Large Thermal Conductivity Research Articles

Study on heat extraction characteristics in a rock fracture for the application of enhanced geothermal systems

Enhanced geothermal systems (EGS) technologies aim to extract geothermal energy from hot dry rock (HDR). Heat extraction performance is of great importance for thermal efficiency and life cycle of EGS. It is intuitively considered that EGS reservoir modeling should be established based on accurate measured data of thermal and hydraulic properties of rock matrix and fluid, such as thermal conductivity, heat capacity and fracture permeability, however, which are time-consuming to obtain in some projects. This paper aims to clarify the influence of these parameters on heat extraction performance, which can provide basis for reservoir model simplification during preparation process of engineering scheme. Numerical simulations were carried out based on a rough fracture of granite rock at core scale, which has been validated by amount of experiments. First, the differences between simulation results and the previous experiment results were presented and analyzed, for variations of heat extraction rate per flow rate. Compared with the method with constant flow rate, the compensating method with various flow rate greatly increased the heat extraction rate and postpone thermal breakthrough time. Therefore, a simplified method was proposed in this paper to optimize the flow rate or predict the heat extraction rate when EGS operation with various flow rates. The method are applicable for geothermal reservoir management for efficient and stable heat extraction. Finally, the effects of properties of rock mass and fracture were studied. The production temperature and heat extraction rate were larger with larger thermal conductivity and heat capacity. It was found that the maximum influence on heat extraction rate were less than 20% and 10% within the common value ranges of thermal conductivity and heat capacity, respectively. There were almost no differences in the heat transfer process in fractures with different fracture permeabilities. It can be concluded that the prediction error of heat extraction performance is tolerable for engineering reservoir simulations, even without precise experimental test for thermal and hydraulic properties variations with temperature or pressure.

Self-Exfoliation of Flake Graphite for Bioinspired Compositing with Aramid Nanofiber toward Integration of Mechanical and Thermoconductive Properties

HighlightsA self-grinding exfoliation strategy that depends on mutual shear friction between flake graphite particles is successfully developed to prepare pristine graphene with largely enhanced yield and productivity.Bioinspired assembly of pristine graphene nanosheets to an interconnected aramid nanofiber network is achieved by a continuous sol-gel-film transformation strategy and generates a flexible yet highly thermoconductive film.Flexible yet highly thermoconductive materials are essential for the development of next-generation flexible electronic devices. Herein, we report a bioinspired nanostructured film with the integration of large ductility and high thermal conductivity based on self-exfoliated pristine graphene and three-dimensional aramid nanofiber network. A self-grinding strategy to directly exfoliate flake graphite into few-layer and few-defect pristine graphene is successfully developed through mutual shear friction between graphite particles, generating largely enhanced yield and productivity in comparison to normal liquid-based exfoliation strategies, such as ultrasonication, high-shear mixing and ball milling. Inspired by nacre, a new bioinspired layered structural design model containing three-dimensional nanofiber network is proposed and implemented with an interconnected aramid nanofiber network and high-loading graphene nanosheets by a developed continuous assembly strategy of sol–gel-film transformation. It is revealed that the bioinspired film not only exhibits nacre-like ductile deformation behavior by releasing the hidden length of curved aramid nanofibers, but also possesses good thermal transport ability by directionally conducting heat along pristine graphene nanosheets.Graphical abstract

Fabrication of energy-saving MgO with large grain size and low thermal conductivity: Towards a new type of magnesia for high-temperature furnaces

The development of advanced refractories with low thermal conductivity and long service life is of great significance for optimizing the energy consumption structure of high-temperature industries. For this purpose, a series of magnesia refractories with large grain size, low thermal conductivity, and high slag resistance had been successfully prepared at 1600 °C, in which Al2O3 and La2O3 were chosen as the additives. The results show that with the introduction of additives, MgAl2O4 and LaAlO3 as the second phase are formed inside and at the grain boundaries of the MgO matrix grains, and further promote the growth of grains and improved certain properties. Meanwhile, the grown grains (from 4.92 to 29.51 μm, increased ∼5 times), the increased density, and the enhanced strength can be attributed to the activated sintering triggered by Al2O3; the lower thermal conductivity (from 18.49 to 15.73 W·m−1·K−1 at 500 °C, decreased ∼15%) and the better slag resistance can be ascribed to the formation of LaAlO3 and MgAl2O4. However, due to the Nener effect, the maximum grain size is obtained at additives of 4 wt%, but the thermal conductivity is not limited by this.

