Showing papers on "Carbide published in 1994"

Buffer structure between silicon carbide and gallium nitride and resulting semiconductor devices

Abstract: A transition crystal structure is disclosed for providing a good lattice and thermal match between a layer of single crystal silicon carbide (25) and a layer of single crystal gallium nitride (24). The transition structure comprises a buffer formed of a first layer of gallium nitride and aluminum nitride (22), and a second layer of gallium nitride and aluminum nitride (23) adjacent to the first layer. The mole percentage of aluminum nitride in the second layer (23) is substantially different from the mole percentage of aluminum nitride in the first layer (22). A layer of single crystal gallium nitride (24) is formed upon the second layer of gallium nitride and aluminum nitride. In preferred embodiments, the buffer further comprises an epitaxial layer of aluminum nitride upon a silicon carbide substrate.

Superconductivity at 23 K in yttrium palladium boride carbide

Abstract: COPPER oxide compounds have dominated superconductivity research since 1986 because of their very high transition temperatures (Tcs). In contrast, no new families of high-Tc intermetallic compounds have been discovered since the A15-type Nb3X compounds were first reported in 19531. The intermetallies with highest Jcs have all been based on niobium, with the highest Tcs being 20.7 K for bulk Nb3Ga and 23.2 K for sputtered films of Nb3Ge (refs 2, 3). Here we report the observation of superconductivity at 23 K in a multiple-phase bulk sample of a quaternary intermetallic, yttrium palladium boride carbide. This is higher than any Tc reported previously for a bulk intermetallic compound. Although the materials are not yet single-phase, the superconducting volume fraction is large. We propose that this compound may represent the first of a new family of superconducting intermetallics with relatively high Tcs.

Solid-state synthesis and characterization of the ternary phase Ti3SiC2

Abstract: Ti3SiC2 is the only true ternary compound in the Ti-Si-C system. It seems to exhibit promising thermal and mechanical behaviour. With the exception of its layered crystal structure, most of its properties are unknown, owing to the great difficulty of synthesis. A new procedure of solid-state synthesis with several steps is proposed, which results in Ti3SiC2 with less than 5 at % of TiC. Ti3SiC2 is stable at least up to 1300 °C. Beyond this temperature, it can decompose with formation of non-stoichiometric titanium carbide and gaseous silicon, with kinetics highly dependent on the nature of the surroundings. As an example, graphite can initiate this process by reacting with silicon, while alumina does not favour the decomposition which remains very slow. The oxidation of Ti3SiC2 under flowing oxygen starts at 400 °C with formation of anatase-type TiO2 film, as studied by TGA, XRD, SEM and AES. Between 650 and 850 °C both rutile and anatase are formed, rapidly becoming protecting films and giving rise to slow formation of SiO2 and more TiO2. The oxidation kinetics is slower than for TiC, owing to a protecting effect of silica. By increasing the temperature, both oxidation processes (i.e. direct reaction and diffusion through oxide layers) are activated and an almost total oxidation is achieved between 1050 and 1250 °C resulting in titania (rutile) and silica (cristobalite).

Role of Eta-carbide Precipitations in the Wear Resistance Improvements of Fe-12Cr-Mo-V-1.4C Tool Steel by Cryogenic Treatment

Abstract: The wear resistance of an Fe-12.2wt%Cr-0.84wt%Mo-0.43wt%V-1.44wt%C alloy tool steel after cold treatment at 223K (subzero treatment) and after cryogenic treatment 93K (ultra-subzero treatment) has been investigated. The wear resistance of steels after cryogenic treatment is superior to that after cold treatment. The effects of cryogenic treatment on the microstructure were also studied by means of X-ray diffraction and transmission electron microscopy methods. Unlike cold treatment, cryogenic treatment improves the preferential precipitation of fine η-carbides instead of e-carbides. These fine carbide particles enhance the strength and thoughness of the martensite matrix and then increase the wear resistance. The formation mechanism of fine η-carbide is discussed.

