechanical and thermal properties as well as the structure–activity relationship of the obtained composites based on the modified filler were investigated under the synergistic effect of non‐covalent and covalent bonding. The experimental results indicated that the modification of GNP by [AMIm]Cl occurred under mild conditions. The dispersibility of [AMIm]Cl@GNP in SBR was enhanced by its participation in vulcanization and the synergistic effect of non‐covalent and covalent cross‐linking networks such as cation‐π, π‐π, and hydrogen bonds. This improvement led to an increase in the interfacial bond strength of the composites and a reduction in the interfacial thermal resistance. These effects resulted in both reinforcement and thermal conductivity properties. The tensile strength and elongation at break of [AMIm]Cl@GNP/SBR composites increased from 18.2 to 22.9 MPa and from 737.9% to 923.5%, respectively, when the modification ratio of [AMIm]Cl was 1% and the loading of [AMIm]Cl@GNP was only 4 phr (mass ratio). At the same time, the thermal conductivity in the vertical direction of the composites increased from 0.25 to 0.44 W/(m·K), and the thermal conductivity in the horizontal direction increased from 0.27 to 0.81 W/(m·K).Highlights Modified powder [AMIm]Cl@graphene nanoplatelets were prepared; [AMIm]Cl@graphene nanoplatelet/styrene‐butadiene rubber blend was prepared; Mechanical properties of blends significantly improved over blank; The horizontal thermal conductivity of the blends increases obviously; The vertical thermal conductivity of the blends was improved.

Novel composite phase change materials supported by oriented carbon fibers for solar thermal energy conversion and storage

Phase change materials (PCMs) have aroused significant interest as promising materials for solar thermal energy conversion and storage. However, the long-standing shortcomings of liquid leakage, low thermal conductivity, and weak solar absorptance limit their practical applications. Herein, novel composite phase change materials (CPCMs) with anisotropic heat conduction are manufactured by mixing continuous carbon fibers (CFs) and palmitic acid (PA)/olefin block copolymer (OBC) mixtures using pressure induction and vacuum treatment. Because the oriented CFs in the vertical direction can offer heat transfer channels inside the composites, the vertical and horizontal thermal conductivities of the CPCMs are 5.84 W·K−1·m−1 and 1.34 W·K−1·m−1, respectively, resulting in a relatively high anisotropic degree of 4.36. Furthermore, to improve the absorption of solar radiation for the composite, carbon black is applied to the upper surface of the CPCMs, achieving a high total solar absorptance of 0.966. Due to the combination of the high solar absorptance of the carbon black and the high vertical thermal conductivity within the composites, the CPCMs exhibit outstanding solar-to-thermal efficiencies of 87.54% ∼ 95.08% at 1– 3 kW·m−2. In addition, stability testing also confirms that the CPCMs have excellent leakage-proof properties, thermal stability, and cyclic stability for long-term utilization. This work can provide an efficient route to synthesize high-performance CPCMs for solar thermal applications.

Physical and mechanical properties of foam-type panels manufactured from recycled cardboard

This study investigated different properties of foam-type composites made from recycled cardboard. Two types of panels, type A (237.36 kg/m3) and type B (284.28 kg/m3) were manufactured varying the share of corn starch as binder. Dimensional stability to water, vertical density profile (VDP), thermal conductivity, bending characteristics, and internal bond strength tests were conducted. The results revealed that the higher corn starch content improved the modulus of elasticity (MOE), modulus of rupture (MOR), and internal bond strength (IB) by 56%, 33%, and 30% for panel type B, respectively. However, no significant difference was observed for the internal bonding strength test. Statistical analysis revealed that no substantial difference in the thermal conductivity property of the two types of panels was determined at a 95% confidence level. The water absorption and thickness swelling increased for both samples after 24 h water immersion. Foam-type panels made from waste cardboard would have the potential to be used for certain applications such as insulation purposes within the perspective of sustainability and a green approach.

