Aerogel-based materials for building applications: Influence of granule size on thermal and acoustic performance

Energy and Buildings 152 (2017) 472–482

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Aerogel-based materials for building applications: Influence of granule size on thermal and acoustic performance Cinzia Buratti a,∗ , Francesca Merli b , Elisa Moretti a a b

Department of Engineering, University of Perugia, Via G. Duranti 67, 06125 Perugia, Italy University of Perugia, CIRIAF Interuniversity Research Center on Pollution and Environment “Mauro Felli”, Via G. Duranti 63, 06125 Perugia, Italy

a r t i c l e

i n f o

Article history: Received 24 February 2017 Received in revised form 12 June 2017 Accepted 25 July 2017 Available online 28 July 2017 Keywords: Silica aerogel Thermal conductivity Acoustic properties Transmission loss (TL) Absorption coefficient (␣) Aerogel-based materials

a b s t r a c t Aerogel was one of the most promising nano-materials for use in buildings and its thermal performance was widely discussed in the literature while an accurate study of acoustic properties was never provided. The aim of the paper is to investigate experimentally the influence of granules size on both thermal and acoustic properties of granular aerogels and aerogel-based solutions (a plaster and a translucent polycarbonate panel) for energy saving in buildings. Several kinds of aerogels were investigated, ranging from small granules (0.01–1.2 mm) to large granules (1–4 mm). For each kind of aerogel, the absorption coefficient (␣) and Transmission Loss (TL) were measured at normal incidence in a traditional impedance tube, taking into account 5 thicknesses (15, 20, 25, 30 e 40 mm) and thermal conductivity (␭) was evaluated by the Heat Flux Meter, setting up an appropriate methodology because of the sample nature. The aerogel granules outperformed the conventional insulating materials: depending on the particle sizes, ␭ varies in 19–23 × 10ˆ(-3) W/(mK) range at 10 ◦ C. The smallest granules (highest density) had the best performance, both in terms of thermal and acoustic insulation: ␣-values and TL better than the ones of rock wool were achieved (␣ = 0.95 and TL = 15 dB at about 1700 Hz). The good acoustic behaviour was confirmed also considering the two aerogel-based solutions for buildings: the peak of the absorption coefficient of the aerogel-based plaster was 0.29 at about 1050 Hz, compared to a value of about 0.1 of conventional plasters; simultaneously, ␭ diminished from 0.7 W/mK to 0.05 W/mK. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In the background of superinsulation materials, aerogels are achieving a more and more important role, especially in the granular form. It is less expensive than the monolithic aerogel; furthermore the difficulty in producing monoliths without defects, cracks, and inhomogeneity can compromise the optical quality of the transparent layer [1]. Granular aerogels are easier to handle, because they could be poured like a powder [1], but their optical and thermal performance is in general worse than the monolithic ones [2,3], due to the air trapped in the inter-granular macropores, even if the granules themselves offer the same thermal conductivity as the monoliths. For this reason, research on monolithic silica aerogel for fenestration applications recently started again and new technics for their preparation were proposed, by means of a rapid supercritical extraction process implemented in an industrial hot press [4].

∗ Corresponding author. E-mail address: [email protected] (C. Buratti). http://dx.doi.org/10.1016/j.enbuild.2017.07.071 0378-7788/© 2017 Elsevier B.V. All rights reserved.

In the last decade the attention of the researchers was focused on the characterization of granular aerogels [2,3,5–17], but a systematic investigation of the influence of the granule size was concentrated only in some of the last studies [18–20]. Aerogels are nanostructured solid materials characterized by low density and high porosity (> 90%); they can be defined as superinsulation materials, due to their low thermal conductivity. Koebel et Al. [1] in 2012 introduced a definition of superinsulation materials basing on the value of the thermal conductivity instead of the total heat transfer coefficient U-value: the limit for superinsulation materials is set at a value of ␭ of 0.020 W/mK and aerogels can be considered in this class of materials. The low values of the thermal conductivity of aerogels are due to the combination of low density and small pores, which contribute to reduce heat transfer. Density of SiO2 aerogels vary in the 80–200 kg/m3 and in general the thermal conductivity increases when density increases, due to the conduction through the silica network and the radiation and gaseous conduction through the air inside the pores. But at the same time the small pores limit conductive and convective gas transport, therefore it contributes to reduce the thermal conductivity, overcoming the effect of the increased density. Thermal conductivity

