Method for preparing tungsten carbide coating on surface of tungsten electrode and tungsten electrode

Abstract

The invention discloses a method for preparing a tungsten carbide coating on the surface of a tungsten electrode and the tungsten electrode, and relates to the technical field of tungsten electrodes. According to the method, the carbonization layer is prepared in the three stages of a carbon attachment stage, a carbon fixation stage and a decarburization stage, by controlling operation parameters (such as voltage) of the three stages and the like, surplus carbon on the surface can be further reacted through the carbon fixation step, the thickness of the carbonization layer can be increased, it can be better guaranteed that the final reaction product is tungsten carbide, and the yield of the product is improved. And a carbonized layer with uniform and stable structure and phase composition is prepared on the surface of the tungsten electrode. The prepared carbonized layer is a loose lamellar structure, the phase structure is pure tungsten carbide, an emission active substance migration channel is provided for the surface of the tungsten-based hot cathode material, diffusion and supplement of active substances of a cathode material working chamber are promoted, the emission performance of the tungsten-based cathode material is improved, and meanwhile long-term stable work of the tungsten-based cathode material is better guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of tungsten electrodes, and in particular to a method for preparing a ditungsten carbide coating on the surface of a tungsten electrode and a tungsten electrode. Background Art

[0002] Doping a certain amount of rare earth oxides or carbides into pure tungsten can effectively reduce the electron work function of the matrix, greatly improving its electron emission performance. At present, the cathode material of magnetrons is mainly thoriated tungsten, and there are many studies on other rare earth tungsten-based materials to replace thoriated tungsten cathodes. In addition, preparing a carbide layer on the surface of the tungsten electrode can effectively improve the emission performance of the cathode material. However, since the melting points of other rare earth materials are mostly lower than that of thorium, there are often problems of volatilization and diffusion imbalance of emission active substances at high temperatures. Low-melting-point rare earth cathode materials often have rapid evaporation and consumption of surface emission active substances, and the internal active substances cannot migrate outward in time to replenish, resulting in a decrease in the emission performance of the magnetron, inability to start oscillation, or even failure. Therefore, low-melting-point rare earth cathode materials have further improved requirements for the surface carbide layer.

[0003] However, the uniformity of the structure and composition of the carbide layer formed by the existing carbide technology cannot be guaranteed. The carbide layer is mainly composed of dense tungsten carbide or a mixture of tungsten carbide and ditungsten carbide, which is not conducive to the migration of the emission active material, thereby affecting the normal use of the cathode material.

[0004] Therefore, there is an urgent need to improve the preparation process of the carbide layer in order to obtain a carbide layer with relatively uniform structure and composition, which is beneficial to improving the emission performance of tungsten-based cathode materials.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing a ditungsten carbide coating on the surface of a tungsten electrode and a tungsten electrode, aiming to form a carbide layer with a phase structure of pure ditungsten carbide and improve the emission performance of a tungsten-based cathode material.

[0007] The present invention is achieved in that:

[0008] In a first aspect, the present invention provides a method for preparing a ditungsten carbide coating on the surface of a tungsten electrode, comprising: sequentially performing a carbon attachment stage, a carbon fixation stage and a decarburization stage;

[0009] In the carbon attachment stage, the tungsten electrode is placed in an atmosphere of carbon-containing hydrocarbon gas and electrically heated to form a carbon layer.

[0010] In the carbon fixation stage, the carbon-containing hydrocarbon gas is discharged and then electrically heated, with the control voltage being 6V to 8V;

[0011] In the decarburization stage, the voltage is increased to 8V to 12V for electric heating;

[0012] The voltage applied in the decarburization stage is controlled to be greater than the voltage applied in the carbon fixation stage.

[0013] In an optional embodiment, during the carbon fixation stage, the carbon-containing hydrocarbon gas is discharged by vacuuming, introducing hydrogen or introducing an inert gas.

[0014] In an optional embodiment, during the carbon fixation stage, the power-on time is controlled to be 50s to 100s.

