Nos últimos anos, materiais baseados em grafeno têm atraído grande interesse na área eletroquímica devido às suas excelentes propriedades eletrônicas. Neste trabalho, apresentamos a obtenção de óxido de grafeno reduzido utilizando métodos eletroquímicos. A redução eletroquímica do óxido de grafeno (OG) foi realizada na superfície de um eletrodo de carbono vítreo (ECV) utilizando voltametria cíclica. O óxido de grafeno reduzido eletroquimicamente (OG-RE) foi caracterizado utilizando espectroscopia Raman, microscopia eletrônica de varredura (MEV), microscopia de força atômica (MFA), espectroscopia de fotoelétrons excitados por raios-X (XPS) e espectroscopia de impedância eletroquímica. A relação ID/IG obtida através dos espectros Raman do OG e do OG-RE foram de 0,98 e 1,15, respectivamente, indicando que o processo de redução resultou em uma maior desorganização estrutural. A espectroscopia de fotoelétrons de raios-X mostrou que a proporção entre C e O no OG foi de 45,7% e, que após a redução eletroquímica essa relação diminuiu para 38,5%. O eletrodo modificado com OG-RE foi empregado para quantificação de furosemida (FUR) em amostras farmacêuticas utilizando voltametria de pulso diferencial e análise por injeção em fluxo com detecção amperométrica. Os limites de detecção e quantificação calculados para o método proposto foram de 0,35 e 1,18 µmol L-1, respectivamente. Visando à aplicação em amostras de interesse clínico, fabricou-se um sensor descartável e de baixo custo para a detecção de FUR utilizando papel sulfite pintado com lápis de desenho e ativado por laser de CO2. Os estudos voltamétricos utilizando o [Ru(NH3)6]Cl3 mostraram que o transporte de massa neste dispositivo foi governado por difusão e que o tratamento com laser diminui a resistência a transferência de carga, assim como resultou em um menor problema relacionado ao envenenamento da superfície do eletrodo / In recent years, graphene-based materials have attracted great interest in the area of electrochemistry due to its excellent electronic properties. In this work, we present the production of reduced graphene oxide using electrochemical methods. The electrochemical reduction of graphene oxide (GO) was carried out on the surface of a glassy carbon electrode (GCE) using the cyclic voltammetry technique. Electrochemically reduced graphene oxide (ER-GO) was characterized using Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Xray excited photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). ID / IG ratio obtained through Raman spectra of GO and ER-GO were 0.98 and 1.15, respectively, indicating that the reduction process resulted in greater structural disorganization. XPS showed that the ratio between C and O in the GO was 45.7% and that after electrochemical reduction this ratio decreased to 38.5%. The ER-GO modified electrode was used as a sensor for furosemide (FUR) in pharmaceutical samples using the techniques of differential pulse voltammetry and flow injection analysis with amperometric detection. The limits of detection and quantification for the proposed method were 0.35 and 1.18 µmol L-1, respectively. Aiming to the application in clinical samples a disposable, low-cost paper-based sensor for the detection of FUR was fabricated using office paper painted with drawing pencil and activated by CO2 laser. Voltammetric studies using [Ru(NH3)6]Cl3 have shown that mass transport in this device was controlled by diffusion and the laser decreases resistance to charge transfer, as well as, avoided the problem with electrode surface poisoning

<p><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:PunctuationKerning /> <w:ValidateAgainstSchemas /> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables /> <w:SnapToGridInCell /> <w:WrapTextWithPunct /> <w:UseAsianBreakRules /> <w:DontGrowAutofit /> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><!--[if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" LatentStyleCount="156"> </w:LatentStyles> </xml><![endif]--><!--[if gte mso 10]> <mce:style><! /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0pt 5.4pt 0pt 5.4pt; mso-para-margin:0pt; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} --><!--[endif] --></p><p class="MsoBodyText" style="margin-bottom: 0.0001pt;">U disertaciji je izvr&scaron;ena voltametrijska karakterizacija i određivanje pet&nbsp;neonikotinoidnih insekticida (imidakloprida, tiametoksama, klotianidina, nitenpirama i&nbsp;acetamiprida) na različitim elektrodama na bazi ugljenika, kao &scaron;to su elektroda od&nbsp;ugljenične paste, elektroda od staklastog ugljenika i bizmut-filmom modifikovani&nbsp;staklasti ugljenik. Pored ispitivanja u model-rastvorima, razvijene visokoosetljive&nbsp;diferencijalne pulsne voltametrijskemetode su primenjene za određivanje neonikotinoida&nbsp;u realnim uzorcima. Neonikotinoidi su određivani u uzorcima rečne vode i komercijalnim&nbsp;formulacijama, a praćena je i njihova fotolitička i fotokatalitička razgradnja. Radi&nbsp;potvrđivanja voltametrijskih rezultata, izvedena su komparativna merenja uz primenu&nbsp;tečne hromatografije sa detektorom od niza dioda i infracrvenespektrometrije. Detekcija&nbsp;pojedinih degradacionih proizvoda nastalih u toku ispitivanih fotodegradacionih procesa&nbsp;je praćena tečnohromatografskom metodom sa tandemskom masenospektrometrijskom&nbsp;detekcijom. Proučavana je povr&scaron;inska struktura CR 5 grafitnog praha, elektrode od&nbsp;ugljenične paste na bazi trikrezil-fosfata i elektrode od staklastog ugljenika pomoću&nbsp;skenirajuće elektronske mikroskopije.