Induktiefgekoppelde plasmas: die rol van die skermgas in hoëdrywingstoerusting

Induktiefgekoppelde-plasmareaktore (IGP’s) het toepassings in verskeie industrieë, insluitend die voorbereiding van metaalpoeiers vir laagvervaardiging. Die skermgas (ook skutgas gnoem) speel ’n belangrike rol in die termiese afskerming van die reaktorwand in ’n IGP. Die energie wat verloor word deur die wand van die reaktor kan verminder word deur die hittesone weg van die wand af te beweeg. Hierdie verplasing van die hittesone word bereik deur ’n skermgas te gebruik wat moeiliker ioniseer as die plasmagas. Die ioniseringsgraad van waterstof is laer as dié van argon weens die hoër elektriese geleidingsvermoë van argon by soortgelyke temperature. Waterstof word dus in klein hoeveelhede in die skutgas gebruik met argon as die hoof bestandeel en hoofplasmagas. Die waterstof voorkom dus plasmavorming naby die wand. Die skutgas het ook ’n heelwat hoër vloeisnelheid en verminder sodoende die beskikbare tyd vir hitte-oordrag na die wand. Die besondere hoë temperature wat in ’n IGP bereik word, belemmer egter die meting van eenvoudige lesings soos vloeisnelheid en temperatuur. Rekenaarmodelle voorsien ons van die geleentheid om die fisiese en chemiese eienskappe van ’n plasma te ondersoek asook die nodige gereedskap om die gedrag van die plasma te analiseer sonder eksperimentele lesings. Daar is verskeie numeriese modelle van IGP-sisteme in die literatuur alhoewel nie een van dié modelle die effek van die skutgassamestelling in ag neem nie. Die hoeveelheid waterstof in die skutgas kan groot newe-effekte hê op die plasmagas a.g.v. die hoër ionisasiepotensiaal van waterstof. ’n Oormaat waterstof in die skutgas is ook ’n verkwisting van voermateriaal. Albei die faktore het ’n invloed op die ekonomiese uitvoerbaarheid van die plasmaproses. Hierdie navorsing het beoog om die optimale skutgassamestelling te vind vir die reaktor wat by Necsa gebruik word vir sferoïedisering. Die werk is uitgevoer met die kommersiële eindige-elementsagtewarepakket COMSOL Multiphysics R. Hierdie rekenaarmodel dui daarop dat die wand beskerm kan word van plasmavorming met ’n waterstof/argon skutgas wat sodoende ook die energieverliese deur die wand verminder. Waterstof verbeter die skutgas se hitte-oordragvermoë, maar verskuif die hittesone weg van die wand af. As gevolg van hierdie twee kompeterende meganismes bestaan daar ’n optimale bedrywingspunt by 3 vol% H2 in die skutgas. Die model is bevestig deur die energiebalans van die model te vergelyk met eksperimentele resultate.Dissertation (MEng)--University of Pretoria, 2020.Advanced Metals Initiative Suid Afrikaanse Akademie vir Wetenskap en KunsChemical EngineeringMEngUnrestricte

Fossil evidence of Archaean life

Evidence for the existence of life during the Archaean segment of Earth history (more than 2500 Myr ago) is summarized. Data are presented for 48 Archaean deposits reported to contain biogenic stromatolites, for 14 such units reported to contain 40 morphotypes of putative microfossils, and for 13 especially ancient, 3200–3500 Myr old geologic units for which available organic geochemical data are also summarized. These compilations support the view that life's existence dates from more than or equal to 3500 Myr ago

A controlled human infection model of Streptococcus pyogenes pharyngitis (CHIVAS-M75): an observational, dose-finding study

Background: Streptococcus pyogenes is a leading cause of infection-related morbidity and mortality. A reinvigorated vaccine development effort calls for new clinically relevant human S pyogenes experimental infection models to support proof of concept evaluation of candidate vaccines. We describe the initial Controlled Human Infection for Vaccination Against S pyogenes (CHIVAS-M75) study, in which we aimed to identify a dose of emm75 S pyogenes that causes acute pharyngitis in at least 60% of volunteers when applied to the pharynx by swab. Methods: This observational, dose-finding study was done in a clinical trials facility in Melbourne (VIC, Australia). Groups of healthy volunteers aged 18–40 years, at low risk of complicated S pyogenes disease, and without high type-specific anti-emm75 IgG antibodies against the challenge strain were challenged and closely monitored as inpatients for up to 6 days, and then as outpatients for 6 months. Antibiotics were started upon diagnosis (clinical signs and symptoms of pharyngitis and a positive rapid molecular test) or after 5 days in those without pharyngitis. Rapid test results were confirmed by standard bacterial culture. After a sentinel participant, cohorts of five and then ten participants were challenged, with protocol-directed dose-escalation or de-escalation for subsequent cohorts. The primary outcome was the proportion of participants at each dose level with pharyngitis by day 5 after challenge. The study is registered with ClinicalTrials.gov, NCT03361163. Findings: Between July 10, 2018, and Sept 23, 2019, 25 healthy adults were challenged with emm75 S pyogenes and included in analyses. Pharyngitis was diagnosed in 17 (85%; 95% CI 62–97) of 20 participants at the starting dose level (1–3 × 105 colony-forming units [CFU]/mL). This high proportion prompted dose de-escalation. At the lower dose level (1–3 × 104 CFU/mL), pharyngitis was diagnosed in one of five participants. Immunological, biochemical, and microbiological results supported the clinical picture, with acute symptomatic pharyngitis characterised by pharyngeal colonisation by S pyogenes accompanied by significantly elevated C-reactive protein and inflammatory cytokines (eg, interferon-γ and interleukin-6), and modest serological responses to streptolysin O and deoxyribonuclease B. There were no severe (grade 3) or serious adverse events related to challenge. Interpretation: We have established a reliable pharyngitis human infection model with reassuring early safety findings to accelerate development of vaccines and other interventions to control disease due to S pyogenes. Funding: Australian National Health and Medical Research Council.SCOPUS: ar.jinfo:eu-repo/semantics/publishe