Toughness Properties of a Silicon Carbide with an in Situ Induced Heterogeneous Grain Structure

Abstract: Toughness characteristics of a heterogeneous silicon carbide with a coarsened and elongated grain structure and an intergranular second phase are evaluated relative to a homogeneous, fine-grain control using indentation–strength data. The heterogeneous material exhibits a distinctive flaw tolerance, indicative of a pronounced toughness curve. Quantitative evaluation of the data reveals an enhanced toughness in the long-crack region, with the implication of degraded toughness in the short-crack region. The enhanced long-crack toughness is identified with crack-interface bridging. The degraded short-crack toughness is attributed to weakened grain or interface boundaries and to internal residual stresses from thermal expansion mismatch. A profound manifestation of the toughness-curve behavior is a transition in the nature of mechanical damage in Hertzian contacts, from classical single-crack cone fracture in the homogeneous control to distributed subsurface damage in the heterogeneous material.

Chemical order in amorphous silicon carbide.

Abstract: While ordering in alloy crystals is well understood, short-range ordering in amorphous alloys remains controversial. Here, by studying computer-generated models of amorphous SiC, we show that there are two principal factors controlling the degree of chemical order in amorphous covalent alloys. One, the chemical preference for mixed bonds, is much the same in crystalline and amorphous materials. However, the other factor, the atomic size difference, is far less effective at driving ordering in amorphous material than in the crystal. As a result, the amorphous phase may show either strong ordering (as in GaAs), or weaker ordering (as in SiC), depending upon the relative importance of these two factors.

Formation of carbon films on carbides under hydrothermal conditions

Abstract: CARBON films find applications in a wide range of fields, ranging from microelectronics to materials science1. In ceramic matrix composites they confer the high strength and toughness needed for applications in aerospace, nuclear and automotive engineering2. Chemical vapour deposition is traditionally used to prepare carbon films, but it is relatively expensive, and not easily adapted to coating samples in the form of whiskers, platelets or powders. Here we report that silicon carbide, the most common component of composite ceramics, can be coated with carbon films of nanometre to micrometre thickness by hydrothermal treatment at 300–800 °C. We have applied the technique to SiC fibres, powders, platelets and single crystals, as well as to other carbides. Our method should provide a general and inexpensive route to high-toughness composites and lubricating coatings.

Drill bit inserts enhanced with polycrystalline diamond

Abstract: A drill bit has means at one end for connecting the bit to a drill string and a plurality of inserts at the other end for crushing the rock to be drilled. The inserts have a cemented tungsten carbide body partially embedded in the drill bit and at least two layers at the protruding drilling portion of the insert. The outermost layer contains polycrystalline diamond and particles of carbide or carbonitride of elements selected from the group consisting of W, Ti, Ta, Cr, Mo, Cb, V, Hf and Zr. The remaining layers adjacent the polycrystalline diamond layer are transition layers each comprising a composite containing diamond crystals, particles of tungsten carbide, and particles of titanium carbonitride. The average size of the diamond particles in the polycrystalline diamond layer is greater than the average size of the carbide or carbonitride particles; and the average size of the diamond particles in the transition layers is greater than the average sizes of the carbide and carbonitride particles. In particular, the transition layers contain particles of carbide and/or carbonitride with average grain sizes of less than one micrometer. The outermost layer of polycrystalline diamond extends along at least a portion of the length of the grip portion of the carbide body embedded in the drill bit.

Electronic Structure of Refractory Carbides and Nitrides

Abstract: 1. Theoretical methods used to calculate the electronic structure and properties of refractory compounds 2. Electronic structure, chemical bonding and properties of binary carbides 3. Electronic structure and interatomic interactions in transition-metal nitrides 4. Electronic structure and properties of nonstoichiometric carbides and nitrides 5. S-, P-element impurities in carbides, nitrides, and their solid solutions 6. Hydrogen-containing carbides and nitrides and their solid solutions 7. Influence of metal sublattice doping on electronic properties of carbides and nitrides 8. Electronic structure of carbides and nitrides surface.

