Formation Of SiC Nanocrystals Research Articles

Enhanced Wear and Corrosion Resistance of AZ91 Magnesium Alloy via Adherent Si-DLC Coating with Si-Interlayer: Impact of Biasing Voltage

Magnesium alloys are the lowest-density structural metals with a wide range of applications, such as aircraft skins, engine casings and automobile hubs. However, its low surface hardness and non-corrosion resistance in natural environments limit its wide range of applications. In this work, Si-DLC coatings (Si: 15 at.%) are fabricated on AZ91 alloy using a hollow cathode discharge combined with a DC bias voltage from 0 to −300 V to increase the deposition rate and modulate the structure and properties of the coatings. The Si interlayer with a thickness of around 0.6 µm is deposited first to enhance the adhesion. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy are used to investigate the effect of DC bias on the microstructure evolution of Si-DLC coatings. Meanwhile, corrosion and wear resistance of the coatings at various bias voltages have been investigated using electrochemical workstations and pin-on-desk wear testers. It is shown that the bias-free coating has a loose structure and is less resistant to corrosion and wear. The bias coating has a compact structure, small carbon cluster size, high chloride ion corrosion resistance, and high wear resistance against Al2O3 spheres. The corrosion potential of the coating bias at −300 V is −0.98 V, the corrosion current density is 1.35 × 10−6 A·cm−2, the friction coefficient is 0.08, and the wear rate is 10−8 orders of magnitude. The formation of SiC nanocrystals and high sp3-C, as well as the formation of transfer films on the surface of their counterparts, are the main reasons for the ultra-high wear resistance of the bias coatings. The wear rate, coefficient of friction, and corrosion rate of the coating are 0.0069 times, 0.2 times, and 0.0088 times that of the AZ91 alloy, respectively. However, the bias coating has only short to medium-term protection against the magnesium alloy and no long-term protection due to cracks caused by its high internal stress.

Thermodynamics of a phase transition of silicon nanoparticles at the annealing and carbonization of porous silicon

The formation of SiC nanocrystals of the cubic modification in the process of high-temperature carbonization of porous silicon has been analyzed. A thermodynamic model has been proposed to describe the experimental data obtained by atomic-force microscopy, Raman scattering, spectral analysis, Auger spectroscopy, and X-ray diffraction spectroscopy. It has been shown that the surface energy of silicon nanoparticles and quantum filaments is released in the process of annealing and carbonization. The Monte Carlo simulation has shown that the released energy makes it possible to overcome the nucleation barrier and to form SiC nanocrystals. The processes of laser annealing and electron irradiation of carbonized porous silicon have been analyzed.

Structural and electrical characterization of annealed Si1−xCx/SiC thin film prepared by magnetron sputtering

Si-rich Si1—xCx /SiC multilayer thin films are prepared using magnetron sputtering, subsequently followed by thermal annealing in the range of 800–1200 °C. The influences of annealing temperature (Ta) on the formation of Si and/or SiC nanocrystals (NCs) and on the electrical characteristics of the multilayer film are investigated by using a variety of analytical techniques, including X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectrometry (FT-IR), current—voltage (I—V) technique, and capacitance-voltage (C—V) technique. XRD and Raman analyses indicate that Si NCs begin to form in samples for Ta ≥ 800 °C. At annealing temperatures of 1000 °C or higher, the formation of Si NCs is accompanied by the formation of SiC NCs. With the increase in the annealing temperature, the shift of FT-IR Si—C bond absorption spectra toward a higher wave number along with the change of band shape can be explained by a Si—C transitional phase between the loss of substitutional carbon and the formation of SiC precipitates and a precursor for the growth of SiC crystalline. The C—V and I—V results indicate that the interface quality of Si1—xCx/SiC multilayer film is improved significantly and the leakage current is reduced rapidly for Ta ≥ 1000 °C, which can be ascribed to the formation of Si and SiC NCs.

