Preparation method of nano tungsten bronze powder

Abstract

The invention belongs to the technical field of infrared wave-absorbing functional materials, and provides a preparation method of nano tungsten bronze powder. The method comprises the following steps: taking ammonium metatungstate and cesium carbonate as raw materials, reacting under a low-temperature heating condition, performing evaporative crystallization to obtain a cesium tungstate precursor, performing coarse grinding to obtain primary fine powder, performing hydrogen reduction calcination to obtain tungsten bronze powder, and finally performing sanding again to reduce the particle size of the powder to nanoscale. Compared with traditional processes such as hydrothermal and solvothermal processes, the method does not need high-temperature and high-pressure conditions, the process is safe and energy-saving, and the prepared nano tungsten bronze powder is uniform in particle size distribution, low in energy consumption, low in cost, high in yield and suitable for large-scale production.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of infrared absorbing functional materials, and in particular relates to a method for preparing nano tungsten bronze powder. Background Art

[0002] The preparation methods for nano-tungsten bronze powder are mainly divided into liquid phase methods (such as hydrothermal method, solvothermal method, sol-gel method, etc.), solid phase method and gas phase method. These methods have their own advantages, but they all have certain limitations in the large-scale preparation of nano-scale, high-performance tungsten bronze powder.

[0003] Traditional liquid-phase methods (hydrothermal / solvothermal) often have limited single-batch yields, making it difficult to produce large quantities of products in a short period of time. For example, hydrothermal methods are limited by reactor volume, resulting in limited output and long reaction cycles. While solvothermal methods can slightly increase reaction rates (particularly with microwave assistance, which only requires minutes to hours), they are limited by the high cost of organic media and are generally only suitable for small-scale production. Consequently, these methods have low production efficiency and are unable to meet the needs of large-scale industrial production. Solid-phase methods inherently react rapidly (high-temperature reactions typically complete within a few hours) and can accommodate large amounts of raw materials in the furnace, resulting in high absolute yields per single synthesis. Therefore, solid-phase methods offer advantages in raw material conversion and mass production, and are often considered high-yield processes. However, tungsten bronze produced by traditional solid-phase methods is in micron-sized lumps, requiring further wet grinding to produce nanopowders. This grinding process, if completed in a single step, is time-consuming and inefficient, reducing overall production efficiency. Gas-phase methods (such as spray pyrolysis) can continuously produce nanoparticles, theoretically offering impressive yields. However, in practice, maintaining product quality often requires a low precursor flow rate. Otherwise, particles will agglomerate severely or the reaction will be incomplete. This limits the amount of product collected per unit time. Furthermore, the high cost of continuous equipment operation also limits economic productivity. Overall, the gas phase method offers good continuity, but this does not necessarily result in high output per unit energy, necessitating a trade-off between yield and quality.

[0004] Traditional methods also present safety issues. The main safety risks of hydrothermal and solvent thermal methods lie in high temperatures, high pressures, and the use of organic solvents. Hydrothermal reactions are often carried out in sealed reactors at pressures exceeding tens of atmospheres. If the seal fails or the process is improperly operated, explosions can occur. Solvothermal reactions use flammable organic solvents, posing both fire and explosion risks under high temperature and high pressure. Furthermore, precautions must be taken during the high-temperature unloading and cleaning processes to prevent burns and organic volatilization. Generally speaking, liquid-phase methods require strict high-pressure containment and ventilation, resulting in a lower safety margin. Solid-phase methods are typically conducted in atmospheric-pressure, high-temperature furnaces, eliminating the need for high-pressure vessels and thus avoiding the risk of explosion. Furthermore, they do not use large amounts of organic solvents, making them inherently safer. The main risk of these methods lies in the use of a reducing atmosphere, such as hydrogen, which can create combustion and explosion hazards if handled improperly. However, hydrogen reduction in high-temperature furnaces is a mature industrial technique, and risks can be mitigated through inert gas dilution and monitoring. Furthermore, traditional solid-phase methods sometimes employ strong reducing agents, such as metallic sodium, for mechanical milling reduction. This method carries the risk of violent reactions and fire, making it unsuitable for large-scale production. Gas-phase methods involve high-temperature flames or plasma, and similarly require careful handling of flammable gases and high-temperature materials. For example, spray pyrolysis often uses a hydrogen / inert gas mixture as a carrier gas and reducing agent, requiring the prevention of hydrogen leaks. Plasma methods, involving high-voltage power supplies and plasma jets, also present certain risks. Overall, gas-phase methods require stringent safety controls, as they present the risk of accidents caused by high-temperature flames or flammable gases.

