Input Gas Ratio Research Articles

Microstructure Evolution and Mechanical Behavior of Mo–Si–N Films

The molybdenum silicon nitride (Mo–Si–N) films were deposited by a radio frequency (RF) magnetron reactive dual-gun co-sputtering technique with process control on input power and gas ratio. Composition variation, microstructure evolution, and related mechanical and tribological behavior of the Mo–Si–N coatings were investigated. The N2/(Ar + N2) flow ratios were controlled at 10/20 and 5/20 levels with the tuning of input power on the Si target at 0, 100, and 150 W. As the silicon contents increased from 0 to 33.7 at.%, the film microstructure evolved from a crystalline structure with Mo2N and MoN phases to an amorphous feature with the Si3N4 phase. The analysis of selected area electron diffraction patterns in TEM also indicated an amorphous feature of the Mo–Si–N films when Si content reached 20 at.% and beyond. The hardness and Young’s modulus changed from 16.5 to 26.9 and 208 to 273 GPa according to their microstructure features. The highest hardness and modulus were attributed to nanocrystalline Mo2N and MoN with Si solid-solution. The crystalline Mo–Si–N films showed a smooth tribological track and less wear failure was found. In contrast, the wear track with severe failures were observed for Mo–N and amorphous Mo–Si–N coatings due to their lower hardness. The ratios of H/E and H3/E2 were intensively discussed and correlated to the wear behavior of the Mo–Si–N coatings.

Microstructure and Properties of Porous SiC Ceramics Modified by CVI‐SiC Nanowires

Sintered porous SiC ceramics are modified with SiC nanowires prepared via chemical vapor infiltration (CVI). SiC nanowires are successfully grown within sintered porous SiC ceramics following vapor–liquid–solid growth. The diameter of the SiC nanowires is in the range of 200 nm–1 μm, and first decreases with increasing input gas ratio (α = 50, 60, 70, and 80) and increases thereafter. The diameter of the nanowires decreases from the surface to the interior areas of the porous SiC ceramics. SiC nanowires effectively improve the mechanical properties of the porous SiC ceramics, and sample Ni‐50 has the highest flexural strength of 33.91 MPa and fracture toughness of 0.79 MPa · m1/2, which increases by 90.4% and 49.1% compared to an unmodified sample, respectively. Additionally, the presence of SiC nanowires leads to porous SiC ceramics with altered porosity and microstructure, and higher thermal conductivity. The porous SiC ceramics modified by CVI SiC nanowires satisfy the requirements of gas filtration applications and the pressure drop increases with decreasing apparent porosity. The porous SiC ceramics modified with CVI SiC nanowire has higher permeability than those resulting from the introduction of CVI‐SiC matrix or CVD‐SiC coating into porous SiC ceramics.

Growth of SiC nanowires by low pressure chemical vapor infiltration using different catalysts

SiC nanowires were synthesized by LPCVI using different catalysts, and the influences of input gas ratio (α) and catalysts were investigated. The average diameter firstly decreased and then increased with increasing α. Under Ni-based catalysis, SiC nanowires were long and thin, and increased with increasing concentration; under Fe-based catalysis, they were short and thick, and the influence of concentration could be neglected. The growth of SiC nanowires was controlled by vapor-liquid-solid (VLS) growth mechanism and the liquid-solid interface between nanowire and metal droplet was the growth plane. At same concentration, the diameter grown under Ni-based catalyst decreased with decreasing diameter of catalyst droplet, while under Fe-based catalyst, the diameters were not affected by catalyst droplet because of the high concentration. SiC nanowires were synthesized in 2D C/SiC composites and could enhance the mechanical properties effectively because of energy consuming from the fracture, pulled up and debond of SiC nanowires.

Wide Gap Microcrystalline Silicon Oxide Emitter for a-SiOx:H/c-Si Heterojunction Solar Cells

This paper reports on the development of phosphorous doped microcrystalline silicon oxide (µc-SiOx:H) films as an emitter window layer in flat p-type silicon heterojunction (SHJ) solar cells featuring intrinsic a-SiOx:H buffer layers. We investigated the material properties of n-type µc-SiOx:H films grown at various input gas ratios and correlated the results of SHJ solar cells utilizing varying oxygen content and thickness of the emitter layer to the corresponding film properties. A maximum efficiency of 19.0% was achieved. The excellent short circuit current of 35.8 mA/cm2 for flat cells was attributed to the low optical losses in the emitter window.

