Experimental investigation on the effective thermal conductivities of different hydrate-bearing sediments

The natural gas hydrate has been regarded as an important future green energy.Significant progress on the hydrate exploitation has been made,but some challenges are still remaining.In order to enhance the hydrate exploitation efficiency,a significant understanding of the effective thermal conductivity(ETC)of the hydrate-bearing sediment has become essential,since it directly controls the heat and mass transfer behaviors,and thereby determines the stability of hydrate reservoir and production rate.In this study,the effective thermal conductivities of various hydrate-bearing sediments were in-situ measured and studied.The impacts of temperature,particle size and type of sediment were investigated.The effective thermal conductivities of the quartz sand sediments before and after hydrate formation were in-situ measured.The results show the weak negative correlation of effective thermal conductivity of the quartz sand sediment on the temperature before and after the hydrate formation.The effective thermal conductivity of the hydrate-bearing sediment decreases with the increase of particle size of the sediment.The dominant effect of the type of porous medium on the characteristics of the effective thermal conductivity of hydrate-bearing sediment was highlighted.The results indicate that both the effective thermal conductivities of hydrate-bearing quartz sand sediment and hydrate-bearing silicon carbide sediment are weakly negatively correlated with temperature,but the effective thermal conductivity of hydrate-bearing clay sediment is weakly positively dependent on the temperature.In addition,the values of the effective thermal conductivities of various hydrate-bearing sediments are in the order of hydrate-bearing silicon carbide sediment>hydrate-bearing quartz sand sediment>hydrate-bearing clay sediment.These findings could suggest that the intrinsic thermal conductivity of porous medium could control the characteristics of effective thermal conductivity of hydrate-bearing sediment.

Strong anharmonicity-assisted low lattice thermal conductivities and high thermoelectric performance in double-anion Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)semiconductors

The thermoelectric properties of layered Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)materials are systematically investigated by first-principles approach.Soft transverse acoustic modes and direct Mo d–Mo d couplings give rise to strong anharmonicities and low lattice thermal conductivities.The double anions with distinctly different electronegativities of Mo_(2)AB_(2)monolayers can reduce the correlation between electron transport and phonon scattering,and further benefit much to their good thermoelectric properties.Thermoelectric properties of these Mo_(2)AB_(2)monolayers exhibit obvious anisotropies due to the direction-dependent chemical bondings and transport properties.Furthermore,their thermoelectric properties strongly depend on carrier type(n-type or p-type),carrier concentration and temperature.It is found that n-type Mo_(2)AB_(2)monolayers can be excellent thermoelectric materials with high electric conductivity,σ,and figures of merit,ZT.Choosing the types of A and B anions of Mo_(2)AB_(2)is an effective strategy to optimize their thermoelectric performance.These results provide rigorous understanding on thermoelectric properties of double-anions compounds and important guidance for achieving high thermoelectric performance in multi-anion compounds.

Molecular dynamics study of thermal conductivities of cubic diamond,lonsdaleite,and nanotwinned diamond via machine-learned potential

Diamond is a wide-bandgap semiconductor with a variety of crystal configurations,and has the potential applications in the field of high-frequency,radiation-hardened,and high-power devices.There are several important polytypes of diamonds,such as cubic diamond,lonsdaleite,and nanotwinned diamond(NTD).The thermal conductivities of semiconductors in high-power devices at different temperatures should be calculated.However,there has been no reports about thermal conductivities of cubic diamond and its polytypes both efficiently and accurately based on molecular dynamics(MD).Here,using interatomic potential of neural networks can provide obvious advantages.For example,comparing with the use of density functional theory(DFT),the calculation time is reduced,while maintaining high accuracy in predicting the thermal conductivities of the above-mentioned three diamond polytypes.Based on the neuroevolution potential(NEP),the thermal conductivities of cubic diamond,lonsdaleite,and NTD at 300 K are respectively 2507.3 W·m^(-1)·K^(-1),1557.2 W·m^(-1)·K^(-1),and 985.6 W·m^(-1)·K^(-1),which are higher than the calculation results based on Tersoff-1989 potential(1508 W·m^(-1)·K^(-1),1178 W·m^(-1)·K^(-1),and 794 W·m^(-1)·K^(-1),respectively).The thermal conductivities of cubic diamond and lonsdaleite,obtained by using the NEP,are closer to the experimental data or DFT data than those from Tersoff-potential.The molecular dynamics simulations are performed by using NEP to calculate the phonon dispersions,in order to explain the possible reasons for discrepancies among the cubic diamond,lonsdaleite,and NTD.In this work,we propose a scheme to predict the thermal conductivity of cubic diamond,lonsdaleite,and NTD precisely and efficiently,and explain the differences in thermal conductivity among cubic diamond,lonsdaleite,and NTD.

