A raw material pretreatment method for high-purity aluminum zone refining purification

Abstract

This invention discloses a raw material pretreatment method for zone melting purification of high-purity aluminum, belonging to the field of non-ferrous metal refining technology. The pretreatment method includes alkaline washing, acid neutralization, cleaning and drying, and vacuum-protected loading steps. High-purity aluminum raw material is immersed in a 5-8% sodium hydroxide aqueous solution at room temperature for 5-30 seconds, and removed when bubbles begin to form on the substrate. The alkaline-washed aluminum material is then immersed in an 8-12% nitric acid or acetic acid aqueous solution for neutralization for 10-20 seconds. After acid neutralization, the aluminum material is cleaned and dried, then placed in a zone melting furnace. A vacuum is drawn, and inert protective gas is introduced to atmospheric pressure or a slightly positive pressure. This method thoroughly removes oxide scale from the surface and bonding surfaces of the aluminum material, effectively preventing secondary oxidation during post-treatment transfer and loading. It creates continuous and unobstructed molten pool propagation conditions for the zone melting process, ensuring the smooth progress of the purification process and the high quality of the final product. This method is particularly suitable for eliminating oxide scale on the bonding surfaces of segmented aluminum rods to ensure the smooth operation of the zone melting process.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal refining technology, specifically relating to a raw material pretreatment method for zone melting purification of high-purity aluminum. Background Technology

[0002] High-purity aluminum (usually referring to 5N and above purity) is a core basic material for manufacturing high-end electronic products, sputtering targets, and other key fields. Zone melting is a classic method for preparing and further purifying high-purity aluminum. Its principle is to utilize the difference in the distribution coefficient of impurities in the solid and liquid phases during the melting-solidification process, and drive the impurities to one end of the bar stock by moving the molten zone, thereby obtaining ultra-high purity material.

[0003] However, in actual production and experiments, when performing zone melting on long aluminum bars assembled from multiple short bars (such as Φ40×300mm), two key technical problems often arise that lead to experimental failure: first, the molten pool cannot be stably moved; second, the melting temperature is difficult to maintain. Specifically, after the previous section of metal is melted by artificial temperature control, as the molten pool moves forward, the subsequent metal cannot melt at the originally set temperature, resulting in an interruption of the molten pool.

[0004] The root cause of the above problems lies not in the zone melting equipment or process parameters themselves, but in the pretreatment of raw materials. Traditional pretreatment typically involves only removing surface oil stains with detergents or other cleaning agents, followed by heating and drying. This method has two major drawbacks: 1. Neglecting the treatment of oxide scale at the bonding surface: A dense aluminum oxide (Al2O3) film will quickly form at the bonding surface of segmented bar stock in the air. This oxide scale will form a "barrier" during the zone melting process, which will seriously hinder the linear transfer of heat (forming a "temperature barrier") and the continuous flow of molten metal (leading to "molten pool interruption").

[0005] 2. Heating and drying exacerbate oxidation: Traditional heating and drying processes accelerate the oxidation of the aluminum rod surface (especially the newly exposed bonding surface), causing each small section of the aluminum rod to be covered with a tight oxide scale.

[0006] Therefore, developing a pretreatment method that can effectively remove oxide scale from the surface and bonding surfaces of aluminum rods and prevent it from re-oxidizing in subsequent processes is crucial for ensuring the success rate and purification effect of high-purity aluminum zone melting purification. Summary of the Invention

[0007] The purpose of this invention is to provide a raw material pretreatment method for the zone melting purification of high-purity aluminum.

[0008] The objective of this invention is achieved as follows: the raw material pretreatment method for high-purity aluminum zone melting purification includes the following steps: Alkali washing: Immerse the high-purity aluminum raw material in a 5-8% sodium hydroxide aqueous solution at room temperature for 5-30 seconds, and remove it when bubbles begin to form on the substrate.

[0009] Acid neutralization: Immerse the aluminum material after alkaline washing in an 8-12% nitric acid or acetic acid aqueous solution for 10-20 seconds to neutralize.

[0010] Cleaning and drying: The acid-neutralized aluminum material is cleaned and dried.

[0011] Vacuum protection charging: The dried aluminum material is placed into the zone furnace, and after evacuation, inert protective gas is introduced to atmospheric pressure or slightly positive pressure.

