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PF aerogel used as high-temperature insulator
Journal Pre-proof Preparation and characterization of organic-inorganic hybrid ZrOC/PF aerogel used as high-temperature insulator Sue Ren, li Hu, Xiutao Li, Jinpeng Fan, Jun Liang PII:
S0272-8842(19)33297-3
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.107
Reference:
CERI 23476
To appear in:
Ceramics International
Received Date: 15 October 2019 Revised Date:
17 October 2019
Accepted Date: 12 November 2019
Please cite this article as: S. Ren, l. Hu, X. Li, J. Fan, J. Liang, Preparation and characterization of organic-inorganic hybrid ZrOC/PF aerogel used as high-temperature insulator, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.107. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Preparation and Characterization of Organic-inorganic Hybrid ZrOC/PF Aerogel used as High-temperature Insulator Sue Ren1, li Hu3, Xiutao Li4, Jinpeng Fan1*, Jun Liang1,2* 1. Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and Structures, Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, China 2. State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China 3. Shanghai Institute of Aerospace System Engineering, Shanghai 201109, China 4. Aerospace Research Institute of Material & Processing Technology, Beijing 100076, China
Corresponding authors: Jun Liang (J. Liang), Jinpeng Fan (J. Fan) E-mail: [email protected] [email protected] TEL: +86-10-68913236
1
Abstract
Organic-inorganic hybrid ZrOC/PF aerogel composite is successfully prepared via one sol-gel polymerization process, using PF and ZrOC sols as precursor, respectively. SEM, BET, FT-IR, and XPS measurements are used to investigate the effect of the mass ratio of ZrOC to PF on physical, microstructure, and thermal properties of the composite aerogels. Results show that the aerogels exhibit relative low density (0.258-0.332 g/cm3), relative high compressive strength (0.32-1.40 MPa), and low thermal conductivity (0.0466-0.0660 W/m·K). SEM images demonstrated that the particles are composed of the tangled clusters of the PF and ZrOC and the particle size ranges from a dozen nm to tens of nm, up to several microns. XPS results indicate that elements C, O, and Zr in the samples are based on C-C and Zr-O valences band binding, respectively.
The
ZrOC/PF
aerogel
with
excellent
physical
property
and
potential
high-temperature resistance may be used in aerospace in the near future.
Keywords: ZrOC/PF aerogel; thermal conductivity; organic-inorganic hybrid composites; sol-gel polymerization
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1. Introduction
Aerogels, as thermal insulator material, have attracted growing attention in aerospace, insulation, other engineering areas in recent years because of low density, low thermal conductivity, and high specific surface area characterization [1, 2]. Usually, their structure is composed of nanoparticles in randomly arrangement and exhibits 3D network characterization. The high-temperature stability of aerogels is closely related to the inner structure. The long period working temperature of organic aerogels (e.g. polyimide, PI, and phenol-formaldehyde, PF) is 300oC and 600oC, respectively, because nanoparticles may be sintered at 700oC or high temperature. Obviously, the organic aerogel is not satisfied aircrafts’ requirement with enhanced high temperature; therefore, improving the aerogel temperature resistance becomes an impressing and tough problem. Ceramics, a type of the inorganic material, exhibit high melting point, high rigidity, and hardness, excellent corrosion resistance, etc. and is widely used in the fields of aerospace, fire protection, filtration, and so on. If the ceramics could be incorporated into organic aerogels, the temperature resistance would be largely improved. Researchers have conducted extensive experiments on organic-inorganic hybrid materials and achieved abundant results. Yu et al. [3, 4] prepared the phenol-formaldehyde-resin/silica composite aerogel with mechanically resilient, using chitosan as a template by the co-polymerization and nanoscale phase separation method. They found that prepared aerogel can resist a high-temperature flame without disintegration and prevents the temperature on the non-exposed side from increasing above the temperature critical for the collapse of the reinforced concrete structure. Feng et al. [5] addressed that compared with that of the phenolic resin (PR) aerogel, the maximum pyrolysis rate (Tmax) of the PR/SiO2 hybrid 3
phenolic aerogel is improved from 539 °C to 602 °C, evidently widened the thermal decomposition zone of phenolic resin. Take phenol-formaldehyde (PF) aerogel, for example, it could not be used at 1000oC or even higher temperature since the components composed of aerogels may be decomposed. Therefore, the temperature resistance of PF may be potentially enhanced by incorporating ceramics into the aerogel matrix. Zr-based compounds possess a high melting point (as high as 3000oC) and low vapor pressure and previous studies have confirmed that Zr-based composites (e.g. ZrO2, ZrC, and ZrB2) exhibited excellent ablation resistance and could be used at 2000oC or even high-temperature environment [6-8]. Researchers both in domestic and overseas have done a plentiful study on Zr-based composites. Dong et al. [9, 10] described that the three-dimensional Cf/SiC-ZrC-ZrB2 composites could be used at as high as 2000oC. Zhang et al. [11, 12] reported that the ZrO2 addition could improve the working temperature of organic aerogels. Park et al. [13, 14] synthesized zirconia-based alumina compound aerogels with enhanced thermal stability. Other researches studies also demonstrated that Zr-based composites exhibit excellent mechanical performance and high-temperature stability. Compared to ZrB2 and ZrO2, ZrC displays a high melting point and excellent ablative resistance and its oxidative products are ZrO2 and CO2 [15-17]. Although researchers have done extensively efforts on preparation and characterization of ZrO2 compounds, the study on ZrC/PF composite is rare. What’s more, previous researches [18] indicated that the ZrO2/PF or ZrO2/C composite may be used as high as 1000oC and ZrC crystalline phase would appear as temperature increased to 1500°C. In this study, we intended to prepare organic-inorganic hybrid PF/ZrOC composite aerogel using novolac phenol-formaldehyde (PF) sol as raw material. To well blend with the PF sol and 4
form a homogenous copolymer solution, ZrOC sol as ZrC precursor is prepared as well. ZrOC/PF aerogel is synthesized through the copolymerization sols method, accompanied by supercritical drying, and hexamethylenetetramine (HMTA) is used as the crosslinker. The effect of the mass ratio of ZrOC to PF on physical and mechanical properties of ZrOC/PF aerogels is investigated. The aerogel with outstanding mechanical and physical performance and high-temperature resistance may be possibly used in aerospace and other fields in the near future.
