Showing papers on "Carbide published in 1997"

Nanocrystalline carbide/amorphous carbon composites

Abstract: Nanocrystalline TiC/amorphous carbon (a-C) composite films were synthesized at near room temperature with a hybrid process combining laser ablation of graphite and magnetron sputtering of titanium. Film microstructure was investigated by x-ray photoelectron spectroscopy, x-ray diffraction analyses, and transmission electron microscopy. Mechanical properties were evaluated from nanoindentation, scratch, and friction tests. The films consisted of 10 nm sized TiC crystallites encapsulated in a sp3 bonded a-C matrix. They had a hardness of about 32 GPa and a remarkable plasticity (40% in indentation deformation) at loads exceeding their elastic limit. They were also found to have a high scratch toughness in addition to a low (about 0.2) friction coefficient. The combination of hardness and ductility was correlated with film phase composition and structural analyses, using concepts of nanocomposite mechanics. The properties of the TiC/a-C composites make them beneficial for surface wear and friction protection.

Chemical reactivity of aluminium with boron carbide

Abstract: The chemical reactivity of boron carbide (B4C) with metallic aluminium (Al) was studied at temperatures ranging from 900 to 1273 K (627–1000 °C). Al–B4C powder mixtures were cold pressed, heated for 1–450 h under 105 Pa of purified argon and characterized by X-ray diffraction (XRD) optical metallography (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Whatever the temperature in the investigated range, B4C has been observed to react with solid or liquid Al. As long as the temperature is lower than 933 K (660 °C), i.e. as long as Al is in the solid state, interaction proceeds very slowly, giving rise to the formation of ternary carbide (Al3BC) and to diboride (AlB2). At temperatures higher or equal to 933 K, Al is in the liquid state and the reaction rate increases sharply. Up to 1141 ± 4 K (868 ± 4 °C), the reaction products are Al3BC and AlB2: at temperatures higher than 1141 K, Al3 BC is still formed while Al3B48C2 (β-AlB12) replaces AlB2. In the three cases, interaction proceeds via the same mechanism including, successively, an incubation period, saturation of aluminium in B and C, nucleation and growth by dissolution–precipitation of Al3BC and a C-poor boride and, finally, the passivation of B4C by Al3BC. These results are discussed in terms of solid–liquid phase equilibria in the Al–B–C ternary system, with reference to the binary invariant transformation: α-AlB12 + L ⇔ AlB2, which has been found to occur at 1165 ± 5 K (892 ± 5 °C).

Preparation of carbide nanorods

Abstract: A process utilizing a supported metal catalyst, a volatile species source, and a carbon source has been developed to produce carbide nanorods with diameters of less than about 100 nm and aspect ratios of 10 to 1000. The volatile species source, carbon source, and supported metal catalyst can be used to produce carbide nanorods in single run, batch, and continuous reactors under relatively mild conditions. The method employs a simple catalytic process involving readily available starting materials.

Silicon carbide metal-insulator semiconductor field effect transistor

Abstract: A silicon carbide metal-insulator semiconductor field effect transistor having a u-shaped gate trench and an n-type silicon carbide drift layer. A p-type region is formed in the silicon carbide drift layer and extends below the bottom of the u-shaped gate trench so as to prevent field crowding at the corner of the gate trench. A unit cell of a metal-insulator semiconductor transistor having a bulk single crystal silicon carbide substrate of n-type conductivity silicon carbide. A first epitaxial layer of n-type conductivity silicon carbide and a second epitaxial layer of p-type conductivity silicon carbide formed on the first epitaxial layer. A first trench is formed which extends downward through the second epitaxial layer and into the first epitaxial layer. A second trench, adjacent the first trench, is also formed extending downward through the second epitaxial layer and into the first epitaxial layer. A region of n-type conductivity silicon carbide is formed between the first and second trenches and having an upper surface opposite the second epitaxial layer. An insulator layer is formed in the first trench where the upper surface of the gate insulator layer formed on the bottom of the first trench is below the lower surface of the second epitaxial layer. A region of p-type conductivity silicon carbide is formed in the first epitaxial layer below the second trench. Gate and source contacts are formed in the first and second trenches respectively and a drain contact is formed on the substrate. Preferably the gate insulator layer is an oxide such that the transistor formed is a metal-oxide field effect transistor.

