Showing papers on "Carbide published in 1999"

Heterostructures of Single-Walled Carbon Nanotubes and Carbide Nanorods

Abstract: A method based on a controlled solid-solid reaction was used to fabricate heterostructures between single-walled carbon nanotubes (SWCNTs) and nanorods or particles of silicon carbide and transition metal carbides. Characterization by high-resolution transmission electron microscopy and electron diffraction indicates that the heterostructures have well-defined crystalline interfaces. The SWCNT/carbide interface, with a nanometer-scale area defined by the cross section of a SWCNT bundle or of a single nanotube, represents the smallest heterojunction that can be achieved using carbon nanotubes, and it can be expected to play an important role in the future fabrication of hybrid nanodevices.

Silicon Oxycarbide Glasses

Abstract: The first attempts to introduce carbon into glass date back to 1951. But up until recently, the use of carbon or carbide raw materials, and the oxidation, volatilization and decomposition that accompany high temperature melting, have limited the synthesis of true silicon oxycarbide glasses. Here, the term silicon-oxycarbide refers specifically to a carbon-containing silicate glass wherein oxygen and carbon atoms share bonds with silicon in the amorphous, network structure. Thus, there is a distinction between black glass, which contains only a second-phase dispersion of elemental carbon, and oxycarbide glasses which usually contain both network carbon and elemental carbon. In addition to exploring the unique properties and applications of these glasses, per se, they are also of interest for developing models of the residual amorphous phases in polymer-derived silicon-carbide and silicon-nitride ceramics. The application of sol/gel techniques to glass synthesis has significantly advanced the development and characterization of silicon oxycarbide glasses. In this approach, alkyl-substituted silicon alkoxides, which are molecular precursors containing oxygen and carbon functionalities on the silicon, can be hydrolyzed and condensed without decomposition or loss of the carbon functional group. A low-temperature (<1000°C) heat-treatment of the gel creates a glassy silicate material whose molecular structure consists of an oxygen/carbon anionic network. In addition, there is always a blackening of the material due to elemental carbon, which forms during pyrolysis and densification of the gel. The nature of the network carbon, and especially the distribution and form of the elemental carbon, are fundamental to the structure and properties of these novel materials. Their chemical and physical characteristics as revealed by NMR, Raman and TEM are discussed in the overview. In addition, the high temperature stability of these glasses (up to 1750°C), and the effect of hot-pressing, are described. It will be shown that the silicon oxycarbide network is stable up to 1000–1200°C. The network carbon is terminated with hydrogen (i.e., CH, =CH2 and –CH3), and with polyaromatic carbon (i.e., nC6Hx) wherein most of the elemental carbon resides. These glasses can be described as molecular composites of polyaromatic graphene-rings dispersed in a silicon oxycarbide network. After heating to temperatures in excess of 1000–1200°C, the oxycarbide network decomposes through the loss of hydrogen, and a two- or three-phase glass-ceramic consisting of nanocrystalline graphite, silicon carbide, and amorphous silica or cristobalite, is created. Some of the properties and applications of these glasses/glass-ceramics for coatings, composites and porous solids are summarized.

Cemented Tungsten Carbides: Production, Properties and Testing

Abstract: Introduction * Crystal Structure and Phase Equilibria * Production of Metal and Carbide Powders * Consolidation of Cemented Carbides * Sintering Behavior of Cemented Carbides * Microstructural Aspects of Cemented Carbides * Mechanical Behavior of Cemented Carbides * Magnetic Properties * Wear and Erosion of Cemented Carbides * Thermal Shock Resistance * Corrosion and Oxidization * Joining of Cemented Carbides * Testing and Quality Control * Classification and Applications * Coatings * Fire Grained and Functionally Graded Cemented Carbides * Reclamation of Cemented Carbides * Toxicity in Cemented Carbide Production.

Ceramic Fibers for Matrix Composites in High-Temperature Engine Applications

Abstract: High-temperature engine applications have been limited by the performance of metal alloys and carbide fiber composites at elevated temperatures. Random inorganic networks composed of silicon, boron, nitrogen, and carbon represent a novel class of ceramics with outstanding durability at elevated temperatures. SiBN(3)C was synthesized by pyrolysis of a preceramic N-methylpolyborosilazane made from the single-source precursor Cl(3)Si-NH-BCl(2). The polymer can be processed to a green fiber by melt-spinning, which then undergoes an intermediate curing step and successive pyrolysis. The ceramic fibers, which are presently produced on a semitechnical scale, combine several desired properties relevant for an application in fiber-reinforced ceramic composites: thermal stability, mechanical strength, high-temperature creep resistivity, low density, and stability against oxidation or molten silicon.

