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Title: Diamond-doped Silica Aerogels for Solar Geoengineering
Authors: Vukajlovic, Jovana
Issue Date: 2021
Publisher: Newcastle University
Abstract: Geoengineering includes many techniques aimed to fight the global warming, which is one of the biggest problems today. Even though aerosol injection into stratosphere is one of the most promising solar geoengineering techniques, sulfate aerosols, which are suggested for such an application, show significant drawbacks such as infrared (IR) absorption and ozone degradation. Here, a novel composite material comprised of diamonds dispersed in a silica aerogel network is investigated and compared to pure silica aerogel. Silica aerogels are ultralight, highly porous, transparent and can host particles, while fulfilling particle size limitation in terms of potential health risks for humans during respiration. In theoretical models, diamond particles with high refractive index (~ 2.41) showed outstanding upscattering among other materials, without IR absorption. Moreover, in recent years, the lowcost production of silica aerogels suitable for large scale production has been developed and diamond powders can be purchased at low cost in ton quantities. Before doping of the silica aerogels, diamond part of the composite was studied since the diamond is outstanding light scatterer and also for future applications of the diamond particles. Two types of diamond particles were used: high pressure high temperature (HPHT) microdiamonds (~ 500 nm in size) and detonation nanodiamonds (DNDs) (~ 3 nm in size). Raw HPHT microdiamonds were treated with 1-undecene and zirconia beads and nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopy were used to detect the alkyl groups on the surface of the treated HPHT microdiamonds. Both HPHT microdiamonds were free of graphitic sp2 carbon (Raman scattering). Nanodiamond colloids, original 4 wt% (as purchased) and diluted 1 wt%, were studied in terms of Zeta potential, pH and dynamic light scattering (DLS) measurements to assess the stability of nanoparticles. The results suggest that nanodiamond colloids are not stable in air or after dilution as the particles tend to agglomerate. Both micro- and nanodiamonds were added to tetraethoxysilane (TEOS)-based silica aerogels. In case of microdiamonds, concentrations of 500, 700 and 900 ppm of diamond particles in the precursor sol before gelation were used. It was observed that the higher the diamond concentration in the sol, the final aerogel contained more wt% of diamond, with the maximum concentration of ~ 3.3 wt% according to X-ray diffraction (XRD). The nanodiamond-doped silica aerogel contained ~ 7.5 wt% of diamond in the final aerogel. It is suggested that the small size of the nanodiamond particles allowed them to incorporate well in the aerogel network and not to be washed away during solvent exchange step. Furthermore, aerogel with nanodiamonds increased the surface area compared to the both pure aerogel and microdiamond-doped silica aerogel, possible due to similar size of diamond to silica particles that did not disturb the gelation of silica. Although DNDs are not suitable for solar scattering due to graphitic sp2 carbon, DNDs were used in order to study the doping of aerogels with diamonds. Additionally, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the porous aerogel formation and doping with diamond particles. The ultraviolet (UV)/Visible (Vis)/Near infrared (NIR) diffuse reflectance showed that composite microdiamond-doped silica aerogel has an improved reflectance compared to microdiamond powder or pure silica alone. The fall speed was calculated to estimate how long the measured materials would stay in the atmosphere before sedimentation. The obtained results are promising and could stimulate further in-depth studies with similar materials with a potential for applications in solar geoengineering
Description: Ph. D. Thesis.
Appears in Collections:School of Engineering

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