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A novel building block material for the generation of non-brittle ceramic composites consisting of micron-sized alumina platelets homogeneously coated with multi-wall carbon nanotubes (MWCNTs) bound to their surface is described. The MWCNT phase is grown in situ from immobilised metal catalyst particles using chemical vapour deposition techniques. In-depth Raman and scanning electron microscope studies revealed that this approach solves the typical issue of MWCNT-agglomeration in ceramic matrices and paves the way to excellent control over MWCNT purity and concentration within the resulting composite material. Moreover, we show that the preparation of the catalyst is the most important factor for the generation of uniformly distributed MWCNTs of high-quality on these platelets. With these MWCNT-coated alumina building blocks, we have manufactured nacre-like biomimetic composites using spark plasma sintering. The resulting composites are electrically conductive and three-point bending tests show a transition from brittle/catastrophic failure to graceful failure, holding great promise towards multifunctional, tough and strong lightweight ceramic composite manufacturing. Image 1 [ABSTRACT FROM AUTHOR]

The high temperature grain growth behaviour of Y 2 O 3 partially stabilised ZrO 2 and ZrO 2 -CNT (0.5 and 2 vol%) composites was investigated in a series of isothermal experiments carried out between 1500 and 1800 °C. The grain growth activation energy was found to be 417 ± 20 kJ mol − 1 for the ZrO 2 , which increased with the addition of CNTs to 509 ± 41 and 497 ± 17 kJ mol − 1 for the 0.5 and 2 vol% composites respectively. A sintering exponent of 2 was observed for the ZrO 2 , corresponding to normal grain growth, whilst the nanocomposites showed a CNT drag dominated sintering mechanism, with exponents of 2.7 and 3.2 for the 0.5 and 2 vol% composites respectively. This work confirms and quantifies the direct influence of CNTs on the grain growth mechanism and activation energy of ZrO 2 -CNT composites. [ABSTRACT FROM AUTHOR]

Polystyrene- (PS-) graphene nanoplatelets (GNP) (0.1, 1, and 10 wt.%) nanofibers were successfully produced via electrospining of dimethyformamide- (DMF-) stabilized GNP and PS solutions. Morphological analysis of the composite nanofibers confirmed uniform fiber formation and good GNP dispersion/distribution within the PS matrix. The good physical properties of GNP produced by liquid exfoliation were transferred to the PS nanofibers. GNP modified PS nanofibers showed a 6-fold increase in the thermal conductivity and an increase of 7-8 orders of magnitude in electrical conductivity of the nanofibers at 10 wt.% GNP loading. [ABSTRACT FROM AUTHOR]

Graphene Nano-Sheets (GNS) with different lateral sizes (193, 373 and 1070 nm) were produced using liquid phase exfoliation and controlled centrifugation. Alumina-GNS (0.8 vol%) composites with different size GNS were consolidated using spark plasma sintering. The fracture toughness, hardness and elastic modulus of the composites decreased with increasing GNS size. Microstructural investigation of the composites suggested that their mechanical properties were directly related to: (1) average number of GNS present per unit volume in composite; and (2) relative size of GNS to alumina grains. Small size GNS resulted in a 35% improvement of fracture toughness of alumina-GNS composite. The mechanism of crack propagation changed as the size of the GNS increased. The smaller size GNS produced various toughening mechanisms, while the larger size GNS, with similar size to the alumina grains, resulted in grain boundary sliding. The composite with larger size GNS was machinable using ordinary tungsten carbide drill bits. [ABSTRACT FROM AUTHOR]

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 Al 2 O 3 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. [ABSTRACT FROM AUTHOR]

Well dispersed silica-graphene nanoplatelet (GNP) and silica-graphene oxide nanoplatelet (GONP) composites were fabricated by optimising their processing conditions. Different processing methods, including colloidal and powder processing, were investigated using different solvents. High temperature and inert environment during SPS resulted in the reduction of the GONP during sintering. The in situ reduction of GONP in the SPS was investigated for various sintering times and temperatures, and characterised using Raman spectroscopy and XRD analysis. The composite sintered at 1200°C, 50MPa pressure and 15min dwell time confirmed the recovery of the crystalline graphitic phase of GNP after reduction without crystallising the silica matrix. GONP was found to inhibit the crystallisation of the silica matrix at higher sintering times and temperatures possibly due to increased viscosity and reduced mobility of the silica particles bound to GNP. [ABSTRACT FROM AUTHOR]

