TRACE IMPURITIES HAVE SYNERGISTIC EFFECTS ON ELECTRONIC STRUCTURE OF OXIDE-NICKEL CATHODES
Using the methods of electron spectroscopy for chemical analysis (ESCA) and characteristic electron energy losses (EEL), it was found that the joint doping of barium oxide crystallites with nickel and such elements as yttrium, rhenium, palladium and strontium leads to a synergistic effect – a decrease in the curvature of the energy bands of crystallites at their surface, that is, to a decrease in the work function of the material.
BaO crystallites, which are formed in the material during fabrication and thermal activation of the cathode, are the main emission-active component of most microwave device cathode materials. Emission properties of pure ВаО crystallites are determined by oxygen vacancies forming acceptor-like surface states on the oxide surface which lead to curvature of energy zones at the oxide surface upwards . In doing so, the vacancy concentration in BaO crystallites after cathode fabrication is in the range of 2–6 . 1025 m–3 in different types of cathodes and reaches 3–5 . 1026 m–3 after cathode activation [2–3]. In  synergistic effect of calcium and strontium impurities on electronic structure of barium oxide crystallites was established for the first time. The aim of this work is to study experimentally the joint effect of nickel impurities and other types of micro-impurities in the barium oxide crystallites on the electronic structure of barium oxide.
RESEARCH METHODS AND RESULTS
To study the microdoping effect on the barium oxide crystallites electronic structure, especially on the volumetric and surface concentration of oxygen vacancies in barium oxide crystallites, as well as to study a possible synergic effect of double doping, the experimental material samples were prepared in the form of pellets dia. 7 mm and of 1 mm thick, obtained by sintering and subsequent pressing of barium carbonate powder mixture and powdered additional components in amount of 10% (w/w) - yttrium oxide, rhenium, palladium and strontium carbonate. The powders were sintered in vacuum at 1200 °C for 2 hours on a nickel plate. Since nickel exhibits considerable volatility at this temperature, sintering results to formation of barium oxide crystallites containing oxygen vacancies, barium oxide crystallites containing oxygen vacancies and alloyed with atoms of an additional component (yttrium, rhenium, palladium, strontium), and barium oxide crystallites containing oxygen vacancies and alloyed with atoms of an additional component and atoms of nickel.
Electronic states of elements in the material samples were examined inspected by electron spectrometry for chemical analysis (ESCA), spectra interpretation was performed by dividing peaks in the ESCA spectra into Gaussian peaks taking into account influence of surrounding atom by other elements on shifts of atomic peaks which depend on electronegativity value of the mentioned elements. As an example, Fig.1 illustrates the structure of 3d5/2-electron level of barium in a 90% BaCO3+10% Y2O3 material sample. Interpretation of electronic states of barium in material samples is given in table 1.
The concentration of oxygen vacancies in the material samples was studied by characteristic electron energy loss spectroscopy (SELE) at a primary electron energy of 1005 eV with a detection step of 0.05 eV. The spectra were digitally differentiated to increase sensitivity of the method.
Since non-stoichiometric barium oxide containing oxygen vacancies is a donor-type semiconductor and other oxide phases in the material are dielectrics, the characteristic electron losses in the cathode material are due to excitation of volume and surface plasmons in barium oxide, which energies are, respectively, ΔEоб and ΔEпов, determined by the expressions:
where e∗ is effective charge of oxygen vacancy, m∗ is effective electron mass of oxygen vacancies, ħ is Planck constant, ε0 is dielectric constant, ε = 3.6 is high frequency permittivity of barium oxide, Nоб is bulk concentration of oxygen vacancies, Nпов is surface concentration of oxygen vacancies . Hence, the total loss ΔE for plasmonic vibration excitation can be represented as:
ΔE = n1Eоб + n2Eпов, (3)
where n1 and n2 are integers.
The position of the NVS valence band edge in relation to the Fermi level EF was determined by ECCA, taking into account that the density of NV(E) electron states near the valence band edge is described by the approximate relation:
In this case the dependence of the square of the ECCA signal intensity on the bonding energy will be a straight line, which extrapolation to the energy axis and specifically allows to determine the EVS parameter relative to the Fermi level. The value of the EVS parameter in its turn makes it possible to determine magnitude and direction of curvature of energy bands V in barium oxide crystallites at the surface.
In table 2 the summary data on the specified parameters of the electronic structure phases, which have been formed in the samples of studied material as a result of their annealing in vacuum are presented. At interpretation of phases in Table 2 results of Table 1 have been taken into account, as well as the values of peak intensities of the characteristic losses.
When studying the upper edge position of the valence band in oxide phases by ECCA, it is not possible to separately determine the valence band edge position for each oxide phase separately in the case where there are several oxide phases. At the same time the total position of the valence band edge determines the value of total curvature of energy bands, i.e. the value of material work function. Table 3 shows the valence zone top position relative to the Fermi level and the value of the total curvature of energy zones V in barium oxide crystallites for the studied materials.
Thus, the results of the study allow us to draw the following conclusions:
the introduction of the doping impurity and nickel in the form of nickel vapor into the cathode material as a result of sample annealing on the nickel substrate, besides BaO(1-x) phase, leads to the formation of Ba(1-y)O(1-x)Niy phase which are crystallites of barium oxide containing oxygen vacancies and dissolved impurity nickel atoms, and also Ba(1-y-z)O(1-x)NiyMez phases where Me is an impurity atom;
it is the latter phase that is characterised by reduced surface concentration of oxygen vacancies relative to the value of bulk concentration of vacancies in the phase volume;
the co-doping of barium oxide crystallites with nickel and such impurity elements as yttrium, rhenium, palladium, strontium leads to decrease of energy zones curvature of barium oxide at its surface, i.e. to decrease of work function value of barium oxide. This shows the synergic effect of the common doping of barium oxide crystallites with two types of impurity atoms.
PEER REVIEW INFO
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Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.