A potassium-doped tungsten nanocrystal material and its preparation method

Abstract

The present invention belongs to the technical field of powder metallurgy, and specifically discloses a potassium-doped nanocrystalline tungsten material and a preparation method thereof, comprising the following steps: putting ammonium paratungstate into a spray granulation device, then adding an aqueous solution of a dopant, mixing, and drying to obtain potassium-doped APT; performing pre-reduction through a converter to obtain ammonium tungsten bronze; performing one-step reduction of the ammonium tungsten bronze through a fifteen-tube furnace to obtain potassium-doped D powder; performing two-step reduction of the ammonium tungsten bronze through a fifteen-tube furnace to obtain potassium-doped B powder; then cleaning with hydrochloric acid and hydrofluoric acid, followed by washing with water, removing water after filtration, drying, crushing and sieving to obtain washed potassium-doped D powder and washed potassium-doped B powder; mixing the two to obtain a mixed powder; then performing isostatic pressing treatment to obtain a tungsten green body; and sintering the tungsten green body in an intermediate frequency induction sintering furnace to obtain the product. The potassium-doped nanocrystalline tungsten material of the present invention has the advantages of high strength, good plasticity, stable high-temperature performance, etc., and the products processed therefrom have a long service life.

Description

Technical Field

[0001] The present invention belongs to the technical field of powder metallurgy, and particularly relates to a nanocrystalline potassium-doped tungsten material and a preparation method thereof. Background Art

[0002] Tungsten is a metal with excellent physical properties. It is silver-white in color and has a steel-like appearance. It has the highest melting point among all metals, and has low saturated vapor pressure, low thermal expansion coefficient, high thermal strength, and good thermal and electrical conductivity. These properties enable tungsten to have a wide range of applications under special conditions such as high temperature and high pressure, wear and corrosion resistance, and strong radiation. However, pure tungsten materials have limitations in some applications, mainly because their recrystallization temperature and ductile-brittle transition temperature are relatively high, and their plasticity is poor, which makes processing relatively difficult.

[0003] To overcome these limitations, doping and dispersion strengthening of tungsten-based materials is an effective method to improve performance. For example, on the basis of traditional powder metallurgy and forming processes, doping with potassium, oxides, carbides or nitrides can effectively improve the processing performance of tungsten materials and enhance plasticity and high-temperature strength. Among them, potassium-doped tungsten materials, as important materials for various heating components, high-precision medical and detection tungsten devices at present, can improve the performance of materials by doping trace amounts of potassium in tungsten, showing densification and thermal conductivity compatible with pure tungsten. The doping of potassium leads to a significant increase in the hardness of the sintered state and the deformed state. Although potassium-doped tungsten materials exhibit many excellent properties, they still face some challenges in practical applications. For example, the existing potassium-doped tungsten materials have coarse crystallization and low potassium content, which may cause cracking in rolling processing, reduce the yield rate, and affect the service life of the final heating components. Therefore, achieving fine and uniformly distributed dispersed phases is the key to obtaining high-performance tungsten materials.

[0004] Chinese Patent CN105903977A discloses a method for producing potassium-doped tungsten powder and a method for producing tungsten bars using the tungsten powder. The method includes the following steps: doping silicon, aluminum, and potassium components into blue tungsten powder, and performing one or two hydrogen reduction processes to produce doped tungsten powder, then performing alkali washing and acid washing on the tungsten powder; pressing the treated doped tungsten powder into blank bars under high pressure; placing the pressed blank bars in a molybdenum boat and performing high-temperature pre-sintering under hydrogen protection: placing the pre-sintered blank bars in a vertical furnace, and heating up by applying electricity according to a program under hydrogen protection until the tungsten bars are densely metallized, thus obtaining the tungsten bars; this patent uses alkali washing and acid washing instead of hydrofluoric acid washing, avoiding the use of hydrofluoric acid, having little environmental pollution, good powder dispersion, moderate potassium content in the finished tungsten bars, and good deep-processing performance; Chinese Patent CN108274017A discloses a potassium-doped tungsten plate and a preparation method thereof. The method includes the following steps: mixing potassium silicate and aluminum nitrate solutions into a container filled with tungsten oxide according to a weight ratio, and obtaining doped tungsten oxide after mixing and drying; performing reduction treatment on the obtained doped tungsten oxide to obtain doped tungsten powder; performing acid washing on the obtained doped tungsten powder until the potassium content in the tungsten powder is 80-160 ppm, the silicon content is 154-281 ppm, and the aluminum content is not higher than 49 ppm; drying, pressing, and sintering the obtained acid-washed tungsten powder, thus obtaining the potassium-doped tungsten plate; the potassium-doped tungsten plate prepared by this patent has good high-temperature anti-deformation performance and normal-temperature anti-bending performance after rolling, and the performance of the products prepared by it is superior to those of products made of pure tungsten plates; however, the potassium-doped tungsten materials disclosed in the above patents still have some deficiencies, and their comprehensive performance such as plasticity, processability, and service life still needs to be improved. Summary of the Invention

