Global ETD Search

1	Determination of Piezoelectric Parameters from Measured Natural Frequencies of a Piezoelectric Circular Plate Chen, Ting-chun 19 July 2010 (has links) Due to the complexity of electro-elastic coupling characteristics in piezoelectric material, some of the elastic, dielectric and piezoelectric parameters are difficult to be measured. Usually, these parameters are determined by assuming that all offer parameters are remained constant during the measurement. However, the interactive effect between material properties makes this assumption be not always true. In this study, the measured natural frequencies of the specified circular piezoelectric plate are used to extract these parameters simultaneously. In other words, all these parameters are determined with considering the interactive electro-elastic coupling effect. The analytic model of free-free circular piezoceramic plate was derived and solved to establish the relationship between natural frequencies and its material parameters, to cover most all the parameters, the out-of-plane(non-symmetric transverse) and in-plane(symmetric extensional) modes are considered. The genetic algorithm is employed to determine most all elastic, dielectric and piezoelectric parameters from a least square error between the calculated and measured natural frequencies. Numerical results derived from the parameters proposed in this work reveal a good agreement with the measured data. In other words, the proposed method to extract the piezoelectric parameters is feasible and effective. piezoelectric constants genetic algorithm piezoelectric material vibration elastic constants piezoelectric circular plate dielectric constants
2	Molten-salt Synthesis Of Nanocrystalline Strontium Antimony Manganese Oxide (Sr2SbMnO6) : A Gaint Dielectric Constant Material Baral, Antara 07 1900 (has links) High dielectric constant materials are of technological importance as they lead to the miniaturization of the electronic devices. For instance, in the case of memory devices based on capacitive components, such as static and dynamic random access memories, the dielectric constant will ultimately decide the level of miniaturization. In this context, the observation of anomalously high dielectric constant (>10) in the double perovskite Sr2SbMnO6 (SSM) over wide frequency (100 Hz1 MHz) and (190373 K) temperature range has attracted a great deal of attention. However, unfortunately their dielectric losses were also high which limit their use for possible capacitor and related applications. The dielectric loss however was known to decrease with decreasing crystallite size in electroceramics. Therefore, the present work has been focused on the synthesis of nanocrystalline SSM powders by moltensalt route. The characterization of the ceramics fabricated from these powders for their microstructural and dielectric properties. A cubic phase of SSM powder was obtained by calcining the as synthesized powders at 900°C/10h by using sulphate flux. The crystallite size was ~ 60 nm. The activation energy associated with the particle growth was found to be 95 ± 5 kJmol-1 . The ceramic sintered at 1075°C/16h exhibited high dielectric constant (>10at 1 kHz) with low loss (0.72 at 1 kHz) at room temperature. The results are interpreted in terms of a twolayer model with conducting grains partitioned from each other by poorly conducting grain boundaries. Using this model, we attributed the two electrical responses in impedance and modulus formalisms to the grain and grain boundary effects, respectively, while the detected Debyelike relaxation and large dielectric constant were explained in terms of MaxwellWagner relaxation. Strontium Antimony Sr2SbMnO6 Dielectric Materials Molten-salt Synthesis High Dielectric Constants High Dielectric Constant Materials Strontium Antimony Manganese Oxide Material Fabrication Technique Dielectric Constant Material Nanocrystalline SSM Materials Science
