Microstructure and mechanical properties of multi-walled carbon nanotubes containing Al2O3–C refractories with addition of polycarbosilane

Carbon nanotubes (CNTs) are a promising reinforcement for fabricating Al₂O₃–C refractories. However, CNTs are prone to agglomerate or react with antioxidants or reactive gaseous phases such as Al (g), Si (g) and SiO (g), etc. at high temperatures. To overcome the problems above, polycarbosilane (PCS) and multi-walled carbon nanotubes (MWCNTs) were firstly mixed with micro-alumina powder in a liquid medium and then incorporated into Al₂O₃–C refractories. Then the microstructure and mechanical properties of Al₂O₃–C refractories fired in the temperature range from 800 °C to 1400 °C were investigated in this work. The results showed that the MWCNTs were well dispersed in the specimens with addition of PCS in contrast to the specimens without PCS due to the PCS adsorption on the surface of MWCNTs during the mixing process. And the mechanical properties, such as cold modulus of rupture (CMOR), flexural modulus (FM), forces and displacements of Al₂O₃–C refractories with PCS were much higher than those without PCS, which was attributed to more homogeneous dispersion of MWCNTs, more residual MWCNTs as well as different morphologies of ceramic whiskers. Meanwhile, the oxidation resistance of Al₂O₃–C refractories with PCS was improved greatly, which was supposed that the in situ formed SiC_xO_y coating prevented the oxidation of MWCNTs to some extent.     

High temperature mechanical properties of Al2O3/TiC micro–nano-composite ceramic tool materials

A type of Al₂O₃-based composite ceramic tool material simultaneously reinforced with micro-scale and nano-scale TiC particles was fabricated by the hot-pressing technology with different contents of cobalt additive. The effects of cobalt on the ambient temperature mechanical properties and high temperature flexural strength were investigated. The flexural strength and fracture toughness of the composite with 3 vol% cobalt as a function of temperature were investigated. Cobalt greatly enhanced the ambient temperature flexural strength and fracture toughness, while further increasing the content of cobalt led to a dramatic strength degradation, especially at high temperature. The flexural strength of the composite containing 3 vol% cobalt decreased as the temperature increased from 20 to 1200 °C, and the fracture toughness decreased as a function of the temperature up to 1000 °C but increased at 1200 °C. The degradation of high temperature flexural strength was ascribed to the change of the fracture mode, the grain and grain boundary oxidation, the decrease of elastic modulus and the grain boundary sliding.     

Sintering,mechanical, electrical and oxidation properties of ceramic intermetallic TiC–Ti3Al composites obtained from nano-TiC particles

The paper discusses the development of a new material system for interconnect application in Solid Oxide Fuel Cells (SOFC) based on TiC–Ti₃Al. Nano-sized TiC powders utilized in this research were synthesized using carbon coated TiO₂ precursors from a patented process. The pressureless sintering of TiC–Ti₃Al in a vacuum was applied at temperatures between 1100 °C and 1500 °C and content of Ti₃Al was varied in the range of 10–40 wt%. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase evaluation and sintering behavior. Relative density increased markedly with increasing sintering temperature because of grain growth and formation of the Ti₃AlC₂ secondary phase. Dense products (>95% TD) were prepared from nanosized TiC powders with 10 and 20 wt% Ti₃Al, but with about 8 to 10% porosity for 30 and 40 wt% Ti₃Al. The mechanical properties were determined from Vickers hardness and fracture toughness calculations. Vickers hardness decreased and fracture toughness increased with increasing Ti₃Al content. The electrical conductivity and oxidation behavior of TiC–Ti₃Al composites were investigated to evaluate the feasibility for SOFC interconnect application. The electrical conductivity measurements in the air at 800 °C for 100 h were made using the Kelvin 4-wire method.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

Load-dependence of Knoop hardness of Al2O3–TiC composites

Nine samples of Al₂O₃–30 wt.% TiC composites were prepared by hot-pressing the Al₂O₃ powder mixed with TiC particles. The average sizes of the TiC particles used for preparing the nine samples were different with each other. Knoop hardness measurements were conducted on these nine samples, respectively, in the indentation load range from 1.47 to 35.77 N. For each sample, the measured Knoop hardness decreases with the increasing indentation load. The classical Meyer's power law and an empirical equation proposed originally by Bückle were verified to be sufficiently suitable for describing the observed load-dependence of the measured hardness. Analysis based on Meyer's law can not provide any useful information about the cause of the observed ISE while true hardness values, which are load-independent, can be deduced from the Bückle's equation. It was found that the deduced true hardness increases with the average size of TiC particles existing in the sample.     

