Oxidation Of Carbon Fibers Research Articles

Modelling high temperature progressive failure in C/SiC composites using a phase field model: Oxidation rate controlled process

High-temperature oxidation damage in C/SiC composite, alongside mechanical failure, has becoming a focal point of developing high performance motor components. However, most of existing models focus on only one field and thus can hardly to simulate a complete process. To address this, a thermodynamically consistent phase field model tailored specifically for C/SiC composites is proposed. This model offers a long-desired capability to encompass both carbon fiber oxidation in oxidation controlled stage and mechanical fracture, as well as their intricate interactions. Instead of relying on predefined fields or empirical knowledge, our model determines the oxygen field distribution and the evolution of new cracks through the differential equations rigorously, thereby providing a more accurate estimation of the location and extent of the failure process. The validity and reliability of our model have been tested through a few numerical studies. The proposed model has successfully captured the intricate characteristics of micro-crack propagation in C/SiC composites, including the saturation of cracks originating from the SiC matrix and the fracture process of carbon fibers after oxidation. As a result, our research is anticipated to be serving as an invaluable foundation for quantitative investigations into the performance of C/SiC composites, paving the way for the development of more robust and reliable high-temperature C/SiC composites.

Modulating Carbon Fiber Surfaces with Vinyltriethoxysilane Grafting to Enhance Interface Properties of Carbon Fiber/Norbornene-Polyimide Composites.

In this study, vinyltriethoxysilane (TEVS) was introduced onto the surface of carbon fiber using liquid-phase oxidation and impregnation methods to incorporate vinyl groups onto the carbon fiber, thereby enhancing the chemical bonding between the carbon fiber and norbornene-polyimide (PI-NA). Through a systematic study of the hydrolysis conditions and concentration of the TEVS solution, the optimal modification conditions were determined. These conditions were used to graft TEVS onto the surface of oxidized carbon fiber to prepare carbon-fiber-reinforced PI-NA composites (CF/PI-NA). The results show that when carbon fiber was treated with a 0.4 wt% TEVS solution, the interlaminar shear strength (ILSS) of the composites reached 65.12 MPa, and the interfacial shear strength (IFSS) reached 88.58 MPa, representing increases of 27.58% and 35.62%, respectively, compared to the CF/PI-NA composite materials prepared from untreated carbon fiber. It is worth noting that the modification method described in the study is simple and easy to implement, and it has the potential for large-scale continuous production applications.

Capillary reinforcement strategy guided construction of robust oxygen functionalized carbon-based porous foam for highly efficient solar evaporation

Recent research on carbon-based porous foams has made great progress to alleviate global clean water scarcity. However, it is still a challenge to produce carbon-based porous foams with both high evaporation performance and good mechanical property because of weak interface interactions between carbon-based material and matrix. Herein, we report an effective capillary enhancement strategy to construct robust oxygen-functionalized 3D carbon-based porous foams (OCPF) with abundant hydrophilic oxygen-functional groups from oxidized carbon fiber (OCF) and graphite oxide (GO). Owing to capillary enhancement strategy, a robust foam can be obtained, such as compression stress increases by 6–70 times, and compression modulus reaches 0.56–8.01 MPa, and the obtained foam also contains abundant oxygen-functionalized carbon materials to modulate water state. As a result, the optimized OCPFs show a high evaporation rate of ∼ 3.08 kg·m−2·h−1 with an efficiency of ∼ 94 % under one sun irradiation, which can be further raised to 7.89 kg·m−2·h−1 (5 cm height with 3 m·s−1 wind speed). Besides, the OCPFs also exhibit satisfied desalination and purification performance in actual saline water, domestic and industrial sewage, corrosive and oily wastewater. The feasible fabrication process, excellent performances, and potential applications of OCPFs could provide a new insight into the future development of high-performance and durable carbon-based solar evaporators for large-scale practical applications.

