In hydrometallurgical copper production plants, titanium-based electrodes coated with a conductive layer of IrO2-Ta2O5 are widely used as anodes in acidic copper electrowinning baths because of their long service life and low overpotential for oxygen evolution. The presence of trace amounts of ions such as fluoride, aluminum, and iron in sulfate-based electrowinning baths is believed to affect the stability of IrO2-Ta2O5 coated anodes. Hence, in this study, the effect of fluoride and metallic cations on the lifetime of IrO2-Ta2O5 coated Ti electrodes in sulfuric acid solutions has been investigated, and a degradation mechanism for IrO2-Ta2O5 coatings in the presence of fluoride has been proposed. Typical lifetime of the conductive ceramic coated anodes is 1 to 2 years. In order to predict electrode performance over this long period, an accelerated laboratory test that can be carried out in a few weeks is often used. This test, known as accelerated lifetime test (ALT), is conducted by electrically stressing the anodes at a current density that is much higher than the current density used for electrowinning while monitoring the change in the cell potential. The time required for the cell voltage to increase by 5 V is taken as the accelerated lifetime of the oxide electrode. In this research, titanium mesh samples coated with mixed iridium oxide-tantalum oxide layers were tested as anodes in 2 M sulfuric solution a constant current density of 0.54 A/cm2. A two-electrode cell with a bare titanium mesh serving as the cathode was used for experiments. In addition to ALTs, anodic polarization measurements were also carried out to study the changes in overpotential for oxygen evolution on electrodes before and after ALTs. Additionally, morphology and chemical composition analyses were performed on electrodes before and after ALTs using various techniques such as scanning electron microscopy (SEM) analysis, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Chemical species that are likely to be present in the electrowinning bath were predicted using the computer software STABCAL and presented in distribution-pH and Pourbaix diagrams. The results of multiple ALTs in the absence and presence of various levels of fluoride indicate that the anode lifetime was greatly reduced by the presence of fluoride in sulfuric acid solutions. The greater the amount of fluoride added, the shorter the anode lifetime. Additionally, both in the absence and presence of fluoride, the molar ratio of IrO2 to Ta2O5 in the coating did not change during dissolution. In studying strategies to prolong the lifetime of the electrode in a fluoride-containing solution, a method of complexing fluoride ions using metallic cations such as Al3+ and Fe3+ was developed and demonstrated. The anode lifetime was successfully prolonged from 200 to over 500 hours with the addition of aluminum ions to a fluoride-containing solution. Compared with ferric ions, aluminum ions are more efficient in complexing with fluoride to extend the lifetime of electrodes.

Abstract Dental caries is a common infectious disease affecting young children living in low-income families in the United States. Efficacious, safe, feasible and cost-effective caries prevention methods for these children are essential. Several studies have shown the efficacy of fluoride varnish and xylitol to improve the oral health of children. The efficiency of caries prevention programs including the use of fluoride varnish or xylitol chewing gum in early childhood has not been well documented in communities with private dental services. The purpose of this study was to determine the caries prevalence and distribution in the primary dentition, and to evaluate the effectiveness of two prevention programs, including applications of fluoride varnish and the use of xylitol chewing gum in randomized groups of preschool children attending Head Start school programs in Northern Florida. The caries preventive effect of fluoride varnish in the primary dentition was evaluated in a sample of 4–6 year-old Head Start schoolchildren in Alachua, Florida (n = 142). Caries progression after nine months was analyzed using dmf(s/t) and ds values. A modified caries scoring system, which differentiates between active and inactive carious lesions, was used to evaluate the effect of fluoride on early noncavitated enamel lesions. The effect of xylitol gum was evaluated by measuring the levels of salivary mutans streptococci before and after a three week chewing period in 3–5 year-old children attending the Head Start school in Starke, Florida (n = 61). This study is in line with earlier reports that caries prevalence is high in Head Start preschool children. This study showed that active noncavitated enamel lesions were common in the primary dentition and that applications of fluoride varnish may offer an effective means of arresting these early enamel lesions. Chewing the xylitol gum reduced the levels of salivary mutans streptococci, thereby possibly reducing the risk for dental caries in these children. While the detection and monitoring of early enamel lesions is critical in determining effectiveness of prevention therapy, this study suggests that fluoride varnish applications may offer an efficient, non-surgical treatment for decay in children. Also, the prevention program with xylitol may provide an additional method to be used in situations where other prevention methods are difficult to implement.

