Aerogel Composite from fly ash and application as thermal insulation material

Hóa học – Sinh học – Môi trường P. Q. Nghiep, L. A. Kien, “Aerogel composite from fly ash thermal insulation material.” 408 AEROGEL COMPOSITE FROM FLY ASH AND APPLICATION AS THERMAL INSULATION MATERIAL Pham Quoc Nghiep*, Le Anh Kien Abstract: Aerogel Composite from fly ash is an environmentally friendly material and applcation as an thermal insulating material. The synthesis of aerogel composite from fly ash using cross linked technique and freeze-drying method. The general condition of the material to be investigated factors such as fly ash concentration and adhesive, assimilation time, reaction temperature, drying condition. Physical properties such as porosity, density, thermal conductivity and mechanical property were determined from the completely dried samples. As a result, this study has synthesized materials with porosity up to 90 %, density less than 0.5 g/cm3, thermal conductivity less than 0.1 W/(m.K). Therefore, composite aerogel from fly ash is a lightweight, highly insulating material. Keywords: Aerogel Composite; Fly Ash; Thermal Insulating Material; Lightweight Material; Freeze-drying. 1. INTRODUCTION An aerogel is a highly porous solid that holds gas inside the porosity of its solid network. They are renowned for their low densities (0.003 to 0.5 g/cm3) and high porosities (70-99.8%) [1-5]. Kistler et al. [6] invented the first aerogel in 1931, which was a relatively transparent silica aerogel. In the same year, they also prepared a variety of different aerogels such as introducing other mediums such as alumina, nickel tartarate, stannic oxide, tungstic oxide, gelatine, agar, nitrocellulose, cellulose, and egg albumin [6]. Their aerogel inventions concerned both the organic and inorganic aspects of the solid substance. In later years, Kistler [7] also initialized the fabrication of aerogels at industrial scale. In addition, aerogels such as carbon aerogels, silica aerogels, and cellulose aerogels have served as absorbents for oils and other organic pollutants [8] . One of the famous applications of aerogels is thermal insulation, as aerogels with thermal conductivity of 0.0089–0.05 W/m.K are among the best known thermal insulation materials [9-11]. Cohen et al. [11] reported a silica aerogel, fabricated by using separated catalysts for hydrolysis, condensation, and gelation steps, with a thermal conductivity of 0.0089 W/m.K under ambient conditions. In contrast, air has a thermal conductivity of around 0.025 W/m.K, which also implies that the low thermal conductivity of the silica aerogel is not the result of the high porosity [12-13] . The low thermal conductivity of silica aerogels can be explained by the Knudsen effect, where the pore size of a material is smaller than the mean free path of air. This results in the material’s thermal conductivity being dramatically reduced [14] . 2. MATERIAL AND METHODS 2.1. Materials and methods The coal fly ash powder was collected from Duyen Hai factory in Vietnam with a specific chemical composition as shown below in table 1. Polyvinyl alcohol (PVA, MW 89,000~98,000g/mol). All the reagents used without further purification. Deionised (DI) water was used for fabrication of samples as well. The reaction solution was prepared by dissolving Polyvinyl Alcohol (PVA) powder into warm deionised (DI) water of 100ml using a magnetic stirrer, at temperature below 80 °C. Following which, CFA was added into the solution with a desired ratio and stirred for 3 hours at 80 °C to facilitate the cross-linking reaction. Then, the mixture is frozen in the refrigerator overnight before thawing and forming a gel