Characterization of Silica Aerogel Synthesized via Ambient Pressure Drying Process

Transcription

1 Characterization of Silica Aerogel Synthesized via Ambient Pressure Drying Process Imtiaz Ali Soomro a *, Muhammad Moazam Baloch a a Mehran University of Engineering and Technology Jamshoro Sindh Pakistan *Corresponding Author imtiaz.soomro@faculty.muet.edu.pk Abstract Aerogels are highly porous structures prepared via a sol-gel process and supercritical drying technology. Among the classes of aerogels, silica aerogel exhibits the most remarkable physical properties, possessing lower density, thermal conductivity, refractive index, and dielectric constant than any solids. In present work low-density silica-based aerogels were synthesized via ambient pressure drying process. The goal was to develop an easy, fast, reliable, non-toxic method, using inexpensive materials. The recipe incorporated the most recent technique in aerogel manufacturing, replacing tetramethylorthosilicate (TMOS) with tetraethylorthosilicate (TEOS). Both chemicals are precondensed silica precursors, with TEOS being the less toxic of the two. First hydrogel was formed using TEOS as precursor by sol-gel method and then reacted with two organic solvent i.e n-hexane and toluene separately to produce two types of alcogel. The resulting alcogels were then dried at 200oC for 60 minutes replacing the conventional complex supercritical drying method. After drying the liquid inside the alcogel was replaced with air resulting inthe formation of aerogel. The aerogel was formed in the form of beads (large size particles). The effect of two organic compounds on physical and chemical properties of the hexane based (HB) and toluene based (TB) aerogel was evaluated. The microstructure and crystallinity of the prepared aerogels were determined using SEM and XRD respectively. Pore size, pore volume and surface area were determined by BET analyzer. Thermal stability and presence of organic compound in aerogel was determined using TGA and FTIR spectroscopy respectively. Keywords: Aerogel, sol-gel process, ambient pressure drying 1.1 Introduction Aerogel has become a material of interest to scientists in recent decades due to its unique physical and chemical properties that give it the potential to improve technologies in a variety of fields [1]. Silicabased aerogels are the simplest and most widely studied type of aerogel, with new uses and applications arising every day. Silica aerogels are highly porous nanostructured materials prepared via a sol-gel process and supercritical drying technology [2]. Among the classes of aerogels, silica aerogel exhibits the most remarkable physical properties such as high specific surface area ( m 2 /g), high porosity (80 99%), lower density (typically 0.03 g/cm3), high thermal insulation, refractive index, and low dielectric constant (k = ) than any solids [3]. Its acoustical property is such that it can absorb the sound waves reducing speed to 100 m/s compared to 332 m/s for air. The unique combination of thermal and structural properties makes it an attractive material for a variety ofapplications ranging from platforms for supercapacitors, catalytic supports, thermos flasks, Internal Confinement Fusion (ICF) targets for thermonuclear fusion reactions, chemical sensors, thermal insulation, radio luminescent devices, drug delivery media, storage mediafor liquid rocket propellants, and as Cerenkov radiation detector media.(4).the structural instability of silica aerogel limits its applications, which demands further research. However, when it comes to commercialization, the result is not as expected. It seems that mass production, particularly in the aerospace industry, has dawdled behind. [1-5] The critical step in preparation of various types of aerogels is drying of wet gels [6]. The quality of aerogels obtained is poor if wet gels are dried under atmospheric pressure. During atmospheric drying,the porous structure of the gels collapses due to capillary pressure resulting in shrinkage of the gel network and there by reduction of porosity. This happens as a result of interfacial tension between the vapor and the liquid phases in the pores. This can be eliminated by supercritical drying, that is, by drying in the absence of a vaporliquid interface [7]. Silica aerogels are produced by removing the entrapped solvent from the wet gel by using a suitable drying technology, usually a supercritical drying process, able to avoid the pore collapse phenomenon and keep intact the porous texture of the wet material while maintaining the

