Near-critical density foam target as well as preparation method and application thereof

Abstract

The invention provides a near-critical density foam target as well as a preparation method and application thereof, and belongs to the technical field of foaming macromolecules. According to the invention, ammonium bicarbonate is used as a foaming agent and is decomposed to generate NH3, CO2 and H2O which are gases or volatile liquids, no solid impurity is left, and the purity of the foam target is improved; by limiting the use amount of ammonium bicarbonate, the foaming amount can be adjusted, so that the foam target reaches the near-critical density, and the laser plasma interaction requirement is met; by limiting the type of the polymer and the use amount of the solvent, the precursor solution can have high initial viscosity, initial bubble expansion is inhibited, and stable growth of bubbles is adapted along with gradual decomposition of ammonium bicarbonate in the heating process.

Description

Technical Field

[0001] This invention belongs to the field of foamed polymer technology, specifically relating to a near-critical density foam target, its preparation method, and its application. Background Technology

[0002] The rapid development of ultrashort and ultraintense laser technology has driven breakthroughs in cutting-edge fields such as laser-driven particle acceleration, inertial confinement fusion, and laboratory astrophysics. Among them, laser-driven particle acceleration, such as proton acceleration, has shown great application potential in fields such as medical radiotherapy, fast ignition of inertial confinement fusion, and high-energy physics due to its advantages such as high acceleration gradient (up to 100 GV / m and above) and compact device.

[0003] The performance of laser-driven particle acceleration (such as particle energy and beam quality) is closely related to the target material properties, especially its density, homogeneity, purity, and structure. Near-critical density (1~100 mg / cm³) 3 Foam targets have become one of the key target materials for improving particle acceleration efficiency because they can effectively regulate laser energy deposition and suppress plasma instability.

[0004] Existing methods for preparing foam targets mainly include the sol-gel method, chemical vapor deposition (CVD), and laser ablation. These methods all have certain limitations: the sol-gel method has a long preparation cycle and is difficult to scale up; CVD is costly, requires complex equipment, and is difficult to prepare large-size targets; laser ablation can only prepare surface-structured targets and cannot obtain bulk foam. Furthermore, traditional foaming agents, such as azodicarbonamide, easily leave solid impurities (such as nitrides) after decomposition, leading to a decrease in target purity and interfering with the particle acceleration process. Summary of the Invention

[0005] The purpose of this invention is to provide a near-critical density foam target, its preparation method, and its applications. The preparation method provided by this invention is simple, requires minimal equipment, and can produce high-purity near-critical density foam targets.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing a near-critical density foam target, comprising the following steps: Ammonium bicarbonate, polymer, and organic solvent are mixed to obtain a precursor solution; The precursor solution is degassed and then injected into a mold for gradient heating to obtain a foam target. The mass ratio of ammonium bicarbonate to polymer is 1:(5~20); The mass ratio of the polymer to the organic solvent is 1:(5~15); The polymer is cellulose triacetate, polyethylene terephthalate, or polystyrene. The gradient heating process is as follows: the temperature is raised from room temperature to 60~90℃ and held for 2~4 hours; then the temperature is raised to 100~120℃ and held for 1~2 hours; finally the temperature is raised to 150~200℃ and held for 1~2 hours.

[0007] Preferably, the mass ratio of the polymer to the organic solvent is 1:(5~8).

[0008] Preferably, the heating rate of the gradient heating is 1~5℃ / min.

[0009] Preferably, the gradient heating is carried out in an inert atmosphere.

[0010] Preferably, the gas pressure for gradient heating is 0.5~0.8 atm.

[0011] Preferably, the degassing is vacuum degassing, the vacuum degree of the vacuum degassing is 0.01~0.1MPa, and the vacuum degassing time is 1~3h.

[0012] Preferably, the mold is made of polytetrafluoroethylene or 420 stainless steel.

[0013] The present invention also provides a near-critical density foam target prepared by the preparation method described above, wherein the density of the near-critical density foam target is 1~100 mg / cm³. 3 The purity is higher than 99.9%.

