Theoretical hardness of the cubic BC2N

Vickers hardness of the cubic BC₂N has been investigated using the microscopic hardness model, in which the parameters have been obtained using first principles calculations. Ionicities of the chemical bonds in the cubic BC₂N depend on their surrounding chemical environments, which are included in the hardness calculations of the cubic BC₂N using the population ionicity scale. For the four selected configurations of the cubic BC₂N, the theoretical Vickers hardness has been found to lie between 70 and 72 GPa, consistent with the experimental value of 76 GPa. According to our calculations, it should be the cubic BC₂N that ranks second among the superhard materials instead of the cubic BN.     

Half metallicity in BC2)N nanoribbons: stability, electronic structures, and magnetism

Nanoribbons are suggested to be among the most promising candidates being considered as building blocks in future electronics. In this study, we use density functional calculations to examine the structures and electronic properties of BC(2)N nanoribbons with bare zigzag-shaped edges (zz-BC(2)NNRs). Four different types of atomistic edge configurations are considered, including ribbons terminated with two C edges, B and N edges, B an C edges, and C and N edges. We find the existence of half-metallicity in the ground state of the zz-BC(2)NNRs with two bare C edges and with bare C and N edges. The other two configurations of the zz-BC(2)NNRs can be either semiconducting or metallic, depending on the specific configuration. We also find that the stability of the zz-BC(2)NNRs are largely dependent on ribbon width. The zz-BC(2)NNRs become energetically more stable when the nanoribbon width exceeds 3.3 nm. It is interesting to find that half-metallic zz-BC(2)NNRs with a width of 0.7 nm are thermodynamically more stable than either metallic or semiconducting counterparts. Therefore, the possibility of synthesizing half-metallic zz-BC(2)NNRs exists.     

Symmetry Properties of Single-Walled BC2N Nanotubes

The symmetry properties of the single-walled BC₂N nanotubes were investigated. All the BC₂N nanotubes possess nonsymmorphic line groups. In contrast with the carbon and boron nitride nanotubes, armchair and zigzag BC₂N nanotubes belong to different line groups, depending on the index n (even or odd) and the vector chosen. The number of Raman- active phonon modes is almost twice that of the infrared-active phonon modes for all kinds of BC₂N nanotubes.     

Atomic structure and mechanical properties of BC2N superlattice

Structural motifs for the BC₂N superlattices were identified from a systematic search based on a greedy algorithm. Using a tree data structure, we have retrieved seven structural models for c-BC₂N 1 × 1 × 1 lattice which were identified previously by Sun et al. [Phys. Rev. B 64, 094108 (2001)]. Furthermore, the atomic structures with the maximum number of C–C bonds for c-BC₂N 2 × 2 × 2, 3 × 3 × 3, and 4 × 4 × 4 superlattices were found by imposing the greedy algorithm in the tree data structure. This new structural motif has not been previously proposed in the literature. A total of up to 512 atoms in the c-BC₂N superlattice are taken into consideration. The atoms in these superlattices are in diamond-like structural form. Furthermore, the C atoms, as well as B and N atoms, form the octahedral motif separately. The octahedral structure consisting of C is bounded with {111} facets, and each facet is interfaced to a neighboring octahedral structure consisting of B and N atoms. The electronic and mechanical properties of newly identified low energy structures were analyzed.     

Economic comparison of several membrane configurations for H2/N2 separation

Several one- and two-stage membrane systems were compared for use in separating H₂/N₂ mixtures from coal gasification processes. Computer models of cross flow membrane modules were used in the evaluations. The processing cost, determined by a discounted cash flow analysis of capital and operating expenses, was used as the basis for the comparison. Membrane properties were those of a poly[etherimide] composite membrane. Hydrogen mole fractions from 34% to 97% in the feed and from 80% to 99.9% in the product (permeate) were examined; in all cases it was required that 95% of the feed H₂ be recovered. Four configurations were evaluated: single module (SM), single module with recycle (SMR), two-module series (SER), and two-stage cascade (CAS). The results showed that for conditions where SM was capable of performing the separation the optimum recycle rate in SMR was zero, and thus SMR and SM were identical. In some conditions, SER also reduced to SM. In general, SM is best for easy separations (where the feed and product compositions are similar), CAS is best for difficult separations (where feed and product compositions are very different), and SER is best for separations of moderate difficulty; in this example, where the H₂ recovery is fixed, SMR is never the best configuration. In some circumstances, it is economically better to treat only a portion of the feed in the membrane system, but to a higher purity than is required, and then to mix the overconcentrated stream with the untreated feed to make a product stream of the desired purity.     

