2.1. Appearance and Properties of the Primary Materials, Aerogels and Honeycomb Materials
In our study, two different types of aerogels were synthesized and characterized. Due to different precursors we used, the aerogels produced exhibit different haptic and mechanical properties. They both are elastic but with different degrees of deformability. The first type is a methyltrimethoxysilane (MTMS) based silica aerogel, which is highly flexible. It is reversible deformable like a marshmallow. This aerogel, we call in the present study SA1 (silica aerogel 1). Due to its high flexibility, we also call it super-flexible. The second type is an methyltrimethoxysilane- [3-(2,3-Epoxypropoxy)-propyl]-trimethoxysilan (MTMS-GPTMS) based aerogel having lower degree of flexibility. This type is rubber-like and more brittle. We name this aerogel SA2 (silica aerogel 2) or low-flexible by reason of its reduced deformability.
The pure SA1 aerogel, shown in the
Figure 1 is plain white and extremely fluffy. When slightly blowing over its surface, aerogel powder comes off and one can hardly feel any counterforce when compressing it by hand. Shrinkage after supercritical drying was below 5%, and is neglected in further considerations. It shows low density (0.037 g·cm
−3) and low thermal conductivity (0.034 W·(m·K)
−1), as given in
Table 1. Due to the high flexibility, its compressive modulus is only 3 kPa. The SEM image in the
Figure 2 shows the microstructure of an SA1 sample with large particles and pore sizes. For flexible aerogels, large pores are responsible for the reversible deformation of the network [
19].
Table 1.
Properties of produced aerogels and honeycombs.
Table 1.
Properties of produced aerogels and honeycombs.
Material | Thermal Conductivity (W·(m·K)−1) | Compressive Modulus a (MPa) | Envelope Density (g·cm−3) | Skeletal Density (g·cm−3) | Porosity (%) | Mean Pore Size (nm) |
---|
Super-flexible aerogel SA1 | 0.034 | 0.003 | 0.037 | 1.38 | 97.3 | 242 |
Low-flexible aerogel SA2 | 0.038 | 0.074 | 0.092 | 1.41 | 93.5 | 113 |
Aramid honeycomb C1-3.2-29 | 0.060 b | 0.030 in-plane 10.7 out-of-plane | 0.029 c | - | - | - |
Aramid honeycomb A10-92-5.2 d | 0.07 b | 0.086 in-plane 12.2 out-of-plane | 0.092 b | - | - | - |
Aramid honeycomb C1-6.4-24 e | 0.08 b | 0.035 in-plane 11.3 out-of-plane | 0.024 c | - | - | - |
Figure 1.
Super-flexible silica aerogel SA1.
Figure 1.
Super-flexible silica aerogel SA1.
Figure 2.
Microstructure of SA1 (magnification of image 1k× and of inset 5k×).
Figure 2.
Microstructure of SA1 (magnification of image 1k× and of inset 5k×).
To characterize the porous structure of an aerogel the porosity and mean pore size were calculated. The porosity was determined by using of Equation (1) from envelope
ρenv. and skeletal
ρskel. densities.
The specific pore volume
vpore is given by
and can be used to calculate the average pore size
dav using [
22]
The SA1 exhibits high porosity of 97.3% and average pore size of about 242 nm.
The low-flexible silica aerogel (SA2), shown in
Figure 3, is much stiffer in contrast to the SA1. Being white as well, with a haptic like an eraser or rubber, it is still slightly flexible and compressible. Though being significantly more robust than the super-flexible aerogels, the SA2 aerogel is still easy breakable into pieces. Its structure consists of small, interconnected particles and pores in the range of 0.5–1 μm as shown in
Figure 4. Compared to SA1 and SA2 aerogels, it appears with a finer structure. The calculation of average pore size confirms that the pores of SA2 are almost two times smaller than SA1. The low-flexible aerogel exhibits higher envelope density (0.092 g·cm
−3) caused by the higher solid concentration. A slightly higher thermal conductivity (0.038 W·(m·K)
−1) and an almost 25 times higher compressive modulus compared to SA1 is measured consequentially, as given in
Table 1. The porosity of SA2 is slightly lower (93.5%).
The envelope density of aramid honeycombs of type C1, shown in
Figure 5, is slightly lower compared to the synthesized aerogels, SA1 and SA2. The envelope density of honeycombs type A10-92-5.2 is similar to SA2 aerogels and much higher than of SA1.