[0005] Aiming at the deficiencies existing in the above-mentioned prior art, the present invention aims to provide a nanocrystalline potassium-doped tungsten material. Using ammonium tungstate salt as a raw material, adding a dopant by spraying, and through optimizing the reduction, mixing, and sintering processes, efficiently preparing a nanocrystalline potassium-doped tungsten material, which has the advantages of high strength, good plasticity, and stable high-temperature performance, and the products processed by it have a long service life. The present invention also provides a preparation method for the above-mentioned nanocrystalline potassium-doped tungsten material.

[0006] A preparation method for a nanocrystalline potassium-doped tungsten material includes the following steps:

[0007] Step I: Put 300-500 parts by weight of ammonium paratungstate (APT) into a spray granulation device. Add an appropriate amount of potassium hydroxide, aluminum nitrate, and potassium silicate to form a dopant aqueous solution in 5-20 parts by weight of water. Add the dopant aqueous solution to ammonium paratungstate by spraying, stir and mix, perform steam drying, and maintain the vacuum degree in the device at -0.02 to -0.05 Mpa to obtain potassium-doped APT. Perform pre-reduction on the potassium-doped APT in a converter at 400-500 °C under a hydrogen atmosphere until the reduction is completed to obtain ammonium tungsten bronze.

[0008] Step II: Prepare potassium-doped D powder with a particle size of 3.2 - 3.5 μm from 50 - 150 parts by weight of the ammonium tungsten bronze obtained in Step I through a fifteen-tube furnace at 800 - 900 °C;

[0009] Step III: Prepare tungsten oxide with a particle size of 2.5 - 2.8 μm from 50 - 150 parts by weight of the ammonium tungsten bronze obtained in Step I through a fifteen-tube furnace at 600 - 700 °C, and then prepare potassium-doped B powder with a particle size of 2.4 - 2.7 μm through a fifteen-tube furnace at 800 - 900 °C;

[0010] Step IV: Wash the potassium-doped D powder obtained in Step II and the potassium-doped B powder obtained in Step III successively with hydrochloric acid with a mass percentage concentration of 3 - 6% and hydrofluoric acid with a mass percentage concentration of 3 - 6% to remove ineffective potassium, silicon, aluminum and other impurities, then wash with water 1 - 3 times, filter, remove water with absolute ethanol, vacuum dry, crush and screen to obtain the washed potassium-doped D powder and the washed potassium-doped B powder respectively;

[0011] Step V: Mix 40 - 60 parts by weight of the washed potassium-doped D powder obtained in Step IV and 40 - 60 parts by weight of the washed potassium-doped B powder obtained in Step IV to obtain a mixed powder, and then perform cold isostatic pressing at 130 - 160 MPa for 150 - 200 s to obtain a tungsten green compact;

[0012] Step VI: Put the tungsten green compact obtained in Step V into a medium-frequency induction sintering furnace, introduce hydrogen simultaneously from above and below, and sinter at 2000 - 2100 °C for 5 - 10 h to obtain the nanocrystalline potassium-doped tungsten material.

[0013] Preferably, in Step I, the doping agent aqueous solution contains, calculated by equivalent, potassium oxide with a weight percentage of 0.2 - 0.4% relative to the weight of ammonium paratungstate, aluminum oxide with a weight percentage of 0.001 - 0.003%, and silicon dioxide with a weight percentage of 0.3 - 0.5%.

[0014] Preferably, the washed potassium-doped D powder obtained in Step IV is the washed potassium-doped D powder with a particle size of 3.2 - 3.5 μm, a potassium content of 60 - 80 ppm, and an oxygen content of less than 100 ppm.

[0015] Preferably, the washed potassium-doped B powder obtained in Step IV is the washed potassium-doped B powder with a particle size of 2.4 - 2.7 μm, a potassium content of 160 - 200 ppm, and an oxygen content of less than 100 ppm.

[0016] The present invention applies nanotechnology to the manufacture of potassium-doped tungsten materials. Through spray granulation of APT and dopants, and the combination of potassium-doped B powder and potassium-doped D powder, medium-frequency low-temperature high-efficiency sintering is achieved. The prepared potassium-doped tungsten material has a moderate effective potassium content. Potassium is not only evenly distributed in the tungsten powder, but also has a better binding effect. The crystal structure reaches the nanoscale, the rolling yield is high, and the high-temperature resistance performance is better. It shows better high-temperature stability performance in heating components, high-precision medical and detection tungsten devices, and has the advantages of long service life and high processing efficiency.

[0017] Preferably, a synergist is also added in step V and mixed with the washed potassium-doped D powder and the washed potassium-doped B powder together.

