SiC Filaments Research Articles

Evolution of mechanical-acoustic-electrical properties on SiC filament underlaying an oxidation treatment

Accurately obtaining the acoustic emission (AE) and electrical resistance (ER) responses of SiC filaments in the damage process is one of the keys to realizing the damage monitoring of SiC/SiC composite structures. A novel mechanical-electrical coupling specimen for fiber filament was designed based on the strength test strategy. During tensile failure, the difference in AE and ER responses between original and oxidation-treated SiC filaments was studied. The ER monitoring results show that the initial resistivity of SiC filaments is negatively correlated with their tensile strength. With increasing oxidation treatment temperature, the tensile strength of SiC filaments gradually decreases while the initial resistivity gradually increases. The AE monitoring results show that the AE signal of SiC fiber fracture has the characteristics of high amplitude, high energy, long duration, short rise time, and low peak frequency. A strong positive correlation is observed between the amplitude and energy of the AE signals of fiber fracture and the tensile strength of SiC filaments; the greater the tensile strength, the greater the amplitude, duration, and energy of the AE signals generated when the fiber fracture. In addition, the tensile modulus and resistance sensitivity coefficient of SiC filaments were measured.

Investigation of Specimen Size Effects on P-Quantile Diagrams and Normal Distributions of Critical Flaw Strengths in Fiber Tows

The present paper proposes a model of the specimen size effect on the critical flaw strength distribution in fiber tows for composite reinforcement. The model is based on the basic assumption of brittle fracture that the failure probability at a given strength increases with specimen size in the p-quantile vs. strength relation and on the normal distribution. Empirical results derived from force–strain curves determined on tows made of 1000 and 500 Nicalon SiC filaments and with various gauge lengths show some discrepancy with predictions using the model. The empirical p-quantile diagrams and cumulative distributions of critical flaw strengths exhibited excellent reproducibility at longer gauge lengths, which suggests the absence of a size effect above a critical tow size. The reproducibility of flaw strength distributions at gauge lengths above 60 mm and the higher strengths obtained at lower gauge lengths despite structural effects were related to the features of the critical flaw distribution in tows of parallel fibers.

Influence of Supersaturation on Growth Behavior and Mechanical Properties of Polycrystalline 3C-SiC on W Wire Substrate

As an important reinforcement for metal matrix composites, the microstructure and mechanical properties of W-core SiC filament have drawn increasing attentions among researchers. In this work, the growth behavior of polycrystalline 3C-SiC on W-wire substrate in the chemical vapor deposition (CVD) process and the evolution of mechanical properties in preparation of W-core SiC filament, were investigated as a function of gas-phase supersaturation. Kinetic studies revealed that the growth of 3C-SiC grains was limited by surface reactions at both 850 °C and 1050 °C, and the deposit experienced similar morphological changes from a porous structure to large clusters, with the increase in supersaturation. Structural analyses and mechanical tests show that the production of pores and the amorphous phase with a low supersaturation, of 9.6 × 107 at 850 °C, resulted in a reduction in the modulus and hardness of the polycrystalline deposits, to 270.3 GPa and 33.9 GPa, while the reduced structural defects (e.g., stacking faults and twins) in highly (111) orientated 3C-SiC grains, as well as the improved surface quality obtained with the medium supersaturation of 1.6 × 107 at 1050 °C, enhanced the tensile strength and the Weibull modulus of W-core SiC filament to 2.88 GPa and 11.2, respectively. During the growth of 3C-SiC grains, the variation in structural defects density is controlled by the critical nucleation energy of the two-dimensional (2D) nucleus.