[0015] In an optional embodiment, during the decarburization stage, the power-on time is controlled to be 20s to 120s.

[0016] In an optional embodiment, the carbon attachment stage includes: introducing a carbon-containing hydrocarbon gas into the carbonization chamber where the tungsten electrode is located, controlling the chamber pressure to -0.1MPa to 1.0MPa, electrically heating the tungsten electrode, controlling the voltage to 5V to 8V, and the power-on time to 50s to 150s.

[0017] In an optional embodiment, the introduced carbon-containing hydrocarbon gas contains at least one of benzene and methane;

[0018] Preferably, the carbon-containing hydrocarbon gas is methane or a methane mixture, the methane mixture contains hydrogen or an inert gas, and the volume proportion of methane in the methane mixture is 8% to 11%;

[0019] When the carbon-containing hydrocarbon gas is methane, the chamber pressure is controlled to be -0.1 to -0.05 MPa.

[0020] In an optional embodiment, before the carbon attachment stage, the tungsten electrode is subjected to surface pretreatment to remove surface dirt and oxide layer;

[0021] Preferably, the surface pretreatment process includes: clamping a tungsten electrode between a positive electrode and a negative electrode of a carbonization chamber, heating the electrode under vacuum, an inert atmosphere or a hydrogen atmosphere, controlling the DC voltage to be 6V to 10V, and the power-on time to be 10s to 60s;

[0022] When the surface pretreatment process adopts vacuum conditions, the control pressure is 1×10 -3 ~1×10 -5 Pa.

[0023] In an optional embodiment, the diameter of the tungsten electrode is 0.1 mm to 2.0 mm, and the length is 5 cm to 15 cm;

[0024] Preferably, rare earth oxide is added to the tungsten electrode, and the rare earth element is selected from at least one of thorium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium;

[0025] Preferably, the added amount of rare earth oxide is 1 wt% to 2 wt%.

[0026] In an optional embodiment, a carbonization device is used to carry out the reactions at each stage, and the carbonization device comprises:

[0027] A carbonization chamber, in which a tungsten electrode is placed during the preparation process;

[0028] An electric control module, used for applying voltage to the tungsten electrode in the carbonization chamber and controlling the power-on time;

[0029] An air intake module, used for introducing gas into the carbonization chamber;

[0030] A vacuum module, used for evacuating the carbonization chamber;

[0031] An exhaust module, used to exhaust the gas in the carbonization chamber;

[0032] Preferably, the exhaust module further comprises an ignition mechanism for igniting the gas.

[0033] In a second aspect, the present invention provides a tungsten electrode, wherein a tungsten carbide coating is provided on the surface of the tungsten electrode, wherein the tungsten carbide layer is a loose lamellar structure, and the loose lamellar structure contains cross-distributed transverse grain boundaries and vertical grain boundaries;

[0034] Preferably, the thickness of the tungsten carbide layer is 40 to 50 μm;

[0035] Preferably, the ditungsten carbide coating is prepared by any of the methods described in the foregoing embodiments.

[0036] The present invention has the following beneficial effects: the present invention is divided into three stages, namely, the carbon attachment stage, the carbon fixation stage and the decarburization stage, to prepare the carbide layer. By controlling the operating parameters (such as voltage) of the three stages, the carbon fixation step can further react the excess carbon on the surface, which can not only increase the thickness of the carbide layer, but also better ensure that the final reaction product is ditungsten carbide, so that the surface of the tungsten electrode is prepared with a carbide layer having a uniform and stable organizational structure and phase composition. The carbide layer prepared by the present invention is a loose lamellar organization, and the phase structure is pure ditungsten carbide, which provides a migration channel for the emission active substances on the surface of the tungsten-based hot cathode material, promotes the diffusion and supplementation of the active substances in the cathode material studio, improves the emission performance of the tungsten-based cathode material, and better ensures the long-term stable operation of the tungsten-based cathode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0038] Figure 1 A schematic diagram of a carbonization device provided in an embodiment of the present invention;

