This article discusses the possible outcome of starch airgel synthesis via ambient pressure drying, based on similar studies conducted previously. 54 Figure 4.15 Thermal conductivity of Tencel fiber-silica composite aerogels prepared via ambient pressure drying and supercritical drying (Markevicius et al., 2017). 56 Figure 4.16 The effect of fiber content on the thermal conductivity of the ATP/silica airgel composite (Li et al., 2017).
INTRODUCTION
- Background
- Problem Statement
- Objectives
- Research Scope
- Thesis Outline
This thesis is divided into five chapters that consist of the overall details of the review paper. In Chapter 1, the introduction of the overview is explained by explaining the background, problem statements, objectives and the scope of the proposed methodology. The calculation for the relevant parameter is also included to cover the detailed explanation of the case.
LITERATURE REVIEW
Introduction
Types of Aerogels and Application
A process called thermal activation is used to prepare this material which involves the controlled combustion of carbon from the airgel structure in an oxidizing atmosphere such as carbon dioxide (Biener et al also reported that through the removal of carbon, new micropores are created within the microstructures and thus resulted in increased overall surface area Another application of carbon aerogels is in the field of green technologies such as electric double layer capacitor, EDLC (Pekala et al., 1998). It was reported that in devices, charge storage was in the form of ions accumulated on the surface of the material and then resulted in the formation of an intermediate between electrostatic capacitors and batteries (Conway, 1991).
Hydrophobic and Hydrophilic Aerogels
In addition to being used as energy storage, carbon aerogels are also used to increase the efficiency of other solid hydrogen storage materials, especially multilayer hybrids (Biener et al also reported that carbon aerogels are one of the most potential candidates for this application due to their tunable porosity, large pore vol. and its ability to adapt the surface characteristics of the carbon framework, Wagh and Ingale (2002) further explained that when the hydrolytically stable Si-R (R=CH3) replaces the Si-OH structure, the airgel will exhibit hydrophobicity as it prevents water adsorption and will not be susceptible to water. There are two methods to achieve hydrophobic aerogels, namely (a) surface chemical modification of aerogels with gaseous reagents and (b) surface modification of colloidal particles by incorporating some hydrophobic reagents in alcohol.
Synthesis of Aerogels
- Sol Gel Process
- Advantages of Sol Gel
This method is capable of large-scale application due to its advantages of simple operation and high safety (Rao et al., 2007). According to Kumar et al. 2015), this method for synthesizing nanomaterials can improve the adhesion between the substrate and the top layer. The materials can be easily formed into a complex geometry due to the gel state (Kumar et al., 2015).
Property of Aerogels
This step is the longest step of the freeze-drying process, as the water in the material is sublimated up to 95% and the pressure is reduced to a very low level, while the temperature in the planks is increased to facilitate the sublimation of solvents (Kremer et al., 2009; Simón-Herrero et al., 2016). To facilitate the removal of residual solvent, the temperature of the shelves is increased to integer values higher than that required in the primary drying step (Zhai et al., 2005). Aerogels are generally described as brittle materials such as glass, and the stress-strain relationship evolves towards fracture like any other elastic material (Woignier et al., 2009).
Biodegradable Aerogels
This material also exhibits low resistance to water vapor diffusion and good fire ratings, which makes it a favorable material for thermal insulation applications (Ganobjak et al further concluded that for thermal insulation construction, aerogels are able to renovate small space or where a thick insulation would change the appearance of the building suddenly.2019) also demonstrated in their studies that aerogels are prone to deteriorate with a prolonged exposure to temperatures above 70°C and at a high relative humidity of more than 90%.
Cellulose Aerogels
- Preparation of Cellulose Aerogels
- Properties of Cellulose Aerogels
In general, according to Long et al. 2018) the preparation of cellulose aerogels includes three steps, namely the dispersion of cellulose or cellulose derivatives, sol-gel process and the drying process to obtain aerogels. According to Long et al. 2018), by using cellulose as the precursor to synthesize aerogels, there are several advantages that can be achieved, namely;. First, the expansion of the liquid is caused by the high dissolution of supercritical carbon dioxide in the liquid sol-gel which consequently causes the spilling of excess liquid from the get network (Long et al., 2018).
Starch Aerogels
- Properties of Starch Aerogels
- Morphology of Starch Aerogels
- Thermal Properties of Starch Aerogels
- Production of Starch Aerogels
- Various Application of Starch Aerogels
In a similar study conducted by Wang et al. 2018) proved that the addition of starch to konjac glucomannan-based aerogels had improved the elasticity of the resulting aerogels. This inference is also consistent with the density of the starch aerogels with the increased starch concentration. The structures of the starch aerogels obtained by Ubeyitogullari and Ciftci (2016) mainly consist of interconnected fibrils and nanopores.
Zhu (2019) mentioned that the crystallinity of native starch can interfere with the gelatinization of aerogels. Cross-linking, which is one of the starch modifications, can change the microstructure of starch aerogels (Abhari et al, 2017). The uniformity of the pore distribution of aerogels can be attributed to the cross-linking of starch (Zhu, 2019).