Drude, Hall and Maximal Conductivities: A Unified Complex Model

By adopting a complex formulation of Ohm’s law, we arrive at combined equations connecting the conductivities of conductors. The horizontal resistivity is equal to the inverse of Drude’s conductivity δo( ), and the vertical resistivity (ρy) is equal to the Hall’s conductivity ( δH). At high magnetic field, the horizontal conductivity becomes exceedingly small, whereas the vertical conductivity equals to Hall’s conductivity. The Hall’s conductivity is shown to represent the maximal conductivity of conductors. Drude’s and Hall’s conductivities are related by δo =δHωC , where ωC is the cyclotron frequency, and is the relaxation time. The quantization of Hall’s conductivity is attributed to the fact that the magnetic flux enclosed by the conductor is carried by electrons each with h/e, where h is the Planck’s constant and e is the electron’s charge. The Drude’s conductance is found to be equal to Hall's conductance provided the magnetic flux enclosed by the conductor is a multiple of h/e.

Influence of Metal Additives on the Electrical Conductivities of the Oxide Ceramics as an Electrode Material

The electrical conductivities are reported for various oxide ceramics and cermets at 1000°C.Adding metalcan greatly enhance the electrical condnctivities of the oxide materials.The conductivity of the ceramic addedwith metal depends on:(1)the conductivity σ_o of the oxides;(2)the content of metal additives;(3)thedispersion of the metal phase among oxide phase.The conductivity of the metal added does not affect the con-ductivity of the metal-containing ceramic.Although the metal-containing ceramic has much higher conductivi-ty than oxide ceramic,the change rate of their conductivities with temperature is similar and is controlled byE_g of the oxide.

On the Electrical and Thermal Conductivities of Cast A356/Al₂O₃Metal Matrix Nanocomposites

To assess the effect of the dispersion of Al2O3 nanoparticles into A356 Al alloy on both the electrical and thermal conductivities, A356/Al2O3 metal matrix nanocomposites (MMNCs) were fabricated using a combination of rheocasting and squeeze casting techniques. Two different sizes of Al2O3 nanoparticles were dispersed into the A356 Al alloy, typically, 60 and 200 nm with volume fractions up to 5 vol%. The effect of the nanoparticles size and volume fraction on the electrical and thermal conductivities was evaluated. The results revealed that the A356 monolithic alloy exhibited better electrical and thermal conductivities than the MMNCs. Increasing the nanoparticles size and/or the volume fraction reduces both the thermal and electrical conductivities of the MMNCs. The maximum reduction percent in the thermal and electrical conductivities, according to the A356 monolithic alloy, were about 47% and 38%, respectively. Such percentages were exhibited by A356/Al2O3MMNCs containing 5 vol% of nanoparticles having 60 and 200 nm, respectively.

Grain Boundary Conductivities of Peridotite at High Temperature and High Pressure

The grain boundary conductivities of dry synthetic peridotite are conducted by virtue of YJ-3000t multi-anvil press and Solartron-1260 Impedance /Gain-phase Analyzer at conditions of 4.0 GPa, 1373 K and frequency ranging from 10-2 to 106 Hz. The Fe2O3+Fe3O4,Ni+NiO,Fe+Fe3O4,Fe+FeO and Mo+MoO2 solid buffers are adopted to adjust

INVESTIGATION ON THE EFFECTIVE CONDUCTIVITIES OF CARBON/CARBON FIBER COMPOSITES WITH INHOMOGENEOUS INTERPHASE

Thermal conductivites of carbon/carbon fiber composites with inhomogeneous interphase are studied in thtis paper. the inhomogeneous interphase is modeled approximately as a multilayered structure consisting of many thin layers haying homogeneous properties, and close-formed solution of the effective conductivities of the composites is obtained by using the Mori-Tanaka mean-fieid

