Preparation method of spherical iridium powder

Abstract

The invention provides a preparation method of spherical iridium powder, and relates to the field of iridium powder preparation. The method comprises the following steps: mixing an iridium raw material and an alkaline reagent to obtain a mixture, and performing first roasting on the mixture to obtain a roasted product; pickling the roasted product to obtain a pickled product; under the mixed atmosphere of argon and hydrogen, plasma spheroidizing is conducted on the product obtained after acid pickling, and a plasma spheroidized product is obtained; the plasma spheroidized product is subjected to atomization treatment, and an atomized product is obtained; the atomized product is subjected to multi-stage cooling, and cooled iridium powder is obtained; and grading the cooled iridium powder, performing secondary roasting in a reducing atmosphere, and cooling to obtain the spherical iridium powder. The spherical iridium powder is uniform in particle size and high in sphericity degree, and the compactness and stability of the spherical iridium powder are improved.

Description

Technical Field

[0001] The present application relates to the field of iridium powder preparation, and particularly to a method for preparing spherical iridium powder. Background Art

[0002] Aerospace attitude and orbit control engines are the core components of modern aerospace vehicles, which are characterized by high working temperatures and large loads. Therefore, materials with high temperature resistance and strong stability are required to ensure their efficient and reliable operation. Currently, high-performance green unit engines are being studied, and the combustion temperature of the engines reaches above 1600°C. After the trial run of traditional metal materials, problems such as metal deformation occur, which cannot meet the requirements of the engines for long-term operation. It is necessary to develop a new type of material to meet the usage requirements of aerospace attitude and orbit control engines.

[0003] Iridium has the characteristics of high melting point, stable chemical properties, and strong high-temperature stability, and has become a key material for the development of a new generation of high-temperature core components.

[0004] The iridium powder prepared by the existing process has problems such as uneven particle size and low sphericity, and there are also certain deficiencies in density and stability.

[0005] Based on this, there is an urgent need to provide an iridium powder preparation process to solve the above problems. Summary of the Invention

[0006] The purpose of the present application is to provide a method for preparing spherical iridium powder to solve the above problems.

[0007] To achieve the above purpose, the present application provides a method for preparing spherical iridium powder, including: Mixing an iridium raw material and an alkaline reagent to obtain a mixture, and subjecting the mixture to a first calcination to obtain a calcined product; Subjecting the calcined product to pickling to obtain a pickled product; Under a mixed atmosphere of argon and hydrogen, subjecting the pickled product to plasma spheroidization to obtain a plasma spheroidized product; subjecting the plasma spheroidized product to atomization treatment to obtain an atomized product; Subjecting the atomized product to multi-stage cooling to obtain cooled iridium powder; Classifying the cooled iridium powder, and subjecting it to a second calcination and cooling under a reducing atmosphere to obtain spherical iridium powder.

[0008] Optionally, the method for preparing spherical iridium powder satisfies at least one of the following conditions: A. The alkaline reagent includes sodium hydroxide and / or sodium peroxide; B. The mass ratio of the iridium raw material to the alkaline reagent is 1:2 - 5.

[0009] Optionally, the temperature of the first roasting is 750°C - 800°C, and the time is 8h - 12h.

[0010] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The acid solution for pickling includes hydrochloric acid and / or nitric acid; B. In the mixed atmosphere, the volume content of hydrogen is 5% - 10%.

[0011] Optionally, the power of the plasma spheroidization is 50 - 80kW, the temperature is 3000°C - 3500°C, and the jet velocity is 200m / s - 300m / s.

[0012] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The pressure of the atomization is 1.0 kPa - 1.5 kPa; B. The energy density of the atomization is 25J / cm 3 -40J / cm 3 ; C. The sheath gas flow rate of the atomization is 20 L / min - 30 L / min; D. Before the atomization, the atomization device is also subjected to vacuum treatment, and the ultimate vacuum degree of the vacuum treatment is ≤5×10 -3 Pa.

[0013] Optionally, the multi-stage cooling includes rapid cooling and slow cooling that are carried out in sequence; The cooling rate of the rapid cooling is 10 4 K / s - 10 5 K / s; The cooling rate of the slow cooling is 10 2 K / s - 10 3 K / s; The end temperature of the rapid cooling is 1000°C - 1500°C.

[0014] Optionally, the particle size of the target iridium powder after classification is 10μm - 40μm.

[0015] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The temperature of the second roasting is 800°C - 1000°C, and the time is 2h - 4h; B. The cooling rate is ≤5°C / min.

[0016] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The purity of the spherical iridium powder is ≥99.9%; B. The sphericity of the spherical iridium powder is ≥95%; C. The D50 of the spherical iridium powder is 15μm - 25μm.

[0017] Compared with the prior art, the beneficial effects of the present application include: The preparation method of the spherical iridium powder provided by the present application shows significant advantages in terms of the sphericity, particle size uniformity, compactness and stability of the iridium powder through multi-step collaborative innovation; First, the iridium raw material is mixed with an alkaline reagent and then subjected to the first calcination. In this step, through the reaction of the alkaline reagent with the iridium raw material, iridium is converted into a soluble salt, effectively separating and removing impurities. After pickling, the purity of the iridium powder can be increased to more than 99.9%, laying a high-purity foundation for subsequent spheroidization; The plasma spheroidization process is carried out in a mixed atmosphere of argon and hydrogen. The reduction effect of hydrogen inhibits the oxidation of the iridium particle surface. With the inert protection of argon, the molten iridium droplets fully contract into regular spheres under the action of surface tension. The sphericity of the obtained iridium powder can reach more than 95%, which is significantly improved compared with the traditional pure inert atmosphere spheroidization technology, and the surface is smooth and has few defects. The sphericity is close to the theoretical optimal state, realizing spherical iridium powder with uniform particle size and high sphericity, improving the compactness and stability of the spherical iridium powder; After plasma spheroidization, through atomization treatment, using precisely controlled atomization pressure, energy density and sheath gas flow rate, the particles are broken into the target particle size range. Combined with the subsequent multi-stage cooling process, the formation of coarse grains is inhibited, forming a metastable dense structure, and then slowly cooled to promote grain boundary fusion and reduce internal pores. Finally, the density of the iridium powder reaches 22.4 g / cm 3As described above, it is close to the theoretical density of iridium, and the compactness is improved by about 2% compared with the traditional single cooling process, effectively enhancing the structural stability of the powder; the grading process further screens out the target particles with a particle size of 10-40 μm, and by controlling D50 to be 15-25 μm and the distribution span ≤1.5, a high degree of particle size uniformity is achieved, meeting the stringent requirements for powder dispersibility in high-end applications; the second calcination is carried out in a reducing atmosphere, which can eliminate the possible residual oxide layer on the particle surface, refine the grains and stabilize the crystal structure, making the iridium powder perform excellently in the corrosion resistance test; from the perspective of process economy, this method is compatible with existing precious metal smelting equipment, the vacuum pretreatment before atomization reduces the introduction of impurities, reduces the subsequent impurity removal cost, and has high energy utilization rate. The overall energy consumption is reduced by 10%-15% compared with the traditional process, and the material utilization rate is increased to more than 95% through precise grading. In practical applications, the characteristics of high sphericity and narrow particle size distribution can significantly increase the electrochemical active area when used as a catalyst carrier, and can reduce the precious metal consumption by 20% when applied to proton exchange membrane fuel cells; the high compactness and corrosion resistance make it perform excellently in the field of aerospace coatings, and the thermal shock resistance is increased by 30%, effectively extending the service life of the coating. In summary, this method achieves a technical breakthrough in the preparation of spherical iridium powder through process innovation, has both high performance and industrial production advantages, and provides a high-quality material basis for related high-end fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application and should not be regarded as limiting the scope of the present application.

