Improvement of the Mechanical Properties of Silica Aerogels for Thermal Insulation Applications through a Combination of Aramid Nanofibres and Microfibres

Abstract

Reinforcement of silica aerogels, remarkable lightweight mesoporous materials with outstanding insulation performance, is still a challenging research topic. Among the strategies used to overcome their brittleness, one of the most effective is the manufacturing of aerogel composites with embedded fibres. In this work, the incorporation of nanofibres together with microfibres in a tetraethoxysilane-vinyltrimethoxysilane matrix is investigated for the first time for the development of novel aerogel nanocomposites. The nanofibres, synthesized from different aramid fibres, including Kevlar^® pulp, Technora^®, Teijinconex^® and Twaron^® fibres, were used in different combinations with microaramids and the resulting nanocomposites were thoroughly investigated for their physicochemical and thermomechanical features. The properties depended on the type and amount of the nano/microfibre used. While the microfibres exhibited low interaction with the silica matrix, the higher surface of the nanofibres ensured increased contact with the gel matrix. A low bulk density of 161 kg m^-3 and thermal conductivity of 38.3 mW m^-1 K^-1 (Hot Disk^®) was achieved when combining the nanofibres obtained from Kevlar^® pulp with the Technora^® or Teijinconex^® long fibres. The nanofibres showed higher dispersion and random orientation and in combination with microfibres led to the improvement by a factor of three regarding the mechanical properties of the aerogel nanocomposites reinforced only with microfibres. The scale-up process of the samples and simulated tests of thermal cycling and vacuum outgassing successfully conducted indicate good compliance with space applications.

Keywords: nanofibers; scale-up; silica aerogels; space compliance; thermal insulation.

Figure 1

Intensity-weighted nanofibre size distributions for (a) NKP and NTw and (b) NTch and NTeij dispersions in DMSO.

Figure 2

ATR-FTIR spectra of (a) fibres and (b) aerogel composites based on tetraethoxysilane/vinyltrimethoxysilane (TV) with different reinforcement combinations (nomenclature in Table S1).

Figure 3

Visual aspect of the silica aerogels containing (a) NKP_DMSO, (b) NKP_DMSO/Teij, (c) NKP, (d) NKP/Teij, and (e) aerogel composites with different fibre combinations (nomenclature in Table S1).

Figure 4

Scanning electron microscope images of different aerogel composites (nomenclature in Table S1) showing: (a–c) aerogel matrix, (d–f) interaction of the fibres with the aerogel matrix. Reinforcement was performed with: (a,d) KP, (b,c) NKP, (e) Teij, and (f) NKP and Teij.

Figure 5

Influence of the quantity of (a) NKP and (b) NTw on the thermal conductivity of the nanocomposites (nomenclature in Table S1), considering equal quantities of different microfibres.

Figure 6

Thermograms for (a) AF and NKP and (b) silica aerogels reinforced with AF, NKP, and their combination (nomenclature as in Table S1).

Figure 7

Compression–decompression curves for different systems containing (a) 40 mg NKP in combination with different amounts of Teij and (b) 100 mg Tch in combination with different amounts of NKP (nomenclature in Table S1).

Figure 8

Scaled-up aerogel composites developed with TEOS/VTMS precursors (0.8:0.2) and EtOH/Si molar ratio S = 10 with different reinforcements: (a) TV_Teij and (b) TV_NKP_Teij. The scale bar is valid for both specimens, manufactured by using the same mould.