Magnetic carbides

Cobalt. There is no U.S. mine production of cobalt. Refining of imported nickel—cobalt mattes has not occurred since the mid-1980s. About 1600 t of secondary cobalt was recycled from scrap by 13 faciUties in the United States representing - 22% of total U.S. consumption. The price of the metal was around 44/kg. Most is imported from Zaire and Zambia. Increasing quantities are coming from Russia. Historically, the price of cobalt has been quite volatile and dependent on the pohtical environment in those countries. Cobalt is used in superaHoys, 40% catalysts, 14% paint driers, 11% magnetic alloys, 10% and cemented carbides and other uses, 16%. 

Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has the formula Fe C, and thus consists of 6.67 wt % carbon and the balance iron. Cementite is very hard and britde. As the hardest constituent of plain carbon steel, it scratches glass and feldspar, but not quart2. It exhibits about two-thirds the induction of pure iron in a strong magnetic field, but has a much lower Curie temperature. 

The Group 4—6 carbides are thermodynamically very stable, exhibiting high heats of formation, great hardness, elevated melting points, and resistance to hydrolysis by weak acids. At the same time, these compounds have values of electrical conductivity. Hall coefficients, magnetic susceptibiUty, and heat capacity in the range of metals (7). 

In addition to chemical analysis a number of physical and mechanical properties are employed to determine cemented carbide quaUty. Standard test methods employed by the iadustry for abrasive wear resistance, apparent grain size, apparent porosity, coercive force, compressive strength, density, fracture toughness, hardness, linear thermal expansion, magnetic permeabiUty, microstmcture, Poisson s ratio, transverse mpture strength, and Young s modulus are set forth by ASTM/ANSI and the ISO. 

Many ceramic applications are high value and small volume, so energy expenditure is high. Ferroelectric magnets, electronic substrates, electrooptics, abrasives such as silicon carbide and diamond, are examples. Diamond is found naturally, and made synthetically by the General Electric Company at high pressure and temperature. Synthetic diamonds for abrasives require less energy to make than the value in Table 4 nevertheless, the market is carefully divided between natural and synthetic diamonds. 

Figure 4. Magnetization (Bohr magnetons/iron atom) as a function of temperature for ZSM-5 (11.1% Fe) (H 6300 Oe (V) reduced (A) carbided (0) used).

Catalyst Production. Supported magnetite particles were produced on Grafoll (Union Carbide), a high surface area form of graphite. The nature of Grafoll and the reasons It is convenient to use In MCssbauer spectroscopy experiments eu e described elsewhere (25). Grafoll is also well suited for magnetic susceptibility experiments. 

Fig. 6.14 Mossbauer spectra of a-Fe nanoparticles on a carbon support. The spectra were obtained at 80 and 300 K with the indicated magnetic fields applied perpendicular to the y-ray direction. The asymmetry in the spectra is due to the presence of a small amount of iron carbide particles. (Reprinted with permission from [58] copyright 1985 by the American Chemical Society)...

Figure 1. Magnetic effect in the Curie point of the -Fe3C carbide.

Cobalt in small amounts is an essential element associated with vitamin B12, but at high levels can be toxic. There are no daily recommended intake levels for cobalt. Intestinal bacteria use cobalt to produce cobalamin, which in turn is an essential component of vitamin B12. Industrially, cobalt is used in pigments, permanent magnets, and as an alloy to harden metals as in tungsten carbide blades or drills. 

Vanadium is added to steel for high resistance to oxidation and to stabilize carbide. Vanadium foil is used for cladding titanium to steel. Vanadium-gallium alloy is used in making superconductive magnets. An important compound of vanadium is pentoxide which has many commercial uses (See Vanadium Pentoxide). 

The metal is most often recovered from its principal ore, zircon. The ore is mined, crushed and preliminary segregation is by gravity, electrostatic, and magnetic separation. Separated ore mixed with carbon is charged into an arc furnace and heated to about 3,500°C. This forms zirconium carbide and silicon monoxide, and the monoxide is driven off as vapor. Zirconium carbide is then placed in a chlorinator and heated with chlorine gas at high temperatures. The carbide is converted to zirconium tetrachloride, ZrCfl. Also, small amounts of hafnium that is always associated with zirconium converts to its tetrachloride, HfCfl. 

Paramagnetism results from unpaired electrons. As a result, most compounds containing transition, rare-earth, and actinide elements, including oxides, nitrides, carbides, and borides, exhibit paramagnetism. Such ceramics are generally not of importance due to their paramagnetism alone, since they often exhibit other types of magnetism, as well. 

Many other atom-probe analyses of different phases in different types of steels exist as steels are one of the most important materials. It is possible to investigate how the magnetic properties of alloys are correlated to the microstructures of different phases in the alloys.57,58,59 The chemical contents, growth process and structures of metallic carbides in different alloy steels have been studied with the field ion microscope and the atom-probe field ion microscope.60 61 62 63 We refer the reader to some of the original papers published on these subjects. 

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