A method for solving pitting pits generated on the surface of a superconducting niobium cavity by buffered chemical polishing and a BCP acid solution

Abstract

The present invention relates to a method for solving the problem of pitting on the surface of a superconducting niobium cavity during buffered chemical polishing, which includes using a BCP acid solution mixed with HF, HNO3, and H3PO4 to react with niobium to form a soluble niobium compound for polishing the surface of the superconducting niobium cavity. Among them, by changing the ratio of the BCP acid solution, the gas solubility of NO gas in the BCP acid solution is increased and the viscosity of the BCP acid solution is reduced to avoid pitting on the surface of the superconducting niobium cavity during BCP polishing. The present invention also relates to a BCP acid solution, where k = V(HF + HNO3) / V(H3PO4) ≥ 2 and n = V(HNO3) / V(HF) > 1.64. According to the method for solving the pitting on the surface of a superconducting niobium cavity during buffered chemical polishing and the BCP acid solution of the present invention, the problem of pitting on the BCP surface of the superconducting niobium cavity can be solved, and both the polished niobium surface and the radio frequency performance of the polished superconducting niobium cavity are very good.

Description

Technical Field

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

[0002] Superconducting radio frequency cavities are the main particle acceleration structures in many modern accelerators, which are generally made of high-purity large-sized niobium with a residual resistivity ratio (RRR) of ~300. The most important radio frequency performance of superconducting niobium cavities can be characterized by the quality factor Q o and the accelerating gradient E acc Two parameters. The larger Q o (~1 / R s ), the smaller the loss of the superconducting niobium cavity and the lower the operating cost of the accelerator; the larger E acc , the higher the energy obtained by charged particles passing through the superconducting niobium cavity, and the more compact the accelerator can be built, thus the lower the cost. In order to improve the radio frequency performance of superconducting niobium cavities, 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 inner surface of the niobium cavity.

[0003] Buffered Chemical Polishing (BCP for short) is a commonly used method for polishing the surface of superconducting niobium cavities internationally. Its principle is to use a mixed acid solution of hydrofluoric acid (HF), nitric acid (HNO3), and phosphoric acid (H3PO4) with a certain volume ratio to react with niobium materials to form soluble niobium compounds, thereby achieving the polishing of the surface of superconducting niobium cavities. Among them, nitric acid dissolves niobium, hydrofluoric acid dissolves niobium oxides, and phosphoric acid slows down or controls the etching rate. Internationally, the commonly used acid solution volume ratio (i.e., the ratio) for BCP polishing of superconducting niobium cavities is HF:HNO3:H3PO4 = 1:1:2 (for convenience of description, hereinafter the corresponding volume ratio of the three acids or the BCP acid solution ratio is represented by a digital ratio). However, this ratio of acid solution sometimes causes a large number of pitted small pits on the surface of superconducting niobium cavities, and these pitting and pits will seriously limit the radio frequency performance of superconducting niobium cavities. So far, little is known internationally about the cause of the pitting and pits and effective solutions. Summary of the Invention

[0004] In order to solve the problem of pitting and pits in the above-mentioned prior art, the present invention provides a method for solving the pitting and pits on the surface of superconducting niobium cavities during buffered chemical polishing and a BCP acid solution.

[0005] Method for solving pitting pits on the surface of superconducting niobium cavities by chemical buffering and polishing according to the present invention, which includes using a BCP acid solution mixed with HF, HNO3, and H3PO4 to react with niobium to form soluble niobium compounds for polishing the surface of superconducting niobium cavities. Among them, by changing the ratio of the BCP acid solution, the gas solubility of NO gas in the BCP acid solution is increased and the viscosity of the BCP acid solution is reduced to avoid pitting pits on the surface of the superconducting niobium cavity during the BCP polishing process.

[0006] Preferably, the method includes the following steps: S1, confirm the NO gas solubility and viscosity of HF, HNO3, and H3PO4 in the BCP acid solution; S2, reduce the volume ratio of H3PO4 in the BCP acid solution and increase the volume ratio of HNO3 so that k = V(HF + HNO3) / V(H3PO4) ≥ 2 in the BCP acid solution; S3, adjust the ratio of HF and HNO3 so that n = V(HNO3) / V(HF) > 1.64 in the BCP acid solution to obtain the BCP polishing acid solution; S4, use the BCP polishing acid solution to perform BCP polishing on the niobium sample; S5, use the BCP polishing acid solution to perform BCP polishing on the inner surface of the superconducting niobium cavity.

