Graphene reinforced alumina nano-composites

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 5 vol% content were densified (>99%) using SPS. The fracture toughness of the material increased by 40% with the addition of only 0.8 vol% 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 ⩾2 vol%. Elastic modulus remained nearly constant for up to 2 vol% and decreased significantly for 5 vol% due to the formation of the inter-connecting graphene network. Fracture toughness measured with the indentation and chevron notch methods were consistent up to 2 vol% and at 5 vol% the percolating network of graphene resulted in easy crack propagation with significant discrepancy between the results for the two methods.     

Effect of graphene addition on the mechanical and electrical properties of Al2O3-SiCw ceramics

This paper presents a study on graphene-reinforced Al₂O₃-SiCw ceramic composites and the relationship between graphene oxide (GO) loading and the resulting mechanical and electrical properties. Well-dispersed ceramic-GO powders were fabricated using a colloidal processing route. Dense composites were obtained via spark plasma sintering, a technique that has the ability to reduce GO to graphene in situ during the sintering process. The mechanical properties of the sintered composites were investigated. The composite with only a small amount of graphene (0.5 vol.%) showed the highest flexural strength (904 ± 56 MPa), fracture toughness (10.6 ± 0.3 MPa·m^1/2) and hardness (22 ± 0.8 GPa) with an extremely good dispersion of graphene within the ceramic matrix. In addition to these exceptional mechanical properties, the sintered composites also showed high electrical conductivity, which allows the compacts to be machined using electrical discharge machining and thus facilitates the fabrication of ceramic components with sophisticated shapes while reducing machining costs.     

Effect of graphite nanoplatelets on the mechanical properties of alumina-based composites

Al₂O₃-based composites using exfoliated graphite nanoplatelets (xGnPs) have been developed by powder metallurgy (PM) route using both conventional as well as spark plasma sintering (SPS) processes. Al₂O₃-0.2, 0.5, 0.8, 3 and 5 vol% xGnP composites have been developed, and the effect of the addition of xGnP on the density, hardness, fracture toughness and wear behaviour of the various Al₂O₃-xGnP composites have been analyzed. Conventional sintering was done at a temperature of 1650 °C for 2, 3 and 4 h in inert atmosphere, whereas SPS was carried out at 1450 °C under 50 MPa pressure for 5 min. A uniform dispersion of the xGnP in the Al₂O₃ matrix was observed in the composites upto the addition of 3 vol% xGnP. Results indicate that a significant improvement in hardness, wear resistance and fracture toughness of the composites could be achieved by using xGnP as nanofiller. The hardness and fracture toughness of the composites developed by both conventional sintering and SPS show an increase upto the addition of 3 and 0.8 vol% xGnP respectively. The wear resistance of the composites also shows significant improvement upto the addition of 3 vol% xGnP. The composites developed by SPS have been found to possess superior mechanical properties as compared to the composites developed by conventional sintering. The improvement in the mechanical properties can be attributed to the strong interaction between the xGnP and the Al₂O₃ matrix at the interfaces and to the toughening mechanisms such as crack bridging and crack deflection.     

Anisotropy of functional properties of SiC composites with GNPs,GO and in-situ formed graphene

This paper reports on anisotropy of functional properties of different silicon carbide-graphene composites due to preferential orientation of graphene layers during sintering. Dense silicon carbide/graphene nanoplatelets (SiC/GNPs) and silicon carbide/graphene oxide (SiC/GO) composites were sintered in the presence of yttria (Y₂O₃) and alumina (Al₂O₃) sintering additives at 1800 °C in vacuum by the rapid hot pressing (RHP) technique. It is found that electrical conductivity of SiC/GNPs and SiC/GO composites increases significantly in the perpendicular direction to the RHP pressing axis, reached up to 1775 S/m in the case of SiC/GO (for 3.15 wt.% of rGO). Also, thermal diffusivity was found to increase slightly by the addition of GNPs in the SiC/GNPs composites in the perpendicular direction to the RHP pressing axis. But, in the parallel direction, the addition of GNPs showed a negative effect. The formation of graphene domains was observed in reference sample SiC-Y₂O₃-Al₂O₃ sintered by RHP, without any addition of graphene. Their presence was confirmed indirectly by increasing electrical conductivity about three orders of magnitude in comparison to the reference sample sintered by conventional hot press (HP). Raman, SEM and TEM analysis were used for direct evidence of presence of graphene domains in RHP reference sample.     

