G. V. Samsonov

Thermionic emission properties of refractory compounds and materials based on them (a review)

Conclusions1.On the basis of an analysis of the thermionic emission properties of solids, it is concluded that a correlation exists between a surface characteristic (electron work function) and volume properties reflecting the thermodynamic stability of their crystal structure. The correlation is attributable to the fact that the level of physical properties is strongly influenced by the strength of interatomic bonds, which detemine the potential energy of the electronic interaction in the lattice.2.As a result of systematic investigations into the thermionic emission properties of refractory carbides in their homogeneity regions and materials based on them, it has been established that the work function depends on the degree of defectiveness of such carbides, growing with increase in the strength of the Me-C and Me-Me bonds.3.It has been found that the electron work functions of boride and suicide phases of the Groups IV and V transition metals increases with rise in the nonmetallic component content of these phases, which is a consequence of an intensification of the electronic interaction between the metal and nonmetal atoms.

Work function of LaB6-based alloys☆

Abstract The concentration dependence of the electron work function φ in LaB6-“ScB6”, LaB6-YB6 alloys and (La, Y, Sc)B6 solid solutions were studied in the temperature range 1400 – 2000 K using the thermal emission technique. We found that the electron work function increased on substituting yttrium and scandium for some of the lanthanum atoms in LaB6, the most passivating effect on LaB6 being exerted by scandium. Assuming that the emission occurred from the hexaboride film, we can explain this by the essentially covalent character of the chemisorption bonds, the ion component of which (and the amount by which φ is decreased) decreases in the direction La → Y → Sc.

Thermionic emission from complex alloys containing lanthanum hexaboride

Conclusions1.A study of the thermionic emission properties of alloys of the LaB6-“ScB6” and LaB6−YB6 systems revealed that emission from these alloys is due to a film mechanism. Values ofθT are given by the expressions 2.66 + 2.0·10−4 T, for LaB6, and 3.57 + 3.5·10−4 T, for YB6.2.The monotonic increase in work function observed on passing from LaB6 to YB6 is a consequence of changes in the chemical composition of the surface layers of the alloys. After high-temperature heating alloys of the LaB6-YB6 system are characterized by a stable state of emission in the temperature range investigated. Prolonged heating at 1780‡K increases the work function of YB6 to a value close to those of YB12 and B as a result of enrichment of the specimen surface layer with boron liberated through the decomposition of part of the YB6 into YB4 and B.3.In the La-rich range alloys of the LaB6-“ScB6” system have been found to exhibit an unstable state of emission. The electron work function of alloys ranging in composition from equimolar to that of “ScB6” remains virtually constant at a level corresponding to the composition “ScB6”; under these conditions, emission becomes stable.

Differences in emission parameters and adsorption properties of single-crystal faces

Published data on the work function and adsorbtion properties of faces of metal and nonmetal single crystals are considered from the viewpoint of a model of solids based on configurational localization. An explanation is proposed for the different values of work function yielded by different faces of a crystal of the same metal.

Behavior of refractory compounds in alkali metal media

ConclusionsAn analysis of published data has shown that refractory compounds can be regarded as potentially useful materials for direct energy conversion devices. Further work would appear to be desirable in the following directions: 1) development of polycrystalline materials of increased purity (for example, pyrolytic coatings) whose adsorption properties could be regulated by varying their composition and hence the spectrum of chemisorption binding; 2) synthesis of single crystals, which may open up wide possibilities for the selection of faces with optimum emission-adsorption characteristics. The existence of such a choice is due to a pronounced anisotropy of properties, brought about by the strongly heterodesmic atomic interactions in single crystals of refractory compounds — a result of the presence of metal- nonmetal and nonmetal-nonmetal bonds as well as of metal-metal interactions.

Golcao calculation of electronic spectra of transition-metal compounds

Conclusions1.Basic concepts are presented on electron-spectrum calculation for crystalline substances using the GOLCAO method.2.Detailed calculations are presented on the electron spectra of transition-metal diborides.3.The calculations have been used to estimate electron structure parameters such as dissociation energy, atomic charge, electron-state filling, and statistical weights of atoms with stable electronic configurations.4.There is a relationship between the trends in the properties of these compounds and the filling of the orbitals in the stable sp configurations of the boron atom and the d configurations of the metal atom.

The electronic spectrum and physical properties of titanium, vanadium, and chromium diborides

Conclusions1.In diborides of the transition metals, the latter reveal a donor capability, decreasing with transition from diborides of group IV to those of V and VI groups.2.Upon transition of a metal atom from the free state to a diboride there occurs a rearrangement of the electron structure along the pattern S2dn→S2−xdn−y3.The results of calculation of the diboride electron spectrum performed by the strong bond method permit clarification of the basic principles of development of the physicochemical properties of this class of compounds.4.Comparison of calculated and experimental data makes possible several assumptions as to the properties of aluminum diboride and complex alloys of diborides.