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Hydraulic fracturing has been widely implemented to enhance hydrocarbon production from shale reservoirs. However, one of the main challenges during hydraulic fracturing is to understand what factor(s) triggers high salinity of flowback water, which sometimes can be up to 300,000 mg/L. While several mechanisms have been proposed to explain the controlling factor behind the high salinity of flowback water, there has been little discussion about the effect of fluid-shale interactions (e.g., mineral dissolution and surface complexation) on the high salinity, and far too attention has been paid to quantify the contribution of fluid-shale interactions. We thus conducted spontaneous imbibition experiments using deionised water and outcrops from Marcellus, Barnett and Eagle Ford shale plays with minor in-situ precipitated salts. We also monitored the pH, electrical conductivity and ion concentrations (Cl-, K+, Ca2+, NO3-, F-, Br- and NH+) of the surrounding water during spontaneous imbibition process in consecut...

The wettability of shale oil reservoirs is an important petrophysical property to determine subsurface multiphase flow and residual oil saturation. While a few studies have been undertaken to under...

A high-performance anion exchange membrane (AEM) is critical for the development of alkaline fuel cell. In this work, AEMs with an interpenetrating polymer network (IPN) are synthesized. An electron microscope clearly reveals a highly efficient “ion channel” network, which is constructed with a small amount of cation exchange groups. This specially designed ion channel leads to extraordinary hydroxide conductivity (e.g., 257.8 mS cm -1 at 80 °C) of IPN AEMs at moderate ion exchange capacity ( IEC = 1.75 mmol g − 1 ), as well as excellent long-term alkaline stability at harsh condition which showed that 81% of original conductivity can be retained after a long time for 1248 hours. Moreover, a remarkable peak power density of 1.20 W cm -2 (0.1 MPa backpressure) with nonprecious metal (FeNx-CNTs) as oxygen reduction reaction (ORR) catalyst in a fuel cell test was achieved. This work offers a general strategy to prepare high-performance AEMs based on IPN structure design.

Thermoelectric devices can convert thermal energy directly into electrical energy. The aim of this study was to develop an approach for integrating computer-aided analysis with an optimization method that could be applied to the design and optimization of thermoelectric generators. The optimization framework consisted of a model generator, a direct solver, and a numerical optimizer. The simplified conjugate-gradient method (SCGM) was used to build the optimizer, and the general-purpose finite-element code was used for the direct solver and model generator. This approach was applied to the multi-objective and multi-parameter optimization of geometric thermoelectric generators to design an optimal structure for both a two-stage bismuth-telluride (BiTe)-based and skutterudite-based thermoelectric generator (TEG) module. The leg length and the ratio between the cross-sectional areas (i.e., footprint) of the semiconductor columns and the TEG module were found to significantly affect the TEG performance; hence, all were incorporated into the present optimization study. Multi-objective optimization was used to realize a design that properly balanced the power output and conversion efficiency so that both improved simultaneously.

A channel is a significant part of a proton exchange membrane fuel cell (PEMFC), and the configuration of the channel has a great effect on the mass transfer of the PEMFC, which directly influences the performance. In this study, a three-dimensional, single-phase, and non-isothermal model of a PEMFC with a single straight channel is developed. Based on the model, genetic algorithm (GA) is adopted to obtain an optimal design of the channel configuration. Power consumption of flow and output power of a PEMFC are considered into the objective function, and the width of bottom and top edges of the channel, in both the anode and cathode sides, are selected as variables. The optimal design obtained is a trapezoidal channel. At an operating potential of 0.4 V, the increment in current density in the optimal design is 10.92% compared to a basic design having a square channel. Moreover, the optimal design shows more uniform distributions of reactants and current density than the basic design.

Super-insulating aerogels are promising materials to improve the energy efficiency of buildings. However, fabricating super-insulating yet mechanically robust and shape-memory aerogels remains challenging. Here, we integrate graphene oxide and a block copolymer to fabricate hybrid aerogels with triple networks and systematically study their mechanical and thermal properties. We show that the first network serves as sacrificial bonds and dissipates energy upon deformation, enabling the aerogels to have a high mechanical performance. The second network allows the aerogels to memorize the original permanent shape, and the third network is able to store strain energy and fix the aerogels in a temporary shape by vitrification. Remarkably, the strong phonon-scattering effect generated by the enormous interfaces between the three networks yields an ultra-low solid thermal conductivity of ∼8 mW m−1 K−1. The multi-functionality makes this class of hybrid aerogels particularly suitable for super-insulation applications on complex surfaces and in small spaces of buildings, industry and spacecrafts.

