The Department Of Electronic Structure And Electronic Properties was organized at the Institute in 1957. Its first Head became the Member of the A.S. of Ukr.SSR A.G. Lesnik. Since 1958 till 1982 the department had being headed by the Honoured Scientist, Dr. Sci. (Tech.), Prof. I.Ya. Dekhtiar.
In 1982 the department was reorganized to the corresponding laboratory. The laboratory affiliated to the Department of Solid-State Spectroscopy, which was headed by the Candidate of Physico-mathematical Sciences V.I. Silantiev till 1989. Since 1989 till 1995 the researchers of the department had been working as a part of Physics of Films Department. Since 1995 the laboratory, and since 1996 the department, has being headed by the Dr. Sci. (Phys.–Math.), Prof. M.M. Nyshchenko.
The fist activity of the department was focused on the study of the dependencies of electric and magnetic properties of metals on structural imperfections. It was shown, that change of form and magnetic properties of metals during cyclic processing is determined by the formation and interaction of the defects of crystal lattice. (I.Ya. Dekhtiar, E.G. Madatova). The peculiarities of the defects influence were revealed on electric properties of alloys of the transitional metals (I.Ya. Dekhtiar, S.G. Sakharova) and formation of the localized magnetic moments in paramagnets (I.Ya. Dekhtiar, R.G. Fedchenko). The scientists studied the influence of structural factors on properties of ferromagnets (I.Ya. Dekhtiar, V.V. Polotniuk).
The period of 1957–1980 was marked by intensive research in the field of physics of radioactive defects aiming on creation of radiation-resistant materials for nuclear power engineering and for application in space. The effect of gamma-quantum on a long-range order at room temperatures in the ordered alloys was researched, radiation-enhanced diffusion in the heterogeneous systems under the action of gamma-radiation and neutrons was studied, and its model was built, taking into account the increase of vacancies` concentration, threshold energy displacements and ionization of atoms (I.Ya. Dekhtiar. A.M. Shalaev).
The effect of interaction of positrons with defects in materials was firstly discovered in 1964 while studying the electron-positron annihilation in nickel and its alloys (I.Ya. Dekhtiar, V.S. Mikhalenkov, D.A. Levina, S.G. Sakharova). I.Ya. Dekhtiar, E.G. Madatova, V.S. Mikhalenkov, S.G. Sakharova and R.G. Fedchenko focused their subsequent activities to disclose its physical nature and laid the principles for development of new direction—study of electronic structure of defects. The Fermi surface shape of metals at room temperatures using positrons was studied for the first time and thereby confirmed the fruitfulness of the experiment idea suggested by A.A. Smirnov and M.A. Krivoglaz. The defect evolution in solids under the action of external factors (deformation, radiation, heat treatment, etc.) was researched, anisotropy of the electronic characteristics of defects was determined. The appearance of vacancy clusters during metal deformation was discovered and the role of admixtures in this process was shown (V.S. Mikhalenkov).
Since 1980 the method of positron annihilation has been used for study of nature of the amorphous state. The electronic criterion of amorphization was determined. It was experimentally grounded. One of the factors is localization of the positrons in defects of free volume type (Bernal hole), the sizes of which are smaller than the sizes of vacancy in crystals, and the other factor is increase of the effective size of the electron shell of an ion core that takes the positron. The first factor is structural connected with increase of surface energy due to the presence of holes, and the second one is electron that compensates the first due to the decrease of electron energy (Fermi), thus stabilizing the amorphous state of the substance at room temperatures. It was also determined that structural relaxation in the amorphous alloys by heating is accompanied with annealing of defects of free volume type, and the initial stage of crystallization, on the contrary, is characterized by intensive defects formation (I.Ya. Dekhtiar, E.G. Madatova, M.M. Nyshchenko, S.P. Likhtorovich).
