1,720,974 research outputs found
Development of single-, few- and multiple-nanowire gas-sensor two-terminal devices on ceramic substrates and characterization by impedance spectroscopy
No full textNano-size effects on optical, structural and phononic properties of VO2 and WO3 by ultrasonic-nebulizer spray pyrolysis technique
Full text linkStudent Number : 0420699F -
MSc Dissertation -
School of Physics -
Faculty of ScienceThis dissertation presents for the first time the conditions for the synthesis of VO2 by ultrasonic
nebula-spray pyrolysis (UNSP) from a precursor solution of NH4VO3+VCl3 optimized as follows:
a carrier gas of argon at a flow rate of 11 liters per minute, a furnace temperature of 400 to 700oC.
This work also incorporates thermodynamic variables of Tpr-P-V into the equations that relate
the mean diameter of droplets, D, to frequency of the exciting ultrasound waves, f, the density of
the precursor solution, and the surface tension, , previously worked on independently by Lang
and Jokanovic. The incorporation results in the diameters of the droplets (and consequently the
collected grains) being smaller as p and Tpr are increased in a non- linear form. The variable V,
however, increases the diameter of the droplets as it is allowed to increase. This study shows the
departures many authors find of the theory from experiment but it also shows that the departure
does not lie in the equations but rather on post- synthesis and annealing effects. From X-ray
diffraction, scanning electron microscopy (SEM) and Raman spectroscopy, this study shows that
as furnace temperature is increased the morphology of the sample surfaces for both VO2 and WO3
transforms from amorphous to crystalline, from spherical grains to plate-like structures, with grain
mean diameter increasing non-linearly in some cases and decreasing non-linearly in other cases
confirming previous findings, the latter enjoying the majority vote. In Raman spectra of the as-
obtained WO3, asymmetric broadening of the Raman peaks was observed in some samples and
a phonon confinement model was employed in the size distribution prediction. These findings
prompted the re- workout of the phonon confinement model. In this dissertation an equation has
been derived based on the Faucet-Campbell equation of the PC model. The new equation relates the
ratio of neighboring peaks in a material’s Raman spectrum to the mean diameter of the grains. The
present modification allows the PCM model to predict the grain size beyond the current limiting
range of 0 to 100 nm. Analysis of the experimental data using this equation unveils two different equations- one for particles of size below 100 nm and the other equation for particles with larger
that 100 nm. Also this analysis has enabled the present study to evaluate the phonon dispersion
relations for WO3
Detection of acetone using nanostructured WO3 for diabetes mellitus monitoring applications
Full text linkThesis (PhD (Biochemistry))--University of Pretoria, 2020.Diabetes mellitus which is characterized by a high levels of blood glucose is a major source of mortality, morbidity and health costs worldwide. Major gaps exist in efforts to comprehend the burden nationally and globally, especially in developing nations, due to a lack of accurate, cheap and non-invasive data and devices for monitoring and surveillance. In Africa, type 2 diabetes mellitus represents 90% of diabetes cases. The disease mainly relies on management and monitoring. Although reliable blood glucose monitoring techniques and devices exist worldwide, the challenge is with the cost, invasiveness, and long sample preparation. Herein this study, the challenge was addressed by synthesizing WO3 materials for the detection of acetone in a simulated human breath. Acetone has been reliably confirmed to be the biomarker of diabetes mellitus. The Gas Chromatography-Mass Spectrometry (GC-MS) was employed to quantify acetone in type 2 diabetes mellitus. A statistically significant correlation (R=0.756) between blood glucose and breath acetone was observed, between blood acetoacetate and breath acetone (R=0,897), and between beta-hydroxybutyrate and breath acetone (R=0,821). Furthermore, we used semiconducting metal oxide (WO3) to investigate its selectivity, sensitivity, and response towards acetone. Semiconducting metal oxides sensor has the potential to detect volatile organic compound (VOCs) at low concentrations as low as 0.1 ppb. Other advantages of semiconducting metal oxides sensors include, facile and cheap device fabrication, portability, real-time analysis, and facile operating principle. We used two synthesis methods for fabrication of acetone sensors namely solvothermal method whereby solvent ratios