Mechanical and electrical properties of hot-pressed borosilicate glass matrix composites containing multi-wall carbon nanotubes

Borosilicate glass matrix composites reinforced with 10 wt% multiwall carbon nanotubes (CNTs) were fabricated using a conventional powder processing route and uniaxial hot pressing. The microstructure of the composites contained aggregates of CNTs which had not been infiltrated by the viscous glass during hot-pressing leaving a ∼9% residual porosity. As a result, the mechanical properties (hardness, elastic modulus, fracture toughness and fracture strength) were not improved in comparison to those of the monolithic glass matrix. However the brittleness index (B), which is the ratio of hardness to fracture toughness, decreased with addition of CNTs, which indicates that the composites should exhibit improved contact damage and wear resistance. Electrical resistivity measurements revealed that the addition of 10 wt% CNTs to the normally insulating borosilicate glass decreased its resistivity to 13 Ω cm in comparison to the high value (10¹⁵ Ω cm) of the monolithic glass.     

Densification process of borosilicate glass powders under hydrothermal hot-pressing conditions

Borosilicate glass powders containing Li₂O over 3.3 wt% were densified by the addition of water under hydrothermal hot-pressing conditions at 25 MPa below 300° C. The initial shrinkage proceeded by a viscous flow mechanism. The glass powders with a high concentration of network-modifying oxides had high densification rates. At low temperature, the glass powders were gently densified after an induction period in which shrinkage was very slow. During the induction period, the glass powders reacted with water and lithium in the glass structure was dissolved in water.     

涂敷含硼硅玻璃SiC涂层的C/SiC复合材料空气氧化行为

以2D C/SiC复合材料为基底, 采用聚合物裂解工艺(Polymer plyen)制备了含硼硅玻璃SiC自愈合涂层。利用扫描电镜对含硼硅玻璃SiC涂层的2D C/SiC复合材料氧化前后的微结构形貌进行了分析。研究了含硼硅玻璃SiC涂层的C/SiC复合材料在静态空气中700℃、 1000℃和1200℃下的氧化行为, 并分析了涂层层数对C/SiC复合材料氧化行为的影响。结果表明: 含硼硅玻璃SiC涂层在该温度下形成的玻璃相可以较好地封填表面缺陷(裂纹和孔洞); 并且随温度升高及涂层层数增加, 试样在氧化过程中质量减少率降低, 氧化后的强度保持率提高。      

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Tribological properties of surface modified nano-alumina/epoxy composites

Nano-sized Al₂O₃ particles grafted with polystyrene or polyarcrylamide were employed as fillers for fabricating epoxy based composites. Curing habit, mechanical properties and tribological performance revealed by sliding wear tests of the composites were investigated. The experimental results indicated that the nanoparticles accelerate curing of epoxy, increase composites' impact strength and decrease wear rate and frictional coefficient of the composites. The surface modification by means of grafting polymerization can further enhance the properties improvement of epoxy due to the increased filler/matrix interfacial interaction. Compared to frictional coefficient, wear rate of epoxy can be decreased more remarkably by the addition of nano-alumina when rubbing against steel. The wear mode changes from severe peeling off of unfilled epoxy to mild micro-ploughing in the case of nano-alumina filled composites.     

Preparation of C-fibre borosilicate glass composites: Influence of the fibre distribution on mechanical properties

Optimum conditions with respect to preparation-determined fibre distribution in borosilicate glass composites were investigated. Continuous C-fibre bundles were impregnated with glass powder in silicon alkoxide solution and wound in parallel to prepregs which were hot pressed into unidirectional composites. The influence of the glass particle size during the impregnation of fibre bundles and during hot pressing on the homogeneity of the fibre distribution was of special interest, as well as the influence of pressure and temperature on the densification of the composites. Optimum conditions were related to optimum values in the bending strength of the resulting composites. Under the optimum hot-pressing conditions the fibre volume content was varied. It has been shown that the fibre distribution was much more homogeneous when fine-grained glass powder was used for impregnation. At high fibre volume concentrations of the composites the distribution was better than at low concentrations. High fibre concentrations were connected with fracture toughness of the sample, as shown by the three-point bending experiments, whereas samples with low C-fibre concentration showed brittle behaviour.     

