938 resultados para conduction bands


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Kinoite Ca2Cu2Si3O10(OH)4 is a mineral named after a Jesuit missionary. Raman and infrared spectroscopy have been used to characterise the structure of the mineral. The Raman spectrum is characterised by an intense sharp band at 847 cm-1 assigned to the ν1 (A1g) symmetric stretching vibration. Intense sharp bands at 951, 994 and 1000 cm-1 are assigned to the ν3 (Eu, A2u, B1g) SiO4 antisymmetric stretching vibrations. Multiple ν2 SiO4 vibrational modes indicate strong distortion of the SiO4 tetrahedra. Multiple CaO and CuO stretching bands are observed. Raman spectroscopy confirmed by infrared spectroscopy clearly shows that hydroxyl units are involved in the kinoite structure. Based upon the infrared spectra, it is proposed that water is also involved in the kinoite structure. Based upon vibrational spectroscopy, the formula of kinoite is defined as Ca2Cu2Si3O10(OH)4•xH2O.

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Demands for delivering high instantaneous power in a compressed form (pulse shape) have widely increased during recent decades. The flexible shapes with variable pulse specifications offered by pulsed power have made it a practical and effective supply method for an extensive range of applications. In particular, the release of basic subatomic particles (i.e. electron, proton and neutron) in an atom (ionization process) and the synthesizing of molecules to form ions or other molecules are among those reactions that necessitate large amount of instantaneous power. In addition to the decomposition process, there have recently been requests for pulsed power in other areas such as in the combination of molecules (i.e. fusion, material joining), gessoes radiations (i.e. electron beams, laser, and radar), explosions (i.e. concrete recycling), wastewater, exhausted gas, and material surface treatments. These pulses are widely employed in the silent discharge process in all types of materials (including gas, fluid and solid); in some cases, to form the plasma and consequently accelerate the associated process. Due to this fast growing demand for pulsed power in industrial and environmental applications, the exigency of having more efficient and flexible pulse modulators is now receiving greater consideration. Sensitive applications, such as plasma fusion and laser guns also require more precisely produced repetitive pulses with a higher quality. Many research studies are being conducted in different areas that need a flexible pulse modulator to vary pulse features to investigate the influence of these variations on the application. In addition, there is the need to prevent the waste of a considerable amount of energy caused by the arc phenomena that frequently occur after the plasma process. The control over power flow during the supply process is a critical skill that enables the pulse supply to halt the supply process at any stage. Different pulse modulators which utilise different accumulation techniques including Marx Generators (MG), Magnetic Pulse Compressors (MPC), Pulse Forming Networks (PFN) and Multistage Blumlein Lines (MBL) are currently employed to supply a wide range of applications. Gas/Magnetic switching technologies (such as spark gap and hydrogen thyratron) have conventionally been used as switching devices in pulse modulator structures because of their high voltage ratings and considerably low rising times. However, they also suffer from serious drawbacks such as, their low efficiency, reliability and repetition rate, and also their short life span. Being bulky, heavy and expensive are the other disadvantages associated with these devices. Recently developed solid-state switching technology is an appropriate substitution for these switching devices due to the benefits they bring to the pulse supplies. Besides being compact, efficient, reasonable and reliable, and having a long life span, their high frequency switching skill allows repetitive operation of pulsed power supply. The main concerns in using solid-state transistors are the voltage rating and the rising time of available switches that, in some cases, cannot satisfy the application’s requirements. However, there are several power electronics configurations and techniques that make solid-state utilisation feasible for high voltage pulse generation. Therefore, the design and development of novel methods and topologies with higher efficiency and flexibility for pulsed power generators have been considered as the main scope of this research work. This aim is pursued through several innovative proposals that can be classified under the following two principal objectives. • To innovate and develop novel solid-state based topologies for