Prognostic analysis of thermal interface material effects on anisotropic heat transfer characteristics and state of health of a 21700 cylindrical lithium-ion battery module

In this study, a semi-empirical computational heat transfer model has been developed to simulate anisotropic thermal characteristics in nickel-rich rechargeable 21700 lithium-ion battery modules with thermal interface materials. A set of conservation equations were developed and numerically solved using a finite-volume-based computational fluid dynamics technique. Experimental measurements of direct current internal resistance, temperature-dependent open circuit potential, and electrochemical impedance were conducted to obtain thermal and electrical data prior to the development of battery heat generation and performance aging models. Detailed reversible and irreversible heat generation rates in battery modeling could be fully addressed and evaluated from the experimental data for post-extensive numerical analysis. After validating the developed battery models against measured experimental data, further numerical analyses were performed to evaluate the effect of thermal interface materials on the battery module temperatures and the state of health. Numerical results showed that the cooling performance of the battery module was improved by reducing interfacial thermal contact resistances after the thermal interfacing application. Moreover, it was found that the parallel thermal conductivity of the interfacing materials to bottom cooling channels is primarily related to the uniform temperature distribution of the battery module, whereas the vertical thermal conductivity is rather associated with maximum temperature. These features are clearly apparent with increased coolant flow rate and thermal interfacing thickness. Lastly, the state of health of the battery module can be enhanced with improved temperature uniformity and decreased maximum temperature of the module when the thermal interfacing materials are employed.

Steppingstone-inspired construction of high vertical thermal conductivity material with low carbon fiber content

Thermal interface materials (TIMs) with good thermal conductivity are paramount in mitigating the heat concentration challenges encountered during the operation of highly integrated components in sophisticated electronic devices. To optimize the comprehensive performance of TIMs, a balance must be struck: minimizing the filler concentration to attenuate materials hardness while maximizing filler content to bolster the thermal conduction pathway. Drawing inspiration from the orientation of tree branches passing through steppingstones in river, a method was proposed in this study. This method exploits the shear effect of carbon fiber (CF), owing to viscosity variances during diameter extrusion, and the differential flow velocity between CF and alumina to induce a significant degree of orientation. Combined with subsequent flipping and bonding, a TIM with vertically oriented CF was prepared. The TIM was obtained with a mere 12.1 wt% CF incorporation, the composite exhibits a through-plane thermal conductivity of 21.29 W/(m·K), representing an enhancement of two orders of magnitude relative to pristine silicone rubber, while retaining its flexibility and deformability. The orientation degree and high efficiency orientation effect of CF have been characterized via scanning electron microscopy (SEM), thermal conductivity test and light-emitting diode (LED) temperature rise test.

Heat transfer in phase change materials for integrated batteries and power electronics systems

An integrated batteries and power electronics system has great potential in improving the compactness, flexibility and multifunctionality in electrical vehicles. However, it needs to overcome thermal management challenges due to different operation temperatures of batteries and power electronics. To tackle this issue, this paper presents the first systematic study on the heat transfer characteristics in phase change materials (PCMs) based thermal management system sandwiched between two significantly different constant heat sources, through an experimentally validated transient three-dimensional heat transfer model. All key parameters of PCMs affecting the system are identified through a new dimensionless formulation, including the ratio of horizontal and vertical thermal conductivity kxy and kz, the aspect ratio (ratio of thickness and length), and Jakob number Ja (ratio of sensible heat and latent heat). Modelling results show that the system operation duration τd increases monotonically with the increase of kxy. However, increasing kz is not always benificial for τd and there is an optimal value for kz when keeping kxy unchanged. In addition, the increase of PCMs thickness can prolong τd monotonically. The reduction of Ja, e.g. increasing latent heat, is always beneficial for τd.

Enhancing vertical thermal conductivity of carbon fiber reinforced polymer composites using cauliflower-shaped copper particles

Carbon fiber reinforced polymer (CFRP) composites are lightweight and mechanically robust, and therefore have received considerable attention with a wide range of applications from the aerospace to automotive industries. However, their low vertical thermal conductivity seriously limits the potential applicability of high-performance CFRPs in metal replacement. Herein, we propose a simple and effective, scalable strategy to fabricate thermally conductive and mechanically robust CFRP composites on the basis of formation of vertical heat transfer pathways using pressure-assisted thermal migration of cauliflower-shaped dendritic copper particles. A new type of CFRP composite was achieved by facile layer-by-layer coating of prepregs with dendritic coppers and subsequent pressure-assisted thermal processing under elevated temperature. The cauliflower-shaped dendritic copper particles efficiently infiltrated into the CFRP matrix and thereby provided an effective heat transfer pathway, resulting in a CFRP composite with an excellent vertical thermal conductivity of 2.321 W m−1 K−1, which is 155% higher than that of pristine CFRP. Moreover, the proposed CFRP composite afforded a high thermal diffusivity of 1.125 × 10−6 m2 s−1 and a low specific heat capacity of 1.267 J g−1 K−1, as well as an excellent flexural strength of about 2.4 GPa.

