Silicon-rich Samples Research Articles

Influence of Multiphase High Silicon Steel (Retained Austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) and Carbon Content of RA-Cγ on Rolling/Sliding Wear

In this research work, heat-treatment processes have been utilized to obtain multiphase microstructure in the silicon rich steel samples, silicon in the steel helps in the development of multiphase microstructure and to keep away from carbide precipitation development through the austempering. The desired multiphase microstructure (Retained austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) consisting of continuous cooling (CC) for 0, 20 and 40 s respectively after austenization followed by austempering at (300, 350 and 400 °C) to form a high wear resistance multiphase steels with microstructure varies amount of F,B, P and RA during continuous cooling. Steels with varies retained austenite up to (5±1.1 to 18±1.9%) along with excellent specific wear rate (2.038 × 10−9-1.061 × 10−8 m3/N-m) were obtained. Further, the rolling/sliding wear rate has been obtained through the disc-on-disc experimental setup. The effect of continuous cooling on retained austenite, carbon content of retained austenite on specific rolling/sliding SWR (specific wear rate) has been studied; phase fraction (P, B and F) and the materials characterization with the help of XRD, AFM and FE-SEM.

Evolution of the sensitized Er(3+) emission by silicon nanoclusters and luminescence centers in silicon-rich silica.

The structural and optical properties of erbium-doped silicon-rich silica samples containing different Si concentrations are studied. Intense photoluminescence (PL) from luminescence centers (LCs) and silicon nanoclusters (Si NCs), which evolves with annealing temperatures, is obtained. By modulating the silicon concentrations in samples, the main sensitizers of Er3+ ions can be tuned from Si NCs to LCs. Optimum Er3+ PL, with an enhancement of more than two, is obtained in the samples with a medium Si concentration, where the sensitization from Si NCs and LCs coexists.

Magnetic and Magnetocaloric Properties of LaFe$_{13-{\rm x}}$(Si$_{1-{\rm w}}$Al$_{\rm w}$)$_{\rm x}$ Alloys and Their Hydrides

The influence of aluminum on the magnetic and magnetocaloric properties of LaFe13-xSix alloys is investigated. For this purpose the magnetic phase diagrams of three quasi-ternary alloy series of LaFe13-x(Si1-wAlw)x with x = 1.14, 1.43, and 1.71 are presented. Samples with high iron-content (x = 1.14) and high aluminum-content show antiferromagnetic (AFM) ordering which changes over to ferromagnetic (FM) ordering as the aluminum-content is decreased. For intermediate Si- and Al-contents samples show both low temperature FM ordering and higher temperature AFM ordering. As the overall iron-content is decreased ( x = 1.43, and 1.71) FM ordering dominates the phase diagram. Upon hydrogenation all samples become ferromagnetic and transition temperatures are increased by about 150 K. A detailed analysis of the hydrogen content shows that aluminum-rich samples take up more hydrogen compared to the silicon-rich samples. This can be linked to larger interstitial voids resulting from the lattice expansion when Si is replaced with Al. The isothermal entropy change of aluminum-substituted alloys is always lower than the one of the silicon-variants at the same iron concentration.

Luminescence-center-mediated excitation as the dominant Er sensitization mechanism in Er-doped silicon-richSiO2films

The structural and optical properties of erbium-doped silicon-rich silica samples containing $12\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ of excess silicon and $0.63\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ of erbium are studied as a function of annealing temperature in the range $600--1200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Indirect excitation of ${\mathrm{Er}}^{3+}$ ions is shown to be present for all annealing temperatures, including annealing temperatures well below $1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for which no silicon nanocrystals are observed. Two distinct efficient $({\ensuremath{\eta}}_{\mathrm{tr}}g60%)$ transfer mechanisms responsible for ${\mathrm{Er}}^{3+}$ excitation are identified: a fast transfer process $({\ensuremath{\tau}}_{\mathrm{tr}}l80\phantom{\rule{0.3em}{0ex}}\mathrm{ns})$ involving isolated luminescence centers (LCs), and a slow transfer process $({\ensuremath{\tau}}_{\mathrm{tr}}\ensuremath{\sim}4--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s})$ involving excitation by quantum confined excitons inside Si nanocrystals. The LC-mediated excitation is shown to be the dominant excitation mechanism for all annealing temperatures. The presence of a LC-mediated excitation process is deduced from the observation of an annealing-temperature-independent ${\mathrm{Er}}^{3+}$ excitation rate, a strong similarity between the LC and ${\mathrm{Er}}^{3+}$ excitation spectra, as well as an excellent correspondence between the observed LC-related emission intensity and the derived ${\mathrm{Er}}^{3+}$ excitation density for annealing temperatures in the range of $600--1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The proposed interpretation provides an alternative explanation for several observations existing in the literature.

