Method for avoiding generation of hard spots and pits on buffered chemically polished surface of superconducting niobium cavity, and BCP acid liquor

Abstract

The present invention relates to a method for avoiding the generation of hard spots and pits on a buffered chemically polished surface of a superconducting niobium cavity, which method comprises: reacting a BCP acid liquor formed by mixing HF, HNO3 and H3PO4 with niobium to form a soluble niobium compound, and polishing a surface of a superconducting niobium cavity by using same, wherein the ratio of the BCP acid liquor is changed to increase the gas solubility of NO gas in the BCP acid liquor and reduce the viscosity of the BCP acid liquor, so as to prevent the generation of hard spots and pits on the surface of the superconducting niobium cavity during a BCP polishing process. The present invention further relates to a BCP acid liquor, wherein k=V(HF+HNO3) / V(H3PO4)≥2, and n=V(HNO3) / V(HF)>1.64. By means of the method for avoiding the generation of hard spots and pits on a buffered chemically polished surface of a superconducting niobium cavity and the BCP acid liquor in the present invention, the problem of hard spots and pits being generated on the BCP surface of the superconducting niobium cavity can be solved, and both the polished niobium surface and the polished superconducting niobium cavity have very good radio frequency performance.

Description

A method for solving pitting on the surface of superconducting niobium cavity buffer chemical polishing and BCP acid solution Technical Field

[0001] The present invention relates to accelerator surface treatment, and more particularly to a method for solving the problem of pitting on the surface of a superconducting niobium cavity buffer chemical polishing, and a BCP acid solution. Background Art

[0002] Superconducting high-frequency cavities are the main particle acceleration structures in many modern accelerators. They are generally made of high-purity bulk niobium with a residual resistivity ratio (RRR) of ~300. The most important radio frequency performance of superconducting niobium cavities can be expressed as the quality factor Q. o and the acceleration gradient E acc Two parameters characterize. Q o (~1 / R s ) is larger, the smaller the superconducting niobium cavity loss is, and the lower the accelerator operating cost is; E acc The larger the diameter, the higher the energy that charged particles can obtain through the superconducting niobium cavity, and the more compact the accelerator can be built, thus reducing the cost. In order to improve the radio frequency performance of the superconducting niobium cavity, it is usually necessary to polish the inner surface of the superconducting niobium cavity to remove the damaged layer, contaminants and defects on the cavity surface to obtain a damage-free, clean, smooth and flat niobium cavity inner surface.

[0003] Buffered chemical polishing (BCP) is a commonly used method for polishing the surface of superconducting niobium cavities internationally. It utilizes a mixture of hydrofluoric acid (HF), nitric acid (HNO3), and phosphoric acid (H3PO4) in a specific volume ratio to react with the niobium material to form a soluble niobium compound, thereby polishing the surface of the superconducting niobium cavity. Nitric acid dissolves niobium, hydrofluoric acid dissolves niobium oxide, and phosphoric acid slows or controls the etching rate. The commonly used acid volume ratio (i.e., formulation) for BCP polishing of superconducting niobium cavities internationally is HF:HNO3:H3PO4 = 1:1:2 (for ease of description, numerical ratios will be used to represent the corresponding volume ratios of the three acids or the BCP acid formulation). However, this acid formulation can sometimes lead to the formation of numerous small, pitted pits on the surface of the superconducting niobium cavity, which can severely limit the RF performance of the cavity. To date, little is known about the causes of these pits and effective solutions.

[0004] Summary of the Invention

[0005] In order to solve the problem of pitting in the prior art, the present invention provides a method for solving the problem of pitting on the surface of a superconducting niobium cavity buffer chemical polishing and a BCP acid solution.

[0006] According to the present invention, a method for resolving pitting on the surface of a superconducting niobium cavity during buffered chemical polishing includes polishing the surface of the superconducting niobium cavity using a BCP acid solution containing a mixture of HF, HNO3, and H3PO4 to react with niobium to form a soluble niobium compound. The method further comprises modifying the ratio of the BCP acid solution to increase the solubility of NO gas in the BCP acid solution and reduce the viscosity of the BCP acid solution, thereby preventing pitting on the surface of the superconducting niobium cavity during the BCP polishing process.

[0007] Preferably, the method includes the following steps: S1, confirming the NO gas solubility and viscosity of HF, HNO3 and H3PO4 in the BCP acid solution; S2, reducing the volume ratio of H3PO4 in the BCP acid solution and increasing the volume ratio of HNO3, so that k in the BCP acid solution is V(HF+HNO3) / V(H3PO4)≥2; S3, adjusting the ratio of HF and HNO3, so that n in the BCP acid solution is V(HNO3) / V(HF)>1.64, to obtain a BCP polishing acid solution; S4, using the BCP polishing acid solution to perform BCP polishing on a niobium sample; S5, using the BCP polishing acid solution to perform BCP polishing on the inner surface of a superconducting niobium cavity.