Electrical resistivity of Cr–Al2O3 and ZrxAly–Al2O3 composites with interpenetrating microstructure

AC and DC resistivity of Cr–Al₂O₃ and Zr_xAl_y–Al₂O₃ composites with varying metal content were measured. A strong percolation behavior was observed in the Cr–Al₂O₃ system, where the AC resistivity varied nine orders of magnitude close to the percolation threshold of 28 vol.%. AC measurements were less dependant on the contact resistance than DC measurements. The best reproducibility was obtained at a frequency of 100 kHz. AC resistivity values of insulating composites differed from DC values and may also be frequency-dependant. DC measurements up to 600 °C indicate that the intermetallic phases ZrAl₃ and ZrAl are PTC conductors. The electrical properties of Zr_xAl_y–Al₂O₃ samples with a metal content of 29 vol.% were anisotropic, with a much higher resistivity in the pressing direction.     

Additive manufacturing of continuous carbon fiber–reinforced silicon carbide ceramic composites

A material extrusion (MEX) technology has been developed for the additive manufacturing of continuous carbon fiber–reinforced silicon carbide ceramic (C_f/SiC) composites. By comparing and analyzing the rheological properties of the slurries with different compositions, a slurry with a high solid loading of 48.1 vol% and high viscosity was proposed. Furthermore, several complex structures of C_f/SiC ceramic composites were printed by this MEX additive manufacturing technique. Phenolic resin impregnation–carbonization process reduces the apparent porosity of the green body and protects the C_f. Finally, the reactive melting infiltration (RMI) process was used to prepare samples with different C_f contents from 0 to 2 K (a bundle of carbon fibers consisting of 1000 fibers). Samples with C_f content of 1 K show the highest bending strength (161.6 ± 10.5 MPa) and fracture toughness (3.72 ± 0.11 MPa·m^1/2) while the thermal conductivity of the samples with the C_f content of 1 K reached 11.0 W/(m·K). This study provides a strategy to prepare C_f/SiC composites via MEX additive manufacturing and RMI.     

Microwave hybrid sintering of Al2O3 and Al2O3–ZrO2 composites,and effects of ZrO2 crystal structure on mechanical properties,thermal properties,and sintering kinetics

Structural ceramics such as Al₂O₃ and Al₂O₃–ZrO₂ composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO₂ crystal structures on Al₂O₃–ZrO₂ composites, especially on the sintering kinetics, when fabricated by microwave sintering.The microwave hybrid sintering of Al₂O₃ and Al₂O₃–ZrO₂ composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an α–alumina matrix, and the mechanical and thermal properties were studied. It was found that Al₂O₃ experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO₂ was added. Al₂O₃–c-ZrO₂ also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 ± 11 kJ/mol for pure Al₂O₃, 319 ± 71 kJ/mol for Al₂O₃–c-ZrO₂, and 342 ± 77 kJ/mol for Al₂O₃–t-ZrO₂. These values indicated that the activation energy was increased by the addition of either type of ZrO₂, with the highest value shown by Al₂O₃–t-ZrO₂.     

Fabrication of WSi2–Al2O3 and W5Si3–Al2O3 composites by combustion synthesis involving thermite reduction

Formation of WSi₂–Al₂O₃ and W₅Si₃–Al₂O₃ composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO₃+2Al and 0.6WO₃+0.6SiO₂+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al₂O₃. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi₂/Al₂O₃ from 0.8 to 4.0 were achieved for the production of the WSi₂–Al₂O₃ composites. Due to the lower formation exothermicity, the W₅Si₃–Al₂O₃ composites were produced with a narrower range of W₅Si₃/Al₂O₃ from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi₂ and unreacted W in the as-synthesized W₅Si₃–Al₂O₃ composites.     