</p> / <p>This thesis is concerned with the voltammetric characterization and&nbsp;determination of five neonicotinoid insecticides (imidacloprid, thiamethoxam,&nbsp;clothianidin, nitenpyram and acetamiprid) at different carbon-based electrodes such as&nbsp;carbon paste, glassy carbon and bismuth-film modified glassycarbon. In addition to the&nbsp;investigations in model solutions, the developed highly sensitive differential pulse&nbsp;voltammetric methods were applied for the determination of neonicotinoids in real&nbsp;samples. Neonicotinoids were determined in river water samples, commercial&nbsp;formulations, and their photolytic and photocatalytic degradation was also monitored. To&nbsp;confirm the voltammetric results, comparative measurements were performed by liquid&nbsp;chromatography with diode-array detector and infrared spectrometry. Detection of some&nbsp;degradation products formed during the photodegradation processes was monitored using&nbsp;liquid chromatography with tandem mass-spectrometric detection. The surface structure&nbsp;of CR 5 graphite powder, tricresyl phosphate-based carbon paste electrode and glassy&nbsp;carbon electrode was investigated by scanning electron microscopy.</p>

Made available in DSpace on 2014-12-17T15:42:11Z (GMT). No. of bitstreams: 1 NathaliaCVN_DISSERT.pdf: 2020842 bytes, checksum: efcd9ca5e5cfd7d63412f72c80546c39 (MD5) Previous issue date: 2013-07-31 / In this paper, the technique of differential pulse voltammetry (DPV) has been studied for monitoring the concentration of oxalic acid (OA) during their electrochemical oxidation (EO) in acidic medium using platinum anode supported on titanium (Ti / Pt). The DPV was standardized and optimized using a glassy carbon electrode modified with cysteine. The modification with cysteine was developed electrochemically, forming a polymeric film on the surface of the glassy carbon electrode. The formation of the polymer film was confirmed by analysis of scanning electron microscope and atomic force microscope, confirming the modification of the electrode. The electrochemical degradation was developed using different current densities 10, 20 30 and 40 mA cm -2 electrode with Ti / Pt observing the degradation of oxalic acid, and monitored using the method of KMnO4 titration. However, the analyzes with DPV showed the same behavior elimination of oxalic acid titration. Compared with the titration method classical observed and DPV could be a good fit, confidence limits of detection and confirming the applicability of the technique electroanalytical for monitoring the degradation of oxalic acid / No presente trabalho, a t?cnica de voltametria de pulso diferencial (DPV) foi estudada para o monitoramento da concentra??o de ?cido ox?lico (OA) durante a sua oxida??o eletroqu?mica (EO) em meio ?cido utilizando o ?nodo de platina suportada em tit?nio (Ti/Pt). A t?cnica de DPV foi padronizada e otimizada usando um eletrodo de carbono v?treo modificado com ciste?na. A modifica??o com ciste?na foi desenvolvida eletroquimicamente, formando um filme polim?rico na superf?cie do eletrodo de carbono v?treo. A forma??o do filme polim?rico foi confirmada por an?lise de Microsc?pio Eletr?nico de Varredura e Microsc?pio de For?a At?mica, confirmando a modifica??o na superf?cie do eletrodo. A degrada??o eletroqu?mica foi desenvolvida usando diferentes densidades de corrente, 10, 20 e 30 mA cm-? com eletrodo de Ti/Pt, observando a degrada??o do ?cido ox?lico, sendo monitorado empregando a metodologia de titula??o com KmnO4. No entanto, as an?lises com DPV mostraram o mesmo comportamento de elimina??o do acido ox?lico, observado com a titula??o. Comparado com o m?todo de titula??o cl?ssica e DPV conseguiu-se um bom ajuste, intervalos de confian?a e limites de detec??o confirmando a aplicabilidade da t?cnica eletroanal?tica para o monitoramento da degrada??o de ?cido ox?lico

<p>U okviru ove doktorske disertacije, razvijene su elektroanalitičke metode za određivanje odabranih pesticida primenom hronopotenciometrije. Ispitana je mogućnost primene elektrode od staklastog ugljenika i tankoslojne živine elektrode kao radnih elektroda za određivanje insekticida imidakloprida i herbicida metamitrona i metribuzina. U cilju optimizacije uslova hronopotenciometrijske tehnike, za svaki ispitivani sistem (pesticid/radna elektroda) ispitan je uticaj najznačajnijih eksperimentalnih parametara na analitički signal pesticida, uključujući mehanizam generisanja analitičkog signala na radnoj elektrodi (oksidacija/redukcija), optimalnu metodu za uklanjanje rastvorenog kiseonika iz analiziranog rastvora, kao i vrstu, pH i koncentraciju pomoćnog elektrolita. Nakon toga definisan je optimalni opseg potencijala i struje redukcije. Pored toga, u okviru validacije metode za svaki ispitivani sistem, ispitan je i definisan opseg linearnosti, određena granica detekcije i granica kvantifikacije, ispitana preciznost, tačnost, robusnost i selektivnost metode. Dobijeni analitički signal za ispitivane pesticide bio je posledica ireverzibilne redukcije analita na radnoj elektrodi, a za svaki analit dobijen je jedan redukcioni pik. U slučaju određivanja metribuzina eksperimenti su pokazali da se analitički signal ne može detektovati na elektrodi od staklastog ugljenika uz kori&scaron;ćenje dostupne instrumentacije. Ispitivanja u model-rastvorima za svaki ispitivani sistem pokazala su dobre karakteristike u pogledu preciznosti, tačnosti, selektivnosti i robusnosti metode. Najveća osetljivost za sva tri pesticida postignuta je uz primenu tankoslojne živine elektrode, a vrednosti granice detekcije iznosile su 0,17 mg/l za imidakloprid, 0,07 mg/l za metamitron i 0,04 mg/l za metribuzin. Nakon optimizacije i validacije, razvijene hronopotenciometrijske metode primenjene su na komercijalne formulacije pesticida i uzorke vode. U cilju dodatne potvrde hronopotenciometrijskih rezultata na uzorcima vode izvedena su komparativna merenja uz primenu hromatografske LC-MS/MS analize.