Microstructure and thermal and electric conductivities of high dense Mo/Cu composites

For electronic packaging applications, Mo/Cu composites with volume fractions of 55%, 60% and 67% Mo were fabricated by the patented squeeze-casting technology. The microstructures and thermal and electric conduction properties of the Mo/Cu composites were investigated. The results show that Mo particles are homogeneous and uniform, and the Mo-Cu interfaces are clean and free from interfacial reaction products and amorphous layers; the densifications of the Mo/Cu composites are higher than 99%. The thermal conductivities of Mo/Cu composites range from 220 to 270W/(m·℃) and decrease with an increase in volume fraction of Mo content. The thermal conductivities agree well with the predicted values of theoretical models. The electric conductivities of Mo/Cu composites are in the range of 2228MS/m and decrease with the increase of Mo content. The achievement of higher thermal and electric conduction is attributed to the full densities and high purity Mo/Cu composites, which are attained through the cost-effective squeeze-casting technology processes.

Evolution of copper nanowires through coalescing of copper nanoparticles induced by aliphatic amines and their electrical conductivities in polyester films

Copper nanowires were synthesized by the wet chemical reduction method using copper sulfate as the copper precursor,aliphatic amines(methylamine,ethanediamine,1,2-propanediamine)as the inducing reagents,and hydrazine hydrate as the reductant through the aging and reduction processes.The high-resolution transmission electron microscopy(HRTEM)images reveal that the copper nanowires were synthesized by coalescing extremely small-sized copper nanoparticles with the particle sizes of1–6 nm in copper complex micelles.A longer aging time period favored the coalescing of the copper nanoparticles to form thinner copper nanowires in the following reduction process.The coalescing extent of copper nanoparticles in copper nanowires was highly enhanced by ethanediamine and 1,2-propanediamine as compared with that by methylamine.The copper nanowire-filled polyester films had higher electrical conductivity than the copper nanoparticle-filled ones.

IONIC CONDUCTIVITIES OF SEGMENTED POLYETHER POLYURETHANEUREA COMPLEXES WITH LiClO_4

Ionic conductivity values for segmented polyether polyurethaneurea （PEUU） complexes with LiClO4 were determined and values as high as～1.1×10-4 S·cm-1 at 353K and～1.0×10-5S·cm-1 at 306K were achieved. The ionic conductivity data were analyzed using the VTF （Vogel-Tamman-Fulcher） equation and WLF （Williams-Landel-Ferry） type equation. Values have been estimated for the "apparent" activation energies of ion transport from VTF equation and they lie in the range 2.70—5.53 kJ·mol-1.

Two-step homogenization for the effective thermal conductivities of twisted multi-filamentary superconducting strand

For the accurate prediction of the effective thermal conductivities of the twisted multi-filamentary superconducting strand,a two-step homogenization method is adopted.Based on the distribution of filaments,the superconducting strand can be decomposed into a set of concentric cylinder layers.Each layer is a two-phase composite composed of the twisted filaments and copper matrix.In the first step of homogenization,the representative volume element(RVE)based finite element(FE)homogenization method with the periodic boundary condition(PBC)is adopted to evaluate the effective thermal conductivities of each layer.In the second step of homogenization,the generalized self-consistent method is used to obtain the effective thermal conductivities of all the concentric cylinder layers.The accuracy of the developed model is validated by comparing with the local and full-field FE simulation.Finally,the effects of the twist pitch on the effective thermal conductivities of twisted multi-filamentary superconducting strand are studied.

Vacuum current-carrying tribological behavior of MoS2-Ti films with different conductivities

Current-carrying sliding is widely applied in aerospace equipment,but it is limited by the poor lubricity of the present materials and the unclear tribological mechanism.This study demonstrated the potential of MoS_(2)-based materials with excellent lubricity as space sliding electrical contact materials by doping Ti to improve its conductivity.The tribological behavior of MoS_(2)-Ti films under current-carrying sliding in vacuum was studied by establishing a simulation evaluating device.Moreover,the noncurrent-carrying sliding and static current-carrying experiments in vacuum were carried out for comparison to understand the tribological mechanism.In addition to mechanical wear,the current-induced arc erosion and thermal effect take important roles in accelerating the wear.Arc erosion is caused by the accumulation of electric charge,which is related to the conductivity of the film.While the current-thermal effect softens the film,causing strong adhesive wear,and good conductivity and the large contact area are beneficial for minimizing the thermal effect.So the moderate hardness and good conductivity of MoS_(2)-Ti film contribute to its excellent current-carrying tribological behavior in vacuum,showing a significant advantage compared with the traditional ones.

