Reactive hot-pressing of ZrB2-ZrC-SiC ceramics via direct addition of SiC

A series of ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B₄C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB₂: 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol%.     

Observation of polytypism among the hydroxysalts of Ni(II) and Co(II)

Layered hydroxysalts derive their crystal structure from mineral brucite. Nickel and cobalt based hydroxysalts with the composition M₃(OH)₄(NO₃)₂ crystallize in trigonal crystal system. On careful examination of the PXRD patterns, it is found that Ni₃(OH)₄(NO₃)₂ crystallizes in hexagonal crystal system while Co₃(OH)₄(NO₃)₂ crystallizes in rhombohedral crystal system. This clearly demonstrates that the crystal structures in layered hydroxysalts are mainly dictated by the nature of the cation occupying the preferential site within the layer.     

Crystal structure of an aliphatic polyoxamide containing methyl side-groups: Poly(2-methyl-1,8-octamethyleneoxamide)

Poly (2-methyl-1,8-octametyleneoxamide) (nylon-MOMD-2) was prepared from dibutyl oxalate and 2-methyl-1,8-octamethylenediamine. The chemical structure was investigated by ¹³C nuclear magnetic resonance (¹³C NMR). The ¹³C NMR spectrum revealed that the methyl groups distribute among the four β-sites at random. The crystal structure and its temperature dependence were also investigated by molecular mechanics calculations, X-ray diffraction, and differential scanning calorimetry. The structure was determined to be monoclinic and a = 6.26 Å, b = 8.80 Å, c = 14.7 Å, and β = 50.7°. The torsion angle of the NH–CH₂ bonds has a skew conformation and there are the two distinct torsion angles in the crystal structure. The crystal structure of nylon-MOMD-2 has a disordered and statistical structure. The crystal structure shows no phase transition on heating until its melting point, although the unit cell expands in the a and b axis directions.     

Novel (Zr,Ti)B2-(Zr,Ti)C-SiC ceramics via reactive hot pressing

Fully dense (Zr, Ti)B₂-(Zr, Ti)C-SiC ceramics were prepared by reactive hot-pressing using ZrB₂, TiC, and SiC as the initial materials for the first time. Effects of SiC addition on the microstructure evolution and mechanical properties were reported. The in-situ reaction between ZrB₂ and TiC as well as the SiC addition leads to the grain refinement. Besides, elongated (Zr, Ti)B₂ plate-like grains are obtained due to the occurrence of a transient liquid phase, which leads to the crack deflection in the matrix effectively. Mechanical properties are improved significantly due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects. The ZrB₂-10 mol%TiC composite with 10 mol% SiC additional exhibits good comprehensive mechanical properties of the hardness of 20.2 GPa, the flexural strength of 803 MPa, and the fracture toughness of 5.7 MPa m^1/2.     

Sub-stoichiometric tungsten oxide by the stearic acid method

The synthesis of tungsten oxide by peroxotungstic route presents challenges, such as low solubility of tungstic acid in water and the fact that calcination product is a mixture of polytungstates. We propose a new route to obtain sub-stoichiometric tungsten oxides, using stearic acid, a non-toxic and low-cost material, as the dispersing and structuring agent for tungstic acid. The interaction between the precursor and the structuring agent was evaluated through chemical and structural analyses. Calcination of the stearic acid/tungstic acid systems produces vacant-rich monoclinic and orthorhombic tungsten oxides. Stearic acid plays a role of growth-directing agent and higher concentration of stearic acid promoted the growth of structures with plaque-shaped morphology in the samples, contributing to obtain sub-stoichiometric tungsten oxides, desired materials for visible light absorption.     

Microstructural and optical properties of SnO2–ZnSnO3 ceramics

This work deals with some physical investigation on SnO₂–ZnSnO₃ ceramics grown on glass substrates at different temperatures (450 °C and 500 °C). Structural and optical properties were investigated using X-Ray diffraction (XRD), Raman, infrared (IR) absorption (FTIR), UV–visible spectroscopy and Photoluminescence (PL) techniques. XRD results revealed the existence of a mixture of SnO₂/ZnSnO₃ phases at different annealing temperatures. Structural analysis showed that both phases are polycrystalline. On the other hand, the optical constants (refractive index, extinction coefficient and the dielectric constants) have been obtained by the transmittance and the reflectance data. The optical band gap energy changed from 3.85 eV to 3.68 eV as substrate temperature increased from 450 °C to 500 °C. Raman, FTIR modes and PL reinforced this finding regarding the existence of biphasic (SnO₂ and ZnSnO₃) which is detected also by X-Ray diffraction analysis. Finally, the Lattice Compatibility Theory was evoked for explaining the unexpected incorporation of zinc ions in a rhombohedral structure within SnO₃ trigonal lattice, rather than the occupation of SnO₂ available free loci. All the results have been discussed in terms of annealing temperature.     