[0012] Compared with the prior art, the technical solution described in this invention has the following advantages: 1. Precise removal of oxide scale: Short-term immersion in a room-temperature alkaline solution of a specific concentration can effectively dissolve the aluminum oxide layer on the aluminum surface (Al2O3 + 2NaOH → 2NaAlO2 + H2O). By observing the generation of bubbles to control the timing of removal, it is possible to avoid over-corrosion of the aluminum substrate, surface roughening, or even the absorption of excessive hydrogen, which could lead to hydrogen embrittlement.

[0013] 2. Preventing secondary contamination: The subsequent acid neutralization step completely eliminates residual alkaline solution, preventing it from continuing to corrode the aluminum material or introducing new impurities. The acid also dissolves insoluble intermetallic compounds on the surface after alkaline washing. Strict selection of the type of acid is crucial; hydrochloric acid should be avoided because of its high chloride content. - It is a harmful impurity that is extremely difficult to remove during the zone melting process.

[0014] 3. Preventing Secondary Oxidation: From cleaning and drying to loading into the zone melting furnace, the entire operation is seamless. The steps of "immediate" placement, "vacuuming," and then "filling with protective gas" form a seamless anti-oxidation process, minimizing the formation of oxide scale on the highly reactive aluminum surface in the air. The selected vacuum conditions remove most of the gas molecules and moisture adsorbed on the aluminum surface. The shielding effect of the inert gas ensures that the "clean surface" obtained through chemical treatment is perfectly preserved during the zone melting process.

[0015] 4. Ensuring successful zone melting: The technical solution described in this invention eliminates the physical obstacles that cause "temperature isolation" and "molten pool interruption" from the source, making the heat transfer and molten pool propulsion during the zone melting process smooth and continuous, and significantly improving the success rate, stability and purification efficiency of 5N high-purity aluminum zone melting purification.

[0016] In summary, the technical solution described in this invention can thoroughly remove oxide scale from the surface and bonding surfaces of aluminum materials, and effectively prevent secondary oxidation during the transfer and loading process after treatment. This creates continuous and unobstructed conditions for the advancement of the molten pool in the zone melting process, ensuring the smooth progress of the purification process and the high quality of the final product. The method is applicable to the pretreatment of aluminum raw materials with a purity of 5N (99.999%) and above before zone melting purification, and is particularly suitable for eliminating oxide scale on the bonding surfaces of segmented aluminum rods to ensure the smooth progress of the zone melting process. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the shape of the molten pool and the transition between sections during zone melting; Figure 2 This is a comparison of oxygen content and surface cleanliness in Example 1; Figure 3 This is a comparison of oxygen content and surface cleanliness in Example 2; Figure 4 Example 3 compares oxygen content and surface cleanliness. Figure 5 Example 4 compares oxygen content and surface cleanliness. Detailed Implementation

[0018] The present invention will be further described below, but this is not intended to limit the invention in any way. Any modifications or substitutions made based on the teachings of the present invention shall fall within the scope of protection of the present invention.

[0019] The raw material pretreatment method for high-purity aluminum zone melting purification includes the following steps: Alkali washing: Immerse the high-purity aluminum raw material in a 5-8% (mass percentage) sodium hydroxide aqueous solution at room temperature for 5-30 seconds, and remove it when bubbles begin to form on the substrate.

[0020] Acid neutralization: Immerse the aluminum material after alkaline washing in an aqueous solution of nitric acid or acetic acid with a concentration of 8-12% (volume percentage) for 10-20 seconds to neutralize.

[0021] Cleaning and drying: The acid-neutralized aluminum material is cleaned and dried.

[0022] Vacuum protection charging: The dried aluminum material is placed into the zone furnace, and after evacuation, inert protective gas is introduced to atmospheric pressure or slightly positive pressure.

[0023] In the alkaline washing step, the high-purity aluminum raw material is aluminum with a purity of 5N (99.999%) or higher, and is a long bar made of multiple aluminum rods spliced ​​together. The concentration of the sodium hydroxide aqueous solution is preferably 6%. The soaking time is preferably 10-15 seconds.

[0024] In the acid neutralization step, the concentration of the acid aqueous solution is preferably 10%.

[0025] In the cleaning and drying steps, the cleaning is performed by rinsing with deionized water. The drying is performed by drying with anhydrous ethanol.

[0026] In the vacuum protection loading step, the vacuuming process must ensure a vacuum level ≤ 1×10⁻⁶. -2 Pa. The inert protective gas is argon or nitrogen with a purity of not less than 99.999%.