Their thermal conductivity is almost two times higher, due to higher heat transfer via the solid and gaseous phases. The aramid fibers form dense walls, with a density of 1.44 g·cm
−3 being much higher than that of aerogels [
23]. Since the heat transfer via the solid backbone is directly proportional to the density of backbone material, the thermal conductivity of aramid is higher. The heat transfer via gaseous phase depends, amongst other parameters, on the pore dimension. In the cells of 3.2–6.4 mm, the diffusive and convective heat transfer is predominant and leads also to a high thermal conductivity [
4]. Filling of cells with aerogel should decrease the heat transfer and lead to better insulating materials.
As expected, the stiffness of aramid honeycomb material is significantly higher than that of aerogels. The honeycomb materials are highly resilient. After releasing a load, they spring back to their initial size and shape. The honeycomb A10-92-5.2 shows the highest density and the highest compressive modulus in both directions. The properties of C1 type honeycombs look similar.
Figure 3.
Low-flexible silica aerogel SA2.
Figure 3.
Low-flexible silica aerogel SA2.
Figure 4.
Microstructure of SA2 (magnification of image 10k× and of inset 24k×).
Figure 4.
Microstructure of SA2 (magnification of image 10k× and of inset 24k×).
Figure 5.
Aramid honeycomb C1-6.4-24 covered with phenolic resin.
Figure 5.
Aramid honeycomb C1-6.4-24 covered with phenolic resin.
2.2. Appearance and Properties of the Aerogel-Honeycomb-Composite Materials
The composite materials, depicted in
Figure 6 and
Figure 7, containing SA1 aerogel show sound adhesion on the honeycomb and surround it thoroughly without any cracks. Some small pores are visible on the surface caused by formation of air bubbles during sol-gel synthesis. These holes (encircled) could negatively affect the composite and cause a weakening.
Figure 8 and
Figure 9 depict the composites with low-flexible SA2 aerogels. Without any defects inside the material, the samples exhibit good adhesion. The firm contact between aerogels and honeycomb material was additionally approved by SEM.
Figure 10 and
Figure 11 show both materials: honeycombs and aerogels. One can see aramid fibers covered with aerogel particles. This confirms a continuous, firm contact between the two materials.
Figure 6.
Super-flexible silica aerogel (SA1) with aramid honeycombs A10-92-5.2.
Figure 6.
Super-flexible silica aerogel (SA1) with aramid honeycombs A10-92-5.2.
Figure 7.
Super-flexible silica aerogel (SA1) with aramid honeycombs C1-3.2-29.
Figure 7.
Super-flexible silica aerogel (SA1) with aramid honeycombs C1-3.2-29.
Figure 8.
Low-flexible silica aerogel (SA2) with aramid honeycombs C1-3.2-29.
Figure 8.
Low-flexible silica aerogel (SA2) with aramid honeycombs C1-3.2-29.
Figure 9.
Low-flexible silica aerogel (SA2) with aramid honeycombs C1-6.4-24.
Figure 9.
Low-flexible silica aerogel (SA2) with aramid honeycombs C1-6.4-24.
Figure 10.
SEM image (magnification 208×) of a cut-off cross-section of the SA1 aerogel-honeycomb composite, showing aramid-fibers, covered with good adhering aerogel particles.
Figure 10.
SEM image (magnification 208×) of a cut-off cross-section of the SA1 aerogel-honeycomb composite, showing aramid-fibers, covered with good adhering aerogel particles.
Figure 11.
Adhesion of SA2 aerogel particles on aramid fiber (magnification 4.0k×).
Figure 11.
Adhesion of SA2 aerogel particles on aramid fiber (magnification 4.0k×).
2.3. Thermal Properties
As already mentioned, heat in solids is transferred via different mechanisms [
24,
25,
26,
27]. The composites with higher density should transfer heat faster through their solid network. The density of the composites produced depends on two parameters. First, SA2 aerogels have higher densities, therefore the composites using SA2 aerogel will have a higher density too. Second, the higher the volume fraction of the honeycomb, the higher the envelope density. Composites with honeycomb type C1-3.2-29 (with the smallest cell size) consist of the highest amount of aramid (volume fraction: 7.3 vol.-%) as shown in the
Table 2. On the other hand, the higher the volume fraction of the aerogel, the lower the thermal conductivity of composites should be.
We calculated the theoretical thermal conductivity with the rule of mixture
where
λ denotes the thermal conductivities and
Φ the volume fraction of components. The results are given in the
Table 2.
Table 2.
Properties of produced honeycomb.
Table 2.
Properties of produced honeycomb.
