A preparation method of spherical iridium powder

Abstract

The present application provides a method for preparing spherical iridium powder, relating to the field of iridium powder preparation. An iridium raw material and an alkaline reagent are mixed to obtain a mixture, and the mixture is subjected to a first calcination to obtain a calcined product; the calcined product is acid-washed to obtain an acid-washed product; the acid-washed product is plasma-spheroidized in a mixed atmosphere of argon and hydrogen to obtain a plasma-spheroidized product; the plasma-spheroidized product is atomized to obtain an atomized product; the atomized product is subjected to multi-stage cooling to obtain cooled iridium powder; the cooled iridium powder is graded, and a second calcination and cooling are performed in a reducing atmosphere to obtain spherical iridium powder. This method achieves spherical iridium powder with uniform particle size and high sphericity, thereby improving the density and stability of the spherical iridium powder.

Description

Technical Field

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

[0002] Space attitude and orbit control engines are core components of modern spacecraft. They operate at high temperatures and under heavy loads, requiring materials with high temperature resistance and strong stability to ensure efficient and reliable operation. Research is currently underway on high-performance green unit engines, which reach combustion temperatures exceeding 1600°C. Conventional metal materials suffer from deformation after test runs, making them inadequate for long-term operation. Therefore, a new material is needed to meet the requirements of space attitude and orbit control engines.

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

[0004] The iridium powder produced by the existing process has problems such as uneven particles and low sphericity, and also has certain deficiencies in density and stability.

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

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

[0007] To achieve the above objectives, the present application provides a method for preparing spherical iridium powder, comprising:

[0008] An iridium raw material and an alkaline reagent are mixed to obtain a mixture, and the mixture is subjected to a first calcination to obtain a calcined product;

[0009] acid-washing the calcined product to obtain an acid-washed product;

[0010] Under a mixed atmosphere of argon and hydrogen, the pickling product is subjected to plasma spheroidization to obtain a plasma spheroidized product; the plasma spheroidized product is subjected to atomization to obtain an atomized product;

[0011] The atomized product is subjected to multi-stage cooling to obtain cooled iridium powder;

[0012] The cooled iridium powder is classified, and then subjected to a second calcination and cooling in a reducing atmosphere to obtain spherical iridium powder.

[0013] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions:

[0014] A. The alkaline reagent includes sodium hydroxide and / or sodium peroxide;

[0015] B. The mass ratio of the iridium raw material to the alkaline reagent is 1:2-5.

[0016] Optionally, the temperature of the first calcination is 750° C.-800° C., and the time is 8 h-12 h.

[0017] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions:

[0018] A. The pickling acid solution includes hydrochloric acid and / or nitric acid;

[0019] B. The volume content of hydrogen in the mixed atmosphere is 5%-10%.

[0020] Optionally, 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.

[0021] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions:

[0022] A. The atomization pressure is 1.0 kPa -1.5 kPa;

[0023] B. The energy density of the atomization is 25 J / cm 3 -40J / cm 3 ;

[0024] C. The sheath gas flow rate of the atomization is 20 L / min-30 L / min;

[0025] D. Before the atomization, the atomization device is also vacuum treated, and the vacuum degree at the end of the vacuum treatment is ≤5×10 -3 Pa.

[0026] Optionally, the multi-stage cooling includes rapid cooling and slow cooling performed sequentially;

[0027] The cooling rate of the rapid cooling is 10 4 K / s -10 5 K / s;

[0028] The cooling rate of the slow cooling is 10 2 K / s -10 3 K / s;

[0029] The terminal temperature of the rapid cooling is 1000°C-1500°C.

[0030] Optionally, the particle size of the target iridium powder after classification is 10 μm-40 μm.

[0031] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions:

[0032] A. The second calcination temperature is 800°C-1000°C, and the time is 2h-4h;

[0033] B. The cooling rate is ≤5°C / min.

[0034] Optionally, the method for preparing the spherical iridium powder satisfies at least one of the following conditions:

[0035] A. The purity of the spherical iridium powder is ≥99.9%;

[0036] B. The sphericity of the spherical iridium powder is ≥95%;

[0037] C. The D50 of the spherical iridium powder is 15 μm-25 μm.

[0038] Compared with the prior art, the advantages of this application include:

[0039] The preparation method of spherical iridium powder provided in the present application shows significant advantages in sphericity, particle size uniformity, density and stability of iridium powder through multi-step collaborative innovation. First, the iridium raw material is mixed with an alkaline reagent and then subjected to a first roasting. In this step, the iridium is converted into a soluble salt through the reaction of the alkaline reagent with the iridium raw material, and impurities are effectively separated and removed. After acid washing, 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 reducing effect of hydrogen inhibits the surface oxidation of the iridium particles. Combined with the inert protection of argon, the molten iridium droplets are filled with water under the action of surface tension. The iridium powder shrinks into regular spheres, and the sphericity of the obtained iridium powder can reach more than 95%, which is a significant improvement over the traditional pure inert atmosphere spheroidization technology. The surface is smooth, with few defects, and the sphericity is close to the theoretical optimal state, achieving spherical iridium powder with uniform particle size and high sphericity, thereby improving the density and stability of the spherical iridium powder. After plasma spheroidization, the particles are broken into the target particle size range by atomization treatment using precisely controlled atomization pressure, energy density and sheath gas flow. Combined with the subsequent multi-stage cooling process, the formation of coarse grains is suppressed, forming a metastable dense structure, and then slowly cooled to promote grain boundary fusion and reduce internal pores, ultimately making the iridium powder density reach 22.4g / cm 3The above is close to the theoretical density of iridium, and the density is improved by about 2% compared with the traditional single cooling process, which effectively enhances the structural stability of the powder; the grading process further screens out the particle size of 10-40μm The target particles are obtained by controlling the D50 value between 15-25μm and the distribution span ≤1.5, achieving a high degree of uniformity in particle size, meeting the stringent requirements of high-end applications for powder dispersibility. The second calcination is carried out in a reducing atmosphere, which can eliminate the oxide layer that may remain on the surface of the particles, refine the grains and stabilize the crystal structure, so that the iridium powder performs well in corrosion resistance tests. 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 and reduces the subsequent impurity removal costs. It also has high energy utilization rate, and the overall energy consumption is reduced by 10%-15% compared with traditional processes. The material utilization rate is increased to more than 95% through precise grading. In practical applications, the high sphericity and narrow particle size distribution enable it to significantly increase the electrochemical active area when used as a catalyst carrier, and can reduce the amount of precious metals used by 20% when used in proton exchange membrane fuel cells. The high density and corrosion resistance make it excel in the field of aerospace coatings, with thermal shock resistance increased by 30%, effectively extending the service life of the coating. In summary, this method has achieved a technological breakthrough in the preparation of spherical iridium powder through process innovation, combining high performance with the advantages of industrial production, and providing a high-quality material foundation for related high-end fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope of the present application.