[0018] Preferably, the addition amount of the synergist is 1-5 parts by weight; more preferably, the addition amount of the synergist is 2-3 parts by weight.

[0019] Preferably, the synergist is composed of titanium nitride@short-cut tungsten fibers and hafnium diboride in a weight ratio of 10:1-5.

[0020] Preferably, the preparation method of the titanium nitride@short-cut tungsten fibers is as follows:

[0021] The short-cut tungsten fibers are added to absolute ethanol according to a solid-liquid ratio of 1 g:5-10 mL and ultrasonically treated for 30-50 min, filtered, and vacuum dried to obtain pretreated short-cut tungsten fibers; 10-20 parts by weight of the pretreated short-cut tungsten fibers are placed on a substrate disk, and a precursor is introduced for chemical vapor deposition. The deposition time is 3-5 h, and the deposition temperature is 500-700 °C to obtain titanium nitride@short-cut tungsten fibers.

[0022] Preferably, the short-cut tungsten fibers are prepared by cutting tungsten fibers with a diameter of 100-200 μm and a tensile strength of 2.5-4 GPa to 1-3 mm.

[0023] Preferably, the precursor is composed of gaseous titanium tetrachloride and ammonia gas, and the flow ratio of gaseous titanium tetrachloride to ammonia gas is 1:3-6.

[0024] The present invention uses titanium nitride@short-cut tungsten fibers and hafnium diboride as synergists. The self-made titanium nitride@short-cut tungsten fibers can effectively improve the toughness of tungsten-based materials. The titanium nitride layer deposited on the fiber surface can make the interface between the fiber and the tungsten matrix present a weak bonding state. When an external force acts and the crack of the tungsten material extends to the vicinity of the fiber, this weak bonding fiber / tungsten matrix interface is conducive to the generation of energy-consuming mechanisms such as crack deflection / bridging, fiber debonding / pulling out, etc., thereby increasing the path of energy expansion and further improving the toughness of the nanocrystalline potassium-doped tungsten material. At the same time, titanium nitride itself has high strength, high hardness, wear resistance, high-temperature chemical stability and high thermal conductivity, which can increase the melting point of the tungsten material, inhibit the growth of grains, make the tungsten material have high density and small grain size, and improve the comprehensive performance and service life.

[0025] Hafnium diboride has the characteristics of high hardness, high melting point, good electrical and thermal conductivity. During the sintering process, hafnium diboride can be dispersed into the tungsten material to improve its intergranular toughness, act as a pinning point for dislocations, inhibit lattice slip, and improve the strength and high-temperature wear resistance of the tungsten material. At the same time, hafnium diboride can play an adhesive role, promote the combination between the titanium nitride@short-cut tungsten fiber potassium-doped powders, and is conducive to the formation of a uniform and dense tungsten material.

[0026] Advantages of the present invention:

[0027] The present invention uses ammonium tungstate salt as a raw material, adds a dopant in a spray manner, and efficiently prepares a nanocrystalline potassium-doped tungsten material by optimizing the reduction, mixing and sintering processes, which has the advantages of high strength, good plasticity, and stable high-temperature performance. The average number of grains at the interface is not less than 10,000 / mm 2 , the average grain size is less than 10 μm, the product has good ductility and a high rolling yield, and can improve the service life of products such as heating components, high-precision medical and detection tungsten devices. Specific embodiments

[0028] The above-mentioned invention content of the present invention will be further described in detail below in conjunction with specific embodiments, but it should not be understood that the scope of the above-mentioned subject matter of the present invention is limited to the following embodiments.

[0029] Example 1

[0030] A preparation method of a nanocrystalline potassium-doped tungsten material, comprising the following steps:

[0031] Step Ⅰ: Put 400 parts by weight of ammonium paratungstate (APT) into a spray granulation device. Add an appropriate amount of potassium hydroxide, aluminum nitrate, and potassium silicate to 10 parts by weight of deionized water to form a doping agent aqueous solution. The doping agent aqueous solution contains, based on equivalent calculation, potassium oxide with a weight percentage of 0.22% relative to the weight of ammonium paratungstate, aluminum oxide of 0.0015%, and silicon dioxide of 0.36%. Add the doping agent aqueous solution to ammonium paratungstate by spraying, stir and mix, and perform steam drying while maintaining the vacuum degree in the device at -0.02 to -0.05 Mpa to obtain potassium-doped APT. Conduct pre-reduction in a converter at 440 °C under a hydrogen atmosphere until the reduction is complete to obtain ammonium tungsten bronze;

[0032] Step Ⅱ: Use a fifteen-tube furnace to produce potassium-doped D powder with a particle size of 3.5 μm from 100 parts by weight of the ammonium tungsten bronze obtained in Step Ⅰ at 860 °C;

