rus
Issue #7-8/2022
V.I.Kapustin, I.P.Li, A.V.Shumanov
ELECTRONIC STRUCTURE OF THE SCANDATE CATHODE VARIANTS
ELECTRONIC STRUCTURE OF THE SCANDATE CATHODE VARIANTS
INTRODUCTION
Since the 1990s scientific laboratories of universities and industrial enterprises in various countries have been carrying out the research aimed at creating scandate cathodes, i.e. metal-porous cathodes containing scandium in their composition. According to the research results it was found that such cathode can provide thermoelectronic emission current density up to 100 A/cm2, and in future up to 400 A/cm2 [1–3]. This fact opens a possibility of creating fundamentally new types of electrovacuum microwave devices. At the same time, the obtained results show poor technological reproducibility of the scandate cathodes emission properties so, until now, no industrial enterprises have mastered mass production of the microwave devices with cathodes of this type. At the same time, a possible synergistic effect of micro impurities in barium oxide crystallites on its electronic structure turned out to be insufficiently studied, and influence of the phase state, in particular, the components containing scandium, on the electronic structure of the emission-active phase of cathode material was also insufficiently studied. The present work is devoted to investigate these issued.
RESEARCH METHODS AND RESULTS
Powder samples with a diameter of 7 mm and thickness of 1 mm were produced by sintering and pressing tablets made from a mixture of tungsten powder (90% by weight), pre-synthesized and ground barium-calcium aluminate powder of 2.5BaO.0.4CaO.Al2O3 composition and additional components in the form of powders of intermetallic Re2Sc, 80%W+20%Re alloy powder and scandium hydride powder ScH2. The powders were also sintered in vacuum at 1200 °C for 2 hours, but on a tungsten plate. As a result of sintering, barium oxide crystallites containing oxygen vacancies as well as barium oxide crystallites containing oxygen vacancies and, depending on composition of the mixtures, doped with tungsten, rhenium, scandium atoms and combination of these elements were formed in the material samples. The powder of Re2Sc intermetallide was obtained by electric arc re-melting of the components in purified argon followed by milling in a ball mill. The powder of scandium hydride ScH2 was obtained by annealing of scandium in a hydrogen medium followed by milling in a ball mill. The alloy powder 80%W+20%Re was made in the Research and Production Complex "Advanced Powder Technology" (Tomsk) by atomizing wire BP-20 (their alloy 80%W+20%Re) by electric explosion. Electronic states of elements in samples of materials were studied by electron spectrometry for chemical analysis (ESCA). The spectra were decoded by division of spectra peaks of ESCA into Gaussian peaks taking into account the influence of the fact that the atom is surrounded by other elements and, thereby, impacts on the shifts of atoms peaks which depend on the value of electronegativity of specified elements. The deciphering of the electronic states of barium samples is given in Table 1.
The oxygen vacancies concentration in the material samples was investigated by characteristic electron energy loss spectroscopy (SELE) at the primary electron energy of 1005 eV at a detection step of 0.05 eV. The spectra were digitally differentiated to increase the method sensitivity.
Since non-stoichiometric barium oxide containing oxygen vacancies is a donor type semiconductor and other oxide phases in the material are dielectrics, the characteristic losses of electrons in the cathode material are caused by excitation of volume and surface plasmons in barium oxide, which energies are, respectively, ΔEоб and ΔEпов determined by the following expressions:
, (1)
. (2)
where e∗ is the effective charge of oxygen vacancy, m∗ is effective electron mass of oxygen vacancies, ħ is the Planck constant, ε0 is the dielectric constant, ε = 3.6 is the high frequency permittivity of barium oxide, Nоб is the bulk concentration of oxygen vacancies, Nпов is the surface concentration of oxygen vacancies [3].Thus, the total loss ΔE for plasmon oscillation excitation can be represented as
, (3)
where n1 and n2 – integers.
Table 2 summarizes parameters of the electronic structure of phases formed in the studied samples of materials as a result of their annealing in vacuum. At deciphering of phases in Table 2, the results of Table 1, and also value of peaks intensities of characteristic losses have been taken into account.
To carry out a comprehensive analysis of the scandium phase state effect in the cathode material on formation of oxygen vacancies in barium oxide crystallites, Fig.1 data shows concentration dependences (on the scandium oxide content) of volume and surface oxygen vacancy concentration in material samples based on tungsten powder and 2.5BaO . 0.4CaO . Al2O3 phase, in which the aluminum oxide was completely or partially replaced with scandium oxide [5].