Preparation of C-fibre borosilicate glass composites: Influence of the fibre type on mechanical properties

Nine different types of carbon fibres were used as the reinforcement component of the borosilicate glass DURAN. Optimum preparation procedures and parameters for the resulting composites were investigated, the mechanical properties were measured in the bending test, and the results compared. It was found that the densification of composites incorporated with high-modulus (hm) C-fibres could be done at lower temperatures than that of composites with high-strength (hs) C-fibres. The utilization of the fibres in the composites with respect to the tensile strength of the fibres was much better for the hm-C-fibres than for the hs-C-fibres. The experimentally obtained values for Young's modulus and the bending strength of the composites were compared with those calculated from the linear mixing rule (LMR). The measured bendover stresses (stress limit of the elastic region in the stress-strain diagrams) were compared with those from the Aveston-Cooper-Kelly model, which calculates stresses at which cracks in the glass matrix occur. It has been shown that good agreement between experimental and calculated values was found if relatively large values were assumed for the interfacial shear strength.     

EPR in a barium borosilicate glass containing titanium ions

EPR spectra were studied in an equimolar barium borosilicate glass containing 0 to 12 mol% titanium ions. The valence concentration ratio, defined as the ratio of the concentration of the lower valence state transition metal ion Ti³⁺ to the total concentration of the transition metal ion Ti³⁺ + Ti⁴⁺, was varied between 0.15 and 0.85 at each concentration. The spectra were studied as a function of temperature down to 15 K. A broad asymmetric absorption peak is observed with effectiveg ~ 1.938 at room temperature. This absorption is attributed to a tetragonally distorted octahedral ligand field around Ti³⁺ ion. Theg value decreases as the temperature decreases, with a distinct transition between 60 and 100 K. Accompanied by this decrease ing is a distinct broadening of the curves with a transition in the same transition range. These and other observations are explained in terms of a lengthening of the spin-lattice relaxation time in this transition range and this transition is analogous to the NMR motional narrowing phenomena in solids. Many observations at low temperatures suggest a non-random distribution of titanium ions in these glasses.     

Photocatalytic properties of composites containing titania nanoparticles on submicron Y2O3 spheres

The photocatalytic properties of TiO₂/Y₂O₃ composites prepared by coating with anatase and η-TiO₂ nanoparticles, Degussa P25, and Hombifine N by simultaneous dispersing Y₂O₃ and TiO₂ powders in a weakly alkaline aqueous medium (procedure I) or Y₂O₃ powder in an acidic titanium-containing hydrosol (procedure II) have been studied for the first time. The results demonstrate that, after sonication, the η-TiO₂/Y₂O₃ and anatase/Y₂O₃ composites have almost identical photocatalytic activities (PCAs), which are however lower than those of anatase and η-TiO₂, whereas the PCA of the samples prepared by procedure II is comparable to that of nano-TiO₂ under UV irradiation and exceeds the activity of Degussa-N,S (Degussa P25 annealed with thiourea) exposed to visible light. The highest PCA is offered by a TiO₂〈Cl〉/Y₂O₃ composite modified with the products of titanium tetrachloride hydrolysis in air.     

