A method for preparing a helical hexagonal attenuator

Abstract

The present invention belongs to the technical field of preparation of space traveling wave tubes, and provides a method for preparing a helical hexagonal attenuator: graphite powder and boron nitride powder are mixed and pressed in different proportions to obtain raw material blocks, and the raw material blocks are sequentially placed in a mold and pressed in the order of increasing or decreasing boron nitride proportions, and then oxygen-isolated calcined, cut, and impedance adjusted to obtain a helical hexagonal attenuator. The processing method provided by the present invention can adjust the resistance value of the attenuator by controlling the graphite proportion in the raw material, does not require complex equipment for coating, and has a simple method, high production efficiency, and low scrap rate.

Description

Technical Field

[0001] The invention belongs to the technical field of space traveling wave tube preparation, and in particular relates to a method for preparing a helical hexagonal attenuator. Background Art

[0002] During the signal transmission process of the traveling wave tube, since the impedance matching of the transmission line cannot reach the ideal value, there are many reflected waves in the tube. The reflected waves are easy to produce self-excited oscillation with the forward signal, thereby affecting the power of the traveling wave tube and increasing the instability of the gain. Helical traveling wave tubes often use attenuators to absorb these reflected waves, suppress self-excited oscillations, and ensure the stability of the traveling wave tube. In the helical traveling wave tube, in order to absorb the reflected waves, the attenuator is required to have not only good mechanical strength and thermal conductivity, but also different impedance values ​​are required to be designed according to the different frequencies of the transmitted signals. At present, graphite films can be deposited on ceramic substrates by pyrolysis of carbon-based compounds. The thickness of the deposited graphite is changed by the deposition time at different positions. However, the graphite thickness gradient effect of the attenuator prepared by this method is not obvious, the processing difficulty is large, and the processing accuracy is not easy to control. Therefore, it is necessary to provide a new processing method for attenuators for improvement. Summary of the invention

[0003] In view of the problems in the preparation of attenuators in the prior art, the present invention provides a method for processing a helical hexagonal attenuator, which has a significant gradual change in the thickness of a graphite film, is easy to process and has high precision.

[0004] To achieve the above purpose, the present invention adopts the following technical solution.

[0005] A method for preparing a helical hexagonal attenuator comprises the following steps:

[0006] (1) Graphite powder and boron nitride powder are mixed in different proportions, sintering aids are added, the mixture is ball-milled, and the mixture is put into a mold and pressed to obtain a raw material block;

[0007] (2) placing the raw material blocks into the mold in the order of increasing or decreasing boron nitride proportions and press-forming to obtain a blank;

[0008] (3) calcining the blank in an oxygen-free environment and cutting it to obtain a semi-finished product;

[0009] (4) The semi-finished product is tested and / or impedance adjusted to obtain a helical hexagonal attenuator.

[0010] The microscopic morphology of the graphite powder is phosphorus flakes, and the average particle size is 500nm; the average particle size of the boron nitride is 1μm.

[0011] The added amount of the graphite powder is (0.2-1) wt % of the total mass of the graphite powder and the boron nitride powder.

[0012] The sintering aid is boron oxide, and the addition amount is 0.2wt%.

[0013] Preferably, the ball milling is performed by wet grinding with anhydrous ethanol as the dispersion medium; the mass ratio of anhydrous ethanol to raw material is (1-1.5):1; and the ball milling time is (10-15) hours. Furthermore, the ball milling further includes drying and crushing steps.

[0014] Preferably, the pressure applied in step (1) is (5-10) MPa; and the pressure applied in step (2) is (150-350) MPa.

[0015] In step (3), the calcination is hot pressing sintering. Preferably, the calcination temperature is (1500-2000)°C; the heating rate is (50-150)°C / min; and the calcination pressure is (30-50)MPa.

[0016] Preferably, the impedance is adjusted by low-temperature oxidation decarbonization.

[0017] Specifically, the semi-finished product with too low resistance is heated by a DC coil, and when the temperature reaches 500°C, a mixed gas of appropriate amount of oxygen and nitrogen is introduced to perform low temperature oxidation and decarbonization.

