DFT characterization of a new possible two-dimensional BN allotrope with a biphenylene network structure.

The pioneering work on the newly experimentally synthesized biphenylene network C has triggered a worldwide tide of research on its family material counterparts. In this study, a biphenylene network BN structure was theoretically characterized by density functional theory (DFT) calculations. Initially, the structure's mechanical and thermal stabilities were evaluated. There were no imaginary frequencies in the phonon dispersion curve, indicating that the structure was mechanically stable. Additionally, the energy barrier for forming a biphenylene network BN structure from perfect pristine 2D h-BN is substantially less than that for forming a biphenylene network C from a perfect graphene sheet, as can be explained from the greater structure distortion in the biphenylene network BN with lower bond stress which thus caused lower energy. The electronic band structure and detailed projected density of states analysis indicated that the biphenylene network BN is a semiconductor with the valence band maximum (VBM) and the conduction band minimum (CBM) states from the pz orbitals of N and B atoms with sp2 hybridization. Finally, a bilayer structure was also proposed. Our obtained results provide more insights into two-dimensional biphenylene network BN based structures and those family materials which could be widely used in relevant nanoelectronic devices.

Stretched three-dimensional white graphene with a tremendous lattice thermal conductivity increase rate.

Despite the increasing interest in the physical properties of the newly synthesized three-dimensional (3D) nano-architectured graphene, there are still few studies on the thermal transport properties of this family of materials. In the present work, heat transport of 3D h-BN and its mechanical response are systematically explored through first principles calculations. It is fascinating to find that the thermal conductivity of the 3D h-BN honeycomb structure could be significantly modulated by mechanical tension. Its lattice thermal conductivity perpendicular to the hole axis increases by 7.2 times at 6% critical strain, compared to only 0.67 times for that of the strained 3D graphene counterpart. The structure's thermal conductivity versus mechanical tension differs quantitatively and qualitatively from the monotonic downward trend of traditional bulk diamond or silicon under tension. This deviation from the classic behavior could be attributed to the modification of the phonon lifetimes, together with the competition between group velocities of low- and high-lying phonons under strain. Finally, the phonon vibrational modes contribution analysis indicates that the BN ribbon atoms contribute mainly at a lower frequency range. Our results provide important insights into potential employment of nano-architectured 3D white graphene for thermal management in relevant industrial applications.

Study on the Application of Sediment-Based Embankment Building and Ultra-High-Performance Concrete (UHPC) Preparation in the Resource Utilization of Yellow River Sediment.

The Yellow River is difficult to control. Little water and a large amount of sediment results in sediment accumulation in its lower reaches as sediment inflow exceeds transport capacity. Reducing this sediment deposition is essential for harnessing the Yellow River. Included in this process is the rational use of the sediment. Many researchers have investigated usage of Yellow River sediment as an aggregate material for concrete production, but there are still some problems (e.g., low resource utilization and low strength of the concrete made from Yellow River sediment). To make up the deficiency in the existing research, this study proposes two methods of sediment utilization. One is to use Yellow River sediment to build embankments, and the other is to use ultra-fine Yellow River sand to prepare ultra-high-performance concrete (UHPC). Test results reveal that the prepared high-strength concrete performs well in each test, including: fluidity, mechanical properties, pore structure, ecological evaluation, microscopic measurement of the interface transition zone, and economic analysis.

Research on thermoeconomic fault diagnosis for marine low speed two stroke diesel engine.

To satisfy the requirements of low fuel consumption, low emission, and high efficiency of the shipping industry, marine diesel engines are developing in the direction of automation and energy-saving, which increases the possibility and complexity of marine diesel engine failures. A one-dimension thermodynamic model for the marine diesel engine is built with AVL Boost software. The model is applied to a low-speed two-stroke 6S50MC diesel engine, and the error between the main performance parameters obtained by simulation and the test bench data is less than 3% under 100% and 75% load. Based on the model, 6 typical single faults and many typical double faults concomitant phenomena of diesel are reproduced. Based on the second law of thermodynamics, the exergy flow among the components and the external environment is analyzed. The thermoeconomic model of a marine diesel engine is established where the "fuel" and "product" of the components are defined according to their function. The fault diagnosis results show that the effects of faults generally propagate through the diesel engine system and affect the behavior of several components, resulting in induced malfunction in normal components. Therefore the malfunction MFi of each component is the superposition of the intrinsic malfunction and the induced malfunction according to the malfunction and dysfunction analysis. The thermoeconomic fault diagnosis method can be used to narrow the search range of abnormal components though it cannot accurately locate the fault.

Study on seafloor hydrothermal systems circulation flow and heat transfer characteristics.

