SiC Nanocomposites Research Articles

Conventional sintering of LAS–SiC nanocomposites with very low thermal expansion coefficient

Very low thermal expansion materials have been developed in a wide temperature range from −150 °C to 450 °C. These very low expansion materials are composed of a lithium aluminosilicate matrix with dispersed silicon carbide nanoparticles. The nSiC content in the most stable composite has resulted to be 27 vol.%. Powder processing and sintering temperature control have been found to be of key importance in order to obtain low porosity sintered nanocomposites. Here it is proposed the sintering by pressureless methods (conventional sintering) in order to obtain a LAS–nSiC nanocomposite with improved mechanical properties and excellent expansion behaviour by means of control of the phase reactions and glass formation during sintering.

Silver and gold nanoparticles in silica matrices: synthesis, properties, and application

Methods of synthesis, optical characteristics, morphology, and catalytic and bactericidal characteristics of composite materials based on silica (films and powders) containing nanoparticles of silver, gold, and their binary compounds with alloy or core–shell structure are examined. The photochemical reduction of Au3+ and Ag+ with a photocatalyst (a film of SiO2 with adsorbed benzophenone) makes it possible to generate stable nanoparticles of gold and silver in solutions for subsequent introduction into adsorbents and catalysts. Examples of the use of nanosized composites in catalysis and in microbiological experiments are presented.

Influence of SiCnp content on the microstructure and mechanical properties of ZrB 2–SiC nanocomposite

A ZrB2–SiC nanocomposite that introduced nanosized SiC particles into a ZrB2 matrix was fabricated by hot-pressing at 1900 °C for 60 min under a 30 MPa uniaxed load. The composite had an intragranular microstructure that was peculiar to this material. Investigation of the mechanical properties revealed a highest flexural strength of 1009.1 ± 43 MPa and a fracture toughness of 6.8 ± 0.2 MPa m1/2 for the ZrB2–20 vol.% SiC nanocomposite. These improved mechanical properties were strongly dependent on the number of intragranular particles.

Influence of various rare-earth oxide additives on microstructure and mechanical properties of silicon nitride based nanocomposites

Influence of rare-earth oxide additives (La2O3, Nd2O3, Y2O3, Yb2O3 and Lu2O3) on the microstructure and mechanical properties of hot-pressed silicon nitride/silicon carbide micro/nanocomposites has been investigated. The results were consequently compared to those obtained on the reference monolithic Si3N4 sintered with the same additives. The composites exhibited finer microstructure compared to monolithic materials and the aspect ratio of β-Si3N4 grains increased with a decreasing ionic radius of rare-earth elements both in monolithic Si3N4 and in Si3N4–SiC nanocomposites. The hardness of both kinds of material increased with a decreasing ionic radius of rare-earth element. Fracture toughness and four-point bending strength also increased with a decreasing ionic radius of rare-earth oxide additives. The fracture toughness of monoliths was higher in comparison with composites due to the coarser microstructures which provide beneficial toughening mechanisms during crack propagation. Ceramics containing smaller size of rare-earth cations exhibited crack deflection more frequently compared to the materials doped with larger ones because of higher aspect ratio of β-Si3N4 grains. The positive influence of finer microstructure of the composites on the strength was not observed due to the present strength degrading processing flaws in their microstructures in the form of agglomerates of SiC and non-reacted carbon areas.

In situ preparation of Al 2O 3–SiC nanocomposites via sol–gel method followed by pressureless sintering

Al 2O 3–SiC nanocomposites containing 5 and 10 vol% SiC were prepared via sol–gel method followed by pressureless sintering. AlCl 3·6H 2O and TEOS were used as precursors for alumina and silicon carbide, respectively. The dried gel was heat treated in the range of 800–1600 °C. Two kinds of powders were used to prepare sintered bodies, which were the calcined powder at 800 °C and the synthesized powder at 1600 °C. Green compacts were prepared by cold isostatic pressing, and then pressureless sintered in Ar–12% H 2 atmosphere at 1700 °C. It was shown that β-SiC was in situ formed by means of mullite and carbon reaction at 1600 °C. The density of samples prepared from calcined powder was more than that of samples prepared from synthesized powder. A homogeneous distribution of nanosized SiC particles was synthesized in situ during sintering. The SiC nanoparticles were located predominantly to the interior of Al 2O 3 matrix grains.