Densification,mechanical and microstructure properties of β-spodumene—alumina composites

New systems consisting of ceramic composites have been recently developed to replace conventional alumina as a substrate for high performance large scale integrated packaging. Few investigations on the β-spodumene—alumina composites have been performed. Accordingly, the effect of β-spodumene content on the sintering behaviour and mechanical properties of the β-spodumene—alumina composites is the aim of the present study. Microstructural characteristics, thermal expansion and phase relations were also discussed. Fine β-spodumene and alumina powders of high purity were synthesized by aqueous sol-gel processing. The gels were dried and calcined and the resulting materials characterized by XRD. Various compositions of β-spodumene—alumina were prepared and conventional sintering studies were conducted at temperatures of 1400°–1600 °C with soaking periods of 1–4 h. The obtained results showed that densification was reduced with the increase of β-spodumene content. Mullite was detected in bodies containing 5% β-spodumene. There was no evidence of mullite presence with higher contents of β-spodumene. Increasing β-spodumene from 5 mass% up to 15 mass% increased the bodies bending strength by ≈43%, while the thermal expansion coefficient was slightly changed from 1.05 to 2.67×10⁻⁶/°C in the range of 20° to 1000 °C.     

Thermal shock resistance of SiC fibrerein-forced borosilicate glass and lithium aluminosilicate matrix composites

Thermal shock resistance of an SiC fibre-(Nicalon®) reinforced borosilicate glass (Pyrex) and lithium aluminosilicate (LAS) matrix composite has been investigated experimentally in the temperature range 0–1000 K. Longitudinal Young's modulus and flexure strength of the composites after thermal shock were obtained as a function of thermal shock temperature. The results are discussed with the observed damage of the composite. The borosilicate glass matrix composite showed multiple cracking of the glass matrix perpendicular to the fibre axis when the thermal shock temperature was above 600 K. Decreases in Young's modulus and flexure strength were also recognized after multiple cracking of the matrix was initiated. On the other hand, the LAS matrix composite showed no damage at thermal shock temperatures below 800 K. However, at 800 K and above, microcracking of the matrix along the fibre axis was observed. After thermal shock, no decrease in the flexure strength was recognized, while the Young's modulus decreased due to microcracking of the matrix when the thermal shock temperatures were 800 K and above. It was found that the major advantage of the composite against thermal shock was to retain non-catastrophic failure properties even after the development of thermally induced damage in the composite.     

Densification and mechanical properties of shock-treated alumina and its composites

The effects of shock treatment and consolidation method on densification behaviour and mechanical properties of Al₂O₃, Al₂O₃-ZrO₂, Al₂O₃-SiC (whisker), and Al₂O₃-ZrO₂-SiC (whisker) have been studied. It was established that shock treatment does not improve the sintering kinetics of alumina or alumina-based composites. On the other hand, partial shock compaction followed by sintering provided higher densities compared to sintering alone. Unshocked and pressureless sintered materials possessed better mechanical properties than shock-treated materials, in general. No significant difference was noted in the mechanical properties of hot-pressed Al₂O₃-ZrO₂ composites with regard to shock treatment. Improved mechanical properties were occasionally found in shock-treated and hot-pressed whisker-reinforced alumina, although a direct relationship with shock pressure was not observed. The improvement was attributed to decreased whisker aspect ratios upon shock treatment, leading to enhanced microstructural uniformity.     

Tensile behaviour of borosilicate glass matrix-Nicalon (silicon carbide) fibre composites

Unidirectional composites consisting of a borosilicate glass (Corning 7740) matrix reinforced with Nicalon (silicon carbide) fibres were fabricated and tested in monotonic tension at temperatures ranging from room temperature to 650 °C. The ultimate tensile strength showed little dependence on temperature up to about 425 °C and failed by longitudinal splitting. There was a significant increase in strength at 540 °C and a slight decrease in strength when tested above this temperature, and the failure involved extensive fibre pull-out. The elastic modulus (stiffness) was found to decrease progressively with increasing temperature. The matrix consists of borosilicate glass within the plies and very fine grains of alpha (low) cristobalite in the inter-ply regions. The behaviour of the composite as a whole was found to be dependent upon the behaviour of the matrix at the temperature of testing.     

