Role of interface on electrical conductivity of carbon nanotube/alumina nanocomposite

Interface of multiwalled carbon nanotube (MWCNT)/alumina (Al₂O₃) nanocomposites have been studied using TEM. At low sintering temperature (T_sin=1500 °C), a 3–5 nm thick amorphous interface region was noticed. Nanocomposite sintered at 1700 °C possessed a well-defined graphene layer coating on matrix grains as the interface between CNT and Al₂O₃. A mechanism of such layered interface formation has been proposed. No traceable chemical reaction product was observed at the interface even after sintering at 1700 °C. It was noticed that while DC electrical conductivity (σ_DC) of 1500 °C sintered 2.4 vol% MWCNT/Al₂O₃ nanocomposite was only~0.02 S/m, it raised to ~21 S/m when sintering was done at 1700 °C. Such 10³ times increase in σ_DC of present nanocomposite at a constant CNT loading was not only resulted from the exceptionally high electron mobility of CNT but the well-crystallized graphene interface on insulating type Al₂O₃ grains also significantly contributed in the overall increase of electrical performance of the nanocomposite, especially, when sintering was done at 1700 °C.     

Strong and ultra-flexible polymer-derived silicon carbonitride nanocomposites by aligned carbon nanotubes

We report the synthesis of flexible ceramic composites with a high tensile strength (536.33±7.23 MPa) using carbon nanotube sheet aligned by mechanically stretching process. The process is based on the infiltration and pyrolysis of liquid ceramic precursor into aligned carbon nanotube sheet. Mechanical properties and microstructure of the resultant composites are investigated. The resultant nanocomposites maintain well-aligned carbon nanotube morphology with high volume fraction (60%) and long pullout (15 µm), contributing to a high degree of load-transfer efficiency and toughening. Flexibility test reveals that such ceramic nanocomposites retain the original mechanical properties and microstructures after one thousand repetitions of 75% bending deformation, showing excellent compliance and durability. Applications requiring materials with high flexibility and mechanical properties can benefit from this research.     

Mechanical properties and rapid consolidation of binderless nanostructured tantalum carbide

The rapid sintering of nanostructured TaC hard material was investigated with a focus on the manufacturing potential of high-frequency induction heated sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. A dense pure TaC hard material with a relative density of up to 96% was produced with simultaneous application of 80 MPa pressure and induced current within 3 min. The finer the initial TaC powder size, the higher the density and the better mechanical properties. The fracture toughness and hardness values obtained from 10 h milled powder were 5.1 ± 0.3 MPa m^1/2 and 22 GPa, respectively, under 80 MPa pressure and 80% output of total power capacity (15 kW).     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

Fabrication and characterization of random chopped fiber reinforced reaction bonded silicon carbide composite

This paper deals with the microstructure and mechanical properties of reaction bonded silicon carbide reinforced with random chopped carbon fibers of 3 mm length. The composites were fabricated by dispersing chopped carbon fibers into bimodal SiC/C suspension, forming green body through slip casting, and then reaction sintering at 1700 °C. The effect of the chopped fiber fraction on microstructure and mechanical properties was evaluated. A significant increase of fracture toughness was obtained as the carbon fiber fraction approaches 30 vol.%. The chopped fibers had reacted with liquid silicon during reaction sintering, so little fiber pullout was observed. Crack deflection and bridging is the predominant mechanism for the composite toughening.     

Microstructure and physicomechanical properties of pressureless sintered multiwalled carbon nanotube/alumina nanocomposites

Multiwalled carbon nanotube (MWCNT)/alumina (Al₂O₃) nanocomposites containing CNT from 0.15 vol.% to 2.4 vol.% have been successfully fabricated by simple wet mixing of as-received commercial precursors followed by pressureless sintering. Extent of densification of nanocomposites sintered at low temperature (e.g. 1500 °C) was <90%, but increased up to ∼99% when sintered at 1700 °C and offered superior performance compared to pure Al₂O₃. Nanocomposites containing 0.3 vol.% MWCNT and sintered at 1700 °C for 2 h in Argon led to ∼23% and ∼34% improvement in hardness and fracture toughness, respectively, than monolithic Al₂O₃. In addition, the highest improvement (∼20%) in bending strength was obtained for 0.15 vol.% MWCNT/Al₂O₃ nanocomposite compared to pure Al₂O₃. Weibull analysis indicated reliability of nanocomposites increased up to 0.3 vol.% MWCNT, whereas, beyond that loading consistency was the same as obtained for pure Al₂O₃. Detailed microstructure and fractographic analysis were performed to assess structure-property relationship of present nanocomposites.     