pulsed power generation • To improve available technologies that have the potential to accommodate solid-state technology by revising, reconfiguring and adjusting their structure and control algorithms. The quest to distinguish novel topologies for a proper pulsed power production was begun with a deep and through review of conventional pulse generators and useful power electronics topologies. As a result of this study, it appears that efficiency and flexibility are the most significant demands of plasma applications that have not been met by state-of-the-art methods. Many solid-state based configurations were considered and simulated in order to evaluate their potential to be utilised in the pulsed power area. Parts of this literature review are documented in Chapter 1 of this thesis. Current source topologies demonstrate valuable advantages in supplying the loads with capacitive characteristics such as plasma applications. To investigate the influence of switching transients associated with solid-state devices on rise time of pulses, simulation based studies have been undertaken. A variable current source is considered to pump different current levels to a capacitive load, and it was evident that dissimilar dv/dts are produced at the output. Thereby, transient effects on pulse rising time are denied regarding the evidence acquired from this examination. A detailed report of this study is given in Chapter 6 of this thesis. This study inspired the design of a solid-state based topology that take advantage of both current and voltage sources. A series of switch-resistor-capacitor units at the output splits the produced voltage to lower levels, so it can be shared by the switches. A smart but complicated switching strategy is also designed to discharge the residual energy after each supply cycle. To prevent reverse power flow and to reduce the complexity of the control algorithm in this system, the resistors in common paths of units are substituted with diode rectifiers (switch-diode-capacitor). This modification not only gives the feasibility of stopping the load supply process to the supplier at any stage (and consequently saving energy), but also enables the converter to operate in a two-stroke mode with asymmetrical capacitors. The components’ determination and exchanging energy calculations are accomplished with respect to application specifications and demands. Both topologies were simply modelled and simulation studies have been carried out with the simplified models. Experimental assessments were also executed on implemented hardware and the approaches verified the initial analysis. Reports on details of both converters are thoroughly discussed in Chapters 2 and 3 of the thesis. Conventional MGs have been recently modified to use solid-state transistors (i.e. Insulated gate bipolar transistors) instead of magnetic/gas switching devices. Resistive insulators previously used in their structures are substituted by diode rectifiers to adjust MGs for a proper voltage sharing. However, despite utilizing solid-state technology in MGs configurations, further design and control amendments can still be made to achieve an improved performance with fewer components. Considering a number of charging techniques, resonant phenomenon is adopted in a proposal to charge the capacitors. In addition to charging the capacitors at twice the input voltage, triggering switches at the moment at which the conducted current through switches is zero significantly reduces the switching losses. Another configuration is also introduced in this research for Marx topology based on commutation circuits that use a current source to charge the capacitors. According to this design, diode-capacitor units, each including two Marx stages, are connected in cascade through solid-state devices and aggregate the voltages across the capacitors to produce a high voltage pulse. The polarity of voltage across one capacitor in each unit is reversed in an intermediate mode by connecting the commutation circuit to the capacitor. The insulation of input side from load side is provided in this topology by disconnecting the load from the current source during the supply process. Furthermore, the number of required fast switching devices in both designs is reduced to half of the number used in a conventional MG; they are replaced with slower switches (such as Thyristors) that need simpler driving modules. In addition, the contributing switches in discharging paths are decreased to half; this decrease leads to a reduction in conduction losses. Associated models are simulated, and hardware tests are performed to verify the validity of proposed topologies. Chapters 4, 5 and 7 of the thesis present all relevant analysis and approaches according to these topologies.