Sample | Aerogel (volume %) | Honeycomb (volume %) | Envelope Density (g·cm−3) | Theoretical Thermal Conductivity λeff (W·(m·K)−1) | Measured Thermal Conductivity (W·(m·K)−1) |
---|
SA1 C1-3.2-29 | 92.7 | 7.3 | 0.069 | 0.036 | 0.038 |
SA1 A10-92-5.2 | 95.0 | 5.0 | 0.062 | 0.036 | 0.039 |
SA1 C1-6.4-24 | 96.2 | 3.8 | 0.073 | 0.036 | 0.036 |
SA2 C1-3.2-29 | 92.7 | 7.3 | 0.091 | 0.040 | 0.044 |
SA2 A10-92-5.2 | 95.0 | 5.0 | 0.092 | 0.040 | 0.044 |
SA2 C1-6.4-24 | 96.2 | 3.8 | 0.091 | 0.040 | 0.039 |
Because of the honeycomb structure, the volume fraction of the filling is over 90%. It depends only on the pores dimension of the honeycomb materials used. As expected, the thermal conductivities calculated for composites are equal, even with different aerogel amounts and different cell sizes of the honeycomb materials. Higher conductivity of SA2 aerogels affects the conductivity of composites, which is slightly higher.
Compared to theoretical values, the thermal conductivities measured for several samples are higher. The differences are negligible and average about 2%–5%. In general, the conductivity in comparison to the honeycomb itself (0.06–0.08 W·(m·K)−1) was substantially decreased. With a larger cell diameter, the volume and therefore the mass fraction of the aerogel increase, which results in a decrease of thermal conductivity for the composite material. The best results could be achieved with SA1 aerogel and honeycombs C1-6.4-24. Here, the thermal conductivity could be successfully reduced by 40 percent in comparison to the honeycomb material itself.
2.4. Mechanical Properties
Important requirements for insulating materials are a sufficient stiffness, a suitable loading capacity and, in addition, a certain flexibility. Flexible insulation can guarantee a perfect contact between the insulated surface and the insulating material, so that no air or other fluids can flow in-between. The extremely soft flexible aerogels satisfy this requirement if they are reinforced e.g., by flexible honeycomb materials.
The mechanical properties of the synthesized composites were tested in-plane and out-of-plane as shown in
Figure 12.
Figure 12.
Schematic representation of conducted compression tests.
Figure 12.
Schematic representation of conducted compression tests.
The effect of the aerogel filling of the honeycombs on the mechanical properties will be discussed on the example of the C1-3.2-29 honeycomb material.
Figure 13,
Figure 14,
Figure 15 and
Figure 16 display the load-displacement data of four representative samples of C1-3.2-29. Each figure compares the pure aerogel SA1 or SA2 as references, the empty honeycombs and the composite material.
The soft and super-flexible SA1 silica aerogel was compressed up to 80%. After reaching 40% of compression, a first small crack was observed. As shown in
Figure 13, they were followed by several other cracks. They indicate irreversible deformation of the material. Further compression leads to densification of the porous structure. The strain increased rapidly and reached 0.015 MPa at 80% compression.
Figure 13.
Compression curves of SA1 with C1-3.2-29 in-plane.
Figure 13.
Compression curves of SA1 with C1-3.2-29 in-plane.
The stress-compression curve of empty honeycomb in-plane shows three regions. The first region is characterized by a constant slope with rising stress. This slope was used to determine the compressive modulus. After reaching a maximum, the region of elastic deformation ends and a plateau is reached, which indicates the second region. Further deformations in the structure are reflected in the long plateau, which extends up to 70%. Many small cracks are characteristic for the deformations of the walls. Finally, when the cell walls touch each other, densification starts and the stress rises [
28].
Figure 14.
Compression curves of SA1 with C1-3.2-29 out-of-plane.
Figure 14.
Compression curves of SA1 with C1-3.2-29 out-of-plane.
Figure 15.
Compression curves of SA2 with C1-3.2-29 in-plane.
Figure 15.
Compression curves of SA2 with C1-3.2-29 in-plane.
Figure 16.
Compression curves of SA2 with C1-3.2-29 out-of-plane.
Figure 16.
Compression curves of SA2 with C1-3.2-29 out-of-plane.
In contrast, the curve of the composite exhibits a higher slope in the first region. The filling of honeycomb cells increases the deformation resistance, even if the filling material is very soft as, in our case, the SA1 aerogel. The compressive modulus of the honeycomb material is increased by factor 2.3 and compared to the pure aerogel by a factor of ten. The roughness of the curve in the plateau region indicates several large cracks. They reflect a rupture inside the composite material.
Nevertheless, some difficulties occurred during testing. As loading progressed, the samples bent slightly so that perfect uniaxial load could not be reached. Thicker samples could help to avoid this problem.
