Electrical conductivity and thermal properties of spark plasma sintered Al2O3-SiC-CNT hybrid nanocomposites

In this study, we report the electrical conductivity and thermal properties of Al₂O₃-SiC-CNT hybrid nanocomposites processed via ball milling (BM) and spark plasma sintering (SPS). The initial powders and consolidated samples were characterized using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM), respectively. A multifunction calibrator and a high-resolution digital multimeter were used to measure the electrical conductivity. The thermal properties were measured using a thermal constants analyser. The SiC and CNT-reinforced alumina hybrid nanocomposites exhibited a significant increase in their room-temperature electrical conductivity, which made them suitable for electrical discharge machining. The Al₂O₃-5SiC-2CNTs had a high electrical conductivity value of 8.85 S/m compared to a low value of 6.87×10⁻¹⁰ S/m for the monolithic alumina. The addition of SiC and CNTs to alumina decreased its room-temperature thermal properties. The increase in temperature resulted in a decrease in the thermal conductivity and thermal diffusivity but an increase in the specific heat of the monolithic alumina and the hybrid nanocomposites. These properties were correlated with the microstructure, and possible transport mechanisms were discussed.     

Role of synthesis method on microstructure and mechanical properties of graphene/carbon nanotube toughened Al2O3 nanocomposites

The effects of hot-pressing (HP) and spark plasma sintering (SPS) methods on the grain size, microstructural features, and mechanical behaviour of graphene nanoplatelet/carbon nanotubes (GNTs) reinforced Al₂O₃ nanocomposites were comprehensively studied. Different graphene nanoplatelet to carbon nanotube ratios were selected as the overall reinforcement content of composites prepared using HP and SPS. Highly densified samples (>98%) were obtained at 1650 °C under 40 MPa in Ar atmosphere, with dwell times of 1 h and 10 min for HP and SPS respectively. Both types of sample showed a mixture of inter- and transgranular fracture behaviour. A 50% grain size reduction was observed for samples prepared by HP compared to SPS samples. Both types of samples achieved a high flexural strength and fracture toughness of >400 MPa and 5.5 MPa m^1/2, whilst SPS samples peaked at relatively lower GNT contents than those for the HP samples. Based on analyses of the morphology, grain sizes and fracture mode, similar toughening mechanisms for both types of sample were observed, involving the complex characteristics of the combined GNT fillers.     

Thermal conductivity of UO2 pellets enhanced by a semi-continuous structure of Ti3SiC2

The current distribution structure of the thermal conductivity (TC) enhancements in UO₂ matrix can be divided into three categories: dispersed structure, continuous structure and oriented structure. This paper develops a new semi-continuous structure to improve the TC of UO₂. Ti₃SiC₂ is selected as the enhancement in this paper due to its excellent irradiation resistance and high TC at elevated temperature. The semi-continuous structure of Ti₃SiC₂ is formed through a simple centrifugal spheroidization of UO₂ and a followed powder mixing process. Both the theoretical prediction and experimental results indicate the semi-continuous structure has a close TC enhancing efficiency to the continuous structure. Meanwhile, the semi-continuous structure has the advantages of simplifying the fabrication process and can be applied to more kinds of enhancements compared to the continuous structure. The TC of Ti₃SiC₂/UO₂ is remarkably improved at elevated temperature compared with pure UO₂, which is beneficial to the fuel safety.     

Sol-gel synthesis,spark plasma sintering,structural characterization,and thermal conductivity measurement of heavily Nb-doped SrTiO3/TiO2 nanocomposites

Heavily Nb-doped strontium titanate (SrTi_1-xNb_xO₃) nanoparticles and SrTi_1-xNb_xO₃/TiO₂ nanocomposite powders were synthesized by a sol-gel method. Structural characterization of the obtained powders was performed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and UV–visible spectroscopy. The powders were densified by spark plasma sintering (SPS) method up to 98% of the relative density. Upon composite production, the thermal conductivity of the un-doped samples was effectively decreased for SrTiO₃/TiO₂ nanocpmposite from 12 to 8 W/m.K. On the other hand, thermal conductivity of the Nb-doped SrTi_0.8Nb_0.2O₃/TiO₂ composite was decreased by about 50% down to 3.4 W/m.K in comparison to SrTiO₃/TiO₂ due to the phonon scattering at the point defects originated from both Nb atoms and TiO₂ nanoparticles.     