2. Experiments 2.1 Materials
All chemical reagents are analytically pure and used as received without further purification, including n-propyl zirconate (ZNP, Zr(CH3CH2CH2O)4, 99.9%, Macklin, China), sucrose (C12H22O11, 99.9%), acetic acid (CH3COOH, 99.9%), hexamethylenetetramine (HMTA, C6H12N4, 99.9%), ethanol (C2H5OH, 99.9%,). The above reagents are purchased from the Tongguang Fine Chemical Company of China. Phenol formaldehyde (PF) and deionized water are supplied by Kuentek cashew Co. Ltd and Shanghai Yishi Chemical Industry Co. Ltd., respectively.
2.2 Preparation
The ZrOC/PF aerogel was prepared through the copolymerization dual sols method, coupled with the supercritical drying, from the PF and ZrOC sol as and the HMTA as the crosslinker and catalyst. Firstly, the PF sols are individually prepared through dissolving the resin in the ethanol solution and the sol-gel process and the concentration of PF sol is 0.20 g/ml. Secondly, the ZrOC sol is synthesized through prepared through the chemical reaction between two solutions
5
containing of A and B at 80oC for 6h. The A solution is obtained through the dissolving amount of the sucrose in the mixture of the acetic acid and the deionized water at 80oC for 0.5h. The B solution is prepared by blending a certain amount of the acetic acid and ZNP under continuous stirring. After the A solution is transparent, then the part of B is poured into the A until complete uniformity. The volume fraction of the deionized water in the solution is 20% and the molar ratio of C to Zr is 4. The source of carbon (C) and zirconium (Zr) is offered by the raw materials of the sucrose and ZNP, respectively. The ZrOC/PF was prepared through mixing the PF and ZrOC sols and the mass ratio of the ZrOC to PF was 0.5:1, 1:1, 1.5:1, and 2:1, and the corresponding samples were label as ZrOC/PF-12, ZrOC/PF-22, ZrOC/PF-32, and ZrOC/PF-42, respectively. Besides, the mass fraction of the HMTA account for the PF solution is 6 wt.%. The PF without ZrOC addition is used as the reference. Fig.1 shows the flowchart of the fabrication of the ZrOC/PF aerogel.
2.3 Characterization
The morphology of the aerogels was observed using a HITACHI S-4800 scanning electron microscope (SEM), equipped with Energy-dispersive X-ray spectroscopy (EDS). Fourier Transform Infrared Spectroscopy (FT-IR) was recorded on Nicolet 5700 spectrophotometer using KBr Pellets containing 1 wt.% of samples. Nitrogen adsorption-desorption measurements were performed at Quantachrome Instruments (ASIQM000-1-MP). The surface area was evaluated using the Brunauer-Emmett-Teller (BET) method from the absoption branch of the isotherm curve. The pore size distributions were calculated according to the Barrett-Joyner-Halenda (BJH) model. XPS measurements are carried out on Thermo ESCALAB 250XI equipment using Al Kα as an excitation source. The quasi-static uniaxial compressive test was on the cylindrical sample using 6
an electronic universal testing machine (INSTRON LEGEND 2367) with a crosshead speed of 0.5 mm/min, according to the National Standard of the People’s Republic of China (GB/T 4740-1999). The compressive strength of the stress-strain curves was determined from the compressive test and Young’s modulus corresponding to the initial slope of the curves is calculated from the complete linear stage of the stress-strain curves.
3. Results and discussion 3.1 Physical properties of the ZrOC/PF aerogel
Table 1 presents the physical properties of the ZrOC/PF aerogels at normal temperature with different mass ratios of ZrOC to PF, respectively. It shows that the bulk density increased from 0.258 g/cm3 to 0.322 g/cm3 with the mass ratios increasing from 0.5:1 to 2:1, and correspondingly, the compressive strength increased from 0.322 MPa to 1.403MPa. The compressive strength values are higher than that of the single, binary or the fiber-reinforced aerogels including SiO2, Al2O3, ZrO2[11, 12], graphene[19], which indicates that the aerogel with the excellent mechanical property may be potentially used in the engineering area. Table 1 The physical properties of the ZrOC/PF aerogel
PF ZrOC/PF(0.5:1) ZrOC/PF(1:1) ZrOC/PF(1.5:1) ZrOC/PF(2:1)
Bulk density (g/cm3)
Compressive strength (MPa)
Thermal conductivity (W/m·K)
Young’s modulus (MPa)
0.245 0.258 0.306 0.313 0.322
1.423 0.322 0.404 0.864 1.403
0.0413 0.0466 0.0511 0.0602 0.0660
52.94 5.12 7.43 17.17 24.42
3.2 Microstructure of the ZrOC/PF aerogel
Figure2 shows the contact angle, dimension size, and microstructure of ZrOC/PF aerogels 7
and to further illustrate the characterization of the ZrOC/PF aerogel, PF aerogel as the reference is also presented. Take ZrOC/PF-22 for example, the contact angle is 148o, beyond a critical value of the wettability angle (90o), indicating that the aerogel exhibited excellent hydrophobic property. What’s more, the skeleton of the porous structure of the composite aerogel is composed of the entangled bead-chains, formed by numerous particles. Each of the chains is intertwined, possessing the characteristic of the “grape string”-like structure, similar to that of the organic aerogels [5]. Though the composite aerogels possess typical features of the aerogels, their microstructure is obviously different from PF, based on the following two reasons. Firstly, the composition is discrepant. PF aerogel is consisted of the only PF particles, while ZrOC/PF is composed of the ZrOC and PF particles, originated from the PF and PF and ZrOC sols, respectively. Secondly, the particle size and morphology is different. The morphology of ZrOC and PF seems like that of the wormlike [20] and ball-shaped, respectively. Besides, the size of the pores channel formed by bead-chains is on nanoscale or micrometer scale. With decreasing of the mass ratio of ZrOC to PF, the diameter of the particles composed of skeleton gradually increased and the bead-chain aggregated tightly. Once active points are motivated, the generated oligomer could assemble to form the particles until accumulating enough to separate from the cluster. The “grape string”-like chains containing of PF and ZrOC is generated during the process and the size of the chain channel become narrow with increasing the mass ratio of ZrOC to PF. The study also indicated that the sol-gel polymerization of linear phenolic resin and HMTA is very similar to the conventional curing process of novolac resin [21]. HMTA plays cross-linking and catalysis dual roles during the process. In the initial stage, the HMTA initially serves as the cross-linker reagent. The HMTA gradually decompose aldehyde and ammonia during the sol-gel 8