Densified multiwalled carbon nanotubes–titanium nitride composites with enhanced thermal properties

Densified multiwalled carbon nanotube (MWNT)–TiN composites with various MWNTs contents were successfully obtained through a spark plasma sintering (SPS) method. The thermal conductivity k was found to increase with the MWNT amount and temperature. In the presence of 5 wt% MWNTs, there was a 97% enhancement in k at 703 K compared with that of TiN. The high thermal conductivity of MWNTs, a good interfacial combination and a homogeneous dispersion of MWNTs are key issues to enhance the thermal conductivity of MWNT–TiN composites.     

Preparation and characterization of Al2O3/TiC micro–nano-composite ceramic tool materials

An Al₂O₃-based composite ceramic tool material reinforced with micro-scale and nano-scale TiC particles was fabricated by a hot-pressing technology with cobalt additive in different sintering processes. The microstructure, indention cracks and phase composition of composites were characterized with scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The experimental results showed that Al₂O₃/TiC_μ/TiC_n micro–nano-composite containing 6 vol% nano-scale TiC and 35 vol% micro-scale TiC, which were sintered under a pressure of 32 MPa at a temperature of 1650 °C in vacuum for 20 min, had optimum mechanical properties. The addition of both nano-scale TiC and Co contributed to the microstructure evolution and the improvement of mechanical properties. Effects of nano-scale TiC on mechanical properties were investigated. The toughening and strengthening mechanisms of micro–nano-composites were discussed.     

Preparation of AlN–Al2O3 composites exhibiting antibacterial and water purification properties

This study aimed to develop new antibacterial and water purification materials without heavy metal contamination. Herein, AlN–Al₂O₃ composites were prepared by changing the content of AlN in raw material. The results showed that AlN, Al₂O₃, aluminum oxynitride, and yttrium aluminum garnet phases were generated by adjusting the AlN: Al₂O₃ ratio. The difference in the ability of AlN and aluminum oxynitride to release substances such as ammonium ions and aluminum hydroxide when reacting with water resulted in remarkably different pH values of the sample immersion solution, which led to an increase in the material antibacterial efficiency with the addition of AlN. Similar results were also obtained with zinc ion absorption. Therefore, AlN–Al₂O₃ composite ceramics can potentially be used as novel antibacterial and water purification materials without heavy metal contamination through the release of ammonium ions for conferring antibacterial effects and of aluminum hydroxide for absorbing heavy metals and suspended impurities.     

Effect of adding Y2O3 on structural and mechanical properties of Al2O3–ZrO2 ceramics

The effects of adding 1–8 wt% Y₂O₃ on phase formation and fracture toughness of Al₂O₃–xZrO₂–Y₂O₃(AZY) ceramics were studied. Phase formations of the samples were characterized by the X-ray diffraction (XRD) technique. It was found that the major phase was rhombohedral-Al₂O₃, while the minor phase consisted of the monoclinic-ZrO₂, tetragonal-ZrO₂ and monoclinic-Y₂O₃. It was found that Y₂O₃ contents did not clearly influence grain shape of AZY ceramics. The results obtained from the microhardness test could be used to evaluate the fracture toughness. It was found that the smaller grains had high fracture toughness. The maximum fracture toughness of 4.827 MPa m^1/2 was obtained from 4 wt% Y₂O₃. Refinement of lattice parameters using Rietveld analysis revealed the quantitative phases of AZY ceramics. This shows that under adding Y₂O₃ conditions the proportion of tetragonal-ZrO₂ phase plays an important role for the mechanical properties of AZY ceramics.     

Densification,microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black

The effect of nano-sized carbon black on densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB₂) – silicon carbide (SiC) ceramic was studied. A ZrB₂-based ceramic matrix composite, reinforced with 20 vol% SiC and doped with 10 vol% nano-sized carbon black, was hot pressed at 1850 °C for 1 h under 20 MPa. For comparison, a monolithic ZrB₂ ceramic and a ZrB₂–20 vol% SiC composite were also fabricated by the same processing conditions. By adding 20 vol% SiC, the sintered density slightly improved to ~93%, compared to the relative density of ~90% of the monolithic one. However, adding 10 vol% nano-sized carbon black to ZrB₂–20 vol% SiC composite meaningfully increased the sinterability, as a relatively fully dense sample was obtained (RD=~100%). The average grain size of sintered ZrB₂ was significantly affected and controlled by adding carbon black together with SiC acting as effective grain growth inhibitors. The Vickers hardness, flexural strength and fracture toughness of SiC reinforced and carbon black doped composites were found to be remarkably higher than those of monolithic ZrB₂ ceramic. Moreover, unreacted carbon black additives in the composite sample resulted in the activation of some toughening mechanisms such as crack deflections.     

