Enthalpy intermetallic compounds

The Miedema s parameters and the Miedema model and formula proved to be useful in an approximate evaluation of the formation enthalpy of alloys, in the estimate of the formation capability of intermetallic compounds, etc. 

One of the simplest calorimetric methods is combustion bomb calorimetry . In essence this involves the direct reaction of a sample material and a gas, such as O or F, within a sealed container and the measurement of the heat which is produced by the reaction. As the heat involved can be very large, and the rate of reaction very fast, the reaction may be explosive, hence the term combustion bomb . The calorimeter must be calibrated so that heat absorbed by the calorimeter is well characterised and the heat necessary to initiate reaction taken into account. The technique has no constraints concerning adiabatic or isothermal conditions hut is severely limited if the amount of reactants are small and/or the heat evolved is small. It is also not particularly suitable for intermetallic compounds where combustion is not part of the process during its formation. Its main use is in materials thermochemistry where it has been used in the determination of enthalpies of formation of carbides, borides, nitrides, etc. 

Fig. 7. Volcano -shaped curve for a series of Ni intermetallic compounds, obtained by plotting the activity for hydrogen evolution (J0 in 9.2 M NaOH at 80 °C) against the calculated enthalpy for the...

The values of AH thus calculated are presented in Table 3.3. The standard enthalpies (heats) of formation of the nickel-aluminium compounds, expressed in kJ mol1 and kJ g-atom, are also given for comparison. It is seen that the most negative value of AH is obtained for the NiAl3 intermetallic compound which is indeed the first to form. 

Here, AH(A-B) is the partial molar net adsorption enthalpy associated with the transformation of 1 mol of the pure metal A in its standard state into the state of zero coverage on the surface of the electrode material B, ASVjbr is the difference in the vibrational entropies in the above states, n is the number of electrons involved in the electrode process, F the Faraday constant, and Am the surface of 1 mol of A as a mono layer on the electrode metal B [70]. For the calculation of the thermodynamic functions in (12), a number of models were used in [70] and calculations were performed for Ni-, Cu-, Pd-, Ag-, Pt-, and Au-electrodes and the micro components Hg, Tl, Pb, Bi, and Po, confirming the decisive influence of the choice of the electrode material on the deposition potential. For Pd and Pt, particularly large, positive values of E5o% were calculated, larger than the standard electrode potentials tabulated for these elements. This makes these electrode materials the prime choice for practical applications. An application of the same model to the superheavy elements still needs to be done, but one can anticipate that the preference for Pd and Pt will persist. The latter are metals in which, due to the formation of the metallic bond, almost or completely filled d orbitals are broken up, such that these metals tend in an extreme way towards the formation of intermetallic compounds with sp-metals. The perspective is to make use of the Pd or Pt in form of a tape on which the tracer activities are electrodeposited and the deposition zone is subsequently stepped between pairs of Si detectors for a-spectroscopy and SF measurements. 

This work is a continuation of our earlier study [1] of the hydrogen interaction with intermetallic compound (IMC) AB2-type Tio.9Zro.1Mn . 3V0.5. The measurements were carried out in twin-cell differential heat-conducting Tian-Calvet type calorimeter connected with the apparatus for gas dose feeding, that permitted us to measure the dependencies of differential molar enthalpy of desorption (AHdes.) and equilibrium hydrogen pressure (P) on hydrogen concentration x (x=[H]/[AB2]) at different temperatures simultaneously. The measurements were carried out at 150°C, 170°C and 190°C and hydrogen pressure up to 60 atm. 

An account of the gaseous species observed by Knudsen effusion mass spectrometry in the eqilibrium vapor of metals, alloys, oxides, halides, and technical systems is given. The fundamentals and recent developments of this method are briefly reported. Dissociation and atomization enthalpies of selected gaseous species are tabulated. Accounts of the equilibrium studies by Knudsen effusion mass spectrometry in order to obtain thermodynamic properties for condensed phases from gas phase data are additionally given for the aforementioned materials. Table 8 shows as an example the enthalpies and Gibbs energies of formation for different solid intermetallic compounds. A special section (Sect. 

Besides the mechanical alloying of elemental powders, ball-milling of an intermetallic compound can also lead to amorphization, as demonstrated for several alloys [3.18, 19, 130, 131] (for more details see Chap. 2). This cannot be explained by the above statements, since in this case no composition-induced destabilization provides the driving force for an interdiffusion reaction. Amorphization by milling starting from powders of crystalline intermetallics is attributed instead to the accumulation of lattice defects - mainly the creation of antiphase boundaries - which raise the free enthalpy of the faulted intermetallic above that of the amorphous alloy. Therefore, there exists some similarity with irradiation-induced amorphization [3.20]. 

Finally, they correlate their resnlts, together with the varions literatnre data for the enthalpies of formation of MX3 intermetallic compounds, with the atomic radii of the involved elements. 

The authors discuss their results in the light of values predicted by semi-empirical models for intermetallic compounds and with earlier experimental literature results for the enthalpy of formation of these and related compounds. 

Other authors like Kubaschewski Alcock (1979) have correlated the enthalpies of formation with the volume contractions upon compound formation. For ionic compounds the more exothermic is the reaction the greater the contraction eind with good confidence one can derive the enthalpy from crystallographic information. For intermetallic compounds the absolute values of the enthalpies of formation are very small and poorly correlated with volume contractions. Then, we can deduce in intermetallic compixinds the volume effocts do reflect a more complex bond situation. 

Crystallization temperature (T ) for several amorphous alloys and formation enthalpies (AH) calculated by means of Miedema s model for the corresponding (hypothetical) intermetallic compounds. The AH values are given in units of kj per mol of alloy (Miedema et al., 1980). 

FICU RE 17.12 Temperature (o) and enthalpy ( ) of the formation of AI12M0 intermetallic compound in mechanically alloyed Al-10 at.% Mo powders as a function of milling time. 

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