RU2627707C1 - Method of producing pressed metal-alloy palladium-barium cathode - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: intermetallide Pd5Ba is produced by melting, is ground in an atmosphere of inert gas or CO₂ to obtain a powder, the resulting powder is mixed with palladium powder and mechanically activated by the planetary or vibrator mill for 5-15 minutes. The powder obtained after mechanoactivation is compressed, and the compact is sintered in an argon atmosphere in a beam of fast electrons at a temperature of (700-800)°C for 25-40 minutes.

EFFECT: increasing the coefficient of secondary electron emission of the pressed metal-alloy cathodes Pd-Ba.

2 tbl, 2 ex

Description

The invention relates to electronic equipment and can be used for the manufacture of effective thermo- and secondary-emission metal alloy cathodes for high-power microwave electronics (traveling wave tubes, magnetrons, etc.).

Among metal alloy cathodes, the most interesting are cathodes based on an alloy of palladium with barium, since they have a unique property - resistance to ion and electron bombardment.

There is a method for producing cathode materials based on platinum group metal and barium by argon-arc melting with a non-consumable tungsten electrode (see: NP Esaulov. Methods of electric melting in the development of special alloys for radio electronics. Electrometallurgy. 2011, No. 4. - P. 30 -33). The specified method is as follows. To conduct the melting process, an arc vacuum furnace (DVP) is used. The furnace includes a fore-vacuum pump VN-2, a vacuum unit VA-0.5; vacuum chamber, water-cooled copper crystallizer, non-consumable tungsten electrode, grade A argon cylinder. Before melting, Pd or Pt powder is compacted (pressed), and paraffin and oil are removed from the barium surface, as well as an oxide layer. The purified barium is placed directly on the bottom of the well of a copper water-cooled crystallizer, and metal is loaded in a compact form on top of it. The working chamber is pumped out, argon is introduced. They supply power to the electrode. The metal melts to form an alloy.

The resulting alloy is two-phase and represents a matrix of a refractory metal (Pd, Pt) with inclusions of an intermetallic compound (Pd ₅ Ba; Pt ₅ Ba).

The main disadvantage of this method is the highly uneven distribution of the intermetallic phase in the metal matrix of the platinum group. This fact leads to a decrease in the secondary electron emission coefficient (CEEE) of the alloy, and a decrease in the efficiency electrovacuum devices (EEC) based on it and reducing the percentage of yield.

Closest to the proposed method (prototype) is a method for producing cathode materials based on platinum group metal and barium, which allows to achieve a more uniform distribution of intermetallic in (see: NP Esaulov. Methods of electric melting in the development of special alloys for radio electronics. Electrometallurgy. 2011, No. 4. - S. 30-33). The specified method is as follows. The argon-arc melting method described above melts the intermetallic compound of palladium and barium - Pd ₅ Ba. This alloy is milled in an atmosphere of inert gases or carbon dioxide, mixed with such an amount of a noble metal, which is necessary to obtain an alloy with a given composition, pressed and sintered.

The disadvantage of this method is the lack of homogeneity of the distribution of intermetallic in the matrix of a noble metal, the high energy intensity of the sintering process.

The technical result of the present invention is the increase of the EEE of the cathode, efficiency and the percentage of yield of devices using Pd-Ba cathodes, lowering the energy intensity of the sintering process.

The specified technical result is achieved by the fact that before the operation of compacting (pressing) a powder mixture consisting of palladium powder and intermetallic powder, the mixture is mechanically activated for 5-15 minutes. After that, the resulting mixture is pressed, after which sintering in an argon atmosphere is carried out in a beam of fast electrons at a temperature of (5-10)% less than the temperature of the eutectic of the intermetallic compound.

The invention consists in the following. The mechanical activation of a mixture of powders of a platinum group metal and an intermetallic compound leads to a decrease in the average particle size by more than 2 times after 5 min of activation and 10 times after 15 min of activation, a several-fold decrease in the mass-average size and a several-fold increase in the specific surface of the powder particles. In addition, the process of mechanical activation leads to an increase in the defectiveness and energy saturation of activated materials due to plastic deformation and crushing.

Joint mechanical activation of the main components of the cathode material (matrix powder and intermetallic powder) already before the pressing stage allows to obtain a uniform distribution of intermetallic in the matrix and increase the activity of interacting powders during sintering.