[0005] Each method has different performance in particle size control: hydrothermal / solvothermal methods excel at directly synthesizing nanoparticles. By adjusting reactant concentrations, temperature, time, and additives, the product morphology and size can often be controlled to within tens to hundreds of nanometers. For example, there are reports that Cs particles of approximately 50 nm can be directly obtained by hydrothermal treatment at 190°C. 0.32WO3 nanoparticles; Solvothermal reaction combined with surfactants can also produce specific morphologies such as nanosheets and nanorods. Generally speaking, liquid-phase methods offer fine particle size control due to the adjustable nucleation and growth processes, resulting in small, narrowly distributed products. However, if the reaction time is too long or post-processing is inappropriate, particles may agglomerate. Furthermore, liquid-phase products often require a drying process, which can lead to the formation of hard agglomerates if care is not taken. Solid-phase methods typically produce directly sintered particles / agglomerates with particle sizes far exceeding the nanometer scale. Because high-temperature solid-phase reactions are diffusion processes, small particles grow and adhere at high temperatures. Adjusting parameters to achieve specific nanometer sizes with solid-phase methods is difficult unless mechanical forces (such as mechanical activation or ball milling) are introduced. For example, the aforementioned solid-phase ball milling reduction method can directly produce particles with a size of 50 nm, but this requires high-energy ball milling for up to 40 hours and carries significant risks. Consequently, particle size control with traditional solid-phase methods is difficult, resulting in a wide particle size distribution and requiring subsequent pulverization to reduce the particle size. Gas-phase methods offer intermediate performance in terms of particle size control. The particles produced by rapid thermal decomposition are often in the nanometer range due to high temperature quenching. However, the residence time and concentration of the particles in the gas phase will affect their degree of agglomeration - when the concentration is too high, collision and growth are likely to occur, while when the concentration is too low, the efficiency is low. By adjusting the precursor concentration, carrier gas flow rate, temperature, etc., the average particle size can be controlled to a certain extent. For example, a study obtained highly crystallized and precisely stoichiometric Cs by optimizing the temperature and hydrogen concentration of spray pyrolysis. 0.32 WO3 nanoparticles exhibit even better near-infrared absorption than those produced by the liquid-phase method. This suggests that particle size and composition can be controlled through vapor-phase conditions. However, precise control of individual particle size is difficult in industrial applications, so powders produced by the vapor-phase method may have a broad particle size distribution, necessitating a classification step.

