RU2494489C1 - Magnetron with selfheated startup with special activation of field-emission cathodes - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: magnetron with selfheated startup comprises alternating field-emission cathodes (FEC), made in the form of rings of tantalum foil with thickness of ~4 mcm and secondary-emission pressed palladium-barium emitters with barium content in the amount of 11-25 wt% and open porosity of 2-27%. Generation of current of field emission, together with emission from the FEC edge, mainly depends on the flow of auto-electrons from spikes of whisker crystals with length of 10-100 nm and dimensions in cross sections - 5-20 nm, formed on the end surface of FEC under certain conditions.

EFFECT: higher stability of magnetron excitation, reliability and durability of its operation.

3 cl, 7 dwg

Description

The invention relates to electronic equipment and can be used in microwave devices of the M type, in particular in magnetrons of the centimeter, millimeter and submillimeter wavelength ranges with non-triggering.

The main problem that arises when creating magnetrons of this class is to ensure the stability and durability of the cathodes under conditions of intense electron and ion bombardment, the effect of which leads to rapid degradation of the parameters of field-effect cathodes and premature failure of the device.

Known magnetron with non-triggering start (Fig. 1) [1; 2], containing the anode (1) and a cathode assembly coaxially placed inside it, consisting of a molybdenum, nickel or niobium core (2); heater (3); screens (4) and alternating autoelectronic cathodes (AEC) (5) made of tantalum foil ~ 4 μm thick and secondary emission cathodes (EEC): metal-porous tungsten-aluminate (MPC), nickel oxide (OK) or “cast” metal alloy cathodes (MSCs) of an alloy of palladium with barium (6).

The main disadvantages of this magnetron are the significant scatter and low level of field emission current, which are:

I _ae ~ 0.1-1.0 mA in magnetrons, in which they are used as the secondary-emission cathode of the MPC and OK (figure 2; table 1, line 1);

I _ae ~ 0.5-3.0 mA in magnetrons, in which cast MSCs from an alloy of palladium with barium are used as VEC (Fig. 2; table 1, line 2);

(The field emission current was measured in magnetrons pumped out to a residual gas pressure of P ~ 5 · 10 ^-6 Pa., With a pulsed anode voltage U _a = 4500 V., pulse duration τ = 6 μs. And duty cycle Q = 1000). In addition, such a magnetron is characterized by irreproducibility of operational parameters and a relatively low percentage of suitable devices, comprising ~ 20-30% with MPC and OK and ~ 40-50% with "cast" palladium - barium secondary cathodes.

Known magnetron with a good start, which is the prototype of this invention. The magnetron (Fig. 1) consists of an anode (1) and a cathode assembly coaxially placed inside it, consisting of a molybdenum core (2), a heater (3), screens (4), alternating autoelectronic cathodes made of tantalum foil ~ 4 μm thick. (5) and secondary-emission pressed palladium-barium cathodes with a length of 0.4-5 mm. from a mixture of powders of emission-active palladium — a barium alloy with a barium content of 11–25% (weight) and palladium in an amount of 75–89% (weight) with an open porosity of 2–27% [3]. The indicated magnetron, along with a sufficiently long service life (more than 5000 hours) and a high percentage of suitable devices (at least 80%), is characterized by a relatively low stability and a large spread of field emission current values, which leads, as a rule, to the need for long-term training of the device in the generator mode (figure 2; table 1, line 3).

To find out the reason for such instability and such a spread in the field emission current, cathodes were studied using an EVO - 40 scanning electron microscope from Zeiss with a CDD X Flash 1106 X-ray energy dispersive spectrometer and a thermogravimetric analysis unit equipped with an ultra-sensitive vacuum balance and a time-of-flight mass spectrometer. The objects of study were AEC and EEC from magnetrons that were pumped out under different modes of activation of the cathode nodes.

As a result of the research, the following data were obtained.

1. When using a combination of tantalum AEC and various types of effective VEC (MPC or OK) in non-heating magnetrons, the generation is initiated by field emission from the sharp edge of the AEC covered completely or partially by evaporation products (barium) from the VEC during pumping and training. The degree of barium coating of the working edge of the autoelectronic cathode depends on the sorption properties of the AEC material, on the temperature and duration of its activation, on the rate of evaporation of barium, etc. Therefore, any errors or inaccuracies in the activation of the cathode assembly determine the instability and spread of field emission properties of AEC.

