Near-net-shape lanthanum hexaboride tungsten-doped cathode and preparation method and application thereof

Abstract

The invention relates to the technical field of illumination, in particular to a near-net-shape lanthanum hexaboride tungsten-doped cathode and a preparation method and application thereof. The invention provides a preparation method of a near-net-shaped lanthanum hexaboride tungsten-doped cathode, which comprises the following steps of: mixing tungsten powder and lanthanum hexaboride powder to obtain a mixture, placing the mixture in a mold of which the inner wall is coated with a carbon powder layer, carrying out first hot pressing sintering at a first temperature, heating to a second temperature, carrying out second hot pressing sintering, and cooling to obtain the near-net-shaped lanthanum hexaboride tungsten-doped cathode. And the near-net-shaped lanthanum hexaboride doped tungsten cathode is obtained. Wherein the first temperature ranges from 1500 DEG C to 2000 DEG C; the second temperature is 2200 to 2400 DEG C; in conclusion, one-time forming of the lanthanum hexaboride-doped tungsten cathode is achieved, the efficient and low-cost preparation method of the lanthanum hexaboride-doped tungsten-based cathode material is developed, meanwhile, a loose lamellar tungsten carbide layer is formed on the surface of the lanthanum hexaboride-doped tungsten cathode, and the electron emission performance of the material is effectively improved.

Description

Technical Field

[0001] The present invention relates to the field of lighting technology, and in particular to a near-net-shape lanthanum hexaboride-doped tungsten cathode and a preparation method and application thereof. Background Art

[0002] Gas discharge lamp is an important light source, widely used in various occasions. The performance of gas discharge lamp directly affects the lighting effect. As an important part of gas discharge lamp, the selection and use of cathode materials directly affect the performance of gas discharge lamp. Thoriated tungsten electrode is widely used as cathode material of gas discharge lamp because it can significantly reduce the electron work function of tungsten. However, because thorium is radioactive, it brings safety risks to the use of this material.

[0003] The lanthanum hexaboride (LaB6) cathode has the characteristics of low electron work function, high density of emitted electrons, resistance to ion bombardment, and good resistance to poisoning, but its melting point is relatively low. Tungsten has a high melting point, good conductivity and chemical stability, etc. Therefore, doping lanthanum hexaboride into tungsten can effectively improve the electron emission ability of tungsten, and the tungsten matrix can further improve its high temperature resistance, thereby improving the performance of gas discharge lamps. However, adding a large amount of lanthanum hexaboride will cause the tungsten matrix to be brittle, and it is easy to crack or even be unable to be processed in subsequent pressure processing. This not only increases the complexity and cost of the preparation process, but also affects the performance of the cathode material. The existing lanthanum hexaboride cathode preparation method is mainly to first prepare a cathode matrix rod, obtain a cathode matrix of a corresponding shape by pressure processing and machining, and then form a tungsten carbide coating on its surface by a surface carburization process. The preparation process is complicated and inefficient, and the obtained carbide layer is mainly a dense tungsten carbide coating, which is not conducive to the migration of active substances during service, resulting in low electron emission performance of the material. Summary of the invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the problems in the prior art that the lanthanum hexaboride cathode forming process is complicated, the efficiency is low, and the surface carbide layer is too dense, resulting in low electron emission performance of the cathode, thereby providing a near-net-shaped lanthanum hexaboride doped tungsten cathode and its preparation method and application.

[0005] The present invention provides a method for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, comprising the following steps: mixing tungsten powder and lanthanum hexaboride powder to obtain a mixture, placing the mixture in a mold with a carbon powder layer coated on the inner wall, performing a first hot pressing sintering at a first temperature, and then heating the mixture to a second temperature for a second hot pressing sintering, so as to obtain the near-net-shape lanthanum hexaboride-doped tungsten cathode;

[0006] Among them, the first temperature is 1500-2000℃; the second temperature is 2200-2400℃.

[0007] Preferably, the first hot pressing sintering pressure is 30-70 MPa, and the time is 10-60 min;

[0008] Preferably, the second hot pressing sintering pressure is 30-70 MPa, and the time is 20-60S;

[0009] Preferably, the heating rate from the first temperature to the second temperature is 300-600°C / min;

[0010] Preferably, a cooling process is further included after the second hot pressing sintering, and the cooling process includes a process of dropping from the second temperature to a third temperature, wherein the third temperature is 1750-1850° C., and the time for dropping from the second temperature to the third temperature is 1-2 minutes.

