A variety of physical experiments, such as central hotspot ignition,
Matter and Radiation at Extremes, Volume. 6, Issue 2, 025901(2021)
Fabrication of solid CH-CD multilayer microspheres for inertial confinement fusion
Deuterated polymer microspheres can be used as a neutron source in conjunction with lasers because thermonuclear fusion neutrons can be produced efficiently by collisions of the resulting energetic deuterium ions. A new type of solid deuterated polymer microsphere with a carbon hydrogen–carbon deuterium (CH-CD) multilayer has been designed for preparing the target for inertial confinement fusion (ICF) experiments. To fabricate these solid CH-CD multilayer microspheres, CH beads are first fabricated by a microfluidic technique, and the CD coating layer is prepared by a plasma polymerization method. Both polystyrene (PS) and poly(α-methylstyrene) (PAMS) are used as the material sources for the CH beads. The effects of the PS and PAMS materials on the quality of the solid CH beads and the resulting CH-CD multilayer polymer microspheres are investigated. The solid PS beads have better sphericity and a smoother surface, but large vacuoles are observed in solid PS-CD multilayer microspheres owing to the presence of residual fluorobenzene in the beads and a glass transition temperature of the solid PS beads that is lower than the temperature of plasma polymerization. Therefore, solid PAMS beads are more suitable as a mandrel for fabricating solid CH-CD multilayer polymer microspheres. Solid CH-CD multilayer microspheres with specified size have been successfully prepared by controlling the droplet size and the CD deposition rate and deposition time. Compared with the design value, the diameter deviation of the inner CH beads and the thickness deviation of the CD layer can be controlled within 20 μm and 2 μm, respectively. Thus, an approach has been developed to fabricate solid CH-CD multilayer microspheres that meet the physical design requirements for ICF.
I. INTRODUCTION
A variety of physical experiments, such as central hotspot ignition,
To fabricate deuterated polymer microspheres meeting the physical requirements for ICF experiments, deuterated polystyrene (DPS) has been synthesized by radical polymerization and purified to remove hydrophilic substances.
A new type of solid deuterated polymer microsphere with a carbon hydrogen–carbon deuterium (CH-CD) multilayer has been designed to investigate the kinetic effects arising in the interpenetration layer between the corona plasma of the compressed bead and the plasma produced by the laser and the inner wall of the hohlraum because the solid CH bead can suppress implosion neutron and hydrodynamic instabilities, while the CD layer can produce a measurable D–D neutron yield.
II. EXPERIMENTAL
A. Materials
PS was purchased from Acros Organics, Inc., and PAMS was obtained from Southwest University of Science Technology, China.
Figure 1.GPC results for (a) PS and (b) PAMS materials.
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B. Preparation of CH-CD multilayer polymer microspheres
The solid CH-CD multilayer polymer microspheres were prepared by a microfluidic technique and plasma polymerization (
Figure 2.Schematic illustration of CH-CD multilayer polymer microspheres.
C. Characterization
The thermal stabilities of the materials and corresponding beads were measured by thermogravimetric analysis (TGA) at 10 °C/min from 50 °C to 600 °C. The flow rate of the nitrogen atmosphere was 40 ml/min. The glass transition temperature was measured by differential scanning calorimetry (DSC) at 10 °C/min from 50 °C to 220 °C. The flow rate of the nitrogen atmosphere was also 40 ml/min. A second scan was also run to eliminate the thermal history after an initial scan that was followed by cooling. The pyrolysis products of the materials and the beads were characterized by pyrolysis–gas chromatography–mass spectrometry (PY-GC-MS) at 500 °C for 30 s. Both the O droplets (which were composed of polymer solution) and the resulting shells were characterized by a digital microscope. The surfaces of the polymer microspheres and their cross-sectional morphologies were characterized by scanning electron microscopy (SEM). The beads were broken in liquid nitrogen, and the fractured surfaces before the SEM characterization were sputter-coated with gold under argon for 1 min. The polymer microspheres were also characterized by a computed tomography technique to observe the locations of vacuoles.
The diameters of the microspheres were measured by a measuring microscope. From the experimental results for the diameter, the out-of-roundness δOOR, defined as half the difference between the maximum and minimum outer diameters of a torus projected from the microsphere in six directions, was used to characterize the sphericity. The surface roughness was measured by both a white light interferometer and a sphere mapper. From the results of the white light interferometry, the average roughness Ra, root-mean-squared roughness Rq, and maximum height roughness Rt were calculated over the entire measured array as follows:
III. RESULTS AND DISCUSSION
A. Fabrication of solid CH beads
To obtain solid CH beads meeting the physical design requirements, an elaborate fabrication scheme was devised. The diameter of the CH beads was required to be 480 µm in this work, and so, according to conservation of mass, the average diameter of the corresponding droplets was calculated to be 1128 µm when the polymer mass fraction of the oil phase was 8.0%.