[0033] Step Ⅲ: Use a fifteen-tube furnace to produce tungsten oxide with a particle size of 2.5 μm from 100 parts by weight of the ammonium tungsten bronze obtained in Step Ⅰ at 680 °C, and then use a fifteen-tube furnace to produce potassium-doped B powder with a particle size of 2.4 μm at 860 °C;

[0034] Step Ⅳ: Wash the potassium-doped D powder obtained in Step Ⅱ and the potassium-doped B powder obtained in Step Ⅲ successively with hydrochloric acid with a mass percentage concentration of 5% and hydrofluoric acid with a mass percentage concentration of 5% to remove ineffective potassium, silicon, aluminum, and other impurities, then wash with deionized water twice, filter, use anhydrous ethanol for water removal, perform vacuum drying, and crush and screen to obtain washed potassium-doped D powder with a particle size of 3.5 μm, a potassium content of 69 ppm, and an oxygen content lower than 100 ppm and washed potassium-doped B powder with a particle size of 2.4 μm, a potassium content of 184 ppm, and an oxygen content lower than 100 ppm;

[0035] Step Ⅴ: Mix 50 parts by weight of the washed potassium-doped D powder obtained in Step Ⅳ and 50 parts by weight of the washed potassium-doped B powder to obtain a mixed powder, and then perform cold isostatic pressing at 150 MPa for 180 s to obtain a tungsten green body;

[0036] Step Ⅵ: Put the tungsten green body obtained in Step Ⅴ into a medium-frequency induction sintering furnace, introduce hydrogen simultaneously from above and below, and sinter at 2055 °C for 7 h to obtain the nanocrystalline potassium-doped tungsten material with an oxygen content of 11 ppm, a potassium content of 98 ppm, an aluminum content of 18 ppm, a silicon content of 7 ppm, and a density of 18.5 g / cm 3 . Use SEM to select three 10 μm × 10 μm regions for observation and perform mathematical statistics at 5000 times magnification. The average number of grains is 15000 grains / mm 2 , and the average grain size is 9 μm. After rolling the above-mentioned nanocrystalline potassium-doped tungsten material to 5 mm multiple times and making it into a mosquito coil-shaped heating component, the service life is increased by 2.0 times compared to pure tungsten.

[0037] Comparative Example 1

[0038] A preparation method of potassium-doped tungsten nanocrystal material, comprising the following steps:

[0039] Step I: Put 400 parts by weight of ammonium paratungstate (APT) into a spray granulation device. Add an appropriate amount of potassium hydroxide, aluminum nitrate, and potassium silicate to 10 parts by weight of deionized water to form a doping agent aqueous solution. The doping agent aqueous solution contains 0.22% potassium oxide, 0.0015% aluminum oxide, and 0.36% silicon dioxide by weight percentage relative to ammonium paratungstate according to equivalent calculation. Add the doping agent aqueous solution to ammonium paratungstate by spraying, stir and mix, dry with steam, and maintain the vacuum degree in the device at -0.02 to -0.05 Mpa to obtain potassium-doped APT. Carry out pre-reduction in a converter at 440°C under a hydrogen atmosphere until the reduction is complete to obtain ammonium tungsten bronze;

[0040] Step II: Obtain potassium-doped D powder with a particle size of 3.5 μm by using a fifteen-tube furnace to process 100 parts by weight of the ammonium tungsten bronze obtained in Step I at 860°C;

[0041] Step III: Wash the potassium-doped D powder obtained in Step II successively with hydrochloric acid with a mass percentage concentration of 5% and hydrofluoric acid with a mass percentage concentration of 5% to remove ineffective potassium, silicon, aluminum, and other impurities, then wash with deionized water twice, filter, use absolute ethanol for water removal, vacuum dry, crush and screen to obtain washed potassium-doped D powder with a particle size of 3.5 μm, a potassium content of 69 ppm, and an oxygen content lower than 100 ppm;

[0042] Step IV: Subject the washed potassium-doped D powder obtained in Step III to cold isostatic pressing at 150 MPa for 180 s to obtain a tungsten green body;

[0043] Step V: Put the tungsten green body obtained in Step IV into a medium-frequency induction sintering furnace, introduce hydrogen simultaneously from above and below, sinter at 2055°C for 7 h to obtain the potassium-doped tungsten nanocrystal material, with a density of 17.9 g / cm 3 . After the above potassium-doped tungsten nanocrystal material is rolled to 5 mm for multiple times and made into a mosquito coil-shaped heating element, its service life is increased by 1.12 times compared with that of pure tungsten.