[0019] Figure 1 SEM image of the circular iridium powder prepared in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] As used herein: "Prepared from" is synonymous with "comprising". As used herein, the terms "comprising", "including", "having", "containing" or any other variation thereof are intended to cover non-exclusive inclusion. For example, a composition, step, method, article or device containing the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article or device.

[0021] The connecting term "consisting of" excludes any unrecited element, step, or component. If used in a claim, this phrase renders the claim closed-ended, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause within the body of a claim rather than immediately following the subject, it only limits the elements described in that clause; other elements are not excluded from the claim as a whole.

[0022] When an equivalent, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper preferred values and lower preferred values, this should be understood to specifically disclose all ranges formed by any pairing of any range upper limit or preferred value with any range lower limit or preferred value, whether or not that range is separately disclosed. For example, when the range "1 - 5" is disclosed, the described range should be interpreted as including the ranges "1 - 4", "1 - 3", "1 - 2", "1 - 2 and 4 - 5", "1 - 3 and 5", etc. When a numerical range is described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0023] In these examples, unless otherwise specified, the parts and percentages are by mass.

[0024] "Parts by mass" refers to the basic measurement unit representing the mass ratio relationship of multiple components. 1 part can represent any unit mass, such as 1 g or 2.689 g, etc. If we say that the mass part of component A is a parts and the mass part of component B is b parts, it means the mass ratio of component A to component B is a:b. Or, it means the mass of component A is aK and the mass of component B is bK (K is any number representing a multiple factor). It should not be misunderstood that, different from the number of mass parts, the sum of the mass parts of all components is not limited to 100 parts.

[0025] "And / or" is used to indicate that either or both of the stated circumstances may occur. For example, A and / or B includes (A and B) and (A or B).

[0026] This application provides a method for preparing spherical iridium powder, comprising: Mixing an iridium raw material and an alkaline reagent to obtain a mixture, and subjecting the mixture to a first calcination to obtain a calcined product; Subjecting the calcined product to pickling to obtain a pickled product; Under a mixed atmosphere of argon and hydrogen, subjecting the pickled product to plasma spheroidization to obtain a plasma spheroidized product; subjecting the plasma spheroidized product to atomization treatment to obtain an atomized product; It should be noted that argon is used as the main carrier gas in the mixed atmosphere to prevent iridium from being oxidized at high temperatures. At the same time, the gas flow rate and pressure are precisely controlled so that the droplets can be quickly cooled and form spherical powders, avoiding powder particle adhesion or irregular shapes. And during the atomization process, the purpose of adding hydrogen is to utilize its high thermal conductivity (0.18 W / m·K) to accelerate the cooling of the droplets, reduce the surface tension, and promote spheroidization to balance the energy transfer efficiency and atomization stability. The atomized product is subjected to multi-stage cooling to obtain cooled iridium powder. The cooled iridium powder is classified, and under a reducing atmosphere, it is subjected to a second calcination and cooling to obtain spherical iridium powder.

[0027] In some embodiments, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The alkaline reagent includes sodium hydroxide and / or sodium peroxide; It should be noted that when both sodium hydroxide and sodium peroxide are used as the alkaline reagent, the following synergistic effects can improve the preparation effect of spherical iridium powder: 1. Strengthening the decomposition and oxidation ability: Sodium hydroxide, as a strong alkaline reagent, mainly undergoes a double decomposition reaction with iridium raw materials (such as iridium oxides, halides, etc.) through the strong alkaline environment in the molten state, destroying the raw material structure and generating soluble iridates (such as Na2IrO3, etc.); Sodium peroxide has both strong alkalinity and strong oxidizing properties. Its peroxide group (O2 2- ) decomposes to produce oxygen and reactive oxygen species (such as ·O - ) during high-temperature calcination, which can further oxidize the low-valent iridium or insoluble impurities (such as metallic iridium particles, carbides, etc.) that may exist in the iridium raw materials, converting them into high-valent compounds (such as salts of Ir 4+ or Ir 6+ ) that are easily soluble in acid. The combined use of the two can significantly improve the decomposition efficiency and oxidation degree of the iridium raw materials, ensuring that the iridium in the calcined product exists in the form of a single high-valent state, laying a foundation for subsequent acid washing and impurity removal. 2. Optimizing the structure of the calcined product: The strong hygroscopicity of sodium hydroxide can make the mixture form a uniform paste before calcination, promoting the full contact between the iridium raw materials and the alkaline reagent; The gas (such as O2) released by sodium peroxide during the melting process can produce a "bubbling" effect, making the calcined product form a porous and loose structure, reducing the phenomenon of melt caking. This porous structure can increase the contact area between the acid solution and the product during acid washing, accelerating the dissolution of impurities (such as unreacted metal oxides, silicates, etc.), and at the same time avoiding the difficulty of leaching iridates due to being wrapped by a dense structure, thereby improving the recovery rate of iridium and reducing the impurity residue. 3. Cooperative impurity removal and stable valence state: The oxidizing property of sodium peroxide can effectively decompose organic impurities or reducing substances (such as carbon, sulfur, etc.) mixed in iridium raw materials, oxidize them into gases such as CO2 and SO2 and let them escape; sodium hydroxide dissolves acidic impurities (such as Al2O3, SiO2, etc.) through an alkaline environment to form soluble sodium salts (such as NaAlO2, Na2SiO3). The combined use of the two can cover more types of impurity removal paths. Especially for impurities that are difficult to be oxidized and dissolved by alkali (such as certain metal alloy phases), more thorough removal can be achieved through the synergistic effect of oxidation-alkali dissolution. In addition, the strong oxidizing environment maintained by sodium peroxide can inhibit iridium from being accidentally reduced to the metallic state during roasting, ensure its existence in a stable high valence state, and avoid the difficulty of melting and spheroidizing of iridium metal particles due to their high melting point (-2443 °C) during subsequent plasma spheroidization, which affects the sphericity of the final powder; 4. Energy consumption reduction and efficiency improvement: The strong oxidizing property of sodium peroxide can reduce the activation energy required for the decomposition of iridium raw materials, so that the first roasting temperature can be controlled in the lower range of 750-800 °C (if only sodium hydroxide is used, a higher temperature may be required to completely decompose some insoluble iridium compounds), and at the same time, the roasting time is shortened to 8-12 hours, thereby reducing energy consumption. In addition, when the two are used in combination, the oxygen-releasing property of sodium peroxide can reduce the material splashing caused by water evaporation during roasting, improve operation safety, and at the same time, by adjusting the ratio of the two (such as a mass ratio of 1:2-5), it can flexibly adapt to the pretreatment requirements of different iridium raw materials (such as iridium-containing waste, crude iridium powder, etc.), enhancing the universality of the process; In summary, the synergistic effect of sodium hydroxide and sodium peroxide significantly improves the efficiency and quality of the pretreatment of iridium raw materials through multiple mechanisms of "alkali dissolution-oxidation-structure regulation-impurity cooperative removal", provides a high-purity and easy-to-process precursor for subsequent steps such as plasma spheroidization, atomization, and cooling classification, and ultimately ensures the particle size uniformity, sphericity, and density of spherical iridium powder.

[0028] B. The mass ratio of the iridium raw material to the alkaline reagent is 1:2-5.

[0029] Optionally, the mass ratio of the iridium raw material to the alkaline reagent can be 1:2, 1:3, 1:4, 1:5 or any value between 1:2-5.

[0030] In some embodiments, the temperature of the first roasting is 750 °C - 800 °C, and the time is 8h - 12h.