</p> / <p>Within this doctoral dissertation, electroanalytical methods for the determination of selected pesticides using chronopotentiometry are developed. The possibility of using glassy carbon electrode and thin film mercury electrode as a working electrode for determination of insecticide imidacloprid and herbicides metamitron and metribuzin is investigated. In order to optimize the conditions of the chronopotentiometric technique for each tested system (pesticide/working electrode), the influence of the most important experimental parameters on the analytical signal of the pesticides is investigated, including the generating mechanism of the analytical signal on the working electrode (oxidation/reduction), the optimal method for removal of dissolved oxygen from the analyzed solution, as well as type, pH, and concentration of the supporting electrolyte. Afterwards, the optimal range of potential and reduction current is defined. Additionally, within the validation of the method for each tested system, the linearity range is determined and defined, limit of detection and quantification are determined, and precision, accuracy, robustness and selectivity of the method are tested. The obtained analytical signal for the investigated pesticides is the result of irreversible reduction of the analyte on the working electrode, and for each analyte one reduction peak is obtained. In the case of metribuzin determination, the experiments have shown that by using the available instrumentation the analytical signal can not be detected on the glassy carbon electrode. Investigation in model solutions for each tested system shows good characteristics in terms of precision, accuracy, selectivity and robustness of the method. The highest sensitivity for all three pesticides was achieved by using thin film mercury electrode, and values of detection limit were 0.17 mg/l for imidacloprid, 0.07 mg/l for metamitron and 0.04 mg/l for metribuzin. After optimization and validation, the developed chronopotentiometric methods are applied to commercial pesticide formulations and water samples. In order to further confirm the chronopotentiometric results on the water samples, comparative measurements are performed using chromatographic LC-MS/MS analysis.</p>

Nos últimos anos, materiais baseados em grafeno têm atraído grande interesse na área eletroquímica devido às suas excelentes propriedades eletrônicas. Neste trabalho, apresentamos a obtenção de óxido de grafeno reduzido utilizando métodos eletroquímicos. A redução eletroquímica do óxido de grafeno (OG) foi realizada na superfície de um eletrodo de carbono vítreo (ECV) utilizando voltametria cíclica. O óxido de grafeno reduzido eletroquimicamente (OG-RE) foi caracterizado utilizando espectroscopia Raman, microscopia eletrônica de varredura (MEV), microscopia de força atômica (MFA), espectroscopia de fotoelétrons excitados por raios-X (XPS) e espectroscopia de impedância eletroquímica. A relação ID/IG obtida através dos espectros Raman do OG e do OG-RE foram de 0,98 e 1,15, respectivamente, indicando que o processo de redução resultou em uma maior desorganização estrutural. A espectroscopia de fotoelétrons de raios-X mostrou que a proporção entre C e O no OG foi de 45,7% e, que após a redução eletroquímica essa relação diminuiu para 38,5%. O eletrodo modificado com OG-RE foi empregado para quantificação de furosemida (FUR) em amostras farmacêuticas utilizando voltametria de pulso diferencial e análise por injeção em fluxo com detecção amperométrica. Os limites de detecção e quantificação calculados para o método proposto foram de 0,35 e 1,18 &#181;mol L-1, respectivamente. Visando à aplicação em amostras de interesse clínico, fabricou-se um sensor descartável e de baixo custo para a detecção de FUR utilizando papel sulfite pintado com lápis de desenho e ativado por laser de CO2. Os estudos voltamétricos utilizando o [Ru(NH3)6]Cl3 mostraram que o transporte de massa neste dispositivo foi governado por difusão e que o tratamento com laser diminui a resistência a transferência de carga, assim como resultou em um menor problema relacionado ao envenenamento da superfície do eletrodo / In recent years, graphene-based materials have attracted great interest in the area of electrochemistry due to its excellent electronic properties. In this work, we present the production of reduced graphene oxide using electrochemical methods. The electrochemical reduction of graphene oxide (GO) was carried out on the surface of a glassy carbon electrode (GCE) using the cyclic voltammetry technique. Electrochemically reduced graphene oxide (ER-GO) was characterized using Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Xray excited photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). ID / IG ratio obtained through Raman spectra of GO and ER-GO were 0.98 and 1.15, respectively, indicating that the reduction process resulted in greater structural disorganization. XPS showed that the ratio between C and O in the GO was 45.7% and that after electrochemical reduction this ratio decreased to 38.5%. The ER-GO modified electrode was used as a sensor for furosemide (FUR) in pharmaceutical samples using the techniques of differential pulse voltammetry and flow injection analysis with amperometric detection. The limits of detection and quantification for the proposed method were 0.35 and 1.18 &#181;mol L-1, respectively. Aiming to the application in clinical samples a disposable, low-cost paper-based sensor for the detection of FUR was fabricated using office paper painted with drawing pencil and activated by CO2 laser. Voltammetric studies using [Ru(NH3)6]Cl3 have shown that mass transport in this device was controlled by diffusion and the laser decreases resistance to charge transfer, as well as, avoided the problem with electrode surface poisoning

