Ya Zhong*abc,
Gaofeng Shaod,
Xiaodong Wua,
Yong Kong
ac,
Xue Wanga,
Sheng Cuiac and
Xiaodong Shenac
aCollege of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China. E-mail: yzhong@njtech.edu.cn
bSuqian Advanced Materials Institute of Nanjing Tech University, Suqian 223800, PR China
cJiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, PR China
dChair of Advanced Ceramic Materials, Technische Universität Berlin, Berlin 10623, Germany
First published on 25th July 2019
Monolithic polymer(resorcinol-formaldehyde) reinforced alumina (RF/Al2O3) aerogel composites were prepared using a sol–gel method and supercritical fluid CO2 drying. The formation mechanism, chemical compositions, pore structures, morphologies, thermal and mechanical performances of RF/Al2O3 aerogel composites with different RF/Al molar ratios were investigated. The results show that the two networks of organic resorcinol-formaldehyde and inorganic alumina are completely independent of one another. The as-synthesized RF/Al2O3 aerogels consist of spherical organic carbon particles and fibrous alumina, which possess low bulk density (0.077–0.112 g cm−3), low shrinkage (1.55–2.76%), low thermal conductivity (0.024–0.028 W m−1 K−1), and high specific surface area (453.26–722.75 m2 g−1). Especially, the sample prepared with molar ratio RF/Al = 1 shows the best network structure with the higher compressive strength (1.83 MPa) and Young's modulus (122.57 MPa). The resulting robust RF/Al2O3 aerogel composites could be potentially used as thermal insulators, catalysts and adsorbents.
To address this issue, various structural reinforcement strategies have been attempted to improve the mechanical properties of aerogels. Incorporation of inorganic fibers,15–22 carbon fiber23–25 or advanced nanomaterials26–31 as supporting skeletons into aerogel matrixes is one of the most convenient and effective methods to overcome their fragility and poor mechanical properties.32,33 Moreover, appropriate fibers could not only strengthen the aerogel materials but also be used as opacifiers to reduce the radiative heat transport in aerogels at high temperature.34 Nevertheless, due to the relatively thicker diameter (5–25 μm) and the brittleness of inlaid inorganic fibers, most of the aerogel matrix usually crack into small fragments with impairing the integrality,35 destroying the microscopic pore structures and decreasing the mechanical properties of the aerogel composites.36
Hybridization of oxide aerogels with polymers is another extensive researched approach to obtain robust aerogels by increasing their tensile strength. Depending on the chemical relationships between the polymers and the surrounding skeletal structures, polymer/sol–gel composites are divided into two categories:37 (1) the polymer and the inorganic framework are completely independent of one another, namely, interpenetrating networks; (2) there is covalent bonding between the polymeric and the inorganic component, namely, crosslinking frameworks. Up to now, various polymers had been successfully integrated with oxide aerogels as reinforcement to improve their mechanical properties. For instance, Leventis et al.38 proposed a method using poly(hexamethylene diisocyanate) as cross-linker to prepare the strong lightweight silica/Di-ISO aerogel monoliths, which are much less hygroscopic than native silica and do not collapse when in contact with liquids. Moghaddas et al.39 developed a method of preparing the silica aerogel/rigid polyurethane foam nanocomposite by ambient pressure drying, which showed efficient thermal insulation (0.0268–0.0314 W m−1 K−1) and good mechanical properties. Hu et al.40 introduced a method of using poly(dimethylsiloxane) as reinforcement to prepare compressible and superhydrophobic polymer/graphene aerogel composites, which showed enhanced compressive strength and a stable Young's modulus. Özbakır et al.41 synthesized the novel monolithic and crack-free PMVE-silica aerogel composites by CO2 supercritical drying and the effect of polymer fraction in solid network on drying was investigated both by experiments and simulations. Li et al.42 fabricated silica aerogel/aramid pulp composites via ambient pressure drying by adding aramid pulps into silica sol directly, which retained the integrality and nice interface adhesion. The compressive strength was enhanced obviously up to 1.2 MPa and the low thermal conductivity of 0.0232–0.0278 W m−1 K−1. Maleki et al.43 introduced a low-cost and time-saving method of using BTMSH and ETESB as cross-linkers to prepare lightweight polymer-reinforced silica aerogels, which showed good compression strength (11–400 kPa) and low thermal conductivity (0.039–0.093 W m−1 K−1). Therefore, the polymer-reinforced aerogel composites exhibit excellent structural integrity and mechanical performance without sacrificing other unique properties. However, the research about polymer reinforced alumina aerogel is limited.
