Increasing demand for high-density memories has necessitated the search for new materials with higher dielectric constants to satisfy the minimum charge storage density requirements. Several materials such as Ta₂O₅, BST¹, BBT² are being investigated to replace the currently used Si based oxide/nitride dielectrics. Among the materials under investigation, Ta₂O₅ is one of the most promising, especially from the fab compatibility point of view. Ta₂O₅ thin films offer a six-fold increase in dielectric constant compared to conventional dielectrics. However, the significant improvement in dielectric constant is offset by higher leakage currents compared to conventional dielectrics. Improvement in both, dielectric and insulating properties is required for the successful integration of Ta₂O₅ thin films into devices. In the current research work, it was demonstrated that by chemically modifying the tantalum pentoxide matrix, significant improvements in its electrical properties can be achieved which would enable the fabrication of a reliable high-density memory device. In the present work, the effects of Al addition on Ta₂O₅ thin films were systematically studied. The structural and electrical properties of these chemically modified thin films were investigated in detail to establish their potential for device applications. The effects on dielectric and insulating characteristics due to incorporation of Al in Ta₂O₅ matrix were studied in capacitor configuration. A metallorganic solution decomposition (MOSD) technique was used to deposit thin films onto Pt coated Si(100) substrates. The capacitors were fabricated by sputter depositing Pt electrodes on the top surface of the films. The dielectric and insulating properties of pure and modified Ta₂O₅ thin films and their dependence on film composition, processing temperature, and the thickness were discussed and an attempt was made to provide theoretical understanding for the experimental observations. The dielectric and insulating properties of Ta₂O₅ were found to be significantly modified by addition of Al. It was observed that Al addition has decreased the leakage currents approximately by an order of magnitude and improved thermal and bias stability characteristics of Ta₂O₅ capacitors. For example, the leakage currents in crystalline pure Ta₂O₅ thin films were found to be 4.5 x 10⁷ A/cm² in a 1MV/cm dc field which decreased to 3.4 x 10⁸ A/cm² for 10% Al modified Ta₂O₅ thin films. A typical dielectric constant of 42.5 was obtained for 10% Al modified Ta₂O₅ thin films. This is significantly higher compared to the commonly reported dielectric constant of 25 to 35 for Ta₂O₅ thin films. This enhancement was attributed to strong (100) orientation exhibited by both pure and modified Ta₂O₅ thin films. The high dielectric constant, low dielectric loss, low leakage currents and low temperature coefficient of capacitance suggest the suitability of Al modified Ta₂O₅ as a capacitor dielectric for future generation DRAM applications. ¹Barium strontium titanate, ²Barium bismuth tantalate / Master of Science

<p>Digital microfluidics as implemented in electrowetting-on-dielectric (EWD) technology has been widely used as a platform for miniaturizing the biomedical or biochemical laboratory on a chip in recent years. DNA pyrosequencing, one of the DNA sequencing-by-synthesis methods, has been successfully integrated on EWD devices. However, this platform requires microliters of reagents and 200~300V of applied voltages, which contributes to higher costs and limits the feasibility of a portable system. This dissertation proposes a low voltage EWD device using multi-layer insulators that can manipulate picoliter droplets on chip. A 300pl droplet was dispensed and actuated at voltages as low as 11.4Vrms and 7.2Vrms respectively on a 95um electrode a EWD device with a 20um SU8 gasket. The stacked insulators in the actuator consisted of 135nm tantalum pentoxide (Ta2O5) and 180nm parylene C films deposited and coated with 70 nm of CYTOP. The physical scaling of electrodes was further demonstrated for 33um and 21um electrode devices, resulting in droplets of 12pl and 5pl respectively in conjunction with 3um gaskets. Manipulation of magnetic beads during dispensing, droplet splitting and merging, and droplet transport were also demonstrated on the scaled EWD devices. The chemiluminescent light produced by the on-chip reaction of 100pl ATP-luciferin and luciferase could be detected with an external cooled CCD camera, but detecting this reaction with smaller-scale droplet reactions was limited by the external detector's sensitivity. Based on fundamental theories and experiments, the actuation voltage and dimensional scaling of EWD devices have been demonstrated, but the use of picoliter droplets in biochemical applications will required improved sensing methods.</p> / Dissertation

