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Laser-induced breakdown spectroscopy applied to pasture, titanium, and bioplastics

Precision agriculture is a farming practice that makes production more efficient. Farmers are able to treat infield variability optimising efficiency, growth, and yield by tailoring the time, rate, and type of fertilizer that is applied. This reduces costs, waste, and environmental side effects such as runoff and leaching caused by overfertilization. Precision agriculture technology measures the nutritional status of crops to inform what, and where, nutrients are needed. The sensors need to be precise, discriminative, and work in real time to ensure that optimal windows for nutrition are not missed. These sensor systems provide aerial imaging, and crop, or soil, colour index maps. A technology that has proven effective on some agricultural specimens is laserinduced breakdown spectroscopy (LIBS). LIBS is an optical emission technique that utilizes a high-powered pulsed laser to create a plasma on the sample surface. As the plasma cools, photons are emitted at distinct wavelengths corresponding to the elemental composition in the plasma, which should represent the sample. This thesis investigates using LIBS as a sensor for precision agriculture. Multiple chemometric methods have been used on the pasture spectra to build calibration models. There are large deviations between spectra belonging to a single sample. This is due to surface inhomogeneity, particle size, lens-to-sample distance, temperature fluctuations between plasmas, and other causes. Temperature corrections were investigated using Boltzmann plots, Saha-Boltzmann plots, and intensity ratios. With limited success in mitigating the variations in pasture spectra, LIBS was used to investigate non-aqueous systems. The ability to selectively sinter the surface of injection moulded titanium was examined. Titanium metal injection moulding allows the creation of complex metal parts that are lightweight, biocompatible, and costs less than machining. Multiple LIBS pulses produced sintering in the ablation crater of injection moulded titanium by sufficiently heating the titanium particles so that fusion occurred. The spectra from LIBS can be used to monitor the extent to which the surface is sintered by measuring the reduction in carbon emissions. An autofocus system, based on the triangulation method, was used to minimise variations caused by lens-to-sample distance (LTSD). With the success of sintering titanium, LIBS was used to investigate non-aqueous organic systems. Employing LIBS to discriminate bioplastics from regular plastics was explored in recycle waste streams. If bioplastics are present in the recovery process of regular plastics the resulting product contains impurities. This study was undertaken to determine the feasibility of incorporating bioplastics in the curbside pickup of recyclables in New Zealand. The common recyclables are plastics, glass, tin cans, and aluminium cans. The setup was designed to emulate a one-shot LIBS detection system in a recycling plant. Models were created using k nearest neighbours and soft independent modelling class analogy from the spectra. 100 % discrimination between bioplastics and regular plastics was achieved. An autofocus system, combining dual lasers, was used to overcome the occlusions produced by sample geometry.
Type of thesis
Jull, H. B. (2018). Laser-induced breakdown spectroscopy applied to pasture, titanium, and bioplastics (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11864
The University of Waikato
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