Graphene nano-platelets (GNP) were synthesised using a liquid phase exfoliation method and then used to produce well dispersed silica-GNP composites. The tribological properties of the silica-GNP composites with different vol% GNP loading were investigated. The wear behaviour at room temperature was studied using a ball on disc technique with alumina and borosilicate (BS) balls as the tribological counterpart. In the case of alumina balls, GNP above a critical concentration decreased the coefficient of friction of the composite by ~20% while increasing the wear resistance of the composite by ~5.5 times. In the case of using BS balls there was an improvement of ~8.5 times in the wear resistance while the coefficient of friction did not change. In both cases a percolating network of GNP above a critical concentration provided a lubricating effect to the silica matrix, resulting in a significant improvement in the tribological properties of the nano-composites. The extent of damage to GNP during tribology testing was quantified using Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Abstract: Graphene was prepared using liquid phase exfoliation and dispersed in an alumina matrix using an ultrasonication and powder processing route. Al2O3–graphene composites with up to 5vol% content were densified (>99%) using SPS. The fracture toughness of the material increased by 40% with the addition of only 0.8vol% graphene. However for higher graphene contents the improvement in fracture toughness was limited. Graphene changed the mechanism of crack propagation for the alumina matrix from inter-granular to trans-granular. The formation of an inter-connecting graphene network promoted easy fracture for concentration ⩾2vol%. Elastic modulus remained nearly constant for up to 2vol% and decreased significantly for 5vol% due to the formation of the inter-connecting graphene network. Fracture toughness measured with the indentation and chevron notch methods were consistent up to 2vol% and at 5vol% the percolating network of graphene resulted in easy crack propagation with significant discrepancy between the results for the two methods. [Copyright &y& Elsevier]

The processing conditions for preparing well dispersed silica–graphene nanoplatelets and silica–graphene oxide nanoplatelets (GONP) composites were optimized using powder and colloidal processing routes. Fully dense silica–GONP composites with up to 2.5 vol% loading were consolidated using spark plasma sintering. The GONP aligned perpendicularly to the applied pressure during sintering. The fracture toughness of the composites increased linearly with increasing concentration of GONP and reached a value of ∼0.9 MPa m1/2 for 2.5 vol% loading. Various toughening mechanisms including GONP necking, GONP pull-out, crack bridging, crack deflection and crack branching were observed. GONP decreased the hardness and brittleness index (BI) of the composites by ∼30 and ∼50% respectively. The decrease in BI makes silica–GONP composites machinable compared to pure silica. When compared to silica–Carbon nanotube composites, silica–GONP composites show better process-ability and enhanced mechanical properties. [ABSTRACT FROM AUTHOR]

Suspensions containing 8 and 10 wt% polyacrylonitrile (PAN) and 1, 2, 3, 5, 7 and 10 wt% graphene oxide (GO) were prepared using a special mixing routine and fibres were generated from these mixtures by pressurised gyration. The combination of pressure and gyration speed was effective in controlling the fibre morphology and fibre diameter which ranged from 1 to 20 μm. Fibres were pyrolysed to remove the polymer and only the 10 wt% PAN fibres survived. The microstructure of the pre- and post-pyrolysis products were characterised by scanning electron microscopy, both with and without focussed ion beam etching, Fourier-transformed infrared and Raman spectroscopies. Pyrolysed fibre electrical conductivities were measured and only those containing 1 and 2 wt% GO were conductive. [ABSTRACT FROM AUTHOR]

