A method of spark plasma sintering for producing a thick target

Abstract

This invention discloses a method and apparatus for preparing thick targets by spark plasma sintering. The method includes: preparing targets selected from... 58 Ni、 nat Ni、 nat Ga or 58 Ni、 nat Ni、 nat Ga compound, alloy, or combination thereof target powder is pretreated and pressed into a green layer; the green layer is placed on a support layer and assembled into a mold system; spark plasma sintering is performed in an argon atmosphere with an oxygen content ≤10 ppm, controlling the heating rate at 100-150℃ / min, the holding time at 10-15 min, and the current density at 3-12 A / mm. 2 Axial pressure of 5-80 MPa; finally, cooling and demolding yield a thick target; the device includes a vacuum and atmosphere system, a pressure and current introduction system, a mold system, and a PLC control system; this invention is applicable to 58 The preparation of thick targets using materials such as Ni and Ga-Ni alloys has resulted in targets with low element loss, high relative density, uniform microstructure, and excellent radiation resistance. These targets are particularly suitable for the preparation of high-performance thick targets required for the production of medical radionuclides.

Description

Technical Field

[0001] This invention relates to the field of radioactive isotope production target preparation technology, and more specifically, to a method for preparing thick targets by spark plasma sintering. Background Technology

[0002] The medical radionuclide Ge-68 serves as the precursor to the positron emission tomography (PET) imaging nuclide Ga-68. Its large-scale production relies on the irradiation preparation of high-performance gallium-nickel (Ga–Ni) alloy targets. Currently, the preparation of Ga–Ni alloy thick targets mainly employs two methods: high-temperature melting and electroplating.

[0003] The high-temperature melting method typically involves melting high-purity nickel and gallium in a vacuum or inert atmosphere in a specific ratio. After melting the nickel at high temperatures, gallium is added, and the mixture is thoroughly stirred and cast into a mold. Densification is then achieved through cold and hot pressing processes. While this method can produce thicker targets, it suffers from complex processes, high energy consumption, and high equipment costs. Furthermore, due to gallium's low melting point and high vapor pressure, elemental segregation and volatilization losses easily occur during melting and pressing, leading to uneven target composition and high porosity. This makes the target susceptible to corrosion, blistering, or cracking under high-current proton beam irradiation. Additionally, the high-temperature melting method often leaves unreacted Ga metal residue, resulting in poor phase-to-phase bonding and lower irradiation stability. The electroplating method, on the other hand, improves the electric field distribution at room temperature using a constant-temperature water bath and stirring device to achieve co-deposition of Ga and Ni on a metal substrate. This process has the advantages of low operating temperature and simple equipment. However, due to the complexity of the electrolyte system and the large differences in the mobility of metal ions, it often leads to segregation of coating composition and uneven thickness. Most existing electroplating devices adopt a vertical structure, and the electric field distribution of the stirring unit is uneven, making it difficult to obtain a coating with uniform thickness and consistent composition over a large area. Usually, the coating thickness is difficult to stably reach the millimeter level or above. At the same time, the density of the electroplated layer is poor, and it is easy to generate pores or microcracks, which reduces the thermal conductivity and thermal stability of the target material and makes it difficult to meet the heat dissipation requirements under high-power beam current.

[0004] In recent years, spark plasma sintering (SPS) technology has been considered a potential alternative process for preparing high-performance target materials due to its advantages such as rapid heating rate, low sintering temperature, and high densification efficiency. This technology achieves rapid densification and grain refinement of powder materials through the coupling effect of mechanical pressure, pulsed current, thermal conduction, and atmosphere control, effectively improving the thermal conductivity and radiation resistance of the target material. The Italian INFN-LNL laboratory has already experimented with using SPS in the LARAMED project. 89 Zr、 52 Mn, 67The preparation of Cu and other nuclide targets verified the feasibility of this technology. However, existing SPS equipment still has significant limitations in the preparation of large-size or complex alloy systems (such as Ga–Ni alloys): (1) The uneven distribution of electric field and temperature field leads to significant differences in the degree of sintering between the center and the edge of the target material; (2) At high temperatures, gallium is prone to volatilization and segregation, making it difficult to obtain an alloy structure with uniform composition; (3) Conventional SPS devices have limited mold cooling capacity and cannot effectively control temperature gradient, which makes the target material prone to stress cracks or interface delamination.

[0005] Furthermore, existing SPS systems are mostly standard structural designs and have not been optimized for the thick target requirements of radioisotope production. Their mold structure, atmosphere control, and closed-loop temperature regulation capabilities are insufficient, making it impossible to simultaneously achieve high densification, low loss, and process repeatability, thus limiting their application in… 58 Practical applications in Ni metal thick targets and Ga–Ni alloy targets.

[0006] In summary, existing Ga–Ni and 58 Ni thick target preparation technology generally suffers from problems such as complex processes, severe component segregation, high element volatilization loss, limited thickness, and insufficient thermomechanical properties, making it difficult to meet the requirements for use under high-power cyclotron irradiation conditions. Therefore, there is an urgent need for a new thick target preparation process and device to achieve the preparation of target materials with high density, high component homogeneity, low element loss, and excellent thermomechanical stability. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for preparing thick targets by spark plasma sintering, so as to solve the problems of complex thick target preparation process, serious element volatilization loss, poor composition uniformity, insufficient density and poor thermomechanical properties in the prior art.