were varied, and the sol-gel method where carbon nanospheres were used as a template and cobalt as a dopant. The sensor fabricated with 51:49 water: ethanol is found to demonstrate high response and good selectivity to 2 ppm level of acetone when compared with the one fabricated with pure ethanol, 18:92 (ethanol: water) and 92% water. Furthermore, the sensor could respond to low concentrations of acetone ranging from 0.5 to 4.5 ppm of acetone at 100 °C. For the sol-gel method, the 0.6 % Co-doped WO3 showed higher response and selectivity towards acetone gas from as low as 0.5 ppm at a very low operating temperature of 50 °C. Contrary, there was a very low response from other gases including toluene, NO2, NH3, CH4 and H2S operating at a similar temperature. This highlights the acetone selectivity of our 0.6 % Co-doped WO3 sample. Based on the two methods used for the synthesis of the acetone sensor, we can conclude that the Co-doped sensor shows better performance as compared to the as-prepared WO3. This is from the findings that the Co-doped WO3 can respond and select acetone concentration at 50 ◦C, which is a very low temperature in comparison to other platforms described in literature. An envisioned portable point of care diabetic device could therefore be operated at 50 ◦C in any point of care setting.Council for Scientific and Industrial ResearchBiochemistryPhD (Biochemistry)Unrestricte
Laser-pyrolysis and flammability testing of graphite flame-retarded polyethylene
Full text linkDissertation (MSc)--University of Pretoria, 2018.The fire behaviour of linear low-density polyethylene composites containing 10 wt.% of different carbon-based fillers was studied. Cone calorimeter tests conducted at a heat flux of 35 kW_m_2 showed that the expandable graphite sample reduced the peak heat release rate by about 50 % while the flake graphite increased the ignition time by about 80 %. Pyrolysis combustion flow calorimetry results were practically identical for all composites. This reveals shortcomings of this bench-scale flammability test method when the flame retardancy mechanisms relies on either the development of physical barrier layer at the surface of the burning sample or on reflecting the incident heat flux. Similarly, it was found that laser pyrolysis-thermogravimetric analysis generated outcomes that did not correlate with the cone calorimeter results at all. In particular, the composite based on expandable graphite performed poorly. The likely explanation is that the aspect ratios of the small samples were such that the barrier effects on which this system relies, was negated by edge effects.Chemical EngineeringMScUnrestricte
Nano-size effects on opto-electronic, structural and vibrational properties of vanadium and tungsten oxides produced by laser and ultrasonic spray pyrolysis techniques
No full textThe thesis reviews two pyrolysis techniques – ultrasonic spray pyrolysis
(USP) and laser pyrolysis (LP). The two techniques - USP at the Physics LP at
the National Laser Centre, CSIR in Pretoria, South Africa – were designed and
assembled by the candidate for the purpose of producing two related
materials - VO2 and WO3. The two smart materials find applications in energy
regulation for air conditioning alternatives, gas sensing for pollution control
purposes, recording industry and computer memory. The thesis shows the
pursuit for small particles of these materials in order to see the change in their
properties at nano-scale. Novel structures that were not expected were foundnanowires
and nanotips of WO3 and nanobelts and nano-ribbons of VO2. The
confinement of the 700 cm-1 and 800 cm-1 optical phonons is reported in WO3
nanowires, enhanced thermochromism of VO2 nano-structures with a
hysteresis width of 80oC is presented, the 145 cm-1 phonon splitting is
reported in VO2 nanoribbons and attributed to surface phonons as a greater
portion of atoms become surface atoms at nanoscale. A number of theoretical
models have been proposed in order to explain some inexplicable
phenomena: the new solid-vapor-solid growth mechanism of the nanowires, a
modified phonon confinement model to suit phononconfinement in nanoribbons
geometry, a model to relate the hysteresis width in the
thermochromism of VO2 to the ribbon thickness and grain size based on
martensitic type of transformations and a simple “charge-up” model to
predict how hot the laser-aerosol interaction zone gets at various laser power
settings. More questions have been unearthed and these are also addressed and the way forward is proposed
Gas sensing properties of nanostructured vanadium oxide semiconductors by chemoresistive and optical methods