We reported an interface-induced synthesis of iron nitrogen (FeNx) enriched carbon nanotubes (FeNx-CNTs) with high activity in alkaline and acidic environment for oxygen reduction reaction (ORR) by coating polypyrrole (PPy) thin layers on iron-doped zeolitic imidazolate framework-8 (Fe-ZIF). The constructed polymer/metal-organic framework (MOF) interface suppresses the agglomeration of iron (Fe) atomic sites during the thermal activation, and thus facilitates carbon nanotubes (CNTs) formations with FeNx coordination in carbon matrix. The produced FeNx-CNTs catalyst shows excellent ORR activities with extremely high peak power density of 1.15 and 1.16 W cm−2 in anion and proton exchange membrane fuel cells, respectively.

Interpenetrating polymer network anion exchange membrane (IPN AEM) consists of cross-linked quaternized poly (vinylbenzyl chloride) and cross-linked poly (vinyl alcohol) is synthesized in this work. Electron microscope clearly reveals the IPN structure, and with this structural design, a highly efficient ion channel within IPN AEMs is constructed. This specially designed structure leads to high hydroxide (OH−) conductivity (e.g., 141.7 mS cm−1 at 80 °C) at a moderate ion exchange capacity (IEC) of 1.61 mmol g−1, and a remarkable peak power density of 0.64 W cm−2 with non-noble metal oxygen reduction reaction (ORR) catalyst in H2/Air (CO2-free) anion exchange membrane fuel cells (AEMFCs) test.

Fluid-shale interactions appear to significantly affect shale micromechanical properties, which regulate fracture network propagation during multistage hydraulic fracturing in shales and thus the gas production. While published works confirm that fluid-shale interactions can reduce Young's modulus and rock strength compared to dry shale samples, few attentions have been paid to unveil the nature of the physics with a combination of geochemical modelling and disjoining pressure isotherm, and fewer works have envisaged the effect of mineralogy (calcite-rich or quartz-rich rock) on shale weakening with presence of aqueous liquids. We hypothesize that fluid-mineral interactions likely generate electrical double layer force between mineral surfaces which would shift the disjoining pressure from strongly negative (mineral-air-mineral) to less negative or even positive (mineral-fluid-mineral) thus triggering Young's modulus reduction. To test the hypothesis, geochemical modellings including minerals dissolution (calcite, dolomite, quartz, pyrite and illite) and surface complexation were performed together with disjoining pressure isotherm using literature experimental data, accounting for the Young's modulus reduction induced by fluid-shale interactions. The geochemical modelling shows that the amount of mineral dissolution at in-situ condition is negligible, and thus may play a minor role in Young's modulus. Disjoining isotherm shows that both calcite and quartz (plane-plane geometry) give a strong negative disjoining pressure generated by van der Waals force and structural force in the presence of air, whereas the disjoining pressure can be shifted from strongly negative to less negative for calcite and even positive for quartz in presence of fluids due to the electrostatic force, suggesting a Young's modulus reduction due to the weaker adhesion in line with literature experimental data. This work provides an overall conceptual framework, which supports the hydraulic fracturing fluid design and treatment in shale reservoirs from geochemical and disjoining pressure isotherm perspectives.