<p>Proces pečenja u proizvodnji keramičkog crepa u većini slučajeva se izvodi u<br />oksidavionoj atmosferi. Dobijena mikrostruktura koja obezbeđuje najvažnije osobine<br />primene zavisi od mineralo&scaron;kog sastava polaznog materijala i temperature termičkog<br />tretmana.<br />Sa stanovi&scaron;ta proizvodnje glinenog crepa, sirovina sa značajnim sadržajem karbonata kao &scaron;to je kop gline u Kanjiži, predstavlja problem te se ne može koristiti u njenom prirodnom sastavu. U oksidacionom termičkom tretmanu, staklasta faza koja se formira razgradnjom lakotopivih glinenih minerala ne kvasi u dovoljnoj meri zemnoalkalne okside nastale razgradnjom karbonata. Očekivane nove kristalne faze, kao &scaron;to su gelenit i anortit, koje predstavljaju osnovu dobrih tehničkih osobina crepa, ne mogu nastati u potrebnoj količini. Jedno od re&scaron;enja ovog problema je pobolj&scaron;anje interakcije između silikata i CaO/MgO promenom kvaliteta atmosfere pečenja.<br />Cilj je bio da se dobije odgovor na pitanje, da li se mogu promenom tehnolo&scaron;kih<br />parametara pečenja pobolj&scaron;ati karakteristike finalnog proizvoda dobijenog od sirovinske sme&scaron;e sa visokim masenim udelom karbonata.<br />Dati su rezultati pečenja u oksidacionoj i redukcionoj atmosferi u pogledu fizičkomehaničkih karakteristika i nastanka novih kristalnih faza. U radu su primenjene metode 57Fe M&ouml;ssbauer spektroskopije, difrakcija X zraka i dilatometrijska analiza za<br />identifikaciju promena faza tokom pečenja u oksidacionoj i redukcionoj atmosferi<br />(CO/N2 atmosfera) na temperaturama od 700-1060&deg;C. Ove promene reflektuju<br />dehidroksilaciju glinenih minerala, dekompoziciju karbonata, denzifikaciju i formiranje<br />novih kristalnih faza (plagioklasi). U cilju analize industrijskih uzoraka, pored 57Fe<br />M&ouml;ssbauer spektroskopije, primenjene su i sledeće metode: skenirajuća elektronska<br />mikroskopija (SEM) sa EDS analizom, živina porozimetrija, fotoelektronska spektroskopija (XPS) i karakterizacija apsorbcije vode, otpornosti na mraz i nosivosti.<br />Primena redukujuće atmosfere pokazala se kao opravdana, dobijen je glineni crep<br />pobolj&scaron;anih karakteristika u odnosu na standardni proizvod pečen u oksidacionoj<br />atmosferi.</p> / <p>The firing process in clay roofing tile production is normally carried out in oxidizing<br />firing atmosphere. The obtained microstructure which provides the most important<br />properties of the application depends on the mineralogical composition of the starting raw material and the firing temperature.<br />Concerning the production of roofing tiles, clay raw materials with a high content of<br />carbonates such as the Kanjiza clay material, possess an inappropriate composition. The melt phase is formed after the decomposition of clay minerals during the common firing process. This liquid phase, due to its extent wetting characteristics, is not able to wet properly the earth alkali oxides obtained after the decomposition of the carbonates. The expected new crystalline phases, like gehlenite and anorthite, which are needed for the favorable technical properties of the roofing tiles, can not be formed in the required quality and quantity. One solution to this problem is the limebonding improvement by changing the conditions of the firing atmosphere.<br />The main aim of this work is to examine the effects of firing conditions on the<br />properties of clay roofing tiles with high carbonate content. The obtained physical and<br />mechanical properties and the obtained new crystalline phases are described in detail.<br />The experiment used dilatometry, X-ray diffractometry, 57Fe M&ouml;ssbauer<br />spectroscopy to identify the changes during the firing process in oxidizing and<br />reducing atmosphere (CO/N2 atmosphere) at temperatures between 700 and 1060&deg;C.<br />Furthermore, for industrial sample analysis, scanning electronmicroscopy with EDS,<br />XPS, Hg porosimetry, water absorption procedure, frost resistance and bending<br />measurements were applied.<br />The application of reducing atmosphere was proved to be valid. The obtained<br />roofing tiles had improved properties compared to the standard production which uses<br />oxidizing firing atmosphere.</p>

The ability to tune the properties over a wide range of values by changing the additives, composition, etc., has made chalcogenide glassy semiconductors, most interesting from both fundamental physics as well as technology point of view. In particular, the occurrence of the two network topological thresholds namely the Rigidity Percolation Threshold (RPT) and the Chemical Threshold (CT) and their influence on various properties of chalcogenide glasses have been of immense interest during the last three decades. The Rigidity Percolation Threshold (also known as the Stiffness Threshold or Mechanical Threshold) corresponds to the composition at which the material transforms from a floppy polymeric glass to a rigid amorphous solid, whereas at Chemical Threshold the sample tends towards an ordered state. Though the rigidity percolation has been considered for long to occur at a critical threshold defined by the constraint’s theory, the recent theoretical and experimental investigations have found the RPT to occur over a range of compositions. In systems exhibiting an extended rigidity percolation, two distinct transitions namely from a floppy to an isostatically rigid phase and from an isostatically rigid to a stressed rigid phase are seen. In