[0031] Optionally, the temperature of the first roasting can be 750 °C, 760 °C, 770 °C, 780 °C, 790 °C, 800 °C or any value between 750 °C - 800 °C, and the time can be 8h, 9h, 10h, 11h, 12h or any value between 8h - 12h.

[0032] It should be noted that the iridium raw materials often have reduced reactivity due to surface oxidation, as well as being wrapped by sulfides or organic substances, and impurities (such as Fe, Al, etc.) will interfere with subsequent reduction or spheroidization processes, etc. In this application, surface activation treatment is carried out on them. At 750°C - 800°C, the organic substances and sulfides in the iridium raw materials can be effectively decomposed, while avoiding excessive oxidation or melting of iridium metal. 8 - 12 hours ensures sufficient reaction of iridium with the alkaline flux to be converted into soluble sodium salts (such as Na2IrO3), which is convenient for subsequent water leaching separation.

[0033] In some embodiments, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The acid solution for pickling includes hydrochloric acid and / or nitric acid; It should be noted that the acid solution can dissolve the roasted iridium raw materials, remove the surface passivation layer, and improve the reactivity. Preferably, when the volume ratio of hydrochloric acid to nitric acid is 3:1, the oxidizing property and dissolution efficiency can be balanced, and excessive passivation of iridium can be avoided. Through the above activation treatment, the surface passivation layer can be effectively removed, and the insoluble impurities can be decomposed, enabling iridium to enter the subsequent process in a highly active form; B. In the mixed atmosphere, the volume content of hydrogen is 5% - 10%.

[0034] Optionally, the volume content of hydrogen can be 5%, 6%, 7%, 8%, 9%, 10% or any value between 5% - 10%.

[0035] It should be noted that when the volume content of hydrogen is 5% - 10%, the high thermal conductivity of hydrogen can be utilized to improve the energy transfer efficiency, while reducing the oxidation risk.

[0036] In some embodiments, the power of the plasma spheroidization is 50 - 80 kW, the temperature is 3000°C - 3500°C, and the jet velocity is 200 m / s - 300 m / s.

[0037] Optionally, the power of the plasma spheroidization can be 50 kW, 60 kW, 70 kW, 80 kW or any value between 50 - 80 kW, the temperature can be 3000°C, 3100°C, 3200°C, 3300°C, 3400°C, 3500°C or any value between 3000°C - 3500°C, and the jet velocity can be 200 m / s, 250 m / s, 300 m / s or any value between 200 m / s - 300 m / s.

[0038] It should be noted that iridium has a high melting point and large density, and the temperature and energy density of the plasma in the conventional plasma atomization technology cannot meet its melting and atomization requirements. Therefore, this application designs specific plasma parameters for the preparation of the product after pickling (iridium powder after pickling), such as higher plasma temperature, better plasma jet velocity and power, etc., to ensure that the product after pickling can be fully melted and evenly dispersed into droplets. Plasma spheroidization requires ensuring that the iridium raw material can be fully melted and evenly dispersed into droplets; when the power of plasma spheroidization is 50-80kW, it can match the high heat capacity requirement of iridium; when the temperature is 3000℃-3500℃, it can ensure the full melting and uniform atomization of the high-density iridium raw material; when the jet velocity is 200m / s - 300m / s, it can promote the dispersion efficiency of iridium droplets and avoid droplet agglomeration caused by excessive density; It should also be noted that the plasma spheroidization process of this application is not a universal technology, but a key process specifically designed for the iridium powder after the first roasting + pickling pretreatment. The two form a technical closed-loop through "pretreatment to create spheroidization conditions - spheroidization parameters matching material properties", and the specific relevance is reflected in the following core dimensions: I. The unique properties imparted to the iridium powder by the pretreatment are the necessary premise for the spheroidization process. The iridium powder without pretreatment faces multiple technical obstacles during direct plasma spheroidization due to the presence of a dense oxide film (such as IrO2, with a thickness reaching the micron level), an impurity coating layer (the content of base metals such as Fe and Al often exceeds 0.5wt%), and a coarse grain structure on the surface. The oxide film causes the plasma energy to first break through the inert surface, and the energy loss rate exceeds 40%. Even when the temperature is raised above 3800℃, it is still difficult to melt evenly; the low-melting-point impurities melt in advance to form liquid-phase adhesion, resulting in droplet agglomeration and the generation of a large number of satellite balls; the coarse grains are prone to local overheating and splashing due to lattice stress concentration, and finally the sphericity is only 80%-85%, and the surface oxygen content exceeds 5000ppm; while this application uses a combined process of alkaline roasting at 750-800℃ + pickling to perform directional modification on the iridium powder: a. Surface activity reconstruction: The synergistic alkali fusion effect of sodium hydroxide and sodium peroxide breaks the original oxide film, and pickling further dissolves the residual alkaline salts and metal oxides to form a nanoscale porous surface (the specific surface area increases from 0.5m 2 / g to 3-5m 2 / g), enabling the plasma energy to directly act on the metallic iridium particles, and the energy absorption rate is increased to more than 90%. Rapid and uniform melting can be achieved at 3000-3500℃, reducing energy consumption by 30% compared with the conventional process; b. Deep impurity removal: The strong oxidizing property of sodium peroxide decomposes non-metallic impurities such as S and C and makes them escape as gases. Acid pickling reduces the content of base metal impurities such as Fe and Al to less than 0.01 wt%, avoiding the formation of low-melting-point liquid phases or brittle intermetallic compounds at high temperatures, and ensuring that iridium droplets shrink into regular spheres by surface tension in the pure metal state; c. Crystal structure optimization: Low-temperature roasting promotes the refinement of iridium grains to 20 - 50 μm and releases lattice stress. The latent heat during melting is reduced by 15%. The standard deviation of the droplet size distribution narrows from ±15 μm to ±5 μm, significantly improving the melting uniformity; II. Plasma spheroidization parameters are precisely customized according to the characteristics of the pretreated materials. The high activity, low impurity, and porous structure characteristics of the pretreated iridium powder determine the unique design of plasma process parameters: d. Temperature and power matching: The thermal conductivity of the pretreated iridium powder is increased by 20%. A stable jet at 3000 - 3500 °C can be maintained at a power of 50 - 80 kW, ensuring that iridium particles complete the entire process of "melting - dispersion - spheroidization" within 10 ms. If used for untreated iridium powder, the temperature distribution is uneven (fluctuating ±200 °C) at the same power, resulting in some particles not melting or excessive evaporation; while increasing the power to above 100 kW can cause melting, but it will trigger grain boundary oxidation due to excessive energy, and the surface oxygen content surges by more than 3 times; e. Jet velocity and dispersion efficiency: The particle size distribution of the pretreated iridium powder is concentrated (D50 = 10 - 30 μm, span ≤ 1.5). Single-particle precise dispersion can be achieved at a jet velocity of 200 - 300 m / s, avoiding droplet adhesion caused by agglomeration (adhesion rate < 5%). Untreated iridium powder is prone to form agglomerates of 50 - 100 μm due to surface impurity adsorption and cannot be effectively dispersed at the same velocity. Additional ultrasonic crushing is required (cost increases by 20%), and more than 10% of the agglomerates still enter the spheroidization stage, resulting in the satellite ball ratio exceeding 20%; f. Synergistic protection of hydrogen - argon atmosphere: The surface activity of the pretreated iridium powder is extremely high. A hydrogen content of 5 - 10% can instantaneously reduce the newly formed oxide layer during the flight of droplets (the reaction rate constant reaches 2.3×10 -4 cm 3 / (mol·s), making the surface tension close to the theoretical value of pure iridium (1.9 N / m vs theoretical value 2.0 N / m), ensuring sufficient spherical shrinkage. If used for un - pickled iridium powder, the residual alkaline salts decompose at high temperatures to produce CO2, which reacts with H2 to generate water vapor, instead exacerbating surface oxidation and the sphericity decreasing by more than 10%; III. Experimental data verifies the inseparable process correlation. The applicant's laboratory data shows that for unpretreated iridium powder, even when using extreme parameters (3800 °C, pure argon atmosphere, 100 kW power), the sphericity is only 87.2%, the surface oxygen content exceeds 4000 ppm, the energy consumption increases by 50% compared to the process of this application, and the iridium volatilization loss rate reaches 4.2%. However, through the synergistic effect of pretreatment and spheroidization in the method of this application, key performance breakthroughs have been achieved, including a sphericity ≥ 95%, a surface oxygen content ≤ 1000 ppm, a 30% reduction in energy consumption, and a recovery rate ≥ 98%. This difference essentially stems from the directional regulation of the physical and chemical properties of iridium powder by pretreatment - only the iridium powder treated by the first calcination + pickling process of this application can achieve "efficient melting - uniform spheroidization - low - loss preparation" under the set plasma parameters. The technical correlation between the two constitutes the core innovation point that differentiates from the existing processes.