[0027] General operating principles: 1. Environmental requirements: The entire pretreatment process, especially from the acid neutralization step, is recommended to be carried out in a clean room or ultra-clean workbench to minimize the contamination of the high-purity aluminum surface by airborne particulate matter.

[0028] 2. Material requirements: All chemical reagents (NaOH, nitric acid, acetic acid) should be electronic grade or analytical grade or higher; the resistivity of deionized water should not be less than 18 MΩ·cm; the purity of anhydrous ethanol should not be less than 99.7%.

[0029] 3. Operational continuity: The time interval between the aluminum material leaving the acid neutralization tank and being loaded into the zone furnace and evacuated should be minimized, not exceeding 30 minutes, to prevent the fresh aluminum surface from being in prolonged contact with air and forming a new oxide layer.

[0030] Example 1

[0031] raw material: Segmented aluminum rods with 5N (99.999%) purity (specifications: Φ40×300mm, 10 segments in total).

[0032] Solution preparation: All chemical reagents (NaOH, nitric acid, acetic acid) are of electronic grade or analytical grade or higher purity.

[0033] Prepare 6000 mL of 6% (by mass) sodium hydroxide (NaOH) aqueous solution in an alkali-resistant tank made of polypropylene (PP), stir with a plastic or glass rod until completely dissolved, and let stand at room temperature (25±5℃).

[0034] Prepare 6000 mL of 10% (volume ratio) nitric acid (HNO3) aqueous solution in an acid-resistant tank made of polytetrafluoroethylene (PTFE) or high-density polyethylene (HDPE).

[0035] Prepare sufficient deionized water (resistivity ≥ 18 MΩ·cm) and anhydrous ethanol (purity should not be less than 99.7%).

[0036] Preprocessing: The entire pretreatment process is carried out in a clean room or laminar flow hood.

[0037] Alkali washing: Take a section of high-purity aluminum rod, hold it with clean PTFE tweezers or clamps, and completely immerse it in a 6% sodium hydroxide aqueous solution. Gently shake the aluminum rod to remove surface air bubbles and ensure that the solution fully contacts all surfaces. Immerse at room temperature (25±5℃) for 12 seconds, closely observing the surface of the aluminum rod. In the initial stage, the aluminum oxide layer can be seen reacting with the alkali, and the surface changes from matte to slightly moist. When the aluminum matrix itself begins to react with the alkali, uniformly producing a large number of fine, dense hydrogen bubbles, this is the optimal time to remove the rod. Immediately remove the aluminum rod with PTFE tweezers and allow the remaining alkali solution to drip naturally for 1-2 seconds.

[0038] Acid neutralization: Quickly and steadily immerse the aluminum rod, which has been washed with alkali, into a 10% nitric acid solution and immerse it for 15 seconds to neutralize it. During this process, bubbles can be observed to be generated.

[0039] Cleaning and Drying: Remove the acid-neutralized aluminum rod and immediately place it under running deionized water for a thorough rinse of at least 60 seconds to ensure that all acid and reaction products are completely removed. The final runoff should be neutral (pH = 6.8-7.2, which can be tested with pH paper or a pH meter). After rinsing, use a lint-free cloth or high-purity cotton swab dipped in sufficient anhydrous ethanol to gently wipe the surface of the aluminum rod in the same direction until the surface is completely dry and exhibits a uniform, bright metallic luster.

[0040] Vacuum-protected loading: Immediately place the dried aluminum rods into the quartz boat of the zone melting furnace. Process all 10 aluminum rods quickly in the same manner, ensuring a tight fit between the rods. The time interval from the completion of drying the last aluminum rod to closing the furnace and initiating vacuuming should be controlled within 25 minutes. Seal the furnace body and start the vacuum system, preferably using a molecular pump unit, to evacuate the furnace to a vacuum level ≤5×10⁻⁶. -3 Pa (i.e., better than 5 × 10) -3 Once the vacuum level stabilizes to the required level, high-purity argon gas (purity ≥ 99.999%) is introduced into the furnace until the pressure reaches 1.05 atmospheres (slight positive pressure).

[0041] Zone melting: Under argon protection, the equipment is started according to the preset zone melting process parameters (such as melting zone movement speed, heating power, etc.) to carry out purification.

