Nanoscale SiC Research Articles

Research on hydrodynamic suspension nanoscale polishing for K9 optical glass

In this paper, a hydrodynamic suspension nanoscale polishing system based on Computer Numerical Control (CNC) is established. A new type of compliant polishing tool system is developed, which can realise nanoscale displacement compensating. Also polishing experiments on K9 optical glass, using the polishing fluid mixed with nanoscale SiC abrasive particles, have been conducted to investigate the fundamental machining mechanism. A high-quality surface without crystallite dislocation and micro cracks is achieved. It is shown that the removal of atomic layers size can be done by colliding nanoscale abrasive particles with the workpiece surface.

Effect of Al 2O 3 particle size on the mechanical properties of alumina-based ceramics

To study the effect of Al 2O 3 particle size on the mechanical properties of alumina-based ceramics, the monolithic nano-scale alumina ceramics and the Al 2O 3μ/Al 2O 3n/SiC n multi-scale nanocomposite materials were fabricated. The fabricating process, microstructure and the mechanical properties of the new materials were investigated in detail. The results showed that nano-scale Al 2O 3 ceramic material may be fabricated at a lower temperature with high density, and the flexural strength was improved compared with micro-scale monolithic alumina ceramics. With the optimal dispersing and fabricating technology, the intra/intergranular microstructure with well-dispersed nano-scale Al 2O 3 and nano-scale SiC was obtained in Al 2O 3μ/Al 2O 3n/SiC n composite. The improved mechanical properties may be mainly attributed to the grain fining, grain boundary reinforcement and transgranular fracture mode. The size matched effect and the different thermal expansion coefficient of the added nano-scale Al 2O 3 particles resulted in the higher mechanical properties compared with the Al 2O 3μ/SiC n composite.

Study on the Multi-Scale Nanocomposite Ceramic Tool Material

An advanced ceramic cutting tool material was developed by means of micro-scale SiC particle cooperating with nano-scale SiC particle dispersion. With the optimal dispersing and fabricating technology, this multi-scale nanocomposite may get both higher flexural strength and fracture toughness than that of the single-scale composite. The improved mechanical properties may be mainly attributed to the inter/intragranular microstructure with a lot of micro-scale SiC particles located on the grain boundary and a few nano-scale SiC particles located in the matrix grain. Because of the thermal expansion mismatch between SiC and Al2O3 resulting in the compressive stress on the SiC/Al2O3 interface, the interface bonding strength between Al2O3 and SiC was reinforced, which can compel the crack propagating into the relatively weak matrix when meeting the SiC particle on the boundary; while the alumina grain boundary is not the same strong as the SiC/Al2O3 interface and the Al2O3 grain, therefore the crack propagates sometimes along the Al2O3 grain boundaries and sometimes through the grains, when reaching to the nano-scale SiC particle inside the matrix, the crack was pinned and then deflected to the sub-grainboundaries. These coexisting transgranular and intergranular fracture mode induced by micro-scale and nano-scale SiC and the fining of matrix grain derived from the nano-scale SiC resulted in the remarkable strengthening and toughening effect.

A nuclear reaction analysis and optical microscopy study on controlled growth of large SiC nanocrystals on Si formed by low-energy ion implantation and electron beam annealing

Ion implantation of 20keV 12C+ ions into (100), p-type silicon with ion fluence of 8×1016at.cm−2 followed by an electron beam annealing under high vacuum conditions has been performed to investigate the formation of crystalline nano-scale SiC features on the silicon surface. Depending on the implantation and annealing conditions, the SiC nanocrystal numbers and average spacing can be controlled by adjusting the implantation and annealing conditions. Typically 300–1000 SiC nanocrystals are produced per 1000μm2 spaced 0.7–1.2±0.1μm. Nuclear reaction analysis measurements using the deuterium induced 12C(d,p)13C reaction show that carbon is present in the implanted and annealed samples and varies only to a small degree by the annealing time. However, by not using a liquid nitrogen trap surrounding the targets during implantation, the carbon contamination on the surface reduces the number of SiC nanocrystals and increases their average distance. Specific results are discussed.

Fabrication of Al-based hybrid composites reinforced with SiC whiskers and SiC nanoparticles by squeeze casting

Hybrid preforms containing SiC whiskers and nanoscale SiC particles were made by wet blending. The Al-based hybrid composites, with 20 vol.% SiC whiskers and 0, 2, 5, 7 vol.% SiC nanoparticles, were fabricated by squeeze casting route. The optimum squeezing casting parameters, especially infiltration pressure for fabricating the (SiCw·SiCp)/Al hybrid composites were obtained from the infiltration tests. The reinforcements distribute homogeneously in the matrix of the as cast composites, and the strength of the composites is found to increase with increasing content of the nanoparticles.