The materials for electrodes of high-power independent current sources— cesium direct thermionic converters of heat energy of nuclear fuel to electric TET on the basis of the refractory alloys, used for on-board power supply of space stations have been developed in the department since 1963. Due to the complex study of physical properties of surfaces and volume the working samples of electrodes with high energy of cesium adsorption and low working function were developed and tested. Under the direction of I.Ya. Dekhtiar researchers V.I. Silantiev, N.A. Shevchenko, V.I. Patoka, L.F. Dubikovsky had developed and experimentally realized the principles of optimal alloying to improve the adsorption interaction of refractory metals` surface with cesium, thus, allowing to reach record operation characteristics for that time. It was revealed that as a result of segregation at 2400K on boundary (110) W and Mo carbon forms from the volume a monolayer (graphene) with vacuum working function φ=5,08 and 4,82 eV correspondingly. At higher temperatures it is removed from the surface, which is cleaned up to atomically clean surface with increase of φ to 5,30 eV and 5,00 eV for W and Ìî correspondingly. These results were principally important to increase the efficiency of TET operation: graphene on the boundary (110)W is a collector that immunizes it and prevents adsorption of the residual gases, reduces the coefficient of elastic reflection of electrons from 45% to 13% and during cesium adsorption reduces the minimal value of φ to 1,32 eV. It was found that energies of maxima in spectra of the reflected electrons from atomically clean faces and after graphene formation are as a square of the serial number of peak and are described with the equation of dimensional quantization of the electron`s energy (M.M. Nyshchenko, M.Ya. Shevchenko).
It was for the first time in 1973 that I.Ya.Dekhtiar and M.M. Nyshchenko set the possibility of usage of laser emission to create the superconductors with high critical parameters. Later this method of processing was widely used in the country and abroad for production both metallic and ceramic superconductors. The results obtained using the Mӧssbauer effect allowed to develop the physical conceptions on kinetics and formation mechanism of metastable crystal and amorphous phases under the laser action and about impact mechanisms on the electron properties. The physical model of these processes based on the competition of fronts of solidification velocities and atoms diffusion in liquids was developed. It was also shown that kinetics of phases` formation is determined by interatomic interaction (M.M. Nyshchenko). Such approach allowed to obtain the epitaxial layers of α–iron disilicide, in which concentration of nonstoichiometric vacancies is specified by emission regimes, during laser alloying on the silicon surface. Reduction of their concentration enlarges the width of the forbidden zone of α-phase and converts from equilibrium metallic state to semiconductor one (M.M. Nyshchenko, S.P. Likhtorovich and R.G. Fedchenko).
The department has started the studies of nanostructural materials and systems since 1998. Firstly, there were fullerenes and triglycerides of fatty acids, in which the nanopores with inner diameter (0,35-0,36 nm) were found. The nanopores were equal external diameter of fullerene Ñ60 (0,355 nm). During the insertion to nanopores it stabilized the structure and increased the products resistance to facture at room temperatures. The further activities were focused on study of inner structure of multi-layer carbon nanotubes (CNT): the annihilation regularities of positrons were determined with conduction electrons, localized with π–electrons in interlayer gaps and covalent σ–electrons within the defect structure of hexagonal layer. Information about relative concentration of free electrons and defects, their electron structure was obtained. The interconnections between these parameters, for instance, in CNT with edge dislocations the localization radius of wave function π–electrons between the layers is increased twice causing the growth of concentration of free electrons and increase of transverse conductivity in one order (M.M. Nyshchenko, Ye.A.Tsapko, V.Yu. Koda, I.Ye.Galstian).