[0008] Preferably, in step S2, HF:HNO3:H3PO4=1:1:1, 2:1:1, 3:1:1, 1:2:1, 1:3:1, 1:4:1, 1:6:1, 2:2:1.

[0009] Preferably, in step S3, the reaction rate is reduced by increasing the volume ratio of HNO3.

[0010] Preferably, in steps S4 and S5, the temperature T of the BCP polishing acid solution is ≤15°C, the acid solution flow rate is ≤30 L / min, and the polishing time is ≥0.5 min.

[0011] Preferably, in steps S4 and S5, the polishing rate is between 0.5-2 μm / min.

[0012] Preferably, step S5 includes: S51, performing a BCP polishing experiment on the inner surface of the superconducting niobium cavity using a BCP polishing acid solution; S52, performing a BCP background treatment on the superconducting niobium cavity using the BCP polishing acid solution; S53, performing a performance test on the superconducting niobium cavity.

[0013] Preferably, the performance test is a vertical test to confirm that the BCP polishing acid meets the surface radio frequency performance requirements of the superconducting niobium cavity.

[0014] According to the BCP acid solution of the present invention, k=V(HF+HNO3) / V(H3PO4)≥2, and n=V(HNO3) / V(HF)>1.64.

[0015] Preferably, the BCP acid solution has a ratio of HF:HNO3:H3PO4=1:3:1.

[0016] According to the present invention, a method for resolving pitting on the surface of a superconducting niobium cavity during buffered chemical polishing is developed by modifying the BCP acid solution ratio to increase the gas solubility of the BCP mixed acid and reduce the mixed acid viscosity, thereby obtaining a new, more optimized acid solution ratio. Using this new, more optimized BCP acid solution while maintaining a low acid solution temperature can resolve the pitting issue on the BCP surface of a superconducting niobium cavity. The BCP acid solution provided by the present invention can resolve the pitting issue on the BCP surface of a superconducting niobium cavity, resulting in excellent radio frequency performance for both the polished niobium surface and the polished superconducting niobium cavity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG1 is a process flow chart of a method for solving the problem of pitting on the surface of a superconducting niobium cavity buffer chemical polishing according to a preferred embodiment of the present invention.

[0018] FIG2 shows the 2K vertical test results of a 1.3 GHz Tesla-type single-cell fine-grained superconducting niobium cavity with BCP background treatment. DETAILED DESCRIPTION

[0019] The preferred embodiments of the present invention are given below in conjunction with the accompanying drawings and described in detail.

[0020] Through extensive BCP polishing experiments on niobium samples and superconducting niobium cavities, the present invention has identified the fundamental reason for the large number of pitting on the surface of niobium samples or superconducting niobium cavities after BCP polishing: the solubility of the product gas in the BCP acid solution with a ratio of 1:1:2 is too low, resulting in the gas (nitric oxide, NO) produced by the reaction between the acid solution and niobium being unable to dissolve and drain away from the niobium surface in a timely and effective manner. Instead, these undissolved gases aggregate and merge on the niobium surface to form bubbles that adhere to the niobium surface, ultimately forming a large number of pitting on the BCP-polished surface. Furthermore, excessive acid viscosity can increase bubble aggregation. Since the pitting is caused by the low gas solubility and high viscosity of the 1:1:2 acid solution, the pitting is caused by the high viscosity of the acid solution.

[0021] Therefore, the present invention can fundamentally solve the problem of pitting on the surface of a superconducting niobium cavity during the BCP polishing process. As a primary factor, the present invention can change the BCP acid solution ratio to increase the solubility of the BCP mixed acid gas. Since the NO gas solubility of an acid solution with a certain ratio is limited, when the gas in the acid solution is supersaturated, the remaining undissolved gas easily aggregates into bubbles and adheres to the surface. The present invention makes the generated NO gas more easily dissolve in the acid, so that it will not aggregate on the niobium surface to produce bubbles. As a secondary factor, the present invention can reduce the viscosity of the mixed acid. Because excessive acid viscosity promotes the aggregation of gases into bubbles, the present invention makes it easier for the generated NO gas to break free from the constraints of the acid and be discharged, so that it will not aggregate on the niobium surface to produce bubbles. In this way, the present invention can obtain a new and more optimal BCP acid solution ratio. Using the new and more optimal BCP acid solution ratio while ensuring a lower acid solution temperature can solve the problem of pitting on the surface of a superconducting niobium cavity during the BCP polishing process. The new BCP acid solution ratio not only has a sufficiently large gas solubility and a low enough viscosity to avoid pitting, but also ensures good surface polishing quality (i.e., flat, smooth, and bright) and a moderate polishing rate, and can be used for actual superconducting niobium cavity BCP polishing.