[0006] Energy consumption and costs vary across methods. Liquid-phase methods, due to their relatively low reaction temperatures (<200°C), may appear energy-efficient. However, the long heating periods maintained under closed conditions, coupled with extensive post-reaction washing and drying, contribute significantly to overall energy consumption. Hydrothermal methods, in particular, often require 12–24 hours of heating and insulation. Solvothermal methods, however, face high solvent vapor pressure at high temperatures, requiring thick-walled containers with low heat transfer efficiency, further increasing energy consumption. Furthermore, organic solvent loss and recovery contribute to the cost. Numerous publications have highlighted the high energy consumption and time-consuming nature of traditional liquid-phase methods. For example, some commentators claim that hydrothermal and solvent-thermal methods, under high temperature and pressure, require long preparation cycles and high energy input, making them unsuitable for energy-sensitive large-scale production. Solid-phase methods, on the other hand, react rapidly at high temperatures and utilize concentrated heat energy. For example, calcination at 750–800°C completes the reaction in just two hours. Despite the high temperatures, the total heating time is short, and if the furnace is well insulated, the heat energy consumption per unit mass of product may not be significantly higher than that of hydrothermal methods. Furthermore, there is no subsequent, lengthy drying step. However, if the energy consumption of the grinding process is added, the overall energy consumption of the traditional solid-phase process increases. The long-term ball milling / sand milling requires motor power input, which indirectly increases costs. Furthermore, if hydrogen is used in solid-phase reduction, the production and safe use of hydrogen itself has certain costs. According to industry reports, large cesium tungsten bronze particles produced by the solid-phase method must be ground to <100nm using high-end dispersion equipment for a long time, resulting in a "significant increase" in cost and energy consumption. Therefore, the energy consumption and cost of the post-processing stage of the traditional solid-phase method are relatively high. The gas phase method generally uses high-temperature instantaneous reactions, which have high energy utilization (reactions are completed in a short time), but requires a continuous high-temperature heat source. For example, flame jets require combustion gas or electric heating, and plasma requires a high-power power supply. This energy consumption is considerable. In addition, gas phase cooling often wastes heat (unless a heat recovery system is used). Therefore, the gas phase method has a high energy consumption per unit of output and is only considered when pursuing special properties or continuous production. Its cost is often higher than that of traditional processes.

[0007] Therefore, the existing technology for the batch preparation of nano-tungsten bronze powder still has the problems of high process risk, high cost and difficulty in large-scale production. It is of great significance to develop a new process for the preparation of nano-tungsten bronze powder that is safe, efficient and suitable for industrialization. Summary of the Invention

[0008] The present invention aims to overcome the shortcomings of the prior art tungsten bronze powder preparation process, which is high in risk and cannot be mass-produced, and to provide a safe and efficient method for preparing nano tungsten bronze powder.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] The preparation method of the nano tungsten bronze powder comprises the following steps:

[0011] A method for preparing nano tungsten bronze powder, characterized by comprising the following steps:

[0012] (1) reacting a tungsten source with a cesium salt under heating conditions to generate a cesium tungstate precursor;

[0013] (2) performing a first wet sand milling on the cesium tungstate precursor to obtain a refined precursor powder;

[0014] (3) calcining the precursor powder in a hydrogen atmosphere to obtain tungsten bronze powder;

[0015] (4) The tungsten bronze powder is subjected to a second wet sand grinding to obtain nano-scale tungsten bronze powder.

[0016] The tungsten source is selected from soluble or insoluble tungsten compounds, including ammonium metatungstate, ammonium orthotungstate, sodium tungstate, tungsten trioxide and tungstic acid.

[0017] The cesium salt is selected from water-soluble cesium compounds, including cesium carbonate, cesium nitrate, cesium chloride and cesium acetate.

[0018] Preferably, when the tungsten source is a soluble tungsten compound, the tungsten source and the cesium salt are dissolved in water respectively to form a mixed solution, and the reaction is carried out by heating at 70-90° C. under stirring conditions.

[0019] Preferably, the mass ratio of the tungsten source to the deionized water is 1:2.5-4, and the mass ratio of the cesium salt to the deionized water is 1:1-1.5.

[0020] Preferably, during the wet sand milling process, the mass ratio of the powder to the organic solvent is 1:1-1.5, the organic solvent is ethanol or isopropanol, and the grinding medium is zirconia beads with a particle size of 0.1 to 0.2 mm.

[0021] The calcination temperature is 550-750° C., and the time is 1.5-2 hours. Preferably, the reduction calcination is performed at a temperature of 650° C. for 2 hours.

[0022] Preferably, the median particle size D50 of the finally obtained nano tungsten bronze powder is 50-100 nm.

[0023] Preferably, the molar ratio of the tungsten element in the tungsten source to the cesium element in the cesium salt is controlled to be 10:2 to 10:5, so that the doping ratio of Cs / W in the final tungsten bronze is 0.2-0.5.

[0024] Through the above scheme, the present invention proposes a systematic solution to the pain points of the existing technology, and successfully takes into account various aspects of performance, with the following advantages:

[0025] This breakthrough breaks through the bottleneck of scale: Previously, hydrothermal methods and other methods were difficult to scale up to mass production, and solid-phase products were difficult to directly use in high-end applications. This invention, through a "two-step sand milling + medium-temperature reduction" process, enables mass production of nano-tungsten bronze, a key step towards industrial practicality and a significant improvement over existing processes.