Influence Of Growth Conditions on Irradiation Induced Defects in 4H-SiC

Nitrogen doped 4H-SiC epitaxial layers grown by hot-wall chemical vapor deposition were investigated by Deep Level Transient Spectroscopy after irradiation with 6 MeV electrons at room temperature. This study is focusing on the influence of nitrogen doping and C/Si ratio on the behaviour of the Z1,2 and EH6,7 levels which occur in already as-grown material but are substantially enhanced by electron and ion irradiation. It was found that both the Z1,2 and EH6,7 concentrations increase with both the nitrogen doping and the C/Si ratio. However, while the Z1,2 concentration increases during post-irradiation thermal treatment the opposite holds for the EH6,7 level especially in silicon rich samples. On the basis of these results, the influence of carbon and nitrogen on the formation of the Z1,2 complex is reconfirmed and a possible identity of the EH6,7 defect is discussed.

Silicon nanoclusters containing nitrogen and sensitization of erbium luminescence in SiO x:Er

Silicon-rich silica samples doped with erbium were grown by PECVD and characterized by photoluminescence, time-resolved photoluminescence and Fourier transform infrared spectroscopy. We observe that upon increased silicon content, the absorption spectrum reveals the formation of a Si N bond. This indicates the possible incorporation of nitrogen from the precursor N 2O gas into the Si nanoclusters. The highest erbium photoluminescence is obtained for the sample with the highest silicon content and its decay characteristics are nearly single exponential with a time constant of 5 ms. In addition to erbium emission, a visible luminescence peak at about 550 nm is observed. This shows multi-exponential decay kinetics with decay times of the order of 10 ns. We propose that this emission is due to small Si nanoclusters covered by a Si N shell. From the measurements, we study a mechanism to explain the erbium excitation in this material.

Luminescence and structural properties of silicon-rich nitride by X-ray absorption spectroscopy

The luminescence and structural properties of silicon-rich nitride thin films were comprehensively examined by X-ray absorption spectroscopy. The near-edge X-ray absorption fine structures demonstrated the formation of silicon clusters in the silicon-rich nitride films. In addition, atomic resolved images of transmission electron microscope evidenced constitution of ultra-small (2–5 nm) silicon clusters. The X-ray absorption near-edge fine structures surveyed in photoluminescence yield and total electron yield configurations revealed that the luminescence are mainly coming from the Si–N bonding sites for lower degree of silicon-rich films. For higher silicon-rich samples, the Si–Si bonding sites increasingly contribute to the band edge absorption in the photoluminescence yield near-edge fine structures. These results strongly suggested that the transitions from the gap states localized in the surface of embedded silicon clusters and quantum confinement states within silicon clusters might contribute to the luminescence. The contribution weightiness of quantum confinement states increase while increasing the silicon-richness. It provides a reasonable explanation for the prolonged debates on the luminescence origins.

Selected properties of EuCo 2(Si 1−xGe x) 2 alloys

EuCo 2(Si 1− x Ge x ) 2, x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 samples were synthesised by induction melting followed by annealing at 900 °C and rapid quenching. X-ray powder diffraction and Auger electron spectroscopy studies revealed that solid solutions are formed only for x⩽0.2 and x⩾0.7. Magnetic susceptibility investigations for the solid solutions revealed a dominant divalent europium valence state in the germanium-rich samples and a dominant trivalent europium component in the silicon-rich samples. In the germanium-rich samples, a long-range antiferromagnetic ordering was observed. In all samples studied, additional magnetic transitions at various temperatures were detected, which could be attributed to small clusters containing different europium chemical surrounding from that in the predominant phase.

Oxygen to silicon ratio determination of SiO xH y thin films

The oxygen to silicon ratio of several SiO xH y thin films deposited by the electron cyclotron resonance plasma method was studied by several methods (heavy ion elastic recoil detection analysis, energy dispersive X-ray spectroscopy, Auger spectroscopy and infrared spectroscopy). Among these methods, other groups found that x scales linearly with the wavenumber of the Si–O–Si stretching vibration ( v st) by the relation x = 0.020 v st − 19.3. This equation has been used by many different groups to determine x of SiO x thin films, but we have found that in our ECR deposited films the above mentioned formula gives accurate results for x values higher than 1.5, but for Si richer films the formula overestimates the x value, with values well outside the 20% accuracy range. A possible explanation of this discrepancy may be the bonded hydrogen of the films: in the plasma deposited samples used in this study the hydrogen content was high, above 20 at.% for silicon-rich samples. As a consequence, the Si–O–Si groups were immersed in a more electronegative matrix than in the usual case (SiO x with a low hydrogen concentration) and thus the variation of the position of the stretching peak was less pronounced.