[0008] To overcome the shortcomings of the prior art, the present invention provides a method for preparing a thick target by spark plasma sintering, comprising the following steps: S1: Pretreatment and Green Forming: The target material powder is pretreated, and the pretreated powder is pressed into a green layer; the target material powder is selected from... 58 Ni、 nat Ni、 nat Ga or 58 Ni、 nat Ni、 nat Compounds, alloys or combinations thereof of Ga; S2: The green layer is placed on a support layer to assemble the assembly. The assembly is placed in the mold system in the vacuum chamber of the spark plasma sintering device. A vacuum is drawn and high-purity argon is introduced to control the oxygen content to ≤10 ppm. S3: Spark Plasma Sintering: Spark plasma sintering is performed within a controllable parameter range using the aforementioned spark plasma sintering apparatus. The parameters include a heating rate of 100-150 ℃ / min, a holding time of 10-15 min, and a current density of 3-12 A / mm². 2 Axial pressure 5-80 MPa; S4: Post-processing: After the discharge plasma sintering in step S3 is completed, the target is cooled and demolded to obtain a thick target.

[0009] Compared with existing technologies, the method for preparing thick targets by spark plasma sintering disclosed in this application has the following advantages: The method of this invention achieves rapid solid-phase densification based on process paths and parameter windows S1–S4 without relying on liquid-phase processes such as melting or electroplating. Specifically, S1 involves selecting from… 58 Ni, natNi, natGa, and their compounds / alloys / combinations powders are pretreated and pressed into a green layer to construct a uniform initial porosity and particle contact state, providing a repeatable starting structure for subsequent densification; in S2, the green layer and support layer are assembled and placed into the mold system of the device, and vacuumed and purged in an inert atmosphere with an oxygen content ≤10 ppm, which significantly suppresses the loss and segregation of volatile / oxidizable elements, stabilizes stoichiometry, and reduces the introduction of impurities; in S3, the heating rate is 100–150 ℃ / min, the holding time is 10–15 min, the current density is 3–12 A / mm², and the axial pressure is 5–80. Within a confined window of MPa, discharge plasma sintering is performed. Through the coordinated control of pulsed current, axial pressure, and temperature field, rapid densification under the coupling of multiple thermal, electrical, and mechanical fields is achieved, realizing the synchronous matching of multiple electrical, thermal, and mechanical fields: the high heating rate promotes rapid neck formation and inhibits abnormal grain growth; the synergistic effect of axial pressure and current density reduces particle contact resistance and increases the densification rate; short-term heat preservation completes the final densification and microstructure stabilization, reducing the edge-to-center difference and residual stress as a whole, and obtaining a uniform microstructure and compositional distribution in the thickness direction and in-plane; in S4, after cooling and demolding, a thick target is obtained. Combining the continuity of the above steps, a comprehensive technical effect of near-full density, low element loss, highly uniform composition and microstructure, and stable integration with the support layer can be achieved, thus meeting the stringent requirements for thermal conductivity, mechanical properties, and long-term stability under high-current-intensity irradiation conditions.

[0010] In one possible implementation, in step S1, the target material raw material powder is... 58 Ni metal powder or Ga-Ni alloy powder, and the target material raw material powder is 58When the Ni metal powder is used, the pretreatment is freeze-dry grinding, and the particle size D90 of the powder is ≤45 μm; when the target material raw material powder is Ga-Ni alloy powder, the pretreatment is to compound nano-sized Ga powder with micron-sized Ni powder.

[0011] Compared with existing technologies, the above-mentioned technical solution can lay a crucial microstructural foundation for subsequent sintering through specific pretreatment for different powder systems. 58 Ni elemental powder, freeze-dry grinding under liquid nitrogen cryogenic protection, enhances sintering driving force by increasing powder specific surface area and surface activity, and effectively suppresses oxide formation during low-temperature brittle grinding, creating favorable conditions for obtaining a clean particle interface and achieving rapid bulk diffusion densification. For the volatile and segregated Ga-Ni alloy system, the combination of nano-sized Ga powder and micron-sized Ni powder constitutes an optimized bimodal particle system. The nanoparticles fill the pores of the micron-sized particles, significantly increasing the initial relative density of the green body. At the same time, the nanoparticles, as short diffusion paths and high surface energy activation points, can significantly reduce the sintering activation energy, inducing rapid formation and growth of the sintering neck. Meanwhile, the micron-sized powder acts as a skeleton to effectively maintain the shape of the green body and prevent excessive shrinkage and deformation in the early stage of sintering.

[0012] In one possible implementation, in step S1, the pressing conditions are: applying a uniaxial pressure of 50–80 MPa at room temperature and holding the pressure for 30–60 seconds, and the thickness / diameter ratio of the green layer is ≤ 0.15.

[0013] Compared with existing technologies, the above-mentioned technical solution can provide an ideal initial structure for subsequent sintering by optimizing the green body forming process parameters. The moderate pressure of 50-80 MPa can achieve effective rearrangement and mechanical interlocking of particles without excessively damaging the powder activity, resulting in a high initial green body density. The holding time of 30-60 seconds ensures stress relaxation and particle position stability. Strictly controlling the green body thickness / diameter ratio to ≤0.15 follows the basic requirement of uniform distribution of current and temperature fields during SPS (spark plasma sintering), effectively avoiding the phenomenon of excessive temperature difference between the center and the edge and uneven densification caused by an excessively large diameter-to-thickness ratio.

[0014] In one possible implementation, in step S2, the material of the support layer is Ta, Ti, Nb or Inconel alloy.

[0015] Compared with existing technologies, the above-mentioned technical solutions enable Ta, Ti, Nb, and Inconel alloys to possess high melting points, excellent high-temperature strength, good thermal conductivity, and thermal expansion coefficients that match the green layer. During the SPS process, this further ensures that the support layer itself does not deform at high temperatures and can form a strong and tough metallurgical interface with the green layer through diffusion under pressure and current. During high-current proton beam irradiation, the excellent thermal conductivity helps to quickly dissipate heat from the target surface and avoid local overheating, while the matching thermal expansion coefficients can effectively alleviate thermal cycling stress and prevent the interface from cracking or peeling due to thermal mismatch.