process, which severs as the base catalyst for accelerating the novel PF resin cross-linking and polymerization. The sol-gel process depends on the nucleation and growth of colloidal polymer clusters. It is shown that the polymer cluster particle with the small size gives the aerogel with mesoporous structure, and vice versa. The proper concentration of the PF and HMTA is crucial to the size of the cluster, which would further determine whether a gel remains an unbroken monolith, or seriously collapsed structure during the ambient pressure drying. What’s more, the size of the cluster is too small or too large would not be beneficial to mechanical performance. Therefore, controlling the cluster size is of virtual important for the preparation of the ZrOC/PF aerogel. The effect of the size of clusters on the mechanical performance of aerogels would be discussed in the following subsection. Fig.3 is the mapping of the main element composed of the aerogels. It shows that the elements containing C, O, and Zr are uniformly distributed in the ZrOC/PF-22 aerogel, indicating that the copolymerization sol method is feasible for the fabrication of ZrOC/PF aerogel and their percentage is 74%, 13%, and 13%, respectively. Obviously, the proportion of the Zr and O is equal but the proportion of the element of C is approximately six times larger than that of the element of Zr or O.
9
Fig.2
10
Fig.3 To investigate the chemical bonds behavior in the samples, we use XPS to analyze the binding of valence bonds among the sample surface. Fig.4 shows the high-resolution XPS spectra of the ZrOC/PF-42 aerogel with fitting curves relative to Zr 3d, C 1s, and O 1s bands, respectively. The results demonstrated that Zr 3d, C 1s, and O 1s all possess large peaks, suggest that the element including Zr, C, and O are present in large amounts in the sample. The sub-peak fitting in Fig.1b corresponding to Zr 3d band is consisted of the spin-orbit doublets (containing Zr 3d5/2 and
11
3d3/2) and the two groups of Zr 3d doublets is detected in the spectrum, with the binding energy difference of E = 2.4eV [22, 23]. Deconvolution of the Zr 3d peak showed that the binding energies of Zr atoms involved are 181.75 eV, 182.4 eV, 184.1 eV, and 184.75 eV, respectively. The Zr 3d doublets including the binding energies of 181.75, 182.4, 184.1 and 184.75 eV are attributed to the Zr-O bond [24]. Fig.4 also indicated that the elemental C and O in the sample is based on the C-C and Zr-O valence band binding, respectively. This result is consistent with the previous study [23].
Fig.4 To further clarify the characteristic of the 3D network structure of the aerogel, the Brunauer-Emmett-Teller (BET) method is used to estimate pore size distribution and the specific surface area of porous monoliths aerogels. Fig.5a and 5b show the N2 the adsorption-desorption isotherms of the ZrOC/PF-42 aerogel, respectively. The isotherms exhibit a sharp increment at low relative pressure, indicating that the substantial micropores are still in the aerogel. As the pressure close to unity (P/P0≈1), the absorption amount rapidly increases, assuming that plenty of
12
macropores may be present in the aerogel. Besides, the adsorption-desorption isotherms show no apparent hysteresis loops, in comparison with other aerogels prepared through supercritical or ambient temperature drying. Fig.4b indicates that the ZrOC/PF aerogel exhibited a relatively wide pore size distribution, ranging from 2 to 200nm. Besides, the hierarchically porous structure with the micropores and macropores is simultaneous in the sample. The microspores maintain the shape-selectivity or size-selectivity advantages for the guest molecules and offer high adsorption capacity [25, 26]. The structural change after the sol-gel process is monitored using the FT-IR method. Fig.5c shows the FT-IR spectra of the ZrOC/PF aerogels with different mass ratios of ZrOC to PF. The peak at 3470 cm-1 is due to the -OH stretching vibration, while the peaks at 2937 cm-1 and 2855 cm-1 resulted from CH2 stretching, attributed to the cross-linked reaction. The angular absorption bond at 1580 cm-1 is supposed as a characteristic peak of the association of ligand with zirconyl [27]. The bands around 1500 cm-1 in the left labeled area corresponding to the aromatic ring stretching vibrations are slightly different after the reaction, possibly due to the mono- or dis-substitution on the aromatic ring. The peak of C-N at 1400cm-1 is originated from the substituted benzylamines, reflecting that the part of N atom inserts into the aerogel network structure. What’s more, the peak at 829 cm-1 and 752 cm-1 in the gray area is attributed to the H atoms in the aromatic rings of different substitutes. The bond around 572 cm-1 may be closely related to the Zr-O vibration in the Zr-O-C chains[28]. The result also indicated that the Zr-O chain has been incorporated into the complex aerogel.