Full text linkThesis (PhD)--University of Pretoria, 2017.The aims of this research thesis are to synthesise VO2, V2O5 and V6O13 nanostructures and apply the materials on sensor electrodes for gas and humidity sensing. These materials were synthesised and optimised using chemical vapour deposition (CVD), microwave assisted and pulse laser deposition (PLD) techniques. Analyses with thermogravimetric (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), Raman and Fourier transform infrared (FTIR) spectroscopy showed VOx phases order as NH4VO3 ? VO2 + V2O5 (150 – 200 °C) ? V6O13 (300 °C) ? V2O5 (above 350 °C). This is when the precursor NH4VO3 was annealed in CVD between 100 – 350 °C in H2 atmosphere for 2 hrs. Adsorption analysis of VOx nanostructures showed a profile of Brunauer-Emmett-Teller (BET) surface areas which increased with the annealing temperature until 300 °C after which the transition occurred. Humidity (%) sensing response of VOx showed high response for V6O13 and V2O5 phase whereas, the Langmuir isotherm plot in the form of the response per BET surface area with respect to different levels of relative humidity showed high response for VO2. Phase evolution diagram based on these properties has been proposed. Thermal CVD annealing of NH4VO3 at 500 °C in N2 atmosphere for 2, 12 and 24 hours produced monoclinic V6O13 (at 2 hrs) and ?-orthorhombic V2O5 (at 12 and 24 hrs) nanorod structures using the above characterization techniques. Gas sensing application of these structures revealed that the H2S gas is selective in adsorption to V6O13 phase with 132 % response magnitude at 350 °C and 60 ppm, this response is 647.2 % higher than that of NH3, CH4, NO2, H2 and CO. The response and recovery times are 32 and 129 s respectively which is remarkably short compared with the data in literatures. This V6O13 sensor was ranked with its V2O5 counterpart and still found to be 238.5 % higher for H2S gas. Density functional theory (DFT) through ab initio molecular dynamics of (110) facet of monoclinic V6O13 and ?- orthorhombic V2O5 also showed high H2S adsorption energy for V6O13 than V2O5 with a profile which simulate the experimental findings. Low temperature microwave assisted synthesis of VOx from NH4VO3 without post-annealing treatment demonstrated small size homogeneous crystallite with high BET surface area and high adsorption and desorption pores. These properties translated to sub-ppm room temperature sensing of the flammable CH4 and odorant NH3 and toxic NO2 with high sensitivity. The VO2 (B) phase produced via the same microwave process applied for humidity sensing in the lateral gate metal oxide semiconductor field effect transistor (MOSFET) configuration for 0, 5, 8, 10, 12 and 15 V gate voltages. An optimum percentage humidity response observed at 5 V showed response and recovery times in the order of 60 – 70 s which is remarkably shorter than the ? 300 s response of the non-gated VO2 humidity sensor reported in this thesis. Statistical information extracted from the non-linear S-curve Hill Dose rate showed that the VO2 (B) sensor is very resilient to relative humidity by showing the humidity level of more than 100% where the response of the sensor could be reduced to 50%. In-situ Raman spectroscopy sensing of NH3 gas at the surface of PLD deposited V2O5 thin film was presented. The film crystal structure, depth profile and oxidation state was studied by cross section scanning electron microscope (SEM), time of flight secondary ion mass spectroscopy (TOF-SIMS), XPS and group theoretical analysis. Recoverable red shift of 194 cm-1 and blue shift of 996 cm-1phonons upon the interaction with the NH3 gas at 25 and 100 °C was observed. Decrease in the Raman scattered photons of the 145 cm-1 phonon was also observed for different levels of NH3 exposure. The responses of these phonon properties in NH3 environment compared to the chemoresistive sensing of the film at 40 ppm showed that the in-situ Raman spectroscopy techniques is not only more sensitive but also demonstrated possibility for selective gas detection via blue and red shift of phonon frequencies.PhysicsPhDUnrestricte
Indigenous natural dyes for Gratzel solar cells : Sepia melanin
Full text linkDye-sensitized Solar Cells (DSSC), also known as Grätzel cells, have been identified as a cost-effective, easy-to-manufacture alternative to conventional solar cells. While mimicking natural photosynthesis, they are currently the most efficient third-generation solar technology available. Among others, their cost is dominated by the synthetic dye which consists of efficient Ruthenium based complexes due to their high and wide spectral absorbance. However, the severe toxicity, sophisticated preparation techniques as well as the elevated total cost of the sensitizing dye is of concern.