Catalytic properties of early transition metal nitrides and carbides: n-butane hydrogenolysis, dehydrogenation and isomerization

Abstract: Phase-pure early transition metal nitrides and carbides were prepared via the temperature programmed reaction of metal oxides with NH3 or a CH4/H2 mixture. The nitrides and carbides were mostly mesoporous with surface areas up to 81 m2/g. Their gravimetric butane conversion rates were generally higher than those for a Pt–Sn/Al2O3 catalyst. Activities for the nitrides and carbides ranged from 0.4×1012 to 10×1012 molecules/cm2 s at 723 K and decreased as follows: γ-Mo2N>W2C≈WC>β-W2N≈WC1−x>β-Mo2C>VN≈V8C7≫NbC≈Nb4N3.92. The metal atom type had the most significant effect on the activity and selectivity. The Group VI metal nitrides and carbides were much more active than the Group V metal compounds. In general, the Group VI metal compounds catalyzed butane hydrogenolysis and dehydrogenation with similar selectivities while the vanadium compounds had dehydrogenation selectivities in excess of 98%. The β-W2N catalyst also catalyzed butane isomerization possibly as a consequence of the presence of oxygen on the surface. The effect of lattice structure was significant and obvious for the tungsten carbides where WC (hex) was almost twice as active as WC1−x (fcc) despite having similar C/W ratios. Nitrides and carbides of the same metal and lattice structure had similar activities suggesting that the effect of the non-metal atom type was small. We believe variations in the catalytic properties of the nitrides and carbides were the result of differences between their electronic structures.

Nanocrystalline WC and WC/a-C composite coatings produced from intersected plasma fluxes at low deposition temperatures

Abstract: Low temperature vacuum deposition of tungsten carbide coatings, W1−yCy with compositions that varied from y=0 to 0.9, was investigated. Special attention was given to the production of nanocrystalline carbides with coatings of y>0.5. Previous attempts at producing WC with excess carbon at near room temperatures resulted in the formation of amorphous phases. In this study, crystalline WC was produced at 45 and 300 °C by the intersection of plasma fluxes from magnetron sputtering of tungsten and laser ablation of graphite. At both temperatures, formation of WC chemical bonding and nanocrystalline cubic β-WC1−x was observed using x-ray photoelectron spectroscopy and grazing angle x-ray diffraction when the carbon content was increased more than 30%. Increasing the substrate temperature to 300 °C did not affect the percentage of WC bonding, but it did promote considerable crystallization of cubic WC. As the carbon content was increased to more than 50%, a second phase consisting of amorphous carbon (a-C) was observed together with amorphitization of β-WC1−x. The a-C phase was identified as amorphous diamond-like carbon (DLC) by Raman spectroscopy. At 60–80 at. % C, a two phase structure was produced, which was composed of nanocrystalline β-WC1−x with 5–10 nm grains and amorphous DLC. The hardness of the WC/DLC composites was about 26 GPa based on nanoindentation tests. Correlation of the chemistry, microstructure, and mechanical properties of WC and WC/a-C coatings is discussed.

Solid state metathesis routes to transition metal carbides

Abstract: Flame initiated (800 °C, 10 s) or bulk thermal (2 days, 1000 °C) reactions of mixed powders of transition metal halides and CaC 2 or Al 4 C 3 produce transition metal carbides (TiC, ZrC, HfC, V 8 C 7 , NbC, TaC, Cr 3 C 2 , Mo 2 C and WC) in good yields. The carbides were characterised by X-ray powder diffraction, SEM/EDX, FTIR, microelemental analysis, TEM, electron diffraction and ELNES.

Characterization of the W2C phase formed during the high velocity oxygen fuel spraying of a WC + 12 pct Co powder

Abstract: A variety of experimental techniques have been used to study a WC-12 pct Co powder and the coatings produced by high velocity oxygen fuel (HVOF) spraying of the powder onto a steel substrate. Many of the structural characteristics of the powder were also found in the coating. However, when the metallic matrix of the powder was melted during thermal spraying, the carbides were partially dissolved and a very heterogeneous liquid phase was produced in which the W/C ratio varied from about 1 to 4. These variations have been linked with oxidation of the liquid phase during spraying. The factors influencing the formation of W2C in the coating have been identified as (1) an in situ transformation of WC into W2C maintaining the original WC faceted morphology and (2) the precipitation of W2C from the W-rich liquid phase matrix as the coating cools. A cobalt containing carbide of the M6C-M12C type has also precipitated from the liquid phase when the W/C and W/Co ratios were high.

