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macroporous silicon carbonitride ceramic using freeze casting method with a silsesquiazane precursor
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Fabrication of hierarchically micro/meso/macroporous silicon carbonitride ceramic using freeze casting method with a silsesquiazane precursor Tae-Hwan Huh, Young-Je Kwark∗ Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul, 06978, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Hierarchically porous structure Silsesquiazane Freeze casting Silicon carbonitride (SiCN) Cross-linking degree
Hierarchically porous silicon carbonitride (SiCN) was prepared using the freeze casting method with silsesquiazane (SSQZ) as a precursor. As SSQZ possesses a highly branched structure, it can provide a higher crosslinking degree than linear polysilazane and thereby, improve the structural stability. The porous SiCN can be produced without collapsing the pore structures by removing the organic groups during the pyrolysis process due to the high cross-linking degree. In this study, we used the freeze casting method to introduce macropores. In addition, the effects of glycidyl-POSS and 1,7-octadiene diepoxide as additional cross-linking agents on the formation of porous ceramics were investigated. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses showed that SSQZ with cross-linking agents demonstrated a higher crosslinking degree and was successfully converted to SiCN ceramics. As a result, the porous SiCN ceramics exhibited an improved structural stability, specific surface area, and pore volume.
1. Introduction Polymer-derived ceramic (PDC) has received considerable attention from many researchers as an alternative route to ceramics [1–5]. In contrast to traditional methods used for the preparation of ceramics involving powder sintering and vapor deposition technologies, the PDC route that employs preceramic polymers can be used to efficiently form various shapes, such as fiber, layers, film, or composite materials [6–16]. Among the different preceramic polymers, polysilazane (PSZ) was used as a precursor to non-oxide silicon carbonitride (SiCN) ceramics. The non-oxide ceramics has unique properties such as higher creep and oxidation resistance compared to oxide ceramics, such as silica (SiO2) and silicon oxycarbide (SiOC) [17–19]. The PDC's route has an advantage of wide spectra of applicable process, such as molding, spin coating, casting, or spinning, over conventional sintering process. However, to maintain its structure during the following pyrolysis, the preceramic polymer needs to be properly cross-linked. Typically, the cross-linking of PSZ proceeds through hydrosilylation, vinyl polymerization, dehydro-coupling, and trans-amination reactions. For this purpose, most PSZs used in PDC process, such as Ceracet® or Durazane®, contain vinyl (C]C) and Si–H as functional groups, and PSZs with other functional groups have been rarely employed. Modifying substituents in the preceramic polymers render a way to modify the atomic composition of the resulting ceramics. For example, phenyl
∗
substituents in preceramic polymers would give higher portion of free carbon phase to enhance thermal stability, electrochemical property, and electromagnetic shielding property [20–22]. The PDCs route involves the ceramization process at high temperatures above 600 °C, during which the organic groups in preceramic polymers are volatilized and rearranged to create ceramic bonds [23]. In this process, micro- and mesopores are generated due to the volatilization of organic groups to form porous ceramic materials [24–30]. However, it is very difficult to produce macropores larger than 50 nm by pyrolysis of preceramic polymers. One of the best approaches to introduce macropores over 50 nm is the freeze casting method. This process can successfully generate macropores due to the little capillary force created during the sublimation of the solidified solvent [31–37]. There are two different types of freeze casting processes, suspension and solution-based freeze casting. Unlike ceramic and metal materials using powder-based suspension freeze casting process, preceramic polymers can adapt the solution-based process to provide pore morphology controlling parameters. However, above all other requirement, the solvent used in the process should be able to dissolve the preceramic polymers, and there have been only a few reports of using camphene, cyclohexane, and tertbutanol as the freeze casting solvent. Yoon et al. fabricated porous silicon carbide (SiC) ceramic by freeze casting process using polycarbosilane as precursor and camphene as solvent. They adjusted the
Corresponding author. Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul, 06978, South Korea. E-mail address: [email protected] (Y.-J. Kwark).
https://doi.org/10.1016/j.ceramint.2020.01.144 Received 3 August 2019; Received in revised form 19 December 2019; Accepted 14 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Tae-Hwan Huh and Young-Je Kwark, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.144
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Fig. 1. (a) Structure of chemicals used in the study and (b) cross-linking mechanism of PSSQZ with water molecule.
PSSQZ as a white powder.
pore width by changing solution concentrations and freeze casting temperature [38]. Xue et al. fabricated porous dendritic porous SiC with improved compressive strength using divinylbenzene as crosslinking agent from polycarbosilane/camphene solution [39]. More recently, Konegger et al. fabricated porous PSZ-derived ceramics through photo-induced thio-ene click reaction and freeze casting of camphene solution [40]. On the other hand, Naviroj et al. investigated preparing porous SiOC from polysiloxane by solution-based freeze casting process using cyclohexane and tert-butanol as solvents [34]. In this work, we prepared SiCN ceramic with micro-/meso-/macropores. Silsesquiazane with phenyl substituents (PSSQZ) was used as a preceramic polymer to SiCN ceramic. Unlike conventional linear PSZs, the branch-structured PSSQZ could introduce highly cross-linked structure during the hydrolysis of the silazane (Si–NH) group and condensation reactions of the generated silanol (Si–OH) groups to form siloxane (Si–O–Si) bonds. Since preliminary study revealed that PSSQZ did not soluble in camphene, we applied benzene as the freeze casting solvent for PSSQZ to introduce the macropores. We also investigated the effect of cross-linking degree on the structural stability of micro/ meso/macropores by employing glycidyl-POSS and 1,7-octadiene diepoxide as additional cross-linking agents. The resulting hierarchically porous SiCN ceramics exhibited improved structural stability and high specific surface area/pore volume.
