Refractory compound formation

Borovinskaya, I. P., Refractory compounds formation during combustion of heterogeneous condensed systems. Proceedings of the Fourth All Union Symposium on Combustion and Explosion (Russian), Moscow, 138 (1977). 

Refractory compound formation is avoided by chemical competition or by use of a high-temperature flame. 

Ranking metal borides as refractory compounds results from the formation of covalent B — B bonds by the electron-deficient B atoms ". As a result the metal lattice may be changed drastically, even for low B contents. 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

Refractory compounds can be determined using a nitrous oxide-acetylene flame. The formation of refractory oxides with gases in flames might not be considered an interference, since it is constant under a given set of conditions but it does decrease the sensitivity markedly so that measurement of the element may not be possible. 

An indication of the trend of the solid phase stability in the alloys of Mn and Re with the different elements of the 4th and 6th rows of the Periodic Table is contained in Table 5.39, where the melting points of selected compounds have been collected. In the Mn series alloys we may notice, here too, the gaps in the pattern of the compound formation. In the case of Re alloys, very high melting points are observed in the compounds with other refractory metals (even if often... 

For strontium determinations samples, standards and blanks are made to 1% (m/v) in lanthanum to prevent the formation of refractory compounds. Analyses for strontium also suffer from ionisation interferences which are discussed later. 

An alternative technique for preventing the analyte forming refractory compounds in the flame is to add a compound to the sample which forms a preferential complex with the analyte. Ethylenediaminetetraacetic acid (EDTA) is often used since it complexes with the cation thus preventing its association with an anion that can lead to the formation of a refractory compound. 

Many of the interelement interferences result from the formation of refractory compounds such as the interference of phosphorous, sulfate, and aluminum with the determination of calcium and the interference of silicon with the determination of aluminum, calcium, and many other elements. Usually these interferences can be overcome by using an acetylene-nitrous oxide flame rather than an acetylene-air flame, although silicon still interferes with the determination of aluminum. Since the use of the nitrous oxide flame usually results in lower sensitivity, releasing agents such as lanthanum and strontium and complexing agents such as EDTA are used frequently to overcome many of the interferences of this type. Details may be found in the manuals and standard reference works on AAS. Since silicon is one of the worst offenders, the use of an HF procedure is preferable when at all possible. 

In some cases, several refractory compounds can result from two or more parallel reactions occurring simultaneously in the combustion wave. A typical example of this type is the Ti-C-B system, where both the Ti+C and Ti-I-2B reactions affect the combustion synthesis and structure formation processes (Shcherbakov and Pityulin, 1983). By adjusting the contents of carbon and boron powders in the reactant mixture, either carbide- or boride-based ceramics can be obtained. 

Although the transition metal ehaleogenides usually are quite refractory, direct reaction is feasible in many cases. In some cases (e.g., W and Mo), the oxide is volatile, making the surface at least accessible to reaction. In addition, the metals often have high rates of diffusion in the compounds, thus reducing the surface passivation effect of compound formation. This is probably because diffusion jumps are more probable in the presence of elements that can change their charge states. This property can be helpful in conversion of an oxide to a sulfide via H2S or CS2. 

Boron nitride is a refractory compound of great strength which is finding application as a fibre material. For its formation by the reaction ... 

Chemical interference, or the chemical combination of the element of interest with other elements in the sample or the flame, is probably the most important interference in flame methods. It directly affects the efficiency of production of neutral atoms in the flame and hence affects both absorption and emission in a similar manner. One of the most common types of chemical interference is the formation of refractory compounds with the test element, usually by an anion in the aspirated solution. The result is a decreased signal. For example, phosphate will react with calcium ions to produce calcium pyrophosphate in the flame. Less frequently, the presence of another cation may result in a decreased signal. For example, aluminum causes low results in the determination of magnesium, owing to the formation of a heat-stable aluminum-magnesium compound. Occasionally, a positive interference will occur in the presence of an interfering substance. The mechanism is not clearly understood, but has to do with the formation of a compound more volatile than the test element. 

Stable compound formation will always cause a depressive effect. Typical examples are the lowering of alkaline earth metal absorbances in the presence of phosphate, aluminate, silicate and some other oxo anions, the low sensitivity of metals which form thermally stable oxides (refractory oxide elements), and the depression of the calcium signal in the presence of proteins. In addition, some refractory oxide elements may also form stable carbides, especially in rich hydrocarbon flames. 

The formation of refractory compounds in the atomizer may suppress or enhance the atomic absorption signal of a given metal. These methods are rapid, but other species present in the sample that might suppress or enhance the absorbance of the metal added cause interference. In order to overcome these interferences, the sample should be treated previously with a cation- or anion-exchanger. The determinations may be performed by adding a constant concentration of the metal ion to the sample solution or by an atomic absorption titration. 

Magnesium can be used for the determination of ortho-, pyro-, tri-, tetra-, and hexaphosphates. The shape of the titration curve for orthophosphate (Figure 98B) suggests the formation of refractory compounds with 2, 3, and 4 magnesium atoms for each phosphorus atom. At any of these points the magnesium-phosphate ratio remains constant, and all these points may serve as an end-point for the titration, even in the presence of sulfate. 

This is perhaps the most common type of chemical interference. The presence of certain anions may cause refractory compounds to be formed with the analyte. As a consequence, its atomisation is hindered and a decrease in response is observed. A weU-known example is the suppression of the response of Ca with increasing concentrations of phosphate or sulfate. When the anion concentration in the flame is increased while keeping the calcium concentration constant, the absorbance decreases to about half its original value which is attributed to the formation of hardly dissociated calcium phosphate or sulfate. At high anion concentrations, the analyte response again becomes independent of the anion concentration. 

The co-occurrence of cations may also contribute to the formation of refractory compounds. The presence of aluminum has such an effect on the determination of magnesium. It is assumed that a refractory aluminium—magnesium oxide is formed which reduces the sensitivity of the magnesium determination. 

Chemical effects may originate from either the sample solution or the flame and represent the major type of interference in flame-AAS. The mechanisms behind chemical interferences can be split into two types. The first of these is where the atomization of the analyte is not complete through occlusion into refractory compounds. Stable compound formation of this type will cause a depression of the signal through physical entrapment of small amounts of analyte in clotlets of matrix oxide in the flame. In premixed laminar flames the volatilization of solid particles begins as soon as they enter the primary reaction zone. The time this process takes is dependent on the size of the particle and so the occurrence of such interferences will depend critically on the observation height in the flame. In the second type, the analyte atoms may react with other atoms or... 

The high plasma temperature ensures complete atomization of the sample (even in the case of refractory compounds) and also prevents the formation of di- or polyatomic species. Therefore, most matrix effects common for flame and electrothermal atomization techniques are eliminated in ICP-AES. 

Here is the energy of dissociation of one mole of Oj or Nj (N = 8) or of atomisation of one mole oi C (N = 4). This correction aUows for the fact that formation of the MX0 75 compound takes place with the starting metalloid dissociated (sublimated) 25%. As distinct from Eyf, is of thermodynamic origin, as it is the energetic part of the changes in the free energy occurring on vacancy formation. Results of E and E calculations for refractory compounds are discussed in Chapter 4. 

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