2 integrity and high porosity of the gel [8]. Due to the complexity and various types of risk involved in supercritical drying method the use of silica aerogels in various applications has been hindered greatly [9]. To overcome the problems of supercritical drying researchers have developed ambient pressure drying (APD) and the freeze drying processes [3,10]. In freeze drying method the phase boundary between the liquid and gas phase does not exist and thus the capillary pressure does not play an important role. In this method, the solvent must be exchanged with a low expansion coefficient and a high sublimation pressure. The pore liquid is frozen and sublimed under vacuum [11]. Material obtained by this way is called a cryogel. Unfortunately, freeze drying has many disadvantages including the fact that the aging period has to be prolonged for stabilization of the network and, in some cases; the network may be destroyed by crystallization of the solvent in the pores[1]. To solve these problems, the APD method has been proposed in order to decrease the synthesis time of aerogels in a cheaper and safer way. The APD process occurs via solvent-exchange with low surface tension solvents (LSTS) and surface modification of the wet gels, in which the silylated surfaces do not participate in condensation reactions as the gel is collapsed by the capillary tension during drying and springs back toward its original porous state after drying. However, solvent-exchange is a lengthy andtedious process, which usually takes several days or weeks [3]. The reasons for solvent-exchange are mainly rely on two aspects: 1) exchange water and ethanol with LSTS in order to reduce capillary pressure; and 2) water should be exchanged with organic solvents so that the hydrophobic modification reagents, such as trimethylchlorosilane (TMCS), can reach to the pore surfaces. Though great efforts have been made in reducing the aging and solvent-exchange time for APD, it's normally takes 24 h [8,10] In present work silica based aerogels were synthesized through sol-gel process in which toluene and n-hexane were used as agent for solvent exchange and TMCS as surface modification agent. The structure-property relationship of the silica aerogels was also investigated. 1.2 Experimental Method: Preparation of silica aerogel via sol-gel process Silica aerogels were synthesized by sol gel process followed by ambient pressure drying. The steps involved in preparation of silica aerogel have been given in flow chart shown in Fig 1.Tetraethylorthosilicate (TEOS) was used as precursor. Other chemicals used were ethanol, hydrochloric acid, ammonium hydroxide, n-hexane, toluene and trimethylchlorosilane (TMCS).In the first step of aerogel synthesis, tetraethylorthosilicate, ethanol, hydrochloric acid and water were mixed in the ratio of 1 : 3 : 1 : respectively and then the solution was stirred for 30 minutes at room temperature resulting in the formation of sol. The sol consisting of nano particles of SiO 2dispersed in liquid solvent was poured in Teflon cylinders. After that ammonium hydroxide solution was added to the sol which cross linked the SiO 2 nano particles into network structure according following reaction. 6