High Temperature Sensors Based on Metal–Insulator–Silicon Carbide Devices

Abstract: High temperature gas sensors based on catalytic metal-insulator-silicon carbide (MISiC) devices are developed both as capacitors and Schottky diodes. A maximum operation temperature of 1000 degrees C is obtained for capacitors based on 4H-SiC, and all sensors work routinely for several weeks at 600 degrees C. Reducing gases like hydrocarbons and hydrogen lower the flat band voltage of the capacitor and the barrier height of the diode. The time constants for the gas response are in the order of milliseconds and because of this good performance the sensors are tested for combustion engine control. For temperatures around 600 degrees C total combustion occurs on the sensor surface and the signal is high for fuel in excess and low for air in excess. At temperatures around 400 degrees C the response is more linear. The high temperature operation causes interdiffusion of the metal and insulator layers in these devices; and this interdiffusion has been studied. At sufficiently high temperatures the inversion capacitance shows different levels for hydrogen free and hydrogen containing ambients, which is suggested to be due to a reversible hydrogen annealing effect at the insulator-silicon carbide interface.

Double cemented carbide composites

Abstract: Double cemented carbide composites comprise a plurality of first regions and a second ductile phase that separate the first regions from each other. Each first region comprises a composite of grains and a first ductile phase bonding the grains. The grains are selected from the group of carbides consisting of W, Ti, Mo, Nb, V, Hf, Ta, and Cr carbides. The first ductile phase is selected from the group consisting of Co, Ni, Fe, alloys thereof, and alloys with materials selected from the group consisting of C, B, Cr, Si, and Mn. A preferred first region comprises tungsten carbide grains that are cemented with a cobalt first binder phase and which are in the form of substantially spherical pellets. The second ductile phase is selected from the group consisting of Co, Ni, Fe, W, Mo, Ti, Ta, V, Nb, alloys thereof, and alloys with materials selected from the group consisting of C, B, Cr, and Mn. A preferred second ductile phase is cobalt. Additionally, additives such as those selected from the group consisting of carbides, nitrides, and borides can be added to the second ductile phase to provide improved properties of wear resistance. The composites are prepared by combining hard phase particles formed from the grains and first ductile phase, with the second ductile phase material under conditions of pressure and heat, and have improved properties of fracture toughness and equal or better wear resistance when compared to conventional cemented tungsten carbide materials.

Reactivities of transition metals with carbon: Implications to the mechanism of diamond synthesis under high pressure

Abstract: The ability for a transition metal to react with carbon increases with its number of electron vacancies in d -orbitals. Elements (e.g. Cu, Zn) with no d vacancies are inert relative to carbon. Elements (e.g. Fe, Co) with few d -vacancies are effective carbon solvents. Elements (e.g. Ti, V) with many d -vacancies are carbide formers. Transition metals with intermediate reactivities can attract carbon atoms in graphite without forming a carbide. Such a moderation of interaction may catalyze the graphite → diamond transition in the diamond stability field and its back conversion in graphite stability field. The catalytic conversion of graphite to diamond, under high pressure in a molten metal, proceeds by nucleation and growth. Graphite will first be disintegrated into flakes by the invasion of liquid metal. These flakes are then puckered by the catalytic action of liquid metal that pulls every other carbon atom away from the basal planes of graphite. The puckering converts a graphite flake into a diamond nucleus that grows by feeding on either carbon atoms dissolved, or graphite flakes suspended in the molten catalyst. The capability of a catalyst to nucleate and grow diamonds under high pressure may be modeled by its atomic size and electronic configuration. This model predicts that the most powerful elemental catalysts are Co, Fe, Mn, Ni and Cr. These transition metals are the most commonly used catalyst components for the commercial production of synthetic diamond under high pressure. The catalytic power, as already determined is based on a microscopic mechanism described in this research, also correlates well with the activation energies calculated from macroscopic kinetics data available in literature

Effect of Carbide Grain Size on the Sliding and Abrasive Wear Behavior of Thermally Sprayed WC-Co Coatings

Abstract: The carbide size and cobalt content of thermally sprayed tungsten carbide/cobalt coatings (WC-Co) can influence their microstructure, fracture strength, friction response and abrasion resistance. In this paper, these properties have been determined for one commercially available and three experimental WC-17 wt.% Co thermally sprayed coatings. The experimental coatings were processed from starting powders containing median carbide size distributions of 1.2, 3.8 and 7.9μm, respectively. All the coatings were produced using a high velocity oxy-fuel (HVOF) spray process. The present results indicate that coatings with a higher percentage of finer carbide size distribution in the starting powder display a higher degree of decomposition of the WC phase to W2C phase and, consequently, display lower fracture toughness and abrasion resistance values. Unidirectional, unlubricated sliding wear tests did not reveal major differences in the sliding wear response of the coatings as a function of carbide size. The micro...

