Processing and Mechanical Properties of Pressureless-Sintered Niobium-Alumina-Matrix Composites

Niobium-alumina (Nb-Al2O3) composites have been fabricated via the pressureless sintering of compacts of intensively milled Nb-Al2O3 mixtures. Strength and fracture toughness of the composites increase as the niobium content increases. For a composite that contains 50 vol% niobium, strengths of up to 680 MPa, with a corresponding fracture toughness of 6.3 ± 0.3 MPam1/2 and hardness of 6.4 GPa, have been obtained.     

Weathering of High-Density Polyethylene-Wood Plastic Composites

Due to the widespread use of wood-plastic composites (WPCs), high-density polyethylene-wood flour composites (HDPE/WF) were studied in order to determine their stability in different application conditions. UV degradation and periodic absorption/desorption of moisture cause damaging changes to material during WPCs’ exterior application, so it is necessary to ensure WPCs’ durability against atmospheric influences. Samples were characterized by FTIR spectroscopy and scanning electron microscopy (SEM) in order to study the degradation after simulated weathering. The degree of water absorption was also determined. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used for the determination of composites’ thermal properties. Results show that the stability of the HDPE/WF composites to UV treatment highly depends on stabilizer content and its dispersion in the polymer matrix. Incompatibility of HDPE and wood particles is a major problem that should be solved to achieve good durability and satisfying properties in use.     

高密度聚乙烯／废橡胶胶粉共混物的改性研究

采用新的改性体系三元乙丙橡胶（EPDM）、硅油、过氧化二异丙苯（DCP）对高密度聚乙烯／胶粉（HDPE／SRP）共混物进行了研究，探讨了各改性剂用量对共混物力学性能的影响。结果表明：EPDM用量为10－15份，硅油用量为4份，DCP用量为0.2份，共混物具有较好的力学性能。与未改性HDPE／SRP共混物相比，改性后共混物（EPDM为10份，硅油为4份，DCP为0.2份）的冲击强度和断裂伸长率分别可以大幅度提高，并且胶粉含量越少，提高的幅度越明显。扫描电镜观察结果表明：改性共混物中弹性体的分散和包覆结构是韧性提高的主要原因。     

Mechanical,Thermal and Morphological Characterization of High-Density Polyethylene and Vermiculate Composites

High-density polyethylene composites containing 7, 10, 15 and 20% w/w vermiculite (VMT) were prepared by extrusion in a twin-screw extruder at various shear rates. The thermal and mechanical properties and morphological characteristics of the composites were evaluated and compared with pure high-density polyethylene. The blend morphologies were determined by scanning electron microscopy (SEM). The thermal properties of the composites were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The TGA results reveal that the use of VMT particles to fill polyethylene increased the thermal stability of the composite. The results obtained indicate that a shear speed of 400 rpm and vermiculite content of 7% showed the best properties of impact resistance. These observations were confirmed by morphology analysis.     

无压烧结碳化硼的研究进展

碳化硼陶瓷具有高硬度、高熔点和低密度的特点,是优异的结构陶瓷,在民用、航空和军事等领域都得到了重要应用。本文综述了无压烧结碳化硼陶瓷的国内外研究进展,阐述了不同的烧结助剂、烧结温度和颗粒尺寸等因素对碳化硼陶瓷性能的影响。     

Mechanical,Thermal, and Microstructure Analysis of Silk- and Cotton-Waste-Fiber-Reinforced High-Density Polyethylene Composites

In this study, composite structures were produced using HDPE polymer with silk and cotton waste as reinforcement fibers in different ratios. Cotton and silk wastes were mixed in the ratios of HDPE/silk or cotton waste 100%/0%, 97%/3%, and 94%/6%. This mixture was prepared with double-screwed extruder. The tests were carried out in terms of tensile strength, % elongation, yield strength, elasticity module, izod impact strength, melt flow index (MFI), heat deflection temperature (HDT), and vicat softening temperature. Materials' thermal transitions were determined with differential scanning calorimetric (DSC) and microstructure properties with scanning electron microscope (SEM).     

Sensitivity of Dielectric Properties to Wear Process on Carbon Nanofiber/High-Density Polyethylene Composites

We examined the correlation of wear effects with dielectric properties of carbon nanofibers (CNFs; untreated and organosilane-treated)-reinforced high-density polyethylene (HDPE) composites. Wear testing for the nanocomposites over up to 120 h was carried out, and then, dielectric permittivity and dielectric loss factor of the polymer composites with the increased wear time were studied. Scanning electron microscope and optical microscope observations were made to analyze the microstructure features of the nanocomposites. The results reveal that there exist approximate linear relationships of permittivity with wear coefficient for the nanocomposites. Composites containing silanized CNFs with the sufficiently thick coating exhibited high wear resistance. The change in permittivity was more sensitive to the increased wear coefficient for the nanocomposites with lower wear resistance. This work provides potential for further research on the application of dielectric signals to detect the effects of wear process on lifetime of polymeric materials.     