2.3. Freeze casting and pyrolysis of PSSQZ In a 20 mL vial, the prepared PSSQZ powder was dissolved in benzene at a solid loading of 30 wt%. After PSSQZ was fully dissolved, a catalyst (DMAPO, 5 mol% to Si–N units in PSSQZ) and a cross-linking agent (glycidyl-POSS, 1 mol% or 1,7-octadiene diepoxide, 5 mol% to Si–N units in PSSQZ) were added to the PSSQZ solution to induce crosslinking with continuous stirring at 60 °C. After 24 h, the solution was transformed to a gel. The prepared gel was frozen at different temperature of 0, -10, and −20 °C for 3 h, and then benzene was sublimated under vacuum (100 mTorr) to obtain the white porous PSSQZ agglomerate. Finally, the prepared porous PSSQZ samples were pyrolyzed in argon at 700 °C for 4 h with ramping at 1 °C min−1 to produce hierarchically porous SiCN ceramics. 2.4. Characterization
Trichloro (phenyl)silane (TCPS), pyridine (anhydrous), and benzene (anhydrous) were purchased from Sigma Aldrich Co., Ltd. 3-dimethylamino-1-propanol (DMAPO) and 1,7-octadiene diepoxide were purchased from Tokyo chemical Industry Co., Ltd. Glycidyl-POSS was purchased from Hybrid Plastics. Ammonia (NH3) gas was purchased from Seong-gang Gas Co., Ltd. Hexane was purchased from Samchun Chemical Co., Ltd. All the chemicals were used as received without further treatment.
Fourier transform infrared (FT-IR, Vertex 70, Bruker) spectroscopy measurements were performed on 32 scans with a spectral resolution of 4 cm−1 using the KBr pallet. Chemical bonding of PSSQZ-derived porous ceramic was confirmed using X-ray photoelectron spectroscopy (XPS, K-alpha+, ThermoFisher Scientific) with Al Kα X-ray source. Chemical composition was confirmed using elemental analyzer (EA, Flash 2000, Thermo Fisher Scientific). The porous structure was observed using field emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss) with a magnification factor of 5.0 K; sample specimens were first sputtered with Pt. Specific Brunauer–Emmett–Teller (BET) surface areas and density functional theory pore size distributions were determined by nitrogen adsorption and desorption isotherms measured at 77 K (Autosorb® iQ-MP, Quantachrome). The specific surface area and pore volume of the samples were calculated based on the nitrogen adsorption and desorption isotherms results. The pore distribution of macropore above 50 nm was confirmed by mercury intrusion porosimetry (MIP, PM33GT, Quantachrome).
2.2. Synthesis of PSSQZ
3. Results and discussion
Pyridine (450 mL) and TCPS (50 mL) were charged into a threeneck flask with stirring, and the mixture was reacted for 4 h at 0 °C under NH3 gas flow. After the reaction, the solution was stirred for 24 h at room temperature under N2 gas flow. The solution was precipitated from excess hexane. The precipitate was then filtered and dried under vacuum condition for 24 h at room temperature to obtain
Fig. 1 shows the chemical structure of the PSSQZ and the crosslinking mechanism of the PSSQZ with water molecule. The Si–N units in PSSQZ can be hydrolyzed by a trace amount of water molecules in air to form Si–OH groups. The Si–OH groups undergo a condensation reaction to form Si–O–Si units. As PSSQZ is of branched structure, the hydrolysis-condensation reaction would form crosslinked structure. The
2. Experimental procedure 2.1. Materials
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and phenyl group peaks disappeared, and new peaks formed at 1600 cm−1 for C]C bonding and at 800–1200 cm−1 for Si–O, Si–C, and Si–N bonding (Fig. 2b). It was speculated that the C]C peaks originated from the free carbon phase produced during the pyrolysis process. Moreover, the Si–H peak was observed at 2200 cm−1, which was produced by the reaction between PSSQZ and H2 gas in mixture gas during the pyrolysis process. Because it was difficult to characterize the chemical composition of Si–O, Si–C, and Si–N bonding in the SiCN ceramics in FT-IR spectra due to the peak overlap, XPS measurements were used for the detailed analysis of the chemical structure. It is observed from the XPS spectra shown in Fig. 3 that the Si 2p peaks of QZ-700 originating from Si–C, Si–N, and Si–O bonding appear at 101.8, 102.6, and 103.5 eV, respectively [43]. Because the XPS analysis was performed on the surface of the sample, large Si–O peaks were observed. However, from the Si–N and Si–C peaks, we could confirmed that the prepared ceramics were mainly composed of SiCN. The C 1s peaks of QZ-700 demonstrate the presence of the C–C bonding at 284.8 eV, which is probably due to the phenyl group in the PSSQZ transforming to free carbon phase. In addition, the C–Si, C–O, C–N, and C]O peaks were detected at 285.9, 286.5, 286.7, and 289.1 eV, respectively [43]. These results indicated that the SiCN ceramic pyrolyzed at 700 °C contained free carbon phase. In this study, we used only SSQZ with phenyl group as a substituent. However, SSQZ that incorporates various substituents, such as methyl, vinyl, and other alkyl groups can be used to control the elemental content of the SiCN ceramics finally achieved [44]. The QZC-700 exhibits an XPS spectrum similar to that of QZ-700, because the catalyst was only involved in promoting the reaction. However, QZCP-700 and QZCD-700 exhibit XPS spectra different from that of QZ-700. In the Si 2p spectra, the Si–N peak of QZCP-700 and QZCD-700 decreased, while the Si–C and Si–O peaks increased from those of QZ-700 and QZC-700. The higher intensities for Si–C and Si–O peaks can be attributed to the presence of the additional cross-linking agent. The Si–C bond in glycidyl-POSS and additional carbon atoms in 1,7-octadiene diepoxide might increase the Si–C content in the pyrolyzed SiCN ceramics. The Si–O peak also increased due to the Si–O–Si bonds in glycidyl-POSS and the bonding between Si–OH in the hydrolyzed SSQZ and epoxy groups in the cross-linking agents, glycidyl-POSS and 1,7-octadiene diepoxide, during pyrolysis. It was also observed from the C 1s that the intensity of C–C peak showed the largest intensity in QZCP-700 and QZCD-700 as in QZ-700. The bulk chemical composition of the SiCN ceramics was confirmed