[0016] In one possible implementation, step S3, the spark plasma sintering process includes the following stages: Preheating and contact activation stage: Heat at a rate of 100-150 ℃ / min while applying an axial pressure of 5-20 MPa and 3-6 A / mm². 2 Current density; High-speed densification and phase control stage: Continue to increase the temperature at a rate of 100-150 ℃ / min to the target sintering temperature; Final densification and microstructure stabilization stage: Hold at the target sintering temperature for 10-15 min, while increasing the axial pressure to 20-50 MPa.

[0017] Compared with existing technologies, the above technical solution achieves efficient densification and microstructure optimization of thick target materials by establishing a staged and precisely controlled sintering kinetic path: in the preheating and contact activation stages, a rapid heating rate of 100-150℃ / min is used in conjunction with relatively low axial pressure (5-20 MPa) and current density (3-6 A / mm²). 2 The pulsed current can rapidly traverse the temperature range where powder surfaces are prone to re-oxidation. Simultaneously, it utilizes the Joule heating effect of the pulsed current to preferentially clean the particle surface and form an initial sintering neck, establishing a good conductive and mass transfer channel for subsequent densification. In the high-speed densification and phase control stage, high-speed heating is maintained to quickly bring the system to the sintering temperature. The thermal shock effect promotes particle boundary diffusion and bulk diffusion, driving the material to achieve bulk shrinkage and rapid elimination of pores. In the final densification and microstructure stabilization stage, by increasing the pressure to 20-50 MPa and holding it at 10-15 min, the residual closed pores are effectively closed by utilizing the plastic flow and creep mechanism of the material at high temperature. At the same time, the synergistic effect of pressure-temperature-time can induce the dispersion precipitation of strengthening phases in the alloy system, constructing a stable multi-scale microstructure.

[0018] In one possible implementation, in step S2, the discharge plasma sintering apparatus includes: A vacuum and atmosphere system, including a vacuum chamber, a high-vacuum pump unit connected to the vacuum chamber, and an inert gas circuit; The pressure and current introduction system includes: an upper pressure head and a lower pressure head arranged opposite each other, the upper pressure head and the lower pressure head being slidably inserted through the top and bottom of the vacuum cavity along the axial direction for introducing axial pressure and pulse current; a hydraulic loading system connected to the upper pressure head and / or the lower pressure head; and a pulse power supply system electrically connected to the upper pressure head and the lower pressure head. A mold system is disposed inside the vacuum cavity and between the upper and lower pressure heads. The mold system includes a mold cylinder and an upper and lower die punches that respectively contact the upper and lower pressure heads. A sintering cavity for accommodating the assembly is formed between the upper and lower die punches. The control system is connected to the pulse power supply system and the hydraulic loading system via signal connection.

[0019] Compared with existing technologies, the vacuum and atmosphere system of this embodiment achieves high vacuum through a series connection of mechanical pumps and molecular pumps. It also utilizes an inert gas loop with an integrated gas purifier and an oxygen analyzer to construct a real-time monitorable ultra-low oxygen dynamic atmosphere (oxygen content ≤10 ppm), suppressing the oxidation and volatilization of high-value, highly reactive powders. The pressure and current introduction system employs a dual-electrode water-cooled pressure head with opposing upper and lower electrodes, achieving uniform distribution of axial pressure and pulsed current across the mold cross-section. This effectively avoids localized overheating or uneven densification caused by current or pressure concentration within the sintered body. The control system, by integrating multiple sensors, achieves real-time monitoring and closed-loop precise control, ensuring the reproducibility of complex sintering process curves and batch-to-batch stability. These subsystems are not simply stacked together but deeply synergistic, forming a highly efficient, stable, and reliable dedicated platform. This provides a solid hardware guarantee for the preparation of high-performance thick targets, effectively overcoming the inherent defects of existing general-purpose SPS equipment in preparing such special materials, such as imprecise atmosphere control, uneven field distribution, and poor process repeatability.

[0020] In one possible implementation, the inner wall of the mold cylinder, the upper and lower die punches, and the end faces that contact the powder are all coated with a boron nitride coating; the high vacuum pumping unit consists of a mechanical pump and a molecular pump connected in series; the inert gas circuit includes a gas source and a gas purifier, and is equipped with an oxygen analyzer for real-time monitoring of the oxygen content in the cavity, the oxygen analyzer being connected to the inert gas circuit.

[0021] Compared with existing technologies, the above technical solution ensures the purity and stability of the sintering environment by constructing a multi-layered protection system. The boron nitride coating forms a stable chemical isolation layer between the mold and the powder. Utilizing the high-temperature stability and low surface energy characteristics of BN material, it effectively blocks highly reactive powders (including...) 58The diffusion reaction and chemical adsorption of Ni and Ga elements with graphite molds at high temperatures; simultaneously, a pumping unit composed of a mechanical pump and a molecular pump connected in series, through a staged vacuuming method, first rapidly removes most of the gas molecules, and then deeply removes the residual gas to ensure a yield of ≤10 -2 The background vacuum of Pa reduces the scattering effect of gas molecules on the pulse current. On this basis, the inert gas circuit of the integrated gas purifier and the oxygen analyzer form a closed-loop control system. The oxygen analyzer monitors the oxygen content in the circuit in real time and adjusts the purity of the intake air accordingly, forming a dynamic inert gas protection barrier.

[0022] In one possible implementation, both the upper and lower pressure heads are internally machined with spiral cooling channels, and flexible graphite pads are provided at the contact interfaces between the upper and lower pressure heads and the upper and lower die punches.

[0023] Compared with existing technologies, the above-mentioned technical solution can ensure the uniformity and stability of the sintering process by optimizing thermal management and pressure transmission paths. The spiral cooling channel processed inside the pressure head significantly improves cooling efficiency by increasing the heat exchange area and guiding the coolant to form turbulence. It can promptly remove a large amount of Joule heat generated by the pulse current, preventing the pressure head from deforming, burning, or sticking to the mold due to overheating. At the same time, the flexible graphite gasket set at the contact interface between the pressure head and the die punch utilizes the compressibility and self-lubricating properties of graphite material to automatically compensate for the parallelism error between the end face of the pressure head and the end face of the die punch, ensuring that the axial pressure and pulse current are uniformly distributed across the cross-section.