[0039] In summary, the plasma spheroidization process of this application is not an independent link, but an organic whole deeply coupled with the previous pretreatment process. Pretreatment creates the necessary conditions of "high - activity surface, low impurity content, and optimized crystal structure" for spheroidization, while the spheroidization parameters are precisely adapted to the material characteristics after pretreatment. Neither can be missing, and together they construct an efficient and high - quality technical path for the preparation of spherical iridium powder.

[0040] In some embodiments, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The atomization pressure is 1.0 kPa - 1.5 kPa; Optionally, the atomization pressure can be 1.0 kPa, 1.1 kPa, 1.2 kPa, 1.3 kPa, 1.4 kPa, 1.5 kPa, or any value between 1.0 kPa - 1.5 kPa; It should be noted that the core step of the atomization process includes the fragmentation of molten metal. When the atomization pressure is 1.0 kPa - 1.5 kPa, it is to maintain a suitable pressure environment in the atomization chamber, avoid the energy attenuation of active particles (such as plasma or high - speed air flow) due to too high pressure, and affect the droplet fragmentation efficiency; by precisely regulating the pressure, ensure that the molten iridium droplets are evenly dispersed under the action of gas kinetic energy, and prevent particle adhesion or uneven size caused by pressure fluctuations; within this pressure range, it not only meets the energy transfer requirements of active particles but also avoids insufficient gas kinetic energy caused by too high vacuum degree; B. The energy density of the atomization is 25 J / cm 3 - 40 J / cm 3 ; Optionally, the energy density of the atomization can be 25 J / cm 3 、30 J / cm 3 、25 J / cm 3 、40 J / cm3 or 25 J / cm 3 -40 J / cm 3 any value between; It should be noted that when the energy density of atomization is 25 J / cm 3 -40 J / cm 3 to match the high heat capacity requirement of iridium, ensuring that iridium particles rapidly absorb heat and completely melt in the high-temperature plasma region; C. The sheath gas flow rate of the atomization is 20 L / min - 30 L / min; Optionally, the sheath gas flow rate of the atomization can be 20 L / min, 25 L / min, 30 L / min or any value between 20 L / min - 30 L / min; D. Before the atomization, the atomization device is also subjected to vacuum treatment, and the ultimate vacuum degree of the vacuum treatment ≤ 5×10 -3 Pa.

[0041] Optionally, the ultimate vacuum degree of the vacuum treatment can be 5×10 -3 Pa, 1×10 -3 Pa, 5×10 -4 Pa or any value ≤ 5×10 -3 Pa.

[0042] It should be noted that vacuum treatment can remove impurity gases such as oxygen and nitrogen in the atomization chamber, completely remove the impurity gases, provide a clean environment for the subsequent process, and reduce its interference with the energy transfer path; reduce the collision probability between gas molecules and high-temperature iridium droplets, and avoid the mixing of impurity elements (such as oxygen and carbon) into the powder to ensure the purity of iridium powder.

[0043] In some embodiments, the multi-stage cooling includes rapid cooling and slow cooling performed in sequence; It should be noted that due to problems such as particle adhesion, stress concentration, and impurity residue in the atomized iridium powder, it is necessary to control the cooling rate of the atomized iridium powder particles. A multi-stage cooling method is adopted. First, the droplets are initially solidified by rapid cooling, and then the residual stress is removed by slow cooling, so as to obtain iridium powder with uniform particle size and high sphericity; The cooling rate of the rapid cooling is 10 4 K / s - 10 5 K / s; Optionally, the cooling rate of the rapid cooling can be 10 4 K / s, 5×10 4 K / s, 10 5 K / s or 10 4 K / s - 10 5 K / s any value between; In some embodiments, the medium for rapid cooling includes argon; It should be noted that after atomization, the iridium powder is prone to secondary collision and fusion due to the difference in droplet kinetic energy. Forced convection of high-speed air flow can accelerate solidification, prevent adhesion into irregular particles, and rapid cooling enables the surface of the atomized droplets to solidify rapidly, forming a dense shell layer to inhibit droplet collision and adhesion; The cooling rate of the slow cooling is 10 2 K / s - 10 3 K / s; Optionally, the cooling rate of the slow cooling can be 10 2 K / s, 5×10 2 K / s, 10 3 K / s or any value between 10 2 K / s - 10 3 K / s; In some embodiments, the medium for slow cooling includes argon; It should be noted that when the high-temperature iridium powder solidifies rapidly, the internal residual stress (about 200 - 500 MPa) will cause microcracks or lattice defects. Gradient cooling can relieve stress concentration, release the internal thermal stress by slow cooling, reduce lattice distortion, and improve the sphericity; The end temperature of the rapid cooling is 1000°C - 1500°C.

[0044] Optionally, the end temperature of the rapid cooling can be 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C or any value between 1000°C - 1500°C.

[0045] In some embodiments, the particle size of the target iridium powder after classification is 10μm - 40μm.

[0046] Optionally, the particle size of the target iridium powder after classification can be 10μm, 20μm, 30μm, 40μm or any value between 10μm - 40μm.

[0047] In some embodiments, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The temperature of the second calcination is 800°C - 1000°C, and the time is 2h - 4h; Optionally, the temperature of the second calcination can be 800°C, 900°C, 1000°C or any value between 800°C - 1000°C, and the time can be 2h, 3h, 4h or any value between 2h - 4h; It should be noted that after classification, there are still internal defects in the iridium powder (due to uneven grain boundary impurities and stress distribution in the classified iridium powder), which are prone to thermal fatigue cracking in high-temperature applications (such as rocket nozzle coatings). Through the linkage of gradient cooling and heat treatment, rapid solidification can inhibit particle adhesion, slow cooling can reduce stress, and heat treatment can further eliminate microdefects, ultimately meeting the performance index requirements of high purity, high sphericity, and uniform particle size of the iridium powder. Moreover, the second calcination can also activate the diffusion of iridium atoms, repair grain boundary defects, avoid excessive grain coarsening, and remove the surface oxide layer and reduce it to its original state. The specific reaction is as follows: IrO2 + 2H2 → Ir + 2H2O↑; B. The cooling rate ≤ 5°C / min.

[0048] Optionally, the cooling rate can be 5°C / min, 4°C / min, 3°C / min, 2°C / min, 1°C / min, or any value ≤ 5°C / min.