[0022] In addition, the present invention can provide a preferred BCP acid solution ratio. The preferred acid solution ratio range or protection range is limited to: (1) the ratio of the sum of the volumes of hydrofluoric acid (HF) and nitric acid (HNO3) to the volume of phosphoric acid (H3PO4), i.e., k = V(HF+HNO3) / V(H3PO4) ≥ 2. After BCP polishing, no pitting or concavity appears on the surface, which indicates that the solubility of the acid solution gas is sufficiently large and the viscosity is sufficiently small; (2) n = V(HNO3) / V(HF) > 1.64 (n = 1.64 is the volume ratio corresponding to the molar ratio of HNO3 (69%) to HF (49%) in the BCP reaction equation). This allows a viscous thin layer composed of the reaction product of niobium and nitric acid (HNO3) to form quickly, which can effectively inhibit the reaction rate and facilitate surface polishing. This ratio not only solves the problem of pitting on the surface of superconducting niobium cavities during BCP polishing, but also ensures excellent radio frequency performance of both the polished surface and the polished superconducting niobium cavity. It can be applied to the BCP polishing of superconducting niobium cavities, their components, accessory workpieces, and niobium samples. For example, an optimal BCP acid ratio of HF:HNO₃:H₃PO₄ is 1:3:1. For example, to avoid pitting while maintaining a slower polishing rate, ratios of 1:4:1, 1:5:1, or 1:5:2 can be achieved.

[0023] Example 1

[0024] As shown in Figure 1, the method for resolving pitting on the surface of a superconducting niobium cavity buffer chemical polishing according to this embodiment first includes determining the solubility and viscosity of the three primary acid gases (nitric oxide, NO) in the BCP. This can be achieved by querying or measuring the corresponding solubility and viscosity values ​​at different temperatures based on the concentrations of the three primary acids used. At room temperature of approximately 20°C, hydrofluoric acid (HF, 48% wt) has a viscosity of ~1.0 mPa.s, nitric acid (HNO3, 60% wt) has a viscosity of ~2.5 mPa.s, and phosphoric acid (H3PO4, 85% wt) has a viscosity of ~45.2 mPa.s. It can be seen that the viscosity of the BCP mixed acid is primarily determined by the volume ratio of phosphoric acid. NO is slightly soluble in water, and as the concentrations of HF and H3PO4 solutes in water increase, the solubility of NO gas decreases. However, its solubility in aqueous nitric acid solution is many times greater than in water, and increases with increasing nitric acid concentration. It can be seen that phosphoric acid gas at a concentration of 85% has the lowest solubility and highest viscosity, while nitric acid gas has the highest solubility.

[0025] As shown in Figure 1, the method for resolving pitting on the surface of a superconducting niobium cavity buffered chemical polishing according to this embodiment includes reducing the volume ratio of phosphoric acid (H3PO4) and increasing the volume ratio of nitric acid (HNO3) in the BCP mixed acid to increase the solubility of the BCP mixed acid gas and reduce its viscosity. When the ratio of the sum of the volumes of hydrofluoric acid (HF) and nitric acid (HNO3) to the volume of phosphoric acid (H3PO4) is k = V(HF + HNO3) / V(H3PO4) ≥ 2, pitting no longer occurs on the surface after BCP polishing, indicating sufficient acid gas solubility and low viscosity. To facilitate adjustment and comparison of acid gas solubility and viscosity, using 1:1:1 as a reference, the phosphoric acid (H3PO4) ratio is reduced and the nitric acid (HNO3) ratio is increased to k ≥ 2, such as 1:1:1, 2:1:1, 3:1:1, 1:2:1, 1:3:1, 1:4:1, and 2:2:1.

[0026] As shown in Figure 1, the method for solving the problem of pitting pits on the surface of superconducting niobium cavity buffer chemical polishing according to the present embodiment then includes adjusting the ratio of hydrofluoric acid (HF) and nitric acid (HNO3) to meet the BCP polishing requirements. On the basis of reducing the phosphoric acid volume ratio and increasing the nitric acid volume ratio, it is also necessary to adjust the ratio of hydrofluoric acid (HF) and nitric acid (HNO3) according to actual needs (such as polishing rate, polishing surface quality). With 1:1:1 as a benchmark, increasing the hydrofluoric acid HF volume ratio, the reaction rate increases significantly, such as 1.5:1:1, 2:1:1, 3:1:1; The inventors found that increasing the nitric acid HNO3 volume ratio, the reaction rate gradually decreases, such as 1:2:1, 1:3:1, 1:4:1; increasing the phosphoric acid volume ratio, the reaction rate gradually decreases, such as 1:3:1, 1:3:2.