Consequently, the current global trend in the field focuses on the exploitation of alternative organic dyes such as natural dyes which have been studied intensively. The main attractive features of natural dyes are their availability, environmental friendly, less toxicity, less polluting and low in cost. This contribution reports on the possibility of using sepia melanin dye for such DSSC application in replacement of standard costly ruthenium dyes.
The sepia melanin polymer has interesting properties such as a considerable spectral absorbance width due to the high degree of conjugation of the molecule. This polymer is capable of absorbing light quantum, both at low and high energies ranging from the infrared to the UV region.
The comprehensive literature survey on Grätzel solar cells, its operating principle, as well as its sensitization by natural dyes focusing on sepia melanin has been provided in this master’s dissertation. The obtained results in investigating the morphology, chemical composition, crystalline structure as well as optical properties of sepia melanin samples using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy x-ray diffraction, X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), Raman spectroscopy, UV-VIS absorption spectroscopy as well as Photoluminescence (PL) for Grätzel solar cell application have been reported.
These results represent an important step forward in defining the structure of melanin. The results clearly show that sepia melanin can be used as natural dye to DSSC sensitization. It is promising for the realization of high cell performance, low-cost production, and non-toxicity. It should be emphasized here that natural dyes from food are better for human health than synthetic dyes.M. Sc. (Physics)Physics1 online resource (xii, 101 leaves) : illustration
Synthesis of nickel oxide/hydroxides and their nanocomposites with carbon materials for supercapacitor and gas sensing applications
Full text linkThesis (PhD)--University of Pretoria, 2017.The goal of this thesis is to produce NiO- and Ni(OH)2-carbon based nanocomposites and
explore their possible adoption as active electrode materials in supercapacitor and gas sensing
applications. Field-emission scanning electron microscopy (FESEM), transmission electron
microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction
(XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermal
gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and gas adsorption
analyses were utilized to evaluate the structure and morphology of all samples in this study.
The major aim of integrating carbon-based nanomaterials (graphene foam, graphene oxide
and activated carbon) into Ni-based oxides and hydroxides in this study is to take advantage
of their outstanding characteristics. These include good electrical conductivity, high corrosion
resistance, large SSA, low-cost, good cyclic and temperature stability, as well as the
capability to serve as a substrate for growth of other materials to form a suitable composite.
The electrochemical evaluation as a potential supercapacitor electrode was employed in a
three (3)-electrode configuration for the as-prepared Ni(OH)2/carbon based electrodes (NiOH)2/graphene foam and Ni(OH)2/graphene oxide electrodes) while the gas sensing
characteristics of NiO/carbon-based electrodes were investigated using NCSM-CSIR gas
sensing station controlled by a KEITHLEY pico-ammeter system. The electrochemical results
of Ni(OH)2/carbon-based electrodes have demonstrated a superior electrochemical
performance as compared to the pristine Ni(OH)2 electrodes with the results comparable and
even better than some earlier related studies available in the literature. Similarly, NiO/carbonbased
electrodes in the form of NiO/graphene foam and NiO/activated carbon electrodes both
exhibited enhanced gas sensing properties in comparison to the pristine NiO electrode due to
the increased specific surface area and electrical conductivity that are linked to its sensing
response, response time and recovery time. Thus, the results obtained from these studies have
clearly established the viability of these carbon-based nanomaterial composites as promising
candidates for electrochemical supercapacitor and gas sensing applications.CSIR-National Centre for Nano-Structured MaterialsSouth African national research foundation (NRF)PhysicsPhDUnrestricte
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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