Thermal transport of graphene-C3B superlattices and van der Waals heterostructures: a molecular dynamics study.

Two-dimensional (2D) materials have attracted more and more attention due to their excellent properties. In this work, we systematically explore the heat transport properties of Graphene-C3B (GRA-C3B) superlattices and van der Waals (vdW) heterostructures using molecular dynamics method. The effects of interface types and heat flow directions on the in-plane interfacial thermal resistance (ITRip) are analyzed. Obvious thermal rectification is detected in the more energy stable interface, GRA zigzag-C3B zigzag (ZZ) interface, which also has the minimum value of ITRip. The dependence of the superlattices thermal conductivity (k) of the ZZ interface on the period length (lp) is investigated. The results show that when thelpis 3.5 nm, thekreaches a minimum value of 35.52 W m-1K-1, indicating a transition stage from coherent phonon transport to incoherent phonon transport. Afterwards, the effects of system size, temperature, coupling strength and vacancy defect on the out-of-plane interfacial thermal resistance (ITRop) are evaluated. With the increase of temperature, coupling strength and vacancy defect, ITRopare found to reduce effectively due to the enhanced Umklapp phonon scattering and increased probability of energy transfer. Phonon density of states and phonon participation ratio is evaluated to reveal phonon behavior during heat transport. This work is expected to provide essential guidance for the thermal management of nanoelectronics based on 2D monolayer GRA and C3B.

A Review of Molecular Models for Gas Adsorption in Shale Nanopores and Experimental Characterization of Shale Properties.

Shale gas, as a promising alternative energy source, has received considerable attention because of its broad resource base and wide distribution. The establishment of shale models that can accurately describe the composition and structure of shale is essential to perform molecular simulations of gas adsorption in shale reservoirs. This Review provides an overview of shale models, which include organic matter models, inorganic mineral models, and composite shale models. Molecular simulations of gas adsorption performed on these models are also reviewed to provide a more comprehensive understanding of the behaviors and mechanisms of gas adsorption on shales. To accurately understand the gas adsorption behaviors in shale reservoirs, it is necessary to be aware of the pore structure characteristics of shale reservoirs. Thus, we also present experimental studies on shale microstructure analysis, including direct imaging methods and indirect measurements. The advantages, disadvantages, and applications of these methods are also well summarized. This Review is useful for understanding molecular models of gas adsorption in shales and provides guidance for selecting experimental characterization of shale structure and composition.

Microscopic Insights and Optimization of the CH4-CO2 Replacement in Natural Gas Hydrates.

Using the CO2 replacement method to exploit natural gas hydrates and store CO2 has great significance in energy access and environmental protection. Herein, the molecular dynamic method is utilized to analyze and evaluate the CH4-CO2 replacement at different constant temperatures and pressures. For optimization, various temperature oscillations are introduced in the CH4-CO2 replacement. It illustrates that increasing the temperature can improve the amounts of CH4 escape and CO2 capture but is unfavorable to the long-term CO2 storage and hydrate stability. The effects of pressure are not as significant and definite as those of temperature. Appropriate temperature oscillations can achieve comprehensive improvements, which benefit from both the deep diffusion of CO2 in the higher temperature stage and the rapid rebuilding of CO2 hydrate within just nanoseconds caused by the memory effects in the lower temperature stage. The results also reveal that the optimal lower temperature duration and frequency should be moderate. Decreasing the lower temperature value can distinctly enhance CO2 capture and hydrate stability. This study can help understand the mechanisms of CH4-CO2 replacement under different temperature and pressure conditions, especially at temperature transitions, and proposes a potentially effective method to achieve large-scale carbon sequestration in the hydrate.

Dynamic Behaviors and Mechanisms of Air-Foam Flooding at High Pressure and Reservoir Temperature via Microfluidic Experiments.