A study of surface-coated SiC whiskers on carbon fiber substrates and their properties for diesel particulate filter applications

β-Silicon carbide (β-SiC) whiskers were synthesized on carbon fiber substrates using a chemical vapor infiltration (CVI) vapor–solid (VS) growth mechanism. An additional SiC surface coating process was utilized after whisker deposition by controlling the input gas ratio of the source gas flow and changing the H2 (hydrogen) diluent gas to N2 (nitrogen) under the same deposition temperature of 1,300°C. As the surface coating deposition time increased, whiskers thickness and spherical blunt tips which were seen at the top edge of the whiskers went thicker. Observing the microstructure of the resulting tips by transmission electron microscopy (TEM) revealed that uncoated whiskers showed few stacking faults, whereas surface-coated whiskers were completely filled with stacking faults. The effect of surface coating deposition time was also evaluated by measuring the properties of a filtration system. Specifically, as the surface coating deposition time increased, gas permeability decreased; however, even at 30min, the gas permeability of the thickest surface coated whisker filters was five times higher than that of cordierite honeycomb, which is currently used in commercial diesel particulate filter (DPF) devices. A specimen that had been surface coated for more than 20min almost completely maintained its prime line density under high-pressure (5MPa) gas. Moreover, we confirmed that SiC surface coating on whiskers and carbon fiber substrates enhanced oxidation resistance and filtration efficiency.

Silicon oxide based n-doped layer for improved performance of thin film silicon solar cells

We propose the use of n-doped silicon oxide as alternative n-layer in thin film Si p-i-n solar cells. By varying input gas ratios, films with a wide range of optical and electrical properties are obtained. Applying these layers in solar cells, good electrical and optical properties are demonstrated. A relative efficiency increase up to 13.6% has been observed on the cells adopting a simple Ag back contact. A similar spectral response as with the cell with standard n-layer plus ZnO/Ag back contact is obtained. The deposition of a buffer layer at the back contact can therefore be avoided.

Chemical vapor infiltration (CVI) SiC whisker on carbon woven fabric for filter applications

β-silicon carbide whiskers have been synthesized on a carbon fabric by a vapor-solid (VS) mechanism using the Chemical Vapor Infiltration (CVI) process. Optimum processing conditions for SiC whisker growth were determined by mapping of SiC deposition behavior. SiC was deposited on a carbon fabric substrate as a film or whiskers, depending on processing conditions. The mean diameter and line density of the whiskers was the highest at an input gas ratio of 50. As temperature increases, the mean diameter of the whiskers increased and the line density of whiskers decreased. In the optimum processing conditions, whiskers grew uniformly with high growth density and formed a networking structure between each of the carbon filaments resulting in a structure suitable for filter applications.

Synthesis of thin diamond films from faceted nanosized crystallites

Diamond films consist of crystallites having nanometer grains were deposited using low methane concentration by hot filament chemical vapor deposition (HFCVD). The results show that films consist of nanodiamond grains with grain sizes ranging from 20 nm to 200 nm having thickness dependent size. Increasing the deposition time, the grain size increases and hence the thickness of the film increases. The diamond nucleation (nucleation density 10 10 cm −2) is comparable to that obtained by biasing the substrate. The use of low methane concentration for the formation of nano crystallites improves the quality of the film as indicated by Raman spectroscopy. The distance between the filament and substrate is increased while maintaining the substrate temperature. The effects of this large separation on the gas phase chemistry are discussed which helps to understand the reduced size of the crystallites under input gas ratios when microcrystallines are obtained.

화학증착법에 의하여 제조된 탄화지르코늄 코팅층의 물성

The ZrC layer instead of SiC layer is a critical and essential layer in TRISO coated fuel particles since it is a protective layer against diffusion of fission products and provides mechanical strength for the fuel particle. In this study, we carried out computational simulation before actual experiment. With these simulation results, Zirconium carbide (ZrC) films were chemically vapor deposited on ZrO₂ substrate using zirconium tetrachloride (ZrCl₄), CH₄ as a source and H₂ dilution gas, respectively. The change of input gas ratio was correlated with growth rate and morphology of deposited ZrC films. The growth rate of ZrC films increased as the input gas ratio decreased. The microstructure of ZrC films was changed with input gas ratio; small granular type grain structure was exhibited at the low input gas ratio. Angular type structure of increased grain size was observed at the high input gas ratio.

In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells

We show that SiO-based intermediate reflectors (SOIRs) can be fabricated in the same reactor and with the same process gases as used for thin-film silicon solar cells. By varying input gas ratios, SOIR layers with a wide range of optical and electrical properties are obtained. The influence of the SOIR thickness in the micromorph cell is studied and current gain and losses are discussed. Initial micromorph cell efficiency of 12.2% (Voc=1.40V, fill factor=71.9%, and Jsc=12.1mA∕cm2) is achieved with top cell, SOIR, and bottom cell thicknesses of 270, 95, and 1800nm, respectively.