Wet-chemical coating of silicon carbide fibers with hexagonal boron nitride layers

Hexagonal BN fiber coatings and BN powders were prepared by pyrolysis of the raw materials boric acid and urea in an atmosphere consisting of hydrogen and nitrogen. The powders were used to determine the appropriate mixing ratio of the raw materials to produce BN with the desired composition and crystal structure. The pyrolysis of boric acid and urea in a molar mixing ratio of 1:2 resulted in a BN that was hexagonal and had a near-stoichiometric composition.To prepare a solution for the coating of fibers, boric acid and urea were dissolved in an ethanol-water mixture. The coating was then applied to SiC filaments using a continuous roll-to-roll dip-coating process. It could be shown by SEM/EDS that BN layers were applied to the fibers. No significant bridging in the fiber bundle was found. Furthermore, it could be demonstrated by grazing incidence x-ray diffraction that the layers were crystalline.

Influence of silicon carbide reinforcement on mechanical behavior of 3D printed polymer composites

Silicon Carbide reinforced ABS based polymer composite has been developed by Fused Deposition Modelling (FDM) technology. The percentage of SiC was varied between 0 and 4%wt in steps of 2%wt. Twin screw extrusion technique was employed to develop ABS + SiC filament suitable for FDM printing process. 3D printed pure ABS and ABS + SiC composites were subjected to tensile test as per ASTM standard. Results shows the SiC filled ABS composite provides remarkable improvement in ultimate tensile strength compared to pure ABS. Further, Ultimate tensile strength increases with increase in SiC content.

Temperature and CH* measurements and simulations of laminar premixed ethylene jet-wall stagnation flames

New experimental 2D measurements are reported to characterise the flame location, shape and temperature of laminar premixed ethylene jet-wall stagnation flames when the equivalence ratio, exit gas velocity and burner-plate separation distance are varied. Bandpass-filtered optical measurements of the CH* chemiluminescence were used to provide information about the shape and location of the flames. Thin filament pyrometry (TFP) using a 14 µm diameter SiC filament was used to make line measurements of the temperature to reconstruct the full 2D temperature field for the first time in premixed, jet-wall stagnation flames. The comparison of CH* measurements with (intrusive) and without (non-intrusive) the presence of the SiC filament showed that the filament resulted in minimal disturbance of the flame when the filament was placed downstream of the flame front. However, the flame was observed to attach to the filament, resulting in more significant disturbance, when it was placed upstream of the flame front. The flames were simulated using both 1D and 2D models. The 2D simulations were used to provide estimates of the velocity, kinematic viscosity and thermal conductivity required to calculate the gas temperature from the TFP data. The 1D simulations showed excellent agreement with the experimentally observed centreline quantities, but required the strain boundary condition to be fitted in order to match the experimentally observed flame location. The 2D simulations showed excellent agreement without the need for any fitting, and correctly predicted the flame shape, location and temperature as the experimental conditions were varied. A comparison of the set of simulated temperature-residence times along different streamlines showed relatively uniform distributions within each flame. However, the most uniform set of temperature-residence time distributions did not correlate with the flattest flame.

Microstructure and mechanical property of high growth rate SiC via continuous hot‐wire CVD

AbstractAn average growth rate of SiC at 3.4‐28.5 μm/min can be achieved via continuous hot‐wire CVD method with input powers of 300‐380 W. Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS) analysis, and X‐ray diffraction (XRD) method, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied to investigate the microstructure of deposits. At 300 W, amorphous SiC and Si were main products in deposit, which were rapidly replaced by crystallite SiC containing high density stacking faults at 340 W and above. Moreover, with the grain size of SiC increasing from ~25 to ~420 nm, the stacking faults probability decreasing from 0.179 to 0.125, as well as the surface morphology changed from loose‐packed granules to a well‐defined faceted structure with strong (111) texture. Structural changes led to the increase of deposit's Young's modulus from 266.3 to 341.5 GPa, and the mean tensile strength of SiC filament from 1.57 to 3.03 GPa. The successive growth of W/SiC interfacial layer above 360 W resulted in the reduction in mean tensile strength and Weibull moduls of SiC monofilaments, which agrees with the prediction from critical interfacial layer thickness theory.