[0039] Figure 2 This is the effect diagram of the carbonization process;

[0040] Figure 3 Metallographic images of the carbonized layer prepared in the embodiment of the present invention; (a), (b) and (c) represent different magnifications;

[0041] Figure 4 for Figure 3 A magnified image of

[0042] Figure 5 The metallographic images of the carbonized layer prepared in the comparative example of the present invention; (a) and (b) represent different magnifications;

[0043] Figure 6 This is the XRD diagram of the carbonized layer prepared in the embodiment of the present invention;

[0044] Figure 7 This is an EPMA quantitative analysis diagram of the carbonized layer prepared in an embodiment of the present invention.

[0045] Icons: 100-Carbonization device; 110-Carbonization chamber; 120-Electronic control module; 121-Power supply; 130-Intake module; 140-Vacuum module; 141-Vacuum pump; 142-Vacuum gauge; 150-Exhaust module; 151-Ignition mechanism; 152-Water tank; 001-Tungsten electrode; DETAILED DESCRIPTION

[0046] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0047] The embodiment of the present invention provides a method for preparing a ditungsten carbide coating on the surface of a tungsten electrode, comprising: sequentially performing surface pretreatment, a carbon attachment stage, a carbon fixation stage and a decarburization stage, wherein the stages are performed at Figure 1 The carbonization is carried out in the carbonization device 100.

[0048] like Figure 1 As shown, the carbonization device 100 includes a carbonization chamber 110, an electric control module 120, an air intake module 130, a vacuum module 140 and an exhaust module 150. The carbonization device 100 can realize carbonization under different pressures.

[0049] The carbonization chamber 110 is the main structure of the carbonization device 100. During operation, the tungsten electrode 001 is clamped between the positive and negative electrodes of the carbonization chamber 110. The electric control module 120 is used to apply voltage to the tungsten electrode 001 and control the power-on time. The electric control module 120 includes a power supply 121 and other structures, which can stably control the applied voltage. During the carbonization process, the air intake module 130 needs to be used to introduce reaction gas, inert gas or hydrogen to meet the process requirements.

[0050] When vacuum operation is required, vacuum is performed using the vacuum module 140. The vacuum module 140 includes a vacuum pump 141 and a vacuum gauge 142. The vacuum pump 141 provides power for vacuuming, and the vacuum gauge 142 detects the pressure in the carbonization chamber 110. The vacuum pump 141 can be a general mechanical pump, a molecular pump, or the like.

[0051] When the gas in the carbonization chamber 110 needs to be exhausted, the exhaust module 150 is turned on. Since many reaction gases are combustible gases, such as hydrogen, methane, etc., a water tank 152 and an ignition mechanism 151 can be further provided. After the gas is transported to the water tank 152, it is passed into the ignition mechanism 151 through a gas pipeline to ignite the gas for further use.

[0052] The following are the four stages of surface pretreatment, carbon attachment, carbon fixation and decarburization:

[0053] S1. Surface pretreatment

[0054] Before the carbonization stage, the tungsten electrode is surface pretreated to remove surface dirt and oxide layer and improve the bonding strength between the carbide layer and the tungsten electrode.

[0055] In some embodiments, the tungsten electrode may be in the form of a wire or a rod, and the diameter of the tungsten electrode is 0.1 mm to 2.0 mm, such as 0.1 mm, 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, etc.; the length of the tungsten electrode is 5 cm to 15 cm, such as 5 cm, 8 cm, 10 cm, 12 cm, 15 cm, etc. In some embodiments, rare earth oxides are added to the tungsten electrode, and the rare earth element is selected from at least one of thorium (Th), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), scandium (Sc) and yttrium (Y), and the rare earth element may be any one or more of the above. The addition amount of rare earth oxide is 1 wt% to 2 wt%, such as 1.0 wt%, 1.3 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, etc.