[0024] In one possible implementation, the control system is a programmable logic controller (PLC), which is connected to a temperature sensor, a pressure sensor, and a displacement sensor. The temperature sensor includes a K-type thermocouple and a non-contact infrared thermometer. The K-type thermocouple is connected to the sintering cavity, and the K-type thermocouple is connected to the non-contact infrared thermometer. The control system also includes a host computer that is communicatively connected to the PLC.

[0025] Compared with existing technologies, the above-mentioned technical solution, by constructing a multi-level, closed-loop intelligent control system, achieves precise perception and proactive intervention of key parameters in the sintering process. The composite temperature sensing system, consisting of K-type thermocouples and non-contact infrared thermometers, overcomes the limitations of single temperature measurement methods through signal connection and data fusion, realizing comprehensive, accurate, and mutually calibrated real-time monitoring of the temperature field inside the sintering cavity and on the mold surface. Pressure and displacement sensors accurately track axial pressure values ​​and indenter displacement, respectively. All sensor data are acquired and processed at high speed by a PLC, and precise closed-loop control of the pulse power supply system and hydraulic loading system is performed according to the preset process curve. The host computer communicates with the PLC and undertakes the tasks of managing the process formula, recording and storing all process data, and quality traceability related to the performance of the finished product.

[0026] In one possible implementation, the cooling and demolding conditions are as follows: the temperature is reduced according to a preset cooling program, the cooling rate is controlled at 50-100 ℃ / min, and after the pressure is released, the sintered target is taken out.

[0027] Compared with existing technologies, the above technical solution precisely controls the cooling rate within the range of 50-100 ℃ / min, avoiding excessive thermal stress caused by uneven thermal shrinkage inside the target due to excessively rapid cooling, which could lead to macroscopic cracks or microscopic damage. It also prevents the sintered body from staying in the high-temperature zone for too long due to excessively slow cooling, which could lead to abnormal grain coarsening or excessive precipitation of unfavorable brittle phases, affecting the strength and toughness of the target material.

[0028] The above-mentioned device system of the present invention is not a simple stacking, but a synergistic optimization of the challenges of low oxygen control, field uniformity and thermal management in thick target preparation. The method and device of the present invention constitute a deeply synergistic dedicated system, and the two are indispensable, together ensuring the repeatability and batch-to-batch stability of target performance. Attached Figure Description

[0029] Figure 1 This is a process flow diagram of the method for preparing thick targets by spark plasma sintering according to the present invention; Figure 2 This is a schematic cross-sectional view of the overall structure of the spark plasma sintering apparatus of the present invention. Figure 3 This is a logical connection block diagram of the mold system and control system of the present invention.

[0030] Figure 4 This is a schematic diagram of the internal structure of the sintering cavity in this invention.

[0031] Reference numerals: 1. Vacuum chamber; 2a. Upper pressure head; 2b. Lower pressure head; 3. Flexible graphite gasket; 4. Mold system; 4a. Upper die punch; 4b. Lower die punch; 4c. Mold cylinder; 5. Sintering chamber; 6. Pulse power supply system; 7. PLC; 8. Infrared thermometer; 9. K-type thermocouple; 10. Pressure sensor; 11. Displacement sensor; 12a. Mechanical pump; 12b. Molecular pump; 13a. Gas source; 13b. Gas purifier; 14. Oxygen analyzer; 15. Host computer; 16. Support layer; 17. Green layer. Detailed Implementation

[0032] First, those skilled in the art should understand that these embodiments are merely used to explain the technical principles of the embodiments of this application and are not intended to limit the scope of protection of the embodiments of this application. Those skilled in the art can make adjustments as needed to adapt to specific application scenarios.

[0033] In the description of the embodiments of this application, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this application based on the specific circumstances.

[0034] In the embodiments of this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0035] This invention provides a method for preparing thick targets by spark plasma sintering, such as... Figure 1 As shown, it includes the following steps: S1: Pretreatment and Green Forming: The target material powder is pretreated, and the pretreated powder is pressed into a green layer; the target material powder is selected from... 58 Ni、 nat Ni、 nat Ga or 58 Ni、 nat Ni、 nat Compounds, alloys or combinations thereof of Ga; S2: The green layer is placed on a support layer to assemble the assembly. The assembly is placed in the mold system in the vacuum chamber of the spark plasma sintering device. A vacuum is drawn and high-purity argon is introduced to control the oxygen content to ≤10 ppm. S3: Spark Plasma Sintering: Spark plasma sintering is performed within a controllable parameter range using the aforementioned spark plasma sintering apparatus. The parameters include a heating rate of 100-150 ℃ / min, a holding time of 10-15 min, and a current density of 3-12 A / mm². 2 Axial pressure 5-80 MPa; S4: Post-processing: After the discharge plasma sintering in step S3 is completed, the target is cooled and demolded to obtain a thick target.

[0036] As a preferred embodiment, in step S1, the target material raw material powder is 58 Ni metal powder or Ga-Ni alloy powder, and the target material raw material powder is 58 When the Ni metal powder is used, the pretreatment is freeze-dry grinding, and the particle size D90 of the powder is ≤45 μm; when the target material raw material powder is Ga-Ni alloy powder, the pretreatment is to compound nano-sized Ga powder with micron-sized Ni powder.

[0037] As a preferred embodiment, in step S1, the pressing conditions are: applying a uniaxial pressure of 50–80 MPa at room temperature and holding the pressure for 30–60 seconds, and the thickness / diameter ratio of the green layer is ≤ 0.15.