[0044] Comparative Example 2

[0045] A preparation method of potassium-doped tungsten nanocrystal material, comprising the following steps:

[0046] Step Ⅰ: Put 400 parts by weight of ammonium paratungstate (APT) into a spray granulation device. Add an appropriate amount of potassium hydroxide, aluminum nitrate, and potassium silicate to 10 parts by weight of deionized water to form a dopant aqueous solution. The dopant aqueous solution contains, based on equivalent calculation, potassium oxide with a weight percentage of 0.22% relative to the weight of ammonium paratungstate, aluminum oxide with 0.0015%, and silicon dioxide with 0.36%. Add the dopant aqueous solution to ammonium paratungstate by spraying, stir and mix, then conduct steam drying, and maintain the vacuum degree in the device at -0.02 to -0.05 Mpa to obtain potassium-doped APT. Conduct pre-reduction in a converter at 440°C under a hydrogen atmosphere until the reduction is complete to obtain ammonium tungsten bronze;

[0047] Step Ⅱ: Obtain tungsten oxide with a particle size of 2.5 μm by using the potassium-doped ammonium tungsten bronze obtained in Step Ⅰ through a fifteen-tube furnace at 680°C, and then obtain potassium-doped B powder with a particle size of 2.4 μm through the fifteen-tube furnace at 860°C;

[0048] Step Ⅲ: Wash the potassium-doped B powder obtained in Step Ⅱ successively with hydrochloric acid with a mass percentage concentration of 5% and hydrofluoric acid with 5% to remove ineffective potassium, silicon, aluminum, and other impurities, then wash with deionized water twice, conduct water removal with absolute ethanol after filtration, conduct vacuum drying, crush and screen to obtain washed potassium-doped B powder with a particle size of 2.4 μm, a potassium content of 184 ppm, and an oxygen content lower than 100 ppm;

[0049] Step Ⅳ: Subject the washed potassium-doped B powder obtained in Step Ⅲ to cold isostatic pressing at 150 MPa for 180 s to obtain a tungsten green body;

[0050] Step Ⅴ: Put the tungsten green body obtained in Step Ⅳ into an intermediate frequency induction sintering furnace, introduce hydrogen simultaneously from above and below, and sinter at 2055°C for 7 h to obtain the nanocrystalline potassium-doped tungsten material. After subjecting the above nanocrystalline potassium-doped tungsten material to multiple rollings to 5 mm and making it into a mosquito coil-shaped heating component, the service life is increased by 1.37 times compared with pure tungsten.

[0051] Example 2

[0052] A preparation method of a nanocrystalline potassium-doped tungsten material, comprising the following steps:

[0053] Step Ⅰ: Put 400 parts by weight of ammonium paratungstate (APT) into a spray granulation device. Add an appropriate amount of potassium hydroxide, aluminum nitrate, and potassium silicate to 10 parts by weight of deionized water to form a doping agent aqueous solution. The doping agent aqueous solution contains, calculated by equivalent, 0.22% potassium oxide, 0.0015% aluminum oxide, and 0.36% silicon dioxide based on the weight percentage of ammonium paratungstate. Add the doping agent aqueous solution to ammonium paratungstate by spraying, stir and mix, and perform steam drying while maintaining the vacuum degree in the device at -0.02 to -0.05 Mpa to obtain potassium-doped APT. Carry out pre-reduction in a converter at 440 °C under a hydrogen atmosphere until the reduction is complete to obtain ammonium tungsten bronze;

[0054] Step Ⅱ: Use a fifteen-tube furnace to produce potassium-doped D powder with a particle size of 3.5 μm from 100 parts by weight of the ammonium tungsten bronze obtained in Step Ⅰ at 860 °C;

[0055] Step Ⅲ: Use a fifteen-tube furnace to produce tungsten oxide with a particle size of 2.5 μm from 100 parts by weight of the ammonium tungsten bronze obtained in Step Ⅰ at 680 °C, and then use a fifteen-tube furnace to produce potassium-doped B powder with a particle size of 2.4 μm at 860 °C;

[0056] Step Ⅳ: Wash the potassium-doped D powder obtained in Step Ⅱ and the potassium-doped B powder obtained in Step Ⅲ successively with hydrochloric acid with a mass percentage concentration of 5% and hydrofluoric acid with a mass percentage concentration of 5% to remove ineffective potassium, silicon, aluminum, and other impurities, then wash with deionized water twice, filter, use absolute ethanol for water removal, vacuum dry, crush and screen to obtain washed potassium-doped D powder with a particle size of 3.5 μm, a potassium content of 69 ppm, and an oxygen content lower than 100 ppm and washed potassium-doped B powder with a particle size of 2.4 μm, a potassium content of 184 ppm, and an oxygen content lower than 100 ppm;

[0057] Step Ⅴ: Mix 50 parts by weight of the washed potassium-doped D powder obtained in Step Ⅳ, 50 parts by weight of the washed potassium-doped B powder, and 2.5 parts by weight of a synergist to obtain a mixed powder, and then perform cold isostatic pressing at 150 MPa for 180 s to obtain a tungsten green compact; The synergist is composed of titanium nitride@short-cut tungsten fibers and hafnium diboride (CAS: 12007-23-7) in a weight ratio of 10:3;