Air injection has been proven to be an effective improved oil recovery technique for deep and light oil reservoirs with low permeability and poor water injectivity. But the efficiency of air injection in highly heterogeneous reservoirs is low due to poor gas sweeping that may lead to early oxygen breakthrough caused by gas channeling and viscous fingering. Foam can be used to assist air injection to overcome the obstacles of early gas breakthrough and to increase the displacement and sweeping efficiency. In this paper, laser-etched visual microscopic pore models were used as microfluidic devices to study the air-foam flooding process in porous media at reservoir temperature and high pressure. The dynamic behaviors and relevant mechanisms of air-foam flooding were investigated. Typical mechanisms of foam generation in porous media are achieved in different parts of the micromodel, which can be listed as follows: lamella leave-behind, lamella division, and snap off. Analysis on flow states of air foam showed that foams migrate in porous media by bursting and regenerating during the flooding process. It can be observed that the flow mode of foam in porous media is the separate flow of gas and liquid through microscopic displacement experiments, suggesting that foam should not be treated as a homogeneous phase in heterogeneous porous media. The pressing, occupying, and selective blocking effects of foam in porous media exhibited different oil displacement performances with the presence of various pore geometries and networks. Tiny foams also showed stripping and carrying effects on larger oil droplets benefiting from the lipophilicity of foam. Through comprehensive analysis on overall and local oil displacement mechanisms, air-foam injection could enhance the microscopic sweep volume and improve the oil displacement efficiency.

Vapor Condensation on Bioinspired Hierarchical Nanostructured Surfaces with Hybrid Wettabilities.

Vapor condensation on bioinspired hierarchical nanostructured surfaces with hybrid wettabilities has been investigated using molecular dynamics simulations. A series of hierarchical surfaces consisting of nanocylinder arrays with hydrophilic top and hydrophobic nanopillar arrays are constructed. The results manifest that the condensed nanodroplets undergo three states in the whole water vapor condensation process, and the total condensed atom number on surfaces increases with the increase of nanocylinder diameter (D), which indicates that the introduction of hydrophilic nanocylinders is conducive to improving the condensation performance compared with that on the hydrophobic surface patterned with homogeneous nanopillars. However, the nucleation sites on hierarchical nanostructured surfaces are covered by the condensed nanodroplets at the end of condensation, which suppresses the further enhancement of condensation performance. To solve these problems, we add a collection region close to the edge of the nanostructured surface. It is noticed that the condensed nanodroplets can roll into the collection regions gradually during the condensation process, which keeps the nucleation sites on nanostructured surfaces exposed effectively, especially for the cases of 20 Å ≤ D ≤ 40 Å. Moreover, the cluster number, the total condensed atom number, and the condensation enhancement efficiency on hierarchical nanostructured surfaces with collection regions at 20 Å ≤ D ≤ 40 Å are higher obviously compared with those on surfaces without collection regions. Our study demonstrates that the bioinspired hierarchical nanostructured surface with the collection region is beneficial to boost the vapor condensation performance, which can bring new insights into water vapor condensation.

Interfacial thermal transport of graphene/ß-Ga2O3 heterojunctions: a molecular dynamics study with a self-consistent interatomic potential.

Graphene/ß-Ga2O3 heterojunctions are widely used in high-power and high-frequency devices, for which thermal management is vital to the device operation and life. Here we apply molecular dynamics simulation to calculate the interfacial thermal resistance (ITR) between graphene and ß-Ga2O3. Based on the rigid ion model, a self-consistent interatomic potential with a set of parameters that can well reproduce the basic physical properties of crystal ß-Ga2O3 is fitted. Using this potential, the effects of model size, interface type, temperature, vacancy defects and graphene hydrogenation on the ITR of graphene/ß-Ga2O3 heterojunctions are evaluated. The results show that there is no obvious dependence of ITR on the size of graphene and ß-Ga2O3. It is reported that the ITR values of the (100), (010) and (001) interfaces are 7.28 ± 0.35 × 10-8 K m2 W-1, 6.69 ± 0.44 × 10-8 K m2 W-1 and 5.22 ± 0.35 × 10-8 K m2 W-1 at 300 K, respectively. Both temperature increase and vacancy defect increase can prompt the energy propagation across graphene/ß-Ga2O3 interfaces due to the enhancement of phonon coupling. In addition, graphene hydrogenation provides new channels for in-plane and out-of-plane phonon coupling, and thus reduces the ITR between graphene and ß-Ga2O3. This study provides basic strategies for the thermal design and management of graphene/ß-Ga2O3 based photoelectric devices.