Improvement of carbon fiber oxidation resistance by thin ceramic coating using silica particles

Carbon fiber-reinforced plastics have become indispensable materials in energy saving and new energy technologies, such as automobiles, wind power generators, and hydrogen tanks for fuel cells. However, the lack of oxidation resistance limits the high-temperature applications of carbon fibers. In this study, we developed a coating technology for carbon fibers to improve their oxidative resistance. Carbon fibers with silicon carbide coatings were obtained by the adsorption of silica colloids on carbon fibers using electro adsorption, followed by heating. The resulting coating has small thickness and strong chemical connections. The thermogravimetric analysis confirmed that the oxidation start temperature of the coated carbon fibers was higher than that of carbon fibers. Moreover, the mechanical properties of the coated carbon fibers were maintained. Therefore, the proposed coating technology can broaden the application of carbon fibers as the alternative to some SiC fiber applications. Further, it is applicable for recycling carbon fibers to increase their value in terms of thermal oxidation stability.

Comparison of dynamic mechanical properties of carbon fiber and graphene oxide grafted carbon fiber modified concrete

Graphene oxide grafted carbon fiber (CF-GO) is successfully prepared by the chemical grafting method of “grafting to”. The physical properties, mechanical properties and dispersion effects of carbon fiber and graphene oxide are comparatively studied. Carbon fiber modified concrete (CFMC) and graphene oxide grafted carbon fiber modified concrete (CF-GOMC) are prepared, and the dynamic mechanical properties and fiber/concrete matrix interfaces of CFMC and CF-GOMC are comparatively studied. The results show that the physical and mechanical properties of CF-GO are improved compared with carbon fiber, and the dispersion effect of CF-GO in concrete matrix is better than that of carbon fiber. When the fiber content is the same, the dynamic mechanical properties of CF-GOMC are better than that of CFMC, and the strain rate effect of dynamic mechanical properties indicatoes is stronger than that of CFMC. The strengthening and toughening effect of CF-GO on concrete is better than that of carbon fiber. The carbon fiber/concrete matrix interface exhibits a relatively large porosity, while the CF-GO/concrete matrix interface is well bonded. The graphene oxide on the surface of CF-GO exerts an “expansion bolt” effect, attracting the deposition of hydration product crystals and regulating their crystal structure.

Static and fatigue performance of graphene nanoplatelets coated bi‐directional carbon fiber‐epoxy composites under bending loads

AbstractThe addition of graphene nanoplatelets as a nanofiller on the surface of pristine and oxidized carbon fiber for making reinforced polymer composites has shown improvement in monotonic flexural properties. However, the fatigue response of these laminates under bending loads has not been investigated so far. The current study aims to examine the monotonic and fatigue behavior of the graphene nanoplatelets (GNPs) coated bi‐directional pristine and oxidized carbon fiber epoxy composites under bending loads. Eight layers of fabrics were used to manufacture the laminates through vacuum‐assisted resin transfer molding technique (VARTM). An optimum amount of GNP addition on oxidized fiber leads to ~830‐fold enhanced fatigue life at higher stresses as compared to pristine laminates. However, excessive GNP addition on pristine as well as oxidized fiber leads to a deterioration in fatigue life. In‐situ optical microscopic examination was done to observe the crack initiation and propagation of all the specimens with respect to the number of cycles coupled with post‐mortem SEM images of the fracture surfaces.

Effect of oxidant concentration on the microwave properties of oxidized carbon fibers

Here, we report how the oxidation of carbon fibers (CFs) with H2O2 and HNO3 affects the CFs’ microwave properties. The CFs were characterized by SEM and TEM, and oxygen-containing surface groups were quantified by thermal analysis methods. Treatment with H2O2 and HNO3 solutions increased the oxygen content to about 6 at% and up to 10 at%, respectively, which decreased the reflection loss and increased the transmission loss at 25.86–37.5 GHz. The microwave properties showed pronounced correlations with the concentration of HNO3 and not with H2O2, indicating that the HNO3 oxidant is more promising for microwave loss tuning.

Effects of OCF content and oxidation treatment on thermal conductivity and corrosion resistance of CF/BN/EPN coating

