Engineered Nanostructures and Thermoelectric Nanomaterials

University dissertation from Stockholm : Materialvetenskap

Abstract: This thesis deals with novel aspects of Nanomaterials,Nanoscience and Nanotechnology. On the materials side, novelmethods were developed for the fabrication of nano-sizedbuilding blocks, such as nanoparticles, nanorods and nanowireswith defined structures and high homogeneity. Nanoparticleswere consolidated to nanostructured materials. The furtherstudy included the fabrication of more complex structures onpatterned or non-patterned surfaces as well as hierarchicalstructures. Size dependent properties of these functionalnanostructures were investigated.Thermoelectric (TE) materials have dual property ofgenerating electrical energy by using a temperature gradient,or cooling/heating when electrical energy is supplied. There isa high promise for significantly improving the figure of meritof TE materials through nanostructuring, because of theselective diffraction of phonons by the high density of grainboundary created leading to a decrease of thermal conductivity.Two primary groups of thermoelectric nanomaterials have beenstudied in this Thesis; Skutterudites and Bismuth telluride.Bi-Te alloys are state of the art TE materials and their figureof merit and mechanical properties can be enhanced bynanostructuring. Skutterudites with their unique host-gueststructure containing rattling atoms, have a promise forachieving exceptionally high ZT values through nanostructuringand doping. Chemical methods have been developed for thesynthesis of the TE nanomaterials. Solution chemical methodswere used for preparing nanosized precursors of each of thesecompounds, with exact composition and high homogeneity byco-precipitation from aqueous solutions. The precursors werefurther processed by thermo-chemical treatments includingcalcination, reduction and alloying at relatively lowtemperatures to produce nano-powders with high purity of TEphase.Comprehensive characterization of several un-doped and dopedTE nanomaterials has been undertaken. Pure phase of TEnanomaterials could be obtained for the different systems.Nanostructured TE materials have shown thermal conductivityabout one order of magnitude lower than those of large graincounterparts and single crystal. Despite the decrease ofelectrical conductivity (s) and Seebeck coefficient (S), anenhanced ZT value of 0.17 has been obtained, which is nearlydouble that of polycrystalline materials with micron size.Molecular engineering of Skutterudites by doping the structurewith atoms of one or two elements, were introduced and showedan enhancement of the electrical conductivity and Seebeckcoefficient. Skutterudites doped with Ni or Te have exhibitedhigher s and S resulting in higher ZT values of 0.3 and 0.37respectively. For Skutterudites doped with both Ni and Te, a ZTof 0.65 was obtained. This is the highest figure of meritreported for unfilled skutterudites, which clearly shows anenhancement of materials performance as a result ofnanostructuring and molecular engineering. Further enhancementcan be achieved though a combination of nanostructuring, dopingand rattling atoms.We also studied the fabrication of building blocks with morecomplex nanostructures by using bottom-up strategies.Core-shell structured nanocomposites are fabricated by atemplate-directed self-assembly method, where colloidal silicaparticles were used as templates. Surface functionalization ofsilica nanospheres using coupling molecules that are capable ofbinding to metallic surfaces. In this approach, metallicnanoparticles could be assembled on the surface of the silicaparticles. With porous shell layer, it was possible to removethe core of the particles and replace it with another material.Templates having an array of nanosized channels were preparedby anodization of aluminum films, and used for the fabricationof nano-objects by electrodeposition and metal nanowires andnanorods to be confined into nano-channels of the template.Another approach for template-assisted nano-engineering thefabrication of hierarchically arranged periodic 3-D structureswere fabricated by the self-assembly of polystyrenenanospheres. A replication of the 3-D structure has beenachieved by filling the voids between the polystyrenenanospheres with a suitable material, followed by the removalof the polystyrene spheres. In this way a macro-poroushierarchical replica of the original structure is obtained.Finally a combine top-down and bottom up approaches has beenused for the preparation of 2D and 3D structures by thedirected assembly of nanoparticles on patterned andnon-patterned surfaces. The work included microcontact printingtechniques, where bifunctional molecules were transferred to asurface by using a polymeric stamp. By employing differentbifunctional groups on pre-assembled surfaces, nanoparticulatemultilayers consisting of same or different kind of materialswere successfully obtained.Keywords:Nanoengineering, Molecular Engineering,Nanoparticles, Core-shell structures, mCP, Self-assembly,Electrodeposition, 3D ordered structures, Thermoelectricmaterials, Electrical conductivity, Seebeck coefficient,Thermal conductivity, Chemical alloying, ZT.

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