同向非对称双螺杆混合石墨增强导热高分子材料性能研究

选用粒径为15 μm的鳞片石墨和3 μm的氧化铝(Al2O3)为导热填料,采用新型同向非对称双螺杆挤出机为加工设备,在石墨填充量为10 %（质量分数,下同）而Al2O3填充量为10 %~50 %范围内,制备聚丙烯(PP)/Al2O3和PP/Al2O3/石墨导热高分子材料并进行性能测试。结果表明,添加少量石墨,在Al2O3填充量低时可增强导热高分子材料的拉伸强度,石墨与Al2O3的混杂减缓了拉伸强度下降的速率,改善了导热高分子材料的弯曲和冲击性能;PP/Al2O3/石墨的熔体流动速率比PP/Al2O3的小,PP/Al2O3/石墨比PP/Al2O3的负载热变形温度升高约15 %。     

Effect of Polyaxial Stress States on Failure Strength of Alumina Ceramics

The effect of polyaxial stress fields on the brittle fracture strength of polycrystalline alumina was investigated through the use of thin-walled cylinders. Combinations of internal pressure, external pressure, and axial loads produced stress states of tension-compression, tension-tension, and compression-compression. The failure envelope was generated for these stress states. The results indicated that biaxial tensile stresses reduced the strength of the material; however, the tensile strength increased at least 50% when a compressive stress existed normal to the tensile direction. Compression strengths as high as 640,000 psi were measured for a biaxial compressive stress state.     

Determination of Biaxial Compressive Strength of a Sintered Alumina Ceramic

The compressive strength of commercial high-density alumina was measured in several compressive biaxial stress states. The compressive strength was only slightly affected by the magnitude of the intermediate principal stress. The average value of the compressive strengths measured in all stress states, excluding the equibiaxial and the near-uniaxial compressive stress states, was 528 ksi. The average of the coefficients of variation was 3%. The tensile strength measured for the material indicates that the ratio of compressive strength to tensile strength is ∼ 18.     

Biaxial and Uniaxial Data for Statistical Comparisons of a Ceramic's Strength

The uniaxial and equibiaxial tensile strengths of a brittle material were measured in bending. Equibiaxial tension was attained by concentric ring loading of disks and uniaxial tension by four-point line loading of plates. The two specimen designs give equal volumes, surface areas, and stress gradients. Ground surfaces and lapped surfaces were tested. The equibiaxial tensile strength of a dense alumina was lower than the uniaxial tensile strengths for both ground and lapped surfaces, 8.5 and 8.1%, respectively. The Batdorf theory of flaw statistics, in which biaxial tensile strengths can be predicted from the statistical distribution of uniaxial tensile strength measurements, agreed with the data.     

Monazite Coatings on Fibers: I, Effect of Temperature and Alumina Doping on Coated-Fiber Tensile Strength

Monazite was continuously coated onto Nextel 720 fibers, using an aqueous precursor and in-line heat treatment at 900°–1300°C. Some experiments were repeated with alumina-doped precursors. Coated fibers were heat-treated for 100 h at 1200°C. Coatings were characterized by optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. Coated-fiber tensile strengths were measured by single-filament tensile tests. The precursors were characterized by X-ray diffractometry, differential thermal analysis/thermogravimetric analysis, and mass spectrometry. Coated-fiber tensile strength was lower for fibers coated at higher deposition temperatures. Heat treatment for 100 h at 1200°C decreased tensile strength further. The coatings were slightly phosphate-rich and enhanced alumina grain growth at the fiber surface, but phosphorus was not detected along the alumina grain boundaries. Fibers with alumina-doped coatings had higher tensile strengths than those with undoped coatings after heat treatment for 100 h at 1200°C. Alumina added as α-alumina particles gave higher strengths than alumina added as colloidal boehmite. Alumina doping slowed monazite grain growth and formed rough fiber–coating interfaces after 100 h of heat treatment at 1200°C. Possible relationships among precursor characteristics, coating and fiber microstructure development, and strength-degradation mechanisms are discussed in this paper.     