Low-temperature extraction-pyrolytic synthesis of nanosized composites based on metal oxides

This article deals with the subject of synthesizing with the use of the extraction pyrolysis method of nanosized composites based on iron, manganese, bismuth, zirconium, europium, terbium oxides, and metal platinum, both in the form of powder and thin coatings on dielectric supports. The composition, morphology, and properties of nanosized synthesized materials are studied.

Analysis of the compaction behavior of Al–SiC nanocomposites using linear and non-linear compaction equations

The compressibility behavior of Al–SiC nanocomposite powders was examined and the density-pressure data were analyzed by linear and non-linear compaction equations. SiC particles with an average size of 50 nm were mixed with gas-atomized aluminum powder (40 μm average size) at different volume fractions (up to 20 vol%) and compacted in a rigid die at various pressures. In order to highlight the effect of reinforcement particle size, the compressibility of micrometric SiC particles of two sizes (1 and 40 μm) was also examined. Analysis of the compressibility data indicated hindering effect of the hard ceramic particles on the plastic deformability of soft aluminum matrix, particularly at high volume fractions. More pronounced effect on the yield pressure was obtained for the nanometric particles compared with the micrometric ones. Nevertheless, better particles rearrangement was taken place when the ultrafine SiC particles were utilized. In light of the experimental and theoretical analysis, the densification mechanism of aluminum matrix composites and the effect of reinforcement particle size and volume fraction are discussed.

EIS Study of Bulk Al-SiC Nanocomposite Prepared by Mechanical Alloying and the Hot Press Method

Metal matrix composites (MMCs) are engineering materials in which a hard ceramic component is dispersed in a ductile metal matrix in order to obtain characteristics such as hardness and corrosion resistance. Corrosion resistance is one of the important properties of nanocomposites; however, the corrosion mechanism of the Al- SiC nanocomposite has not yet been determined. .In this study, bulk Al-5% SiC nanocomposite was prepared using mechanical alloying and the hot press method. Corrosion behavior was then investigated using EIS techniques such as Nyquist and the Bod diagram. A larger charge transfer resistance was found for the Al- SiC nanocomposite by the EIS diagrams, confirming its corrosion resistance in a 3.5wt% NaCl solution.

Characterization of the Corrosion Behavior of Hot Pressed Nanocomposites Al-SiC Powder

The object of the present work is the characterization of the corrosion behavior of Al-5%SiC nanocomposites prepared by mechanical alloying and the hot pressing method. Al/SiC metal matrix composite (MMCp) powder with volume fraction of 5% SiC was synthesized by high-energy milling of the blended micro component powder. The mechanically mixed nanocomposite powder was consolidated into bulk compacts using hot pressing at 1 GPa pressure for thirty seconds at three different temperatures of 440, 480 and 520°C. The microstructure was investigated by Secondary electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The corrosion behavior of the coating was evaluated using electrochemical techniques such as potansiodynamic polarization and EIS. It was found that the corrosion resistance of Al-5%SiC nanocomposite when hot pressed at 480°C, in 3.5% NaCl solution was higher than pure Al and Al-SiC nanocomposite that had been hot pressed at other temperatures.

Investigation of planetary milling for nano-silicon carbide reinforced aluminium metal matrix composites

High-energy planetary milling was used for mixing aluminium powders with 1 vol.% of silicon carbide (SiC) nanoparticles. A number of milling parameters were modified for constituting the relationship between the energy input from the balls and the hardness of the bulk nanocomposite materials. It was shown that mixing characteristics and reaction kinetics with stearic acid as process control agent can be estimated by normalised input energy from the milling bodies. For this, the additional parameter characterising the vial filling was determined experimentally. Depending on the ball size, a local minimum in filling parameter was found, laying at 25 or 42% filling of the vial volume for the balls with diameter of 10 and 20 mm, respectively. These regions should be avoided to achieve the highest milling efficiency.After a hot compaction, fourfold difference of hardness for different milling conditions was detected. Therewith the hardness of the Al–1 vol.% nanoSiC composite could be increased from 47 HV 0.5 of pure aluminium to 163 HV 0.5 when milling at the highest input energy levels.

Confocal fluorescence microscopy in alumina-based ceramics: Where does the signal come from?

