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1.
The adjustment of strength and ductility of high-temperature ceramics is still a big challenge. Although Mo-based silicides are promising high-temperature materials, the influence of Mo concentration on the mechanical and electronic properties of Mo-based silicides is unclear. In addition, it is necessary to explore the novel Mo-based silicides. In this paper, we present results of novel phases, mechanical and electronic properties of the stable Mo-based silicides within various stoichiometries. Two new Mo-based silicides: MoSi (Cmcm and Pnma) and Mo2Si (I4/mcm) are predicted. The calculated results show that the volume deformation resistance of Mo-based silicides increases with increasing Mo concentration. MoSi2 shows the strongest elastic stiffness and shear deformation resistance due to the strong Mo-Si bonds. The calculated intrinsic hardness of MoSi2 (37.7 GPa) is much larger than that of other Mo-based silicides. In particular, MoSi2 and MoSi show brittle behavior. However, other silicides exhibit ductility. We further find that high concentration of Mo can improve the electronic properties of Mo-based silicides because of the formation of Mo-Mo metallic bond. Finally, our works indicate that the adjustment of the Mo stoichiometric ratio to improve the mechanical and electronic properties of Mo-based silicides.  相似文献   

2.
Transition-metal aluminium nitrides widely used as protective tool coatings are a class of materials with a combination of high hardness, outstanding wear resistance as well as good chemical stability. In this work, through a well developed structure searching method, the ground-state phase of V0.5Al0.5N is verified and systematically studied on its mechanical and electronic properties by comparing with Ti0.5Al0.5N and W0.5Al0.5N via first-principles calculations. Our results show that the ground-state phase of V0.5Al0.5N adopts a hexagonal structure of P63/mmc symmetry. Mechanical property studies demonstrate the hexagonal phase has a surprisingly improved hardness of about 38 GPa and enhanced ideal strengths relative to its well-known metastable cubic B1 phase whose hardness is only about 20 GPa. This mechanical enhancement greatly expands the upper limit of the strength and hardness for this type of Al-containing ternary systems. Meanwhile, detailed analysis on strength and elastic anisotropy indicates it also exhibits much better mechanical isotropy. Underlying mechanism of the mechanical enhancement is explored by the electronic analysis in-depth. The position of the EF is tuned by the introduction of the Al element and this electronic tuning leads to a metallic-to-semiconductor transformation from B1 to the hexagonal phase and the strengthening of the bonds between the metal elements and the N atoms.  相似文献   

3.
Transition metal silicides are promising advanced functional materials. However, the structure and relevant properties of Mo5Si3 are not well understood. In this work, we investigate the crystal structure, elastic properties, Vickers hardness, elastic anisotropy, electronic and thermodynamic properties of Mo5Si3 by using the first-principles calculations. Three structures: tetragonal, hexagonal and orthorhombic structures are considered. The calculated results show that those structures are thermodynamically stable. In particular, we firstly predict that Mo5Si3 with hexagonal (P63/mcm) structure is a stable phase. The calculated electronic structure shows that Mo5Si3 exhibits better electronic properties because of the charge overlap between Mo-4d state and Si-3p state near the Fermi level. Importantly, Mo5Si3 shows the strong deformation resistance and high elastic stiffness in comparison to other TM5Si3. Mo5Si3 with tetragonal structure has the smaller percentage anisotropy in compressibility and high percentage anisotropy in shear. We further find that the Debye temperature and heat capacity of tetragonal structure are larger than that of hexagonal structure. The high-temperature thermodynamic properties of Mo5Si3 are attributed to the vibration of Si atom.  相似文献   

