LiF target preparation process based on interface diffusion anchoring

Abstract

The invention discloses a LiF target preparation process based on interface diffusion anchoring, which comprises the following steps: preheating a metal substrate to 200-300 DEG C, depositing a LiF film on the surface of the substrate through a vacuum evaporation process, and carrying out in-situ annealing to obtain a LiF target with high chemical purity, high surface uniformity and high film-substrate binding force. According to the method, the substrate preheating process is introduced, the mutual diffusion capacity between LiF molecules deposited on the surface of the substrate and substrate metal atoms is improved, the vacuum evaporation process parameters of the LiF film are integrally optimized, it is ensured that the LiF molecules have enough time to be mutually dissolved and expanded with the substrate metal atoms before cooling, and crystallization rearrangement is completed; therefore, a firm diffusion anchoring interface structure is formed, and the film-substrate binding force is remarkably enhanced. By introducing an in-situ annealing process, thermal stress generated due to mismatch of thermal expansion coefficients between the LiF film layer and the substrate is released, and it is ensured that the film layer of the LiF target is not prone to falling off under high-energy proton bombardment.

Description

Technical Field

[0001] This invention belongs to the field of accelerator neutron source target preparation technology, and particularly relates to a LiF target preparation process based on interface diffusion anchoring. Background Technology

[0002] In accelerator-based neutron source generation methods, proton bombardment of a LiF target is used ( 7 Li(p,n) 7 LiF (Be) is the mainstream approach for generating monoenergetic neutrons. Current technologies mainly employ vacuum evaporation or magnetron sputtering to deposit LiF layers hundreds of nanometers thick onto metal substrates.

[0003] While vacuum evaporation can ensure high chemical purity and surface uniformity of the film, the difference in thermal expansion coefficients between LiF and metal substrates such as Ag, along with low surface energy matching, leads to poor film-substrate adhesion, making it prone to peeling under prolonged beam bombardment. Magnetron sputtering, although it can improve adhesion, easily introduces impurity gases, thereby reducing the monoenergeticity of the neutron energy spectrum generated when protons bombard the LiF target. Therefore, to improve the film-substrate adhesion strength of LiF targets and solve the problem of film detachment due to thermal stress concentration under beam bombardment, a LiF target preparation process with high chemical purity, high surface uniformity, and high film-substrate adhesion is urgently needed. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a LiF target preparation process based on interfacial diffusion anchoring, which can produce LiF targets with high surface uniformity, high chemical purity, and strong film-substrate adhesion.

[0005] This invention discloses a LiF target preparation process based on interfacial diffusion anchoring, comprising the following steps: S1. Vacuum pumping The substrate and vapor deposition material are placed into the vacuum chamber, and the chamber pressure is evacuated to ≤10. -4 Pa; S2. Substrate preheating The substrate is heated to 200~300℃ and the substrate temperature is kept constant. S3. Evaporation deposition The raw material for vapor deposition is preheated by resistance heating. After the evaporation rate stabilizes, a LiF film is deposited on the substrate surface until the target thickness is reached. Then, the resistance heating is turned off, and the evaporation deposition is completed. S4. In-situ annealing In a vacuum environment, the substrate is cooled to room temperature in situ with the furnace to obtain a LiF target.

[0006] Furthermore, the substrate is a metal such as Ta, Ag, Mo, Pt, or Al.

[0007] Furthermore, before the substrate is placed into the vacuum chamber, it is ultrasonically cleaned and dried sequentially using acetone, anhydrous ethanol, and deionized water.

[0008] Furthermore, the deposition distance between the substrate and the evaporation source is 20~30cm.

[0009] Furthermore, in step S2, the substrate is heated to 220°C.

[0010] Furthermore, in step S3, the rotating substrate stage is turned on, and the substrate rotates at a speed of 5~15 rpm.

[0011] Furthermore, in step S3, the deposition rate of the LiF film is 0.1~1 nm / s.

[0012] Furthermore, in step S4, the cooling rate of the substrate during in-situ cooling with the furnace is 1~5℃ / min.

[0013] Furthermore, the LiF film thickness of the LiF target is 100~1200nm.