reaction is known to be catalyzed by adding polar molecule (DMAPO) [41]. Also, multi-functional epoxy compounds (glycidyl POSS with 8 epoxy groups, 1,7-octadiene diepoxide with 2 epoxy groups) can react with the Si–OH groups to form additional cross-linking points. In this work, we prepared four different crosslinked PSSQZs; PSSQZ only (QZ), PSSQZ with DMAPO (QZC), PSSQZ with DMAPO and glycidyl POSS (QZCP), and PSSQZ with DMAPO and 1,7-octadiene diepoxide (QZCD). Protic polar solvents cannot be adapted as a solvent for PSZs because they can react with Si center to induce hydrolysis reaction. Instead, PSSQZ is soluble in many organic non-polar and aprotic polar solvents, such as tetrahydrofuran, chloroform, toluene, dimethyl sulfoxide, and dimethyl formamide. However, since conventional free casting casting solvents for preceramic polymers, such as camphene, cyclohexane, and tert-butanol, do not dissolve PSSQZ, we used benzene as the solvent for PSSQZ. Since benzene has high triple point pressure and sublimation vapor pressure, it is widely used in freeze drying of organic compounds. Yang et al. also used benzene as a freeze casting solvent in preparing TiO2 foam with micron channel [42]. All the ingredients were dissolved in benzene and set to form gel after 24 h at 60 °C. The prepared PSSQZ gels were freeze-dried at 0 ~ −20 °C, and then pyrolyzed in argon at 700 °C to get SiCN ceramics in powder form (ceramic yield ~ 65%). We also synthesized polymethylphenylsilazane (PMPSZ), a linear polysilazane with phenyl group as a substituent, to verify that cross-linking was effectively performed by the branched structure of PSSQZs (Fig. S1). PMPSZ was not gelled under the same process conditions and showed a very low ceramic yield of less than 10% after pyrolysis, indicating that the branched structure of PSSQZs was advantageous for improving crosslinking degree. The chemical structure of the prepared porous PSSQZs and the pyrolyzed ceramics was confirmed by FT-IR analysis. Before pyrolysis, the FT-IR spectra show the absorption bands of the N–H stretching peak, Si–NH–Si peak, and the Si-phenyl peak at approximately 3,400, 900, and 700 cm−1, respectively (Fig. 2a). In the case of the PSSQZ without any additive (QZ), the FT-IR spectra shows weak intensity of the absorption bands at approximately 1000 cm−1 corresponding to the Si–O–Si peak. However, for the PSSQZs with a catalyst (QZC) and additional cross-linking agent (QZCP and QZCD), the intensity of the Si–O–Si peak increases and the free O–H stretching peaks are also observed at approximately 3600 cm−1. This indicates that the higher level of the cross-linking reaction is probable due to the catalyst and additional cross-linking agent. After the pyrolysis, the Si-phenyl, Si–NH–Si,
Fig. 2. FT-IR spectra of (a) PSSQZs and (b) pyrolyzed SiCN ceramics. 3
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Fig. 3. XPS analysis of SiCN ceramics prepared under different cross-linking conditions. Si 2p spectrum of (a) QZ-700, (b) QZC-700, (c) QZCP-700, (d) QZCD-700, and C 1s spectrum of (e) QZ-700, (f) QZC-700, (g) QZCP-700, (h) QZCD-700.
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IV nitrogen isotherm with a hysteresis loop in the relative pressure range of 0.8–1.0 [45]. However, unlike the most common type IV nitrogen isotherm, the adsorption amount tended to increase without saturating, because a large number of macropores were formed during the freeze casting process. The samples with cross-linking agent, QZCP and QZCD, exhibited adsorbed volumes higher than those of QZ and QZC. These results indicated that the cross-linking degree affected the formation of meso- and macropores. According to the pore size distribution, the major pore size of QZ and QZC were estimated to be approximately 2–10 nm. In contrast, the samples with additional crosslinking agent, QZCP and QZCD, demonstrated a large number of mesopores over a size of 10 nm. As a result, QZ and QZC possessed a specific surface area (SSA) of 6.26 and 9.20 m2 g−1, respectively, whereas QZCP and QZCD possessed a higher SSA of 28.52 and 19.52 m2 g−1, respectively (Table 2). In contrast, the SiCN ceramics pyrolyzed at 700 °C showed a large amount of nitrogen adsorption at low relative pressure range, indicating the presence of micropore formed by the volatilization of organic groups in PSSQZ during pyrolysis (Fig. 5c). In addition, QZCP-700 and QZCD-700 exhibited an adsorption volume higher than those of QZ-700 and QZC-700. It was presumed that the higher cross-linking degree of QZCP-700 and QZCD700 using the cross-linking agent resulted in stable pores that dis not collapse during pyrolysis. In the pore size distribution, QZCP-700 and QZCD-700 formed more pores in the size range of over the 10 nm than QZ-700 and QZC-700 (Fig. 5d). Particularly, QZCD-700 exhibited more pores in the range of 2–10 nm than other SiCN ceramics. As shown in Table 2, the SSA and a pore volume (Vtotal) of the SiCN ceramics are larger than those of the PSSQZs due to the micropores generated during pyrolysis. In particular, QZCP-700 and QZCD-700 exhibited a high SSA of 735.1 and 954.2 m2 g−1, respectively, and Vtotal of 0.69 and 0.85 cm3 g−1, respectively. On the other hand, mean pore size decreased during the pyrolysis, giving values of approximately 20–40 nm for PSSQZs and 2.5–4 nm for the SiCN ceramics as micro/mesopores were generated during the pyrolysis process. In conclusion, we prepared SiCN ceramics with high SSA and pore volume values by improving the cross-linking degree and structural stability of PSSQZs by using glycidyl-POSS and 1,7-octadiene diepoxide as additional crosslinking agents.