Increasing the retrogradation time resulted in an increase in thermal conductivity via the reduction of the specific surface area (Druel et al., 2017). The network and mechanical stability of the starch aerogels are significantly reduced as drying increases. The hardness and crystallinity of the starch aerogels can be increased by cross-linking.
The mechanical and electrical properties of the PEDOT materials were significantly improved by the aerogels.
Executive Summary
METHODOLOGY
- Introduction
- Materials
- Synthesis of Starch Aerogels
- Hydrogel Formation
- Solvent Exchange and Esterification
- Ambient Pressure Drying
- Sample Characterization
- Scanning Electron Microscopy
- Density Measurement
- Thermal Conductivity Test
To prevent water loss, samples were sealed with parafilm and placed in a refrigerator for retrogradation for 48 h. In this process, the water in the monolith was replaced by ethanol using a one-step solvent exchange process. The hydrogel is weighed on a digital balance and the density is calculated using equation (3.5).
It is measured using the heat flow method in accordance with ASTM E1225-99 Standard Test Method for Thermal Conductivity of Solids by the shielded comparative longitudinal heat flow technique as shown in Figure 3.3 and Figure 3.4. The solidified hydrogel sample in the holder is inserted into the testing area of the thermal conductivity unit as shown in Figure 3.5. The instrument used is the PA Hilton H112A Linear Heat Conduction Unit found in the Basic Thermodynamics Lab.
First, the hydrogel sample and the container are placed in the two copper terminals of the conduction unit. The interface temperature of the hot (Th) and cold (Tc) ends of the sample is calculated by extrapolating the values of all the thermocouples (T1-T8). The effective thermal conductivity of the hydrogel sample (WmK-1), Kc can be calculated using this formula;.
The gap temperature (the temperature between the hot and cold region) can be calculated using equation 3.3.
EXPECTED RESULT AND DISCUSSION
Introduction
Structural, physicochemical and functional properties of wheat starch-based
- Effect of drying technique: Ambient pressure drying
- Density and shrinkage
- Mechanical properties of wheat starch aerogels
- Thermal Conductivity of Starch-based Aerogel
The imbalance will eventually result in the collapse of the porous structure (Wang et al., 2005). In another study conducted by Ganesan et al. 2016) suggested that another factor attributed to the microstructure of the aerogels is the liquid-vapor interface. The dotted line corresponds to the theoretical specific surface (details in text) (Markevicius et al., 2017).
As reported by Markevicius et al. 2017), the calculation of the theoretical specific surface area was based on a simple "mixing rule" based on the specific surface area of pure silica aerogels. The addition of silylation agent contributed to the preservation of the porous structure in the sample shown in Figure 4.7 by avoiding irreversible pore collapse during ambient pressure drying. Supercritical drying method avoids the creation of capillary stress and the total shrinkage of the material essentially occurs during sample drying (Markevicius et al., 2017).
This is due to the large difference in relative density and the pores and nanofibril distribution in the cell walls of the material. According to Zhang et al. 2017) the incorporation of starch into the aerogels led to a more reinforced network of the hybrid airgel while helping to reduce the relaxation of the polymer chains. As stated by Illera et al. 2018), the mean free path of the gas molecules enclosed in the pores and the pore sizes influence the thermal conductivity in the gas phase.
According to Druel et al. 2017), the thermal conductivity of starch-based aerogels is related to the type of starch and the processing condition. Based on Figure 4.16, it is evident that as the concentration of ATP in the matrix increases, it is attributed to the thermal conductivity of the matrix. From this comparison and analysis, it can also be deduced that improving the mechanical properties of starch-based airgel may compromise the thermal conductivity of the material.
CONCLUSION AND FUTURE RESEARCH DIRECTION
This paper chose a different drying route, namely ambient pressure drying, compared to other conventional drying mechanisms, namely supercritical drying with carbon dioxide and freeze drying. To date, there are a limited number of studies in which aerogels have been synthesized via drying at ambient pressure. Based on previous studies, there are several factors that determine these parameters for dried aerogels at ambient pressure.
Ambient pressure drying: a successful approach for the preparation of silica and silica-based mixed oxide aerogels. Facile and novel approach to ambient pressure drying for the synthesis and physical characterization of cellulose-based aerogels. Strong, machinable, and insulating chitosan-urea aerogels: toward ambient pressure drying of biopolymer airgel monoliths.
Improved mechanical and thermal insulation properties of monolithic attapulgite nanofiber/silica airgel composites dried at ambient pressure. Reduction of processing time by mechanical shaking of dried TEOS-based TEOS granules at ambient pressure. Structural characteristics and thermal conductivity of ambient pressure-dried silica aerogels by one-step solvent exchange/surface modification.
One-step eco-friendly fabrication of classical monolithic silica aerogels via water solvent system and ambient pressure drying.
