Structural and electronic properties of Si nanocrystals embedded in amorphous SiC matrix

Si-rich hydrogenated amorphous silicon carbide thin films were prepared by plasma-enhanced chemical vapor deposition technique. As-deposited films were subsequently annealed at 900 °C and 1000 °C to form Si nanocrystals embedded in amorphous SiC matrix. Raman spectra demonstrate the formation of Si nanocrystals with size around 7–9 nm. For the sample annealed at 1000 °C, the crystallinity can be reached to 70%. As increasing the annealing temperature, the dark conductivity is increased accompanying with the increase of crystallinity of the film. The dark conductivity reaches to 1.2 × 10^?6 S cm^?1 for the sample annealed at 1000 °C, which is 4 orders of magnitude higher than that of as-deposited film. It is found that the carrier transport process is dominated by the thermally activated transport process according to the temperature-dependent conductivity results.     

Oxidation protection and behavior of C/C composites with an in situ SiC nanowire–SiC–Si/SiC–Si coating

An in situ SiC nanowire–SiC–Si/SiC–Si protective coating was prepared on C/C composites by pack cementation and heat treatment. SiC nanowires suppressed the cracking of the coating by nanowire pullout and bridging and microcrack deflection, avoiding the oxidation of C/C composites. Results showed that the oxidation of the samples was a continuous weight gain process. The oxidation behavior was fitted to the parabolic–linear model and the final weight gain was 1.8% during thermogravimetric analysis from 50 to 1600 °C. The oxidation behavior was fitted to the parabolic model and the final weight gain was 0.51% during isothermal oxidation at 800 °C.     

Melting behavior of nanosized lead particles embedded in an aluminum matrix

Molecular dynamics (MD) simulations, employing semi-empirical glue-type (similar to embedded atom method) many-body potentials, have been used to model the melting behavior of nanosized Pb particles embedded in an Al matrix. All the Pb particles studied melt well above the bulk melting temperature, with the melting point elevation displaying a damped periodic trend with increasing Pb particle size. By inspecting snapshots derived from the simulations, it is also possible to conclude that melting occurs via nucleation at {1 0 0} interfaces. A phenomenological model, based on the consideration of interfacial energy, volume expansion and lattice mismatch, has been developed to describe this interesting melting behavior. Our model leads to the conclusion that the periodic variation of melting point with size is due to an oscillatory strain energy contribution arising from the large size mismatch of Pb and Al atoms.     

Martensite fraction-temperature diagram of TiNi wires embedded in an aluminum matrix

The martensite fraction-temperature diagrams of TiNi wires embedded in a pure aluminum matrix were studied by differential scanning calorimetry (DSC) measurement. Results showed that the initial phase constitution at the deformation temperature affects significantly the transformation behaviors of the constrained TiNi wires. However, for all composites with different phase constitution, the DSC curves are similar in the second and following heating cycles, where only one peak appears on each curve that corresponds to the reverse transformation of the self-accommodating martensite. There is always some amount of preferentially oriented martensite left in the TiNi wire, but its reverse transformation cannot be detected by conventional DSC.     

Characterization of Sn and Si nanocrystals embedded in SiO2 matrix fabricated by magnetron co-sputtering

Sn and Si nanocrystals were prepared by depositing Sn-Si-rich SiO₂ films using a co-sputtering process and a subsequent annealing. The microstructure and optical properties of Sn and Si nanocrystals were characterized by scanning electron microscopy (SEM), Raman spectra, X-ray diffraction and photoluminescence spectra. The crystallization of Sn has started at the annealing temperature of 400 °C, and was accomplished at 700 °C. However, the phase of amorphous Si starts to transform into nanocrystal Si when the annealing temperature is higher than 700 °C. These results illustrate that Sn existence as an element may played an important role in lowering the crystallization temperature of Si, and the crystallization rate of Si will be enhanced when Sn atom serves as the nucleation centre. Because quantum confinement effects are expected at relatively large radius for nanocrystal Sn, the redshift of high-energy PL peak may result from quantum confinement effects of nanocrystal Sn. However, the low-energy PL peak may be attributed to defects.     