s-Process Zirconium in Presolar Silicon Carbide Grains

Abstract: The isotopic composition of zirconium in silicon carbide grains from the Murchison meteorite was measured by resonant ionization mass spectrometry of laser-ablated neutral atoms. These grains are condensates from the atmospheres of red giant stars that existed before the formation of our sun and solar system, and they contain records of nucleosynthesis in these stars. The r-process–dominated isotope zirconium-96 was depleted by more than a factor of 2 compared with the s-process–dominated isotopes zirconium-90, zirconium-91, zirconium-92, and zirconium-94, in agreement with expectations for neutron capture nucleosynthesis in asymptotic giant branch stars.

Polycrystalline diamond cutter with integral carbide/diamond transition layer

Abstract: A composite body cutting instrument formed of a polycrystalline diamond layer sintered to a carbide substrate with a carbide/diamond transition layer. The transition layer is made by creating carbide projections perpendicular to the plane of the carbide substrate face in a random or nonlinear orientation. The transition layer manipulates residual stress caused by both thermal expansion and compressibility differences between the two materials and thus increases attachment strength between the diamond and carbide substrate by adjusting the pattern, density, height and width of the projections.

Surface phonon dispersion in graphite and in a lanthanum graphite intercalation compound

Abstract: Using high-resolution electron energy-loss spectroscopy the surface-phonon dispersion of graphite has been determined in the \ensuremath{\Gamma}K direction over the whole energy range and the whole Brillouin zone. Born\char21{}von Karman model calculations are used to describe the dispersion relations. An unexpected result is the splitting of the ZA and ZO mode at the K point. Following a previously introduced procedure to form in situ rare-earth graphite intercalation compounds (GIC), which for lanthanum results in an intermediate carbide phase, we prepared this carbidic phase and the final GIC-like phase. The carbide shows five dispersionless features that may be attributed to Einstein modes of graphite islands. The phonon dispersion of the final phase shows the same modes as graphite shifted in energy: softening of the optical and stiffening of the acoustical phonons occurs. This is described within a Born\char21{}von Karman model by weakening the nearest-neighbor interaction and strengthening the second-nearest-neighbor interaction. The evolution of the phonon dispersion gives a first hint that the GIC-like phase may develop in two stages: first a monolayer graphene on top of the carbide and then the very thin GIC layer.

Silicon carbide semiconductor device with trench

Abstract: A silicon carbide semiconductor device having a high blocking voltage, low loss, and a low threshold voltage is provided. An n+ type silicon carbide semiconductor substrate 1, an n- type silicon carbide semiconductor substrate 2, and a p type silicon carbide semiconductor layer 3 are successively laminated on top of one another. An n+ type source region 6 is formed in a predetermined region of the surface in the p type silicon carbide semiconductor layer 3, and a trench 9 is formed so as to extend through the n+ type source region 6 and the p type silicon carbide semiconductor layer 3 into the n- type silicon carbide semiconductor layer 2. A thin-film semiconductor layer (n type or p type) 11a is extendedly provided on the surface of the n+ type source region 6, the p type silicon carbide semiconductor layer 3, and the n- type silicon carbide semiconductor layer 2 in the side face of the trench 9. A gate electrode layer 13 is disposed through a gate insulating layer 12 within the trench 9. A source electrode layer 15 is provided on the surface of the p type silicon carbide semiconductor layer 3 and on the surface of the n+ type source region 6, and a drain electrode layer 16 is provided on the surface of the n+ type silicon carbide semiconductor substrate 1.

Magnetron-sputtered superhard materials

Abstract: Superhard materials such as nanocrystalline cubic boron nitride (c-BN) and β -silicon carbide ( β -SiC) as well as amorphous boron carbide (B 4 C) and highly tetrahedral amorphous carbon (ta-C) are produced by radio frequency (RF) unbalanced magnetron sputtering in combination with intense ion plating in a pure argon discharge. As a result of energy and mass analysis the film-forming fluxes Φ n consist of sputtered atomic target components and the plating flux Φ Ar + of argon ions. Subplantation, ion-plating-induced increase of surface mobility and substrate-temperature-induced crystallisation are the three main parameters affecting the formation of superhard phases with strong covalent bonding. Knock-on subplantation allows the formation of B 4 C with hardness up to 72 GPa at a flux ratio Φ Ar + / Φ n of 3 for a plating energy of 75 eV. Also c-BN and ta-C can be produced with similar parameters. In the case of SiC, densification is diminished by preferential sputtering of Si and consequently stochiometry and hardness are adversely affected. However, intense ion plating with a low ion energy of 25 eV and small film-forming fluxes shift the temperature of the phase transition from amorphous to nanocrystalline β -SiC from the usual value of >900 °C to about 420 °C. Furthermore, investigations of the formation of superhard materials in the ternary system boron–carbon–nitrogen are reported.