Pie-rolling-inspired construction of vertical carbon fiber high thermal conductivity hybrid networks

Carbon fiber (CF) with a high thermal conductivity (Tc) of 1100 W m−1 K−1 along its one-dimensional (1D) direction is considered a promising filler for fabricating high-performance thermal interface material (TIM). However, Tc in the radial direction of CF is far less than 10 W m−1 K−1 determines that Tc highly depends on the orientation of CF. In this study, polydimethylsiloxane (PDMS)/short carbon fibers (SCFs)/Al spherical particle (PDMS/SCFs/Al) composite is firstly prepared. SCFs are then arranged into horizontal (0°), inclined (45°), and vertical (90°) orientations, respectively, utilizing a convenient “pie-rolling” method that does not rely on any specific instrument. As a result, the vertically oriented SCFs together with Al spherical particles establish an effective thermal conductivity-three-dimensonal (3D) network, and the Tc of the through-plane is as high as 10.46 W m−1 K−1, while the in-plane Tc is 6.23 W m−1 K−1 measured from a steady-state method. The anisotropic thermal conductivity is also verified by a hot-disk method. The working mechanism and thermal conductivity of oriented SCFs and Al spherical particles composites are being studied using finite element simulation. In addition, the change in surface temperature of the composites during heating and cooling stage is observed using an infrared thermal imaging camera. A 16 ℃’s temperature decline demonstrates that the high-efficiency heat transfer along the vertical orientated carbon fiber-based 3D network was successfully realized in SCF-90 when it was used as a TIM between a bare die and heat pipe of a laptop. This work illustrates the prospect of using SCFs to prepare a high thermally conductive 3D network could be used in the future thermal management of electronic devices.

Simultaneously improving vertical thermal conductivity and oil resistance of carbon fiber reinforced composite through layer-by-layer plasma surface treatment

Carbon fiber reinforced composite (CFRC) has attracted considerable attention as a robust high-performance material with outstanding strength-to-weight ratio and is widely used in various applications from aerospace to automotive industries. However, this composite commonly suffers from low vertical thermal conductivity and poor oil resistance, thereby limiting the applicability of lightweight CFRC in metal replacement. Here, we present a facile and effective approach to simultaneously improve vertical thermal conductivity and oil resistance of CFRC on the basis of formation of surficial and interfacial hydrophilic oxide layers in the stacked prepregs using the layer-by-layer plasma surface treatment. Thermally conductive and oil-resistant composite was achieved by simple layer-by-layer plasma surface treatment on individual prepregs and subsequent high-pressure compression of plasma-treated prepregs under elevated temperature. It is experimentally revealed that densely interlocked laminated CFRC with rough and oxidized multiple interlayers where interpenetrated oxide moieties are vertically distributed minimizes interfacial thermal resistance between stacked prepregs, thereby offering an efficient heat dissipation path in a through-thickness direction. Furthermore, the plasma-induced hydrophilic modification of both surface and interlayer endows the CFRC with excellent oil resistance. Consequently, this facile approach affords high vertical thermal conductivity and remarkable oil resistance, as well as an excellent mechanical strength. Compared to conventional CFRC, our versatile composite accomplishes excellent vertical thermal conductivity with 135% enhancement after oil absorption and outstanding oil resistance with 240% improvement.

Enhancement of the vertical thermal conductivity in conductive silicone composites using hybrid fillers and double-sided coating process

Herein, we propose an effective method for improving the vertical thermal conductivity of conductive silicone composites using thermally conductive hybrid fillers and a double-sided coating process. By applying the resin mixing and screen-printing method instead of the conventional spray coating, it was possible to prevent the shrinkage of the conductive composite fabric. It was found that a higher graphene content in the hybrid filler resulted in a higher thermal conductivity for the conductive composites. Moreover, a double-sided coating of conductive silicone additionally improved the vertical thermal conductivity of the fabric due to the formation of a double-sided heat transfer network.

