Papers by Author: Andréi L. Kholkin

Abstract: This work analyzes the processing of Pb(Zr,Ti)O3 (PZT) thin films directly on copper-coated polymer films. PZT thin film deposition was performed onto the metallized Kapton® films using a single RF plasma jet. In order to reduce the interaction of PZT and Cu during the initial growth stage, an ultrathin amorphous TiO2-x seeding layer was sputter-deposited prior to PZT deposition. The film texture was a mixture of (111)-oriented perovskite nanocrystals, rutile and pyrochlore. Topography and piezoelectric in-plane and out-of-plane response of the films were evaluated using a commercial AFM adapted for piezoforce measurements. The as-deposited films were self-polarized with polarization pointing at the surface of the sample. Polarization was switchable and a piezoelectric hysteresis was obtained.

Abstract: Investigation of crystal structure and magnetic properties of the diamagnetically- substituted Bi1-xAxFeO3-x/2 (A= Ca, Sr, Pb, Ba; x= 0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 giving rise to the appearance of weak ferromagnetism.

Abstract: In an attempt to gain a better understanding of the lattice dynamics in lanthanum doped SrTiO3 (STO) ceramics, with general formulae LaxSr(1-1.5x)TiO3, we have carried out a detailed Raman study of the x=0.0133 ceramic in the temperatures ranging from 10K to room temperature. Our results show that lanthanum substitution gives rise to new features in the Raman spectra. The Rayleigh band and the extra Raman-forbidden infrared active modes observed provide evidence for the existence of disorder in the system, apparently associated with grain structure, and lattice defects. Moreover, the absence of a TO1 polar soft mode supports the non-existence of a ferroelectric phase at low temperatures. The shift of the para-antiferrodistorsive transition temperature towards higher temperatures clearly evidences that the replacement of Strontium by Lanthanum occurs at the lattice A-site.

Abstract: AgxNi1-x (x=0.0-1.0) films were grown on Cu substrates by electrodeposition method. The films were found to be a nanocrystalline mixture of pure silver and nickel. The grain sizes were determined by X-ray diffraction and electron microscopy techniques. The minimal value was 3.3 m for the alloy with 70 wt% concentration of Ni. The stability of the grown films upon heating in ir and in vacuum was examined. An increase in the grain size was found to begin at 150°C.

Abstract: Cadmium chalcogenide quantum dots (QD’s) were synthesised using a single source approach while zinc oxide QD’s were obtained by a colloidal technique. In both situations the dots were surface capped with tri-octylphosphine oxide (TOPO) hence leading to nanodispersed systems in organic solvents such as toluene. The organically capped QD’s (CdSe, CdS and ZnO) were used as building-units to fabricate LbL (layer-by-layer) films on glass and quartz substrates. A linear increase in the visible light absorbance (due to the QD’s) with the number of deposited layers indicates that multi-layered systems have been fabricated. In order to investigate the effect of the LbL manipulation on the integrity of the QD’s, comparative studies on the optical properties of the starting QD’s and the nanostructured films have been performed. The observation of quantum size effects in both cases suggests minimal degradation of the QD’s though clustering had probably occurred, a point which was further confirmed by AFM analysis.

Abstract: Lead-free relaxor ceramics based on sodium-bismuth titanate, (1-x)(Na1/2Bi1/2)TiO3 - xLa(Mg1/2Ti1/2)O3 [(1-x)NBT-xLMT] (0≤x≤0.25), were prepared by both the conventional mixed oxide method and by the Pechini route, and their crystal structure and dielectric properties were investigated. All the compositions studied were found to possess a rhombohedrally distorted crystal lattice at room temperature. A distortion degree of the unit cell decreases and its volume increases with LMT content. The temperature of the dielectric permittivity maximum does not change with increasing substitution rate. As x is increased, the frequency-dependent dielectric peaks are flattened. The compositional evolution of structure and dielectric characteristics of the ceramics obtained are analysed and discussed in respect to size, charge and polarizability of the cations involved.

Abstract: A quantitative model for the calculation of spontaneous polarization of BLSF ceramics as a function of the degree of texture is presented using a general formalism based on the texture analysis via an orientation distribution function. The March-Dollase equation was selected to fit the measured texture distribution because its fitting parameters can be related to experimentally measurable stereological values obtained from SEM images. The results are applied to the SrBi2Ta2O9 (SBT) system, which is a well-known member of the BLSF family. The textured SBT ceramics were fabricated by templated grain growth (TGG) and the microstructure and degree of texture were evaluated by SEM analysis, which allowed correlating the texture development to the sintering parameters. Enhanced ferroelectric properties were measured perpendicularly to the uniaxial pressing direction of the ceramics, revealing the influence of the grains orientation, anisotropy and volume fraction of textured material on the ferroelectric properties. Finally, the predicted values for the spontaneous polarization as a function of the degree of texture were compared with those measured from the hysteresis loops.