1,720,983 research outputs found
Evaluation of Bi2O3 and Sb6O13 as oxidants for silicon fuel in time delay detonators
Get PDFDissertation (MSc)--University of Pretoria, 2008.This study considered bismuth (III) oxide (Bi2O3) and antimony hexitatridecoxide (Sb6O13) as potential substitutes for the red lead (Pb3O4) and barium sulphate (BaSO4) oxidants currently used in time delay compositions for detonator assemblies. Fine silicon powders with a specific surface area of 2 - 10 m2/g were used as fuels. Some experiments were also done with a coarse manganese powder as fuel. Bi2O3 was synthesised by the thermal decomposition of (BiO) 2CO3 by heating at 460°C for 15 hours. The yield was near quantitative, ie. close to the 91,4% expected based on the complete conversion of the carbonate to the oxide. Sb6O13 was obtained by heating colloidal antimony pentoxide (Sb2O5) for 8 hours at 315°C. This resulted in a ca. 20 % mass loss and yielded a reactive black powder. In the Si-Bi2O3 system, compositions in the range 5 - 40% by mass Si were ignitable with shock tubing. Burn rates measured in lead tubes varied between 15 and 155 mm/s. This highest burn rate was obtained with 20% silicon. Addition of additives such as KMnO4 and boric oxide had little effect on the burn rate. The fast burning Si-Bi2O3 system is a potential replacement for the commercial Si - red lead system. The burning rate decreased with increasing compaction of the samples. Burn rate also decreased when the aluminium instead of lead tubes were used. This is attributed to a greater heat loss with the former. The combustion products were characterised using DTA, FT-IR, XRD and SEM. The results show that the combustion reactions led to reduction of the oxidant to the corresponding metal form. The Sb6O13-Si system requires an initiating composition such as Bi2O3-50%Si (Type 4). It is slow burning and thus a possible replacement for the commercial BaS04-Si system. The lowest sustainable and reproducible burn rate, in the absence of additives, was 4,8 mm/s. It was achieved using 10% silicon Type 4. Adding small amounts of fumed silica <2%) increased the burn rate. This is attributed to better mixing and compaction. However, lower burn rates (~2 mm/s) are possible if more fumed silica is added as inert diluent. Replacing the silicon fuel with manganese powder gave more exothermic and even slower burning compositions.Graduate School of Technology Management (GSTM)unrestricte
Coating processes towards selective laser sintering of energetic material composites
Get PDFDissertation (MSc (Applied Science:Chemical Technology))--University of Pretoria, 2019.This research aims to contribute to the safe methodology for additive manufacturing (AM) of
energetic materials. Coating formulation processes were investigated to find a suitable method
that may enable selective laser sintering (SLS) as the safe method for fabrication of high
explosive (HE) compositions. For safety and convenience reasons, the concept demonstration
was conducted using inert explosive simulants with properties quasi-similar to the real HE.
Coating processes for simulant RDX-based microparticles by means of PCL and 3,4,5-
trimethoxybenzaldehyde (as TNT simulant) are reported. These processes were evaluated for
uniformity of coating the HE inert simulant particles with binder materials to facilitate the SLS
as the adequate binding and fabrication method. The critical constraints being the coating
effectiveness required, spherical particle morphology, micron size range (>20 μm) and a good
powder deposition and flow, and performance under SLS to make the method applicable for
HEs.