[0041] Figure 1 This is the SEM image of the round iridium powder prepared in Example 1. DETAILED DESCRIPTION

[0042] As used herein:

[0043] "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0044] The conjunction "consisting of" excludes any unspecified element, step, or component. If used in a claim, this phrase renders the claim closed, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the subject matter, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0045] When an amount, 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 as specifically disclosing all ranges formed by any pairing of any range upper limit or preferred value with any range lower limit or preferred value, regardless of whether the range is disclosed alone. For example, when a range of "1 to 5" is disclosed, the described range should be interpreted as including the range "1 to 4", "1 to 3", "1 to 2", "1 to 2 and 4 to 5", "1 to 3 and 5", etc. When a numerical range is described herein, unless otherwise stated, the range is intended to include its end values ​​and all integers and fractions within the range.

[0046] In these examples, parts and percentages are by mass unless otherwise indicated.

[0047] "Parts by mass" refers to the basic unit of measurement used to express the mass ratio of multiple components. One part can represent any unit of mass, such as 1g or 2.689g. For example, if we say that the parts by mass of component A are a parts and the parts by mass of component B are b parts, this means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, we could say that the mass of component A is aK and the mass of component B is bK (K is an arbitrary number representing a multiplication factor). It's important to note that, unlike parts by mass, the sum of the parts by mass of all components is not limited to 100 parts.

[0048] "And / or" is used to indicate that one or both of the stated situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0049] The present application provides a method for preparing spherical iridium powder, comprising:

[0050] An iridium raw material and an alkaline reagent are mixed to obtain a mixture, and the mixture is subjected to a first calcination to obtain a calcined product;

[0051] acid-washing the calcined product to obtain an acid-washed product;

[0052] Under a mixed atmosphere of argon and hydrogen, the pickling product is subjected to plasma spheroidization to obtain a plasma spheroidized product; the plasma spheroidized product is subjected to atomization to obtain an atomized product;

[0053] It is important to note 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 to enable the droplets to cool quickly and form spherical powders, avoiding adhesion or irregular shape of powder particles. Furthermore, the purpose of adding hydrogen during atomization is to utilize its high thermal conductivity (0.18 W / m·K) to accelerate droplet cooling, reduce surface tension, and promote sphericity, thereby balancing energy transfer efficiency and atomization stability.

[0054] The atomized product is subjected to multi-stage cooling to obtain cooled iridium powder;

[0055] The cooled iridium powder is classified, and then subjected to a second calcination and cooling in a reducing atmosphere to obtain spherical iridium powder.

[0056] In some embodiments, the method for preparing spherical iridium powder satisfies at least one of the following conditions:

[0057] A. The alkaline reagent includes sodium hydroxide and / or sodium peroxide;

[0058] It should be noted that when sodium hydroxide and sodium peroxide are used as alkaline reagents at the same time, the two can improve the preparation effect of spherical iridium powder through the following synergistic effects:

[0059] 1. Enhanced decomposition and oxidation ability: Sodium hydroxide, as a strong alkaline reagent, mainly undergoes double decomposition reaction with iridium raw materials (such as iridium oxides, halides, etc.) in a strong alkaline environment under the molten state, destroying the raw material structure and generating soluble iridium salts (such as Na2IrO3, etc.); Sodium peroxide has both strong alkalinity and strong oxidizing properties, and its peroxide (O2 2- ) decomposes during high temperature calcination to produce oxygen and active oxygen species (such as O - ), which can further oxidize the low-valent iridium or insoluble impurities (such as metal iridium particles, carbides, etc.) that may exist in the iridium raw material, and convert them into high-valent compounds (such as Ir 4+ or Ir 6+ The combination of the two can significantly improve the decomposition efficiency and oxidation degree of the iridium raw material, ensuring that the iridium in the product after roasting exists in a single high-valent state, laying the foundation for subsequent pickling and impurity removal;

[0060] 2. Optimize the structure of the calcined product: The strong hygroscopicity of sodium hydroxide can make the mixture form a uniform paste before calcination, promoting full contact between the iridium raw material 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 agglomeration of the melt. This porous structure can increase the contact area between the acid and the product during pickling, accelerate the dissolution of impurities (such as unreacted metal oxides and silicates, etc.), and prevent the iridium salt from being difficult to leached due to being wrapped in a dense structure, thereby improving the recovery rate of iridium and reducing residual impurities;

[0061] 3. Synergistic impurity removal and valence state stabilization: The oxidizing property of sodium peroxide can effectively decompose organic impurities or reducing substances (such as carbon and sulfur) mixed in the iridium raw material, oxidizing them into gases such as CO2 and SO2. Sodium hydroxide dissolves acidic impurities (such as Al2O3 and SiO2) in an alkaline environment to generate soluble sodium salts (such as NaAlO2 and Na2SiO3). The combination of the two can cover a wider range of impurity removal pathways, especially for impurities that are difficult to oxidize and difficult to dissolve in alkali (such as certain metal alloy phases). More thorough removal can be achieved through the synergistic effect of oxidation and alkaline dissolution. In addition, the strong oxidizing environment maintained by sodium peroxide can inhibit the accidental reduction of iridium to a metallic state during the roasting process, ensuring its existence in a stable high-valence state, and preventing the metal iridium particles from being difficult to melt and spheroidize during subsequent plasma spheroidization due to their high melting point (-2443°C), which affects the sphericity of the final powder.

[0062] 4. Reduce energy consumption and improve efficiency: The strong oxidizing property of sodium peroxide can reduce the activation energy required for the decomposition of iridium raw materials, allowing the first roasting temperature to be controlled in the lower range of 750-800℃ (if sodium hydroxide is used alone, higher temperatures may be required to completely decompose certain insoluble iridium compounds). At the same time, the roasting time is shortened to 8-12 hours, thereby reducing energy consumption. In addition, when the two are used together, the oxygen-releasing properties of sodium peroxide can reduce material splashing caused by water evaporation during the roasting process, improving operational safety. 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, coarse iridium powder, etc.), enhancing the universality of the process;

[0063] In summary, the synergistic effect of sodium hydroxide and sodium peroxide significantly improves the efficiency and quality of iridium raw material pretreatment through the multiple mechanisms of "alkaline dissolution-oxidation-structure regulation-coordinated removal of impurities", providing high-purity and easy-to-process precursors for subsequent plasma spheroidization, atomization and cooling classification steps, and ultimately ensuring the particle size uniformity, sphericity and density of spherical iridium powder.

[0064] B. The mass ratio of the iridium raw material to the alkaline reagent is 1:2-5.

[0065] 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 and 5.

[0066] In some embodiments, the first calcination is performed at a temperature of 750° C. to 800° C. for a time of 8 hours to 12 hours.