[0014] The present invention also provides the application of the near-critical density foam target described above in the generation of high-energy particles by ultra-short and ultra-intense laser ablation.

[0015] Preferably, the wavelength of the ultrashort, ultra-intense laser is 355~1064nm, the pulse width is 10fs~100ps, and the focusing intensity is 10. 18 ~10 21 W / cm 2 .

[0016] This invention provides a method for preparing a near-critical density foam target, comprising the following steps: mixing ammonium bicarbonate, a polymer, and an organic solvent to obtain a precursor solution; degassing the precursor solution and injecting it into a mold for gradient heating to obtain a foam target; the mass ratio of ammonium bicarbonate to polymer is 1:(5~20); the mass ratio of polymer to organic solvent is 1:(5~15); the polymer is cellulose triacetate, polyethylene terephthalate, or polystyrene; the gradient heating is as follows: heating from room temperature to 60~90℃ and holding for 2~4 hours; then continuing to heat to 100~120℃ and holding for 1~2 hours; finally, continuing to heat to 150~200℃ and holding for 1~2 hours. This invention uses ammonium bicarbonate as a foaming agent, which decomposes to produce NH3, CO2, and H2O, all of which are gases or volatile liquids with no solid impurities, thus improving the purity of the foam target. By limiting the amount of ammonium bicarbonate, the foaming amount can be adjusted to achieve a near-critical density in the foam target, matching the requirements of laser-plasma interaction. By limiting the type of polymer and the amount of solvent, the precursor solution can have a high initial viscosity, suppressing the initial bubble expansion. During the heating process, as ammonium bicarbonate gradually decomposes, it adapts to the steady growth of bubbles. After the polymer dissolves, it exhibits "molecular chain entanglement" characteristics. As the solvent evaporates, the molecular chain spacing decreases, the entanglement density increases, and the system viscosity increases exponentially, effectively "capturing" the decomposition of ammonium bicarbonate. The generated gas is prevented from escaping. Gradient heating is used in the first stage to suppress the decomposition of ammonium bicarbonate at low temperatures, while allowing a large amount of solvent to evaporate, further increasing the system viscosity and better suppressing the decomposition of ammonium bicarbonate, thus avoiding the problem of uncontrollable decomposition. In the second stage, medium temperature allows ammonium bicarbonate to begin slow decomposition, and the generated gas forms initial bubbles in the viscous system. The micro-airflow generated by solvent evaporation helps the bubbles to distribute evenly and avoids local aggregation. In the third stage, as the solvent completely evaporates, the polymer also forms a continuous skeletal structure. High temperature causes complete decomposition of ammonium bicarbonate, further expanding within the polymer skeleton to meet the density requirements of the foam target. Simultaneously, high temperature also solidifies the polymer skeleton, locking the foam structure and preventing bubble collapse. The results of the examples show that the foam target prepared by the method provided by this invention has a density of 1~100 mg / cm³. 3 Controllable, with impurity content below 0.1 wt%. Detailed Implementation

[0017] All raw materials used in this invention are not particularly limited in their source; they can be purchased from the market or prepared using conventional methods known to those skilled in the art.

[0018] There are no particular restrictions on the purity of any of the raw materials used in this invention, but analytical grade raw materials are preferred.

[0019] This invention provides a method for preparing a near-critical density foam target, comprising the following steps: Ammonium bicarbonate, polymer, and organic solvent are mixed to obtain a precursor solution; The precursor solution is degassed and then injected into a mold for gradient heating to obtain a foam target. The mass ratio of ammonium bicarbonate to polymer is 1:(5~20); The mass ratio of the polymer to the organic solvent is 1:(5~15); The polymer is cellulose triacetate, polyethylene terephthalate, or polystyrene. The gradient heating process is as follows: the temperature is raised from room temperature to 60~90℃ and held for 2~4 hours; then the temperature is raised to 100~120℃ and held for 1~2 hours; finally the temperature is raised to 150~200℃ and held for 1~2 hours.

[0020] This invention involves mixing ammonium bicarbonate, a polymer, and an organic solvent to obtain a precursor solution.