Influence of silicon and carbon excesses on the aqueous dispersion of SiC nanocrystals for optical application

The dispersion behaviour of laser-synthesized silicon carbide nanoparticles (npSiC) in water is investigated by photon correlation spectroscopy (PCS). With regard to previous studies and due to an application in the processing of optical materials, this paper concerns low npSiC contents (from 0.05 to 10 wt.%). The role played by the particle surface state is be pointed out through the consideration of stochiometric (C/Si = 1), carbon-rich (C/Si > 1) and silicon-rich (C/Si < 1) nanopowders. Suspensions made from stoichiometric and silicon-rich nanopowders are easily dispersed and stable with time. The PCS measurements reveal in this case more than 95% of isolated nanoparticles, pointing out the key role of the oxidized layer covering the grain of silicon-rich samples. At the opposite, the carbon-rich powders are hardly dispersed in pure water, correlated with the presence of a relatively inert graphitic carbon layer at the grain surface. However, by addition of a commercial polymeric dispersant, all nanopowders induce high quality suspensions. In particular, the carbon-rich samples are easily dispersed, and possible dispersion mechanisms of npSiC in presence of a polymeric surfactant are discussed. The influence of the npSiC loading and the time evolution of the suspension are also presented. By considering stoichiometric, as well as carbon- and silicon-rich samples, this paper demonstrates the possibility to achieve high quality dispersions of SiC nanoparticles, whatever the chemical composition of the powder, as an easy step for optical material processing.

Rhenium ohmic contacts on 6H-SiC

Rhenium (Re) thin-film contacts (100-nm thick) were deposited on carbon-rich, nominally stoichiometric, and silicon-rich 6H–SiC surfaces, which were moderately doped with nitrogen (1.28×1018cm−3). Morphology (Dektak), phase formation (x-ray diffraction), chemistry (Auger electron spectroscopy), and electrical properties (I–V) were characterized for the as-deposited and annealed (120min, 1000°C, vacuum &amp;lt;1×10−6Torr) contacts. As-deposited films were nonohmic. Films grown on carbon-rich surfaces were nonspecular, granular, and often delaminated during characterization. At room temperature in air, the Re films on stoichiometric SiC remained optically specular reflecting for 3h, but then became hazy from oxidation. The Re films on silicon-rich surfaces, stored in air at room temperature, resisted ex situ oxidation for approximately 24h. The annealed samples remained specular without visible signs of oxidation. The annealing resulted in a reduction in surface roughness for all the films regardless of substrate chemistry. The phase separation between carbon and rhenium was observed based on the formation of interfacial Re clusters and a ∼10-nm graphite surface layer after annealing. Auger data showed that Si layers (5–10nm) deposited to create Si-rich surfaces were partially consumed to form rhenium silicide during annealing, and the sharp Re∕Si∕SiC interface became more diffused with Re detected ∼50nm deeper into the structure. The annealing of Re films on moderately doped (1.28×1018cm−3) SiC resulted in ohmic contacts with an average specific contact resistance of 7.0×10−5Ωcm2 for stoichiometric and 1.6×10−5Ωcm2 for silicon-rich samples. The annealed contacts on carbon-rich surfaces remained rectifying.

High-silica SAPO-5 with preferred orientation: synthesis, characterization and catalytic applications

Silicon-rich SAPO-5 samples with preferred orientation were synthesized with a simple template of diethylamine at the crystallization temperature of 573 K and a short crystallization time. 29Si MAS NMR spectra showed that only SM2 occurred for the sample with Si/Al ratio of 0.25, while SM2 + SM3 occurred for the sample with Si/Al ratio of 0.42. Though there were very few strong acid sites in SAPO-5 framework with Si/Al ratio of 0.25, it showed high n-alkane hydrocracking activity. It also showed unusual catalytic behavior for longer chain hydrocarbons ( n-C12, n-C16), which could be adsorbed within SAPO-5 channels below 573 K, while hydrocracked at temperatures higher than 598 K. The 12-ring channels structure of SAPO-5 with preferred orientation is used to explain the unusual results.

