Multifunctional Lightweight Structures of Silicon Carbide Nanowires

Silicon carbide (SiC) as a type of ceramic material possesses unique properties such as high hardness, good high temperature strength, and excellent oxidation resistance. However, the intrinsic shortcomings of ceramic-based materials, such as high brittleness, low recoverable compressibility, and low fatigue resistance, prevent their utilisations as structural or functional components. To overcome these issues, highly porous lightweight and flexible SiC ceramics constructed by nanowires are promising alternatives for advanced engineering applications. The aim of this thesis is therefore to fabricate highly porous lightweight and flexible SiC nanowire structures by three novel approaches: (1) in-situ chemical-blowing; (2) melamine foam-based replica template; (3) electrospinning and explore their properties towards different applications. The overview, including the aims and objectives of this thesis is outlined in Chapter 1. The existing knowledge about lightweight SiCNW structures including crystallography, synthesis approaches, physical properties (mechanical strength, thermal conductivity, high temperature stability), and well-developed energy and environment-related applications (piezoresistive sensors, catalyst support, absorbers, and filters) is documented in Chapter 2. The generic information of the starting materials, synthesis techniques, equipment, and method used for the fabrication of 3D SiCNW structures, characterisation of their microstructural features, and evaluation of the various aspects of their multifunctionalities is descripted in Chapter 3. To identify suitable techniques to assemble SiC nanowires (SiCNWs) into 3D architectures, Chapter 4 provides a selection of advanced manufacturing approaches for lightweight SiCNW structures with easy and precise control of the overall shape and growth of SiCNWs. Followed with the demonstration of the exciting properties of the as-obtained three SiCNW structures including mechanical properties, thermal insulation performance, thermo-oxidation resistance, and fire-retardance in Chapter 5. Finally, based on their own characteristics, the applications of the SiCNW structures such as piezoresistive sensors, catalyst support, and efficient absorbents for oil and organic solvents are present in Chapter 6. A guidance in the manufacturing of advanced ceramic nanowire structures with desired microstructures and properties tailored for specific applications will be eventually provided. I first demonstrated the creation of SiCNW sponges by a facile template/catalyst-free sugar-blowing technique, by reacting SiO2 with sustainable kitchen sugar, using NH4Cl as a blowing agent. The as-grown, highly porous SiCNW sponges exhibited a core-shell structure, the core part with a density of 115-125 mg cm-3 was comprised of short and tangled SiC whiskers with SiC flakes embedded, while the shell layer with an ultralow density of ~25 mg cm-3 consisted of numerous smooth SiCNWs. These sponges exhibited a compressive modulus of ~389 kPa, recoverability under cyclic compression loading for 100 cycles at a strain of 20% and a thermal conductivity of 42-92 mW m-1K-1. Secondly, I reported the fabrication of SiCNW scaffolds with tuneable microstructures, densities, and therefore properties, by regulating the solid loading content in the reticulated melamine foam (MF) template. The resulting samples exhibited high strength (modulus up to ~167.3 kPa), good recoverability (11% residual strain and 72% maximum stress after 100 compressive cycles at a ε = 20%), and low thermal conductivity of 32-54 mW m-1K-1. Finally, I successfully created 3D SiCNW aerogels by using a Mille crêpe stacking and sintering of the electrospun PAN/SiO2 fibres for the first time. The resulting aerogels made of interconnected SiCNWs displayed an ultralight density of 29 mg cm-3, excellent compressive recoverability and fatigue resistance. Meanwhile, the SiCNW aerogels exhibited a thermal conductivity of 24 mW m-1K-1, even lower than that of the air, suggesting its superinsulation capability. Benefitting from intrinsic properties of SiC, experimental results have shown that all the as-obtained SiCNW structures exhibited good thermal insulation performance, exceptional high-temperature stability, fire-retardance, and temperature-invariant elasticity. Furthermore, I have explored the best-suited functional applications for each SiCNW structure. The SiCNW sponges and aerogels with better compressive recoverability and mechanical stability exhibited interesting electromechanical sensing capability. The sponge-based sensor exhibited a gauge factor up to 87 and stable wide-range compression-resistance responses. Whilst the aerogel-based strain sensor with higher recoverable strains presented stable sensing behaviour at different strains, frequencies, elevated temperatures over 200 °C and excellent repeatability over 2000 cycles. Owing to the cellular structure with the co-existence of SiC nanowires and struts, good interconnectivity, and competent mechanical strength and stability, the SiCNW scaffolds demonstrated the exclusive suitability as excellent support for MOF-derived TiO2-C catalyst, with ~35% enhanced in-situ loading of the catalyst, enabling a superior photocatalytic performance and good repeatability for at least 3 cycles. I further examined the SiCNW structures as organic solvent/oil absorbent. They exhibited rapid absorption of various organic solvents and oils. Typically, the SiCNW aerogels possess the highest absorption capacity of 32-86 g g-1, as well as robust recoverability. Meanwhile, the absorbed content can be easily removed by squeezing, distillation, and combustion, while the SiCNW structures remain unchanged. These features have shown that the SiCNW structures are promising for applications for the potential removal of chemical spills and oil leakage, with the advantage of easy recycling. All these remarkable findings will not only provide an important opportunity to advance the understanding of lightweight SiCNWs structures and make original contributions to utilise them as multifunctional devices, but also bring us the new ways to reshape the manufacturing of porous ceramics for future energy and environment-related applications

Gaussian Mixtures Based IRLS for Sparse Recovery With Quadratic Convergence

In this paper, we propose a new class of iteratively re-weighted least squares (IRLS) for sparse recovery problems. The proposed methods are inspired by constrained maximum-likelihood estimation under a Gaussian scale mixture (GSM) distribution assumption. In the noise-free setting, we provide sufficient conditions ensuring the convergence of the sequences generated by these algorithms to the set of fixed points of the maps that rule their dynamics and derive conditions verifiable a posteriori for the convergence to a sparse solution. We further prove that these algorithms are quadratically fast in a neighborhood of a sparse solution. We show through numerical experiments that the proposed methods outperform classical IRLS for l_p-minimization with p\in(0,1] in terms of speed and of sparsity-undersampling tradeoff and are robust even in presence of noise. The simplicity and the theoretical guarantees provided in this paper make this class of algorithms an attractive solution for sparse recovery problems

An efficient algorithm for total variation regularization with applications to the single pixel camera and compressive sensing

In this thesis, I propose and study an efficient algorithm for solving a class of compressive sensing problems with total variation regularization. This research is motivated by the need for efficient solvers capable of restoring images to a high quality captured by the single pixel camera developed in the ECE department of Rice University. Based on the ideas of the augmented Lagrangian method and alternating minimization to solve subproblems, I develop an efficient and robust algorithm called TVAL3. TVAL3 is compared favorably with other widely used algorithms in terms of reconstruction speed and quality. Convincing numerical results are presented to show that TVAL3 is suitable for the single pixel camera as well as many other applications