[0011] Preferably, the first hot pressing sintering and the second hot pressing sintering process are carried out in a hot pressing sintering furnace;

[0012] Preferably, the mass percentage of lanthanum hexaboride in the mixture is 0.5 to 10%wt;

[0013] The preparation process of the mixture includes mechanically mixing tungsten powder and lanthanum hexaboride powder;

[0014] Preferably, the mechanical mixing is selected from ball milling or V-type powder mixing;

[0015] Preferably, the mechanical mixing time is 24 to 72 hours;

[0016] Preferably, the particle size of the tungsten powder is 3-6 μm, and the particle size of the lanthanum hexaboride powder is 1-3 μm.

[0017] Preferably, the mechanical mixing is selected from ball milling; wherein the ball-to-material ratio in the ball milling is (8-12):1; and the ball milling speed is 40-60RPM.

[0018] Preferably, the carbon powder layer on the inner wall of the mold is formed by spraying a carbon nanopowder coating on the inner wall of the mold;

[0019] Preferably, the carbon nanopowder has a particle size of 100 to 200 nm;

[0020] Preferably, the carbon nanopowder coating has a thickness of 50 to 200 μm.

[0021] Preferably, the mold cavity of the mold is a columnar structure, and the columnar structure includes a cylindrical section and a conical section located at one end of the cylindrical section;

[0022] Preferably, the length of the columnar structure is 12 to 30 mm;

[0023] Preferably, the cone angle of the cone section is 15-50°;

[0024] Preferably, the diameter of the cylindrical section is 5-15 mm.

[0025] The present invention also provides a cathode prepared by the above-mentioned preparation method, wherein the outermost layer of the cathode is ditungsten carbide.

[0026] Preferably, the thickness of the ditungsten carbide is 20-80 μm;

[0027] Preferably, the cathode is a cylinder, and the cylinder includes a cylindrical section and a conical section located at one end of the cylindrical section;

[0028] Preferably, the length of the column is 12 to 30 mm;

[0029] Preferably, the cone angle of the cone section is 15-50°;

[0030] Preferably, the diameter of the cylindrical section is 5-15 mm.

[0031] The present invention also provides a mold for preparing the above-mentioned near-net-shape lanthanum hexaboride-doped tungsten cathode, wherein the mold cavity of the mold is used to prepare the near-net-shape lanthanum hexaboride-doped tungsten cathode;

[0032] Preferably, the mold cavity of the mold is a columnar structure, and the columnar structure includes a cylindrical section and a conical section located at one end of the cylindrical section;

[0033] Preferably, the mold comprises an upper pressing head, a lower pressing head, and a concave mold; the concave mold has a through hole; the upper pressing head and the lower pressing head are arranged opposite to each other in the through hole and form a mold cavity with the side wall of the through hole; the upper pressing head and / or the lower pressing head are movably arranged in the through hole;

[0034] Preferably, a forming portion is provided on the lower pressure head; the forming portion corresponds to the conical section of the columnar structure;

[0035] Preferably, the material of the upper pressure head is selected from a graphite mold or a pure tungsten mold; and / or, the material of the lower pressure head is selected from a graphite mold or a pure tungsten mold; and / or, the material of the concave mold is selected from a graphite mold or a pure tungsten mold.

[0036] Optionally, the mold includes multiple pairs of upper and lower pressure heads; the concave mold has multiple through holes; the multiple pairs of upper and lower pressure heads and the multiple through holes form multiple mold cavities correspondingly.

[0037] It can be understood that there are multiple through holes in the die, and each through hole corresponds to a pair of upper and lower pressure heads.

[0038] Optionally, the upper pressing head is connected to the upper pressing head fixing seat with the side facing away from the lower pressing head;

[0039] Optionally, the lower pressure head is connected to a lower pressure head fixing seat on a side facing away from the upper pressure head.

