Pyrocarbon Layer Research Articles

Kinetics of pyrocarbon formation on the surface of carbon fiber filament

A kinetic model of the formation of a pyrocarbon layer on the surface of filaments of carbonized PAN fiber is proposed. The resulting kinetic model is confirmed by experimental data. Models for the formation of a pyrocarbon layer and an increasing of thickness of a pyrocarbon layer on a bundle of carbon fiber filaments after a long cycle of CVD are predicted.

Pyrocarbon Coating on Granular Al2O3 for HTGR-Type Power Reactor

Fourth-generation nuclear power systems are based on high-temperature gas-cooled reactors, in which the pebble fuel is the primary energy carrier. In this regard, applying protective pyrocarbon coatings on granulated fuel is an essential problem in ensuring the reliability of nuclear power plants. The article’s main idea is to research rational technological parameters of forming a pyrocarbon protective coating on the granules of a nuclear fuel model. For this purpose, granulated Al2O3 with the protective pyrocarbone coating was applied as a fuel model. The article’s aim is to study the effect of thermophysical parameters on applying a protective pyrocarbon coating on granulated Al2O3. During the experimental studies, thermal imaging of the pyrolysis process was used. The scientific novelty of the work is the equilibrium curves for the systems Al2O3:CH4, Al2O3:CH4:N2, and Al2O3:CH4:Ar. Their analysis allowed for evaluating rational thermochemical parameters of the pyrolysis process. As a result, rational thermophysical parameters of coating granulated Al2O3 with a pyrocarbon layer were evaluated, and the practical possibility of applying the pyrocarbon coating to granulated Al2O3 in the electrothermal fluidized bed was experimentally proven. It was shown that nitrogen does not significantly affect the target reaction product under a temperature of less than 1500 K. Also, the rational conditions for the pyrocarbon coating at a pressure of 0.1 MPa were realized at a temperature of 900–1500 K and using argon. Moreover, pyrocarbon was precipitated from hydrocarbon at 1073–1273 K. Overall, the need to add an inert gas for reducing the carbon black formation was proven to prevent a reduction of natural gas efficiency.

Microstructural heterogeneity of the buffer layer of TRISO nuclear fuel particles

Tristructural isotropic (TRISO) nuclear fuel particles contain a layered spherical shell designed to retain fission products; however, failure occurs in rare cases—commonly initiated in the porous pyrocarbon buffer layer. Achieving a comprehensive understanding of the buffer-initiated failure mechanisms requires detailed characterization of the buffer porosity and its heterogeneous distribution across multiple length scales. We performed FIB-SEM tomography across the buffer layer thickness (∼100 µm) to produce 3D reconstructions of the buffer microstructure with 50 nm spatial resolution. We found an average overall porosity of ∼14%, which does not solely account for the low density of the buffer (50% of the theoretical density). Additionally, the local porosity and its fluctuation increase from the kernel interface towards the inner pyrocarbon (IPyC) layer, which we attribute to the chemical vapor deposition process conditions during the TRISO particle fabrication. Detailed characterization of the porous microstructure—including analysis of the pore size, distribution, shape, and orientation—provides insight into the process-structure-property-performance relations of TRISO nuclear fuel particles and will inform multiscale models designed to predict the failure of TRISO particles under irradiation.

Comparison of Adsorbents Containing Carbon Nanotubes for Express Pre-Concentration of Volatile Organic Compounds from the Air Flow

New composite adsorbents including silica supports (silica, aerosilogel, and diatomite) and carbon materials (multiwall carbon nanotubes and pyrolytic carbon) have been prepared and characterized. The analytical capabilities of the produced sorbents have been evaluated by their efficiency in the express pre-concentration of volatile organic compounds (butanol and phenols) from the air stream. The prepared surface-layered adsorbents containing multiwall carbon nanotubes placed onto the surface of aerosilogel by use of the carbon vapor deposition method with preloading cobalt nanostructures as a catalyst were found significantly more efficient than traditionally used graphitic carbon-based adsorbents Carbopacks B, C, and X. Additionally, a new adsorbent composed of diatomite Porochrome-3 support coated with a pyrocarbon layer was prepared. This low surface area composited adsorbent allowed both quantitative pre-concentration of phenol and isomeric cresols from the air and their thermal desorption. The developed adsorbents provided fast pre-concentration of selected phenols with a concentration factor of 2 × 103 in 5 min and were used for gas chromatographic determination of analytes in the air at low concentration levels starting from several μg/m3 with a flame ionization detector.