화학증착법을 이용한 삼중 코팅 핵연료 제조에 관한 연구

TRISO coated fuel particle is one of the most important materials for hydrogen production using HTGR (high temperature gas cooled reactors). It is composed of three isotropic layers: inner pyrolytic carbon (IPyC), silicon carbide (SiC), outer pyrolytic carbon (OPyC) layers. In this study, TRISO coated fuel particle layers were deposited through CVD process in a horizontal hot wall deposition system. Also the computational simulations of input gas velocity, temperature profile and pressure in the reaction chamber were conducted with varying process variable (i.e temperature and input gas ratios). As deposition temperature increased, microstructure, chemical composition and growth behavior changed and deposition rate increased. The simulation showed that the change of reactant states affected growth rate at each position of the susceptor. The experimental results showed a close correlation with the simulation results.

A study of the effect of excessive free carbon on mean whisker diameters grown by chemical vapor deposition

Silicon carbide whiskers were grown by chemical vapor deposition without a metallic catalyst at a temperature ranging between 1000 and 1100 °C, and at a constant pressure of 5 Torr with input gas ratios, α [H 2/MTS (methyltrichlorosilane)], of 30 to 50. The mean diameter of the whiskers changed as temperature and input gas ratios were varied. To determine why the diameter of the whiskers changed, we investigated the effect of these two parameters, temperature and gas ratio, on the stoichiometry of the deposit, both from thermodynamical calculations and from XPS measurements on the as grown whiskers. We demonstrated that the amount of free carbon increases as the temperature increases and the input gas ratio decreases. We showed the correction between the amount of free carbon and the mean whisker diameter.

Densification of SiC f/SiC composite by the multi-step of whisker growing and matrix filling

The whisker growing and matrix filling process was applied to reduce the canning effect during the CVI process. Grown whiskers may serve to divide the large natural pores between fibers or bundles and then matrix filling may be efficiently carried out through the modified pore structure. In this study, the multi-step of the in situ whisker growing and matrix filling process were performed to investigate the densification behaviors of SiC f/SiC composite. MTS (CH 3SiCl 3) and H 2 were used as source and diluent gases, respectively. The process cycles were repeated three times. The whisker growth and densification behaviors with the number of the process cycles were evaluated. SiC whiskers were grown in the SiC fiber preforms at the input gas ratio of 60 and the total reaction pressure of 5 Torr. As the number of process cycles increased, the maximum size of the large voids decreased and homogeneity of the voids distributions increased.

Influence of deposition conditions on mechanical properties of low-pressure chemical vapor deposited low-stress silicon nitride films

The effect of deposition temperature, deposition pressure, or input gas ratio (SiH2Cl2:NH3) on film stress was determined for low-pressure chemical vapor deposited silicon nitride films. Wafer curvature measurements were performed for films deposited on single crystal silicon and amorphous silica wafer substrates to determine film stress σdep, biaxial modulus Ef+, and coefficient of thermal expansion αf. Apparent plane strain film modulus Ēf′ and hardness H were measured using depth-sensing indentation. Ellipsometry was used to measure film thickness tf and refractive index n. Infrared spectroscopy, x-ray photoelectron spectroscopy (XPS), forward recoil energy spectroscopy (FReS), and Rutherford backscattering spectroscopy (RBS) experiments were performed to determine film composition. Although film deposition stress varied from −135 MPa (compressive) to 235 MPa (tensile) Ef+, Ēf′, H, and αf remained nearly constant. Infrared spectroscopy resolved only Si-N species for all films, and results from FReS on three films confirmed that the hydrogen content was negligible. RBS and XPS indicated that Si/N increased with increased compressive σdep. Ellipsometry and RBS indicated that all films were silicon-rich, to a greater extent with increased compressive σdep. As RBS indicated that atomic density decreased with increased compressive deposition stress, it was concluded that the deposition conditions changed both thermal and intrinsic deposition stress for all films. In particular, intrinsic stress was tensile, and became increasingly tensile for increased Si/N and decreased atomic density. Assuming thermal stress was similar for all films examined here, the intrinsic stress must have varied from changes dependent on the deposition conditions.

The field emission properties of silicon carbide whiskers grown by CVD

Silicon carbide whiskers and films were grown by chemical vapor deposition without a metallic catalyst in the temperature range between 1000 °C and 1150 °C, and at a constant pressure and input gas ratio[H 2/MTS(Methyltrichlorosilane)] of 5 Torr and 30, respectively. The mean diameter of whiskers increased from 96 nm to 1.24 μm as the deposition temperature increased up to 1100 °C. Further increasing of the growth temperature made the growth mechanism transfer from whisker growth to film growth and the surface morphology adopted a pebble-like structure. Silicon carbide whiskers and films showed cold field emission properties and the whiskers, which were obtained, at a temperature of 1050 °C showed relatively good field emission properties.