The effects of oxidation time and content of oxidized carbon fiber (OCF) on the thermal conductivity (TC) and corrosion resistance of the OCF/BN/EPN coating are investigated. The results show that the oxidation treatment does not change the chemical organization of OCF, while the amount of carboxyl group grafting and the roughness of OCF surface increase with the increase in the oxidation time. Moreover, SEM results show that the higher content OCF fillers causes the more severe contact in the coatings. As the oxidation time increases, the TC gradually enhances, whereas the EIS results increase firstly and then decrease. This can be attributed to the dual effect of the oxidation treatment and content of OCF. The positive effect of oxidation treatment is that it enhances the interfacial compatibility between the coating and the OCF, improving the thermal conductivity and corrosion resistance of the coating. The downside is that the increase in the amount of carboxyl groups on the surface of the OCF with the oxidation time increases the hydrophilicity of the coating, which reduces the corrosion resistance. In terms of content, it significantly shortens the transmission distance of phonons between fillers and improves phonon transmission efficiency but leads to the increase in the number of defects between the filler and resin interfaces, thus enhancing the thermal conductivity of the coating and reducing the corrosion resistance. As a result, when the oxidation time of the OCF is 1.5 h and 15 wt% OCF is added, the best comprehensive performance can be obtained, i.e., and the TC of the coating reaches 1.4 W/m·K while the impedance modulus at 0.1 Hz is 4.6 × 109 Ω·cm2. This work provides an important guidance for the design of heat exchanger coatings with excellent TC and corrosion resistance.

One‐step in‐situ growth of CNTs on oxidized carbon fiber: Effects of catalyst concentration and growth time on mechanical properties of carbon fiber

AbstractChemical vapor deposition (CVD) method was performed to grow carbon nanotubes (CNTs) on oxidized carbon fiber (CF). First, CF was oxidized by a simple and gentle way to increase the number of active groups and the optimal treatment time was determined. Then, effects of catalyst concentration and growth time on morphologies and loading amount of CNTs, the properties of CF/CNTs reinforcements, and the interlayer properties of CF/CNTs composites were explored. When the catalyst concentration and growth time were controlled at a reasonable level, CNTs can be uniformly grown on the CF surface. The interlaminar shear strength (ILSS) was characterized and the fracture surfaces of CF/CNTs composites were observed. The results demonstrated that when the catalyst precursor was kept at 0.05 mol/L and the growth time was controlled at 10 min, CNTs had the best strength effect and the ILSS increased by 30.25% compared to that of composites without CNTs. The fracture surface showed that CNTs can penetrate the matrix at the interface and enhance the interface through mechanical interlocking effect. The CNTs‐induced interphase allows the stress inside the composites to propagate continuously between resin and fibers, thereby heightening the load carrying capacity.Highlights A simple and gentle way was performed to activate CF without strength reduction. The influence mechanism of various parameters was revealed in detail. The mechanism of CNTs enhancing the interface was revealed.

The effects of carbon fiber surface treatment by oxidation process for enhanced mechanical properties of carbon fiber/epoxy composites for biomedical application

In this research, the production of carbon fiber composite (CFC) with epoxy resin was carried out for biomedical application. The surface of the carbon fibers was previously oxidized with concentrated nitric acid at a temperature of 100 °C for 30–120 min to create a rough surface impression on the carbon fibers to enhance interfacial bonding in the composite, increase surface area, and reduce surface tension. The carbon fiber/epoxy composite was fabricated using the vacuum assisted resin infusion method. Characterization of the oxidized carbon fibers and the composite products was performed using a digital microscope, scanning electron microscope, and Fourier Transform Infrared (FTIR) analysis. FTIR analysis results indicated that the carbon fiber oxidation process introduced new chemical functional groups, such as –CN and –CO groups. Mechanical characterizations included tensile testing of non-oxidized and oxidized carbon fiber and tensile testing of carbon fiber/epoxy composite. The results showed that the composite formed from oxidized carbon fibers/epoxy resin exhibited higher tensile strength compared to non-oxidized CFC. The longer the carbon fiber oxidation process, the higher the tensile strength values obtained.

STRUCTURE PARAMETER OF INTERFACIAL INTERACTION IN Cf/C COMPOSITES AND ITS CHANGE DURING PORE FORMATION PROCESS

The C<sub>f</sub>/C composite oxidation of polyacrylonitrile-based carbon fibers with a combined carbon matrix at 600°C up to a weight loss of 80 wt% has been investigated. The changes in the open porosity, specific surface area, apparent and true densities of the composite, as well as the ultimate strength and elastic modulus during three-point transverse bending tests have been analyzed. An analytical model for obtaining qualitative and semi-quantitative estimates of the change in integral pore size during oxidation of heterophase materials has been developed. The change in pore size is estimated through the evolution of open porosity P and materials specific surface area S. The model has been validated in the C<sub>f</sub>/C composite oxidation study. The composite's mechanical properties dependences on the value of the integral pore size are obtained and analyzed. The structural parameter P/S characterizes the change in the interphase interaction during pores formation and development. It is established that the failure mechanism depends on the value of the parameter P/S and dynamically changes with the oxidation of C<sub>f</sub>/C composite. The use of P/S parameter as a criterion for estimation and control of composite materials structural integrity in open systems is proposed.