CO2 huff-n-puff process appears to be important to unlock hydrocarbon resources from shale oil reservoirs after multi-stage hydraulic fracturing. While existing literature shows that CO2 diffusion plays a significant role in kinetics of oil recovery, few studies have been able to draw on any systematic research into fluid-shale interaction due to water uptake of CO2 in connate water (water carbonation), which governs fluids flow in fractures thus CO2 huff-n-puff performance. We therefore hypothesized that water carbonation would depress ion exchange process between oil and organic matter (OM) thus forming a more water-wet system. Moreover, water carbonation would also decrease the electrostatic forces between oil and edge charge on OM, thereby further promoting water-wet system. To test the hypothesis, we measured contact angles in non‑carbonated high salinity brine (HS), carbonated high salinity brine (CO2 HS) and carbonated low salinity brine (CO2 LS) at temperature of 25 °C and under pressure of 3000 psi. Moreover, a geochemical modelling was conducted to evaluate ion exchange and surface complexation reactions in three different brines. Our contact angle measurements showed that HS gave a contact angle of 130°, while CO2 HS and CO2 LS resulted a contact angle of 23.5° and 23.0°, suggesting a more water-wet system. Geochemical modelling shows that ion exchange reactions between oil and shale surfaces are dramatically depressed in carbonated brine. In particular, the bridges number between oil and shale surfaces decreases from 5.2 × 10−4 μmol/m2 to 5.3 × 10−6 μmol/m2 in carbonated brines, supporting the contact angle measurements. Moreover, the computed surface potential at oil and rock surfaces increases from around −40 mV to 150 mV, suggesting more repulsive forces in the presence of carbonated brine, further supporting contact angle results. This work reveals for the first time that water carbonation during CO2 huff-n-puff process likely triggers a hydrophilic shale surface, which may significantly affects multiphase flow in natural and hydraulic fractures in particular.

Subcritical crack growth in calcite-bearing reservoirs plays a vital role in rock deformation. Published experimental results show that fluid-rock interaction likely affects rock surface energy and triggers fracture propagation. However, much of research up to now has been only descriptive in nature, impairing a substantial interpretation to predict subcritical crack growth. In this study, we developed a physicochemical model to relate fluid-calcite interaction in particular surface potential to surface energy in light of capacitance theory. To test the model, we calculated calcite surface chemical species and surface potential as a function of pH, ion type and fluid salinity using surface complexation modeling. Moreover, we compared the predicted surface energy with Bergsaker et al.'s experimental measurements. Our results confirm that fluid chemistry would affect calcite surface species distribution and surface energy. At high acidic condition, lowering salinity increases surface potential of brine-calcite. At alkaline condition, lowering salinity decreases surface potential in the presence of MgSO4 and MgCl2 but increases that of Na2SO4. Furthermore, pH would affect the level of bonding-capable divalent such as Ca2+ through the equilibrium of calcite dissolution-precipitation, and thus indirectly influence surface energy. Our results unveil the importance of fluid-rock interaction on subcritical crack growth and shed light on hydrocarbon exploitation and CO2 capture and storage in carbonate reservoirs. These findings also delineate the inner connection between geochemical and geo-mechanical properties at subsurface.

Unlike conventional optics, plasmonics enables unrivaled concentration of optical energy well beyond the diffraction limit of light. However, a significant part of this energy is dissipated as heat. Plasmonic losses present a major hurdle in the development of plasmonic devices and circuits that can compete with other mature technologies. Until recently, they have largely kept the use of plasmonics to a few niche areas where loss is not a key factor, such as surface-enhanced Raman scattering and biochemical sensing. Here, we discuss the origin of plasmonic losses and various approaches to either minimize or mitigate them based on understanding of fundamental processes underlying surface plasmon modes excitation and decay. Along with the ongoing effort to find and synthesize better plasmonic materials, optical designs that modify the optical powerflow through plasmonic nanostructures can help in reducing both radiative damping and dissipative losses of surface plasmons. Another strategy relies on the development of hybrid photonic–plasmonic devices by coupling plasmonic nanostructures to resonant optical elements. Hybrid integration not only helps to reduce dissipative losses and radiative damping of surface plasmons, but also makes possible passive radiative cooling of nanodevices. Finally, we review emerging applications of thermoplasmonics that leverage Ohmic losses to achieve new enhanced functionalities. The most successful commercialized example of a loss-enabled novel application of plasmonics is heat-assisted magnetic recording. Other promising technological directions include thermal emission manipulation, cancer therapy, nanofabrication, nanomanipulation, plasmon-enabled material spectroscopy and thermo-catalysis, solar water treatment, and thermophotovoltaics., United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-FG02-02ER45977), United States. Defense Advanced Research Projects Agency (Grant DE-AR0000471), Solid-State Solar-Thermal Energy Conversion Center (Award DE-SC0001299), Solid-State Solar-Thermal Energy Conversion Center (Award DE-FG02-09ER46577)

