RESEARCHING CONDITIONS FOR FORMATION OF FILAMENTOUS NANOSCALE CRYSTALS ON SURFACES OF FIELD EMISSION CATHODES OF MAGNETRONS WITH INITIAL START
The launch of magnetrons with instant start into the generation mode is due to the current of field emission from nanoscale crystals, formed on the surface of tantalum field emisson cathodes during the cathode-heating unit (CHU) activation. When the current level is not sufficient to launch the magnetron, cathodes should additionally trained in diode. This process increases magnetron cycle time and includes the use of extra production resources. This work reflects the results of studies of the influence of surface defects of tantalum washers artificially created by ion and chemical etching on the growth processes of nanowhiskers. Possibility of using tungsten and niobium washers as autocathodes for magnetrons with "cold" cathode is considered.
Original paper
RESEARCHING CONDITIONS FOR FORMATION OF FILAMENTOUS NANOSCALE CRYSTALS ON SURFACES OF FIELD EMISSION CATHODES OF MAGNETRONS WITH INITIAL START
N.E.Ledentsova1, 2, Cand. of Sci. (Tech), Head of Laboratory, ORCID: 0000-0002-7859-2048
D.V.Bychkov1, Leading Engineer, ORCID: 0009-0008-4792-0417
I.P.Li1, 2, Doct. of Sci. (Tech), Deputy Director, ORCID: 0009-0007-7731-2327 / i.li@pluton.msk.ru
A.V.Shumanov1, Director, ORCID: 0000-0002-6284-2700
V.I.Kapustin1, Doct. of Sci. (Physics and Mathematics), Prof., Chief Specialist, ORCID: 0009-0002-5807-1429
Abstract. The launch of magnetrons with instant start into the generation mode is due to the current of field emission from nanoscale crystals, formed on the surface of tantalum field emisson cathodes during the cathode-heating unit (CHU) activation. When the current level is not sufficient to launch the magnetron, cathodes should additionally trained in diode. This process increases magnetron cycle time and includes the use of extra production resources. This work reflects the results of studies of the influence of surface defects of tantalum washers artificially created by ion and chemical etching on the growth processes of nanowhiskers. Possibility of using tungsten and niobium washers as autocathodes for magnetrons with "cold" cathode is considered.
Keywords: field emission, nanowhiskers, etching, Pd-Ba cathodes, magnetron
For citation: N.E. Ledentsova, D.V. Bychkov, I.P. Li, A.V. Shumanov, V.I. Kapustin. Researching conditions for formation of filamentous nanoscale crystals on surfaces of field emission cathodes of magnetrons with initial start. NANOINDUSTRY. 2023. V. 16, no. 6. PP. 370–377. https://doi.org/10.22184/1993-8578.2023.16.6.370.377.
INTRODUCTION
The magnetron technical purpose is to generate of electromagnetic radiation in the microwave range. JSC "Pluton" takes the leading position in serial production of magnetrons with instantaneous readiness, which are part of the group of EVD microwave devices and stand out among others by the start-up time. In these magnetrons start-up in the generation mode is provided by auto-electron emission current (AEC), created by auto-emission cathode (AEC), and maintenance of this generation is due to secondary-electron emission from Pd-Ba secondary-emission cathodes (SEC). Fig.1 schematically represents the "cathode-anode" node of such a magnetron [1].
It is known that the working surface of autoemission cathodes is the sharp edges of Ta rings, which in the process of cathode activation are covered with BaO film, which, as a consequence, reduces the electron work function from the value of 4.12 eV (Ta) [2] to 2.1–2.3 eV (BaO). After conducting more studies in JSC "Pluton", it was found out that on the side surface of tantalum autocathodes in the process of cathode activation during pumping and training of the device formation of a system of palladium spicules – whiskers, partially or completely covered with BaO, capable of providing a sufficient level of autoemission to start the device in the generation mode occurs [3]. Formation of the nanowhisker system occurs on dislocations on the tantalum surface. In this regard, an important task is to find ways to develop and multiply such defects on autoelectron emitters before putting them into the device, in order to increase the number of nanowhiskers, their height and concentration, which can lead to an increase in autoelectron emission current. The work will also study the possibility of using other materials such as niobium and tungsten as autocathode materials.
The search for methods of formation of the palladium spike system plays an important role in obtaining the required autoemission parameters of the cathode and is a promising and important task in development of new high-power devices.