No description available.

A nonuniform annealing rate of color centers in LiF is attributed to a nonuniform density of dislocations which, in turn, is a direct consequence of the cleaving process. The tensile deformation properties of annealed LiF crystals were determined over the temperature range – 196 ⁰C. to +60 ⁰C. Several tests were also carried out on ɣ - irradiated specimens at ambient temperatures. A transition occurs in the work hardening rate during stage II deformation. Experiments involving both strain-rate and temperature cycling were performed over the limited temperature range of - 60 to +60 ⁰C. The results were analyzed in terms of rate theory expressions and indicated that the rate controlling mechanism for dislocation motion in stage IIA is probably the nonconservative motion of jogs in screw dislocations. Stage IIB hardening is more likely controlled by dislocation intersections. Evidence is presented which indicated that stress relaxation experiments may provide an extremely easy technique for the determination of the internal flow stress. Values obtained from such experiments on LiF agree remarkably well with those obtained from rate theory experiments. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The magneto-optical Faraday effect was used, to observe the spin-bath relaxation in Europium doped CaF₂ crystals at helium temperatures from 1.5 to 4.2° K. Theory shows that the magnitude of the Faraday rotation is proportional to the difference in population of the ground state doublet. The experimental method used in this thesis is the same as that which was successfully used by Glattli and by Griffiths. Pulsed microwave power at X-band frequencies was employed to disturb the equilibrium between the spin system and the bath. The observed relaxation time t was expected to have a T⁻¹ temperature dependence hut because of the orientation of the crystal in our dc magnetic field which resulted in the seven transition lines being close together, cross-relaxation effects produced a temperature dependence of approximately T⁻². It is suggested that exchange coupled pairs of Eu²⁺ and clusters involving Eu³⁺ may account for this temperature dependence rather than there being a phonon bottleneck present. The field dependence of the relaxation time was found not to be directly related to the resonance spectrum, The Eu²⁺ concentration of the crystals used in this thesis was .2% / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Orientador: Marinês Nobre dos Santos Uchôa / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Odontologia de Piracicaba / Made available in DSpace on 2018-08-22T09:08:01Z (GMT). No. of bitstreams: 1 Rodrigues_LiviaPagotto_M.pdf: 2190671 bytes, checksum: 59ef6221009e42c7c0c783952e447920 (MD5) Previous issue date: 2013 / Resumo: Este estudo teve como objetivo verificar in situ se a irradiação do esmalte com laser de dióxido de carbono (CO2) com comprimento de onda 10,6 ?m e densidade de energia de 20,0 J/cm2 associada ao emprego de um adesivo ortodôntico seria capaz de reduzir a perda mineral do esmalte ao redor de bráquetes ortodônticos, submetidos à situação de alto desafio cariogênico. Para isto, 80 dentes terceiros molares humanos retidos, foram cortados, obtendo-se ao final 160 espécimes de 5 x 5 x 2 mm, os quais foram divididos entre os grupos. Durante duas fases de 14 dias cada, 20 voluntários (idade média de 24 anos) utilizaram um dispositivo intraoral palatino contendo quatro espécimes de esmalte dentário humano, que foram aleatoriamente divididos em quatro grupos: (1) Adesivo + Dentifrício não fluoretado (DNF) (Controle), (2) Laser de CO2 + Adesivo + Dentifrício não