Comparison of the oxidation behaviour of two dense hot isostatically pressed tantalum carbide (TaC and Ta2C) Materials

Abstract: Isothermal oxidation of dense HIPed tantalum carbide materials TaC and Ta2C, has been performed in flowing oxygen between 750 and 850 °C. The behaviour of the two carbides: i.e. TaC (NaCl type structure) and Ta2C (hexagonal type), is characterized by the growth of a non-protective oxide scale which, on square section samples, forms a maltese cross. X-Ray diffraction analysis has only shown the formation of tantalum hemipentoxide βTa2O5. The oxidation of TaC proceeds by an interfacial reaction process. For Ta2C, the mechanism could be more complex due to the presence of an intermediate oxycarbide layer TaCxOy which has been detected at the Ta2C-Ta2O5 interface. Indeed, in this case, it is not possible to exclude a diffusion limiting process through this oxynitride sublayer of constant thickness with time.

Modelling precipitation sequences in power plant steels Part 1 – Kinetic theory

Abstract: The ability of steels to resist creep deformation depends on the presence in the microstructure of carbides and intermetallic compounds which precipitate during tempering or during elevated temperature service. The precipitation occurs in a sequence which leads towards thermodynamic equilibrium. The present paper deals with an extension of the Johnson-Mehl-Avrami theory for overall transformation kinetics. The modification permits the treatment of more than one precipitation reaction occurring simultaneously, afeature which isfound to be essential for representing the reactions observed experimentally in a wide range of secondary hardening steels.

Electrically assisted synthesis of particles and films with precisely controlled characteristics

Abstract: The present invention relates to methods of manufacturing oxide, nitride, carbide, and boride powders and other ceramic, organic, metallic, carbon and alloy powders and films and their mixtures having well-controlled size and crystallinity characteristics. This invention relates, more particularly, to a development in the synthesis of fine ceramic, metallic, composite, carbon and alloy nanometer-sized particles with precisely controlled specific surface area, or primary particle size, crystallinity and composition. The product made using the process of the present invention and the use of that product are also claimed herein.

Transition metal carbides, nitrides, and borides for electronic applications

Abstract: Compounds of transition metals from Groups IV and V with carbon, nitrogen, or boron (e.g., NbC, TiN, and ZrB2) are electronically conductive but are also very hard and have high melting points. These materials resist electromigration and prevent diffusion because their strong interatomic bonding makes the activation energy for diffusion very high. Carbides and nitrides form the NaCl crystal structure, but are nonstoichiometric with nonmetal atom vacancies that scatter electrons. This defect-controlled resistivity can be eliminated with an order-disorder transformation at a specific nonmetal/metal ratio. The diborides are essentially sentially stoichiometric and have low resistivities. These metallic ceramics can be deposited as thin films to form interconnects and diffusion barriers in ultralarge-scale integrated circuits.

Palladium black as model catalyst in the hydrogenolysis of CCl2F2 (CFC-12) into CH2F2 (HFC-32)

Abstract: Palladium black is applied as a model catalyst for the hydrogenolysis of CCl 2 F 2 into CH 2 F 2 over carbon supported palladium. The performance of palladium black is comparable with that of palladium on activated carbon. Fresh and used samples are characterized with X-ray diffraction (XRD), temperature programmed reduction (TPR), and temperature programmed oxidation (TPO) in a differential scanning calorimeter (DSC). Under reaction conditions, at temperatures as low as 423 K, palladium is converted into palladium carbide (PdC 0.15 ). The amount of carbon on the catalytic surface is negligible. Fluorine present has no catalytic effect. Methane treatment (523 K) and ethene reatment (448 K) also lead to the formation of palladium carbide. In those cases carbon is deposited on the surface of the palladium. The amount of hydrogen in fresh palladium, determined by measuring the temperature of decomposition of palladium hydride at different partial pressures of hydrogen and by measuring the heat of decomposition in DSC, is as PdH 0.6 . This amount is in agreement with the value as obtained by TPR and as reported in literature.