[0049] In some embodiments, the method for preparing the spherical iridium powder satisfies at least one of the following conditions: A. The purity of the spherical iridium powder ≥ 99.9%; Optionally, the purity of the spherical iridium powder can be 99.9%, 99.99%, 99.999%, or any value ≥ 99.9%; B. The sphericity of the spherical iridium powder ≥ 95%; Optionally, the sphericity of the spherical iridium powder can be 95%, 96%, 97%, 98%, 99%, or any value ≥ 95%; C. The D50 of the spherical iridium powder is 15μm - 25μm.

[0050] Optionally, the D50 of the spherical iridium powder can be 15μm, 20μm, 25μm, or any value between 15μm - 25μm.

[0051] The following will describe the implementation plan of the present application in detail with specific embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0052] Example 1 This embodiment provides a method for preparing spherical iridium powder, and the specific steps are as follows: S1: Clean the iridium raw material. After cleaning, mix it with an alkaline reagent to obtain a mixture. The mass ratio of the iridium raw material to the alkaline reagent is 1:3. Among them, the alkaline reagent is sodium hydroxide and sodium peroxide with a mass ratio of 1:1. Perform the first roasting (temperature: 780 °C, time: 10 h) on the mixture to obtain a roasted product; S2: Pickle the roasted product with aqua regia (volume ratio of concentrated hydrochloric acid to nitric acid is 3:1) to obtain a pickled product; S3: Under a mixed atmosphere of argon and hydrogen (hydrogen volume content is 7%), perform plasma spheroidization on the pickled product. Among them, the power of plasma spheroidization is 70 kW, the temperature is 3200 °C, and the jet velocity is 250 m / s to obtain a plasma spheroidized product; Perform vacuum treatment on the atomization device until the ultimate vacuum degree of the vacuum treatment is 2×10 -3 Pa. Then, introduce a mixed gas of argon and hydrogen (hydrogen volume content is 7%, total gas flow rate is 10 L / min) into the system, raise the chamber pressure to 1.2 kPa, and perform atomization treatment on the plasma spheroidized product. The energy density of atomization is 30 J / cm 3 , and the sheath gas flow rate is 25 L / min to obtain an atomized product; S4: First, quickly cool the atomized product with argon (cooling rate is 5×10 4 K / s) to preliminarily solidify the droplets until reaching 1200 °C, and then slowly cool it with argon (cooling rate is 5×10 2 K / s) to obtain cooled iridium powder; S5: Classify the cooled iridium powder to obtain a target classified product with a particle size of 15 μm - 25 μm. Under a hydrogen atmosphere, perform the second roasting at a temperature of 900 °C for 3 h, and then cool it at a rate of 5 °C / min to obtain spherical iridium powder.

[0053] Performance detection of spherical iridium powder: Purity: 99.95% (ICP-MS); Sphericity: 96.8% (SEM image analysis, proportion of particles with roundness factor ≥ 0.95); D50: 20 μm, particle size distribution span 1.3 (laser particle size analyzer); Density: 22.43 g / cm 3 (Archimedes drainage method, close to the theoretical density of 22.56 g / cm 3 ); Surface oxygen content: 750 ppm (XPS detection), weight loss rate after soaking in aqua regia for 24 h is 0.03%.

[0054] The SEM of the spherical iridium powder is as Figure 1As shown, the surface of the spherical iridium powder is smooth and defect-free, with uniform particle size, and the typical particle roundness factor reaches 0.97 (scale bar 50μm).

[0055] Example 2 The difference from Example 1 is that the alkaline reagent is sodium hydroxide.

[0056] Example 3 The difference from Example 1 is that the power of plasma spheroidization is 50kW, the temperature is 3000°C, and the jet velocity is 200m / s.

[0057] Comparative Example 1 The difference from Example 1 is that no alkaline reagent is added.

[0058] The core difference between this comparative example and Example 1 is that the alkaline reagent is completely omitted, and the crude iridium powder containing 90% IrO2 is directly calcined in an air atmosphere at 780°C for 10h, attempting to prepare spherical iridium powder through subsequent processes. However, this omission leads to multiple technical bottlenecks in the entire process, which are specifically manifested as follows: There are double failures of impurity encapsulation and surface passivation in the pretreatment stage. The calcination process without the participation of the alkaline reagent can only cause physical sintering of the iridium raw material and cannot break the surface oxide film and impurity encapsulation through chemical reactions. Among them, the intractability of the oxide film is manifested as follows: the IrO2 oxide film (thickness ≥ 200nm) on the surface of the iridium raw material is not converted into soluble Na2IrO3 under alkali-free conditions. XPS detection shows that the Ir 4f7 / 2 peak position after calcination still corresponds to IrO2 (71.2eV), and more than 50% of the oxide film remains after pickling, forming an "inert shell layer" that hinders subsequent reactions.

[0059] There is also residual impurity solidification. The Fe and Al impurities in the raw material (original contents 0.8wt% and 0.5wt%) do not react with the alkali, but instead form an Ir-Fe-Al alloy phase with iridium at high temperature (Fe3Ir and AlIr2 diffraction peaks are detected by XRD) and cannot be dissolved by pickling; sulfides (such as IrS2) and organic substances, due to the lack of oxidation by sodium peroxide, respectively wrap the iridium particles in the form of solid sulfur (S content 0.5wt%) and graphitized carbon film (C content 0.8wt%). TEM shows that the thickness of the carbon film reaches 50 - 100nm; and the alkali-free calcination product forms a dense block structure with a porosity < 10% and a specific surface area of only 0.8m² / g (4.1m² / g in Example 1) due to the lack of the "bubbling" effect of the alkali. The acid penetration efficiency during pickling is low, and it takes 3 times longer to reach the same impurity removal effect. 2 / g), and the acid penetration efficiency during pickling is low, and it takes 3 times longer to reach the same impurity removal effect; In the plasma spheroidization stage, there are energy barriers and spherical shrinkage obstacles. Pretreatment defects directly lead to failure of plasma energy transfer and abnormal droplet behavior. Specifically, the melting is insufficient, the oxide film and the impurity coating layer form thermal resistance, and the actual heating temperature of the iridium particles at the same power (70kW) is only 2800°C, which is lower than the softening point of iridium (2800°C). 30% of the particles show the phenomenon of "external melting and internal solidification". SEM shows that the proportion of unmelted cores is as high as 25%, and the surface of the particles is uneven and attached with unmelted debris; the surface tension is disordered, and the residual graphitized carbon film reduces the surface tension of the droplets to 1.5N / m (1.9N / m in Example 1), and the SO2 gas produced by the decomposition of sulfides forms bubbles inside the droplets, resulting in defects such as "bulging" and "tailing" during the spherical shrinkage process. The sphericity rate drops sharply to 78.2%, and the proportion of satellite balls is as high as 25.3% (Example 1 Only 3.2%); Oxidation intensified: pretreatment without alkaline reagents cannot reduce the oxygen binding state in the raw materials, and the IrO2 generated in the plasma cannot be effectively reduced by H2 (the carbon film hinders gas diffusion), and the surface oxygen content soars to 5800ppm (750ppm in Example 1), forming a thick oxide layer (thickness>50nm); As a result, the performance of the final product is completely deteriorated. The lack of synergistic effect of the alkaline reagent causes the key performance indicators of the spherical iridium powder to be far lower than those in Example 1. Specifically, the purity is insufficient: the total residual impurities such as Fe, Al, S, and C reach 1.5wt%, and the purity is only 98.5% (99.95% in Example 1), which cannot meet the stringent purity requirements of high-end catalysis or electronic packaging; the density defect: due to the presence of unmelted cores and bubbles inside, the density drops to 21.85g / cm 3 (Example 1 is 22.43 g / cm 3 ), the proportion close to the theoretical density dropped from 99.4% to 96.8%, and through cracks appeared in the thermal shock resistance test (10 cycles at 1000°C); the chemical stability was poor: the weight loss rate after immersion in aqua regia for 24 hours was as high as 1.2% (0.03% in Example 1), and the residual Ir-Fe alloy phase and carbide were preferentially corroded, resulting in the collapse of the powder structure.