[0026] Integrated Innovation: While the individual unit operations (evaporation crystallization, hydrogen reduction, and sand milling) are not entirely novel, their organic integration into a novel process route creates synergistic effects and addresses the shortcomings of each traditional process. This process integration is not simply a patchwork, but rather a clever arrangement of key nodes in particle size control, demonstrating specialized technical thinking and a creative combination of innovations.

[0027] Improved Performance: The products produced using this method achieve high levels of particle size distribution and near-infrared shielding performance (high cesium doping ratio, small particle size, high purity, low agglomeration, and excellent NIR shielding), while also achieving a more energy-efficient and safer production process. The simultaneous achievement of these goals demonstrates the unique overall design of this technical solution, which cannot be simply derived from existing technologies.

[0028] The present invention is an improved invention of the process route, and its innovation lies mainly in the optimized combination of the process flow, rather than the discovery of a completely new substance or reaction mechanism. However, since no literature has previously reported exactly the same process, and this process provides an effective solution for large-scale preparation, it has obvious practical value and improvement effects, and has substantial creative progress. In summary, the technical solution of the present invention shows obvious advantages over traditional methods through comparison, and has made innovative improvements in realizing the preparation of industrial nano tungsten bronze powder. This solution has improved key indicators such as production efficiency, safety and controllability, particle size quality and energy consumption cost, and has positive significance for the large-scale application of tungsten bronze materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 1 is a schematic diagram of the process for preparing nano tungsten bronze powder according to Example 1 of the present invention. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to the accompanying drawings and examples.

[0031] Example:

[0032] A preparation process of nano tungsten bronze powder comprises the following steps (see Figure 1 ):

[0033] Step 1: Preparation of cesium tungstate precursor

[0034] (1) 300 g of ammonium metatungstate was mixed and dissolved with deionized water at a solid-liquid mass ratio of 1:2.5 to prepare an ammonium metatungstate aqueous solution;

[0035] (2) dissolving 62.6 g of cesium carbonate in deionized water (mass ratio 1:1) to prepare a cesium carbonate solution;

[0036] (3) Pour the cesium carbonate solution into a reactor with stirring and heating functions, heat it to 70°C and add an appropriate amount of dispersant, then slowly add the above-mentioned ammonium metatungstate solution while stirring, heating and stirring until no more bubbles are generated in the solution, to obtain a uniform cesium tungstate precursor solution;

[0037] (4) The cesium tungstate precursor solution obtained after the reaction is placed in a drying oven, and the water is evaporated and dried under normal pressure. The resulting white crystals are hydrated cesium tungstate polycomplex salts. These crystals are important intermediates for the subsequent hydrogen reduction synthesis of tungsten bronze powder.

[0038] Step 2: First sanding (rough sanding)

[0039] (1) mixing the white cesium tungstate crystal powder obtained in step 1 with anhydrous ethanol in a mass ratio of 1:1, adding an appropriate amount of dispersant, and stirring evenly to form a stable slurry;

[0040] (2) The slurry is added to a sand mill for wet grinding. The grinding medium is zirconia beads with a diameter of about 0.2 mm. The powder is ground to a particle size D50 of about 150 to 200 nm to obtain a primary fine powder slurry.

[0041] Step 3: Hydrogen reduction calcination

[0042] (1) drying the wet powder slurry obtained in step 2 at low temperature in a drying oven to recover the ethanol solvent and obtain a dry precursor powder;

[0043] (2) The dried powder was placed in a tubular furnace with hydrogen, and reduction calcined at 650°C for 2 hours. After cooling, blue-black tungsten bronze powder was obtained.

