A study on nanophase functional materials : high T<SUB>c</SUB>superconductors and Zinc Oxide

Abstract: Nanophase materials are an active materials research topicthat has received a great deal of attention over the past fewyears. Tlus mesis deals mainly with the synthesis, processing,and characterization of nanophase functional materials. Twotypes of functional materials were considered: high temperaturesuperconducting oxides (HTSC) and Wnc oxide. By using nanosizeparticles, the performance of both materials is greatlyenbanced. The synthesis is done mainly by chemical solutionmethods. Precipitation technique has been used for thepreparation of nanosized particles. Tbc former material, whhcomplex but requires exact tomposition, while the later has arelahvely simple tomposition but emphasis is given to conwolledmorphology.Thermodynamic modeling of chemical reactions has beenapplied to all systems studied in this thesis and extensivelyused for determming the optimal conditions for the formation ofthe solid powder with the exact required tomposition. It allowsthe examination of the influence of the varration of theconcennation of the different components on the tomposition orthe morphology of the formed solid powder. Chemical control ofthe tomposition, particles sim and morphology has achieved viaconuolling the chemical reactions in solution, either in bulkor confined zones (droplet), e.g., in microemulsion and flowinjection synthesis.Nanosized Y- and Bi-based superconducting precursors havebeen synthesized by means of oxalate coprecipitation in bulksolution. Microemulsion method has also been used to prepareY-based precursor. To carry out the reactions in droplet, hasthe advantage of controlhng not only of the chemicaltomposition, but also the physical powder characteristics bylimiting particle size to the droplet diameter. Chemical andmorphological properties of these powders have been evaluatedby different techniques, e.g., ICP, TGA, XRD, BBT, SEM, EDS,and particle sim. measurement The influence of the homogeneityand the small gram sim distribution of the nanophase powders onthe properties of the materials have been noted.A study of the processing of the superconducting precursorinto the crystalline oxide materials fornred uponthermomechanical treatment was undertaken. Bi-based preeursorswere used to fabricate of tapes by powder-in-tube technique(PIT). A systematic analysis of the fabrication of high qualitysuperconducting tapes, the effects of different annealing timesand temperatums, compacting pressures and the number ofpressing-annealing cycles performed were investigated.Nanophase zinc oxide powder was prepared by differenttechniques, e.g., precipitation in solution, flow injectionsynthesis technique (FIS), and laser vaporization technique. Bycontrolled precipitation in aqueous solutions, using ammoniumcarbamate, nanosized ZnO particles with new rod-shapemorphology was obtained. FIS technique has been developed forthe synthesis of nanosized ZnO with less agglomeration andhighly uniform powder. ZnO prepared by laser vaporizationtechnique have a web-like structure and much smaller particlesize (few nanometers).Doped and undoped ZnO powders was used to fabricatenanostructured film elecuodes, in a Grätzel-type solarcell t evaluated the photo-tonversion properties. For filmsconsisting of 15 nm crystallites ZnO, a maximum monochromaticcurrent tonversion effrciency of 58% was obtained and theoverall solar energy tonversion efficiency was 2%, which isabout 400% higher than reported value.Key words: nanophase materials, chemical methods, highT, superconductors, zinc oxide, modeling of aqueous solution,precipitation, microemulsion, flow injection synthesis,critical current, dye-sensitized solar cell, electrode,photoconversion efficiency.