[0014] This invention provides a LiF target preparation process based on interfacial diffusion anchoring. Compared with existing technologies, the preparation process of this invention has at least the following outstanding advantages: This invention optimizes the vacuum evaporation process for LiF films, particularly by introducing substrate preheating and in-situ annealing processes. This promotes the diffusion of LiF molecules on the substrate surface and micro-diffusion at the interface, significantly enhancing the physical and chemical bonding strength between the film and the substrate while ensuring high chemical purity and surface uniformity of the LiF film. Scratch tests show that the critical load for the film-substrate bonding force of the LiF target prepared by this invention is approximately 2 N, which is about 4 to 5 times higher than that of the substrate-free preheating process. The LiF target prepared by this invention can meet the requirements for generating a stable monoenergetic neutron radiation field. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 The image shows the acoustic emission signal spectrum during the scratching process of the LiF film in Example 1 of this invention. Figure 2This is a super depth-of-field microscope image of the scratches on the LiF film in Example 1 of the present invention; Figure 3 This is a microstructure image of the LiF film scratches characterized by SEM-EDS in Example 1 of the present invention, wherein... Figure 3 (a) is the energy spectrum of F element characterized by EDS. Figure 3 (b) is the energy spectrum of Ag element characterized by EDS. Figure 3 (c) shows the microstructure characterized by SEM; Figure 4 The image shows the acoustic emission signal spectrum of the LiF film during the scratching process in Comparative Example 1 of this invention. Figure 5 This is a super-depth-of-field microscope image of the scratches on the LiF film in Comparative Example 1 of this invention; Figure 6 This is a microstructure image of the LiF film scratches in Comparative Example 1 of this invention, characterized by SEM-EDS. Figure 6 (a) is the energy spectrum of F element characterized by EDS. Figure 6 (b) is the energy spectrum of Ag element characterized by EDS. Figure 6 (c) shows the microstructure characterized by SEM; Figure 7 This is a schematic diagram of the membrane-substrate interface structure of the LiF film of the present invention, wherein... Figure 7 (a) is a schematic diagram of the membrane-based interface structure in Comparative Example 1. Figure 7 (b) is a schematic diagram of the membrane-based interface structure in Example 1. Detailed Implementation

[0017] The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] Unless otherwise specified, all temperatures mentioned herein are in degrees Celsius, and the preferred embodiments can be freely combined as needed. Those skilled in the art will understand that the data and parameters described in the examples are merely exemplary and do not constitute a limitation of the invention. All components used in the following examples and comparative examples are compounds known in the art, and all equipment used is equipment publicly known in the art. All components and equipment used in this invention can be obtained commercially or prepared using known techniques.

[0019] This invention provides a LiF target preparation process based on interface diffusion anchoring, comprising the following steps: S1. Vacuum pumping The substrate is selected from metals such as Ta, Ag, Mo, Pt, and Al, with Ag being preferred; before the substrate is placed into the vacuum chamber, it is ultrasonically cleaned and dried in sequence using acetone, anhydrous ethanol, and deionized water. The substrate and lithium fluoride deposition material were placed into the vacuum chamber, and the deposition distance between the substrate and the evaporation source was set to 20-30 cm. The vacuum pump was turned on, and the chamber pressure was evacuated to ≤10 ℃. -4 Pa ensures that residual air impurities are eliminated from affecting purity and bonding strength; S2. Substrate preheating Turn on the heating device and heat the substrate to 200~300°C, preferably to 220°C, and keep the substrate temperature constant. The substrate heating temperature range of the present invention is the optimal process window that takes into account both the crystallinity of LiF and the diffusion of LiF molecules at the substrate interface. The substrate heating temperature of the present invention enables the migration energy and crystallization rate of LiF molecules deposited on the substrate surface to reach the optimal balance, and significantly enhances the film-substrate adhesion through the interface anchoring effect. S3. Evaporation deposition Turn on the rotating substrate stage and rotate the substrate at a speed of 5-15 rpm; cover the evaporation source with a shield and preheat the evaporation material using resistance heating. After the evaporation rate stabilizes, remove the shield from the evaporation source and deposit a LiF film on the substrate surface. The deposition rate is preferably 0.1-1 nm / s until the target thickness of 100-1200 nm is reached; cover the evaporation source with a shield, turn off the resistance heating, and end the evaporation process. The LiF molecules deposited on the substrate surface in this way can obtain sufficient migration energy, thereby forming a tight interface anchor. S4. In-situ annealing In a vacuum environment, the substrate is cooled to room temperature in situ with the furnace, with a cooling rate preferably of 1~5℃ / min, to obtain a LiF target. The present invention effectively releases the thermal mismatch stress between the film and substrate interfaces through in-situ annealing, thereby preventing cracks caused by thermal stress concentration due to beam bombardment during subsequent use.