3 The reaction results in gelation of the sol. The hydrogel formed is then aged for 24 hours at room temperature. The hydrogel was then placed in ethanol for 24 hours to replace water present hydrogel pore structure leading to the formation alcogel [7].The alcogel was then placed in n-hexane and toluene separately and then heated at 60 o C for 24 hours on water bath toreplaceorganic solvent present in alcogel. The surface chemical modification of the gel samples were carried out by adding the TMCS as silylating agent under the condition of heating at 45 C on water bath for 12 h. Finally, the alcogel samples were dried at 200 o C in furnace to get the aerogel Characterization Fourier transform infrared spectroscopy (FTIR)was used to determine the surface properties of the aerogel. Total surface area and the pore size distribution were determined by Barrett-Joyner- Halenda (BJH)adsorption cumulative pore volume methods using Quanta-sorb Sorption System (Quantachrome Instruments, by Brunauer, Emmett and Teller (BET) and UK) with N 2 at 273 K assorbate. Total specific pore volume was evaluated from N 2 uptake at P/P0 = 0.99.Before analysis, the samples were out gassed for 2 hours at 220 o C. The microstructure and morphology of the aerogel samples was observed using scanning electron microscopy (JEOL). The samples were prepared for SEMusing gold nanoparticles as the surface coating agent. The X-ray diffractometer (XRD) was used to analyze the structure of the aerogel. Thermal gravimetric analyses(tga) technique was used to determine thermal stability of the aerogel. 1.3 Results and Discussion Effect of solvent on physical properties of silica aerogel Pore size distribution of the silica aerogel samples derived from theisotherm with the BJH method is shown in Fig.2.The Fig.2 shows that pore radius increases with increasing cumulative pore volume. The samples were composed of mesopores and macropores with pore radius in the range of3-100 nm. This can be explained by collapsing of some of the pore structure due to capillary forces exerted on the walls of the pores by n-hexane and toluene evaporation during drying. During ambient dying when the temperature reaches at 100 o C the volume of the gel decreases and shrinkage occurs due to the evaporation of the organic solvent resulting in the breakage of some of the pores. As drying progresses the gel volume increases and regain to its original size when the temperature reaches up to 150 o C and then remains constant for further increase up to 200 o C. This increase of the gel volume due to increase in temperature depends many factors such as surface tension, viscosity, and vapor pressure of the pure solvents and the heating rate. A similar phenomenon has been observed by the Rao. A.P and Rao A.V [4]. Fig.1: Flow Chart of silica aerogel preparation Drying at interior regions of the gel is accomplished by the diffusion of the solvents molecules to the surface where they vaporize depending on the vapor pressure of the solvent.toluene is less volatile solvent compared to hexane so the effect of 7

4 evaporation was expected to be less pronounced, while its viscosity is similar to that of hexane provided similar hydrodynamic behavior. Hence, a significant increase in evaporation rate at higher temperatures is expected in hexane. Also surface tension of the organic solvents decreases with increasing temperature resulting in the higher vapor pressure and easier for the molecules to come out from gel. This can account for the reason that at low temperature both solvents has higher surface tension which develops strong capillary pressure within pores of the gel which results in shrinkage at initial stages of the drying. Some of the important properties of the n-hexane and toluene are given in the table 1.Above 100 o C, there is a repulsive force due to nonreactive O Si R groups on the pore surface of the gel and this leads the gel to regain its original volume. Hence, the gel springs back to its original volume. Also hexane has lower molecular weight ( g/mol) compared to toluene ( g/mol) due to which it vaporizes first and exerts higher pressure to the pore walls in the initial stage of the drying. It was found from the Fig.1that there is a small PSD in the n-hexane aerogel compared to toluene sample. This is due to difference in capillary pressure developed within the pore structure of the gel during drying. Hexane has low surface tension and higher vapor pressure which exerts low pressure on the pore walls. On contrary, toluene having high surface tension, low vapor pressure, higher molecular weight and longer chain length results in higher shrinkage of the gels and also the contact angle between the pore walls and solvent is high which results in large PSD. Fig.2: PSD of silica aerogel samples Table 1: Chemical and physical properties of the solvents Solvent Vapor Pressure (mm of Hg) at 25 o C Surface Tension (mn/m) at 25 o C Molecular weight (g/mol) Boiling Point ( o C) Viscosity centipose (cp) at 25 o C Relative Evaporation Rate compared to butyl acetate (1) Vapor Density compared to Air (1) Density g/cc at 25 o C n- Hexane (fast) Toluene (slow)