钕铁硼粉末热压成型工艺与组织研究

利用PW1700型x射线衍射仪和QUANTA400型扫描电子显微镜研究了钕铁硼粉末热压成型中的工艺影响因素,在ZKRY-90型真空热压炉中进行热压成型。结果表明,粉末热压成型工艺主要受温度影响,压力次之。制样后对组织和取向进行了分析,发现测得的NdFeB磁体颗粒取向趋于一致,并以(006)取向为主要取向。     

Enhanced sodium ion conduction of highly dense bulk zeolite X prepared by hydrothermal hot-pressing

Highly dense bulk zeolite X with translucency was successfully fabricated by hydrothermal hot-pressing of zeolite powder at 150 °C. Densification was achieved by addition of a small amount of distilled water or a 1 mol/L NaOH solution. Enhanced sodium ion conduction was observed for the densified bulk ceramic bodies. The ion conductivities obtained were beyond 0.001 S/cm, and are comparable to those of practical solid electrolytes applied in electrochemical devices such as batteries and chemical sensors.     

Effect of pressure on the crystal structure of ettringite

X-ray diffraction and infrared data have been collected from a sample of ettringite from ambient pressure to 6.4 GPa. The sample was found to reversibly transform to an amorphous phase at 3 GPa. The isothermal bulk modulus of ettringite was found to be 27(7) GPa and the incompressibilities of the lattice parameters were found to be 71(30) GPa along a and 108(36) GPa along c.     

The density of cement phases

The densities of principal crystalline phases occurring in Portland cement are critically assessed and tabulated, in some cases with addition of new data. A reliable and self-consistent density set for crystalline phases was obtained by calculating densities from crystallographic data and unit cell contents. Independent laboratory work was undertaken to synthesize major AFm and AFt cement phases, determine their unit cell parameters and compare the results with those recorded in the literature. Parameters were refined from powder diffraction patterns using CELREF 2 software.A density value is presented for each phase, showing literature sources, in some cases describing limitations on the data, and the weighting attached to numerical values where an averaging process was used for accepted data. A brief discussion is made of the consequences of the packing of water to density changes in AFm and AFt structures.     

Crystal structure of poly(pentamethylene 2,6-naphthalate)

The crystal structure of poly(pentamethylene 2,6-naphthalate) (PPN) was determined by using X-ray diffraction and molecular modeling. The unit cell of PPN was found to be triclinic ( space group) with dimensions of a=0.457 nm, b=0.635 nm, c=2.916 nm, =121.6°, β=90.4°, γ=87.6°, and the calculated crystal density was 1.311 g cm⁻³. The unit cell contains one polymer chain with two repeating units. In the unit cell, the PPN backbone takes gauche/gauche conformation in the middle part of each pentamethylene unit, and two naphthalene rings are in face-to-face arrangement.     

Preparation of tungsten borides by combustion synthesis involving borothermic reduction of WO3

An experimental study on the preparation of two tungsten borides, WB and W₂B₅, was conducted by self-propagating high-temperature synthesis (SHS), during which borothermic reduction of WO₃ and elemental interaction of W with boron proceeded concurrently. Powder mixtures with two series of molar proportions of WO₃:B:W = 1:5.5:x (with x = 1.16–2.5) and 1:7.5:y (with y = 0.5–1.33) were adopted to fabricate WB and W₂B₅, respectively. The starting stoichiometry of the reactant compact substantially affected the combustion behavior and the phase composition of the final product. The increase of metallic tungsten and boron reduced the overall reaction exothermicity, leading to a decrease in both combustion temperature and reaction front velocity. The initial composition of the reactant compact was optimized for the synthesis of WB and W₂B₅. In addition to small amounts of W₂B and W₂B₅, the powder compact of WO₃ + 5.5B + 2 W produced WB dominantly. Optimum formation of W₂B₅ was observed in the sample of WO₃ + 7.5B + 0.85W. Experimental evidence indicates that an excess amount of boron about 10–13% is favorable for the formation of WB and W₂B₅.     