[0020] This invention adds substrate preheating and in-situ annealing steps, and optimizes the overall vacuum evaporation process parameters for depositing LiF films. By depositing the film at an extremely low deposition rate and combining it with surface diffusion at a specific temperature, it ensures that LiF molecules have sufficient time and migration energy for crystal rearrangement and diffusion anchoring, thereby forming a high-density and low-stress interface-anchored LiF film structure.

[0021] The present invention will now be described in more detail with reference to exemplary embodiments. The following embodiments or experimental data are intended to illustrate the present invention by way of example, and those skilled in the art should understand that the present invention is not limited to these embodiments or experimental data.

[0022] Example 1 The preparation of LiF targets based on interface diffusion anchoring by vacuum evaporation includes the following steps: S1. Vacuum pumping The substrate was selected as high-purity Ag with a purity of 99.99%. The substrate was ultrasonically cleaned and dried in sequence using acetone, anhydrous ethanol, and deionized water. The substrate and lithium fluoride deposition material were placed into the vacuum chamber, and the deposition distance between the substrate and the evaporation source was set to 25 cm. The vacuum pump was turned on, and the chamber pressure was evacuated to ≤10 ℃. -4 Pa; S2. Substrate preheating Turn on the heating device to heat the substrate to 220°C and keep the substrate temperature constant; S3. Evaporation deposition Turn on the rotating substrate stage and rotate the substrate at 10 rpm. Cover the evaporation source with a shield and preheat the evaporation material using resistance heating. After the evaporation rate stabilizes, remove the shield from the evaporation source and deposit a LiF film on the substrate surface. Evaporation and deposition for 20 minutes yields a LiF film with a thickness of approximately 115 nm and a deposition rate of approximately 0.1 nm / s. Cover the evaporation source with a shield, turn off the resistance heating, and end the evaporation process. S4. In-situ annealing In a vacuum environment, the substrate is cooled to room temperature in situ with the furnace at a cooling rate of 3℃ / min to obtain a LiF target.

[0023] Example 2 The preparation of LiF targets based on interface diffusion anchoring by vacuum evaporation includes the following steps: S1. Vacuum pumping The substrate was selected as high-purity Ag with a purity of 99.99%. The substrate was ultrasonically cleaned and dried in sequence using acetone, anhydrous ethanol, and deionized water. The substrate and lithium fluoride deposition material were placed into the vacuum chamber, and the deposition distance between the substrate and the evaporation source was set to 30 cm. The vacuum pump was turned on, and the chamber pressure was evacuated to ≤10 ℃. -4 Pa; S2. Substrate preheating Turn on the heating device to heat the substrate to 200°C and keep the substrate temperature constant; S3. Evaporation deposition Turn on the rotating substrate stage and rotate the substrate at 15 rpm. Cover the evaporation source with a shield and preheat the evaporation material using resistance heating. After the evaporation rate stabilizes, remove the shield from the evaporation source and deposit a LiF film on the substrate surface. Evaporation and deposition for 20 minutes yields a LiF film with a thickness of approximately 600 nm and a deposition rate of approximately 0.5 nm / s. Cover the evaporation source with a shield, turn off the resistance heating, and end the evaporation process. S4. In-situ annealing In a vacuum environment, the substrate is cooled to room temperature in situ with the furnace at a cooling rate of 5°C / min to obtain a LiF target.

[0024] Example 3 The preparation of LiF targets based on interface diffusion anchoring by vacuum evaporation includes the following steps: S1. Vacuum pumping The substrate was selected as high-purity Ag with a purity of 99.99%. The substrate was ultrasonically cleaned and dried in sequence using acetone, anhydrous ethanol, and deionized water. The substrate and lithium fluoride deposition material were placed into the vacuum chamber, and the deposition distance between the substrate and the evaporation source was set to 20 cm. The vacuum pump was turned on, and the chamber pressure was evacuated to ≤10 ℃. -4 Pa; S2. Substrate preheating Turn on the heating device to heat the substrate to 300°C and keep the substrate temperature constant; S3. Evaporation deposition Turn on the rotating substrate stage and rotate the substrate at 5 rpm. Cover the evaporation source with a shield and preheat the evaporation material using resistance heating. After the evaporation rate stabilizes, remove the shield from the evaporation source and deposit a LiF film on the substrate surface. Evaporation and deposition for 20 minutes yields a LiF film with a thickness of approximately 1180 nm and a deposition rate of approximately 1 nm / s. Cover the evaporation source with a shield, turn off the resistance heating, and end the evaporation process. S4. In-situ annealing In a vacuum environment, the substrate is cooled to room temperature in situ with the furnace at a cooling rate of 1℃ / min to obtain a LiF target.