[0040] Optionally, the upper pressure head and the lower pressure head are connected to an external power source for heating;

[0041] Optionally, the upper pressure head and the lower pressure head move relative to each other to reduce the volume of the mold cavity.

[0042] The present invention also provides a gas discharge lamp, wherein the cathode of the gas discharge lamp is the cathode prepared by the preparation method described above or the cathode described above.

[0043] The technical solution of the present invention has the following advantages:

[0044] The present invention provides a method for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, comprising the following steps: mixing tungsten powder and lanthanum hexaboride powder to obtain a mixture, placing the mixture in a mold with an inner wall coated with a carbon powder layer, performing a first hot pressing sintering at a first temperature, and heating to a second temperature to perform a second hot pressing sintering to obtain the near-net-shape lanthanum hexaboride-doped tungsten cathode; wherein the first temperature is 1500-2000°C; and the second temperature is 2200-2400°C.

[0045] In the present invention, a mixed material is formed in a mold with an inner wall coated with a carbon powder layer, and a first hot pressing sintering is performed at a first temperature, and a second hot pressing sintering is performed at a second temperature, so that a lanthanum hexaboride-doped tungsten cathode is formed in one step, and an efficient and low-cost preparation method of a lanthanum hexaboride-doped tungsten-based cathode material is developed. A loose lamellar tungsten carbide layer is formed on the surface of the prepared lanthanum hexaboride-doped tungsten cathode, and the tungsten carbide layer is divided into loose lamellar shapes by cross-distributed lateral grain boundaries and vertical grain boundaries, which significantly increases the number of grain boundaries, provides more migration channels for rare earth active substances, and effectively improves the electron emission performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0047] Figure 1 is a longitudinal cross-sectional view of the mold used in Example 1 of the present invention;

[0048] Figure 2 is a cross-sectional view of the concave mold used in Example 1 of the present invention;

[0049] Figure 3 This is a metallographic image of the carbide layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode prepared in Example 1 of the present invention;

[0050] Figure 4 This is a phase composition analysis diagram of the carbide layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode prepared in Example 1 of the present invention;

[0051] Figure 5 This is a metallographic image of the carbide layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode prepared in Comparative Example 1 of the present invention;

[0052] Figure 6 This is a scanning electron microscope image of the carbide layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0053] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0054] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0055] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0056] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0057] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0058] Example 1

[0059] This embodiment provides a method for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0060] (1) Lanthanum hexaboride with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed at a weight ratio of 1:50, the mixed powders are loaded into a ball mill with a ball-to-material ratio of 10:1, and ball milling is performed at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0061] (2) The mixed material is placed in a pure tungsten mold; the longitudinal section of the mold is as follows Figure 1 As shown, the mold includes a concave mold 300 ( Figure 2 is a cross section of a die 300), the die 300 has seven through holes 301; the mold further comprises seven pairs of upper rams 200 and lower rams 400 (one through hole 301 corresponds to one upper ram 200 and one lower ram 400), wherein the lower ram 400 is provided with a forming portion 401; the upper ram 200 and the lower ram 400 are movably placed in the through hole 301, and the upper ram 200, the forming portion 401 and the side wall of the through hole 301 form a mold cavity 600; the mold cavity is a cylindrical structure with a diameter of 10 mm and a total length of 50 mm, the cylindrical structure comprises a cylindrical section and a conical section at one end of the cylindrical section, and the cone angle of the conical section is 40°; wherein the seven upper rams 200 are fixed on the upper ram fixing seat 100, and the seven lower rams 400 are fixed on the lower ram fixing seat 500;

[0062] Before loading the mixture into the mold, spray a carbon nanopowder coating on the inner wall of each cavity, the carbon powder particle size is 100nm; the coating thickness is 80μm. Then add 30g of the mixture prepared in step (1) into each cavity;

[0063] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper press head 200, the lower press head 400 and the heating power supply for heating, and applying a pressure of 60 MPa to the mixture through the relative movement of the upper press head 200 and the lower press head 400, and sintering at 2000°C for 30 minutes; keeping the pressure unchanged, then raising the temperature to 2400°C at 400°C / min, sintering for 40S, then cooling to 1800°C after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops below 100°C, taking out the mold and cooling it naturally to room temperature, and then demolding to obtain the near-net-shape lanthanum hexaboride-doped tungsten cathode.