the category of chalcogenide glasses, tellurides have been found to exhibit interesting properties including the phenomenon of electrical switching which finds applications in Phase Change Memories (PCM). Studies on various thermal, electrical and photoelectrical properties of glassy tellurides help us in identifying suitable materials for different technological applications. This thesis deals with Differential Scanning Calorimetric (DSC) & Temperature Modulated Alternating Differential Scanning Calorimetric (ADSC) studies, electrical switching investigations, photoconductivity & photothermal measurements on certain metal doped telluride glasses. The composition dependence of properties such as glass transition & crystallization temperatures, switching voltage, thermal diffusivity, photosensitivity, etc., have been analyzed to obtain information about topological thresholds, thermally reversing window, etc. The first chapter of thesis provides an overview of properties of amorphous semiconductors, in particular chalcogenide glasses. The local & defect structure, the electronic band structure & electrical properties, electrical switching behavior, etc., are discussed in detail. The theoretical aspects related to the experiments undertaken in this thesis work have also been described. The instrumentation used for various experiments conducted to measure thermal, electrical, photoelectrical and photothermal properties have been discussed in chapter two. The chapter three deals with the photocurrent measurements on As40Te60-xInx (7.5 ≤ x ≤ 16.5) glasses. In these samples, it has been found that the photocurrent increases with illumination, which is understood on the basis of the large dielectric constant and also due to the presence of a large number of positively charged defect states. Further, the composition dependence of the conductivity activation energy and the photosensitivity exhibit a maximum at x = 12.5 (<r> = 2.65) and a minimum at x = 15.0 (<r> = 2.70) which has been identified to be the Rigidity Percolation Threshold (RPT) and the Chemical Threshold (CT) respectively. The results of electrical switching, DSC and Photothermal Deflection (PTD) studies on As20Te80-xGax (7.5 ≤ x ≤ 18.5) glasses, undertaken to elucidate the network topological thresholds, are described in chapter four. It has been found that all the As20Te80-xGax glasses studied exhibit memory type electrical switching. The switching voltage (VT) of these glasses increases monotonically with x, in the composition range 7.5 ≤ x ≤ 15.0. The increase in VT with gallium addition leads to a local maximum at x = 15.0 and VT decreases with x thereafter, reaching a distinct minimum at x = 17.5. Based on the variation with composition of the electrical switching voltages, the composition x = 15.0 and x = 17.5 have been identified to be the rigidity percolation and chemical thresholds of the As20Te80-xGax glassy system respectively. Further, the DSC studies indicate that As20Te80-xGax glasses exhibit a single glass transition (Tg) and two crystallization reactions (Tc1 & Tc2) upon heating. There is no appreciable change in Tg of As20Te80-xGax glasses with the addition of upto about10 atom% of Ga, whereas a continuous increase is seen in the crystallization temperature (Tc1). It is interesting to note that both Tg and Tc1 exhibit a maximum at x = 15.0 and a minimum at x = 17.5, the compositions identified to be the RPT and CT respectively by the switching experiments. The composition dependence of thermal diffusivity estimated from the PTD signal, indicate the occurrence of an extended stiffness transition in As20Te80-xGax glasses, with the compositions x = 9.0 and x = 15.0 being the onset and the completion of an extended rigidity percolation. A maximum and a minimum are seen in the thermal diffusivity respectively at these compositions. Further, a second maximum is seen in the thermal diffusivity of As20Te80-xGax glasses, the Chemical Threshold (CT) of the glassy system. The fifth chapter of the thesis describes the ADSC, electrical switching and photocurrent measurements on Ge15Te85-xInx (1 ≤ x ≤ 11) glasses. It is found there is not much change in the Tg of Ge15Te85-xInx glasses in the composition range 1 ≤ x ≤ 3. An increase is seen in Tg beyond x = 3, which continues until x = 11. Further, the composition dependence of non-reversing enthalpy shows the presence of a thermally reversing window in the compositions range x = 3 and x = 7. Electrical switching studies indicate that Ge15Te85-xInx glasses exhibit threshold type of switching at input currents below 2 mA. It is observed that switching voltages decrease initially with indium addition, exhibiting a minimum at x = 3, the onset of the extended rigidity percolation as revealed by ADSC. An increase is seen in VT above x = 3, which proceeds till x = 8, with a change in slope (lower to higher) seen around 7 atom% of indium which corresponds to the completion of the stiffness transition. The reversal in trend exhibited in the variation of VT at x = 8, leads to a well defined minimum around x = 9, the chemical threshold of the Ge15Te85-xInx glassy system. Photocurrent measurements indicate that there is no photodegradation in Ge15Te85-xInx glasses with x < 3, whereas samples with x ≥ 3 show photodegradation behavior. The composition dependent variation in the glass transition temperature has been attributed for this behavior. Further, the composition dependence of photo sensitivity has been found to show the signatures of the extended rigidity percolation and the chemical threshold in Ge15Te85-xInx glasses. The last chapter of thesis (chapter six) summarizes the results obtained and also the scope of future work to be undertaken.

