Porous tungsten matrix for hot cathode and preparation method thereof

Abstract

The invention relates to the technical field of thermionic cathode materials, and discloses a porous tungsten matrix for a thermionic cathode and a preparation method thereof, and the preparation method comprises the following steps: S1, preparing a tungsten powder raw material; s2, pressing into a tungsten blank; s3, hydrogen sintering is conducted, specifically, the tungsten blank is placed in the hydrogen atmosphere at the temperature of 900-1550 DEG C to be sintered for the first time; s4, vertical sintering: electrifying two ends of the tungsten blank, heating and sintering to form a tungsten porous body; s5, infiltrating copper: in a hydrogen atmosphere, adding a copper source into the tungsten porous body in batches, and carrying out graded heating and sintering to obtain a copper-containing tungsten alloy matrix; s6, machining to form a cathode substrate; and S7, copper removal treatment is conducted, and the porous tungsten matrix for the hot cathode is obtained. The porous tungsten matrix for the hot cathode is uniform in pore and uniform in tungsten grain size, and is very suitable for being used as the porous tungsten matrix in the hot cathode.

Description

Technical Field

[0001] The present invention relates to the technical field of hot cathode materials, and in particular to a porous tungsten substrate for a hot cathode and a preparation method thereof. Background Art

[0002] Microwave vacuum electronic devices are power-type emission sources commonly used in radar, communications, remote sensing, telemetry, electronic countermeasures, and precision guidance equipment. Among them, the hot cathode, the core component of microwave vacuum electronic devices, is its electron emission source. The quality and performance of the hot cathode directly affect the working efficiency and service life of microwave vacuum electronic devices and systems. The hot cathode is usually required to have low operating temperature, high current density, high reliability, and long service life. Specifically, hot cathodes are widely used in technical fields such as plasma cutting, plasma arc welding, and plasma coating. The high-temperature plasma generated by the hot cathode is used to melt and blow away metal materials to achieve cutting, melt two pieces of metal to form a weld, or spray metal or non-metal powder onto a substrate to form a thin film.

[0003] The hot cathode mainly includes the following parts: cathode material, heating element, support structure and electrode lead-out assembly. Among them, the hot cathode porous tungsten matrix is ​​a special and important metal hot cathode matrix in the hot cathode. It has a special structure and performance material and plays an important role in many fields. The hot cathode porous tungsten matrix usually has a three-dimensional network structure with high porosity. This structure is composed of many tiny tungsten particles connected to each other, forming a large number of pores, and the size and distribution of these pores can be regulated by the preparation process.

[0004] The preparation methods of the hot cathode porous tungsten substrate mainly include the following:

[0005] (1) Powder metallurgy method:

[0006] Tungsten powder is mixed with an appropriate binder and made into a porous tungsten matrix through processes such as pressing and sintering. This method is one of the more common methods. The porosity and pore size of the tungsten matrix can be controlled by process parameters, but the preparation process is relatively complicated and requires strict control of process conditions.

[0007] (2) Chemical vapor deposition method:

[0008] At high temperature, a tungsten film is deposited on a substrate through a chemical reaction. By controlling the reaction conditions, a porous tungsten matrix with a specific porosity and structure can be prepared. This method can prepare a high-quality porous tungsten matrix, but the process is difficult and the production cost is high.

[0009] For example, the patent with publication number CN108251734A discloses a tungsten substrate for hot cathode and its preparation method, which uses narrow-grained, medium-fine-grained tungsten or tungsten alloy powder classified by wet method as raw material, and undergoes vacuum degassing, screening, cold isostatic pressing, composite sintering, and then hot isostatic pressing, re-pressing and re-firing modification, full-dense copper infiltration, and finally machining and high-temperature vacuum copper removal to obtain a tungsten substrate for hot cathode containing pure tungsten, tungsten-iridium, tungsten-osmium or tungsten-rhenium alloy.

[0010] The existing preparation method of the tungsten substrate for the hot cathode as mentioned above has a complicated preparation process, a high degree of technical difficulty, and is not easy to control product quality, resulting in problems such as uneven pores in the product and uneven tungsten particle size. Summary of the invention

[0011] The technical problems to be solved by the present invention are:

[0012] At present, powder metallurgy or chemical vapor deposition is usually used to produce hot cathode tungsten substrates in microwave vacuum electronic devices. However, the existing preparation methods not only have complicated processes, difficult processes, and high production costs, but the products produced also have defects such as uneven porosity and uneven tungsten particle size.