Microstructure and dielectric properties of glass/Al2O3 composites with various low softening point borosilicate glasses

The effects of various low softening point borosilicate glasses on both the sintering behavior and dielectric properties of glass/Al₂O₃ composites were investigated by FTIR, DSC, XRD, SEM and EDS. Results show that the addition of alkali oxides and PbO can decrease the glass softening temperature. While, the addition of Al₂O₃ and more SiO₂ content in the glass can increase the continuity of glass network and further increase the glass softening temperature of samples. Glass with lower softening temperature has more time to flow to finish the densification of samples, and that can contribute to get better sintered composites. By contrast, CaO–PbO–B₂O₃–SiO₂–Na₂O–K₂O glass/Al₂O₃ composites sintered at 875?°C for 15?min exhibit better properties of a bulk density of 3.06?g/cm³, a porosity of 0.17?%, a λ value of 2.47?W/m?K, a ε _r value of 8.05 and a tan δ value of 8.8?×?10^?4 at 10?MHz.     

Synthesis of SiC/BN nanocomposite powders by carbothermal reduction and nitridation of borosilicate glass, and the properties of their sintered composites

A new synthetic method for the fabrication of SiC/BN nanocomposites was devised to attain strong machinable ceramics. SiC/BN nanocomposites that contained 10, 20, and 30?vol% hexagonal BN (h-BN) were successfully fabricated by sintering SiC-BN nanocomposite powders by carbothermal reduction and nitridation of borosilicate glass powders. Homogeneous mixtures of silica (SiO(2)), boric acid (H(3)BO(3)), and carbon powder were heated in a nitrogen atmosphere to synthesize SiC-BN nanocomposite powders. Borosilicate glass was obtained by reacting SiO(2) and B(2)O(3) above 800?°C, and SiC and turbostratic BN (t-BN) were obtained by reacting borosilicate glass with carbon powder and nitrogen gas at 1500?°C. Carbothermal reduction followed by nitridation yielded SiC-BN nanocomposite powder composed of nanosized SiC and t-BN. By hot-pressing nanocomposite SiC-BN powders containing 7?wt% Al(2)O(3) and 2?wt% Y(2)O(3), machinable SiC/BN nanocomposites were obtained without a significant decrease in their fracture strength.     

Nanoscale structure and local mechanical properties of fiber-reinforced composites containing MWCNT-grafted hybrid glass fibers

Carbon nanotubes (CNTs) were grown from the surface of glass fibers by chemical vapor deposition, and these hybrid fibers were individually dispersed in an epoxy matrix to investigate the local composite structure and properties near the fiber surface. High-resolution transmission electron microscopy revealed the influence of infiltration and curing of a liquid epoxy precursor on the morphology of the CNT “forest” region, or region of high CNT density near the fiber surface. Subsequent image analysis highlighted the importance of spatially dependent volume fractions of CNTs in the matrix as a function of distance from the fiber surface, and nanoindentation was used to probe local mechanical properties in the CNT forest region, showing strong correlations between local stiffness and volume fraction. This work represents the first in situ measurements of local mechanical properties of the nano-structured matrix region in hybrid fiber-reinforced composites, providing a means of quantifying the reinforcement provided by the grafted nanofillers.     

Densification and mechanical properties of liquid phase sintered tantalum

This paper has as main objective to investigate the influence of nickel (Ni), iron (Fe) and copper (Cu) addition on the tantalum (Ta) powder processing, by promoting a liquid phase sintering (LPS). The role of these metals is to lower the Ta sintering temperature, maintaining good densification and mechanical properties. Ni, Fe and Cu 1wt% additions to Ta powder were performed. Samples were cold pressed at 350 MPa. Sintering was carried out at 1300 to 2000 °C, for 1 hour, under a vacuum of 10^‐6 Pa. Density, linear shrinkage and activation energy were measured and calculated. Hardness and compression tests were also conducted. Ni was the most promissing Ta LPS activator, once it enabled the best results of densification and mechanical properties.