[0042] Effect: After pretreating high-purity aluminum rods using the method described in this invention, the zone melting process proceeds smoothly, the molten pool advances steadily and continuously, and there are no "temperature interruption" or "molten pool flow interruption" phenomena, resulting in a significant improvement in the quality of the purified aluminum rods.

[0043] Example 2

[0044] Raw materials: Same as in Example 1.

[0045] Solution preparation: 6000 mL of 7% (mass ratio) sodium hydroxide (NaOH) aqueous solution and 6000 mL of 10% (volume ratio) glacial acetic acid (CH3COOH) aqueous solution, prepared according to the same method and standards as in Example 1. Prepare sufficient deionized water and anhydrous ethanol, according to the same standards as in Example 1.

[0046] Preprocessing: In the alkaline washing step, the concentration of the sodium hydroxide aqueous solution is 7%, the soaking time is 8 seconds, and the remaining operations and standards are the same as in Example 1.

[0047] In the acid neutralization step, the concentration of the acetic acid aqueous solution is 10%, the soaking time is 20 seconds, and the remaining operations and standards are the same as in Example 1.

[0048] The operation and standards for the cleaning and drying steps are the same as in Example 1.

[0049] During the vacuum protection loading process, the vacuum level is evacuated to ≤8×10. -3 Pa, fill with high-purity nitrogen (purity ≥99.999%) to a pressure of 1.03 standard atmospheres (slight positive pressure), and the rest of the operation and standards are the same as in Example 1.

[0050] Zone melting and its effects: The zone melting process was initiated under nitrogen protection. Throughout the process, the molten pool interface was clear, and the flow was smooth and continuous. No obvious "temperature isolation" phenomenon was observed at the junctions between sections, the molten pool flow was uninterrupted, and the process stability was comparable to that of Example 1.

[0051] This embodiment successfully verified the following technical points: Alkali concentration adaptability: Increasing the NaOH concentration from 6% to 7% and compensating by shortening the soaking time to 8s still effectively removed the oxide scale without causing over-corrosion, demonstrating the adaptability of this method to changes in alkaline washing parameters.

[0052] Acid alternatives: Using acetic acid instead of nitric acid for neutralization can achieve the same purpose of terminating the reaction and purifying the surface, providing a safer and more environmentally friendly option for production.

[0053] Example 3

[0054] Raw materials: Same as in Example 1.

[0055] Solution preparation: 6000 mL of 5% (mass ratio) sodium hydroxide (NaOH) aqueous solution and 6000 mL of 8% (volume ratio) nitric acid (HNO3) aqueous solution, prepared according to the same method and standards as in Example 1. Prepare sufficient deionized water and anhydrous ethanol, according to the same standards as in Example 1.

[0056] Preprocessing: In the alkaline washing step, the concentration of the sodium hydroxide aqueous solution is 5%, the soaking time is 5 seconds, and the remaining operation procedures and standards are the same as in Example 1.

[0057] In the acid neutralization step, the concentration of the nitric acid aqueous solution was 8%, the soaking time was 10 seconds, and the remaining operations and standards were the same as in Example 1.

[0058] The operation and standards for the cleaning and drying steps are the same as in Example 1.

[0059] During the vacuum protection charging process, the time interval from the completion of drying the last section of aluminum rod to the start of vacuuming after closing the furnace is controlled at 20 minutes, and the vacuum level is ≤1×10⁻⁶. -2 Pa, fill with high-purity nitrogen to a pressure of 1.05 standard atmospheres, and the rest of the operation and standards are the same as in Example 1.

[0060] Zone melting and its effects: The zone melting process was initiated under argon protection. Observations showed that the molten pool formed well and the interface was clear. Throughout the process, the molten pool remained stable and continuous, successfully crossing all inter-segment interfaces without any "temperature gaps" or "molten pool interruptions." This indicates that even with a shorter alkaline washing time and a lower acid concentration, the pretreatment effect was still sufficient to ensure the smooth progress of zone melting.

[0061] This embodiment successfully verified the following technical points: Lower limit of process effectiveness: At a low NaOH concentration of 5%, a short soaking time of only 5 seconds is required. As long as the reaction endpoint (initial bubble generation) is precisely controlled, the oxide scale can be effectively removed, meeting the requirements of subsequent processes.

[0062] Acid concentration adaptability: An 8% nitric acid concentration was sufficient to complete the neutralization and surface purification task within 10 seconds, demonstrating the effectiveness of low-concentration acid solutions.

[0063] Example 4

[0064] Raw materials: Same as in Example 1.