fluoretado (DNF), (3) Adesivo + Dentifrício fluoretado (DF) e (4) Laser de CO2 + Adesivo + Dentifrício fluoretado. Para simular o alto desafio cariogênico os voluntários gotejaram sobre os espécimes dentais, solução de sacarose a 20%, oito vezes ao dia, em horários pré-determinados. Após cada fase, foi coletado o biofilme ao redor dos bráquetes e analisada a concentração de flúor no biofilme seco empregando-se eletrodo específico para íon flúor. A perda mineral do esmalte ao redor dos bráquetes ortodônticos foi avaliada indiretamente pela análise de microdureza. Os resultados da concentração de flúor no biofilme seco foram analisados pelo teste de Lilliefors e teste de Kruskal-Wallis, seguidos do teste de Student Newman-Keuls (? = 0,05); e os resultados da perda mineral foram analisados pelo teste de Lilliefors para verificação da normalidade dos dados seguidos do teste ANOVA. A análise da concentração de flúor no biofilme seco mostrou que os grupos 1 e 2 diferiram dos grupos 3 e 4, sendo que nos grupos 3 e 4 a concentração de flúor foi significativamente maior que aquelas observadas nos grupos 1 e 2. Nas condições experimentais empregadas nesta pesquisa, o sistema adesivo mostrou-se efetivo em inibir a desmineralização, in situ, ao redor de bráquetes ortodônticos, mediante alto desafio cariogênico. Assim, a presença ou não do laser de CO2 e do dentifrício fluoretado não teve influência quanto à perda mineral do esmalte ao redor de bráquetes ortodônticos / Abstract: The aim of this study was to verify whether the irradiation of carbon dioxide (CO2) laser with a wavelength of 10.6 ?m and density of 20.0 J/cm2 combined with the use of an orthodontic adhesive could reduce enamel demineralization around orthodontic brackets, subjected to a situation of high cariogenic challenge in the oral environment. For this, 80 human third molars retained were cut into 160 specimens of 5 x 5 x 2 mm, which were divided into the groups. During two phases of 14 days each, 20 volunteers (mean age 24 years) used a device intraoral palatal containing four specimens of human enamel, which were randomly divided into 4 groups: 1- Adhesive + Non Fluoride Toothpaste (NFT) (control), 2- CO2 Laser + Adhesive + Non Fluoride Toothpaste (NFT), 3- Adhesive + Fluoride Toothpaste (FT) and 4- CO2 Laser + Adhesive + Fluoride toothpaste. To simulate a high cariogenic challenge, volunteers dripped on the slabs a 20% sucrose solution eight times a day at predetermined times. After each phase, the biofilm around the brackets was collected and the fluoride concentration in the dry biofilm was analyzed, using an ion specific electrode. The mineral loss around orthodontic brackets was determined by cross-sectional microhardness measurement. The results of the fluoride concentration in dried biofilm were analyzed by Lilliefors and Kruskal-Wallis test followed by the Student Newman-Keuls test (? = 0.05) and the results of the mineral loss were analyzed by Lilliefors test followed by ANOVA. Regarding the fluoride concentration in the dry biofilm, the data showed a significant difference between groups 1 and 2 when compared to groups 3 and 4, and the groups 3 and 4 showed fluoride concentrations in the biofilm higher than those observed for groups 1 and 2. Under the experimental conditions employed in this study, the adhesive system was showed to be effective in reducing demineralization in situ around orthodontic brackets in a high cariogenic challenge situation. Thus, the presence or absence of CO2 laser and fluoride toothpaste showed to have no influence on the enamel mineral loss around orthodontic brackets / Mestrado / Odontopediatria / Mestra em Odontologia