2. In the case of using cast or pressed palladium-barium emitters as an EEC, the mechanism for generating field emission current differs significantly from the mechanism described in paragraph 1.

2.1. Autoelectronic emission from the working edge of AEC activated by barium makes up only a small part (no more than 5-10%) of the total current of autoelectronic emission.

2.2. The field emission current is predominantly due to emission from whisker tips [4], possibly formed from tantalum crystals with palladium, fully or partially coated with barium. Figure 3 (a; b; c) shows micrographs of the AEC fragment at different magnifications:

- figure 3-a with an increase of M ~ 100 x shows fragments of a secondary-emission pressed palladium-barium cathode (VEC) and the surface of the field-effect cathode (AEC) with whiskers;

- figure 3-b shows a fragment of the surface of the AEC with whiskers with an increase of M ~ 30000 x;

- figure 3-c with an increase in M ~ 50,000 x shows a fragment of AEC with whiskers (N.K.) with a length of ~ 50-100 nm and a cross-sectional size of ~ 5-20 nm.

2.3. The growth rate and the degree of surface filling with whiskers depends on the vapor pressure of palladium near the AEC surface, depending on the rate of palladium evaporation. Figure 4 shows the temperature dependence of the rate of evaporation of palladium from the surface of the pressed palladium-barium emitter. From this dependence it follows that the optimum temperature for the formation of whiskers is the cathode temperature range T _to ~ 700-1100ºС. (~ 970-1370K). At lower cathode temperatures, due to the low rate of palladium evaporation, the process of crystal formation (growth) becomes ineffective and can stretch for a long period of time. At high cathode temperatures, due to the crystal structure of the pressed emitter, an irreversible change in geometric dimensions (expansion) occurs, causing a deterioration in the electrical parameters of the magnetron.

2.4. On the surface of the AEC, when compact “cast” palladium-barium emitters are used as an EEC, nuclei of whiskers are formed, but due to the low palladium flux, their growth rate is significantly lower than in the case of pressed emitters. In pressed emitters, palladium particles are not densely packed and therefore, during the heat treatment, a crystal lattice rearrangement is likely to occur, accompanied by the formation of “worm” formations on the surface, causing an increase in the effective evaporation surface, and,

hence the flow of palladium. Figure 5 shows a micrograph of the surface of the emitter with "worm" formations after activation at a temperature of T ~ 1000ºC. (~ 1270K) for 60 minutes

2.5. The rate of formation and completeness of filling the end surface of the AEC with whiskers can be controlled by selecting the cathode activation temperature in the range T _k ~ 700-1100ºС (~ 970-1370К) and the corresponding value of time t from the temperature-time relation: T · t ~ 1.0 · 10 ^-6 ÷ 1.5 · 10 ⁷ (K · sec).

2.6. When the cathode is activated in an electric field, for example, when the anode voltage is turned on, the direction of the whiskers can be ordered and oriented along the field, i.e. towards the anode, due to which the conditions for the formation of a flow of field electrons are facilitated.

2.7. During the thermal treatment of the cathode assembly, the processes of thermal diffusion dissociation of the Pd ₅ Ba phase occur in the volume of the pressed palladium-barium emitter with the formation of free barium, which, when it enters the surface of the AEC, is sorbed on the sharp edge of the AEC and on the surface of whiskers, thereby increasing the emission activity of AEC . It should be noted that in cast palladium-barium emitters, unlike pressed emitters, porosity is practically absent. Therefore, the delivery of barium from the volume of the EEC occurs only due to its migration along grain boundaries and individual defects. As a result of this, due to the low concentration of barium on the surface of cast VEC, the process of activation of AEC by barium is difficult and causes the instability of the field emission current. In pressed palladium-barium emitters, the process of delivering barium from volume to surface is significantly facilitated and is ensured both by migration along grain boundaries and defects, and through pore channels (migration and Knudsen transfer).

2.8. The rate of formation of free barium also depends on the magnitude of the external electric field. When an external electric field (E ~ 1 × 10 ³ ÷ 5 × 10 ⁴ B / cm) is applied in the EEC volume, the processes of electrothermodiffusion dissociation of the Pd ₅ Ba phase occur, ultimately leading to an increase in the concentration of free barium on the EEC surface, and This ensures a higher activation rate of AEC.