5 The physical properties of the silica aerogel samples made with two organic solvent are given in the Table2. The specific surface area and pore volume of the silica aerogel made with tolueneis higher than aerogel made n-hexane. Table 2: Physical Properties of synthesized aerogels Sampl e no Solvent used Surface area (m 2 /g) Pore diameter (nm) Pore volume (cc/g) 1 n-hexane Toluene The nitrogen adsorption isotherms of the porous silica aerogel samples are shown in the Fig. 3. The isotherms are typically type IV which according to the IUPAC classification are the characteristic feature of the mesoporous materials (2-50 nm) [4,11].It was observed from the Fig. 3 that the volume of the gas adsorbed in the porous silica aerogel samples based on hexane and toluene increased rapidly initially at P/P o up to 1 and then increase gradually with increase of P/P o up to 0.8 and then rise sharply with further increase of P/Po to 0.95 and remain constant up to 1.The higher surface area and volume of N 2 absorption in toluene based sample can be attributed to the fact that the gels at higher temperature (about 150 o C) shrink less than at lower temperature (about 100 o C) resulting in lower capillary pressure on the pore walls leading to large pore size Structural and thermal properties of silica aerogel The SEM images of silica aerogel samples are shown in Fig. 4.It can be seen in the Fig that the silica aerogels have a sponge like microstructure and exhibit a three dimensional nano porous network of silica nanoparticles.fig.5shows the XRD pattern of both n-hexane based and toluene based silica aerogels which confirms the amorphous structure of the silica nano particles having characteristic peak between 20 and 30 (2θ) in both graphs.tga curves of the prepared silica aerogel samples in the temperature range of C, in nitrogen atmosphere at a heating rate 10 C per minute are shown in Fig.6. It was observed that initially weight loss occurs within 100 o C due to loss of water present within pores. The second weight loss occurs at 383 o C in n- hexane based sample which is due to oxidation of --- C 2H 5group. At third degradation peak was observed at 477 o C corresponding to the degradation of methyl (--CH 3) groups which is related to the hydrophobicity of the silica aerogels. Above this temperature, the silica aerogels become hydrophilic due to the oxidation of the methyl groups responsible for the hydrophobicity of aerogels.fig.7 and Fig.8 shows the FTIR spectra of n-hexane and toluene based silica aerogel respectively. The absorption peaks at 3350 cm 1 and 3250 cm 1 are related to the hydroxyl (OH) groups. The absorption peaks at 1800 (CH 3 stretching) and 852 (Si-CH 3 bending) cm 1 are attributed to surface modification of hydrogels by TMCS[27-29]. The peaks between 1200and 2000 cm - 1 correspond to the IR absorption by Si CH 3 groups [20,21] present in the both type of aerogels which confirms the surface modification process. The observed strong peaks at 458, 1055, 1385 and 1642 cm 1 are related to the bending of the O Si O and Si O Si vibrations, Si O Si and Si-H 2O vibrations, respectively [30]. The intensity of absorption peaks of O H at 3432 and 1642 cm 1 decreased for 24h of aging period due to replacement of hydroxyl with tri-methyl groups [30, 31] Fig.3: Adsorption isotherms of the aerogels with various solvents 9

6 A B 4 Fig.4: SEM images of n-hexane and Toluene based silica aerogel 10

7 Fig.5: XRD pattern of n-hexane and Toluene based silica aerogel Fig.6: TGA curve of n-hexne and Toluene based silica aerogel Fig.7: FTIR spectrum of Toluene based Fig.8: FTIR spectrum of Toluene based silica aerogel silica aerogel 1.4 Conclusion: Silica aerogels were synthesized by sol gel process followed by ambient pressure drying. Two organic liquids i.e., n-hexane and toluene were used for solvent exchange process. Effect of the two solvents on physical, structural and thermal properties of the synthesized aerogels was determined. The specific surface area and pore volume of the silica aerogel made with toluene (i.e., 445 m 2 /g) was higher than 11 aerogel made n-hexane (330 m 2 /g).it was also found that there is a small PSD in the n-hexane based aerogel compared to toluene based. This is due to difference in capillary pressure developed within the pore structure of the two types of gel during drying. It was also found from TGA results that toluene based silica aerogel have higher thermal stability compared to n-hexane based. FTIR results also confirm that toluene based aerogel samples have better optical transparency compared to n-hexane based. Acknowledgment: The authors would like to thank Centre of Excellence in Analytical Chemistry University of Sindh Jamshoro Pakistan, for providing the FTIR facility

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