Multifunctional bioceramic-based composites reinforced with silica-coated carbon nanotube core-shell structures

Carbon nanotube (CNT) possesses eminent mechanical properties and has been widely utilized to toughen bioceramics. Major challenges associated with CNT-reinforced bioceramics include the inhomogeneous dispersion of CNTs and the insufficient interfacial strength between the two phases. To address such issues, this research describes the first use of silica-coated CNT (S-CNT) core-shell structures to reinforce bioceramics using hydroxyapatite (HA) as a representative matrix. HA-based composites with 0.1–2 wt% S-CNT are sintered by spark plasma sintering to investigate their mechanical and biological properties. It is found that when 1 wt% raw CNT (R-CNT) is added, very limited increases in fracture toughness (K_IC) is observed. By contrast, the incorporation of 1 wt% S-CNT increased the K_IC of HA by 101.7%. This is attributed to more homogeneously dispersed fillers and stronger interfacial strengths. MG63 cells cultured on the 1 wt% S-CNT/HA pellets are found to proliferate faster and possess significantly higher alkaline phosphatase activities than those grown on the HA compacts reinforced with 1 wt% R-CNT, probably by virtue of the released Si ions from the SiO2 shell. Therefore, the S-CNT core-shell structures can improve both mechanical and biological properties of HA more effectively than the conventionally used R-CNTs. The current study also presents a novel and effective approach to the enhancement of many other biomedical and structural materials through S-CNT incorporation.     

Influence of carbon nanotubes on thermal and electrical conductivity of zirconia-based composite

Carbon nanotubes (CNTs) are widely used in ceramic-matrix composites (CMC) as a filler. An individual carbon nanotube exhibits extremely high thermal conductivity, however, the influence of CNTs on the thermal conductivity of CMCs is moderate. In contrast, even a small quantity of CNTs significantly increases the electrical conductivity of CMCs. The present paper studies this contradictory influence for ZrO₂-CNTs composites with 3, 5, 10 and 20 vol% multi-wall carbon nanotubes (MWCNTs). Their thermal and electrical conductivity was studied by the laser flash method and electrochemical impedance spectroscopy. The analysis reveals that the moderate influence of MWCNTs on the thermal conductivity of composites originates from the similar thermal conductivity of MWCNTs in a bundle and zirconia. On the other hand, the substantial difference in the electrical conductivity of MWCNTs and zirconia leads to an exponential increase in the electrical conductivity of the ZrO₂-CNTs composite even with small additions of nanotubes.     

Microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte prepared by spark plasma sintering

In this paper, the microstructure and ionic conductivity of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolytes prepared by spark plasma sintering (SPS) were investigated by XRD, SEM, TEM and EIS, respectively. The results showed that as the sintering temperature was increased, both the relative density and the ionic conductivity of the sintered LAGP samples first increased and then decreased, achieving a maximum value of 97% and 2.12 × 10⁻⁴ S cm⁻¹ simultaneously at 700 °C. At the same time, the crystallinity of the sintered samples was improved, while a few impurity phases, such as AlPO₄ and GeO₂, appeared in the samples. It was also found that carbon contamination and oxycarbide gas was be brought in during SPS. Carbon contamination could produce an extra grain boundary impedance to the samples and could be removed by annealing at 500 °C in an air atmosphere. Oxycarbide gas could affect the relative density of the sintered LAGP samples and could be mitigated by choosing a suitable SPS process. Moreover, the shear modulus of the sintered LAGP was measured to be 49.6 GPa, which exceeded the minimum value of 8.5 GPa that was necessary to suppress Li dendrite growth.     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

Evaluation of effective thermal conductivity for carbon nanotube/polymer composites using control volume finite element method

Effective thermal conductivity of the polymeric composites filled with carbon nanotubes (CNTs) is predicted by using the asymptotic expansion homogenization technique (AEH), which makes it possible to localize and homogenize a heterogeneous medium. In the present study, CNT embedded epoxy composites are taken into account as the heterogeneous system. The representative volume element (RVE) employed in the homogenization process is constructed by assuming that the CNTs are dispersed homogeneously in the polymer matrix. It is presumed that the RVE contains a single CNT and that there is no direct interaction between neighboring CNTs. The dispersion state of CNTs in the composites is morphologically characterized by the field emission scanning electronic microscope (FESEM). In order to consider the orientation state of CNTs, the bounding approach is adopted by using the orientation tensor. It is found that the numerically homogenized thermal conductivity is higher than that obtained by the analytic model. Predicted conductivities are also compared with experimental results as well as analytic results. The homogenization technique yields the effective thermal conductivity accordant with experimental results. In the case that a heterogeneous material has anisotropic properties or geometrical complexity, the homogenization technique is an efficient method to obtain averaged material properties equivalent to those of the real heterogeneous medium.     