The Effect of Temperature on the Surface Energetic Properties of Carbon Fibers Using Inverse Gas Chromatography

This paper constitutes an original and new methodology for the determination of the surface properties of carbon fibers in two forms, namely, oxidized and untreated, using the inverse gas chromatography technique at infinite dilution based on the effect of temperature on the surface area of various organic molecules adsorbed on the carbon fibers. The studied thermal effect showed a large deviation from the classical methods or models relative to the new determination of the surface properties of carbon fibers, such as the dispersive component of their surface energy, the free surface energy, the free specific energy, and the enthalpy and entropy of the adsorption of molecules on the carbon fibers. It was highlighted that the variations in the London dispersive surface energy of the carbon fibers as a function of the temperature satisfied excellent linear variations by showing large deviations between the values of γsd (T), calculated using different models, which can reach 300% in the case of the spherical model. All models and chromatographic methods showed that the oxidized carbon fibers gave larger specific free enthalpy of adsorption whatever the adsorbed polar molecules. The obtained specific enthalpy and entropy of the adsorption of the polar solvents led to the determination of the Lewis acid–base constants of the carbon fibers. Different molecular models and chromatographic methods were used to quantify the surface thermodynamic properties of the carbon fibers, and the results were compared with those of the thermal model. The obtained results show that the oxidized carbon fibers gave more specific interaction energy and greater acid–base constants than the untreated carbon fibers, thus highlighting the important role of oxidization in the acid–base of fibers. The determination of the specific acid–base surface energy of the two carbon fibers showed greater values for the oxidized carbon fibers than for the untreated carbon fibers. An important basic character was highlighted for the two studied carbon fibers, which was larger than the acidic character. It was observed that the carbon fibers were 1.4 times more acidic and 2.4 times more basic. The amphoteric character of the oxidized fibers was determined, and it was 1.7 times more important than that of the untreated fibers This tendency was confirmed by all molecular models and chromatographic methods. The Lewis acid and base surface energies of the solid surface, γs+ and γs−, as well as the specific acid–base surface energy γsAB of the carbon fibers at different temperatures were determined. One showed that the specific surface energy γsAB of the oxidized fibers was 1.5 times larger than that of the untreated fibers, confirming the above results obtained on the strong acid–base interactions of the oxidized carbon fibers with the various polar molecules.

Elucidation of Surface Functional Groups Deposited by Electrochemical Surface Treatment of Discontinuous Carbon Fiber by NEXAFS and XPS.

Control of carbon fiber heteroatom (oxygen and nitrogen) functionalization using electrochemical oxidation is explored in a variety of electrolyte solutions. Results of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy indicate that most electrolytes in aqueous and anodic conditions are limited to heteroatom surface content of no more than 13 atomic percent (at %) with a majority C-O and/or C-N moieties; the remaining moieties include an oxidative sequence of carbon (alcohol to ketone to carboxylate) and more complex O- and N-containing groups. The pH of the electrolyte solution was found to be crucial in controlling the ratio of the amount of oxygen to nitrogen functionalities, with the increased basicity of solution resulting in higher nitrogen deposition. The oxidative (and/or thermal) decomposition of many electrolytes during electrochemical treatment can have a major impact on functionalization through changes to pH. Oxidation of carbon fiber in some electrolyte solutions showed higher surface concentrations of heteroatoms (25-30 at %) than most electrolytes (13 at %). Mechanisms were proposed to explain how some electrolytes can exceed 13 at % of heteroatom deposition. Specifically, we hypothesized that electrolytes that contain organic ions with chelation capabilities and moieties that produce additional sites of functionalization can overcome that threshold.