Figure 3.(a) Diameter distributions of solid PAMS beads (purple bars) and PS beads (green bars). (b) Optical micrograph of PAMS beads of 484
The sphericity, surface roughness, and residual solvent of the solid CH beads were determined to investigate the effect of the PS and PAMS materials on the quality of the beads. For both PS and PAMS beads, all the δOOR values were less than 1.0 µm, which meant that the sphericity was higher than 99.6%, i.e., all the beads had good sphericity. The PS beads showed better sphericity than the PAMS beads [
Figure 4.Effect of PS and PAMS materials on the quality of solid CH beads: (a) sphericity; (b) power spectrum; (c) surface roughness; (d) thermal properties.
The differences in sphericity and surface roughness are probably due to the different molecular weights and molecular structures of PS and PAMS. The viscosity of the PAMS solution is higher than that of the PS solution, and the PAMS chain length is greater, because the weight-average molecular weight of the PAMS is higher. Generally, the initial droplets are not ideal spheres, and they take some time to become spherical. Owing to the higher viscosity, the time to reach a spherical shape is shorter for PAMS droplets, and their ability to regain ideal sphericity is also weaker, which is unfavorable for improving sphericity. The higher viscosity also makes the rearrangement of PAMS chains more difficult. Moreover, there is one more side group (a methyl group) in the PAMS chain compared with the PS chain, which imposes more restrictions on the torsion about σ bonds in the backbone of the chain, and so the steric exclusion increases and the PAMS backbone becomes less flexible.
Moreover, our previous work has confirmed that complete removal of the FB remaining in the beads is a challenging task. The amount of residual solvent can be evaluated by TGA.
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Figure 5.Pyrolysis products of PAMS material at 500 °C for 30 s: (a) GC plot; (b) mass spectrum for
B. Fabrication of solid CH-CD multilayer polymer microspheres
To fabricate solid CH-CD multilayer polymer microspheres, a CD layer was deposited on the polymer beads by plasma polymerization with a working pressure of 10 Pa, a power of 15 W, and a D2/CD4 gas flow of 2/1. The average diameter of the PAMS-CD multilayer microspheres was 516 µm, and the variation coefficient of the diameter was less than 1.0%. Thus, the average wall thickness of the CD layer was 16.0 µm. The CD deposition rate v can be calculated by
Figure 6.Micrographs of solid CH-CD multilayer polymer microspheres: (a) optical micrography; (b) cross-sectional SEM images; (c) computed tomography image of PS-CD microspheres from two directions.
As shown in
Figure 7.DSC first–scan curves of the polymer materials and corresponding beads: (a) PS material and beads; (b) PAMS material and beads.
Figure 8.Effect of PS and PAMS materials on the quality of CH-CD multilayer polymer microspheres: (a) sphericity; (b) surface roughness; (c) SEM micrographs of microsphere surface.
IV. CONCLUSION
Solid CH-CD multilayer polymer microspheres were successfully prepared by a microfluidic technique and plasma polymerization. The diameter variation of the beads and the variation coefficient in each batch were less than 5 µm and 1.0%, respectively, indicating good monodispersity. The diameter of the solid CH beads and the wall thickness of the CD layer were controlled by the size of the corresponding droplets, as well as the CD deposition rate and deposition time, respectively. The effects of the PS and PAMS materials on the qualities of both the solid CH beads and the resulting CH-CD multilayer polymer microspheres were investigated. The PS and PAMS beads showed no significant differences in monodispersity and amount of residual solvent, but the PS beads had better sphericity and a smoother surface, probably owing to the differences in molecular structure and molecular weight between the PS and PAMS. Many vacuoles with 5 µm–30 µm diameter appeared in the central region of the PS-CD multilayer microspheres, because the glass transition temperature of the PS beads was lower than the temperature of plasma polymerization and there was some FB remaining in the beads. Therefore, the PAMS beads are more suitable for use as a mandrel for fabricating CH-CD multilayer polymer microspheres. The surface quality was reduced after coating with the CD layer. Further work is therefore necessary to find ways to improve the quality of solid CH-CD multilayer polymer microspheres.
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Meifang Liu, Xing Ai, Yiyang Liu, Qiang Chen, Shuai Zhang, Zhibing He, Yawen Huang, Qiang Yin. Fabrication of solid CH-CD multilayer microspheres for inertial confinement fusion[J]. Matter and Radiation at Extremes, 2021, 6(2): 025901
Category: Inertial Confinement Fusion Physics
Received: Oct. 25, 2020
Accepted: Jan. 5, 2021
Published Online: Apr. 22, 2021
The Author Email: Yin Qiang (qyin839@sina.com)