In recent years, as biological information analysis technology rapidly develops in hematology, biochemistry and microbiology areas, demand for portable measurement systems become more and more important. This study makes efforts in developing disposable hydrogen and potassium ion sensor and microsystem for analysis application. The measured ion concentration data by this analysis microsystem provide people a judgement on their health condition, and furthermore an important reference for medical treatment for patients. There are several advantages in using IC or MEMS technology to manufacture portable measurement system, the advantages are down-scaling, short reaction time, trace chemical analysis, low power dissipation, and low cost. So the thesis uses extended gate field effect transistor, in order to measure multiple ions at the same time, multiple transistors are manufactured on the same chip with an ion selective membrane on top of the gate sensitive layer. This allows the measurement result of the multiple ion be shown at the same time. The main processing steps of the ion sensor developed in this study involve at least four photolithographic and three thin-film deposition processes. Based on the measurement result, the hydrogen ion sensor¡¦s sensitivity is 30.7 mV/decade for a sensing range pH1 ~ pH13. The sensitivity of the potassium ion sensor is 11.5 mV/decade for a sensing range 10-1M to 10-3M.

The gradual depletion of and dependence on fossil fuels, air pollution and global warming have all accelerated the development of alternative energy systems which use hydrogen as an energy carrier. The hybrid sulphur cycle (HyS) is the foremost electrothermochemical process that can produce hydrogen as the energy carrier. The HyS cycle consists of two units, namely the sulphuric acid decomposition reactor and the sulphur dioxide electrolyser (SDE). The SDE is responsible for the SO2 electrooxidation to sulphuric acid and protons at the anode and the electro–reduction of protons to hydrogen at the cathode. This research study focuses on the kinetic data collected from the prepared catalysts for SO2 electro–oxidation at the anode. Platinum dispersed on carbon, niobium pentoxide, tantalum pentoxide and zirconium dioxide as electrocatalysts were prepared using sodium borohydride as a reducing agent. These electrocatalysts were characterized using transmission electron microscopy and x–ray diffraction. Cyclic voltammetry was used to study the electrochemical active surface area (EAS) and the results showed that Pt/ZrO2–C had a higher EAS area than Pt/Ta2O5–C, Pt/Nb2O5–C and Pt/C. The high EAS of Pt/ZrO2–C can be explained by the low crystal size however after a series of linear polarisation scans Pt/ZrO2–C experiences a much greater area loss than all the other catalysts. Linear polarisation scans for each of the catalysts revealed that the influence of increased temperature and sulphuric acid concentration were showed improved results. Levich and Koutecky–Levich plots revealed that the SO2 oxidation is a multistep reaction on all the prepared catalysts and that there are regions which are kinetic and diffusion controlled and diffusion–only controlled. Pt/Ta2O5–C catalysts exhibited superior catalytic activity and stability compared Pt/Nb2O5–C, Pt/ZrO2–C and Pt/C. The Pt/ZrO2–C exhibited the most inferior catalytic activity and stability. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011.

The gradual depletion of and dependence on fossil fuels, air pollution and global warming have all accelerated the development of alternative energy systems which use hydrogen as an energy carrier. The hybrid sulphur cycle (HyS) is the foremost electrothermochemical process that can produce hydrogen as the energy carrier. The HyS cycle consists of two units, namely the sulphuric acid decomposition reactor and the sulphur dioxide electrolyser (SDE). The SDE is responsible for the SO2 electrooxidation to sulphuric acid and protons at the anode and the electro–reduction of protons to hydrogen at the cathode. This research study focuses on the kinetic data collected from the prepared catalysts for SO2 electro–oxidation at the anode. Platinum dispersed on carbon, niobium pentoxide, tantalum pentoxide and zirconium dioxide as electrocatalysts were prepared using sodium borohydride as a reducing agent. These electrocatalysts were characterized using transmission electron microscopy and x–ray diffraction. Cyclic voltammetry was used to study the electrochemical active surface area (EAS) and the results showed that Pt/ZrO2–C had a higher EAS area than Pt/Ta2O5–C, Pt/Nb2O5–C and Pt/C. The high EAS of Pt/ZrO2–C can be explained by the low crystal size however after a series of linear polarisation scans Pt/ZrO2–C experiences a much greater area loss than all the other catalysts. Linear polarisation scans for each of the catalysts revealed that the influence of increased temperature and sulphuric acid concentration were showed improved results. Levich and Koutecky–Levich plots revealed that the SO2 oxidation is a multistep reaction on all the prepared catalysts and that there are regions which are kinetic and diffusion controlled and diffusion–only controlled. Pt/Ta2O5–C catalysts exhibited superior catalytic activity and stability compared Pt/Nb2O5–C, Pt/ZrO2–C and Pt/C. The Pt/ZrO2–C exhibited the most inferior catalytic activity and stability. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011.