The absence of long-range order in glassy chalcogenides provides the convenience of changing the elemental ratios and hence the properties over a wide range. The interesting properties exhibited by chalcogenide glasses make them suitable materials for Phase Change Memories (PCM) and other applications such as infrared optical devices, photo-receptors, sensors, waveguides, etc. One of the most remarkable properties of chalcogenides is their electrical switching behavior. Reversible (threshold type) or irreversible (memory type) switching from a high resistance OFF state to a low resistance ON state in glassy chalcogenides occurs at a critical voltage called the threshold/switching voltage (VT). Investigations on the switching behavior and its composition dependence throw light on the local structural effects of amorphous chalcogenide semiconductors and also help us in identifying suitable samples for PCM applications. Thermal analysis by Differential Scanning Calorimetry (DSC) has been extensively used in glass science, particularly for measurements of thermal parameters such as enthalpy of relaxation, specific heat change, etc., near glass transition. Quite recently, the conventional DSC has been sophisticated by employing a composite temperature profile for heating, resulting in the Temperature Modulated DSC (TMDSC) or Alternating DSC (ADSC). Measurements made using ADSC reveal thermal details with enhanced accuracy and resolution, and this has lead to a better understanding of the nature of glass transition. The thermal parameters obtained using DSC/ADSC are also vital for understanding the electrical switching behavior of glassy chalcogenides. The motivation of this thesis was twofold: The first was to develop a novel, high voltage programmable power supply for electrical switching analysis of samples exhibiting high VT, and second to investigate the thermal and electrical switching behavior of certain Se-Te based glasses with Ge and Sb additives. The thesis contains seven chapters: Chapter 1: This chapter provides an overview of amorphous semiconductors (a-SC) with an emphasis on preparation and properties of glassy chalcogenides. The various structural models and topological thresholds of a-SC are discussed with relations to the glass forming ability of materials. The electronic band models and defect states are also dealt with. The essentials of electrical switching behavior of chalcogenides are discussed suggesting the electronic nature of switching and the role of thermal properties on switching. Chapter 2: The second chapter essentially deals with theory and practice of the experimental techniques adopted in the thesis work. The details of the melt-quenching method of synthesizing glassy samples are provided. Considering the importance, the theory of thermal analysis by DSC & ADSC, are discussed in detail, highlighting the advantages of the latter method adopted in the thesis work. The instrumentation and electronics, developed and used for electrical switching analysis are also introduced at a block diagram level. Finally, the methods used for structural analysis are briefed. Chapter 3: This chapter is dedicated to the design and development details of the programmable High Voltage dc Power Supply (HVPS: 1750 V, 45 mA) undertaken in the thesis work. The guidelines used for power supply topology selection, the specifications and block diagram of the HVPS are provided in that sequence. The operation of the HVPS is discussed using the circuit diagram approach. The details of software control are also given. The performance validations of the HVPS, undertaken through voltage & current regulation tests, step & frequency response tests are discussed. Finally, the sample-test results on the electrical switching behavior of representative Al20As16Te64 and Ge25Te65Se10 samples, obtained using both the current & voltage sweep options of the HVPS developed are illustrated. Chapter 4: Results of the thermally induced transitions governed by structural changes which are driven by network connectivity in the GexSe35-xTe65 (17 ≤ x ≤ 25) glasses, as revealed by ADSC experiments, are discussed in this chapter. It is found that the GexSe35-xTe65 glasses with x ≤ 20 exhibit two crystallization exotherms (Tc1 & Tc2), whereas those with x ≥ 20.5, show a single crystallization reaction upon heating (Tc). The glass transition temperature of GexSe35-xTe65 glasses is found to show a linear, but not-steep increase, indicating a progressive and not an appreciable build-up in network connectivity with Ge addition. The exothermic reaction at Tc1 has been found to correspond to the partial crystallization of the glass into hexagonal Te and the reaction at Tc2 is associated with the additional crystallization of rhombohedral Ge-Te phase. It is also found that the first crystallization temperature Tc1 of GexSe35-xTe65 glasses of lower Ge concentrations (with x ≤ 20), increases progressively with Ge content and eventually merges with Tc2 at x = 20.5 (<r> = 2.41); this behavior has been understood on the basis of the reduction in Te-Te bonds of lower energy and an increase in Ge-Te bonds of higher energy, with increasing Ge content. Chapter 5: This chapter deals with the electrical switching studies on GexSe35-xTe65 (17 ≤ x ≤ 25) glasses, with an emphasis on the role of network connectivity/rigidity on the switching behavior. It is found that the switching voltage (VT) increases with Ge content, exhibiting a sudden jump at x=20, the Rigidity Percolation Threshold (RPT) of the system. In addition, the switching behavior changes from memory to threshold type at the RPT and the threshold switching is found to be repetitive for more than 1500 cycles. Chapter 6: In this chapter, the results of thermal analysis (by ADSC) and electrical switching investigations on SbxSe55-xTe45 (2 ≤ x ≤ 9) are discussed. It is found that the addition of trivalent Sb contributes very meagerly to network growth but directly affects the structural relaxation effects at Tg. Further, SbxSe55-xTe45 glasses exhibit memory type electrical switching, which is understood on the basis of poor thermal stability of the samples. The metallicity factor is observed to outweigh the network factor in the composition dependence of VT of SbxSe55-xTe45 glasses. Chapter 7: The chapter 7 summarizes the results obtained in the thesis work and provides the scope for future work. The references are cited in the text along with the first author’s name and year of publication, and are listed at the end of each chapter in alphabetical order.