When calculating the surface and volume concentration of oxygen vacancies the effective mass values of electrons on oxygen vacancies and the effective charge of vacancies in pure barium oxide crystals and in crystals doped with one type of micro-impurities were taken from [4-5] where they were determined experimentally by optical absorption. Values of indicated parameters for barium oxide crystallites doped with two types of micro-impurities have not been determined experimentally at present. Therefore, for the mentioned case of double doping in calculations by relations (1) and (2) the values of effective mass and effective charge for the component which is contained in material in the maximal concentration were taken. Such a choice could lead to a certain inaccuracy when calculating the absolute values of surface and bulk vacancy concentrations, but did not affect the most important parameter – the ratio of surface and bulk vacancy concentrations in each sample.
During annealing of barium-calcium aluminate in vacuum in the presence of other phases, the established volume concentration of oxygen vacancies in barium oxide crystallites depends not only on the initial phase composition of the material but also on temperature and annealing time, material porosity and granulometric composition of powder components. In fact, optimization of particle size distribution, porosity, temperature and annealing time causes activation of the cathode material which results in formation of the required concentration of oxygen vacancies in the volume of barium oxide crystallites. However, the upper monolayer of the barium oxide crystallites is in thermodynamic equilibrium with the crystallite volume at any given time, with:
surface vacancy concentration, as first shown in [6] and seen from the results of this work, is determined not only by the equilibrium between the volume and surface of the crystal, but also by the presence of dopant (impurity) atoms in the upper monolayer of the crystal;
it is surface concentration of oxygen vacancies which determines the value of the energy zones distortion at the barium oxide crystallite surface: the smaller distortion value, the smaller value of the work release [6].
In [6] theory of scandium cathodes was proposed, according to which the low output work of the barium oxide crystallite can be provided by barium oxide nanocrystallite formation, in the upper monolayer of which barium atoms are completely or partially replaced with scandium atoms. An indication of formation of such structure is a decrease in the surface concentration of oxygen vacancies compared with the bulk concentration of oxygen vacancies. The condition required for formation of such a structure is the size factor of the alloying element – its ionic radius must be about 0.60 of the ionic radius of barium. Based on the theoretical approach [13], we can also formulate more general conditions which ensure reduction of the surface concentration of oxygen vacancies in barium oxide crystallites relative to its bulk concentration:
the formation enthalpy of the alloying element oxide must be higher than the enthalpy of the barium oxide formation, i.e. the energy of the "oxygen ion – alloying element ion" bond must be higher than the energy of the "oxygen ion – barium ion" bond;
for effective alloying of barium oxide crystallites by other element, such element at the initial phase should be in a poorly bound form, for example, as a part of intermetallide, hydride or nanocrystalline oxide with high excess surface energy;
for segregation of the alloying element exactly in the upper monolayer of the barium oxide crystal, such crystal must be in the form of nanocrystals in which there is a significant difference of interplanar distance between the first and second monolayers and monolayers in the volume of the nanocrystal. This is where the size factor of the alloying element comes into play.
Precisely these conditions, as can be seen from Table 2, are satisfied with introduction of additional components in the form of intermetallic Re2Sc and ScH2 hydride in initial barium carbonate with a small content of an additional component in the form of scandium oxide and are violated at high concentrations of scandium oxide in the material. The same conditions are fulfilled also in case of joint alloying of barium oxide with nickel and strontium which, actually, make the basis of high thermal emission properties of the nickel oxide cathodes.
CONCLUSIONS
Thus, the following conclusions can be drawn from the results of the work:
1. High thermal emission properties of the scandium cathode can be provided for formation of barium oxide crystallites in the cathode material where which barium atoms in the upper monolayer are completely or partially replaced with scandium atoms.
2. An effective doping of crystallites with scandium atoms can be achieved by using scandium in the scandium intermetallic, scandium hydride or scandium oxide in a nanocrystalline state as the cathode material.
1. To ensure segregation of scandium atoms exactly in the upper monolayer of barium oxide crystallites is possible by formation of the mentioned crystallites at the stage of cathode activation in the form of the barium oxide nanocrystallites.
2. Formation of barium oxide nanocrystallites exactly at the stage of cathode activation is possible by using an activator of barium-calcium aluminate decomposition (tungsten) in the form of tungsten nanocrystals or lower oxide tungsten vapour supplied to the aluminate surface from the cathode volume through pores in the aluminate.