[0038] As a preferred embodiment, in step S2, the material of the support layer is Ta, Ti, Nb or Inconel alloy.

[0039] As a preferred embodiment, step S3 includes the following stages in the spark plasma sintering process: Preheating and contact activation stage: Heat at a rate of 100-150 ℃ / min while applying an axial pressure of 5-20 MPa and 3-6 A / mm². 2 Current density; High-speed densification and phase control stage: Continue to increase the temperature at a rate of 100-150 ℃ / min to the target sintering temperature; Final densification and microstructure stabilization stage: Hold at the target sintering temperature for 10-15 min, while increasing the axial pressure to 20-50 MPa.

[0040] As a preferred embodiment, in step S2, the discharge plasma sintering apparatus is as follows: Figure 2-4 As shown, it includes: The vacuum and atmosphere system includes a vacuum chamber 1, a high-vacuum pump unit 12 connected to the vacuum chamber 1, and an inert gas circuit 13. The pressure and current introduction system includes: an upper pressure head 2a and a lower pressure head 2b arranged opposite each other, the upper pressure head 2a and the lower pressure head 2b being slidably inserted axially through the top and bottom of the vacuum cavity 1 for introducing axial pressure and pulse current; a hydraulic loading system connected to the upper pressure head 2a and / or the lower pressure head 2b; and a pulse power supply system 6 electrically connected to the upper pressure head 2a and the lower pressure head 2b. A mold system 4 is disposed inside the vacuum cavity 1 and located between the upper pressure head 2a and the lower pressure head 2b. The mold system 4 includes a mold cylinder 4c and an upper die punch 4a and a lower die punch 4b that are in contact with the upper pressure head 2a and the lower pressure head 2b, respectively. A sintering cavity 5 for accommodating the assembly is formed between the upper die punch 4a and the lower die punch 4b. The control system 7 is connected to the pulse power supply system 6 and the hydraulic loading system via signals.

[0041] As a preferred embodiment, the inner wall of the mold cylinder 4c, the end faces of the upper die punch 4a and the lower die punch 4b that contact the powder are all coated with a boron nitride coating; the high vacuum pumping unit 12 is composed of a mechanical pump 12a and a molecular pump 12b connected in series; the inert gas circuit 13 includes a gas source 13a and a gas purifier 13b, and is equipped with an oxygen analyzer 14 for real-time monitoring of the oxygen content in the cavity, the oxygen analyzer 14 being connected to the inert gas circuit 13.

[0042] This invention deeply couples a dedicated spark plasma sintering device with optimized process steps, forming a synergistic and efficient complete technical solution. After the green body preparation is completed in step S1, the assembly and loading process in step S2 directly depends on the core structure of the dedicated device: the assembly is placed in the sintering cavity 5) formed by the mold cylinder, upper die punch, and lower die punch. All surfaces of the mold system that come into contact with the powder are coated with boron nitride, which effectively prevents the reaction and adhesion of the highly active target powder to the mold during subsequent high-temperature sintering. At the same time, the vacuum and atmosphere system of the device starts to work. The high-vacuum pumping unit 12, which consists of a mechanical pump and a molecular pump connected in series, first pumps the cavity to a high vacuum. Then, high-purity argon is introduced through the inert gas circuit 13 of the integrated gas purifier, and real-time monitoring and feedback are achieved through the oxygen analyzer 14. This establishes and maintains an ultra-low oxygen environment with an oxygen content of ≤10 ppm for the sintering in step S3, which is a key prerequisite for achieving extremely low element volatilization and oxidation loss. In the core step S3, the synergistic effect of the device is further demonstrated: the pressure and current introduction system and control system 7) strictly execute the three-stage sintering procedure designed for thick targets; the upper and lower dual-electrode water-cooled pressure head, driven by the hydraulic loading system, precisely applies axial pressure, while its internal spiral cooling channel and the flexible graphite gasket 3 at the end jointly ensure the thermal stability of the pressure head under long-term high current and the uniform distribution of pressure and current between it and the mold end face; the pulse power supply system 6, under the instruction of the programmable logic controller PLC 7, outputs pulse current of a specific mode, and its synergistic change with the pressure directly drives the activation, densification and phase transformation of powder particles. The PLC system monitors and controls the entire process in real time through integrated temperature sensors (including infrared thermometer 8 and K-type thermocouple 9), pressure sensor 10 and displacement sensor 11), ensuring the accuracy and repeatability of the process. All data is stored in the host computer 15 for quality traceability.

[0043] In summary, this invention achieves a high degree of matching and mutual enhancement between process and equipment by executing innovative process steps in a specially optimized device. The steps provide a platform for the device's functions, while the device provides precise, stable, and reliable physical guarantees for achieving the method's objectives. Together, they ultimately overcome the technical challenges of low loss, high density, uniform structure, and strong bonding in thick target preparation.

[0044] As a preferred embodiment, both the upper pressure head 2a and the lower pressure head 2b are internally machined with spiral cooling channels, and flexible graphite pads 3 are provided at the contact interfaces between the upper pressure head 2a and the lower pressure head 2b and the upper die punch 4a and the lower die punch 4b.

[0045] As a preferred embodiment, the control system is a programmable logic controller (PLC7). The PLC7 is connected to a temperature sensor, a pressure sensor 10, and a displacement sensor 11. The temperature sensor includes a K-type thermocouple 9 and a non-contact infrared thermometer 8. The K-type thermocouple 9 is connected to the sintering chamber 5, and the K-type thermocouple 9 is connected to the non-contact infrared thermometer 8. The control system also includes a host computer 15 that is communicatively connected to the PLC7.

[0046] As a preferred embodiment, the cooling and demolding conditions are as follows: the temperature is reduced according to a preset cooling program, the cooling rate is controlled at 50-100 ℃ / min, and after the pressure is released, the sintered target is taken out.