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can in fact be reduced also by compression of the granular aerogel bed, which reduces the volume fraction of air in the inter-granular macroporosity [1,21]. Thermal conductivity of aerogels is in general declared in the 0.012–0.020 W/mK range [1,8,13], but in some cases it can reach a value of 0.008 W/mK [7]. When compared to other insulation and superinsulation materials, aerogels show excellent fire resistance, effect as vapour barrier, resistance to direct sunlight, durability, and sound absorption [13]. Neugebauer et al. [14] describe a technique for compacting a bed of granular aerogel P100 (Cabot Corporation) in order to reduce the thermal conductivity. The particle size distribution is measured by means of a tower of sieves agitated by a motorized sieve shaker. The density of the initial naturally settled bed was about 68 kg/m3 and the thermal conductivity 24 × 10ˆ(-3) W/(mK). Different degrees of compression were applied and the corresponding thermal conductivity was measured. Results showed that as the compression (and the density) increases, the thermal conductivity decreases, reaching a minimum value of about 13 × 10ˆ(-3) W/(mK) at a bed density of about 150–165 kg/m3 (or a compressive strain between 55 and 59%). The optical properties of both granular and monolithic aerogel were studied since many years [7,2,3]. The solar and visible transmission of monolithic silica aerogels is in general higher than the granular one, while the U-values are comparable in non evacuated conditions. The difference in the optical properties depends on the parameters of the sol-gel process, especially for the monolithic panes, and on the granule size for the granular form, but the influence of the granule size was not highlighted in the these studies. Furthermore the monolithic pane allows the view through it, but it scatters the transmitted light resulting in a hazy picture and modified colour rendering: the light reflected appears in fact bluish, while the transmitted one is slightly reddened. Granular aerogel, on the contrary, does not allow the view through it, resulting in a completely diffuse transmission able to avoid glare. Less studies are available on the acoustic properties of aerogels [22–25]. In a first study in 1998 [22] the acoustic propagation in aerogels and alcogels was investigated, while the acoustic reflection coefficient, the attenuation, and the sound velocity were studied in [23] and were compared with those of a glass wool sample. Small and large granule aerogels were considered and it was found that the glass wool exhibits better absorption properties than large granule aerogels, while small granules have a higher attenuation than glass wool. Furthermore it was demonstrated that the best behaviour of aerogel granules as audible sound absorbers was found when used in coupled layers with different granule sizes. It is confirmed in [24], where the attenuation of three layer absorbers was investigated in an impedance tube: the first two layers act as an impedance matcher, while the third one acts as an absorber. Three samples of granular aerogels of 1 mm, 3 mm, and 80 ␮m diameters were arranged in different orders and thicknesses; the results could be related to the transmission loss, even if data inside the impedance tube is overestimated. A maximum attenuation of about 60 dB was obtained for a sample constituted by three layers 2 cm thick of decreasing diameter (3 mm, 1 mm, and 80 ␮m). Granular aerogels with very low granule size (powders) were also used to simulate the snow behaviour, in order to use the acoustic emissions to predict avalanches [25]. In the recent years (2014–2016) a systematic investigation of the influence of the granule size on the optical, thermal, and acoustic performance of aerogels was carried out [18–20]. Gao et al. [18] used silica aerogels granules of 3–5 mm and prepared aerogel granules with smaller particle sizes by grinding and sieving the received granules. Three glazing units were assembled with two clear glass panels (thickness 4 mm each) and an intermediate gap (thickness 14 mm), filled respectively with air, large granules, and small gran-

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ules, for a total thickness of the glazing units of 22 mm. The thermal and optical properties were investigated and the results showed that they are strongly correlated to the particle size. The glazing unit with small granules showed lower U-values with respect to the one with large granules (1.05 W/m2 K instead of 1.19 W/m2 K), but also lower visible light transmittance and solar factor (␶v = 0.15 and g = 0.27 for small granules; ␶v = 0.50 and g = 0.57 for large granules). The same values for the glazing unit with air in the gap were U = 2.86 W/m2 K, ␶v = 0.81, and g = 0.75, therefore the systems with small and large granules allowed improved thermal insulation performance having a reduction in heat losses of about 63% and 58% respectively. Ihara et al. [15] investigated the thermal properties of glazing systems assembled with two types of aerogel granules. Small granules (diameter 0.7–1.2 mm) and large granules (diameter 1.2–4.0 mm) were used, in order to verify the tendency (highlighted in other studies [26]) of the granules of different size to produce some sort of line pattern (as clouds), due to their biased distribution. Nevertheless in this study it was not observed and the appearance of the tested samples was quite uniform. The samples were assembled with 6-mm-thick heat strengthened glasses, with 30 and 60 mm gap thickness filled with aerogel granules. The Uvalues were measured by means of hot box tests with various tilt angles, in order to evaluate the influence of the convection on the thermal performance. Results showed that convection in the granular cavity does not affect the U-values, which are approximately the same (within the measurement uncertainty) for vertical and horizontal position of the glazing system. The center-of-glass U values were about 0.52 and 0.28 W/m2 K respectively for the samples with 30 and 60 mm thick granular cavities, resulting in a calculated thermal conductivity of the granular layer of about 17–19 W/mK. The influence of the granule size was investigated only in terms of moisture permeance and results showed no significant difference. Sachithanadam and Joshi [19] investigated the effect of granule size on the acoustic properties of silica aerogel composites; the absorption coefficient and the transmission loss were measured by the impedance tube method both for different thick layers of granules (10 and 15 mm) and protein-based silica aerogel composites. Silica aerogel granule sizes varied in the 0.50–3.35 mm range, distributed into six groups of nominal size. Results showed that for the granule layers the absorption coefficient tends to decrease with the increase in granule size and with the increase in layer thickness; the smaller granules tend to be more compact, therefore the higher silica content contributes to the tortuous path of the sound waves to propagate through the nano-pores of the granules. A further investigation on layers of 5 cm thickness was carried out in order to find the peak value of the absorption coefficient; it was equal to 0.86 for the sample with the granules diameter in the 1.00–1.40 mm range and to 0.81 for the one with the granules diameter in the 1.40–2.00 mm range, both at 980 Hz. The same samples showed also the best values of the average transmission loss, equal to 14.8 and 15.4 dB respectively. In the last year the Authors started a systematic study on the influence of the granule size on the thermal and acoustic properties of aerogels. In a preliminary step thermal conductivity and transmission loss of three granule sizes were investigated [20]: large (0.7–4.0 mm), medium (0.7–2.0 mm), and small (0.01–1.2 mm) granues, with densities of 65–70, 70–75, and 80–85 kg/m3 respectively. Results showed that the small granules have the best performance both in terms of thermal and acoustic properties. Depending on the granule size, the thermal conductivity varies in the 19–22 × 10ˆ(-3) W/(mK) range at 10 ◦ C, while a Transmission Loss value of 13 and 19 dB at 6400 Hz were obtained for small granule layers 20 and 40 mm thick respectively. The maximum values of the Transmission Loss of medium and large granules are in the 10–14 dB range.

