Refractory carbide-forming element

As discussed earlier, tungsten furnaces, as first proposed by Sychra et al. [175a], are useful for the determination of refractory carbide forming elements, which in the case of a graphite furnace may suffer from poor volatilization, but they are more... 

Catalysts for the chemical industry have to be characterized with respect to their trace impurities and major components. Not only is their composition when they are used initially in chemical reactors important, but also their alteration in the course of time. As carbide forming elements such as V and Ti are often used, atomic absorption spectrometry could be problematic. This also applies to catalysts for exhaust gas detoxification in cars. Noble metals such as Pt, Pd and Rh are fixed on alumina supports often also containing cerium compounds. Both for the determination of the stoichiometry but also for the monitoring of the noble metal contents in used catalysts, AAS suffers from problems because of the need for sample dissolution as well as for the requirement to determine refractory oxide forming elements. 

In-situ thermal destruction of the matrix or selective volatilization can be applied. The latter has proved useful in geological samples [123]. Selective volatilization of volatile elements from refractory matrices is useful spectral interference from matrices with complex spectra can be avoided. The approach can also be used for the volatilization of refractory oxide- or carbide-forming elements, which form volatile halogenides (e.g., Ti). Here, substances such as AgCl or polytetra-fluoroethylene (PTFE) powder can be used as thermochemical reagents [121]. In the case of Ti in the presence of C and PTFE, the following reactions have to be considered ... 

Electrothermal atom cells have changed radically since their inception in the late 1950s. The majority of electrothermal devices have been based on graphite tubes that are heated electrically (resistively) from either end. Modifications such as the West Rod Atomizer (a carbon filament) were also devised but were later abandoned. Tubes and filaments made from highly refractory metals such as tungsten and tantalum have also been made, but they tend to become brittle and distorted after extended use and have poor resistance to some acids. Their use continues, however, in some laboratories that need to determine carbide-forming elements. For example, silicon reacts with the graphite tube to form silicon carbide, which is both very refractory and very stable. The silicon is therefore not atomized and is lost analytically. Use of a metal vaporizer prevents this. 

Several other types of atomizer have been developed. Some of these are based on the design of the West rod, but others have made tubular atomizers from extremely refractory metals such as tungsten, tantalum and molybdenum. This latter class of atomizers tend to be made in-house by some laboratories and, at present, do not have any commercial suppliers. They have the advantage of being inert and non-porous so there is little interaction with the analyte, so that they can be used for the determination of elements which form refractory carbides. However, after extended use and in the presence of some acids, many of these atomizers become brittle and distorted. 

The interstitial or refractory carbides are the most important of the three classes these are formed by the transition metals, the most stable being the carbides of the metals in Groups IVa, Va, and Via. Such carbides may be made from the elements at high temperatures under pressure and resemble the metals themselves. They tend, however, to be much harder and higher melting than the parent metals. The melting points, (admittedly approximate) of tantalum carbide, TaC, (4200° C) and zirconium carbide, ZrC, (3800° C) may be compared with those of tungsten... 

Diamond can nucleate on foreign surfaces, notably elements that form refractory carbides (Si, Mo, Ta, W), without pretreatment. 

Over the atomization temperature range ( 1200°-3000°C), carbide formation between many metal oxides and graphite is favored thermodynamically (28). While carbide formation has been used to explain poor sensitivity for elements such as boron or tungsten, which form refractory carbides, little attention was initially devoted to the interference caused by carbide formation with other elements. However,... 

T able 2.1 Periodic Table of the Elements Showing Their Electron ativity and Elements Forming Refractory Carbides... 

Note Elements in Box A form refractory interstitial carbides and elements in Box B form re actory covalent carbides... 

The third factor governing the structure of nitrides is the nature of the bond between the nitrogen atom and the other element forming the compound. As mentioned in Ch. 2, the bond is the force of attraction that holds together the atoms of a molecule.l The bonds in refractory carbides can be ionic (saltlike nitrides), covalent (covalent nitrides), or a combination of metallic, covalent, and ionic (interstitial nitrides) (for a discussion of electronic bonding, see Ch. 2, Sec. 5.0). 

It is not essential that, for combustion synthesis, the reactants are elements and initially solid (at least one of them). Refractory carbides and nitrides with complex chemical compositions (metal halogenides, organometallics, etc.) are also formed in combustion of gaseous systems. The special features of combustion synthesis in the gas phase were analyzed in the recent review by Brezinsky (5). The synthesis in a gaseous system proceeds at a stationary combustion front with moving flows of reactants and product. Consequently, these processes differ in essence from combustion of condensed systems by the conditions of nucleation and growth of the new phase, which manifest themselves in the product morphologies. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

Plutonium forms several binary compounds that are of interest because of their refractory character and stability 1 at high lemperatures, These include the carbide, nitride, silicide. and sulfide of the element. 

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