Thermal stability of low-oxygen silicon carbide fibers (Hi-Nicalon) in carbon monoxide

The effect of CO treatments on thermal stability of low-oxygen SiC fibers (Hi-Nicalon) was examined at 1273–1773 K using mass change measurements, X-ray diffraction (XRD) analysis, Auger electron spectroscopy (AES) analysis, resistivity measurements, scanning electron microscopy (SEM) observation and tensile tests. The fiber properties remained unchanged by heating below 1573 K. In addition to the grain growth of SiC, reduction of resistivity and degradation of strength above 1573 K, mass loss was observed above 1673 K. AES analysis showed carbon film formation on fiber surfaces at high temperature. The carbon film was formed by the following reaction:CO(g) + SiC(s) = SiO(g) + 2C(s)A Tensile strength of 1.83 GPa was retained even after exposure at 1773 K for 10 h, owing to the suppressing effect of the carbon film on the thermal decomposition of SiC _X O _Y phase.     

Effect of CO2 partial pressure on oxidation of low-oxygen SiC fibers (Hi-Nicalon) in Ar-CO2 gas mixtures

The oxidation behavior and thermal stability of Si—C fibers (Hi-Nicalon) in Ar-CO₂ gas mixtures were investigated at 1773 K, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile tests. Mass gain and cristobalite formation were observed at p _{CO 2} 10³ Pa, showing the occurrence of passive-oxidation of the fibers. On the other hand, the active-oxidation was characterized by the mass loss, no formation of SiO₂ film and a marked increase in resistivity at p _{CO 2} 5 × 10² Pa. The oxygen potential for the active-to-passive oxidation transition in Ar-CO₂ gas mixtures was nearly identical to that in Ar-O₂ gas mixtures. About 50% of the strength in the as-received state was retained after the active-oxidation in Ar-CO₂ gas mixtures.     

Silica coating of vapor grown carbon fibers

Silica coatings have been applied to vapor grown carbon fibers (VGCFs) by a liquid phase deposition process. Unlike the coating of single walled carbon nanotubes, the addition of a surfactant to “solubilize” the VGCFs results in an extremely non-uniform coating consisting of spherical silica aggregated around the tubes. As was observed for fullerenes, hydroxylation of the surface of the VGCF appears to be key to the formation of a uniform silica coating. Irrespective of the type of VGCF, heating a suspension of VGCFs in nitric acid to dryness (Type II) gave us the best results in terms of silica growth around the VGCF and there is a correlation between the percent of hydroxyls present on the VGCF surface and on the type of growth that occurs on the VGCF. Nitric acid treatment of VGCFs for 1 day in solution were precipitated with acetone (Type III treatment), and then coated with silica. This acid treatment made the coated fibers highly soluble in EtOH.     

High temperature exposure of low-oxygen SiC fibers (Hi-Nicalon) in CO-CO2 gas mixtures

The oxidation behavior and thermal stability of Si-C fibers (Hi-Nicalon) in CO-CO₂ gas mixtures were investigated at 1773 K, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile test. Mass gain and cristobalite formation were observed at 2 × 10³ Pa, showing the occurrence of passive-oxidation of the fibers. The active-oxidation, which was characterized by the mass loss, involved the formation of carbon film at 5 × 10² Pa. 65–90% of the strength in the as-received state was retained after the active-oxidation at 10² Pa and after the passive-oxidation at 5 × 10⁴ Pa. Surface imperfections produced very large decrease in tensile strength in the CO₂ partial pressure range of 5×10² to 10⁴ Pa.     

碳纤维表面电镀镍研究

采用连续电镀法对碳纤维进行镀镍处理,利用扫描电子显微镜、能谱仪和热重分析等方法研究了镀镍碳纤维的物理化学结构及性能的变化.结果表明:镀层为纯净的镍,结合强度较好;镀层厚度的增加与电镀时间及电流密度分别呈线性正比关系.镀镍碳纤维的导电性和抗氧化性能得到了明显提高,电阻率可降低到0.74×10-6Ω·m,起始氧化温度提高了...     