Resistivity and thermal conductivity of an organic insulator β′–EtMe3Sb[Pd(dmit)2

A finite residual linear term in the thermal conductivity at zero temperature in insulating magnets indicates the presence of gapless excitations of itinerant quasiparticles, which has been observed in some candidate materials of quantum spin liquids (QSLs). In the organic triangular insulator β′–EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature thermal conductivity depends on the cooling process and the finite residual term is observed only in samples with large thermal conductivity. Moreover, the cooling rate dependence is largely sample dependent. Here we find that, while the low-temperature thermal conductivity significantly depends on the cooling rate, the high-temperature resistivity is almost perfectly independent of the cooling rate. These results indicate that in the samples with the finite residual term, the mean free path of the quasiparticles that carry the heat at low temperatures is governed by disorders, whose characteristic length scale of the distribution is much longer than the electron mean free path that determines the high-temperature resistivity. This explains why recent X-ray diffraction and nuclear magnetic resonance measurements show no cooling rate dependence. Naturally, these measurements are unsuitable for detecting disorders of the length scale relevant for the thermal conductivity, just as they cannot determine the residual resistivity of metals. Present results indicate that very careful experiments are needed when discussing itinerant spin excitations in β′–EtMe3Sb[Pd(dmit)2]2.

Graphene oxide: Synthesis, Characterization and its Applications

Graphene oxide, one of the allotropes of carbon, has sp2 hybridized carbon atoms arranged in a two-dimensional honeycomb lattice. In last three to four decades, lot of research interest has been shown in it by virtue of its large surface area, high thermal and electrical conductivity. It has versatile and tunable carbon backbone which can lead to facile functionalization and incorporation into a variety of products. It has been extensively studied for applications in electronics, catalysis, sensors, energy conversion and storage etc. Besides these versatile applications and properties, graphene itself cannot achieve the goal, it needs incorporation of different other nanomaterials and vice- versa. Hereby we tried to give a mini but critical analysis of graphene oxide synthesis, its structure elucidation, characterization and application in various fields.

Thermodynamic effects in a gas modulated Invar-based dual Fabry–Pérot cavity refractometer

By measuring the refractivity and the temperature of a gas, its pressure can be assessed from fundamental principles. The highest performing instruments are based on Fabry–Perot cavities (FPC). Gas modulation refractometry (GAMOR) is a methodology that has the ability to reduce the influence of disturbances to such an extent that high-precision (sub-parts-per-million) assessments of pressure can be made by the use of FPCs of Invar. To allow for high accuracy assessments, it is of importance to assess the uncertainty contribution from the thermodynamic effects that are associated with the gas filling and emptying of the cavity (pV-work). This paper presents a detailed scrutiny of the influence of the gas exchange process on the assessment of gas temperature on an Invar-based dual-FPC (DFPC) instrumentation. It is shown that by virtue of a combination of a number of carefully selected design entities (a small cavity volume with a bore radius of 3 mm, a spacer material with high heat capacitance, large thermal conductivity, and no regions that are connected with low thermal conductance, i.e. no heat islands, and a continuous assessment of temperature of the cavity spacer) the system is not significantly affected by pV-work. Simulations show that 10 s after the filling all temperature gradients in the system are well into the sub-mK range. Experiments support that refractivity assessments initiated after 40 s are not significantly affected by the pV-work. The analysis given in this work indicates that an upper limit for the influence of pV-work on the Invar-based DFPC system using 100 s long gas modulation cycles is 0.5 mK/100 kPa (or 1.8 ppm/100 kPa). Consequently, thermodynamic effects will not be a limiting factor when the Invar-based DFPC GAMOR system is used for assessments of pressure or as a primary pressure standard up to atmospheric pressures.

Valence Disproportionation of GeS in the PbS Matrix Forms Pb5Ge5S12 Inclusions with Conduction Band Alignment Leading to High n-Type Thermoelectric Performance.

