A temperature-programmed desorption and oxidation investigation of wear debris from carbon/carbon composite aircraft brakes

Wear debris obtained from pilot scale disc-disc friction tests carried out at different temperatures and sliding speeds were characterized by simultaneous thermogravimetric and mass-spectrometric measurements. The original C/C material consisted of an ex-PAN preform densified by CVI. Friction tests cycles consisted of a large number of identical braking strokes with experimental parameters corresponding to taxiing. Temperature-programmed desorption spectra of CO₂, CO and H₂O present similarities with those published in the literature for a wide variety of oxidized activated carbons. However the amount of oxygen expressed per unit surface is generally higher for debris than for oxidized activated carbons. The formation of the corresponding oxygen groups is attributed to the mecanochemical actions (rupture, oxidation, and compaction) that carbon debris undergoes on sliding surfaces before being ejected from the disc/disc contact. In addition, temperature-programmed oxidation of debris under diluted oxygen showed that they are much more oxidable than the original C/C material.     

Generation of H2 gas from polystyrene and poly(vinyl alcohol) by milling and heating with Ni(OH)2 and Ca(OH)2

A two-step process to generate H₂ gas; first by milling polystyrene (PS) or poly(vinyl alcohol) (PVA) with Ni(OH)₂ and Ca(OH)₂, followed by heating of the milled product in the second-step was performed in this work. Polymer and hydroxide mixtures obtained after milling for 60 min and heating to 700 °C showed H₂, CH₄, H₂O, CO, and CO₂ as the main gaseous products with H₂ as the dominant gas generated between 350 and 500 °C. Analysis of the gaseous products by TG-MS and gas-chromatography, and solid products by TG-DTA and XRD shows that CO₂ gas was fixed as CaCO₃ at temperatures between 350 to 600 °C allowing generation of H₂ gas with concentrations over 95% for PS and over 98% for PVA. The results in this study show that milling of solid based hydrocarbon compounds with nickel and calcium hydroxides allows dispersion of nickel to hydrocarbon surfaces and facilitates C-C bond rupture in polymer(s) during heating at temperatures below 500 °C, at the same time calcium adsorbs CO₂. This process could be developed to treat hydrocarbon based wastes such as plastics, biomass or combinations at low temperatures avoiding syngas purification and separation steps.     

Oxidation behaviour of C/C–SiC brake discs tested on full-scale bench rig

C/C–SiC composites are considered to be strong candidates for the new generation of high-speed train brake discs. To achieve a better application, it is necessary to improve understanding of the oxidation behaviour of C/C–SiC brake discs after a full-scale bench test rig. In this study, full-scale braking bench tests for C/C–SiC self-mated brake pairs were conducted under a braking speed of 350–420 km/h and a braking pressure of 17–28 kN. Moreover, the oxidation behaviour and mechanisms of the C/C–SiC brake discs during the practical braking process were investigated. The results indicate that the oxidation behaviour is highly dependent on the friction surface region of the C/C–SiC brake disc owing to the distribution of microcracks, the formation of friction films, the difference in temperature, and the contact content with O₂. Specifically, the oxidation depths of the friction layer on the inner circumferential surface, middle friction surface, and outer circumferential surface were 278.3, 252.1, and 359.9 μm, respectively. Furthermore, the oxidation reaction preferentially occurs in the active area of the C fibre and pyrolytic carbon (PyC) during the braking process.     