3. The sign of formation of the scandate cathode effective structure a reduced surface concentration of oxygen vacancies in the barium oxide nanocrystallites in relation to its volume concentration.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
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.
Since the 1990s scientific laboratories of universities and industrial enterprises in various countries have been carrying out the research aimed at creating scandate cathodes, i.e. metal-porous cathodes containing scandium in their composition. According to the research results it was found that such cathode can provide thermoelectronic emission current density up to 100 A/cm2, and in future up to 400 A/cm2 [1–3]. This fact opens a possibility of creating fundamentally new types of electrovacuum microwave devices. At the same time, the obtained results show poor technological reproducibility of the scandate cathodes emission properties so, until now, no industrial enterprises have mastered mass production of the microwave devices with cathodes of this type. At the same time, a possible synergistic effect of micro impurities in barium oxide crystallites on its electronic structure turned out to be insufficiently studied, and influence of the phase state, in particular, the components containing scandium, on the electronic structure of the emission-active phase of cathode material was also insufficiently studied. The present work is devoted to investigate these issued.
RESEARCH METHODS AND RESULTS
Powder samples with a diameter of 7 mm and thickness of 1 mm were produced by sintering and pressing tablets made from a mixture of tungsten powder (90% by weight), pre-synthesized and ground barium-calcium aluminate powder of 2.5BaO.0.4CaO.Al2O3 composition and additional components in the form of powders of intermetallic Re2Sc, 80%W+20%Re alloy powder and scandium hydride powder ScH2. The powders were also sintered in vacuum at 1200 °C for 2 hours, but on a tungsten plate. As a result of sintering, barium oxide crystallites containing oxygen vacancies as well as barium oxide crystallites containing oxygen vacancies and, depending on composition of the mixtures, doped with tungsten, rhenium, scandium atoms and combination of these elements were formed in the material samples. The powder of Re2Sc intermetallide was obtained by electric arc re-melting of the components in purified argon followed by milling in a ball mill. The powder of scandium hydride ScH2 was obtained by annealing of scandium in a hydrogen medium followed by milling in a ball mill. The alloy powder 80%W+20%Re was made in the Research and Production Complex "Advanced Powder Technology" (Tomsk) by atomizing wire BP-20 (their alloy 80%W+20%Re) by electric explosion. Electronic states of elements in samples of materials were studied by electron spectrometry for chemical analysis (ESCA). The spectra were decoded by division of spectra peaks of ESCA into Gaussian peaks taking into account the influence of the fact that the atom is surrounded by other elements and, thereby, impacts on the shifts of atoms peaks which depend on the value of electronegativity of specified elements. The deciphering of the electronic states of barium samples is given in Table 1.
The oxygen vacancies concentration in the material samples was investigated by characteristic electron energy loss spectroscopy (SELE) at the primary electron energy of 1005 eV at a detection step of 0.05 eV. The spectra were digitally differentiated to increase the method sensitivity.
Since non-stoichiometric barium oxide containing oxygen vacancies is a donor type semiconductor and other oxide phases in the material are dielectrics, the characteristic losses of electrons in the cathode material are caused by excitation of volume and surface plasmons in barium oxide, which energies are, respectively, ΔEоб and ΔEпов determined by the following expressions:
, (1)
. (2)
where e∗ is the effective charge of oxygen vacancy, m∗ is effective electron mass of oxygen vacancies, ħ is the Planck constant, ε0 is the dielectric constant, ε = 3.6 is the high frequency permittivity of barium oxide, Nоб is the bulk concentration of oxygen vacancies, Nпов is the surface concentration of oxygen vacancies [3].Thus, the total loss ΔE for plasmon oscillation excitation can be represented as
, (3)
where n1 and n2 – integers.
Table 2 summarizes parameters of the electronic structure of phases formed in the studied samples of materials as a result of their annealing in vacuum. At deciphering of phases in Table 2, the results of Table 1, and also value of peaks intensities of characteristic losses have been taken into account.
To carry out a comprehensive analysis of the scandium phase state effect in the cathode material on formation of oxygen vacancies in barium oxide crystallites, Fig.1 data shows concentration dependences (on the scandium oxide content) of volume and surface oxygen vacancy concentration in material samples based on tungsten powder and 2.5BaO . 0.4CaO . Al2O3 phase, in which the aluminum oxide was completely or partially replaced with scandium oxide [5].