[0060] Comparative Example 1 confirms that omitting the alkaline agent will trigger a chain reaction of "impurity solidification → surface passivation → spheroidization failure". Even if the same plasma spheroidization parameters are used, it is impossible to make up for the fundamental defects caused by the lack of pretreatment.

[0061] Comparative Example 2 The difference from Example 1 is that no pickling is performed.

[0062] In Comparative Example 2, the pickling process is skipped when preparing spherical iridium powder, and the product after the first roasting is directly subjected to plasma spheroidization. This seems to simplify the process, but in fact it triggers a series of chain reactions, resulting in overall deterioration of product performance.

[0063] In the product after the first calcination, there are a large amount of unreacted sodium hydroxide and sodium peroxide remaining, as well as undissolved impurities. In the high-temperature environment of plasma spheroidization, these alkaline substances rapidly melt and volatilize. Sodium hydroxide boils and decomposes to produce sodium oxide gas at about 1275 °C, and sodium oxide reacts with hydrogen to form sodium hydroxide aerosol. These aerosols not only pollute the plasma atmosphere but also adsorb on the surface of iridium droplets. The strongly polar sodium hydroxide interface layer seriously hinders the normal contraction of the surface tension of the droplets, causing the iridium droplets that should originally shrink into spheres under the action of surface tension to finally form serrated and irregular particle morphologies, resulting in a significant decrease in the sphericity rate. It can be found through scanning electron microscope observation that the proportion of particles with a roundness factor less than 0.8 in this comparative example exceeds 40%, while this proportion is extremely low in Example 1 (the proportion of particles less than 0.8 is less than 1%).

[0064] At the same time, impurities such as iron oxide and aluminum oxide generated during the first calcination process were not removed from the surface of the iridium particles due to the lack of an acid washing step. These impurities tightly wrap the iridium particles in the form of nanoscale particles, forming a "core-shell" structure. In the plasma spheroidization stage, the energy of the plasma needs to first melt this layer of impurity shell before it can act on the iridium particles, which undoubtedly greatly increases the difficulty of energy transfer. It is detected that compared with Example 1, the melting time of the iridium particles in this comparative example is extended by 5 ms, and there are still 30% of the particles with "unmelted core" defects, seriously affecting the quality of the iridium powder.

[0065] In addition, the lack of acid washing fails to effectively break the oxide film on the surface of the iridium particles. The iridium dioxide in the calcination product only undergoes lattice relaxation and is not converted into soluble sodium iridate. In subsequent processing, even under the same acid washing conditions, more than 50% of the oxide film remains. XPS detection shows that the 4f 7 / 2 peak position of iridium in this comparative example corresponds to iridium dioxide, confirming the existence of the oxide film. The remaining oxide film significantly increases the thermal resistance, making the central temperature of the iridium particles 150 - 200 °C lower than that in Example 1 under the same 70 kW power, and the energy absorption rate also drops from 90% to 60%, further exacerbating the problem of insufficient melting.

[0066] Finally, the spherical iridium powder prepared in Comparative Example 2 is far inferior to that in Example 1 in many key indicators. In terms of purity, due to the residual impurities such as iron and aluminum, the purity is only 99.2%, which is 0.75 times lower than that in Example 1; the sphericity drops to 82.3%, a decrease of 14.5 percentage points; the surface oxygen content soars to 3800 ppm, more than 5 times that in Example 1; the density decreases by 1.5%, and the content of excessive impurity sodium exceeds the standard by more than 500 times. These data fully prove that the pickling process is a necessary step to remove alkaline impurities and break the oxide film. Without this link, even if the parameters of subsequent processes such as plasma spheroidization are the same, high-quality spherical iridium powder cannot be prepared.

[0067] Comparative Example 3 The difference from Example 1 is that the temperature of the first roasting is 700 °C and the time is 6 h.

[0068] The main reasons for the failure mechanism of the low-temperature and short-time group (700 °C / 6 h) are incomplete chemical reactions and insufficient activation of the structure. The specific analysis of the incomplete chemical reaction is as follows: Insufficient iridium conversion: The reaction of NaOH with IrO2 (IrO2 + 2NaOH → Na2IrO3 + H2O↑) has a reaction rate constant k = 0.02 min at 700 °C -1 (k = 0.15 min at 800 °C -1 ). XRD shows that 25% of the unreacted IrO2 remains in the roasted product (less than 5% remains in Example 1), resulting in an iridium recovery rate of only 85% during subsequent pickling (98% in Example 1).

[0069] Incomplete decomposition of impurities: Fe and Al impurities exist in the form of FeO and Al2O3 (not converted into soluble NaFeO2 and NaAlO2). After pickling, the residual amount of Fe reaches 0.2 wt% (less than 0.01 wt% in Example 1). During plasma spheroidization, Fe-Ir alloy particles are formed, resulting in a decrease in sphericity.

[0070] The specific analysis of the insufficient activation of the structure is as follows: Limited specific surface area: The porosity of the low-temperature roasted product is only 15% (30% in Example 1), and the specific surface area is 2.3 m 2 / g (4.1 m 2 / g in Example 1). The plasma energy absorption rate drops to 70%. At the same power (70 kW), the melting time of iridium particles is extended to 12 ms (10 ms in Example 1), resulting in a sphericity of only 88.2%.

[0071] Residual oxide film: A 50 - 100 nm thick IrO2 film remains on the surface of iridium particles (the peak position of Ir4f7 / 2 detected by XPS is 71.2 eV), which hinders the reduction of H2, and the surface oxygen content reaches 2500 ppm (750 ppm in Example 1).

[0072] Comparative Example 4 The difference from Example 1 is that the power of plasma spheroidization is 40 kW, the temperature is 2500 °C, and the jet velocity is 150 m / s.

[0073] The specific analysis of the failure mechanism of the low - energy group (40 kW / 2800 °C / 150 m / s) in this comparative example is as follows: 1. Incomplete melting and insufficient energy: Temperature below the softening point: 2800 °C is lower than the softening point of the pretreated iridium powder (about 2800 °C), resulting in only a thin surface layer of iridium particles melting, and the interior remains solid (DSC detection shows that the melting enthalpy is only 60% of the theoretical value). SEM observes that 50% of the particles are in the shape of "hemispheres" or "dumbbells", and the diameter of the unmelted core reaches 5 - 10 μm.

[0074] Insufficient jet breaking force: The dynamic pressure generated by a jet velocity of 150 m / s is only 0.3 MPa (0.6 MPa in Example 1), which cannot effectively break the agglomerates of iridium particles larger than 30 μm, resulting in a wide droplet size distribution after atomization (D50 = 35 μm, span is 2.5), and the proportion of satellite balls reaches 22%.

[0075] 2. Surface oxidation and deterioration of sphericity: Decrease in hydrogen reduction efficiency: At low temperatures, the activation energy of H2 is insufficient (reaction rate constant k = 0.1×10 -4 cm 3 / (mol・s), 2.3×10 -4 cm 3 / (mol・s) in Example 1), the thickness of the surface oxide layer of iridium droplets reaches 20 nm (5 nm in Example 1), and the uneven surface tension results in a sphericity rate of only 82.1%.

[0076] Concentration of cooling stress: The difference in thermal expansion coefficients between the unmelted core and the molten layer (solid iridium: 6.5×10 -6 / °C vs liquid iridium: 7.2×10 -6 / °C) causes radial cracks, 40% of the particles have through - defects, and the density drops to 22.0 g / cm 3 (22.43 g / cm 3 in Example 1).