[0018] The present invention has the following advantages:

[0019] The present invention provides a method for processing a helical hexagonal attenuator. A carbon film boron nitride attenuator with a gradual resistance is prepared by mixing graphite and boron nitride powder at high temperature and high pressure and based on the principle of carbon segregation. The resistance of the attenuator can be adjusted by controlling the proportion of graphite in the raw materials. Complex equipment for coating is not required. The method is simple, the production efficiency is high, and the scrap rate is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the helical hexagonal attenuator of Example 1 and the resistance variation range;

[0021] Figure 2 This is a picture of the finished product obtained in Example 1;

[0022] Figure 3 Schematic diagram of the helical hexagonal attenuator of Example 1 and the resistance variation range. DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with examples, but the present invention is not limited by the following examples.

[0024] Example 1 Preparation of helical hexagonal attenuator

[0025] (1) 500 nm flaky graphite powder and 1 μm boron nitride powder were mixed in a mass ratio of 1:99 to obtain powder A, and 2% of the mass of powder A was added with boron oxide as a sintering aid to obtain powder B.

[0026] Then, the powder B was added into a ball mill, an equal amount of anhydrous ethanol was added, and grinding balls of twice the weight of the powder were added for 10 hours of wet ball milling to obtain a slurry; the slurry was dried at 100°C to constant weight, crushed and ground, and passed through a 300-mesh sieve to obtain a powder C.

[0027] According to the above method, powders with a graphite:boron nitride mass ratio of 0.9:99.1, 0.8:99.2, 0.7:99.3, 0.6:99.4, and 0.5:99.5 were obtained;

[0028] Then, the graphite powder is injected into the mold in descending order and pressed at 10 MPa for 3 minutes to form a 1 mm thick sheet-shaped raw material block, and the pure boron nitride powder is pressed into a 50 mm thick block-shaped material block;

[0029] (2) stacking the raw material blocks into a mold in descending order of the graphite powder ratio, and cold isostatically pressing them at 200 MPa for 5 min to obtain a blank;

[0030] (3) Place the blank into the furnace and evacuate it to a vacuum degree higher than 1×10 -1 Pa, nitrogen was filled in to a pressure of 30 MPa, and then the temperature was raised to 1560°C at 100°C / min for high temperature sintering for 1 hour, and then the heating program was turned off and the pressure was released. After cooling, the attenuator semi-finished products with a length of 50 mm, a width of 10 mm, and a thickness of 0.6 mm were cut;

[0031] (4) In accordance with Figure 1 The resistance value change shown is as follows: the resistance value change range of 0-2mm is (500±50)Ω, 2mm-4mm is (700±50)Ω, and 4mm-6mm is (1200±50)Ω. Check whether the impedance of the semi-finished product is qualified; the unqualified semi-finished product with too low resistance is heated with a DC coil, and when the temperature reaches 500℃, a proper amount of mixed gas of oxygen and nitrogen is introduced for low-temperature oxidation and carbon removal to obtain the following Figure 2 The finished product of the spiral hexagonal attenuator shown in the figure has a gradual distribution of carbon in the horizontal direction; after ten batches of trial production, the scrap rate was 8.32%.

[0032] Comparative Example 1 Attenuator prepared by conventional method

[0033] (1) Add 2% boron oxide as a sintering aid to boron nitride powder with a particle size of 1 μm, add the mixture into a ball mill, add an equal amount of anhydrous ethanol, and then add grinding balls with a mass twice that of the powder for wet ball milling for 10 hours to obtain a slurry; bake the slurry at 100°C to constant weight, crush and grind it, pass it through a 300-mesh sieve to obtain a powder, inject it into a mold, and cold isostatically press it at 200 MPa for 5 minutes to form a 50 mm thick block;

[0034] (2) Place the material block in the furnace and evacuate it to a vacuum degree higher than 1×10 -1 Pa, nitrogen was filled in to a pressure of 30 MPa, and then the temperature was raised to 1560 °C at 100 °C / min for high temperature sintering for 1 h, and then the heating program was turned off and the pressure was released. After cooling, it was cut into clamping rods of 50 mm in length, 10 mm in width, and 6 mm in thickness;

[0035] (3) Deposition of carbon film by vacuum pyrolysis: In a vacuum evaporation chamber, a vacuum pump is used to evacuate it to a low vacuum of less than 1Pa, and 500Pa of n-heptane gas is filled in; then a direct current is passed through the heating coil to generate Joule heat, and at 900°C, the n-heptane gas undergoes thermal decomposition to generate carbon atoms, which are deposited on a nearby substrate to form a carbon film; after ten batches of trial production, the scrap rate was 49.8%. It can be seen that due to the difficulty in controlling the method of depositing the carbon film, the scrap rate is much higher than that in Example 1.