[0058] The preparation method of the titanium nitride@short-cut tungsten fibers is as follows:

[0059] The chopped tungsten fibers are added to absolute ethanol at a solid-liquid ratio of 1 g:7 mL and ultrasonically treated for 45 min, filtered, and vacuum dried to obtain pretreated chopped tungsten fibers; 15 parts by weight of the pretreated chopped tungsten fibers are placed on a base disk, and a precursor is introduced for chemical vapor deposition. The deposition time is 4.5 h, and the deposition temperature is 600 °C. The precursor consists of gaseous titanium tetrachloride and ammonia, and the flow rate ratio of gaseous titanium tetrachloride (CAS: 7550-45-0) to ammonia is 1:5 to obtain titanium nitride@chopped tungsten fibers. The chopped tungsten fibers are obtained by cutting tungsten fibers with a diameter of 100 μm and a tensile strength of 3 GPa to 1.5 mm;

[0060] Step VI: Put the tungsten green compact obtained in Step V into an intermediate frequency induction sintering furnace, introduce hydrogen simultaneously from above and below, and sinter at 2055 °C for 7 h to obtain the nanocrystalline potassium-doped tungsten material.

[0061] Example 3

[0062] A preparation method of a nanocrystalline potassium-doped tungsten material, comprising the following steps:

[0063] Step I: Put 400 parts by weight of ammonium paratungstate (APT) into a spray granulation device. An aqueous doping agent solution composed of appropriate amounts of potassium hydroxide, aluminum nitrate, and potassium silicate is added to 10 parts by weight of deionized water. The aqueous doping agent solution contains 0.22% potassium oxide, 0.0015% aluminum oxide, and 0.36% silicon dioxide by weight percentage relative to ammonium paratungstate according to equivalent calculation. The aqueous doping agent solution is added to ammonium paratungstate by spraying, stirred and mixed, steam dried, and the vacuum degree in the device is maintained at -0.02 to -0.05 Mpa to obtain potassium-doped APT, and pre-reduced in a converter at 440 °C under a hydrogen atmosphere until the reduction is complete to obtain ammonium tungsten bronze;

[0064] Step II: 100 parts by weight of the ammonium tungsten bronze obtained in Step I are used to produce potassium-doped D powder with a particle size of 3.5 μm in a fifteen-tube furnace at 860 °C;

[0065] Step III: 100 parts by weight of the ammonium tungsten bronze obtained in Step I are used to produce tungsten oxide with a particle size of 2.5 μm in a fifteen-tube furnace at 680 °C, and then potassium-doped B powder with a particle size of 2.4 μm is produced in a fifteen-tube furnace at 860 °C;

[0066] Step Ⅳ: The potassium-doped D powder obtained in Step Ⅱ and the potassium-doped B powder obtained in Step Ⅲ are successively washed with hydrochloric acid with a mass percentage concentration of 5% and hydrofluoric acid with a mass percentage concentration of 5% to remove ineffective potassium, silicon, aluminum and other impurities, then washed twice with deionized water, filtered, dehydrated with absolute ethanol, vacuum dried, crushed and sieved to obtain the washed potassium-doped D powder with a particle size of 3.5 μm, a potassium content of 69 ppm and an oxygen content of less than 100 ppm, and the washed potassium-doped B powder with a particle size of 2.4 μm, a potassium content of 184 ppm and an oxygen content of less than 100 ppm;

[0067] Step Ⅴ: 50 parts by weight of the washed potassium-doped D powder obtained in Step Ⅳ, 50 parts by weight of the washed potassium-doped B powder obtained in Step Ⅳ and 2.5 parts by weight of a synergist are mixed to obtain a mixed powder, and then cold isostatically pressed at 150 MPa for 180 s to obtain a tungsten green compact; the synergist is titanium nitride@short tungsten fibers;

[0068] The preparation method of the titanium nitride@short tungsten fibers is as follows:

[0069] The short tungsten fibers are added to absolute ethanol according to a solid-liquid ratio of 1 g:7 mL and ultrasonically treated for 45 min, filtered, and vacuum dried to obtain pretreated short tungsten fibers; 15 parts by weight of the pretreated short tungsten fibers are placed on a substrate disk, and a precursor is introduced for chemical vapor deposition. The deposition time is 4.5 h and the deposition temperature is 600 °C. The precursor consists of gaseous titanium tetrachloride and ammonia, and the flow ratio of gaseous titanium tetrachloride (CAS: 7550-45-0) to ammonia is 1:5 to obtain titanium nitride@short tungsten fibers. The short tungsten fibers are obtained by cutting tungsten fibers with a diameter of 100 μm and a tensile strength of 3 GPa to 1.5 mm;

[0070] Step Ⅵ: The tungsten green compact obtained in Step Ⅴ is put into an intermediate frequency induction sintering furnace, and hydrogen is introduced simultaneously from above and below, and sintered at 2055 °C for 7 h to obtain the nanocrystalline potassium-doped tungsten material.