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The main features of the examined samples are reported in Table 1. The samples were weighed with an electronic precision balance and the density was estimated: the range of values obtained is in agreement with data supplied by the manufacturer. Sample S shows the highest bulk density, in the 80–85 kg/m3 range, while increasing the size of granules, a lower specific weight for the same volume was estimated, due to a greater quantity of voids (bulk density estimated in the 65–75 kg/m3 range). 2.2. Thermal characterization

Fig. 1. The investigated samples for thermal and acoustic characterization.

In the present paper, an extended experimental campaign starting from the preliminary results obtained in [20] was carried out on four types of granular aerogels (named S, M, L1, and L2), different for applications and granule sizes, ranging from small granules (0.01–1.2 mm) to large ones (1–4 mm). In [20] S, M, and L1 were investigated only in terms of thermal conductivity and Transmission Loss. In the present paper a fourth sample, L2, was added and for all the samples the acoustic absorption coefficient was also measured. In particular, thermal conductivity of the four samples was measured at 10 and 23 ◦ C with the Heat Flow Meter method. The acoustic properties (absorption coefficient ␣ and Transmission Loss TL) were measured at normal incidence in a traditional impedance tube (Kundt’s tube), by means of the two (␣) and a four (TL) microphones configuration; five different thicknesses of the granules layer were investigated: 15, 20, 25, 30, and 40 mm. Due to the granular nature of the sample, it was necessary to develop a specific measurement methodology, by fixing the impedance tube in vertical position. Furthermore, considering the interesting properties found for the loose granules, the same were incorporated in to two innovative aerogel-based solutions for energy saving in buildings: a plaster and a translucent polycarbonate panel. A comparison between the thermal and acoustic performance of the aerogel-based samples and the ones of conventional materials was also carried out. Thermal conductivity was compared with the one of inorganic fibrous (glass or rock wool), natural (cork, kenaf or wood fiber), and synthetic materials (expanded polystyrene, extruded polyurethane or polystyrene), while acoustic properties with the ones of rock wool and of an innovative panel (made of basalt fibres).

The thermal properties of the samples were evaluated with Fox 314 HFM apparatus (Heat Flow Meter method) [20,28,29], able to measure the steady-state heat transfer through flat materials, according to ASTM Standard C518 [30] and EN ISO 12667 [31]. The sample was placed between two flat plates controlled to a specified constant temperature (Fig. 2. (a)): thermocouples fixed in the plates measure the temperature drop cross the sample and wireless thermal flux meters (HFMs) embedded in each plate measure the heat flow through the specimen. The thermal conductivity (␭ inW/ (m · K)) was calculated according to Fourier’s law, by measuring the heat flux (q in W/m2 ), the temperature difference across the sample (T in K), and the thickness of the specimen (s in m), in steady-state condition, as: ␭ = (s · q) /T[W/m · K]

(1)