Fracture Origin and Strength in Advanced Pressureless-Sintered Alumina

Advanced raw materials and shaping approaches enable the production of pressureless-sintered alumina parts where, in bending, the average maximum stress at the fracture origin is as high as 800 MPa. In individual specimens that fracture at lower stresses (450–600 MPa), failure often originates at volume flaws, as known for hot-pressed alumina with a similar strength. Also, transgranular and intergranular fracture modes along the crack path are the same as those observed in hot-pressed alumina. If the size and the frequency of volume flaws are reduced, fracture initiates at smaller defects in the ground surfaces and bodies with a bending strength of >800 MPa are produced without hot pressing. The grain-size dependence of grinding-induced surface damage contributes to a grain-size effect for the strength.     

Pressureless Sintering of High-Density ZrB2–SiC Ceramic Composites

Ultra-high-temperature ceramic composites of ZrB₂ 20 wt%SiC were pressureless sintered under an argon atmosphere. The starting ZrB₂ powder was synthesized via the sol–gel method with a small crystallite size and a large specific surface area. Dry-pressed compacts using 4 wt% Mo as a sintering aid can be pressureless sintered to ∼97.7% theoretical density at 2250°C for 2 h. Vickers hardness and fracture toughness of the sintered ceramic composites were 14.82±0.25 GPa and 5.39±0.13 MPa·m^1/2, respectively. In addition to the good sinterability of the ZrB₂ powders, X-ray diffraction and scanning electron microscopy results showed that Mo formed a solid solution with ZrB₂, which was believed to be beneficial for the densification process.     

Effects of Defibration Conditions on Mechanical and Physical Properties of Wood Fiber/High-Density Polyethylene Composites

Defibration conditions influence wood fiber characteristics and thereby properties of fiber-based materials. In this study, the effects of several defibration conditions on mechanical and physical properties of fiber-based wood-plastic composites (WPCs) are illustrated. Various WPCs were tested containing different thermo-mechanical pulps (TMPs) or groundwood pulp (GWP), whereby material composition (50 wt% wood fibers, 47 wt% polymer, 3 wt% coupling agent) and the production process (internal mixer, injection molder) were kept consistent. The data from the experiment revealed that differing defibration conditions led to statistically significant differences in the tested flexural, tensile, and impact properties as well as in the water absorption of WPC. Overall, the GWP and the TMP which was produced under the mildest defibration conditions performed best in fiber-based WPCs. Therefore, grinders and refiners may be equally suitable to produce pulp for WPC usage. As a side-effect within this study, the reinforcing effect of fiber application on flexural and tensile properties was on an extraordinarily high level.     

Preparation of High-Density Ceria-Yttria Ceramics

Fine Ce-Y oxide powders were prepared by coprecipitation of the carbonate, followed by calcination at 620°C. Hydrolysis of trichloroacetic acid was used to precipitate most of the material from a homogeneous solution; addition of ammonium bicarbonate and ripening of the precipitate were used to increase the yield to 68 to 70%. The precipitate apparently consisted of more than one phase and had an overall composition which suggested that it was a possible mixture of hydrated carbonate, hydroxycarbonate, and hydroxide. The calcined oxide powder was used to prepare an yttria-doped ceria ceramic by hot-pressing. The ceramic had the fluorite structure phase with a lattice constant of 0.541088 nm and a composition of Ce_0.914Y_0.086O_1.957. The bulk density of the material was 6.96 g/cm³, or 99.4% of theoretical density. The ceramic had equiaxed grains, with an average dimension of 1 to 4 μm and with residual porosity mainly along the grain boundaries. Ac impedance properties of the chemically prepared oxide are compared with those of mechanically mixed and fired Y-doped CeOZ.     