[0007] 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.

[0008] Preferably, in step S3, the reaction rate is reduced by increasing the volume ratio of HNO3.

[0009] Preferably, in steps S4 and S5, the temperature T of the BCP polishing acid solution ≤ 15 °C, the acid solution flow rate ≤ 30 L / min, and the polishing time ≥ 0.5 min.

[0010] Preferably, in steps S4 and S5, the polishing rate is between 0.5 - 2 μm / min.

[0011] Preferably, step S5 includes: S51, use the BCP polishing acid solution to perform BCP polishing experiment on the inner surface of the superconducting niobium cavity; S52, use the BCP polishing acid solution to perform BCP background treatment on the superconducting niobium cavity; S53, perform performance testing on the superconducting niobium cavity.

[0012] Preferably, the performance test is a vertical test to confirm that the BCP polishing acid solution meets the surface radio frequency performance requirements of the superconducting niobium cavity.

[0013] For the BCP acid solution according to the present invention, k = V(HF + HNO3) / V(H3PO4) ≥ 2 and n = V(HNO3) / V(HF) > 1.64 therein.

[0014] Preferably, the HF:HNO3:H3PO4 of the BCP acid solution is 1:3:1.

[0015] According to the method for solving the problem of pitting on the surface of superconducting niobium cavities during buffered chemical polishing according to the present invention, by changing the ratio of the BCP acid solution to increase the solubility of the BCP mixed acid gas and decrease the viscosity of the mixed acid, a new and better acid solution ratio is obtained, and the new and better BCP acid solution ratio is used. At the same time, ensuring a lower acid solution temperature can solve the problem of pitting on the BCP surface of superconducting niobium cavities. The BCP acid solution provided by the present invention can solve the problem of pitting on the BCP surface of superconducting niobium cavities, and both the polished niobium surface and the radio frequency performance of the polished superconducting niobium cavity are very good. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a process flow chart of the method for solving the problem of pitting on the surface of superconducting niobium cavities during buffered chemical polishing according to a preferred embodiment of the present invention.

[0017] Figure 2 Shows the 2K vertical test results of the 1.3GHz Tesla type single-cell fine-grained superconducting niobium cavity - BCP background treatment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] The following is combined with the drawings to give the preferred embodiments of the present invention and describe them in detail.

[0019] Through a large number of niobium sample and superconducting niobium cavity BCP polishing experiments, the present invention has found the root cause of a large number of pitting pits 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 small, resulting in the gas (nitric oxide NO) generated by the reaction of the acid solution and niobium being unable to be dissolved and discharged from the niobium surface in a timely and effective manner. These undissolved gases will aggregate and merge on the niobium surface to form bubbles and adhere to the niobium surface, and finally a large number of pitting pits will be formed on the BCP polished surface. In addition, the too high viscosity of the acid solution will also increase the coalescence of bubbles. Since the reason for the formation of pitting pits is that the gas solubility of the 1:1:2 acid solution is too small and the viscosity is too high.

[0020] Therefore, the present invention can fundamentally solve the problem of pitting on the surface of superconducting niobium cavities during BCP polishing. As the main factor, the present invention can change the BCP acid solution ratio to increase the solubility of the BCP mixed acid gas. Since the solubility of NO gas in an acid solution with a certain ratio is limited, when the gas in the acid solution is supersaturated, the remaining undissolved gas is likely to coalesce into bubbles and adhere to the surface. The present invention makes the generated NO gas more easily dissolve in the acid, so that bubbles will not accumulate on the niobium surface. As a secondary factor, the present invention can reduce the viscosity of the mixed acid. Because a too high acid solution viscosity promotes the coalescence of gas into bubbles, the present invention enables the generated NO gas to more easily break free from the bondage of the acid and be discharged, so that bubbles will not accumulate on the niobium surface. In this way, the present invention can obtain a new and better BCP acid solution ratio. Using the new and better BCP acid solution ratio and ensuring a lower acid solution temperature at the same time can solve the problem of pitting on the surface of superconducting niobium cavities during BCP polishing. The new BCP acid solution ratio not only has a large enough gas solubility and a small enough viscosity to avoid pitting, but also can ensure good surface polishing quality (i.e., flat, smooth, and bright), a moderate polishing rate, and can be used for the BCP polishing of actual superconducting niobium cavities.