[0067] Optionally, the temperature of the first calcination can be 750°C, 760°C, 770°C, 780°C, 790°C, 800°C or any value between 750°C and 800°C, and the time can be 8h, 9h, 10h, 11h, 12h or any value between 8h and 12h.

[0068] It should be noted that the reaction activity of iridium raw materials is often reduced due to surface oxidation and coating with sulfides or organic matter, and impurities (such as Fe, Al, etc.) will interfere with subsequent reduction or spheroidization processes. This application performs surface activation treatment on them, and 750℃-800℃ can effectively decompose the organic matter and sulfides in the iridium raw materials, while avoiding excessive oxidation or melting of iridium metal. 8-12 hours ensure that iridium fully reacts with the alkaline flux and is converted into a soluble sodium salt (such as Na2IrO3), which is convenient for subsequent water leaching separation.

[0069] In some embodiments, the method for preparing spherical iridium powder satisfies at least one of the following conditions:

[0070] A. The pickling acid solution includes hydrochloric acid and / or nitric acid;

[0071] It should be noted that the acid solution can dissolve the calcined iridium raw material, remove the surface passivation layer, and enhance the reaction activity. Preferably, when the volume ratio of hydrochloric acid to nitric acid is 3:1, it can balance the oxidizing property and dissolution efficiency, and avoid excessive passivation of iridium. Through the above activation treatment, the surface passivation layer can be effectively removed, and the insoluble impurities can be decomposed, so that the iridium can enter the subsequent process in a highly active form.

[0072] B. The volume content of hydrogen in the mixed atmosphere is 5%-10%.

[0073] Optionally, the volume content of hydrogen can be 5%, 6%, 7%, 8%, 9%, 10% or any value between 5% and 10%.

[0074] It should be noted that when the volume content of hydrogen is 5%-10%, the high thermal conductivity of hydrogen can be used to improve energy transfer efficiency while reducing the risk of oxidation.

[0075] In some embodiments, the plasma spheroidization has a power of 50-80 kW, a temperature of 3000° C.-3500° C., and a jet velocity of 200 m / s-300 m / s.

[0076] Optionally, the power of 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 and 3500°C, and the jet velocity can be 200 m / s, 250 m / s, 300 m / s or any value between 200 m / s and 300 m / s.

[0077] It should be noted that iridium has a high melting point and high density. The temperature and energy density of the plasma in conventional plasma atomization technology cannot meet its melting and atomization requirements. Therefore, this application designs specific plasma parameters for the preparation of the pickling product (iridium powder after pickling), such as higher plasma temperature, better plasma jet velocity and power, etc., to ensure that the pickling product can be fully melted and evenly dispersed into droplets. Plasma spheroidization must ensure 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 that the high-density iridium raw material is fully melted and evenly atomized; when the jet velocity is 200m / s-300m / s, it can promote the dispersion efficiency of the iridium droplets and avoid droplet agglomeration due to excessive density;

[0078] It should also be noted that the plasma spheroidization process in this application is not a universal technology, but a key process designed specifically for iridium powder after the first roasting and pickling pretreatment. The two form a technical closed loop through "pretreatment to create spheroidization conditions - spheroidization parameters matching material properties". The specific correlation is reflected in the following core dimensions:

[0079] 1. Pretreatment imparts unique properties to iridium powder, which is a necessary prerequisite for the spheroidization process. Iridium powder without pretreatment faces multiple technical obstacles when directly subjected to plasma spheroidization due to the presence of a dense oxide film (such as IrO2, with a thickness of up to micrometers), an impurity coating (the content of base metals such as Fe and Al often exceeds 0.5wt%) on the surface, and a coarse grain structure. The oxide film causes the plasma energy to first break through the inert surface, resulting in an energy loss rate of over 40%. Even if the temperature is raised to above 3800°C, it is still difficult to melt uniformly. Low-melting-point impurities melt prematurely to form liquid phase adhesion, causing droplets to agglomerate and form a large number of satellite spheres. Coarse grains are prone to local overheating and splashing due to lattice stress concentration. The final spheroidization rate is only 80%-85%, and the surface oxygen content exceeds 5000ppm. This application uses a combined process of alkaline roasting at 750-800°C + acid washing to carry out directionally modified iridium powder:

[0080] a. Surface activity reconstruction: The synergistic alkali melting of sodium hydroxide and sodium peroxide breaks the original oxide film, and the acid washing further dissolves the residual alkaline salts and metal oxides to form a nano-scale porous surface (specific surface area from 0.5m2 / g increased to 3-5m 2 / g), so that the plasma energy directly acts on the metallic iridium particles, the energy absorption rate is increased to more than 90%, and rapid and uniform melting can be achieved at 3000-3500℃, reducing energy consumption by 30% compared with conventional processes;

[0081] b. Deep impurity removal: The strong oxidizing property of sodium peroxide decomposes non-metallic impurities such as S and C and causes them to escape as gas. Pickling reduces base metal impurities such as Fe and Al to below 0.01wt%, avoiding the formation of low-melting-point liquid phases or brittle intermetallic compounds at high temperatures, and ensuring that the iridium droplets shrink into regular spherical shapes under surface tension in the pure metallic state;

[0082] c. Crystal structure optimization: Low-temperature calcination refines iridium grains to 20-50μm and releases lattice stress, reducing the latent heat of melting by 15% and narrowing the standard deviation of droplet size distribution from ±15μm to ±5μm, significantly improving melting uniformity.

[0083] 2. Plasma spheroidization parameters are precisely customized according to the characteristics of the pre-treated material. The high activity, low impurity and porous structure of the pre-treated iridium powder determine the unique design of the plasma process parameters:

[0084] d. Matching temperature and power: After pretreatment, the thermal conductivity of iridium powder increases by 20%. A stable jet flow of 3000-3500°C can be maintained at a power of 50-80kW, ensuring that the iridium particles complete the entire "melting-dispersion-spheroidization" process within 10ms. If used for untreated iridium powder, the temperature distribution is uneven (fluctuation of ±200°C) at the same power, resulting in some particles not melting or excessive evaporation. While increasing the power to above 100kW can still melt, the excess energy will cause grain boundary oxidation, and the surface oxygen content will increase by more than 3 times.