Characterisation of thin silica films deposited on carbon fibre by an atmospheric pressure non-equilibrium plasma (APNEP)

Low modulus carbon fibres were used to investigate the potential of a low cost novel atmospheric pressure microwave plasma technique for continuously depositing silica coatings onto carbon fibres, in a tow form. The objective was to improve the interfacial properties of aluminium/carbon fibre composites produced by liquid metal infiltration techniques. Amorphous silica coatings, approximately 50–400 nm thick, were successfully produced in a continuous process. The nature and morphology of the coatings were determined using transmission electron microscopy. Squeeze cast and gas pressure infiltrated samples were manufactured to investigate the fibre/matrix interface using tensile tests and a short beam interlaminar shear test. The tensile samples displayed brittle fracture with ultimate tensile stresses of 223 and 251 MPa for the uncoated and coated fibres, respectively. The shear samples did not show any interlaminar shear, but failed in tension. It is concluded that the silica coating did not have a significant effect on improving the carbon fibre/aluminium matrix interface. However, the microwave plasma technique was very successful and can be modified to produce low cost alternative coatings for future development.     

Processing of pharmaceutical materials by electrospraying under reduced pressure

Context: Electrospraying was used in drug particle production.

Objective: The aim of the research was to evaluate the possibilities to produce drug particles with desired pharmaceutical properties by electrospraying. In particular, the effect of drying pressure on particle properties was studied.

Materials and methods: A poorly water soluble model drug (budesonide) was dissolved in chloroform, and the solution was atomized by electrospraying. Following this, the charged droplets were neutralized and dried in a drying chamber. The pressure in the drying chamber was varied. The dried particles were collected and analyzed.

Results: The pressure reduction had a slight impact on particle size distribution. The particles produced in reduced pressure turned out to be notably more porous than the particles produced in atmospheric pressure. The pressure reduction also affects the degree of crystallinity of the product. The dissolution of the particles produced in reduced pressures was faster to a certain extent than that of the particles produced in atmospheric pressure.

Discussion and conclusions: A setup for electrospraying materials in a reduced pressure was presented. The pressure reduction had a notable impact on particle morphology. The possibilities to tailor the particle properties during electrospraying were studied.     

Thermal stability of SiO2-coated SiC fiber (Hi-Nicalon) under argon atmosphere

Thermal stability of low-oxygen SiC fiber (Hi-Nicalon) coated with SiO₂ film was investigated. The SiO₂ film of same thickness but different crystal structure was formed by heating low-oxygen SiC fiber (Hi-Nicalon) under different oxidation conditions. The oxidation treatment and the subsequent exposure at 1773 K in argon caused very little loss of strength for unoxidized core. For as-oxidized fiber with SiO₂ film which containedimperfections, further loss of strength was caused after exposure in argon. There was little degradation of core strength on being exposed repeatedly at rapid heating and cooling rate in argon. The fiber oxidized at 1773 K kept high level of strength even after exposure at 1823 K. This is because the change in crystal structure of SiO₂ film before and after exposure in argon, which was the controlling factor in the degradation of strength, was diminished with increasing oxidation temperature.     

碳纤维表面磁控溅射镀铜研究

以碳纤维T- 700为原料,对其进行低温等离子预处理,实验表明,尽管预处理对碳纤维拉伸强度有一定影响,但表面粗糙度增加.经预处理后的碳纤维在室温下采用磁控溅射法在其表面溅射镀铜,利用扫描电子显微镜( SEM)、原子力显微镜(AFM)分别对镀铜碳纤维的表面形态进行表征,用VC9804A+型万用电表测量了碳纤维的导电性能....     

Improvement of anti-oxidation properties of carbon fibers by SiC/SiO2 ceramic coating

To improve the anti-oxidation properties of carbon fibers (CFs), the sol-gel method followed by pyrolysis was used to coat CFs with SiC/SiO2 ceramic coatings. The SiO2 sol-gel coating was performed by dip coating a PAN(polyacrylonitrile)-based stabilized fiber (PSF) in a silica sol prepared by the polycondensation of tetraethylorthosilicate (TEOS) in the presence of an acidic catalyst. The PSF coated with SiO2 sol then underwent heat treatments at high temperatures in an inert atmosphere to deposit the SiC/SiO2 and carbonize the deposited fibers. The surface morphology of the CFs deposited with SiC/SiO2 was characterized using a scanning electron microscope (SEM). The relative oxidation resistance of the SiC/SiO2 layer deposited on the CFs was determined by the weight loss due to the use of a thermogravimetric analyzer (TGA) under flowing air, and the data were used to calculate the activation energies through an Arrhenius plot.     