[0027] As shown in Figure 1, the method for resolving pitting on the surface of a superconducting niobium cavity buffered chemical polishing according to this embodiment includes selecting a BCP acid solution ratio of HF:HNO3:H3PO4 = 1:3:1 based on the desired BCP polishing rate and post-BCP polishing surface quality. This ratio, k = V(HF+HNO3) / V(H3PO4) = 4 > 2, ensures sufficient acid gas solubility and low viscosity to prevent pitting. Furthermore, increasing the proportion of nitric acid (HNO3) effectively reduces the acid reaction rate. Finally, n = V(HNO3) / V(HF) = 3 > 1.64 (n = 1.64 is the volume ratio corresponding to the molar ratio of nitric acid (HNO3) to hydrofluoric acid (HF) in the BCP reaction equation), which allows for the rapid formation of a viscous thin layer composed of the reaction products of niobium and nitric acid (HNO3). This viscous thin layer effectively suppresses the reaction rate and facilitates surface polishing.

[0028] As shown in FIG1 , the method for solving the problem of pitting on the surface of the buffered chemical polishing of the superconducting niobium cavity according to this embodiment then includes conducting a BCP polishing experiment on a niobium sample. The niobium sample is a niobium sample with RRR ≥ 300. The size of the niobium sample is generally small, and the niobium sample and the superconducting niobium cavity are from the same piece (batch) of plate. The BCP polishing of the niobium sample is performed by a sample BCP polishing device. The optical inspection of the niobium sample can be carried out using a mobile phone microfocus lens, an optical microscope, a metallographic microscope, a laser confocal microscope, a scanning tunneling microscope and other morphological observation instruments and equipment. The BCP polishing rate of the niobium sample is calculated from the thickness measurement data before and after polishing and the polishing time. The BCP polishing process of the niobium sample is observed and recorded, and this step is performed by a camera device (such as a camera, a mobile phone). Specifically, the steps include: preparing 50L of mixed acid in a ratio of 1:3:1, namely, adding 10L of hydrofluoric acid (HF), 30L of nitric acid (HNO3), and 10L of phosphoric acid (H3PO4), and then evenly mixing the three acid solutions; taking a niobium sample (sample size 5mm*8mm*2.8mm), numbering, weighing, thickness measuring, and optical inspection before BCP polishing, and recording the surface morphology of the sample before BCP polishing; taking 400mL of BCP mixed acid in a ratio of 1:3:1 and injecting it into the sample BCP polishing device, adjusting the acid temperature T≤8°C, the acid flow rate ~1cm / s and other parameters, and then placing the sample in the sample. Polishing was performed in a BCP polishing apparatus for 10 minutes, while the entire polishing process was observed and recorded using a camera. After BCP polishing, the niobium sample was rinsed with pure water until neutral, then transferred to a cleanroom for another ultrasonic clean and air-dried. The dried niobium sample was weighed, thickness measured, and optically inspected, and the surface morphology after 10 minutes of BCP polishing was observed and recorded. Based on the weighing and thickness measurements before and after 10 minutes of BCP polishing, the polishing rate (the thickness of the niobium cavity or niobium sample that can be etched away per minute during BCP polishing, a specific manifestation of the reaction rate mentioned above) was calculated to be approximately 1-2 μm / min. The optical inspection results before and after 10 minutes of BCP polishing were compared with the BCP polishing process records to confirm that the polished surface with the new BCP acid ratio of 1:3:1 was free of pitting and pits, resulting in a flat, smooth, and bright surface. The above experimental process was repeated multiple times for the niobium sample, varying polishing process parameters such as temperature, flow rate, and polishing time. The recommended acid temperature T is ≤ 15°C, the acid flow rate is ≤ 5 cm / s, and the polishing time is ≥ 0.5 min. BCP polishing experiments on these niobium samples showed that, while the polishing rate varied under different polishing process parameters, the polishing rate was moderate (approximately 1-2 μm / min). The niobium samples were free of pits and pits, and the polished surfaces were flat, smooth, and bright. This confirmed that the new BCP acid ratio of 1:3:1 met the BCP polishing requirements for niobium samples under different polishing process parameters.