Mechanical and thermal properties of spark plasma sintered Al2O3-graphene-SiC hybrid composites

Monolithic Al₂O₃ and Al₂O₃-graphene-SiC hybrid composites were prepared by spark plasma sintering (SPS) under vacuum atmosphere. The results show that the hybrid composites were almost completely dense (>97%). SiC content has a significant effect on the microstructure of the composites. With the increase of SiC content, the average grain size of alumina decreased gradually. The addition of SiC to alumina changed fracture mode from inter-granular fracture to mixed fracture mode of inter-granular fracture and trans-granular fracture. The Al₂O₃-0.4 wt%graphene-5 wt% SiC hybrid composite has the highest bending strength and hardness, which were 57% and 19.22% higher than those of the monolithic alumina, respectively. The room temperature (RT) thermal conductivity of the monolithic Al₂O₃ (25.5 W/m·K) was the highest. The thermal conductivity and thermal diffusivity coefficient of the composites decreased with the increase in temperature, while the specific heat of monolithic alumina and composites increased with the increase in temperature and additives. These properties were related to the microstructure of materials and the possible transport mechanisms were discussed.     

Rapid and low temperature spark plasma sintering synthesis of novel carbon nanotube reinforced titanium matrix composites

Multi–walled carbon nanotube (MWCNT) reinforced titanium matrix composites were synthesized using a spark plasma sintering method at a low sintering temperature of 550 °C. The effects of the weight fraction of MWCNTs on the microstructures and the mechanical and thermal properties of the composites were investigated. No reaction products were detected in the composites, indicating that the MWCNTs in the composites maintained their structural integrity after sintering, and thus, because of their advantageous properties, could reinforce the titanium matrix. As a result, the compressive strength of the composite containing 0.4 wt.% MWCNTs reached 1106 MPa, which was an increase of 61.5% compared to that of pure titanium under at the same conditions. In addition, the results revealed that compressive strength of the bulk compacts increased initially and then decreased with an increase in weight fraction of MWCNTs. However, compressive strain of the sintered composites continued to fall at a slow rate. The microhardness and thermal diffusivity of the composites rose steadily with an increasing content of MWCNTs. When the weight fraction of MWCNTs in the composites exceeded 0.8%, the compressive strength of the composites declined significantly due to the increasing aggregation of the MWCNTs.     

A review on the processing technologies of carbon nanotube/silicon carbide composites

Due to the extraordinary electronic, mechanical, chemical, thermal, magnetic, and optical properties, carbon nanotube (CNT), an excellent one-dimensional nano-material, has been considered as a new filler for polymer, metal, and ceramic matrix composites with the main purpose of improving their mechanical performance, fracture behavior, and functional features. In the silicon carbide (SiC) ceramic field, there are many CNT reinforced SiC ceramic matrix composites and CNT/SiC hybrid structures, which have been investigated successfully using various of methods. This paper reviews the current status of researches and describes all different routes for effectively dispersing CNTs throughout SiC ceramic matrix, densifying composites, and synthesizing hybrid structures.     

Improvement of Vickers hardness measurement on SWNT/Al2O3 composites consolidated by spark plasma sintering

Dense alumina composites with different carbon nanotube content were prepared by colloidal processing and consolidated by Spark Plasma Sintering (SPS). Single-wall carbon nanotubes (SWNTs) were distributed at grain boundaries and also into agglomerates homogeneously dispersed. Carrying out Vickers hardness tests on the cross-section surfaces instead of top (or bottom) surfaces has shown a noticeable increase in the reliability of the hardness measurements. This improvement has been mainly attributed to the different morphology of carbon nanotube agglomerates, which however does not seem to affect the Vickers hardness value. Composites with lower SWNT content maintain the Vickers hardness of monolithic alumina, whereas it significantly decreases for the rest of compositions. The decreasing trend with increasing SWNT content has been explained by the presence of higher SWNT quantities at grain boundaries. Based on the results obtained, a method for optimizing Vickers hardness tests performance on SWNT/Al₂O₃ composites sintered by SPS is proposed.     