It is generally known that interpenetrating inorganic sol–gel networks with polymers have been pursued mainly for preventing the shrinkage and cracking problems encountered upon drying of the wet inorganic gels. In this study, monolithic polymer(resorcinol-formaldehyde) reinforced alumina (RF/Al2O3) aerogel composites were prepared using sol–gel method and supercritical fluid CO2 drying. In addition, the two networks of organic resorcinol-formaldehyde and inorganic alumina were completely independent of one another. Furthermore, the details of synthesis and discussion of the effects of RF/Al molar ratios on the microstructures evolution and physicochemical properties of RF/Al2O3 aerogel composites are given below.
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Fig. 2 Photographs of RF/Al2O3 aerogels composites prepared with different RF/Al molar ratios (S1: RF/Al = 0.5, S2: RF/Al = 0.67, S3: RF/Al = 1.0, S4: RF/Al = 1.5, S5: RF/Al = 2.0). |
Sample | Gelation time (min) | Linear shrinkage (%) | Bulk density (g cm−3) | Surface areas (m2 g−1) | Average pore size (nm) | Compressive strength (MPa) | Young's modulus (MPa) | Thermal conductivity (W (m−1 K−1), 25 °C) |
---|---|---|---|---|---|---|---|---|
S1 | 120 | 1.55 | 0.094 | 453.26 | 48.47 | 0.74 | 49.56 | 0.025 |
S2 | 75 | 2.14 | 0.103 | 658.43 | 39.12 | 1.29 | 86.41 | 0.027 |
S3 | 50 | 2.76 | 0.112 | 722.75 | 32.08 | 1.83 | 122.57 | 0.028 |
S4 | 135 | 1.97 | 0.096 | 643.81 | 21.57 | 1.14 | 75.08 | 0.026 |
S5 | 350 | 1.73 | 0.077 | 517.39 | 17.29 | 0.56 | 37.19 | 0.024 |
Fig. 3 shows the compressive stress versus compressive strain curves of RF/Al2O3 aerogel composites prepared with different RF/Al molar ratios. Table 1 lists the values of compressive strength and Young's modulus of the as-prepared RF/Al2O3 aerogel composites. The as-synthesized RF/Al2O3 aerogel composites exhibit an excellent mechanical property, which is one of the highest compressive Young's modulus of pure aerogels without using the structural reinforcement materials (bulk density of about 0.10 g cm−3) ever reported. Additionally, it is worth mentioning that the mechanical properties of RF/Al2O3 aerogel composites with interpenetrating organic/inorganic network structures are closely related to the RF/Al molar ratios. As shown in Fig. 3, the sample with molar ratio RF/Al = 1 shows the best mechanical property, and the values of compressive strength and Young's modulus are 1.83 MPa and 122.57 MPa, respectively, which is mainly caused by more uniform internal framework structure and larger bulk density. It was reported that the aerogels with equal magnitude bulk density about 0.15–0.30 g cm−3, such as SiO2,46 Al2O3,47 TiO2,48 C2–50/SiO2,49 layer/SiO2 (ref. 50) aerogels, exhibited poor mechanical properties (compressive Young's modulus of 3.88 MPa, 11.4 MPa and 3.5 MPa, 23–52 MPa, 8.77 MPa, respectively), which could be due to the low densities as well as the disordered porous network morphology of aerogel materials.
Fig. 4 shows the XRD patterns of RF/Al2O3 aerogel composites prepared with different RF/Al molar ratios. For all the as-prepared samples, they display relative broad or weak diffraction peaks, indicating the presence of amorphous organic carbon and alumina in RF/Al2O3 aerogel composites. With the decrease of RF/Al molar ratios, the broad diffraction peak at 22° gradually disappears, meanwhile, the visible characteristic diffraction peaks of boehmite emerge out. The broad diffraction peaks with 2θ values of 15°, 28°, 38°, 49°, 65° and 72° correspond to crystal planes of (020), (120), (031), (200), (002), (251) of pseudo-boehmite (AlO(OH), PDF no. 83-2384), respectively. It exists as polycrystalline boehmite instead of the amorphous phase. Furthermore, the ever-present weak peaks at 37°, 43°, 63°, and 76° are due to the poor crystallization γ-Al2O3 phase in RF/Al2O3 aerogel composites.
