Direct Carburization Research Articles

Properties of Carbide-Metal Cermets Prepared from Composite Powders by Direct Reduction and Carburization Process

The transition-metal carbide cemented by metal has excellent combined properties. In this study, cermets were prepared by vacuum sintering from carbide-metal composite powders. The transition- metal oxides (Cr2O3, MoO3, V2O5, Nb2O5 and TiO2), cementing-metal oxides (Co3O4 and NiO), and carbon black were used as raw materials to pre-synthesize composite powders such as Cr3C2-Co, Mo2C-Co, VC-Co, NbC-Co and TiC-Ni, by a direct reduction and carburization process in vacuum. Results show that the participation of Co3O4 and NiO as well as the vacuum circumstance were greatly propitious to the carburization of transition-metal oxides into carbides. The carbothermal condition was greatly improved by the direct reduction and carburization process.

New synthesis of tungsten carbide particles and the synergistic effect with Pt metal as a hydrogen oxidation catalyst for fuel cell applications

Some new tungsten carbides (WCs) were synthesized by the carburizing of tungsten nitrides (W2N) and tungsten sulfides (WS2) as precursors, which were prepared from different starting materials. The carbides were characterized by BET surface area measurements, XRD, SEM and TEM. WCs synthesized by these procedures afforded higher surface areas than those obtained from a conventional direct carburization of WO3. WC containing a small amount of Pt metal was prepared by a wet impregnation method and evaluated as an anode catalyst for the electrochemical hydrogen oxidation reaction. The property of WC which is associated with its synthetic pathway exerted a significant influence on the catalyst performance. The activity of a binary Pt-WC catalyst was greatly improved by choosing a suitable WC and modifying the preparative method. As a result, it turned out that the mass activity of the Pt-WC catalyst system was superior to the current commercial Pt catalyst although only one-tenth the amount of Pt metal was used. It is thought that there is a synergistic effect between Pt and WC in terms of activity, leading to a drastic reduction in the use of Pt metal. A Pt-WC catalyst system may be regarded as one of the candidates for cost-effective catalysts for the future fuel cell devices.

Synthesis and characterization of high-surface area tungsten carbides and application to electrocatalytic hydrogen oxidation

In analogy with W 2 N and WO 3 , WS 2 and WP were found to undergo the CH 4 carburization to produce α-WC. α-WC obtained from WS 2 and W 2 N exhibited a high specific surface area compared with other routes. The introduction of a small amount of Pt into the WCs prepared in these ways contributed to a remarkable activity enhancement for the hydrogen electro-oxidation reaction. Tungsten carbides (WC) with different crystalline phases prepared from WO 3 as the starting material showed electrocatalytic activity in the order of α-WC > β-WC 1− x > β-W 2 C for the hydrogen oxidation reaction. α-WC obtained by methane carburizing of tungsten nitride (W 2 N), which was prepared from nitridation of WO 3 , produced much higher surface area than that from the direct carburization of WO 3 . We first found capable of carburize tungsten sulfide (WS 2 ) and tungsten phosphide (WP) to produce α-WC though they require severe carburization reaction. In particular, α-WC obtained from WS 2 exhibited a high specific surface area as well as α-WC by way of W 2 N. The XPS analysis revealed that the surface properties of α-WC are significantly dependent on the preparative methods. A large amount of graphite carbon was deposited on the surface of α-WC from direct carburization, whereas, there was far less on α-WC by way of W 2 N and WS 2 .The electrocatalytic activities of WCs obtained in these ways were greatly improved. The binary catalysts of WC promoted with Pt were evaluated in comparison with the current Pt catalyst. The introduction of a small amount of Pt onto the WC contributed to a remarkable anodic current enhancement. It turned out that the specific activity (activity based on the surface area of Pt metal) was higher than that of the commercial catalyst. It is concluded that the interaction Pt with α-WC enhance the utilization of Pt for the hydrogen electro-oxidation reaction.