[0021] In this invention, the polymer is cellulose triacetate, polyethylene terephthalate, or polystyrene. The solution viscosity of the above polymer decreases with increasing temperature, while it has a high viscosity at room temperature, which can suppress initial bubble expansion; moreover, it has good film-forming properties and chemical stability, and its hydrocarbon composition is suitable for proton acceleration requirements.

[0022] In this invention, the mass ratio of ammonium bicarbonate to polymer is 1:(5~20), preferably 1:(10~15); as one embodiment of this invention, the mass ratio of ammonium bicarbonate to polymer can be 1:5, 1:8, 1:10, 1:12, 1:15, 1:16, or 1:20. Ammonium bicarbonate, as a foaming agent, exhibits a high gas yield after thermal decomposition; 1g of ammonium bicarbonate can produce 700cm³ of gas. 3 Standard gas; when the mass ratio of ammonium bicarbonate to polymer is within the above range, the density of the foam target can be kept near the critical density range.

[0023] In one embodiment of the present invention, when the polymer is cellulose triacetate, the organic solvent can be dichloromethane or chloroform; when the polymer is polyethylene terephthalate, the organic solvent can be o-chlorophenol, trifluoroacetic acid, or a mixed solvent of phenol and tetrachloroethane in a volume ratio of 1:1; when the polymer is polystyrene, the solvent can be acetone.

[0024] In this invention, the mass ratio of the polymer to the organic solvent is 1:(5~15), preferably 1:(5~8); as one embodiment of this invention, the mass ratio of the polymer to the organic solvent can be 1:5, 1:8, 1:10, 1:12, or 1:15. The polymer is dissolved in the organic solvent, and gas is generated by a foaming agent to form a foam material; when the mass ratio of the polymer to the organic solvent is within the above range, the precursor solution can have a higher viscosity, suppressing the expansion of the initial bubbles.

[0025] In one embodiment of the present invention, the mixing can be carried out by stirring at 30~40°C for 2~3 hours.

[0026] After obtaining the precursor solution, the present invention degassing the precursor solution and injecting it into a mold for gradient heating to obtain a foam target.

[0027] In this invention, the degassing is preferably vacuum degassing; the vacuum degree of the vacuum degassing is preferably 0.01~0.1 MPa, more preferably 0.03~0.08 MPa; as one embodiment of this invention, the vacuum degree of the vacuum degassing can be 0.03 MPa, 0.05 MPa, 0.06 MPa, or 0.08 MPa. Degassing can remove dissolved air and microbubbles from the precursor solution, preventing them from acting as heterogeneous nucleation sites and causing localized abnormal foaming; a vacuum degree within the above range is beneficial for reducing microbubbles in the solution and further improving foaming stability.

[0028] In this invention, the vacuum degassing time is preferably 1-3 hours, more preferably 1.5-2.5 hours, and even more preferably 2 hours. A vacuum degassing time within this range helps reduce microbubbles in the solution and further improves foaming stability.

[0029] In one embodiment of the present invention, the degassing can be carried out in a vacuum drying oven.

[0030] In this invention, the mold is preferably made of polytetrafluoroethylene or 420 stainless steel. The molds made of these materials have moderate thermal conductivity, which is beneficial for uniform heating of the material inside the mold and further improves the stability of the foaming process.

[0031] In one embodiment of the present invention, the mold can be flat, conical, or spherical. Molds with these shapes can ensure the molding accuracy of the foam target, facilitate uniform heating of the material within the mold, and further improve the stability of the foaming process.

[0032] In this invention, the gradient heating is as follows: the temperature is raised from room temperature to 60-90°C and held for 2-4 hours; then the temperature is raised to 100-120°C and held for 1-2 hours; finally, the temperature is raised to 150-200°C and held for 1-2 hours. This invention, through gradient heating, firstly suppresses the decomposition of ammonium bicarbonate at low temperatures, allowing the solvent to evaporate significantly and further increasing the system viscosity, thus better suppressing the decomposition of ammonium bicarbonate and avoiding the problem of uncontrollable decomposition. In the second stage, at a moderate temperature, ammonium bicarbonate begins to decompose slowly, and the generated gas forms initial bubbles in the viscous system. The micro-airflow generated by the solvent evaporation helps the bubbles to distribute evenly and avoids local aggregation. In the third stage, as the solvent completely evaporates, the polymer also forms a continuous skeletal structure. The high temperature causes the ammonium bicarbonate to completely decompose and further expand within the polymer skeleton, meeting the density requirements of the foam target. Simultaneously, the high temperature also solidifies the polymer skeleton, locking the foam structure and preventing bubble collapse.