Converting waste heat into useful electricity using solid-state thermoelectrics has a potential for enormous global energy savings. Lead chalcogenides are among the most prominent thermoelectric materials, whose performance decreases with an increase in chalcogen amounts (e.g., PbTe > PbSe > PbS). Herein, we demonstrate the simultaneous optimization of the electrical and thermal transport properties of PbS-based compounds by alloying with GeS. The addition of GeS triggers a complex cascade of beneficial events as follows: Ge2+ substitution in Pb2+ and discordant off-center behavior; formation of Pb5Ge5S12 as stable second-phase inclusions through valence disproportionation of Ge2+ to Ge0 and Ge4+. PbS and Pb5Ge5S12 exhibit good conduction band energy alignment that preserves the high electron mobility; the formation of Pb5Ge5S12 increases the electron carrier concentration by introducing S vacancies. Sb doping as the electron donor produces a large power factor and low lattice thermal conductivity (κlat) of ∼0.61 W m-1 K-1. The highest performance was obtained for the 14% GeS-alloyed samples, which exhibited an increased room-temperature electron mobility of ∼121 cm2 V-1 s-1 for 3 × 1019 cm-3 carrier density and a ZT of 1.32 at 923 K. This is ∼55% greater than the corresponding Sb-doped PbS sample and is one of the highest reported for the n-type PbS system. Moreover, the average ZT (ZTavg) of ∼0.76 from 400 to 923 K is the highest for PbS-based systems.

Simulation of CO2 Capture Process in Flue Gas from Oxy-Fuel Combustion Plant and Effects of Properties of Absorbent

Oxy-fuel combustion technology is an effective way to reduce CO2 emissions. An ionic liquid [emim][Tf2N] was used to capture the CO2 in flue gas from oxy-fuel combustion plant. The process of the CO2 capture was simulated using Aspen Plus. The results show that when the liquid–gas ratio is 1.55, the volume fraction of CO2 in the exhaust gas is controlled to about 2%. When the desorption pressure is 0.01 MPa, desorption efficiency is 98.2%. Additionally, based on the designability of ionic liquids, a hypothesis on the physical properties of ionic liquids is proposed to evaluate their influence on the absorption process and heat exchanger design. The process evaluation results show that an ionic liquid having a large density, a large thermal conductivity, and a high heat capacity at constant pressure is advantageous. This paper shows that from capture energy consumption and lean circulation, oxy-fuel combustion is a more economical method. Furthermore, it provides a feasible path for the treatment of CO2 in the waste gas of oxy-fuel combustion. Meanwhile, Aspen simulation helps speed up the application of ionic liquids and oxy-fuel combustion. Process evaluation helps in equipment design and selection.

Responsive Liquid Metal Droplets: From Bulk to Nano.

Droplets exist widely in nature and play an extremely important role in a broad variety of industrial processes. Typical droplets, including water and oil droplets, have received extensive attention and research, however their single properties still cannot meet diverse needs. Fortunately, liquid metal droplets emerging in recent years possess outstanding properties, including large surface tension, excellent electrical and thermal conductivity, convenient chemical processing, easy transition between liquid and solid phase state, and large-scale deformability, etc. More interestingly, liquid metal droplets with unique features can respond to external factors, including the electronic field, magnetic field, acoustic field, chemical field, temperature, and light, exhibiting extraordinary intelligent response characteristics. Their development over the past decade has brought substantial breakthroughs and progress. To better promote the advancement of this field, the present article is devoted to systematically summarizing and analyzing the recent fundamental progress of responsive liquid metal droplets, not only involving droplet characteristics and preparation methods, but also focusing on their diverse response behaviors and mechanisms. On this basis, the challenges and prospects related to the following development of liquid metal droplets are also proposed. In the future, responsive liquid metal droplets with a rapid development trend are expected to play a key role in soft robots, biomedicine, smart matter, and a variety of other fields.