2. Materials and methods

In order to carry out the measurements, a sample holder (Fig. 2. (b)) was realised, with external dimensions of 0.3 × 0.3 m: it was in compliance with ASTM C687-12 [32], which includes granular types such as vermiculite and perlite and pelletized products, comparable to granular aerogel. The box container was built with a wood frame (thickness of the wood borders equal to 5 mm), and two very thin steel plates (total thicknesses of 0.4 mm) used to cover the top and the bottom of the frame. The steel plates had a negligible thermal resistance and they ensured perfectly adherence to the Heat Flow meter plates (hot and cold sides), so that the influence on the total thermal conductivity of the samples could be considered negligible. The box was filled up manually with aerogel granules (Fig. 2(c)) and a thickness of 40 mm was considered: the thickness of the loose-fill materials shall be at least 10 times the mean dimension of grains, according to the reference standard [31]. The specimen thickness was determined by a digital system integrated in the Heat Flow Meter apparatus: a mean value was supplied by the system. The thermal conductivity was measured at mean reference temperatures of 10 ◦ C and 23 ◦ C and the test duration was at least 10 h for each sample. In order to estimate the thermal conductivity, the thickness of the steel plates was subtracted.

2.1. The investigated samples

2.3. Acoustic characterization

Fig. 1 shows the four types of the translucent granular silica aerogel investigated in the paper: material was supplied by Cabot Corporation, USA [27]. The samples differ for granule sizes and for the applications, such as suggested by the Company in its technical sheets: they optimized the nature and the granule size distribution, which affect the scattering and the light transmission. Sample S and sample L2 are employed for incorporation into composites (such as plasters, concretes) or in the production of materials at high thermal performance, as insulation blankets, and they are characterized by diameter of the particles respectively in the 0.01–1.2 mm and 1.2–4.0 mm range (measured by a centesimal caliber). The intermediate grain sizes (sample M and sample L1) are instead specific for insulating daylighting applications, that require maximum light transmission.

The acoustic characterization of granular aerogel was carried out by measuring sound absorption (normal incidence absorption coefficient, ␣) and sound insulation (Transmission Loss, TL) properties in an impedance tube (Kundt’s tube, Brüel & Kjær, model 4206; 1/4 inch microphones Brüel & Kjær, model 4187), using a two (␣) and a four (TL) microphones configuration respectively. The normal incidence absorption coefficient was measured by means of the transfer function method, according to ISO 10534-2 standard [33]. For the absorption coefficient measurements, the environmental parameters of the room (atmospheric pressure, air temperature, and relative humidity) were firstly measured and microphones calibration was performed. Then, after the sample positioning, the sound pressures were measured at the same time in two fixed positions and the transfer function between them was calculated

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Table 1 Characteristics of the examined granular silica aerogel. Sample

Estimated diameter of granules (mm)

Estimated bulk density (kg/m3 )

Sample features

Applications

S

0.01–1.2

80–85

Granules used in the production of plasters or materials with high thermal performance (blankets, concretes).

M

0.7–2.0

70–75

L1

0.7–4.0

65–70

L2

1.2–4.0

65–70

Regular shape. Small particles that form a dust. Regular shape. Uniform particles. Irregular shape. Flat particles. Irregular shape. Voluminous particles.

Granules designed for insulating daylighting applications (maximum light transmission). Granules designed for insulating daylighting applications (maximum light transmission). Granules used in the production of plasters or materials with high thermal performance (blankets, concretes).

Fig. 2. (a) The Heat Flow Meter apparatus; (b) the sample holder; (c) aerogel in the wood box container.

[20,28,29]. The reflection factor (r) was given by Eq. (2) by measuring the transfer function between the two fixed microphone positions (H12 ), the transfer function for the incidence (HI ) and the reflected (HR ) wave, the wavenumbers (K0 ), and the distance between the top of the sample surface and microphone position x1 ; the normal incidence absorption coefficient (␣) was obtained as shown in (3): r = [(H12 − HI )/ (HR − H12 )]exp (2iK0 x1 ) ˛ = 1 − |r|

2

Table 2 Thermal conductivity of the samples: influence of the temperature and of the granule size. Temperature(◦ C)

10 23

Thermal Conductivity (10ˆ(-3) W/(mK)) S

M

L1

L2

18.6 19.6

20.2 21.2

21.4 22.9

22.5 24.4

(2) (3)

Transmission Loss is a key quantification of the acoustic insulation properties of materials and it is related to the sound transmission coefficient (␶) as:



TL = 10 · log 1/



(4)

As for the absorption measurements, the environmental settings of the laboratory were measured before starting each test (they are used in the elaboration by Brüel & Kjær PULSE LabShop) and the microphones were calibrated in order to avoid channels phase displacement errors. The measurements of the Transmission Loss were carried out with the two-load method, by acquiring the sound pressure in four fixed microphone positions [34–36]: two consecutive acquisitions were determined for each sample, by modifying the characteristics of the tube extremity (a reflective and an absorbing material were installed). Cylindrical samples with a diameter of 100 mm (large tube) and 29 mm (small tube) for frequencies in the 100–1600 Hz and in the 400–6400 Hz range respectively were considered for absorption measurements: they were combined with a software to cover the entire range frequencies (100–6400 Hz). Only the large tube (100 mm, 100–1700 Hz) was used for the Transmission Loss measurements, because of the nature of the sample and the difficulty of positioning. The experimental apparatus in the two configurations is shown in Fig. 3 (absorption Fig. 3(a) and TL configurations Fig. 3(b)). For each particle size, 5 different thicknesses were considered (15, 20, 25, 30, and 40 mm), according to the typical thickness used in glazing solutions. At least three measurements for each sample were carried out by filling the tube each time and a mean value of the ␣ and TL was considered.