褐煤热压脱水实验研究

为提高褐煤脱水率,以锡林郭勒褐煤为研究对象,采用高温反应釜进行了褐煤热压脱水实验,考察了温度、压力和保压时间对褐煤脱水率的影响,并利用静态氮吸附仪对热压脱水前后煤样的孔隙结构进行测定。结果表明:温度250℃、压力3 MPa、保压60 min时褐煤脱水率最大可达到93.67%。在热力协同作用下,加压可使褐煤实现"攥拳式"脱水,压缩褐煤碳骨架的同时,挤出内水,改变褐煤孔隙结构及分布。热压脱水后,褐煤比表面积、总孔体积、平均孔径分别降低了0.70192 m2/g、0.00405 cm3/g和1.38593 nm,煤样孔径分布明显改变,微孔比例增加5.57%,中孔比例增加4.97%,大孔比例减小10.54%。实验证明以气体加压方式进行褐煤热压脱水可行。     

Crystal structure of Kuzel's salt 3CaO·Al2O3·1/2CaSO4·1/2CaCl2·11H2O determined by synchrotron powder diffraction

The crystal structure of Kuzel's salt has been successfully determined by synchrotron powder diffraction. It crystallizes in the rhombohedral R3? symmetry with a = 5.7508 (2) Å, c = 50.418 (3) Å, V = 1444.04 (11) ų. Joint Rietveld refinement was realized using three X-ray powder patterns recorded with a unique wavelength and three different sample-to-detector distances. Kuzel's salt is the chloro-sulfoaluminate AFm phase and belongs to the layered double hydroxide (LDH) large family. Its structure is composed of positively charged main layer [Ca₂Al(OH)₆]⁺ and negatively charged interlayer [Cl_0.50·(SO₄)_0.25·2.5H₂O]⁻. Chloride and sulfate anions are ordered into two independent crystallographic sites and fill successive interlayer leading to the formation of a second-stage compound. The two kinds of interlayer have the compositions [Cl·2H₂O]⁻ and [(SO₄)_0.5·3H₂O]⁻. The crystal structure explains why chloride and sulfate anions are not substituted and why the formation of extended solid solution in the chloro-sulfate AFm system does not occur.     

Global optimization of reactive distillation networks using IDEAS

In this work, we present a methodology for the global optimization of reactive distillation (RD) networks, through the Infinite DimEnsionAl State-space (IDEAS) approach. Within the IDEAS framework, network synthesis is formulated as an infinite dimensional linear optimization problem. The IDEAS conceptual framework is realized through solution of a series of finite dimensional linear programs whose optimum values converge to the infinite program’s infimum. The proposed optimal design methodology is demonstrated on a case study involving reactive distillation-based synthesis of methyl tert-butyl ether (MTBE) from isobutene and methanol.     

Effect of mechanochemical treatment on petroleum coke-CO2 gasification

The effect of mechanochemical treatment during the grinding of petroleum coke on its gasification by CO₂ was studied. An additive derived by drying the black liquor in papermaking industry is adopted in grinding process. Results show that the gasification reactivity of petroleum coke is effectively improved by grinding, and the activation by wet grinding is more noticeable than that by dry grinding. Besides, by wet grinding petroleum coke and additive together, the active metal species in additive are not easily volatilized in gasification, and retain a high catalytic reactivity to the coke-CO₂ reaction throughout most of the conversion range. Changes in crystal structure of the petroleum coke induced by mechanochemical treatment is related to its gasification reactivity. In general, the crystalline-amorphous phase transition is the tendency of long time mechanical grinding, while a crystal structure re-formation stage is observed after wet grinding of petroleum coke with and without additive for some time. Similar phenomenon has also been found in the reported data, but not given attention. Some discussion is made in the paper, and more work should be undertaken to disclose the mechanism.     

A simple route to produce tungsten carbide powders by high-energy ball milling and annealing

Tungsten carbide powders are extensively used to provide wear-resistance properties to metal-matrix coatings. Here, we propose a simple and efficient route for the production of WC powders from elemental W and C by room-temperature high-energy ball milling (HEBM) and subsequent annealing. WC powders were obtained, with a minor (<4%wt.) iron contamination, with only 4 h of milling of a W/C atomic mixture of 42/58 and 90min anneal at 1000 °C. This innovative and simple route to produce WC powders can be used to produce feedstock of reinforcing particles intended for wear resistant coatings. We explored the impact of the milling times (from 6min to 96 h), compositions (W:C ratio of 1, 1.17 and 1.38) and annealing conditions ranging from 4 h at 850 °C to 90 min at 1000 °C. The samples were analyzed for phase distribution and composition using XRD, SEM with EDX mapping, XRF and TGA. As expected, milling induces amorphization and introduces iron from the milling media. We show that an excess of C can efficiently suppress the formation of deleterious tungsten carbides.