The stress-compression curves out-of-plane show another progression in the
Figure 14. The loading capacity in that direction is much higher. A first deformation seen as a bending of the stiff walls is observed after reaching 0.4 MPa. Then, the resistance becomes weaker and the walls start bending at several positions. After 80% compression, irreversible densification of the honeycomb material takes place.
The curve of the composite material looks quite similar. The delayed rise of the curve for the out-of-plane measurements is caused by protruding aerogel, which could not be cut perfectly without damaging the composites structure, resulting in an offset. For the honeycomb material, the nearly linear behavior of the first part, which can be found for all samples, indicates elastic behavior over a large range of deformation. The linear relation between stress and compression ends with a maximum followed by a region of almost constant stress. In out-of-plane, the compressive modulus decreased by 5%. The weakening of the composite could be caused by defects in the material. As shown in
Figure 7, air bubbles were formed between cell walls and aerogel, so that a continuous contact is not given in the composite. One can assume that, under loading, the cracks will start at these positions.
The compression test of SA2 aerogel is shown in
Figure 15. A short linear region at the beginning, where the compression was determined, is followed by two jumps in the curve. Compared to SA1, we can see a steeper slope in the Hookean region, which speaks for a higher compressive modulus. When the compression of 30% is exceeded, irreversible deformations occur in the aerogel material. After first fracturing of pore walls, which is reflected in the jumps, the stress starts to rise. The aerogel loses porosity and becomes a compact material. The compressive modulus of the composite material is four times higher than that of the empty honeycomb material itself. Under in-plane loading, several cracks along the walls arise. As seen in
Figure 17, the contact between the two materials of the composite under uniaxial load is the weakest point. To improve the strength of adhesion, a pre-treatment of the surface of the honeycomb with e.g., surfactants, should be performed.
Figure 17.
Compression test of SA2 with C1-3.2-29 honeycombs in-plane. Several cracks and holes arose during compression.
Figure 17.
Compression test of SA2 with C1-3.2-29 honeycombs in-plane. Several cracks and holes arose during compression.
The out-of-plane compression curves of the composite materials containing SA2 show similar behavior as the ones with SA1 aerogel. The stiffness of these composites is improved by 13%.
The compression curves of all other composites produced, combining the two types of aerogel SA1 and SA2 with various honeycomb materials, look similar to the given examples. The results of the compression tests are summarized in
Figure 18. In all cases, the in-plane compressive moduli of the composite materials are increased. Due to higher stiffness of SA2 aerogel, the corresponding composite materials are stiffer too. The highest values are reached with the medium cell size A10-92-5.2 honeycomb from HEXEL
®. Honeycombs of type C1-6.4-24 with bigger cell size and therefore highest aerogel amount showed the lowest stiffness.
Figure 18.
Compressive moduli of empty and field honeycombs in two directions. Since we could observe defects in the manufactured materials (air bubbles, holes), these weak points should be prevented. Research along this line is currently in progress.
Figure 18.
Compressive moduli of empty and field honeycombs in two directions. Since we could observe defects in the manufactured materials (air bubbles, holes), these weak points should be prevented. Research along this line is currently in progress.
The results of the out-of-plane compression tests are similar. The highest improvement is carried out using A10-92-5.2 honeycomb material with a cell size of 6.4 mm. No increase is reached for both types of aerogel. In general, weakening of a honeycomb material is caused by the high capability of moisture absorption of aramid [
29]. The cells of the honeycomb materials consist of aramid fibers, which take up humidity by capillary forces. For honeycomb materials with cell size of 3.2 mm, the moisture absorption is 1.3% and, for 4.8 mm, it is 1.7% [
20]. Therefore, to avoid moisture absorption, all honeycomb materials of aramid are covered by phenolic resins. As soon as the honeycombs are cut, the protecting layer is broken and moisture can be absorbed via the cut surfaces.
To summarize the thermal and mechanical properties of the silica-aerogel aramid honeycomb composite materials, we point out that the lowest thermal conductivity could be reached with SA1 aerogels and C1-6.4-24 honeycomb materials. In contrast, the same composite possesses poor mechanical properties. The highest improvement in terms of stiffness could be achieved with SA2 and SA1 aerogels and A10-92-5.2 type honeycomb materials. The honeycombs A10-92-5.2 possess the highest strength (0.09 MPa in-plane and 12.2 MPa out-of-plane) compared with other honeycomb materials. It can be expected that the combining of A10-92-5.2 with aerogels would lead to highest values. On the other side, the ratio of both materials in the composite plays also an important role. The hybrid with the largest cell size (C1-6.4-24) contains a high amount of soft aerogel, which reduces the compressive strength. Obviously, the cell size of A10-92-5.2 represents the best ratio of combined materials.