[0056] In some embodiments, the surface pretreatment process includes: clamping the tungsten electrode 001 between the positive electrode and the negative electrode of the carbonization chamber 110, heating it with electricity in a vacuum, inert atmosphere or hydrogen atmosphere, controlling the DC voltage to 6V~10V, and the power-on time to 10s~60s. Under this voltage condition, the tungsten electrode can reach a surface temperature of about 1200-1500°C, so that the tungsten oxide on the surface is decomposed and removed at high temperature.

[0057] Specifically, during the surface pretreatment process, the controlled DC voltage can be 6V, 7V, 8V, 9V, 10V, etc., and the power-on time can be 10s, 20s, 30s, 40s, 50s, 60s, etc.

[0058] Furthermore, the surface pretreatment can be carried out under vacuum conditions or in an inert gas or hydrogen atmosphere. When the surface pretreatment process is carried out under vacuum conditions, the pressure is controlled to be 1×10 -3 ~1×10 -5 Pa, such as 1×10 -3 , 1×10 -4 , 1×10 -5 Pa et al.

[0059] S2, Carbon attachment stage

[0060] In the carbon attachment stage, the tungsten electrode is placed in an atmosphere of carbon-containing hydrocarbon gas and heated by electricity to form a black carbon layer. During the reaction, methane and other carbon-containing hydrocarbon gases are decomposed into carbon and hydrogen after being heated by the filament. The reaction formula is as follows:

[0061] CH4(g)=C(s)+H2(g);

[0062] In the carbon attachment stage, after the carbon atmosphere is electrically heated, a primary reaction occurs on the surface of the tungsten electrode (wire / rod), and a black carbide layer is attached to the surface, which is a mixture of tungsten carbide, ditungsten carbide and carbon. That is:

[0063] W(s)+C(s)=WC(s);

[0064] 2W(s)+C(s)=W2C(s);

[0065] When the primary reaction occurs, the surface of the tungsten electrode (wire / rod) is covered with dense black carbon particles. The carbonized layer is thin, with a thickness of 5-10μm and a dense structure.

[0066] In actual operation, the carbon attachment stage includes: introducing carbon-containing hydrocarbon gas into the carbonization chamber where the tungsten electrode is located, controlling the chamber pressure to -0.1MPa~1.0MPa, and heating the tungsten electrode (wire / rod) again with a controlled voltage of 5V~8V and a power-on time of 50s~150s, so that a black carbon layer is formed on the surface of the tungsten electrode (wire / rod).

[0067] Specifically, in the carbon deposition stage, the controlled chamber pressure can be -0.01MPa, -0.05MPa, -0.03MPa, -0.01MPa, 0.01MPa, 0.10MPa, 0.50MPa, 1.00MPa, etc.; the controlled voltage can be 5V, 6V, 7V, 8V, etc.; the controlled power-on time can be 50s, 80s, 100s, 120s, 150s, etc.

[0068] Furthermore, the carbon-containing hydrocarbon gas contains at least one of benzene and methane, and may contain benzene, methane, or benzene and methane.

[0069] In a preferred embodiment, the carbon-containing hydrocarbon gas is methane or a methane mixture, the methane mixture contains hydrogen or inert gas, and the volume proportion of methane in the methane mixture is 8% to 11%, such as 8%, 9%, 10%, 11%, etc.

[0070] The amount of methane gas introduced is controlled by adjusting the chamber pressure. When the methane mixed gas is introduced, the chamber pressure can reach 1.0MPa. When the carbon-containing hydrocarbon gas is methane, it is appropriate to control the chamber pressure to -0.1 to -0.05MPa to prevent the amount of methane gas introduced from being too large, affecting the reaction uniformity or making the carbon layer too thick.

[0071] S3, Carbon fixation stage

[0072] In the carbon fixation stage, after the carbon-containing hydrocarbon gas is discharged, the tungsten electrode (wire / rod) is powered on and heated again, with the voltage controlled to 6V to 8V and the power-on time to 50s to 100s, so that the carbon layer on the surface can react further.