[0025] Comparative Example 1 The LiF target was prepared by vacuum evaporation. The preparation process was the same as in Example 1, except that step S2 was omitted and the substrate was not preheated. The substrate was directly evaporated and deposited at room temperature. The other process parameters were exactly the same as in Example 1.

[0026] Comparative Example 2 The LiF target was prepared by vacuum evaporation. The preparation process was the same as in Example 1, except that the substrate preheating temperature in step S2 was 90°C. All other process parameters were exactly the same as in Example 1.

[0027] Comparative Example 3 The LiF target was prepared by vacuum evaporation. The preparation process was the same as in Example 1, except that the substrate preheating temperature in step S2 was 350°C. All other process parameters were exactly the same as in Example 1.

[0028] The adhesion between the LiF target and the film substrate was tested using a scratch test. Scratch tests were performed on the LiF targets prepared in Examples 1-3 and Comparative Examples 1-3. The film-substrate adhesion was characterized by scratch testing using a BRUKER UMTTriboLab tribometer. A linearly increasing load mode was used, with the normal load gradually increasing from 0.5 N to 15 N. The scratch length was 5 mm, and the test time was 750 s. To further analyze the failure behavior of the LiF films, the scratched areas were characterized by ultra-depth-of-field microscopy and SEM-EDS.

[0029] Figure 1 The image shows the acoustic emission signal collected during the scratching process of the LiF film in Example 1. The acoustic emission signal showed significant fluctuations in the range of approximately 2 to 3 N. Figure 2 The image shown is a super-depth-of-field microscope image of the scratches on the LiF film in Example 1. Figure 2 The scratches, from left to right, represent loads ranging from 0.5 N to 15 N, indicating that the film did not peel off at the initial location. Elemental analysis and microstructure analysis of the film surface were performed using EDS and SEM. Figure 3 As shown in (a)~3(c) Figure 3 The scratches from left to right represent loads ranging from 15N to 0.5N. Figure 3 As shown in (a) and (b), the F element energy spectrum signal is significant in the 0–2N range, while the substrate Ag element energy spectrum signal is extremely weak. In the 2–3N range, the substrate Ag element energy spectrum signal begins to increase, while the F element energy spectrum signal begins to decrease. This indicates that the LiF film maintains good integrity in the 0–2N range, but partial peeling begins in the 2–3N range. Figure 3 (c) The depth of the scratches can be seen to vary from 0.5 to 15 N, so the critical load for the film-substrate bonding force of the LiF target is about 2 N.

[0030] Figure 4 The image shows the acoustic emission signal collected from the LiF film during the scratching process in Comparative Example 1. Figure 4 The film did not exhibit clearly identifiable critical failure characteristics, which is usually associated with early failure of the film under extremely low loads. Figure 5 The image shown is a super-depth-of-field microscopic image of the scratches on the LiF film in Comparative Example 1. Figure 5 The scratches, from left to right, represent loads ranging from 0.5 N to 15 N. Microscopic images show that significant peeling occurs at the initiation point of each scratch. Elemental analysis and microstructure analysis of the film surface were performed using EDS and SEM. Figure 6 As shown in (a)~6(c), Figure 6 The scratches from left to right represent loads ranging from 15N to 0.5N. Figure 6As shown in (a) and (b), the energy spectrum signal of Ag in the substrate is clearly distinguishable throughout the entire scratched area, while the energy spectrum signal of F is almost undetectable within the detection limit. This indicates that the LiF film has been completely peeled off in the scratched area. Figure 6 (c) It can also be seen that the entire scratch area is clearly distinguishable, indicating that the critical load of the LiF target film-substrate bonding force is ≤0.5N.