Isomerization of n-Heptane on an Oxygen-Modified Molybdenum Carbide Catalyst

Abstract: The isomerization of n-heptane in the presence of hydrogen has been carried out over a molybdenum carbide catalyst modified by an oxygen treatment. Heptane was isomerized selectively to isoheptanes, a reaction which is difficult over traditional bifunctional catalysts due to extensive cracking. The C[sub 7] products were mainly monomethylhexanes, 2-methylhexane and 3-methylhexane, in close to equilibrium ratios. A typical bifunctional catalyst (Pt supported on an acidic zeolite) gave similar isomerization products, but mostly propane and isobutane as the cracked products. The selectivity over the oxidized carbide was found to be a function of pressure but independent of the conversion; increased the hydrogen pressure led to a decrease in the C[sub 7] selectivity. This was found to be different from the Pt/zeolite catalyst, over which the selectivity was a function of the conversion; a high selectivity was only obtained at low conversions. The active carbide-based catalyst was probably an oxycarbide of molybdenum. The results obtained over the oxidized carbide catalyst are discussed in terms of a bond-shift mechanism via a metallocyclobutane intermediate.

Tensile Creep Behavior of Alumina/Silicon Carbide Nanocomposite

Abstract: Tensile creep and creep rupture behaviors of alumina/17 vol% silicon carbide nanocomposite and monolithic alumina were investigated at 1,200 to 1,300 C and at 50 to 150 MPa. Compared to the monolithic alumina, the nanocomposite exhibited excellent creep resistance. The minimum creep rate of the nanocomposite was about three orders of magnitude lower and the creep life was 10 times longer than those of the monolith. The nanocomposite demonstrated transient creep until failure, while accelerated creep was observed in the monolith. It was revealed that rotating and plunging of intergranular silicon carbide nanoparticles into the alumina matrix increased the creep resistance with grain boundary sliding.

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

Abstract: The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M7C3 carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M7C3 and V6C5 carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M23C6 carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M7C3 carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.

Cemented carbide with binder phase enriched surface zone and enhanced edge toughness behavior and process for making same

Abstract: Cemented carbide inserts are available containing WC and cubic phases of carbide and/or carbonitride in a binder phase based on cobalt and/or nickel with a binder phase enriched surface zone The binder phase content along a line essentially bisecting the rounded edge surfaces increases toward the edge and cubic phase is present As a result, the edge toughness of the cutting inserts is improved

Synthesis of Nanoscale Metal Oxide Particles Using Laser Vaporization/Condensation in a Diffusion Cloud Chamber

Abstract: A novel method which combines laser vaporization of metals with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide and carbide particles (10-20 nm) of homogeneous size and well-defined composition. The following oxides have been synthesized: ZnO, SiO 2 , Fe 2 O 3 , Bi 2 O 3 , PdO, NiO, AgO, TeO, Sb 2 O 3 , TiO 2 , ZrO 2 , Al 2 O 3 , CuO, In 2 O 3 , SnO, V 2 O 5 , and MgO. The microscale structures of the SiO 2 and Al 2 O 3 particles exhibit secondary open-structure arrays of interesting weblike matrices with significant volumes of voids in the aggregate structure. These materials may have special applications in catalysis and as reinforcing agents for liquid polymers

Multiple grade cemented carbide articles and a method of making the same

Abstract: The present invention provides multiple grade, composite cemented carbide articles and a method of making such articles The cemented carbide articles comprise carbides of different grades (or different compositions and/or microstructures) and, therefore, correspondingly different properties at different locations in the same article The method of the present invention comprises filling different areas or portions of a die with metallurgical powders having different compositions and/or microstructures The powder is then compressed as a single compact in the die cavity The compressed compact is subsequently sintered to produce a multigrade cemented carbide article having composition and/or microstructural variations within the volume of the article