[0033] In this invention, the heating rate of each stage of the gradient heating is preferably 1~5℃ / min independently; as one embodiment of this invention, the heating rate of the gradient heating can be 2℃ / min, 3℃ / min, or 4℃ / min. A heating rate within the above range is beneficial for uniform heating of the material, further improving foaming stability.

[0034] In this invention, the gradient heating is preferably carried out in an inert atmosphere. As one embodiment of the invention, the inert atmosphere can be nitrogen or argon. Heating in an inert atmosphere helps maintain the stability of the polymer and further improves the purity of the foam target.

[0035] In one embodiment of the present invention, the gradient heating can also be carried out in an air atmosphere. The atmosphere for gradient heating is determined by the matching relationship between the polymer's thermal stability (oxidative resistance), the solvent's volatility characteristics (boiling point, evaporation rate), and the decomposition temperature window of the foaming agent. The core objective is to synchronize the solvent evaporation rate with the ammonium bicarbonate decomposition rate, avoid polymer / foaming product oxidation, and ultimately ensure the structural integrity and purity of the foam target.

[0036] As one embodiment of the present invention, when the gradient heating is carried out in an inert atmosphere, it can be carried out in a vacuum drying oven; when the gradient heating is carried out in an air atmosphere, it can be carried out in a forced-air drying oven.

[0037] In this invention, the preferred gradient heating pressure is 0.5~0.8 atm; as one embodiment of this invention, the gradient heating pressure can be 0.5 atm, 0.6 atm, 0.7 atm, or 0.8 atm. A gradient heating pressure within the above range can accelerate solvent evaporation, increase the decomposition activation energy of ammonium bicarbonate, and further improve the stability of the foaming process.

[0038] In one embodiment of the present invention, after the gradient heating is completed, the obtained product is subjected to vacuum drying. The parameters of the vacuum drying can be: vacuum degree of 0.001~0.01MPa, and time of 2~8h. Vacuum drying can remove residual H2O and trace amounts of incompletely decomposed gases, further improving the purity of the foam target.

[0039] As one embodiment of the present invention, the foam target can be precision machined. The precision machining can be carried out by using femtosecond laser cutting or mechanical precision cutting to process the foam target to the target size, with the dimensional accuracy controlled within ±5μm.

[0040] This invention uses ammonium bicarbonate as a foaming agent, which decomposes to produce NH3, CO2, and H2O, all of which are gases or volatile liquids with no solid impurities, thus improving the purity of the foam target. By limiting the amount of ammonium bicarbonate, the foaming amount can be adjusted to achieve a near-critical density in the foam target, matching the requirements of laser-plasma interaction. By limiting the type of polymer and the amount of solvent, the precursor solution can have a high initial viscosity, suppressing the initial bubble expansion. During the heating process, as ammonium bicarbonate gradually decomposes, it adapts to the steady growth of bubbles. After the polymer dissolves, it exhibits "molecular chain entanglement" characteristics. As the solvent evaporates, the molecular chain spacing decreases, the entanglement density increases, and the system viscosity increases exponentially, effectively "capturing" the gases produced by the decomposition of ammonium bicarbonate and preventing gas escape. Through gradient heating, the low temperature in the first stage suppresses the carbonation of the foam. The decomposition of ammonium bicarbonate causes a large amount of solvent to evaporate, further increasing the viscosity of the system and better suppressing the decomposition of ammonium bicarbonate, thus avoiding the problem of uncontrollable decomposition of ammonium bicarbonate. In the second stage, the medium temperature causes ammonium bicarbonate to begin to decompose slowly, and the generated gas forms initial bubbles in the viscous system. The micro-airflow generated by the evaporation of the solvent can help the bubbles to be evenly distributed and avoid local aggregation. In the third stage, as the solvent evaporates completely, the polymer also forms a continuous skeleton structure. The high temperature causes ammonium bicarbonate to decompose completely and expand further in the polymer skeleton to meet the density requirements of the foam target. At the same time, the high temperature also solidifies the polymer skeleton and locks the foam structure to prevent the bubbles from collapsing. The ammonium bicarbonate used in this invention is inexpensive, and the preparation process is based on mature processes such as solution mixing and heating foaming, which is easy to mass-produce and meets the demand for a large number of targets in high repetition frequency laser devices.