Synthesis of magadiite using a natural diatomite material

BACKGROUND: This study investigated the synthesis of magadiite from a natural diatomite material. The influence of key reaction parameters, including reaction time, temperature and molar ratios of Na₂O/SiO₂ and H₂O/Na₂O, on the formation of magadiite were investigated. The as‐synthesized magadiite was characterized by X‐ray powder diffraction, scanning electron microscopy, infrared spectroscopy, thermogravimetry and differential thermal analysis. RESULTS: The well crystallized magadiite with a rosette like morphology was prepared from a dispersion with the molar ratio H₂O/Na₂O = 28.15 and Na₂O/SiO₂ = 0.15 by heating at 160 °C for 42 h. The basal space of the synthesized magadiite is 15.5 Å and the stretching and bending frequencies of the SiO₄ units making up the magadiite layer were recorded. The layered structure was destroyed when the temperature rose above 250 °C, and combining the results of thermogravimetry and differential thermal analysis suggested the structural alteration may be due to the condensation of silanol groups. CONCLUSION: Diatomite has been used to synthesize magadiite with a high purity and well crystallized. The cost of synthesis has been reduced allowing its use in conventional industrial applications, thus expanding the commercial utilization of diatomite. Copyright © 2009 Society of Chemical Industry     

Reactions of coal with the chlorides of iron,chromium and copper

The study of X-ray diffraction patterns indicates the possibility of the formation of intercalation compounds in several coals after reactions with FeCl₃, FeCl₃·6H₂O, CuCl₂, CuCl₂·2H₂O, and CrCl₂· 6H₂O. These reactions were carried out without solvent at temperatures ranging from 215 to 250 °C. X-ray evidence suggests that washing of the products, obtained from these reactions, with dilute acid returns the coal starting material substantially unchanged. X-ray and chemical evidence shows that reaction of FeCl₃ and FeCl₃·6H₂O with coal results in the reduction of some Fe(III) to Fe(II).     

Comparative investigations of coal pyrolysis under inert gas and H2 at low and high heating rates and pressures up to 10 MPa

Five German hard coals of 6–36 wt% volatile matter yield (maf) were pyrolysed at pressures up to 10 MPa, using two different apparatuses, which mainly differ in the heating rates. One consists of a thermobalance where a coal sample of ≈ 1.5 g is heated at a rate of 3 K min ^?1 under a gas flow of 3 I min^?1. The other apparatus is constructed for rapid heating (10²?10³ K s^?1) of a small sample of ≈10 mg of finely-ground coal distributed as a layer between the folded halfs of a stainless-steel screen, heated by an electric current. The product gas composition was determined by quantitatively analysing for H₂, CH₄, C₂H₄, C₂H₆, CO, CO₂ and H₂O. The amounts of tar and char were measured by weighing. The heating rate, pressure and gas atmosphere were varied. Under an inert gas atmosphere, high heating rates result in slightly higher yields of liquid products, e.g. tar. The yields of light hydrocarbon gases remain the same. With increasing pressure, the thermal cracking of tar is intensified resulting in high yields of char and light hydrocarbon gases. Under H₂, pyrolysis is influenced strongly at elevated pressure. Additional amounts of highly aromatic products are released by hydrogenation of the coal itself, particularly between 500 and 700°C. This reaction is less effective at higher heating rates because of the shorter residence time and diffusion problems of H₂. The yield of light gaseous compounds CH₄ and C₂H₆ increases markedly under either heating condition owing to gasification of the reactive char.     

MoSi2-based cylindrical susceptor for rapid high-temperature induction heating in air

Conductive susceptors are necessary for heating non-conductive materials in induction heating systems. Susceptor materials should have sufficient electrical conductivity and thermal/chemical stability under a range of environmental conditions. However, many susceptor materials oxidize in high-temperature environments, resulting in degradation and poor durability. Here, we used intermetallic MoSi₂ ceramics as a susceptor material and designed a cylindrical susceptor to apply to rapid high-temperature induction heating in an oxidizing environment. MoSi₂ was prepared by self-propagating high-temperature synthesis (SHS), and then used to fabricate cylindrical susceptors by slip casting. The optimal thickness of the susceptor was controlled by modelling. A MoSi₂-based cylindrical susceptor with SiO₂ protective layer showed higher heating rate (4.2–26.8 °C/s at 0.5–2.5 kW) than a commercial rod-type susceptor (7.7–13.6 °C/s at 1.0–2.5 kW) of the same material. In addition, our susceptor endured high temperatures below 1700 °C and severe thermal cycle (700–1600 °C, heating for 2min and cooling for 1min) during 36 cycles. In general, these results demonstrate that the MoSi₂-based susceptor can be applied to a rapid induction heating furnace that can be used in air at a high temperature of 1600 °C (equal to the available temperature of a commercial graphite susceptor in H₂).     

