TRANSITION METAL NITRIDES AND CARBIDES
5.2 APPLICATIONS
Nitrides and carbides are also considered among the hardest materials. Table 5.2 shows data for the measured Vickers microhardness for these compounds. Such measurements are performed using a diamond indenter with square geometry. The indenter is forced towards the surface of the material and the diagonal of the micro-indentation is measured. In all cases, carbides and nitrides are significantly harder than the pure metals and are also comparable or superior to that of ceramic materials.
5.1.2 Electrical and Magnetic Properties
In contrast to the physical properties, transition metal carbides and nitrides possess electric and magnetic properties that are often similar to metals. For example, electrical resistivities of Ti or W are 39 and 5.39 mV cm at room temperature, while their respect-ive carbides have only slightly higher resistivities of 68 and 22 mV cm. For comparison the electrical resistivity of the hard SiC ceramics is significantly higher (1000 mV cm).
Carbides and nitrides of Fe, Co, and Ni are ferromagnetic as their parent metals.
The other compounds which are paramagnetic, show typically lower susceptibilities compared to the pure metals.
temperatures up to 25008C is crucial. In particular VC and TiC retain high strengths up to 18008C and therefore can be used as high-temperature structural materials.
5.2.1.2 Coatings on Cutting Tools Many transition metal nitrides and carbides are applied commercially as hard coatings. For example, TiN is commonly used in coating of steel drills to increase the lifetime and hardness. One can easily recognize TiN coatings because they have a shiny golden color, which also clearly reflects their metallic character (Fig. 5.3). In another case such coatings may be applied to press stamps and significantly prolong their lifetime.
5.2.1.3 Catalysis In 1972 Boudart (2) for the first time demonstrated that transition metal carbides and nitrides can be used as catalysts in reactions that are specific to the expensive transition metals like platinum or palladium (prices of platinum and WC differ by orders of magnitude). Since then many potential applications in catalysis have been reported for isomerization, hydrotreating (HDN, hydrodenitrogenation;
HDS, hydrodesulfurization) and dehydrogenation reactions. For example, HDN and HDS processes are very important in the petrochemical industry for removal of sulfur and nitrogen compounds from gasoline and diesel. Nitrides and carbides were also found to be more resistant to poisoning than industrially used Co-Mo sulfide catalysts. However, the first commercial application came in 1997, when MoOxNywas used as a catalyst for the hydrazine decomposition in satellite thrusters and outper-formed a standard catalyst based on iridium (3).
N2H4 ! N2þ 2H2
The similar catalytic properties of transition metal carbides and nitrides to those of noble metals can be attributed to their similar electronic properties and structure. The valence electron count of WC is similar to that of Pt (1).
5.2.1.4 Carbide-Derived Carbons (CDC) An interesting property of the tran-sition metal carbides is that the metal can be selectively leeched from the carbide net-work by chlorine to form a nanosolid porous carbide-derived carbon structure (CDC)
Figure 5.3 TiN-coated drill.
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with high specific surface areas and pore diameters between 0.3 and 6 nm. Such materials can be used in the separation processes and immobilization of large mol-ecules (enzymes), and their properties can be tuned by using different metals and different metal removal conditions.
TiC(solid)þ 2Cl2(gas)600!10008C C(solidporous)þ TiCl4(gas)
One can also define the shape of CDC material on the nanoscale by changing the morphology of the carbide precursor (MCx) (Fig. 5.4).
Another possibility of carbon formation is the hydrothermal treatment of carbides (100 MPa, 3008C to 4008C in H2O). However, in this case a thin, uniform carbon film is produced on the surface of the carbide. Recently, it was also shown that SiC can be transformed into nanocrystalline diamond during silicon etching with Cl2(by proper adjustment of the reaction parameters). It has not yet been tested if it also can be applied on the transition metal carbides.
Carbide-derived carbons are considered interesting materials for electrodes and hydrogen storage.
5.2.2 Nanosized Transition Metal Nitrides and Carbides: Advantages and Specific Applications
5.2.2.1 Nanopowders Reduction of the particle size has a strong effect on the properties of materials. In the powder form, the specific surface area increases dramati-cally with decreasing particle size. Typidramati-cally, the relation between the particle size and specific surface area can be predicted based on simple geometric considerations assuming cubic or spherical particle shape (Fig. 5.5).