[0047] Overall, the method for preparing thick targets by spark plasma sintering of the present invention has the following technical advancements: First, the material utilization rate is high and the economic benefits are significant: This invention eliminates the wall adhesion loss caused by the reliance on the lateral surrounding mold in traditional powder sintering. Combined with short cycle, low process temperature and ultra-low oxygen partial pressure (≤10 ppm) conditions, it effectively suppresses the loss of volatile / oxidizable elements, making the comprehensive utilization rate of high-value enriched isotope powders significantly higher than that of electroplating and conventional pressureless sintering. The low loss also allows the target material that has not participated in the reaction to be recycled and reused as powder, forming a closed-loop material flow and further reducing the unit product cost. Second, it has excellent mechanical properties: through low-temperature fine powder processing and rapid densification, the porosity of the target is strictly controlled, achieving near-full densification; under the coupling of electricity, heat and force, a robust interface is formed with reliable mechanical interlocking as the main function and diffusion bonding characteristics, enabling the target to withstand long-term repeated bombardment by high-intensity particle beams without cracking or peeling off. Its thermomechanical stability and service life are significantly better than traditional methods. Third, it has high production efficiency and good batch stability: the sintering cycle of this invention can be controlled in the tens of minutes range, which is an order of magnitude improvement over electroplating (tens of hours) and traditional sintering (several hours); the process adopts closed-loop control to accurately control and track key parameters such as temperature, pressure, current and displacement throughout the process, with small batch fluctuations and good repeatability, meeting the quality consistency requirements of large-scale production. Fourth, it has strong scalability and flexible process: The method of this invention supports multi-target stacking and parallel sintering, and can obtain multiple target parts simultaneously in a single cycle, thereby increasing the unit time capacity and significantly reducing costs, and has good production line expansion and adaptability. Fifth, the microstructure can be precisely controlled: While achieving high densification, the method of this invention can induce the dispersion precipitation of beneficial strengthening phases through the design of thermal cycling and residence time, and construct a conductive chain with high density, multi-scale strengthening phase, high wear resistance, and radiation resistance. By matching the parameter window, the generation of unfavorable intermetallic phases in the interface region can be suppressed, avoiding the resulting strength reduction and early failure, thereby obtaining a high-performance interface and overall structure with fine microstructure and good consistency in thickness and in-plane.

[0048] The present application will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0049] Example 1: High density 58 Preparation of Ni thick targets S1: Pretreatment and Green Forming Take 200 grams of [amount] with an enrichment degree >99.5%. 58 Ni powder was placed in a cryogenic planetary ball mill under high-purity argon protection, with an appropriate amount of cemented carbide grinding balls added at a ball-to-powder ratio of 10:1. Liquid nitrogen was introduced to cool the powder to -160°C, and the powder was then freeze-dried at this temperature for 3 hours. After grinding, the powder D90 decreased to 38 μm, and the particle size distribution became more concentrated.

[0050] After processing 58 Ni powder was filled into a cylindrical graphite mold system 4 (cavity diameter 30 mm) in which the inner wall of the mold cylinder 4c and the end faces of the upper and lower die punches were coated with boron nitride. A uniaxial pressure of 70 MPa was applied at room temperature and held for 45 seconds to obtain a green body with a thickness of 4.0 mm. The calculated relative density of the green body was 78%, which meets the requirement of thickness / diameter ratio ≤ 0.15.

[0051] S2: Assembly and Loading Prepare a tantalum metal disc with a diameter of 34 mm and a thickness of 3 mm as a support layer, and polish the surface. Place the green blank layer in the center of the support layer to assemble the assembly, and place the assembly in the sintering cavity 5 of the mold system 4 of the spark plasma sintering device.

[0052] Start the vacuum and atmosphere system: First, the high-vacuum pumping unit, consisting of mechanical pump 12a and molecular pump 12b connected in series, evacuates the vacuum chamber to 5 × 10⁻⁶ ppm. -2 After Pa, high-purity argon gas supplied by gas source 13a and purified by gas purifier 13b is dynamically introduced. The oxygen analyzer 14 installed in the loop monitors and provides feedback in real time to control the oxygen partial pressure in the atmosphere to be stable at ≤3 ppm.

[0053] S3: Spark Plasma Sintering The sintering program is set through the control system, the core of which is a programmable logic controller (PLC7). Preheating and contact activation stage: Heating is carried out at a heating rate of 120 ℃ / min, while an axial pressure of 10 MPa is applied to the upper pressure head via a hydraulic loading system; the pulse power supply system 6 applies pulsed current through the upper and lower pressure heads. During this process, the K-type thermocouple 9 and the non-contact infrared thermometer 8 jointly monitor the temperature, and their signals are fed back to the PLC 7. The spiral cooling channels inside the upper and lower pressure heads, as well as the flexible graphite gaskets 3 between their end faces and the upper and lower die punches, ensure the uniformity of thermal management and current / pressure distribution. High-speed densification and phase control stage: Continue heating at a rate of 120 ℃ / min to reach the target sintering temperature of 1250℃; PLC7 monitors the densification shrinkage process based on the signal from displacement sensor 11; Final densification and microstructure stabilization stage: The temperature was held at 1250 ℃ for 12 min, while the axial pressure was increased to 35 MPa; pressure sensor 10 ensured pressure control accuracy. All process parameters throughout the sintering process were recorded by PLC7 and transmitted to the host computer 15 for storage and quality traceability. S4: Post-processing After sintering, the target is cooled to below 200°C under vacuum at a rate of 80°C / min according to the preset cooling program. After the pressure is released, the sintered target is taken out.

[0054] The final target had a diameter of 29.8 mm and a thickness of 3.7 mm; its relative density was determined to be 99.5% by weighing. Proton beam bombardment tests were conducted on the TR-FLEX 30 MeV cyclotron at a beam intensity of 200 μA for 6 hours; after the test, the target surface remained intact, without cracks or peeling.