[0077] Comparative Example 5 The differences from Example 1 are as follows: Instead of performing the stepwise cooling in Step S4, direct rapid cooling is carried out, omitting the slow cooling stage in Step S4. After atomization, the product is only rapidly cooled (cooling rate: 5×10 4 K / s) to room temperature, completely skipping the stepwise process of "rapidly cooling to 1200°C and then slowly cooling". Other process parameters are the same as those in Example 1.

[0078] The defect analysis of omitting slow cooling in this comparative example is as follows: 1. Structure defects caused by thermal stress concentration: Stress generation mechanism: Rapid cooling (5×10 4 K / s) causes the surface of the iridium droplets to solidify within 1 ms to form a rigid shell layer, but the interior remains in a high-temperature liquid state (core temperature ≥ 2000°C). The difference in thermal expansion coefficients between the liquid iridium and the solid shell layer (liquid: 7.2×10 -6 / °C; solid: 6.5×10 -6 / °C) leads to a sudden increase in radial thermal stress. Calculations show that the maximum stress reaches 800 MPa (less than 50 MPa after slow cooling in Example 1); Defect manifestations: SEM observations show that more than 60% of the particles have through microcracks (crack width: 1 - 5 μm), and some particles are broken into fragments due to stress release (fragmentation rate is about 15%); X-ray diffraction (XRD) shows that the degree of lattice distortion (Δd / d) increases from 0.05% in Example 1 to 0.3%, indicating the presence of a high density of dislocations inside.

[0079] 2. Deterioration of densification and sphericity: Internal pores are retained. Rapid cooling inhibits the feeding process of the liquid iridium, and the unfrozen liquid iridium inside forms irregular pores due to volume shrinkage (CT scans show that the porosity increases from 1.2% in Example 1 to 6.5%). The density decreases from 22.43 g / cm 3 to 22.0 g / cm 3 , and the proportion close to the theoretical density decreases from 99.4% to 97.5%.

[0080] Increase in surface roughness: Driven by stress, local plastic deformation occurs in the solid shell layer, and "orange peel-like" protrusions appear on the particle surface (roughness Ra increases from 0.5 μm to 1.8 μm). The sphericity rate decreases from 96.8% to 89.2%, and the proportion of particles with a roundness factor ≥ 0.95 decreases by more than 20%.

[0081] 3. Decline in performance stability: Chemical stability: Microcracks and pores provide channels for corrosive media. The weight loss rate after soaking in aqua regia for 24 h increases from 0.03% to 0.2%. IrCl3 solution is generated due to electrochemical corrosion at the crack, resulting in the collapse of the powder structure.

[0082] Mechanical strength: Vickers hardness tests showed that the hardness of the iridium powder in Comparative Example 5 was unevenly distributed (400 - 600 HV, while that in Example 1 was 480 ± 10 HV). In the compression test, an obvious yield plateau appeared in the stress-strain curve, indicating the presence of a large number of mobile dislocations. In Example 1, due to sufficient stress release, it showed elastic deformation until fracture.

[0083] Comparative Example 6 The difference from Example 1 was that the classification in Step S5 was not carried out, and the iridium powder with mixed particle sizes after cooling (containing 15% fine powder < 10 μm, 12% coarse powder > 40 μm, and 8% irregular particles) was directly subjected to the second calcination, resulting in the failure of the overall process optimization effect, and the second calcination was directly carried out.

[0084] Performance analysis of the spherical iridium powder in this comparative example omitting the classification process: 1. Imbalance in calcination efficiency caused by uneven particle size: Due to the large particle size span of the unclassified iridium powder (D50 = 25 μm, span of 2.8), significant differences in heat conduction were presented during the second calcination. The reduction inside the coarse particles (> 40 μm) was incomplete: there were unfused nuclei or pores inside (CT scan showed a porosity of 8%). Heat needed to penetrate a solid shell layer of 20 - 40 μm, resulting in a temperature lag of 100 - 150 °C in the central region compared to the surface. XPS detection showed that the oxygen content in the center of the coarse particles reached 2500 ppm (the oxygen content in the center of Example 1 < 500 ppm). The reduction reaction (IrO2 + 2H2 → Ir + 2H2O↑) only remained on the particle surface, forming a "pure outside and impure inside" sandwich structure.

[0085] Agglomeration and contamination of fine particles (< 10 μm): The specific surface area of the fine powder was as high as 8 m 2 / g, with extremely strong surface activity. During calcination, it agglomerated into lumps of 50 - 100 μm due to van der Waals forces. The diffusion of hydrogen inside the lumps was blocked (the effective diffusion coefficient decreased from 0.1 cm 2 / s to 0.01 cm 2 / s), resulting in an increase in the local oxygen partial pressure, which instead promoted the formation of IrO2. The oxygen content in the center of the lumps suddenly increased to 4000 ppm, becoming an impurity enrichment area.

[0086] 2. Chain destruction of irregular particles and satellite balls: The 15% irregular particles (such as dumbbells and flakes) and 8% satellite balls that were not removed caused multiple defects during the roasting process, and the sphericity was irreversibly damaged: the irregular particles were worn due to the offset of the center of gravity when rolling and colliding in the roasting furnace. SEM showed jagged notches on their edges. The proportion of particles with a roundness factor <0.9 increased sharply from 3.2% in Example 1 to 30%, and the sphericity rate plummeted from 96.8% to 85.3%. Satellite balls (small particles attached to the main particles) slipped at the grain boundaries at the junction with the main particles at high temperatures, forming "tailing" or "tumor-like" protrusions, destroying the overall spherical symmetry.

[0087] Impurity enrichment and strength reduction at grain boundaries: The interface between the satellite ball and the main particle was originally enriched with impurities such as Na and Fe (EDS detected Na content of 1000ppm). During calcination, the diffusion rate of impurities along the grain boundary increased by 3 times, resulting in a decrease in grain boundary strength. In the compression test, the proportion of intergranular fractures soared from 10% in Example 1 to 45%, and the fracture surface showed an obvious rock candy-like morphology, indicating that the grain boundary has become the origin of brittle fracture.

[0088] 3. The performance of the final product has completely deteriorated. The lack of refined screening in the grading process has greatly deteriorated the key performance indicators of the iridium powder and caused the particle size distribution to be out of control: after the unclassified mixed powder is calcined, the fine powder agglomerates into coarse particles, and the surface of the coarse particles peels off to produce new fine powder. The particle size distribution span deteriorates from 1.3 in Example 1 to 2.8, which cannot meet the requirements of high-end catalyst carriers for particle size uniformity (D90-D10) / D50≤1.5).

[0089] Purity and surface quality decreased: the residual oxide layer inside the coarse particles and the impurities introduced by the agglomeration of fine powder caused the purity to drop from 99.95% to 99.6%; the surface roughness increased from Ra0.5μm to 1.2μm due to collision wear, the exposure rate of active sites decreased by 23% when used as a catalyst carrier, and the catalytic efficiency decreased by 18% when used for the first time.

[0090] Application performance failure: In the proton exchange membrane fuel cell test, the unclassified iridium powder caused the catalyst layer thickness to fluctuate due to uneven particle size, and the peak power density of the single cell decreased from 1.2 W / cm in Example 1 to 2 Down to 0.9W / cm 2 In aviation coating applications, irregular particles increase the porosity of the coating, causing the thermal shock resistance to drop from 50 cycles without cracks to 10 cycles with the appearance of network cracks.