Table 1 Elemental analysis for pyrolyzed SiCN ceramics. Sample
QZ-700 QZC-700 QZCP-700 QZCD-700
Elemental Composition (wt%)
Empirical Formula (in mol)
Si
C
N
H
O
49.8 54.9 63.6 57.1
33.6 33.2 25.9 32.0
11.7 5.0 4.5 4.9
3.2 2.7 2.8 2.7
1.7 4.3 3.3 3.3
SiC1.58N0.47H1.77O0.06 SiC1.41N0.18H1.34O0.14 SiC0.95N0.14H1.21O0.09 SiC1.31N0.17H1.32O0.10
by EA (Table 1). Without catalyst and additional crosslinking agent, the resulting ceramics (QZ-700) is of lower oxygen content giving N/O ratio of 7.83. However, when catalyst (QZC-700) and additional crosslinking agent (QZCP-700 and QZCD-700) were used, the oxygen content increased to give the N/O ratio of 1.26 to 1.70. The higher oxygen content is related to the higher level of the crosslinking degree and also the high oxygen content in the additional crosslinking agent. From the aforementioned FT-IR, XPS, and EA results, we confirmed that the SiCN ceramics were successfully manufactured. Fig. 4 show the FE-SEM micrographs corresponding to the freezecasted PSSQZs and SiCN ceramics. Before pyrolysis, all samples formed macropores through the freeze casting method. As we did not use uniaxial freezing method, the prepared PSSQZs and SiCN ceramics possessed an isotropic pore structure. The freeze casting of PSSQZs was carried out at −10 and −20 °C as well as 0 °C to confirm the change of morphology of macropore with freezing temperature. From Fig. S2, it was observed that the size of the macropores got smaller as the freezing temperature was lowered, as reported in the previous work of solutionbased freeze casting of polycarbosilane in camphene [38]. Since we targeted to get macropores in bigger size, we used the freezing temperature of 0 °C for the following experiments. After pyrolysis, while QZ-700 and QZC-700 demonstrated the collapse of macropores, QZCP700 and QZCD-700 with additional cross-linking agents maintained the macropore structures. As evidenced with the data from the FTIR and XPS analysis, the cross-linking agent of QZCP-700 and QZCD-700 increased the cross-linking degree, which resulted the structural stability to exhibit maintaining the macropores after pyrolysis. In addition, macropore size distributions of QZCD-700 determined using mercury intrusion porosimetry showed much more pore in the range from 0.1 to 10 μm compared to QZ-700 (Fig. S3). Moreover, QZCD-700 with higher cross-linking degree was able to produce monolith structure by freezecasting process, and the monolith structure was not collapsed after pyrolysis to give ~40% of volume shrinkage (Fig. S4). The use of the additional cross-linking agents affects the formation of micro/mesopores as well. Fig. 5a and c displays the multipoint BET nitrogen gas adsorption isotherm and corresponding pore size distribution for the freeze-casted PSSQZs. All the samples exhibited a type
4. Conclusions Hierarchically porous SiCN ceramics were prepared by applying the freeze casting method and PSSQZ as a preceramic polymer. After gelation of PSSQZ in benzene, the freeze casting process was applied to form macro-sized pore structure. To improve the pore structure stability during the pyrolysis, we used glycidyl-POSS or 1,7-octadiene epoxide as additional cross-linking agents. The freeze casted PSSQZ was pyrolyzed at 700 °C, and micro/mesopores were formed by evaporating organic
Fig. 4. FE-SEM micrographs of (a) QZ, (b) QZC, (c) QZCP, (d) QZCD, (e) QZ-700, (f) QZC-700, (g) QZCP-700, and (h) QZCD-700. 5
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Fig. 5. Nitrogen adsorption isotherms for (a) PSSQZs, and (b) pyrolyzed SiCN ceramics, and pore size distribution plots for (c) PSSQZs, and (d) pyrolyzed SiCN ceramics from gas sorption analyzer.
Funding
Table 2 Porosity features of PSSQZs and pyrolyzed SiCN ceramics. Sample
Specific Surface Area (m2 g−1)
Vtotal (cm3 g−1)
Mean Pore Size (nm)
QZ QZC QZCP QZCD QZ-700 QZC-700 QZCP-700 QZCD-700
6.26 9.20 28.52 19.52 605.9 642.6 735.1 954.2
0.043 0.046 0.28 0.11 0.43 0.47 0.69 0.85
27.48 19.78 38.65 23.14 2.82 2.95 3.78 3.58
This work was supported by the Defense Acquisition Program Administration and Agency for Defense Development under grant number 17-CMMA-21. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data
groups in PSSQZ. After pyrolysis, the FE-SEM analysis of QZCP-700 and QZCD-700 showed the maintenance of the macropores due to the improved cross-linking degree by adding additional cross-linking agent, while the macropores of QZ-700 and QZC-700 appeared collapsed. QZCP-700 and QZCD-700 respectively demonstrated a high SSA of 735.1 and 954.2 m2 g−1 and a high pore volume of 0.69 and 0.85 cm3 g−1, respectively. From the above results, it is expected that the hierarchically porous SiCN ceramics can be effective in several fields, including catalyst support, scaffold, filters, and membrane applications.