Strength of a Ceramic at High Temperatures Under Biaxial and Uniaxial Tension

A design is presented for a fixture to test concentric-ring loaded disks and four-point loaded plates at high temperatures. In this fixture the loading plunger is guided by flexure plates to determine the load path with accuracy. The uniaxial and equibiaxial tensile strengths of a brittle material were measured at 982°C with this fixture. For lapped surfaces, the equibiaxial tensile strength of dense, isostatically pressed, sintered alumina was 16.0% lower than the uniaxial tensile strength. The uniaxial strength at 982°C was 42.3% lower than the previously reported value for comparable tests at room temperature. The effect of frictional forces and lateral constraint, of specimen and loading geometry, and of local stresses on the nominal stress are discussed.     

Static and Cyclic Fatigue Behavior of a Polycrystalline Alumina

The fatigue behavior of a polycrystalline alumina was investigated. Stress conditions consisted of a static tensile stress and a static tensile stress with superposed sinusoidal cyclic stress. The alumina exhibited the expected static fatigue behavior; a cyclic fatigue effect characterized by a frequency and amplitude dependence was also observed. Possible mechanisms of cyclic fatigue in brittle materials are discussed.     

Verwendung von Kohlenstoff-Fasern im Apparatebau

Use of carbon fibres in equipment constuction . Carbon fibres are characterized by high tensile strengths, high modulus of elasticity, and good corrosion resistance. They are therefore especially suitable for reinforcement of components made of plastics (CFC). Carbon fibre reinforced parts made of resin-impregnated graphite withstand higher mechanical stress and offer greater reliability in operation. CFP materials is an economical alternative to high-alloyed steel for use in corrosive media. Apart from graphite CFC is one of the few materials of construction suitable for use up to temperatures of over 2000 °C. It has also been sucessfully used for highly corrosion-resistant equipment such as grids and piping.     

Bend Strength versus Tensile Strength of Fiber-Reinforced Ceramics

The bending strength of fiber-reinforced glasses and ceramics is often observed to be higher than their tensile strength; the difference varies, however, from one material to another. To gain an understanding of the relationship between these two measure of strength, we have carried out an analysis of bending which accounts for the deviations from linearity that occur on the tensile side of the beam. The results of this analysis indicate that the strength ratio (bending strength/tensile strength) depends most sensitively on the rate at which the stress drops after the ultimate tensile strength. In particular, composites failing gracefully (with a gradual decay in stress) tend to have comparatively higher strengths in bending. A method of inferring the: tensile strength from simply the load-deflection curve in bending is proposed. In addition, by accounting for the weakness in interlaminar shear, we can predict the variation in bend strength with beam aspect ratio. The various theories are compared with experimental data.     

Fracture of composite alumina/yttria-stabilized zirconia joints

Four-point bend tests have been performed on samples consisting of yttria-stabilized zirconia containing 0–80% alumina joined by plastic deformation to the same or different composition. The fracture strength of joints between the same composition was equal to the strength of the monolithic material. Fracture of joints made between different compositions occurred at the position of maximum tensile residual stress, as determined by finite-element analysis, not at the interface. Measured strengths were in accord with fracture mechanics and the calculated residual stresses.     

Strength and Notch Sensitivity of Porous-Matrix Oxide Composites

The effects of matrix strength on the notched and unnotched tensile properties of a family of porous-matrix oxide composites are examined both experimentally and theoretically. Experiments are performed on three composites, distinguished from one another by the amount of binding alumina within the matrix. Increases in alumina concentration produce elevations in unnotched tensile and shear strengths, but the benefits are offset by an increase in notch sensitivity. The degree of notch sensitivity is rationalized on the basis of a model that accounts for interactions between notch tip tensile and shear bands. The model predictions are cast in terms of the ratio of the notch length to a characteristic bridging length scale. These results, in turn, form the basis for a simple analytical formula for notched strength, accounting for effects of elastic anisotropy and finite sample size. The utility of this formula in predicting notched strength is assessed. Issues associated with bridging law shapes and bridging length scales are addressed. The effect of alumina concentration on notch sensitivity is discussed in terms of its influence on the bridging length scale, dictated by the interplay between the unnotched tensile strength, the longitudinal Young's modulus, the degree of in-plane elastic anisotropy, and the fracture energy. The net result is a decreasing bridging length scale and hence increasing notch sensitivity as the matrix is strengthened with alumina.     

