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Thin shell aerogel fabrication for FIREX-I targets using high viscosity (phloroglucinol carboxylic acid)/formaldehyde solution

Published online by Cambridge University Press:  22 July 2008

H. Yang ,
K. Nagai ,
M. Nakai  and
T. Norimatsu
H. Yang
Affiliation:
Institute of Laser Engineering, Osaka University, Osaka, Japan
K. Nagai*
Affiliation:
Institute of Laser Engineering, Osaka University, Osaka, Japan
M. Nakai
Affiliation:
Institute of Laser Engineering, Osaka University, Osaka, Japan
T. Norimatsu
Affiliation:
Institute of Laser Engineering, Osaka University, Osaka, Japan
*
Address correspondence and reprint requests to: Keiji Nagai, Institute of Laser Engineering, Osaka University, Yamada Oka 2–6, Suita, Osaka, Japan. E-mail: knagai@ile.osaka-u.ac.jp
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Abstract

Capsules with a thin aerogel shell were prepared by the OO/W/OI emulsion process. (Phloroglucinol carboxylic acid)/formaldehyde (PF) was used as the water phase (W) solution to form the shell of the capsule. PF is a linear polymer prepared from phloroglucinol carboxylic acid. The viscosity of the PF solution can reach a high level of 9×10−5 m2/s without gelation while resorcinol/formaldehyde (RF) gelates at ~3–4×10−5 m2/s. Using the viscous PF solution, capsule with a 17 µm gel shell was fabricated. This thickness satisfies the specification of the first phase of Fast Ignition Realization Experiment (FIREX-I) at Osaka University. When PF gel was extracted to remove the organic solvent, shrinkage of 9% occurred. The final density of the PF aerogel was 145 mg/cm3. Both the shell thickness and density can satisfy the specification of FIREX-I. The pore size of the PF aerogel was less than 100 nm while that of RF was 200–500 nm. The SEM showed that PF had particle-like foam structure while RF had fibrous-like foam structure.

Keywords

Fast ignitionLow densityThin shell thicknessFusion targetPhenol formaldehyde resin
Type
Research Article
Information
Laser and Particle Beams , Volume 26 , Issue 3 , September 2008 , pp. 449 - 453
Copyright
Copyright © Cambridge University Press 2008