Table 1 row 4 (figure 2) shows data on the magnitude of the field emission current and the duration of training of magnetrons with pressed

palladium-barium emitters after activation in an electric field E ~ 5 · 10 ³ V / cm, temperature T ~ 1250ºK for 60 minutes

(T · t ~ 4.5 · 10 ⁶ K · sec). As can be seen from the table, these magnetrons, compared with others, have advantages both in field emission current and in training duration in dynamic mode.

Thus, based on the foregoing, the subject of the invention is:

1. Magnetron with non-triggering starting, similar in design to the prototype, characterized in that to increase the stability and reproducibility of the initiation of generation, the cathode is subjected to special activation in the temperature range T ~ 970-1370 K subject to the condition T · t ~ 1.0 · 10 ⁶ ÷ 1.5 · 10 ⁷ (K · sec). In this case, palladium whiskers with a length of ~ 50-100 nm and cross-sectional sizes of ~ 5-20 nm, partially or completely coated with barium, are grown on the end surface of the AEC. The presence of whiskers on the surface of the AEC significantly increases the magnitude of the field emission current, and thereby ensures the reliability of the magnetron.

2. The cathode assembly of the magnetron, shown in Fig.6, consists of alternating AEC (5) and EEC (6), between which are pressed palladium-barium washers (7) made of a mixture of powders of emission-active palladium-barium alloy containing barium in an amount of 11-25% (wt.) and palladium in an amount of 75-89% (wt.) with an open porosity of 2-27% and a length of ~ 0.1-1 mm, which are activators of AEC. During the activation of the cathode, the evaporated components from the washer, adsorbed on the surface of the AEC, cause the formation of whiskers. To increase the efficiency of the formation of whiskers, additives in the form of powders of platinum, tungsten, tantalum or rhenium in the amount of 1-50% (weight) are added to the composition of the washer material. In addition to palladium-barium emitters, various metals and alloys with stable secondary emission properties and resistant to ion and reverse electron bombardments, for example, iridium, platinum, osmium, intermetallic compounds, platinum with barium, are used as an EEC in this cathode design , iridium with lanthanum, iridium with cerium, osmium with lanthanum, etc.

Briefly about the mechanism of magnetron generation excitation.

When a pulsed anode voltage is turned on, the total field current from the tips of the whiskers and the working edge of the AEC forms the initial electron stream, which, under the influence of crossed electric and magnetic fields, bombards the surface of the VEC, "knocking out" the secondary electrons. As a result, an electron avalanche is formed in the interaction space of the magnetron, initiating the generation of the magnetron.

Example 1

Autoelectronic emission current of a magnetron with nonstop triggering with a cathode assembly consisting of alternating AECs and pressed palladium-barium emitters with a porosity of ~ 12% after activation at a temperature of T ~ 1270K for 90 min (T · t ~ 6.9 · 10 ⁶ K · Sec.), Amounted to ~ 30 mA. The magnetron was introduced into the generation mode without glow and trained to obtain stable parameters for τ ~ 4 hours. After testing for a service life of 5,000 hours, the parameters of the device remained virtually unchanged.

Example 2

Autoelectronic emission current of a magnetron with a non-triggering start with a cathode assembly consisting of alternating AEC, pressed palladium-barium washers with porosity of ~ 10% and secondary emission emitters (pressed palladium-barium bushings with porosity of ~ 5%) after activation at a temperature of T ~ 1200K for 150 min (T ~ t ~ 1.1 · 10 ⁷ K · sec.), amounted to ~ 40 mA. The magnetron was introduced into the generation mode without glow and trained to obtain stable parameters for τ ~ 2 hours. The service life of such a magnetron exceeded 5000 hours.

Note.

It should be noted that along with the stated materials, the following data were obtained in the course of the work:

a) on the end surface of the AEC, when pressed platinum-barium emitters are used as an EEC, whiskers of tantalum with platinum are formed, partially or completely coated with barium;

b) when powders, for example, tungsten, tantalum, rhenium, and other refractory metals are added to the composition of pressed palladium-barium emitters, the process of changing the surface structure occurs much more intensively and at lower temperatures. The micrograph (Fig. 7) shows a fragment of the surface of a pressed palladium-barium emitter with an additive of tungsten powder (~ 10 wt.%) After activation for 60 min at a temperature of T ~ 1170 K.