Theoretical study of edge states in BC2N nanoribbons with zigzag edges

In this paper, electronic properties of BC₂N nanoribbons with zigzag edges are studied theoretically using a tight binding model and the first-principles calculations based on the density functional theories. The zigzag BC₂N nanoribbons have the flat bands when the atoms are arranged as B-C-N-C along the zigzag lines. In this arrangement, the effect of charge transfer is averaged since B and N atoms are doped in same sublattice sites. This effect is important for not only the formation of flat bands but also for the validity of the tight binding model for such system.     

Structure and mechanical properties of cubic BC2N crystals within a random solid solution model

Within a random solid solution model based on diamond lattice, the formation energies, lattice constants, and elastic properties of BC₂N crystals were calculated using first-principles methods. We found that the degree of BN–C₂ mixture defined by the ratio of bond contents is the key for the lattice parameter, relative stability, and physical properties in the BC₂N solid solution. BC₂N crystals with the most probable degree of mixture possess larger lattice parameters and less elastic moduli with respect to cubic-BN, and the high formation energy can be compensated by the contribution of configuration entropy under high-temperature conditions. The present model can explain the controversial experimental results on BC₂N crystals synthesized under different conditions.     

Crystallization Behavior of Amorphous Si2BC3N Ceramic Monolith Subjected to High Pressure

The crystallization behavior of amorphous Si₂BC₃N monoliths by heating at 1000°C–1400°C and 5 GPa was investigated with the special attention to the nucleation mechanisms of β‐SiC and BN(C) phases. Nanoscale puckered structures arising in particle bridging areas were found and its evolution behavior well reflected the nucleation process of nanocrystallites. The temperature‐dependent crystallization of amorphous Si₂BC₃N monoliths at 5 GPa passes through four stages: The material remains amorphous below 1100°C. It undergoes partial phase segregation (1100°C–1200°C), followed by initiation of nucleation (1200°C–1250°C), and then nucleation and growth of β‐SiC and turbostratic BN(C) crystallites (>1250°C). The first principles calculation indicates the nucleation precedence of BN(C) phase over β‐SiC. BN(C) nucleates preferentially at bridges between ceramic particles causing SiC to concentrate in particle interiors thus forming capsule‐like structures.     

Direct Decomposition of N2O over Cesium-doped CuO Catalysts

A series of Cs promoted copper oxide catalysts were prepared by co-precipitation method and tested for the direct decomposition of nitrous oxide (N₂O). The Cs promoted catalysts were more active particularly with a molar ratio of Cs/Cu at 0.1 compared to bulk CuO. Methods of XRD, BET, XPS, H₂-TPR, and N₂O-TPD were used to characterize these catalysts to evaluate structure activity relationship. The characterization results indicated that the addition of Cs could improve the reduction of Cu²⁺–Cu⁰ by facilitating the desorption of adsorbed oxygen species, during the N₂O decomposition. The influences of oxygen and steam on N₂O decomposition over these catalysts were also studied.     

High-pressure synthesis of TaN compacts with high hardness and thermal stability

High-hardness TaN compacts with a diameter and height of 6 mm were synthesized through the phase transformation of CoSn-type into WC-type TaN under experimental conditions of high temperature and high pressure. The Vickers hardness of WC-type TaN compacts obtained at the pressure of 5 GPa and the temperature of 1873 K is found to be ~21.5 ± 1.5 GPa with a load of 3 Kg, which is comparable to that of the pure tungsten carbide compact (~20 GPa) and higher than those of ultrahigh temperature ceramics ZrB₂ and HfB₂ under the same load. The high hardness of WC-type TaN compacts presented in this work is attributed to the deviatoric strain and well-bonded nanograins. The synthesized WC-type TaN compacts also possess a high oxidation temperature (1072 K). These results suggest that the WC-type TaN with high hardness and high thermal stability holds great promise for industrial applications.     

Metal Exchanged ZSM-5 Zeolite Based Catalysts for Direct Decomposition of N2O

Co+Pt/ZSM-5 and Ag+Pt/ZSM-5 type catalysts were prepared by ion exchange method followed by calcination. These Co and Ag based catalysts, promoted by a small amount of Pt have been studied for their catalytic activity towards N₂O decomposition. Both the catalysts show high catalytic activity, however, cobalt–platinum based catalyst shows relatively better activity at higher temperature. At 550 °C almost 100% conversion of N₂O is achieved over Co+Pt/ZSM-5 with a maximum of 0.08479 mmole of N₂O decomposed per gram of the catalyst per unit time. These catalytic materials have been characterized for their structure, composition, morphology and other details, using XRD, SEM, EDX, ICP, BET techniques. Much improved catalytic activity for the bimetallic zeolite than the mono-metal containing compositions clearly demonstrate the synergistic effect of these transition metals, while high surface area of ZSM-5 is also responsible for the improved N₂O decomposition activity.     