Method of making composite cermet articles

Abstract: Methods for making, methods for using and articles comprising cermets, preferably cemented carbides and more preferably tungsten carbide, having at least two regions exhibiting at least one property that differs are discussed. Preferably, the cermets further exhibit uniform or controlled wear to impart a self-sharpening character to an article. The multiple-region cermets are particularly useful in wear applications. The cermets are manufactured by juxtaposing and densifying at least two powder blends having different properties (e.g., differential carbide grain size or differential carbide chemistry or differential binder content or differential binder chemistry or any combination of the preceding). Preferably, a first region of the cermet comprises a first ceramic component having a relatively coarse grain size and a prescribed binder content and a second region, juxtaposing or adjoining the first region, comprises a second ceramic component, preferably carbide(s), having a grain size less than the grain size of the first region, a second binder content greater than the binder content of the first region or both. These articles have an extended useful life in such applications as, for example, wear. The multiple region cermets of the present invention may be used with articles comprising tools for materials manipulation or removal including, for example, mining, construction, agricultural, and metal removal applications.

Supported polycrystalline diamond compact having a cubic boron nitride interlayer for improved physical properties

Abstract: Metal carbide supported polycrystalline diamond (PCD) compacts having improved shear strength and impact resistance properties, and a method for making the same under high temperature/high pressure (HT/HP) processing conditions. A sintered polycrystalline cubic boron nitrite (PCBN) compact interlayer is provided to be bonded at a first interface to a sintered PCD compact layer, and at a second interface to a cemented metal carbide support layer comprising particles of a metal carbide in a binder metal. The supported compact is characterized as having a substantially uniform sweep through of the binder metal from the cemented metal carbide support layer, which sweep through bonds the sintered PCD compact layer to the sintered PCBN interlayer, and the sintered PCBN interlayer to the cemented metal carbide support layer.

Influence of microstructure on tensile properties of spheroidized ultrahigh-carbon (1.8 Pct C steel

Abstract: Ultrahigh-carbon steel (UHCS) containing 1.8 pct carbon was processed to create microstructures consisting of fine-spheroidized carbide particles (0.2- to 1.5-μm size range) within a fine-grained ferrite matrix (0.3- to 5-μm range) through a variety of thermomechanical processing and heat-treatment combinations. Tensile ductility, yield, and fracture strengths, and strain-hardening behavior were evaluated at room temperature. Yield strengths ranged from 640 to 1450 MPa, and uniform tensile elongation ranged from 3 to 23 pct. Quantitative analyses revealed that a Hall-Petch type relationship exists between the yield strength and the ferrite grain size and carbide particle size within grain interiors. The fracture strength, on the other hand, was found to be uniquely dependent on the coarse carbide particle size typically found at grain boundaries. Data from other investigators on spheroidized carbon steels were shown to correlate well with the data for the UHCS (1.8 pct C) material. It was shown that the tensile ductility will increase when the difference between the fracture strength and the yield strength is increased and when the strain-hardening rate is decreased. The basis for the trends observed is that the tensile ductility is limited by the fracture process that appears to be dictated by the nucleation of cracks at large carbide particles. The results obtained indicate that UHCSs have significant potential for sheet applications where high strength and good ductility are primary requirements.

Effects of carbon content on mechanical properties of 5%Mn steels exhibiting transformation induced plasticity

Abstract: A series of highly ductile, high strength steels exhibiting transformation induced plasticity due to retained austenite was developed by varying the carbon content in the range 0·01–0·4 wt-% in 5 wt-%Mn based steel. For up to 0·l%C the mechanical properties are insensitive to cooling rate after intercritical heating, but afurther increase in carbon content causes a large sensitivity to the cooling rate, owing to carbide precipitation occurring during slow cooling. By suppressing this carbide precipitation with water quenching after the intercritical holding, an excellent combination of tensile strength (1580 MN m−2) and uniform elongation (21%) was attained at 0·3%C in this series.MST/1964