Smart heating devices with reliable self-regulating performances and high efficiency, combined with additional properties like mechanical flexibility, are of particular interest in healthcare, soft robotics, and smart buildings. Unfortunately, the development of smart heaters necessitates managing normally conflicting requirements such as good self-regulating capabilities and efficient Joule heating performances. Here, a simple and universal materials design strategy based on a series connection of different conductive polymer composites (CPC) is shown to provide unique control over the pyroresistive properties. Hooke's and Kirchhoff's laws of electrical circuits can simply predict the overall pyroresistive behavior of devices connected in series and/or parallel configurations, hence providing design guidelines. An efficient and mechanically flexible Joule heating device is hence designed and created. The heater is characterized by a zero temperature coefficient of resistance below the self-regulating temperature, immediately followed by a large and sharp positive temperature coefficient (PTC) behavior with a PTC intensity of around 106. Flexibility and toughness is provided by the selected elastomeric thermoplastic polyurethane (TPU) matrix as well as the device design. The universality of the approach is demonstrated by using different polymer matrices and conductive fillers for which repeatable results are consistently obtained. [ABSTRACT FROM AUTHOR]

A novel processing methodology that allows combined preheating and Flash- SPS ( FSPS) of silicon carbide-based materials has been developed. Beta-SiC (+10 wt% B4C) powders were densified (Ф 20 mm) up to 96% of their theoretical density in 17 s under an applied pressure of 16 MPa (5 kN). The flash event was attributed to the sharp positive temperature dependence of the electrical conductivity (thermal runaway) of SiC, and a sudden increase in electric power absorption (Joule heating) of the samples after a sufficient preheating temperature (>600°C) was reached. The microstructural evolution was analyzed by examining materials densified by FSPS in the range of 82%-96% theoretical densities. FEM modeling results suggest that the FSPS heating rate was of the order of 8800°C/min. A comparative analysis was done between FSPS and reference samples (sintered using conventional SPS in the temperature range of 1800°C-2300°C). This allowed for a better understanding of the temperatures generated during FSPS, and in turn the sintering mechanisms. We also demonstrated the scalability of the FSPS process by consolidating a large α-SiC disk (Ф 60 mm) in about 60 s inside a hybrid SPS furnace equipped with an induction heater, which allowed us to achieve sufficient preheating (1600°C) of the material to achieve FSPS. [ABSTRACT FROM AUTHOR]

Ultra High Temperature SPS (UHTSPS) was used to sinter pure α-SiC at 2450 °C. Such a high temperature and partial vacuum conditions (1200 Pa) promoted SiC sublimation and condensation reactions. In the presence of an electric field, materials with graded porosity could be produced by using UHTSPS. At high temperature, the condensation of the gaseous species was controlled by the polarity of the applied electric field. Preferential condensation of SiC occurred on the negative electrode (cooler surface) due to the Peltier effect associated with the n-type thermoelectric behaviour of SiC. In the absence of an electric field, condensation was driven by gravity and it resulted in dense SiC monoliths. [ABSTRACT FROM AUTHOR]

Cylindrical and bamboo-like boron nitride nanotubes (BNNTs) have been used to reinforce brittle amorphous borosilicate glass matrix materials prepared by spark plasma sintering. The mechanical properties, such as hardness, Young's modulus, fracture toughness, and scratch resistance of the materials have been investigated. The fracture toughness of the composites showed an improvement of ~30% compared to the pure amorphous glass. BNNTs pull-out, crack bridging, stretching, and crack deflection toughening mechanisms were observed in the reinforced glass matrix composites. Extensive pull-out of the BNNTs (>400nm) was observed in the form of the telescopic "sword-in-sheath" mechanism, resulting in poor energy dissipation due to the weak Van der Waals force between the inner walls of the BNNTs. The scratch resistance was significantly improved (~26%) after the addition of the BNNTs, and the results correspond well with the brittleness index of the materials. [ABSTRACT FROM AUTHOR]

Pure ZrB2 powder was Flash sintered in an SPS furnace ( FSPS). The samples were densified up to 95.0% in 35 s under an applied pressure of 16 MPa. Compared to Conventional SPS ( CSPS), the newly developed FSPS technique resulted in an unprecedented energy and time savings of about 95% and 98% respectively. ZrB2 monoliths obtained by CSPS and FSPS were compared with respect to microstructures, densification behavior, and grain growth. The developed methodology might find application to a wide range of highly conductive ceramics as such refractory borides and carbides. [ABSTRACT FROM AUTHOR]