[0073] Specifically, in the carbon fixation stage, the DC voltage can be 6V, 7V, 8V, etc., and the power-on time can be 50s, 60s, 70s, 80s, 90s, 100s, etc.

[0074] It should be noted that in the carbon fixation stage, after the carbon-containing hydrocarbon gas is subsequently evacuated, the tungsten electrode (wire / rod) is heated at a low temperature, so that the carbon black particles attached to the surface of the tungsten electrode further undergo a secondary reaction, further increasing the thickness of the carbonized layer. The reaction formula is as follows:

[0075] W2C(s)+C(s)=2WC(s);

[0076] When the secondary reaction occurs, the carbon particles on the surface of the tungsten electrode (wire / rod) are further reduced, the color turns gray visibly, the thickness of the carbonized layer increases further, and the surface layer remains a dense structure.

[0077] In some embodiments, during the carbon fixation stage, the carbon-containing hydrocarbon gas is discharged by vacuuming, introducing a large amount of hydrogen, or introducing a large amount of inert gas. The method of discharging the carbon-containing hydrocarbon gas can be any of the above methods.

[0078] S4, decarbonization stage

[0079] During the decarburization stage, the voltage is increased to 8V~12V for electric heating. The voltage applied in the decarburization stage is controlled to be greater than the voltage applied in the carbon fixation stage. The power-on time of the decarburization stage is controlled to be 20s~120s. After this stage, the surface of the tungsten electrode (wire / rod) exhibits metallic luster.

[0080] Specifically, the DC voltage passed during the decarbonization stage can be 8V, 9V, 10V, 11V, 12V, etc.; the power-on time can be 20s, 50s, 80s, 100s, 120s, etc.

[0081] In the decarburization stage, the tungsten electrode (wire / rod) is heated again at high temperature, and a tertiary reaction occurs on the surface of the tungsten electrode. Tungsten carbide continues to react into ditungsten carbide, namely:

[0082] WC(S)+W(S)=W2C(S);

[0083] When the tertiary reaction occurs, the surface of the tungsten electrode (wire / rod) presents a metallic luster, and the surface is a uniform single-layer carbonized layer with a thickness of 30 to 60 μm.

[0084] The present invention provides a tungsten electrode, which is prepared by the method provided in an embodiment of the present invention. By controlling the carbon content and voltage during the carbonization process, a carbonized layer with uniform and stable organizational structure and phase composition is prepared on the surface of the tungsten electrode, and the phase structure is pure tungsten carbide.

[0085] It should be noted that the tungsten carbide coating prepared by the method provided in the embodiment of the present invention is a loose lamellar structure, which contains cross-distributed transverse grain boundaries and vertical grain boundaries, has a relatively neat organization, and has a larger number of grain boundaries, providing more migration channels for rare earth active substances. The thickness of the tungsten carbide layer is 40 to 50 μm, such as 40 μm, 43 μm, 45 μm, 48 μm, 50 μm, etc.

[0086] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0087] Example 1

[0088] This embodiment provides a method for preparing a tungsten carbide coating on the surface of a tungsten electrode. The tungsten electrode (wire / rod) used has a diameter of 0.5 mm and a length of 12 cm and is doped with lanthanum oxide in an amount of 2 wt%. The steps are as follows:

[0089] (1) Clamp the tungsten electrode (wire / rod) between the positive and negative electrodes of the carbonization chamber, and evacuate the carbonization chamber to 3×10 - 4 Pa, the tungsten electrode (wire / rod) is electrically heated through the electric control module, the DC voltage is 8V, the power-on time is 40s, and the oxide layer on the surface of the tungsten wire is removed.

[0090] (2) Pure methane (purity greater than 99%) is injected into the carbonization chamber, and the methane pressure in the carbonization chamber is controlled to be -0.08 MPa. The tungsten electrode (wire / rod) is powered on again for heating at a voltage of 6 V for 100 s, and a black carbon layer is formed on the surface of the tungsten electrode (wire / rod).