We demonstrate the agreement between first-principles calculations and experimental measurements of size effects in thermal transport in SiGe alloys without fitting parameters. Transient thermal grating (TTG) is used to measure the effect of the grating period on the temperature decay. The virtual crystal approximation under the density-functional-theory framework combined with impurity scattering is used to determine the phonon properties for the exact alloy composition of the measured samples. With these properties, classical size effects are calculated for the experimental geometry of reflection mode TTG using the recently developed variational solution to the phonon Boltzmann transport equation, which is verified against established Monte Carlo simulations. We find agreement between theoretical predictions and experimental measurements in the reduction of thermal conductivity (as much as fourfold of the bulk value) across grating periods spanning one order of magnitude. This paper provides a framework for the study of size effects in thermal transport in opaque materials.

Significance Thermoelectric technology can boost energy consumption efficiency by converting some of the waste heat into useful electricity. Heat-to-power conversion efficiency optimization is mainly achieved by decreasing the thermal conductivity in many materials. In comparison, there has been much less success in increasing the power factor. We report successful power factor enhancement by improving the carrier mobility. Our successful approach could suggest methods to improve the power factor in other materials. Using our approach, the highest power factor reaches ∼106 μW⋅cm −1 ⋅K −2 at room temperature. Such a high power factor further yields a record output power density in a single-leg device tested between 293 K and 868 K, thus demonstrating the importance of high power factor for power generation applications.

Tailoring the thermal conductivity of polymers is central to enlarge their applications in the thermal management of flexible integrated circuits. Progress has been made over the past decade by fabricating materials with various nanostructures, but a clear relationship between various functional groups and thermal properties of polymers remains to be established. Here, we numerically study the thermal conductivity of single-stranded carbon-chain polymers with multiple substituents of hydrogen atoms through atomic mass modification. We find that their thermal conductivity can be tuned by atomic mass modifications as revealed through molecular dynamics simulations. The simulation results suggest that heavy homogeneous substituents do not assist heat transport and trace amounts of heavy substituents can in fact hinder heat transport substantially. Our analysis indicates that carbon chain has the biggest contribution (over 80%) to the thermal conduction in single-stranded carbon-chain polymers. We further demonstrate that atomic mass modifications influence the phonon bands of bonding carbon atoms, and the discrepancies of phonon bands between carbon atoms are responsible for the remarkable drops in thermal conductivity and large thermal resistances in carbon chains. Our study provides fundamental insight into how to tailor the thermal conductivity of polymers through variable substituents., National Science Council (China) (51376069), National Key Basic Research Program of China (2013CB228302)

et al., Studying thermal transport at the nanoscale poses formidable experimental challenges due both to the physics of the measurement process and to the issues of accuracy and reproducibility. The laser-induced transient thermal grating (TTG) technique permits non-contact measurements on nanostructured samples without a need for metal heaters or any other extraneous structures, offering the advantage of inherently high absolute accuracy. We present a review of recent studies of thermal transport in nanoscale silicon membranes using the TTG technique. An overview of the methodology, including an analysis of measurements errors, is followed by a discussion of new findings obtained from measurements on both >solid> and nanopatterned membranes. The most important results have been a direct observation of non-diffusive phonon-mediated transport at room temperature and measurements of thickness-dependent thermal conductivity of suspended membranes across a wide thickness range, showing good agreement with first-principles-based theory assuming diffuse scattering at the boundaries. Measurements on a membrane with a periodic pattern of nanosized holes (135nm) indicated fully diffusive transport and yielded thermal diffusivity values in agreement with Monte Carlo simulations. Based on the results obtained to-date, we conclude that room-temperature thermal transport in membrane-based silicon nanostructures is now reasonably well understood., The work done at MIT was supported as part of the “Solid State Solar-Thermal Energy Conversion Center (S3TEC),” an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001299/DEFG02-09ER46577. The contribution by A.V.-F. and J. J. A.-G. was partially supported by Project 192 “Fronteras de la ciencia” and Project 251882 “Investigacion Científica Basica.” A. V.-F. also appreciates support from Conacyt through normal and mixed scholarships. MS and CMST acknowledge support from the Spanish program Severo Ochoa (Grant SEV-2013-0295), projects PHENTOM (FIS2015-70862-P) and nanoTHERM (CSD2010-00044), as well as from the EU project MERGING (309150). Z.L. and E.N.W. further acknowledge support and funding from the Air Force Office of Scientific Research (AFOSR), and are grateful to program manager Dr. Ali Sayir.

