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.     

Effect of citrate to nitrate ratio on the sol-gel synthesis of nanosized α-Al2O3 powder

This paper studied the effect of the variation of citrate to nitrate ratio on the sol-gel synthesis of nanocrystalline α-alumina powder. The process involves the formation of a sol using aluminium nitrate and citric acid which then transferred to a transparent gel and finally to a powdery mass. The prepared alumina powder was thoroughly characterised by different techniques such as phase analysis, thermal analysis, microstructure study, etc. Variation in citrate to nitrate molar ratio (C/N) influenced the thermal decomposition behaviour, particle size distribution, phase transformation temperature and specific surface area of the developed powder. The complete formation of α- Al₂O₃ was obtained at around 950 °C for C/N=1.5. The unimodal distribution of the particles within a narrow size range and higher particle to particle linking through solid bridging are the characteristics of the powder prepared at C/N=1.5. It was found higher citrate to nitrate ratio in the fuel rich condition is responsible for better properties of synthesised alumina powder.     

Increased electrical conductivity and the mechanism of samarium‐doped ceria/Al2O3 nanocomposite electrolyte

To further enhance the electrical conductivity of doped ceria, the samarium‐doped ceria (SDC)/Al₂O₃ nanocomposites were prepared through sintering the coprecipitated powders in 1100°C‐1300°C. The grain sizes of all composites are less than 100 nm and decrease with alumina addition. Besides the main phases of SDC and Al₂O₃, the SmAlO₃ can precipitate in the composites if sintered at higher temperatures or for longer dwell time. The deviations of SDC diffraction peak positions demonstrate the solid solution of alumina into SDC lattice. The total electrical conductivities of the composites increase with alumina content until 30% alumina is added. The SDC/30%Al₂O₃ presents the higher total conductivity than the pure SDC by about five times. Specifically, the grain interior conductivity generally decreases with the alumina addition while the grain‐boundary conductivity increases with that. The introduction of the conductive SDC/Al₂O₃ interface can contribute to the rise of total conductivity, yet the excessive alumina addition also blocks the oxygen ion conduction. The SmAlO₃ precipitation is detrimental to the ion conduction for it consumes part of alumina and leads to the decrement in Sm concentration of SDC grain. Appropriate alumina addition not only enhances the conductivity of SDC but also lowers the material cost.     

Electrical conductivity of spark plasma sintered Al2O3–SiC and Al2O3-carbon nanotube nanocomposites

The electrical conductivity of alumina-silicon carbide (Al₂O₃–SiC) and alumina-multiwalled carbon nanotube (Al₂O₃-MWCNT) nanocomposites prepared by sonication and ball milling and then consolidated by spark plasma sintering (SPS) is reported. The effects of the nanophase (SiC and MWCNTs) and SPS processing temperature on the densification, microstructure, and functional properties were studied. The microstructure of the fabricated nanocomposites was investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The phase evolution was determined using X-ray diffraction (XRD). The room-temperature direct current (DC) electrical conductivity of the monolithic alumina and nanocomposites was determined using the four-point probe technique. The EDS mapping results showed a homogenous distribution of the nanophases (SiC and MWCNTs) in the corresponding alumina matrix. The room-temperature DC electrical conductivity of monolithic alumina was measured to be 6.78 × 10⁻¹⁰ S/m, while the maximum electrical conductivities of the alumina-10 wt%SiC and alumina-2wt%MWCNT samples were 2.65 × 10⁻⁵ S/m and 101.118 S/m, respectively. The electrical conductivity increased with increasing nanophase concentration and SPS temperature. The mechanism of electrical conduction and the disparity in the electrical performance of the two investigated nanocomposite systems (alumina-SiC and alumina-MWCNT) are clearly described.     

Fabrication and electromagnetic interference shielding effectiveness of Ti3Si(Al)C2 modified Al2O3/SiC composites

A dense alumina fiber reinforced silicon carbide matrix composites (Al₂O₃/SiC) modified with Ti₃Si(Al)C₂ were prepared by a joint process of chemical vapor infiltration, slurry infiltration and reactive melt infiltration. The conductive Ti₃Si(Al)C₂ phase introduced into the matrix modified the microstructure of Al₂O₃/SiC. The refined microstructure was composed of conductive phase, semiconductive phase and insulating phase, which led to admirable electromagnetic shielding properties. Electromagnetic interference shielding effectiveness (EMI SE) of Al₂O₃/SiC and Ti₃Si(Al)C₂ modified Al₂O₃/SiC were investigated over the frequency range of 8.2–12.4 GHz. The EMI SE of Al₂O₃/SiC-Ti₃Si(Al)C₂ exhibited a significant increase from 27.6 to 42.1 dB compared with that of Al₂O₃/SiC. The reflection and absorption shielding effectiveness increased simultaneously with the increase of the electrical conductivity.     