Fracture Toughness of Spray-Dried Powder Compacts

The strengths and fracture toughness values were measured for alumina powder compacts containing two different binder systems. Diametral compression was used to measure both the tensile strength and the fracture toughness (through-thickness notch). This methodology was very useful in linking processing parameters, such as binder choice and compaction stress, to the quality of the green bodies. Observations of the compact structure before and after fracture showed that the binders segregated to the region between the spray-dried granules. The presence of the excess binder in this region was linked to both the failure mode and the creation of secondary cracks.     

Graphite Fibre-Epoxy Matrix Interface Interactions

Studies of fibre-epoxy resin matrix model composites show that the “tensile debond” test is not applicable to carbon or graphite fibres. Fibre fracture occurs under the compression strains involved relieving interface stresses and precluding subsequent debond. Calculated minimum bond strengths for pitch-based graphite fibres are similar to results for boron and glass fibres. Interfacial failure is obtained with the “shear debond” test for low and intermediate modulus graphite fibres, but compression fracture also occurs first with high modulus fibres. Pitch-based graphite fibres show a decreasing adhesive interaction with epoxy resin the more oriented the fibre, but results compare favorably with those of other fibres. Surface characterisation shows that all pitch-based graphite fibres exhibit a surface-oriented skin, although surface roughness increases with fibre modulus. The fibres all exhibit similar apparent surface energy characteristics which suggests that wettability does not play a significant role in determining interfacial bond strengths.     

Reliability Analysis of Structural Ceramics Subjected to Biaxial Flexure

Sintered alumina and silicon nitride were tested in uniaxial (four-point and three-point bend) and biaxial (uniformpressure-on-disk) flexure tests in inert conditions. Fracture origins were identified to be surface flaws in alumina and subsurface pores in silicon nitride. Batdorf's statistical fracture theory and two different fracture criteria, the critical normal stress criterion and a noncoplanar strain energy release rate criterion, were used to examine size and stress-state effects on fracture strengths of the two ceramics. Size effects assessed in four-point and three-point bend tests were in good agreement with the theoretical predictions for both ceramics. Measured biaxial strengths of alumina were in good agreement with the prediction when a noncoplanar strain energy release rate criterion and random surface flaw orientations were assumed. On the other hand, biaxial fracture strength of the silicon nitride was consistent with a prediction based on preferred flaw orientation (i.e., normal to the principal stress in the disks) and the normal stress fracture criterion. Orientation distributions of the fracture planes assessed from the fracture patterns of the disks supported the assumptions of random flaw orientations (alumina) and the preferred flaw orientations (silicon nitride), respectively, for the two ceramics. The preferred flaw orientation in silicon nitride is suggested to originate at subsurface pores as a result of crack nucleation in the plane of maximum tensile stress concentration, i.e., a diametral plane normal to the maximum principal stress.     

Low-Cost Melt Formed Siliconized Silicon Carbide Radiant Tube Materials

This paper describes a new low-cost, melt infiltration method for manufacturing silicon carbide–silicon (SiC─Si) and graphite–silicon (G─Si) materials for radiant tube applications, and the physical and mechanical characteristics of these materials. Characterization of the microstructures and various physical and mechanical properties of three SiC─Si and one G─Si materials which were produced by this newly developed processing method are discussed in detail. The C-ring and O-ring specimens tested in compression showed that at all test temperatures (room to 1350°C), the SiC─Si materials are substantially stronger than the G─Si material, which actually contained 8.5 vol% SiC due to reaction of the graphite with the molten Si during processing. The strengths as determined by the C-ring and O-ring tests for each of the materials were similar, thus indicating that the inherent flaws on the inside and outside surfaces of the tubes are similar in severity. Since these materials showed significant deformation when tested at and above 1000°C, the strength of the G─Si material and one of the SiC─Si materials was also measured at room and at elevated temperatures, using a tensile test procedure. Results of the tensile tests illustrate that, due to stress redistribution at 1000°C and above, the C-ring and O-ring data overestimate the actual fracture strengths of these materials. The fracture toughness of the G─Si material measured at room temperature and 1000°C was much less than that of the SiC─Si materials. This behavior was explained by the presence of low-strength graphite grains, which did not inhibit crack propagation as effectively as the SiC grains.     

