UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

618 IMMC 2018 | 19th International Metallurgy & Materials Congress

Preparation of Silica Aerogel by Ambient Pressure Drying Process Using Diatomite Powder

Reyhan Arslan, Görey Eğribel, Emre Mudam, Nil Toplan

Sakarya University, Engineering Faculty, Metallurgical and Materials Engineering Departmant, Esentepe Campus, 54186 Sakarya, Turkey

Abstract

Silica aerogel is synthesized by atmospheric pressure drying using natural silica source raw materials such as diatomite. Diatomite refers to the light-colored sedimentary rock that is composed of the remains of one-celled algae known as diatoms. Diatomite is highly siliceous, has a low specific gravity, and is very porous. Due to the porous structure and the unique microstructure consisting of nanometer sized particles, there is a striking number of application areas of silica aerogels. The produced aerogels are dried sol-gel materials which prevent pore collapse. The silica source powder was dissolved by boiling in the sodium hydroxide solution for a specified time. The pH of the solution was adjusted to 6-7 by neutralizing with 1M H2SO4 acid solutions prepared. The pH neutralized solution was allowed to age at room temperature after being filtered. Surface modification and drying are the most critical steps in sol-gel production. The bonds in the structure are strengthened and the material strength is increased by surface modification with TEOS (tetraethylorthosilicate). The aging process was supported by treatment of gels with solutions prepared using ethanol and TEOS chemicals at 50 � for 1 day. The gel was washed several times with n-heptane to remove the ethanol/TEOS solution. Solvent exchange with n-heptane was performed before drying in atmospheric pressure and finally the silica aerogel powders were dried at 90°C for 1 day at atmospheric pressure. The obtained silica aerogel powders were analyzed with SEM, FESEM-EDS, XRD, FTIR devices and the results were discussed.

1. Introduction

The aerogels are the unique materials with low density, high porosity and specific surface area. Aerogels can be prepared from any oxides and their mixtures. At the moment the SiO2-basedaerogels are well-studied materials. The synthesis routes and properties of silica aerogels, as well as the area of their potential applications can be found in many reviews and original papers [1].

Silica aerogels are the lightest and extremely porous manmade solids ever known. Because of their fascinating properties the aerogels find potential applications in superthermal insulators, catalyst supports and dielectric materials. Aerogels are usually prepared by supercritical drying of wet silica gels. Supercritical drying process can avoid capillary stress and associated drying shrinkage, which are usually prerequisite of obtaining aerogel structure. However, supercritical drying process is so energy intensive and dangerous that real practice and commercialization of the process is difficult. An alternative cost-effective process is very important for commercial success of the aerogel. The ambient pressure drying technique is one of the alternative cost-effective processes of aerogel synthesis. In order to obtain highly porous aerogel structure, elimination of capillary stress during ambient pressure drying is very important. Liquid evaporation from wet gel during drying creates a capillary tension and that tension is balanced by the compressive stress on the solid network, causing shrinkage of the gel. Strengthening of gel network, surface modification and solvent exchange of the wet gel are necessary to suppress such type of capillary tension and shrinkage during ambient pressure drying. The gel should be aged in silane solution to increase the strength and stiffness of it. Finally, pore liquid must be replaced by low surface tension solvent to reduce capillary stress and associated drying shrinkage. The conventional method of silica aerogel preparation is sol-gel process using organic silicon monomer as precursors. However, such organic precursors are so expensive that aerogel production in an industrial scale is not economically viable [2-9]. In the present study, we report upon our efforts to synthesize silica aerogel through ambient pressure drying using diatomite as the silica source.

2. Experimental

Diatomite was mixed with 250 ml 1 mol.l–1NaOH aqueous solution. The mixture was refluxed for 2 h. Most of the diatomite was dissolved in NaOH solution. The solution was

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

61919. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

filtered to remove the undissolved residues. The sodium silicate solution was neutralized using 250 ml 1 mol.l–1 H2SO4 solutions to form silica gel at pH 7. The prepared gel was aged at room temperature for 24 h under sealed condition. The aged gel was washed using de-ionized water to remove Na and Cl. Subsequently, the silica gel was soaked in a solution of 20 vol% H2O/ethanolfor 24 h at 50oC followed by aging with ethanol at same condition. The ethanol treated gel was aged in a solution of 70 vol% TEOS/ethanol for 24 h at 70oC. Finally, modified gels were aged for another 24 h inside n-heptane at room temperature before air drying. The gel was dried in 48 h interval at 90-120oC for H2SO4 with partially covered condition. Fourier transform infrared spectroscopy (FTIR, Spectrum RX-1, Perkin Elmer) was employed to investigate the chemical bonding state of surface modifying agent with aerogels.

3. Results and Discussion

The morphology of the silica aerogels was characterized by scanning electron microscopy (SEM) and field emission scanning electron microscope (FESEM). The silica aerogel exhibited a pearl necklace morphology and a three-dimensional network which was constructed by nanometer-sized silica particles.