Viscous behaviour of Bi2O3–B2O3–ZnO glass composites with ceramic fillers

Abstract

Variation in the viscous flow behaviour, nature and extent of glass fluidity in glass/filler composites are addressed with respect to various factors such as filler type, content, size, density and migration distance. The characterisation of a glass (Bi₂O₃–B₂O₃–ZnO) composite consisting of two different fillers (cordierite and willemite) was determined using hot stage microscopy, a differential scanning calorimeter and a flow button test. The microstructure was analysed using a scanning electron microscope. The apparent viscosity of the glass composites increased on increasing concentration and density of the filler. The variation in the viscosity is due to the diffusion of the glass matrix through channels in the cordierite filler of the composite. Based on the calculated migration distance of the filler in the glass matrix, the present work suggests that the interfacial behaviour and the density of the filler play a significant role in determining the viscous flow of the glass composites.     

Effect of Al2O3 particles on mechanical and tribological properties of Al–Mg dual-matrix nanocomposites

This article aims to manufacture homogenous dual-matrix Al–Mg/Al₂O₃ nanocomposite from their raw materials and give insight into the correlation between powder morphology, crystallite structure and their mechanical and tribological properties. Al–Mg dual-matrix reinforced with micro/nano Al₂O₃ particles was manufactured by a novel double high-energy ball milling process followed by a cold consolidation and sintering. Microstructure and phase composition of the prepared samples were characterized using FE-SEM, EDS and XRD inspections. Mechanical and wear properties were characterized using compression and sliding wear tests. The results showed that a milling of Mg with Al₂O₃ particles in an initial step before mixing with Al has the beneficial of well dispersion of Al₂O₃ nanoparticles in Al–Mg dual matrix. The Al–Mg dual matrix reinforced with nano-size Al₂O₃ showed 3.29-times smaller crystallite size than pure Al. Moreover, the hardness and compressive strength are enhanced by adding nano-size Al₂O₃ with Al–Mg dual matrix composite while the ductility is maintained relatively high. Additionally, the wear rate of this composite was reduced by a factor of 2.7 compared to pure Al. The reduced crystallite size, the dispersion of Al₂O₃ nanoparticles and the formation of (Al–Mg)_ss were the main improvement factors for mechanical and wear properties.     

Severe wear mechanisms in Al2O3–AlON ceramic composites

Severe wear mechanisms in Al₂O₃-AlON ceramic composites during their friction against a bearing steel were investigated and analysed by different techniques, mainly transmission electron microscopy (TEM). It was shown that ceramic damages correspond not to a classical intergranular cracking but to a breakdown of alumina–alumina grain boundaries leading to their pull off. Consequently, a new model of the severe wear of ceramics, based on a dielectric approach is proposed. Moreover, the existence of AlON located at such boundaries induces a delaying effect of this damage and seems to participate in the forming and the stability in the contact of a third body essentially constituted by iron oxides.     

Fabrication and dielectric properties of transparent PZN–BT ceramic

Transparent ceramic of 0.85Pb(Mg_1/3Nb_2/3)O₃–0.15BaTiO₃ has been successfully prepared by a two-stage sintering method using conventional raw materials. The ceramics exhibited an excellent crystallinity, high density and clean grain boundary. The transmittance keeps about4 0% from visible to near infrared regions. The frequency dependence of T_m and relaxor behavior has also been investigated using Vogel–Fulcher model and Power model.     

Properties of cement–sand-based piezoelectric composites with carbon nanotubes modification

Carbon nanotubes (CNTs) were dispersed in a cement–sand-based piezoelectric composite as conductive fillers to improve its poling efficiency. Specimens were prepared by mixing PZT powders, cement and sand with CNTs. The effect of CNTs ranging from 0 to 0.9 vol% on properties of the composite, including its piezoelectric coefficient, dielectric constant and loss, and sensing characteristic, were characterized. It was found that the addition of CNTs facilitated effective poling under a low electric field of 1 MV/m at room temperature and improved the piezoelectric and dielectric properties of the composite. The composite modified by CNTs achieved optimal properties when the CNTs content was 0.6 vol% and this was verified by the investigation of sensing effects of the composite through compressive tests.