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A simple phenomenological model for the relationship between structure and composition of the high Tc cuprates is presented. The model is based on two simple crystal chemistry principles: unit cell doping and charge balance within unit cells. These principles are inspired by key experimental observations of how the materials accommodate large deviations from stoichiometry. Consistent explanations for significant HTSC properties can be explained without any additional assumptions while retaining valuable insight for geometric interpretation. Combining these two chemical principles with a review of Crystal Field Theory (CFT) or Ligand Field Theory (LFT), it becomes clear that the two oxidation states in the conduction planes (typically d8 and d9) belong to the most strongly divergent d-levels as a function of deformation from regular octahedral coordination. This observation offers a link to a range of coupling effects relating vibrations and spin waves through application of Hund’s rules. An indication of this model’s capacity to predict physical properties for HTSC is provided and will be elaborated in subsequent publications. Simple criteria for the relationship between structure and composition in HTSC systems may guide chemical syntheses within new material systems.

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In order to mimic the formation of archerite in cave minerals, the mineral analogue has been synthesised. The cave mineral is formed by the reaction of the chemicals in bat guano with calcite substrates. X-ray diffraction proves that the synthesised archerite analogue was pure. The vibrational spectra of the synthesised mineral are compared with that of the natural cave mineral. Raman and infrared bands are assigned to H2PO4-, OH and NH stretching and bending vibrations. The Raman band at 917 cm-1 is assigned to the HOP stretching vibration of the H2PO4- units. Bands in the 1200 to 1800 cm-1 region are associated with NH4+ bending modes. Vibrational spectroscopy enables the molecular structure of archerite to be analysed.

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The phosphate mineral brazilianite NaAl3(PO4)2(OH)4 is a semi precious jewel. There are almost no minerals apart from brazilianite which are used in jewellery. Vibrational spectroscopy was used to characterize the mol. structure of brazilianite. Brazilianite is composed of chains of edge-sharing Al-O octahedra linked by P-O tetrahedra, with Na located in cavities of the framework. An intense sharp Raman band at 1019 cm-1 is attributed to the PO43- sym. stretching mode. Raman bands at 973 and 988 cm-1 are assigned to the stretching vibrations of the HOPO33- units. The IR spectra compliment the Raman spectra but show greater complexity. Multiple Raman bands are obsd. in the PO43- and HOPO33- bending region. This observation implies that both phosphate and hydrogen phosphate units are involved in the structure. Raman OH stretching vibrations are found at 3249, 3417 and 3472 cm-1. These peaks show that the OH units are not equiv. in the brazilianite structure. Vibrational spectroscopy is useful for increasing the knowledge of the mol. structure of brazilianite.

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The multianion mineral gartrellite PbCu(Fe3+,Cu)(AsO4)2(OH,H2O)2 has been studied by a combination of Raman and infrared spectroscopy. The vibrational spectra of two gartrellite samples from Durango and Ashburton Downs were compared. Gartrellite is one of the tsumcorite mineral group based upon arsenate and sulphate anions. Crystal symmetry is either triclinic in the case of an ordered occupation of two cationic sites, triclinic due to ordering of the H bonds in the case of species with 2 water molecules per formula unit, or monoclinic in the other cases. Characteristic Raman spectra of the minerals enable the assignment of the bands to specific vibrational modes. These spectra are related to the structure of gartrellite. The position of the hydroxyl and water stretching vibrations are related to the strength of the hydrogen bond formed between the OH unit and the AsO4 anion.

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Organic solar cells based on bulk heterojunction between a conductive polymer and a carbon nanostructure offer potential advantages compared to conventional inorganic cells. Low cost, light weight, flexibility and high peak power per unit weight are all features that can be considered a reality for organic photovoltaics. Although polymer/carbon nanotubes solar cells have been proposed, only low power conversion efficiencies have been reached without addressing the mechanisms responsible for this poor performance. The purpose of this work is therefore to investigate the basic interaction between carbon nanotubes and poly(3-hexylthiophene) in order to demonstrate how this interaction affects the performance of photovoltaic devices. The outcomes of this study are the contributions made to the knowledge of the phenomena explaining the behaviour of electronic devices based on carbon nanotubes and poly(3-hexylthiophene). In this PhD, polymer thin films with the inclusion of uniformly distributed carbon nanotubes were deposited from solution and characterised. The bulk properties of the composites were studied with microscopy and spectroscopy techniques to provide evidence of higher degrees of polymer order when interacting with carbon nanotubes. Although bulk investigation techniques provided useful information about the interaction between the polymer and the nanotubes, clear evidence of the phenomena affecting the heterojunction formed between the two species was investigated at nanoscale. Identifying chirality-driven polymer assisted assembly on the carbon nanotube surface was one of the major achievements of this study. Moreover, the analysis of the electrical behaviour of the heterojunction between the polymer and the nanotube highlighted the charge transfer responsible for the low performance of photovoltaic devices. Polymer and carbon nanotube composite-based devices were fabricated and characterised in order to study their electronic properties. The carbon nanotube introduction in the polymer matrix evidenced a strong electrical conductivity enhancement but also a lower photoconductivity response. Moreover, the extension of pristine polymer device characterisation models to composites based devices evidenced the conduction mechanisms related to nanotubes. Finally, the introduction of carbon nanotubes in the polymer matrix was demonstrated to improve the pristine polymer solar cell performance and the spectral response even though the power conversion efficiency is still too low.