The significantly smaller particle sizes of the platinum group metal powder and the intermetallic compound after the mechanical activation process, the high specific surface area of the particles, as well as its increased activity due to the release of stored energy in the technological process, allow for a more uniform distribution of the Pd ₅ Ba intermetallic compound in the Pd matrix and due to this increase the cEEE of the cathode, efficiency and the percentage of suitable devices based on this cathode.

The replacement of conventional thermal sintering by radiation thermal (RTS) in a fast electron beam is due to the substantially lower energy intensity of the latter and higher sintering quality (see: 1).

In RTS, in addition to the temperature factor, such an important factor as radiation-stimulated diffusion also acts. Due to this, sintering occurs both at lower temperatures and in a shorter time.

Example 1. By the method of argon arc melting, a stoichiometric alloy Pd ₅ Ba was obtained (20.51% by weight of Ba content). The resulting alloy was crushed in a cast-iron mortar to achieve an average particle size of 500-600 microns.

After this procedure, 6.5 g of the obtained Pd ₅ Ba alloy powder was mixed on a mixer with 29.63 g of Pd powder of the PPd1 grade PdAP-1 STO 00195200 - 040-2008 (to obtain a Pd-Ba compound containing 4.5% wt. . content of Ba).

The resulting mixture of powders was pressed into a staff under a pressure of 5.0 t / cm ² . Next, pressing the mixture (Pd-Pd ₅ Ba) was placed in the oven. The pump chamber of the arc melting unit was pumped out to a pressure of 10 ^-2 Pa. Then argon was injected to a pressure of 1.5 atm. Next, the stove was heated to a temperature of 800 ° C and the pressing was sintered at this temperature for 2.5 hours.

Samples with a thickness of h = 500-1000 μm were cut from the resulting Pd-Ba material. Five cathodes for magnetrons were made from different parts of the obtained sample. So the samples were prepared in accordance with the prototype. For each cathode, the HEEE was measured. The results were averaged and thus obtained the value of the EEE of the sample prepared in accordance with the prototype.

Next, samples were prepared in accordance with the proposed method. A stoichiometric Pd ₅ Ba alloy (20.51% wt. Ba content) was obtained by argon arc melting. The resulting alloy was crushed in a cast-iron mortar to achieve an average particle size of 500-600 microns.

After this procedure, 6.5 g of the obtained Pd ₅ Ba alloy powder was mixed on a mixer with 29.63 g of Pd powder of the PPd1 grade PdAP-1 STO 00195200 - 040-2008 (to obtain a Pd-Ba compound containing 4.5% wt. . content of Ba). The resulting mixture of powders was loaded into an AGO-2C planetary mill and mechanically activated for 8 minutes. The mechanical activation process was carried out using steel balls and a steel drum.

After mechanical activation, the resulting mixture was pressed into a staff under a pressure of 5.0 t / cm ² . The resulting powder compact (Pd-Pd ₅ Ba) was placed in a specially designed cell for PTC, connected to the output window of the electron accelerator. Air was pumped out of the cell to a pressure of 10 ^-2 Pa, after which argon was injected to a pressure of 1.5 atm. Then, the ILU-6 electron accelerator was switched on (beam energy E = 2.5 MeV) and the sample temperature was monitored. The temperature of the samples during the radiation treatment was controlled by a Pt-Pt / Rh thermocouple. In order to eliminate interference from the electron beam in the thermocouple, a third platinum electrode was used, one end of which was welded to the working junction, and the opposite one was grounded. Within 20 minutes of operation of the accelerator, a temperature of 800 ° C was reached. This sample temperature was maintained due to the operation of the accelerator for 30 minutes. After this, the accelerator was turned off, and the sample was naturally cooled to room temperature.

Samples with a thickness of h = 500-1000 μm were cut out from the resulting sintered Pd-Ba material. Eight cathodes for magnetrons were made from different parts of the obtained sample. For each cathode, the HEEE was measured. The results were compared with the test results of the cathodes and magnetrons obtained by the technology of the prototype. The results are presented in table. one.

Example 2. By the method of argon arc melting, a stoichiometric alloy Pd ₅ Ba was obtained (20.51% wt. Of the content of Ba). The resulting alloy was crushed in a cast-iron mortar to achieve an average particle size of 500-600 microns.