[0064] Figure 3 In order to prepare the metallographic diagram of the near-net-shape lanthanum hexaboride-doped tungsten cathode, it can be seen from the figure that the thickness of the outermost carbide layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode is about 30 to 40 μm, and the carbide layer is divided into loose lamellar layers by the cross-distributed transverse grain boundaries and vertical grain boundaries. The more grain boundaries there are, the more migration channels are provided for the rare earth active substances.

[0065] Figure 4 From the phase composition analysis diagram for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, it can be seen that the composition of the carbide layer is ditungsten carbide, and there is no tungsten carbide phase.

[0066] Example 2

[0067] This embodiment provides a method for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0068] (1) Lanthanum hexaboride powder with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed at a weight ratio of 1:20, the mixed powders are loaded into a ball mill with a ball-to-material ratio of 10:1, and ball milling is performed at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0069] (2) The mixed material is loaded into a mold; the mold is similar to the mold used in Example 1, and the upper pressing head 200, the forming portion 401 and the side wall of the through hole 301 form a mold cavity; the mold cavity is a cylindrical structure with a diameter of 10 mm and a total length of 50 mm, and the cylindrical structure includes a cylindrical section and a conical section at one end of the cylindrical section, and the cone angle of the conical section is 40°;

[0070] Before loading the mixture into the mold, spray a carbon nanopowder coating on the inner wall of each mold cavity, the carbon powder particle size is 100nm; the coating thickness is 80μm; then add 25g of the mixture prepared in step (1) into each mold cavity;

[0071] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper press head 200, the lower press head 400 and the heating power supply for heating, and applying a pressure of 60 MPa to the mixture through the relative movement of the upper press head 200 and the lower press head 400, and sintering at 1800°C for 30 minutes; keeping the pressure unchanged, then raising the temperature to 2300°C at 600°C / min, sintering for 40S, then cooling to 1800°C after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops below 100°C, taking out the mold and cooling it naturally to room temperature, and then demolding to obtain the near-net-shape lanthanum hexaboride-doped tungsten cathode.

[0072] Example 3

[0073] This embodiment provides a method for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0074] (1) Lanthanum hexaboride with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed in a weight ratio of 1:10, the mixed powders are loaded into a ball mill with a ball-to-material ratio of 10:1, and ball milling is performed at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0075] (2) placing the mixed material into a mold; the mold is the same as the mold used in Example 1;

[0076] Before loading the mixture into the mold, a carbon nanopowder coating is sprayed on the inner wall of each mold cavity, and the carbon powder particle size is 100nm; the coating thickness is 80μm; then 20g of the mixture prepared in step (1) is added to each mold cavity;

[0077] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper pressing head 200, the lower pressing head 400 and the heating power supply for heating, and applying a pressure of 60 MPa to the mixture at 1600°C through the relative movement of the upper pressing head 200 and the lower pressing head 400, and sintering for 30 minutes; keeping the pressure unchanged, then raising the temperature to 2200°C at 600°C / min, sintering for 50S, then cooling to 1800°C after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops to below 100°C, taking out the mold, cooling it naturally to room temperature, and then demolding it to obtain the near-net-shape lanthanum hexaboride-doped tungsten cathode.

[0078] Comparative Example 1

[0079] This comparative example provides a method for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0080] (1) Lanthanum hexaboride with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed at a weight ratio of 1:50, the mixed powders are loaded into a ball mill with a ball-to-material ratio of 10:1, and ball milling is performed at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0081] (2) Loading the mixture into a mold; the mold is the same as the mold in Example 1. Before loading the mixture into the mold, spray a carbon nanopowder coating on the inner wall of each mold cavity, the carbon powder particle size is 100nm; the coating thickness is 80μm; then add 30g of the mixture prepared in step (1) into each mold cavity;

[0082] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper press head 200, the lower press head 400 and the heating power supply for heating, and applying a pressure of 60 MPa to the mixture through the relative movement of the upper press head 200 and the lower press head 400, and sintering at 2000°C for 30 minutes; then cooling to 1800°C after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops below 100°C, taking out the mold and cooling it naturally to room temperature, and then demolding to obtain the near-net-shape lanthanum hexaboride-doped tungsten cathode.