Mechanical properties of the SiCf/SiC composites reinforced with KD-I and KD-II fibers fabricated assisted by a microwave heating method

Abstract SiCf/SiC composites using KD-I and KD-II SiC fibers braided preforms as the reinforcements were fabricated by applying the polymer impregnation and pyrolysis (PIP) technique with a microwave heating assistance. The microwave heating temperature was 1100 °C, 1200 °C, 1300 °C, and 1400 °C, respectively. Microstructures, flexure properties, and fracture behaviors of the composites were investigated. The KD II SiCf/SiC composites exhibited higher flexure properties and improved non-brittle fracture characteristics than those of the KD-I SiCf/SiC composites. The differences in the flexural properties, fracture behaviors and microstructures between the KD-I and KD II SiCf/SiC composites were discussed based on the tensile properties of the SiC filaments and the interfacial bonding statues in the composites.

Combustion characteristics of butanol isomers in multiphase droplet configurations

This study reports results of experiments on the isolated droplet burning characteristics of butanol isomers (n-, iso-, sec-, and tert-) under standard atmosphere conditions in an environment that promotes spherical combustion. The data are compared with predictions from a detailed numerical model (DNM) that incorporates complex combustion chemistry, radiative heat transfer, temperature dependent variable fluid properties, and unsteady gas and liquid transport. Computational predictions are generated using the high temperature kinetic models of Sarathy et al. (2012) and Merchant et al. (2013).The experiments were performed in a free-fall facility to reduce the effects of buoyancy and produce spherical droplet flames. Motion of single droplets with diameters ranged from 0.52mm to 0.56mm was eliminated by tethering them to two small-diameter SiC filaments (∼14µm diameter). In all the experiments, minimal sooting was observed, offering the opportunity for direct comparison of the experimental measurements with DNM predictions that neglect soot kinetics.The experimental data showed that the burning rates of iso- and sec-butanol are very close to that of n-butanol, differing only in flame structure. The flame stand-off ratios (FSR) for n-butanol flames are smaller than those for the isomers, while tert-butanol flames exhibited the largest FSR. DNM predictions based upon the kinetic model of Sarathy et al. over-predict the droplet burning rates and FSRs of all the isomers except n-butanol. Predictions using a kinetic model based upon the work of Merchant et al. agree much better with the experimental data, though relatively higher discrepancies are evident for tert-butanol simulation results. Further analyses of the predictions using the two kinetic models and their differences are discussed. It is found that the disparity in transport coefficients for isomer specific species for Sarathy et al. model fosters deviation in computational predictions against these newly acquired droplet combustion data presented in this study.

Correlation between microstructures of SiC-reinforced titanium matrix composite and liquid route processing parameters

A new procedure for filamentary metal matrix composite processing is described here. It consists in running carbon-coated SiC filaments through a liquid titanium bath in levitation. The liquid metal/carbon interaction must be significant enough to enable filament wetting and sufficiently low to avoid composite embrittlement. To insure both requirements, different configurations of the initial ceramic filament can be used: (1) SiC(C) filament free of any other coating, (2) SiC(C) filament coated with a carbide obtained by reactive chemical vapour deposition (R-CVD), or (3) SiC(C) previously coated by a first metal layer. In order to choose the best conditions for developing the process, the different processing configurations were studied through modelling and numerical simulations of the filament/matrix interaction. The microstructure of the interfacial zone between filament and matrix was investigated through SEM and Auger electron spectroscopy (AES) analyses. The microstructure of the interfacial zone between filament and matrix was investigated through SEM and AES analyses. The results show that in comparison to the first processing configuration, the best way to obtain filamentary composite semi-products without excessive fibre/matrix interaction is to use the second configuration. However, the latter requires preliminary R-CVD operations, while the third configuration leads to moderate carbon embrittlement effect without requiring additional equipment.