1. INTRODUCTION

Fuel target technology (Nagai et al., Reference Nagai, Norimatsu, Izawa and Nalwa2004a) is a key issue in fast ignition research (Yamanaka, Reference Yamanaka1983; Tabak et al., Reference Tabak, Hammer, Glinsky, Kruer, Wilks, Woodworth, Campbell, Perry and Mason1994; Fernandez et al., Reference Fernandez, Hegelich, Cobble, Flippo, Letzring, Johnson, Gautier, Shimada, Kyrala, Wang, Wetteland and Schreiber2005; Hora, Reference Hora2007; Nobile et al., Reference Nobile, Nikroo, Cook, Cooley, Alexander, Hackenberg, Necker, Dickerson, Kilkenny, Bernat, Chen, Xu, Stephens, Huang, Haan, Forsman, Atherton, Letts, Bono and Wilson2006). The invention of the hollow cone shell geometry (Kodama et al., Reference Kodama, Norreys, Mima, Dangor, Evans, Fujita, Kitagawa, Krushelnick, Miyakoshi, Miyanaga, Norimatsu, Rose, Shozaki, Shigemori, Sunahara, Tampo, Tanaka, Toyama, Yamanaka and Zepf2001; Nakamura et al., Reference Nakamura, Sakagami, Johzaki, Nagatomo and Mima2006; Sakagami et al., Reference Sakagami, Johzaki, Nagatomo and Mima2006) was critical in demonstrating the heating of a deuterated hydrocarbon plasma to a temperature of about 1 keV using Gekko XII and the petawatt laser at Osaka University (Kodama et al., Reference Kodama, Shiraga, Shigemori, Toyama, Fujioka, Azechi, Fujita, Habara, Hall, Izawa, Jitsuno, Kitagawa, Krushelnick, Lancaster, Mima, Nagai, Nakai, Nishimura, Norimatsu, Norreys, Sakabe, Tanaka, Youssef, Zepf and Yamanaka2002, Reference Kodama, Azechi, Fujita, Habara, Izawa, Jitsuno, Jozaki, Kitagawa, Krushelnick, Matsuoka, Mima, Miyanaga, Nagai, Nagatomo, Nakai, Nishimura, Norimatsu, Norreys, Shigemori, Shiraga, Sunahara, Tanaka, Tampo, Toyama, Tsubakimoto, Yamanaka and Zepf2004; Kitagawa et al., Reference Kitagawa, Sentoku, Akamatsu, Mori, Tohyama, Kodama, Tanaka, Fujita, Yoshida, Matsuo, Jitsuno, Kawasaki, Sakabe, Nishimura, Izawa, Mima and Yamanaka2002). Even for the fast ignition scheme, highly compressed implosion is required (Azechi et al., Reference Azechi, Jitsuno, Kanabe, Katayama, Mima, Miyanaka, Nakai, Nakai, Nakaishi, Nakatsuka, Nishiguchi, Norrays, Setsuhara, Takagi, Yamanaka and Yamanaka1991), and fuel shell structure is necessary. In the first phase of fast ignition realization experiment (FIREX-I) (Tanaka et al., Reference Tanaka, Kodama, Kitagawa, Kondo, Mima, Azechi, Chen, Fujioka, Fujita, Johzaki, Lei, Matsuoka, Miyanaga, Nagai, Nagatomo, Nishimura, Norimatsu, Shigemori, Shiraga, Tanpo, Tohyama, Yabuuchi, Zheng, Izawa, Norreys, Stephens and Hatchett2004; Kodama et al., Reference Kodama, Azechi, Fujita, Habara, Izawa, Jitsuno, Jozaki, Kitagawa, Krushelnick, Matsuoka, Mima, Miyanaga, Nagai, Nagatomo, Nakai, Nishimura, Norimatsu, Norreys, Shigemori, Shiraga, Sunahara, Tanaka, Tampo, Toyama, Tsubakimoto, Yamanaka and Zepf2004; Johzaki et al., Reference Jonzaki, Sakagami, Nagatomo and Mima2007), a cryogenic deuterium tritium (DT) fuel target (Norimatsu et al., Reference Norimatsu, Nagai, Takeda, Mima and Yamanaka2003, Reference Norimatsu, Harding, Stephens, Nikroo, Petzoldt, Yoshida, Nagai and Izawa2006; Mima et al., Reference Mima, Tanaka, Kodama, Johzaki, Nagatomo, Shiraga, Miyanaga, Murakami, Azechi, Nakai, Norimatsu, Nagai, Taguchi and Sakagami2007; Azechi et al., Reference Azechi2006) was designed. The target includes a ~500 µm capsule which has a 20 µm thick low density foam shell (Nagai et al., Reference Nagai, Azechi, Ito, Iwamoto, Izawa, Johzaki, Kodama, Mima, Mito, Nakai, Nemoto, Norimatsu, Ono, Shigemori, Shiraga and Tanaka2005) to support liquid or solid DT (Iwamoto et al., Reference Iwamoto, Maekawa, Mito, Sakagami, Motojima, Nakai, Nagai, Fujimura, Norimatsu, Azechi and Mima2007) to develop a uniform, shell-shape-tailored fuel layer. In FIREX-I, the density of this foam shell is allowed to be 100 mg/cm3. Various materials have been studied for this support foam shell (Streit et al., Reference Streit and Schroen2003; Khalenkov et al., Reference Khalenkov, Borisenko, Kondrashov, Merkuliev, Limpouch and Pimenov2006; Nemoto et al., Reference Nemoto, Nagai, Ono, Tanji, Tanji, Nakai and Norimatsu2006; Yamanaka et al., Reference Yamanaka, Nagai, Nemoto, Nomura, Shimoyama, Tanji, Tanji, Nakai and Norimatsu2007; Borisenko et al., Reference Borisenko, Khalenkov, Kmetik, Limpouch, Merkuliev and Pimenov2007) in which phenol-formaldehyde resin has been used and investigated because of its transparency in the visible region.

By using the OO/W/OI emulsion process to fabricate capsules for these cryogenic targets, a high viscosity of the shell material allows for thin shell thickness (Ito et al., Reference Ito, Nagai, Nakai, Norimatsu, Nikitenko, Tolokonnikov, Koresheva, Fujimura, Azechi and Mima2006a). It is difficult to reach the high viscosity needed for thin shell fabrication with the traditionally used resorcinol/formaldehyde (RF) because of its cross-linking structure. Therefore, the thickness of RF shells are 50–100 µm (Lambert et al., Reference Lambert, Overturf, Wilemski, Letts, Schroen and Cook1997; Nikroo et al., Reference Nikroo, Czechowicz, Paguio, Greenwood and Takagi2004; Ito et al., Reference Ito, Nagai, Nakai and Norimatsu2006b; Paguio et al., Reference Paguio, Takagi, Thi, Hund, Nikoo, Paguio, Luo, Greenwood, Acenas and Chowdhury2007). (Phloroglucinol carboxylic acid)/formaldehyde (PF), another phenolic resin, is a linear polymer, and the viscosity of its solution can reach a high value without gelation. In a previous study, PF was used as an additional precursor for RF solution to adjust its viscosity (Ito et al., Reference Ito, Nagai, Nakai, Norimatsu, Nikitenko, Tolokonnikov, Koresheva, Fujimura, Azechi and Mima2006a), but the properties of PF solution can satisfy the requirement better for the fabrication of thin shell capsules.