A comparative study of the lattice structure,optical band gap,electrical conductivity and polarization at different stages of the heat treatment of chemical routed Al(OH)3

Aluminium hydroxide (Al(OH)₃) was prepared by chemical reaction of the Al(NO₃)₃ in alkaline medium. The as-prepared powder was heated in the temperature range 250 °C to 1250 °C for studying the structural phase transformation at different stages of the heat treatment. The synchrotron x-ray diffraction patterns confirmed a structural transformation of Al(OH)₃ through different (Boehmite, γ, θ, δ, and α) polymorphic phases of Al₂O₃ on increasing the heat treatment temperature. The samples in Boehmite (γ-AlOOH) and α- Al₂O₃ phases showed Raman active modes, whereas the intermediate (meta-stable) multi-phased structure showed weak Raman active peaks. The analysis of UV–visible spectra of the samples indicated two optical band gap energy values in the high energy range 4.50–4.73 eV and low energy range 3.06–3.84 eV. The voltage dependence of current, capacitance and electrical polarization were recorded to study electrical properties in heat treated samples. The capacitance value, derived from the polarization, showed a usual increasing trend on decreasing the measurement frequency (inverse of the time) of driving electric voltage. The measured electrical polarization in the samples was found to be highly correlated to their electrical conductivity and the results are helpful to understand the role of electrical conductivity on exhibiting the apparently ferroelectric properties in high conductive and low polarizable dielectric oxides.     

Al2O3 Colloidal Nanocrystals with Strong UV Emission

A facile sol–gel procedure has been developed for the synthesis of colloidal alumina nanocrystals. For the first time, optical characterization procedures were employed to study the quantum confinement effects in optical properties of the prepared Al₂O₃ sol. Accordingly, the hyperbolic band model was used to determine the optical band gap of colloidal alumina nanocrystals. X‐Ray diffraction pattern was used to study the crystallographic phase of the dried gel. Morphological characterization was performed using scanning electron microscopy (SEM). Inductively Coupled Plasma (ICP) emission spectroscopy was used to determination purity of the Al₂O₃ powder. High‐resolution TEM showed that the diameter of colloidal nanocrystals is about 10 nm. Photoluminescence spectroscopy demonstrated that quantum yields for colloidal nanocrystals are 68% with 300 nm excitation wavelength. The experimental observations confirm that highly stable alumina sol with strong UV emission was synthesized. The mentioned optical properties have not been reported before.     

Simultaneously enhanced mechanical properties and thermal properties of ultrahigh‐molecular‐weight polyethylene with polydopamine‐coated α‐alumina platelets

Polydopamine (PDA) was employed to modify micrometric Al₂O₃ platelets to improve the interfacial compatibility between α‐Al₂O₃ powder and ultrahigh‐molecular‐weight polyethylene (UHMWPE). The structure of PDA‐coated Al₂O₃ and UHMWPE composites was investigated via Fourier transform infrared spectroscopy, scanning electron microscopy and X‐ray photoelectron spectroscopy. The thermal stability and mechanical performance of the samples were also evaluated. It is clear that UHMWPE/PDA‐Al₂O₃ composites exhibit better mechanical properties, higher thermal stability and higher thermal conductivity than UHMWPE/Al₂O₃ composites, owing to the good dispersion of Al₂O₃ powder in the UHMWPE matrix and the strong interfacial force between the macromolecules and the inorganic filler caused by the presence of PDA. The tensile strength and the tensile elongation at break of UHMWPE/PDA‐Al₂O₃ composite with 1 wt% PDA‐Al₂O₃ are 62.508 MPa and 462%, which are 1.96 and 1.98 times higher than those of pure UHMWPE, respectively. The thermal conductivity of UHMWPE/PDA‐Al₂O₃ composite increases from 0.38 to 0.52 W m^?1 K^?1 with an increase in the dosage of PDA‐Al₂O₃ to 20 wt%. The results show that the prepared PDA‐coated Al₂O₃ powder can simultaneously enhance the mechanical properties and thermal conductivity of UHMWPE. © 2018 Society of Chemical Industry     