[0083] Figure 5 This is a metallographic image of the carbide layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode prepared in Comparative Example 1. It can be seen from the figure that the outermost layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode is a dense tungsten carbide layer ( Figure 6 The scanning electron microscope image of the carbide layer of the near-net-shape lanthanum hexaboride-doped tungsten cathode prepared in Comparative Example 1 is shown in Table 1. Figure 6 The energy spectrum data in the box shows that the atomic ratio of carbon and tungsten is close to 1:1, and the outermost layer of the nearly net-shaped lanthanum hexaboride-doped tungsten cathode is mainly tungsten carbide. The thickness of the tungsten carbide layer is about 30 to 40 μm, and the structure is blocky, without any lamellar channels.

[0084] Table 1

[0085] element Wt% At% CK 5.56 41.88 OK 2.05 11.58 LaL 6.83 4.45 WL 85.56 42.09 Matrix Correction ZAF

[0086] Comparative Example 2

[0087] This comparative example provides a method for preparing a lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0088] (1) Lanthanum hexaboride with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed at a weight ratio of 1:50, the mixed powders are loaded into a ball mill with a ball-to-material ratio of 10:1, and ball milling is performed at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0089] (2) The mixture is loaded into a mold; the mold is the same as the mold in Example 1; before loading the mixture into the mold, a carbon nanopowder coating is sprayed on the inner wall of each mold cavity, the carbon powder particle size is 100nm; the coating thickness is 80μm; then 30g of the mixture prepared in step (1) is added to each mold cavity;

[0090] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper pressing head 200, the lower pressing head 400 and the heating power supply for heating, and applying a pressure of 60 MPa to the mixture through the relative movement of the upper pressing head 200 and the lower pressing head 400 at 2400°C for sintering for 30 minutes; then cooling to 1800°C after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops below 100°C, taking out the mold and cooling it naturally to room temperature, and then demolding to obtain the lanthanum hexaboride-doped tungsten cathode.

[0091] Comparative Example 3

[0092] This comparative example provides a method for preparing a lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0093] (1) Lanthanum hexaboride with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed at a weight ratio of 1:50, the mixed powders are loaded into a ball mill with a ball-to-material ratio of 10:1, and ball milling is performed at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0094] (2) The mixture is loaded into a mold; the mold is the same as the mold in Example 1; before loading the mixture into the mold, a carbon nanopowder coating is sprayed on the inner wall of each mold cavity, the carbon powder particle size is 5 μm; the coating thickness is 80 μm; then 30 g of the mixture prepared in step (1) is added to each mold cavity;

[0095] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper pressing head 200, the lower pressing head 400 and the heating power supply for heating, and applying a pressure of 60 MPa to the mixture through the relative movement of the upper pressing head 200 and the lower pressing head 400 at 2400°C for sintering for 30 minutes; then cooling to 1800°C after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops below 100°C, taking out the mold and cooling it naturally to room temperature, and then demolding to obtain the lanthanum hexaboride-doped tungsten cathode.

[0096] Comparative Example 4

[0097] This comparative example provides a method for preparing a lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0098] (1) Lanthanum hexaboride with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed at a weight ratio of 1:50, the mixed powders are loaded into a ball mill with a ball-to-material ratio of 10:1, and ball milling is performed at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0099] (2) placing the mixture into a mold; the mold is the same as the mold in Example 1; 30 g of the mixture prepared in step (1) is added to each mold cavity, and the inner wall of the mold cavity of the mold is not sprayed with a carbon nanopowder coating;

[0100] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper pressing head 200, the lower pressing head 400 and the heating power supply for heating, and applying a pressure of 60 MPa to the mixture through the relative movement of the upper pressing head 200 and the lower pressing head 400 at 2000°C, and sintering for 30 minutes; keeping the pressure unchanged, then raising the temperature to 2400°C at 400°C / min, sintering for 40S, then cooling to 1800°C after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops below 100°C, taking out the mold, cooling it naturally to room temperature, and then demolding to obtain the lanthanum hexaboride-doped tungsten cathode.