In this study, the viscous PF solution was used to fabricate thin shell capsules. The gel shell was extracted by CO2 supercritical fluid to remove the solvent, resulting in the foam structure filled only with air, which is called aerogel. The nano-structure of PF aerogel shell was compared with that of RF aerogel shell obtained the same way. The fine nano-structure endows PF and RF aerogel with not only the ability to serve as a fuel supporter, but also provides good transparency in visible region to observe fueling with DT and to allow the routine characterization of shells using optical techniques.

2. EXPERIMENT DETAILS

2.1. Materials

Phloroglucinol carboxylic acid (1.25 g, Tokyo Chemical Industry, Co., LTD) was uniformly dispersed into 12 ml of pure water by ultrasonication, and then 8 ml of 1 mol/l NaOH (Sigma Aldrich Japan) and 1.71 ml of 37% formaldehyde (Nacalai Tesque, Inc.) were added and stirred at 70°C. Sixty-five minutes later, the solution was cooled down in an ice bath for 30 min. Then the PF solution was kept at room temperature while waiting for the viscosity to increase.

The viscosity of the PF solution was checked during the gelation process. When the viscosity reached 9×10−5 m2/s, it was used as the water phase solution to fabricate capsules with the OO/W/OI emulsion process. Encapsulation of the PF solutions to form a compound emulsion was accomplished using a triple-orifice droplet generator (Nagai et al., Reference Nagai, Nakajima, Norimatsu, Izawa and Yamanaka2000; Norimatsu et al., Reference Norimatsu, Takagi, Izawa and Mima1998; Lambert et al., Reference Lambert, Overturf, Wilemski, Letts, Schroen and Cook1997; Yang et al., Reference Yang, Han, Zhao, Nagai and Gu2006). The fabrication conditions were as following: the flow rate of W, OI, and OO liquid are 0.083 ml/min, 0.130 ml/min, and 93 ml/min, respectively.

The shell of the obtained capsules made of the PF solution then gelated during the rotation in a drum. The PF gel shell was washed by 1×10−6 m2/s silicon oil (Shinetsu, Japan) and then exchanged with acetone solutions (Vacetone:Vwater = 1:3, Vacetone:Vwater = 1:1, Vacetone:Vwater = 3:1 and 100% acetone step by step) to remove water gradually (every solution for 1 h). The completely exchanged PF gel was extracted by supercritical CO2 to remove the acetone in the gel porous structures to form the aerogel. The extraction condition was 80 atm and 333 K for 24 h.

2.2. Measurements

The viscosities of the PF solutions were measured by a digital viscometer, DV M-E, Tokyo Keiki Co. LTD, Japan. The SEM image was taken with a JEOL JSM 7400 FS. The PF gel was extracted by ISCO SFX 220 supercritical fluid extractor.

3. RESULTS AND DISCUSSION

3.1. The Viscosity Change and Minimum Shell Thickness

In order to get capsules with 20 µm thick shells, shell material solution with a viscosity higher than 9×10−5 m2/s is required in the OO/W/OI emulsion process (Ito et al., Reference Ito, Nagai, Nakai, Norimatsu, Nikitenko, Tolokonnikov, Koresheva, Fujimura, Azechi and Mima2006a). The time-dependent viscosity of the PF precursor solution is shown in Figure 1. From the figure, we can see that after a reaction time of 40 h, the viscosity of PF solution reaches 9×10−5 m2/s.

Fig. 1. Shown is the viscosity change of PF solution at room temperature as a function of time. The precursor PF solution was obtained by stirring the uniform mixture of 1.25 g phloroglucinol carboxylic acid, 1.71 ml formaldehyde, 12 ml H2O and 8 mmol NaOH at 70°C for 65 min.

The catalyst concentration of 0.4 mmol/ml was chosen to fabricate hollow spherical shells because the rate of viscosity increase with this catalyst concentration was suitable for capsule fabrication using OO/W/OI emulsion process and the droplet generator. By using a PF solution with the viscosity of 9×10−5 m2/s, the minimum gel shell thickness obtained was 17 µm.