Role of incorporated hydrogen in non-stoichiometric photo-deposited silicon nitride films

Silicon nitride films were deposited by mercury-sensitized photochemical vapor deposition utilizing a gaseous mixture of Si 2H 6 (2% in Ar) and NH 3 under 253.7 nm ultraviolet light irradiation. Non-stoichiometric silicon-rich and nitrogen-rich samples were deposited by varying gas flow ratio. High partial pressure of disilane results in the formation of clusters, with the incorporation of hydrogen in the form of SiH and NH stretching modes below the detection level of Fourier transform infrared spectroscopy. In the nitrogen-rich sample the bonded hydrogen concentration is 0.56×10 18 cm −2. After annealing the samples at 350 °C for 3 h in a pressure of 50 mTorr, the interface electronic state density decreases for silicon-rich sample and increases for nitrogen-rich sample.

Evolution of non-Fermi liquid state from U 2Pd 1− xSi 3+ x spin glasses

We study the competition between RKKY and Kondo interactions in the U 2Pd 1− x Si 3+ x ( x=0, ±0.1, ±0.2, ±0.3, ±0.4, ±0.5) compounds in which Pd and Si atoms are randomly distributed at the same site. With increasing x, the spin-glass freezing temperatures, T f, detected by the cusps of the M/ H curves decrease gradually from 21 to 0 K. The silicon-rich samples of x=0.4, 0.5 exhibit the non-Fermi liquid behaviors of C/ T∝ χ∝ T −1+ λ below 10 K.

Compositional and structural properties of deuterated plasma enhanced chemical vapour deposited silicon-carbon alloys

Abstract Hydrogenated and deuterated amorphous silicon carbon films were prepared by plasma enhanced chemical vapour deposition (PECVD) starting from silane and deuterated methane gas mixtures. The gas percentages was varied in order to produce films with different carbon and silicon content. The elemental composition was determined by Rutherford backscattering and elastic recoil detection analysis and the bonding structure by infrared spectroscopy. The hydrogen plus deuterium atomic fraction, in the grown films, is about 0.36, almost independent of the film composition. However, the concentration of hydrogen or deuterium depends on the carbon content. In silicon-rich samples both hydrogen and deuterium atoms are contained in the films; with increasing carbon content, the hydrogen concentration decreases and the deuterium concentration increases. At the highest carbon concentration (0.28) the resulting films are fully deuterated with the deuterium atoms attached both to silicon and carbon. From infrared absorption analysis, information on plasma chemistry and surface or bulk reactions during film growth was obtained. For the first time the experimental determination of the origin of the bonded hydrogen in amorphous SiC: H films, grown by methane-silane PECVD, is reported. Comparison of elemental composition and infrared spectra of films grown from hydrogenated and deuterated methane shows that hydrogen exchange occurs between carbon and silicon atoms. Moreover, as the films approach stoichiometry the hydrogen incorporated into the sample originates mainly from methane gas.

Synthesis and characterization of SAPO-41: effect of the silicon content and the crystal size on the hydroisomerization of n-octane over Pt–Pd/SAPO-41

Silicoaluminophosphate SAPO-41 was synthesized either in the presence of surfactant or without added surfactant. Under the synthesis conditions with added surfactant small-size molecular sieve crystals were formed, while larger crystals were obtained in the absence of surfactant. Magic-angle spinning nuclear magnetic resonance spectroscopic studies showed that silica-rich domains existed for silicon-rich samples. Infrared spectra in the OH stretching vibrations and temperature-programmed desorption of ammonia revealed the presence of Brønsted acid sites. Samples prepared in the presence of surfactant exhibited higher acid strength. The hydroconversion of n-octane was perfomed over Pt–Pd-containing SAPO-41 samples. These catalysts showed high selectivity for the hydroisomerization reaction. Among the C8 isomers, monomethylheptanes were preferentially formed. The selectivity towards hydroisomerization at constant conversion level did not change with the SAPO-41 crystal size. However, a small-crystal SAPO-41 sample was considerably more active than large-crystal SAPO-41. The shape selectivity of the tubular medium-pore SAPO-41 played the major role. Pore mouth catalysis at the interior of the pore is also discussed.