[Display omitted] • Unique pyroresistive behaviour, self-regulating at a temperature far below melting point, hence lowering the risk of polymer degradation. • The composite exhibits one of the highest reported positive temperature coefficient switching temperatures of 200 °C. • The co-polyester thermoplastic elastomer guaranteed some mechanical flexibility, which allowed the composite to Joule heat even when bent to radii of curvature smaller than 15 mm. Pyroresistive polymer composites are smart materials with great promise in numerous fields of electronic devices, including temperature and strain sensors, self-regulating heating, and over-current/over-temperature protective devices. However, the operational window of these materials is limited by the phase transition temperatures of the polymer matrix and the brittleness of the composites. Herein, a conductive polymer composite based on a co-polyester thermoplastic elastomer reinforced with graphene nanoplatelets (GNPs) was developed, which exhibits a positive temperature coefficient (PTC) switching temperature of 200 °C. This system shows a more gradual increase in resistivity with temperature and a less pronounced drop in resistivity past the critical transition temperature than other PTC composites. Moreover, Joule heating experiments revealed the ability to regulate at a predefined maximum temperature (within 150 – 200 °C), tuneable through parameters such as filler concentration, applied voltage and thermal boundary conditions, as also demonstrated by multiphysics simulations. Due to the more gradual PTC curve, this composite can produce steady state heating far below the PTC peak, unlike almost all other PTC materials: a unique advantage for self-regulating heating devices as the polymer is not in a melt state, reducing polymer degradation, filler mobility and increasing safety margin before onset of NTC behaviour. [ABSTRACT FROM AUTHOR]

Abstract: Highly transparent pure alumina (65.4%) with an average grain size of 200nm was fabricated using high-pressure (>400MPa) spark plasma sintering. For the first time, highly transparent alumina ceramics were produced using low cost carbon fiber composite for the pressing punches. The high pressure device was also used to fabricate fully dense nanostructured alpha alumina with average grain size of 107nm. The developed low cost high pressure sintering apparatus might be applied to produce a wide range of pore free nanostructured materials. [Copyright &y& Elsevier]

Abstract: 45S5 Bioglass® powder has been densified at low temperatures using Spark Plasma Sintering (SPS). By employing SPS with pressures ranging from 70 to 300MPa, it was possible to achieve fully dense and completely amorphous 45S5 Bioglass® samples at temperatures as low as 500–550°C. By increasing the sintering temperature up to 600°C the dense samples crystallized to Na2CaSi2O6 phase rather than Na2Ca2Si3O9 which is usually achieved at higher temperature. Acellular in vitro test results in Simulated Body Fluid (SBF) revealed that SPSed samples sintered at 600°C resulted in faster formation of hydroxyapatite compared to pressureless sintered ones (1050°C), which were highly crystallized. Amorphous Bioglass® and/or Na2CaSi2O6 phase prepared by SPS could potentially produce Bioglass® composites with increased bioactivity. [Copyright &y& Elsevier]

Abstract: Liquid exfoliation of graphene generally results in flakes with lateral size of one micron or less on average, too small for many applications. In this paper we describe a method to separate an existing dispersion with mean flake length of ∼1μm into fractions, each with different mean flake size. The initial dispersion is centrifuged at a high centrifugation rate, separating small flakes in the supernatant from large flakes in the sediment. Redispersion of the sediment, followed by successive centrifugation, separation and redispersion cycles can be used to separate the flakes by size so long as the centrifugation rate is decreased with each cycle. This procedure results in a range of dispersions with mean flake length varying from 1μm for the highest final centrifugation rate to 3.5μm for the sample whose final centrifugation rate was 500rpm. [Copyright &y& Elsevier]

Two Ag-containing microwave dielectric ceramics AgCa 2 B 2 V 3 O 12 (B = Mg, Zn) with garnet structure were prepared through solid-state reaction method. Dense ceramics were obtained at low sintering temperatures, 665 °C for AgCa 2 Zn 2 V 3 O 12 and 730 °C for AgCa 2 Mg 2 V 3 O 12 . Their microwave dielectric properties were characterized for the first time and analyzed by means of packing fraction, bond valence, octahedral distortion, Raman spectra and infrared reflectivity spectra. Both compounds displayed high chemical compatibility with Ag electrodes. Additionally, thermally stable ceramics with near-zero temperature coefficients of resonance frequency ( τ f ) were achieved by forming ceramic composites with CaTiO 3 . [ABSTRACT FROM AUTHOR]