Wear of carbon fiber reinforced carbon matrix composites: Study of abrasive,oxidative wear and influence of humidity

The wear of C/C composites has been studied using a subscale aircraft brake dynamometer linked with a mass spectrometer. A disc-on-disc configuration allowed for simulation of various aircraft landing energy conditions (12.5%, 25%, 50%, and 100% of normal landing energy of a Boeing 737 aircraft) performed at 50% and 90% relative humidity levels. The microstructure of composite brakes was altered by applying three different heat treatment temperatures: 1800, 2100, and 2400 °C, respectively. A mass spectrometer linked to an environmental chamber of the subscale dynamometer was utilized to measure the in situ CO₂ release during the wear tests. The relationships between microstructure, hardness of individual components of composites and wear performance at varied conditions are presented. Carbons obtained at higher heat treatment temperatures are most vulnerable to abrasive wear, while the less ordered carbons, typical for samples heat treated at lower temperatures, showed significant amount of oxidative wear. Oxidative wear was related to excessive heating of materials. Optimization of wear behavior of C/C composite is only possible by understanding the mechanisms of the microstructural changes of materials, corresponding mechanical properties and the nature of wear under various environmental conditions.     

The pyrolysis of cellulose derivatives

The thermal degradation in vacuo of ethyl cellulose and cellulose acetate in the form of very thin films or bulk material between 230° and 320°C has been studied. With the ethyl cellulose films, volatilization (as measured by weight loss) was a first-order process up to about 50% reaction, with an activation energy of 208 kJ/mole. This is about the same as that associated with the initial drop in intrinsic viscosity of the solid during bulk pyrolysis, in which very high molecular weight material, probably crosslinked, was formed at a later stage. The volatile products from ethyl cellulose included H₂O, CO, CO₂, C₂H₄, C₂H₆, C₂H₅OH, CH₃CHO, unsaturated aliphatic compounds, and furan derivatives. Acetic acid and acetyl derivatives of D -glucose were produced from cellulose acetate. It is suggested that the polymers degrade by radical chain mechanisms, and a number of possible elementary steps are proposed.     

Braking Behavior of C/SiC Composites Prepared by Chemical Vapor Infiltration

Carbon fiber-reinforced silicon carbide matrix composites have the potential to overcome the shortcoming of the currently used carbon/carbon friction materials in aircraft brakes. In this article, the carbon/silicon carbide (C/SiC) composites were prepared by chemical vapor infiltration method, and the brake disks with different densities and component content were finally obtained. The friction coefficient and friction stability can be significantly improved by increasing both material density and carbon content. When the density of C/SiC composite is 2.3 g/cm³, the coefficient of friction measured is 0.23, the coefficient of friction stability remains about 0.43, the liner wear rate is less than 9.3 μm/cycle, and the weight wear rate is less than 9.1 μm/cycle. The rapid increase of friction coefficient approaching the end of braking is mainly related to the increasing of surface temperature in a short time and the enhanced adhesion and abrasion of contact conjunctions and asperities. The C/SiC composites exhibited a good stability of braking against fading versus the braking number and surface temperature. The surfaces of C/SiC brake disks were covered with wear debris including the fragment of carbon fibers after the braking tests. The wear on the surfaces is significantly determined by cyclic mechanical and thermal stresses, which result in the micro-cracks in the SiC matrix, the thin flakes of the surface materials as well as the grooves.     