Transformation pathways of silica under high pressure

Network-forming oxides with rigid polyhedral building blocks often possess significant capacity for densification under pressure owing to their open structures. The high-pressure behaviour of these oxides is key to the mechanical properties of engineering materials and geological processes in the Earth's interior. Concurrent molecular-dynamics simulations and first-principles calculations reveal that this densification follows a ubiquitous two-stage mechanism. First, a compact high-symmetry anion sublattice forms, as controlled by strong repulsion between the large oxygen anions, and second, cations redistribute onto the newly created interstices. The same mechanism is observed for two different polymorphs of silica, and in the particular case of cristobalite, is corroborated by the experimental finding of a previously unidentified metastable phase. Our simulations not only clarify the nature of this phase, but also identify its occurrence as key evidence in support of this densification mechanism.     

The individual effect of coating thicknesses and deposition temperatures on Raman spectra of carbon films deposited on silica glass fibers by thermal chemical vapor deposition

Abstract

Carbon films are deposited on silica glass fibers by thermal chemical vapor deposition using pure methane as the precursor gas, and the individual effect of coating thicknesses and deposition temperatures on Raman spectra of carbon films is investigated. The results show that if the temperature is fixed at 950°C, the D peak position, the full‐width‐at‐half maximum of D band, and the integrated intensity ratio of the D band to the G band increase with increasing the coating thickness. This is because the size and number of particles grown on the carbon film surface increase with increasing the coating thickness and the carbon film becomes more disordered. Alternatively, if the coating thickness is fixed at 1200 nm, the D peak position, the full‐width‐at‐half maximum of D band, and the integrated intensity ratio of the D band to the G band decrease with increasing the deposition temperature. This indicates that the carbon film becomes more ordered and its nano‐grain size increases as the deposition temperature increases from 925 to 1025°C.     

Development of vapor grown carbon fibers (VGCF) reinforced carbon/carbon composites

C/C composites are developed using vapor grown carbon fibers (VGCF) with two types of pitches as matrix precursor. The composites are carbonized at 1000°C by applying the isostatic pressure throughout the carbonization process and further heat treated at different temperatures up to 2500°C in the inert atmosphere. By applying iso-static pressure one can able to developed VGCF based C/C composites possessing the very high bulk density (1.80 g/cm³) and apparent density (2.01 g/cm³) only by heat treatment up to 2500°C without any densification cycle. This high value of density is due to the extremely strong fiber-matrix interactions and self sintering between the VGCF fibers during carbonization process under iso-static pressure. From the SEM study it reveals that, fiber-matrix interactions are strong and fiber boundaries merges with each other, also there is not a evidence of matrix shrinkage cracks in case 1500°C heat treated composites. On the other hand, in 2500°C heat treated composites, there is evidence of uniform fiber-matrix interfacial cracks and porosity in nanometer dimensions. This is due to the change in fiber morphology above HTT 1500°C. But the formation of nano width cracks does not affect on the mechanical properties of composites. The compressive strength increases from 95MPa of 1500°C to 105 MPa of 2500°C heat treated composites. However, hardness decreases due to the increase in the degree of graphitization of composites on 2500°C. The study reveals that by controlling processing condition and the uniform dispersion of VGCF fibers in the matrix phase, it can be possible to developed nano porosity at fiber-matrix interface.     