3	Optical Properties of Organic Semiconductors: from Submonolayers to Crystalline Films / Optische Eigenschaften organischer Halbleiter: von Submonolagen zu kristallinen Filmen Nitsche, Robert 12 April 2006 (has links) (PDF) We have measured the optical properties of films of the organic semiconductors PTCDA (3,4:9,10-perylene-tetracarboxylic dianhydride) and HBC (peri-hexabenzocoronene), prepared by Organic Molecular Beam Expitaxy (OMBE), on different substrates by means of Differential Reflectance Spectroscopy (DRS). The optical setup enables us to directly follow the thickness dependent optical properties of the organic films, starting from submonolayer coverage up to thicker films on the order of 20 monolayers (ML) film thickness. Due to the different optical nature of the different substrates used, i.e., mica, glass, Au(111), and HOPG, the direct interpretation of the DRS signal is not feasible. Therefore, we have proposed a method by which the calculation of the optical constants n (index of refraction) and k (absorption index) of thin films on arbitrary substrates from just one spectral measurement (in our case the DRS) becomes possible. The results fulfill a priori a Kramers-Kronig consistency and no specific model is needed to express the spectral behavior of the optical constants. Based on our method, we have successfully calculated the optical constants, and therefore the absorption behavior, of films of different thickness of PTCDA on mica, glass, Au(111), and HOPG, as well as of HBC on mica, glass, and HOPG. Extrinsic effects due to island growth or the presence of a polarizable substrate (screening) have been accounted for. We have introduced a finite dipole model which considers the extended geometry and anisotropy of the organic molecules. The calculated absorption behavior is discussed in great detail in terms of spectral changes with varying film thickness, different growth modes, degree of ordering of the films, interactions with the substrates and oscillator strength. A direct observation of a monomer-dimer transition in solid films could be observed for the first time. Our results indicate an exciton delocalization over about 4 molecules for both molecules. Absorption Au(111) Differentielle Reflexionsspektroskopie Dipolmodell Exzitonen Glas Glimmer Graphit HBC HOPG Kramers-Kronig Oszillatorstärke PTCDA dünne Filme optische Eigenschaften optische Konstanten Au(111) Differential Reflectance Spectroscopy Glass HBC HOPG Kramers-Kronig PTCDA absorption dielectric constants dipole model excitons graphite mica optical constants optical properties oscillator strength thin films ddc:530 rvk:UP 3050 Absorption Dipol Dünne Schicht Exziton Glimmer Gold Graphit Optische Eigenschaft Optische Konstante Oszillatorstärke Perylendianhydrid Reflexionsspektroskopie
4	Optical Properties of Organic Semiconductors: from Submonolayers to Crystalline Films Nitsche, Robert 23 November 2005 (has links) We have measured the optical properties of films of the organic semiconductors PTCDA (3,4:9,10-perylene-tetracarboxylic dianhydride) and HBC (peri-hexabenzocoronene), prepared by Organic Molecular Beam Expitaxy (OMBE), on different substrates by means of Differential Reflectance Spectroscopy (DRS). The optical setup enables us to directly follow the thickness dependent optical properties of the organic films, starting from submonolayer coverage up to thicker films on the order of 20 monolayers (ML) film thickness. Due to the different optical nature of the different substrates used, i.e., mica, glass, Au(111), and HOPG, the direct interpretation of the DRS signal is not feasible. Therefore, we have proposed a method by which the calculation of the optical constants n (index of refraction) and k (absorption index) of thin films on arbitrary substrates from just one spectral measurement (in our case the DRS) becomes possible. The results fulfill a priori a Kramers-Kronig consistency and no specific model is needed to express the spectral behavior of the optical constants. Based on our method, we have successfully calculated the optical constants, and therefore the absorption behavior, of films of different thickness of PTCDA on mica, glass, Au(111), and HOPG, as well as of HBC on mica, glass, and HOPG. Extrinsic effects due to island growth or the presence of a polarizable substrate (screening) have been accounted for. We have introduced a finite dipole model which considers the extended geometry and anisotropy of the organic molecules. The calculated absorption behavior is discussed in great detail in terms of spectral changes with varying film thickness, different growth modes, degree of ordering of the films, interactions with the substrates and oscillator strength. A direct observation of a monomer-dimer transition in solid films could be observed for the first time. Our results indicate an exciton delocalization over about 4 molecules for both molecules. info:eu-repo/classification/ddc/530 ddc:530

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