[0013] The technical solution adopted by the present invention is:

[0014] The present invention provides a method for preparing a porous tungsten substrate for a hot cathode, comprising the following steps:

[0015] S1 prepares tungsten powder raw material;

[0016] S2 is pressed into tungsten blank;

[0017] S3 low temperature sintering: place the tungsten blank in a hydrogen atmosphere at 900-1550℃ for low temperature sintering;

[0018] S4 vertical melting sintering: electricity is applied to both ends of the tungsten blank to generate heat and sinter to form a tungsten porous body;

[0019] S5 copper infiltration: in a hydrogen atmosphere, copper source is added to the tungsten porous body in batches, and heated and sintered in stages to obtain a copper-containing tungsten alloy matrix;

[0020] S6 is machined into cathode substrate;

[0021] S7: copper removal treatment to obtain the porous tungsten substrate for the hot cathode.

[0022] Preferably, in step S4, the current is 2000-5700A and the sintering time is 10-50min.

[0023] Preferably, in step S5, sintering is performed in a plurality of high temperature zones including 1320-1350°C, 1380-1400°C, 1440-1460°C, and 1490-1500°C.

[0024] Preferably, when entering each high temperature zone, an equal amount of copper source is added, the copper source can be selected from one or more of copper powder, copper powder compact, copper foil, and the total amount of copper source added is 1-3wt% of the porous tungsten substrate.

[0025] Preferably, a push boat furnace is used for copper infiltration treatment, and the push boat speed is 2.5-50 mm / min, and the total sintering time in the high temperature zone is 10-120 min.

[0026] Preferably, in step S6, the machining accuracy of the tungsten copper end face of the cathode emission surface should be lower than 0.6 μm, and the machining accuracy of the circumferential surface and end face of the non-emitting surface should be lower than 1.0 μm.

[0027] Preferably, in step S7, a chemical method is used to remove copper, including the following steps: cleaning the cathode substrate, immersing it in an acid solution, taking it out and rinsing it; then immersing it in water and heating it, taking it out and dehydrating and drying it, so as to obtain the porous tungsten substrate for the hot cathode.

[0028] Preferably, in step S7, a vacuum method is used to remove copper, and the process conditions are: vacuum degree is lower than 5×10-3 Pa, processing temperature is 1550-1650° C., and processing time is 50-100 min.

[0029] Preferably, in steps S1 and S2, the particle size of the tungsten powder raw material is 0.5-7 μm, and it is pressed into a tungsten blank at 50-240 MPa for 1-20 min.

[0030] The porous tungsten substrate for hot cathode prepared by the above preparation method has a total porosity of 4-50%, an average pore size of 0.5-5μm, a pore uniformity of ±2μm, a tungsten particle size of 1-10μm, and a tungsten grain uniformity of ±3μm.

[0031] The beneficial effects of the present invention are as follows:

[0032] The method for preparing a porous tungsten substrate for a hot cathode proposed in the present invention simplifies the necessary steps considered in the existing production process of a porous tungsten substrate for a hot cathode, and improves specific technical means and process conditions such as sintering methods and copper infiltration treatment. A porous tungsten substrate material with uniform and dense pores and uniform tungsten particle size can be prepared through a more convenient production method. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a scanning electron microscope image of the porous tungsten substrate in Example 1 magnified 2000 times;

[0034] Figure 2 This is a scanning electron microscope image of the porous tungsten material in Comparative Example 1 magnified 2000 times. DETAILED DESCRIPTION

[0035] 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.

[0036] The present invention provides a method for preparing a porous tungsten substrate for a hot cathode, comprising the following steps:

[0037] (1) Preparation of raw materials:

[0038] Take tungsten powder and sieve it to obtain tungsten powder raw material with suitable particle size;

[0039] The sieve is selected to be 100-500 mesh to obtain 0.5-7μm tungsten powder raw material.