Morphological analysis of zirconium nuclear fuel retaining rods braided with SiC: Quality assurance and defect identification

In the after-Fukushima world, the stability of materials under extreme conditions is an important issue for the safety of nuclear reactors. Among the methods explored currently to improve zircaloys’ thermal stability in off-normal conditions, using a protective coat of the SiC filaments is considered because silicon carbide is well known for its remarkable chemical inertness at high temperatures. A typical SiC fiber contains ∼50,000 individual filaments of 5–10μm in diameter. In this paper, an effort was made to develop and apply mathematical morphology to the process of automatic defect identification in Zircaloy-4 rods braided with the protective layer of the silicon carbide filament. However, the issues of the braiding quality have to be addressed to ensure its full protective potential. We present the original mathematical morphology algorithms that allow solving this problem of quality assurance successfully. In nuclear industry, such algorithms are used for the first time, and could be easily generalized to the case of automated continuous monitoring for defect identification in the future.

Study on the influence of reaction temperature on the preparation of C-Core SiC filaments

SiC filaments were widely used as reinforcements in metal-matrix and ceramic-matrix composites. C-core SiC filaments were prepared by CVD process using DC electrically heating for the first time in domestic. The effects of reaction temperature on the morphologies, deposition rate, microstructure, grain size and composition for the C-core SiC filaments coating prepared by CVD process in CH3SiHCl2-H2-Ar system, were explored by means of SEM, XRD, Raman, and AES. The experimental results show that with increasing reaction temperature the deposition rate increases rapidly in an exponential manner, surface kinetic processes become stronger, and Si/C atomic ratio decreases. At low temperatures, Si phase co-deposites with SiC, inhibiting the grain growth, while at high temperatures, the grain growth results in rough surface, and the stress changed from tensile to compressive.

Investigation of Defects in Solar Cells and Wafers by Means of Magnetic Measurements

Elaborated characterization tools play an important role for the further improvement of solar material and the development of solar cells. Besides the huge variety of highly advanced methods mainly based on optical and electrical measurements, the direct measurement of surface currents by the detection of their induced magnetic fields has gained less attention. The novel method current-analysis-by-inductive-coils (CAIC) based on an inductive coil detector and is reviewed and compared with already established methods, which are light-beam-induced-current (LBIC) and dark-lock-in-thermography (LIT). The detector reveals complementary information at high resolution. The LIT measurements depicted shunting defects in forward and reverse current. Because of the high spatial resolution of the CAIC measurement technique it was determinable that in some cases the positions of current sinks in the CAIC maps are not corresponding with the microstructure. The analyses of the superpositions reveals macroscopic precipitates like SiC and Si3N4 filaments and clusters as an origin of some of the shunts.

Effect of Carbon Coating on Tensile Strength of SiC Filament by Chemical Vapor Deposition

The tensile strengths of carbon-coated and non carbon-coated SiC filament by Chemical Vapor Deposition were tested, respectively, which were analyzed according to double-parameter Weibull distribution. Various techniques including XRD and SEM were also used to study the phase composition and microstructure of SiC filament. The result shows that carbon coating plays a very important role on increasing the tensile strength.

Efficient methods for detection of SiC and Si3N4 precipitates and filaments in multi‐crystalline silicon wafers and solar cells

AbstractTwo easy and efficient methods are demonstrated to detect SiC and Si3N4 filaments and precipitates in multi‐crystalline silicon (mc‐Si) wafers and solar cells by Optical Light (OLM) Microscopy and Scanning Electron Microscopy (SEM). In the case of OLM the image is created in the dark field mode which clearly visualizes the grain boundaries decorated by filaments in acidic etched silicon solar cells. In the case of SEM the image is created by a combination of Secondary Electrons (SE) imaging and Back Scattered Electrons (BSE) imaging. By using this method a clear discrimination of precipitates with different chemical compositions is possible. Note that both methods make it possible to investigate and analyze relatively large sample areas in a comparatively short period of time. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Substrate Material on Tensile Behavior and Fracture Characteristics of SiC by Chemical Vapour Deposition

SiC filament was prepared by Chemical Vapour Deposition method using W wire as the substance material. The combination between W wire and SiC became a key element influencing tensile strength of SiC. In this paper, tensile fracture morphology of SiC filament was analyzed, from which interface combination between SiC and W wire was discussed under different deposition temperature and the influence of interface layer’s thickness on final tensile of SiC filament was given. How to control the thickness of interface layer is a key factor for improving performance of SiC filament.