13
Fig.5
3.3 Mechanical and thermal insulation properties of the ZrOC/PF aerogel
Because the aerogel may confront various force and thermal extreme environment, the mechanical and thermal insulation performance as one of the fundamental feathers of the aerogel material becomes rather important. What’s more, the mechanical strength may reflect the monolithicity and processability of the monolithic materials. Fig.6a shows the strain-stress curves of the ZrOC/PF aerogel with the mass ratios of ZrOC to PF increasing from 1:2 to 4:2. It can be seen that the aerogel with a density of 0.258-0.322 possessed the compressive strength of 0.32-1.40 MPa, lower than that of the PF aerogel. The compressive behavior of the aerogels is similar to that of most of the aerogels [11, 12] and three well-defined stages namely elastic, plateau, and fracture stages are observed during the compressive process. In the elastic stage, the stress increased approximately linearly with the strain and ended at peak stress, which was defined as the compressive strength of the sample. Then, the sample came into the second stage of the 14
plateau stage and the stress value that the sample can withstand gradually decreased with the increase of strain, differently from typical stress-strain curves. It is obvious that the zigzag shape is present in the curves, similar to the PF aerogels. Lastly, with the strain further increasing, the stress value that the sample can withstand continuous decreased and correspondingly, the sample had damaged until complete failure. It is noted that compared with that of the PF or carbon aerogels obtained by ambient pressure drying, the compressive strength of the aerogel in this study is low. Such low compressive strength may be closely related to their microstructure. Microscopically, the ZrOC/PF aerogel possessed grape string”-like chain structure composed of the PF and ZrOC secondary nanoparticles. The fragility of the aerogel may be attributed to the weaker connection forces between the necks. The brittleness of the aerogel framework may result from the inter-particle connection zone, namely the necks shown in Fig.4b. It is known that the aerogel is a type of porous material with the porosity as high as 90%, therefore, when they undertake the stress the porous in the sample would be firstly compacted. Once the stress values are higher than that of the aerogel could afford, the neck connected by the particles may debond or fracture, resulting in the destruction of the “grape string”-like chains and further failure of the aerogel. Based on the above-mentioned reasons, we know that the compressive strength can be improved by strengthening the inter-particle neck acting force without generating a severe weight penalty. The “necks” are formed by coagulation of polymer particles on slow gelation and growth to a certain point by dissolution and re-precipitation of polymer clusters during the aging process (see Fig.6b). In the study, the formed clusters in the sol gradually grow by the polymerization reaction, resulting in evolving into particles with various sizes on different chemical reaction 15
conditions. The obtained particles are tightly connected with others by the inter-particle neck. Therefore, it is reasonable that prepared ZrOC/PF aerogel could exhibit excellent mechanical properties simultaneous with low density. Table 1 also shows that the total thermal conductivity of the aerogel increased from 0.0466 to 0.0660 W/m·K with increasing the mass ratio of ZrOC to PF from 0.5:1 to 2:1, slightly higher than that of the PF. What’s more, the compressive strength of the organic-inorganic hybrid aerogels exhibits relatively high compressive strength than of that of the organic ones. The ZrOC/PF possessed low density (0.258-0.322 g/cm3) and a homogeneous three-dimensional network structure consisting of the particles on the nanometer or micron scale. Because the diameter of the particle is less than 10 µm, the contribution to thermal conduction among the particles to thermal conductivity is minor. Therefore, the solid-gas thermal conductivity is the main way in the aerogels composite. The thermal coupling between the gas phase and the solid particles cannot be neglected since the pores are smaller than 10 µm [29]. As shows that the thermal conductivity rose from 0.0466 to 0.066 W/m·K with the increase of the amount of the ZrOC. Although the thermal conductivity of ZrOC/PF aerogel is higher than that of PF, their values are much lower than that of high-temperature thermal insulation materials [11, 12]. Furthermore, compared with the organic aerogel, the composite aerogel containing the non-ablative Zr element may be suitable for the high-temperature environment, which provides a new possible organic-inorganic hybrid material with lightweight and ablative resistance.
16
Fig.6
Conclusion Organic-inorganic hybrid ZrOC/PF aerogel with excellent physical, mechanical, and thermal properties is successfully prepared, using the ZrOC and PF as the precursor through the polymerization-induced phase separation method in this study. The results indicate that the aerogels possessed relatively low bulk density, high compressive strength, and low thermal conductivity. The SEM images reveal that the microstructure of the aerogel is composed of the entangled “grape string”-like chains, formed by the PF and ZrOC cluster particles, contributing to the robust mechanical performance of the aerogel. The diameter of the cluster particles ranges from a dozen of nm to tens of nm, up to microns. XPS results indicate that the sub-peak fitting corresponding to Zr 3d band is based on the Zr-O bond. FT-IR results show that the bond around 572 cm-1 is related to the Zr-O vibration in the entangled chains. Compressive behavior of aerogels is similar to that of the cellular materials and the elastic, plateau, and fracture stages are observed in the strain-stress curves. The ZrOC/PF aerogel with the outstanding feature may be the promising candidate used in aerospace engineering in the future.
Acknowledgments The research work is supported by the National Natural Science Foundation of China (Project 17
No.11732002, 51872029, 11902201, and 11902033) and China Postdoctoral Science Foundation (Project No: 2018M631350).