[0040] (2) Pressing into blanks:

[0041] The tungsten powder raw material is loaded into a soft rubber mold for cold isostatic pressing to obtain a tungsten blank;

[0042] The pressure of cold isostatic pressing is 50-240MPa, the pressing time is 1-20min, and during the pressing process, the soft rubber mold is preferably fixed and limited by a rigid sleeve to obtain a uniform pressed green sheet with a regular shape, so as to reduce the rough turning process of the pre-sintered green sheet.

[0043] (3) Low temperature sintering:

[0044] The tungsten blank is placed in a hydrogen atmosphere for low temperature sintering;

[0045] High-purity hydrogen is used, the dew point of hydrogen is not higher than -60°C, the low-temperature sintering temperature is 900-1550°C, and the low-temperature sintering time is 20-100min.

[0046] (4) Vertical melting sintering:

[0047] Electricity is passed through both ends of the tungsten blank at a current of 2000-5700A, so that the tungsten blank itself acts as a resistor to generate heat, generating high temperature at a faster heating rate for high-temperature sintering. The sintering time is 10-50 minutes to obtain a tungsten porous body.

[0048] (5) Copper infiltration:

[0049] A copper source is added to the tungsten porous body, and copper infiltration treatment is performed in a high-purity hydrogen atmosphere to obtain a copper-containing tungsten alloy matrix;

[0050] The copper source can be selected from copper powder, copper powder compact, copper foil, etc., and the total addition amount of the copper source is 1-3wt% of the porous tungsten matrix. Fully dense copper infiltration is adopted, and the tungsten alloy porous body is continuously infiltrated with a push-boat furnace in a high-purity hydrogen atmosphere. The pushing speed is 2.5-50mm / min, and the boat is pushed slowly at a uniform speed. The temperature in the high temperature zone is 1300-1500℃, the total sintering time in the high temperature zone is 10-120min, and the inner diameter length of the high temperature zone furnace is approximately 5-25% of the length of the tungsten alloy porous body.

[0051] (6) Machining:

[0052] The copper-tungsten alloy substrate is machined to obtain a cathode substrate of a desired size;

[0053] Among them, the machining accuracy of the tungsten copper end face of the cathode emission surface should be less than 0.6μm, and the machining accuracy of the circumferential surface and end face of the non-emitting surface should be less than 1.0μm.

[0054] (7) Copper removal:

[0055] The cathode substrate is subjected to copper removal treatment to obtain a porous tungsten substrate for a hot cathode;

[0056] Copper removal can be carried out by chemical method or vacuum method. The process steps of chemical copper removal are: cleaning the substrate, immersing the substrate in acid solution for acid corrosion treatment, taking out and rinsing, immersion heating, taking out and dehydrating and drying; the process conditions of vacuum copper removal are: vacuum degree is less than 5×10 -3 pa, the processing temperature is 1550-1650℃, and the processing time is 50-100min.

[0057] The porous tungsten substrate for hot cathode in the present invention can be mainly used for porous tungsten substrate components in low-temperature, high-current, high-reliability and long-life hot cathodes. Its total porosity is 4-50%, the average pore diameter (mercury intrusion method) is 0.5-5μm, the pore size is 0.2-7μm, the pore uniformity is ±2μm, the tungsten grain uniformity is ±3μm, the closed porosity is <0.2%, the average tungsten grain size is 1-10μm, and the skeleton tensile strength is 300-700MPa. That is, the porous tungsten substrate for hot cathode has uniform pores and uniform tungsten grain size, and is very suitable for use as a porous tungsten substrate in a hot cathode.

[0058] Example 1

[0059] Take tungsten powder with an average particle size of about 3.8μm, pass it through a 200-mesh sieve, and then evenly load the sieved tungsten powder into a 30mm Φ soft rubber mold with a certain taper fixed by a steel sleeve, and cold isostatically press it at a pressure of 200MPa for 5 minutes to obtain a tungsten blank with a relatively regular appearance; place the tungsten blank in a push boat furnace, fill it with high-purity hydrogen until all the air is exhausted, and sinter it at a low temperature of 1400℃ for 60 minutes; then take it out and cool it, roughly process it to the required size, and then place it in a vertical melting furnace and turn on the power. Heat, control the maximum vertical melting current to 5700A, keep warm and sinter for 20 minutes to obtain a tungsten porous body; take out and cool, add copper powder to the tungsten porous body, and place it in a push boat furnace, slowly push the boat under a high-purity hydrogen atmosphere, keep warm at 1300℃ for 120 minutes, and then freely process it according to the required size; finally, remove the copper by chemical method, that is, first clean the substrate, immerse the substrate in acid solution for acid corrosion treatment, take out and rinse, immerse and heat, take out and dehydrate and dry to obtain a porous tungsten substrate.