Nitride formation in synthesis of titanium aluminide matrix composite coatings by reactive RF plasma spraying

This paper presents an in situ process to form intermetallic matrix composite coatings by reactive radio frequency (RF) plasma spraying with premixed elemental powder. The typical splat morphology of impinged titanium droplets on a stainless steel substrate is a disk with an outer peripheral fringe. If the supplied titanium powder size becomes finer or the nitrogen partial pressure in the plasma gas increases, splats containing prominent asperities with a smaller flattening ratio appear along with the plain disk type. An increase in nitrogen content is detected in all the splats sprayed with finer titanium powder and/or higher nitrogen partial pressure. The splats containing prominent asperities, which correspond to TiN, are twice as high in nitrogen content than the plain disk type. Aluminum splats are also classified into two categories: a disk type with an irregular outer periphery and a seminodular type. Oxygen exists on the splat surfaces, on which there are nitrogen concentrated areas corresponding to AlN. Consequently, the nitride formation proceeds on titanium and aluminum droplets during the flight as well as on the substrate. If the substrate temperature is higher than 873 K just before spraying with premixed titanium and aluminum powder, the formation of TiAl and Ti₂AlN proceeds on the substrate because of negligible mutual collisions during the flight. Titanium aluminide matrix in situ composites sprayed with premixed titanium and aluminum powder contain more nitrides than those sprayed with TiAl compound powder, because of the higher nitrogen absorption in titanium and aluminum droplets that results in an exothermic reaction.     

Friction and wear behavior of plasma assisted chemical vapor deposited nanocomposites made of metal nanoparticles embedded in a hydrogenated amorphous carbon matrix

Nanocomposite coatings consisting of preformed silver or chromium nanoparticles embedded into a hydrogenated amorphous carbon matrix (a-C:H) were synthesized by Electron Cyclotron Resonance plasma assisted Chemical Vapor Deposition (ECR-CVD). In a first step, the nanoparticles were distributed on silicon substrates by dipping in an ethanol suspension. In a second step, the ECR-CVD deposition of the a-C:H layer was done. The effect of the incorporation and the concentration on the friction and wear behavior was derived from unlubricated reciprocating sliding tests performed in ambient air. A decrease in the coefficient of friction, more intense with Cr incorporation, is induced by the preferential metal interaction with environment. In addition, for both metals, the coefficient of friction becomes lower as the metal concentration increases. A gradual increase in the coefficient of friction is detected for increasing the number of sliding cycles, which is attributed to the combined effect of surface smoothing and oxidation in the sliding contact. In conclusion, the valuable protective properties of the fullerene-like a-C:H coatings are enhanced by metal addition. As a consequence, a considerable reduction of the surface roughness and the volume loss in the wear tracks is especially noticeable for 10,000 cycles tests.     

Cu clustering and Si partitioning in the early crystallization stage of an Fe73.5Si13.5B9Nb3Cu1 amorphous alloy

Solute clustering and partitioning behavior in the early crystallization stage of an Fe_73.5Si_13.5B₉Nb₃Cu₁ amorphous alloy have been studied by employing a three-dimensional atom probe (3DAP) and a high resolution electron microscope (HREM). Results from the 3DAP have clearly shown that Cu atom clusters are present in the amorphous state after annealing below the crystallization temperature. The density of these clusters is in the order of 10²⁴/m³, which is comparable to that of the α-Fe grains in the optimum nanocrystalline microstructure. In the early stage of primary crystallization, Cu clusters are in direct contact with the α-Fe nanocrystals, suggesting that the α-Fe primary particles are heterogeneously nucleated at the site of Cu clusters. In the early stage of crystallization, the concentration of Si is lower in the primary crystal than in the amorphous matrix phase, unlike in the late stage of the primary crystallization, where Si partitions into the α-Fe phase with a composition of approximately 20 at.%.     

Preparation,characterization, and adsorption-photocatalytic activity of nano TiO_2 embedded in diatomite synthesis materials

Nano TiO_2 embedded in diatomite(NTED)synthesis materials were prepared by a sol-gel method in absolute ethyl alcohol suspension. NTED synthesis materials have a higher specific surface area and larger pore volume using micromeritics ASAP 2020. The result of characterization was measured by X-ray diffraction(XRD),Fourier transform infrared(FT-IR) spectroscopy, scanning electron microscope(SEM), and energy dispersive spectroscopy(EDS). The cyanide wastewater was employed to evaluate the adsorption capacity and photocatalytic activity of prepared NTED synthesis materials. The better photocatalytic activity and adsorption capacity of NTED synthesis materials are attributed to their high surface area and higher UV absorption intensity for the cyanide wastewater.     