Of the coating processes investigated, suspension system and single emulsion methods gave
required particle near spherical morphology, size and uniform coating. The suspension process
appears to be suitable for the SLS of HE mocks and potential formulation methods for active
HE composites. The density was estimated to be comparable with the current HE compositions
and plastic bonded explosives (PBXs) such as C4 and PE4, produced from traditional methods. The formulation method developed and the understanding of the science behind the processes
paves the way toward safe SLS of the active HE compositions and may open avenues for further
research and development of munitions of the future.Chemical EngineeringMSc (Applied Science:Chemical Technology)Unrestricte
Robustness of Bifidobacterium longum LMG 13197 encapsulated in lyophilized Vegetal BM 297 ATO-inulin lipid-based synbiotic microparticles
Get PDFDissertation (MSc)--University of Pretoria, 2015.Consumers are currently concerned about improving their health, and therefore demand foods that are beneficial to overall health. This has caused the rising interest in probiotics, which are live microorganisms which when ingested in sufficient amounts, restore balance in the gastrointestinal tract and consequently improve health. Probiotic bacteria have been incorporated into various food products which are now referred to as functional foods, and represent about 65% of the world‟s food market. Probiotics are sensitive to various environmental factors such as oxygen, moisture, pH and temperature. It is of great importance that probiotics remain viable and alive throughout the stages of processing, storage in food products and during gastrointestinal transit in order for them to confer health benefits. The use of prebiotics and microencapsulation to protect and ensure viability of probiotics has been used in food industries. Challenges faced when using most microencapsulation techniques include the need for a food grade encapsulating material, stability of the probiotic cells during encapsulation processes and storage, the need to minimize negative effects they might have on the organoleptic properties of foods into which they are incorporated. The freeze drying technique, which is known to be suitable for the preservation of probiotic cells, avoids heat induced injuries to cells and also slows down detrimental chemical reactions, was used in the current study to prepare microparticles encapsulating probiotic bifidobacteria. Due to limited reports on the use of lipid based food grade encapsulating materials for the microencapsulation of probiotics, this study explored the use of such materials and developed a lipid based synbiotic material which is expected to protect and improve probiotic viability. A lipid based excipient Vegetal BM 297 ATO and various concentrations of the prebiotic inulin were used to prepare different formulations, followed by an investigation to determine which concentration of inulin resulted in better protection and survival of Bifidobacterium longum LMG 13197 during the freeze drying process. Bifidobacterium longum LMG 13197 was successfully encapsulated in Vegetal using freeze drying method. It was observed that the formulation prepared with 2% (w/v) inulin resulted in better protection of B. longum LMG 13197 during the encapsulation process. Characterization of the microparticles revealed that they contained high numbers of bacterial cells resulting from relatively high encapsulation efficiency. The presence of inulin resulted in microparticles with an acceptable size which is desirable for food applications. These results led to further investigation of the potential of Vegetal-inulin matrix to protect bifidobacteria in simulated gastrointestinal fluids and improve shelf life under different storage conditions. This study demonstrates that the Vegetal-inulin matrix protected B. longum LMG 13197 during transit in the simulated gastric fluid (SGF) and subsequently released the cells in the simulated intestinal fluid (SIF). In comparison with the unencapsulated cells, the number of cells released in SIF was higher, which suggests that the Vegetal-inulin matrix has the potential to release probiotics in the colon for health benefits to be exerted. The shelf life of encapsulated B. longum LMG 13197 powders stored in glass bottles was investigated under two different storage temperatures for 6 weeks. The study demonstrates that although there was a high loss of viable probiotic cells during storage at 25°C, Vegetal-inulin matrix improved survival of probiotics for 3 weeks as opposed to the unencapsulated cells. On the other hand, encapsulation with Vegetal did not offer improved survival of bacteria when compared to the unencapsulated cells at 4°C, but the addition of inulin offered better protection for up to 5 weeks. Therefore, better shelf life of Vegetal-inulin microparticles containing B. longum LMG 13197 can be achieved at 4°C than at 25°C.tm2015Microbiology and Plant PathologyMScUnrestricte
Enhanced oral bioavailability of the antiretroviral efavirenz encapsulated in poly(epsilon-caprolactone) nanoparticles by a spray-drying method
Get PDFAIM: To encapsulate efavirenz (EFV) within poly(epsilon-caprolactone) (PCL) nanoparticles (NPs) and compare the oral pharmacokinetics with that of EFV-loaded micelles and pure EFV NPs. MATERIALS & METHODS: EFV-loaded PCL NPs were produced by a double-emulsion/spray-drying method. RESULTS: NPs displayed a hydrodynamic diameter of 200-250 nm. The encapsulation efficiency was 86-93% and the mass recovery was above 60%. X-ray diffraction indicated that drug and PCL underwent amorphization during the spray-drying process. Encapsulation within NPs significantly increased the maximum concentration in plasma and the bioavailability. CONCLUSION: EFV-loaded PCL NPs represent a promising platform to develop scalable pharmaceuticals with improved (bio)pharmaceutic performance.Fil: Tshweu, Lesego. Council for Scientific & Industrial
Research; SudáfricaFil: Katata, Lebogang. North-West University. Faculty of Agriculture, Science & Technology. Chemistry Department; Sudáfrica. Council for Scientific & Industrial
Research; SudáfricaFil: Kalombo, Lonji. Council for Scientific & Industrial
Research; SudáfricaFil: Chiappetta, Diego Andrés. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Tecnología Farmacéutica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Höcht, Christian. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Farmacología; ArgentinaFil: Sosnik, Alejandro Dario. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Tecnología Farmacéutica; Argentina. Technion - Israel Institute of Technology; IsraelFil: Swai, Hulda. Council for Scientific & Industrial
Research; Sudáfric
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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