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Fig. 4 XRD patterns of RF/Al2O3 aerogels composites prepared with different RF/Al molar ratios (S1: RF/Al = 0.5, S2: RF/Al = 0.67, S3: RF/Al = 1.0, S4: RF/Al = 1.5, S5: RF/Al = 2.0). |
Fig. 5 presents the TG curves of the as-dried RF/Al2O3 aerogel composites prepared with different RF/Al molar ratios heat-treated to 1200 °C in flowing argon. The thermogram profile can be divided into three main regions. The first stage (below 100 °C) is caused by the evolution of physically adsorbed H2O, CO2 and residual solvent. In general, because of the nano-sized porous structure and high porosity, H2O, CO2 and solvent adsorbed in the porous structure of the sample can not be removed completely during the supercritical CO2 drying process. At the second stage (100–500 °C), an obvious weight loss of all the samples occurs due to the continuous thermal decomposition of the polymer(resorcinol-formaldehyde) in RF/Al2O3 aerogel composites. For the last stage (above 500 °C), all the TG curves of the RF/Al2O3 aerogel composites tend to smooth and stabilization. The mass remaining of the samples (S1–S5) at 1200 °C are 56%, 54%, 53%, 50% and 45% of the original, respectively. According to previous reports,51 the TG curves of the samples (S4, S5) with molar ratios (RF/Al = 1.5, 2.0) similar to the pure RF aerogel under the same conditions, which gives the initial mass loss of adsorbed solvents below 100 °C and only one additional step above 400 °C, yielding at 700 °C a carbon aerogel with a mass loss of ∼50% of the original. In addition, unlike the other mutually interpenetrating resorcinol-formaldehyde/metal oxide (RF/MOX, M: Fe, Cu) networks,52 the TG curves of as-synthesized RF/Al2O3 aerogel composites suggest that there is no reaction takes place between RF and Al2O3.
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Fig. 5 TG of curves of RF/Al2O3 aerogels composites prepared with different RF/Al molar ratios (S1: RF/Al = 0.5, S2: RF/Al = 0.67, S3: RF/Al = 1.0, S4: RF/Al = 1.5, S5: RF/Al = 2.0). |
Fig. 6 presents the FT-IR spectrum of polymer (RF), Al2O3 aerogel and RF/Al2O3 aerogel composites. As shown in Fig. 6, all the characteristic peaks of the RF/Al2O3 aerogel composites relatively draw close to each other. Moreover, the characteristic peaks of RF/Al2O3 aerogel composites are caused by the pure polymer (RF) and Al2O3 aerogels, and no obvious wavenumber shifts can be observed. The presence of water molecules is evidenced by the bands at 3426 cm−1 and 1621 cm−1, and there are no significant changes of the intensity with the decrease of RF/Al molar ratios. The two weak bands at 2973 cm−1 and 2898 cm−1 are related to the C–H stretching vibration of hydrocarbon groups. The unique band only existed in S5 at 1720 cm−1 is associated with stretching of the CO bond of carbonyl or carboxyl groups. The bonds at 1460 cm−1 and 1474 cm−1 are due to the –CH2– stretching vibration. The two bonds of 1293 cm−1 and 1232 cm−1 belong to the stretching vibration of C–O–C hydroxymethyl ether bond. It is well known that the lamellar structure of AlOOH has been previously reported by Yarbrough and Roy.53 The band at 1069 cm−1 is assigned to the Al–O–H stretching vibration of boehmite. The OH groups within the structure could form zigzag chains between the planes of oxygen ions, which could lead to the OH stretching modes due to their crystallographically inequivalent coupling effect.54 The bands at 885 cm−1, 769 cm−1, 623 cm−1 and 484 cm−1 are attributed to the Al–O structural vibration of boehmite. It is worth noting that the intensity of the bands corresponding to boehmite weakens gradually with the increase of RF/Al molar ratios, which is caused by the presence of polymer (RF) in its environment. Thus, the above FTIR analysis is consistent with the XRD and TGA results.