EQUIPMENT AND RESULTS
In this work, the cathode activation process was carried out in a high-vacuum unit for study of autoemission properties, shown in Fig.2. The chamber with the anode block equipped with the cathode-heating unit was pumped out by a forevacuum pump, then by a magnetodischarge pump NMD-0.25 up to the chamber pressure ≈ 3 ∙ 10–8 Torr. Pressure in the chamber was monitored by a magnetic-discharge vacuum gauge PMM 32 placed directly under the sample. During continuous degassing of the chamber, activation of the cathode was performed. The autoemission current was measured using a high-voltage pulse source with a voltage pulse amplitude up to 5 kV, pulse duration (variable) 0.5/1/6 μs, and pulse ratio is 1000. Registration of Ua and Iae values by time was performed on a personal computer in the Excel program package. The design of the first CHU is presented in Fig.3. The cathode node was activated according to the mode similar to the processing mode of serial CHUs.
After activation, cathode autoemission current was at the level of experimental error. Fig.4 shows an image of the disassembled assembly obtained using a scanning electron microscope by "ZEISS EVO 40". For convenience of the study, the autoemitters were bent to one side.
Microphotographs of the surfaces of tungsten, niobium and tantalum autocathodes and their elemental composition, respectively, are presented below in Figs.5–8.
The surfaces of the tungsten and niobium washers are covered with a palladium film, but no whisker growth is observed (Figs.5, 6).
Summarizing the obtained results of the studies for the first cathode assembly the following can be noted:
whiskers on the tungsten surface and niobium autocathodes were not detected. Probably, this effect is caused by absence of microdefects on their surface, which are leveled, presumably, during recrystallization annealing during foil rolling;
on the surface of etched and unetched tantalum autocathodes, whisker nuclei are clearly visible, and their number on the etched surface is somewhat higher. Probably, low autoemission was caused by the insufficiently developed structure of nanowhiskers, i.e., the cathode was not sufficiently activated.
Due to the fact that whiskers grow on microdefects, further research work will be carried out on CHU 2, whose autocathodes will be etched not only in ion plasma but also by chemical-technological treatment methods. The design of CHU No. 2, which was assembled for a similar experiment, is presented below.
CHU No. 2 consisted of cathode leg 1, screen 2, seven palladium-barium emitters 3 and 4, fourteen tungsten washers 5, tantalum autocathode, 0.004 mm thick, six tantalum autocathodes, 0.01 mm thick, processed as follows:
"0.01" – 0.01 mm thick tantalum degreased in trichloroethylene with ultrasonication;
"i1" and "i2" – tantalum with ion etching in nitrogen plasma for 15 min and 30 min, respectively;
"h1" – tantalum chemically treated in a solution of HNO3 and 5 % HF (40 %);
"h2" – tantalum treated by chemical engineering method for 2 min in a mixture of HNO3 and H2SO4 (3:1) with ultrasound;
"0.01UZ" – tantalum 0.01 mm thick, degreased in trichloroethylene with ultrasound and additionally ultrasonically treated in absolute alcohol.
CHU No. 2 was activated according to the cathodes activation modes of commercially available magnetrons in the unit for study of autoemission properties. Thermal emission current corresponds to the TEE level of current obtained from the cathodes of commercially available products.
After activation, the CHU No. 2 was also dismantled and, similarly to CHU No. 1, was examined in a ZEISS EVO 40 scanning electron microscope. For convenience of the study, the auto emitters were bent to one side. The obtained results are presented in Figs.10–11.
The elemental composition of all presented tantalum autocathodes is similar to composition of the sample "0.01" shown in Fig.11c. It is also evident from the obtained images that nanoscale filamentous crystals are present on all autocathodes, however, more whiskers were formed on the surface of the "0.01" AEC, as well as after chemical etching. It is impossible not to emphasize different distribution density of nanowhiskers and their shape on different samples: on sample "0.01" – flat, wide in a large number; on sample "h1" – in a large number, but not evenly arranged; on sample "h2" – thin, elongated, in a large number; on sample "i1" – long, thin, in a relatively small number; on samples "0.004" and "0.01UZ" – in a relatively small number, compared to others.
CONCLUSIONS
The results of this work have contributed to identification of the positive effect of artificial development of microdefects on the tantalum surface on the growth and number of whiskers, which undoubtedly can serve as a new promising direction in magnetrons design with instantaneous readiness.
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.