Characterization of slurry phase iron catalysts for Fischer-Tropsch synthesis

Abstract: The study of Iron Fischer–Tropsch catalysts by conventional powder X-ray diffraction (XRD) is complicated by the number and type of phases present (α-Fe, various iron carbides, Fe x C: 2 3 O 4 ). Peak overlap in the diffraction patterns and differences in the X-ray scattering ability of each phase can make quantitation difficult. This has led to the consensus that activity for Fischer–Tropsch (F–T) synthesis does not correlate with the bulk composition of the iron catalyst, as seen by X-ray diffraction. As we demonstrate in this paper, some of the problems with sample analysis arise from the difficulty in preserving microstructures and composition intact, as the sample is removed from the reactor and prepared for analysis. Our results indicate that Soxhlet extraction, a commonly used procedure to remove the wax from a catalyst, can cause changes in catalyst phase composition. We present a study of a slurry-phase iron F–T synthesis catalyst, where samples have been removed under inert atmosphere and care was taken to preserve the catalyst constituents intact. Quantitative Rietveld structural refinement, combined with step-scanned X-ray diffraction data, allows us to determine changes in composition and morphology in the working catalyst. We conclude that, in its most active form, this Fe catalyst contains the Fe 7 C 3 carbide with small amounts of alpha-iron (α-Fe), while the χ-carbide (hereafter designated as Fe 5 C 2 ), also present in differing amounts during each run, appears to be less active. Major changes in carbide particle size also occur during the course of the F–T synthesis run. The active catalyst contains a significant amount of highly dispersed carbide particles.

Studies on the Carbothermal Preparation of Titanium Carbide from Different Gel Precursors

Abstract: Different mixed organic–inorganic gels as precursors for the synthesis of titanium carbide and oxycarbides have been prepared in aqueous and organic solutions starting either from rutile or titanium alkoxide. The sol–gel processes have been controlled by complexing additives such as H 2 O 2 , acetic acid or ethyl aceto-acetate. Upon pyrolysis up to ∼600°C, composites of finely divided particles of amorphous titania and carbon are formed. Monitoring of the high-temperature treatment by TG, XRD and nitrogen adsorption up to 1600°C revealed a three-step carbothermal reduction mechanism through lower titanium oxides and oxycarbides. The intimate mixture of titania and carbon leads to a considerable lowering of the onset of the reaction temperature in comparison with rutile/carbon black and rutile/gel mixtures. Delayed reactions in the final steps, however, may be due to the interruption of the intimate contact of the reactants by pore generation. During the high temperature processes microporous and mesoporous intermediate materials have been prepared; the final products at 1600°C are finely divided particles of oxycarbides with oxygen contents of 1·2–3·4 wt% and grain sizes lower than 1 μ m .

The influence of alloying elements on the chlorine‐induced high temperature corrosion of Fe‐Cr alloys in oxidizing atmospheres