Taking into attention that application of CNT on practice foresees the development of technologies of their consolidation, it was revealed that properties of single and compacted nanotubes differ due to the attraction between themselves. If for isolated nanotube electric conductivity is realized only along its axis, so for the ordered nanotubes` array it is also possible in transverse direction under the condition of their compression in the same direction. As a result the transition to metal state (Ä-Ì) occurs and the jump of electric conductivity in 6-10 orders at room temperatures is fixed. At that, the density of CNT array, under which the transition Ä-Ì takes place, is reduced due to increase of nanotubes defects. The existence of the reverse relaxation transition (Ì-Ä) is determined during further unloading. Its high steepness is necessary for creation of supersensitive deformation sensors for seismic and industrial control, inertial navigation systems. The mechanism of deformation effect on electric conductivity of CNT array is connected with local changes of electronic structure and the transfer character of charge carriers in tangency points of nanotubes, which are nanodimensional gates with extremely low Young modulus (< 1 GPà). The transition of Ì-Ä in the CNT array has perspective for application only under the condition to minimize inelastic interactions (M.M. Nyshchenko, G.Yu. Mikhailova, V.Yu. Koda).
It was determined that maximum radial electric conductivity is one order higher due to the opening of the conductivity channels within dislocations for charge carriers which are inside the CNT in multi-layer CNT with edge dislocations. It causes the appearance of two- step transition (Ä-Ì) on 3 and 4 orders of magnitude: first is connected with the increase of the amount and general area of contacts through which tunneling takes place, and the second is connected with the change of the mechanism of electrons transfer between nanotubes. Firstly it is tunnel through dielectric gap, then it is straightening transition through energy barrier and ohm (barrier-free transition). The collective quant effects as oscillations of electric conductivity appear during intensive deformation (M.M. Nyshchenko, G.Yu. Mikhailova).
In order to increase concentration of free electrons in CNT, having the 3 orders higher mobility than metals, the additions of metallic microparticles with lower working function have been inserted, thus allowing to enhance the transversal electric conductivity of composites in dozens of times. The revealed effect is connected with a process of CNT array ordering with metallic particles and shifting of electrons from metal to CNT (M.M. Nyshchenko, G.Yu. Mikhailova).
While studying the effect of low energy electrons (<200 eV) on work function of PTFE with CNT the maximum was found on nonlinear dependence of work function on electrons` energy allowing to record and cancel information by electrons. Their length of de Broglie wave is ~1nm that is 2 orders lower the waves` length of optical modern data recording systems. Researching the composite with the help of method of positron spectroscopy showed that CNT are acceptors of electrons, which increase the work function ϕ, although at Å>80 eV reduction of ϕ occurs as a result of secondary electron-electron emission (M.M. Nyshchenko, N.A. Shevchenko. Ye.A. Tsapko, V.I. Patoka).
With purpose to find out the possibilities of CNT usage in space, nuclear reactors and radioactive waste storages their electric conductivity and thermal e.m.f. after irradiation by high-energy (2 and 21 MeV) electrons and gamma quanta (1,2 MeV) with doze to 2·1017 el/sm2 have been researched. It is found that both radiation and initial (growth) defects increase thermal e.m.f. Effect of the first is one order higher: after radiation the Seebeck coefficient (α) is increased about in 2 times as a result of formation of radioactive defects. The presence of the initial defects in CNT reduces the effect of radiation on electric conductivity and allows to compensate it. The minimum dependence of the elasticity threshold of the CNT array on irradiation doze is connected with carrying out of two processes: formation of radiation defects and healing of defects (M.M. Nyshchenko, G.Yu. Mikhailova, I.M. Sidorchenko, Yu.F. Suskaia, B. Kovalchuk, V.V. Anikeev, V.I. Patoka).
Application of CNT as cathode showed that emissive current under the action of the concentrated solar radiation is controlled by the ionization processes and concentration growth of charge carriers between electrodes, appearance of space negative charge that creates the retarding field and secondary ion-electron emission. The physical principle of the emissive transformation of concentrated solar energy to electric one at temperatures < 600°Ñ is formulated, the essence of which lies in ionization and formation of plasma, separation of charges in the field of contact difference of potentials on points of CNT and neutralization of positive ions by adsorption on points of CNT-cathode (M.M. Nyshchenko, N.A. Shevchenko, Ye.A. Tsapko, I.M. Sidorchenko).
| © G.V. Kurdyumov Institute for Metal Physics, N.A.S.U., 2024 |