[0021] In addition, the present invention can provide a better BCP acid solution ratio. The range or protection range of the better acid solution ratio is limited as follows: (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 appears on the surface, indicating that the gas solubility of the acid solution is large enough and the viscosity is small enough; (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 can enable the rapid formation of a viscous thin layer composed of the reaction products of niobium and nitric acid (HNO3), and this viscous thin layer can effectively inhibit the reaction rate and is beneficial to surface polishing. This ratio can not only solve the problem of pitting on the surface of superconducting niobium cavities during BCP polishing, but also the polished surface and the radio frequency performance of the polished superconducting niobium cavities are very good, and it can be applied to the BCP polishing of actual superconducting niobium cavities, superconducting niobium cavity parts, accessory workpieces, niobium samples, etc. For example, a better BCP acid solution ratio is HF:HNO3:H3PO4 = 1:3:1. For instance, if one wants to avoid pitting while requiring a slower polishing rate, ratios such as 1:4:1, 1:5:1, 1:5:2, etc. can be obtained.

[0022] Example 1

[0023] As Figure 1As shown, the method for solving the pitting on the surface of superconducting niobium cavities during buffered chemical polishing according to this embodiment first includes confirming the solubility and viscosity of the three original acid gases (nitric oxide NO) in BCP. This can be obtained by querying or measuring the corresponding solubility and viscosity values at different temperatures according to the concentrations of the three original acids used. At around room temperature of 20°C, the viscosity of hydrofluoric acid (HF, 48% wt) is ~1.0 mPa·s, the viscosity of nitric acid (HNO3, 60% wt) is ~2.5 mPa·s, and the viscosity of phosphoric acid (H3PO4, 85% wt) is ~45.2 mPa·s. It can be seen therefrom 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 solute concentrations of HF and H3PO4 in water increase, the solubility of NO gas decreases. However, its solubility in nitric acid aqueous solution is many times greater than that in water and increases with the increase of nitric acid concentration. It can be seen that the solubility of phosphoric acid gas with a concentration of 85% is the smallest and the viscosity is the largest, while the solubility of nitric acid gas is the largest.

[0024] As Figure 1 shown, the method for solving the pitting on the surface of superconducting niobium cavities during buffered chemical polishing according to this embodiment then includes reducing the volume ratio of phosphoric acid (H3PO4) in the BCP mixed acid and increasing the volume ratio of nitric acid (HNO3) to increase the gas solubility of the BCP mixed acid and reduce the 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), i.e., k = V(HF + HNO3) / V(H3PO4) ≥ 2, no pitting will appear on the surface after BCP polishing, and the gas solubility of the acid solution is large enough and the viscosity is small enough. For the convenience of adjusting and comparing the gas solubility and viscosity of the acid solution, taking 1:1:1 as a reference, reduce the proportion of phosphoric acid (H3PO4) and increase the proportion of nitric acid (HNO3) to k ≥ 2, such as 1:1:1, 2:1:1, 3:1:1, 1:2:1, 1:3:1, 1:4:1, 2:2:1.

[0025] As Figure 1 shown, the method for solving the pitting on the surface of superconducting niobium cavities during buffered chemical polishing according to this embodiment then includes adjusting the ratio of hydrofluoric acid (HF) and nitric acid (HNO3) to meet the requirements of BCP polishing. On the basis of reducing the volume ratio of phosphoric acid and increasing the volume ratio of nitric acid, it is also necessary to adjust the ratio of hydrofluoric acid (HF) and nitric acid (HNO3) according to actual requirements (such as polishing rate, polishing surface quality). Taking 1:1:1 as a benchmark, increasing the volume ratio of hydrofluoric acid HF will significantly increase the reaction rate, such as 1.5:1:1, 2:1:1, 3:1:1; the inventor found that increasing the volume ratio of nitric acid HNO3 will gradually reduce the reaction rate, such as 1:2:1, 1:3:1, 1:4:1; increasing the volume ratio of phosphoric acid will gradually reduce the reaction rate, such as 1:3:1, 1:3:2.