[0029] As shown in Figure 1, the method for resolving pitting and denting during buffered chemical polishing of a superconducting niobium cavity according to this embodiment then includes conducting a BCP polishing experiment on the inner surface of the superconducting niobium cavity. BCP-In polishing of the superconducting niobium cavity refers to inner surface polishing, which is divided into inner surface polishing (BCP-In) and outer surface polishing (BCP-Out). BCP-In polishing of the superconducting niobium cavity is performed using a BCP-In polishing device. Optical inspection of the superconducting niobium cavity is performed using a Japanese superconducting niobium cavity optical inspection system. The BCP polishing rate of the superconducting niobium cavity is calculated based on weighing data and polishing time. Specifically, the step includes: preparing 200L of BCP mixed acid in a ratio of 1:3:1 (i.e., adding 40L of hydrofluoric acid (HF), 120L of nitric acid (HNO3), and 40L of phosphoric acid (H3PO4), and then evenly mixing the three acid solutions); selecting a 1.3GHz single-cell superconducting niobium cavity for a BCP-In polishing experiment, and weighing, measuring thickness, and optically inspecting the inner surface of the superconducting niobium cavity before polishing; installing the superconducting niobium cavity on a BCP-In polishing device, and then setting parameters such as the polishing acid temperature and flow rate (i.e., flow velocity), performing BCP-In polishing on the superconducting niobium cavity for 10 minutes, and finally rinsing and drying in a clean room (HPR); weighing, measuring thickness, and optically inspecting the inner surface of the niobium cavity after drying; and calculating, based on the weighing and thickness measurement results before and after the 10-minute BCP-In polishing of the superconducting niobium cavity, a polishing rate of approximately 1.5μm / min for the 10-minute BCP-In polishing of the superconducting niobium cavity is obtained. Comparison of optical inspection results of the inner surface of the superconducting niobium cavity before and after 10 minutes of polishing confirmed that the polished surface with the new BCP acid ratio of 1:3:1 was free of pitting and had a smooth, bright, and flat surface. The above polishing experiments for the superconducting niobium cavity BCP-In were repeated multiple times, varying polishing process parameters such as temperature, flow rate, and polishing time. Recommended polishing parameters include an acid temperature T ≤ 15°C, an acid flow rate ≤ 30 L / min, and a polishing time ≥ 0.5 min. These polishing experiments for the superconducting niobium cavity BCP-In showed that, while the polishing rate varied under different polishing process parameters, it remained moderate (approximately 1-2 μm / min). The inner surface of the superconducting niobium cavity was free of pitting and had a smooth, bright surface. This confirms that the new BCP acid ratio of 1:3:1 meets the process requirements for superconducting niobium cavity BCP-In under various polishing parameters.

[0030] As shown in FIG1 , the method for solving the pitting pits on the surface of the buffered chemical polishing of the superconducting niobium cavity according to this embodiment includes the following steps: (1) BCP re-polishing 150 μm to 250 μm (new cavity) or BCP re-polishing 40 to 60 μm (old cavity); (2) heat treatment at 800 to 900°C for 3 hours; (3) BCP light polishing 20 μm. The heat treatment refers to baking the superconducting niobium cavity at a set temperature in a special heat treatment vacuum furnace to remove hydrogen (H2). The superconducting niobium cavity ultra-clean assembly refers to installing the flange, antenna, angle valve and other accessories required for the superconducting niobium cavity vertical test in a Class 10 clean room after the surface treatment of the superconducting niobium cavity is completed, and then vacuuming. The superconducting niobium cavity vertical test refers to installing and connecting the assembled superconducting niobium cavity to a vertical test liquid helium dewar specially used for testing superconducting niobium cavities, thereby obtaining the radio frequency performance of the superconducting niobium cavity in the superconducting state. Specifically, a 1.3GHz Tesla-type single-cell fine-grained superconducting niobium cavity was selected for BCP background treatment (including optical inspection of the inner surface of the superconducting niobium cavity before and after BCP heavy polishing and light polishing), then ultra-clean assembly was performed, and finally vertical testing was performed to obtain the radio frequency performance of the superconducting niobium cavity after BCP background treatment (see Figure 2); the vertical test performance and inner surface optical inspection of the cavity BCP background treatment were excellent, so the new BCP acid ratio of 1:3:1 was determined to be the optimal BCP acid ratio. According to the QE curve of the superconducting cavity measured at temperature T = 2K in Figure 2, it can be seen that after the 1.3G single-cell fine-grained superconducting niobium cavity was polished with a 1:3:1 BCP acid solution, the acceleration gradient E acc When <24MV / m, quality factor Q o >1.0×10 10 , maximum Q o More than 2.0×10 10 , maximum E acc The results show that the optimal BCP acid solution ratio of 1:3:1 obtained by the method of the present invention can not only avoid the formation of pits, but also achieve excellent superconducting performance in the superconducting cavity polished with it.

[0031] As shown in Figure 1, the method for resolving pitting on the surface of a superconducting niobium cavity during buffered chemical polishing according to this embodiment finally includes providing a BCP acid solution in a ratio of 1:3:1. This ratio not only resolves the pitting problem on the surface of the superconducting niobium cavity during BCP polishing, but also improves the post-polishing surface quality and radio frequency performance of the superconducting niobium cavity. Therefore, it can be applied to the BCP polishing of superconducting niobium cavities, their components, accessory workpieces, niobium samples, and other applications.