Structural properties and mechanical behavior of SWCNTs and MWCNTs reinforced Al2O3 fabricated by spark plasma sintering

Carbon nanotubes due to their structural and mechanical properties are good candidates as the second phase to improve the mechanical properties of alumina-based ceramics. In the present study, the effects of single wall and multi-wall carbon nanotubes on structural and mechanical properties of alumina were investigated. SWCNTs and MWCNTs were dispersed in alumina powder via a conventional method using 1 wt % PVA water solution as media. Sintering process for two different composite powders, alumina-2 wt. % SWCNTs and alumina 2 wt % MWCNTs was performed by spark plasma sintering technique at 1500 °C and 20 MPa for 10 min. Results showed that the presence of CNTs in alumina caused a considerable amount of porosity in final bodies. SEM images of fracture surfaces revealed the agglomeration of SWCNTs which played a dominant role in the deterioration of mechanical properties. MWCNTs reinforced alumina obtained higher Vickers hardness and bending strength values (12.91 GPa and 291 MPa, respectively) compared to that of SWCNTs (9.18 GPa and 276 MPa, respectively), due to sever agglomerate of SWCNTs throughout sintered composites. Typical load-displacement (P/h) curves were obtained from bending strength test and discussed. It was concluded that the addition of MWCNTs to alumina represented better densification and mechanical properties compared to SWCNTs.     

Microstructure and properties of Al2O3-TiC-Ti3SiC2 composites fabricated by spark plasma sintering

Abstract

Abstract

Highly densified Al₂O₃-TiC-Ti₃SiC₂ composites were fabricated by spark plasma sintering technique and subsequently characterised. From fracture surface observation, it is found that Al₂O₃ is 0·2-0·4?μm, TiC is 1-1·5?μm and Ti₃SiC₂ is 1·5-5?μm in grain size. With the increase in Ti₃SiC₂ volume contents, Vickers hardness of the composites decreases because of the low hardness of monolithic Ti₃SiC₂. The fracture toughness rises remarkably when the contents of Ti₃SiC₂ increase, which is attributed to the pullout and microplastic deformation of Ti₃SiC₂ grains. At the same time, the flexural strength of the composites shows a considerable improvement as well. The electrical conductivity rises significantly as the Ti₃SiC₂ contents increase because of the formation of Ti₃SiC₂ network and the increase in conductive phase contents.     

Influence of conductive secondary phase on thermal gradients development during Spark Plasma Sintering (SPS) of ceramic composites

Spark Plasma Sintering (SPS) has attracted a lot of interest in recent years owing to its ability to enable the densification of a broad range of materials in a very short processing time. It is well documented in the literature that the very high heating rates that can be applied with this technology can lead to the apparition of large thermal gradients in the tool and thus affect the homogeneity of the compact.In the present study, the influence of the compact thermal and electrical properties on the thermal gradients was studied. Al₂O₃, AlN and TiC powders were used to produce series of Al₂O₃-TiC and AlN-TiC composites (0, 25, 50, 75, 100 vol%TiC) showing different electrical and thermal conductivities. Two pyrometers were used in order to observe and measure the thermal gradients and the percolation of the current during sintering at a high heating rate and without insulation.Electrical conductivity measurements were carried out on samples presenting different relative densities. This samples were obtained through interrupted sintering cycles at temperatures below and above the identified percolation threshold temperature.It was shown that high thermal gradients can appear during SPS depending on the processing parameters (dimensions of the die and heating rate) but also on the composition of the compact (proportion of conductive phase) and on its density.     

Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Mechanical and thermo-mechanical properties of carbon nanotube reinforced composites

A study on the mechanical and thermo-mechanical properties of carbon nanotube (CNT) reinforced nanocomposites is presented in this article. Mori–Tanaka method is used for modeling the effective stiffness and coefficient of thermal expansion. Regression formulas were developed to describe the effects of CNT orientation, aspect ratio, and CNT volume fraction. Given the statistical distributions of CNT orientations and aspect ratios, the effective properties can be conveniently derived by numerical integration using these formulas.     

Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites

Nanostructured modification of polymers has opened up new perspectives for multi-functional materials. In particular, carbon nanotubes (CNTs) have the potential to realise electrically conductive polymers with improved or retaining mechanical performance. This study focuses on the evaluation of both, the electrical and thermal conductivity of nanoparticulate filled epoxy resins. We discuss the results with regard to the influence of the type of carbon nanotube (SWCNT, DWCNT and MWCNT), the relevance of surface-functionalisation (amino-functionalisation), the influence of filler content (wt% and vol%), the varying dispersibility, the aspect ratio and the specific surface area.