Fabrication of Nano-Sized WC/Co Composite Powder by Direct Reduction and Carburization with Carbon

Direct reduction and carburization process was thought one of the best methods to make nano-sized WC powder. The oxide powders were mixed with graphite powder by ball milling in the compositions of WC-5,-10wt%Co. The mean size of mixed powders was about 250 nm after milling. The mixture was heated at the temperatures of 600~800°C for 5 hours in Ar. The reaction time of the reduction and carburization was decreased as heating temperatures and cobalt content increased. The mean size of WC/Co composite powders was about 260 nm after the reactions. And the mean size of WC grains in WC/Co composite powder was about 38 nm after the reaction at 800°C for 5 hours.

Effects of Inhibitors on the Synthesization of WC/Co Composite Powders by Mechanochemical Method

Nano-sized WC particles in WC/Co composite powders were synthesized by mechanochemical method. The raw powders(WO3, Co3O4, VC, Cr3C2 and graphite) were mixed by planetary milling with ethyl alcohol for 30 hours. The compositions were WC-10 and -20 wt% Co after reduction and carburization. The powders of VC and Cr3C2 were added as the inhibitors to inhibit the formation of nano-sized WC particles. The direct reduction and carburization of the oxides with carbon were carried at 900°C for 1 to 3 hours under flowing Ar gas. The resultant powders were compacted and sintered at 1300~1360°C for 0.5 hour. The degree of the direct reduction and carburization of the mixed powders increased with the reaction time. The mean size of WC particles in WC/Co composite powders was about 16 nm, and the mean size of WC/Co composite powders was about 240 nm. In WC-10 and -20 wt% Co, the sintered compacts shrank by 16% and 24%, respectively. The mean sizes of WC in WC/Co composite powders were about 50 and 30 nm.

ＷＯ３の直接炭化反応によるナノＷＣ粉末の製造

Nano-sized WC powder that is expected as the raw material for nano-sized WC-Co hardmetals with high hardness and strength was developed. The reaction path and morphological change in the reaction process of WO3 with carbon powder (direct carburization) were firstly investigated by thermogravimetric analysis. The proper reaction condition examined by the thermogravimetric analysis was developed to trial production by direct carburization facilities.The direct carburization reaction from WO3 to WC occurred through the generation of several intermediate products in the order of WO3 → WO2.72 → WO2 → W → W2C → WC. The nano-sized particles were generated at the stage from WO2.72 to WO2. We succeeded in the development of nano-sized WC powder less than 100 nm by controlling the reaction conditions for each intermediate products.

Synthesis and characterization of molybdenum carbides using propane as carbon source

Bulk face-centered-cubic (fcc)-based η-MoC 1− x and hexagonal-close-packed (hcp)-based β-Mo 2C have been prepared using C 3H 8/H 2 by temperature-programmed reaction method and a rapid heating method. In this work, direct carburization of MoO 3 produces η-MoC 1− x or MoO x C y with excess carbon, different from that with CH 4/H 2 or C 2H 6/H 2 as carburization reagent. A successive post-treatment by hydrogen causes the phase transformation from fcc-based η-MoC 1− x or MoO x C y to hcp-based β-Mo 2C. Amorphous SiO 2-supported β-Mo 2C is also successfully prepared by the two methods and passes through the same route as the bulk one. HRTEM, BET surface area measurements and thiophene hydrodesulfurization reaction are conducted for the comparison of the two methods. The results indicate that different ramping rates bring slight difference in specific surface area and initial catalytic activity but obvious difference in particle size to the final product supported β-Mo 2C.