[0101] Comparative Example 5

[0102] This comparative example provides a method for preparing a lanthanum hexaboride-doped tungsten cathode, comprising the following steps:

[0103] (1) Thorium oxide powder with a Fisher particle size of 1 μm and tungsten powder with a Fisher particle size of 3 μm are mixed at a weight ratio of 1:50, the mixed powders are placed in a ball mill with a ball-to-material ratio of 10:1, and the mixture is ball milled at a rotation speed of 50 RPM for 72 hours to obtain a mixed material;

[0104] (2) placing the mixture into a mold; the mold is the same as the mold in Example 1; adding 30 g of the mixture prepared in step (1) into each mold cavity;

[0105] (3) placing the mold containing the mixture in a hot pressing sintering furnace, connecting the upper pressing head 200, the lower pressing head 400 and the heating power supply to heat, and applying a pressure of 60 MPa to the mixture through the relative movement of the upper pressing head 200 and the lower pressing head 400, and sintering at 2000° C. for 30 minutes; then heating to 2400° C. at 400° C. / min, sintering for 40 seconds, then cooling to 1800° C. after 1 minute, and then turning off the power to cool naturally; after the temperature of the hot pressing sintering furnace drops below 100° C., taking out the mold and cooling it naturally to room temperature, and then demolding to obtain the thoriated tungsten doped tungsten cathode precursor;

[0106] (4) Butyl acetate, ethyl cellulose and graphite powder were mixed in a mass ratio of 6:0.5:0.8 and uniformly coated on the surface of the thoriated tungsten cathode precursor obtained in step (3) to a total thickness of 1 mm. After drying, the mixture was heated to 1800° C. under vacuum conditions and kept at this temperature for 5 min. The mixture was then cooled under argon protection to obtain a thoriated tungsten cathode. The thickness of the ditungsten carbide layer was 79.76 μm.

[0107] Test Case

[0108] The carbonized layer thickness and organizational morphology of the cathodes prepared by gold-relative Examples 1-3 and Comparative Examples 1-5 were analyzed, and the analysis results are shown in Table 2.

[0109] Table 2

[0110]

[0111]

[0112] It can be seen from Table 2 that Examples 1 to 3 can obtain loose lamellar carbonized layer structures, while the carbonized layer structures of the samples that have not undergone two hot pressing sintering processes (Comparative Examples 1 to 2) and that use coarse carbon powder (Comparative Example 3) are mainly dense block-like.

[0113] The cathodes prepared in Examples 1-3 and Comparative Examples 1-5 were assembled into 1KW or 2KW xenon lamps, and the flicker-free working time and saturated emission current density were tested in an indoor environment.

[0114] The test results are shown in Table 3.

[0115] Table 3

[0116]

[0117] It can be seen from Table 3 that the cathode surface of Example 1 is a loose lamellar structure, the cathode can be flicker-free for more than 1200 hours, and the saturated emission current is relatively high. However, since Comparative Examples 1 to 2 are dense block-layer or double-layer structures, the electrode emission is relatively unstable, the working time with flicker is also short, and the saturated emission current is also low. The carbonized layer on the cathode surface of Comparative Examples 3 to 4 is too thin or even without a carbonized layer, the emission performance of the cathode is further reduced, and flicker is more likely to occur. The thoriated tungsten electrode used in Comparative Example 5 has a significantly lower saturated emission current density than the lanthanum hexaboride-doped tungsten electrode of Example 1. This is because the electron work function of thorium is higher than that of lanthanum hexaboride. In addition, although the carbide layer structure obtained in Comparative Example 5 is also loose and lamellar, there are no obvious lateral grain boundaries and vertical grain boundaries, and it is impossible to provide more migration channels for rare earth active materials, resulting in a further reduction in the saturated emission current density.

[0118] In Example 1, the ratio of lanthanum hexaboride to tungsten powder is 1:50, and in Examples 2 and 3, the ratio of lanthanum hexaboride to tungsten powder reaches 1:20 and 1:10. It can be seen from the table that with the increase of the lanthanum hexaboride doping amount, the emission current density of the electrode is also significantly improved. Since the melting point of lanthanum hexaboride is relatively low, the cathodes of Examples 2 and 3 are more suitable for use with lower power xenon lamps.