[0032] Example 2

[0033] The method for solving the problem of pitting on the surface of a superconducting niobium cavity buffer chemical polishing according to this embodiment first includes determining the solubility and viscosity of the three primary acid gases (nitric oxide, NO) of the BCP. At room temperature of approximately 20°C, the viscosity of hydrofluoric acid (HF, 48% wt) is approximately 1.0 mPa.s, the viscosity of nitric acid (HNO3, 60% wt) is approximately 2.5 mPa.s, and the viscosity of phosphoric acid (H3PO4, 85% wt) is approximately 45.2 mPa.s. It can be seen that the viscosity of the BCP mixed acid is mainly determined by the volume ratio of phosphoric acid. NO is slightly soluble in water, and when the concentration of HF and H3PO4 solutes in water increases, the solubility of NO gas decreases. However, its solubility in aqueous nitric acid solution is many times greater than that in water, and increases with increasing nitric acid concentration. It can be seen that phosphoric acid gas with a concentration of 85% has the lowest solubility and the highest viscosity, while nitric acid gas has the highest solubility.

[0034] According to this embodiment, the method for resolving pitting on the surface of a superconducting niobium cavity buffered chemical polishing process involves reducing the volume ratio of phosphoric acid (H3PO4) and increasing the volume ratio of nitric acid (HNO3) in the BCP mixed acid. When the ratio of the sum of the volumes of hydrofluoric acid (HF) and nitric acid (HNO3) to the volume of phosphoric acid (H3PO4)—that is, k = V(HF + HNO3) / V(H3PO4) ≥ 2—pitting no longer occurs on the surface after BCP polishing, indicating that the acid gas solubility is sufficiently high and the viscosity is sufficiently low. To facilitate adjustment and comparison of the acid gas solubility and viscosity, using 1:1:1 as a reference, the phosphoric acid (H3PO4) ratio is reduced and the nitric acid (HNO3) ratio is increased to k ≥ 2, such as 1:1:1, 2:1:1, 3:1:1, 1:2:1, 1:3:1, 1:4:1, 1:6:1, and 2:2:1.

[0035] The method for resolving pitting on the surface of a superconducting niobium cavity buffer chemically polished according to this embodiment then includes adjusting the ratio of hydrofluoric acid (HF) to nitric acid (HNO3). Based on a 1:1:1 ratio, increasing the volume ratio of hydrofluoric acid to HF significantly increases the reaction rate, such as to 1.5:1:1, 2:1:1, and 3:1:1. Increasing the volume ratio of nitric acid to HNO3 gradually decreases the reaction rate, such as to 1:2:1, 1:3:1, 1:4:1, and 1:6:1. Increasing the volume ratio of phosphoric acid gradually decreases the reaction rate, such as to 1:3:1 and 1:3:2.

[0036] According to this embodiment, the method for resolving pitting on the surface of a superconducting niobium cavity buffered chemical polishing process includes selecting a BCP acid solution ratio of HF:HNO3:H3PO4 = 1:6:1. This ratio, k = V(HF+HNO3) / V(H3PO4) = 7 > 2, ensures sufficient solubility and low viscosity of the acid solution to prevent pitting. Furthermore, increasing the proportion of nitric acid (HNO3) effectively reduces the acid reaction rate. Finally, n = V(HNO3) / V(HF) = 6 > 1.64 (n = 1.64 is the volume ratio corresponding to the molar ratio of nitric acid (HNO3) to hydrofluoric acid (HF) in the BCP reaction equation), which allows for the rapid formation of a viscous thin layer composed of the reaction products of niobium and nitric acid (HNO3). This viscous thin layer effectively suppresses the reaction rate and facilitates surface polishing.

[0037] The method for solving the pitting pits on the surface of the superconducting niobium cavity buffered chemical polishing according to this embodiment then includes conducting a BCP polishing experiment on a niobium sample. Specifically, the steps include: preparing 48L of mixed acid in a ratio of 1:6:1, namely, adding 6L of hydrofluoric acid (HF), 36L of nitric acid (HNO3), and 6L of phosphoric acid (H3PO4), and then evenly mixing the three acid solutions; taking a niobium sample (sample size 5mm*8mm*2.8mm), numbering, weighing, thickness measurement, and optical inspection before BCP polishing, and recording the surface morphology of the sample before BCP polishing; taking 400mL of BCP mixed acid in a ratio of 1:6:1 and injecting it into the sample BCP polishing device, adjusting the acid temperature T≤8℃ and the acid flow rate to 1 cm / s and other parameters. The sample was then placed in a sample BCP polishing apparatus and polished for 10 minutes, while the entire polishing process was observed and recorded with a camera. After BCP polishing, the niobium sample was rinsed with pure water until neutral, then transferred to a cleanroom for another pure water ultrasonic cleaning and air-dried. The dried niobium sample was weighed, thickness measured, and optically inspected, and the surface morphology after 10 minutes of BCP polishing was observed and recorded. Based on the weighing and thickness measurement results before and after 10 minutes of BCP polishing, the polishing rate for the niobium sample after 10 minutes of BCP polishing was calculated to be approximately 0.5-1.5 μm / min. The optical inspection results before and after 10 minutes of BCP polishing of the niobium sample were compared with the BCP polishing process records. It was determined that the polished surface with the new BCP acid ratio of 1:6:1 was free of pitting and pits, and the polished surface was flat, smooth, and bright. The above experimental process for the niobium sample was repeated multiple times, varying various polishing process parameters such as temperature, flow rate, and polishing time. Recommended acid temperature T ≤ 15°C, acid flow rate ≤ 5 cm / s, and polishing time ≥ 0.5 minutes are recommended. The BCP polishing experiments on these niobium samples showed that although the polishing rates varied under different polishing process parameters, the polishing rates were moderate (approximately 0.5-1.5 μm / min), the surfaces of the niobium samples were free of pits, and the polished surfaces were flat, smooth, and bright. This confirmed that the new BCP acid solution ratio of 1:6:1 met the BCP polishing requirements for niobium samples under different polishing process parameters.