4.
The mechanical properties of newly synthesized Ca2C3 and Ca2C under pressure have been studied by using the first-principles calculations with generalized gradient approximation. The equilibrium geometry, elastic stiffness constants, various moduli, and Pugh's ratio of the C2/m phase of Ca2C3 and the C2/m and Pnma phases of CaC2 are systematically studied. The elastic stiffness constants of C2/m-Ca2C3 under 0–30GPa, C2/m-Ca2C under 0–7.5 GPa, and Pnma-Ca2C under 7.5–30 GPa satisfy the Born?Huang mechanical criteria. The three phases of calcium carbides exhibit ductile characteristics. The surface constructions of bulk and Young's moduli illustrate the mechanical anisotropy of Ca2C3 and Ca2C. Our results are consistent with previously obtained experimental and theoretical data and have significant implications for the application of calcium carbides.  相似文献   

5.
Lanthanide orthophosphate ceramics with monazite structure gained broad interest for several industrial applications. The crystallization processes, compressibility and sinterability of monazite-type lanthanum orthophosphate powder hydrothermally synthesized at 200 °C as well as mechanical properties of the sintered compacts were investigated. Based on a combination of thermo- and surface area analyses, X-ray diffraction as well as scanning electron microscopy studies it was found that the crystallization process occurs at ∼500 °C and the final crystallization of LaPO4 monoclinic phase takes place at 1400 °C. The sintered pellets are characterized by a density of 98% of theoretical density, a Vickers hardness of 5.7 ± 0.1 GPa and fracture toughness of 1.4 ± 0.1 MPa m0.5.  相似文献   

6.
The structural, electronic and optical properties of six WB2 diborides with hP3, hP6, hP12, oP6, hR9 and hR18 structures were systematically investigated using the first-principles calculation based on density functional theory. The optimized atomic coordinates and lattice parameters agree well with the corresponding experimental and theoretical results. All WB2 are energetically stable, and hP6-WB2 has the best phase stability and hP3-WB2 shows the worst phase stability. The results of density of states and the charge density differences indicate that WB2 have the strong W–B and B–B covalent bonds. The hardness was obtained from the Mulliken population. The predicted values of absorption coefficient α(ω) and reflectivity R(ω) reveal that the laser with a longer wavelength is recommended during the synthesis of WB2 coatings on the substrate surface using the Nd-YAG laser. Finally, the anisotropy in optical properties for WB2 was discussed via the polycrystalline and directional static dielectric constants ε1(0) and static refractive indexes n(0).  相似文献   

7.
Novel Sm-based enantiomeric pair, generally formulated Sm(DBM)3L (LR,R in 1, LS,S in 2, DBM = dibenzoylmethanate) have been successfully prepared via the reaction of Sm(DBM)3·2H2O with chiral ligands LR,R (−)-4,5-pinene bipyridine and LS,S (+)-4,5-pinene bipyridine (Scheme 1), respectively. The crystal structure analysis of 1 and 2 reveal that they crystallize in chiral space group P21 of monoclinic system. The central Sm(III) ion is octacoordinate with six β-diketonate oxygen atoms and two chiral pinene bipyridine nitrogen atoms, forming a coordination polyhedron best described as the distorted square antiprism (SA). The CD spectra (Fig. S1) further confirm that 1 and 2 are enantiomers. The photoluminescence investigations of 1 and 2 demonstrate that they display deep-red luminescence characteristic of the Sm3+.  相似文献   

8.
    
《Ceramics International》2017,43(12):8763-8768
Although ZrB2 is a promising ultrahigh-temperature ceramic, the intrinsic brittleness and low fracture toughness are the main bottlenecks. To solve these key problems, by means of first-principles calculations, we predict ZrB2 new phases, and investigate the influence of new phase on the mechanical properties and electronic properties of ZrB2. The calculated results show that two new ZrB2 phases: RuB2-type with orthorhombic structure (Pmmn, No.59) and ReB2-type with hexagonal structure (P63/mmc, No.194) are dynamical stability at the ground state. Although RuB2-type and ReB2-type structures weaken the volume deformation resistance and shear deformation resistance of ZrB2, it results in brittle-to-ductile transition due to the formation of weak Zr-B bond along the shear direction. Importantly, ReB2-type structure improves the electronic properties of ZrB2 because of the strong charge overlap between conduction band and the valence band near Fermi level (EF). Therefore, our work can open up a new clue to improve the ductility and electronic properties of ZrB2.  相似文献   