[0026] As Figure 1As shown, the method for solving the pitting pits on the surface of superconducting niobium cavities by buffered chemical polishing according to this embodiment next includes selecting the BCP acid solution ratio as HF:HNO3:H3PO4 = 1:3:1 according to requirements such as the required BCP polishing rate and the surface quality after BCP polishing. For this ratio, k = V(HF + HNO3) / V(H3PO4) = 4 > 2, which can ensure that the gas solubility of the acid solution is large enough and the viscosity is small enough, and will not cause the formation of pitting pits; secondly, increasing the proportion of nitric acid (HNO3) can effectively reduce the reaction rate of the acid solution; 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) can enable the rapid formation of a viscous thin layer composed of the reaction products of niobium and nitric acid (HNO3). This viscous thin layer can effectively inhibit the reaction rate and is beneficial to surface polishing.

[0027] As Figure 1As shown, according to the method for solving the problem of pitting and pitting on the surface of the buffered chemical polishing of the superconducting niobium cavity in this embodiment, the following includes a niobium sample BCP polishing experiment. 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 come from the same piece (batch) of plate. The niobium sample BCP polishing is performed by a sample BCP polishing device. The niobium sample optical inspection can be carried out using a mobile phone micro-focus lens, an optical microscope, a metallographic microscope, a laser confocal microscope, a scanning tunneling microscope and other morphological observation instruments and equipment. The niobium sample BCP polishing rate is calculated from the thickness measurement data before and after polishing and the polishing time. The step of observing and recording the BCP polishing process of the niobium sample is performed by a camera device (such as a camera, a mobile phone). Specifically, the step includes: 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 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 1:3:1 and injecting it into the sample BCP polishing device, adjusting the acid temperature T≤8°C, the acid flow rate of ~1cm / s and other parameters, and then placing the sample in the sample Polishing was performed in a BCP polishing device for 10 minutes, and the entire polishing process was observed and recorded by a camera. After the BCP polishing of the niobium sample was completed, it was washed with pure water until it was neutral, and then transported to a clean room for another pure water ultrasonic cleaning and dried. The dried niobium sample was weighed, thickness measured and optically inspected, and the surface morphology of the sample after BCP polishing for 10 minutes was observed and recorded. According to the weighing and thickness measurement results before and after BCP polishing of the niobium sample for 10 minutes, the polishing rate of the sample after BCP polishing for 10 minutes (the thickness of the niobium cavity or niobium sample that can be etched and removed per minute by BCP polishing is the specific manifestation of the reaction rate mentioned above) was calculated to be about 1-2μm / min. The surface optical inspection results of the niobium sample before and after BCP polishing for 10 minutes were compared, and combined with the BCP polishing process record, it was determined that the polishing surface of the new BCP acid solution ratio was 1:3:1 without pits and pits, and the polished surface was flat, smooth and bright. The above niobium sample experimental process was repeated many times, and different polishing process parameters such as temperature, flow rate, polishing time and other parameters were changed. Among them, it is recommended that the acid temperature T≤15℃, the acid flow rate≤5cm / s, and the polishing time≥0.5min. The BCP polishing experiments of these niobium samples show that although the polishing rate varies under different polishing process parameters, the polishing rate is moderate (about 1-2μm / min), the surface of the niobium samples has no pits, and the polished surface is flat, smooth and bright. It is determined that the new BCP acid ratio of 1:3:1 meets the BCP polishing requirements of niobium samples under different polishing process parameters.