In situ synthesis,microstructure, mechanical properties and thermal shock resistance of (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composites

Dense (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composites with adjustable content of (ZrB₂ + SiC) reinforcements (0–30 vol.%) were prepared by in situ hot-pressing. The microstructure, room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composites were investigated and compared with monolithic Zr₂[Al(Si)]₄C₅ ceramic. ZrB₂ and SiC incorporated by in situ reaction significantly improve the mechanical properties of Z₂[Al(Si)]₄C₅ by the synergistic action of many mechanisms including particulate reinforcement, crack deflection, branching, bridging, “self-reinforced” microstructure and grain-refinement. With (ZrB₂ + SiC) content increasing, the flexural strength, toughness and Vickers hardness show a nearly linear increase from 353 to 621 MPa, 3.88 to 7.85 MPa·m^1/2, and 11.7 to 16.7 GPa, respectively. Especially, the 30 vol.% (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composite retains a high modulus up to 1511 °C (357 GPa, 86% of that at 25 °C) and superior strength (404 MPa) at 1300 °C in air. The composite shows higher thermal conductivity (25–1200 °C) and excellent thermal shock resistance at ΔT up to 550 °C. Superior properties render the composites a promising prospect as ultra-high-temperature ceramics.     

Effects of individual and combined additions of phosphorus, boron and cerium on primary and eutectic silicon in an Al-30Si alloy

The modification of silicon in an Al-30Si alloy was studied using optical microscopy, electron probe micro-analysis, transmission electron microscopy and differential scanning calorimetry. It is found that phosphorus master alloys combined with boron master alloys have good modification effect on primary silicon but no evident modification effect on eutectic silicon, while boron combined with cerium has good modification effect on eutectic silicon. The results of differential scanning calorimetry show that phosphorus, boron or cerium addition and their combined addition have different undercooling effects on eutectic silicon. Many scholars thought that AlP particles were the nuclei of eutectic silicon when phosphorus was enough in the alloy. Our results show that β-(Al,Si,Fe) can still be the nucleus of plate-like eutectic silicon while AlP is the nucleus of primary silicon when there is enough phosphorus in the melt. In addition, the mechanism about the modification was also discussed.     

Corrosion studies of rebar in contact with saturated solution produced by leaching of Portland Type-II cement: examining the effects of chloride,carbonation and an amorphous silica additive

This study examined the effects of silica addition on electrochemical behavior of rebar in native pH 12.5 saturated solutions prepared by leaching of Portland Type-II cement, and in solutions reduced by CO₂ to pH 9. Cyclic polarizations showed that at pH 12.5 the silica additive increased the threshold concentration required for pitting from 100 to 105mM, and from 0.3 mM to 0.5 mM at pH 9. Passivation kinetic exponents were consistently larger for solutions with either pH when silica was present. The silica additive provided the passive film with generally increased resistance and lower capacitance, as confirmed by electrochemical impedance spectroscopy. Charge carrier densities calculated from Mott-Schottky plots were on the order of 1020 (cm^-3), increasing significantly after the passive film breakdown. Raman spectroscopy of rebar samples immersed in saturated cement solutions showed films predominantly composed of iron oxyhydroxides with a new shoulder attributed to amorphous silica.     

The in situ synthesis and wear performance of a metal matrix composite coating reinforced with TiC-TiB2 particulates, formed on Ti-6Al-4V alloy by a low oxygen partial pressure fusing technique

A metal matrix composite coating reinforced with TiC-TiB₂ particulates has been successfully fabricated utilizing the in situ reaction of Al, Ti and B₄C by the low oxygen partial pressure fusing technique to improve the wear resistance of Ti-6Al-4V alloy. The results show that increasing the B₄C content is adverse to forming the coating for the formation of interfacial stress; however, the addition of TiC powder as a diluent can favor the formation of this coating and the addition of small amounts of Y₂O₃ can greatly improve the adhesion of the coating. After a pin-on-disc wear test, the wear mass loss of the coating is only about 1/12 that of the Ti-6Al-4V alloy and the wear mechanism of coating is a mixed type of slight peeling-off, adhesion and abrasion.