The effect of doping level and sintering temperature on Jc(H) performance in nano-SiC doped and pure MgB2 wires

Nanoscale SiC doped Fe∕MgB2 wire samples were prepared by an in situ reaction technique using SiC doping levels of 0, 5, 10, and 15 wt %. Samples were heat treated at different temperatures using different temperature profiles. The effects of doping level and sintering temperature on superconducting properties of wire samples were investigated. The important finding of this study was that the enhancement in Jc(H) by nano-SiC doping can be achieved at different field regions by appropriate compromising of the doping level and sintering temperature.

Fabrication and mechanical properties of Al2O3/Ti(C0.7N0.3) nanocomposites

Two kinds of Al2O3/Ti(C0.7N0.3) nanocomposites were fabricated with traditional hot pressed sintering and repetitious-hot-pressing technology, one is added with nano-scale SiC, and the other is without SiC. The results showed that the mechanical properties of the former are higher than that of the latter, especially the fracture toughness can reach up to 8.3MPam1/2. Although the fracture toughness remains high, repetitious-hot-pressing results in the reduction of flexural strength. The improvement of the mechanical properties is interpreted from the different microstructure and fracture mode. The microstructure shows that the addition of nano-scale Ti(C0.7N0.3) and nano-scale SiC lead to the refinement of matrix grain, and the inter/intragranular microstructure can be formed instead of the intergranular microstructure in monolithic alumina. The higher fracture toughness resulted mainly from the transgranular fracture mode.

The effect of ZrSi2 and SiC doping on the microstructure andJc–B properties ofPIT processed MgB2 tapes

We investigated the effect of ZrSi2 and SiC doping on the microstructure, critical current densityJc and flux pinningof Fe-sheathed MgB2 tapes prepared by the in situ powder-in-tube method. Heat treatment was performed at a low temperatureof 650 °C for 1 h. The phases, microstructures and flux pinning were characterized by means of x-raydiffraction, scanning electron microscope, magnetic and transport property measurements. It wasfound that the tapes doped with nanoscale SiC had the best pinning performance, while theZrSi2 powder showed a similar improved field dependence ofJc compared withundoped samples. Jc values for the SiC doped samples were enhanced by two orders ofmagnitude at 4.2 K in magnetic fields above 8 T. At 4.2 K and 10 T, theJc reached ∼1.5 × 104 A cm−2. Moreover, the critical temperature for the doped tapes decreased slightly(<1.2 K). Microstructural analysis shows that very good grain connections or/and grainrefinement were obtained for the doped tapes. The mechanism of the enhancement of theflux pinning is also discussed.

Effect of Nanoscale SiC Particles on Strength Distribution of Joined Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; to Inconel Alloy

Two-layered Si3N4/SiC nano-composites with 50vol.% SiC have been fabricated by two-step sintering of a powder mixture of a-Si3N4 and carbon powder with a mean size of 13nm, and 5wt.% Y2O3. Nano-sized SiC particles were formed through reactions: carbon and surface SiO2 on the Si3N4 particles, and carbon and Si3N4 particles. To combine the specific advantages of nanoscale ceramics with that of metals, they are often used together within one composite component. In this study, the fabrication and mechanical properties of a nanoscale SiC layer brazed with an Inconel alloy were investigated. It was shown that, with a variation of strain rate, the joints have a bending strength of 330-380MPa, and the deflection of the interlayer increases with increasing strain rate. The fracture types are classified into three groups; cracks grow into the metal-brazing filler layer, the ceramicbrazing filler layer or inside the ceramic.

Mismatched Heteroepitaxy of Tetrahedral Semiconductors with Si via ZrB2 Templates

We demonstrate integration of cubic SiC (heterostructures and nanostructures) and assemblies of Ge nanoscale islands with Si substrates via a conductive and reflective ZrB2 buffer layer. Hexagonal ZrB2 is grown on cubic Si(111) via a coincidence-misfit mechanism in which the strain is accommodated by edge dislocations along the interface. Ge islands with uniform sizes and strain-free microstructures were grown on ZrB2/Si(111) at 500 °C via thermolysis of Ge2H6, circumventing the strain-driven (Stranski−Krastanov) island formation on Si and associated limitations. Heteroepitaxy between ZrB2(0001) and Ge(111) is obtained via alignment of four lattice rows of Ge with every five rows of ZrB2, (i.e., “magic mismatch”) despite the large difference in lattice constants. Cubic SiC layers with monocrystalline microstructures and atomically abrupt interfaces are grown on ZrB2/Si(111) via single source molecular beam epitaxy of C2(SiH3)2 at 800 °C. Nanoscale SiC islands with perfectly coherent zinc blende structures...