Effects of pyrocarbon on morphology stability of SiC nanowires at high temperatures

AbstractTo understand the effect of pyrocarbon (PyC) on the morphology stability of SiC nanowires at high temperatures (1800‐2100°C), a PyC layer was prepared to wrap the 3C‐SiC nanowire by chemical vapor deposition. The results showed that the existence and thickness of PyC layer played a crucial role in stabilizing the structures and morphology of SiC nanowires. The SiC nanowires without PyC layer transformed to platelet‐shaped structures above 1800°C. The SiC nanowires with a ~1.5 μm PyC layer could keep their structures and morphology at 2100°C, while the SiC nanowires with a ~1 μm PyC layer could not maintain their microstructures at this temperature. The stability of SiC nanowires at high temperature might be related to the vapor phase nucleation, thermodynamics phase transformation and the stacking faults (SFs) expansion, which could be limited by the PyC layer. Therefore, preparing a protective PyC layer with an appropriate thickness might be a potential method to extend the practical applications of SiC nanowires at high temperatures.

Chemical vapor infiltration of pyrocarbon from propene into model capillaries

7 dead-end parallel capillaries 1.0 mm in diameter, 31.0 mm in length and 10 mm separation were aligned perpendicular to the axis of a gas conducting tube and infiltrated at 1 223 K and 40 kPa (N2:C3H6=9:1) for 75 h with a gas residence time of 0.56 s within the tube. The thickness of the pyrocarbon layers produced was determined along each capillary at 15h intervals. Results indicate that the pyrocarbon deposition rate decreases from the mouth to the end of the capillaries and the rate gradients are lower in the capillaries nearer to the gas inlet. Pyrocarbon deposition in the capillaries was simulated by a deposition model using the lumped reaction mechanism. The gas composition in the capillary mouths was obtained from computation by assuming that gas-phase reactions are homogeneous and the tube is a plug flow reactor. The calculated deposition rate profiles agree well with the experimental ones. Both the experimental and theoretical results indicate that pore blockage is relieved by reducing the gas residence time within the conducting tube.

Internal friction behaviour of carbon fibre reinforced multilayered (PyC–SiC)n matrix composites

Abstract Carbon fibre reinforced multilayered (PyC–SiC) n matrix (C/(PyC–SiC) n ) composites were prepared by alternate deposition of pyrocarbon (PyC) and SiC inside preforms via chemical vapour infiltration. The matrix microstructures and internal friction behaviours of C/(PyC–SiC) n composites (n = 1, 2 and 4) under different testing conditions of frequency, strain amplitude and temperature were studied. The results show that with increasing the number of sequences (n), the internal friction increases due to the enhancement of interfacial internal friction. The internal friction of C/(PyC–SiC) n composites increases with the increase of frequency related to thermoelastic mechanism, but exhibits anomalous amplitude effect in the testing amplitude range. Effect of temperature on internal friction behaviours is attributed to combined effects of carbon fibres, PyC–SiC matrices and interfaces between fibres and PyC, adjoining PyC and SiC layers. It is also found that internal friction is sensitive to microstructural defects induced by damage. These results indicate that internal friction is an effective and efficient method to characterize the structural evolution and internal damage of C/(PyC–SiC) n composites non-destructively.

Proximal interphalangeal (PIP) joint replacements with pyrolytic carbon implants in the hand.

Until the late 1980s, proximal interphalangeal (PIP) joint reconstruction had been almost exclusively performed by the use of monobloc silicone spacers and associated with acceptable to good clinical outcomes.More recently, new materials such as metal-on-polyethylene and pyrocarbon implants were proposed, associated with good short-term and mid-term results.Pyrocarbon is a biologically inert and biocompatible material with a low tendency to wear. PIP pyrolytic implants are characterised by a graphite core, visible on radiographs and covered by a radiolucent outer layer of pyrocarbon.New surgical techniques and better patient selection with tailored rehabilitative protocols, associated with the knowledge arising from the long-term experience with pyrocarbon implants, has demonstrated noteworthy clinical outcomes over the years, as demonstrated by recent studies. Cite this article: EFORT Open Rev 2017;2:21–27. DOI: 10.1302/2058-5241.2.160041