Improved thermoelectric properties of WS2-WSe2 phononic crystals: insights from first-principles calculations.

Recently, two-dimensional transition metal dichalcogenide (TMDC) monolayers have attracted much attention owing to their excellent physical properties. In the present study, we systematically investigate the thermoelectric properties of different WS2-WSe2 phononic crystals by utilizing first-principles calculations. First, the thermal properties of all phononic crystals with superlattices (SL1 and SL2) and their individual components (WS2 and WSe2) are evaluated, in which the lattice thermal conductivities (kph) of WS2 and WSe2 monolayers present isotropic behaviors, while the values of SL1 and SL2 monolayers reveal weak anisotropic behaviors. It can be observed that the kph values of WS2 and WSe2 monolayers are larger than those of SL1 and SL2 monolayers, which can be attributed to the decreasing phonon group velocity and phonon lifetime. Moreover, we calculate the electronic band structures of all monolayers, indicating that all monolayers are semiconductors. Afterwards, the electrical conductivities, the Seebeck coefficients, the power factors, the electronic thermal conductivities, and the ZT values at different temperatures are evaluated. The ZTmax values of WS2, WSe2, SL1, and SL2 monolayers with p-type doping are 0.43, 0.37, 0.95, and 0.66 at 1000 K. It can be proved that the SL1 monolayer possesses the largest ZT, which is at least two times higher than those of the WS2 and WSe2 monolayer. Finally, we build two kinds of phononic crystals with periodic holes (PCH1 and PCH2) and evaluate the thermoelectric properties. It can be observed that the PCH2 structure shows the best thermoelectric performance. The ZTmax values of the PCH2 structure can reach 2.53 and 4.54 with p-type doping along the x and y directions, which are 2.66 and 6.75 times higher than those of the SL1 monolayer. This work provides a new strategy to obtain higher thermoelectric performance and demonstrates the potential applications of phononic crystals in TMDC-based nanoelectronic devices.

Experimental Study on Ultrasonic Vibration-Assisted WECDM of Glass Microstructures with a High Aspect Ratio.

With the rapid development of micro-electro-mechanical systems (MEMSs), the demand for glass microstructure is increasing. For the purpose of achieving high quality and stable machining of glass microstructures with a high aspect ratio, ultrasonic vibration is applied into the micro-wire electrochemical discharge machining (WECDM), which is proposed as ultrasonic vibration-assisted WECDM with a micro helical electrode. Firstly, the formation of a gas film on the surface of the helical electrode in WECDM machining is simulated, meaning the thickness of the gas film can be reduced by adding suitable ultrasonic amplitude, thus reducing the critical voltage, then the machining localization and stability were enhanced. Then, the micro helical electrode with a diameter of 100 µm is used to carry out sets of experiments that study the influence of ultrasonic amplitude, machining voltage, duty factor, pulse frequency, and feed rate on the slit width. The experimental results show that the machining stability and quality are significantly improved by adding suitable ultrasonic amplitude. When the amplitude was 5.25 µm, the average slit width was reduced to 128.63 µm with a decrease of 20.78%. Finally, with the optimized machining parameters, micro planar coil structure and microcantilever structure with a high aspect ratio were fabricated successfully on the glass plate. It is proved that ultrasonic vibration-assisted WECDM with the micro helical electrode method can meet the requirements of high aspect ratio microstructure machining for hard and brittle materials.

Microfluidic Technology for Antibacterial Resistance Study and Antibiotic Susceptibility Testing: Review and Perspective.