TiC Nanocrystal Formation from Carburization of Laser-Grown Ti/O/C Nanopowders for Nanostructured Ceramics

Refractory carbide ceramics (TiC and ZrC) raise interest as promising materials for high-temperature applications such as structural materials for the future generation of nuclear reactors. In this context, nanostructured ceramics are expected to exhibit improved thermomechanical properties as well as better behavior under irradiation when compared to conventional materials. It is therefore necessary to synthesize carbide nanocrystals of such materials to elaborate the ceramics. We report here the formation study of TiC nanocrystals through the direct carburization of Ti/O/C nanopowders grown by laser pyrolysis. A spray of titanium tetraisopropoxide was laser pyrolyzed with ethylene as the sensitizer, leading to Ti/O/C nanopowders with various C contents controlled by the synthesis conditions. Annealing treatments performed on these nanopowders under an inert atmosphere without any C addition enabled the formation of TiC grains through the carburization of the oxide phase by free C incorporated during the synthesis. The powders were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The final TiC grain size was about 80 nm, and the grains were monocrystalline. The influence of the free C content on the grain growth during the annealing step, together with its effects on the densification of the ceramics after sintering by high-pressure flash sintering, was examined. A 93% densification was finally achieved.

Carburized Carbon Content of Reduced Iron and Direct Carburization in Carbon Composite Iron Ore Pellets Heated at Elevated Temperature

Using coal char, coke and graphite, carbon composite iron ore pellets of 18 mm in diameter with the mixing ratio of [reducible oxygen mol(O)]/[fixed carbon mol(C)]=1 were prepared inside a rotating tire. The pellets were heated in nitrogen at 1 150-1 380°C until reactions just finished. The reacted pellet was separated into a shell part and a core part. The pulverized shell and core parts were repeatedly cleaned in ethanol under ultrasonic vibration to thoroughly remove attached carbon particles. The content of carbon dissolved in reduced iron was measured by analyzing them by the combustion method. The direct carburization through direct contact points is the major reaction and the gaseous carburization is the minor one. Coal char bearing pellets showed carbon contents much less than the others; less than 0.5% in shell and increased with temperature at last exceeding solidus line at 1 300°C in core. Increase in the mixing amount of char showed no change in the carbon content. But roughening the char significantly increased the content. Coke bearing pellets showed carbon contents near to solidus line in shell and between solidus and liquidus lines in core. Graphite bearing pellets showed carbon content almost agreed with solidus line in shell and above liquidus line in core. Free particles of coke and coal char were observed to drop from the void formed inside reacted pellets at 1 300°C or more. Free graphite particles were never observed from reacted pellets in any conditions. Recommended conditions for melting the reduced iron at low temperature were proposed.

Reactions, Coalescence of Reduced Iron Particles, and Liberation of Carbon Particles in Carbon Composite Iron Ore Pellets

Reactions in carbonaceous material composite iron ore pellets of 18 mm heated at elevated temperatures were studied. The overall reduction rate is controlled by a complex combination of direct reactions and indirect reactions and never by gasification reaction. The final reduction of iron oxide is controlled by carburized carbon. The carburization of the reduced iron is mainly progressed by the direct carburization reaction, C(s)=[C]. As the carbon content approaches to the solidus line of the phase diagram of Fe-C system, the reduced iron particles are gradually forced to coalesce to grow in much larger particles. The coalescing rate is increased with temperature. Its driving force is the large surface energy of reduced iron due to its large specific surface area. Due to the coalescence, the particles of carbon are apt to lose the contact state with the reduced iron particles and to lose the ability to carburize the reduced iron. Therefore, the carburization exceeding the solidus line can be severely depressed. Consequently, the carburization up to the solidus line can be achieved at relatively lower temperatures, i.e. 1 200-1 250°C. As a result, the carbon content of reduced iron decreases with increasing temperature. Since the coal char has a very poor ability to directly carburize reduced iron, the carbon contents of reduced iron were very low and increased with increasing temperature. Graphite and coke possess the strongest and the intermediate carburizing abilities, respectively. And the carbon contents of the shell part and core part of reduced iron decreases with increasing temperature.