[0041] The present invention also provides a near-critical density foam target prepared by the preparation method described above, wherein the density of the near-critical density foam target is 1~100 mg / cm³. 3 The purity is higher than 99.9%.

[0042] In one embodiment of the present invention, the bubble pore size of the near-critical density foam target can be 0.1~10μm, and the standard deviation of the pore size distribution is ≤1μm.

[0043] In one embodiment of the present invention, the near-critical density foam target can be a flat plate, a cone, or a sphere; the thickness of the flat plate can be 1~100μm.

[0044] In another embodiment of the present invention, the near-critical density foam target can also be a multi-layer composite structure, and the number of layers in the multi-layer composite structure can be two; the preparation method can be to hot-press two layers of flat foam target together at 60°C.

[0045] The near-critical density foam target provided by this invention has a uniform bubble structure, which can improve the stability and experimental repeatability of laser-plasma interaction; it also has good self-support and mechanical strength, and can be directly used for target frame installation and precise positioning without the need for additional support structures, thus avoiding support interference.

[0046] The present invention also provides the application of the near-critical density foam target described above in the generation of high-energy particles by ultra-short and ultra-intense laser ablation.

[0047] In this invention, the wavelength of the ultrashort, ultra-intense laser is preferably 355-1064 nm, more preferably 500-800 nm; as one embodiment of this invention, the wavelength of the ultrashort, ultra-intense laser can be 400 nm, 600 nm, 800 nm, or 1064 nm. Wavelengths within the above ranges are advantageous for generating high-energy particles.

[0048] In this invention, the pulse width of the ultrashort, ultra-intense laser is preferably 10 fs to 100 ps, ​​more preferably 30 fs to 60 ps; as one embodiment of this invention, the pulse width of the ultrashort, ultra-intense laser can be 30 fs, 50 fs, 80 fs, 30 ps, ​​50 ps, ​​or 80 ps. A pulse width within the above range is beneficial for generating high-energy particles.

[0049] In this invention, the preferred focusing intensity of the ultrashort, ultra-intense laser is 10. 18 ~10 21 W / cm 2 More preferably 10 19 ~10 20 W / cm 2As one embodiment of the present invention, the focusing intensity of the ultrashort, ultra-intense laser can be 2 × 10⁻⁶. 19 W / cm 2 5×10 19 W / cm 2 8×10 19 W / cm 2 2×10 20 W / cm 2 Or 8×10 20 W / cm 2 The focusing intensity of ultrashort, ultra-intense lasers within the above-mentioned range is conducive to the generation of high-energy particles.

[0050] In one embodiment of the present invention, the high-energy particle can be a proton, a carbon ion, or an aluminum ion; the energy range of the high-energy particle can be 10~500MeV.

[0051] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0052] Example 1 A method for preparing a flat TAC near-critical density foam target, comprising the following steps: (1) Raw material mixing: Mix 10g of cellulose triacetate (TAC) with 1g of ammonium bicarbonate, add to 100g of dichloromethane, stir at 30℃ for 2h until completely dissolved to form a uniform precursor solution; (2) Degassing treatment: The precursor solution was placed in a vacuum drying oven and degassed for 2 hours under a vacuum of 0.05 MPa to remove air bubbles from the solution; (3) Molding and foaming: The degassed precursor solution is injected into a polytetrafluoroethylene flat mold (size 50mm×50mm×1mm), placed in a forced-air drying oven, heated to 60℃ at a rate of 2℃ / min, held for 2h, then heated to 120℃, held for 1h, then heated to 150℃, held for 1h; during this process, ammonium bicarbonate decomposes to produce gas, and TAC gradually solidifies to form a foam structure; (4) Post-processing: The foam structure was transferred to a vacuum drying oven and dried at a vacuum of 0.005 MPa for 4 hours to remove residual moisture and gas, resulting in a TAC foam target with a thickness of 500 μm. (5) Precision machining: The foam target is machined into a 10mm×10mm flat plate using femtosecond laser cutting, with dimensional accuracy controlled within ±3μm.