Si and SiC nanocrystals in an amorphous SiC matrix: Formation and electrical properties

AbstractLayers of Si nanocrystals in a dielectric matrix have promising properties to be implemented as the absorber layer in a top cell of a Si‐based tandem solar cell. Si nanocrystals in SiC are produced by plasma deposition of Si rich a‐SiC:H and subsequent solid phase crystallization by thermal annealing at temperatures between 800°C and 1000°C. The Si rich a‐SiC:H films were doped with boron by addition of diluted diborane (B2H6 in H2) to the plasma process. The microstructure was investigated for different gas fluxes and annealing procedures and the films were found to consist of amorphous or polycrystalline SiC with embedded Si nanocrystals. The microstructural results are then correlated with the electrical and optoelectronic properties. By choosing appropriate deposition parameters, the films micro‐structure could be modified such that the crystallization of both Si and SiC nanocrystals is favoured and the conductivity is enhanced. X‐ray diffraction (XRD) patterns show the formation of both Si and SiC nanocrystals after annealing at 900 °C, if the hydrogen flux in the plasma is large enough. The formation of SiC nanocrystals is confirmed by Fourier Transformed IR (FTIR) spectra where a transition from a Gaussian shaped peak of the amorphous SiC‐phase to a blueshifted Lorentzian peak of nanocrystalline SiC is observed. Dark dc IV measurements reveal the increase of conductivity with diluted diborane flux from 9.32 10–6 Ω–1cm–1 to 1.98 10–2 Ω–1cm–1 for a 1000 °C anneal. Increase of annealing temperature at constant doping also increases conductivity by about two orders of magnitude. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-fluence Si-implanted diamond: Formation of SiC nanocrystals and sheet resistance

The sheet resistance and structural properties of high-fluence Si-implanted diamond were investigated. In order to minimize the radiation damage and to facilitate SiC formation the implantation was performed at 900 °C. All samples were subsequently annealed in a rf-heated furnace at 1500 °C for 10 min in order to remove defects and thermally unstable phases. X-ray diffraction, infrared absorption spectrometry, and high-resolution cross-sectional transmission electron microscopy revealed the formation of a buried layer inside the implanted diamond, which contains SiC nanocrystallites. These SiC nanocrystals have a cubic structure and are nearly perfectly aligned with the diamond lattice. Raman spectroscopy was applied to analyze radiation-damage-induced graphitization in dependence on the implantation conditions. The sheet resistance of the samples was measured as function of temperature by four point probe technique in van-der-Pauw geometry. The decrease of the sheet resistance with increasing ion fluence unambiguously shows the influence of implantation-induced damage. The behavior of the sheet resistance can strongly be modified by additional nitrogen implantation. The resulting higher conductivity is interpreted as partial incorporation of the nitrogen donor into the SiC nanocrystals. However, when the Si fluence exceeds a critical value of 5.3×1017 Si+ cm−2 at 900 °C the diamond is irreversibly damaged and defect related conductivity dominates.

Growth Control And Characterization Of Wide Band Gap Silicon-Carbon Films

AbstractSilicon-carbon films have been grown by reactive hydrogen magnetron sputtering at a substrate temperature of 730°C, with different values of carbon-to-silicon sputtered area ratio, rc. The layers were investigated by infrared spectroscopy, Raman scattering, x-ray photoelectron spectroscopy and optical absorption. For rc below 30%, almost only Si nanocrystallites were formed with a few fraction of amorphous SiC, whereas for rc exceeding 30%, a drastic change was noticed, leading to the achievement of SiC crystals in the layers. These latter were found of near-stoichiometric composition with an atomic ratio C/Si ˜1.04. The results suggest that the excess C is of graphitic-like configuration being likely located in the intergrain regions, in addition to some silicon-oxygen bonds. These features are accompanied by an abrupt widening of the band gap in the transition region that is consistent with the formation of SiC nanocrystals. The large value measured for the band gap (≥3 eV) is thought to be due to more than one origin, such as size effect of SiC, Si-O bonds and possible presence of different SiC polytypes.

Silicon carbide synthesis with energy pulses

Polycrystalline SiC layers were synthesized through nanosecond pulse heating of thin carbon films deposited on single-crystalline silicon wafers. The samples were submitted to electron beam irradiation (25 keV, 50 ns) at various current densities in vacuum (∼10−4mbar) and to XeCl excimer laser pulses (308 nm, 15ns) in air. Rutherford backscattering spectrometry (RBS) showed that in the e-beam annealed samples mixing of the elements at the interface starts at current densities of about 1200 A/cm2. The mixed layer thickness increases almost linearly with current density. From the RBS spectra a composition of the intermixed layers close to the SiC compound was deduced. Transmission electron microscopy (TEM) and electron diffraction studies clearly evidenced the formation of SiC polycrystals. Using the XeCl excimer laser, intermixing of the deposited C film with the Si substrate was observed after a single 0.3 J/cm2 pulse. Further analysis evidenced the formation of SiC nanocrystals, embedded in a diamond film.