[0028] like Figure 1As shown, according to the present embodiment, the method for solving the problem of pitting on the surface of the buffered chemical polishing of the superconducting niobium cavity includes the following steps: performing a BCP polishing experiment on the inner surface of the superconducting niobium cavity. The superconducting niobium cavity BCP-In polishing refers to the inner surface polishing, and the superconducting niobium cavity BCP polishing is divided into inner surface polishing (BCP-In) and outer surface polishing (BCP-Out). The superconducting niobium cavity BCP-In polishing is performed by a superconducting niobium cavity BCP-In polishing device. The superconducting niobium cavity optical inspection is performed by using a Japanese superconducting niobium cavity optical inspection system. The superconducting niobium cavity BCP polishing rate is calculated from the weighing data and the polishing time. Specifically, the step includes: preparing 200L of BCP mixed acid according to 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 temperature and flow rate (i.e., flow velocity) of the polishing acid solution, and 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 BCP-In polishing of the superconducting niobium cavity for 10 minutes, a polishing rate of about 1.5μm / min of the BCP-In polishing of the superconducting niobium cavity for 10 minutes is obtained. The optical inspection results of the inner surface of the superconducting niobium cavity before and after polishing for 10 minutes were compared to determine that the new BCP acid ratio of 1:3:1 produced no pits on the polished surface, and the polished surface was flat, smooth and bright; the above superconducting niobium cavity BCP-In polishing experimental process was repeated many times, and different polishing process parameters such as temperature, flow rate, polishing time and other parameters were changed. Among them, it is recommended that the acid temperature T≤15℃, the acid flow rate≤30L / min, and the polishing time≥0.5min. These superconducting niobium cavity BCP-In polishing experiments show that although there are differences in polishing rate under different polishing process parameters, the polishing rate is moderate (about 1-2μm / min), the inner surface of the superconducting niobium cavity has no pits, and the polished surface is flat, smooth and bright. It is determined that the new BCP acid ratio of 1:3:1 meets the process requirements of superconducting niobium cavity BCP-In under different polishing parameters.

[0029] like Figure 1As shown, the method for solving the pitting and pits on the surface of superconducting niobium cavities during buffered chemical polishing according to this embodiment next includes the BCP background treatment of superconducting niobium cavities and vertical testing. The BCP background treatment of superconducting niobium cavities includes the following steps: (1) BCP heavy polishing of 150 μm to 250 μm (for new cavities) or BCP heavy polishing of 40 to 60 μm (for old cavities); (2) heat treatment at 800 to 900 °C for 3 hours; (3) BCP light polishing of 20 μm. The heat treatment means that the superconducting niobium cavity is baked in a dedicated heat treatment vacuum furnace at a set temperature to remove hydrogen gas (H2). The ultra-clean assembly of the superconducting niobium cavity means that after the surface treatment of the superconducting niobium cavity is completed, accessories such as flanges, antennas, and angle valves required for the vertical testing of the superconducting niobium cavity are installed in a Class 10 cleanroom, and then the vacuum is pumped. The vertical testing of the superconducting niobium cavity means that the assembled superconducting niobium cavity is installed and connected to a vertical test liquid helium dewar dedicated to testing superconducting niobium cavities, so as to obtain the radio frequency performance of the superconducting niobium cavity in the superconducting state. Specifically, a 1.3 GHz Tesla type single-cell fine-grained superconducting niobium cavity is selected for BCP background treatment (where the inner surface of the niobium of the superconducting niobium cavity is optically inspected before and after BCP heavy polishing and light polishing), then ultra-clean assembly is carried out, and finally vertical testing is carried out to obtain the radio frequency performance of the superconducting niobium cavity after BCP background treatment of the superconducting niobium cavity (see Figure 2 ); the vertical testing performance and the inner surface optical inspection of the BCP background treatment of this cavity are relatively good, so the new BCP acid solution ratio of 1:3:1 is determined as the relatively good BCP acid solution ratio. According to Figure 2 , for the superconducting cavity measured at a temperature T = 2K, it can be seen that after the 1.3G single-cell fine-grained superconducting niobium cavity is polished with a BCP acid solution of 1:3:1, when the acceleration gradient E acc < 24 MV / m, the quality factor Q o > 1.0×10 10 , the maximum Q o exceeds 2.0×10 10 , the maximum E acc reaches 26 MV / m, and its superconducting performance is very good, reaching the world-class level. This result shows that the relatively good BCP acid solution ratio of 1:3:1 obtained by the method of the present invention can not only avoid the formation of pitting and pits, but also the superconducting performance of the superconducting cavity polished with it is excellent.