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1329-1342. https://doi.org/10.1007/s10765-009-0617-z
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Figure Captions Fig. 1 Flowchart of preparation the ZrOC/PF aerogel Fig. 2 The dimension size and the contact angle of the ZrOC/PF-42 aerogel and SEM images of the ZrOC/PF aerogels with the mass of ZrOC to PF of (a)0.5;1, (b)1;1, (c)1.5:1, and (d)2:1, respectively Fig. 3 The mapping of the main elements including composed of the ZrOC/PF-42 aerogel Fig. 4 XPS of the ZrOC/PF-42 aerogel Fig. 5 N2 adsorption-desorption isotherms curve, DFT pore size distributions and FT-IR images of the ZrOC/PF aerogels Fig. 6 Strain-stress curves of the ZrOC/PF aerogels and the schematic illustration of the dual networks and necks
23
Dear editors: We declare that no conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
You sincerely Jun Liang
S0272-8842(19)33297-3
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.107
Reference:
CERI 23476
To appear in:
Ceramics International
Received Date: 15 October 2019 Revised Date:
17 October 2019
Accepted Date: 12 November 2019
Please cite this article as: S. Ren, l. Hu, X. Li, J. Fan, J. Liang, Preparation and characterization of organic-inorganic hybrid ZrOC/PF aerogel used as high-temperature insulator, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.107. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Preparation and Characterization of Organic-inorganic Hybrid ZrOC/PF Aerogel used as High-temperature Insulator Sue Ren1, li Hu3, Xiutao Li4, Jinpeng Fan1*, Jun Liang1,2* 1. Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and Structures, Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, China 2. State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China 3. Shanghai Institute of Aerospace System Engineering, Shanghai 201109, China 4. Aerospace Research Institute of Material & Processing Technology, Beijing 100076, China
Corresponding authors: Jun Liang (J. Liang), Jinpeng Fan (J. Fan) E-mail: [email protected] [email protected] TEL: +86-10-68913236
1
Abstract
Organic-inorganic hybrid ZrOC/PF aerogel composite is successfully prepared via one sol-gel polymerization process, using PF and ZrOC sols as precursor, respectively. SEM, BET, FT-IR, and XPS measurements are used to investigate the effect of the mass ratio of ZrOC to PF on physical, microstructure, and thermal properties of the composite aerogels. Results show that the aerogels exhibit relative low density (0.258-0.332 g/cm3), relative high compressive strength (0.32-1.40 MPa), and low thermal conductivity (0.0466-0.0660 W/m·K). SEM images demonstrated that the particles are composed of the tangled clusters of the PF and ZrOC and the particle size ranges from a dozen nm to tens of nm, up to several microns. XPS results indicate that elements C, O, and Zr in the samples are based on C-C and Zr-O valences band binding, respectively.
The
ZrOC/PF
aerogel
with
excellent
physical
property
and
potential
high-temperature resistance may be used in aerospace in the near future.
Keywords: ZrOC/PF aerogel; thermal conductivity; organic-inorganic hybrid composites; sol-gel polymerization
2
1. Introduction
Aerogels, as thermal insulator material, have attracted growing attention in aerospace, insulation, other engineering areas in recent years because of low density, low thermal conductivity, and high specific surface area characterization [1, 2]. Usually, their structure is composed of nanoparticles in randomly arrangement and exhibits 3D network characterization. The high-temperature stability of aerogels is closely related to the inner structure. The long period working temperature of organic aerogels (e.g. polyimide, PI, and phenol-formaldehyde, PF) is 300oC and 600oC, respectively, because nanoparticles may be sintered at 700oC or high temperature. Obviously, the organic aerogel is not satisfied aircrafts’ requirement with enhanced high temperature; therefore, improving the aerogel temperature resistance becomes an impressing and tough problem. Ceramics, a type of the inorganic material, exhibit high melting point, high rigidity, and hardness, excellent corrosion resistance, etc. and is widely used in the fields of aerospace, fire protection, filtration, and so on. If the ceramics could be incorporated into organic aerogels, the temperature resistance would be largely improved. Researchers have conducted extensive experiments on organic-inorganic hybrid materials and achieved abundant results. Yu et al. [3, 4] prepared the phenol-formaldehyde-resin/silica composite aerogel with mechanically resilient, using chitosan as a template by the co-polymerization and nanoscale phase separation method. They found that prepared aerogel can resist a high-temperature flame without disintegration and prevents the temperature on the non-exposed side from increasing above the temperature critical for the collapse of the reinforced concrete structure. Feng et al. [5] addressed that compared with that of the phenolic resin (PR) aerogel, the maximum pyrolysis rate (Tmax) of the PR/SiO2 hybrid 3
phenolic aerogel is improved from 539 °C to 602 °C, evidently widened the thermal decomposition zone of phenolic resin. Take phenol-formaldehyde (PF) aerogel, for example, it could not be used at 1000oC or even higher temperature since the components composed of aerogels may be decomposed. Therefore, the temperature resistance of PF may be potentially enhanced by incorporating ceramics into the aerogel matrix. Zr-based compounds possess a high melting point (as high as 3000oC) and low vapor pressure and previous studies have confirmed that Zr-based composites (e.g. ZrO2, ZrC, and ZrB2) exhibited excellent ablation resistance and could be used at 2000oC or even high-temperature environment [6-8]. Researchers both in domestic and overseas have done a plentiful study on Zr-based composites. Dong et al. [9, 10] described that the three-dimensional Cf/SiC-ZrC-ZrB2 composites could be used at as high as 2000oC. Zhang et al. [11, 12] reported that the ZrO2 addition could improve the working temperature of organic aerogels. Park et al. [13, 14] synthesized zirconia-based alumina compound aerogels with enhanced thermal stability. Other researches studies also demonstrated that Zr-based composites exhibit excellent mechanical performance and high-temperature stability. Compared to ZrB2 and ZrO2, ZrC displays a high melting point and excellent ablative resistance and its oxidative products are ZrO2 and CO2 [15-17]. Although researchers have done extensively efforts on preparation and characterization of ZrO2 compounds, the study on ZrC/PF composite is rare. What’s more, previous researches [18] indicated that the ZrO2/PF or ZrO2/C composite may be used as high as 1000oC and ZrC crystalline phase would appear as temperature increased to 1500°C. In this study, we intended to prepare organic-inorganic hybrid PF/ZrOC composite aerogel using novolac phenol-formaldehyde (PF) sol as raw material. To well blend with the PF sol and 4
form a homogenous copolymer solution, ZrOC sol as ZrC precursor is prepared as well. ZrOC/PF aerogel is synthesized through the copolymerization sols method, accompanied by supercritical drying, and hexamethylenetetramine (HMTA) is used as the crosslinker. The effect of the mass ratio of ZrOC to PF on physical and mechanical properties of ZrOC/PF aerogels is investigated. The aerogel with outstanding mechanical and physical performance and high-temperature resistance may be possibly used in aerospace and other fields in the near future.