Effects of carbon fillers on tensile and flexural properties in polypropylene‐based resins

A potential application for conductive resins is in bipolar plates for use in fuel cells. The addition of carbon filler can increase the electrical and thermal conductivities of the polymer matrix but will also have an effect on the tensile and flexural properties, important for bipolar plates. In this research, three different types of carbon (carbon black, synthetic graphite, and carbon nanotubes) were added to polypropylene and the effects of these single fillers on the flexural and tensile properties were measured. All three carbon fillers caused an increase in the tensile and flexural modulus of the composite. The ultimate tensile and flexural strengths decreased with the addition of carbon black and synthetic graphite, but increased for carbon nanotubes/polypropylene composites due to the difference in the aspect ratio of this filler compared to carbon black and synthetic graphite. Finally, it was found that the Nielsen model gave the best prediction of the tensile modulus for the polypropylene based composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The Effect of Applied Stress on the Densification of a Low-Temperature Cofired Ceramic-Filled Glass System Under Constrained Sintering

The effect of uniaxial stress on the mechanical response and densification behavior of a low-fire borosilicate glass (BSG)+alumina system during constrained sintering of a multilayer BSG+alumina/alumina laminate has been investigated. Compared with free sintering, the pressure-less constrained sintering of BSG+alumina exhibits poorer densification, and larger porous bulk viscosity at a given temperature. This is caused by the in-plane tensile stress and anisotropic development generated in the transverse directions of the laminate during constrained sintering. The applied uniaxial stress required in the thickness direction to densify BSG+alumina under constrained sintering varies in the range of 50–400 kPa at 700°–800°C. The above results are in agreement with those calculated using the viscous analogy for the constitutive relationships of a porous sintering compact.     

Stress Development during Constrained Sintering of Alumina/Glass/Alumina Sandwich Structure

Stress development during the constrained sintering of a sandwich structure of alumina/glass/alumina has been studied. Stress distribution in the glass layer was calculated using a finite element method. The shear stress exhibited a maximum at the edge of the alumina/glass interface. The in-plane tensile stress, formed during constrained sintering, decreased from the interface of the alumina/glass to the middle of the glass in the z -direction, but increased from the edge to the center of glass in the x − y direction. This in-plane tensile stress reduces the driving force of densification, resulting in a larger x − y shrinkage and higher densification in the middle of the glass.     

Graphite Fibre-Epoxy Matrix Interface Interactions

Studies of fibre-epoxy resin matrix model composites show that the “tensile debond” test is not applicable to carbon or graphite fibres. Fibre fracture occurs under the compression strains involved relieving interface stresses and precluding subsequent debond. Calculated minimum bond strengths for pitch-based graphite fibres are similar to results for boron and glass fibres. Interfacial failure is obtained with the “shear debond” test for low and intermediate modulus graphite fibres, but compression fracture also occurs first with high modulus fibres. Pitch-based graphite fibres show a decreasing adhesive interaction with epoxy resin the more oriented the fibre, but results compare favorably with those of other fibres. Surface characterisation shows that all pitch-based graphite fibres exhibit a surface-oriented skin, although surface roughness increases with fibre modulus. The fibres all exhibit similar apparent surface energy characteristics which suggests that wettability does not play a significant role in determining interfacial bond strengths.     

Ceramic Composites with Three-Dimensional Architectures Designed to Produce a Threshold Strength—II. Mechanical Observations

Finite element modeling and linear elastic fracture mechanics are used to model the residual stresses and failure stress of ceramic composites consisting of polyhedral alumina cores surrounded by thin alumina/mullite layers in residual compression. This type of composite architecture is expected to exhibit isotropic threshold strength behavior, in which the strength of the composite for a particular assumed flaw will be constant and independent of the orientation of tensile loading. The results of the modeling indicate that the strengths of such architectures will be higher than those of laminates of similar architectural dimensions that were previously found to exhibit threshold strength behavior for a particular flaw type. Flexural testing of the polyhedral architectures reveals that failure is dominated by processing defects found at junctions between the polyhedra. Fractography revealed the interaction of these defects with the residual stresses in the compressive layers that separate the polyhedra.