CVD Treatment of Carbon Fibers and Evaluation of Their Dispersion in CFRC

Carbon-fiber-reinforced cement-based (CFRC) composites is a promising functional material which can be used both in the military and civil fields against electromagnetic interference. However, it is essential to make carbon fibers dispersed uniformly during the preparation of CFRC. In this work, short carbon fibers were treated through Chemical Vapor Deposition (CVD) process at high temperature between 900°C and 1200°C under the protection of diluted nitrogen gas N2 to modify the surface of carbon fibers to further strengthen the bonding between carbon fibers and cement matrix. Natural gas (98% CH4) was used as a precursor. It was decomposed to produce an uneven layer of pyrocarbon that was deposited on the surface of carbon fibers. CVD-treated carbon fibers were pre-dispersed by using ultrasonic wave. Both hydroxyethyl cellulose (HEC) and silicon fume were used as dispersants and as admixtures. They helped CVD-treated carbon fibers distribute uniformly. The mass fraction of HEC was around 1.78% in the aqueous solution. Four methods, namely, the simulation experiment (SE) method, the scanning electron microscopy (SEM) method, the fresh mixture (FM) method, and the electrical resistivity measurement (ERM) method were, respectively, applied to evaluate fiber dispersion degree. Each method indicated its own advantages and disadvantages and it therefore catered for different conditions. Of the four evaluation methods, the SE method was the most convenient way to determine the pre-dispersion state prior to the preparation of CFRC composites. This method was helpful for predicting the subsequent dispersion state of carbon fibers in the cement matrix because it economized a large quantity of raw materials and time.

The effect on the tensile properties of PIP-processed SiC/SiC composite of a chemical vapor-infiltrated SiC layer overlaid on the pyrocarbon interface layer

A chemical vapor-infiltrated (CVI) SiC layer is often deposited on the pyrocarbon (PyC) fiber–matrix interface layer in SiC fiber-reinforced SiC matrix (SiC/SiC) composites. It is normally applied to protect the PyC layer from reacting with molten Si or sintering aids during manufacturing, and to guard against the effects of high temperature, oxidation and moisture during use. In this study, we investigated the effect of this SiC layer on the tensile properties of a composite. Tensile tests of our composite samples showed the SiC layer to have no noticeable effects on its ultimate load or fracture strain, whereas it decreased the load-to-strain ratio and proportional limit. The test results were analyzed by carrying out element tests on filaments and fiber bundle samples, fracture mirror analysis of pullout fibers, and finite element analysis (FEA) of residual thermal stress around the interface.

First high temperature safety tests of AGR-1 TRISO fuel with the Fuel Accident Condition Simulator (FACS) furnace

Three TRISO fuel compacts from the AGR-1 irradiation experiment were subjected to safety tests at 1600 and 1800°C for approximately 300h to evaluate the fission product retention characteristics. Silver behavior was dominated by rapid release of an appreciable fraction of the compact inventory (3–34%) at the beginning of the tests, believed to be from inventory residing in the compact matrix and outer pyrocarbon (OPyC) prior to the safety test. Measurable release of silver from intact particles appears to become apparent only after ∼60h at 1800°C. The release rate for europium and strontium was nearly constant for 300h at 1600°C (reaching maximum values of approximately 2×10−3 and 8×10−4 respectively), and at this temperature the release may be mostly limited to inventory in the compact matrix and OPyC prior to the safety test. The release rate for both elements increased after approximately 120h at 1800°C, possibly indicating additional measurable release through the intact particle coatings. Cesium fractional release from particles with intact coatings was <10−6 after 300h at 1600°C or 100h at 1800°C, but release from the rare particles that experienced SiC failure during the test could be significant. However, Kr release was still very low for 300h 1600°C (<2×10−6). At 1800°C, krypton release increased noticeably after SiC failure, reflecting transport through the intact outer pyrocarbon layer. Nonetheless, the krypton and cesium release fractions remained less than approximately 10−3 after 277h at 1800°C.