Abstract: The effect of the alloying elements Al, Cr, Mn, Mo, Si and Ti on the corrosion behaviour of ferritic Fe-15Cr model alloys was studied in a N2/He-5 vol.% O2 gas mixture with and without additions of 500–1500 vppm HCl at 600°C. The main corrosion mechanism is “active oxidation”, characterized by the formation of volatile metal chlorides at the metal/oxide interface. Volatilization and subsequent conversion of the chlorides into oxides results in the formation of porous and poorly adherent oxide scales. Large mass gains were observed for Fe-15Cr, Fe-35Cr and Fe-15Cr with additions of 5 wt.% Ti, 10 wt.% Mn or 10 wt.% Mo. The specific morphology of the corrosion products depends strongly on the alloying elements. For the Fe-Cr alloys, a model for the formation of the scales, which are characterized by alternating dense and porous layers, is presented. The addition of 5 wt.% Si or Al to Fe-15Cr leads to much better corrosion resistance by the formation of protective Cr2O3/Al2O3-layers, however in the case of Al addition the behaviour depends strongly on the experimental conditions, as surface treatment and flow velocity. In Fe-15Cr-10Mo preferential removal of the more reactive metals Fe and Cr was observed resulting in a Mo-enriched porous metal zone underneath the metal-oxide interface. The effect of carbon on the corrosion behaviour was examined by addition of 0.3–0.8 wt.% C to the model alloys. Cr-rich M23C6-carbides were attacked preferentially while Mo-rich M6C-carbides are very stable relative to the matrix and the attack occurs in regions surrounding the carbides. Der Einflus von Legierungselementen auf die chlorinduzierte Hochtemperaturkorrosion von Fe-Cr-Legierungen in oxidierender Atmosphare Der Einflus der Legierungselemente Al, Cr, Mn, Mo, Si und Ti auf das Korrosionsverhalten ferritischer Fe-15Cr-Modellegierungen wurde in einer N2/He-5 vol.% O2 Gasmischung mit und ohne Zusatz von 500–1500 vppm HCl bei 600°C untersucht. Der wesentliche Korrosionsmechanismus ist die „aktive Oxidation“, bei der sich fluchtige Metallchloride an der Metall/Oxid-Grenzflache bilden. Verdampfen und anschliesende Umsetzung der Chloride in Oxide fuhrt zu porosen und schlecht haftenden Oxidschichten. Hohe Massenzunahmen wurden beobachtet bei Fe-15Cr, Fe-35Cr und Fe-15Cr mit Zusatzen von 5 gew.% Ti, 10 gew.% Mn oder 10 gew.% Mo. Die spezifische Morphologie der gebildeten Oxidschichten hangt stark von den jeweiligen Legierungselementen ab. Im Falle der Fe-Cr Legierungen wird ein Modell fur die Bildung der Oxidschichten, die aus abwechselnd kompakten und porosen Schichten bestehen, vorgestellt. Der Zusatz von 5 gew.% Si oder Al fuhrt zu stark verbesserten Korrosionswiderstanden durch die Bildung von schutzenden Cr2O3/Al2O3-Schichten; im Falle des Aluminiums ist dies allerdings kritisch abhangig von den experimentellen Bedingungen, wie Oberflachenbearbeitung und Stromungsgeschwindigkeit. Bei Fe-15Cr-10Mo wird eine bevorzugte Reaktion der reaktiveren Metalle Fe und Cr beobachtet, was zu einer Mo-angereicherten porosen Metallzone unterhalb der Metall/Oxid-Grenzflache fuhrt. Der Effekt von Kohlenstoff auf das Korrosionsverhalten wurde durch Zulegieren von 0,3–0,8 gew.% C zu den Modellegierungen untersucht. Cr-reiche M23C6-Karbide werden relativ zur Matrix bevorzugt angegriffen, wahrend Mo-reiche M6C-Karbide sehr bestandig sind und der Angriff bevorzugt in den umgebenden Metallbereichen erfolgt.

The effect of Ti and TiO2 additions on the pressureless sintering of B4C

Abstract: The effect of TiO2 and of Ti additions on the sintering behavior of B4C was studied in the 1800 °C to 2190 °C temperature range. According to thermodynamic predictions that were verified by the experimental results, TiB2 is formed in situ in both instances. Titanium oxide is reduced by carbon that originates in B4C and decreases its carbon content. The activity of carbon in the boron carbide phase is reduced significantly with decreasing carbon content. By sintering at 2160 °C for 1 hour, a mixture of fine-sized B4C with 40 wt pct TiO2 is transformed into a 95 pct dense two-phase composite that consists of substoichiometric boron carbide and TiB2 and displays a bending strength of 420 MPa. The significantly improved sintering behavior of this composite ceramic is attributed to the enhanced mass transport processes that takes place in substoichiometric B4C. The low carbon activity values in substoichiometric B4C ensure that no deleterious carbide phases are produced by reaction with molten metals.

Polycrystalline diamond cutter with an integral alternative material core

Abstract: A cutting element, insert or compact, is provided for use with drills used in the drilling and boring of subterranean formations or in machining of metal, composites or woodworking. This new insert besides having a superabrasive layer on the surface of a substrate, also may have one or more superabrasive core element sections incorporated in the substrate to provide improved internal residual stress characteristics. By so manipulating residual stresses, this invention provides cutting elements, which are more fracture resistant thereby providing improved work life. Also, by providing additional superabrasive material in the substrate, this invention improves the cutting efficiency of the compact after the compact has undergone significant wear. Another embodiment of this invention employs one or more carbide core regions within a superabrasive region, which covers the majority of the outer surface of the insert.

Precipitation of an intermetallic phase with Pt2Mo-Type structure in Alloy 625

Abstract: The microstructure of Alloy 625, which has undergone prolonged (∼70,000 hours) service at temperatures close to but less than 600 °C, has been characterized by transmission electron microscopy. The precipitation of an intermetallic phase Ni2(Cr, Mo) with Pt2Mo-type structure has been observed in addition to that of the γ″ phase. Six variants of Ni2(Cr, Mo) precipitates have been found to occur in the austenite grains. These particles exhibit a snowflake-like morphology and are uniformly distributed in the matrix. They have been found to dissolve when the alloy is subjected to short heat treatments at 700 °C. The occurrence of the Ni2(Cr, Mo) phase has been discussed by taking the alloy chemistry into consideration. Apart from the intermetallic phases, the precipitation of a M6C-type carbide phase within the matrix and the formation of near continuous films, comprising discrete M6C/M23C6 carbide particles, at the austenite grain boundaries have been noticed in the alloy after prolonged service.