Due to the granular nature of the sample, a specific measurement methodology was developed (Fig. 4): the impedance tube was fixed in vertical position, and in the Transmission Loss configuration the granules were inserted in sample holders purposely realised (cylindrical steel cable supports closed at the bottom by a thin porous ply), which had a negligible influence on the experimental results, as demonstrated by preliminary tests.

3. Experimental results 3.1. Thermal performance Thermal conductivity results are reported in Table 2 at the different mean reference temperatures (10 ◦ C and 23 ◦ C). Values of ␭ in the 18.6–22.5 × 10ˆ(-3) W/(mK) range at 10 ◦ C were obtained, depending on the particle sizes. As expected, due to the structure of the aerogel, the conduction in the solid silica was very low being the solid fraction less than 10% and the convection in the pores was reduced because of their dimensions (5–20 nm); neverthless the thermal conductivity of the granules is higher than the one of monolithic pane, due to the air in the inter-granules voids. For smaller granules (sample S), a reduction of thermal conductivity of about 17% was obtained with respect to the larger ones (samples L1 and L2), while the medium sizes aerogel (sample M) showed an intermediate behaviour: when the granule sizes decreases, the voids have a smaller size so the heat transfer by convection and radiation is reduced.

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Fig. 3. The Impedance tube: (a) absorption measurements configuration in large (left) and small (right) tube;(b) TL configuration.

Fig. 4. Details of the measurement methodology developed to test the samples.

Considering the influence of the temperature on the thermal conductivity, the ␭-value increases of about 5–8% when the mean temperature increases from 10 ◦ C to 23 ◦ C. 3.2. Acoustic performance The average normal incidence absorption coefficient (␣) trends (combination of the large and the small tube measurements, 100–6400 Hz) for each sample are reported in Fig. 5, depending on

the thickness. As expected, when the thickness increases, the first peak of the absorption curve is moved to lower frequencies for all the samples, according to [37], due to the higher tortuosity for the samples with higher thickness. Moreover, the first peak is higher for the larger particle size (samples L1 and L2, ␣ ∼ = 1 for both), while decreases for S and M samples (equal to 0.86 and 0.88 respectively). The influence of the granule size was evaluated with the same thickness: Fig. 6 shows the ␣-values of the four samples for the thickness of 15 mm, typically used in building applications (simi-

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477

Fig. 5. Normal incidence absorption coefficient of S (a), M (b), L1 (c) and L2 (d) samples for different thicknesses.

Fig. 6. Normal incidence absorption coefficient of the four samples for the thickness of 15 mm.

lar behaviour were found for the other thicknesses). Samples with similar particle size (S and M samples, L1 and L2 samples) are characterized by similar sound absorption performance: in porous granular materials, they highly depend on the interactions between air and grains, which are the biggest responsible of sound energy dissipation. As observed for thermal properties, smaller granules show better performance from the sound absorption perspective (␣ = 1 at about 3200 Hz), whereas the absorption coefficient decreases to about 0.9 at 4200 Hz for the L1 and L2 samples: the more the density, the more the absorption coefficient. In this paper, only the Transmission Loss (TL) levels measured with large tube configuration (100–1700 Hz) are discussed, since

the internal dimensions of the tube (about 100 mm) allowed easier preparation and positioning of the sample. The influence of the thickness was investigated (Fig. 7): an increasing value of the Transmission Loss is observed at a fixed frequency when the thickness of the samples and the surface mass raises, while at the same thickness, TL increases as the frequency grows. These behaviours are in agreement with the Low Mass, that relates sound insulation, frequency and surface mass. For each sample, the TL increases (2–4 dB) with thickness of the bed, according to Literature data [35,36]. Due to the smaller fraction which formed a very fine dust in the sample S, the TL-value is overestimated for the greater thicknesses (25 mm, 30 mm, and 40 mm). Finally, the influence of the particle size is assessed at a thickness of 15 mm (Fig. 8); similar behaviours are found for the other investigated thicknesses (20, 25, 30, and 40 mm). According to the absorption results, the specimens with similar particle size have the same trend of TL. Samples S and M are characterized by the best sound insulation properties (TL=7 dB at about 1700 Hz), whereas for L1 and L2 samples the maximum value is about 4 dB. The attenuation increases and the velocity of sound in the smaller granules decreases, because of the decrement of the air volume fraction, as reported in the Literature [14,15]. 4. Discussion 4.1. Thermal performance The thermal performance of the samples was compared to the one of alternative insulating materials, taking into account the thermal conductivity (Table 3): all aerogel granules demonstrated excellent thermal insulation properties (␭=18.6–22.5 × 10ˆ(-

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Fig. 7. Normal incidence Transmission Loss of S (a), M (b), L1 (c) and L2 (d) samples for different thicknesses.