DECOMPOSITION OF HYDRAZINE BY TUNGSTEN OXYCARBIDES: THE INFLUENCE OF MACROPOROSITY AND MECHANICAL STRENGTH ON A 2 N MICROTHRUSTER ENGINE

The aim of this work was to prepare tungsten oxycarbides by Temperature Programmed Reaction (TPR), starting with previously molded macroporous tungsten oxide (WO3). The tungsten oxycarbides were prepared by one of two different methods: direct carburization with CH4/H2 to obtain WC or preliminary formation of nitrides with NH3 to obtain W2N, followed by carburization to W2C and WC with CH4/H2. Results suggested that the carburization of the W2N was incomplete, resulting in an oxycarbonitride, or did not occur at all, with the end material being W2N. From among the materials prepared, two catalysts were selected as a function of their mechanical properties, and their performances were compared with a 2 N microthruster using the Shell 405 commercial catalyst in the reaction of hydrazine decomposition.

Remarkable support effect of ZrO2 upon the CO2 reforming of CH4 over supported molybdenum carbide catalysts

In reforming of CH4 with CO2 over molybdenum carbide catalysts, the catalytic performance of unsupported hexagonal Mo2C prepared by direct carburization of MoO3 was considerably different from a similar composition, cubic MoC1−x (x≈0.5), prepared through nitriding before carburization. The conversion levels over MoC1−x were substantially higher than those over Mo2C, although the turnover frequencies were lower. X‐ray diffraction analysis indicated that Mo2C deactivated by conversion to MoO2 during the reaction, but the MoC1−x was transformed to the hexagonal Mo2C and remained stable. The activity of Mo2C dispersed on various supports for the CH4–CO2 reaction was also investigated. The performance depended strongly on the property of supports, with the ZrO2‐supported Mo2C catalyst exhibiting the highest activity and durability for this reaction. Moreover, deactivation of Mo2C/ZrO2 at ambient pressure was suppressed by decreasing the loading amount of Mo2C.

Preparation and Characterization of Niobium Carbide and Carbonitride

Unsupported niobium carbide powders of NbC were prepared by carburization of commercial niobium pentoxide in flowing CH4–H2mixtures. Several routes of synthesis were investigated, leading to final carbides with specific surface areas ranging from 10 to 50 m2g−1. During direct carburization, after the first step of reduction of Nb2O5to NbO2, a long induction period which was succeeded by an acceleratory period, was observed for the reduction/carburization to NbC. In the presence of a catalyst of methane decomposition, this induction period was eliminated but the superficial carbon free contamination of the final carbide was increased. However, this contamination can be removed by a reducing treatment and CO chemisorption can be observed. The preparations through niobium oxynitride lead to face-centered cubic carbonitrides with specific surface areas ranging from 27 to 49 m2g−1. Chemical composition and cell parameter of these cabonitrides vary slightly with the synthesis conditions. After a reducing treatment, as for NbC, CO chemisorption was also observed.

Carburization of high chromium alloys

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Preparation and characterization of alumina-supported molybdenum carbide

Molybdenum carbide on alumina was studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature-programmed surface reaction (TPSR) with hydrogen. The molybdenum carbide was prepared by nitridation of 12.5 wt% MoO 3/Al 2O 3 in a flow of NH 3 at 700°C, followed by carburization in a flow of 20% CH 4/H 2 also at 700°C for 3 h. The sample was compared to an unsupported material prepared from MoO 3 in the same manner. The CH 4 formation profile during TPSR was deconvoluted into three peaks with maxima at 490°C, 690°C, and 810°C. The peak at 490°C was assigned to reduction of carbidic carbon, and the peaks at 690°C and 810°C were assigned to the removal of pyrolytic and graphitic carbon, respectively, as indicated by XPS and XRD measurements. Overall, the results suggest that molybdenum carbide was formed on the alumina supported by the carburization treatment at 700°C, in the same manner as with an unsupported reference sample. Prenitridation before carburization resulted in the formation of a carbide with a larger surface area and less free carbon, compared to the carbide formed by direct carburization.