[0031] Comparing Example 1 and Comparative Example 1, it can be seen that Example 1 preheated the substrate to 220°C, thereby improving the interdiffusion ability between LiF molecules and Ag atoms on the substrate. The controlled deposition rate in Example 1 ensured that the LiF molecules reaching the substrate had sufficient time to complete crystallization rearrangement before cooling, resulting in a dense and low-stress film structure formed by the interaction between LiF molecules and substrate metal atoms. The above process can be achieved through… Figure 7 To provide a more vivid explanation, such as Figure 7 As shown in (a), for the substrate in Comparative Example 1 that was not preheated, the Ag atoms in the substrate have a neat and compact lattice arrangement. The LiF molecules that evaporate and deposit onto the substrate surface are deposited onto the substrate surface through physical adsorption, resulting in weak film-substrate adhesion. Figure 7 As shown in (b), in Example 1, the substrate was preheated to 220°C, which to some extent disrupted the metallic bonds and lattice structure of Ag atoms on the substrate surface, thereby enhancing the mutual diffusion ability between LiF molecules and Ag atoms on the substrate. The controlled deposition rate in Example 1 ensured that the LiF molecules reaching the substrate had sufficient time to mutually dissolve and expand with Ag atoms on the substrate before cooling and to complete crystal rearrangement. The LiF molecules and Ag atoms at the film-substrate interface cooperated to form a dense and low-stress diffusion-anchored film structure, as shown in... Figure 7 As shown in (b), this crystal rearrangement following mutual dissolution and diffusion transforms the physical adsorption at the film-substrate interface into chemical bonding, thereby forming a strong interfacial anchor between the LiF film and the substrate, significantly enhancing the film-substrate adhesion. Combined with the in-situ annealing process of this invention, the thermal stress generated between the LiF film and the Ag substrate due to the mismatch in thermal expansion coefficients is effectively released, ensuring that the LiF target is not easily detached under high-energy proton bombardment and remains robust and durable.

[0032] Scratch tests were performed on the LiF targets prepared in Examples 2 and 3 of the present invention, and the test results were similar to those of Example 1 and significantly better than those of Comparative Example 1. The LiF films had critical load strengths of approximately 1.9 N and 1.8 N, respectively, indicating that the process of the present invention can stably support the preparation of LiF targets with different thicknesses of 100~1200 nm.

[0033] Scratch tests were conducted on the LiF targets prepared in Comparative Examples 2 and 3 of this invention. The critical load strength of Comparative Example 2 was only 0.8 N. Compared with Comparative Example 1, the film-substrate adhesion of Comparative Example 2 was improved, but the improvement was not significant due to insufficient preheating temperature. The film-substrate adhesion of Comparative Example 3 was significantly improved, with the critical load reaching 1.9 N. Although the higher substrate temperature improved the film-substrate adhesion, it promoted LiF grain growth. The excessively large LiF grains led to the deterioration of the surface inhomogeneity of the film layer, and even surface defects appeared. Therefore, excessively high preheating temperature is not conducive to the preparation of LiF targets with high surface uniformity. This invention achieves the optimal process balance point through overall optimization of the evaporation process parameters, thereby preparing LiF targets with high chemical purity, high surface uniformity, and high film-substrate adhesion. The LiF targets of this invention can generate a stable monoenergetic neutron radiation field.

[0034] All materials used in this invention are commercially available and can be purchased from retail sources. The above description is merely a preferred embodiment of the invention and is not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A LiF target fabrication process based on interfacial diffusion anchoring, characterized in that, Includes the following steps: S1. Vacuum pumping The substrate and vapor deposition material are placed into the vacuum chamber, and the chamber pressure is evacuated to ≤10. -4 Pa; S2. Substrate preheating The substrate is heated to 200~300℃ and the substrate temperature is kept constant. S3. Evaporation deposition The raw material for vapor deposition is preheated by resistance heating. After the evaporation rate stabilizes, a LiF film is deposited on the substrate surface until the target thickness is reached. Then, the resistance heating is turned off, and the evaporation deposition is completed. S4. In-situ annealing In a vacuum environment, the substrate is cooled to room temperature in situ with the furnace to obtain a LiF target.

2. The LiF target preparation process according to claim 1, characterized in that, The substrate is a metal such as Ta, Ag, Mo, Pt, or Al.

3. The LiF target preparation process according to claim 1, characterized in that, Before the substrate is placed into the vacuum chamber, it is ultrasonically cleaned and dried in sequence using acetone, anhydrous ethanol, and deionized water.

4. The LiF target preparation process according to claim 1, characterized in that, The deposition distance between the substrate and the evaporation source is 20~30cm.

5. The LiF target preparation process according to claim 1, characterized in that, In step S2, the substrate is heated to 220°C.

6. The LiF target preparation process according to claim 1, characterized in that, In step S3, the rotating substrate stage is turned on, and the substrate rotates at a speed of 5~15 rpm.

7. The LiF target preparation process according to claim 1, characterized in that, In step S3, the deposition rate of the LiF film is 0.1~1 nm / s.

8. The LiF target preparation process according to claim 1, characterized in that, In step S4, the cooling rate of the substrate during in-situ cooling with the furnace is 1~5℃ / min.

9. The LiF target preparation process according to claim 1, characterized in that, The LiF film thickness of the LiF target is 100~1200nm.

Images