[0044] Step 4: Second sanding (fine grinding)

[0045] (1) Mix the tungsten bronze powder obtained in step 3 with anhydrous ethanol in a mass ratio of 1:1, and add a dispersant to form a uniform slurry;

[0046] (2) the slurry is further ground in a sand mill, preferably using zirconium oxide grinding beads with a diameter of about 0.1 mm, until the powder particle size D50 reaches 50 to 100 nm;

[0047] (3) Drying the slurry obtained by final grinding and collecting it to obtain the nano tungsten bronze powder product.

[0048] The scheme of this embodiment combines the ideas of solid-phase and mechanical grinding, and embeds the particle size refinement into the process flow through "two-step sand milling", thereby taking into account both output and particle size. First, solution evaporation crystallization is adopted in the precursor formation stage, and a large amount of cesium tungstate precursor can be crystallized in normal pressure equipment (this step is mainly limited by the size of the evaporator or drying box, and it is easy to scale up industrially). Secondly, solid-phase hydrogen reduction is used to convert most of the precursor into tungsten bronze in one step, and the reaction time is only about 2 hours and the conversion rate is high. Although two sand milling steps are added, a large amount of slurry can be processed in the large tank of the sand mill each time, and the two-stage grinding improves efficiency: the first time the crystals are crushed to less than 200nm to shorten the reduction time, and the second time the finished powder is finely ground to less than 100nm. This process can prepare nano tungsten bronze in batches without being limited by the high pressure of the reaction vessel, effectively solving the problem of difficulty in mass production of tungsten bronze. Therefore, the method of the present invention has a significant improvement in production efficiency and output scale compared to small-scale hydrothermal / solvothermal synthesis; compared to the traditional solid-phase method which requires lengthy grinding, this strategy of splitting the grinding and embedding it before and after the reaction improves the overall efficiency and yield.

[0049] Secondly, the method of the present invention focuses on reducing dangerous processes: first, the precursor synthesis is carried out at normal pressure, with only 70-90°C heating, avoiding high-pressure operations; weak base salts such as cesium carbonate are used instead of strong reducing agents such as metallic sodium, and the reaction is mild and controllable. Secondly, although hydrogen is used in the reduction step, it is carried out at medium temperature (550-750°C) in a professional tubular furnace, which is much lower than the 800°C or above of the conventional solid-phase method, and the precursor fine powder reacts rapidly during hydrogenation, reducing the time it stays in the hydrogen environment. The entire set of hydrogen reduction equipment is mature and reliable in industry, and safety can be guaranteed as long as the hydrogen concentration and ventilation are well controlled. In addition, the two sand milling processes are carried out in a closed sand mill using ethanol, and are operated in a general inert environment, with fire prevention and controllable. This process does not involve dangerous operations such as high temperature and high pressure, and is highly safe. Compared with the hydrothermal process that often involves dozens of atmospheres of pressure and the violent reaction of the solid-phase ball milling sodium method, this solution greatly reduces the potential risks. Therefore, in terms of safety, the solution of the present invention reduces the risk of the production process to a lower level through low pressure, moderate temperature and controllable hydrogen reduction, and is a safer preparation process.

[0050] Thirdly, the present invention's solution utilizes a "double grinding" process for particle size control, ensuring a uniform final particle size of 50-100 nm. In contrast, during the precursor stage, the hydrated cesium tungstate polycomplex salt obtained by solution crystallization forms a white microcrystalline powder. The particle size is determined by the crystallization conditions and can be influenced by controlling the evaporation rate. A subsequent sand milling process uniformly pulverizes the powder to 150-200 nm, effectively pre-standardizing the raw material particle size. This particle size is sufficiently small to facilitate rapid subsequent reduction while avoiding the excessively long time required for direct grinding to a smaller size. During the reduction stage, after a single sand milling step, the precursor powder is fine and uniform. This makes calcination at 550-750°C less prone to large sintering agglomerations due to the small initial particle size and relatively low reaction temperature. The calcined tungsten bronze product inherits the fine-grained characteristics of the precursor, avoiding the problem of abnormal particle growth encountered in conventional solid-phase methods. Even with some sintering, the particle size remains roughly around 100 nanometers. Secondary sand grinding stage: The nano-scale tungsten bronze powder is re-grinded to further reduce the D50 from 150-200nm to 50-100nm. Since the initial powder is relatively loose and the particle size is close to the required target, this step of grinding takes a relatively short time and can achieve a very uniform distribution. In summary, the process of the present invention achieves strict control of the particle size through chemical crystallization + two-step physical grinding, so that the final powder particle size distribution is uniform. In contrast, hydrothermal / solvothermal methods require complex chemical conditions to achieve similar sizes, and the solid-phase method can hardly directly obtain such small and uniform particles. Therefore, in terms of particle size control, the solution of the present invention not only achieves the small particle size advantage of the liquid phase method, but also ensures the uniformity of distribution through mechanical means, solving the pain point of the solid-phase method with excessively large particles.