[0053] The density of the foam target was measured to be 20 mg / cm³. 3 The bubble pore size is 2~5μm, the standard deviation of pore size distribution is 0.8μm, and the impurity content is 0.05wt%.

[0054] Density: Based on the improved Archimedes principle, small molecule inert gas (helium) is used to penetrate the open / closed pores of the foam and completely fill the pore space of the target material. The "true total volume (skeleton + pores)" of the target material is calculated by measuring the change in gas volume. Combined with high-precision mass measurement, the density (density = mass / total volume) is obtained.

[0055] Core equipment: Helium specific gravity bottle (Micromeritics AccuPyc series).

[0056] Operating steps: Sample pretreatment: Cut the foam target into regular small pieces (5mm×5mm×5mm, to avoid breakage), wipe the surface quickly with anhydrous ethanol (to remove dust), and vacuum dry (50℃, 2h) to eliminate the influence of surface adsorbed water vapor / solvent residue on quality.

[0057] Mass measurement: Weigh the sample mass (m) using a 1 / 1,000,000 precision analytical balance (Mettler Toledo XS205) and record it to 0.1 μg.

[0058] Volume measurement: Place the sample into the sample chamber of the specific gravity bottle, evacuate (<1Pa) to remove air, and introduce high-purity helium (99.999%). After the system pressure stabilizes, the instrument automatically calculates the total volume of the sample (V, including pores).

[0059] Density calculation: ρ=m / V, perform 3 parallel measurements and take the average value (relative error must be <2% to meet the repeatability requirements of laser experiments).

[0060] Aperture: Scanning electron microscopy (SEM) + image analysis (intuitive, measuring surface / near-surface pore size) Principle: By observing the microscopic morphology of the foam target through SEM and capturing the bubble outline, the pore size and distribution are statistically analyzed using image analysis software (ImageJ), which is the core method for evaluating "pore size uniformity" (laser experiments require a narrow pore size distribution to avoid density fluctuations).

[0061] Core equipment: Field emission scanning electron microscope (FE-SEM, Zeiss Sigma 300, resolution 1nm), gold spraying instrument (polymer foam is non-conductive, requiring spraying of 5~10nm gold film).

[0062] Operating steps: Sample preparation: The foam target was cut into thin slices (thickness <100μm), fixed to the sample stage with conductive adhesive, and then sputtered with gold (vacuum degree <10). -3 Pa, to avoid damaging the foam structure).

[0063] SEM observation: Select an accelerating voltage of 5~10kV (low voltage reduces sample charging effect) and take images in different fields of view (at least 5 non-overlapping regions, including the center and the edge) (magnification of 500~5000 times, adapted to micro / submicron pore size).

[0064] Pore ​​size statistics: Open the SEM image with ImageJ, use the "Particle Analysis" function (set grayscale threshold to distinguish between bubbles and polymer skeleton) to count the equivalent diameter of at least 200 bubbles (e.g., the equivalent diameter of a circle), draw a histogram of pore size distribution, and calculate the average pore size (d-avg) and standard deviation (CV<15% is considered uniform).

[0065] Detection of volatile impurities: Gas chromatography-mass spectrometry (GC-MS, for solvent / foaming agent residue determination) Detection targets: dichloromethane (solvent), ammonium bicarbonate decomposition residues (trace amounts of NH3, organic amines), volatile components such as polymer oligomers.