2. Experiments 2.1 Materials
All chemical reagents are analytically pure and used as received without further purification, including n-propyl zirconate (ZNP, Zr(CH3CH2CH2O)4, 99.9%, Macklin, China), sucrose (C12H22O11, 99.9%), acetic acid (CH3COOH, 99.9%), hexamethylenetetramine (HMTA, C6H12N4, 99.9%), ethanol (C2H5OH, 99.9%,). The above reagents are purchased from the Tongguang Fine Chemical Company of China. Phenol formaldehyde (PF) and deionized water are supplied by Kuentek cashew Co. Ltd and Shanghai Yishi Chemical Industry Co. Ltd., respectively.
2.2 Preparation
The ZrOC/PF aerogel was prepared through the copolymerization dual sols method, coupled with the supercritical drying, from the PF and ZrOC sol as and the HMTA as the crosslinker and catalyst. Firstly, the PF sols are individually prepared through dissolving the resin in the ethanol solution and the sol-gel process and the concentration of PF sol is 0.20 g/ml. Secondly, the ZrOC sol is synthesized through prepared through the chemical reaction between two solutions
5
containing of A and B at 80oC for 6h. The A solution is obtained through the dissolving amount of the sucrose in the mixture of the acetic acid and the deionized water at 80oC for 0.5h. The B solution is prepared by blending a certain amount of the acetic acid and ZNP under continuous stirring. After the A solution is transparent, then the part of B is poured into the A until complete uniformity. The volume fraction of the deionized water in the solution is 20% and the molar ratio of C to Zr is 4. The source of carbon (C) and zirconium (Zr) is offered by the raw materials of the sucrose and ZNP, respectively. The ZrOC/PF was prepared through mixing the PF and ZrOC sols and the mass ratio of the ZrOC to PF was 0.5:1, 1:1, 1.5:1, and 2:1, and the corresponding samples were label as ZrOC/PF-12, ZrOC/PF-22, ZrOC/PF-32, and ZrOC/PF-42, respectively. Besides, the mass fraction of the HMTA account for the PF solution is 6 wt.%. The PF without ZrOC addition is used as the reference. Fig.1 shows the flowchart of the fabrication of the ZrOC/PF aerogel.
2.3 Characterization
The morphology of the aerogels was observed using a HITACHI S-4800 scanning electron microscope (SEM), equipped with Energy-dispersive X-ray spectroscopy (EDS). Fourier Transform Infrared Spectroscopy (FT-IR) was recorded on Nicolet 5700 spectrophotometer using KBr Pellets containing 1 wt.% of samples. Nitrogen adsorption-desorption measurements were performed at Quantachrome Instruments (ASIQM000-1-MP). The surface area was evaluated using the Brunauer-Emmett-Teller (BET) method from the absoption branch of the isotherm curve. The pore size distributions were calculated according to the Barrett-Joyner-Halenda (BJH) model. XPS measurements are carried out on Thermo ESCALAB 250XI equipment using Al Kα as an excitation source. The quasi-static uniaxial compressive test was on the cylindrical sample using 6
an electronic universal testing machine (INSTRON LEGEND 2367) with a crosshead speed of 0.5 mm/min, according to the National Standard of the People’s Republic of China (GB/T 4740-1999). The compressive strength of the stress-strain curves was determined from the compressive test and Young’s modulus corresponding to the initial slope of the curves is calculated from the complete linear stage of the stress-strain curves.
3. Results and discussion 3.1 Physical properties of the ZrOC/PF aerogel
Table 1 presents the physical properties of the ZrOC/PF aerogels at normal temperature with different mass ratios of ZrOC to PF, respectively. It shows that the bulk density increased from 0.258 g/cm3 to 0.322 g/cm3 with the mass ratios increasing from 0.5:1 to 2:1, and correspondingly, the compressive strength increased from 0.322 MPa to 1.403MPa. The compressive strength values are higher than that of the single, binary or the fiber-reinforced aerogels including SiO2, Al2O3, ZrO2[11, 12], graphene[19], which indicates that the aerogel with the excellent mechanical property may be potentially used in the engineering area. Table 1 The physical properties of the ZrOC/PF aerogel
PF ZrOC/PF(0.5:1) ZrOC/PF(1:1) ZrOC/PF(1.5:1) ZrOC/PF(2:1)
Bulk density (g/cm3)
Compressive strength (MPa)
Thermal conductivity (W/m·K)
Young’s modulus (MPa)
0.245 0.258 0.306 0.313 0.322
1.423 0.322 0.404 0.864 1.403
0.0413 0.0466 0.0511 0.0602 0.0660
52.94 5.12 7.43 17.17 24.42
3.2 Microstructure of the ZrOC/PF aerogel
Figure2 shows the contact angle, dimension size, and microstructure of ZrOC/PF aerogels 7
and to further illustrate the characterization of the ZrOC/PF aerogel, PF aerogel as the reference is also presented. Take ZrOC/PF-22 for example, the contact angle is 148o, beyond a critical value of the wettability angle (90o), indicating that the aerogel exhibited excellent hydrophobic property. What’s more, the skeleton of the porous structure of the composite aerogel is composed of the entangled bead-chains, formed by numerous particles. Each of the chains is intertwined, possessing the characteristic of the “grape string”-like structure, similar to that of the organic aerogels [5]. Though the composite aerogels possess typical features of the aerogels, their microstructure is obviously different from PF, based on the following two reasons. Firstly, the composition is discrepant. PF aerogel is consisted of the only PF particles, while ZrOC/PF is composed of the ZrOC and PF particles, originated from the PF and PF and ZrOC sols, respectively. Secondly, the particle size and morphology is different. The morphology of ZrOC and PF seems like that of the wormlike [20] and ball-shaped, respectively. Besides, the size of the pores channel formed by bead-chains is on nanoscale or micrometer scale. With decreasing of the mass ratio of ZrOC to PF, the diameter of the particles composed of skeleton gradually increased and the bead-chain aggregated tightly. Once active points are motivated, the generated oligomer could assemble to form the particles until accumulating enough to separate from the cluster. The “grape string”-like chains containing of PF and ZrOC is generated during the process and the size of the chain channel become narrow with increasing the mass ratio of ZrOC to PF. The study also indicated that the sol-gel polymerization of linear phenolic resin and HMTA is very similar to the conventional curing process of novolac resin [21]. HMTA plays cross-linking and catalysis dual roles during the process. In the initial stage, the HMTA initially serves as the cross-linker reagent. The HMTA gradually decompose aldehyde and ammonia during the sol-gel 8
