Microstructures and tribological properties of carbon/carbon-boron nitride composites fabricated by powdered additives and chemical vapor infiltration

The carbon fiber reinforced/carbon-boron nitride (C/C-BN) dual matrix composites were fabricated via adding hexagonal boron nitride (h-BN) powders into the needled carbon felt and subsequent chemical vapor infiltration (CVI) process. An experimental investigation was performed to study the influences of BN volume content on the microstructures and tribological properties of C/C-BN composites. The results indicate that the pyrolytic carbon (PyC) in the C/C-BN composites is regenerative laminar (ReL) due to the inducement of BN powders during CVI process, whereas the PyC in the C/C composite is classic smooth laminar. Additionally, the friction coefficients of C/C-BN composites with three different BN contents in volume fractions (4.5, 9 and 13.5 vol%) are all higher compared to the reference C/C composite (0.22). Note that the highest coefficient of friction (0.29) is obtained when the BN volume content in the C/C-BN composite is 9 vol%. Moreover, the linear and mass wear rates of C/C-BN composites as well as the 30CrSiMoVA counterparts are significantly decreased with the increase of BN volume content. The favorable friction and wear properties of C/C-BN composites are attributed to the synergistic effect induced by the ReL PyC and BN. The microstructural variation of C/C composites modified by h-BN could improve the compatibility between the C/C-BN composites and 30CrSiMoVA counterpart, resulting in an enhanced adhesive attraction between the wear debris and the surface of 30CrSiMoVA counterpart. Furthermore, the investigations concerning the friction surfaces indicate that the formation of sheet-like friction films with large areas are more easily to occur on the surfaces of 30CrSiMoVA counterparts mating with the C/C-BN composites rather than mating with the C/C composite.     

Liquid reagent CVD of carbon. II. Kinetic experiments and heat and mass transport analysis

Liquid reagent CVD of carbon was performed where the substrate was immersed in liquid benzene and cyclohexane. A kinetic analysis showed the deposition rates obeyed the Arrhenius relationship with apparent activation energies of 280 and 263 kJ/mole, respectively. Deposition rates were highest for benzene. A simple analysis of heat and mass transport based on film boiling theory was performed to achieve fundamental understanding of the process and to define the rate limiting mechanism. It was demonstrated that the hydrogen inhibition effect can be neglected for the liquid reagent CVD process and a high precursor concentration near the substrate surface is achievable, which permits a high deposition rate.     

RHEOLOGICAL CHARACTERIZATION OF PYROLYTIC WOOD DERIVED OILS: EXISTENCE OF A COMPENSATION EFFECT

Detailed rheological study was made for sixteen pyrolytic wood derived oils provided by different laboratories and obtained from a wide range of liquefaction processes. Molecular characterization of these oils has been performed through gel permeation chromatography (GPC) and intrinsic viscosity [η] measurements.

All pyrolytic wood derived oils exhibit an essentially Newtonian behavior in the range of shear rate examined (10^-1 to 10³s^-1). The variation of viscosity with temperature follows an Arrhenius-type relationship. GPC chromatograms and [ η] measurements have shown the existence of a Mark-Houwink relationship between [ η] and molecular weights of the pyrolytic oils with a Mark-Houwink exponent of the order of 0.58 when tetrahydrofuran (THF) is used as solvent. Therefore the pyrolytic oils despite the diversity of the liquefaction processes all belong to a same family. Moreover, a compensation effect has been found. Because of the Newtonian character of these oils, the compensation effect allows the determination of the temperature dependence of viscosity from only one measurement of viscosity at a given temperature     

Chemical vapor infiltration of C/C composites: Fast densification processes and matrix characterizations

Fast densification processes have been developed to improve the fabrication of C/C composite materials. In this work, a comparison is made between two techniques: the film boiling technique with a liquid reagent and the gas infiltration method. In both methods, the same home-made reactor was used. For the film boiling technique, the preform is either wrapped or not with a porous thermal barrier.Two different substrates have been densified, a carbon felt (RVC-2000^® from Le Carbone-Lorraine), and a 3D carbon cloth (Novoltex^® from Snecma). In situ temperature gradients and their temporal changes during the infiltration process have been recorded together with the delivered power necessary to maintain a constant deposition temperature. From these experiments, we have concluded about the following main points:

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the analysis of in situ parameters, powers and temperatures, and the associated profiles of the pyrocarbon deposits,

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the matrix quality with their associated microstructures as characterized by helium density, optical microscopy and Raman scattering experiments,

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the key role of the evolutive preforms as heat and mass exchangers during the process, and the assisted thermal fluxes inside the reactor.

This paper presents results which should allow to control automatically the process at an industrial scale.     