[0055] Example 2: Preparation of Ga-Ni alloy thick target S1: Pretreatment and Green Forming Micron-sized Ni powder (particle size distribution 1-15 μm) and nano-sized Ga powder (average particle size about 80 nm) were mixed in a V-type mixer under argon protection at a mass ratio of 70:30. The compound powder was placed in a low-temperature planetary ball mill under high-purity argon protection, and an appropriate amount of cemented carbide grinding balls were added, with a ball-to-powder ratio of 10:1. Liquid nitrogen was introduced to cool the mixture to -160°C, and the mixture was ground at this temperature for 3 hours. The treated powder is filled into the mold system 4 (the inner wall of the mold cylinder 4c, the upper die punch and the end face of the lower die punch are all coated with boron nitride coating) (cavity diameter 50 mm), and pressed into shape under a pressure of 60 MPa, with a green blank thickness of 5.0 mm.

[0056] S2: Assembly and Loading Prepare a niobium metal disc with a diameter of 34 mm and a thickness of 3 mm as a support layer, and polish the surface. Place the green blank layer in the center of the support layer to assemble the assembly, and place the assembly in the sintering cavity 5 of the mold system 4 of the spark plasma sintering device.

[0057] Start the vacuum and atmosphere system: The vacuum chamber is evacuated to 5 × 10⁻⁶ by mechanical pump 12a and molecular pump 12b. -2 After Pa, high-purity argon gas (from gas source 13a) purified by gas purifier 13b is dynamically introduced, and the oxygen partial pressure is monitored and controlled to be ≤3 ppm by oxygen analyzer 14.

[0058] S3: Spark Plasma Sintering The sintering program is set and executed through a control system based on a PLC7: Preheating and contact activation stage: heating at a heating rate of 150 ℃ / min while applying an axial pressure of 8 MPa; High-speed densification and phase control stage: Continue heating at a rate of 150 ℃ / min to reach the target sintering temperature of 1180 ℃; K-type thermocouple 9 and infrared thermometer 8 provide temperature feedback, and displacement sensor 11 monitors shrinkage; Final densification and microstructure stabilization stage: The temperature was maintained at 1180 ℃ for 10 min, while the axial pressure was increased to 25 MPa; pressure sensor 10 ensured accurate pressure. All process data was collected by PLC7 and stored in the host computer 15.

[0059] S4: Post-processing After sintering, the target is cooled according to the preset cooling program. After the pressure is released, the sintered target is taken out.

[0060] Testing revealed that the target's relative density was 98%. Precise weighing of the target material before and after sintering, combined with analysis of volatile deposits, showed a Ga volatilization loss of 5%, far below the target value. Phase analysis indicated the formation of the expected strengthening phase within the alloy matrix. Thermal load testing at a 200 μA proton beam showed the target remained stable without hot spots. In summary, the successful implementation of Examples 1 and 2 fully validates the effectiveness and superiority of the method and apparatus of this invention. Example 1, through freeze-dry grinding pretreatment of 58Ni powder, optimized green compact forming, and three-stage precise sintering in a dedicated apparatus, successfully obtained a target with a relative density as high as 99.5% and excellent radiation resistance. 58 Ni thick target; Example 2, through the compound pretreatment of nano / micro Ga-Ni powder and the corresponding rapid sintering process, obtained a uniform alloy thick target with a relative density of 98% and the formation of the expected strengthening phase while effectively controlling the Ga element volatilization loss to 5%; The above examples together illustrate the core technical principle of the present invention, namely, through the process path of powder pretreatment-assembly and loading-multi-stage sintering-controllable cooling, in deep collaboration with a special device integrating vacuum atmosphere, pressure current introduction, mold and intelligent control system, using rapid heating to cross the oxidation temperature zone, driving efficient densification through the dynamic combination of pulse current and axial pressure, and maximally suppressing element volatilization and oxidation in an ultra-low oxygen environment, and finally stabilizing the microstructure through controllable cooling, the technical difficulties of high density, low loss, uniform composition and strong bonding in the preparation of thick targets are overcome, providing a complete, efficient and economical technical solution for the preparation of high performance medical isotope production thick targets.

[0061] Figure 4 This is a schematic diagram of the multi-target sintering layout of the mold system 4 of the present invention; to improve efficiency in industrial production, the present invention provides various mold layout schemes; such as... Figure 4 As shown, the mold system 4 can be arranged in the following three basic ways according to production needs: Figure 4In the middle, the left figure shows a serial arrangement structure, that is, multiple mold cavities are arranged sequentially along the axial direction. Each mold cavity is provided with a support layer 16 and a green layer 17 in sequence, and each cavity forms an independent sintering cavity 5. In the same discharge plasma sintering cycle, multiple target bodies can be stacked and sintered sequentially. Figure 4 The middle figure shows a parallel arrangement structure, in which multiple mold units are arranged side by side in the radial direction. Each mold unit also includes an assembly composed of a support layer 16 and a green layer 17. Each unit is defined by an independent mold cylinder 4c, but shares the same set of upper and lower pressure heads and current paths to achieve synchronous densification of multiple targets. Figure 4 The right figure shows a hybrid arrangement structure that combines the above-mentioned serial and parallel layouts. Each arrangement unit includes a forming structure of support layer 16 and green layer 17. Through the coordinated arrangement of multiple rows and columns of mold units, multiple workpieces can be formed synchronously in a single sintering cycle. Through the above-mentioned multi-target stacking and multi-cavity parallel design, the production capacity and equipment utilization of spark plasma sintering can be significantly improved. The specific layout of the mold system 4 can be adapted to the radial dimension and axial thickness of the target. Its overall structural dimensions should match the effective working space between the upper and lower pressure heads to ensure the feasibility of equipment operation, the balance of force, and the uniformity of current distribution.