Studies of the radiative properties of thin films and near-field radiation transfer in layered structures are important for applications in energy, near-field imaging, coherent thermal emission, and aerospace thermal management. A comprehensive study is performed on the optical constants of dielectric tantalum pentoxide (Ta₂O₅) and hafnium oxide (HfO₂) thin films from visible to the far infrared using spectroscopic methods. These materials have broad applications in metallo-dielectric multilayers, anti-reflection coatings, and coherent emitters based on photonic crystal structures, especially at high temperatures since both materials have melting points above 2000 K. The dielectric functions of HfO₂ and Ta₂O₅ obtained from this work may facilitate future design of devices with these materials. A parametric study of near-field TPV performance using a backside reflecting mirror is also performed. Currently proposed near-field TPV devices have been shown to have increased power throughput compared to their far-field counterparts, but whose conversion efficiencies are lower than desired. This is due to their low quantum efficiency caused by recombination of minority carriers and the waste of sub-bandgap radiation. The efficiency may be improved by adding a gold mirror as well as by reducing the surface recombination velocity, as demonstrated in this thesis. The analysis of the near-field TPV and proposed methods may facilitate the development or high-efficiency energy harvesting devices. Many near-field devices may eventually utilize metallo-dielectric structures which exhibit unique properties such as negative refraction due to their hyperbolic isofrequency contour. These metamaterials are also called indefinite materials because of their ability to support propagating waves with large lateral wavevectors, which can result in enhanced near-field radiative heat transfer. The energy streamlines in such structures are studied for the first time. Energy streamlines illustrate the flow of energy through a structure when the fields are evanescent and energy propagation is not ray like. The energy streamlines through two semi-infinite uniaxially anisotropic effective medium structures, separated by a small vacuum gap, are modeled using the Green’s function. The lateral shift and penetration depth are calculated from the streamlines and shown to be relatively large compared to the vacuum gap dimension. The study of energy streamlines in hyperbolic metamaterials helps understand the near-field energy propagation on a fundamental level.

No description available.

[pt] A ustulação cloretante é um processo amplamente utilizado no âmbito da metalurgia extrativa, principalmente no que se diz respeito à obtenção de metais. Estudos demonstram que a partir de um agente cloretante gasoso e a incorporação no sistema de um agente redutor, tanto a cinética quanto a termodinâmica das reações são estimuladas. Neste contexto, compostos organoclorados, como CCl4 e C2Cl4, despontam como candidatos promissores à substituição do Cl2. O presente estudo realiza uma avaliação termodinâmica dos processos a partir de diagramas de especiação para o equilíbrio, assim como a modelagem de dados cinéticos associados à cloração do óxido de cobre (CuO - 923 K a 1173 K) e do pentóxido de tântalo (Ta2O5 - 1073 K a 1223 K), em atmosfera de C2Cl4 diluído em N2, mediante o emprego de equações já consolidadas no âmbito da modelagem de reações gás-sólido (núcleo não reagido, auto-catalítico e Avrami). Os modelos do núcleo não reagido com controle difusional pela camada de cinzas e controle químico, foram os dois que apresentaram ajustes de melhor qualidade. O modelo difusional apresentou energia de ativação global para o CuO de 71,5592 mais ou menos 10 kJ.mol(-1) e de 62,2606 mais ou menos 10 kJ.mol (-1) para o Ta2O5, enquanto que com controle químico, para o CuO, obteve-se o valor de 118,0049 mais ou menos 10 kJ.mol(-1) e para o Ta2O5 um valor de 119,131 mais ou menos 10 kJ.mol(-1). Valores consistentes com o que é apresentado na literatura, sendo superiores em modelos com premissa química do que em modelos difusionais. Aspectos físicos também foram considerados e mediante ao número de Reynolds encontrado (Re=0,26 – escoamento laminar), reforçou-se um controle de natureza mista possível para ambos os óxidos. / [en] Chloride roasting is a process widely used in the field of extractive metallurgy, especially with regard to obtaining metals. Studies show that from a gaseous chlorinating agent and the incorporation of a reducing agent in the system, both the kinetics and the thermodynamics of the reactions are stimulated. In this context, organochlorine compounds, such as CCl4 and C2Cl4, stand out as promising candidates for the replacement of Cl2. The present study performs a thermodynamic evaluation of the processes from speciation diagrams for equilibrium, as well as the modeling of kinetic data associated with the chlorination of copper oxide (CuO - 923 K to 1173 K) and tantalum pentoxide (Ta2O5 - 1073 K to 1223 K), in an atmosphere of C2Cl4 diluted in N2, using equations already consolidated in the context of gas-solid reaction modeling (shrinking core, auto-catalytic and Avrami). The models of the shrinking core with diffusional control by the ash layer and chemical control, were the two that presented better quality adjustments. The diffusional model showed global activation energy for the CuO of 71.5592 plus-minus 10 kJ.mol (-1) and 62.2606 plus-minus 10 kJ.mol (-1) for Ta2O5, while with chemical control, for CuO, 118.0049 plus-minus 10 kJ.mol (-1) was obtained and for Ta2O5 a value of 119,131 plus-minus 10 kJ.mol (-1). Values consistent with what is presented in the literature, being higher in models with chemical premise than in diffusion models. Physical aspects were also considered and, based on the Reynolds number found (Re = 0,26 - laminar flow), a control of mixed nature possible for both oxides was reinforced.