[0085] e. Jet velocity and dispersion efficiency: After pretreatment, the particle size distribution of iridium powder is concentrated (D50 = 10-30μm, span ≤ 1.5). At a jet velocity of 200-300m / s, single particle precision dispersion can be achieved, avoiding droplet adhesion caused by agglomeration (adhesion rate <5%). Untreated iridium powder is prone to forming 50-100μm agglomerates due to surface impurities adsorption. It cannot be effectively dispersed at the same velocity and requires additional ultrasonic crushing (increasing costs by 20%). In addition, more than 10% of the agglomerates still enter the spheroidization stage, resulting in a satellite sphere ratio of over 20%;

[0086] f. Hydrogen-argon atmosphere synergistic protection: The surface activity of iridium powder after pretreatment is extremely high, and the 5-10% hydrogen content can instantly reduce the new oxide layer during the flight of the droplet (the reaction rate constant is 2.3×10 -4 cm 3 / (mol·s), making the surface tension close to the theoretical value of pure iridium (1.9N / m vs theoretical value 2.0N / m), ensuring sufficient spherical shrinkage. If used on un-acid-washed iridium powder, the residual alkaline salt will decompose at high temperature to produce CO2, which reacts with H2 to produce water vapor, which will aggravate surface oxidation and reduce the sphericity by more than 10%;

[0087] 3. Experimental data confirms the inseparability of process relevance. The applicant's laboratory data shows that even with extreme parameters (3800°C, pure argon atmosphere, 100kW power), the sphericity of untreated iridium powder is only 87.2%, the surface oxygen content exceeds 4000ppm, the energy consumption increases by 50% compared with the process of this application, and the iridium volatilization loss rate reaches 4.2%. However, the method of this application achieves key performance breakthroughs of sphericity ≥95%, surface oxygen content ≤1000ppm, energy consumption reduction of 30%, and recovery rate ≥98% through the synergistic effect of pretreatment and spheroidization. This difference essentially stems from the targeted regulation of the physical and chemical properties of iridium powder by pretreatment. Only the iridium powder that has undergone the first roasting + acid washing treatment of this application can achieve "efficient melting - uniform spheroidization - low-loss preparation" under the set plasma parameters. The technical relevance of the two constitutes the core innovation that distinguishes it from existing processes.

[0088] In summary, the plasma spheroidization process of the present application is not an independent link, but an organic whole deeply coupled with the previous pretreatment process. The pretreatment creates the necessary conditions for spheroidization: "highly active surface, low impurity content, and optimized crystal structure", while the spheroidization parameters are precisely adapted to the properties of the material after pretreatment. Both are indispensable and together constitute an efficient and high-quality technical path for the preparation of spherical iridium powder.

[0089] In some embodiments, the method for preparing spherical iridium powder satisfies at least one of the following conditions:

[0090] A. The atomization pressure is 1.0kPa-1.5kPa;

[0091] Optionally, the atomization pressure may 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 and 1.5 kPa;

[0092] It should be noted that the core step of the atomization process involves the fragmentation of the molten metal. When the atomization pressure is 1.0kPa-1.5kPa, a suitable pressure environment is maintained in the atomization chamber to prevent the active particles (such as plasma or high-speed airflow) from attenuating their energy due to excessive pressure, which affects the droplet fragmentation efficiency. By precisely controlling the pressure, the molten iridium droplets are evenly dispersed under the action of the gas kinetic energy, preventing particle adhesion or uneven size due to pressure fluctuations. Within this pressure range, the energy transfer requirements of the active particles are met while insufficient gas kinetic energy due to excessive vacuum is avoided.

[0093] B. The energy density of the atomization is 25 J / cm 3 -40J / cm 3 ;

[0094] Optionally, the energy density of the atomization can be 25 J / cm 3 、30J / cm 3 35J / cm 3 , 40J / cm 3 or 25J / cm 3 -40J / cm 3 Any value between

[0095] It should be noted that when the energy density of atomization is 25J / cm 3 -40J / cm 3 To match the high heat capacity requirement of iridium, the iridium particles are ensured to quickly absorb heat and completely melt in the high temperature area of ​​the plasma;

[0096] C. The sheath gas flow rate of the atomization is 20 L / min-30 L / min;

[0097] Optionally, the sheath gas flow rate for nebulization can be 20 L / min, 25 L / min, 30 L / min, or any value between 20 L / min and 30 L / min;

[0098] D. Before the atomization, the atomization device is also vacuum treated, and the vacuum degree at the end of the vacuum treatment is ≤5×10 -3 Pa.

[0099] Optionally, the vacuum degree at the end of the vacuum treatment can be 5×10 -3 Pa, 1×10 -3 Pa, 5×10 -4 Pa or ≤5×10 -3 Any value of Pa.

[0100] It should be noted that vacuum treatment can remove impurity gases such as oxygen and nitrogen in the atomization chamber, completely remove impurity gases, provide a clean environment for subsequent processes, and reduce their interference with the energy transfer path; reduce the probability of collision between gas molecules and high-temperature iridium droplets, avoid impurity elements (such as oxygen and carbon) from mixing into the powder, and ensure the purity of iridium powder.

[0101] In some embodiments, the multi-stage cooling includes rapid cooling and slow cooling performed sequentially;

[0102] It should be noted that the atomized iridium powder has problems such as particle adhesion, stress concentration, and impurity residue. Therefore, it is necessary to control the cooling rate of the atomized iridium powder particles and adopt a multi-stage cooling method. 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.

[0103] The cooling rate of the rapid cooling is 10 4 K / s -10 5 K / s;

[0104] Optionally, the cooling rate for rapid cooling may be 10 4 K / s, 5×10 4 K / s, 10 5 K / s or 10 4 K / s -10 5 Any value between K / s;

[0105] In some embodiments, the rapid cooling medium includes argon;

[0106] It should be noted that after atomization, iridium powder is prone to secondary collision and fusion due to the difference in kinetic energy of droplets. The forced convection of high-speed airflow can accelerate solidification and prevent adhesion into irregular particles. Rapid cooling causes the surface of the atomized droplets to solidify quickly, forming a dense shell layer, which inhibits droplet collision and adhesion.

[0107] The cooling rate of the slow cooling is 10 2 K / s -10 3 K / s;

[0108] Optionally, the cooling rate for slow cooling may be 10 2 K / s, 5×10 2 K / s, 10 3 K / s or 10 2 K / s -10 3 Any value between K / s;

[0109] In some embodiments, the slow cooling medium comprises argon;

[0110] It should be noted that when high-temperature iridium powder rapidly solidifies, the internal residual stress (about 200-500 MPa) will cause microcracks or lattice defects. Gradient cooling can relieve stress concentration and release internal thermal stress by slowly cooling, thereby reducing lattice distortion and improving sphericity.

[0111] The terminal temperature of the rapid cooling is 1000°C-1500°C.

[0112] Optionally, the endpoint temperature of the rapid cooling may be 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C or any value between 1000°C and 1500°C.

[0113] In some embodiments, the particle size of the classified target iridium powder is 10 μm-40 μm.

[0114] Optionally, the particle size of the target iridium powder after classification may be 10 μm, 20 μm, 30 μm, 40 μm, or any value between 10 μm and 40 μm.