[0091] (3) A large amount of argon gas is introduced into the carbonization chamber to discharge the methane in the chamber, and the tungsten electrode (wire / rod) is powered on and heated again at a voltage of 7 V for 80 seconds to allow the carbon layer on the surface to react further.

[0092] (4) The voltage is increased to 10 V and the power-on time is 80 s. The surface of the tungsten electrode (wire / rod) shows metallic luster.

[0093] Example 2

[0094] This embodiment provides a method for preparing a tungsten carbide coating on the surface of a tungsten electrode. The tungsten electrode (wire / rod) used has a diameter of 0.5 mm and a length of 12 cm and is doped with lanthanum oxide in an amount of 2 wt%. The steps are as follows:

[0095] (1) The tungsten electrode (wire / rod) is clamped between the positive and negative electrodes of the carbonization chamber, a large amount of argon gas is introduced into the carbonization chamber, and the tungsten electrode (wire / rod) is electrically heated by the electric control module at a voltage of 8 V for 60 seconds to remove the oxide layer on the surface of the tungsten wire.

[0096] (2) Adjust the ratio of methane and argon to 1:10 and inject them into the cavity at the same time. Keep the gas under normal pressure for 1 minute to ensure that the two gases in the cavity are evenly distributed. Then close the gas outlet and continue to inject gas. The gas pressure in the cavity is 0.05MPa. The tungsten electrode (wire / rod) is heated again with a voltage of 6V and a power-on time of 120s. A black carbon layer is formed on the surface of the tungsten electrode (wire / rod).

[0097] (3) Turn off the methane gas, continue to introduce a large amount of argon gas into the carbonization chamber to discharge the methane in the chamber, and then energize and heat the tungsten electrode (wire / rod) again at a voltage of 7 V for 60 seconds to allow the carbon layer on the surface to react further.

[0098] (4) The voltage is increased to 9 V and the power-on time is 100 s. The surface of the tungsten electrode (wire / rod) shows a metallic luster.

[0099] Example 3

[0100] The only difference from Example 1 is that in step (2), the methane pressure in the carbonization chamber is -0.05 MPa.

[0101] Example 4

[0102] The only difference from Example 1 is that the voltage in step (2) is 8V.

[0103] Example 5

[0104] The only difference from Example 1 is that the voltage in step (4) is 8V.

[0105] Example 6

[0106] The only difference from Example 1 is that the time in step (4) is 60 seconds.

[0107] Example 7

[0108] The only difference from Example 2 is that the ratio of methane to argon in step (2) is 1:5.

[0109] Example 8

[0110] The only difference from Example 2 is that the gas pressure in step (2) is 0.01 MPa.

[0111] Example 9

[0112] The only difference between this embodiment and embodiment 1 is that the tungsten electrode (wire / rod) used has a diameter of 0.8 mm and a length of 8 cm and is doped with thorium oxide in an amount of 1 wt %.

[0113] Example 10

[0114] The only difference between this embodiment and embodiment 1 is that the tungsten electrode (wire / rod) used has a diameter of 0.3 mm and a length of 15 cm and is doped with yttrium oxide in an amount of 1.5 wt %.

[0115] Comparative Example 1

[0116] The only difference from Example 1 is that step (3) is not performed.

[0117] Comparative Example 2

[0118] The only difference from Example 1 is that the voltage in step (3) is 5V.

[0119] Comparative Example 3

[0120] The only difference from Example 1 is that the voltage in step (3) is 10V.

[0121] Test Example 1

[0122] The carbonized layer prepared in Example 1 was subjected to SEM, metallographic, XRD and EPMA element quantitative analysis. Figure 2 , Figure 3 , Figure 4 , Figure 5 and Figure 6 shown.

