Publikationsserver der RWTH Aachen University
Synthese und Charakterisierung von ferroelektrischen Xerogelen
Abstract
dc:descriptionThe central topic of the research presented is the synthesis and characterization of ferroelectric materials derived from the sol-gel method. Special emphasis is put on the nonlinear optical properties, especially second-harmonic generation (SHG). The sol-gel method is a wet-chemical technique and the chemical reaction starts upon addition water to a solution of an alcoxide, alcohol and a catalyst. Colloidal nanoparticles are formed and dispersed in the solution. The particles agglomerate due to Brownian motion and establish a three dimensional open porous network. The liquid in the pores can be removed by special drying methods yielding so called xerogels or aerogels. In this research a focus was set on the synthesis of LiTaO3, LiNbO3 and KNbO3 using lithium-, tantalum,- niobium,- and potassiumethoxide as starting materials. The wet gels are dried at 40°C-50°. During the drying the gels shrink in volume. Since these materials exhibit a porosity between 5%-50% they are called xerogels. Xerogels consist of networked amorphous particles with sizes in the nanometer range. To crystallize the amorphous nanograins the materials are sintered at different temperatures (700°C-1150°C) and for various times (2h-192h). During the sintering process the grains crystallize, the porosity sinks and their density increases. The grain sizes vary from approximately 250nm-1µm. During the synthesis the chemical composition of the xerogels can be adjusted by the proportion of the ethoxides used. The samples are characterized by thermogravimetric (TGA), nitrogen adsorption (BET- and BJH-methods), x-ray diffraction (XRD) and scanning electron microscopy (SEM). The aero- and xerogels typically have a specific surface area as measured by BET of 3-200m²/g, pore sizes between 5-20nm. During drying water and organic radicals evaporate until completion around 700°C. X-ray diffraction and evaluation by the Rietveld method shows that in stoichiometric samples the expected ferroelectric phases appear on sintering. In non-stoichiometric samples additional non-ferroelectric phases appear. At very high sintering temperatures the samples start to decompose at the surface. For second-harmonic generation measurements a pulsed infrared laser was focused on the ferroelectric xerogels and the SHG-signal was measured in transmission. The sintered xerogels show a strong frequency doubling effect. The samples generate diffuse light because of the randomized electric polarization of each grain and the porous structure. The SHG-energy was measured as a function of the infrared energy, the sample thickness, the chemical composition and the grain sizes. The SHG-energy generated can be described by a polynomial of the incident infrared energy having a linear and a quadratic term. The linear response confirms for the first time theoretical models for the SHG effect in random media. The intensity of the generated light grows linearly with the sample thickness for constant incident energies. The SHG-efficiency increases with the grain size. In conclusion, ferroelectric xerogels can be prepared by the sol-gel method. The diameter of the ferroelectric grains can be increased by varying the sintering temperature and time. The SHG-efficiency can be adjusted by increasing the grain sizes, the sample length and the degree of stoichiometry.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Lisinski, Susanne
- Contributors dc:contributor
-
- Ratke, Lorenz
Subjects
dc:subject × 10Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger