Browsing by Author "Du Plessis, A."

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    Accurate laboratory wavelengths of the e 3 Σ–(ν' = 5) – X 1 Σ+(ν'' = 0) band of 12C16O
    (IOP Science, 2010) Dickenson, G. D.; Nortje, A. C.; Steenkamp, C. M.; Rohwer, E. G.; Du Plessis, A.
    The forbidden singlet-triplet transitions of carbon monoxide (CO) are important in the interpretation of vacuum ultraviolet interstellar absorption spectra and in particular for the measurement of large CO column densities. Twenty rovibronic lines of the e 3Σ–(ν' = 5) – X 1Σ+(ν'' = 0) band of 12 C 16O for which laboratory wavelengths were previously unavailable were identified in laser-induced fluorescence excitation spectra. Wavelengths were assigned to five rovibronic transitions to an average accuracy of 0.0028 Å. A further 15 lines could not be fully resolved and average wavelengths were measured for these groups of closely spaced lines. A wavelength difference of 0.011 ± 0.0028 Å between the measured wavelengths and the calculated wavelengths in the atlas of Eidelsberg & Rostas demonstrates the need for more experimental data on CO.
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    Experimental conditions for vacuum ultraviolet laser spectroscopy
    (Academy of Science for South Africa, 2005) Du Plessis, A.; Steinmann, C. M.; Rohwer, E. G.
    An efficient wavelength-tunable vacuum ultraviolet (VUV) laser source has been developed at our institute in recent years. This laser source works on the principle of sum-frequency mixing of dye laser pulses in a magnesium vapour medium, which is prepared in a crossed heat pipe system. The laser source is coupled to a pulsed supersonic expansion and is in use for spectroscopic measurements. In this paper, we present a complete characterization of the experimental setup. The source yielded 25-ns-long pulses of VUV radiation with peak power of approximately 1 mW. The wavelength is currently tunable over the range 142.7-146.7 nm with a spectral bandwidth of 0.25 ± 0.05 cm -1. Suggested refinements of the experimental technique include an increase in the repetition rate of the measurements and shorter gas pulses.
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    Femtosecond pump probe spectroscopy for the study of energy transfer of light-harvesting complexes from extractions of spinach leaves
    (Academy of Science of South Africa (ASSAf), 2009-10) Ombinda-Lemboumba, S.; Du Plessis, A.; Sparrow, R. W.; Molukanele, P.; Botha, L. R.; Rohwer, E. G.; Steenkamp, C. M.; Van Rensburg, L.
    Measurements of ultrafast transient processes, of temporal durations in the picosecond and femtosecond regime, are made possible by femtosecond pump probe transient absorption spectroscopy. Such an ultrafast pump probe transient absorption setup has been implemented at the CSIR National Laser Centre and has been applied to investigate energy transfer processes in different parts of photosynthetic systems. in this paper we report on our first results obtained with Malachite green as a benchmark. Malachite green was chosen because the lifetime of its excited state is well known. We also present experimental results of the ultrafast energy transfer of light-harvesting complexes in samples prepared from spinach leaves. Various pump wavelengths in the range 600-680 nm were used; the probe was a white light continuum spanning 420-700 nm. The experimental setup is described in detail in this paper. Results obtained with these samples are consistent with those expected and achieved by other researchers in this field.
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    High-resolution vacuum ultraviolet laser spectroscopy of molecules in a free supersonic jet : in search of rare CO isotopomers and CO-Ar van der Waals molecules
    (Academy of Science for South Africa, 2005) Steinmann, C. M.; Du Plessis, A.; Rohwer, E. G.
    Tunable vacuum ultraviolet radiation (in the range 142.7-146.7 nm) from a novel laser source was used to probe the electronic excitation spectrum of cold carbon monoxide molecules in a supersonic noble gas jet. Rotationally resolved spectra of 12C16O,13C16O, as well as the rare but astronomically important 12C18O and 12C17O isotopomers, were recorded. Evidence was obtained for the formation of CO-containing van der Waals complexes in the jet.
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    An improved experimental setup for high-resolution vacuum ultraviolet laser spectroscopy
    (Academy of Science for South Africa, 2005) Steenkamp, C. M.; Du Plessis, A.; Rohwer, E. G.
    The experimental setup for vacuum ultraviolet (VUV) laser spectroscopy at Stellenbosch University's Laser Research Institute, has been improved by the addition of a VUV monochromator. The VUV source has been characterized over the extended wavelength range 144.2-157.7 nm, and simultaneous measurement of laser-induced fluorescence and absorption spectra has been achieved. We discuss current investigations of carbon monoxide molecules and complexes, facilitated by the narrow bandwidth of the VUV light and the low temperature collision-free conditions in a supersonic jet.
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    Looking inside votive creatures : computed tomography (CT) scanning of ancient Egyptian mummified animals in Iziko Museums of South Africa : a preliminary report
    (Stellenbosch University, Department of Ancient Studies, 2012) Cornelius, I.; Swanepoel, L. C.; Du Plessis, A.; Slabbert, Ruhan
    The ancient Egyptians mummified many more animals than humans. The study of ancient Egyptian animal mummies is varied and extensive. Currently new methodologies and modern technology are being used to unlock the secrets of animal mummies. Recently five animal mummies housed in the Egyptian collection of Iziko Museums of South Africa in Cape Town were scanned using a state of the art computed tomography (CT) scanner at Stellenbosch University. Preliminary results revealed two complete bird skeletons, a claw, a fake and the partial skeleton of what appears to be a cat.