A first-principle study on the structure, stability and hardness of cubic BC2N

The hardness of cubic BC₂N (c-BC₂N) has sparked considerable debate in the literature. First-principle calculations, guided by bond counting rules, were engaged to investigate the correlation between the stability of an alloy configuration and its hardness by searching through alloy configurations in all distinct unit cells of size up to 12 atoms. The existence of many low-energy and high-density alloy configurations with formation energies in the order of 100–200 meV/atom suggests that a mixture of high-density alloy configurations in the experimental samples is likely to occur due to the high temperature and non-equilibrium conditions offered by modern growth techniques. Theoretical analysis on elastic stiffness and Vickers hardness confirm that these stable high-density c-BC₂N alloy configurations are indeed harder than c-BN.     

Density-modified structure and mechanical properties of amorphous Si2BC3N by ab initio molecular dynamics calculations

Density-modified structural features and mechanical properties of the amorphous Si₂BC₃N are studied by ab initio molecular dynamics simulations. The chemical bonds of Si₂BC₃N are insensitive to the density variation, which is reflected by the negligible changes in bond lengths, peak locations in density of states and Bader charge values under different densities. Instead, the composition of chemical bonds is altered. Meanwhile, high-density condition induces the transition of polyhedral units from the sp²-like trigonal configuration to the tetrahedral configuration, especially for B, C, and N. This variation is the main structural responding mechanism of Si₂BC₃N to increased density, which is different from the stretching and/or shrinkage of bonds as occurred in crystals. The increased tetrahedron content shall further benefit the amorphous structure stability of Si₂BC₃N by impeding the separation of turbostratic BN(C), and enhance the second-order elastic constants, elastic moduli and tensile/shear strengths of Si₂BC₃N. The high density may decrease the debonding capability of the fiber/Si₂BC₃N interface but will not change the preference for crack deflection at interfaces. These results suggest the promising prospect of mechanical property optimization of SiBCN ceramics through density tailoring in experiments.     

Enhanced mechanical properties,thermal shock resistance and ablation resistance of Si2BC3N ceramics with nano ZrB2 addition

The mechanical properties, thermal shock resistance, and ablation resistance of nano ZrB₂ modified Si₂BC₃N ceramics were investigated. The results show that ZrB₂ stimulated microstructure evolution obviously. Therefore, the maximum strength and fracture toughness reach 559.6 MPa and 6.77 MPa·m^1/2, which are improved by 61.0% and 29.4%, respectively. Furthermore, the residual strengths of 10 wt% ZrB₂ containing composites tested at 1000 ℃ retain 363.6 MPa, which is much higher than 97.7 MPa of pristine Si₂BC₃N ceramics. Besides, the ablation resistance of ZrB₂ modified Si₂BC₃N ceramics at 3000 ℃ is enhanced remarkably and the linear and mass ablation rates of ZrB₂-10 are only 0.009 mm/s and 1.91 mg/s, respectively. The ablation in the ultra-high temperature zone is totally dominated by the ZrB₂ component, and the thermochemical erosion is determined by the oxidation resistance of ZrB₂ in the thermal affected zone.     

Highly Efficient Fe-silicalite Zeolite in Direct Propane Ammoxidation with N2O and O2

A novel process for the direct ammoxidation of propane over steam-activated Fe-silicalite at 723–823 K is reported. Yields of acrylonitrile (ACN) and acetonitrile (AcCN) below 5% were obtained using N₂O or O₂ as the oxidant. Co-feeding N₂O and O₂ boosts the performance of Fe-silicalite compared to the individual oxidants, leading to AcCN yields of 14% and ACN yields of 11% (propane conversions of 40% and products selectivity of 25–30%). The beneficial effect of O₂ on the propane ammoxidation with N₂O contrasts with other N₂O-mediated selective oxidations over iron-containing zeolites (e.g. hydroxylation of benzene and oxidative dehydrogenation of propane), where a small amount of O₂ in the feed dramatically reduces the selectivity to the desired product. It is shown that the productivity of ACN and especially AcCN, expressed as mol product h⁻¹ kg_cat⁻¹, is significantly higher over Fe-silicalite than over active propane ammoxidation catalysts reported in the literature. Our results open new perspectives to improve the performance of alkane ammoxidation catalysts.     