Studium transportu náboje v Ta2O5 oxidových nanovrstvách se zaměřuje především na objasnění vlivu defektů na vodivost těchto vrstev. Soustředíme se na studium oxidových nanovrstev Ta2O5 vytvořených pomocí anodické oxidace. Proces výroby Ta2O5 zahrnuje řadu parametrů, jež ovlivňují koncentraci defektů (oxidových vakancí) v této struktuře. Vrstva oxidu Ta2O5 o tloušťce 20 až 200 nm se často používá jako dielektrikum pro tantalové kondenzátory, které se staly nedílnou součástí elektrotechnického průmyslu. Kondenzátory s Ta2O5 dielektrickou vrstvou lze modelovat jako strukturu MIS (kov – izolant – polovodič). Anodu tvoří tantal s kovovou vodivostí, katodu potom MnO2 či vodivý polymer (CP), které jsou polovodiče. Hodnoty elektronových afinit, respektive výstupních prací, jednotlivých materiálů potom určují výšku potenciálových bariér vytvořených na rozhraních kov-izolant (M – I) a izolant-polovodič (I – S). Dominantní mechanizmy transportu náboje lze určit analýzou I-V charakteristiky zbytkového proudu. Dominantní mechanizmy transportu náboje izolační vrstvou jsou ohmický, Poole-Frenkelův, Shottkyho a tunelování. Uplatnění jednotlivých vodivostních mechanismů je závislé na teplotě a intenzitě elektrického pole v izolantu. Hodnota zbytkového proud je významným indikátorem kvality daného izolantu. Ten závisí na technologii výroby kondenzátoru, významně především na parametrech anodické oxidace a na materiálu katody. I-V charakteristiky zbytkového proudu se měří v normálním a reversním módu, tj. normální mód značí kladné napětí na anodě a reversní mód záporné napětí na anodě. I-V charakteristika je výrazně nesymetrická, a proto tyto kondenzátory musí být vhodně polarizovány. Nesymetrie I-V charakteristiky se snižuje s klesající teplotou, při teplotě pod 50 K a je možno některé kondenzátory používat jako bipolární součástky. Z analýzy I-V charakteristiky lze určit řadu parametrů, jako tloušťku izolační vrstvy a koncentraci defektů v izolační Ta2O5 vrstvě a dále lze odhadnout parametry MIS modelu kondenzátoru - stanovit hodnotu potenciálových bariér na rozhraních M – I a I – S. Měření C-V charakteristik při různých teplotách v rozsahu 10 až 300 K je využíto pro určení výšky potenciálové bariéry na rozhraní I – S, závislosti kapacity na teplotě a dále pro výpočet efektivní plochy elektrod. Z výbrusu vzorků na skenovacím elektronovém mikroskopu byly určeny tloušťky dielektrika Ta2O5 pro jednotlivé vyhodnocované řady kondenzátorů.