집광형 태양열 발전 시스템에서의 태양열유속 분포의 변화에 따른 열전달 수치해석 연구

Concentrating solar power system (CSP) is one of the renewable energies which accumulates solar energy concentrated by condensing mirrors and reflectors in solar absorbers. Boiled water then operates steam turbines to generate electricity. So far, many studies have been conducted for optimization design of CSP which are mostly focused on comparing the materials and dimensions of the absorber to increase thermal efficiency. In this study, we tried to compare heat transfer in solar receiver from different heat flux distributions considering daily change of the azimuthal angle of the solar radiation. A Numerical study was performed using ANSYS FLUENT and appropriate temperature boundary condition was enforced at the receiver inlet using User-Defined-Function. Energy balance was modeled at the receiver inlet considering convectional and radiational loss to the surrounding. The amount of heat transfer to the receiver and heat loss to the surrounding from several characteristic heat flux distributions are compared. In addition, effects from the convective heat transfer coefficient, thermal conductivity, and porosity of the solar absorber are investigated. It has been found that eccentric distribution of the solar heat flux deteriorates thermal efficiency considerably. It is also found that heat transfer becomes larger with small convective heat transfer coefficient and large thermal conductivity. With lowering the porosity, the amount of heat transfer generally increases. But the gain is rather small at very low value with significantly high pressure drop.

Thermoelectric Properties of Pnma and Rocksalt SnS and SnSe

Thermoelectric materials convert waste heat to electricity and are part of the package of technologies needed to limit global warming. The tin chalcogenides SnS and SnSe are promising candidate thermoelectrics, with orthorhombic SnSe showing some of the highest figures of merit ZT reported to date. As for other Group IV chalcogenides, SnS and SnSe can form rocksalt phases under certain conditions, but the thermoelectric properties of these phases are largely unexplored. We have applied a fully ab initio modelling protocol to compare the ZT of the orthorhombic and rocksalt phases of SnS and SnSe. Electronic structures from hybrid density-functional theory were used to calculate the three electrical transport properties, including approximate models for the electron relaxation times, and lattice dynamics calculations were performed to model the phonon spectra and lattice thermal conductivities. We obtained good estimates of the ZT of the well-studied orthorhombic phases. The rocksalt phases were predicted to show larger electrical conductivities and similar Seebeck coefficients to the orthorhombic phases, resulting in higher thermoelectric power factors, but these were offset by larger thermal conductivities. These results therefore motivate further investigation of the recently discovered “π-cubic” phases of SnS and SnSe, which are based on distorted rocksalt supercells, to establish their thermoelectric performance.

Hydrodynamically enhanced thermal transport due to strong interlayer interactions: A case study of strained bilayer graphene

Manipulating interlayer interactions in two-dimensional (2D) materials is a widely used and effective method for regulating heat transport, which is important for thermal management of electronics. In this paper, we find that thermal transport can be enhanced by strong interlayer interactions. For instance, 2D bilayer materials with interlayer bonding have larger thermal conductivity than their monolayer counterparts. To further verify the conjecture, we take bilayer graphene (BLG) as a study case to gradually strengthen the interlayer interactions in BLG by applying a series of compressive strains along the out-of-plane direction. It is found that the thermal conductivity of BLG first decreases and then anomalously increases when the strain is larger than the threshold of 12%. For the decreasing trend, it is mainly due to the strain-induced reduction of phonon group velocity and lifetime. While at larger strain (>12%), the anomalous increase of thermal conductivity is found to be caused by hydrodynamic phonon transport and weak phonon anharmonicity. Thus, enhanced interlayer interactions can enhance the hydrodynamic phonon transport and thus improve the heat transfer performance. In this paper, we not only report the hydrodynamic enhancement of thermal transport through enhanced interlayer interactions but also provide solid evidence. These findings will deepen the understanding of thermal transport in layered materials and heterostructures, which will benefit the applications in thermal management.

Detection of Debonding Defects Between Radar Absorbing Material and CFRP Substrate by Microwave Thermography

Microwave thermography (MWT) is a non-destructive testing method based on the principle of microwave heating, which is widely used in the detection of debonding defects of various composite materials. Based on the principle of Microwave thermography (MWT), a resonant cavity microwave thermography detection system is proposed in this paper. The temperature variation characteristics of debonding defects in carbon Fiber Reinforced Polymer (CFRP) and radar absorbing material (RAM) are studied. This system consists of a resonator microwave source, an infrared thermal imager, a fan and two computers. This system successfully detects such defects at the pixel level by thermally loading the test piece, collecting the temperature change data of the test piece by the thermal imager, analyzing the temperature difference between the normal area and the defect area. In this paper, the advantages of microwave on the radar absorbing material (RAM) with large specific heat capacity and high thermal conductivity, high heating efficiency, large heating area and negligible damage to the radar absorbing material are verified, and the high performance of the detection system is verified. The experimental results show that microwave thermography can realize high-precision debonding detection and positioning of multilayer heterogeneous radar absorbing material (RAM).