CuAlO2 thermoelectric compacts by SPS and thermoelectric performance improvement by orientation control

We fabricated CuO/Al₂O₃ green compacts from plate-like Al₂O₃ and granular CuO powders by multi-press forming and investigated the alumina orientation using Lotgering's method. The results showed that Al₂O₃ particles preferentially aligned perpendicular to the pressure direction and the orientation degree increased as the forming pressure was increased. We proposed a model describing the movement of the alumina particles to explain the pressure effect on their orientation. The orientation calculation was in good agreement with those by Lotgering's method. Furthermore, we prepared the CuAlO₂ compacts by regular or spark plasma sintering (SPS). However, the compacts sintered by SPS exhibited higher orientation degree and density than those produced by regular sintering. The electrical conductivity values of the orientation-controlled compacts sintered by SPS reached 770 S m⁻¹ at 928 K, which was close to that of CuAlO₂ single crystal. The power factor of the CuAlO₂ compacts with highest orientation degree is as high as 5.95 × 10⁻⁵ W m⁻¹ K⁻¹ at 928 K. Therefore, we can conclude that orientation control is an effective method to enhance the thermoelectric performance of compact polycrystalline CuAlO₂ bulks.     

Comparison of the homemade and commercial graphene in heightening mechanical properties of Al2O3 ceramic

Graphene has been successfully fabricated by a novel method, using graphite powder and NMP (N-Methyl Pyrrolidone) as the raw materials based on the principles of liquidoid exfoliation and mechanical milling. SEM, TEM and Raman spectrum were utilized to characterize the morphology of the homemade graphene, illustrating the few defects and rare layers were endowed in this study. Afterwards, the homemade and commercial graphene were doped into Al₂O₃ powder with the mass ratio of 0%, 1%, 2%, and 3% to reinforce the mechanical properties of the matrix. The composites were processed at 1600 °C, pressure of 30 MPa and soaking time of 1 h by vacuum hot pressing. The test results illustrated the bending strength and fracture toughness tended to be intensive at first and subdued afterwards, achieving the optimal performance of 625.4±18.2 MPa and 6.07±0.22 MPa m^1/2 at 2 wt% prepared graphene additive, and the commercial grapheme owned the best heighten effect in 3 wt% graphene/Al₂O₃ composites. Compared to the blank Al₂O₃ sintered samples, the graphene/Al₂O₃ specimens (both prepared and commercial additive) behaved evident increase in mechanical properties, even upon 30% enhanced in fracture toughness and bending strength generally by the prepared grapheme. Moreover, the prepared graphene had better improvement effect than commercial graphene in enhancing mechanical properties of Al₂O₃ ceramic.     

Fabrication and properties of reduced graphene oxide reinforced yttria-stabilized zirconia composite ceramics

Fully dense yttria-stabilized zirconia (YSZ) ceramics reinforced with reduced graphene oxide (RGO) were fabricated by spark plasma sintering (SPS), and their electrical, thermal, and mechanical properties were investigated. Graphene oxide (GO) was exfoliated by a short sonification in dimethylformamide (DMF)/water solution and uniformly mixed with ZrO₂ powders. The microstructure of the composites showed that undamaged RGO sheets were homogeneously distributed throughout matrix grains. The electrical conductivity of YSZ composites drastically increased with the addition of RGO, and it reached 1.2 × 10⁴ S/m at 4.1 vol.%. However, the thermal diffusivity increased only 12% with RGO addition. The hardness decreased slightly with RGO addition, whereas the fracture toughness significantly increased from 4.4 to 5.9 MPa^1/2. The RGO pull-out and crack bridging contributed to the improved fracture toughness.     

Improvement of wetability,sinterability, mechanical and electrical properties of Al2O3-Ni nanocomposites prepared by mechanical alloying

The wetability improvement and particle size reduction of alumina/Ni composites through mechanical alloying were addressed. Their effect on the sinterability (at high temperature), mechanical and electrical properties were studied. Al₂O₃ matrix nanocomposites reinforced with different volume fractions of Ni up to 10 vol% were prepared by mechanical alloying. The milled powders were cold pressed and sintered at different firing temperatures up to 1600 °C. The morphology of powders and the microstructure of sintered bodies were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), respectively. Furthermore, relative density, apparent porosity, mechanical properties and electrical resistivity of the sintered composites were investigated. The results revealed that Al₂O₃ matrix was successfully coated with Ni thin film through mechanical alloying; the thickness of coat was increased with increasing the Ni content. Moreover, the increasing of both Ni content and sintering temperature up 1600 °C, led to a remarkable increase in the relative density and facture toughness of the sintered specimen. On the other hand, microhardness and elastic modulus were decreased with increasing of Ni content, while they increased significantly with the increase of sintering temperature. The electrical resistivity was decreased with increasing Ni content and sintering temperature.     