[0030] As Figure 1 shown, the method for solving the pitting and pits on the surface of superconducting niobium cavities during buffered chemical polishing according to this embodiment finally includes providing this BCP acid solution ratio of 1:3:1. Using this ratio can not only solve the problem of pitting and pits on the surface of superconducting niobium cavities during BCP polishing, but also the surface quality and the radio frequency performance of the superconducting niobium cavity after polishing are very good. Therefore, it can be applied to the BCP polishing of actual superconducting niobium cavities, superconducting niobium cavity parts, accessory workpieces, niobium samples, etc.

[0031] Embodiment 2

[0032] The method for solving the pitting on the surface of the superconducting niobium cavity during buffered chemical polishing according to this embodiment first includes confirming the solubility and viscosity of the three original acid gases (nitric oxide NO) in BCP. At about room temperature of 20 °C, the viscosity of hydrofluoric acid (HF, 48% wt) is ~1.0 mPa·s, the viscosity of nitric acid (HNO3, 60% wt) is ~2.5 mPa·s, and the viscosity of phosphoric acid (H3PO4, 85% wt) is ~45.2 mPa·s. It can be seen from this 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 solute concentrations of HF and H3PO4 in water increase, the solubility of NO gas decreases. However, its solubility in nitric acid aqueous solution is many times greater than that in water and increases with the increase of nitric acid concentration. It can be seen from this that the solubility of phosphoric acid gas with a concentration of 85% is the smallest and the viscosity is the largest, while the solubility of nitric acid gas is the largest.

[0033] The method for solving the pitting on the surface of the superconducting niobium cavity during buffered chemical polishing according to this embodiment then includes reducing the volume ratio of phosphoric acid (H3PO4) in the BCP mixed acid and increasing the volume ratio of nitric acid (HNO3). 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, no pitting will appear on the surface after BCP polishing, and the gas solubility of the acid solution is large enough and the viscosity is small enough. For the convenience of adjusting and comparing the gas solubility and viscosity of the acid solution, taking 1:1:1 as a reference, reduce the proportion of phosphoric acid (H3PO4) and increase the proportion of nitric acid (HNO3) 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, 2:2:1.

[0034] The method for solving the pitting on the surface of the superconducting niobium cavity during buffered chemical polishing according to this embodiment then includes adjusting the ratio of hydrofluoric acid (HF) and nitric acid (HNO3). Based on 1:1:1, increasing the volume ratio of hydrofluoric acid HF will significantly increase the reaction rate, such as 1.5:1:1, 2:1:1, 3:1:1; increasing the volume ratio of nitric acid HNO3 will gradually decrease the reaction rate, such as 1:2:1, 1:3:1, 1:4:1, 1:6:1; increasing the volume ratio of phosphoric acid will gradually decrease the reaction rate, such as 1:3:1, 1:3:2.

[0035] The method for solving the pitting pits on the surface of the superconducting niobium cavity during buffered chemical polishing according to this embodiment next includes selecting the BCP acid solution ratio as HF:HNO3:H3PO4 = 1:6:1. For this ratio, k = V(HF + HNO3) / V(H3PO4) = 7 > 2, which can ensure that the gas solubility of the acid solution is large enough and the viscosity is small enough, and will not cause the formation of pitting pits; secondly, increasing the proportion of nitric acid (HNO3) can effectively reduce the reaction rate of the acid solution; 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) can enable the rapid formation of a viscous thin layer composed of the reaction products of niobium and nitric acid (HNO3). This viscous thin layer can effectively inhibit the reaction rate and is beneficial to surface polishing.