SiC addition for MgB/sub 2/ superconducting wire by suspension spinning

The effect of nanoscale SiC addition on the field dependence of J/sub c/ for MgB/sub 2/ superconducting wire by suspension spinning was examined to enhance the flux pinning. The suspension spinning of commercially available MgB/sub 2/ and SiC powders was examined to fabricate a long superconducting MgB/sub 2/ wire by using the spinning medium of poly(vinyl alcohol) (PVA) or polyacrylonitrile (PAN). The as-drawn filaments were uniaxially pressed under 20 MPa at 200/spl deg/C for 8 h to remove volatile components and connect the MgB/sub 2/ grains. The filamentary samples were enveloped by an iron sheet with a pellet of mixed powder of Mg and B to prevent Mg loss, and vacuum-sealed in a fused quartz tube and sintered. The field dependence of J/sub c/ for the samples was examined at 4.2 K in magnetic fields up to 14 T. The J/sub c/ of the sample was strongly dependent on the spinning medium and sintering condition. Addition of 5 at% SiC in the sample spun by PVA medium attained the maximum J/sub c/ value more than 1000 A/cm/sup 2/ at 4.2 K by applying the field of 14 T.

Computer Simulation of the Early Stages of Nano Scale SiC Growth on Si

Solid source molecular beam epitaxy was applied to create silicon carbide nanoclusters on silicon. The island size distribution can be controlled by an appropriate substrate temperature, carbon fluxes and process times. Rate equation computer simulation was applied to simulate the experimental obtained nano scale nuclei properties.

High transport critical current density and largeHc2 andHirr in nanoscaleSiC doped MgB2 wires sintered at low temperature

We report a systematic study on the effect of sintering temperature on the phase formation,critical current density, upper critical field and irreversibility field of nanoscale SiC dopedMgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizeshave been made and heat treated at temperatures ranging from 580 to1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR,Jc,Hc2,and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature havea very fine and well consolidated grain structure while samples sintered at a high temperaturecontain large grains with easily distinguishable grain boundaries. Low temperature sinteringresulted in a higher concentration of impurity precipitates, larger resistivity, higherJc up to 15 T andlower Tc values. Thesesamples show higher Hc2 and Hirr at T nearTc butlower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced bynano-precipitates and grain boundary structure are the dominant mechanism responsible for higherHc2 atT near Tc. However, higher impurity scattering due to C substitution is responsible for higherHc2 in the low temperature regime for samples sintered at a higher temperature. In addition to highHc2, it is also proposed that the large number of nano-impurities serveas pinning centres and improve the flux pinning, resulting in higherJc values at high magnetic fields up to 15 T.

Microstructure and Mechanical Properties of two Kinds of Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;/SiC Nanocomposites

Two kinds of Al2O3 /SiC nanocomposites with different alumina powder are developed, one is fabricated only by nano-scale alumina powders, the other is by micro-scale with partial nano-scale alumina powders. Both of the two composites may get higher flexural strength and fracture toughness than that of micro-scale monolithic alumina ceramics, but the latter is more preferable than the former. The microstructure and the strengthening and toughening mechanisms of the new ceramic tool materials are investigated, the improvement of mechanical properties may be mainly attributed to the transgranular fracture mode induced by the added nano-scale SiC, while adding nano-scale alumina powder to micro-scale powder, both of the nano-scale Al2O3 and nano-scale SiC may strengthen the micro-scale Al2O3 matrix, that means the nano-scale Al2O3 acted as the dispersed phase.

Thermomechanics of shape memory polymer nanocomposites

Shape memory polymers (SMPs) have the capacity to recover large strains when pre-deformed at an elevated temperature, cooled to a lower temperature, and reheated. The thermomechanical behavior of SMPs can be tailored by modifying the molecular structure of the polymer, or by using the polymer as a matrix for multiphase composites. Here we study the thermomechanics of a SMP polymer and its composites made by adding nano-scale SiC reinforcements. Our experimental study focuses on the thermomechanical behavior in three-point flexure. The results show that the SMP nanocomposites have a higher elastic modulus and are capable of generating higher recovery forces as compared to the SMP resin. When pre-deformed at a temperature well above the glass transition temperature, T g, the stress–strain response at the pre-deformation temperature governs the relationship between the recovery stress/strain and the corresponding constraining strain/stress. When pre-deformed at a temperature below T g, the recoverable stress/strain is not governed by the stress–strain response at the pre-deformation temperature. Rather, a peak recovery stress, which is less than the pre-deformation stress, appears at a temperature near T g. Ramifications of the results on future research efforts and emerging applications of SMPs and their composites are discussed.