[0123] The morphology of the three stages of step (2) carbon attachment, step (3) carbon fixation, and step (4) decarbonization in Example 1 is as follows: Figure 2 As shown. In the carbon attachment stage, the surface of the tungsten electrode (wire / rod) is covered with dense black carbon particles, and the organization structure is dense; in the carbon fixation stage, the carbon particles on the surface of the tungsten electrode (wire / rod) are further reduced, and the color becomes gray visible to the naked eye, the thickness of the carbonized layer is further increased, and the surface is still a dense organization; in the decarburization stage, the surface of the tungsten electrode (wire / rod) presents a metallic luster, and the surface is a uniform single-layer carbonized layer.

[0124] Figure 3 and Figure 4 This is the metallographic image of the tungsten electrode of Example 1. It can be seen that the tungsten electrode has an obvious double-layer structure, in which the core is round fine tungsten grains and the outside is a loose carbide layer. The thickness of the carbide layer is about 40 to 50 μm. The microstructure of the carbide layer is a loose lamellar structure. The loose lamellar structure contains cross-distributed transverse grain boundaries and vertical grain boundaries. The organization is relatively neatly arranged, and the number of grain boundaries is greater, providing more migration channels for rare earth active substances. Figure 4 In the figure, long arrows are used to represent transverse grain boundaries (i.e., transverse lines in the figure), and short arrows are used to represent vertical grain boundaries (i.e., vertical lines in the figure).

[0125] Figure 5This is a metallographic image of a comparative tungsten electrode. It can be seen that the comparative tungsten electrode also has a double-layer structure, with a tungsten matrix as the core, a dense block structure on the outside of the carbide layer, significantly reduced longitudinal grain boundaries, and no lamellar structure.

[0126] Figure 6 This is the XRD phase composition analysis result of the carbonized layer. It can be seen that there is only W2C phase in the surface layer and no WC phase is detected.

[0127] Figure 7 This is the result of EPMA element quantitative analysis. The atomic mass ratio of carbon and oxygen is 1:2, confirming that the phase composition is W2C.

[0128] Test Example 2

[0129] The performance of the tungsten electrodes prepared in the examples and comparative examples was tested, and the results are shown in Table 1.

[0130] Table 1 Performance test results of tungsten electrodes prepared in Examples and Comparative Examples

[0131] Group Saturation emission current / A Example 1 1.15 Example 2 1.08 Example 3 1.03 Example 4 0.98 Example 5 0.93 Example 6 1.03 Example 7 0.91 Example 8 0.95 Example 9 0.98 Example 10 0.84 Comparative Example 1 0.81 Comparative Example 2 0.89 Comparative Example 3 0.83

[0132] As can be seen from Table 1, adding a carbon fixation step can further increase the saturation emission current of the tungsten electrode, while the pressure, voltage and time in the carbon attachment process and the temperature and time control in the decarburization process will affect the emission performance of the cathode. Example 10 Since the electrode is changed to a slender specification, the resistance of the electrode is large, and the electron emission performance of yttrium oxide is correspondingly reduced.

[0133] In summary, the embodiments of the present invention provide a method for preparing a tungsten carbide coating on the surface of a tungsten electrode and a tungsten electrode. The provided preparation method is simple and easy to implement, and has the following advantages:

[0134] (1) Through the three-step carbonization method of carbon attachment-carbon fixation-decarbonization, the reaction between carbon and tungsten can be more effectively controlled by controlling the carbon content and temperature. The carbon fixation step can further react the excess carbon on the surface, which can not only increase the thickness of the carbide layer, but also better ensure that the final reaction product is ditungsten carbide.

[0135] (2) The carbonized layer formed in the embodiment of the present invention is a lamellar structure with a uniform organizational structure, a wider longitudinal channel, and a denser and more uniform transverse channel, which is more conducive to the migration of rare earth active substances and can better ensure the balance of evaporation and diffusion of low-melting-point rare earth active substances.

[0136] (3) The phase component of the carbide layer formed in the embodiment of the present invention is ditungsten carbide. The carbon-deficient phase of ditungsten carbide can reduce the further reaction of carbon and tungsten during high-temperature service, thereby better improving the stability of cathode use.