Formation of N-containing gas-phase species from char gasification in steam

The formation of N-containing products during char-steam gasification has been investigated in a laboratory scale fixed bed reactor. Experiments were conducted at 1000 °C, 0.1-1.0 MPa, and 6-46% of H₂O in He base flow. Two very different coal chars, which were prepared from the rapid heating of Australian bituminous and sub-bituminous coals, were studied. The nitrogen-containing products released during the gasification were measured using an FTIR spectrometer (NH₃, HCN and HNCO) and gas chromatography (N₂). The major N-containing products formed during char-steam gasification are NH₃, HCN and N₂. Reactions of HCN in the same reactor were also studied; these experiments were conducted with HCN alone, HCN/steam, and HCN/steam/char. The results are consistent with a mechanism in which HCN is the primary N-containing product of the char-steam reaction, and the additional products result from further reactions of HCN either in the gas phase or promoted by the surface of the reactor or the char. Increasing concentrations of steam significantly influence the distribution of char-N to N-containing gas-phase products, resulting in the increase of NH₃ at the expense of N₂. Some differences in char behaviour are also observed, particularly on the distribution of N-containing products at 0.1 MPa total pressure.     

Study on combustion mechanism of asphalt binder by using TG-FTIR technique

The combustion mechanism of asphalt binder was investigated by using thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG-FTIR) in a mixed gas environment of 21% oxygen and 79% nitrogen. The results show that the combustion process of asphalt binder consists of three main consecutive stages at a low heating rate. The combustion reaction becomes more and more intense from the 1st to 3rd stage. The release of volatiles occurs mainly at 300-570 °C, and the gaseous products in each stage are different. The main products in the 1st stage are CO₂, CO, H₂O, hydrocarbons, formaldehyde, tetrahydrofuran, formic acid, aromatic compounds, etc. In the next stage, the combustion products mentioned above keep on increasing, but some new volatiles such as alcohols, phenols, styrene, etc. are present. In the last stage, the CH and CO bonds continue to fracture and aromatization reaction occurs, and the release amount of CO₂, CO, and H₂O reaches the maximum. But the content of other products decreases or even disappears due to burning. Among the above volatiles, CO₂ is the dominant gaseous product in the whole combustion process. The concentration of CO₂ and CO keeps increasing, and reaches the maximum intensity at about 520 °C. The evolution of H₂O, CH₄, and formic acid exhibits the trend of increase first, and then decrease. Over 570 °C, there are few products released at the end of the combustion process. Asphalt binder combustion process includes two modes of complete and incomplete combustion, and the latter may be main combustion mode of asphalt binder.     

Tribological behavior and wear mechanism of C/C–SiC brake discs based on third-body layer

C/C–SiC composites are promising candidates for heavy-duty tracked vehicle brake discs. A third-body layer (TBL) can be formed on the surface of C/C–SiC self-mated brake discs, which has an important impact on tribological behavior and wear mechanism of brake discs. Herein, the formation conditions and evolution process of TBL and its effect on friction and wear properties were investigated. An appropriate braking pressure and speed (P and V) are beneficial to the cutting of asperities and refinement of wear debris on the contact surface, which are preconditions for the formation of original TBL. The original TBL can be formed under the P·V of 12, 15, and 16, which effectively improve braking stability and reduce the wear rate. During the continuous braking process, the original TBL undergoes growth, stabilization, destruction, and regeneration. Under the frictional heat and compressive stress, wear debris gradually evolves into a uniform and dense TBL. The average coefficient of friction and wear rate reach to the lowest value of .446 and 38.5 × 10⁻³ cm³/MJ, respectively. A continuous high temperature in the later stages of braking leads to severe oxidative wear. The newly formed TBL covers the original surface to form a multilayered structure, indicating the TBL undergoes destruction and regeneration.     