(a) (b) Fig. 1. SEM photographs of produced silica aerogels powders

(a)

Element a . % O 28.17 Na 1.20 Si 69.94 Cl 0.68

Fig. 2. FESEM photographs and EDS analysis of silica aerogel powders

Fig. 1 shows SEM photographs of produced silica aerogels powder and Fig. 2 shows FESEM photographs and EDS analysis of silica aerogel powders. The extent of Na removal was measured by analyzing Na-content of dried gel using FESEM-EDS. The sodium content of the dry gel was about 1.2% as shown in Fig. 2. Particle size of powders changed between about 10-30 nm.

The strength and stiffness of the wet silica gel was enhanced by aging it in TEOS/ethanol solution. Prior to this aging, the gel was thoroughly washed with ethanol. The strength and stiffness of gel may also increase by ethanol washing due to dissolution of silica from the particles and reprecipitation into the necks between the particles. However, it has been reported that the increase in strength and stiffness by such washing was not sufficient to avoid shrinkage during drying and hence, further aging in TEOS solution

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

620 IMMC 2018 | 19th International Metallurgy & Materials Congress

was necessary. TEOS aging causes silica to precipitate from the aging solution onto the silica network. Precipitation of silica gives an increase in the density of the wet gel and corresponding strengthening and stiffening of the gel network. During aging hydrolysis of TEOS and condensation of silica gel occurs [2].

This shows that Figure 3 shows FTIR spectra of the aerogel heated at different temperatures. There are characteristic bands of silica aerogel at ~1098, ~804 and ~471 cm–1. The ~1098 cm–1 band is associated with the Si-O-Si asymmetric bond stretching vibration while the band at ~804 cm–1is assigned to a network Si-O-Si symmetric bond stretching vibration. The bands at ~471 cm–1 are associated with a network Si-O-Si bond bending vibration.

(a)

(b)Fig. 3. FTIR spectra of a. Diatomite and b. SiO2aerogel powder

The phase purity and crystal structures of diatomite raw powder and produced silica based aerogel powder been determined by XRD and the obtained results were shown in Fig. 4 a-b. It is found that powders exist an obvious diffraction peak at the position of 20 –30 . This indicated that both raw powder and produced aerogel powder were amorphous structure.

Fig. 4. a. Diatomite and b. silica aerogel powder

4. Conclusions

The present paper demonstrated a cost-effective process for the production of silica aerogel using diatomite precursor via ambient pressure drying. The surface modification and strengthening of wet gel was obtained by aging it in TEOS/ethanol solution. Low surface tension liquid n-heptane was used to suppress capillary stresses and associated shrinkage during ambient pressure drying of the gel. Using this route, it was possible to obtain the silica aerogel with low density (0.67 g.cm–3). The process of aerogel production from diatomite by ambient pressure drying method is very important from the industrial point of view and it will significantly widen the commercial exploitation of the silica aerogel.

Acknowledgement This work has been supported by Commission for Scientific Research Projects (BAPK) in Sakarya University (FBYLTEZ, project number: 2016-50-01-041).

References

1. SHALYGIN, Anton S., et al. The impact of Si/Al ratio on properties of aluminosilicate aerogels. Microporous and Mesoporous Materials, 2017, 251: 105-113.

2. NAYAK, J. P.; BERA, J. Preparation of silica aerogel by ambient pressure drying process using rice husk ash as raw material. Transactions of the Indian Ceramic Society, 2009, 68.2: 91-94.

3. SHEWALE, Poonam M.; RAO, A. Venkateswara; RAO, A. Parvathy. Effect of different trimethyl silylating agents on the hydrophobic and physical properties of silica aerogels. Applied Surface Science, 2008, 254.21: 6902-6907.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

62119. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

4. RAO, A. Parvathy; RAO, A. Venkateswara; PAJONK, G. M. Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents. Applied surface science, 2007, 253.14: 6032-6040.

5. SHI, Fei; WANG, Lijiu; LIU, Jingxiao. Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process. Materials Letters, 2006, 60.29-30: 3718-3722.

6. KIM, Chul Eui; YOON, Jong Seol; HWANG, Hae Jin. Synthesis of nanoporous silica aerogel by ambient pressure drying. Journal of sol-gel science and technology, 2009, 49.1: 47.

7. CARLSON, G., et al. Aerogel commer-cialization: technology, markets and costs. Journal of non-crystalline solids, 1995, 186: 372-379.

8. THAPLIYAL, Prakash C.; SINGH, Kirti. Aerogels as promising thermal insulating materials: An overview. Journal of materials, 2014, 2014.

9. BANGI, Uzma KH; RAO, A. Venkateswara; RAO, A. Parvathy. A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying. Science and technology of advanced materials, 2008, 9.3: 035006.

10. DUAN, Yannan, et al. Hydrophobic silica aerogels by silylation. Journal of Non-Crystalline Solids, 2016, 437: 26-33.