[0038] The method for solving the problem of pitting on the buffered chemical polishing surface of the superconducting niobium cavity according to this embodiment includes performing a BCP polishing experiment on the inner surface of the superconducting niobium cavity. Specifically, the steps include: preparing 200 L of BCP mixed acid (i.e., adding 25 L of hydrofluoric acid (HF), 150 L of nitric acid (HNO3), and 25 L of phosphoric acid (H3PO4) in a ratio of 1:6:1, and then evenly mixing the three acids); selecting a 1.3 GHz single-cell superconducting niobium cavity for a BCP-In polishing experiment, and weighing, measuring thickness, and optically inspecting the inner surface of the superconducting niobium cavity before polishing; installing the superconducting niobium cavity on a BCP-In polishing device, and then setting parameters such as the polishing acid temperature and flow rate (i.e., flow velocity), performing BCP-In polishing on the superconducting niobium cavity for 10 minutes, and finally rinsing and drying in a clean room (HPR); weighing, measuring thickness, and optically inspecting the inner surface of the niobium cavity after drying; and calculating, based on the weighing and thickness measurement results before and after the 10-minute BCP-In polishing of the superconducting niobium cavity, a polishing rate of approximately 0.9 μm / min for the 10-minute BCP-In polishing of the superconducting niobium cavity is obtained. Comparison of optical inspection results of the inner surface of the superconducting niobium cavity before and after 10 minutes of polishing confirmed that the polished surface with the new BCP acid ratio of 1:6:1 was free of pitting and smooth, with a flat, bright, and shiny finish. The above polishing process for the superconducting niobium cavity BCP-In was repeated multiple times, varying polishing parameters such as temperature, flow rate, and polishing time. Recommended polishing parameters include an acid temperature T ≤ 15°C, an acid flow rate ≤ 30 L / min, and a polishing time ≥ 0.5 min. These BCP-In polishing experiments demonstrated that, while the polishing rate varied under different polishing parameters, it remained moderate (approximately 0.8-1.3 μm / min). The inner surface of the superconducting niobium cavity was free of pitting and smooth, resulting in a bright, flat, and shiny finish. This confirms that the new BCP acid ratio of 1:6:1 meets the process requirements for superconducting niobium cavity BCP-In under various polishing parameters.

[0039] The method for resolving pitting on the surface of a superconducting niobium cavity after buffered chemical polishing according to this embodiment includes BCP background treatment and vertical testing of the superconducting niobium cavity. A 1.3GHz Tesla-type single-cell fine-grained superconducting niobium cavity was selected for BCP background treatment (including optical inspection of the cavity's niobium interior surface before and after BCP heavy and light polishing). Ultraclean assembly was then performed, followed by vertical testing to determine the RF performance of the BCP-treated cavity. The vertical testing performance and internal surface optical inspection of the cavity were excellent, resulting in the determination of a new BCP acid solution ratio of 1:6:1 as the optimal BCP acid solution.

[0040] The method for resolving pitting on the surface of a superconducting niobium cavity during buffered chemical polishing according to this embodiment finally includes providing the BCP acid solution in a ratio of 1:6:1. This ratio not only resolves the pitting problem on the surface of the superconducting niobium cavity during BCP polishing, but also improves the post-polishing surface quality and radio frequency performance of the superconducting niobium cavity. Therefore, it can be applied to the BCP polishing of superconducting niobium cavities, their components, accessory workpieces, niobium samples, and the like.

[0041] The present invention can fundamentally solve the problem. Based on the fundamental cause of pitting on the surface of the superconducting niobium cavity after BCP polishing (the acid solution has low gas solubility and high viscosity, which leads to bubbles adhering and causing pitting), a targeted solution is proposed: the BCP acid solution ratio is changed to increase the BCP mixed acid gas solubility and reduce the mixed acid viscosity to obtain a new and more optimal acid solution ratio. Using this new and more optimal BCP acid solution ratio while maintaining a low acid solution temperature can solve the pitting problem on the BCP surface of the superconducting niobium cavity.