Simultaneous improvements in flexural strength and ductility of carbon nanotube-doped carbon/carbon composites by depositing a pyrocarbon layer on carbon fibers

Introducing carbon nanotubes (CNTs) on carbon fibers (CFs) can significantly improve the matrix-dominated mechanical strength of carbon/carbon composites (C/Cs). As for the CF-dominated mechanical properties such as flexural property, however, the direct introduction of CNTs on CFs always leads to a humble improvement of flexural strength and a brittle fracture mode of the composites. To overcome this limitation, in this work, a thin medium-textured pyrocarbon (PyC) coating, as an interphase layer, was deposited on CFs by isothermal chemical vapor infiltration (ICVI) and its effects on flexural property of CNT-doped C/Cs (CNT-C/Cs) were investigated. Results show that CNT-C/Cs with ICVI PyC layer (PCCT) have a 1100% improvement in flexural ductility and a 44.3% improvement in flexural strength, compared with CNT-C/Cs without ICVI PyC layer (CCT), which are attributed to the toughening role of CFs through their pullouts and the strengthening role of CNTs on the composites. This work could pave a meaningful way for the development of CNT-C/Cs with high mechanical strength and good toughness.

Microstructure and Mechanical Properties of Multilayer-textured 2D Carbon/Carbon Composites

Two-dimensional (2D) carbon/carbon (C/C) composites with multilayered texture, especially with different thickness of high-textured (HT) pyrocarbon layer, were prepared by isothermal, isobaric chemical vapor infiltration (CVI) technique. The influence of matrix microstructure on mechanical properties of C/C composites was investigated by polarized light microscopy, scanning electron microscopy and three-point bending test. The results show that the samples with multilayer-textured pyrocarbon matrix own a higher flexural strength than the one with pure medium-textured structure, which is attributed to multiple crack deflection and interfacial sliding between different textured pyrocarbon layers and between sub-layers within HT layer. The increase in thickness of HT pyrocarbon layer improves the plasticity of the samples and renders the fracture in pseudo-plastic behavior.

Thorium fuel performance assessment in HTRs

Thorium as a nuclear fuel is receiving renewed interest, because of its widespread availability and the good irradiation performance of Th and mixed (Th,U) oxide compounds as fuels in nuclear power systems. Early HTR development employed thorium together with high-enriched uranium. After 1980, most HTR fuel systems switched to low-enriched uranium. After completing fuel development for AVR and THTR with BISO coated particles, the German program expanded efforts on a new program utilizing thorium and high-enriched uranium TRISO coated particles for advanced HTR concepts for process heat applications (PNP) and direct-cycle electricity production (HHT). The combination of LTI inner and outer pyrocarbon layers surrounding a strong, stable SiC layer greatly improved manufacturing conditions and the subsequent contamination and defective particle fractions in production fuel elements. In addition, this combination provided improved mechanical strength and a higher degree of solid fission product retention, not known previously with HTI-BISO coatings. The improved performance of the HEU (Th,U)O2 TRISO fuel system was successfully demonstrated in three primary areas of development: manufacturing, irradiation testing under normal operating conditions, and accident simulation testing. In terms of demonstrating performance for advanced HTR applications, the experimental failure statistic from manufacture and irradiation testing are significantly below the coated particle requirements specified for PNP and HHT designs at the time. Covering a range to 1300°C in normal operations and 1600°C in accidents, with burnups up to 13% FIMA and fast fluences to 8×1025m−2 (E>16fJ), the results exceed the design limits on manufacturing and operational requirements for the German HTR Modul concept, which were: <6.5×10−5 for manufacturing; <2×10−4 for normal operating conditions; and <5×10−4 for accident conditions. These performance statistics for the HEU (Th,U)O2 TRISO fuel system are in good agreement with similar results for the LEU UO2 TRISO fuel system.

A measurement method for density of HTR coated fuel particles porous pyrocarbon layer

The density of porous pyrocarbon layer of coated fuel particles is one of the important parameters of the coated fuel particles. It directly affects the performance of fuel elements. A measurement method, particle size analyzer method, to analyze the density of porous pyrocarbon was developed, and the standard uncertainty of this method was analyzed. Results showed that the particle size analyzer method has the advantages of fast measurement, large sampling number and high measurement accuracy. This method is very efficient for engineering application and is a promising candidate method to measure the density of porous pyrocarbon of coated fuel particles.

A high-temperature radar absorbing structure: Design, fabrication, and characterization

Abstract In this investigation, a new sandwich structure called the SiC f /SiC sandwich radar absorbing structure (SRAS) for high-temperature radar absorption was designed using SiC f /SiC composites. Samples with the base design and optimized design were then fabricated through the PIP process, and the free space reflectivity was measured. The experimental result agrees with the calculated result, exhibiting excellent absorbing properties. The high-temperature reflectivity of the SiC f /SiC SRAS was also measured and the variation with temperature is found to be in a small range, indicating excellent absorbing properties at elevated temperatures. The factors determining the variation of reflectivity with temperature were also investigated, and the sheet resistance of the SiC fiber fabrics (particularly, the resistance characteristics of the pyrocarbon layer of the SiC fiber has a negative temperature coefficient) used as the lossy layer in the SRAS is found to be primarily responsible for the thermal variation of reflectivity.