Fabrication of Micro- and Nanoscale SiC Structures Using Selective Deposition Processes

ABSTRACTA method to fabricate nanometer scale SiC beams and nanoporous SiC shells using conventional microlithographic techniques combined with selective APCVD has been developed as an alternative to nanolithographic patterning and electrochemical etching. The process involves the selective deposition of poly-SiC films on patterned SiO2/polysilicon/SiO2 thin film multilayers on (100) Si substrates using a carbonization-based 3C-SiC growth process. This technique capitalizes on significant differences in the nucleation of SiC on SiO2 and polysilicon surfaces in order to form mechanically durable and chemically stable structures.

懸濁紡糸法によるＭｇＢ２線材の作製

Suspension spinning of commercially available MgB2 powder was examined to fabricate a long superconducting MgB2 wire. The MgB2 powders were suspended in a nonaqueous poly(vinyl alcohol) solution. The viscous spinning dope was extruded as a filament into a precipitating medium of methyl alcohol and coiled on a winding drum. The as-drawn filaments were pressed and heated in order to remove volatile compounds. The filamentary samples were then cold-pressed, enveloped by an iron sheet with a pellet of mixed powder of Mg and B or with Mg powders, vacuum-sealed in a fused quartz tube and sintered. The Jc of the sample was strongly dependent on the starting materials and sintering condition. Although the transport Jc value was low, such as 4500 A/cm2 at 4.2 K and self-fields, a Jc value of more than 200 A/cm2 was maintained by applying a field of 10 T. Doping of 5 at% nanoscale SiC resulted in an improvement in the Jc and superconductivity at 4.2 K was maintained by applying a field of 14 T.

Low pressure plasma-sprayed Al2O3 and Al2O3/SiC nanocomposite coatings from different feedstock powders

This paper describes a preliminary investigation of a nanocomposite ceramic coating system, based on Al2O3/SiC. Feedstock Al2O3/SiC nanocomposite powder has been manufactured using sol-gel and conventional freeze-drying processing techniques and then low pressure plasma sprayed onto stainless steel substrates using a CoNiCrAlY bond coat. Coatings of a commercial Al2O3 powder have also been manufactured as a reference for phase transformations and microstructure. The different powder morphology and size distribution resulting from the different processing techniques and their effect on coating microstructure has been investigated. Phase analysis of the feedstock powders and of the as-sprayed coatings by X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) showed that the nano-scale SiC particles were retained in the composite coatings and that equilibrium α-Al2O3 transformed to metastable γ- and δ-Al2O3 phases during plasma spraying. Other minority phases in the sol-gel Al2O3/SiC nanocomposite powder such as silica and aluminosilicate were removed by the plasma-spraying process. Microstructure characterisation by scanning electron microscopy (SEM) of the as-sprayed surface, polished cross-section, and fracture surface of the coatings showed evidence of partially molten and unmolten particles incorporated into the predominantly lamella microstructure of the coating. The extent of feedstock particle melting and consequently the character of the coating microstructure were different in each coating because of the effects of particle morphology and particle size distribution on particle melting in the plasma.

The effect of nanoscale SiC particles on the microstructure of Al 2O 3 ceramics

The hot-pressing temperature, matrix grain size, fracture mode and the distribution of SiC were studied to reveal the effect of SiC inclusions on the microstructure of Al 2O 3. Al 2O 3–SiC composite powder was prepared by the precipitation method. Nanoscale SiC particles were coated with Al 2O 3. After hot pressing, most of nanoscale SiC particles were randomly located within the Al 2O 3 grains. SiC inclusions raised the hot-pressing temperature, decreased Al 2O 3 grain size, and inhibited the abnormal grain growth of Al 2O 3. The intergranular fracture for Al 2O 3 was transformed to the intragranular fracture for Al 2O 3–SiC nanocomposites because of the addition of SiC. All those microstructure changes increased the mechanical properties of Al 2O 3-composites by 40%.

Self-organized growth of zero-, one-, and two-dimensional nanoscale SiC structures by oxygen-enhanced hydrogen plasma sputtering

A variety of nanoscale SiC structures, in particular the grain, whisker, and flake, displaying, respectively, zero, one, and two dimensions, has been grown by hydrogen plasma sputtering of a SiC target in the presence of a small amount of oxygen. Growth of the different nanoscale structures takes place by various mechanisms. The SiC whiskers are initiated by SiO2 seed crystals in the SiC film matrix at temperatures around 700 °C. On the other hand, the grains and flakes are separated by insertions of amorphous and graphitic carbon in the films at higher temperatures, around 950 °C. Both these processes result from the reaction of oxygen with the growing SiC film.