Photocurrents in crystal-amorphous hybrid stannous oxide/alumina binary nanofibers

Suppression of charge recombination by thin amorphous alumina layers on metal oxide semiconductors has demonstrated a vital role in electronic appliances beside its role as an insulator. This study reports effect of amorphous alumina (Al₂O₃) on the structural, electrical, and optical properties of stannous oxide (SnO₂). The samples for the present study are prepared as nanofibers by electrospinning a polymeric solution containing aluminum and stannous precursors and subsequent annealing; six samples with varying concentrations of aluminum and stannous are considered. A crystal-amorphous SnO₂/Al₂O₃ hybrid system was confirmed by both XRD and XPS analysis. Both BET and Mott-Schottky analysis showed increase in the surface area and conduction band minimum of the sample with increase in the Al content, however, at the expense of its electrical conductivity. The electron lifetime of the sample increased with increase in the Al content, but the electron transport time increase with decrease in the electrical conductivity of the sample. Both Urbach energy measurement and Stoke's shift showed generation of deeper trap state with increase in the Al content. Investigation on sample photovoltaic performance showed that the loss in electrical conductivity of the sample can be compensated by the improved surface area to a certain extent. Interestingly, a composite nanofiber containing equal molar fraction of aluminum and stannous showed orders of magnitude higher photocurrent despite its similar resistivity as that of pure alumina fibers, which is shown to originate from a Fermi energy gradient at the Al₂O₃/SnO₂ interface.     

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.     

Electrical properties of La2Mo2O9/Al2O3 nanocomposites fabricated by microwave sintering method

High density La₂Mo₂O₉/Al₂O₃ nanocomposite electrolytes were successfully fabricated by microwave sintering method from the nano-scaled La₂Mo₂O₉/Al₂O₃ powders. Phase formation, microstructure, grain size and electrical properties of the specimens were examined using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and AC impedance spectroscopy. It was confirmed that the composite electrolytes consist of nanocrystalline La₂Mo₂O₉ phase with an average grain size of 70–150 nm and a small amount of crystalline α-Al₂O₃ phase at grain boundary. With increasing alumina content, the total conductivity at 600 °C of the composite specimens increases at first and then decreases with a maximum value of about 0.024 S/cm appearing at 2.25 mol% alumina. It was suggested that the enhancement of total conductivity of the composites may be ascribed to the improvement of both grain and grain-boundary conduction introduced by the addition of appropriate amount of alumina.     

Hybrid microwave sintering of alumina and 3 mol% Y2O3-stabilized zirconia in a multimode cavity – Influence of the sintering cell

In microwave (MW) sintering, samples directly heat by absorption of the electromagnetic field, leading to a fast volumetric heating. Thus, the materials are heated thanks to their own dielectric properties. In multimode cavities, samples are always heated with a sintering cell in order to get an insulation of the samples and a heating homogeneity. However, to date, very limited research has been carried out to study the effects of materials used in the sintering cell. In this paper the microstructure and densification of Al₂O₃ and 3Y-TZP were investigated and compared between three different sintering cells. The used sintering cell contains three main elements: thermal insulators, a SiC susceptor and a protective mullite tube. Higher final densities (98.3 ± 0.6% and 98.6 ± 0.6% of T.D. for Al₂O₃ and 3Y-TZP respectively), lower densification temperatures and better microstructure homogeneity were obtained by using the sintering cell containing both mullite tube and SiC susceptor. But, sintering using the sintering cell without the SiC susceptor was also possible, especially for the lowest lossy material,i.e., Al₂O₃. This can be explained by a possible susceptor effect of the mullite tube. However, the microstructural observations showed a difference of homogeneity in the microstructure for Al₂O₃ and 3Y-TZP with some sintering cells. It can be linked to the difference of dielectric properties of the two materials. The influence of the sintering cell is less critical for 3Y-TZP, which couples better with MW than Al₂O₃. It was also observed that densification curves were switched to higher temperatures during sintering with the presence of SiC susceptor and without mullite tube. These results were justified by an error in the temperature measurement of the IR-pyrometer.     