[0071] Example 4

[0072] A preparation method of a nanocrystalline potassium-doped tungsten material, comprising the following steps:

[0073] Step Ⅰ: Put 400 parts by weight of ammonium paratungstate (APT) into a spray granulation device. Add an appropriate amount of potassium hydroxide, aluminum nitrate, and potassium silicate to 10 parts by weight of deionized water to form a doping agent aqueous solution. The doping agent aqueous solution contains 0.22% potassium oxide, 0.0015% aluminum oxide, and 0.36% silicon dioxide by weight percentage relative to ammonium paratungstate according to equivalent calculation. Add the doping agent aqueous solution to ammonium paratungstate by spraying, stir and mix, dry with steam, and maintain the vacuum degree in the device at -0.02 to -0.05 Mpa to obtain potassium-doped APT. Carry out pre-reduction in a converter at 440 °C under a hydrogen atmosphere until the reduction is complete to obtain ammonium tungsten bronze;

[0074] Step Ⅱ: Put 100 parts by weight of the ammonium tungsten bronze obtained in Step Ⅰ into a fifteen-tube furnace to prepare potassium-doped D powder with a particle size of 3.5 μm at 860 °C;

[0075] Step Ⅲ: Put 100 parts by weight of the ammonium tungsten bronze obtained in Step Ⅰ into a fifteen-tube furnace to prepare tungsten oxide with a particle size of 2.5 μm at 680 °C, and then prepare potassium-doped B powder with a particle size of 2.4 μm at 860 °C;

[0076] Step Ⅳ: Wash the potassium-doped D powder obtained in Step Ⅱ and the potassium-doped B powder obtained in Step Ⅲ successively with hydrochloric acid with a mass percentage concentration of 5% and hydrofluoric acid with a mass percentage concentration of 5% to remove ineffective potassium, silicon, aluminum, and other impurities, then wash with deionized water twice, filter, use anhydrous ethanol for water removal, vacuum dry, crush and sieve to obtain washed potassium-doped D powder with a particle size of 3.5 μm, a potassium content of 69 ppm, and an oxygen content lower than 100 ppm and washed potassium-doped B powder with a particle size of 2.4 μm, a potassium content of 184 ppm, and an oxygen content lower than 100 ppm;

[0077] Step Ⅴ: Mix 50 parts by weight of the washed potassium-doped D powder obtained in Step Ⅳ, 50 parts by weight of the washed potassium-doped B powder, and 2.5 parts by weight of a synergist to obtain a mixed powder, and then carry out cold isostatic pressing at 150 MPa for 180 s to obtain a tungsten green body; the synergist is hafnium diboride;

[0078] Step Ⅵ: Put the tungsten green body obtained in Step Ⅴ into an intermediate frequency induction sintering furnace, introduce hydrogen simultaneously from top and bottom, and sinter at 2055 °C for 7 h to obtain the nanocrystalline potassium-doped tungsten material.

[0079] Test Example 1

[0080] Refer to GB / T 228.2-2015 to measure the fracture elongation of the tensile specimens of the nanocrystalline potassium-doped tungsten materials in each example at different temperatures. The test temperature corresponding to when the fracture elongation exceeds 5% is the tensile ductile-brittle transition temperature. And measure the ultimate tensile strength of the nanocrystalline potassium-doped tungsten materials at 800 °C. There are 5 parallel groups in each example, and the average value is taken.

[0081] Table 1. Test Results of Transition Temperature and Tensile Strength

[0082]

[0083]

[0084] Test Example 2

[0085] The nanocrystalline potassium-doped tungsten materials in the above examples were processed into wire materials with a diameter of 200 μm, cut for 300 min under the condition of a current of 10.5 A. After the test, the remaining diameter of the wire materials was measured. There were 5 parallel groups in each example, and the average value was taken. The larger the remaining diameter after the end, the better the high-temperature resistance and anti-wear performance of the alloy.

[0086] Table 2. Test Results of High-Temperature Anti-Wear Performance

[0087] Initial diameter / μm Remaining diameter / μm Example 1 200 187 Example 2 200 198 Example 3 200 194 Example 4 200 191