Nitrogen trifluoride (NF3) is frequently used as a source of fluorine in the electronics device manufacturing industry as a dry etchant during plasma assisted etching of silicon wafers, or during the plasma cleaning of chemical vapor deposition chambers. As a result of the electrochemical synthesis procedures in which carbon anodes are used in a fluorine-rich environment, NF3 product streams are frequently contaminated with ppm-amounts of carbon tetrafluoride (CF4). The electronics manufacturing industry, however, requires NF3 of exceptional purity, i.e. so-called VLSI-grade (very large scale integration) NF3, with CF4 concentrations of 20 ppm and below. Due to the close chemical and physical similarities of the two compounds, the removal of CF4 from NF3 has proven to be rather difficult, and current NF3 purification technologies are relatively inefficient. Although membrane gas separation has proven to be competitive in terms of operating costs and energy efficiency, its use for the purification of NF3 seems to have remained unexplored to date. In this study, the use of high free volume glassy perfluoropolymers of Teflon AF2400, Teflon AF1600, and Hyflon AD60 was therefore investigated. To be able to measure the pure and mixed gas permeabilities and selectivities of the solution-cast membranes towards NF3 and CF4, a custom built experimental setup was used, in which a newly developed gas chromatographic (GC) analysis method was implemented. Using divinylbenzene-styrene co-polymer stationary phases in the form of Super Q, a reliable quantification of mixtures of NF3 and CF4 were achieved without requiring additional fluorocarbon liquid stationary phases, as is commonly used in NF3 production environments. Furthermore, by implementing a dual-channel configuration it was possible to quantify a wide range of NF3 and CF4 concentrations. Using the newly developed technique, NF3 and CF4 concentrations of ca. 1 mol% and upwards could be quantified using a Thermal Conductivity Detector (TCD) on one channel, and NF3 and CF4 concentrations of between ca. 40 vppm and 4000 vppm could be measured using a Pulsed Discharge Helium Ionisation Detector (PDHID) on the second channel of the GC method. The glassy perfluoropolymer membranes of Teflon AF2400, Teflon AF1600, and Hyflon AD60 were prepared by a solution casting method, and it was found that annealing at sufficiently high temperatures (170 – 200 °C) ensured optimum permeability selectivity. In contrast, thermal analysis of the solution-cast Hyflon AD60 membranes that were heated to only 95 °C confirmed that the polymer matrix was significantly swollen due to a residual amount of the casting solvent. Consequently, considerably reduced selectivity and increased permeability of both NF3 and CF4 were observed for such solvent-swollen Hyflon AD60 membranes in comparison with the non-swollen membranes that were annealed at 170 °C. Nonetheless, the measured He/N2 permeability and permeability selectivity of all the membranes studied compared favourably with literature values, and selectively permeated NF3 rather than CF4 wherein the pure and mixed gas permeability selectivity displayed a clear dependence on the fractional free volume (FFV) of the polymer matrices. Thus, in accordance with the decreasing FFV of the perfluoropolymers in the order Teflon AF2400 > Teflon AF1600 > Hyflon AD60, the NF3 permeability decreased from 227 Barrer for Teflon AF2400, to 29 Barrer for Teflon AF1600, to 1.9 Barrer for Hyflon AD60. In contrast, the NF3/CF4 selectivity, α(NF3/CF4), increased inversely from 4.5 for Teflon AF2400, to 6.0 for Teflon AF1600, to the highest selectivity of 12 which was obtained using Hyflon AD60. To elucidate the mechanism of separation, the transport properties of NF3 and CF4 in Teflon AF2400 and Teflon AF1600 w.r.t. diffusion and solubility were studied using Molecular Dynamics (MD), Grand Canonical Monte Carlo (GCMC), and statistical thermodynamic techniques. The results indicated that NF3/CF4 diffusion selectivity (DNF3/DCF4) was favoured by the lower free volume of Teflon AF1600, whereas poor correlation was achieved between the GCMC calculated sorption isotherms of CF4 and the experimentally determined isotherms as reported in the literature. Consequently, the non-equilibrium lattice fluid (NELF) model, which more accurately described the sorption isotherms of CF4, was used to evaluate the solubility selectivity. It was found that by adjusting the NELF model interaction parameter, Ψ, favourable NF3/CF4 solubility selectivities (SNF3/SCF4) were predicted. Furthermore, by combining the solubility selectivity values with the diffusion selectivities calculated from the MD results, permeability selectivity predictions that correlated well with the experimentally determined values were obtained. Based on a semi-quantitative technological evaluation, it was concluded that although good NF3/CF4 mixed gas permeability selectivity was obtained with Hyflon AD60, further research into improving the NF3 solubility, and hence permeability will aid in the development of an efficient membrane gas separation process for the purification of NF3. / PhD (Chemistry),North-West University, Potchefstroom Campus, 2013.