[0036] The method for solving the pitting pits on the surface of the superconducting niobium cavity buffer chemical polishing according to this embodiment then includes conducting a niobium sample BCP polishing experiment. Specifically, this step includes: 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 with a ratio of 1:6:1 and injecting it into the sample BCP polishing device, adjusting the acid temperature T≤8℃, the acid flow rate~ 1cm / s and other parameters, then put the sample into the sample BCP polishing device and polish for 10 minutes, and use a camera to observe and record the entire polishing process; after the BCP polishing of the niobium sample is completed, it is washed with pure water until neutral, and then transported to the clean room for pure water ultrasonic cleaning again and dried; the dried niobium sample is weighed, thickness measured and optically inspected, and the surface morphology of the sample after BCP polishing for 10 minutes is observed and recorded; according to the weighing and thickness measurement results before and after BCP polishing of the niobium sample for 10 minutes, the polishing rate of the sample BCP polishing for 10 minutes is calculated to be about 0.5-1.5μm / min. The surface optical inspection results of the niobium sample before and after BCP polishing for 10 minutes are compared, and combined with the BCP polishing process record, it is determined that the polishing surface of the new BCP acid ratio of 1:6:1 has no pits and pits, and the polished surface is flat, smooth and bright; repeat the above niobium sample experimental process many times, and change different polishing process parameters such as temperature, flow rate, polishing time and other parameters. Among them, it is recommended that the acid temperature T≤15℃, the acid flow rate≤5cm / s, and the polishing time≥0.5min. The BCP polishing experiments of these niobium samples showed that although the polishing rates were different under different polishing process parameters, the polishing rates were moderate (about 0.5-1.5 μm / min), there were no pits on the surface of the niobium samples, and the polished surface was flat, smooth and bright. It was determined that the new BCP acid solution ratio of 1:6:1 met the BCP polishing requirements of niobium samples under different polishing process parameters.

[0037] 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 step includes: preparing 200L of BCP mixed acid according to a ratio of 1:6:1 (i.e., adding 25L of hydrofluoric acid (HF), 150L of nitric acid (HNO3), and 25L 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 temperature and flow rate (i.e., flow velocity) of the polishing acid solution, and 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 BCP-In polishing of the superconducting niobium cavity for 10 minutes, a polishing rate of about 0.9μm / min is obtained after the BCP-In polishing of the superconducting niobium cavity for 10 minutes. The optical inspection results of the inner surface of the superconducting niobium cavity before and after polishing for 10 minutes were compared to determine that the new BCP acid ratio of 1:6:1 produced no pits on the polished surface, and the polished surface was flat, smooth and bright; the above superconducting niobium cavity BCP-In polishing experimental process was repeated many times, and different polishing process parameters such as temperature, flow rate, polishing time and other parameters were changed. Among them, it is recommended that the acid temperature T≤15℃, the acid flow rate≤30L / min, and the polishing time≥0.5min. These superconducting niobium cavity BCP-In polishing experiments show that although there are differences in polishing rate under different polishing process parameters, the polishing rate is moderate (about 0.8-1.3μm / min), the inner surface of the superconducting niobium cavity has no pits, and the polished surface is flat, smooth and bright. It is determined that the new BCP acid ratio of 1:6:1 meets the process requirements of superconducting niobium cavity BCP-In under different polishing parameters.

[0038] 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 includes BCP background treatment and vertical testing of the superconducting niobium cavity. A 1.3GHz Tesla single-cell fine-grained superconducting niobium cavity is selected for BCP background treatment (wherein the inner surface of the superconducting niobium cavity is optically inspected before and after BCP heavy polishing and light polishing), and then ultra-clean assembly is performed, and finally vertical testing is performed to obtain the radio frequency performance of the superconducting niobium cavity after BCP background treatment of the superconducting niobium cavity; the vertical test performance and inner surface optical inspection of the cavity BCP background treatment are better, so the new BCP acid liquid ratio of 1:6:1 is determined to be the better BCP acid liquid ratio.

[0039] The method for solving the pitting and pit formation on the surface of superconducting niobium cavities by chemical buffering polishing according to this embodiment finally includes providing the BCP acid solution ratio of 1:6:1. Using this ratio can not only solve the problem of pitting and pit formation on the surface of superconducting niobium cavities during BCP polishing, but also the surface quality after polishing and the radio frequency performance of superconducting niobium cavities are very good. Therefore, it can be applied to the BCP polishing of actual superconducting niobium cavities, superconducting niobium cavity parts, accessory workpieces, niobium samples, etc.