[0051] Finally, the solution of the present invention strikes a compromise and balance in terms of energy consumption control. First, the precursor crystallization step is carried out at 70-90°C, consuming almost no high-grade energy, and mainly consuming latent heat of evaporation, which can be achieved through waste heat or conventional heating. Secondly, the hydrogen reduction calcination temperature is 550-750°C, which is significantly lower than the traditional solid-phase temperature of over 800°C. Thanks to the small size and high reactivity of the precursor, the heat energy required for reduction and embedding is reduced. In addition, the calcination time of 2 hours is also relatively low. Thirdly, although the sand milling process consumes electricity, the present invention completes the grinding in two steps, and the time for each step is relatively controllable, unlike the long one-time grinding from micron to 50nm. This progressive grinding reduces the energy density requirement for each step. In short, this method has low energy consumption and lower production costs than the traditional liquid phase method. Even compared with the solid phase method that requires long ball milling, the overall energy consumption is lower due to the reduction in total grinding time and the reduction in calcination temperature.

[0052] The above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the same. Although the present invention has been described in detail in conjunction with the above embodiments, persons of ordinary skill in the art may modify or substitute equivalents for the process details and certain technical features without departing from the spirit and scope of the present invention, and such modifications or substitutions shall be deemed to fall within the scope of protection of the present invention.

Claims

1. A method for preparing nano tungsten bronze powder, characterized in that: The following steps are involved: (1) reacting a tungsten source with a cesium salt under heating conditions to generate a cesium tungstate precursor; (2) performing a first wet sand milling on the cesium tungstate precursor to obtain a refined precursor powder; (3) calcining the precursor powder in a hydrogen atmosphere to obtain tungsten bronze powder; (4) The tungsten bronze powder is subjected to a second wet sand grinding to obtain nano-scale tungsten bronze powder.

2. The method according to claim 1, characterized in that The tungsten source is selected from soluble or insoluble tungsten compounds, including ammonium metatungstate, ammonium orthotungstate, sodium tungstate, tungsten trioxide and tungstic acid.

3. The method according to claim 1, characterized in that The cesium salt is selected from water-soluble cesium compounds, including cesium carbonate, cesium nitrate, cesium chloride and cesium acetate.

4. The method according to claim 2, characterized in that When the tungsten source is a soluble tungsten compound, the tungsten source and the cesium salt are dissolved in water respectively to form a mixed solution, and the reaction is carried out by heating at 70-90° C. under stirring conditions.

5. The method according to claim 4, characterized in that The mass ratio of the tungsten source to the deionized water is 1:2.5-4, and the mass ratio of the cesium salt to the deionized water is 1:1-1.

5.

6. The method according to claim 1, characterized in that During the wet sand milling process, the mass ratio of powder to organic solvent is 1:1-1.5, the organic solvent is ethanol or isopropanol, and the grinding medium is zirconia beads with a particle size of 0.1 to 0.2 mm.

7. The method according to claim 1, characterized in that The calcination temperature is 550-750℃ and the time is 1.5-2h.

8. The method according to claim 1, characterized in that The median particle size D50 of the finally obtained nano tungsten bronze powder is 50-100 nm.

9. The method according to claim 1, characterized in that The molar ratio of the tungsten element in the tungsten source to the cesium element in the cesium salt is controlled to be 10:2 to 10:5, so that the doping ratio of Cs / W in the final tungsten bronze is 0.2-0.5.

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