The microstructural effect of chemically vapor infiltrated SiC whiskered thin film on the green body of SiC/C composites

SiC films have been grown by a chemical vapor infiltration and deposition method. Different microstructures, so-called ‘whisker-type’ SiC films, have been fabricated, and the deposition and infiltration conditions of whisker growth have been studied. By connecting the SiC/C particles of the green body, fractural strength has been increased and, subsequently, the open pore structure of the body has been modified by an infiltrated whiskering process without a canning effect. Without using a metallic catalyst, silicon carbide whiskers have been obtained at an input gas ratio (H 2/MTS) above 20, and at temperatures below 1200 °C. The microstructure and composition of the whiskered SiC have been investigated by means of SEM and XPS.

Growth and microhardness of SiC films by plasma-enhanced chemical vapor deposition

SiC films were deposited on Si substrate by a plasma-enhanced chemical vapor deposition technique using a gaseous mixture of SiCl 4, CH 4, H 2 and Ar. The deposition behavior of silicon carbide films was investigated by varying the deposition temperature, RF power and input gas ratio, R x [CH 4/(CH 4+H 2)]. The plasma-enhanced chemical vapor deposition method effectively enhanced the deposition rate and crystallinity of SiC films compared with thermal chemical vapor deposition. A small amount of free silicon was co-deposited with SiC phase in this process; however, the content of free silicon was negligible on increasing the deposition temperature above 1270 °C and decreasing the R x value below 0.04. A monolithic 3C-SiC film with a hardness value of approximately 28 GPa could be obtained at a deposition temperature of 1270 °C, RF power of 60 W and an input gas ratio of 0.04.

Fabrication of silicon carbide whiskers and whisker-containing composite coatings without using a metallic catalyst

Silicon carbide has been focused on by many researchers because of its outstanding mechanical and chemical properties. Silicon carbide whiskers are important for the reinforcement of ceramic matrix composites. However, metallic catalysts, which are necessary for the growth of whiskers, can cause degradation of their properties. Thus, we have made silicon carbide whiskers without using the metallic catalyst. Whiskers were obtained with an input gas ratio of above 20, and their diameter decreased as the input gas ratio increased. As the input gas ratio [H 2(+N 2)/MTS] increases, a transition from whisker growth to film growth occurred at a higher pressure. We also deposited whisker-containing coatings based on the above conditions by alternating the whisker growth and the matrix filling process. The coatings showed pebble-like structures, and their morphologies differed according to the growth condition of the whiskers in the coating layer.

Surface morphologies and electrical properties of antimony-doped tin oxide films deposited by plasma-enhanced chemical vapor deposition

Antimony-doped tin oxide (ATO) films were deposited on Corning glass 1737 substrates by a plasma-enhanced chemical vapor deposition (PE-CVD) technique using a gas mixture of SnCl 4–SbCl 5–O 2–Ar. Electrical properties and surface morphologies of these films were studied by varying the deposition temperature, input gas ratio, R[=(P SbCl5/P SnCl4)], and RF power. The PE-CVD method effectively enhanced the deposition rate and also improved the surface roughness of the deposit compared with thermal CVD. The antimony doped tin oxide films which had relatively good electrical properties were obtained at a deposition temperature of 450°C, an input gas ratio of R=1.12, and a RF power of 30 W. In addition, the studies on the morphological development of the films by AFM analysis suggested that higher input gas ratio and lower deposition temperature led to a decrease in the surface roughness of the deposited films.

The effect of input gas ratio on the growth behavior of chemical vapor deposited SiC films

In an effort to protect a RBSC (reaction-bonded silicon carbide) reaction tube, SiC films were chemically vapor deposited on RBSC substrates. SiC films were prepared to investigate the effect of the input gas ratios (dilute ratio, α = PH2/PMTS = QH2/QMTS) on the growth behavior using MTS (metyltrichlorosilane, CH3SiCly3) as a source in hydrogen atmosphere. The growth rate of SiC films increased and then decreased with the decrease of the input gas ratio at the deposition temperature of 1250°C. The microstructure and preferred orientation of SiC films were changed with the input gas ratio; Granular type grain structure exhibited the preferred orientation of (111) plane in the high input gas ratio region (α = 3–10). Faceted columnar grain structure showed the preferred orientation of (220) plane at the low input gas ratios (α = 1–2). The growth behavior of CVD SiC films with the input gas ratio was correlated with the change of the deposition mechanism from surface kinetics to mass transfer.