Mechanical and electrical properties of carbon nanofibers reinforced aluminum nitride composites prepared by plasma activated sintering

High density carbon nanofibers (CNFs) reinforced aluminum nitride (AlN) composites were successfully fabricated by plasma activated sintering (PAS) method. The effects of CNFs on the microstructure, mechanical and electrical properties of the AlN composites were investigated. The experimental results showed that the grain growth of AlN was significantly inhibited by the CNFs. With 2 wt.% CNFs added into the composites, the fracture toughness and flexural strength were increased, respectively to 5.03 MPa m^1/2 and 354 MPa, which were 20.9% and 13.4% higher than those of monolithic AlN. The main toughening mechanisms were CNFs pullout and bridging, and the main reason for the improvements in strength should be the fine-grain-size effect caused by the CNFs. The DC conductivity of the composites was effectively enhanced through the addition of CNFs, and showed a typical percolation behavior with a very low percolation threshold at the CNFs content of about 0.93 wt.% (1.51 vol.%).     

Enhanced electrical properties of vertically aligned carbon nanotube-epoxy nanocomposites with high packing density

During their synthesis, multi-walled carbon nanotubes can be aligned and impregnated in a polymer matrix to form an electrically conductive and flexible nanocomposite with high backing density. The material exhibits the highest reported electrical conductivity of CNT-epoxy composites (350 S/m). Here, we show how conductive atomic force microscopy can be used to study the electrical transport mechanism in order to explain the enhanced electrical properties of the composite. The high spatial resolution and versatility of the technique allows us to further decouple the two main contributions to the electrical transport: (1) the intrinsic resistance of the tube and (2) the tunneling resistance due to nanoscale gaps occurring between the epoxy-coated tubes along the composite. The results show that the material behaves as a conductive polymer, and the electrical transport is governed by electron tunneling at interconnecting CNT-polymer junctions. We also point out the theoretical formulation of the nanoscale electrical transport between the AFM tip and the sample in order to derive both the composite conductivity and the CNT intrinsic properties. The enhanced electrical properties of the composite are attributed to high degree of alignment, the CNT purity, and the large tube diameter which lead to low junction resistance. By controlling the tube diameter and using other polymers, the nanocomposite electrical conductivity can be improved.     

Broadband electromagnetic shielding performance of carbon nanotube-carbon fibre/silicon carbide cross-scale laminated composites

A carbon nanotube-carbon fibre/silicon carbide (CNT-CF/SiC) laminated composite, with a density of 1.61 g/cm³, thickness of 2.7–3.0 mm and conductivity of 6.10 S/cm, was prepared by densifying a single layer with boron-modified phenolic resin and then welding it with resin-derived carbon layer by layer. This laminated composite was alternately composed of a relatively dense CNT buckypaper/SiC composite layer and a relatively porous three-dimensional needled CF felt/SiC composite layer. The CF felt with a laminated constructure produced a laminated substructure nested within the layers. Expanded graphite with laminated structures produced laminated substructures nested within the interfaces. The average total shielding efficiency values of the composites with 5 layers (CNT-CF/SiC-5), 4 layers and a CNT buckypaper/SiC composite layer on the top surface, and 4 layers and a CF felt/SiC composite layer on the top surface were 45.14, 37.70 and 38.85 dB, respectively, throughout the X-band and were 52.31, 45.56 and 43.54 dB, respectively, throughout the Ku-band. The transmission coefficient of CNT-CF/SiC-5 was as low as 10^?5?10^?6 orders of magnitude over the entire frequency range of 8.2–18 GHz except for very few frequency points. The optimal number of layers for this multilevel and multiscale laminated composite is believed to be 5.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

Microstructure and properties of needle punching chopped carbon fiber reinforced carbon and silicon carbide dual matrix composite

Chopped carbon fiber preform reinforced carbon and SiC dual matrix composites (C/C–SiC) were fabricated by chemical vapor infiltration (CVI) combined with liquid silicon infiltration. The preform was fabricated by repeatedly overlapping chopped carbon fiber web and needle punching technique. A geometry model of the pore structure of the preform was built and reactant gas transportation during the CVI was calculated. The microstructure and properties of the C/C–SiC composites were investigated. The results indicated that the CVI time for densification of the preform decrease sharply, and the model showed the permeability of the preform decreased with the increase of its density. The C/C–SiC exhibited good mechanical characteristics, especially excellent compressive behavior, with the vertical and parallel compressive strength reached to 359(±40) MPa and 257(±35) MPa, respectively. The coefficient of friction (COF) decreased from 0.60 (at 8 m/s) with the increase of sliding velocity, and finally stabilized at ~0.35 under the velocity of 20 m/s and 24 m/s, and the variations of COF were not sensitive to the sliding distance. The wear rates were between 0.012 cm³/MJ and 0.024 cm³/MJ under different velocities. These results showed that the chopped carbon fiber preform reinforced C/C–SiC are promising candidates for high-performance and low-cost friction composites.     