[0060] After testing, the total porosity of the porous tungsten matrix of the product is 13%, the average pore is 2.34μm, the pore uniformity is within the range of ±2μm, the closed porosity is <0.05%, the tungsten grain size is within the range of 4.3-10.5μm, the average tungsten grain is 7μm, the tungsten grain uniformity is within the range of ±3μm, and the skeleton tensile strength is 687MPa.

[0061] Example 2

[0062] Take tungsten powder with an average particle size of about 3.7μm, pass it through a 200-mesh sieve, and then evenly load the sieved tungsten powder into a Ф30mm soft rubber mold with a certain taper fixed by a steel sleeve, and cold isostatically press it at a pressure of 200MPa for 5 minutes to obtain a tungsten blank with a relatively regular appearance; place the tungsten blank in a push boat furnace, fill it with high-purity hydrogen until the air is exhausted, and sinter it at a low temperature of 1400℃ for 60min; then take it out and cool it, rough-process it to the required size, and then place it in a vertical melting furnace, turn on the power for heating, control the maximum current of the vertical melting to 4900A, and sinter it for 30min to obtain a tungsten porous body; take it out and cool it, add copper powder to the tungsten porous body, and place it in a push boat furnace, slowly push the boat in a high-purity hydrogen atmosphere, and keep it at 1300℃ for 120min, and then freely process it according to the required size; finally, remove the copper by chemical method to obtain a porous tungsten matrix.

[0063] After testing, the total porosity of the porous tungsten matrix of the product is 18%, the average pore size is 2.58μm, the pore uniformity is within the range of ±2μm, the closed porosity is <0.03%, the tungsten grain size is within the range of 3.5-8.4μm, the average tungsten grain size is 4.6μm, the tungsten grain uniformity is within the range of ±3μm, and the skeleton tensile strength is 536MPa.

[0064] Example 3

[0065] Take tungsten powder with an average particle size of about 3.7μm, pass it through a 200-mesh sieve, and then evenly load the sieved tungsten powder into a Ф30mm soft rubber mold with a certain taper fixed by a steel sleeve, and cold isostatically press it at a pressure of 200MPa for 5 minutes to obtain a tungsten blank with a relatively regular appearance; place the tungsten blank in a push boat furnace, fill it with high-purity hydrogen until the air is exhausted, and sinter it at a low temperature of 1400℃ for 60min; then take it out and cool it, rough-process it to the required size, and then place it in a vertical melting furnace, turn on the power for heating, control the maximum current of the vertical melting to 4300A, and sinter it for 30min to obtain a tungsten porous body; take it out and cool it, add copper powder to the tungsten porous body, and place it in a push boat furnace, slowly push the boat in a high-purity hydrogen atmosphere, and keep it at 1300℃ for 80min, and then freely process it according to the required size; finally, remove the copper by chemical method to obtain a porous tungsten matrix.

[0066] After testing, the total porosity of the porous tungsten matrix of the product is 22.8%, the average pore size is 2.8μm, the pore uniformity is within the range of ±2μm, the closed porosity is <0.03%, the tungsten grain size is within the range of 2.1-6.3μm, the average tungsten grain size is 3.1μm, the tungsten grain uniformity is within the range of ±3μm, and the skeleton tensile strength is 423MPa.