Method of making composite cermet articles and the articles

Abstract: Methods for making, methods for using and articles comprising ferromagnetic cermets, preferably cemented carbides and more preferably tungsten carbide, having at least two regions exhibiting at least one property that differs are discussed. The multiple-region cermets are particularly useful in wear applications. The cermets are manufactured by juxtaposing and densifying at least two powder blends having different properties (e.g., differential carbide grain size or differential carbide chemistry or differential binder content or differential binder chemistry or differential magnetic saturation or any combination of the preceding). Preferably, a first region of the cermet comprises a first hard component having a prescribed binder content and a first magnetic saturation and a second region, juxtaposing or adjoining the first region, comprising a second binder content different than the binder content of the first region and a second magnetic saturation different than that of the first region. These articles have an extended useful life relative to the useful life of monolithic cermets in such applications as, for example, wear. The multiple region cermets of the present invention may be used with articles comprising tools for materials manipulation or removal including, for example, mining, construction, agricultural, and machining applications.

Effect of solidification conditions on MC carbides in a nickel-base superalloy IN 738 LC

Abstract: Transition metal carbides (MC) formed in the solidification process of cast nickel-base superalloys were found to affect greatly the mechanical properties and boundary performance of alloys. Thus, investigations of their growth behavior have received considerable concern from metallurgists. Earlier studies on such carbides were mostly confined to normal solidification conditions, i.e. equiaxed and directionally solidified conditions. Recently, some researchers also investigated the carbides with the rapid solidification method. However, there is lack of systematic study of growth characters of MC carbides under various solidification conditions. The intent of this paper is to establish a complete description for carbide growth. Accordingly, the variations of morphology, size, amount and distribution of MC carbides in the nickel-base superalloy Inconel 738 LC will be studied under a broad range of cooling rates from 10[sup [minus]2] to 10[sup 5][degree]C/s by use of directional solidification, investment casting, chill casting and splat cooling methods.

Correlation between Volatility of Rare-Earth Metals and Encapsulation of Their Carbides in Carbon Nanocapsules

Abstract: Encapsulation of metals in multilayered graphitic capsules has been studied for all the rare-earth elements (Sc, Y, and Ln = La, Ce, ..., Lu) excluding Pm by using electric arc discharge. Electron microscopy and X-ray diffraction of carbonaceous products revealed that most of rare-earth metals (Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu) were encapsulated in the form of carbides, but the others, Sm, Eu, and Yb, were not. The metals in the former group that were encapsulated had vapor pressures definitely lower than those in the latter group. In the case of thulium (Tm), whose vapor pressure is intermediate between the two groups, only a trace amount of encapsulated carbide was formed. Correlation of volatility of metals with encapsulation was clearly found, suggesting that the volatility of a metal plays an important role in a process of the metal encapsulation. 25 refs., 4 figs., 1 tab.

Multi-layer reflection mirror for soft X-ray to vacuum ultraviolet ray

Abstract: A multi-layer reflection mirror for soft X-ray to vacuum ultraviolet ray, comprises a substrate, a plurality of first layers, and a plurality of second layers formed on the substrate alternately with the first layers. The first layers primarily consists of at least one of single elements, such as ruthenium, or of a boride carbide, silicate, nitride oxide of a transition metal. The second layers primarily consists of at least one of compounds of carbon, silicon (e.g. carbide, nitride and oxide of silicon), boron (e.g. carbide, nitride and oxide of boron), beryllium (e.g. carbide, nitride and oxide of beryllium) and aluminum (e.g. carbide, nitride and oxide of aluminum).