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Fabrication of hierarchically micro/meso/macroporous silicon carbonitride ceramic using freeze casting method with a silsesquiazane precursor Tae-Hwan Huh, Young-Je Kwark∗ Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul, 06978, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Hierarchically porous structure Silsesquiazane Freeze casting Silicon carbonitride (SiCN) Cross-linking degree
Hierarchically porous silicon carbonitride (SiCN) was prepared using the freeze casting method with silsesquiazane (SSQZ) as a precursor. As SSQZ possesses a highly branched structure, it can provide a higher crosslinking degree than linear polysilazane and thereby, improve the structural stability. The porous SiCN can be produced without collapsing the pore structures by removing the organic groups during the pyrolysis process due to the high cross-linking degree. In this study, we used the freeze casting method to introduce macropores. In addition, the effects of glycidyl-POSS and 1,7-octadiene diepoxide as additional cross-linking agents on the formation of porous ceramics were investigated. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses showed that SSQZ with cross-linking agents demonstrated a higher crosslinking degree and was successfully converted to SiCN ceramics. As a result, the porous SiCN ceramics exhibited an improved structural stability, specific surface area, and pore volume.
1. Introduction Polymer-derived ceramic (PDC) has received considerable attention from many researchers as an alternative route to ceramics [1–5]. In contrast to traditional methods used for the preparation of ceramics involving powder sintering and vapor deposition technologies, the PDC route that employs preceramic polymers can be used to efficiently form various shapes, such as fiber, layers, film, or composite materials [6–16]. Among the different preceramic polymers, polysilazane (PSZ) was used as a precursor to non-oxide silicon carbonitride (SiCN) ceramics. The non-oxide ceramics has unique properties such as higher creep and oxidation resistance compared to oxide ceramics, such as silica (SiO2) and silicon oxycarbide (SiOC) [17–19]. The PDC's route has an advantage of wide spectra of applicable process, such as molding, spin coating, casting, or spinning, over conventional sintering process. However, to maintain its structure during the following pyrolysis, the preceramic polymer needs to be properly cross-linked. Typically, the cross-linking of PSZ proceeds through hydrosilylation, vinyl polymerization, dehydro-coupling, and trans-amination reactions. For this purpose, most PSZs used in PDC process, such as Ceracet® or Durazane®, contain vinyl (C]C) and Si–H as functional groups, and PSZs with other functional groups have been rarely employed. Modifying substituents in the preceramic polymers render a way to modify the atomic composition of the resulting ceramics. For example, phenyl
∗
substituents in preceramic polymers would give higher portion of free carbon phase to enhance thermal stability, electrochemical property, and electromagnetic shielding property [20–22]. The PDCs route involves the ceramization process at high temperatures above 600 °C, during which the organic groups in preceramic polymers are volatilized and rearranged to create ceramic bonds [23]. In this process, micro- and mesopores are generated due to the volatilization of organic groups to form porous ceramic materials [24–30]. However, it is very difficult to produce macropores larger than 50 nm by pyrolysis of preceramic polymers. One of the best approaches to introduce macropores over 50 nm is the freeze casting method. This process can successfully generate macropores due to the little capillary force created during the sublimation of the solidified solvent [31–37]. There are two different types of freeze casting processes, suspension and solution-based freeze casting. Unlike ceramic and metal materials using powder-based suspension freeze casting process, preceramic polymers can adapt the solution-based process to provide pore morphology controlling parameters. However, above all other requirement, the solvent used in the process should be able to dissolve the preceramic polymers, and there have been only a few reports of using camphene, cyclohexane, and tertbutanol as the freeze casting solvent. Yoon et al. fabricated porous silicon carbide (SiC) ceramic by freeze casting process using polycarbosilane as precursor and camphene as solvent. They adjusted the
Corresponding author. Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul, 06978, South Korea. E-mail address: [email protected] (Y.-J. Kwark).
https://doi.org/10.1016/j.ceramint.2020.01.144 Received 3 August 2019; Received in revised form 19 December 2019; Accepted 14 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Tae-Hwan Huh and Young-Je Kwark, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.144
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Fig. 1. (a) Structure of chemicals used in the study and (b) cross-linking mechanism of PSSQZ with water molecule.
PSSQZ as a white powder.
pore width by changing solution concentrations and freeze casting temperature [38]. Xue et al. fabricated porous dendritic porous SiC with improved compressive strength using divinylbenzene as crosslinking agent from polycarbosilane/camphene solution [39]. More recently, Konegger et al. fabricated porous PSZ-derived ceramics through photo-induced thio-ene click reaction and freeze casting of camphene solution [40]. On the other hand, Naviroj et al. investigated preparing porous SiOC from polysiloxane by solution-based freeze casting process using cyclohexane and tert-butanol as solvents [34]. In this work, we prepared SiCN ceramic with micro-/meso-/macropores. Silsesquiazane with phenyl substituents (PSSQZ) was used as a preceramic polymer to SiCN ceramic. Unlike conventional linear PSZs, the branch-structured PSSQZ could introduce highly cross-linked structure during the hydrolysis of the silazane (Si–NH) group and condensation reactions of the generated silanol (Si–OH) groups to form siloxane (Si–O–Si) bonds. Since preliminary study revealed that PSSQZ did not soluble in camphene, we applied benzene as the freeze casting solvent for PSSQZ to introduce the macropores. We also investigated the effect of cross-linking degree on the structural stability of micro/ meso/macropores by employing glycidyl-POSS and 1,7-octadiene diepoxide as additional cross-linking agents. The resulting hierarchically porous SiCN ceramics exhibited improved structural stability and high specific surface area/pore volume.