Mechanically strong and bioactive carbon fiber-SiC nanowire-hydroxyapatite-pyrolytic carbon composites for bone implant application

Carbon fiber-SiC nanowire-hydroxyapatite-pyrolytic carbon composites (CHS) composed of carbon fiber, hydroxyapatite (HA), SiC nanowires (SiCnws) and pyrolytic carbon (PyC) were prepared by sol-gel, heat treating, electrochemical deposition (ECD) and isothermal chemical vapor infiltration (ICVI) for bone implant application. The morphology, microstructure and the composition of CHS were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The mechanical properties of CHS were tested and compared with the traditional bone implant materials. The in-vitro bioactivity was evaluated by testing the cell morphology, cell proliferation and cell differentiation. The results show that SiCnws and HA are distributed uniformly inside the CHS. SiCnws are attached on the carbon fibers. The diameter of the SiCnws is around 250 nm and the length is more than 100 μm. HA could grow on the surface of both SiCnws and carbon fibers, forming a brush shape. The flexural strength, shear strength, out-of-plane and in-of-plane compressive strength of CHS are 127 MPa, 90 MPa, 169 MPa and 213 MPa, respectively. The elastic modulus of CHS is 7.2 GPa. The mechanical properties of CHS are similar to those of cortical bone. In-vitro cell test shows that CHS has excellent cell proliferation and cell differentiation. CHS possessing good mechanical and biological performances may have potential to be applied for bone implant materials.     

Selective determination of L-dopa in the presence of uric acid and ascorbic acid at a gold nanoparticle self-assembled carbon nanotube-modified pyrolytic graphite electrode

Gold nanoparticle-functionalized carbon nanotubes (AuNP-CNT) have been prepared by a novel self-assembly method. The new material has been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) and utilized for constructing AuNP-CNT-modified pyrolytic graphite electrode (AuNP-CNT/PGE) to investigate the electrochemical behavior of L-dopa in neutral phosphate buffer solution. Compared to bare PG electrode, AuNP-CNT/PGE shows novel properties towards the electrochemical redox of L-dopa in phosphate buffer solution at pH 7.0. The oxidation potential of L-dopa shows a significant decrease at the AuNP-CNT/PGE. The oxidation current of L-dopa is about 5-fold higher than that of the unmodified PGE. Using differential pulse voltammetry (DPV) method, the oxidation current is well linear with L-dopa concentration in the range of 0.1-150 μM, with a detection limit of about 50 nM (S/N = 3). The proposed electrode can also effectively avoid the interference of ascorbic acid and uric acid, making the proposed sensor suitable for the accurate determination of L-dopa in both pharmaceutical preparations and human body fluids.     

Deposition rates during the early stages of pyrolytic carbon deposition in a hot-wall reactor and the development of texture

Pyrolytic carbon layers were deposited from methane/oxygen/argon mixtures on planar substrates (silicon wafers) at a total pressure of 100 kPa, a maximum gas residence time of 2 s and a temperature of 1100 °C. The depositions were performed in a hot-wall reactor with the substrate oriented parallel to the gas flow. Particular attention was paid to factors that influence the reproducibility of the deposited layers. Scanning and transmission electron microscopy were applied to study the thickness profiles and the texture of the carbon layers. The surface topography was investigated by atomic force microscopy. For pyrolytic carbon deposited without oxygen, an alteration from medium- to high-textured carbon is observed with increasing residence time. Islands are observed on the surface of the layer whose size increases with the texture. For pyrolytic carbon deposited with 3% oxygen, lower deposition rates were obtained and a strong modification of the texture is found compared to gas mixtures without oxygen.     

Flash co-pyrolysis of biomass with polyhydroxybutyrate: Part 1. Influence on bio-oil yield, water content, heating value and the production of chemicals

Bio-oil obtained via flash pyrolysis shows potential to be applied as a renewable fuel. However, bio-oil often contains high amounts of water, which is a major drawback for its application. The influence of a biopolymer - polyhydroxybutyrate (PHB) on the pyrolysis of willow is investigated using a semi-continuous home-built pyrolysis reactor. The flash co-pyrolysis of willow/PHB blends (w/w ratio 7:1, 3:1, 2:1 and 1:1) clearly shows particular merits: a synergetic increase in pyrolysis yield, a synergetic reduction of the water content in bio-oil, an increase in heating value, and a production of easily separable chemicals. The occurrence of synergetic interactions is observed based on a comparison between the actual pyrolysis results of the willow/PHB blends, the theoretical pyrolysis results calculated from the reference pyrolysis experiments (pure willow and pure PHB) and their respective w/w ratio. The co-pyrolysis of 1:1 willow/PHB shows the best overall results.     

Status of reversible electrodeposition electrochromic devices

The status of electrochromic devices based upon the reversible electrodeposition of thin bismuth–copper films is reported. The electrochemistry and interface chemistry of the system relevant to information display applications are briefly described. Low information content displays are currently being produced in small quantities with saturated black and white contrast ratio of 25 : 1 and lifetimes of greater than ten million cycles. Electrochemical scanning tunneling microscopy (ECSTM) studies on highly ordered pyrolytic graphite (HOPG) substrates were undertaken as a first step in the unraveling of the detailed nucleation and growth behavior of the Bi–Cu system at the atomic level. The results obtained support previous studies using scanning electron microscopy and surface spectroscopy. Preliminary results on the plating current efficiency of the system using a quartz crystal microbalance (QCM) are reported.