The increased specific surface area is very important in heterogeneous catalysis since reactions take place at the gas-solid or liquid-solid interface. For example, the dehydrogenation rate of butane over VN significantly improves with the increasing Figure 5.4 Different morphologies of carbide-derived carbons (CDC) inherited from their metal carbide precursors.
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specific surface area (Table 5.3; Reference 4).
Due to the large specific surface area, nanopowders are also more reactive than micrometer-sized materials and some of the nano-sized nitrides and carbides can be completely oxidized if directly exposed to air. For example, Mo2N with specific sur-face area of 120 m2g21can be rapidly oxidized upon exposure to air (powder starts to glow red), while low specific surface area material (,1 m2g21) will show no reaction to the same treatment. In order to prevent rapid oxidation, nanoparticulate materials can be slowly passivated in inert gas (N2, He) containing low amounts of oxygen (0.5% to 1%). In such conditions only a thin layer of oxide is formed and nitrogen cools down the system in order to slow down the reaction. It is known that Figure 5.5 Specific surface area Sg(m2g21) as a function of VN particle diameter d (nm) assuming density r¼ 6.1 g cm23.
TABLE 5.3 Butane Dehydrogenation Rate over VN Catalyst with Different Specific Surface Areas (4)
Sg(m2g21) dS(nm) R (nmol g21s)
14 70 89
29 34 166
59 18 256
Sg, specific surface area; dS, nanoparticle diameter calculated according to the equation presented in Figure 5.5; R, gravimetric reaction rate.
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nanopowders of metals are explosive and should be handled with care. The same con-cerns the nanopowders of the other transition metal carbides and nitrides.
Recently, nanocrystalline VN has been discovered as an excellent material for supercapacitors (5). Compared to standard capacitors (where charge is stored due to the pure electrostatic attraction) in supercapacitors energy is stored at the electro-lyte/solid interface in the form of the electrical double layer and due to the reversible redox reaction on the nanoparticle surface (Fig. 5.6):
VNxOyþ OH ! VNxOykOHþ VNxOyOH
where VNxOykOH2represents the electrical double layer formed by hydroxyl ions adsorbed on nonspecific sites, wherein a large increase in the specific capacitance arises primarily due to the successive oxidation by hydroxyl species (OH2from elec-trolyte). While the charge capacities in standard units are typically at the level of microfarad (1026), modern supercapacitors with similar external dimensions can reach capacities of kilofarad (103). They are of extreme interest, since they can be used to effectively store electric energy for mobile applications. Supercapacitors clearly outperform lithium-based batteries on the charging speed and cycle lifetime and are planned to be used as electrical energy storage units for city buses in Shanghai. In this respect, VN can be an economic alternative for the expensive super-capacitors that are base on Ru nanoparticles.
5.2.2.2 Bulk Nanomaterials One of the most effective ways to densify nanopow-ders into bulk nanomaterials is SPS (spark plasma sintering). The powder is com-pressed and a high density current is passed through it in order to heat it up to the
Figure 5.6 Schematic drawings of standard capacitor (a) and supercapacitor (b).
TRANSITION METAL NITRIDES AND CARBIDES 118
desired sintering temperature (Fig. 5.7). In this respect advantage can be taken of the metallic character of transition metal nitrides and carbides and they can be heated easily by passing current through them. Moreover, very fast heating ramps can be achieved using this method, which is extremely important for proper densification.
Nanopowders can be sintered at lower temperatures since they are energy-rich systems (high specific surface area) and generate high intrinsic stress:
s¼ 2g r
where s is the sintering stress, g is the surface tension, and r is the pore radius (smaller pores are present between nanoparticles). The estimated sintering stress is of the order of 500 MPa, compared to that of5 MPa for conventional micrometer-sized powder compact. The externally applied pressure of 100 MPa used in most of the densification processes fails further to reasonably enhance the sintering kinetics.
The bulk sintered nanomaterials are known to have different mechanical properties with respect to materials with micrometer-sized grains (6). In principle, most of the reports suggest higher hardness with decreasing grain size, but also enhanced plas-ticity and toughness can be observed. Interestingly, the effect of superplasplas-ticity can appear for some of the bulk nanomaterials and they can be deformed beyond the point where they would normally break. It is of great importance how the nanopow-ders are prepared before sintering. The correct processing is crucial to achieve proper mechanical properties and a proper preparation is necessary. No large pores (surrounded by many particles) should be present in the compacted powder since they are extremely difficult to densify and if they remain in the material they can sig-nificantly lower its mechanical properties.
Figure 5.7 Spark plasma sintering (SPS) of nanopowders.
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