Juan José Gómez Fernández

Modification of coal-tar pitch by air-blowing — I. Variation of pitch composition and properties

Abstract Pitch oxidation by air-blowing has been studied as a possible method to modify pitch properties for composite preparation. Two commercial coal-tar pitches, an impregnating pitch and a binder pitch, were air-blown at temperatures between 250–300 °C, for 18 and 14 h, respectively, to produce pitches of similar softening point ranging from 148 to 223 °C. Parent and treated pitches were characterized by elemental analysis, FTIR, thermal analysis (TMA, TGA, and DTA), solubility in toluene and quinoline, XPS, X-ray diffraction, and optical microscopy. Additionally, parent pitches were also characterized by extrography. Pyrolysis products of parent and treated pitches, obtained at 450 °C, were characterized by optical microscopy to monitor the influence of air-blowing on mesophase formation. Results show that pitch modification by air-blowing produces a significant increase in carbon yield, without restricting the fluidity necessary to pitch for impregnation. During air-blowing, the reactions of dehydrogenative polymerization and crossing-linking of oligomers occur.

High frequency characteristics and modelling of p-type 6H-silicon carbide MOS structures

Abstract This paper presents the high frequency electrical characteristics and modelling of Al/SiO 2 / p -type 6HSiC structures. The oxide was thermally grown under dry conditions. Capacitance and conductance vs bias and frequency measurements have been performed in daylight and exposing the capacitors to u.v. light. The experimental C m - V g and G m - V g characteristics show hysteresis effects, which are more important when the samples are exposed to 254 nm u.v. light. This behaviour can be explained in terms of interface traps. The MOS structure modelling is based on an interface trap model in which the interface trap levels are considered to be continuously distributed in the SiC bandgap and only charge exchange between interface trap levels and the SiC bands is allowed. From this formulation and from the G m - f characteristics, the interface state density and the interface trap time constant have been determined.

Effects of carbon black addition on the carbonization of mesophase pitch

Abstract Carbonization of a mesophase pitch produced catalytically from naphthalene with HF/BF3 was examined by blending carbon black particles to suppress its severe expansion just before the resolidification that was its major disadvantage for the application as a composite binding substance. The carbon blacks were found effective to suppress the expansion of the mesophase pitch, the particle of large surface area or high oil absorption ability in the pitch such as Ketjen Black (KB) being most effective. The KB particles were found well dispersed in the whole area of the pitch, extending the chain structure, breaking the molecular stacking into smaller optical units (fine mozaic), and hindering the graphitization at the calcined and graphitized stages of the mesophase pitch. Such dispersed particles are expected to release the evolved gases from the molten mesophase pitch along their surface. It is noted that the slurry of the mesophase pitch with carbon blacks maintained the viscosity low enough for smooth impregnation before the carbonization, although the blending certainly increased apparent viscosity.

Modification of coal-tar pitch by air-blowing. II: Influence on coke structure and properties

Abstract This study is concerned with the use of air-blowing to improve pitch properties as matrix precursor of carbon-carbon composites. Two commercial coal-tar pitches, an impregnating pitch and a binder pitch, and their air-blown products obtained at temperatures between 250 and 300 °C, were pyrolyzed in a horizontal tube furnace at 1000 °C. Cokes were characterized by light microscopy to monitor the influence of air-blowing on their optical texture. Density and porosity of cokes were determined by helium densimetry and mercury porosimetry, and their strength and reactivity by a microstrength test and thermogravimetric analysis, respectively. Results show that air-blowing of the impregnating pitch produces a decrease of the optical texture of coke, while in the case of the binder pitch there is an improvement of the microcrystalline structure. Moreover, air-blowing brings about a substantial increase in the density and strength of cokes, and a decrease in their porosity and reactivity.

The role of carbon black/coal-tar pitch interactions in the early stage of carbonization

Abstract This study was undertaken to gain further insight into the types of interaction between pitch and carbon black (CB) occurring during thermal treatment in the initial stages of carbonization, and the effects on subsequent coke structure. A commercial coal-tar pitch was blended with CB and then thermally treated at temperatures between 400–450 °C, for 5 hours—except for 430 °C, where the time of treatment was for 10 hours. The same thermal treatments were applied in the absence of CB to test the effects of temperature alone. Parent and treated pitches were characterized by elemental analysis, optical microscopy, thermomechanical analysis (TMA) and sequential solvent extraction. Additionally, some of the fractions were characterized by FTIR, GC and 1 H-NMR. Cokes obtained at 900 °C were characterized by optical microscopy in terms of their porosity and optical texture. Results show that the type of CB/pitch interactions are temperature dependent, these interactions being more significant at lower temperatures (below 425 °C). Hydrogenation and polymerization reactions have successively occurred along the range of temperatures used. CB produced an increase of pitch carbon yield without affecting pitch fluidity at the lower temperatures. The effect on the reduction of coke porosity was pronounced at the initial stages of the treatment. Coke optical texture was also affected by the presence of CB showing smaller sizes.