[0067] Example 4

[0068] Take tungsten powder with an average particle size of about 2.5μm, pass it through a 150-mesh sieve, and then evenly load the sieved tungsten powder into a Ф30mm soft rubber mold with a certain taper fixed by a steel sleeve, and cold isostatically press it at a pressure of 200MPa for 5 minutes to obtain a tungsten blank with a relatively regular appearance; place the tungsten blank in a push boat furnace, fill it with high-purity hydrogen until the air is exhausted, and sinter it at a low temperature of 1400℃ for 60min; then take it out and cool it, rough-process it to the required size, and then place it in a vertical melting furnace, turn on the power for heating, control the maximum current of the vertical melting to 4400A, and sinter it for 20min to obtain a tungsten porous body; take it out and cool it, add copper powder to the tungsten porous body, and place it in a push boat furnace, slowly push the boat in a high-purity hydrogen atmosphere, and keep it at 1350℃ for 50min, and then freely process it according to the required size; finally, remove the copper by chemical method to obtain a porous tungsten matrix.

[0069] After testing, the total porosity of the porous tungsten matrix of the product is 12.3%, the average pore size is 1.2μm, the pore uniformity is within the range of ±2μm, the closed porosity is <2.3%, the tungsten grain size is within the range of 7.3-13.2μm, the average tungsten grain size is 9.2μm, the tungsten grain uniformity is within the range of ±3μm, and the skeleton tensile strength is 653MPa.

[0070] Comparative Example 1

[0071] Take tungsten powder with an average particle size of about 3.7μm, pass it through a 200-mesh sieve, and then evenly load the sieved tungsten powder into a Ф30mm soft rubber mold with a certain taper fixed by a steel sleeve, and cold isostatically press it at a pressure of 200MPa for 5 minutes to obtain a tungsten blank with a relatively regular appearance; place the tungsten blank in a push boat furnace, fill it with high-purity hydrogen until the air is exhausted, and sinter it at a low temperature of 1400℃ for 60min; then take it out and cool it, rough-process it to the required size, and then transfer it to a medium-frequency induction furnace for radiation heating treatment to obtain a tungsten porous body; take it out and cool it, add copper powder to the tungsten porous body, and place it in a push boat furnace, slowly push the boat in a high-purity hydrogen atmosphere, and keep it at 1300℃ for 80min, and then freely process it according to the required size; finally, remove the copper by chemical method to obtain a porous tungsten material.

[0072] After testing, the total porosity of the porous tungsten matrix of the product is 22.8%, the pores and tungsten grains are obviously uneven, and there are both very fine pores and coarse pores.

[0073] Test example

[0074] Samples: Examples 1-4, Comparative Example 1

[0075] (1) Electron microscopy observation

[0076] like Figure 1 This is an electron microscope scanning image of the porous tungsten substrate prepared in Example 1, and the view magnification is 2.00KX, and the working distance WD is 9.5mm; Figure 2 This is an electron microscope scanning image of the porous tungsten material prepared in Comparative Example 1, and the view magnification is 2000X, and the working distance WD is 7.6mm. It can be seen from the figure that the pores of the porous tungsten matrix in Example 1 are uniform and dense, while the pores of the porous tungsten material in Comparative Example 1 are obviously uneven, which means that the preparation method of the present invention can solve the problem of uneven pores of the porous tungsten matrix in the prior art.

[0077] (2) Structure and performance testing

[0078] The samples of Examples 1 to 4 and Comparative Example 1 were respectively taken, and the porosity and other structural properties of the samples were observed and calculated using a scanning electron microscope. The skeleton tensile strength of the samples was measured according to the method specified in ISO 6892-1 Metallic Material Tensile Test Part 1. The results are summarized in Table 1 below:

[0079] Table 1 Material properties test results of different samples

[0080] Sample Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Total porosity (%) 13 18 22.8 12.3 22.8 Average pore size (mercury intrusion method) (μm) 2.34 2.58 2.80 1.20 2.9 Pore ​​size (μm) 1.3-5.2 1.2-4.6 1.2-4.5 0.4-2.6 0.3-12 Pore ​​uniformity (μm) ±2 ±2 ±2 ±2 ±6 Closed cell rate (%) 0.05 0.03 0.03 2.30 0.6 Average tungsten particle size (μm) 7 4.6 3.1 9.2 3.1 Tungsten particle size (μm) 4.3-10.5 3.5-8.4 2.1-6.3 7.3-13.2 0.5-13 Tungsten particle size uniformity (μm) ±3 ±3 ±3 ±3 ±7 Skeleton tensile strength (MPa) 687 536 423 653 236