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    Manufacturing and characterization of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu by laser powder bed fusion
    (Elsevier, 2020) Vilardell, A. M.; Yadroitsev, I.; Yadroitsava, I.; Albu, M.; Takata, N.; Kobashi, M.; Krakhmalev, P.; Kouprianoff, D.; Kothleitne, G.; Du Plessis, A.
    Biofunctionalization of Ti6Al4V alloy with metallic agents like Ag or Cu is a promising approach to add anti-bacterial properties and thus to reduce the risk of implant failure. This research investigates the in-situ alloying of Ti6Al4V(ELI) with 3 at.% Cu powders using Laser Powder Bed Fusion (L-PBF). The morphology and geometrical characteristics of the single tracks and layers were studied. Laser powers of 170 W and 340 W, and scanning speeds ranging from 0.4 to 1.4 m/s and 0.8–2.8 m/s were implemented. Single track results showed balling effect and humping at high scanning speeds, 1.4 m/s and 1.6 m/s, for each laser powder respectively. Conversely, keyhole formation occurred at lower scanning speeds of 0.4–0.6 m/s for 170 W laser power, and below and 0.8 m/s for 340 W laser power. For both laser powers, single layers resulted in smoother surfaces at lower scanning speeds. These results were used for the development of optimal process parameters for 3D cubes with 99.9 % density. Optimal process parameters were found for 170 W and 340 W laser powders at 0.7−0.9 and 1.0–1.2 m/s scanning speeds, respectively. In-situ alloying by L-PBF was challenging and a homogeneous distribution of Cu within the alloy was hard to achieve. The increase in laser power from 170 to 340 W resulted in small increase in homogenization. Microstructural analyses after stress-relieving treatment showed the presence of α’ and β phases, as well as CuTi₂ intermetallic precipitates. The finer microstructure together with CuTi₂ intermetallic precipitates resulted in an increase in hardness. This study demonstrates the potential for printing in-situ alloyed Ti6Al4V(ELI)- 3 at.% Cu for biomedical applications. However, further studies are required to determine the effectiveness of antibacterial properties.
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    MicroCT-based bulk density measurement method for soils
    (South African Institution of Civil Engineering, 2019) Le Roux, S. G.; Du Plessis, A.; Clarke, C. E.
    High-resolution micro-computed tomography (microCT) is a method growing rapidly in popularity and has been applied to various soil studies with great success, especially for 3D characterisation of pore spaces or mineral distributions. However, microCT is not typically used for soil bulk density measurements, probably due to relatively simple and fast alternatives. Besides the complex process of image analysis from microCT scans, the method is also limited in resolution, which can result in incorrect total porosity estimation. This is especially true for granular materials, such as soils with small pore spaces between particles. In this work we demonstrate a different, yet very simple methodology for microCT adapted to overcome these limitations by using only volumetric measurements of the samples, and not segmentation of pore spaces or density calibrations. This method allows accurate bulk density determination for soil clods and cores. The method is faster than tradition methods, and it allows for additional analyses, such as surface area, macro-porosity, connected pore network and macro-particle shape analysis. The method is tested and directly compared for the same samples to the traditional waxing Archimedes method, with good correlation. The microCT scans of waxed samples also indicate sources of possible error in the waxing Archimedes method by visualising trapped air and wax penetration into open pore spaces. The method is then applied to cores and local bulk density measurements, and their variability down the cores is demonstrated, which can be very useful in complex soil profiles. The method is robust in varying resolution and image blur as it makes use only of volumetric measurements of the entire sample, not image grey-value calibration or segmentation of pore spaces.
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    A pilot study investigating the presence of voids in bulk fill flowable composites
    (South African Dental Association, 2017) Mulder, R.; Mohammed, N.; Du Plessis, A.; Le Roux, S. G.
    OBJECTIVE: To investigate the presence of voids in bulk fill flowable composites. METHODS: This study investigated two well-known bulk-fill flowable composites, Smart Dentin Replacement (SDR) (Dentsply/Caulk, Milford, Germany) and Filtek bulk fill flowable (FBF) (3M ESPE, Minnesota, USA). Three ampules of each material were randomly selected. The ampules were subjected to 3D Micro-CT (General Electric Phoenix V|Tome|X L240) reconstruction in order to assess the presence of any voids within the ampules. RESULTS: Voids were present in all the ampules. The total void percentage for each group of three ampules was found to be SDR : 1.147 % and FBF : 0.0424 %. There was a significant difference between the volume of voids for SDR and FBF, p-value=0.003924. CONCLUSION: Voids were found in the randomly selected samples of bulk-fill flowable composites. This is undesirable and manufacturers should be urged to ensure that no voids are present, or at least are minimized in the ampules of material.
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    Tensile and high cycle fatigue properties of annealed TI6AL4V (ELI) specimens produced by direct metal laser sintering
    (Southern African Institute for Industrial Engineering, 2018-11-08) Malefane, L. B.; Du Preez, W. B.; Maringa, M.; Du Plessis, A.
    ENGLISH ABSTRACT: The tensile and high cycle fatigue (HCF) properties of high temperature annealed (HTA) direct metal laser sintered (DMLS) Ti6Al4V extra low interstitial (ELI) machined and polished specimens were investigated. The HTA heat treatment of the specimens resulted in the nucleation and growth of the alpha and beta grains from the acicular 𝛼 martensite grains, improving their elongation to failure. The specimens were micro-CT scanned in an attempt to relate the pores in the specimens to their fatigue properties. The micro-CT pore information from suspected crack initiation pores on the surfaces of eventual fracture was used to calculate the stress intensity factors, which correlated well with the decreasing cycles to failure of the fatigue test specimens for all three build directions. Three representative specimens were analysed, and the ‘killer pore’ was identified in each micro-CT scan and fractograph, all of which were proximal to the surface of the specimen.