Mechanical and tribological assessment of composite AlCrN or a-C:Ag-based thin films for implant application

The present study focuses on the comparison of cathodic arc deposited AlCrN (ternary coating) and Ag alloyed a-C (amorphous carbon base coating) on chrome nitride (CrN) medical grade 316 LVM stainless steel. The work comprises of morphological, structural, nanomechanical and tribological evaluation in physiological simulated body fluid (SBF) lubrication following conditions pertaining to simulated hip joint. According to the findings, H/E, H³/E² and E_coating/E_substrate significantly effect the nanomechanical and tribological properties of the coatings. While a-C:Ag/CrN exhibited better L_y value compared to AlCrN/CrN due to better surface quality, the later has shown higher L_c2 value during nanoscratch test attributed to lower H³/E² and higher plastic work done. Inspite of lower friction coefficient, a-C:Ag/CrN observed higher wear rate during simulated tribotest attributed to low hardness, separate graphitic structure due to Ag doping and sudden increase of friction coefficient ascribed to severe abrasive delamination of a-C:Ag top layer. The wear mechanism observed under SEM microscopy indicate severe adhesion of Ti6Al4V counterbody on AlCrN/CrN coated surface. The size of wear debris obtained with AlCrN/CrN-Ti6Al4V tribopair was larger in size compared to a-C:Ag/CrN-Ti6Al4V tribopair. Nevertheless, despite inferior surface quality and lower L_y value and larger wear debris size, AlCrN/CrN coating performed better in nanoscratch (at L_c2 value) and demonstrated lower wear in simulated tribotest in physiological SBF condition.     

Carnallite decomposition into magnesia,hydrochloric acid and potassium chloride: A thermal analysis study; formation of pure periclase

A thermal analysis study of the one-stage decomposition of small (mg) quantities of carnallite, KCl. MgCl₂, 6H₂O → KCl + MgO + 5H₂O ↑ + 2HCl ↓, showed that dehydration and hydrolysis require temperatures above 300 °C. Decomposition of greater amounts, and analysis of the products, showed that at 450 °C, 88 to 90% of the magnesium occurs as chloride-free high-purity periclase. At higher temperatures increasing amounts of chlorides remain in the periclase and are difficult to separate. Unglazed porcelain is a suitable material for the decomposition vessel.     

The tribological behavior of micrometer and nanometer TiO2 particle‐filled poly(phthalazine ether sulfone ketone) composites

Micrometer and nanometer TiO₂ particle‐filled poly(phthalazine ether sulfone ketone) (PPESK) composites with various filler volume fractions from 0.5 to 7.5 vol % were prepared by heating compression molding. The friction and wear behaviors of the PPESK composites were evaluated using the block‐on‐ring test rig by sliding PPESK‐based composite blocks against a mild carbon steel ring under dry friction conditions. The wear debris and the worn surfaces of the PPESK composites filled with micrometer and nanometer TiO₂ particles were investigated by using a scanning electron microscope (SEM), while the structures of PPESK composites and wear debris were analyzed with IR spectra. Experimental results show that antiwear properties of the PPESK composites can be improved greatly by filling nanometer TiO₂ particles, and the friction coefficient decreases when the filler volume fraction is below 2.5%, but when the filler volume fraction is above 2.5% the friction coefficient increases gradually with increasing filler volume fraction. In the case of micrometer TiO₂ filler, wear rates increase with increasing filler volume fractions under identical test conditions, and the friction coefficients are less sensitive to the filler volume fraction. It was also found that the wear mechanism of micrometer TiO₂ particle‐filled PPESK is mainly severe adhesion and abrasive wear, while that of nanometer TiO₂ particle‐filled PPESK is mainly slight abrasive wear. In the former case, there are no transfer film formed on the surface of the counterpart steel, and wear debris are in the form of long and large ribbon. While in the latter case, the wear debris was granule and their size was about 10 μm. In case of 1 vol % nanometer TiO₂ particle‐filled PPESK composites, the transfer film was fairly thinner and smoother, and the transfer film provided better coverage on the surface of steel ring, while that of 7.5 vol % was thicker and discrete. These account for the different friction and wear behavior of micrometer and nanometer TiO₂ particle‐filled PPESK composite. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 906–914, 2004     