As can be seen from the comparison of microphotographs (Fig. 5; 7), the characteristic features of these formations are similar to each other, but structural changes occur at a lower temperature (the temperature difference is about 100 K.).

Information sources

1. Kopylov M.F., Bondarenko B.V., Makhov V.K., Nazarov V.A. Magnetron. RF patent No. 2007777, priority dated April 15, 1992

2. Pipko Yu.A., Semenov L.A., Galaktionova I.A., Eremeeva G.A., Esaulov N.P., Ilyin V.N., Margolis L.M. Magnetron with a non-cathode cathode. RF patent №2019877, priority 17.04. 1991 year

3. Li I.P., Dubois B.Ch., Kashirina N.V., Komissarchik S.V., Lifanov N.D., Zybin M.N. Magnetron with a non-cathode cathode. RF patent No. 2380784, priority of 10.24.2008

4. N.V. Cherepnin. Sorption phenomena in vacuum technology. M. 1973, Ch. 6.

Explanations for illustration.

1. Figure 1. Schematic representation of a magnetron with non-triggering starting (prototype):

1 - anode; 2 - core of the cathode; 3 - heater; 4 - screens; 5 - auto-electronic cathodes; 6 - secondary emission emitters.

2. Figure 2. (Table 1): data on the field emission current and duration of training in the dynamic mode of magnetrons with non-triggering starting with various types of VEC.

3. Figure 3. Micrographs of fragments of the end surface of the AEC with whiskers of palladium at different magnifications:

3-a: with increasing M ~ 1000 x (inside the image: VEC - secondary emission cathode; AEC - field-emission cathode);

3-b: with increasing M ~ 30,000 x;

3rd: with an increase in M ~ 50,000 x (inside the image: N.K. - whiskers).

4. Figure 4. The temperature dependence of the rate of evaporation of palladium from the surface of the pressed palladium-barium emitter;

5. Figure 5. Micrograph of a surface fragment of a pressed palladium-barium emitter after activation at a temperature of T ~ 1270 K for 60 minutes (M ~ 10000 x);

6. Figure 6. Schematic illustration of a patentable magnetron: 1 - anode; 2 - core of the cathode; 3 - heater; 4 - screens; 5 - auto-electronic cathodes; 6 - secondary emission emitters; 7 - washers activators AEC;

7. Figure 7. Micrograph of a surface fragment of a pressed palladium-barium emitter with an additive of tungsten powder in an amount of ~ 10% by weight. after activation at a temperature of T ~ 1170 K for 60 min (M ~ 12000 x).

Claims (3)

1. Magnetron with non-triggering starting, containing an anode and a cathode coaxially placed inside it, containing at least one field-effect cathode (AEC) and at least one secondary emission cathode (VEC), made in the form of a pressed porous sponge formed from a mixture of emission powder active palladium-barium alloy with a barium content of 11-25 wt.% and 75-89 wt.% palladium with an open porosity of 2-27%, characterized in that to increase the stability, reproducibility and stability of initiation of magnetron generation at the end face A controlled structure consisting of directional palladium whiskers with a length of ~ 10-100 nm and a cross-sectional size of ~ 5-20 nm partially or completely coated with barium is grown by special activation of the cathode during the evacuation of the magnetron with an electric field E ~ 1 · 10³-5 · 10^four V / cm in the temperature range T = 970-1370 ° K for a time t, and the optimal completeness of filling the surface of the AEC with whiskers is achieved when the relation T · t ~ 1.0 · 10⁶- 1,510⁷ ° K · s 2. The magnetron according to claim 1, characterized in that to increase the life of the magnetron between the AEC and the EEC is placed pressed palladium-barium washer with a porosity of 2-27% with a barium content of 11-25%, designed to activate the AEC, and as an EEC, Along with pressed palladium, a barium emitter with a porosity lower than that of the washer, a sleeve made of a material with stable secondary emission properties and resistant to ion and reverse electron bombardments, such as iridium, platinum, osmium, can be used. , Intermetallic compounds: platinum and barium; iridium with lanthanum; iridium with cerium; osmium with lanthanum or osmium with barium. 3. The magnetron according to claim 2, characterized in that in order to increase the efficiency of the formation of whiskers on the surface of the AEC, platinum, tungsten, tantalum or rhenium powder in an amount of 1-50 wt.% Is added to the composition of the activator washer.

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