[0066] Principle: GC separates volatile components, MS qualitative (by characteristic ion peaks) + quantitative (external standard method) with sensitivity down to ppb level, and can precisely control solvent residue (the laser target must have dichloromethane residue <5ppm to avoid volatilization affecting the vacuum environment).

[0067] Core equipment: Gas chromatography-mass spectrometry (Agilent 7890A-5975C), headspace sampler (to avoid direct injection contamination).

[0068] Operating steps: Sample pretreatment: Take 100mg of foam target, cut it into pieces and put it into a headspace vial. Add 5mL of high-purity N,N-dimethylformamide (DMF), seal and keep at 60℃ for 30min (to allow volatile impurities to evaporate into the headspace).

[0069] Headspace injection: Inject 1 mL of headspace gas into the GC column. Use an HP-5MS column (weakly polar, for separating halogenated hydrocarbons and amines). Column temperature program: 40℃ for 5 min, then increase to 200℃ at 5℃ / min (for separating dichloromethane).

[0070] Quantitative calculation: Prepare dichloromethane / DMF standard solutions (concentration 0.1~10ppm), plot a standard curve, and calculate the residual amount by the peak area of ​​dichloromethane in the sample (parallel measurement 3 times, RSD<5%).

[0071] Metal impurity detection: Inductively coupled plasma mass spectrometry (ICP-MS, trace metal measurement) targets: metals introduced from raw materials (such as Fe and Na in ammonium bicarbonate), contamination from the preparation process (such as Cr and Ni from equipment wear), and the laser target must have metal impurities <1ppm to avoid affecting the purity of particle acceleration.

[0072] Principle: After the sample is digested into a solution, ICP ionizes metal ions into plasma, and MS detects characteristic ions (such as Ti). 48 Fe 56 With a sensitivity of up to ppt level, it is the gold standard for trace metal detection.

[0073] Operating steps: Sample digestion: Take 500mg of foam target, put it into a polytetrafluoroethylene (PTFE) digestion vessel, add 5mL of high-purity nitric acid (68%, MOS grade) + 2mL of hydrogen peroxide (30%, electronic grade), microwave digestion (program: 120℃ / 5min, 180℃ / 20min), cool and then make up to 50mL with ultrapure water (avoid introducing contamination).

[0074] ICP-MS detection: The instrument sensitivity was optimized using tuning solutions (Li, Y, Ce, Tl), the concentration of target metal ions in the sample solution was determined, and a blank experiment (reagent digestion only) was performed to subtract background and calculate the impurity content: impurity content = (solution concentration × final volume) / sample mass.

[0075] The foam target was used in a laser ablation experiment: laser wavelength 800 nm, pulse width 30 fs, focused intensity 5 × 10⁻⁶. 19 W / cm 2 The maximum proton energy was measured to be 80 MeV, and the beam divergence angle was ≤10 mrad.

[0076] Example 2 A method for preparing a cone-shaped PET near-critical density foam target, comprising the following steps: (1) Raw material mixing: Mix 15g of polyethylene terephthalate (PET) with 1g of ammonium bicarbonate, add it to 150g of trifluoroacetic acid, and stir at 40℃ for 3h until completely dissolved to form a precursor solution; (2) Degassing treatment: The precursor solution was placed in a vacuum drying oven and degassed for 1.5 h under a vacuum of 0.03 MPa; (3) Molding and foaming: The precursor solution is injected into a 420 stainless steel conical mold (cone angle 30°, bottom diameter 10mm, height 20mm), placed in a vacuum drying oven, heated to 60°C at a rate of 3°C / min under a nitrogen atmosphere (0.6atm), held for 2h, then heated to 120°C, held for 1h, then heated to 150°C, held for 1h; a foam structure is formed. (4) Post-processing: The foam structure was dried under a vacuum of 0.001 MPa for 6 hours to obtain a cone-shaped PET foam target; (5) Precision machining: The cone tip is precisely machined to ensure the dimensional accuracy of the cone tip is ±5μm.

[0077] The density of the foam target was determined to be 35 mg / cm³ (using the same method as in Example 1). 3 The bubble pore size is 3~7μm, the standard deviation of pore size distribution is 0.9μm, and the impurity content is 0.08wt%.