Phase-field model of char oxidation in ablative thermal protection system materials

We develop a phase field model of char oxidation during atmospheric entry in the ablation zone of the Phenolic Impregnated Carbon Ablator (PICA). The phase field model is coupled to a model of heat transport, to capture the interaction between the oxidation and temperature, and it is based on our previous model of carbon fiber oxidation. We explore the char oxidation behavior using three 2D examples. First, we simulate oxidation with an imposed temperature gradient for char structures with increasing amounts of initial porosity. The oxidation time decreases with increasing porosity and the oxidation rate fluctuates as pores are opened to oxygen transport. Next, we predict the decrease in temperature due to the endothermic oxidation reactions in thermally-insulated systems with and without transport of oxygen through the top boundary. In both cases, the temperature decreases until oxidation stops; it stops due to the decrease in temperature in the case open to oxygen transport and due to lack of oxygen in the case closed to oxygen. Finally, we explore the oxidation behavior under different heat fluxes. The oxidation reactions compensate for the heat flux until the char is fully consumed and then the temperature increases. Our results demonstrate the importance of considering the interactions between the oxidation reactions and the temperature when modeling char oxidation.

Corrosion resistance of fluorescent carbon fiber reinforced FEVE and fluorescence detection of coating surface

Fluorocarbon resin coating plays a significant role in marine severe corrosion protection. However, seawater has a great influence on the corrosion rate of materials, so it is necessary to strengthen the monitor of marine materials during service. In this study, using solvothermal method, the rare earth (RE) europium nitrate, 1.10-phenoline and concentrated nitric acid oxidized carbon fiber (CF) were chemically doped in the reactor to prepare a kind of carbon fiber with fluorescent characteristics. Then, the RE modified CF was mechanically mixed with fluorocarbon resin to prepare a fluorescent indicator anti-corrosive coating with intrinsic red light emission guided by Eu3+ ultra-sensitive electric dipole transition (5D0→7F2). The correlation between corrosion time and fluorescence spectroscopy could be established to detect the corrosion resistance of the coating, it provides a reference for the anti-corrosion intelligent indication of the coating.

Carbon fiber cloth loaded core-shell structured Pd@AgCl-Ag as an efficient bifunctional electrode for direct hydrogen peroxide-hydrogen peroxide fuel cells

A bifunctional Pd@AgCl-Ag/O-CFC electrode was prepared on oxidized carbon fiber cloth (O-CFC) by hydrothermal method and Galvanic reaction and used for catalytic H2O2 reduction reaction (HPRR) and oxidation reaction (HPOR). The catalyst on electrode surface presents a particular core-shell structure with Ag and AgCl as the core and Pd and AgCl as the shell, facilitating the synergistic interaction between the bimetals and the exposure of the primary active substance of Pd. The Pd@AgCl-Ag/O-CFC electrode exhibited excellent catalytic performance for both HPRR and HPOR with current densities of − 733.3 mA cm−2 and 708.0 mA cm−2, respectively. For the kinetic study, the activation energy of HPRR catalyzed by the Pd@AgCl-Ag/O-CFC electrode was low (7.59 kJ mol−1), and both the electroreduction and electrooxidation reaction processes were controlled by a hybrid of substance diffusion and kinetics. The electroreduction and electrooxidation reactions of H2O2 on the surface of the Pd@AgCl-Ag/O-CFC electrode are both irreversible processes, and their reaction levels are 1 and 0.5, respectively. The direct hydrogen peroxide-hydrogen peroxide fuel cell (DPPFC) was assembled with Pd@AgCl-Ag/O-CFC as a bifunctional electrode, and the open circuit potential reached 0.75 V and power density reached 61.9 mW cm−2 at 338.15 K. In addition, due to the stability of the structure and catalytic performance of the Pd@AgCl-Ag/O-CFC electrode, the DPPFC can operate stably for a long time. This work indicates that the as-prepared Pd@AgCl-Ag/O-CFC can be a promising electrode for DPPFC.

Degradation of the Three-Phase Boundary Zone of Carbon Fiber Anodes in an Electrochemical System.

The electrochemical recycling nanoarchitectonics of graphene oxide from carbon fiber reinforced polymers (CFRPs) is a promising approach due to its economic and environmental benefits. However, the rapid degradation of the CFRP anode during the recycling process reduces its overall efficiency. Although previous studies have investigated the electrochemical oxidation of carbon fibers (CFs) and bonding of CFs to the matrix, few researchers have explicitly studied the electrochemical activity of CFs and the possible fracture caused by strong electrochemical reactions. To address this gap, this study investigates the degradation mechanism of CF anodes by analyzing changes in overall mechanical properties, hardness, elastic modulus, functional groups, and elemental composition of individual fibers. The experimental results demonstrate that the three-phase boundary region experiences the most severe degradation, primarily due to the number of oxygen-containing functional groups, which is the most important factor affecting the degree of degradation. This continuous decrease in the hardness and elastic modulus of individual fibers eventually leads to the fracture of CF anodes.