Abstract: A low‐permittivity dielectric ceramic Li2GeO3 was prepared by the solid‐state reaction route. Single‐phase Li2GeO3 crystallized in an orthorhombic structure. Dense ceramics with high relative density and homogeneous microstructure were obtained as sintered at 1000‐1100°C. The optimum microwave dielectric properties were achieved in the sample sintered at 1080°C with a high relative density ~ 96%, a relative permittivity εr ~ 6.36, a quality factor Q × f ~ 29 000 GHz (at 14.5 GHz), and a temperature coefficient of resonance frequency τf ~ −72 ppm/°C. The sintering temperature of Li2GeO3 was successfully lowered via the appropriate addition of B2O3. Only 2 wt.% B2O3 addition contributed to a 21.2% decrease in sintering temperature to 850°C without deteriorating the dielectric properties. The temperature dependence of the resonance frequency was successfully suppressed by the addition of TiO2 to form Li2TiO3 with a positive τf value. These results demonstrate potential applications of Li2GeO3 in low‐temperature cofiring ceramics technology. [ABSTRACT FROM AUTHOR]

Bio-based and eco-friendly materials have gained a great amount of attention, thanks to the increased awareness of sustainable development and global environments. The use of natural fibres in composites can reduce greenhouse gas emissions and the carbon footprint compared to many synthetic fibres. However, some challenges and concerns remain in natural fibre-reinforced plastics, such as the high moisture absorption and high energy consumption during their manufacturing stage. To tackle these challenges, this study developed an energy-efficient out-of-oven manufacturing method based on a conductive biopolymer nanocomposite film to fabricate natural fibre-reinforced composites with integrated multifunctionalities throughout their life cycle. The smart nanocomposite layer works as an autonomous self-regulating heating element to cure the laminates at the desired temperature without the risk of overheating. Extremely high energy efficiency has been achieved with a significantly reduced energy consumption (a 95% reduction compared with traditional oven curing), which has been attained through the direct heat conduction from the surface layer to the laminates. The embedded nanocomposite surface film on the cured laminate subsequently becomes an integrated multifunctional layer, providing integrated real-time deformation and damage sensing capabilities with enhanced water barrier properties to prolong the service life of natural fibre composites. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Different amounts (0.5, 1, 2.5 and 5wt%) of hollow “cylindrical” and “bamboo-like” boron nitride nanotubes (BNNTs) have been used to reinforce 3Y-TZP zirconia ceramics via spark plasma sintering. No significant influence of different morphologies of BNNTs on the mechanical properties at the macro-scale (elastic modulus, hardness, and fracture toughness) has been observed. The fracture toughness increased continuously with the increasing amount of the BN nanotubes up to 2.5%, resulted in the improvement of ∼100% compared to the reference ZrO2. A direct influence of BNNTs on the toughening of ZrO2 has been recognized. The BNNTs strengthen the zirconia grain boundaries resulting in the alteration in fracture mode from inter- to trans-granular. The BNNTs also promoted the transformation toughening of zirconia. Their influence on the bridging and pull out has been confirmed by the investigation of the composites with the amorphous borosilicate matrix. [Copyright &y& Elsevier]

Abstract: Spark plasma sintering (SPS) was employed to join monolithic β-SiC with or without titanium as intermediate joining material. Both the localized and rapid heating contributed to the inherent energy saving of electric current assisted joining technique. The effects of uniaxial pressure and surface preparation were analyzed independently with respect to the flexural strength and the morphology of the joints. In particular samples polished down to 1μm and joined at 1900°C for 5min achieved the strength of the as received material. The failure occurred outside the joining interface, confirming the optimum quality of the joint. Pressure in combination with surface preparation was necessary to achieve perfect adhesion and pore free direct joining of SiC. The use of Ti foil as a joining material and pressure allowed joining of unpolished SiC. [Copyright &y& Elsevier]