Highly conductive and high-strength carbon nanotube film reinforced silicon carbide composites

Film formed by carbon nanotubes is usually called carbon nanotube film (CNT_f). In the present study, CNT_f fabricated by floating catalyst method was used to prepare CNT_f/SiC ceramic matrix composites by chemical vapor infiltration (CVI). Mechanical and electrical properties of the resulting CNT_f/SiC composites with different CVI cycles were investigated and discussed, and the results revealed that the CNT_f has a good adaptability to CVI method. Tensile test demonstrated an excellent mechanical performance of the composites with highest tensile strength of 646 MPa after 2 CVI cycles, and the strength has a decline after 3 CVI cycles for an excessively dense matrix. While, the elastic modulus of the composite increased with the CVI cycles and reached 301 GPa after 3 CVI cycles. Tensile fracture morphologies of the composites were analyzed by scanning electron microscope to study the performance change laws with the CVI cycles. With SiC ceramic matrix infiltrated into the CNT_f, enhanced electrical conductivity of the CNT_f/SiC composite compared to pure CNT_f was also obtained, from 368 S/cm to 588 S/cm. Conductivity of the SiC matrix with free carbon forming in the CVI process was considered as the reason.     

Toughening and strengthening zirconia through the addition of a transient solid solution additive

A very small amount of nickel oxide (NiO), 0.18 mol%, could dissolve into yttria-stabilized zirconia (YSZ) during sintering in air at elevated temperature. The presence of Ni solutes enhances both the densification and grain growth of YSZ specimens. By heat-treating the NiO-doped YSZ specimen in a reducing atmosphere, nano-sized Ni particles are produced at the grain boundaries. The NiO thus acts as a transient solid solution additive for the YSZ-Ni nanocomposite. The formation of Ni nano-particles introduces an extra ferromagnetic performance into the YSZ specimen. Furthermore, the toughness and strength of YSZ are enhanced respectively by 120% and 40%. The toughness enhancement shows strong dependence on the size of ZrO₂ grains. Nevertheless, the strengthening is contributed by many factors.     

Mechanical and thermophysical properties of carbon/carbon composites with hafnium carbide

Carbon/carbon (C/C) composites with addition of hafnium carbide (HfC) were prepared by immersing the carbon felt in a hafnium oxychloride aqueous solution, followed by densification and graphitization. Mechanical properties, coefficients of thermal expansion (CTE), and thermal conductivity of the composites were investigated. Results show that mechanical properties of the composites decrease dramatically when the HfC content is greater than 6.5 wt%. CTE of the composites increases with the increase of HfC contents. The composites with addition of 6.5 wt% HfC show the highest thermal conductivity. The high thermal conductivity results from the thermal motion of CO in the gaps and pores, which can improve phonon–defect interaction of the C/C composites. Thermal conductivities of the composites decrease when the HfC content is greater than 6.5 wt%, which is due to formation of a large number of cracks in the composites. Cracks increase the phonon scattering and hence restrain heat transport, which results in the decrease of thermal conductivity of the composites.     

Mechanical,electrical and thermal properties of ZrC-ZrB2-SiC ternary eutectic composites prepared by arc melting

ZrC-ZrB₂-SiC composites were prepared by arc-melting in Ar atmosphere using ZrC, ZrB₂ and SiC as starting materials. The ternary eutectic composition of 20ZrC-30ZrB₂-50SiC (mol%) was first identified. SiC about 7?μm in length and 500?nm in diameter, ZrC about 4 μm in length and 1 μm in diameter, in rod-like microstructure, were uniformly dispersed in ZrB₂ matrix of eutectic composite. The eutectic temperature of ZrC-ZrB₂-SiC composite was approximately 2550?K. The Vickers Hardness and fracture toughness of eutectic composite was 23?GPa and 6.2?MPa?m^1/2, respectively. The electrical conductivity decreased from 7.2?×?10⁷ to 1.75?×?10⁶?S?m^?1 with the temperature increasing from 287 to 800?K. The thermal conductivity decreased from 85 to 61?W?K^?1?m^?1 with increasing temperature from 287 to 973?K.     

Oxidation behaviors and shear properties of z-pinned joint made of two-dimensional carbon fiber reinforced silicon carbide composite

Chemical vapor infiltration has been introduced for preparing z-pinned joint, which is made of two-dimensional carbon fiber reinforced silicon carbide composite. The effects of oxidation on the shear properties of the joint were investigated. The results showed that the joint strength increases with the increase of oxidation temperature, which is consistent with the oxidation consumption of the carbon phases. An exponential relationship is presented between the weight loss and the joint strength. In contrast, linear relationships are presented between the weight loss and the mechanical properties of the composite. The exponential relationship results from the coupled shear and bending stress states of the pin, according to the failure mechanisms of the joint. It is observed that in-plane and intra-layer cracks are formed under the shear stress. And these cracks are bridged by the fibers under the bending stress. Accordingly, the fiber bridging mechanism contributes to the joint strength before and after oxidation. For the conditions of this study, the joint strength can be roughly estimated as the plus of the in-plane shear strength and the tensile matrix cracking stress.     