Confocal Cr 3+ fluorescence microscopy is an ideal technique for investigating residual stresses in alumina-based ceramics. Due to their transparency, however, it is important to understand where the collected signal comes from by characterising the probe response function (PRF). Here, a PRF is proposed that captures all the relevant physical effects, including a newly identified consequence of scattering by pores and grain boundaries. The new PRF describes the response of a range of alumina-based ceramics to depth scanning in a high resolution confocal fluorescence microscope in a manner that balances physical significance with the accuracy of empirical fitting. The results showed that measurements could be made deep within single crystals of sapphire and ruby, although refraction degraded the depth resolution from about 3 μm at the surface to 25 μm at a depth of 500 μm. Scattering and absorption limited the depth to which polycrystalline alumina could be probed to ∼15 μm. This was further reduced to ∼4 μm for an alumina–10 vol.% SiC nanocomposite. However, the absorption increased the accuracy of near surface measurements in these materials by preventing contamination from subsurface fluorescence.

Effect of Reinforcement Volume Fraction on the Mechanical Properties of Al—SiC Nanocomposites Produced by Mechanical Alloying and Consolidation

Al—(1, 3, 5, 7, 10 vol%) SiC nanocomposites were produced by mechanical alloying (MA) and double pressing/sintering route. The characteristics of the milled powders and the consolidate specimens were examined using high resolution scanning electron microscopy and X-ray diffraction method. Compression and hardness tests were used to study the effect of SiC volume fraction on the strength of Al—SiC nanocomposites. It was shown that with increasing the SiC volume fraction, finer particles with narrower size distribution and smaller crystallite size are obtained after MA. During sintering close to the melting point of aluminum, the presence of nanometer-scaled SiC particles was found to hinder the grain growth significantly. The Al matrix with a higher SiC content exhibited more potential for grain boundary pinning, i.e., smaller grain size was obtained at higher SiC volume fractions. Consequently, an improved mechanical strength was obtained. The processing method (MA/pressing/sintering) can be used for fabrication of near-net shape Al matrix nanocomposites.

Polymer-derived mullite–SiC-based nanocomposites

Ceramic mullite–SiC nanocomposites were successfully produced at temperatures below 1500 °C by the polymer pyrolysis technique. An alumina-filled poly(methylsilsesquioxane) compound was prepared by mechanically mixing and cross-linking via a catalyst prior to pyrolysis. Heat treatment of warm pressed alumina/polymer bulk samples under the exclusion of oxygen (inert argon atmosphere) up to 1500 °C initiated crystallization of mullite even at pyrolysis temperatures as low as 1300 °C. The influence of the filler and of the pyrolysis temperature on the crystallization behavior of the materials has been investigated. Based on thermal analysis in combination with elemental analysis and X-ray powder diffraction studies four polymer mixtures differing in type and content of nano-alumina powders were examined. Nano-sized γ-Al 2O 3 powders functionalized at the surface by octylsilane groups proved to be more reactive towards the preceramic polymer leading to the formation of a larger weight fraction of mullite crystals at lower processing temperatures (1300 °C) as compared to native nano-γ-Al 2O 3 filler. Moreover, the functionalized nano-alumina particles offer an enhanced homogeneity of the distribution of alumina nano-particles in the starting polysiloxane system. In consequence, the received ceramic samples exhibited a nano-microstructure consisting of crystals of mullite with an average dimension in the range of 60–160 nm and silicon carbide crystals in the range of 1–8 nm.

Fracture Toughness of Si<sub>3</sub>N<sub>4</sub> Based Ceramics with Rare-Earth Oxide Sintering Additives

Fracture toughness of hot-pressed silicon nitride and Si3N4+SiC nanocomposites prepared with different rare-earth oxides (La2O3, Sm2O3, Y2O3, Yb2O3, Lu2O3) sintering additives have been investigated by Chevron Notched Beam, Indentation Strength and Indentation Fracture techniques. The fracture toughness values of composites were lower due to the finer microstructures and the lack of toughening mechanisms. In the Si3N4 with higher aspect ratio (Lu or Yb additives) crack deflection occurred more frequently compared to the Si3N4 doped with La or Y, which was responsible for the higher fracture toughness.