[0040] The present invention can fundamentally solve the problem. Starting from the essential reasons for the formation of pitting and pits on the surface of superconducting niobium cavities after BCP polishing (the solubility of acid solution gas is too small and the viscosity is too large, resulting in the attachment of bubbles to form pitting and pits), a targeted solution is proposed: changing the BCP acid solution ratio to increase the solubility of BCP mixed acid gas and reduce the viscosity of the mixed acid, obtaining a new and better acid solution ratio, and using the new and better BCP acid solution ratio, while ensuring a lower acid solution temperature, can solve the problem of pitting and pit formation on the BCP surface of superconducting niobium cavities.

[0041] The present invention gives a method for optimizing the BCP acid solution ratio. It not only gives a method for solving the problem of pitting and pit formation on the surface of superconducting niobium cavities after BCP polishing, but also gives a method and optimization criteria for optimizing the BCP acid solution ratio, so as to obtain a better BCP acid solution ratio according to different needs. This ratio may be different for different laboratories and different countries. For example, different concentrations of the three original acids used, different required polishing rates, and different required surface quality requirements for BCP polishing will all lead to differences in the obtained better acid solution ratio.

[0042] The present invention provides a better BCP acid solution ratio (such as 1:3:1). This ratio is obtained by the above method, which not only provides a good example and reference, but also this ratio can solve the problem of pitting and pit formation on the BCP surface of superconducting niobium cavities, and the radio frequency performance of the polished niobium surface and the polished superconducting niobium cavity are very good.

[0043] The method of the present invention is simple. Only by changing the BCP acid solution ratio can the problem of pitting and pit formation be solved, without the need to additionally add other auxiliary instruments, equipment or modify the original BCP polishing equipment.

[0044] The present invention has wide applicability. Since only the BCP acid solution ratio needs to be adjusted, this method is applicable to different BCP polishing equipment and different BCP polishing objects all over the world.

[0045] The proposal of the method of the present invention also helps to promote the further research on BCP, a basically mature chemical polishing method, in the field of accelerators.

[0046] The present invention is not limited to solving the problem of pitting on the surface of superconducting niobium cavities during BCP polishing and proposing a preferred BCP acid solution ratio (1:3:1), but can also be widely applied to how to adjust and optimize the BCP acid solution ratio and the optimization of other chemical polishing processes.

[0047] 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.

[0048] The above-mentioned are only the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various changes can be made to the above embodiments of the present invention. That is, all simple, equivalent changes and modifications made according to the claims and the content of the specification of the present invention application fall within the scope of protection of the claims of the present invention patent. Those not described in detail in the present invention are all conventional technical contents.

Claims

1. A method for solving the problem of pitting and pits on the surface of superconducting niobium cavities during buffered chemical polishing, characterized in that, The method includes reacting niobium with a mixed BCP acid solution of HF, HNO3, and H3PO4 to form a soluble niobium compound for surface polishing of a superconducting niobium cavity. Among them, the method includes the following steps: S1. Provide a BCP acid solution, where the ratio of HF: HNO3: H3PO4 in the BCP acid solution is 1:3:1; S2. Perform BCP polishing on the niobium sample using the BCP acid solution with a temperature T ≤ 15°C; S3. Perform BCP polishing on the inner surface of the superconducting niobium cavity using the BCP acid solution with a temperature T ≤ 15°C.

2. The method according to claim 1, characterized in that, In steps S2 and S3, the flow rate of the BCP acid solution ≤ 30 L / min, and the polishing time ≥ 0.5 min.

3. The method according to claim 1, wherein In steps S2 and S3, the polishing rate is between 0.5 - 2 μm / min.

4. The method according to claim 1, wherein Step S3 includes: S31. Perform a BCP polishing experiment on the inner surface of the superconducting niobium cavity using the BCP acid solution; S32. Perform BCP background treatment on the superconducting niobium cavity using the BCP acid solution; S33. Perform performance testing on the superconducting niobium cavity.

5. The method according to claim 4, wherein The performance testing is a vertical test to confirm that the BCP acid solution meets the surface radio frequency performance requirements of the superconducting niobium cavity.

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