at 60 °C for 30 minutes. The mixture was then homogenised by stirring it at room temperature for 2 hours and then Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san Hội thảo Quốc gia FEE, 10 - 2020 409 frozen to form a uniform distribution. Lastly, the frozen sample was then placed in the freeze dyer for 48 hours to remove all the solvent in order to produce a CFA aerogel. The base case ratio is set at 2.5/2 wt% as with consistent testing, a ratio of 2.5/2 wt% is able to produce the most stable aerogel in terms of quality. Following which, ratios of PVA or CFA were altered in creation of subsequent aerogel samples for comparison and determination of the best CFA/PVA ratio. Table 1. Chemical composition of a fly ash, Duyen Hai factory, Vietnam. Chemical Composition Content (wt%) SiO2 55.8 Al2O3 25.8 Fe2O3 7.4 MgO 1.3 CaO 1.1 SO3 0.1 K2O 4.3 Na2O 0.4 TiO3 0.8 C < 3 Total 100 2.2. Characterization of structure and morphology The morphology of the CFA aerogel samples were investigated using a Field Emission Scanning Electron Microscopy (FESEM, Model S-4800 Hitachi, Japan). The density of the CFA aerogels were determined via the measurements of the weight and volume of the samples using a micro weighing scale and a vernier caliper. Porosity Φ (%), is the measure of void spaces in a material and commonly expressed as a percentage of the total volume. Thermal gravimetric analysis (TGA) measures the mass of a sample as a function of sample temperature or time (TA Instruments Q500 Thermogravimetric Analyzer). The C- Thern Tci Thermal Conductivity Analyzer (C-Therm Technologies, Canada) was used to provided highly accurate thermal characteristics of the CFA aerogels’ thermal conductivities. Mechanical properties of CFA aerogels samples can be examined via compressive testing on an Instron 5500 microtester (USA), according to ASTM D1621-10. 3. RESULTS AND DISCUSSION 3.1. Morphologies and structures of CFA aerogel The morphology and structure of the CFA aerogel is displayed in figure 1. Further, samples of the CFA aerogel has an open porous network structure with the CFA powder distributing uniformly in the PVA matrix, figure 1f. This indicates that the CFA powder had successfully self-assembled to form a three-dimensional porous network. 3.1.1. Effects of Fly ash powder concentration As show in table 2 below, the morphology change and control of the CFA aerogels were successfully achieved by changing the fly ash powder concentration. The results suggest that increasing fly ash concentration from 1.25 to 5 wt% leads to an increased density in the structure. CFA contributes a significant amount towards the size of the sample’s density, which shows that an increase in concentration leads to a rise in density. Hóa học – Sinh học – Môi trường P. Q. Nghiep, L. A. Kien, “Aerogel composite from fly ash thermal insulation material.” 410 Table 2. Increasing FA concentration in comparison with density and porosity. FA/PVA Ratio (wt%) Density, ρa (g/cm3) Porosity, Φ (%) 1.25/2 0.103±0.002 87.5±0.5 2.5/2 0.118±0.003 85.5±0.7 5/2 0.188±0.003 76.7±0.8 (a) (b) (c) (d) (e) (f) Figure 1. Morphology of fly ash aerogel: (a) FA-PVA 1.25-2; (b) FA-PVA 2.5-2; (c) FA-PVA 5-2; (d) FA-PVA 2.5-1; (e) FA-PVA 2.5-4; (f) Appearance of fly ash aerogel. 