[0036] Example 2 Preparation of helical hexagonal attenuator

[0037] (1) 500 nm flaky graphite powder and 1 μm boron nitride powder were mixed in a mass ratio of 1:99 to obtain powder A, and 1% by mass of boron oxide was added as a sintering aid to obtain powder B.

[0038] Then, the powder B was added into a ball mill, an equal amount of anhydrous ethanol was added, and grinding balls of twice the weight of the powder were added for 10 hours of wet ball milling to obtain a slurry; the slurry was dried at 100°C to constant weight, crushed and ground, and passed through a 300-mesh sieve to obtain a powder C.

[0039] According to the above method, powders with a graphite:boron nitride mass ratio of 0.8:99.2, 0.6:99.4, 0.4:99.6, 0.2:99.8, and 0.1:99.9 were obtained;

[0040] Then, the graphite powder is injected into the mold in descending order and pressed at 15 MPa for 3 minutes to form a 2 mm thick sheet-shaped raw material block, and the pure boron nitride powder is pressed into a 40 mm thick block;

[0041] (2) stacking the raw material blocks into a mold in descending order of the graphite powder ratio, and cold isostatically pressing them at 200 MPa for 5 min to obtain a blank;

[0042] (3) Place the blank into the furnace and evacuate it to a vacuum degree higher than 1×10 -1 Pa, nitrogen was filled in to a pressure of 50 MPa, and then the temperature was raised to 1850 ° C at 100 ° C / min for high temperature sintering for 1 hour, and then the heating program was turned off and the pressure was released. After cooling, the attenuator semi-finished products with a length of 40 mm, a width of 10 mm, and a thickness of 0.6 mm were cut;

[0043] (4) In accordance with Figure 3 The resistance value changes shown are as follows: the resistance range of 0-2mm is (550±50)Ω, 2mm-4mm is (750±50)Ω, and 4mm-6mm is (1800±50)Ω. The impedance of the semi-finished product is tested to see if it is qualified. The unqualified semi-finished product with too low resistance is heated with a DC coil, and when the temperature reaches 500℃, a proper amount of mixed gas of oxygen and nitrogen is introduced, and low-temperature oxidation and carbon removal are performed to obtain the finished product of the helical hexagonal attenuator. After ten batches of trial production, the scrap rate is 9.01%.

Claims

1. A method for preparing a helical hexagonal attenuator, characterized in that: The following steps are involved: (1) Graphite powder and boron nitride powder are mixed in different proportions, sintering aids are added, the mixture is ball-milled, and the mixture is put into a mold and pressed to obtain a raw material block; (2) placing the raw material blocks into the mold in the order of increasing or decreasing boron nitride proportions and press-forming to obtain a blank; (3) calcining the blank in an oxygen-free environment and cutting it to obtain a semi-finished product; (4) Testing and adjusting the impedance of the semi-finished product to obtain a helical hexagonal attenuator; The method for adjusting the impedance is low-temperature oxidation decarbonization. The low-temperature oxidation decarbonization step is to heat the semi-finished product with too low resistance with a DC coil, and when the temperature reaches 500°C, a mixed gas of appropriate amount of oxygen and nitrogen is introduced.

2. The preparation method according to claim 1, characterized in that: The microscopic morphology of the graphite powder is phosphorus flakes, with an average particle size of 500nm; the average particle size of the boron nitride is 1μm; The added amount of the graphite powder is (0.2-1) wt% of the total mass of the graphite powder and the boron nitride powder; the added amount of the sintering aid is 0.2 wt%.

3. The preparation method according to claim 2, characterized in that: The sintering aid is boron oxide.

4. The preparation method according to claim 1, characterized in that: The ball mill uses anhydrous ethanol as a dispersion medium; the mass ratio of anhydrous ethanol to raw materials is (1-1.5):1; and the ball milling time is (10-15) hours.

5. The preparation method according to claim 1, characterized in that: After ball milling, drying and crushing steps are also included.

6. The preparation method according to claim 1, characterized in that: The pressure applied in step (1) is (5-10) MPa; the pressure applied in step (2) is (150-350) MPa.

7. The preparation method according to claim 1, characterized in that: In step (3), the calcination is hot pressing sintering; the calcination temperature is (1500-2000)°C; the heating rate is (50-150)°C / min; and the calcination pressure is (30-50)MPa.

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