Dislocation Loops in Pressureless-Sintered Undoped BaTiO3 Ceramics

Dislocation loops in pressureless-sintered undoped BaTiO₃ ceramics have been analyzed by transmission electron microscopy. The Burgers vector of the loops and its sense b =+1/2[010] were determined using the g·b =0 invisibility criteria, combined with the inside–outside contrast technique using ( g·b ) s _g >0 or<0, keeping the deviation parameter s _g >0. The edge-vacancy nature was further ascertained by determining the loop habit plane normal n =[0 1 0]. Weak-beam dark-field imaging reveals that loops contained no stacking fault fringes; they are edge-vacancy partial dislocation loops lying in {020} or {010} where parts of the TiO₂ or BaO layer are vacant. It is suggested that the extrinsic defects of both cations and oxygen vacancies generated by non-stoichiometry have condensed during sintering in air and are responsible for the formation of such vacancy loops.     

High-Density Yttria for Practical Ceramic Applications

This paper describes a process by which highly reactive yttria powder can be sintered to high density at a relatively low firing temperature to form a variety of useful ceramic items. The process utilizes an aqueous polymer to precipitate a yttrium precursor. The yttrium precursor is thermally treated to form a friable oxide ash. After a double-calcination-grind procedure, high-density yttria items such as substrates, crucibles, and nozzles can be fabricated.     

高密度烃燃料合成进展

总结了液体高密度烃燃料合成的最新进展,从原料、合成路线、使用性能等方面评价了多环高密度烃、高张力笼状烃、纳米添加剂等的优缺点及发展前景,分析了高密度烃燃料合成中存在的问题,介绍了作者所在课题组在此领域的工作进展。     

Electrical Properties of High-Density Iron-Deficient Nickel-Zinc Ferrites

The dc resistivity of nickel-zinc ferrite was studied as a function of nickel/zinc ratio, apparent density, temperature, and grain size. Resistivities of Ni_0.40Zn_0.51Fe_1.90O₄– and Ni_0.35Zn_0.65-Fe_1.90O₄– are similar. Evaluation of samples sintered between 1100° and 1220°C showed that densification proceeds rapidly for sintering temperatures 1170°C; for these specimens the room temperature resistivity increases to an equilibrium value with sintering time. Samples sintered to 99+% of theoretical density at lower temperatures densify slowly; the resistivity is invariant with sintering time. The Seebeck coefficient for the p -type ferrites is 550 μV/°C from 200° to 700°C; the dielectric constant varies from 17.3 at 0.5 MHz to 16.4 at 15 MHz.     

Microstructure and Densification of Pressureless-Sintered Al2O3/Si3N4-Whisker Composites

Composite densification was studied by performing slip casting and sintering experiments on an Al₂O₃ matrix and Si₃N₄ whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si₃N₄ affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.     

Ultrasonic Evaluation of High-Density Silicon Carbide Ceramics

Nondestructive ultrasound testing has been evaluated as a technique for analyzing isolated bulk defects and microstructural inhomogeneities in silicon carbide (SiC). Three SiC samples of varying thickness, two of which were fabricated by hot pressing and a third that was fabricated by chemical vapor deposition (CVD), were characterized using pulse–echo ultrasound characterization at a frequency of 75 MHz. Point analysis techniques were utilized to measure variations in time-of-flight (TOF), or ultrasound travel time through each sample, for calculation of regional differences in material velocity and elastic properties. C-scan imaging was used to evaluate differences in both TOF and reflected signal amplitude over the area of each sample. Area-under-the-curve (AUTC) and full-width at half-maximum (FWHM) data were obtained from normalized histograms to establish trends for direct sample comparison. It was determined that lower AUTC and FWHM values correlated to higher density samples with fewer inhomogeneities. However, the histogram tail area and distribution were also important features, providing information about specific inhomogeneities and their distributions.     

Some Physical Properties of High-Density Thorium Dioxide

The values for a number of physical properties are reported for a very high density form of thorium dioxide. When specimens of a mixture of 99½% ThO² and ½% CaO, by weight, were hydrostatically pressed at 30,000 lb. per sq. in. and heat-treated for 1 hour at 1800°C., they attained 99.0% of theoretical density. All the test specimens were extremely brittle. Physical-property values determined at room tempera- ture were the following: lattice constant; bulk and theoretical (X-ray) densities; compressive and impact strengths; Knoop hardness; modulus of rupture and Young's modulus, determined by a static method; Young's modulus and the shear modulus, determined by a dynamic method; Poisson's ratio and the bulk modulus, calculated from the dynamic-test data; and the velocity of sound through the material. The properties determined at elevated temperatures were the following : linear thermal expansion modulus of rupture and Young's modulus, determined by a static method; Young's modulus and the shear modulus, determined by a dynamic method; and Poisson's ratio, calculated from the elevated-temperature dynamic-test data. "Martin's diameter" grain counts were taken for the material both before and after heat-treatment.