[0042] This invention provides a method for optimizing the BCP acid ratio. It not only addresses the problem of pitting on the surface of superconducting niobium cavities after BCP polishing, but also provides methods and criteria for optimizing the BCP acid ratio, thereby achieving an optimal BCP acid ratio based on specific needs. This ratio may vary between laboratories and countries, for example, due to differences in the concentrations of the three primary acids used, the required polishing rates, and the desired BCP-polished surface quality.

[0043] The present invention provides an optimal BCP acid-liquid ratio (e.g., 1:3:1). This ratio, obtained using the method, not only provides a good example and reference, but also solves the problem of pitting on the BCP surface of a superconducting niobium cavity. The polished niobium surface and the polished superconducting niobium cavity both exhibit excellent radio frequency performance.

[0044] The method of the present invention is simple and can solve the pitting problem by simply changing the BCP acid solution ratio, without the need to add other auxiliary instruments and equipment or modify the original BCP polishing equipment.

[0045] The present invention has wide applicability. Since only the BCP acid solution ratio needs to be adjusted, the method is applicable to different BCP polishing equipment and different BCP polishing objects around the world.

[0046] The method of the present invention is also helpful to promote further research on BCP, a basically mature chemical polishing method, in the accelerator field.

[0047] The present invention is not limited to solving the problem of pitting on the surface of a superconducting niobium cavity during BCP polishing, and proposing a more optimal BCP acid ratio (1:3:1), but can also be widely used in how to adjust and optimize the BCP acid ratio and other chemical polishing optimizations.

[0048] The implementation method of the present invention is not limited to surface treatment in the accelerator field, but can also be extended and applied to surface treatment research in other fields.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Various modifications are possible to the above embodiments of the present invention. In other words, any simple, equivalent changes and modifications made in accordance with the claims and description of the present invention are within the scope of protection of the present invention. Anything not fully described in this invention constitutes conventional technology.

Claims

1. A method for solving the problem of pitting on the surface of chemical polishing of superconducting niobium cavity buffer, characterized in that: The method comprises the steps of using a mixed BCP acid solution of HF, HNO3 and H3PO4 to react with niobium to form a soluble niobium compound for surface polishing of a superconducting niobium cavity, wherein the gas solubility of NO gas in the BCP acid solution is increased and the viscosity of the BCP acid solution is reduced by changing the ratio of the BCP acid solution to avoid pitting on the surface of the superconducting niobium cavity during the BCP polishing process.

2. The method according to claim 1, characterized in that The method comprises the following steps: S1, confirm the NO gas solubility and viscosity of HF, HNO3 and H3PO4 in BCP acid solution; S2, reducing the volume ratio of H3PO4 in the BCP acid solution and increasing the volume ratio of HNO3, so that k in the BCP acid solution is V(HF+HNO3) / V(H3PO4)≥2; S3, adjusting the ratio of HF and HNO3 so that n in the BCP acid solution is greater than V(HNO3) / V(HF)>1.64, and obtaining a BCP polishing acid solution; S4, performing BCP polishing of the niobium sample using a BCP polishing acid solution; S5, performing BCP polishing on the inner surface of the superconducting niobium cavity using a BCP polishing acid solution.

3. The method according to claim 2, characterized in that In step S2, HF:HNO3:H3PO4=1:1:1, 2:1:1, 3:1:1, 1:2:1, 1:3:1, 1:4:1, 1:6:1, 2:2:

1.

4. The method according to claim 2, characterized in that: In step S3, the reaction rate is reduced by increasing the volume ratio of HNO3.

5. The method according to claim 2, characterized in that: In steps S4 and S5, the temperature of the BCP polishing acid solution is T≤15°C, the acid solution flow rate is ≤30L / min, and the polishing time is ≥0.5min.

6. The method according to claim 2, characterized in that In steps S4 and S5, the polishing rate is between 0.5-2 μm / min.

7. The method according to claim 2, characterized in that Step S5 includes: S51, BCP polishing experiment on the inner surface of superconducting niobium cavity was carried out using BCP polishing acid; S52, using BCP polishing acid to perform BCP background treatment on the superconducting niobium cavity; S53, performance test of superconducting niobium cavity.

8. The method according to claim 7, characterized in that The performance test is a vertical test to confirm that the BCP polishing acid meets the surface RF performance requirements of the superconducting niobium cavity.

9. A BCP acid solution, characterized in that: In the BCP acid solution, k=V(HF+HNO3) / V(H3PO4)≥2, and n=V(HNO3) / V(HF)>1.

64.

10. The BCP acid solution according to claim 9, characterized in that: The BCP acid solution has HF:HNO3:H3PO4=1:3:1.

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