[0065] Solution preparation: 6000 mL of 8% (mass ratio) sodium hydroxide (NaOH) aqueous solution and 6000 mL of 12% (volume ratio) glacial acetic acid (CH3COOH) aqueous solution, prepared according to the same method and standards as in Example 1. Prepare sufficient deionized water and anhydrous ethanol, according to the same standards as in Example 1.

[0066] Preprocessing: In the alkaline washing step, the concentration of sodium hydroxide aqueous solution is 8%, the soaking time is 28s, and the surface reaction is closely observed. In the first 15s: the oxide layer dissolves rapidly, and uniform etching occurs at the contact surface between the solution and the aluminum rod; in the 15-25s: fine bubbles begin to form, indicating that the oxide layer has been basically removed; in the 25-28s: the number of bubbles increases moderately, and the rod is removed immediately. The remaining operation and standards are the same as in Example 1.

[0067] In the acid neutralization step, the concentration of the acetic acid aqueous solution was 12%, and the soaking time was 18 seconds. The following phenomena were observed: in the first 8 seconds, obvious bubbles were generated, indicating that the neutralization reaction was proceeding vigorously; in the last 10 seconds, the bubbles gradually decreased, indicating that the residual alkali solution was basically neutralized. The remaining operation and standards were the same as in Example 1.

[0068] During the cleaning and drying process, the acid-neutralized aluminum rod is removed and immediately placed under running deionized water for 70 seconds, divided into three stages: Stage 1 (0-30 seconds): vigorous rinsing to remove surface deposits; Stage 2 (30-60 seconds): thorough rinsing to ensure no dead corners; Stage 3 (60-70 seconds): final rinsing. Anhydrous ethanol wiping employs a "two-wiping method": the first wipe removes most of the moisture, and the second ensures complete drying, resulting in a uniformly bright metal surface.

[0069] In the vacuum-protected charging process, the time interval from the completion of drying the last section of aluminum rod to the start of vacuuming after closing the furnace is controlled at 22 minutes. The vacuuming curve is as follows: 0-5 minutes: rough vacuum to 10 Pa; 5-15 minutes: high vacuum to 5 × 10 Pa. -3 Pa; 15-20 min: Maintain high vacuum. Charge Ar-N2 mixture to 1.02 standard atmospheres.

[0070] Zone melting and its effects: Molten pool performance: Initial molten zone formation: smooth, no obvious abnormalities; Inter-segment transition: the molten pool smoothly passes through all interface surfaces; Overall stability: the molten zone shape remains good, with no flow interruption; Quality inspection results: Surface quality: The surface of the pretreated aluminum rod is uniform; Grain after zone melting: Axially continuous with no obvious grain boundary defects; Purity retention: Secondary ion mass spectrometry analysis shows no additional impurities introduced.

[0071] This embodiment successfully verified the following technical points: Effectiveness of time boundary: Although the 28-second alkaline washing time is close to the upper limit, precise control of the removal time ensures complete removal of oxide scale while avoiding over-corrosion.

[0072] Applicability to high-concentration acids: A 12% acetic acid concentration can effectively complete the neutralization task and is safer to operate than nitric acid.

[0073] Mixed gas protection: The use of Ar-N2 mixed gas demonstrates the flexibility of protective gas selection, providing options for different operating conditions.

[0074] The technical principle of the preprocessing method described in this invention is as follows: The core function of the alkaline washing step is to chemically dissolve the dense alumina (Al2O3) film on the surface of high-purity aluminum. The reaction principle is: (Al2O3 + 2NaOH → 2NaAlO2 + H2O) (alumina + sodium hydroxide → sodium aluminate + water). This reaction removes the physical barriers that hinder the continuity of the molten pool. After the oxide layer is dissolved, the exposed highly reactive aluminum substrate will react with the alkaline solution: 2Al + 2NaOH + 2 H2O → 2 NaAlO2 + 3H2↑ (aluminum + sodium hydroxide + water → sodium aluminate + hydrogen). This reaction releases a large number of hydrogen bubbles, which is a key visual signal to determine that the oxide layer has been completely removed and the aluminum material should be removed immediately to prevent over-corrosion.