[0137] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a ditungsten carbide coating on the surface of a tungsten electrode, characterized in that: include: The carbon attachment stage, carbon fixation stage and decarbonization stage are carried out in sequence; In the carbon attachment stage, the tungsten electrode is placed in an atmosphere of carbon-containing hydrocarbon gas and electrically heated to form a carbon layer; In the carbon fixation stage, the carbon-containing hydrocarbon gas is discharged and then electrically heated, with the control voltage being 6V to 8V; In the decarburization stage, the voltage is increased to 8V to 12V for electric heating; The voltage applied in the decarbonization stage is controlled to be greater than the voltage applied in the carbon fixation stage.

2. The method according to claim 1, characterized in that During the carbon fixation stage, the carbon-containing hydrocarbon gas is discharged by vacuuming, introducing hydrogen or introducing an inert gas.

3. The method according to claim 1 or 2, characterized in that: During the carbon fixation stage, the power-on time is controlled to be 50s to 100s.

4. The method according to claim 1, characterized in that In the decarburization stage, the power-on time is controlled to be 20s to 120s.

5. The method according to claim 1, characterized in that The carbon attachment stage includes: introducing the carbon-containing hydrocarbon gas into the carbonization chamber where the tungsten electrode is located, controlling the chamber pressure to be -0.1MPa to 1.0MPa, and electrically heating the tungsten electrode with a control voltage of 5V to 8V for a power-on time of 50s to 150s.

6. The method according to claim 5, characterized in that The introduced carbon-containing hydrocarbon gas contains at least one of benzene and methane; Preferably, the carbon-containing hydrocarbon gas is methane or a methane mixed gas, the methane mixed gas contains hydrogen or an inert gas, and the volume proportion of methane in the methane mixed gas is 8% to 11%; When the carbon-containing hydrocarbon gas is methane, the chamber pressure is controlled to be -0.1 to -0.05 MPa.

7. The method according to claim 1, characterized in that Before the carbon attachment stage, the tungsten electrode is subjected to surface pretreatment to remove surface dirt and oxide layer; Preferably, the surface pretreatment process includes: clamping a tungsten electrode between a positive electrode and a negative electrode of a carbonization chamber, heating the electrode under vacuum, an inert atmosphere or a hydrogen atmosphere, controlling the DC voltage to be 6V to 10V, and the power-on time to be 10s to 60s; When the surface pretreatment process adopts vacuum conditions, the control pressure is 1×10 -3 ~1×10 -5 Pa.

8. The method according to claim 1, characterized in that The diameter of the tungsten electrode is 0.1 mm to 2.0 mm, and the length is 5 cm to 15 cm; Preferably, rare earth oxide is added to the tungsten electrode, and the rare earth element is selected from at least one of thorium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium; Preferably, the added amount of the rare earth oxide is 1 wt% to 2 wt%.

9. The method according to claim 1, characterized in that: The reaction at each stage is carried out using a carbonization device, the carbonization device comprising: A carbonization cavity, in which the tungsten electrode is placed during the preparation process; An electric control module, used for applying voltage to the tungsten electrode in the carbonization chamber and controlling the power-on time; An air intake module, used for introducing gas into the carbonization chamber; A vacuum module, used for evacuating the carbonization chamber; An exhaust module, used to exhaust the gas in the carbonization chamber; Preferably, the exhaust module further comprises an ignition mechanism for igniting the gas.

10. A tungsten electrode, characterized in that: The surface of the tungsten electrode is provided with a ditungsten carbide coating, the ditungsten carbide layer is a loose lamellar structure, and the loose lamellar structure contains cross-distributed transverse grain boundaries and vertical grain boundaries; Preferably, the thickness of the tungsten carbide layer is 40-50 μm; Preferably, the ditungsten carbide coating is prepared by the method according to any one of claims 1-9.

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