Effects of particulate metallic phase on microstructure and mechanical properties of carbide reinforced alumina ceramic tool materials

Alumina-based composite ceramic tool materials reinforced with carbide particles were fabricated by the hot-pressing technology. Choice of metallic phase added into the present composite ceramic was based on the distribution of residual stress in the composite. The effects of metallic phase on microstructure and mechanical properties of composites were investigated. The metallic phase could dramatically improve room temperature mechanical properties by refining microstructure, filling pores and enhancing interfacial bonding strength. However, it also led to sharp strength degradation at high temperature because the metallic phase was easier to be oxidized and get soft at high temperature in air. The effects of metallic phase on strengthening and toughening were discussed. The improved fracture toughness of composite with metallic phase was attributed to the lower residual tensile stress in the matrix and the interaction of more effective energy consuming mechanisms, such as crack bridged by particle, crack deflection and intragranular grain failure.     

Low-temperature exfoliated graphene oxide incorporated with different types of natural rubber latex: Electrical and morphological properties and its capacitance performance

A simple with cost-effective method in the production and fabrication of graphene-based rubber nanocomposites as electrode materials is still remain a global challenge. In this work, we proposed one- and two-step approaches to fabricate an exfoliated graphene oxide (GO) as nanofiller in three different types of rubber latex polymer, namely, low ammonia natural rubber latex (NRL), radiation vulcanized NRL (RVNRL), and epoxy NRL 25 (ENRL 25). The electrical conductivity and capacitive behavior of nanocomposite samples were investigated under a four-point probe and cyclic voltammetry measurements, respectively. Meanwhile, the morphological properties were observed using field emission scanning electron microscopy, energy dispersive X-ray, optical polarization microscope, high-resolution transmission electron microscopy, Fourier-transform infrared spectroscopy, micro-Raman spectroscopy, and X-ray diffraction. The thermal stabilities of the nanocomposites were also investigated by thermogravimetric analysis. Among all, the GO/RVNRL polymer nanocomposite samples performed a better homogeneity with an improved electrical conductivity (~8.6 × 10⁻⁴ Scm⁻¹) as compared with the GO/ENRL 25 (~3.1 × 10⁻⁴ Scm⁻¹) and GO/NRL (~2.6 × 10⁻⁴ Scm⁻¹) polymer nanocomposite samples. In addition, the GO/RVNRL polymer nanocomposite electrodes showed acceptable specific capacitance (5 Fg^-1). The successfully fabricated conductive GO-based rubber nanocomposites are suitable for new supercapacitor electrodes.     

Mechanical,electrical and thermal properties of HfC-HfB2-SiC ternary eutectic composites prepared by arc melting

HfC-HfB₂-SiC composites were prepared by arc melting using HfC, HfB₂ and SiC powder as raw materials. The ternary eutectic composition of 16HfC-17HfB₂-67SiC (mol%) was first identified, showing a complicated maze microstructure of HfC, HfB₂ and SiC approximately 500 nm in thickness. The eutectic temperature of the HfC-HfB₂-SiC composite was nearly 2760 K. The Vickers hardness and fracture toughness of the HfC-HfB₂-SiC ternary eutectic composite were 20.8 GPa and 7.7 MPa m^1/2, respectively. With increasing temperature from 300 to 800 K, the electrical conductivity decreased from 8.8 × 10⁵ to 4.3 × 10⁵ Sm⁻¹, whereas the thermal conductivity increased from 28 to 32 W m⁻¹ K⁻¹.     

Remarkable improvement in tribological behavior of plasma sprayed carbon nanotube and graphene nanoplatelates hybrid reinforced alumina nanocomposite coating

Addition of 0.5?wt% of graphene nanoplatelates (GNPs) and 1?wt% carbonnanotube (CNTs) in plasma sprayed Al₂O₃ coating showed the reduction of 93.25% in wear volume loss and 90.94% in wear rate. This could be attributed to the simultaneous effect of enhanced densification, presence of the transferred layer from the counterpart, strong interface between Al₂O₃, GNP and CNTs and toughening offered by the GNPs and CNTs. The lowest COF value of 0.27 was recorded on addition of 0.5?wt% of GNP in Al₂O₃ coating, which could be attributed to the graphitic lubrication on the worn track during the wear.