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Fig. 6 FT-IR spectrum of RF, Al2O3 aerogel and RF/Al2O3 aerogels composites (S1: RF/Al = 0.5, S2: RF/Al = 0.67, S3: RF/Al = 1.0, S4: RF/Al = 1.5, S5: RF/Al = 2.0). |
Fig. 7 shows the microstructures of the RF/Al2O3 aerogel composites prepared with different RF/Al molar ratios. All the samples exhibited porous structures of a typical colloidal gel, which is consisted of polymer (RF), Al2O3 nanoparticles and nanopores. It is found that with the increase of RF/Al molar ratios, the morphology gradually changes from pearl-necklace networks to spherical particles. In addition, the as-prepared samples are comprised of interconnected spherical particles with diameters in the 5–15 nm range of polymer (RF) and Al2O3 aerogel. Furthermore, the nanoparticles of the as-prepared samples progressively adjoin closely to each other and thus possesses the smallest nanopores with diameter at around 10–20 nm for the sample with RF/Al = 2, when compared with the other samples. There are some large pores in the samples with RF/Al = 0.5, 0.67 (S1, S2), and some agglomeration particles appear in the samples with RF/Al = 1.5, 2.0 (S4, S5), which is not beneficial for large specific surface areas. In contrast, the sample with RF/Al = 1 (S3) exhibits a significant homogeneous pore structures with diameters in the range of approximately 30–40 nm.
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Fig. 7 SEM images of RF/Al2O3 aerogels composites prepared with different RF/Al molar ratios (S1: RF/Al = 0.5, S2: RF/Al = 0.67, S3: RF/Al = 1.0, S4: RF/Al = 1.5, S5: RF/Al = 2.0). |
Transmission electron microscopy (TEM) was employed to further investigate the microstructure of the selected RF/Al2O3 composite (Fig. 8, molar ratio RF/Al = 1). The TEM image shows that alumina aerogels exhibit randomly interconnected networks made up of nanometer-sized fibrous alumina (dark field), which is similar to leaflets or sheets (2–5 nm wide, varying lengths), and RF aerogels consisted of interconnected amorphous spheroidal particles surrounding the fibrous alumina aerogels. Due to the fibrous alumina existed in RF/Al2O3 aerogel composites, the mechanical properties of RF/Al2O3 aerogel composites are further improved. The mechanism could be explained by the similar phenomena occurred in fiber-reinforced system. Intriguingly, unlikely to the other aerogel composites reinforced by thick fibers, the nano-scaled fibrous alumina particles are beneficial to enhancing the mechanical performances of the as-prepared RF/Al2O3 aerogel composites instead of destroying the internal pore structures.
The adsorption/desorption isotherms and pore size distribution curves of the samples are shown in Fig. 9. They are type IV curves with type H1 hysteresis loop in the IUPAC classification, which is characteristic of a mesoporous structure with cylindrical pores. The desorption cycles of the isotherms show a hysteresis loop for the five samples, which is generally attributed to the capillary condensation that occurs in the mesopores. It is found that with the increase of RF/Al molar ratios, the ranges of the pore size distribution curves are changed from 0–100 nm to 0–30 nm. As shown in Table 1, the values of specific surface areas undergo the trend of first increase and then decrease with the increase of RF/Al molar ratios. Meanwhile, the average pore size of the samples gradually become smaller from 48.47 nm to 17.29 nm. This is because the fact that with the increase of RF/Al molar ratios, the pore structures of the RF/Al2O3 aerogel composites become more homogenous and some macropore with diameters above 100 nm appear in the composites (S1, S2, S3), which is favorable to increasing the specific surface areas of the composites. By contrast, the sample with RF/Al = 1 shows the preferable framework structure with the highest specific surface areas of 722.75 m2 g−1 (as shown in Fig. 7 and Table 1). However, some agglomeration particles generate in the composites (Fig. 7) with the continuous increase of RF/Al molar ratio, resulting in the decrease of specific surface area.
This journal is © The Royal Society of Chemistry 2019 |