Design, preparation and microwave absorbing properties of resin matrix composites reinforced by SiC fibers with different electrical properties

Abstract One kind of sandwich structure microwave absorbing structures (SSMASs) derived from Salisbury absorbers is reported. The impedance characteristics of SSMASs are analyzed, and the mechanisms of broadening microwave absorbing bandwidth are interpreted by Smith chart. In order to realize SSMASs, high electrical resistivity SiC fibers with Si–C–O surface layers and low electrical resistivity SiC fibers with pyrocarbon surface layers are employed and analyzed by SEM, XPS, AES and HRTEM. The conductive model of SiC fibers with pyrocarbon layers is built and electrical resistivity simulation is done. The SSMASs are fabricated by employing plain woven SiC fiber fabrics with high and low electrical resistivity as reinforcements of dielectric layers and lossy layer, respectively. The microwave absorbing properties of SSMASs are measured and compared with simulated results. The results show that the experimental and simulated results are in good agreement, the SSMASs have better wideband microwave absorbing properties, and the microwave absorbing bandwidth at reflectivity below −10 dB can reach 11.6 GHz.

Hafnium and silicon carbide multilayer coatings for the protection of carbon composites

A solution to protect the surface of a carbon/carbon composite from oxidation at high temperature is to combine refractory carbides, such as hafnium and silicon carbides (HfC and SiC). Their depositions have been studied on flat substrates and present major protection against oxidation at high temperature (several minutes at 2000°C under air). The infiltration of these carbides layers has been observed inside the open porosities of the carbon substrate which enhance the adhesion of the protection.The low pressure chemical vapor deposition (LPCVD) process developed here allows multilayer HfC/SiC depositions. The HfC coatings have various morphologies and thicknesses depending on the experimental conditions (temperature, pressure, dilution). This carbide has been firstly deposited over flat graphite substrates and carbon single fibers. Secondly, the multilayer coating deposition over a C/C composite and over carbon single fibersis studied. Finally, HfC layers have been infiltrated inside carbon fiber tows. To avoid the notch effect on the carbon fibers, a thin layer of pyrocarbon (50nm thick) has been deposited prior to the growth of the carbides. All the coatings were done in a hot wall CVD reactor and their morphologies and chemical compositions characterized by scanning electron microscopy.

Microstructures and formation mechanism of headstand pyrocarbon cones developed by electromagnetic-field-assisted CVD

Novel headstand pyrocarbon cones (HPCs) with hollow structure were developed on the surfaces of pyrocarbon layers of the carbon/carbon (C/C) composites at 650-750 °C by the electromagnetic-field-assisted chemical vapor deposition in the absence of catalysts. The fine microstructures of the HPCs were characterized by high-resolution transmission electron microscopy. The results show that the textural features of the HPCs directly transfer from turbostratic structure in roots to a well-ordered high texture in stems. And the degree of high texture ordering decreases gradually from the stem to the tail of the HPCs. The formation mechanism of the HPCs was inferred as the comprehensive effect of polarization induction on electromagnetic fields and particle-filler property under disruptive discharge.

Effects of a predeposited pyrocarbon layer on the microstructure and mechanical properties of carbon/carbon composites

Carbon/carbon (C/C) composites with multiple matrixes were fabricated by chemical vapor infiltration (CVI) and an impregnation and carbonization step. To study the effects of predeposited pyrocarbon on microstructure and the mechanical properties of C/C composites, different predeposition times (3h, 5h, 7h and 9h) were tested. This caused the predeposited pyrocarbon layer to have a large effect on the mechanical properties of C/C composites. C/C composites with 3h predeposition exhibited the worst mechanical properties because the predeposited pyrocarbon layer is thin, which does not prevent deformation of the preform during impregnation with liquid pitch. For composites with a 5h predeposition time, a pyrocarbon layer with proper thickness played an important role in protecting the preform; this sample possessed better mechanical properties. However, as the predeposition time was increased to 7h and 9h, due to the increased thickness of the deposited pyrocarbon, several pores were generated in the C/C composites that decreased their density and worsened their mechanical properties.