3.1.2. Effects of Polyvinyl alcohol concentration The increase of PVA content from 1 to 4 wt% also leads to increasing density of the aerogels (table 3). This increase is due to the increase in the concentration of PVA, holding the fly ash powder concentration constant. The pore structure of the aerogel decreases with the increase of PVA content, shown by the decrease of porosity from 90.4 to 82.4 %. Table 3. Increasing PVA concentration in comparison with density and porosity. FA/PVA Ratio Density, ρa (g/cm3) Porosity, Φ (%) 2.5/1 0.100±0.002 90.4±0.5 2.5/2 0.118±0.003 85.5±0.7 2.5/4 0.145±0.003 82.4±0.5 3.2. Thermal Gravimetric Analysis(TGA) of CFA aerogel The results are displayed in figure 2 below. Figure 2. Thermal stability of CFA aerogel with different concentration of (a) fly ash powder holding 2 wt% PVA and (b) PVA holding 2.5 wt% fly ash powder. Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san Hội thảo Quốc gia FEE, 10 - 2020 411 In figure 2a, the increase of fly ash concentration in the aerogel leads to decrease in percentage of weight lost that has decomposed with rising temperature. The aerogel with 5 wt% fly ash and 2 wt% PVA show the lowest weight loss of about 35 wt% at 1000 °C. In figure 2b, the increase of PVA concentration in the aerogel leads to increase in percentage of weight lost that has decomposed with rising temperature as well. The aerogel with 2.5 wt% fly ash and 1 wt% PVA show the lowest weight loss of about 35 wt% at 1000 °C. 3.3. Thermal conductivity of CFA aerogel The measured thermal conductivity of various CFA aerogel samples are listed in table 4 below. Thermal conductivity of CFA aerogel exhibits in the range of 0.034-0.036 W/m.K. The range of the thermal conductivity is competitive and comparable to many commercial insulation materials as mineral wool (0.03-0.04 W/m.K), glass wood (0.03- 0.04 W/m.K) and foam glass (0.04-0.05 W/m.K), shown below in table 4 [15, 16]. Nevertheless, the low thermal conductivity and eco-friendliness of the CFA aerogel makes it a promising material for future thermal-insulation applications. Table 4. Thermal conductivity of CFA aerogel. FA/PVA Thermal conductivity, K (W/m.K) FAA1.25/2 0.036±0.001 FAA2.5/2 0.035±0.001 FAA5/2 0.036±0.001 FAA2.5/1 0.034±0.001 FAA2.5/4 0.034±0.001 3.4. Mechanical properties of CFA aerogel The compressive strain–stress curves of the fabricated CFA aerogels are displayed in figure 3 below. For figure 3a, when keeping the PVA concentration constant, the increase of CFA concentration (1.25 to 5 wt%) leads to increase in hardness of the aerogels, shown by the rise in compressive young’s modulus from 49.5 kPa to 152.7 kPa. This is due to the observation explained where rise in CFA concentration results in compactness and eventually, increased hardness of the CFA aerogel samples. Similarly, for figure 3b, the increase of PVA concentration (1 to 4 wt%) leads to increase in hardness of the aerogels, show by the rise in compressive young’s modulus from 22.1 to 153.5 kPa. Figure 3. Mechanical properties of CFA aerogel with different concentration of (a) fly ash powder holding 2 wt.% PVA and (b) PVA holding 2.5 wt.% fly ash powder. 4. CONCLUSION In conclusion, CFA aerogels were successfully developed from recycled coal fly ash powder using a freeze-drying process followed by MTMS coating. Investigations on the various properties of the CFA aerogels were carried out on samples with varying coal fly Hóa học – Sinh học – Môi trường P. Q. Nghiep, L. A. Kien, “Aerogel composite from fly ash thermal insulation material.” 