Five-layer reverse tape casting of IT-SOFC

Tape casting is a well-established method for manufacturing thin ceramic layers with controllable thickness and porosity. This study investigates the potential of 10Sc1CeSZ material for the electrolyte and anode layers for intermediate-temperature solid oxide fuel cells (IT-SOFC) in an anode-supported cell (ASC) geometry. In order to use La_0.6Sr_0.4Co_0.2Fe_0.8 Oxide (LSCF) cathode material, a Gd_0.2Ce_0.8 Oxide (GDC) barrier layer is needed; however, thermal expansion coefficient mismatch results in delamination of the GDC from the electrolyte during high temperature sintering when fabricated by conventional tape casting procedures. For the first time, ASCs have been manufactured by a five-layer tape casting technique; barrier layer, novel composite layer, electrolyte, anode functional layer, and anode substrate. Ni-ScCeSZ composite cells were tested between 650 and 800°C in H₂:N₂ fuel (85% H₂) on the anode and air on the cathode to yield a maximum power density of .46 W/cm². These results demonstrate the feasibility of this new five-layer tape casting technique to produce IT-SOFC.     

Thermal Decomposition of Energetic Materials 56. On the fast thermolysis mechanism of ammonium nitrate and its mixtures with mangnesium and carbon

Fast thermolysis studies of ammonium nitrate (AN) and its mixtures with magnesium and activated charcoal have been carried out by the Fourier transform infrared spectroscopy/temperature profiling technique. When subjected to rapid heating (ca. 80°C/s), AN Sublimes/decomposes around 300°. Sublimation dominates at ambient pressures. The IR-active products of decomposition are NH₃, NO₂, N₂O and H₂O. Reaction schemes accounting for the products are proposed which involve proton transfer leading to NH₃ by the decomposition products of HNO₃. The decomposition of AN is significantly enhanced when AN is mixed with magnesium powder or charcoal, and occurs at as low a temperature as 135°C. Whereas NH₃ is the major product of decomposition of AN Mg mixtures, no NH₃ is observed from AN C mixtures. The results are explained by the reaction of HNO₃ and NH₃ with Mg or C.     

Generation of hydrogen gas from polyethylene mechanically milled with Ni-doped layered double hydroxide

A process for generating hydrogen gas from polyethylene (PE) by milling and heating with Ni-doped layered double hydroxide (LDH), which was prepared also by a mechanochemical route of two-step milling operation, was reported in this work. A mixture of PE and the prepared Ni-doped LDH was first milled in a planetary ball mill for 1 h followed by heating the milled product to 700 °C under He/Ar gas environment for hydrogen emission. Characterizations by a set of analytical methods of X-ray diffraction (XRD), thermogravimetry-mass spectroscopy (TG-MS) and gas chromatography (GC) were performed on the milled and heated samples to monitor the process. Gaseous products obtained during heating mainly consisted of H₂, CH₄, CO, CO₂ with H₂ concentration over 80% between 450 and 550 °C. Such a process could be developed to treat hydrocarbon based solid wastes for hydrogen generation.     

Fuel processing in direct propane solid oxide fuel cell and carbon dioxide reforming of propane over Ni-YSZ

This work is aimed at understanding the reaction mechanism of propane internal reforming in the solid oxide fuel cell (SOFC). This mechanism is important for the design and operation of SOFC internal processing of hydrocarbons. An anode-supported SOFC unit with Ni-YSZ anode operating at 800 °C was tested with direct feeding of 5% propane. CO₂ reforming of propane was carried out in a reactor with Ni-YSZ catalyst to simulate internal propane processing in SOFC. The performance of this direct propane SOFC is stable. The C specie formed over the anode functional layer of SOFC can be completely removed. The major gas products of SOFC are H₂, CO, CH₄, C₂H₄ and CO₂. Pseudo-steady-state internal processing of propane in the anode catalytic layer of SOFC is associated with a CO₂/C₃H₈ molar ratio of about 1.26 and basically CO₂ reforming of propane. CO₂ dissociation to produce the O species to oxidize the C species from dehydrogenation and dissociation of propane and its fragments should be the major reaction during CO₂ reforming of propane.