[0115] In some embodiments, the method for preparing spherical iridium powder satisfies at least one of the following conditions:

[0116] A. The second calcination temperature is 800°C-1000°C, and the time is 2h-4h;

[0117] Optionally, the temperature of the second calcination may be 800° C., 900° C., 1000° C., or any value between 800° C. and 1000° C., and the time may be 2 h, 3 h, 4 h, or any value between 2 h and 4 h;

[0118] It should be noted that after classification, iridium powder still has internal defects (due to grain boundary impurities and uneven stress distribution), which makes it prone to thermal fatigue cracking in high-temperature applications (such as rocket nozzle coatings). Through the linkage of graded cooling and heat treatment, rapid solidification can inhibit particle adhesion, slow cooling can reduce stress, and heat treatment can further eliminate microscopic defects, ultimately achieving the performance index requirements of high purity, high sphericity and uniform particle size of iridium powder. In addition, the second roasting 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. The specific reaction is as follows:

[0119] IrO2+ 2H2→ Ir + 2H2O↑;

[0120] B. The cooling rate is ≤5°C / min.

[0121] Optionally, the cooling rate may be 5°C / min, 4°C / min, 3°C / min, 2°C / min, 1°C / min, or any value ≤5°C / min.

[0122] In some embodiments, the method for preparing spherical iridium powder satisfies at least one of the following conditions:

[0123] A. The purity of the spherical iridium powder is ≥99.9%;

[0124] Optionally, the purity of the spherical iridium powder may be 99.9%, 99.99%, 99.999% or any value ≥99.9%;

[0125] B. The sphericity of the spherical iridium powder is ≥95%;

[0126] Optionally, the sphericity of the spherical iridium powder may be 95%, 96%, 97%, 98%, 99% or any value ≥95%;

[0127] C. The D50 of the spherical iridium powder is 15 μm-25 μm.

[0128] Optionally, the D50 of the spherical iridium powder may be 15 μm, 20 μm, 25 μm, or any value between 15 μm and 25 μm.

[0129] The embodiments of the present application will be described in detail below in conjunction with specific examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, the conditions are carried out according to conventional conditions or manufacturer recommendations. The reagents or instruments used are not specified by the manufacturer and are conventional products that can be purchased commercially.

[0130] Example 1

[0131] This embodiment provides a method for preparing spherical iridium powder, and the specific steps are as follows:

[0132] S1: washing an iridium raw material, mixing the washed raw material with an alkaline reagent to obtain a mixture, wherein the mass ratio of the iridium raw material to the alkaline reagent is 1:3, wherein the alkaline reagent is sodium hydroxide and sodium peroxide in a mass ratio of 1:1, and performing a first calcination of the mixture (temperature 780° C., time 10 h) to obtain a calcined product;

[0133] S2: pickling the calcined product with aqua regia (concentrated hydrochloric acid and nitric acid in a volume ratio of 3:1) to obtain a pickled product;

[0134] S3: In a mixed atmosphere of argon and hydrogen (hydrogen volume content of 7%), the pickling product was subjected to plasma spheroidization, wherein the power of the plasma spheroidization was 70kW, the temperature was 3200℃, and the jet velocity was 250m / s to obtain a plasma spheroidized product; the atomizing device was subjected to vacuum treatment until the vacuum degree at the end of the vacuum treatment was 2×10 -3Pa, 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, increase the chamber pressure to 1.2 kPa, and atomize the plasma spheroidization product. The energy density of the atomization is 30 J / cm 3 , the sheath gas flow rate was 25 L / min, and the atomized product was obtained;

[0135] S4: The atomized product was first rapidly cooled by argon (cooling rate of 5×10 4 K / s) to initially solidify the droplets until they reach 1200 °C, and then slowly cool them with argon (cooling rate of 5 × 10 2 K / s), and the cooled iridium powder was obtained;

[0136] S5: The cooled iridium powder is classified to obtain a target graded product with a particle size of 15 μm-25 μm, and a second calcination is performed under a hydrogen atmosphere at a temperature of 900° C. for 3 hours, and then the temperature is lowered at a rate of 5° C. / min to obtain spherical iridium powder.

[0137] Performance test of spherical iridium powder: Purity: 99.95% (ICP-MS);

[0138] Sphericity: 96.8% (SEM image analysis, percentage of particles with a roundness factor ≥ 0.95);

[0139] D50: 20 μm, particle size distribution span 1.3 (laser particle size analyzer);

[0140] Density: 22.43g / cm 3 (Archimedes drainage method, close to the theoretical density of 22.56g / cm 3 );

[0141] Surface oxygen content: 750ppm (XPS detection), weight loss rate after immersion in aqua regia for 24 hours: 0.03%.

[0142] The SEM of the spherical iridium powder is as follows Figure 1 As shown, the surface of the spherical iridium powder is smooth and defect-free, the particle size is uniform, and the typical particle roundness factor reaches 0.97 (scale 50μm).

[0143] Example 2

[0144] The difference from Example 1 is that the alkaline reagent is sodium hydroxide.

[0145] Example 3

[0146] The difference from Example 1 is that the power of plasma spheroidization is 50 kW, the temperature is 3000° C., and the jet velocity is 200 m / s.

[0147] Comparative Example 1

[0148] The difference from Example 1 is that no alkaline reagent is added.

[0149] The core difference between this comparative example and Example 1 is that the alkaline reagent is completely omitted. The crude iridium powder containing 90% IrO2 is directly calcined in an air atmosphere at 780°C for 10 hours, and spherical iridium powder is attempted to be prepared through subsequent processes. However, this omission causes the entire process to fall into multiple technical bottlenecks, as shown below:

[0150] In the pretreatment stage, there are dual failures of impurity encapsulation and surface passivation. The calcination process without the participation of alkaline reagents can only trigger the physical sintering of the iridium raw material, and cannot break the surface oxide film and impurity encapsulation through chemical reactions. Among them, the stubbornness of the oxide film is manifested in that the IrO2 oxide film (thickness ≥ 200nm) on the surface of the iridium raw material is not converted into soluble Na2IrO3 under alkaline conditions. XPS detection shows that the Ir 4f7 / 2 peak position still corresponds to IrO2 (71.2eV) after calcination. After pickling, more than 50% of the oxide film remains, forming an "inert shell" that hinders subsequent reactions.