Theory of phase stabilities and bonding mechanisms in stoichiometric and substoichiometric molybdenum carbide

Abstract: First principles, total energy methods have been applied to predict the relative stabilities of the four experimentally verified MoC phases: the cubic delta(NaCl) phase and the three hexagonal gamm ...

Comparison of the group V and VI transition metal carbides for methane dry reforming and thermodynamic prediction of their relative stabilities

Abstract: The group V and VI transition metal carbides have been prepared by CH4 TPR, and tested for the dry reforming of methane with carbon dioxide, at elevated pressure. Mo2C and WC were the most stable catalysts, while the group V metal carbides showed the stability order: vanadium \({\text{ >}}\) niobium \({\text{ >}}\) tantalum. Catalyst deactivation was due to carbide oxidation with CO2, while stability was associated with the reaction of metal oxide (from deactivation) with CH4, giving the metal carbide. Calculation of the Gibbs free energy for this reaction resulted in a predicted catalyst stability trend similar to that obtained experimentally.

Kinetic control of silicon carbide/metal reactions

Abstract: The kinetic features governing SiC/metal reactions have been investigated to identify the operating mechanisms and diffusion characteristics. With respect to the contact metal components, the reaction characteristics were identified by two separate modes-formation of silicides and free carbon (Mode I), or formation of carbides and silicides (Mode II). The analysis was confirmed by comparing SiC/Ni and SiC/Cr reactions. The diffusion pathways for both reactions were examined in terms of a chemical potential framework. The application of the analysis has been extended to in-situ interface reactions of SiC/Cu/Ni and SiC/Cr/Ni. New reaction modes representing SiC/metal reactions are considered as well as a strategy to control the reaction pathway.

Current status of silicon carbide based high-temperature gas sensors

Abstract: Silicon carbide (SiC) based field effect gas sensors can be operated at very high temperatures. Catalytic metal-insulator-silicon carbide (MISiC) Schottky diodes respond very fast to a change between a reducing and an oxidizing atmosphere, and cylinder-specific combustion engine monitoring has been demonstrated. The sensors have also been suggested for high-temperature electronic nose applications. Car applications and other harsh environments put very strong requirements on the long-term stability of the sensors. Here we review the current status of the field of SiC based Schottky diode gas sensors with emphasis on the work in our group. Basic work on understanding of the detection mechanism and the influence of interfacial layers on the long-term stability of the sensors is reviewed. The direction of future research and device development in our group is also discussed.

Method for growing beta-silicon carbide nanorods, and preparation of patterned field-emitters by chemical vapor depositon (CVD)

Abstract: A method and an apparatus have been developed to grow beta-silicon carbide nanorods, and prepare patterned field-emitters using different kinds of chemical vapor deposition methods. The apparatus includes graphite powder as the carbon source, and silicon powder as silicon sources. Metal powders (Fe, Cr and/or Ni) are used as catalyst. Hydrogen was the only feeding gas to the system.

Phase transformations in non-stoichiometric vanadium carbide

Abstract: X-ray diffraction, differential scanning calorimetry and reflected light microscopy studies are made of disorder-order phase transformations in the region of homogeneity of a non-stoichiometric cubic vanadium carbide, (0.66 < y < 0.88). It is found that according to the composition of the carbide, both an ordered phase with monoclinic or trigonal symmetry and a cubic ordered phase may form in this carbide at a temperature below 1450 K. Consideration is given to the effect that non-stoichiometry and the ordering of structural vacancies have on the heat capacity of the carbide. Temperatures and heats of reversible equilibrium disorder-order transitions are determined. The ordering transformations and are shown to be first-order phase transitions. An equilibrium phase diagram of the V-C system is constructed which allows for the formation of ordered phases in a non-stoichiometric vanadium carbide. The order parameter functional method is used for calculation of phase equilibria in the region of ordering of non-stoichiometric cubic carbide.

Epitaxial silicon carbide charge particle detectors

Abstract: The radiation detection properties of Schottky detectors made on epitaxial layers of 4H silicon carbide were evaluated. Exposure to 5.48 MeV alpha particles from a 241 Am source in vacuum led to robust signals from the detectors. The collection of the charge carriers was found to increase linearly with the square root of the applied reverse bias.