Fig. 8. Normal incidence Transmission Loss of the four samples for the thickness of 15 mm. Table 3 Comparison with some traditional and natural insulating materials in terms of thermal conductivity. Thermal Conductivity ␭ (10ˆ(-3) W/(mK)) Glass wool Rock wool Rigid polyurethane (PUR) Expanded polystyrene (EPS) Extruded polystyrene (XPS) Wood fiber Cork (panels) Kenaf fiber Aerogel granules

40 40 24 35 30 39 41 38 19–23 (at 10 ◦ C, depending on the particle sizes)

3) W/(mK), depending on the particle sizes), better than the ones of many conventional insulating materials, such as inorganic fibrous (glass or rock wool, ␭=40 × 10ˆ(-3) W/(mK)), natural (cork, kenaf or wood fiber, ␭=38–41 × 10ˆ(-3) W/(mK)) or synthetic materials (expanded polystyrene, extruded polyurethane or polystyrene, ␭=24–35 × 10ˆ(-3) W/(mK)) [28]. Moreover, the obtained results are in agreement with the Literature [18]: for glazing systems with silica aerogel in interspace (14 mm thickness), the reduction in heat losses (U-value) was about 58% for large aerogel granules (particle size 3–5 mm) and it increases up to about 63% for the smaller ones (particle size <0.5 mm) with respect to a conventional double glazing. At the same time, when compared to conventional solutions, small granules show a significantly lower light transmittance (- 81%) with respect to large ones (- 38%). Finally, the thermal conductivity depends on the compaction of bed silica aerogel small granules [14] decreasing from 24 × 10ˆ(-3) W/(mK) (bulk density 68 kg/m3 , uncompressed bed) to 13 × 10ˆ(-3) W/(mK) (bulk density 150–165 kg/m3 , compressive strain of about 55%-59%), approaching to the performance of monolithic aerogel. For these reasons, a further experimental campaign should be carried out, by measuring thermal conductivity with the different filling of the wood frame. 4.2. Acoustic performance In order to easier compare the acoustic properties of the tested samples, three quantitative indexes were calculated for each granule size and thickness (Table 4). The Noise Reduction Coefficient NRC is a mathematically average, rounded to the nearest multiple of 0.05, of the absorption coefficient values in the one octave

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479

Fig. 9. Comparison aerogel samples (30 mm) with rock wool (30 mm) and basalt fiber (27 mm): (a) Absorption coefficient; (b) Transmission Loss.

Table 4 ␣average , NRC, and SAA for the tested samples. Sample

15 mm

S M L1 L2

20 mm

25 mm

30 mm

40 mm

␣med

NRC

SAA

␣med

NRC

SAA

␣med

NRC

SAA

␣med

NRC

SAA

␣med

NRC

SAA

0.62 0.61 0.52 0.49

0.27 0.23 0.14 0.13

0.27 0.24 0.14 0.14

0.64 0.63 0.53 0.49

0.40 0.36 0.22 0.20

0.39 0.35 0.23 0.21

0.69 0.66 0.55 0.51

0.48 0.46 0.35 0.33

0.47 0.45 0.34 0.32

0.69 0.70 0.62 0.58

0.52 0.51 0.42 0.40

0.52 0.50 0.39 0.38

0.69 0.70 0.66 0.65

0.58 0.57 0.47 0.44

0.58 0.57 0.48 0.45

band at the frequencies 250, 500, 1000, and 2000 Hz. It represents the sound energy absorbed by surface when it is achieved by a sound wave. SAA (Sound Absorption Average) index, defined in the ASTM C423-09A Standard [38], is a number rating of sound absorption properties at the twelve 1/3 octave bands from 200 Hz to 2500 Hz and rounded to the nearest multiple of 0.01. Also, the average values of the coefficient of absorption (␣med ) is defined as the area under the absorption curves normalized over the frequency range and determined as shown in equation (5), where f1 is the lower frequency at 100 Hz and f2 is the upper frequency at 6400 Hz, according to [19]:



␣med = 1/ (f2 − f1 )



f2 ␣ (f) df

(5)

f1

The acoustic performance of the investigated insulating materials was compared to the one of a traditional inorganic fibrous material (rock wool) and of a new technology panel (basalt fiber panels), which were investigated by using the same experimental facility and methodology described in Section 2 (Material and methods) [28]. In the comparison, a conventional rock wool panel 3 cm thick and 95 kg/m3 density and a basalt fiber panel (200 kg/m3 , 27 mm thickness) were considered. The sound absorption properties of the aerogel granules (considering 30 mm thickness) are comparable to the ones of the rock wool (Fig. 9(a)) as peak value and frequencies (␣ = 0.95 at about 1800 Hz), while the absorption coefficient of the basalt fiber panel is lower, also due to the lower thickness, equal to 27 mm. At medium-high frequencies (2200–5500 Hz), ␣-value of rock wool is higher than granular silica aerogel: as expected, a porous materials assume absorption values constant after the pick, whereas granular materials have a typical sinusoidal behaviour. However, NRC-values for smaller granules (thick 30 mm) are about equal to the ones of rock wool and basalt fiber, while NRC for L1 and L2 is about 24%–18% lower (Fig. 10). The experimental results are in agreement with Literature data of a natural fiber, with similar conformation of aerogel samples, constituted by granules with interconnected cavities: a cork sample

Fig. 10. NRC values: comparison between conventional materials (rock wool and basalt fiber panel) and the tested samples.

(30 mm thickness) shows a sinusoidal behaviour of the absorption coefficient with a pick of 0.9 at about 1600 Hz [39]. Considering the Transmission Loss measurements, similar results are obtained (Fig. 9(b)): sound insulation properties of rock wool are better than the ones of L1 and L2 samples (TL is about 2 dB less with respect to the opaque material). TL-values obtained for the M granule samples are comparable to those of rock wool, whereas for the smaller granules (sample S), TL is 5–6 dB higher and comparable to the one of the basalt fiber panel. Furthermore, it is important to notice that the density of rock wool (95 kg/m3 ) is greater than the one of aerogel: it is comparable to the one of small granules (85 kg/m3 ), which have higher TL. 5. Building applications Due to their interesting properties, aerogel granules were applied in some innovative commercial solutions for energy saving in buildings: aerogel-incorporated concrete and aerogel based renders have been developed for building refurbishment. Moreover,

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Fig. 11. Products for building applications: (a) conventional plaster; (b) aerogel plaster; (c) polycarbonate; (d) polycarbonate with aerogel.

Fig. 12. Comparison of normal incidence absorption coefficient of a conventional and an aerogel-based plasters.

due to the very low thermal conductivity and good light transmittance, highly energy-efficient translucent windows with granular silica aerogel in interspace have been appearing in the market. Two innovative aerogel-based solutions for energy saving in buildings (a plaster and a translucent polycarbonate panel) were therefore investigated in terms of acoustic and thermal performance and the results were compared to those of conventional solutions (Fig. 11). The considered aerogel plaster was developed in a previous work [40]: it is manufactured by manually mixing natural calk with granular aerogel (large granules, L2) in different percentage, allowing the absorption of air in the mix. Previous thermal investigations by means of the heat flow meter apparatus [40,41] showed that, with respect to conventional plasters, the thermal conductivity decreased of 90%–97% (depending on the % in volume of aerogel), allowing ␭ = 0.05 W/mK at about 80% in volume and a decreasing in density up to −95%. Moreover, with respect to traditional solutions (␭ = 0.29–0.7 W/mK), excellent in-situ thermal performance was obtained: depending on the building wall considered, thermal transmittance was reduced up to 20% when 5 mm of natural plaster with granular aerogel was applied. In the present paper, the acoustic properties of this solution were investigated considering a plaster with about 80–90% in volume of aerogel (␳ = 300–275 kg/m3 , ␭ = 0.05–0.045 W/mK). The sound absorption coefficient at normal incidence was measured for a sample of 28 mm thickness (Fig. 11. (b)) and it was compared to a conventional plaster (thickness 30 mm, Fig. 11. (a)). The aerogel-based plaster shows better acoustic performance: the peak of absorption is 0.29 at about 1100 Hz, compared to a value of about 0.1 for conventional plasters, and NRC-values is about three times higher (Fig. 12). Finally, a multi-sheet polycarbonate panel was investigated: it can be considered as an advanced glazing solution, especially for non-residential buildings, due to its excellent intrinsic properties (high transparency, low weight, and competitive costs) with

Fig. 13. Normal incidence Transmission Loss of a polycarbonate: the influence of aerogel in interspace.

the ones of aerogel (thermal and acoustic insulation, good light transmittance) [17]. A multi-sheet polycarbonate panel (thickness 16 mm) with granular aerogel in interspace (Large granules, L1 sample) was supplied by RODA, Germany [42] and a cylindrical sample with a diameter of 100 mm was made (Fig. 11(c)). The sound insulation properties of this sample were investigated in the Impedance Tube (paragraph 2.3) (Fig. 13), considering also the sample with air in the interspace in order to perform a comparison. When considering aerogel in interspace (Fig. 11(d)), the TL increases at about 3–5 dB in the 280–1200 Hz frequency range, while a negligible difference is observed when the frequency raises (about 1–2 dB). These results are in agreement with the ones provided by the manufacturer [42]: the sound insulation index increases up to 3 dB and thermal transmittance significantly decreases (- 45%); however, at the same time, the light transmittance is reduced of about 20% only. The results are also confirmed by in-lab studies and in-field investigations on aerogel windows [2,12]. 6. Conclusion Granular silica aerogel is one of the most promising nanomaterials for energy saving in buildings and some aerogel-based solutions for building refurbishments have been appearing in the market. In general, thermal and optical performance of silica aerogels are deeply discussed in the Literature, whereas the investigation of acoustic properties is still limited. In the present paper thermal and acoustic properties of silica granular aerogels were investigated, taking into account the influence of granule size (from small, 0.01-1.2 mm, to large granules, 1–4 mm) and the density of the granule bed on the material performance. In order to contain the granules during the experimental campaign, a proper methodology was set up, adjusting the proce-