A thermal conductivity sensor based on mixed carbon material modification for hydrogen detection.

In order to overcome the many shortcomings of traditional hot-wire thermal conductivity sensor design, a new design method was proposed in which a graphene-composite carbon nanotube mixed carbon material was used as a thermal conductivity sensor carrier instead of nano-alumina particles. Taking advantage of the large specific surface area and high thermal conductivity of graphene, as well as the characteristics of a large number of gas transport channels modified by carbon nanotubes, a high-efficiency gas heat exchange medium is made. In order to improve the consistency of the product, electrochemical preparation of an aluminum oxide film material is used to make the chip substrate of the thermal conductivity sensor by MEMS process technology, and the heating sensitive electrode of the sensor is made by a thick film process. Experiments show that the sensor prepared by this method has high sensitivity and zero point stability and has greatly improved the detection accuracy and response time. The sensitivity of the sensor to hydrogen detection increases to 3.287 mV/1%H2, and the response time is shorter than 5.4s. The research results have good application prospects.

Review on benefits, toxicity, challenges, and future of graphene-based face masks in the prevention of COVID-19 pandemic

The COVID-19 pandemic caused by SARS-CoV-2 has become a global public health concern. Recently, vaccines have been developed to treat this infectious disease. However, these newly developed vaccines are not widely available and not suitable for all age groups. In such circumstances, it is wise to wear personal protective equipment (PPE) such as masks, gloves, and gowns to better protect against COVID-19. Face masks have long been recommended as a means of preventing respiratory infections. However, inappropriate use of masks may undermine their effectiveness. The antimicrobial and antiviral properties of graphene have sparked interest in the development of medical devices such as face masks, gloves, and gowns with extra filtering ability to curb the effects of the coronaviruses. Their hydrophobicity, nanosize, large surface area, high electrical and thermal conductivities, and virulence are notable features that reduce the transmission of viruses from person to person via respiratory routes. Graphene-enhanced face masks are intended to encourage travelers to wear them at work and during recreational activities. Moreover, graphene can pose health hazards if inhaled during respiration. In this review, we summarize the current status of graphene and its promising applications for combating COVID-19. Additionally, this review aims to explore the quality of this biomaterial and possible suggestions for the better and safer use of graphene structured respirators.

Thin films of topological nodal line semimetals as a candidate for efficient thermoelectric converters

Thermoelectric materials intrigue much interest due to their wide range of application such as power generators and refrigerators. The efficiency of thermoelectric materials is quantified by the figure of merit, and a figure greater than unity is desired. To achieve this, a large Seebeck coefficient and low phonon thermal conductivity are required. We show that this can be achieved with a thin film of topological nodal line semimetals. We also discusses the correlation effect and spin current induced by a temperature gradient. The obtained results provide insight for the improvement of thermoelectric materials.

Enhanced thermal conductivity of silicone rubber via synergistic effects of polydopamine modification and silver deposition on boron nitride

It is great significance to develop polymeric composites with effective heat dissipation and high electrical insulation for devices in electronics fabrication. Herein, we fabricated silicone rubber (SR) composites with large thermal conductivity (TC) and high electrical insulation by utilizing the synergistic effects of polydopamine (PDA) modification and Ag nanoparticles deposition on the surface of BN sheets, denoted as BN-PDA-Ag/SR composites. Finally, the fabricated BN-PDA-Ag/SR composite at a filler content of 30 vol% exhibited a TC up to 0.75 W/(mK), which was about 5.76 times of pure SR, while it holded a low AC conductivity of 1.89 × 10-11 S/cm at 100 Hz. Overall, the synergistic effects of PDA modification and Ag deposition on BN sheets proposed in this work is an easy and eco-friendly avenue for the preparation of high-performance thermally conductive and insulating polymeric composites for advance electronic equipments.