Preparation of nanosize WC-Co composite powders by plasma

Many researchers have given considerable attention 0 to preparing nanosize powders. Among various kinds of preparation methods, the plasma technique has been widely used to prepare nanosize material powders because of its unique advantages. Nanosize -1 powders of carbides, nitrides, borides and oxides have been prepared by a plasma technique [1]. Ultrafine grain cemented carbides made by nanosize carbide powders have high hardness, -2 strength, toughness and wear resistance, and are widely used in industry. The key for making ultrafine ~._ hard metal is to prepare nanosize tungsten carbide powders. -~ -3 O The preparation of nanosize powders of tungsten g carbide (WC) has been studied by Ronsheim and ...J coworkers using argon as the plasma working gas, with tungsten and graphite as reactants [2]. The -4 results were not ideal because of the high boiling point of tungsten. The main phase of the powders is tungsten. Most of the tungsten powders had not been evaporated and reacted. Using low boiling point -5 WO3 instead of tungsten and CH4 as a carbon source, the result was improved: The main phase is Il-WC .or W2C [21. High purity methane is commonly used as a -6 carbon source while using plasma to prepare carbides. There are two problems when using high purity methane: (a) high purity methane is expensive and results in high production costs; (b) according to the C H equivalence diagram, Fig. 1 [3], methane decomposes into many components at high temperarare. At temperatures 2000-4000K, the main product is C2H2. With increasing temperature the carbon atom concentration increases. Thus lower cost C2H2 has been used as a carbon source instead of high cost methane in our experiments to provide the carbon atoms which the carburization reaction needs. In this method, a hydrogen4-argon plasma generator, hot wall reactor and quenching technique have been used. The preparation of nanosize W C Co powders was studied by direct reduction and carburization using a DC arc plasma, CoWO4 as raw material and C2H2 as a carbon source. Nanosize WC-Co composite of powders of average particle size 40 nm have been synthesized. The reaction temperature was about 4000 K. The preparation procedure for nanosize WC-Co powders can be divided into three steps. (1) Reduction: or Because of the low boiling point of CoWO4 and the high heat transfer ability of hydrogen plasma (which reaches a maximum at 3000-4000 K) when CoWO4 particles are injected into the plasma reactor, they are rapidly heated and evaporated. Most particles exist in vapor form. Meanwhile, hydrogen is of

Study of the Preparation of Bulk Powder Tungsten Carbides by Temperature Programmed Reaction with CH4+ H2Mixtures

The synthesis of bulk tungsten carbides by carburization of W metal or of WO3with mixtures of CH4in hydrogen at various pressures has been studied in temperature programmed experiments. The resulting solids have been characterized by elemental analysis, X-ray diffraction, XPS analysis, and specific surface area measurements. The carburization occurs in two distinct steps: W2C is formed in the first step taking place at about 650°C at atmospheric pressure with a 20% CH4–H2mixture, while the formation of WC occurs only at higher temperatures. During carburization some free carbon is deposited, the importance of which is very much dependent on CH4partial pressure and on the temperature of carburization. It has also been shown that direct carburization of WO3by CH4–H2does not take place, but that the carburization occurs via the reduction of WO3to W metal. The rate of reduction of WO3and that of carburization of W metal are very much dependent on, respectively, hydrogen partial pressure and CH4partial pressure. The extent of reduction of WO3into W metal required for carburization which takes place also depends on CH4partial pressure, indicating a competition between carburization of W metal at the surface and diffusion of W metal into the bulk of the solid.