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dures typically used for solid materials: for thermal campaign with Hot Flow Meter method a wood frame was used, whereas cylindrical steel cable supports closed at the bottom by a thin porous ply allowed the acoustic characterization in a Kundt’s tube. The small granules, which have the highest density (80–85 kg/m3 ), have the best performance both in terms of thermal and acoustic properties. Depending on the granule size, the thermal conductivity at 10 ◦ C varies in the 19–23 × 10ˆ(-3) W/(mK) range and smaller granules showed a thermal conductivity reduction of about 17% with respect to the larger ones. A very good acoustic performance was in general achieved: the best sound insulation properties are observed for the smaller granules (TL=17 dB at about 1700 Hz for 40 mm thickness), whereas for larger granules the maximum values are in the 6–7 dB range. The normal incidence absorption coefficient values followed a sinusoidal behaviour typical of granular materials, with a peak of about 1 at a frequency which diminishes when the granule size diminishes. The excellent thermal and acoustic properties of silica aerogels are evident, especially when compared to other conventional materials: a bed of 30 mm of aerogel of the smaller size shows a better acoustic behaviour than a same thickness of rock wool, with an increasing in the Transmission Loss of about 5–6 dB. Moreover, the excellent acoustic properties of the smaller granules are particularly evident also considering a new technology as basalt fiber panels. Based on the experimental results, a great interest in aerogel is expected in the near future, also due to the decreasing cost, especially for the building refurbishment sector, as superinsulation opaque aerogel-based panels. The experimental characterization of an aerogel-based plaster (80–90% in volume of aerogel) shows a peak of the absorption coefficient equal to 0.29 at about 1050 Hz, compared to a value of about 0.1 of conventional plasters; simultaneously, ␭ diminished from 0.7 W/mK to 0.05 W/mK. Finally, aerogels have the advantage of being translucent, therefore they can be assembled in glazing systems, such as polycarbonate or other glazing solutions, allowing better acoustic and thermal performance together with a moderate reduction in light transmittance. Acknowledgements Authors wish to thank Cabot Corporation for supplying the aerogel samples and Fabrizio Agosti, dr. Andrea Maria Piermatti and dr. Paolo Fiumara for their precious contribution during the experimental campaign. References [1] M. Koebel, A. Rigacci, P. Achard, Aerogel-based thermal superinsulation: an overview, J. Sol-Gel Sci. Technol. 63 (2012) 315–339. [2] C. Buratti, E. Moretti, Experimental performance evaluation of aerogel glazing systems, Appl. Energ. 97 (2012) 430–437. [3] C. Buratti, E. Moretti, Glazing systems with silica aerogel for energy savings in buildings, Appl. Energ. 98 (2012) 396–403. [4] Md. M.H. Bhuiya, A.M. Anderson, M.K. Carrol, B.A. Bruno, J.L. Ventrella, B. Silbermann, B. Keramati, Preparation of monolithic silica aerogel for fenestration applications: scaling up, reducing cycle time, and improving performance, Ind. Eng. Chem. Res. 55 (2016) 6971–6981. [5] S. Spagnol, B. Lartigue, A. Trombe, V. Gibiat, Modeling of thermal conduction in granular silica aerogels, J. Sol-Gel Sci. Technol. 48 (2008) 40–46. [6] C. Buratti, E. Moretti, Lighting and energetic characteristics of transparent insulating materials: experimental data and calculation, Indoor Built Environ. 20 (4) (2011) 400–401. [7] R. Baetens, B.P. Jelle, A. Gustavsen, Aerogel insulation for building applications: a state-of-the-art review, Energy Build. 43 (2011) 761–769. [8] S.B. Riffat, G. Qiu, A review of state-of-the-art aerogel applications in buildings, Int. J. Low-Carbon Technol. 8 (1) (2012) 1–6. [9] C. Buratti, E. Moretti, Silica nanogel for energy-efficient windows, in: F. Pacheco Torgal, M.V. Diamanti, A. Nazari, C.G. Granqvist (Eds.), Nanotechnology in Eco-efficient Construction, Woodhead Publishing Limited, Cambridge, 2013, pp. 207–235.

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