Fabrication of titania-coated silica fibers and effect of substrate shape on coating growth rate

Titania coating was fabricated on fused silica glass fibers of 4-6 μm in diameter by the hydrolysis of Ti-tetraethoxide in ethanol at 20 °C. Changes in the coating thickness with the deposition time were examined by scanning electron microscopy and compared with those on flat soda-lime glass substrate examined by atomic force microscopy. Uniform titania coating was obtained, but severe cracking occurred on coatings thicker than 150 nm due to isotropic shrinkage during the drying process. The faster growth rate of coating on fiber than that on flat substrate could be explained by employing an ‘adhesion growth model.’ X-ray diffractometry showed that titania coating on fused silica glass plate was transformed to anatase-phase TiO₂ by annealing in air at 300 °C. This indicates that the titania coating on a fused silica glass substrate becomes crystalline after annealing at that temperature.     

Evolution of strength silica optical fibers under various moisture conditions

The reliability and the expected lifetime of optical links are closely related to the chemical action of the water molecules on silica network. However, polymeric coating also contributes largely to the mechanical properties of the fiber. Dynamic fatigue measurements were performed on silica optical fibers, either bare or coated, using two-point bending bench. Water concentration in test atmosphere was 50% relative humidity or very dry atmosphere. When water content is a few ppm, the dynamic fatigue curve, which expresses the breaking strength as a function of stress rate deviates from linearity at high stress rate. The same behaviour is observed in ambient atmosphere for very high testing speeds. The comparison between bare and coated fibers shows that the fatigue behaviour at high stressing rate is influenced by the polymer coating.     

Investigations on coating durability and tenso-resistive effect of carbon black-coated polycaprolactam fibers

A novel coating method (dissolving-coating method) is designed to provide electrically conductive fibers with lower electrical resistivity and permanent conductivity. In the dissolving-coating method, no adhesive was used, but a solvent of fiber substrate was used. Carbon black (CB)-coated polycaprolactam (PA6) fibers application for electrically conducting sensor were prepared by the new coating method. The durability of CB coatings on PA6 fibers was tested through washing and stretching test. To make a study on the electromechanical behavior of the CB-coated PA6 fibers under tensile load, the effects of various factors that are responsible that correlates the resistance change with the applied strain, strain rate (S.R), the environmental temperature, and the relative humidity (RH) were discussed.     

Catalytic graphitization of carbon fibers with electrodeposited Ni–B alloy coating

Catalytic graphitization of carbon fibers with electrodeposited Ni–B alloy coating was studied. Carbon fibers with and without electrodeposited Ni–B coating were heat treated at different temperatures and the structural changes were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscope (SEM). The results show that much better graphitization can be achieved for carbon fibers at lower heat treatment temperature (HTT) when the Ni–B coating is present. Moreover, the degree of graphitization of the Ni–B-coated carbon fibers increases with the increasing of B content in the Ni–B coating at the same HTT. The mechanism of catalytic graphitization was discussed.     

Effect of reduced pressure on oxidation and thermal stability of polycarbosilane-derived SiC fibers

To clarify the effects of reduced pressure on the thermal stability of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon and Hi-Nicalon S), the heat-treatments were conducted at 1623 and 1723 K under total pressures (p _T) of 1–10⁵ Pa. The oxidation behavior and thermal stability of the fibers were investigated through examinations of mass change, grain growth, specific resistivity, fiber morphology and tensile strength. All the fibers were definitely oxidized in the active-oxidation regime at p _T 10² Pa and T = 1723 K. Nicalon and Hi-Nicalon S were subjected to serious degradation of fiber strength. Hi-Nicalon had a strength of 1.2 GPa even after heat-treatment at p _T = 1 Pa.     

Characteristics of vibro-fluidization for fine powder under reduced pressure

A cohesive powder (Geldart group C) was fluidized under reduced pressure (P = 0:5 - 10 kPa) with vibration. The fluidized powder was composed of glass beads 6 μm in diameter. The bed pressure drop was measured by decreasing gas velocity and the flow patterns in the bed were observed. A slanting particle flow, which was not observed at atmospheric pressure in a previous study, appeared at a lower pressure than about P = 20 kPa and with a larger vibration strength than the critical vibration strength, A_cr. Under the above conditions, the pressure drop curve changed abnormally due to the occurrence of this slanting particle flow. On the other hand, when the vibration strength was smaller than A_cr, a typical pressure drop curve was obtained. In light of these results, the interrelation between the slanting particle flow and the change in the pressure drop curve was examined.