[0151] There are also solidified impurities. The Fe and Al impurities in the raw materials (original content 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 (XRD detects Fe3Ir and AlIr2 diffraction peaks), which cannot be dissolved by acid washing. Sulfides (such as IrS2) and organic matter, 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 carbon film thickness reaches 50-100nm. Moreover, the alkali-free calcined product forms a dense block structure with a porosity of <10% due to the lack of the "bubbling" effect of the alkali, and the specific surface area is only 0.8m² / g (4.1m² / g in Example 1). 2 / g), the acid penetration efficiency is low during pickling, and the same impurity removal effect cannot be achieved even after extending the pickling time by 3 times;

[0152] During the plasma spheroidization stage, there are energy barriers and spherical shrinkage obstacles. Pretreatment defects directly lead to the 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 25%, 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 generated 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 failed to reduce the oxygen-bound state in the raw materials. The IrO2 generated in the plasma could not be effectively reduced by H2 (the carbon film hindered gas diffusion). The surface oxygen content soared to 5800ppm (750ppm in Example 1), forming a thick oxide layer (thickness > 50nm);

[0153] As a result, the performance of the final product is generally degraded. The lack of synergistic effects of the alkaline reagent results in key performance indicators of the spherical iridium powder being far lower than those in Example 1. Specifically, the purity is insufficient: the total residual impurities such as Fe, Al, S, and C reach 1.5 wt%, 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 is defective: due to the presence of unmelted nuclei and bubbles inside, the density drops to 21.85 g / cm 3 (Example 1 is 22.43g / 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 carbides preferentially corroded, resulting in the collapse of the powder structure.

[0154] Comparative Example 1 confirms that omitting the alkaline reagent 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.

[0155] Comparative Example 2

[0156] The difference from Example 1 is that no pickling is performed.

[0157] In Comparative Example 2, the acid washing step was skipped when preparing the spherical iridium powder, and the product after the first roasting was directly subjected to plasma spheroidization. This seemed to simplify the process, but in fact it triggered a series of chain reactions, resulting in overall deterioration of product performance.

[0158] A large amount of unreacted sodium hydroxide and sodium peroxide, as well as undissolved impurities, remain in the product after the first calcination. In the high-temperature environment of plasma spheroidization, these alkaline substances rapidly melt and volatilize. Sodium hydroxide boils and decomposes at around 1275°C to produce sodium oxide gas, which in turn reacts with hydrogen to form sodium hydroxide aerosols. These aerosols not only pollute the plasma atmosphere but also adsorb on the surface of the iridium droplets. The highly polar sodium hydroxide interface layer severely hinders the normal contraction of the droplet surface tension, causing the iridium droplets, which should have contracted into spheres under the action of surface tension, to ultimately form jagged, irregular particle morphologies, resulting in a significant decrease in sphericity. Scanning electron microscopy observations show that the particles in this comparative example have a roundness factor less than 0.8, accounting for more than 40%, while this proportion is extremely low in Example 1 (particles with a roundness factor less than 0.8 account for less than 1%).

[0159] At the same time, impurities such as iron oxide and aluminum oxide produced in the first roasting process cannot be removed from the surface of the iridium particles due to the lack of a pickling step. These impurities tightly wrap the iridium particles in the form of nanoscale particles, forming a "core-shell" structure. During the plasma spheroidization stage, the energy of the plasma needs to melt this impurity shell before it can act on the iridium particles, which undoubtedly greatly increases the difficulty of energy transfer. According to detection, compared with Example 1, the melting time of the iridium particles in this comparative example is extended by 5ms, and 30% of the particles still have "unmelted core" defects, which seriously affects the quality of the iridium powder.

[0160] In addition, the lack of pickling means that the oxide film on the surface of the iridium particles cannot be effectively broken. The iridium dioxide in the calcined product only undergoes lattice relaxation and is not converted into soluble sodium iridate. In subsequent treatment, even if the same pickling conditions are used, more than 50% of the oxide film remains. XPS detection shows that the 4f 7 / 2 peak of iridium in this comparative example corresponds to iridium dioxide, confirming the presence of the oxide film. The residual oxide film greatly increases the thermal resistance, so that at the same power of 70kW, the center temperature of the iridium particles is 150-200°C lower than that of Example 1, and the energy absorption rate also drops from 90% to 60%, further exacerbating the problem of insufficient melting.

[0161] Ultimately, the spherical iridium powder produced in Comparative Example 2 was significantly inferior to that of Example 1 in several key performance indicators. In terms of purity, due to residual impurities such as iron and aluminum, the purity was only 99.2%, 0.75 times lower than that of Example 1. The sphericity rate dropped to 82.3%, a decrease of 14.5 percentage points. The surface oxygen content soared to 3800 ppm, more than five times that of Example 1. The density decreased by 1.5%, and the impurity sodium content exceeded the standard by more than 500 times. These data clearly demonstrate that the pickling step is essential for removing alkaline impurities and breaking down the oxide film. Without this step, subsequent processes such as plasma spheroidization, even with the same parameters, cannot produce high-quality spherical iridium powder.

[0162] Comparative Example 3

[0163] The difference from Example 1 is that the temperature of the first calcination is 700° C. and the time is 6 h.

[0164] The main reasons for the failure mechanism of the low-temperature short-time group (700℃ / 6h) are incomplete chemical reaction and insufficient structural activation. The specific analysis of incomplete chemical reaction is as follows:

[0165] 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=0.15min at 800℃ -1 ), XRD showed that 25% of unreacted IrO2 remained in the calcined product (<5% in Example 1), resulting in an iridium recovery rate of only 85% during subsequent acid washing (98% in Example 1).

[0166] Impurity decomposition is not complete: Fe and Al impurities exist in the form of FeO and Al2O3 (not converted into soluble NaFeO2 and NaAlO2). The residual Fe content after pickling reaches 0.2wt% (Example 1 <0.01wt%), and Fe-Ir alloy particles are formed during plasma spheroidization, resulting in a decrease in the sphericity.

[0167] The specific analysis of insufficient structural activation is as follows:

[0168] Limited specific surface area: The porosity of the low-temperature calcined product is only 15% (30% in Example 1), and the specific surface area is 2.3m 2 / g (Example 1 is 4.1m 2 / g), the plasma energy absorption rate dropped to 70%, and the melting time of the iridium particles was extended to 12ms (10ms in Example 1) at the same power (70kW), resulting in a spherical rate of only 88.2%.

[0169] Oxide film residue: A 50-100 nm thick IrO2 film remains on the surface of the iridium particles (XPS detection of the Ir4f7 / 2 peak at 71.2 eV), which hinders H2 reduction, and the surface oxygen content reaches 2500 ppm (750 ppm in Example 1).

[0170] Comparative Example 4

[0171] 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.

[0172] The failure mechanism of the low-energy group (40kW / 2800℃ / 150m / s) in this comparative example is analyzed as follows:

[0173] 1. Insufficient melting and insufficient energy:

[0174] The temperature is lower than the softening point: 2800℃ is lower than the softening point of the pretreated iridium powder (about 2800℃), resulting in only a thin layer of surface melting of the iridium particles, while the interior remains solid (DSC detection shows that the melting enthalpy is only 60% of the theoretical value). SEM observations show that 50% of the particles are "hemispherical" or "dumbbell-shaped", and the diameter of the unmelted core is 5-10μm.