process, which severs as the base catalyst for accelerating the novel PF resin cross-linking and polymerization. The sol-gel process depends on the nucleation and growth of colloidal polymer clusters. It is shown that the polymer cluster particle with the small size gives the aerogel with mesoporous structure, and vice versa. The proper concentration of the PF and HMTA is crucial to the size of the cluster, which would further determine whether a gel remains an unbroken monolith, or seriously collapsed structure during the ambient pressure drying. What’s more, the size of the cluster is too small or too large would not be beneficial to mechanical performance. Therefore, controlling the cluster size is of virtual important for the preparation of the ZrOC/PF aerogel. The effect of the size of clusters on the mechanical performance of aerogels would be discussed in the following subsection. Fig.3 is the mapping of the main element composed of the aerogels. It shows that the elements containing C, O, and Zr are uniformly distributed in the ZrOC/PF-22 aerogel, indicating that the copolymerization sol method is feasible for the fabrication of ZrOC/PF aerogel and their percentage is 74%, 13%, and 13%, respectively. Obviously, the proportion of the Zr and O is equal but the proportion of the element of C is approximately six times larger than that of the element of Zr or O.
9
Fig.2
10
Fig.3 To investigate the chemical bonds behavior in the samples, we use XPS to analyze the binding of valence bonds among the sample surface. Fig.4 shows the high-resolution XPS spectra of the ZrOC/PF-42 aerogel with fitting curves relative to Zr 3d, C 1s, and O 1s bands, respectively. The results demonstrated that Zr 3d, C 1s, and O 1s all possess large peaks, suggest that the element including Zr, C, and O are present in large amounts in the sample. The sub-peak fitting in Fig.1b corresponding to Zr 3d band is consisted of the spin-orbit doublets (containing Zr 3d5/2 and
11
3d3/2) and the two groups of Zr 3d doublets is detected in the spectrum, with the binding energy difference of E = 2.4eV [22, 23]. Deconvolution of the Zr 3d peak showed that the binding energies of Zr atoms involved are 181.75 eV, 182.4 eV, 184.1 eV, and 184.75 eV, respectively. The Zr 3d doublets including the binding energies of 181.75, 182.4, 184.1 and 184.75 eV are attributed to the Zr-O bond [24]. Fig.4 also indicated that the elemental C and O in the sample is based on the C-C and Zr-O valence band binding, respectively. This result is consistent with the previous study [23].
Fig.4 To further clarify the characteristic of the 3D network structure of the aerogel, the Brunauer-Emmett-Teller (BET) method is used to estimate pore size distribution and the specific surface area of porous monoliths aerogels. Fig.5a and 5b show the N2 the adsorption-desorption isotherms of the ZrOC/PF-42 aerogel, respectively. The isotherms exhibit a sharp increment at low relative pressure, indicating that the substantial micropores are still in the aerogel. As the pressure close to unity (P/P0≈1), the absorption amount rapidly increases, assuming that plenty of
12
macropores may be present in the aerogel. Besides, the adsorption-desorption isotherms show no apparent hysteresis loops, in comparison with other aerogels prepared through supercritical or ambient temperature drying. Fig.4b indicates that the ZrOC/PF aerogel exhibited a relatively wide pore size distribution, ranging from 2 to 200nm. Besides, the hierarchically porous structure with the micropores and macropores is simultaneous in the sample. The microspores maintain the shape-selectivity or size-selectivity advantages for the guest molecules and offer high adsorption capacity [25, 26]. The structural change after the sol-gel process is monitored using the FT-IR method. Fig.5c shows the FT-IR spectra of the ZrOC/PF aerogels with different mass ratios of ZrOC to PF. The peak at 3470 cm-1 is due to the -OH stretching vibration, while the peaks at 2937 cm-1 and 2855 cm-1 resulted from CH2 stretching, attributed to the cross-linked reaction. The angular absorption bond at 1580 cm-1 is supposed as a characteristic peak of the association of ligand with zirconyl [27]. The bands around 1500 cm-1 in the left labeled area corresponding to the aromatic ring stretching vibrations are slightly different after the reaction, possibly due to the mono- or dis-substitution on the aromatic ring. The peak of C-N at 1400cm-1 is originated from the substituted benzylamines, reflecting that the part of N atom inserts into the aerogel network structure. What’s more, the peak at 829 cm-1 and 752 cm-1 in the gray area is attributed to the H atoms in the aromatic rings of different substitutes. The bond around 572 cm-1 may be closely related to the Zr-O vibration in the Zr-O-C chains[28]. The result also indicated that the Zr-O chain has been incorporated into the complex aerogel.