2.3. Freeze casting and pyrolysis of PSSQZ In a 20 mL vial, the prepared PSSQZ powder was dissolved in benzene at a solid loading of 30 wt%. After PSSQZ was fully dissolved, a catalyst (DMAPO, 5 mol% to Si–N units in PSSQZ) and a cross-linking agent (glycidyl-POSS, 1 mol% or 1,7-octadiene diepoxide, 5 mol% to Si–N units in PSSQZ) were added to the PSSQZ solution to induce crosslinking with continuous stirring at 60 °C. After 24 h, the solution was transformed to a gel. The prepared gel was frozen at different temperature of 0, -10, and −20 °C for 3 h, and then benzene was sublimated under vacuum (100 mTorr) to obtain the white porous PSSQZ agglomerate. Finally, the prepared porous PSSQZ samples were pyrolyzed in argon at 700 °C for 4 h with ramping at 1 °C min−1 to produce hierarchically porous SiCN ceramics. 2.4. Characterization
Trichloro (phenyl)silane (TCPS), pyridine (anhydrous), and benzene (anhydrous) were purchased from Sigma Aldrich Co., Ltd. 3-dimethylamino-1-propanol (DMAPO) and 1,7-octadiene diepoxide were purchased from Tokyo chemical Industry Co., Ltd. Glycidyl-POSS was purchased from Hybrid Plastics. Ammonia (NH3) gas was purchased from Seong-gang Gas Co., Ltd. Hexane was purchased from Samchun Chemical Co., Ltd. All the chemicals were used as received without further treatment.
Fourier transform infrared (FT-IR, Vertex 70, Bruker) spectroscopy measurements were performed on 32 scans with a spectral resolution of 4 cm−1 using the KBr pallet. Chemical bonding of PSSQZ-derived porous ceramic was confirmed using X-ray photoelectron spectroscopy (XPS, K-alpha+, ThermoFisher Scientific) with Al Kα X-ray source. Chemical composition was confirmed using elemental analyzer (EA, Flash 2000, Thermo Fisher Scientific). The porous structure was observed using field emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss) with a magnification factor of 5.0 K; sample specimens were first sputtered with Pt. Specific Brunauer–Emmett–Teller (BET) surface areas and density functional theory pore size distributions were determined by nitrogen adsorption and desorption isotherms measured at 77 K (Autosorb® iQ-MP, Quantachrome). The specific surface area and pore volume of the samples were calculated based on the nitrogen adsorption and desorption isotherms results. The pore distribution of macropore above 50 nm was confirmed by mercury intrusion porosimetry (MIP, PM33GT, Quantachrome).
2.2. Synthesis of PSSQZ
3. Results and discussion
Pyridine (450 mL) and TCPS (50 mL) were charged into a threeneck flask with stirring, and the mixture was reacted for 4 h at 0 °C under NH3 gas flow. After the reaction, the solution was stirred for 24 h at room temperature under N2 gas flow. The solution was precipitated from excess hexane. The precipitate was then filtered and dried under vacuum condition for 24 h at room temperature to obtain
Fig. 1 shows the chemical structure of the PSSQZ and the crosslinking mechanism of the PSSQZ with water molecule. The Si–N units in PSSQZ can be hydrolyzed by a trace amount of water molecules in air to form Si–OH groups. The Si–OH groups undergo a condensation reaction to form Si–O–Si units. As PSSQZ is of branched structure, the hydrolysis-condensation reaction would form crosslinked structure. The
2. Experimental procedure 2.1. Materials
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and phenyl group peaks disappeared, and new peaks formed at 1600 cm−1 for C]C bonding and at 800–1200 cm−1 for Si–O, Si–C, and Si–N bonding (Fig. 2b). It was speculated that the C]C peaks originated from the free carbon phase produced during the pyrolysis process. Moreover, the Si–H peak was observed at 2200 cm−1, which was produced by the reaction between PSSQZ and H2 gas in mixture gas during the pyrolysis process. Because it was difficult to characterize the chemical composition of Si–O, Si–C, and Si–N bonding in the SiCN ceramics in FT-IR spectra due to the peak overlap, XPS measurements were used for the detailed analysis of the chemical structure. It is observed from the XPS spectra shown in Fig. 3 that the Si 2p peaks of QZ-700 originating from Si–C, Si–N, and Si–O bonding appear at 101.8, 102.6, and 103.5 eV, respectively [43]. Because the XPS analysis was performed on the surface of the sample, large Si–O peaks were observed. However, from the Si–N and Si–C peaks, we could confirmed that the prepared ceramics were mainly composed of SiCN. The C 1s peaks of QZ-700 demonstrate the presence of the C–C bonding at 284.8 eV, which is probably due to the phenyl group in the PSSQZ transforming to free carbon phase. In addition, the C–Si, C–O, C–N, and C]O peaks were detected at 285.9, 286.5, 286.7, and 289.1 eV, respectively [43]. These results indicated that the SiCN ceramic pyrolyzed at 700 °C contained free carbon phase. In this study, we used only SSQZ with phenyl group as a substituent. However, SSQZ that incorporates various substituents, such as methyl, vinyl, and other alkyl groups can be used to control the elemental content of the SiCN ceramics finally achieved [44]. The QZC-700 exhibits an XPS spectrum similar to that of QZ-700, because the catalyst was only involved in promoting the reaction. However, QZCP-700 and QZCD-700 exhibit XPS spectra different from that of QZ-700. In the Si 2p spectra, the Si–N peak of QZCP-700 and QZCD-700 decreased, while the Si–C and Si–O peaks increased from those of QZ-700 and QZC-700. The higher intensities for Si–C and Si–O peaks can be attributed to the presence of the additional cross-linking agent. The Si–C bond in glycidyl-POSS and additional carbon atoms in 1,7-octadiene diepoxide might increase the Si–C content in the pyrolyzed SiCN ceramics. The Si–O peak also increased due to the Si–O–Si bonds in glycidyl-POSS and the bonding between Si–OH in the hydrolyzed SSQZ and epoxy groups in the cross-linking agents, glycidyl-POSS and 1,7-octadiene diepoxide, during pyrolysis. It was also observed from the C 1s that the intensity of C–C peak showed the largest intensity in QZCP-700 and QZCD-700 as in QZ-700. The bulk chemical composition of the SiCN ceramics was confirmed