A review on microfluidic technology for antibacterial resistance study and antibiotic susceptibility testing (AST) is presented here. Antibiotic resistance has become a global health crisis in recent decades, severely threatening public health, patient care, economic growth, and even national security. It is extremely urgent that antibiotic resistance be well looked into and aggressively combated in order for us to survive this crisis. AST has been routinely utilized in determining bacterial susceptibility to antibiotics and identifying potential resistance. Yet conventional methods for AST are increasingly incompetent due to unsatisfactory test speed, high cost, and deficient reliability. Microfluidics has emerged as a powerful and very promising platform technology that has proven capable of addressing the limitation of conventional methods and advancing AST to a new level. Besides, potential technical challenges that are likely to hinder the development of microfluidic technology aimed at AST are observed and discussed. To conclude, it is noted that (1) the translation of microfluidic innovations from laboratories to be ready AST platforms remains a lengthy journey and (2) ensuring all relevant parties engaged in a collaborative and unified mode is foundational to the successful incubation of commercial microfluidic platforms for AST.

Machine learning interatomic potential developed for molecular simulations on thermal properties of ß-Ga2O3.

The thermal properties of ß-Ga2O3 can significantly affect the performance and reliability of high-power electronic devices. To date, due to the absence of a reliable interatomic potential, first-principles calculations based on density functional theory (DFT) have been routinely used to probe the thermal properties of ß-Ga2O3. DFT calculations can only tackle small-scale systems due to the huge computational cost, while the thermal transport processes are usually associated with large time and length scales. In this work, we develop a machine learning based Gaussian approximation potential (GAP) for accurately describing the lattice dynamics of perfect crystalline ß-Ga2O3 and accelerating atomic-scale simulations. The GAP model shows excellent convergence, which can faithfully reproduce the DFT potential energy surface at a training data size of 32 000 local atomic environments. The GAP model is then used to predict ground-state lattice parameters, coefficients of thermal expansion, heat capacity, phonon dispersions at 0 K, and anisotropic thermal conductivity of ß-Ga2O3, which are all in excellent agreement with either the DFT results or experiments. The accurate predictions of phonon dispersions and thermal conductivities demonstrate that the GAP model can well describe the harmonic and anharmonic interactions of phonons. Additionally, the successful application of our GAP model to the phonon density of states of a 2500-atom ß-Ga2O3 structure at elevated temperature indicates the strength of machine learning potentials to tackle large-scale atomic systems in long molecular simulations, which would be almost impossible to generate with DFT-based molecular simulations at present.

Molecular dynamic simulation of thermal transport in monolayer C3B x N1-x alloy.

Recently, two-dimensional (2D) monolayers C3B and C3N attract growing research interest due to the excellent physical properties. In this work, the thermal conductivities (k) of the monolayer C3B x N1-x alloy and the special C3B0.5N0.5 superlattice (C3B0.5N0.5-SL) alloy are systematically evaluated by using molecular dynamic simulation. First, the k of monolayer C3B x N1-x alloy presents a U-shaped profile with the increasing random doping ratio (x), in which the lowest k exists in x = 0.5. Second, we further calculate the thermal conductivity of C3B0.5N0.5-SL. The result shows an initial decreasing and then rising trend, and the coherent length is 5.11 nm which occupies the minimum thermal conductivity. Furthermore, to uncover the phonon thermal transport mechanism, we calculate the spatiotemporal thermal transport, phonon density of states, phonon group velocity, participation ratio and the phonon wave packet simulations in monolayer alloy system. We note that on account of the random doping atoms, the enhancive phonon-impurity scattering and phonon localization reduce the thermal conductivity in monolayer C3B x N1-x alloy. In C3B0.5N0.5-SL, when the period length is smaller than the coherent length, coherent phonon modes emerge because of the phonon interference, in which the superlattice can be regarded as a 'newly generated material'. However, when the period length is larger than the coherent length, the decreasing number of the interface in superlattice lessens phonon-interface scattering and cause the increasing thermal conductivity. This work contributes the fundamental knowledge for thermal management in 2D monolayer C3B x N1-x alloy based nanoelectronics.

Molecular dynamics study of convective heat transfer mechanism in a nano heat exchanger.