Enhancing copper infiltration into alumina using spark plasma sintering to achieve high performance Al2O3/Cu composites

Al₂O₃/Cu (with 30 wt% of Cu) composites were prepared using a combined liquid infiltration and spark plasma sintering (SPS) method using pre-processed composite powders. Crystalline structures, morphology and physical/mechanical properties of the sintered composites were studied and compared with those obtained from similar composites prepared using a standard liquid infiltration process without any external pressure. Results showed that densities of the Al₂O₃/Cu composites prepared without applying pressure were quite low. Whereas the composites sintered using the SPS (with a high pressure during sintering in 10 min) showed dense structures, and Cu phases were homogenously infiltrated and dispersed with a network from inside the Al₂O₃ skeleton structures. Fracture toughness of Al₂O₃/Cu composites prepared without using external pressure (with a sintering time of 1.5 h) was 4.2 MPa m^1/2, whereas that using the SPS process was 6.5 MPa m^1/2. These toughness readings were increased by 18% and 82%, respectively, compared with that of pure alumina. Hardness, density and electrical resistivity of the samples prepared without pressure were 693 HV, 82.5% and 0.01 Ω m, whereas those using the SPS process were 842 HV, 99.1%, 0.002 Ω m, respectively. The enhancement in these properties using the SPS process are mainly due to the efficient pressurized infiltration of Cu phases into the network of Al₂O₃ skeleton structures, and also due to high intensity discharge plasma which produces fully densified composites in a short time.     

Microstructure,fracture behaviour and mechanical properties of conductive alumina based composites manufactured by SPS from graphenated Al2O3 powders

Conductive Al₂O₃/graphene composites were manufactured by SPS from Al₂O₃ powders coated with a few graphene layers. Composite powders with a total carbon content of 0.1, 0.6 and 1.0 wt. % were manufactured by chemical vapour deposition. The effect of the graphene content on the microstructure, mechanical and electrical properties of the compacts were studied. Graphene, homogenously located along the grain boundaries, dramatically hindered the Al₂O₃ grain growth. The continuous interconnected graphene network enhanced electrical properties, achieving percolation threshold as low as 0.6 wt. % of graphene. The content of 1 wt. % of graphene increased electroconductivity by 13 orders of magnitude as compared to the monolithic alumina. The indentation fracture toughness increased by 20 % in specimens with 0.6 wt. % graphene content as compared to pure alumina. The presence of 1.0 wt. % of graphene resulted in a slight decrease of elastic modulus and hardness, but strength decreased by 40 %.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Heat transport enhancement of thermal energy storage material using graphene/ceramic composites

Graphene/ceramic composites are proposed by directly depositing graphene on the insulating Al₂O₃ particles by chemical vapor deposition without any metal catalysts. Carbothermic reduction occurring at the Al₂O₃ surface is vital during the initial stage of graphene nucleation and the graphene sheet can connect with neighboring sheets to completely cover Al₂O₃ particles. The quality and layer number of graphene on Al₂O₃ can be finely tailored by changing the growth temperature and gas ratio. Graphene coated Al₂O₃ (G-Al₂O₃) composites are used as effective fillers of stearic acid (SA) to increase the thermal transport property. By the optimization of the layer number of graphene, size of Al₂O₃ particles and ratio of G-Al₂O₃/SA in a quantitative, their thermal conductivities significantly increase up to 11 folds from 0.15 to 1.65 W m⁻¹ K⁻¹. The great improvement is attributed to the high thermal transfer performance of graphene and excellent wettability between graphene and SA. When the G-Al₂O₃/SA composites with the graphene coated porous Al₂O₃ foam, the thermal conductivity further reaches to 2.39 W m⁻¹ K⁻¹, and the corresponding latent heat is 38 J g⁻¹. It demonstrates the potential applications of graphene in thermal transport and thermal energy storage devices.     

B6O materials with Al2O3/Y2O3 additives densified by FAST/SPS and HIP

Fully densified B₆O materials with Al₂O₃/Y₂O₃ sintering additives amounts systematically varied between 0 and 15 vol.% and Al₂O₃/(Al₂O₃ + Y₂O₃) molar ratios of 0.05–1 were prepared by FAST/SPS and HIP at sintering temperatures between 1725 °C and 1900 °C. Their densification and microstructure were correlated with measured mechanical properties. The addition of low additive amounts in the range of 2–3 vol.% was found to increase the fracture toughness and strength from 2.0 MPa m^1/2 (SEVNB) and 420 MPa for pure B₆O to about 3.0 MPa m^1/2 and 540 MPa, but it had no effect on the hardness, which remained at a high level of 30–36 GPa (HV_0.4). Higher additive contents did not yield a further improvement in the toughness but resulted in a reduction in hardness and strength.     

Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites

The Graphene/Mn_0.7Zn_0.3Fe₂O₄ composites were synthesized by coprecipitation and sintered by a spark-plasma-sintering (SPS) method. The thermoelectric properties of the sintered composites were evaluated in the temperature range of 343–973 K. The effect of graphene on the thermoelectrical properties of Mn_0.7Zn_0.3Fe₂O₄ was investigated. The dispersion of 2 wt% graphene in Mn_0.7Zn_0.3Fe₂O₄ effectively enhanced the electrical conductivity and the absolute value of Seebeck coefficient, while thermal conductivity was decreased. The results showed that the maximum ZT value of 0.035 at 973 K was obtained in the composite with 2 wt% graphene.     

Preparation and mechanical properties of alumina composites reinforced with nickel-coated graphene

This paper discusses the influence of nickel–phosphorus coated graphene (Gn–Ni–P) and uncoated graphene (Gn) addition to an alumina matrix and its impact on the mechanical properties of obtained composites. The composites are prepared via powder processing and consolidated using the Spark Plasma Sintering (SPS) method. The effects of the addition of coated graphene and coating thickness on mechanical properties were evaluated. Physical properties such as relative density, hardness and fracture toughness were analyzed. Significant improvement of the fracture toughness (60%) for the composites with 2 vol% Gn–Ni–P compared to reference sample was observed. Moreover, 35% higher K_IC was noticed for Gn–Ni–P reinforced composites than for Al₂O₃–Gn.     

Synthesis,microstructure and electrical conductivity of carbon nanotube–alumina nanocomposites

Carbon nanotube–alumina (CNT–Al₂O₃) nanocomposites have been synthesized by direct growth of carbon nanotubes on alumina by chemical vapor deposition (CVD) and the as-grown nanocomposites were densified by spark plasma sintering (SPS). Surface morphology analysis shows that the CNTs and CNT bundles are very well distributed between the matrix grains creating a web of CNTs as a consequence of their in situ synthesis. Even after the SPS treatment, the CNTs in the composite material are still intact. Experimental result shows that the electrical conductivity of the composites increases with the CNT content and falls in the range of the conductivity of semiconductors. The nanocomposite with highest CNT content has electrical conductivity of 3336 S/m at near room temperature, which is about 13 orders of magnitude increase over that of pure alumina.     

Effects of Pr6O11 on the microstructure and mechanical properties of Ti/Al2O3 composites prepared by pressureless sintering

Ti/Al₂O₃ composites with different volume percentages of Pr₆O₁₁ added (0–12.0  vol.%) were prepared by pressureless sintering at 1600 °C for 1.5 h. The influences of Pr₆O₁₁ on the composition, microstructure and mechanical properties of the composites were characterized and investigated. The results showed that Pr₆O₁₁ could promote the sintering of the composites by generating some new interfacial reaction products, such as AlTiO₂, Pr₂Ti₂O₇ and PrAlO₃. Pr₆O₁₁ could also inhibit the production of TiAl and Ti₃Al by the same mechanism. Additionally, Pr₆O₁₁ changed hexagonal alumina to tetragonal alumina. The latter could improve the mechanical properties of the composites by the effects of crack deflection and particle pullout when it was present in proper amounts. Composites showed satisfactory comprehensive properties when the content of Pr₆O₁₁ was no more than 3.0 vol.%.     

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.     

Fabrication and properties of monolayer graphene reinforced Al2O3/TiN ceramic tool material

Al₂O₃/TiN/graphene ceramic tool materials were prepared by spark plasma sintering technology and the strengthening and toughening mechanisms were studied. The influence of monolayer graphene content on the mechanical properties and microstructure of the composite material were analyzed and the strengthening and toughening mechanisms were researched. The results showed that with an addition of .5 vol.% graphene the mechanical properties of the material reached the best. The bending strength, hardness, and fracture toughness were 624 MPa, 23.24 GPa, and 6.53 MPa·m^1/2, respectively. Graphene existed in the forms of few-layer and multilayer. The toughening mechanism of few-layer graphene was mainly graphene breaking, and that of multilayer graphene included graphene breaking and pulling-out. Graphene could contribute to the uniform growth of grains due to the excellent electrical conductivity and the high thermal conductivity. The addition of nano-TiN introduced many endocrystalline structures and graphene promoted this phenomenon. Micro-TiN grains made the crack extension show a combination of transgranular fracture, intergranular fracture, crack bridging, and crack deflection, while graphene introduced weak grain interfaces and made the crack appear more branches. The layered graphene made the material fracture change from two-dimension to three-dimension.