General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production

Ultrafine WC/Co hard metals (average WC grain sizes ≤ 0.5 μm) can be successfully and reliably obtained by conventional hard metal manufacturing techniques. In this paper, some of the crucial aspects of conventional powder manufacture, powder milling and liquid phase sintering are discussed. Conventional ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization can be carried out. However, inherent to the powder processing techniques used and the particle growth mechanisms involved (oxide precursors used, reduction and carburization history), there exists a lower limit beyond which-finer WC powders cannot be produced. This limit lies in the particle size range of 50–150 nm (0.05–0.15 μm). Powder milling is carried out to obtain an even dispersion of the Co binder in the ultrafine WC matrix. The more uniform the phase distribution (WC, Co, grain growth inhibitor) within the green powder compact, the more uniform will be the material transport during sintering, and hence the uniformity of the WC grain growth/growth inhibition during sintering. Enhanced WC grain growth occurs early in the sintering cycle, even below the temperature at which the liquid phase is formed. This growth can be largely restricted by the addition of VC. However, effective grain growth inhibition has to take place already during this early period of solid-state sintering. The ‘early’ availability of the grain growth inhibitor at the WC/Co interface can, therefore, determine the degree of growth inhibition. Ultrafine hard metals are in particular prone to discontinuous grain growth of the WC. Different reasons for this local growth mode are propounded relating to both the chemical as well as the geometrical departures from uniformity in the green powder compact. While it is still not possible to predict exactly an ultimate WC grain size limit, below which WC grain growth can no longer be restricted, even with proper inhibitor additions, experimental evidence indicates that this average WC grain size limit lies in the range of 200–300 nm. This limit is inherent to the existing conventional processing techniques (powder manufacture, milling, liquid phase sintering) and the WC growth mechanisms involved and can be overcome only by establishing a completely new route in hard metal manufacture.

Reactions of neopentane, methylcyclohexane, and 3,3-dimethylpentane on tungsten carbides: The effect of surface oxygen on reaction pathways

High surface area tungsten carbides with WC and β-W 2C structure were prepared by direct carburization of W0 3 in CH 4H 2 mixtures. Their surfaces appear devoid of excess polymeric carbon and adsorb between 0.2 and 0.4 monolayers of CO and H. These materials are very active in neopentane hydrogenolysis. Chemisorbed oxygen inhibits hydrogenolysis reactions and leads to the appearance of isopentane among the reaction products. Neopentane isomerization to isopentane occurs only on Pt, Ir, and An surfaces. Thus, oxygen-exposed tungsten carbides catalyze reactions characteristic of noble metal catalysts. 3,3-Dimethylpentane isomerizes much faster than neopentane on oxygen-exposed carbides; the isomer distribution suggests that isomerization proceeds via a methyl shift mechanism rather than through the C 5-ring hydrogenolysis pathways characteristic of highly dispersed Pt. The apparent involvement of 3,3-dimethyl-l-pentene reactive intermediates is consistent with carbenium-type methyl shift pathways. Secondary carbon atoms, capable of forming stable carbenium ions, are present in 3,3-dimethylpentane but not in neopentane; they account for the high 3,3-dimethylpentane isomerization rate and selectivity on oxygen-exposed tungsten carbide powders. Both dehydrogenation and isomerization reactions of methylcyclohexane occur on these carbide powders. These results suggest the presence of a bifunctional surface that catalyzes dehydrogenation and carbenium ion reactions typically occurring on reforming catalysts.

Preparation and benzene hydrogenation activity of supported molybdenum carbide catalysts

Alumina-supported molybdenum carbide catalysts have been prepared from Mo0 3/A l2O 3 by (i) reduction with H2 followed by carburization, (ii) direct carburization with CH 4/H 2, and (iii) nitriding with NH 3 followed by carburization. The temperature required for the complete reduction Of MoO3/A1 2O 3 depended strongly on the reductant employed. Temperature-programmed reduction coupled with on-line mass spectrometry and X-ray diffraction revealed reactions involved in the reduction and carburization of Mo0 3. The supported molybdenum carbide catalysts exhibited high catalytic activity in benzene hydrogenation comparable to those of Pt or Ru. Complete reduction of molybdenum was essential for the high activity. Metallic molybdenum was much less active and less stable than its carbides, probably due to too strong adsorption of benzene.