Nitrogen trifluoride (NF3) is frequently used as a source of fluorine in the electronics device manufacturing industry as a dry etchant during plasma assisted etching of silicon wafers, or during the plasma cleaning of chemical vapor deposition chambers. As a result of the electrochemical synthesis procedures in which carbon anodes are used in a fluorine-rich environment, NF3 product streams are frequently contaminated with ppm-amounts of carbon tetrafluoride (CF4). The electronics manufacturing industry, however, requires NF3 of exceptional purity, i.e. so-called VLSI-grade (very large scale integration) NF3, with CF4 concentrations of 20 ppm and below. Due to the close chemical and physical similarities of the two compounds, the removal of CF4 from NF3 has proven to be rather difficult, and current NF3 purification technologies are relatively inefficient. Although membrane gas separation has proven to be competitive in terms of operating costs and energy efficiency, its use for the purification of NF3 seems to have remained unexplored to date. In this study, the use of high free volume glassy perfluoropolymers of Teflon AF2400, Teflon AF1600, and Hyflon AD60 was therefore investigated. To be able to measure the pure and mixed gas permeabilities and selectivities of the solution-cast membranes towards NF3 and CF4, a custom built experimental setup was used, in which a newly developed gas chromatographic (GC) analysis method was implemented. Using divinylbenzene-styrene co-polymer stationary phases in the form of Super Q, a reliable quantification of mixtures of NF3 and CF4 were achieved without requiring additional fluorocarbon liquid stationary phases, as is commonly used in NF3 production environments. Furthermore, by implementing a dual-channel configuration it was possible to quantify a wide range of NF3 and CF4 concentrations. Using the newly developed technique, NF3 and CF4 concentrations of ca. 1 mol% and upwards could be quantified using a Thermal Conductivity Detector (TCD) on one channel, and NF3 and CF4 concentrations of between ca. 40 vppm and 4000 vppm could be measured using a Pulsed Discharge Helium Ionisation Detector (PDHID) on the second channel of the GC method. The glassy perfluoropolymer membranes of Teflon AF2400, Teflon AF1600, and Hyflon AD60 were prepared by a solution casting method, and it was found that annealing at sufficiently high temperatures (170 – 200 °C) ensured optimum permeability selectivity. In contrast, thermal analysis of the solution-cast Hyflon AD60 membranes that were heated to only 95 °C confirmed that the polymer matrix was significantly swollen due to a residual amount of the casting solvent. Consequently, considerably reduced selectivity and increased permeability of both NF3 and CF4 were observed for such solvent-swollen Hyflon AD60 membranes in comparison with the non-swollen membranes that were annealed at 170 °C. Nonetheless, the measured He/N2 permeability and permeability selectivity of all the membranes studied compared favourably with literature values, and selectively permeated NF3 rather than CF4 wherein the pure and mixed gas permeability selectivity displayed a clear dependence on the fractional free volume (FFV) of the polymer matrices. Thus, in accordance with the decreasing FFV of the perfluoropolymers in the order Teflon AF2400 > Teflon AF1600 > Hyflon AD60, the NF3 permeability decreased from 227 Barrer for Teflon AF2400, to 29 Barrer for Teflon AF1600, to 1.9 Barrer for Hyflon AD60. In contrast, the NF3/CF4 selectivity, α(NF3/CF4), increased inversely from 4.5 for Teflon AF2400, to 6.0 for Teflon AF1600, to the highest selectivity of 12 which was obtained using Hyflon AD60. To elucidate the mechanism of separation, the transport properties of NF3 and CF4 in Teflon AF2400 and Teflon AF1600 w.r.t. diffusion and solubility were studied using Molecular Dynamics (MD), Grand Canonical Monte Carlo (GCMC), and statistical thermodynamic techniques. The results indicated that NF3/CF4 diffusion selectivity (DNF3/DCF4) was favoured by the lower free volume of Teflon AF1600, whereas poor correlation was achieved between the GCMC calculated sorption isotherms of CF4 and the experimentally determined isotherms as reported in the literature. Consequently, the non-equilibrium lattice fluid (NELF) model, which more accurately described the sorption isotherms of CF4, was used to evaluate the solubility selectivity. It was found that by adjusting the NELF model interaction parameter, Ψ, favourable NF3/CF4 solubility selectivities (SNF3/SCF4) were predicted. Furthermore, by combining the solubility selectivity values with the diffusion selectivities calculated from the MD results, permeability selectivity predictions that correlated well with the experimentally determined values were obtained. Based on a semi-quantitative technological evaluation, it was concluded that although good NF3/CF4 mixed gas permeability selectivity was obtained with Hyflon AD60, further research into improving the NF3 solubility, and hence permeability will aid in the development of an efficient membrane gas separation process for the purification of NF3. / PhD (Chemistry),North-West University, Potchefstroom Campus, 2013.