[0119] Obviously, the above embodiments are merely examples for clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from them are still within the protection scope of the invention.

Claims

1. A method for preparing a near-net-shape lanthanum hexaboride-doped tungsten cathode, characterized in that: The method comprises the following steps: mixing tungsten powder and lanthanum hexaboride powder to obtain a mixture, placing the mixture in a mold with an inner wall coated with a carbon powder layer, performing a first hot pressing sintering at a first temperature, heating to a second temperature and performing a second hot pressing sintering to obtain the near-net-shape lanthanum hexaboride-doped tungsten cathode; Among them, the first temperature is 1500-2000℃; the second temperature is 2200-2400℃.

2. The preparation method according to claim 1, characterized in that: The first hot pressing sintering pressure is 30-70MPa and the time is 10-60min; Preferably, the second hot pressing sintering pressure is 30-70 MPa, and the time is 20-60S; Preferably, the heating rate from the first temperature to the second temperature is 300-800° C. / min.

3. The preparation method according to claim 1 or 2, characterized in that: The second hot pressing sintering also includes a cooling process, which includes a process of dropping from the second temperature to the third temperature, wherein the third temperature is 1750-1850° C., and the time for dropping from the second temperature to the third temperature is 1-2 minutes.

4. The preparation method according to any one of claims 1 to 3, characterized in that The mass percentage of lanthanum hexaboride in the mixture is 0.5 to 10%wt; The preparation process of the mixture includes mechanically mixing tungsten powder and lanthanum hexaboride powder; Preferably, the mechanical mixing is selected from ball milling or V-type powder mixing; Preferably, the mechanical mixing time is 24 to 72 hours; Preferably, the particle size of the tungsten powder is 3-6 μm, and the particle size of the lanthanum hexaboride powder is 1-3 μm.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The process of forming the carbon powder layer on the inner wall of the mold is to spray a carbon nano powder coating on the inner wall of the mold; Preferably, the carbon nanopowder has a particle size of 100 to 200 nm; Preferably, the carbon nanopowder coating has a thickness of 50 to 200 μm.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The mold cavity of the mold is a columnar structure, and the columnar structure includes a cylindrical section and a conical section located at one end of the cylindrical section; Preferably, the length of the columnar structure is 12 to 30 mm; Preferably, the cone angle of the cone section is 15-50°; Preferably, the diameter of the cylindrical section is 5-15 mm.

7. A cathode prepared by the preparation method according to any one of claims 1 to 6, characterized in that: The outermost layer of the cathode is a tungsten carbide layer.

8. The cathode according to claim 7, characterized in that The thickness of the ditungsten carbide layer is 20-80 μm.

9. A mold for preparing the near-net-shape lanthanum hexaboride-doped tungsten cathode according to any one of claims 1 to 4, characterized in that: The mold cavity of the mold is used to prepare the near-net-shape lanthanum hexaboride-doped tungsten cathode; Preferably, the mold cavity of the mold is a columnar structure, and the columnar structure includes a cylindrical section and a conical section located at one end of the cylindrical section; Preferably, the mold comprises an upper pressing head, a lower pressing head, and a concave mold; the concave mold has a through hole; the upper pressing head and the lower pressing head are placed in the through hole and form a mold cavity with the side wall of the through hole; the upper pressing head and / or the lower pressing head are movably arranged in the through hole; Preferably, a forming portion is provided on the lower pressure head; the forming portion corresponds to the conical section of the columnar structure; Preferably, the mold comprises a plurality of pairs of upper and lower pressing heads; the concave mold has a plurality of through holes; the plurality of pairs of upper and lower pressing heads and the plurality of through holes form a plurality of mold cavities correspondingly; Preferably, the material of the upper pressure head is selected from a graphite mold or a pure tungsten mold, and the material of the lower pressure head is selected from a graphite mold or a pure tungsten mold; and / or, the material of the concave mold is selected from a graphite mold or a pure tungsten mold.

10. A gas discharge lamp, characterized in that: The cathode of the gas discharge lamp is a cathode prepared by the preparation method described in any one of claims 1 to 6 or a cathode described in any one of claims 7 to 8.

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