Effects of Y 2O 3 additives and powder purity on the densification and grain boundary composition of Al 2O 3/SiC nanocomposites

Sub-micron sized SiC additions can be used to increase the wear resistance and change the fracture mode of Al 2O 3. However, these additions also restrict sintering. Al 2O 3 and Al 2O 3–5%SiC ‘nanocomposites’ were prepared from alumina powders of high purity and of commercial-purity, with or without the addition of Y 2O 3. The effects of these compositional variables on sintering rate, final density and grain boundary composition were investigated. A direct comparison with Al 2O 3–SiO 2 composites was also made, as it has been proposed that SiC partially oxidises during processing of Al 2O 3–SiC nanocomposites. The addition of 5 vol.% SiC to Al 2O 3 hindered densification, as did addition of 0.15 wt.% Y 2O 3 or 0.1 wt.% SiO 2. In contrast, the addition of 0.15 wt.% Y 2O 3 to Al 2O 3–5% SiC nanocomposites improved densification. The composition of Al 2O 3–Al 2O 3 grain boundaries in these materials was studied using STEM and EDX microanalysis. The addition of SiC and SiO 2 caused segregation of Si, and Y 2O 3 addition caused segregation of Y. The segregation of each element was equivalent to <10% of a monolayer at the grain boundary. However, if SiC and Y 2O 3 were simultaneously added the segregation increased to 40% of a monolayer. The enhanced segregation was attributed to increased oxidation of SiC in the presence of Y 2O 3 allowing formation of a SiO 2–Al 2O 3–Y 2O 3 eutectic phase or a segregated layer which may explain the improvement in sintering rate when Y 2O 3 was added to nanocomposites.

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.

Mg–6Zn/1.5%SiC nanocomposites fabricated by ultrasonic cavitation-based solidification processing

Mg–6Zn/1.5%SiC nanocomposites were successfully fabricated by ultrasonic cavitation-based dispersion of SiC nanoparticles in Mg–6Zn alloy melt. As compared to un-reinforced Mg–6Zn alloy matrix, the mechanical properties of the nanocomposites including the tensile strength and yield strength of the Mg–6Zn/1.5%SiC nanocomposites were significantly higher; the good ductility of Mg–6Zn alloy matrix was retained. Nanoparticles were dispersed well though there were still some SiC microclusters in the microstructure. Also, the grain size of Mg–6Zn alloy was reduced considerably by the addition of 1.5%SiC nanoparticles.

Mechanical Properties and Microstructure of Mg∕SiC Nanocomposites Fabricated by Ultrasonic Cavitation Based Nanomanufacturing

Magnesium, the lightest structural metal, is of significance to improve energy efficiency in various applications. Mg∕SiC nanocomposites were successfully fabricated by ultrasonic cavitation based dispersion of SiC nanoparticles in Mg melts. As compared to pure magnesium, the mechanical properties including tensile strength and yield strength of the Mg∕SiC nanocomposites were improved significantly, while the good ductility of pure Mg was retained. The grain size of the pure magnesium was refined significantly when SiC nanoparticles were dispersed in the Mg matrix. In the microstructure of Mg∕SiC nanocomposites, while there were still some SiC microclusters, most of the SiC nanoparticles were dispersed very well. Transmission electron microscopy study of the interface between SiC nanoparticles and magnesium matrix indicates that SiC nanoparticles bond well with Mg without forming an intermediate phase.

WITHDRAWN: Effect of reinforcement volume fraction on the mechanical properties of Al–SiC nanocomposites produced by mechanical alloying and sintering

WITHDRAWN: Effect of reinforcement volume fraction on the mechanical properties of Al–SiC nanocomposites produced by mechanical alloying and sintering

Fabrication of nano-sized oxide composite coatings and photo-electric conversion/electron storage characteristics

Titanium dioxide TiO 2 can be used as a photo-anode to give generated electrons to the metal substrate under illumination. The transition metal oxide such as iron oxide Fe 2O 3 can be used to store electrons generated by the photo-electric conversion function of TiO 2 under the illuminated situation while the electrons are discharged from the transition metal oxide to the metal substrate in the dark. In this paper, coatings of nano-sized composite of TiO 2 and Fe 2O 3 were fabricated by the Warm Spray process, in which the feedstock powder is accelerated by a supersonic gas jet with speed above 1.0 km s - 1 and temperature between 800 and 2500 K, and then impacted onto the target substrate continuously to form coatings. The coatings of TiO 2 and Fe 2O 3 nano-composite fabricated by Warm Spray showed no thermal deterioration such as phase transformation and particle growth of the feedstock during the spray process. The coatings fabricated by the Warm Spray had larger photo-current and the electron charge/discharge capacity than that by a conventional HVOF process. In addition, these characteristics were improved by decreasing the primary particle size of TiO 2 and Fe 2O 3.