No description available.

No description available.

Platelets permeabilized by high voltage electric discharges have provided a valuable model system in which to analyse the roles of Ca²⁺ ions and guanine nucleotides in the regulation of secretion by exocytosis. In the present study, the effects of fluoride or fluoroaluminate and of vanadate or pervanadate on secretion of platelet dense granule constituents, and the roles of activation of phospholipase D (PLD), phospholipase C (PLC) and protein kinase C (PKC) in secretion, have been investigated. Electropermeabilized human platelets containing [¹⁴C]5-HT in their dense granules were suspended in a glutamate medium containing ATP and incubated for 10 min at 25°C with, variously, Ca²⁺ buffers, KF/AlCl₃, vanadate/H₂O₂, guanine nucleotides and phorbol 12-myristate 13-acetate (PMA). KF/AlCl₃, which activates heterotrimeric G proteins but not low-M, GTP-binding proteins, caused a Ca²⁺ -dependent [¹⁴C]5-HT secretion; maximal effects were obtained with 10 mM KF plus 10 μM AlCl₃ at a pCa of 6, when 53% of [¹⁴C]5-HT was released. Secretion induced by KF/AlCl₃ in the presence of Ca²⁺ correlated with the stimulation of [3H]diacylglycerol accumulation in permeabilized platelets containing [³H]arachidonate-labelled phospholipids. KF/AlCl₃ also stimulated the phosphorylation of pleckstrin (P47) in permeabilized platelets incubated with [γ-³²P]ATP, indicating the activation of PKC. In the absence of Ca²⁺ (pCa > 9), KF/AlCl₃ caused none of the above effects. These actions of KF/AlCl₃ were attnbutable to the activation of PLC, since KF/AlCl₃ also stimulated the formation of [³H]inositol phosphates in [³H]inositol-labelled permeabilized platelets in the presence of Ca²⁺. PLD activity, measured as the formation of[³H]phosphatidylethanol (PEt) from [³H]arachidonate-labelled phospholipids in the presence of ethanol, could not be detected after stimulation of platelets by KF/AlCl₃ in the absence or presence of Ca²⁺. However, KF/AlCl₃ inhibited the [³H]PEt formation (PLD activity) induced by GTPγS. In the absence of Ca²⁺ (pCa >9), the inhibitory effects of KF/AlCl₃ on [¹⁴C]5-HT secretion induced by GTPγS alone or GTPγS plus PMA correlated well with their inhibitory effects on [³H]PEt formation. At pCa 6, KF/AlCl₃ had only a small inhibitory effect on GTPγS-induced secretion and inhibited GTPγS-induced PLD activity more strongly than GTPγS-induced PLC activity. These results suggest that PLD is important for Ca²⁺ -independent secretion, and that, although both PLD and PLC may play roles in Ca²⁺ -dependent secretion, PLC is likely to be the more important. In the presence of Ca²⁺, either vanadate or H₂O₂ caused concentration-dependent stimulations of [¹⁴C]5-HT secretion, [³H]DAG formation and [³H]PEt formation. At pCa 6, low concentrations of vanadate and H₂O₂, which would be expected to form pervanadate, acted synergistically to stimulate [¹⁴C]5-HT secretion, which correlated with [³H]DAG formation. However, vanadate with H₂O₂ had a biphasic effect on PLD activity that did not correlate with secretion. In addition, at pCa 6, GTPγS-induced PLD activity was abolished by vanadate with H₂O₂, whereas GTPγS-induced secretion and PLC activity were only partially inhibited. These results support the idea that both PLC and PLD are involved in the regulation of secretion but have different contributions to Ca²⁺ dependent and Ca²⁺ -independent secretion. The results are consistent with activation of platelet PLC by a heterotrimeric G protein, but suggest that different mechanisms, possibly involving a low-M, GTP-binding protein, may be involved in the regulation of PLD activity. / Thesis / Master of Science (MSc)

Magister Scientiae Dentium - MSc(Dent) / BACKGROUND: Fluorine is an abundant trace element in the Earth’s crust. The presence of fluorine in bedrocks is the primary source fluoride in ground water, though fluoride occurrence varies with the different rock types. High concentrations of fluoride ions are found in regions of volcanic activities. Volcanoes are the main persistent natural source of fluoride in ground water and in the atmosphere. The occurrence of high concentrations of fluoride in ground water has been reported worldwide. Dental fluorosis is a developmental disturbance of enamel that results from ingestion of high amounts of fluoride during tooth mineralization. Consumption of untreated fluoridated ground water remains the main cause of dental fluorosis in rural settlements. AIM: To establish the risk factors and knowledge of dental fluorosis in three communities of the Far North Region of Cameroon. METHODOLOGY: This study was descriptive and cross-sectional study with an analytic component and was conducted in three communities in Far North Region of Cameroon. The study population was a convenience sample irrespective of age, but who must have been living in the selected study sites for a minimum of 6 to 8 years. Structured questionnaires were administered to elicit socio-demographic characteristics, perceptions dental fluorosis and oral health practices. Oral examination was conducted to determine the extent of dental fluorosis that was scored according to the Thylstrup and Fejerskov index. Participants declared their water sources and water samples were obtained from these sources. Samples of commercial bottled mineral water was purchased from various local outlets. These samples were sent to a certified laboratory for fluoride analysis and are reported in milligrams of fluoride per litre.