Effectof epoxy functionalized elastomer modified carbon fiber on mechanical properties and interfacial adhesion of chloroprene rubber (CR)/natural rubber (NR) composites

AbstractThe carbon fibers were oxidized with concentrated hydrochloric acid, and subsequently modified in two steps. First, the oxidized carbon fibers were grafted with γ‐aminopropyltriethoxysilane (KH550) with the purpose of building a platform on the fiber surface, then the epoxy functionalized elastomer (ethylene‐vinyl acetate‐glycidyl methacrylate terpolymer, EVMG) was grafted on the carbon fiber surface. Chloroprene rubber/natural rubber/carbon fiber (CR/NR/CF) composites were prepared by mechanical blending process. Carbon fibers were characterized by different techniques such as X‐ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA). The results show that EVMG can enhance the interface adhesion between fiber and rubber. It was observed that the surface of the carbon fiber is covered with a layer of the macromolecular film by EVMG, which increases the surface roughness and introduces epoxy groups. On the one hand, carbon fiber and rubber can be entangled through macromolecular chains, and on the other hand, interfacial bonding can be enhanced by chemical interactions. Furthermore, modification by EVMG impacted tensile strength of CF composite increase by 11.05%, the 300% constant tensile stress increased by 21.87%, and the wear resistance increased by 29.8%, respectively.

Nitric acid oxidation and urea modification of carbon fibres as biofilm carriers

ABSTRACT Carbon fibres (CF) are commonly used as carriers in biofilm-based wastewater treatment. The surface properties of the CF are herein modified using a combination of nitric acid oxidation and urea to optimise the carrier to immobilise bacterial cells. The capacity of the CF carriers to immobilise bacterial cells and activated sludge is evaluated using bacterial cell adhesion and sludge immobilisation tests. The total interaction energy profiles between the CF supports and bacterial cells were calculated according to the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory to explain the mechanism by which these modifications enhance this immobilisation capacity. CF-U has a high capacity for immobilising bacterial cells and activated sludge (3.7 g-sludge/g-CF supports) owing to its low total interaction energy. Nitric acid oxidation reduced the diiodomethane contact angle of CF from 55.1° to 38.5°, which reduced the Lifshitz-van der Waals interaction energy, while urea modification further increased the zeta potential of CF from 12.8 mV to −0.7 mV, thereby reducing the electrostatic interaction energy. Experiments and DLVO theory both determined that a combination of nitric acid oxidation and urea modification significantly enhanced the ability of CF to immobilise microorganisms.

Microscopy analysis of carbon fiber oxidation in composite coupons burned and smoldered at three different heat fluxes

Carbon fiber composites consisting of woven carbon fiber layers embedded in a matrix of resin have replaced traditional materials used in modern aircraft. Composite materials respond uniquely from aluminum and titanium alloys in a fire in that they smolder after the flames are extinguished. Flame-out occurs spontaneously once the resin is vaporized or turns to carbonaceous residues or char during the burn. During smolder, resin char and fibers oxidize, producing additional heat and emitting partial combustion products. Previous work investigated oxidation properties and chemical emissions from two sets of carbon fiber composite coupons each with different resin systems when burned and allowed to smolder for 30 min at three heat fluxes (35, 50, 85 kW/m2) in a cone calorimeter. In that study, an increase in heat flux correlated with increased composite decomposition and emissions during smolder. Carbon fibers in the middle layers from the composite samples were analyzed via microscopy, and the results indicated that fiber diameter was similar across all samples, regardless of resin or heat flux. In the present study, carbon fibers in the upper layers of the previously burned and smoldered composites were analyzed via microscopy. The analysis indicated that in the upper delaminated composite layers closest to the heat source, fiber diameter was reduced at 50 and 85 kW/m2 versus 35 kW/m2 and compared to fiber diameter in the middle layers of composites at all heat fluxes. This phenomenon was limited to the upper layers of the composite, which provides evidence to support the concept that temperature and oxygen transport dictate smolder reaction kinetics and are limited in the in-depth direction between fiber layers.