Significantly enhanced thermally conductive epoxy composite composed of caterpillar-like structured expanded graphite/ boron nitride nanotubes

The trend toward miniaturization, integration and multifunctionality of modern electronics has led to a rapid increase in power density, which makes heat dissipation a critical issue. Despite the great potential of graphite-related nanocomposites in dissipating excess heat to ensure high efficiency and long lifetime of electronic devices, the practical application of these composites is limited by the ultra-low vertical thermal conductivity due to the interfacial thermal resistance between graphite layers. Here, a caterpillar-like hybrid filler was fabricated by the in situ intercalation of boron nitride nanotubes (BNNTs) between expanded graphite (EG) layers based on chemical vapor deposition technology. Owing to the optimized interfacial thermal resistance by forming covalent C-N bonding at the interface of EG and BNNT, the through-plane thermal conductivity of epoxy-based nanocomposites can be up to 5.18 Wm−1 K−1. In addition, the composite possessed electromagnetic interference shielding performance of 33.34 dB while maintaining electrical insulation due to the hierarchical structure. This work provided a new strategy for fabricating polymer-based composites with excellent through-plane thermal conductivity in thermal management applications.

Improved thermal conductivities of vertically aligned carbon nanotube arrays using three-dimensional carbon nanotube networks

Vertically aligned carbon nanotube (VACNT) arrays are considered promising thermal interface materials (TIM) due to their superior vertical thermal conductivity. However, the weak interaction between adjacent carbon nanotubes (CNT) leads to the change in orientation of the CNT during polymer infiltration. Therefore, the thermal conductivity of a CNT array/polymer composite does not reach the expected value. Simultaneously, the air gap between adjacent CNT hinders the in-plane heat transfer, and both of the above aspects significantly reduce the thermal conductivities of VACNT-based TIM. In this study, we proposed a novel three-dimensional CNT (3DCNT) network structure comprising VACNT crosslinked by randomly-oriented secondary CNT to provide several in-plane phonon paths and maintain the structural stability of the CNT arrays during polymer infiltration. The network structures formed by the secondary CNT were compared for two different growth times: 10 and 20 min. The 3DCNT network that grew for 20 min exhibited a denser network structure compared to the network that grew for 10 min. The vertical thermal conductivity and horizontal thermal conductivity of the 3DCNT20/Polydimethylsiloxane (3DCNT20/P) composite was 544.77% and 56.06% higher than those of the IVACNT/P composites, respectively.

Chloroform-Assisted Rapid Growth of Vertical Graphene Array and Its Application in Thermal Interface Materials.

With the continuous progress in electronic devices, thermal interface materials (TIMs) are urgently needed for the fabrication of integrated circuits with high reliability and performance. Graphene as a wonderful additive is often added into polymer to build composite TIMs. However, owing to the lack of a specific design of the graphene skeleton, thermal conductivity of graphene‐based composite TIMs is not significantly improved. Here a chloroform‐assisted method for rapid growth of vertical graphene (VG) arrays in electric field‐assisted plasma enhanced chemical vapor deposition (PECVD) system is reported. Under the optimum intensity and direction of electric field and by introducing the highly electronegative chlorine into the reactor, the record growth rate of 11.5 µm h−1 is achieved and VG with a height of 100 µm is successfully synthesized. The theoretical study for the first time reveals that the introduction of chlorine accelerates the decomposition of methanol and thus promotes the VG growth in PECVD. Finally, as an excellent filler framework in polymer matrix, VG arrays are used to construct a free‐standing composite TIM, which yields a high vertical thermal conductivity of 34.2 W m−1 K−1 at the graphene loading of 8.6 wt% and shows excellent cooling effect in interfacial thermal dissipation of light emitting diode.