High thermoelectric properties realized in earth abundant Bi2S3 bulk materials via Se and Cl co-doping in solution synthesis process

• Se and Cl co-doped nanostructured Bi 2 S 3 powders were successfully synthesized via hydrothermal. • A high σ value of 483 S cm −1 is obtained at 300 K for the Bi 2 S 2.4 Se 0.4 Cl 0.20 sample. • A high PFave. of 411 μW m −1 K −2 is achieved for the Bi 2 S 2.4 Se 0.4 Cl 0.20 . • The highest ZT value of 0.66 is achieved at 673 K for the Bi 2 S 2.4 Se 0.4 Cl 0.20 sample. Bi 2 S 3 -based alloys are considered promising thermoelectric materials due to their large Seebeck coefficient and low lattice thermal conductivity. However, low electrical conductivity usually leads to poor electrical transport properties, which seriously restricts their further application in thermoelectric refrigeration and/or power generation. In this work, Bi 2 S 3 with high electrical transport properties is synthesized hydrothermally via Se and Cl co-doping. The maximum electrical conductivity value of 483 S cm −1 was obtained for the Bi 2 S 2.4 Se 0.4 Cl 0.20 sample at room temperature. The significant improvement of electrical conductivity gives rise to a high average power factor of 411 μW m −1 K −2 during the measuring temperature range and a peak value of 456 μW m −1 K −2 at 673 K. Benefiting from the largely improved electrical transport properties, a superior ZT value of approximately 0.66 and ZT ave . of 0.36 were obtained for Bi 2 S 2.4 Se 0.4 Cl 0.20 , and the theoretically calculated conversion efficiency reached 5.7%. The results indicate that Bi 2 S 3 is a promising candidate for thermoelectric applications at medium temperatures. A high ZT value in Bi2S3 system of ∼0.66 was achieved at 673 K for Bi2S3 with Se and Cl co-doped through a facile hydrothermal followed by spark plasma sintering.

Ballistic electrical-thermal transport properties and their applications in graphene-nanoribbon-stacked heterojunctions

The thermal, electrical and thermoelectric properties are investigated in graphene-nanoribbon-stacked heterojunctions by applying the density functional theory combined with the nonequilibrium Green’s function technique. Our calculations show that layer-layer interaction (LLI) can form a large current and electron thermal conductance, and the current and electron thermal conductance are highly sensitive to the geometry details of the layer-stacked structure. Especially, negative differential resistance (NDR) behaviors can be found in the stacked nanoribbon structures. More importantly, different from two-dimensional materials that possess massless Dirac fermions (Zhou et al. (2016), the LLI in stacked quasi-monolayer nanoribbon structures can lead not only to low ratio Ke/(GT) (which is much lower than the constant L0 =2.44× 10−8 WΩ K−2) but also to the high seebeck coefficient near μ =0, which leads to very high figure-of-merit value: the figure-of-merit will be over 6.5 in 4-3.6-2.5-ZGNR and over 3.5 in 4-3.0-2.1-ZGNR at room temperature. The results shed light on its potential application in a multifunction device by applying the LLI, containing NDR effect and thermoelectric technologies.

Improved phase-field-based lattice Boltzmann method for thermocapillary flow.

In this paper, we present an improved phase-field-based lattice Boltzmann (LB) method for thermocapillary flows with large density, viscosity, and thermal conductivity ratios. The present method uses three LB models to solve the conservative Allen-Cahn equation, the incompressible Navier-Stokes equations, and the temperature equation. To overcome the difficulty caused by the convection term in solving the convection-diffusion equationfor the temperature field, we first rewrite the temperature equationas a diffuse equationwhere the convection term is regarded as the source term and then construct an improved LB model for the diffusion equation. The macroscopic governing equationscan be recovered correctly from the present LB method; moreover, the present LB method is much simpler and more efficient. In order to test the accuracy of this LB method, several numerical examples are considered, including the planar thermal Poiseuille flow of two immiscible fluids, the two-phase thermocapillary flow in a nonuniformly heated channel, and the thermocapillary Marangoni flow of a deformable bubble. It is found that the numerical results obtained from the present LB method are consistent with the theoretical prediction and available numerical data, which indicates that the present LB method is an effective approach for the thermocapillary flows.