In this work, the numerical simulation study of the hydrothermal flow and heat transfer process in the porous rock under 30 MPa pressure was developed. The flow and heat transfer characteristics of hydrothermal in rocks with different porosities are studied by changing the porosity of the rock. The simulation results show that the average flow velocity decreases and the average temperature increases when the porosity decreases. The velocity field and temperature field are coupled due to the nonlinear thermophysical properties of hydrothermal. The velocity field and temperature field have strongly interacted in the range of 400-450 ℃ and the effect of temperature on velocity is gradually diminishing outside the range. Most of the fluid will be "squeezed" into the crevice and the average velocity is almost three times the no-creviced case when a crevice is present. The existence of the crevice makes the total heat flux decrease from an overall perspective, and the crevice makes a large temperature gradient at the entrance and export of the crevice from a local perspective. These results provide theoretical support for the utilization of submarine hydrothermal fluid shallow circulation heat energy.

Synergistic denitrification and phosphorus removal performance of a biofilm-microflocculation system and its microbial community variations: A pilot-scale study for a wastewater treatment plant.

AIMS: For upgrading and reconstructing a municipal wastewater treatment plant, a biofilm-microflocculation filter system was designed and established towards synergistic improvement of denitrification and phosphorus removal from the secondary effluent. METHODS AND RESULTS: The establishment of the biofilm-microflocculation filter system underwent several processes, including sludge inoculation, biofilm formation and polyaluminum chloride (PAC) addition as flocculating agent. Microbial community analysis indicated that the dominant denitrification bacteria of the biofilm filter were in the phylum Proteobacteria and the genera Hydrogenophaga and Dechloromonas. On the basis of the initiation of filter system under optimal parameters such as C/N ratio of 5.3, hydraulic retention time of 1.06 h and PAC of 5 mg L-1 , approximately 75% COD, 80% TN and 75% TP could be effectively removed to satisfy discharge standards. Comparing the variations of microbial community structure at the genus level during the operating period of the filter system, it was found that the relative abundance of denitrification bacteria merely shifted from 53.14% to 48.76%, demonstrating that the effect of PAC addition on the main micro-organisms is marginal. CONCLUSIONS: From the above results, it can be verified that the established biofilm-microflocculation filter system has practical and reliable performance for simultaneous biological denitrification and phosphorus removal. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides a reference method for improving the advanced treatment of wastewater plant secondary effluent.

Grand Canonical Monte Carlo Simulations on Phase Equilibria of Methane, Carbon Dioxide, and Their Mixture Hydrates.

The cage occupancy plays a crucial role in the thermodynamic stability of clathrate hydrates and is an important quantity for understanding the CO2-CH4 replacement phenomenon. In this work, the occupancy isotherms of pure CH4, pure CO2, and their mixture in sI and sII hydrates are studied by GCMC + MD simulations. The adsorption of CH4 and CO2 + CH4 in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated occupancy ratio θL/θS and the abundance ratio of CO2 to CH4 vary with the temperature and pressure, which provide the prerequisite information for the prediction of CH4 recovery yield at different conditions in the CO2-CH4 gas exchange process. The phase equilibria of clathrate hydrates of pure gases and mixtures are explored and the corresponding heat of dissociation and hydration numbers are determined. The current investigation provides new perspectives to understand the mechanism behind the gas adsorption behavior of clathrate hydrates.

Ab Initio Studies on the Clathrate Hydrates of Some Nitrogen- and Sulfur-Containing Gases.

Ab initio calculations are performed to investigate the host-guest interactions and multiple occupancies of some sulfur- (H2S, CS2) and nitrogen-containing (N2, NO, and NH3) molecules in dodecahedral, tetrakaidecahedral, and hexakaidecahedral water cages in this work. Five functionals in the framework of density functional theory are compared, and the M06-2X method appears to be the best to predict the binding energies as well as the geometries. Results show that N2 and NO molecules are more stable in the 51264 cage, while NH3 and H2S prefer to stabilize in the 51262 cage. This suggests that the sI hydrates of NH3 and H2S exhibit higher stability than the sII structures and that sII NO hydrate is more stable than sI NO hydrate. N2 is found to be more stable in type II structure with single occupancy and to form type I hydrate with multiple occupancy, which is consistent with the experimental observations. As to the guest molecule CS2, it may undergo severe structural deformation in the 512 and 51262 cage. For multiple occupancies, the 512, 51262, and 51264 water cages can trap up to two N2 molecules, and the 51264 water cage can accommodate two H2S molecules. This work is expected to provide new insight into the formation mechanism of clathrate hydrates for atmospherically important molecules.