Residual Stresses in Polycrystalline Diamond Compacts

Abstract: The effects of residual stress on the integrity of polycrystalline diamond compact (PDC) cutters was investigated. When the compact cooled from the sintering temperature to room temperature, very high radial compressive stresses were induced in the diamond table, and (generally) much lower radial tensile stresses were induced in the cemented tungsten carbide backing. The magnitudes of these residual stresses were not affected very much by the diameter of the compact. However, the residual stresses were affected significantly by the thickness ratio of the carbide layer to the diamond layer. The higher this ratio the greater was the radial compressive stress in the diamond and the lower was the radial tensile stress in the carbide. This same effect was obtained by sintering a relatively thin layer of tungsten carbide on top of the diamond table.

Emission of blue light from hydrogenated amorphous silicon carbide

Abstract: THE development of new electroluminescent materials is of current technological interest for use in flat-screen full-colour displays1. For such applications, amorphous inorganic semiconductors appear particularly promising, in view of the ease with which uniform films with good mechanical and electronic properties can be deposited over large areas2. Luminescence has been reported1 in the red-green part of the spectrum from amorphous silicon carbide prepared from gas-phase mixtures of silane and a carbon-containing species (usually methane or ethylene). But it is not possible to achieve blue luminescence by this approach. Here we show that the use of an aromatic species—xylene—as the source of carbon during deposition results in a form of amorphous silicon carbide that exhibits strong blue luminescence. The underlying structure of this material seems to be an unusual combination of an inorganic silicon carbide lattice with a substantial 'organic' π-conjugated carbon system, the latter dominating the emission properties. Moreover, the material can be readily doped with an electron acceptor in a manner similar to organic semiconductors3, and might therefore find applications as a conductivity- or colour-based chemical sensor.

Low‐frequency, high‐temperature conductance and capacitance measurements on metal‐oxide‐silicon carbide capacitors

Abstract: Low‐frequency capacitance and conductance measurements have been extensively performed from 1 Hz to 100 kHz and in the 293–673 K temperature range, on metal‐oxide‐semiconductor (MOS) capacitors made on silicon carbide material. The energy distribution of the trap time constants, capture cross sections, and interface‐state density are presented. It is shown that only low‐frequency and high‐temperature measurements may provide the ability to scan the midgap region of the forbidden band gap. The experimental results fully confirm the feasibility of MOS devices on silicon carbide material. Furthermore, conductance measurements at high temperature indicate the presence of deep bulk levels.

On the Homogeneous Chemistry of the Thermal Decomposition of Methyltrichlorosilane Thermodynamic Analysis and Kinetic Modeling

Abstract: Equilibrium gas-phase calculations for the Si/C/Cl/H deposition system are performed over the range of conditions used to deposit silicon carbide (SiC) through the thermal decomposition of methyltrichlorosilane (MTS). The compounds that exist in significant quantities in the gas phase, and thus may influence significantly the chemistry of the process, as well as the main deposition precursors are identified. The effect of temperature, pressure, and initial composition of the reacting mixture on the equilibrium composition of the gas phase is determined, and process conditions that may lead to stoichiometric silicon carbide films are suggested. Based on the results of the equilibrium calculations, a kinetic model for the homogeneous chemistry of the decomposition of MTS is proposed. Several reaction sequences leading to the generation of carbon and silicon deposition precursors are considered and their effects on the chemistry of the system are examined. Finally, the mechanism is incorporated into the reaction and transport model of a plug flow hot-wall reactor and the overall model is used to obtain the spatial variation of the composition of the gas phase under conditions typically encountered in chemical vapor deposition reactors.

Preparation of Silicon Carbide Fiber from Activated Carbon Fiber and Gaseous Silicon Monoxide

Abstract: Crystalline silicon carbide (SiC) fiber was produced by a new, simple procedure. Activated carbon fiber (ACF) was reacted with gaseous silicon monoxide and was converted to SiC fiber at elevated temperatures as low as 1,473 K. The reacted fiber consisted of submicrometer particles which were not observed in the original ACF. The SiC crystal size in the reacted fiber was approximately 30 nm. The microstructure of the fiber became dense after it was heat-treated in air at 1,573 K or nitrogen gas at 1,873 K.