[0062] In the description of the embodiments of this application, it should be noted that the terms "inner" and "outer" and other terms indicating direction or positional relationship are based on the direction or positional relationship shown in the drawings. This is only for the convenience of description and does not indicate or imply that the device or component must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this application.

[0063] In the description of this application, the references to terms such as "an embodiment," "some embodiments," "in this embodiment," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0064] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing a thick target by spark plasma sintering, characterized in that, Includes the following steps: S1: Pretreatment and Green Forming: The target material powder is pretreated, and the pretreated powder is pressed into a green layer; the target material powder is... 58 Ni metal powder, or a combination of Ga powder and Ni powder; S2: The green layer is placed on a support layer to assemble it into an assembly. The assembly is placed in the mold system in the vacuum chamber of the spark plasma sintering device, and a vacuum is drawn and high-purity argon is introduced to control the oxygen content to ≤10ppm. S3: Spark Plasma Sintering: Spark plasma sintering is performed within a controllable parameter range using the aforementioned spark plasma sintering apparatus. The parameters include a heating rate of 100-150 ℃ / min, a holding time of 10-15 min, and a current density of 3-12 A / mm². 2 Axial pressure 5-80 MPa; S4: Post-processing: After the discharge plasma sintering in step S3 is completed, the target is cooled and demolded to obtain a thick target.

2. The method for preparing a thick target by spark plasma sintering according to claim 1, characterized in that, In step S1, when the target material raw material powder is 58 When using Ni metal powder, the pretreatment is freeze-dry milling, and the particle size D90 of the powder is ≤45 μm; when the target material raw material powder is a combination of Ga powder and Ni powder, the pretreatment is to compound nano-sized Ga powder with micron-sized Ni powder.

3. The method for preparing a thick target by spark plasma sintering according to claim 1, characterized in that, In step S1, the pressing conditions are: applying a uniaxial pressure of 50–80 MPa at room temperature and holding the pressure for 30–60 seconds to form the green layer, and the thickness / diameter ratio of the green layer is ≤ 0.

15.

4. The method for preparing a thick target by spark plasma sintering according to claim 1, characterized in that, In step S2, the material of the support layer is Ta, Ti, Nb or Inconel alloy.

5. The method for preparing a thick target by spark plasma sintering according to claim 1, characterized in that, In step S3, the spark plasma sintering process includes the following stages: Preheating and contact activation stage: Heat at a rate of 100-150 ℃ / min while applying an axial pressure of 5-20 MPa and 3-6 A / mm². 2 Current density; High-speed densification and phase control stage: Continue to increase the temperature at a rate of 100-150 ℃ / min to the target sintering temperature; Final densification and microstructure stabilization stage: Hold at the target sintering temperature for 10-15 min, while increasing the axial pressure to 20-50 MPa.

6. The method for preparing a thick target by spark plasma sintering according to claim 1, characterized in that, In step S2, the discharge plasma sintering apparatus includes: The vacuum and atmosphere system includes a vacuum chamber (1), a high vacuum pump unit (12) connected to the vacuum chamber (1), and an inert gas circuit (13). The pressure and current introduction system includes: an upper pressure head (2a) and a lower pressure head (2b) arranged opposite each other, the upper pressure head (2a) and the lower pressure head (2b) being slidably inserted axially through the top and bottom of the vacuum cavity (1) for introducing axial pressure and pulse current; a hydraulic loading system connected to the upper pressure head (2a) and / or the lower pressure head (2b); and a pulse power supply system (6) electrically connected to the upper pressure head (2a) and the lower pressure head (2b). A mold system (4) is disposed inside the vacuum cavity (1) and located between the upper pressure head (2a) and the lower pressure head (2b). The mold system (4) includes a mold cylinder (4c) and an upper die punch (4a) and a lower die punch (4b) respectively in contact with the upper pressure head (2a) and the lower pressure head (2b). A sintering cavity (5) for accommodating the assembly is formed between the upper die punch (4a) and the lower die punch (4b). The control system (7) is connected to the pulse power supply system (6) and the hydraulic loading system via signal connection.

7. The method for preparing a thick target by spark plasma sintering according to claim 6, characterized in that, The inner wall of the mold cylinder (4c), the upper die punch (4a), and the lower die punch (4b) are all coated with boron nitride coatings on their end faces that contact the powder. The high vacuum pumping unit (12) is composed of a mechanical pump (12a) and a molecular pump (12b) connected in series. The inert gas circuit (13) includes a gas source (13a) and a gas purifier (13b), and is equipped with an oxygen analyzer (14) for real-time monitoring of the oxygen content in the cavity. The oxygen analyzer (14) is connected to the inert gas circuit (13).

8. The method for preparing a thick target by spark plasma sintering according to claim 6, characterized in that, Both the upper pressure head (2a) and the lower pressure head (2b) are machined with spiral cooling channels inside, and flexible graphite pads (3) are provided at the contact interfaces between the upper pressure head (2a) and the lower pressure head (2b) and the upper die punch (4a) and the lower die punch (4b).

9. The method for preparing a thick target by spark plasma sintering according to claim 6, characterized in that, The control system is a programmable logic controller (PLC) (7). The PLC (7) is connected to a temperature sensor, a pressure sensor (10), and a displacement sensor (11). The temperature sensor includes a K-type thermocouple (9) and a non-contact infrared thermometer (8). The K-type thermocouple (9) is connected to the sintering chamber (5), and the K-type thermocouple (9) is connected to the non-contact infrared thermometer (8). The control system also includes a host computer (15) that is communicatively connected to the PLC (7).

10. The method for preparing a thick target by spark plasma sintering according to claim 1, characterized in that, The cooling and demolding conditions are as follows: the temperature is reduced according to the preset cooling program, the cooling rate is controlled at 50-100 ℃ / min, and after the pressure is released, the sintered target is taken out.

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