[0088] It can be seen from the above results that the nanocrystalline potassium-doped tungsten material prepared by the present invention has good toughness, strength and high-temperature stability. Especially in Example 2, titanium nitride@short-cut tungsten fiber and hafnium diboride are used as synergists at the same time. The self-made titanium nitride@short-cut tungsten fiber can effectively improve the toughness of the tungsten-based material. The titanium nitride layer deposited on the fiber surface can make the interface between the fiber and the tungsten matrix present a weak bonding state. When an external force acts to make the crack of the tungsten material extend to the vicinity of the fiber, this weak bonding fiber / tungsten matrix interface is conducive to the generation of energy-consuming mechanisms such as crack deflection / bridging, fiber debonding / pulling out, etc., so as to increase the path of energy expansion and further improve the toughness of the nanocrystalline potassium-doped tungsten material; at the same time, titanium nitride itself has high strength, high hardness, wear resistance, high-temperature chemical stability and high thermal conductivity, which can increase the melting point of the tungsten material, inhibit the growth of grains, make the tungsten material have high density and small grain size, and improve the comprehensive performance and service life. Hafnium diboride has the characteristics of high hardness, high melting point, good electrical and thermal conductivity. During the sintering process, hafnium diboride can be dispersed into the tungsten material to improve its grain boundary toughness, act as a pinning point for dislocations, inhibit lattice slip, and improve the strength and high-temperature anti-wear performance of the tungsten material; at the same time, hafnium diboride can play a bonding role to promote the combination between the titanium nitride@short-cut tungsten fiber potassium-doped powder, which is beneficial to the formation of a uniform and dense tungsten material.

[0089] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations according to the concept of the present invention without creative work. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field of the present invention based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art should be within the protection scope determined by the claims.

Claims

1. A method for preparing a nanocrystalline potassium-doped tungsten material, characterized in that: The following steps are involved: Step I: put ammonium paratungstate into a spray granulation device, add potassium hydroxide, aluminum nitrate and potassium silicate into water to form a doping agent aqueous solution, add the doping agent aqueous solution into the ammonium paratungstate by spraying, stir and mix, steam dry to obtain potassium-doped APT, and pre-reduce it in a converter at 400-500°C in a hydrogen atmosphere to obtain ammonium tungsten bronze; Step II, the ammonium tungsten bronze obtained in step I is passed through a fifteen-tube furnace at 800-900° C. to prepare potassium-doped D powder with a particle size of 3.2-3.5 μm; Step III, the ammonium tungsten bronze obtained in step I is passed through a fifteen-tube furnace at 600-700° C. to prepare tungsten oxide with a particle size of 2.5-2.8 μm, and then passed through a fifteen-tube furnace at 800-900° C. to prepare potassium-doped B powder with a particle size of 2.4-2.7 μm; Step IV, respectively washing the potassium-doped D powder obtained in step II and the potassium-doped B powder obtained in step III with hydrochloric acid and hydrofluoric acid in sequence, then washing with water, filtering and removing water with anhydrous ethanol, vacuum drying, crushing and sieving, to obtain washed potassium-doped D powder and washed potassium-doped B powder; Step V, mixing the washed potassium-doped D powder and the washed potassium-doped B powder obtained in step IV with a synergist to obtain a mixed powder, and then subjecting the mixed powder to a cold isostatic pressing treatment at 130-160 MPa for 150-200 seconds to obtain a tungsten green body; Step VI, placing the tungsten green body obtained in step V into a medium frequency induction sintering furnace, using hydrogen to be introduced simultaneously from top to bottom, and sintering at 2000-2100° C. for 5-10 hours to obtain a nanocrystalline potassium-doped tungsten material; The synergist is titanium nitride@chopped tungsten fiber and / or hafnium diboride; The preparation method of the titanium nitride@short-cut tungsten fiber is as follows: The short-cut tungsten fibers are added to anhydrous ethanol, ultrasonicated, filtered, and vacuum dried to obtain pretreated short-cut tungsten fibers; The pretreated short-cut tungsten fiber is taken and a precursor is introduced into the pretreatment short-cut tungsten fiber for vapor deposition. The precursor is composed of gaseous titanium tetrachloride and ammonia to obtain titanium nitride@short-cut tungsten fiber.

2. The method for preparing the nanocrystalline potassium-doped tungsten material according to claim 1, characterized in that: The dopant aqueous solution in step I contains 0.2-0.4% potassium oxide, 0.001-0.003% aluminum oxide, and 0.3-0.5% silicon dioxide in weight percentage relative to the weight of ammonium paratungstate according to equivalent calculation.

3. The method for preparing the nanocrystalline potassium-doped tungsten material according to claim 1, characterized in that: After washing in step IV, the potassium-doped D powder has a particle size of 3.2-3.5 μm, a potassium content of 60-80 ppm, and an oxygen content of less than 100 ppm.

4. The method for preparing nanocrystalline potassium-doped tungsten material according to claim 1, characterized in that: After washing in step IV, the potassium-doped B powder has a particle size of 2.4-2.7 μm, a potassium content of 160-200 ppm, and an oxygen content of less than 100 ppm.

5. The method for preparing nanocrystalline potassium-doped tungsten material according to claim 1, characterized in that: The synergist consists of titanium nitride@short-cut tungsten fiber and hafnium diboride.

6. A nanocrystalline potassium-doped tungsten material, characterized in that: The method is prepared by any one of claims 1 to 5.

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