Influence of DepositionTemperature on Tensile Behavior and Fracture Characteristics of SiC Filament by Chemical Vapor Deposition

CH3SiCl3-H2-Ar system was adopted to prepare SiC Filament of high tensile strength by high-frequency induction during Chemical Vapor Deposition (CVD) method. Room-temperature tensile strength of SiC filament prepared under different temperature was test. Crystal structure, surface morphology and fracture character of SiC filament was analyzed with “X-ray diffractometer and scanning electron microscope. Influence of temperature to final tensile behavior of SiC filament was discussed synthetically. The result shows that tensile strength is very sensitive to temperature. From 1000°C to 1400°C, tensile strength increases and then decreases as temperature increases. At 1200°C, tensile strength reaches its max value, the reason ofwhich is composed of three parts. The first one is that the deposited product is β-SiC. The second one is that surface of SiC filament is smooth. The third one is that fracture surface shows ductile dimples fracture, brittle fracture and cleavage fracture patterns. 1200°Cis considered as one reasonable temperature in our laboratory to prepare SiC filament.

Soot and thin-filament pyrometry using a color digital camera

A simple and compact temperature and soot volume fraction diagnostic technique based on ratio pyrometry has been studied. Two different consumer digital single lens reflex cameras were evaluated for use as pyrometers. The incandescence from soot and a SiC filament was imaged at the three wavelengths of each camera’s color filter array (CFA). After characterization of the detector’s signal response curves, temperatures were calculated by two-color ratio pyrometry using a lookup table approach. A SiC filament with known emissivity was shown to provide an absolute light intensity calibration, which further allows the soot volume fraction to be determined. Measurements were carried out on four different flames with varying levels of soot loading. The filament-derived gas temperature and soot temperature measurements have been compared with computational results and overall good agreement has been shown. Soot volume fraction measurements have been compared with previous LII results, with excellent agreement for both cameras tested.

Heat and Mass Transfer Modeling and Simulation during Liquid Route Processing of SiC/Ti Filamentary Composites

To process SiC/Ti filamentary composites using a liquid route method, it is first necessary to overcome various major difficulties such as, high speed filament/matrix coupling, liquid titanium wetting of filament surfaces, and reduction of filament/matrix interaction. All of these requirements depend mainly on the heat and mass transfer, which occurs as the filament runs through a liquid titanium bath. Consequently, these transfers were modeled and simulated numerically during the different processing steps, particularly the cooling step. The results describe the physical phenomena which occur during the process: the carbon transfer from the carbon coated SiC filament to the liquid titanium, heat exchanges, formation of the TiC interphase at the filament surface, and, finally, the solidification of the titanium coating. Numerical simulation has shown the strong influence of running speed which governs the wettability of the filament by the liquid metal. Furthermore, the effects of an additional specific cooling device have been highlighted.

A TEM study of SiC particles and filaments precipitated in multicrystalline Si for solar cells

The microstructure of SiC particles and SiC filament-type precipitates found in block-cast multicrystalline Si was studied in detail by transmission electron microscopy (TEM). TEM investigations showed that the SiC particles are single crystalline and the SiC filaments are microcrystalline. Both types of precipitates consist of cubic SiC. However, a high density of planar defects was found in the filaments. Very wavy and rough interface between SiC filaments and silicon (Si) was revealed by high-resolution TEM. In addition, SiC filaments do not show a special orientation relationship with respect to the Si matrix. The growth mechanisms of SiC precipitates are discussed. Finally, the influence of SiC inclusions in terms of device performance is considered.