13
Fig.5
3.3 Mechanical and thermal insulation properties of the ZrOC/PF aerogel
Because the aerogel may confront various force and thermal extreme environment, the mechanical and thermal insulation performance as one of the fundamental feathers of the aerogel material becomes rather important. What’s more, the mechanical strength may reflect the monolithicity and processability of the monolithic materials. Fig.6a shows the strain-stress curves of the ZrOC/PF aerogel with the mass ratios of ZrOC to PF increasing from 1:2 to 4:2. It can be seen that the aerogel with a density of 0.258-0.322 possessed the compressive strength of 0.32-1.40 MPa, lower than that of the PF aerogel. The compressive behavior of the aerogels is similar to that of most of the aerogels [11, 12] and three well-defined stages namely elastic, plateau, and fracture stages are observed during the compressive process. In the elastic stage, the stress increased approximately linearly with the strain and ended at peak stress, which was defined as the compressive strength of the sample. Then, the sample came into the second stage of the 14
plateau stage and the stress value that the sample can withstand gradually decreased with the increase of strain, differently from typical stress-strain curves. It is obvious that the zigzag shape is present in the curves, similar to the PF aerogels. Lastly, with the strain further increasing, the stress value that the sample can withstand continuous decreased and correspondingly, the sample had damaged until complete failure. It is noted that compared with that of the PF or carbon aerogels obtained by ambient pressure drying, the compressive strength of the aerogel in this study is low. Such low compressive strength may be closely related to their microstructure. Microscopically, the ZrOC/PF aerogel possessed grape string”-like chain structure composed of the PF and ZrOC secondary nanoparticles. The fragility of the aerogel may be attributed to the weaker connection forces between the necks. The brittleness of the aerogel framework may result from the inter-particle connection zone, namely the necks shown in Fig.4b. It is known that the aerogel is a type of porous material with the porosity as high as 90%, therefore, when they undertake the stress the porous in the sample would be firstly compacted. Once the stress values are higher than that of the aerogel could afford, the neck connected by the particles may debond or fracture, resulting in the destruction of the “grape string”-like chains and further failure of the aerogel. Based on the above-mentioned reasons, we know that the compressive strength can be improved by strengthening the inter-particle neck acting force without generating a severe weight penalty. The “necks” are formed by coagulation of polymer particles on slow gelation and growth to a certain point by dissolution and re-precipitation of polymer clusters during the aging process (see Fig.6b). In the study, the formed clusters in the sol gradually grow by the polymerization reaction, resulting in evolving into particles with various sizes on different chemical reaction 15
conditions. The obtained particles are tightly connected with others by the inter-particle neck. Therefore, it is reasonable that prepared ZrOC/PF aerogel could exhibit excellent mechanical properties simultaneous with low density. Table 1 also shows that the total thermal conductivity of the aerogel increased from 0.0466 to 0.0660 W/m·K with increasing the mass ratio of ZrOC to PF from 0.5:1 to 2:1, slightly higher than that of the PF. What’s more, the compressive strength of the organic-inorganic hybrid aerogels exhibits relatively high compressive strength than of that of the organic ones. The ZrOC/PF possessed low density (0.258-0.322 g/cm3) and a homogeneous three-dimensional network structure consisting of the particles on the nanometer or micron scale. Because the diameter of the particle is less than 10 µm, the contribution to thermal conduction among the particles to thermal conductivity is minor. Therefore, the solid-gas thermal conductivity is the main way in the aerogels composite. The thermal coupling between the gas phase and the solid particles cannot be neglected since the pores are smaller than 10 µm [29]. As shows that the thermal conductivity rose from 0.0466 to 0.066 W/m·K with the increase of the amount of the ZrOC. Although the thermal conductivity of ZrOC/PF aerogel is higher than that of PF, their values are much lower than that of high-temperature thermal insulation materials [11, 12]. Furthermore, compared with the organic aerogel, the composite aerogel containing the non-ablative Zr element may be suitable for the high-temperature environment, which provides a new possible organic-inorganic hybrid material with lightweight and ablative resistance.
16
Fig.6
Conclusion Organic-inorganic hybrid ZrOC/PF aerogel with excellent physical, mechanical, and thermal properties is successfully prepared, using the ZrOC and PF as the precursor through the polymerization-induced phase separation method in this study. The results indicate that the aerogels possessed relatively low bulk density, high compressive strength, and low thermal conductivity. The SEM images reveal that the microstructure of the aerogel is composed of the entangled “grape string”-like chains, formed by the PF and ZrOC cluster particles, contributing to the robust mechanical performance of the aerogel. The diameter of the cluster particles ranges from a dozen of nm to tens of nm, up to microns. XPS results indicate that the sub-peak fitting corresponding to Zr 3d band is based on the Zr-O bond. FT-IR results show that the bond around 572 cm-1 is related to the Zr-O vibration in the entangled chains. Compressive behavior of aerogels is similar to that of the cellular materials and the elastic, plateau, and fracture stages are observed in the strain-stress curves. The ZrOC/PF aerogel with the outstanding feature may be the promising candidate used in aerospace engineering in the future.
Acknowledgments The research work is supported by the National Natural Science Foundation of China (Project 17
No.11732002, 51872029, 11902201, and 11902033) and China Postdoctoral Science Foundation (Project No: 2018M631350).
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Figure Captions Fig. 1 Flowchart of preparation the ZrOC/PF aerogel Fig. 2 The dimension size and the contact angle of the ZrOC/PF-42 aerogel and SEM images of the ZrOC/PF aerogels with the mass of ZrOC to PF of (a)0.5;1, (b)1;1, (c)1.5:1, and (d)2:1, respectively Fig. 3 The mapping of the main elements including composed of the ZrOC/PF-42 aerogel Fig. 4 XPS of the ZrOC/PF-42 aerogel Fig. 5 N2 adsorption-desorption isotherms curve, DFT pore size distributions and FT-IR images of the ZrOC/PF aerogels Fig. 6 Strain-stress curves of the ZrOC/PF aerogels and the schematic illustration of the dual networks and necks
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Dear editors: We declare that no conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
You sincerely Jun Liang