3.2. Microstructure Morphology of the PF Aerogel Shell

Although both RF and PF aerogel have three-dimensional (3D) nano-networks, there are two differences between their microstructures. First, the pore size of RF is about 200–500 nm (Fig. 2a) while the pore size of PF is smaller than 100 nm (Fig. 2b). Besides, the pore size uniformity of PF aerogel is better than that of RF aerogel (Fig. 3). The average diameter of the pores of RF and PF foam were 114±54 nm and 32±10 nm.

Fig. 2. SEM images: (a) RF aerogel (b) PF aerogel. The pore size of PF aerogel is smaller than 100 nm. The pore size of RF aerogel is 200–500 nm.

Fig. 3. Shown are the distribution of the pore diameters of (a) RF foam and (b) PF foam along linear lines of 4 µm.

Second, the network morphology of RF and PF aerogel is different. The pore of RF is formed by strings of the gel network (Fig. 2a). This structure has clear junctions (Ito et al., Reference Ito, Nagai, Nakai, Norimatsu, Nikitenko, Tolokonnikov, Koresheva, Fujimura, Azechi and Mima2006a, Reference Ito, Nagai, Nakai and Norimatsu2005). The pore of PF was formed by polymer particles sticking together. There is no fibrous-shaped structure but just particle-like structure (Fig. 2b). This is because the gelation mechanism of RF and PF is different. Resorcinol has only four subsistent groups and thus it is possible to form cross-links with formaldehyde via methylene groups (-CH2-) (Pekala et al., Reference Pekala1989; Ruben et al., Reference Ruben, Pekala, Tillotson and Hrubesh1992) which are inert. Phloroglucinol carboxylic acid has only two subsistent groups on the benzene ring and can only form a linear polymer with formaldehyde. The linear polymers are linked via non-covalent interactions such as hydrogen bonding which is labile and thus it is possible to reorient the 3D network structure. In general, particle-like structures are more stable because they minimize the surface energy between the phases of polymer and solvent. Therefore, in the case of labile bonding network, such a particle structure would be more stable than a fibrous structure. Such particle-like structure has been observed in the more labile bonding case (van der Waals interaction) of PMP (poly (4-methyl-1-pentene)) aerogel (Nagai et al., Reference Nagai, Norimatsu and Izawa2004b). Pores of 100 nm minimize light scattering so that the final dried gel shell is transparent, allowing one to observe the fuel layer formation optically.

3.3. The Volume Shrinkage During Organic Solvent Extraction

We monitored the volume change during extraction. Here is shown one example. Before the extraction, the gel volume was 2.45 mm3 while after the extraction the aerogel volume was 2.28 mm3. The shrinkage due to extraction was about 9% in volume. The density was calculated to be 145 mg/cm3 from the volume and the mass (0.33 mg). This shrinkage rate was smaller than that of RF aerogel.

4. CONCLUSION

(Phloroglucinol carboxylic acid)/formaldehyde is a linear polymer and linked by non-covalent interactions at gelation. Therefore, the viscosity of the precursory PF solution before gelation can be controlled at a high value of 9×10−5 m2/s. By fabricating capsules using such a viscous solution, the shell thickness can be reduced to 17 µm which satisfies the requirement for fast ignition cryogenic targets. The PF gel during extraction had shrinkage of 9% in volume resulting in a final density of 145 mg/cm3. The SEM of PF aerogel showed a fine uniform porous structure with a pore size smaller than 100 nm. Because of different crosslink bonds in the RF and PF foam structure, the inert bond of methylene in RF and the labile bond of hydrogen bonding in PF, RF has a string-like gel network while PF has a particle-like structure.

ACKNOWLEDGEMENT

A part of this work was supported by a Grant-in-Aid for Scientific Research from MEXT Japan.

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Fig. 1. Shown is the viscosity change of PF solution at room temperature as a function of time. The precursor PF solution was obtained by stirring the uniform mixture of 1.25 g phloroglucinol carboxylic acid, 1.71 ml formaldehyde, 12 ml H2O and 8 mmol NaOH at 70°C for 65 min.

Figure 1

Fig. 2. SEM images: (a) RF aerogel (b) PF aerogel. The pore size of PF aerogel is smaller than 100 nm. The pore size of RF aerogel is 200–500 nm.

Figure 2

Fig. 3. Shown are the distribution of the pore diameters of (a) RF foam and (b) PF foam along linear lines of 4 µm.