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Stringhamite CaCuSiO4·H2O is a hydrated calcium copper silicate and is commonly known as a significant ‘healing’ mineral and is potentially a semi-precious jewel. Stringhamite is a neosilicate with Cu2+ in square planar coordination. Vibrational spectroscopy has been used to characterise the molecular structure of stringhamite. The intense sharp Raman band at 956 cm−1 is assigned to the ν1 (A1g) symmetric stretching vibration. Raman bands at 980, 997, 1061 cm−1 are assigned to the ν3 (A2u, B1g) antisymmetric stretching vibrations. Splitting of the ν3 vibrational mode supports the concept that the stringhamite SiO4 tetrahedron is strongly distorted. The intense bands at 505 and 519 cm−1 and at 570 cm−1 are assigned to the ν2 and ν4 vibrational modes. The question arises as to whether the mineral stringhamite can actually function as a healing mineral. An estimation of the solubility product at pH < 5 shows that the cupric ion can be released. The copper ion is a very powerful antibiological agent and thus the mineral stringhamite may well function as a healing mineral.

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Whelanite Ca5Cu2(OH)2CO3,Si6O17•4H2O is a hydrated hydroxy mixed anion compound with both silicate and carbonate anions in the formula. The structural characterisation of the mineral whelanite remains incomplete. Whelanite is probably a neosilicate with Cu2+ in square planar coordination. Two Raman bands at 1070 and 1094 cm-1 are assigned to the ν1 symmetric stretching modes of the CO32- units. The observation of two symmetric stretching modes supports the concept of two non-equivalent CO32- units in the whelanite structure. The intense sharp Raman band at 1006 cm-1 is assigned to the ν1 (A1g) symmetric stretching vibration of the Si6O17 units. The splitting of the ν3 vibrational mode offers support to the concept that the SiO4 tetrahedron in whelanite is strongly distorted. A very intense Raman band observed at 666 cm-1 with a shoulder at 697 cm-1 is assigned to the ν4 vibrational modes. Intense Raman bands at 3534, 3556, 3550 and 3595 cm-1 are assigned to the stretching vibrations of the OH units. Low intensity Raman bands at 2910, 3187 and 3453 cm-1 are assigned to water stretching modes. Thus, vibrational spectroscopy has been used to characterise the molecular structure of whelanite. Whelanite is a mineral that could be conceived as a healing mineral

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Planchéite Cu8Si8O22(OH)4•H2O is a hydrated copper hydroxy silicate. The objective of this work is to use Raman and infrared spectroscopy to determine the molecular structure of planchéite. Raman bands of planchéite at around 1048, 1081 and 1127 are described as the ν1 –SiO3 symmetric stretching vibrations; Raman bands at 828, 906 are attributed to the ν3 –SiO3 antisymmetric stretching vibrations. The Raman band at 699 cm-1 is assigned to the ν4 bending modes of the -SiO3 units. The intense Raman band at 3479 cm-1 is ascribed to the stretching vibration of the OH units. The Raman band at 3250 cm-1 is evidence for water in the structure. A comparison of the spectra of planchéite with that of shattuckite and chrysocolla.