A2N2双污泥系统反硝化除磷工艺的启动与稳定

采用低C/N比实际生活污水,以A₂N₂-SBR（厌氧/硝化/缺氧/硝化）双污泥系统为研究对象,重点考察了A₂N₂系统启动过程中的脱氮除磷特性。试验结果表明：采用在A²/O-SBR和N-SBR单元分别接种种泥,分开培养驯化聚磷菌污泥和硝化菌生物膜,并利用A²/O-SBR单元的出水作为N-SBR单元的进水,25 d好氧硝化菌生物膜挂膜成功,氨氮去除率稳定在93%以上;A²/O-SBR单元采用先厌氧/好氧（A/O）后厌氧/缺氧（A/A）的运行方式,43 d成功培养富集了反硝化聚磷菌（DPAOs）,DPAOs占PAOs的67.81%,反硝化除磷率在77.9%以上;启动成功后原水中约73%和13%的COD分别在A²/O-SBR单元的厌氧段和N-SBR单元曝气过程中被去除,系统出水COD、NH⁺₄-N、PO₄^3--P、TN浓度分别为40.6、0、0.4、13.5 mg·L^-1,达到国家《城镇污水处理厂污染物排放标准》（GB 18918-2002）一级A排放标准。     

Thermal stability and hardness of ReB2 type hexagonal OsB2 with the addition of W

This work attempts to understand the effect of W addition on microstructure, thermal stability, and hardness of ReB₂ type hexagonal osmium diboride (h-OsB₂). h-OsB₂ samples with W atomic concentration of (Os+W) from 0% to 30% were synthesized by mechanochemical method combines with pressure-less sintering. The XRD patterns of the as-synthesized powders indicate the formation of Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solution, which has a ReB₂-type hexagonal structure. After being high temperature sintered, part of the h-OsB₂ phase of the pure OsB₂ transformed to orthorhombic (o) phase, while the h-OsB₂ phase was maintained with the addition of W, which suggests that the thermal stability of the sample was remarkably improved. A macroscopically homogeneous structure with some pores can be found from all groups of the as-sintered Os_1-xW_xB₂ (x?=?0, 0.1, 0.2, 0.3) samples, with some B-rich areas distributed in the W doped samples. The lattice parameters of the Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solutions linearly decreased with the increase of the W concentration. The micro-hardness of the OsB₂ sintered samples is 25?±?2?GPa under an applied load of 0.49?N, which increased to 34?±?2?GPa, 38?±?2 and 37?±?2?GPa, respectively when the W concentration increased from 10, 20 and 30?at%. The increased hardness of the h-OsB₂ can be mainly attributed to the improvement of thermal stability with the addition of W.     

Synthesis and characterization of polycrystalline Mo2BC ceramic

Single-phase polycrystalline Mo₂BC ceramic bulks were synthesized successfully from molybdenum, boron, and graphite powders using the spark plasma sintering method. Herein, it was established that the synthesis temperature of the Mo₂BC ceramic could be as low as 1300 °C. Transmission electron microscopy (TEM) characterization confirmed that the crystal structure of the Mo₂BC ceramic was comparable to that of the MoAlB ceramic. The Vickers hardness of the Mo₂BC ceramic was measured to be 18.1 GPa. Additionally, the compressive strength, flexural strength, and fracture toughness were determined to be 1.74 GPa, 457.72 MPa, and 3.26 MPa· m^1/2, respectively. The Mo₂BC bulk exhibited typical brittle features, in which intergranular and transgranular fractures were the main failure modes.     

Synthesis and structural characterization of N,N,N-trimethyl chitosan

N,N,N-trimethyl chitosan (TMC) with molar mass 29 ~ 136 kg/mol and degree of quaternization 70% ~ 82% was synthesized from chitosan via N,N-dimethyl chitosan (DMC) by a two-step method. Size exclusion chromatography (SEC) coupled with multiangle laser light scattering was employed to characterize the molar mass and chain conformation of DMC and TMC. Nearly no degradation was found for DMC, but degradation by ~50% was observed from TMC. Apart from degradation, the shift of SEC peak toward higher elution volume for N-methylated chitosan was attributed to the increase in chain flexibility. It was shown that chitosan, DMC, and TMC took a random coil conformation in 200 mM acetate buffer, with persistence length decreased from 10 nm for chitosan, to 5.7 nm for DMC and 3.2 nm for TMC. The significant increase in chain flexibility upon N-methylation is most probably due to the breakage of intramolecular hydrogen bonds between chitosan repeating units.     

Direct synthesis of SrTaO2N from SrCO3/Ta3N5 involving CO evolution

SrTaO₂N powder was prepared by a novel reaction between Ta₃N₅ and SrCO₃, instead of the traditional nitridation of SrTaO_3.5. The phase evolution and reaction mechanism during the reaction were investigated. The mixed gas evolution of CO, CO₂ and N₂ were observed in the heat-treatment of Ta₃N₅/SrCO₃. The SrTaO₂N product possessed an average particle size of 330 nm in a narrow distribution with the stoichiometric oxygen and nitrogen content. Its ceramics with various relative density were also fabricated by pressureless sintering of the as-prepared SrTaO₂N powder, showing a high sinterability. The fully-annealed SrTaO₂N bulks with a RD = 82.8% showed a relatively large dielectric constant and an improved dielectric loss.