[0175] Insufficient jet fragmentation force: The dynamic pressure generated by the jet velocity of 150 m / s is only 0.3 MPa (0.6 MPa in Example 1), which cannot effectively break up iridium particle agglomerates larger than 30 μm. As a result, the droplet size distribution after atomization is wide (D50 = 35 μm, span of 2.5), with satellite balls accounting for 22%.

[0176] 2. Surface oxidation and sphericity deterioration:

[0177] Hydrogen reduction efficiency decreases: the activation energy of H2 is insufficient at low temperatures (reaction rate constant k = 0.1×10 -4 cm 3 / (mol・s), Example 1 is 2.3×10 -4 cm 3 / (mol・s)), the thickness of the oxide layer on the surface of the iridium droplet reached 20nm (5nm in Example 1), and the uneven surface tension resulted in a sphericity rate of only 82.1%.

[0178] Cooling stress concentration: the difference in thermal expansion coefficient between the unmelted core and the molten layer (solid iridium: 6.5×10 -6 / ℃ vs liquid iridium: 7.2×10 -6 / ℃) caused radial cracks, 40% of the particles showed penetrating defects, and the density dropped to 22.0g / cm 3 (Example 1 is 22.43g / cm 3 ).

[0179] Comparative Example 5

[0180] The difference from Example 1 is that the step cooling in step S4 is not performed, and the product is directly cooled rapidly, and the slow cooling stage in step S4 is omitted. The atomized product is only cooled rapidly (cooling rate 5×10 4 K / s) to room temperature, completely skipping the grading process of "rapid cooling to 1200°C followed by slow cooling", and the other process parameters were consistent with those in Example 1.

[0181] The defect analysis of omitting slow cooling in this comparative example is as follows:

[0182] 1. Thermal stress concentration causes structural defects:

[0183] Stress generation mechanism: Rapid cooling (5×10 4 K / s) causes the surface of the iridium droplet to solidify within 1ms to form a rigid shell, but the interior is still in a high-temperature liquid state (core temperature ≥ 2000°C). The difference in thermal expansion coefficient between the liquid iridium and the solid shell (liquid: 7.2×10 -6 / ℃; solid state: 6.5×10 -6 / ℃) leads to a sudden increase in radial thermal stress, and calculations show that the maximum stress reaches 800MPa (<50MPa after slow cooling in Example 1);

[0184] Defect manifestation: SEM observation showed that more than 60% of the particles had penetrating microcracks (crack width 1-5μm), and some particles broke into fragments due to stress release (breakage rate of about 15%). X-ray diffraction (XRD) showed that the degree of lattice distortion (Δd / d) increased from 0.05% in Example 1 to 0.3%, indicating the presence of a high density of dislocations inside.

[0185] 2. Deterioration of density and sphericity:

[0186] Internal pores are retained, and rapid cooling inhibits the shrinkage process of liquid iridium. The unsolidified liquid iridium inside shrinks to form irregular pores (CT scanning shows that the porosity increases from 1.2% in Example 1 to 6.5%), and the density increases from 22.43 g / cm 3 Reduced to 22.0g / cm 3 , the proportion close to theoretical density dropped from 99.4% to 97.5%.

[0187] Increased surface roughness: Driven by stress, the solid shell undergoes local plastic deformation, and "orange peel-like" protrusions appear on the particle surface (roughness Ra increases from 0.5μm to 1.8μm). 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%.

[0188] 3. Decreased performance stability:

[0189] Chemical stability: Microcracks and pores provide channels for corrosive media. The weight loss rate increases from 0.03% to 0.2% after immersion in aqua regia for 24 hours. IrCl3 solution is generated at the cracks due to electrochemical corrosion, causing the powder structure to collapse.

[0190] Mechanical strength: Vickers hardness testing showed that the iridium powder in Comparative Example 5 had an uneven hardness distribution (400-600 HV, compared to 480±10 HV in Example 1). A significant yield plateau appeared in the stress-strain curve during the compression test, indicating the presence of a large number of movable dislocations. In contrast, Example 1 exhibited sufficient stress release, exhibiting elastic deformation until fracture.

[0191] Comparative Example 6

[0192] The difference from Example 1 is that the classification in step S5 is not performed, and the iridium powder with mixed particle size after cooling (containing 15% fine powder <10 μm, 12% coarse powder >40 μm and 8% irregular particles) is directly subjected to the second calcination, resulting in the failure of the optimization effect of the whole process, and the second calcination is directly performed.

[0193] Performance analysis of spherical iridium powder in this comparative example in which the classification process is omitted:

[0194] 1. Unbalanced roasting efficiency caused by uneven particle size:

[0195] Due to the large particle size span (D50 = 25 μm, span of 2.8), the unclassified iridium powder showed significant differences in heat conduction during the second roasting. The internal reduction of the coarse particles (> 40 μm) was not complete: there were unmelted cores or pores inside (CT scan showed a porosity of 8%). Heat needed to penetrate the 20-40 μm solid shell, resulting in the temperature of the central area lagging behind the surface by 100-150°C. XPS testing showed that the central oxygen content of the coarse particles reached 2500 ppm (the central oxygen content in Example 1 was <500 ppm), and the reduction reaction (IrO2 + 2H2 → Ir + 2H2O↑) only remained on the surface of the particles, forming a "pure outside and impure inside" sandwich structure.

[0196] Fine particle (<10μm) agglomeration pollution: fine powder specific surface area up to 8m 2 / g, with extremely strong surface activity, it aggregates into 50-100μm agglomerates due to van der Waals forces during calcination, and the diffusion of hydrogen inside the agglomerates is hindered (the effective diffusion coefficient is from 0.1cm 2 / s down to 0.01cm 2 / s), the local oxygen partial pressure increases, which in turn promotes the formation of IrO2. The oxygen content in the center of the agglomerate suddenly increases to 4000ppm, becoming an impurity-rich area.

[0197] 2. Chain destruction of irregular particles and satellite balls:

[0198] The 15% of irregular particles (such as dumbbells and flakes) and 8% of satellite particles that were not removed caused multiple defects during the roasting process, resulting in irreversible damage to sphericity. Due to the shift in their center of gravity, the irregular particles suffered surface wear from rolling and colliding in the roasting furnace. SEM images revealed jagged notches on their edges. The proportion of particles with a roundness factor <0.9 surged from 3.2% in Example 1 to 30%, and the sphericity rate plummeted from 96.8% to 85.3%. Satellite particles (small particles attached to the main particles) experienced grain boundary sliding at the junction with the main particles at high temperatures, forming "tails" or "lumpy" protrusions that disrupted the overall spherical symmetry.

[0199] Impurity enrichment at grain boundaries and decreased strength: The interface between the satellite sphere and the main particle was originally enriched with impurities such as Na and Fe (EDS analysis revealed a Na content of 1000 ppm). During calcination, the diffusion rate of impurities along the grain boundaries increased threefold, resulting in a decrease in grain boundary strength. During compression testing, the proportion of intergranular fractures soared from 10% in Example 1 to 45%, with the fracture surface exhibiting a distinct rock candy-like morphology, indicating that the grain boundaries had become the source of brittle fracture.