The phonon Boltzmann transport equation (BTE) is a powerful tool for studying non-diffusive thermal transport. Here, we develop a new universal variational approach to solving the BTE that enables extraction of phonon mean free path (MFP) distributions from experiments exploring non-diffusive transport. By utilizing the known Fourier solution as a trial function, we present a direct approach to calculating the effective thermal conductivity from the BTE. We demonstrate this technique on the transient thermal grating (TTG) experiment, which is a useful tool for studying non-diffusive thermal transport and probing the mean free path (MFP) distribution of materials. We obtain a closed form expression for a suppression function that is materials dependent, successfully addressing the non-universality of the suppression function used in the past, while providing a general approach to studying thermal properties in the non-diffusive regime., Comment: 17 pages, 2 figures

Superlattices are promising low-dimensional nanomaterials for thermoelectric technology that is capable of directly converting low-grade heat energy to useful electrical power. In this work, the thermal conductivities of GaAs/Ge superlattice nanostructures were investigated systematically in relation to their morphologies and interfaces. Thermal conductivities were measured using ultrafast time-domain thermoreflectance and were found to decrease with increasing interface densities, consistent with our understanding of microscopic phonon transport in the particle regime. Lower thermal conductivities were observed in (GaAs)0.77(Ge2)0.23 alloys; transmission electron microscopy study reveals phase separation in the alloys. These alloys can be interpreted as fine nanostructures, with length scales comparable to the periods of very thin superlattices. Our experimental findings help gain fundamental insight into nanoscale thermal transport in superlattices and are also useful for future improvement of thermoelectric performance using superlattice nanostructures., Comment: 11 pages, 5 figures

A single polyethylene chain was reported to have a high metal-like thermal conductivity (TC), which stands in sharp contrast to the thermally insulating feature of common bulk polyethylene materials. This work numerically investigates the impact of torsion and stretching on the TC of polyethylene strands by using equilibrium molecular dynamics simulations. The simulation results show that torsion slightly reduces the TC of a single polyethylene chain. In contrast, the heat conduction of polyethylene strands could be slightly enhanced under torsional loading with a specific torsional angle. Particularly, an apparent improvement of TC of polyethylene strands is achieved by combining torsion and stretching functions. It is found that the TC of torsional polyethylene strands is sensitive to torsional patterns. Our study proposes a specific torsional pattern of polyethylene strands that significantly enhances the heat conduction of the original counterpart. This study will play an essential role in guiding the...

The phonon Boltzmann transport equation (BTE) is widely utilized to study non-diffusive thermal transport. We find a solution of the BTE in the thin film transient thermal grating (TTG) experimental geometry by using a recently developed variational approach with a trial solution supplied by the Fourier heat conduction equation. We obtain an analytical expression for the thermal decay rate that shows excellent agreement with Monte Carlo simulations. We also obtain a closed form expression for the effective thermal conductivity that demonstrates the full material property and heat transfer geometry dependence, and recovers the limits of the one-dimensional TTG expression for very thick films and the Fuchs-Sondheimer expression for very large grating spacings. The results demonstrate the utility of the variational technique for analyzing non-diffusive phonon-mediated heat transport for nanostructures in multi-dimensional transport geometries, and will assist the probing of the mean free path (MFP) distribution of materials via transient grating experiments., Comment: 19 pages, 4 figures

The impact of boundary scattering on non-diffusive thermal relaxation of a transient grating in thin membranes is rigorously analyzed using the multidimensional phonon Boltzmann equation. The gray Boltzmann simulation results indicate that approximating models derived from previously reported one-dimensional relaxation model and Fuchs-Sondheimer model fail to describe the thermal relaxation of membranes with thickness comparable with phonon mean free path. Effective thermal conductivities from spectral Boltzmann simulations completely free of any fitting parameters are shown to agree reasonably well with experimental results. These findings are important for improving our fundamental understanding of non-diffusive thermal transport in membranes and other nanostructures., Comment: 15 pages, 3 figures