Titanium, Zirconium and Hafnium Nitrides

Titanium tetrakis(dialkylamide) compounds, Ti(NR2)4 (R= Me, Et, n-Pr, n-Bu) have been investigated in the CVD of TiN [144, 154-161]. Their decomposition, carried out in the presence of NH3(g), proceeds under relatively mild conditions and yields TiN films of good quality. Other precursors which have been used for the CVD of TiN include cyclopentadienyl-cycloheptatrienyl titanium, CpTi(C7H7) (15) [161], f-BuTi(NMe)3 [156, 161], [Ti(/i-N-r-Bu)(NMe2)2]2 (16) [156, 161] and Ti(f-BuDAD)2, bis(N,N -di-t-butyl-l,4-diaza-l,3-butadiene) titanium (17) [161]. The mixed amide 

The cubic phases of zirconium- and hafnium nitride display properties quite similar to those of TiN. These compositions have been studied to some extent, however, the number of reports dealing with the CVD of ZrN and HfN is considerably lower than those describing the CVD of TiN. Zirconium and hafnium nitride can be prepared by the decomposition of the appropriate metal tetrakis(dialkylamide) compounds [155]. Zirconium and hafnium nitrides of the stoichiometry M3N4 also have been prepared by CVD. The compounds are formed when M(NEt2)4 is reacted with NH3 at 200-450°C [144]. The films thus produced are crystalline, yellow, transparent and insulating. It has been suggested that these phases are related to the MN phase by a rhombohedral distortion. 

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Fix, R. Gordon, R. G. and Hoffman, D. M., Chemical vapor deposition of titanium, zirconium, and hafnium nitride thin films. Chem. Mater. 3 (1991) 1138-1148. 

The transition metal carbides and nitrides have often been called interstitial compounds [70] however, this is somewhat misleading. The small boron, carbon, or nitrogen atoms certainly occupy octahedral or trigonal prismatic voids of the metal sublattice, but the arrangement of the metal atoms themselves is different from that of the element. In the monocarbides the transition metal atoms show cubic close packing. However, titanium, zirconium, and hafnium are packed hexagonally and vanadium, niobium, and tantalum are body centered cubic [1]. Thus, these monocarbides are inorganic compounds with their individual crystal structures and they should not be considered as an interstitial compound of a transition metal host lattice. 

Christensen AN (1990) A neutron diffraction investigation on single crystals of titanium oxide, zirconium carbide, and hafnium nitride. Acta Chem Scand 44 851-852... 

Of a series of powdered refractory compounds examined, only lanthanum hexa-boride, hafnium carbide, titanium carbide, zirconium carbide, magnesium nitride, zirconium nitride and tin(II) sulfide were dust explosion hazardous, the 2 latter being comparable with metal dusts. Individual entries are ... 

The nitrides and carbides of titanium and zirconium and the carbide of hafnium are extremely hard substances, resembling metals both in appearance and in electrical conductivity. Their formulae approach AxBh but some departure from stoichiometry is possible. Each of these refractory substances has the sodium chloride structure, described alternately (p. 190) as cubic close-packed arrays of metal atoms with the small nonmetal atoms in the octahedral holes. Note, however, that the parent metals themselves do not have cubic close-packed structures. Thus, the older view of such nitrides and carbides as lattices of the parent metals that are expanded to accommodate nitrogen or carbon atoms in the holes (interstices) is not admissible. The nature of the bonding in such refractory nitrides and carbides appears to be linked to the nature of bonding in metals in general, an important and interesting topic, but best pursued in more advanced works. 

Zirconium metal (mp 1855°C 15°C), like titanium, is hard and corrosion resistant, resembling stainless steel in appearance. It is made by the Kroll process (Section 17-A-l). Hafnium metal (mp 2222°C 30°C) is similar. Like titanium, these metals are fairly resistant to acids, and they are best dissolved in HF where the formation of anionic fluoro complexes is important in the stabilization of the solutions. Zirconium will burn in air at high temperatures, reacting more rapidly with nitrogen than with oxygen, to give a mixture of nitride, oxide, and oxide nitride (Zr2ON2). 

Nitrides of elements, including titanium (Ti), silicon (Si), aluminum (Al), boron (B), zirconium (Zr), hafnium (HI), niobium (Nb), and tantalum (Ta) can be produced by conversion of their chlorides in plasma in the presence of hydrogen and nitrogen, or ammonia NH3 (Neuenschwander, Schnet, Scheller, 1965 Murdoch Hamblyn, 1967 DeVink, 1970, 1971a,b Dale, 1974). This process takes place in the gas phase and, therefore, is quite effective. As an example, the yield of Si3N4 produced in this way is 80%. 

The RM device consists of a high-index substrate ( 1 mm thick lead glass, = 1.72825), a thin low-index spacer (about 1000 nm of magnesium fluoride or silica) and a very thin monomode waveguiding layer (about 100 nm of titanium oxide, zirconium oxide, hafnium oxide or silicon nitride). It can be used to monitor re-... 

In February, 1980, Sumitomo from Japan filed the patent Sintered compact for a machining tool and a method of producing the compact [161]. This patent basically covers any compact with 10-80 vol% cBN and a balance of binder material that can comprise any carbides, nitrides, borides, or silicides of metals of groups IVa, Va, or Via. Specifically mentioned are titanium, zirconium, hafnium, vanadium, niobium. 

The early transition metal nitrides of titanium, zirconium, hafnium, tantalum, and tungsten as well as titanium and tantalum carbide are effective diffusion... 

Because hafnium has an elevated absorption cross section for thermal neutrons (almost 565 times that of zirconium), it is extensively used for producing nuclear-reactor control rods. On the other hand, hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides m.p. 3310 C). To a lesser extent, hafnium is used in gas-filled and incandescent lamps as an efficient getter for scavenging oxygen and nitrogen, and alloying with iron, titanium, niobium, and other refractory metal alloys. 

Nitrides are used for wear-resistant applications, most notably surface engineering of cemented carbide cutting tools and tool steels. Titanium nitride (TiN) is the most frequently employed coating, but titanium car-bonitride (TiCN), hafnium carbide (HfC), titanium aluminum nitride (TiAlN), titanium zirconium nitride (TiZrN), and chromium nitride (CrN) have also been used commercially. These coatings are applied by vapor deposition techniques (Fig. 2). 

It must be pointed out that, due to the lower ionization potential of the oxygen atom compared with nitrogen, the proportion of ionic bond in metal oxides having high acceptor characteristics (titanium, zirconium, hafnium, vanadium) is rather less than in the corresponding nitrides, and this must be particularly pronounced for the lower oxides. 

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