9.
Herein, biomimetic Cf/ZrB2-SiC ceramic composites with bouligand structures are fabricated by combining precursor impregnation, coating, helical assembly and hot-pressing sintering. First, Cf/ZrB2-SiC ceramic films are achieved through a precursor impregnation method using polycarbosilane (PCS). Second, the PCS-Cf/ZrB2-SiC ceramic films are coated with ZrB2 and SiC ceramic layers. Finally, hot-pressing sintering is employed to densify helical assembly Cf/ceramic films with a fixed angle of 30°. The microstructures and carbon fiber content on the mechanical properties of biomimetic Cf/ZrB2-SiC ceramic composites are analyzed in detail. The results show that the coated ceramic layer on PCS-Cf/ZrB2-SiC films can heal the cracks formed by pyrolysis of PCS, and the mechanical properties are obviously improved. Meanwhile, the mechanical properties could be tuned by the contents of the carbon fiber. The toughening mechanisms of Cf/ZrB2-SiC ceramic composites with bouligand structures are mainly zigzag cracks, crack deflection, multiple cracks, carbon fiber pulling out and bridging.  相似文献   

10.
ZTM ceramics were successfully derived from coal gangue. The effect of zirconium source (ZrO2 and ZrOCl2?8H2O) and content on properties of the ZTM ceramics has been studied. The phase composition, density, and microscopic morphology were characterized with X-ray diffraction (XRD), Archimedes method and scanning electron microscopy (SEM). The influence of zirconium source, sintering temperature, zirconia content on flexural properties and fracture toughness was studied. The sample, with ZrOCl2?8H2O added 12% zirconia (Z2TM12) sintered at 1475 °C for 3 h has the highest density of 2.83 g/cm3. Partially stable t-ZrO2 was present in samples with zirconium oxychloride (ZrOCl2?8H2O) as the zirconium source, due to the constraints of mullite crystals. Therefore, Z2TM12 had both microcrack toughening and stress phase transformation toughening mechanisms. The flexural strength was increased from 162.40 MPa to 285.04 MPa, while the fracture toughness was improved to 3.55 MPa m1/2 from 2.38 MPa m1/2. Our achievement can be used as a reference to fabricate ZTM ceramics from coal gangue with high-value additions.  相似文献   

11.
Hydrophobic association hydrogels (HA-gels) were successfully prepared through micellar copolymerization of acrylamide (AM) and a small amount of octylphenol polyoxyethylene acrylate (OP-4-AC) in an aqueous solution containing sodium dodecyl sulfate (SDS). HA-gels exhibited excellent mechanical properties and transparency. Especially, HA-gels possessed the capability of re-forming, such as self-healing and molding. From Fourier transform infrared, swelling behavior and re-forming capability of HA-gels, the network structure was established. On the basis of the micellar copolymerization theory, the statistical molecular theory of rubber elastic, and using uniaxial stretching data, the length of the hydrophobic microblocks, the effective network chain density and the molecular weight of the chain length between cross-linking points were evaluated for all HA-gels; furthermore, they were also evaluated for the region of medium deformation by the Mooney-Rivlin theory. For HA-gels, we investigated in detail the effects of the content of compositions in the initial solution, OP-4-AC, SDS and AM, on their tensile mechanical properties on the basis of the proposed network structure. The results clearly indicate their tensile strength, fracture energy, elastic modulus, and elongation strongly depended on their composition content.  相似文献   