[0081] As can be seen from Table 1 above, the pore size range of the samples of Examples 1 to 4 is significantly smaller than the pore size range of the samples in Comparative Example 1, and the pore uniformity is also significantly reduced, indicating that the pore size of the samples of Examples 1 to 4 is small and narrowly distributed, and the pore distribution is significantly more uniform; further, the tungsten particle size range and tungsten particle size uniformity range of the samples of Examples 1 to 4 are also significantly smaller than the tungsten particle size span of the samples of Comparative Example 1, which can be explained that the tungsten particle size of the samples of Examples 1 to 4 is more uniform and the distribution is more uniform. Therefore, the above experiments can illustrate that the preparation method of the porous tungsten matrix proposed in the present invention can effectively overcome the problems of uneven pores and uneven tungsten particle size of the porous tungsten matrix, and other properties remain comparable, so that it can be better applied to hot cathode materials.

[0082] The above description is only a preferred embodiment of the present invention and is 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 porous tungsten substrate for a hot cathode, characterized in that: The steps include: S1 prepares tungsten powder raw material; S2 is pressed into tungsten blank; S3 Hydrogen sintering: Place the tungsten blank in a hydrogen atmosphere at 900-1550℃ for sintering; S4 vertical melting sintering: electricity is applied to both ends of the tungsten blank to generate heat and sinter to form a tungsten porous body; S5 copper infiltration: in a hydrogen atmosphere, copper source is added to the tungsten porous body in batches, and heated and sintered in stages to obtain a copper-containing tungsten alloy matrix; S6 is machined into cathode substrate; S7: copper removal treatment to obtain the porous tungsten substrate for the hot cathode.

2. The method for preparing a porous tungsten substrate for a hot cathode according to claim 1, characterized in that: In step S4, the current is 2000-5700A, and the sintering time is 10-50min.

3. The method for preparing a porous tungsten substrate for a hot cathode according to claim 1, characterized in that: In step S5, sintering is performed in a plurality of high temperature zones including 1320-1350°C, 1380-1400°C, 1440-1460°C, and 1490-1500°C.

4. The method for preparing a porous tungsten substrate for a hot cathode according to claim 3, characterized in that: When entering each high temperature zone, an equal amount of copper source is added. The copper source can be selected from one or more of copper powder, copper powder compact, and copper foil, and the total amount of copper source added is 1-3wt% of the porous tungsten substrate.

5. The method for preparing a porous tungsten substrate for a hot cathode according to claim 3, characterized in that: A push boat furnace is used for copper infiltration treatment, and the push boat speed is 2.5-50mm / min, and the total sintering time in the high temperature zone is 10-120min.

6. The method for preparing a porous tungsten substrate for a hot cathode according to claim 1, characterized in that: In step S6, the machining accuracy of the tungsten copper end face of the cathode emission surface should be lower than 0.6 μm, and the machining accuracy of the circumferential surface and end face of the non-emitting surface should be lower than 1.0 μm.

7. The method for preparing a porous tungsten substrate for a hot cathode according to claim 1, characterized in that: In step S7, a chemical method is used to remove copper, including the following steps: cleaning the cathode substrate, immersing it in an acid solution, taking it out and rinsing it; then immersing it in water and heating it, taking it out and dehydrating and drying it, so as to obtain the porous tungsten substrate for the hot cathode.

8. The method for preparing a porous tungsten substrate for a hot cathode according to claim 1, characterized in that: In step S7, vacuum method is used to remove copper, and the process conditions are: vacuum degree is less than 5×10 -3 pa, the processing temperature is 1550-1650℃, and the processing time is 50-100min.

9. The method for preparing a porous tungsten substrate for a hot cathode according to claim 1, characterized in that: In steps S1 and S2, the particle size of the tungsten powder raw material is 0.5-7 μm, and it is pressed into a tungsten blank at 50-240 MPa for 1-20 min.

10. A porous tungsten substrate for hot cathode prepared by the preparation method according to any one of claims 1 to 9, characterized in that: The porous tungsten substrate for hot cathode has a total porosity of 4-50%, an average pore size of 0.5-5 μm, a pore uniformity of ±2 μm, a tungsten particle size of 1-10 μm, and a tungsten grain uniformity of ±3 μm.

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