reaction is known to be catalyzed by adding polar molecule (DMAPO) [41]. Also, multi-functional epoxy compounds (glycidyl POSS with 8 epoxy groups, 1,7-octadiene diepoxide with 2 epoxy groups) can react with the Si–OH groups to form additional cross-linking points. In this work, we prepared four different crosslinked PSSQZs; PSSQZ only (QZ), PSSQZ with DMAPO (QZC), PSSQZ with DMAPO and glycidyl POSS (QZCP), and PSSQZ with DMAPO and 1,7-octadiene diepoxide (QZCD). Protic polar solvents cannot be adapted as a solvent for PSZs because they can react with Si center to induce hydrolysis reaction. Instead, PSSQZ is soluble in many organic non-polar and aprotic polar solvents, such as tetrahydrofuran, chloroform, toluene, dimethyl sulfoxide, and dimethyl formamide. However, since conventional free casting casting solvents for preceramic polymers, such as camphene, cyclohexane, and tert-butanol, do not dissolve PSSQZ, we used benzene as the solvent for PSSQZ. Since benzene has high triple point pressure and sublimation vapor pressure, it is widely used in freeze drying of organic compounds. Yang et al. also used benzene as a freeze casting solvent in preparing TiO2 foam with micron channel [42]. All the ingredients were dissolved in benzene and set to form gel after 24 h at 60 °C. The prepared PSSQZ gels were freeze-dried at 0 ~ −20 °C, and then pyrolyzed in argon at 700 °C to get SiCN ceramics in powder form (ceramic yield ~ 65%). We also synthesized polymethylphenylsilazane (PMPSZ), a linear polysilazane with phenyl group as a substituent, to verify that cross-linking was effectively performed by the branched structure of PSSQZs (Fig. S1). PMPSZ was not gelled under the same process conditions and showed a very low ceramic yield of less than 10% after pyrolysis, indicating that the branched structure of PSSQZs was advantageous for improving crosslinking degree. The chemical structure of the prepared porous PSSQZs and the pyrolyzed ceramics was confirmed by FT-IR analysis. Before pyrolysis, the FT-IR spectra show the absorption bands of the N–H stretching peak, Si–NH–Si peak, and the Si-phenyl peak at approximately 3,400, 900, and 700 cm−1, respectively (Fig. 2a). In the case of the PSSQZ without any additive (QZ), the FT-IR spectra shows weak intensity of the absorption bands at approximately 1000 cm−1 corresponding to the Si–O–Si peak. However, for the PSSQZs with a catalyst (QZC) and additional cross-linking agent (QZCP and QZCD), the intensity of the Si–O–Si peak increases and the free O–H stretching peaks are also observed at approximately 3600 cm−1. This indicates that the higher level of the cross-linking reaction is probable due to the catalyst and additional cross-linking agent. After the pyrolysis, the Si-phenyl, Si–NH–Si,
Fig. 2. FT-IR spectra of (a) PSSQZs and (b) pyrolyzed SiCN ceramics. 3
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Fig. 3. XPS analysis of SiCN ceramics prepared under different cross-linking conditions. Si 2p spectrum of (a) QZ-700, (b) QZC-700, (c) QZCP-700, (d) QZCD-700, and C 1s spectrum of (e) QZ-700, (f) QZC-700, (g) QZCP-700, (h) QZCD-700.
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IV nitrogen isotherm with a hysteresis loop in the relative pressure range of 0.8–1.0 [45]. However, unlike the most common type IV nitrogen isotherm, the adsorption amount tended to increase without saturating, because a large number of macropores were formed during the freeze casting process. The samples with cross-linking agent, QZCP and QZCD, exhibited adsorbed volumes higher than those of QZ and QZC. These results indicated that the cross-linking degree affected the formation of meso- and macropores. According to the pore size distribution, the major pore size of QZ and QZC were estimated to be approximately 2–10 nm. In contrast, the samples with additional crosslinking agent, QZCP and QZCD, demonstrated a large number of mesopores over a size of 10 nm. As a result, QZ and QZC possessed a specific surface area (SSA) of 6.26 and 9.20 m2 g−1, respectively, whereas QZCP and QZCD possessed a higher SSA of 28.52 and 19.52 m2 g−1, respectively (Table 2). In contrast, the SiCN ceramics pyrolyzed at 700 °C showed a large amount of nitrogen adsorption at low relative pressure range, indicating the presence of micropore formed by the volatilization of organic groups in PSSQZ during pyrolysis (Fig. 5c). In addition, QZCP-700 and QZCD-700 exhibited an adsorption volume higher than those of QZ-700 and QZC-700. It was presumed that the higher cross-linking degree of QZCP-700 and QZCD700 using the cross-linking agent resulted in stable pores that dis not collapse during pyrolysis. In the pore size distribution, QZCP-700 and QZCD-700 formed more pores in the size range of over the 10 nm than QZ-700 and QZC-700 (Fig. 5d). Particularly, QZCD-700 exhibited more pores in the range of 2–10 nm than other SiCN ceramics. As shown in Table 2, the SSA and a pore volume (Vtotal) of the SiCN ceramics are larger than those of the PSSQZs due to the micropores generated during pyrolysis. In particular, QZCP-700 and QZCD-700 exhibited a high SSA of 735.1 and 954.2 m2 g−1, respectively, and Vtotal of 0.69 and 0.85 cm3 g−1, respectively. On the other hand, mean pore size decreased during the pyrolysis, giving values of approximately 20–40 nm for PSSQZs and 2.5–4 nm for the SiCN ceramics as micro/mesopores were generated during the pyrolysis process. In conclusion, we prepared SiCN ceramics with high SSA and pore volume values by improving the cross-linking degree and structural stability of PSSQZs by using glycidyl-POSS and 1,7-octadiene diepoxide as additional crosslinking agents.