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Arsenogorceixite BaAl3AsO3(OH)(AsO4,PO4)(OH,F)6 belongs to the crandallite mineral subgroup of the alunite supergroup. Arsenogorceixite forms a continuous series of solid solutions with related minerals including gorceixite, goyazite, arsenogoyazite, plumbogummite and philipsbornite. Two minerals from (a) Germany and (b) from Ashburton Downs, Australia were analysed by Raman spectroscopy. The spectra show some commonality but the intensities of the peaks vary. Sharp intense Raman bands for the German sample, are observed at 972 and 814 cm−1 attributed to the ν1 PO43− and AsO43− symmetric stretching modes. Raman bands at 1014, 1057, 1148 and 1160 cm−1 are attributed to the ν1 PO2 symmetric stretching mode and ν3 PO43− antisymmetric stretching vibrations. Raman bands at 764 and 776 cm−1 and 758 and 756 cm−1 are assigned to the ν3 AsO43− antisymmetric stretching vibrations. For the Australian mineral, the ν1 PO43− band is found at 973 cm−1. The intensity of the arsenate bands observed at 814, 838 and 870 cm−1 is greatly enhanced. Two low intensity Raman bands at 1307 and 1332 cm−1 are assigned to hydroxyl deformation modes. The intense Raman band at 441 cm−1 with a shoulder at 462 cm−1 is assigned to the ν2 PO43− bending mode. Raman bands at 318 and 340 cm−1 are attributed to the (AsO4)3−ν2 bending. The broad band centred at 3301 cm−1 is assigned to water stretching vibrations and the sharper peak at 3473 cm−1 is assigned to the OH stretching vibrations. The observation of strong water stretching vibrations brings into question the actual formula of arsenogorceixite. It is proposed the formula is better written as BaAl3AsO3(OH)(AsO4,PO4)(OH,F)6·xH2O. The observation of both phosphate and arsenate bands provides a clear example of solid solution formation.

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Three wardite mineral samples from different origins have been analysed by vibrational spectroscopy. The mineral is unusual in that it belongs to a unique symmetry class, namely the tetragonal-trapezohedral group. The structure of wardite contains layers of corner-linked –OH bridged MO6 octahedra stacked along the tetragonal C-axis in a four-layer sequence and linked by PO4 groups. Consequentially not all phosphate units are identical. Thus, two intense Raman bands observed at 995 and 1051 cm-1 are assigned to the ν1 PO43- symmetric stretching mode. Intense Raman bands are observed at 605 and 618 cm-1 with shoulders at 578 and 589 cm-1 are assigned to the ν4 out of plane bending modes of the PO43-. The observation of multiple bands supports the concept of non-equivalent phosphate units in the structure. Sharp infrared bands are observed at 3544 and 3611 cm-1 are attributed to the OH stretching vibrations of the hydroxyl units. Vibrational spectroscopy enables subtle details of the molecular structure of wardite to be determined.

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The two minerals borickyite and delvauxite CaFe3+4(PO4,SO4)2(OH)8•4-6H2O have the same formula. Are the minerals identical or different? The minerals borickyite and delvauxite have been characterised by Raman spectroscopy. The minerals are related to the minerals diadochite and destinezite. Both minerals are amorphous. Delvauxite appears to vary in crystallinity from amorphous to semi-crystalline. The minerals are often X-ray non-diffracting. The minerals are found in soils and may be described as ‘colloidal’ minerals. Vibrational spectroscopy enables an assessment of the molecular structure of borickyite and delvauxite. Bands are assigned to phosphate and sulphate stretching and bending modes. Multiple water bending and stretching modes imply that non-equivalent water molecules in the structure exist with different hydrogen bond strengths. The two minerals show differing spectra and must be considered as different minerals.

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In this paper, we report on many phosphate containing natural minerals found in the Jenolan Caves - Australia. Such minerals are formed by the reaction of bat guano and clays from the caves. Among these cave minerals is the montgomeryite mineral [Ca4MgAl4(PO4)6.(OH)4.12H2O]. The presence of montgomeryite in deposits of the Jenolan Caves - Australia has been identified by X-ray diffraction (XRD). Raman spectroscopy complimented with infrared spectroscopy has been used to characterize the crystal structure of montgomeryite. The Raman spectrum of a standard montgomeryite mineral is identical to that of the Jenolan Caves sample. Bands are assigned to H2PO4-, OH and NH stretching vibrations. By using a combination of XRD and Raman spectroscopy, the existence of montgomeryite in the Jenolan Caves - Australia has been proven. A mechanism for the formation of montgomeryite is proposed.

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The mineral brushite has been synthesised by mixing calcium ions and hydrogen phosphate anions to mimic the reactions in a Cave. The vibrational spectra of the synthesised brushite were compared with that of the natural Cave mineral. Bands attributable to the PO43- and HPO42- anions are observed. Brushite, both synthetic and natural, is characterised by an intense sharp band at 985 cm-1 with a shoulder at 1000 cm-1. Characteristic bending modes are observed in the 300 to 600 cm-1 region. The spectra of the synthesised brushite matches very well the spectrum of brushite from the Moorba Cave, Western Australia.