After this procedure, 3.0 g of the obtained Pd ₅ Ba alloy powder was mixed with a mixer with 29.37 g of Pd powder of the PPd1 grade PdAP-1 STO 00195200 - 040-2008 (to obtain a Pd-Ba compound with a content of 1.9% wt. . content of Ba). The resulting mixture of powders was loaded into an AGO-2C planetary mill and mechanical activation was performed for 12 minutes. The mechanical activation process was carried out using steel balls and a steel drum.

After mechanical activation, the resulting mixture was pressed into a staff under a pressure of 5.0 t / cm ² . The resulting powder compact (Pd-Pd ₅ Ba) was placed in a specially designed cell for PTC, connected to the output window of the electron accelerator. Air was pumped out of the cell to a pressure of 10 ^-2 Pa, after which argon was injected to a pressure of 1.5 atm. Then, the ILU-6 electron accelerator was switched on (beam energy E = 2.5 MeV) and the sample temperature was monitored. The temperature of the samples during the radiation treatment was controlled by a Pt-Pt / Rh thermocouple. In order to eliminate interference from the electron beam in the thermocouple, a third platinum electrode was used, one end of which was welded to the working junction, and the opposite one was grounded. Within 25 minutes of operation of the accelerator, a temperature of 850 ° C was reached. This sample temperature was maintained due to the operation of the accelerator for 37 minutes. After 37 minutes, the accelerator turned off and the sample naturally cooled to room temperature.

Samples with a thickness of h = 500-1000 μm were cut out from the resulting sintered Pd-Ba material. Eight cathodes for magnetrons were made from different parts of the obtained sample. For each cathode, HECE was measured. The results were compared with the test results of the cathodes and magnetrons obtained by the technology of the prototype. The results are presented in table. 2.

As can be seen from the table. 1 and table 2, the operation of mechanical activation of a mixture of powders of palladium and intermetallic based on it and barium significantly affects the characteristics of the cathodes based on them. HEEE of pressed metal alloy cathodes obtained using the proposed method is 15-17% higher than HEEE of cathodes obtained by the technology of the prototype.

Limitations on the amount of time for mechanical activation are selected based on the following considerations. When mechanically activating the powder of the initial sample for less than 5 min, the effect of omechanical activation does not give a noticeable result. At the same time, when the mechanical activation time is more than 15 minutes, no further qualitative increase in the properties of the cathode alloy is observed, but the appearance of grinding material (drum material and grinding media) in the cathode begins to affect.

RTS time limits are selected from the following considerations. When the duration of the RTS pressing is less than 25 minutes, the effect of increasing the QEE of the obtained alloy is insignificant. The duration of the RTS for more than 40 minutes leads to a drop in HEEE.

Limitations on the temperature of radiation-thermal sintering are selected from the following considerations. When the temperature of the PTC is less than 700 ° C, the resulting effect is not sufficient. At the same time, conducting RTS at temperatures above 800 ° C leads to a decrease in the effect due to the evaporation of part of barium from the compact.

Sources

one). Surzhikov A.P., Pritulov A.M. Radiation-thermal sintering of ferrite ceramics. - Moscow: Energoatomizdat, 1998 .-- 217 p.

2). Kostishin V.G., Korovushkin V.V., Panina L.V. et al. Magnetic structure and properties of MnZn ferrites obtained by radiation thermal sintering. Inorganic materials. 2014, t. 50, No. 12. - S. 1352-1356.

3). Kostishin V.G., Andreev V.G., Korovushkin V.V. and others. Obtaining radiation-thermal ceramics of the brand 2000НН by the method of radiation-thermal sintering according to the complete and short technological schemes. Inorganic materials. 2014, t. 50, No. 12. - S. 1387-1392.

four). Kiselev B.G., Kostishin V.G., Komlev A.S. Substantiation of economic advantages of radiation thermal sintering technology of ferrite ceramics. Non-ferrous metals. 2015, No4. - S. 7-11.

Claims (1)

A method for producing a pressed metal-alloy palladium-barium cathode, including the production of a Pd ₅ Ba intermetallic smelting method, its grinding in an inert gas or CO ₂ atmosphere, mixing with a given amount of palladium powder to obtain Pd-Ba material with a given barium concentration, pressing and sintering in the atmosphere inert gas, characterized in that before the pressing and sintering operation, the resulting powder mixture is mechanically activated for 5-15 minutes, and sintering is carried out in a beam of fast electrons for 25-40 minutes ut at a temperature of 700-800 ° C.