Influence of pitch air‐blowing and thermal treatment on the microstructure and mechanical properties of carbon/carbon composites

This paper discusses the optimization of pitch properties as a matrix precursor of C/C composites by subjecting the pitches to different pretreatments to promote polymerization of the molecular components of pitch, which could improve carbon yield and density of the resultant carbon materials. Two series of pitches were prepared by air‐blowing of impregnating coal‐tar pitches at temperatures between 523 and 573 K, and thermal treatment at temperatures between 673 and 703 K, in an inert atmosphere. Effects of pitch treatments on resultant coke texture and porosity were monitored by polarized light microscopy. Unidirectional laminar composites were prepared using the original and treated pitches and commercial PAN‐based carbon fibres. Effects of pitch treatments on the main microstructural features of composites are determined by scanning electron microscopy and polarized light microscopy, and related to the mechanical properties of composites. Results show an improvement of the mechanical properties of composites in both series, particularly for pitch treatments at 523 K/air in the air‐blown series, and at 698 K–5 h in the thermally treated one.

Modifications to carbonization of mesophase pitch by addition of carbon blacks

The addition of carbon blacks with large surface area or high oil absorption ability such as Ketjen Black (KB) was found very effective in suppress the swelling of mesophase pitch during carbonization. A good correlation between the swelling suppression and the dibutylphthalate (DBF) absorption ability indicates that a large effective volume of carbon black in the pitch system is a key factor in suppressing the swelling. The hollow structure and developed chain-like aggregate of carbon black particles are the origins of such large effective volume. Carbon black with large effective volume is well dispersed in mesophase pitch to initiate the pyrolysis of mesophase pitch at the lower temperature range, avoiding the intense pyrolysis and gas evolution in the narrow temperature range which leads to severe swelling of mesophase pitch. The carbon black dispersed in mesophase pitch is also effective to release the pyrolyzed gas smoothly from almost solidifying mesophase pitch of very high viscosity. Airand CO2-gasification of a carbon black (MA600) improved its ability of swelling suppression comparable to that of KB. The air-gasification introduced a graphitic shell and hollow core in the particles of MA600 as observed with KB, significantly increasing the surface area at the low burn-off rate. The CO2-gasification decreased the nodule size, increasing the surface area proportionally with the burn-off rate. Such a structural change appeared to improve the ability for suppressing the swelling of mesophase pitch.

Montecarlo simulation of ion implantation into SiC-6H single crystal including channeling effect

Abstract An ion implantation simulator for single crystal 6H-SiC is presented. This simulator uses a Montecarlo method together with either physically based or semiempirical models to calculate the slowing down of the incoming ions in the crystal. Channeling effect, which appears not to be negligible in SiC single crystal for standard ion implantation conditions, arises naturally as a consequence of the crystal structure and valence electrons distribution. The effect of a native oxide, dynamical amorphization and thermal vibration of the lattice atoms, which affect the channeling behavior of the ions, are also included in the simulation. Recent models for electronic stopping are also used. After calibration, the simulation profiles show good agreement with published SIMS profiles.

Structures of soot generated by laser induced pyrolysis of metal-graphite composite targets

The first results of a novel laser induced pyrolysis (LIP) method for high yield production of fullerene and single walled nanotubes are reported. The pyrolysis process of graphite targets is carried out by means of a continuous CO 2 laser under a flowing argon atmosphere. The vaporization of the targets and the deposition of the generated soot take place in a vertical experimental set-up.

A new lateral IGBT for high temperature operation

Abstract The analysis of a new LIGBT with special emphasis on high temperature behaviour is discussed. A comprehensive experimental characterisation of the static characteristics over the temperature range 300–423 K is reported. Two-dimensional (2-D) numerical simulations are used to explain the observed behaviour and to get a physical insight into the effects of temperature on LIGBT performance. Simulation results show a peculiar latch-up mechanism in the proposed new modified structure different from the conventional IGBT structure. The novel LIGBT structure, proposed here, has been compared with LIGBT structures previously reported. All these structures have been fabricated. The experimental latch-up current density of the proposed LIGBT is four times higher than in the other fabricated structures at high temperature. The dynamic latch-up during the LIGBT turn-off process has also been analysed.