Table 1 Elemental analysis for pyrolyzed SiCN ceramics. Sample
QZ-700 QZC-700 QZCP-700 QZCD-700
Elemental Composition (wt%)
Empirical Formula (in mol)
Si
C
N
H
O
49.8 54.9 63.6 57.1
33.6 33.2 25.9 32.0
11.7 5.0 4.5 4.9
3.2 2.7 2.8 2.7
1.7 4.3 3.3 3.3
SiC1.58N0.47H1.77O0.06 SiC1.41N0.18H1.34O0.14 SiC0.95N0.14H1.21O0.09 SiC1.31N0.17H1.32O0.10
by EA (Table 1). Without catalyst and additional crosslinking agent, the resulting ceramics (QZ-700) is of lower oxygen content giving N/O ratio of 7.83. However, when catalyst (QZC-700) and additional crosslinking agent (QZCP-700 and QZCD-700) were used, the oxygen content increased to give the N/O ratio of 1.26 to 1.70. The higher oxygen content is related to the higher level of the crosslinking degree and also the high oxygen content in the additional crosslinking agent. From the aforementioned FT-IR, XPS, and EA results, we confirmed that the SiCN ceramics were successfully manufactured. Fig. 4 show the FE-SEM micrographs corresponding to the freezecasted PSSQZs and SiCN ceramics. Before pyrolysis, all samples formed macropores through the freeze casting method. As we did not use uniaxial freezing method, the prepared PSSQZs and SiCN ceramics possessed an isotropic pore structure. The freeze casting of PSSQZs was carried out at −10 and −20 °C as well as 0 °C to confirm the change of morphology of macropore with freezing temperature. From Fig. S2, it was observed that the size of the macropores got smaller as the freezing temperature was lowered, as reported in the previous work of solutionbased freeze casting of polycarbosilane in camphene [38]. Since we targeted to get macropores in bigger size, we used the freezing temperature of 0 °C for the following experiments. After pyrolysis, while QZ-700 and QZC-700 demonstrated the collapse of macropores, QZCP700 and QZCD-700 with additional cross-linking agents maintained the macropore structures. As evidenced with the data from the FTIR and XPS analysis, the cross-linking agent of QZCP-700 and QZCD-700 increased the cross-linking degree, which resulted the structural stability to exhibit maintaining the macropores after pyrolysis. In addition, macropore size distributions of QZCD-700 determined using mercury intrusion porosimetry showed much more pore in the range from 0.1 to 10 μm compared to QZ-700 (Fig. S3). Moreover, QZCD-700 with higher cross-linking degree was able to produce monolith structure by freezecasting process, and the monolith structure was not collapsed after pyrolysis to give ~40% of volume shrinkage (Fig. S4). The use of the additional cross-linking agents affects the formation of micro/mesopores as well. Fig. 5a and c displays the multipoint BET nitrogen gas adsorption isotherm and corresponding pore size distribution for the freeze-casted PSSQZs. All the samples exhibited a type
4. Conclusions Hierarchically porous SiCN ceramics were prepared by applying the freeze casting method and PSSQZ as a preceramic polymer. After gelation of PSSQZ in benzene, the freeze casting process was applied to form macro-sized pore structure. To improve the pore structure stability during the pyrolysis, we used glycidyl-POSS or 1,7-octadiene epoxide as additional cross-linking agents. The freeze casted PSSQZ was pyrolyzed at 700 °C, and micro/mesopores were formed by evaporating organic
Fig. 4. FE-SEM micrographs of (a) QZ, (b) QZC, (c) QZCP, (d) QZCD, (e) QZ-700, (f) QZC-700, (g) QZCP-700, and (h) QZCD-700. 5
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Fig. 5. Nitrogen adsorption isotherms for (a) PSSQZs, and (b) pyrolyzed SiCN ceramics, and pore size distribution plots for (c) PSSQZs, and (d) pyrolyzed SiCN ceramics from gas sorption analyzer.
Funding
Table 2 Porosity features of PSSQZs and pyrolyzed SiCN ceramics. Sample
Specific Surface Area (m2 g−1)
Vtotal (cm3 g−1)
Mean Pore Size (nm)
QZ QZC QZCP QZCD QZ-700 QZC-700 QZCP-700 QZCD-700
6.26 9.20 28.52 19.52 605.9 642.6 735.1 954.2
0.043 0.046 0.28 0.11 0.43 0.47 0.69 0.85
27.48 19.78 38.65 23.14 2.82 2.95 3.78 3.58
This work was supported by the Defense Acquisition Program Administration and Agency for Defense Development under grant number 17-CMMA-21. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data
groups in PSSQZ. After pyrolysis, the FE-SEM analysis of QZCP-700 and QZCD-700 showed the maintenance of the macropores due to the improved cross-linking degree by adding additional cross-linking agent, while the macropores of QZ-700 and QZC-700 appeared collapsed. QZCP-700 and QZCD-700 respectively demonstrated a high SSA of 735.1 and 954.2 m2 g−1 and a high pore volume of 0.69 and 0.85 cm3 g−1, respectively. From the above results, it is expected that the hierarchically porous SiCN ceramics can be effective in several fields, including catalyst support, scaffold, filters, and membrane applications.
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