Orderedisorder transition and thermal conductivities of the(NdSmEuGd)_((1-x)/2)Dy_(2x)Zr_(2)O_(7)series

Rare-earth zirconates with pyrochlore and fluorite structures have recently been identified as promising thermal barrier coating materials owing to their low thermal conductivities.In this study,six samples with the general formula(NdSmEuGd)_((1-x)/2)Dy_(2x)Zr_(2)O_(7)were synthesized to further reduce the thermal conductivity.X-ray diffraction and Raman spectroscopy showed that the transition from an ordered pyrochlore to a disordered fluorite structure is due to cation and anion disorder.Transmission electron microscopy showed that anion disorder occurred before cation disorder.A modified mass disorder parameter was introduced into this system,which can describe the change in thermal conductivity well.This parameter can be a basis for designing more complex materials with lower thermal conductivities.

Simultaneously enhancing the electronic and ionic conductivities of Li_(2)ZnTi_(3)O_(8)via modification with polyacrylonitrile-derived carbon for high-performance anodes

Polyacrylonitrile(PAN)with C≡N bonds can be converted to nitrogen-doped carbon during carbonization,which enhances electronic conductivity by compensating for the deficiency of the Li_(2)ZnTi_(3)O_(8)(LZTO)anode.In this study,LZTO was modified by carbonizing a homogeneous PAN/LZTO powder mixture at approximately 800℃for 5 h in nitrogen stream to uniformly coat nitrogen-doped carbon around the LZTO particles and to yield nitrogen-doped LZTO.PAN-60 exhibited a capacity retention of 74.8%as the current density increased from 0.1 to 1.6 A g−1,and had charge/discharge capacities of 250.1/250.8 mAh g−1 even after 1100 cycles at 0.5 A g−1.Structural and compositional analysis along with electrochemical tests showed that the uniform nitrogen-doped carbon coating and the nitrogen-doped LZTO favor electron transfer,while the defects induced by nitrogen-doping in LZTO promote Li-ion migration.The enhanced electronic and ionic conductivities are favorable to alleviate the polarization during cycling,and thus are responsible for the optimized performance.

Enhanced conductivity and weakened magnetism in Pb-doped Sr_(2)IrO_(4)

Group IV element Pb has been selected as the dopant to dope at the Sr site of Sr_(2)IrO_(4). It is exciting to find that the single-phase crystal structure could be maintained with a high Pb doping level of up to x=0.3 in Sr_(2-x)Pb_(x)IrO_(4). The mapping data obtained from energy-dispersive x-ray spectroscopy analyses give solid evidence that the Pb ions are uniformly distributed in the Sr_(2)IrO_(4) matrix. The incorporation of Pb leads to a moderate depression of the canted antiferromagnetic ordering state. The electrical conductivity could be greatly enhanced when the Pb doping content is higher than x=0.2.The present results give a fresh material base to explore new physics in doped Sr_(2)IrO_(4) systems.

Wide frequency phonons manipulation in Si nanowire by introducing nanopillars and nanoparticles

The combination of different nanostructures can hinder phonons transmission in a wide frequency range and further reduce the thermal conductivity(TC).This will benefit the improvement and application of thermoelectric conversion,insulating materials and thermal barrier coatings,etc.In this work,the effects of nanopillars and Ge nanoparticles(GNPs)on the thermal transport of Si nanowire(SN)are investigated by nonequilibrium molecular dynamics(NEMD)simulation.By analyzing phonons transport behaviors,it is confirmed that the introduction of nanopillars leads to the occurrence of lowfrequency phonons resonance,and nanoparticles enhance high-frequency phonons interface scattering and localization.The results show that phonons transport in the whole frequency range can be strongly hindered by the simultaneous introduction of nanopillars and nanoparticles.In addition,the effects of system length,temperature,sizes and numbers of nanoparticles on the TC are investigated.Our work provides useful insights into the effective regulation of the TC of nanomaterials.