Double-layered montmorillonite/MoS2 aerogel with vertical channel for efficient and stable solar interfacial desalination

The shortage of fresh water is becoming a great challenge all over the world, solar desalination is an emerging green and sustainable strategy to produce clean water from seawater or waste water by utilization of renewable solar energy. However, the accumulation of salt will depress the long-term sustainable use of solar evaporators. In this work, a hydrogel evaporator with montmorillonite (Mt) and MoS2 integrated layered structure has been fabricated for stable and efficient solar steam generation. The perfect hydrophilicity of Mt. can facilitate water transmission and salt redissolution speed. The vertical radial porous structure and low thermal conductivity of Mt. hydrogel ensures the shortest water transport path and superior heat localization performance. The upper MoS2 layer has a light absorption rate of up to 92% in the entire solar spectrum (200–2500 nm), which enables an excellent solar to thermal conversion efficiency. Under a low solar irradiation intensity of only 1 kW·m−2, the Mt./MoS2 double-layer aerogel can achieve a high evaporation rate of 1.31 kg·m−2·h−1. More importantly, the Mt./MoS2 double-layer aerogel can produce freshwater with salt ions (Na+, K+, Mg2+, Ca2+) rejection rate over than 99.9%. The as-prepared solar evaporator aerogel is turned out to show good mechanical strength, superior water supply, excellent durability and outstanding desalination performances. As a result, the developed Mt./MoS2 double-layer aerogel with low cost and high-quality of condensed water may provide a good prospect in solar desalination application and wastewater treatment.

Preparation and Thermal Conductivity of FG/PP Composites

The surface modified flake graphite (FG) was filled into polypropylene (PP) and mixed by a double-roll open mill. FG/PP composites with high thermal conductivity were prepared by compression molding. Studies had shown that the modification of FG by coupling agent was beneficial to the dispersion in FG/PP composites. The effect was better when the amount of NDZ-201 was 0.7wt%. With the increase of FG particle size, the thermal conductivity of FG/PP composites increased obviously. Compared with 17μm FG, the average, vertical and parallel thermal conductivity of FG/PP composites prepared with 148μm FG increased by 65.9%, 87.7% and 47.0% respectively. The thermal conductivity of FG/PP composites were improved with the increase of FG content. When FG content was 70wt%, the thermal conductivity increased significantly, the average, vertical and parallel thermal conductivity reached 4.16 W·m-1·K-1, 0.938 W·m-1·K-1 and 18.4 W·m-1·K-1.

Disordered graphite platelets in polypropylene (PP) matrix by spherical alumina particles: Increased thermal conductivity of the PP/flake graphite composites

The strategy of filler hybriding has been widely used to enhance the thermal or electrical conductivities of polymer composites. Flake graphite (FG) has a very high thermal conductivity, but the flaky structure makes it prone to generate planar orientation in polymer matrix during melt processing. This inevitably leads to significant difference in thermal conductivities along the in-plane and through-plane directions of molded sheet samples. The arrangement of FG in parallel induces the limited thermal conductivity along the vertical direction and this poses a great limitation for the real application. In this work, spherical alumina (Al2O3) was introduced into the highly filled (≥30 wt%) polypropylene (PP)/FG composites. It was found that a small content of alumina replacing the FG significantly increases the thermal conductivity on the through-plane direction of the sheet samples. The analysis indicated that spherical alumina particles effectively destroyed the orderly alignment of graphite platelets in the matrix, especially in the composites with high FG loadings. More interconnection between the FG platelets was therefore achieved on the thickness direction of the composites and the composites exhibited enhanced thermal conductivity along the through-plane direction. This work endows a new strategy for composites with two-dimensional functional fillers to improve the vertical thermal conductivity.

Evaluating Variability of Ground Thermal Conductivity within a Steep Site by History Matching Underground Distributed Temperatures from Thermal Response Tests

The variability of ground thermal conductivity, based on underground conditions, is often ignored during the design of ground-source heat pump systems. This study shows a field evidence of such site-scale variations through thermal response tests in eight borehole heat exchangers aligned at a site on a terrace along the foothills of mountains in northern Japan. Conventional analysis of the overall ground thermal conductivity along the total installation length finds that the value at one borehole heat exchanger is 2.5 times that at the other seven boreholes. History matching analysis of underground distributed temperature measurements generates vertical partial ground thermal conductivity data for four depth layers. Based on the moving line heat source theory, the partial values are generally within a narrow range expected for gravel deposits. Darcy velocities of groundwater are estimated to be 74–204 m/y at the borehole with high conductivity, increasing in the shallow layers above a depth of 41 m. In contrast, the velocities at the other seven boreholes are one-to-two orders of magnitude smaller with no trend. These high and low velocity values are considered for the topography and permeability. However, the relatively slow groundwater velocities might not apparently increase the partial conductivity.