12.
The effects of holding time in Bridgman furnace on mechanical properties of injection molded silica-based ceramic cores for directional solidification of SX superalloy have been investigated. The cylindrical samples (S0) were sintered at 1000 ℃ for 60 min, and some of the sintered samples (S1, S2, S3) were treated by heat treatment at 1500 ℃ to simulate the directional solidification process. Isothermal uniaxial compression tests of ceramic core samples were conducted on a Gleeble-1500D mechanical simulator. Weibull approach was used to describe the strength distribution of silica-based ceramic cores. As a result, the mean compressive strength of the sample (S0) is 40.43 MPa. The mean compressive strengths of the samples (S1, S2, S3) with heat treatment at 1500 ℃ are 54.34 MPa, 53.60 MPa and 53.81 MPa, respectively, which are significantly larger than that of S0. The mean elastic moduli of the samples (S1, S2, S3) with heat treatment at 1500 ℃ are 2726.39 MPa, 2855.91 MPa and 2797.14 MPa, respectively, which is significantly higher than that of S0. The refractory particle evolution of the ceramic core during the directional solidification process is analyzed, and the microstructural investigations show that the crack propagation of ceramic core sintered at 1000 ℃ is mainly through the sintering necks between particles. However, the crack propagation of ceramic core holding at 1500 ℃ is extended through the entire large particles. The re-sintering process of ceramic core holding at 1500 ℃ compensates the negative effect of cracks due to the volume contraction during β- to α-phase transformation and the rapid cooling process, and improves the ceramic core uniformity and mechanical properties.  相似文献   

13.
Tungsten borides with excellent mechanical properties have been recognized as a class of ultrahard compounds in various industrial applications. Here, motivated by the recent experimental work, a quantitative comparison analysis on the structure, mechanical strength, and electronic structure of the tantalum-strengthened superhard W0.5Ta0.5B monoboride and its parent material WB has been studied by first-principles calculations. Excellent agreements of the calculated lattice parameters and simulated X-ray diffraction between present results and experimental data have confirmed the crystal structure of the synthesized W0.5Ta0.5B. Compared to the WB, the calculated stress-strain curves show an enhanced shear strength and improved ductility of W0.5Ta0.5B on (100) and (010) crystal planes, originating from the reduction of antibonding states between the W-eg states which enables strenuous sliding of metal bilayer in W0.5Ta0.5B. Furthermore, the lattice instability of W0.5Ta0.5B under large shear strain with an intriguing sequential bond-breaking mode that is derived from the first breaking of zigzag B chains and the subsequent collapsing of WB7 and TaB7 polyhedrons by simultaneously breaking of B–W and B–Ta bonds. These findings shed strong light on the strengthening mechanism of W0.5Ta0.5B and the design for novel ultra-incompressible and superhard solids in transition metal monoborides.  相似文献   

14.
The mechanical properties and self-monitoring capability of cement mortar containing nano-SiO2 or nano-Fe2O3 were experimentally studied and compared with that of plain cement paste. The results showed that the compressive and flexural strengths measured at the 28th day of cement mortar containing nano-SiO2 or nano-Fe2O3 were both higher than that of plain cement mortar with the same water-binder ratio (w/b). Furthermore, the self-monitoring capability of cement mortar with nano-Fe2O3 is also presented in this paper.  相似文献   

15.
Polycrystalline YAl3C3 ceramic was fabricated by an in situ hot-pressing process using YH2, Al, and C powders as starting materials. The crystal structure of YAl3C3 was determined by X-ray diffraction and ab initio calculations to be a hexagonal space group P63/mmc. The lattice parameters are a = b = 3.424 Å, c = 17.291 Å, and the atomic positions are Y at 2a (0, 0, 0), Al1 at 2d (1/3, 2/3, 3/4), Al2 at 4f (1/3, 2/3, 0.138), C1 at 2c (1/3, 2/3, 1/4), and C2 at 4f (1/3, 2/3, 0.6045). The reaction synthesis path for YAl3C3 was presented. Microstructure observations revealed that YAl3C3 ceramic had plate-like grain morphology. The elastic, mechanical and thermal properties of YAl3C3 were investigated. Most significantly of all, the Young’s modulus of YAl3C3 could retain 276 GPa (85% of that at room temperature) at 1500 °C, suggesting that YAl3C3 is a promising lightweight high temperature structural material.  相似文献   