When calculating the surface and volume concentration of oxygen vacancies the effective mass values of electrons on oxygen vacancies and the effective charge of vacancies in pure barium oxide crystals and in crystals doped with one type of micro-impurities were taken from [4-5] where they were determined experimentally by optical absorption. Values of indicated parameters for barium oxide crystallites doped with two types of micro-impurities have not been determined experimentally at present. Therefore, for the mentioned case of double doping in calculations by relations (1) and (2) the values of effective mass and effective charge for the component which is contained in material in the maximal concentration were taken. Such a choice could lead to a certain inaccuracy when calculating the absolute values of surface and bulk vacancy concentrations, but did not affect the most important parameter – the ratio of surface and bulk vacancy concentrations in each sample.
During annealing of barium-calcium aluminate in vacuum in the presence of other phases, the established volume concentration of oxygen vacancies in barium oxide crystallites depends not only on the initial phase composition of the material but also on temperature and annealing time, material porosity and granulometric composition of powder components. In fact, optimization of particle size distribution, porosity, temperature and annealing time causes activation of the cathode material which results in formation of the required concentration of oxygen vacancies in the volume of barium oxide crystallites. However, the upper monolayer of the barium oxide crystallites is in thermodynamic equilibrium with the crystallite volume at any given time, with:
surface vacancy concentration, as first shown in [6] and seen from the results of this work, is determined not only by the equilibrium between the volume and surface of the crystal, but also by the presence of dopant (impurity) atoms in the upper monolayer of the crystal;
it is surface concentration of oxygen vacancies which determines the value of the energy zones distortion at the barium oxide crystallite surface: the smaller distortion value, the smaller value of the work release [6].
In [6] theory of scandium cathodes was proposed, according to which the low output work of the barium oxide crystallite can be provided by barium oxide nanocrystallite formation, in the upper monolayer of which barium atoms are completely or partially replaced with scandium atoms. An indication of formation of such structure is a decrease in the surface concentration of oxygen vacancies compared with the bulk concentration of oxygen vacancies. The condition required for formation of such a structure is the size factor of the alloying element – its ionic radius must be about 0.60 of the ionic radius of barium. Based on the theoretical approach [13], we can also formulate more general conditions which ensure reduction of the surface concentration of oxygen vacancies in barium oxide crystallites relative to its bulk concentration:
the formation enthalpy of the alloying element oxide must be higher than the enthalpy of the barium oxide formation, i.e. the energy of the "oxygen ion – alloying element ion" bond must be higher than the energy of the "oxygen ion – barium ion" bond;
for effective alloying of barium oxide crystallites by other element, such element at the initial phase should be in a poorly bound form, for example, as a part of intermetallide, hydride or nanocrystalline oxide with high excess surface energy;
for segregation of the alloying element exactly in the upper monolayer of the barium oxide crystal, such crystal must be in the form of nanocrystals in which there is a significant difference of interplanar distance between the first and second monolayers and monolayers in the volume of the nanocrystal. This is where the size factor of the alloying element comes into play.
Precisely these conditions, as can be seen from Table 2, are satisfied with introduction of additional components in the form of intermetallic Re2Sc and ScH2 hydride in initial barium carbonate with a small content of an additional component in the form of scandium oxide and are violated at high concentrations of scandium oxide in the material. The same conditions are fulfilled also in case of joint alloying of barium oxide with nickel and strontium which, actually, make the basis of high thermal emission properties of the nickel oxide cathodes.
CONCLUSIONS
Thus, the following conclusions can be drawn from the results of the work:
1. High thermal emission properties of the scandium cathode can be provided for formation of barium oxide crystallites in the cathode material where which barium atoms in the upper monolayer are completely or partially replaced with scandium atoms.
2. An effective doping of crystallites with scandium atoms can be achieved by using scandium in the scandium intermetallic, scandium hydride or scandium oxide in a nanocrystalline state as the cathode material.
1. To ensure segregation of scandium atoms exactly in the upper monolayer of barium oxide crystallites is possible by formation of the mentioned crystallites at the stage of cathode activation in the form of the barium oxide nanocrystallites.
2. Formation of barium oxide nanocrystallites exactly at the stage of cathode activation is possible by using an activator of barium-calcium aluminate decomposition (tungsten) in the form of tungsten nanocrystals or lower oxide tungsten vapour supplied to the aluminate surface from the cathode volume through pores in the aluminate.
3. The sign of formation of the scandate cathode effective structure a reduced surface concentration of oxygen vacancies in the barium oxide nanocrystallites in relation to its volume concentration.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
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.
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