Processing and characterization of fiber-reinforced and layered alumina - graphene composites

The aim of the present contribution is the processing and characterization of fiber-reinforced and layered alumina - graphene composites, prepared by the combination of electrospinning, calcination, chemical vapor deposition (CVD) and spark plasma sintering (SPS). The fiber-reinforced composite contains homogenously distributed graphene-coated polycrystalline alumina microfibers in the Al₂O₃ matrix. The layered composites contain Al₂O₃ layers and layers of graphene-coated alumina microfibers or layers of graphene-coated alumina grains of submicron size. The systems with high density, 99.5–99.9 %, show different grain sizes of Al₂O₃ in their constituents, changing from 0.08 to 1.9 μm in comparison to the monolithic alumina with the average grain size of 2.6 μm. The composites and their layers show increased electrical conductivity, hardness, and fracture toughness by approximately five orders of magnitude, 31 %, and 8%, respectively, in comparison to the monolithic alumina due to the presence of graphene layers, small grain-sized alumina, and microfibers in the composites.     

Effects of artificial aging on the biaxial flexural strength of Ce-TZP/Al2O3 and Y-TZP after various occlusal adjustments

The aim of this study was to determine the effect of aging on the biaxial flexural strength (BFS) of Ce-TZP/Al₂O₃ and Y-TZP after occlusal adjustment. NanoZr block (Ce-TZP/Al₂O₃ nanocomposite) and Katana zirconia block (Y-TZP) were prepared by milling with the aid of CAD/CAM into disk-shaped specimens. For each type of zirconia, 16 specimens were prepared without grinding for the control group (diameter of 16 mm and thickness of 1.20±0.05 mm, mean±SD), while 48 specimens were prepared for 3 experimental groups (n=16 each; 16 mm in diameter and 1.50±0.05 mm thick) with different types of surface grinding: superfine diamond bur (group I), zirconia stone bur (group II), and zirconia stone and fine polishing bur (group III). These specimens underwent an aging process in a steam autoclave for 5 h at 0.2 MPa and 134 °C, and then X-ray diffractometry was applied along with measurements of surface roughness and BFS. After occlusal adjustment, the monoclinic phase percentage increased in 3 experimental groups. Overall the increase was greater for Ce-TZP/Al₂O₃ than for Y-TZP. The Ra value showed similar changes for both types of zirconia. Following the aging process, Y-TZP showed a greater increase in the monoclinic phase percentage, but the change was not statistically significant. The Ra value showed similar changes in both types of zirconia, with no significant differences between before and after the aging process. The results of the BFS test showed that applying the aging process after grinding significantly increased the strength of both types of zirconia, with Ce-TZP/Al₂O₃ being significantly stronger than Y-TZP. The specimens treated by a superfine diamond bur exhibited the highest BFS in the four tested groups. Ce-TZP/Al₂O₃ had a higher BFS and greater resistance to low-temperature degradation than did Y-TZP.     

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.     

3Y-TZP ceramics with improved hydrothermal degradation resistance and fracture toughness

Different factors such as the way of incorporating the Y₂O₃ stabilizer, alumina addition and sintering temperature were assessed with the goal to improve the low temperature degradation (LTD) resistance of 3Y-TZP without compromising on the mechanical properties. The degradation of hydrothermally treated specimens was studied by X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy.Decreasing the sintering temperature decreased the LTD susceptibility of 3Y-TZPs but did not allow to obtain a LTD resistant 3Y-TZP with optimized mechanical properties. Alumina addition along with the use of Y₂O₃ stabilizer coated starting powder allowed to combine both an excellent toughness and LTD resistance, as compared to alumina-free and stabilizer co-precipitated powder based equivalents. Transmission electron microscopy revealed that the improved LTD resistance could be attributed to the segregation of Al³⁺ at the grain boundary and the heterogeneously distributed Y³⁺ stabilizer.     

Preparation of β"-Al2O3 electrolytes by freeze-drying and SPS

Na-ionic conductivity and mechanical strength are two important parameters for β"-Al₂O₃ ceramics used as electrolyte materials. In this study, the ice-templating and freeze-drying processes were used to prepare uniformly porous Al₂O₃ powder with a certain degree of crystallographic orientation. With the help of spark plasma sintering (SPS) process, solid samples were fabricated from the porous power at a low temperature (1100 °C). Finally, highly conductive and highly strong β"-Al₂O₃ electrolytes were prepared after the solid samples were subject to further treatment at a high temperature (1600 °C). The scanning electron microscopy (SEM) results indicate that integration of the initial SPS process and further high-temperature sintering treatment have turned the freeze-dried Al₂O₃ powder to β"-Al₂O₃ samples whose grain structure is similar to the “brick-bridge-mortar” structure. Regarding such β"-Al₂O₃ samples, the bending strength parallel to the pressure direction is up to 273 MPa; the degree of crystallographic orientation, 0.25, and the ionic conductivity perpendicular to the pressure direction, 0.239 S cm^?1 at 350 °C. Therefore, it can be concluded that the combination of freeze-drying, SPS and an additional high-temperature sintering process has helped generate a structure that improves the ion mobility and bending strength of the β"-Al₂O₃ ceramic, and thereby improving its strength and ionic conductivity.     