[0091] 4. The pre-purification function of the classification process is irreplaceable. The failure of Example 6 is essentially that the classification process is not only a particle size screening, but also a key step for the removal of defective particles and material homogenization; Guarantee of heat conduction consistency: In Example 1, by grading, the particle size is controlled within 15 - 25 μm, ensuring that the Fourier number (Fo = 0.8 - 1.2) is consistent during roasting, and the depth of the reduction reaction is uniform; while the wide particle size distribution results in a Fourier number difference of up to 3 times, leading to the coexistence of "over-reduction" and "under-reduction".

[0092] Blocking of defect chain reaction: The satellite balls and irregular particles removed by grading are the main sources of spherical destruction and grain boundary defects during the roasting process. In Example 1, by grading, the proportion of defective particles is controlled below 5%, cutting off the "defect amplification - performance deterioration" chain from the source.

[0093] In summary, Comparative Example 6 proves from the opposite side that the grading process is the key bridge connecting cooling and roasting. Its absence will cause the subsequent processes to fail to achieve the design goals, further highlighting the creative value of the "grading and screening of target particle size" and "multi-stage cooling - roasting" in the method of this application.

[0094] Comparative Example 7 The difference from Example 1 is that the temperature of the second roasting is 1200 °C and the time is 6 h.

[0095] The high-temperature and long-time second roasting causes drastic changes in the internal structure of the iridium powder. 1200 °C has exceeded the critical temperature for significant coarsening of iridium grains (>1000 °C). Under the action of high temperature for up to 6 h, the average size of iridium grains increases sharply from 100 nm in Example 1 to 800 nm. The excessive coarsening of the grains seriously damages the microstructure of the iridium powder, resulting in a sharp drop in its hardness from Vickers hardness of 480 HV to 280 HV, a decrease of 41.7%, and at the same time, the toughness is significantly reduced, and problems such as fragmentation and cracking are likely to occur during subsequent processing.

[0096] At the same time, the excessive temperature and long time exacerbate the oxidation and volatilization of the iridium powder surface. Although under a hydrogen protection atmosphere, iridium still reacts with trace residual oxygen at 1200 °C. XPS detection shows that the surface oxygen content surges from 750 ppm in Example 1 to 2200 ppm, and the thickness of the oxide layer increases by nearly 2 times. Moreover, the volatilization loss rate of iridium soars from 1.5% in Example 1 to 6.8%, not only causing waste of precious metals, but also reducing the purity of the iridium powder to 99.2%. In addition, the reaction equilibrium of hydrogen reduction of iridium oxide at high temperature shifts reversely, and some of the reduced iridium is re-oxidized, further reducing the quality of the iridium powder.

[0097] In terms of macroscopic properties, the sphericity of the iridium powder prepared in Comparative Example 7 was severely damaged. Due to the decrease in the surface tension of the iridium droplets caused by high temperature and the coarsening of the grains, which destroyed the uniformity of the internal structure, the sphericity rate decreased significantly from 96.8% in Example 1 to 83.5%. A large number of pits and protrusions formed due to volatilization and oxidation appeared on the particle surface, and the surface roughness Ra increased from 0.5 μm to 2.1 μm, greatly affecting the dispersibility and fluidity of the iridium powder. In practical applications, when this deteriorated iridium powder was used as a catalyst support, the exposure rate of active sites decreased by 35%, and the catalytic efficiency was reduced by 28% compared to the iridium powder prepared in Example 1; when used for high-temperature coatings, due to the loose structure and increased impurities, its thermal shock resistance decreased significantly. In the 1000 °C cyclic test, large-area spalling occurred after only 15 cycles, far inferior to the excellent performance of Example 1, which had no cracks after 50 cycles.

[0098] It can be seen that the parameter setting of the second calcination temperature of 1200 °C and time of 6 h not only fails to improve the performance of spherical iridium powder, but instead, due to excessive high temperature and long-time action, a series of negative effects such as grain coarsening, increased oxidation, and increased volatilization loss are caused, resulting in a comprehensive decline in the quality of iridium powder, fully demonstrating the necessity and irreplaceability of the second calcination parameter range in Example 1 for ensuring the quality of iridium powder.

[0099] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit them; although the present application has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing examples, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the various examples of the present application.

[0100] In addition, those skilled in the art can understand that although some of the examples herein include certain features included in other examples but not others, the combination of the features of different examples means that it is within the scope of the present application and forms different examples. For example, in the above claims, any one of the claimed examples can be used in any combination. The information disclosed in this background art section is only intended to deepen the understanding of the overall background art of the present application, and should not be regarded as an admission or any form of suggestion that this information constitutes prior art known to those skilled in the art.

Claims

1. A method for preparing spherical iridium powder, characterized in that, Including: Mixing iridium raw material and an alkaline reagent to obtain a mixture, and subjecting the mixture to a first calcination to obtain a calcined product; Subjecting the calcined product to pickling to obtain a pickled product; Under a mixed atmosphere of argon and hydrogen, subjecting the pickled product to plasma spheroidization to obtain a plasma spheroidized product; Subjecting the plasma spheroidized product to atomization treatment to obtain an atomized product; Subjecting the atomized product to multi-stage cooling to obtain cooled iridium powder; Classifying the cooled iridium powder, and under a reducing atmosphere, performing a second calcination and cooling to obtain spherical iridium powder.

2. The preparation method of the spherical iridium powder according to claim 1, characterized in that, Satisfying at least one of the following conditions: A. The alkaline reagent includes sodium hydroxide and / or sodium peroxide; B. The mass ratio of the iridium raw material to the alkaline reagent is 1:2 - 5.

3. The preparation method of the spherical iridium powder according to claim 1, wherein, The temperature of the first calcination is 750°C - 800°C, and the time is 8h - 12h.

4. The method for preparing spherical iridium powder according to claim 1, wherein, Satisfying at least one of the following conditions: A. The acid solution for pickling includes hydrochloric acid and / or nitric acid; B. In the mixed atmosphere, the volume content of hydrogen is 5% - 10%.

5. The preparation method of the spherical iridium powder according to claim 1, characterized in that, The power of the plasma spheroidization is 50 - 80kW, the temperature is 3000°C - 3500°C, and the jet velocity is 200m / s - 300m / s.

6. The preparation method of the spherical iridium powder according to claim 1, characterized in that, Satisfying at least one of the following conditions: A. The pressure of the atomization is 1.0 kPa - 1.5 kPa; B. The energy density of the atomization is 25 J / cm 3 -40 J / cm 3 ; C. The sheath gas flow rate of the atomization is 20 L / min - 30 L / min; D. Before performing the atomization, the atomization device is also subjected to vacuum treatment, and the ultimate vacuum degree of the vacuum treatment is ≤ 5×10 -3 Pa.

7. The preparation method of the spherical iridium powder according to claim 1, characterized in that, The multi-stage cooling includes rapid cooling and slow cooling performed in sequence; The cooling rate of the rapid cooling is 10 4 K / s - 10 5 K / s; The cooling rate of the slow cooling is 10 2 K / s - 10 3 K / s; The end temperature of the rapid cooling is 1000°C - 1500°C.

8. The method for preparing spherical iridium powder according to claim 1, wherein, The particle size of the target iridium powder after classification is 10μm - 40μm.

9. The preparation method of the spherical iridium powder according to claim 1, wherein Satisfying at least one of the following conditions: A. The temperature of the second calcination is 800°C - 1000°C, and the time is 2h - 4h; B. The cooling rate ≤ 5°C / min.

10. The preparation method of the spherical iridium powder according to any one of claims 1-9, characterized in that, Satisfying at least one of the following conditions: A. The purity of the spherical iridium powder ≥ 99.9%; B. The sphericity of the spherical iridium powder ≥ 95%; C. The D50 of the spherical iridium powder is 15μm - 25μm.

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