[0200] 3. The performance of the final product has completely deteriorated. The lack of refined screening in the grading process has caused a significant deterioration in the key performance indicators of the iridium powder, and the particle size distribution is 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 particle size uniformity requirements of high-end catalyst carriers (D90-D10) / D50≤1.5).

[0201] Purity and surface quality decreased: impurities introduced by residual oxide layers inside coarse particles and agglomeration of fine powders 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.

[0202] Application performance failure: In the proton exchange membrane fuel cell test, the uneven particle size of the unclassified iridium powder caused the catalyst layer thickness to fluctuate, and the peak power density of the single cell decreased from 1.2 W / cm 2 Reduced to 0.9W / cm 2 In aviation coating applications, irregular particles cause an increase in coating porosity, reducing thermal shock resistance from no cracks after 50 cycles to the appearance of network cracks after 10 cycles.

[0203] 4. The pre-purification function of the classification process is irreplaceable. The failure of Comparative Example 6 is essentially because the classification process is not only for particle size screening, but also a key step for removing defective particles and homogenizing the material.

[0204] Guaranteeing consistent thermal conductivity: In Example 1, the particle size is controlled within 15-25 μm through classification, ensuring a consistent Fourier number (Fo=0.8-1.2) during calcination and a uniform depth of reduction reaction. However, a wide particle size distribution results in a difference of up to 3 times in the Fourier number, leading to the coexistence of "over-reduction" and "under-reduction".

[0205] Blocking the defect chain reaction: The satellite balls and irregular particles removed by classification are the main sources of spherical destruction and grain boundary defects during the roasting process. Example 1 controls the proportion of defective particles to less than 5% through classification, cutting off the "defect amplification-performance degradation" chain from the source.

[0206] 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 process to be unable to achieve the design goals, further highlighting the creative value of the coordinated optimization of "grading screening target particle size" and "multi-stage cooling-roasting" in the method of this application.

[0207] Comparative Example 7

[0208] The difference from Example 1 is that the second calcination temperature is 1200° C. and the time is 6 hours.

[0209] The high-temperature, long-term second calcination causes drastic changes in the internal structure of the iridium powder. 1200°C exceeds the critical temperature (>1000°C) for significant coarsening of iridium grains. After six hours of high temperature, the average iridium grain size increased dramatically from 100nm in Example 1 to 800nm. This excessive grain coarsening severely disrupts the iridium powder's microstructure, causing its hardness to drop sharply from 480HV Vickers to 280HV, a 41.7% decrease. This also significantly reduces toughness, making it highly susceptible to breakage and cracking during subsequent processing.

[0210] At the same time, excessively high temperatures and excessively long times exacerbate the oxidation and volatilization of the iridium powder surface. Although iridium still reacts with trace amounts of residual oxygen at 1200°C under a hydrogen protective atmosphere, XPS detection shows that the surface oxygen content surges from 750ppm in Example 1 to 2200ppm, and the thickness of the oxide layer increases nearly 2 times. In addition, the volatilization loss rate of iridium soars from 1.5% in Example 1 to 6.8%, which not only causes waste of precious metals, but also causes the purity of iridium powder to drop to 99.2%. In addition, the reaction equilibrium of hydrogen reduction of iridium oxide at high temperature moves in the opposite direction, and part of the reduced iridium is re-oxidized, further reducing the quality of the iridium powder.

[0211] In terms of macroscopic performance, the sphericity of the iridium powder prepared in Comparative Example 7 is seriously impaired. Due to the high temperature, the surface tension of the iridium droplets is reduced, and the grain coarsening destroys the uniformity of the internal structure. The sphericity rate drops sharply from 96.8% in Example 1 to 83.5%. A large number of pits and protrusions formed by volatilization and oxidation appear on the surface of the particles, and the surface roughness Ra increases from 0.5μm to 2.1μm, which greatly affects the dispersibility and fluidity of the iridium powder. In actual applications, when this degraded iridium powder is used as a catalyst carrier, the active site exposure rate is reduced by 35%, and the catalytic efficiency is reduced by 28% compared with 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 is significantly reduced. In the 1000℃ cycle test, large-scale peeling occurs after only 15 cycles, which is far inferior to the excellent performance of Example 1 without cracks after 50 cycles.

[0212] It can be seen that the parameter setting of the second calcination temperature of 1200°C and time of 6h not only fails to improve the performance of the spherical iridium powder, but also causes a series of negative effects such as grain coarsening, increased oxidation, and increased volatilization loss due to excessive high temperature and long time, resulting in an overall decline in the quality of the iridium powder. This fully proves the necessity and irreplaceability of the second calcination parameter range in Example 1 for ensuring the quality of iridium powder.

[0213] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

[0214] Furthermore, those skilled in the art will appreciate that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and to form distinct embodiments. For example, in the claims above, any of the claimed embodiments may be used in any combination. The information disclosed in this background section is intended solely to enhance understanding of the overall background of this application and should not be construed as an admission or any implication that such information constitutes prior art known to those skilled in the art.

Claims

1. A method for preparing spherical iridium powder, characterized in that: include: An iridium raw material and an alkaline reagent are mixed to obtain a mixture, and the mixture is subjected to a first calcination to obtain a calcined product; acid-washing the calcined product to obtain an acid-washed product; Under a mixed atmosphere of argon and hydrogen, the acid-washed product is subjected to plasma spheroidization to obtain a plasma spheroidized product; atomizing the plasma spheroidization product to obtain an atomized product; The atomized product is subjected to multi-stage cooling to obtain cooled iridium powder; The cooled iridium powder is classified, and then subjected to a second calcination and cooling in a reducing atmosphere to obtain spherical iridium powder.

2. The preparation method of spherical iridium powder according to claim 1, wherein At least one of the following conditions is met: 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 spherical iridium powder according to claim 1, wherein The temperature of the first calcination is 750° C.-800° C., and the time is 8 h-12 h.

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

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

6. The method for preparing spherical iridium powder according to claim 1, wherein At least one of the following conditions is met: A. The atomization pressure is 1.0 kPa -1.5 kPa; B. The energy density of the atomization is 25 J / 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 vacuum treated, and the vacuum degree at the end of the vacuum treatment is ≤5×10 -3 Pa.

7. The method for preparing spherical iridium powder according to claim 1, wherein The multi-stage cooling includes rapid cooling and slow cooling performed sequentially; 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 terminal 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 10 μm-40 μm.

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

10. The method for preparing spherical iridium powder according to any one of claims 1 to 9, characterized in that: At least one of the following conditions is met: 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 15 μm-25 μm.

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