Multi-contact hybrid thermal conductive filler Al2O3@AgNPs optimized three-dimensional thermal network for flexible thermal interface materials

In this work, a multi-contact Al₂O₃@AgNPs hybrid thermal conductive filler was synthesized by in-situ growth method to fill high thermal conductivity polydimethylsiloxane (PDMS)-based composites to prepare TIMs. And the thermal conductivity, electrical conductivity, and mechanical properties of the composite materials were studied. During the synthesis process of the multi-contact hybrid filler, different concentrations of silver ions were reduced to generate silver nanoparticles and attached to the surface of Al₂O₃. Al₂O₃@AgNPs/PDMS thermally conductive composites were prepared by changing the filler addition. Using SEM, XPS, and XRD is used to characterize the morphology and chemical composition of Al₂O₃@AgNPs hybrid filler. The thermal conductivity of PDMS-based composites with different AgNPs content under 70 wt% filler loading was studied. The results show that the thermal conductivity of PDMS-based composites filled with 7owt%Al₂O₃@3AgNPs/PDMS multi-contact hybrid filler is 0.67 W/m·K, which is 3.72 times that of pure PDMS, and is higher than that of unmodified Al₂O₃ with the same addition amount. /PDMS composite material has a high thermal conductivity of 24%. This work provides a new idea for the design and manufacture of high thermal conductivity hybrid fillers for TIMs.     

Dry-sliding wear behavior of 3Y-TZP/Al2O3-NbC nanocomposites produced by conventional sintering and spark plasma sintering

This work presents the initial results of the dry-sliding wear behavior of 3 mol% yttria-stabilized zirconia reinforced with 5 vol% alumina-niobium carbide (3Y-TZP/5 vol% Al₂O₃-NbC) nanocomposites sintered by conventional sintering and spark plasma sintering methods in the temperature range of 1350-1450°C. The reinforcement of 3Y-TZP matrix with hard nanoparticles aimed to improve wear strength of the composites. Wear tests were performed by the ball-on-disc method using alumina (Al₂O₃) and tungsten carbide with 6 wt% cobalt cermet (WC-6%Co) balls as counter-materials, a load of 15 N, a sliding distance of 2000 m, and a sliding speed of 0.1 m/s. Wear behavior was evaluated in terms of wear rate and FE-SEM micrograph analysis of the wear tracks. The nanocomposite sintered at 1450°C by conventional sintering exhibited the least wear when tested with the WC-6%Co ball. Generally, the wear mechanism showed evidence of severe wear regime with both counter-materials.     

Design and fabrication of Al2O3f/SiCN composite with excellent microwave absorbing and mechanical properties

A new kind of structural and functional integration ceramic matrix composite material was prepared from high-performance alumina (Al₂O₃) fibers and absorbing silicon carbonitride (SiCN) ceramics via a combination of polymer infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) methods. The Al₂O₃ fiber annealed at its cracked temperature had enhanced permittivity, because the sizing agent on the Al₂O₃ fiber surface was cracked into pyrolysis carbon. For PIP + CVI Al₂O_3f/SiCN composites, PIP SiCN matrix with low conductivity was used as the matching phase, while CVI SiCN matrix with medium permittivity and dielectric loss was regarded as the reinforcing phase distributed in porous PIP SiCN matrix and inter-bundles of Al₂O₃ fiber to improve their mechanical and microwave absorption properties. The fracture toughness and flexural strength of Al₂O_3f/SiCN composite were determined to be 9.4 ± 0.5 MPa m^1/2 and 279 ± 28 MPa, respectively. Based on the design principles for impedance matching, the Al₂O_3f/SiCN composites before and after oxidation were used as loss and impedance layers, respectively. It was found that the optimized composite had the lowest reflection coefficient (RC) of −70 dB and the effective absorption bandwidth covering the whole X-band. In conclusion, Al₂O_3f/SiCN composite can serve as a high-temperature structural material with excellent microwave absorption properties for aerospace applications.