16.
A. Raya  M.A. Mora 《Polymer》2004,45(18):6391-6397
A quantum-chemical study at the Hartree-Fock, (HF), second order Møeller-Plesset perturbation theory, (MP2), and density functional theory, (DFT), levels was performed on perfluorinated oligothiophenes with the aim to predict the potential utility of these materials in the development of electronic devices based on organic n-type semiconductors. The electronic properties analyzed, such as ionization potential, HOMO-LUMO energy difference and electron affinities suggest that perfluorinated oligothiophenes are more difficult to oxidize, and have a larger band gap in comparison with their non-substituted parent compounds. Structural changes on bond lengths and bond angles between perfluorinated and non-substituted oligothiophenes were also observed. Thus, the incorporation of fluorine atoms into oligomers structure could be an effective way to design materials with n-type conductivity.  相似文献   

17.
The polycrystalline samples of multiferroic Bi1−xDyxFeO3 (x = 0, 0.1, and 0.2) were prepared by a modified solid state reaction method and characterized by X-ray diffraction, scanning electron microscopy, differential thermal analysis, dielectric and magnetic measurements. It was shown that the introduction of the Dy3+ ions stabilizes the perovskite structure and improves phase purity. The coexistence of the rhombohedral and orthorhombic phases was found to exist within the investigated concentration range 0.1  x  0.2. The changes and anomalies observed in dielectric response over a wide frequency range were correlated with the structural evolution and the development in microstructure. The SQUID measurements of the field-dependent magnetization at different temperatures demonstrated Dy doping to be a very effective method for inducing a weakly ferromagnetic state in the ferroelectric R3c phase of BiFeO3 in the absence of an external magnetic field.  相似文献   

18.
对影响铝炭复合材料机械性能的诸因素,如气孔率、结合剂种类及用量、焙烧温度、石墨用量等进行了实验研究。  相似文献   

19.
Mg(1-x)ZnxTa2O6 (x = 0.00?0.08) dielectric ceramics were synthesized via the traditional solid-state reaction method. We used XRD and Rietveld refinement to demonstrate that a pure Mg(1-x)ZnxTa2O6 phase with trirutile structure was formed. Zn2+ substitution helped to decrease the Raman full width at half width of the A1g mode at 703 cm?1, which resulted in an increase in the order and rigidity of the TaO6 octahedron, this in turn contributed to improving the Q×f values. Additionally, the introduction of Zn2+ significantly promoted grain growth and increased the dense, and the molecular polarizability, these factors lead to a higher permittivity. Moreover, enhanced Ta-O bond energy resulted in a more stable TaO6 octahedron in the Mg(1?x)ZnxTa2O6 system, which contributed to enhanced τf values via substitution of Zn2+ doped on the A-site. Correspondingly, the microwave dielectric properties were significantly improved for 0.04-doped samples, obtaining: εr = 27, Q × f = 185,000 GHz (at 7.47 GHz), τf =32 ppm/°C.  相似文献   

20.
First-principles calculations using quantum-mechanical density functional theory (DFT) are carried out to investigate the geometrical structure and electronic properties for hydrogen terminated nanometer-sized diamonds. The results reveal that the size dependent feature in the electronic structures for nanodiamonds is different from that of Si clusters. The field emission properties for nanodiamonds are also explored, and it is found that under applied electric field Mulliken charges redistribute and accumulate on the emission side. Furthermore, the emission currents from the occupied orbitals for nanodiamond are calculated and it is revealed that the largest emission current comes from the third highest occupied molecular orbital.  相似文献   

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