412 ash or polyvinyl alcohol concentration. The CFA aerogels demonstrated low densities (0.1-0.188 g/cm3), high porosity (76.7-90.4 %), low thermal conductivities (0.035-0.036 W/m.K) and high elasticities with low Young’s moduli (~150 kPa). In addition, they exhibited decent performance in its acoustic absorption and thermogravimetric analysis, sustaining thermal degradation of up to ~500 oC. These results showed promising thermal which can be used for a variety of potential applications in different sectors. REFERENCES [1]. Dorcheh, A.S.; Abbasi, M. “Silica aerogel; synthesis, properties and characterization”. J. Mater. Process. Technol. Vol 199, 10–26, (2008). [2]. Herrmann, G., Iden, R., Mielke, M., Teich, F., & Ziegler, B. “On the way to commercial production of silica aerogel”. Journal of non- crystalline solids, 186, 380-387, (1995). [3]. Dorcheh, A. S., & Abbasi, M. H. “Silica aerogel; synthesis, properties and characterization”. Journal of materials processing technology, 199(1-3), 10-26, (2008). [4]. Schmidt, M., & Schwertfeger, F. “Applications for silica aerogel products”. Journal of non-crystalline solids, 225, 364-368, (1998). [5]. Cai, J., Liu, S., Feng, J., Kimura, S., Wada, M., Kuga, S., & Zhang, L. “Cellulose– silica nanocomposite aerogels by in situ formation of silica in cellulose gel”. Angewandte Chemie, 124(9), 2118-2121, (2012). [6]. S. S. Kistler, “Coherent expanded aerogels and jellies”, Nature, 127, 741, (1931). [7]. S. S. Kistler, “Treatment of aerogels to render them waterproo”f, U. S. Pat. No. 2, 589, 705, (1952). [8]. Z. Y. Wu, C. Li, H. W. Liang, Y. N. Zhang, X. Wang, J. F. Chen, S. H. Yu, “Carbon nanofiber aerogels for emergent cleanup of oil spillage and chemical leakage under harsh conditions”, Sci. Rep., 4, 4079, (2014). [9]. B. E. Yoldas, M. J. Annen, J. Bostaph, “Chemical engineering of aerogel morphology formed under nonsupercritical conditions for thermal insulation”, Chem. Mater., 12, 2475-2484, (2000). [10]. J. Cai, S. Liu, J. Feng, S. Kimura, M. Wada, S. Kuga, L. Zhang, “Cellulose-Silica Nanocomposite Aerogels by In Situ Formation of Silica in Cellulose Gel”, Angew. Chem. Int. Ed., 51, 2076-2079, (2012). [11]. E. Cohen, L. Glicksman, “Thermal Properties of Silica Aerogel Formula”, J. Heat Transfer, 137, 081601-081608, (2015). [12]. A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C. N. Lau, “Superior thermal conductivity of single-layer graphene”, Nano Lett., 8, 902- 907, (2008). [13]. A.-M. Tang, Y.-J. Cui, T.-T. Le, “A study on the thermal conductivity of compacted bentonites”, Appl. Clay Sci., 41, 181-189, (2008). [14]. S. Kistler, A. G. Caldwell, “Thermal conductivity of silica aerogel”, Ind. Eng. Chem., 26, 658-662, (1934). [15]. Cabeza, L. F., Castell, A., Medrano, M., Martorell, I., Pérez, G., & Fernández, I. “Experimental study on the performance of insulation materials in Mediterranean construction”. Energy and Buildings, 42(5), 630-636, (2010). [16]. Sequeira, S., Evtuguin, D. V., & Portugal, I. “Preparation and properties of cellulose/silica hybrid composites”. Polymer composites, 30(9), 1275-1282, (2009). Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san Hội thảo Quốc gia FEE, 10 - 2020 413 TÓM TẮT AEROGEL COMPOSITE TỪ TRO BAY ỨNG DỤNG LÀM VẬT LIỆU CÁCH NHIỆT Aerogel composite từ tro bay là một vật liệu thân thiện với môi trường được ứng dụng làm vật liệu cách nhiệt. Việc tổng hợp aerogel composite từ tro bay bằng kỹ thuật liên kết ngang và phương pháp đông khô. Điều kiện tổng hợp vật liệu nghiên cứu các yếu tố như nồng độ và chất kết dính tro bay, thời gian đồng hóa, nhiệt độ phản ứng, điều kiện sấy. Các tính chất vật lý của vật liệu như độ xốp, mật độ, độ dẫn nhiệt và tính chất cơ học được xác định từ các mẫu khô hoàn toàn. Kết quả là nghiên cứu này đã tổng hợp các vật liệu có độ xốp lên tới 90%, mật độ nhỏ hơn 0,5g/cm3, độ dẫn nhiệt nhỏ hơn 0,1 W/(m.K). Do đó, aerogel composite từ tro bay là vật liệu nhẹ, có khả năng cách nhiệt cao. Từ khóa: Aerogel Composite; Tro bay; Vật liệu cách nhiệt; Vật liệu nhẹ; Sấy đông khô. Received 29th July 2020 Revised 5th October 2020 Published 5th October 2020 Địa chỉ: Viện Nhiệt đới môi trường/Viện Khoa học và Công nghệ quân sự. *Email: pqnghiep1354@gmail.com.