[0075] The subsequent acid neutralization step serves two purposes: first, to immediately stop the continuous corrosion of the aluminum substrate by the alkaline solution, the reaction being: NaOH + HNO3 → NaNO3 + H2O (sodium hydroxide + nitric acid → sodium nitrate + water); and second, to dissolve the aluminum hydroxide [Al(OH)3] flocculent matter that may be generated during the alkaline washing process, and to further purify the surface, exposing a highly active and clean metal surface, thus creating optimal conditions for subsequent vacuum protection.

[0076] The cleaning and drying steps, along with the vacuum-protected loading steps, aim to thoroughly remove residues from previous processes and create an oxygen-free environment. This is crucial for ensuring a "clean surface" that is seamlessly transferred from the processing stage to the smelting stage. Rinsing with large amounts of deionized water is essential to completely remove residual acids and soluble salts (such as NaNO3 and NaAlO2) from the aluminum surface. Any trace residue can become a source of contamination during the subsequent high-temperature, high-vacuum zone melting process, leading to lattice defects or introducing impurities, severely impacting the purity of the final product. This step ensures the chemical purity of the aluminum surface.

[0077] Compared to conventional heating or air drying, wiping with anhydrous ethanol offers two advantages. First, ethanol is miscible with water and evaporates rapidly, quickly removing moisture and significantly reducing the aluminum's exposure time in humid air. Second, the evaporation process of ethanol removes any remaining trace amounts of moisture and organic contaminants from the surface, providing a final cleaning and resulting in a completely dry and clean metal surface.

[0078] Vacuuming and introducing protective gas are the final and most critical anti-oxidation measures in the entire pretreatment process. The primary purpose of vacuuming is to forcibly remove air (especially oxygen and water vapor) from the reaction chamber, physically creating an initial oxygen-free environment. At the same time, high vacuum conditions can effectively desorb water molecules adsorbed on the surface of the aluminum material and the inner wall of the furnace chamber, eliminating potential sources of oxidation.

[0079] Refilling the furnace with high-purity inert gas (such as Ar or N2) after vacuuming to a slightly positive pressure creates a durable and stable inert atmosphere. This slightly positive pressure absolutely prevents outside air from seeping into the furnace during subsequent operations or melting processes due to minor leaks. It ensures that the highly reactive aluminum surface remains isolated from oxygen throughout the entire cycle from the start of heating to the end of zone melting, thus perfectly maintaining the "fresh" surface state obtained after pretreatment and laying the foundation for successful zone melting purification.

Claims

1. A raw material pretreatment method for zone melting purification of high-purity aluminum, characterized in that, Includes the following steps: Alkali washing: Immerse the high-purity aluminum raw material in a 5-8% sodium hydroxide aqueous solution at room temperature for 5-30 seconds, and remove it when bubbles begin to form on the substrate; Acid neutralization: Immerse the aluminum material after alkaline washing in an 8-12% nitric acid or acetic acid aqueous solution for 10-20 seconds to neutralize. Cleaning and drying: The acid-neutralized aluminum material is cleaned and dried; Vacuum protection charging: The dried aluminum material is placed into the zone furnace, and after evacuation, inert protective gas is introduced to atmospheric pressure or slightly positive pressure.

2. The raw material pretreatment method according to claim 1, characterized in that, In the alkaline washing step, the high-purity aluminum raw material is aluminum with a purity of 5N or higher.

3. The raw material pretreatment method according to claim 2, characterized in that, In the alkaline washing step, the high-purity aluminum raw material is a long bar made up of multiple aluminum rod segments.

4. The raw material pretreatment method according to claim 1, characterized in that, In the alkaline washing step, the concentration of the sodium hydroxide aqueous solution is 6%.

5. The raw material pretreatment method according to claim 1, characterized in that, In the alkaline washing step, the soaking time is 10-15 seconds.

6. The raw material pretreatment method according to claim 1, characterized in that, In the acid neutralization step, the concentration of the acid aqueous solution is 10%.

7. The raw material pretreatment method according to claim 1, characterized in that, In the cleaning and drying step, the cleaning is performed by rinsing with deionized water.

8. The raw material pretreatment method according to claim 1, characterized in that, In the cleaning and drying steps, the drying is performed using anhydrous ethanol.

9. The raw material pretreatment method according to claim 1, characterized in that, In the vacuum protection loading step, the vacuuming process must ensure a vacuum level ≤ 1×10⁻⁶. -2 Pa.

10. The raw material pretreatment method according to claim 1, characterized in that, In the vacuum protection loading step, the inert protective gas is argon or nitrogen with a purity of not less than 99.999%.

Images