Corrosion properties of thin molybdenum silicide films

The corrosion properties of sputtered molybdenum and molybdenum silicide films in hydrochloric acid (HCl) have been studied by means of potentiodynamic measurements. Contributions from the substrate to the corrosion behaviour was avoided by depositing the films on inert aluminium oxide (Al 2O 3). The compositions studied were Mo, MoSi 0.58, MoSi 1.04, MoSi 1.4 and MoSi 1.9–2.1. Characterisation of the sampies was made by X-ray diffraction (XRD) and scanning electron microscopy (SEM) before and after corrosion. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to analyse the polarised films. Corrosion of M 3Si was found in the molybdenum-rich samples (MoSi 0.58) containing the two phases Mo 3Si and Mo 5Si 3. Polarisation curves for these films showed one passivation peak at 228 mV vs. the saturated calomel electrode (SCE). The MoSi 1.9–2.1 films had the best corrosion properties of the films studied. This composition had three passivation peaks, at about 154, 305 and 1850 mV SCE, respectively. In the silicon-rich samples, containing the phases MoSi 2 and Mo 5Si 3, preferentiai corrosion of Mo 5Si 3 was found. All the samples containing the disilicide phase showed at least two passivation peaks. XPS and AES studies on the passive films formed on the samples at the two first passivation peaks indicate that both peaks are due to oxidation of silicon- and molybdenum-containing species. The amount of molybdenum in the outermost layer is increased after the second peak.

Rare-Earth Doped Silicon-Rich Silica: Evidence for Energy Transfer between Silicon Microclusters and Rare-Earth Ions

ABSTRACTWe report the fabrication of rare-earth doped silicon-rich silica thin films by PECVD. The films exhibit absorption edges in the visible region of the optical spectrum consistent with the presence of silicon microclusters. Weak visible photoluminescence due to silicon microclusters is observed. In addition, strong luminescence from the rare-earth ion is obtained even when excited away from characteristic absorption bands; indeed, the luminescence intensity is largely independent of excitation wavelength below 514 nm. We ascribe this to excitation of silicon microclusters followed by an efficient transfer of energy to the rare-earth ions.The very broad absorption of this material opens up the possibility for flashlamp-pumped optoelectronic devices. In addition, we report the fabrication of silicon-rich silica films by PECVD. We show that the optical properties of these films are consistent with the presence of silicon microclusters and show absorption spectra similar to those of the rare-earth doped silicon-rich silica samples. This supports the hypothesis that the principal absorbing species in the rare-earth doped films is microclustered silicon

Eliipsometric Study of Rapid Thermal Annealed PECVD SiN Films

ABSTRACTA conventional parallel plate plasma enhanced chemical vapor deposition system using SiH4-NH3-N2 at 13.56MHz and 300°C was used to deposit SiN films on silicon substrates. Special care was used to assure an oxide free surface before deposition. The silane flow was varied to obtain stoichiometric Si3 N4 and silicon-rich films. Rapid thermal anneal (RTA) was performed for 30 sec at temperatures in the range 600°C — 900°C. Spectroscopic ellipsometry was used to characterize the films. Data was taken for 44 wavelengths (3200–7500Å range) at three angles of incidence on each sample. The structural and optical properties were obtained using several structural models of the film. The models assumed an effective medium approximation for the following three combinations of film constituents: Si3N4 and voids; Si3N4 and amorphous silicon (a-Si); Si3N4, voids and a-Si. In general, the three constituent model gave the best fits to the experimental data. In all films, we observed a decrease in the film thickness and void fraction as the RTA temperature increased. The a-Si percentage did not change vs. the RTA temperature. The reduction in void fraction for the silicon-rich samples was larger than for the Si3N4 film. The results are comparable to those measured by other methods on regular furnace annealed SiN films, i.e. annealing times of 30 minutes or more.

Physical properties of amorphous silicon-carbon alloys produced by different techniques

Results of a study of compositional, optical, electrical, and structural properties of hydrogen amorphous silicon carbide (a-SiC:H) prepared, respectively, by glow-discharge (GD) and reactive sputtering (SP) techniques at power densities varying between 1.25 · 10−2 and 1.25 · 10−1 W · cm−2 for GD samples are presented. Measurements are reported on the composition, optical and IR spectroscopy, and on the temperature dependence of electrical conductivity. All experimental observations suggest that the power density only slightly affects the physical properties of GD silicon-rich samples, whereas those of the carbon-rich SP samples depend more strongly on this deposition parameter. Finally, it is shown that the GD technique can provide films with better characteristics, whereas samples of similar composition prepared by sputtering have higher compositional disorder and are more inhomogeneous.