Characterization of mechanical, thermal and acoustic properties of novel aerogel composites
Date of Issue2016
School of Mechanical and Aerospace Engineering
Silica aerogels (SA) are very light and highly porous materials that are intriguingly and complexly networked with large internal surface area, high hydrophobicity with extremely low density and thermal conductivity. These features make them an ideal choice for applications as thermal and acoustics insulators or as optical, electrical and energy storing devices. However, their exploitation for a wider spectrum of applications is inhibited primarily by their brittleness, which also makes processing and handling of SA difficult. While there are hybrid SAs doped with polymers, ceramics or metals in the market, the improvements in their properties are often compromised with significant increase in density and reduction in thermal insulation performance. The aim of this PhD research is to develop SA based composites fabricated via binder treatment methods namely, ‘froth and mix’ (FM) and ‘freeze drying’ (FD) that are hydrophobic, lightweight with enhanced mechanical, thermal and acoustics properties while retaining the unique properties of SA granules through careful selection of constituent materials and additives to customize significant improvement in individual property. In this research, Gelatin, a water-soluble polymer (WSP) with numerous side chains, which has the versatility to be functionalized with other materials, was used as the main binder. Additives such as sodium dodecyl sulfate (SDS) and COOH functionalized multi wall carbon nanotubes (FMWNTs) were added to achieve specific results. The gelatin silica aerogel (GSA) based composites have shown; i) an increase in strain recovery of up to 90% under 45% compressive strain; ii) thermal conductivities in the range from 0.016 to 0.030 W/m-K resulting in a reduction of 7% as compared to SA granules; iii) super-hydrophobicity averaging 155±10° in contact angle when doped FMWNT; and, iv) balanced acoustic absorber and barrier qualities with up to 20 decibels reduction in transmission. The above outcomes were achieved through numerous experiments validated through the development of predictive and statistical models for the composites properties. Negligible amounts of up to 0.67 wt% SDS and up to 0.084 wt% FMWNTs were added during fabrication of GSA composites with mass fractions ranging from 0.1/0.9 wt% to 0.5/0.5 wt%. The dependency of process variables involved were studied and investigated in the development of the associated mechanical properties through Analysis of Variance (ANOVA) model. The optimal strain recovery with the associated compressive modulus, strength and density were established using the experimental data. The optimized composition of 0.56 wt% SDS regardless of gelatin content were tested and validated with the derived statistical models. The test data presented from these composites was analogous to ‘creep-like’ behavior of a material, typically identified as the primary, secondary and tertiary stages. The rationale and mechanisms behind the ‘creep-like’ behavior were explained using various schematic diagrams and Field Emission Scanning Electron Microscopy (FESEM) with Energy Dispersive X-ray Spectrometer (EDX) images. X-ray Photoelectron Spectroscopy/Electron Spectroscopy for Chemical Analysis (XPS/ESCA) was studied to determine the active elements that facilitate the binding between the aerogels and gelatin/SDS mixture. In the next phase of study, a two-term Gaussian function predictive model was developed as a function of the distribution of aerogel granule size and was extended to derive the predictive models for the thermal properties of the composites. The effects of FMWNT on the GSA and GSA-SDS composite properties were studied and compared. Hydrophobicity, an intrinsic feature for water-resistant and self-cleaning material was observed to be highly prevalent in FMWNT composites. Finally, the acoustic absorption characteristics of the SA granules, GSA-SDS and GSA-SDS/FMWNT composites were investigated. The best absorptive SA granules were then used in fabricating the GSA-SDS and GSA-SDS/FMWNT with single configuration but in various thicknesses. It was observed that maximum absorption occur between 2300 and 2950 Hz which coincides with the most sensitive frequency range for the human ears. From the results, an ‘Inferential Method’ was developed to determine the transmission loss of the specimens and compared with sound meter measurements.
DRNTU::Engineering::Materials::Material testing and characterization