A Generic Strategy to Create Mechanically Interlocked Nanocomposite/Hydrogel Hybrid Electrodes for Epidermal Electronics

Stretchable electronics are crucial enablers for next-generation wearables intimately integrated into the human body.As the primary compliant conductors used in these devices,metallic nanostructure/elastomer composites often struggle to form conformal contact with the textured skin.Hybrid electrodes have been consequently developed based on conductive nanocomposite and soft hydrogels to establish seamless skin-device interfaces.However,chemical modifications are typically needed for reliable bonding,which can alter their original properties.To overcome this limitation,this study presents a facile fabrication approach for mechanically interlocked nanocomposite/hydrogel hybrid electrodes.In this physical process,soft microfoams are thermally laminated on silver nanowire nanocomposites as a porous interface,which forms an interpenetrating network with the hydrogel.The microfoam-enabled bonding strategy is generally compatible with various polymers.The resulting interlocked hybrids have a 28-fold improved interfacial toughness compared to directly stacked hybrids.These electrodes achieve firm attachment to the skin and low contact impedance using tissue-adhesive hydrogels.They have been successfully integrated into an epidermal sleeve to distinguish hand gestures by sensing mus-cle contractions.Interlocked nanocomposite/hydrogel hybrids reported here offer a promising platform to combine the benefits of both materials for epidermal devices and systems.

Highly Thermoconductive,Strong Graphene‑Based Composite Films by Eliminating Nanosheets Wrinkles

Graphene-based thermally conductive composites have been proposed as effective thermal management materials for cooling high-power electronic devices.However,when flexible graphene nanosheets are assembled into macroscopic thermally conductive composites,capillary forces induce shrinkage of graphene nanosheets to form wrinkles during solution-based spontaneous drying,which greatly reduces the thermal conductivity of the composites.Herein,graphene nanosheets/aramid nanofiber(GNS/ANF)composite films with high thermal conductivity were prepared by in-plane stretching of GNS/ANF composite hydrogel networks with hydrogen bonds andπ-πinteractions.The in-plane mechanical stretching eliminates graphene nanosheets wrinkles by suppressing inward shrinkage due to capillary forces during drying and achieves a high in-plane orientation of graphene nanosheets,thereby creating a fast in-plane heat transfer channel.The composite films(GNS/ANF-60 wt%)with eliminated graphene nanosheets wrinkles showed a significant increase in thermal conductivity(146 W m^(−1)K^(−1))and tensile strength(207 MPa).The combination of these excellent properties enables the GNS/ANF composite films to be effectively used for cooling flexible LED chips and smartphones,showing promising applications in the thermal management of high-power electronic devices.

Spatiotemporal variations of sand hydraulic conductivity by microbial application methods

The spatiotemporal distributions of microbes in soil by different methods could affect the efficacy of the microbes to reduce the soil hydraulic conductivity.In this study,the specimens of bio-mediated sands were prepared using three different methods,i.e.injecting,mixing,and pouring a given microbial so-lution onto compacted sand specimens.The hydraulic conductivity was measured by constant-head tests,while any soil microstructural changes due to addition of the microbes were observed by scan-ning electron microscope(SEM)and mercury intrusion porosimetry(MIP)tests.The amount of dextran concentration produced by microbes in each type of specimen was quantified by a refractometer.Results show that dextran production increased exponentially after 5-7 d of microbial settling with the supply of culture medium.The injection and mixing methods resulted in a similar amount and uniform dis-tribution of dextran in the specimens.The pouring method,however,produced a nonuniform distri-bution,with a higher concentration near the specimen surface.As the supply of culture medium discontinued,the dextran content near the surface produced by the pouring method decreased dramatically due to high competition for nutrients with foreign colonies.Average dextran concentration was negatively and correlated with hydraulic conductivity of bio-mediated soils exponentially,due to the clogging of large soil pores by dextran.The hydraulic conductivity of the injection and mixing cases did not change significantly when the supply of culture medium was absent.