[0078] For laser oblique incidence experiments: laser wavelength 1064nm, pulse width 50ps, focused intensity 2×10 20 W / cm 2 The measured proton energy was 150 MeV, and the beam quality was excellent.

[0079] Example 3 A method for preparing a multilayer PS near-critical density foam target, comprising the following steps: First layer preparation: (1) Raw material mixing: Mix 8g of polystyrene (PS) with 0.5g of ammonium bicarbonate, add to 80g of acetone, and stir at 30°C for 2 hours until completely dissolved to form a precursor solution; Steps (2) to (5) are the same as in Example 1; A density of 50 mg / cm³ was obtained. 3 A 100μm thick PS foam layer; Second layer preparation: (1) Raw material mixing: Mix 8g PS with 1.5g ammonium bicarbonate, add to 80g acetone, stir at 30℃ for 2h until completely dissolved to form a precursor solution; Steps (2) to (5) are the same as in Example 1; A density of 10 mg / cm³ was obtained. 3 A 100μm thick PS foam layer; Composite molding: Two PS foam layers were hot-pressed together at 60°C to obtain a multi-layer PS near-critical density foam target.

[0080] The impurity content of the foam target was found to be 0.06 wt% (using the same method as in Example 1).

[0081] The foam target was used in a laser ablation experiment: laser wavelength 800 nm, pulse width 30 fs, focused intensity 5 × 10⁻⁶. 19 W / cm 2 The maximum energy of the proton was measured to be 200 MeV.

[0082] As can be seen from the above embodiments, the preparation method provided by the present invention can prepare high-purity near-critical density foam targets.

[0083] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a near-critical density foam target, comprising the following steps: mixing ammonium bicarbonate, a polymer and an organic solvent to obtain a precursor solution; degassing the precursor solution and then injecting it into a mold for gradient heating to obtain a foam target; the mass ratio of the ammonium bicarbonate to the polymer is 1: (5-20) ; the mass ratio of the polymer to the organic solvent is 1: (5-15) ; the polymer is cellulose triacetate, polyethylene terephthalate or polystyrene; the gradient heating is as follows: heating from room temperature to 60-90 ℃, maintaining for 2-4 h; then continuing to heat to 100-120 ℃, maintaining for 1-2 h; finally, continuing to heat to 150-200 ℃, maintaining for 1-2 h. 2.The method according to claim 1, wherein the mass ratio of the polymer to the organic solvent is 1: (5-8). 3.The method according to claim 1 or 2, wherein the heating rate of the gradient heating is 1-5 ℃ / min. 4.The method according to any one of claims 1-3, wherein the gradient heating is carried out in an inert atmosphere. 5.The method according to any one of claims 1-4, wherein the gradient heating is carried out at a gas pressure of 0.5-0.8 atm. 6.The method according to any one of claims 1-5, wherein the degassing is vacuum degassing, the vacuum degree of the vacuum degassing is 0.01-0.1 MPa, and the vacuum degassing time is 1-3 h. 7.The method according to any one of claims 1-6, wherein the material of the mold is polytetrafluoroethylene or 420 stainless steel.

2. The production method according to claim 1, characterized by, 8.A near-critical density foam target prepared by the method according to any one of claims 1-7.

3. The method of claim 1, wherein, 9.Use of the near-critical density foam target according to claim 8 in the generation of high-energy particles by ultra-short and ultra-intense laser target shooting.

4. The production method according to claim 1 or 3, characterized by, ​ 5. The preparation method according to claim 4, characterized in that, ​ 6. The method of claim 1, wherein, ​ 7. The preparation method according to claim 1, characterized in that, ​ 8. The near-critical density foam target produced by the method of any one of claims 1 to 7, characterized in that, The near-critical density foam target has a density of 1 to 100 mg / cm 3 with a purity higher than 99.9%. ​ 10. Use according to claim 9, characterized in that, The wavelength of the ultra-short and ultra-strong laser is 355-1064nm, the pulse width is 10fs-100ps, and the gathering intensity is 10 18 ~10 21 W / cm 2 .