With the rapid development of micro/nano electro-mechanical systems, the convective heat transfer at the micro/nanoscale has been widely studied for the thermal management of micro/nano devices. Here we investigate the convective heat transfer mechanism of a nano heat exchanger by the employment of molecular dynamics simulation with a modified thermal pump method. First, the temperature jump and velocity slip are observed at the wall-fluid interfaces of the nano heat exchanger. Moreover, the larger Kapitza resistance in the entrance region weakens the convective heat transfer. Second, the heat transfer performance of the nano heat exchanger can be improved by increasing the surface wettability of the solid walls owing to more fluid atoms being involved in heat transport at the walls when the wall-fluid interaction is enhanced. Meanwhile, the strong surface wettability results in the appearance of the quasi-solid fluid layers, which improves the heat transfer between walls and fluids. Finally, we point out that when the surface wettability of the nano heat exchanger is weak, the heat transfer of the hot fluid side is better than that of the cold fluid side, while the convective heat transfer performances of the cold and hot fluid sides are reversed when the surface wettability is strong. This is because of the feebler temperature jump of the hot fluid side when wall-fluid interaction is small and the greater velocity slip of the cold fluid side for walls with large wall-fluid interaction.

Mechanical properties of molybdenum diselenide revealed by molecular dynamics simulation and support vector machine.

Despite the spurring interests in two-dimensional transition metal dichalcogenide (TMDC) materials, knowledge on the mechanical properties of one of their important member, i.e., molybdenum diselenide (MoSe2) is scarce and remains an open topic. In this work, the mechanical properties of h-MoSe2 and t-MoSe2 were systematically investigated using classical molecular dynamics (MD) simulations combined with machine learning (ML) techniques. The effects of chirality, temperature and strain rate on fracture strain, fracture strength and Young's modulus were characterized in both armchair and zigzag directions. For h-MoSe2, the fracture strengths were 13.6 and 13.0 GPa for armchair and zigzag chiralities, respectively, at 1 K and strain rate of 5 × 10-4 ps-1; the corresponding fracture strains were 0.23 and 0.27. The Young's moduli in armchair and zigzag directions exhibited similar values of 100.9 and 99.5 GPa, respectively. For t-MoSe2, much lower fracture strengths of 6.1 and 6.3 GPa, fracture strains of 0.13 and 0.15, and Young's moduli of 83.7 and 83.0 GPa were predicted under the same conditions. A total of 700 MD simulation cases were calculated under different impact factors and initial conditions, which were subsequently fed into the support vector machine (SVM) algorithm for ML modeling. After training, the ML model could predict the mechanical properties of both MoSe2 types given only the input features such as chirality, temperature and strain rate.

Phonon thermal conduction in a graphene-C3N heterobilayer using molecular dynamics simulations.

Two-dimensional (2D) graphene (GRA) and polyaniline (C3N) monolayers are attracting growing research interest due to their excellent electrical and thermal properties. In this work, in-plane and out-of-plane phonon thermal conduction of GRA-C3N heterobilayer are systematically investigated by using classical molecular dynamics simulations. Effects of system size, temperature and interlayer coupling strength on the in-plane thermal conductivity (k) and out-of-plane interfacial thermal resistance (R) are evaluated. Firstly, a monotonic increasing trend of k with increasing system size is observed, while a negative correlation between thermal conductivity and temperature is revealed. The interlayer coupling strength is found to have a weak effect on the in-plane thermal conductivity of the heterobilayer. Secondly, at T = 300 K and χ = 1, the predicted R of GRA â†’ C3N and C3N â†’ GRA are 1.29 × 10-7 K m2 W-1 and 1.35 × 10-7 K m2 W-1, respectively, which indicates that there is no significant thermal rectification phenomenon. It can also be observed that R decreases monotonically with increasing temperature and coupling strength due to the enhanced Umklapp phonon scattering and the phonon transmission probability across the interface. Phonon density of states, phonon dispersions and participation ratios are evaluated to reveal the mechanism of heat conduction in the heterobilayer. This work contributes the valuable thermal information to modulate the phonon behaviors in 2D heterobilayer based nanoelectronics.