Papers by Author: Witold Łojkowski

Abstract: Mechanically alloyed Al65Cu20Ti15 amorphous alloy powder with 10 wt.% nano-TiO2 dispersion was consolidated by high pressure sintering (HPS) at room temperature and/or 300- 450oC for about 1 min using a uniaxial pressure of 8GPa. The sintered material was examined by XRD and TEM. Excellent hardness and strength were obtained in selected sintered samples of 8.34 GPa and 1600 MPa respectively, although ductility was very low (< 1%).

Abstract: Nanoparticles of the wide band gap oxides doped with rare earth (RE) ions are prospective materials for application in optoelectronics as phosphors in a new generation of light sources. In this paper the mechanisms of the excitation of efficient 4f-4f intra-shell transitions in RE doped nanoparticles are discussed. These mechanisms either enhance the rate of host to impurity energy transfer or stimulate the intra-shell transitions of RE ions.

Abstract: Y3Al5O12:Nd3+ nanoceramics were obtained by sintering nanocrystalline Y3Al5O12:Nd3+ powders at 450 oC under different pressures between 2 and 8 GPa. The structural purity, residual stress, strain, texture, and crystallite size of the products were analysed with X-ray powder diffraction. The results indicate that the average crystallite size remains unchanged at about 30 nm in spite of the heating and the pressure applied. No texture was observed. On the other hand, the microstrains increase with increasing pressure, while the residual stresses are at their maximum in the green body and the sample processed at 8 GPa. Increasing strains were observed to increase the width of the luminescence lines and to decrease the luminescence decay times.

Abstract: Time-resolved luminescence characteristics have been studied for cerium doped YAG nanopowders (NP) and nanostructured ceramics (NC). The results obtained have been compared to the luminescence characteristics for the well studied YAG:Ce3+ single crystal (SC). It was detected that the luminescence decay kinetics of Ce3+ related emission in nanocrystals can be closely approximated by two exponents, whereas a single exponential decay was observed in the single crystal. It was also found that the luminescence decay time and light yield of Ce3+ emission are strongly dependent on the cerium concentration and an unusual concentration quenching of Ce3+ emission was observed in YAG nanocrystals. The origin of the differences of luminescence characteristics for cerium doped nanocrystals and SC are discussed. It is suggested that there are special Ce3+ sites which are closely related to the surface states and that they have a strong influence on the luminescence properties of cerium doped YAG nanocrystals.

Abstract: This paper presents the results of studies on ZrO2 containing 10% Eu3+ as a nanofiller in polyurethane nanocomposites with luminescent properties. The nanocomposites, which are potential materials for electro-optical-electronic applications, were prepared by in-situ polymerization. Emission spectra, thermodegradation, thermal analysis and mechanical properties of polyurethane ZrO2/10% Eu3+ were investigated and the structure examined using HRSEM. The aim was to investigate the influence of the distribution of nanofillers in the composite and the structure of hard domains of polyurethanes on the relevant properties for opto-electronic applications.

Abstract: Microwave driven hydrothermal synthesis and hydrothermal synthesis were used to obtain ZrO2 nanopowders. Their production with varying phase composition, the characterisation and selected optical properties concerning their potential use as luminescence oxygen sensors are reported. It was found that the powders obtained by the microwave driven hydrothermal method and annealed at 750 0C in air show experiment repeatability within an accuracy of 6 %.

Abstract: Nanoparticles of wide band gap II-VI compounds doped with transition metal (TM) or rare earth (RE) ions are perspective phosphor materials and fluorescence labels for optoelectronic, biology and medical applications. The efficiency of 3d-3d and 4f-4f intra-shell transitions is shown to be enhanced in TM, RE doped nanoparticles. Two mechanisms of emission enhancement related to spin dependent interactions of free carriers with impurities are discussed. These interactions enhance the TM, RE intra-shell transitions by increasing the rate of host to impurity energy transfer. It is shown also that Al doping increases the intensity of light emission from ZnO nanoparticles.

Abstract: The aim of the work was to examine the influence of pH, high power ultrasound, surfactant and dopant quantity on the particle size distribution of ZrO2:Pr3+, with praseodymium content varying between 0.05 and 10 %. The nanopowders were obtained via a hydrothermal microwave driven process. To establish if the dopant was located on the surface of the zirconia nanoparticles, the particle size distribution, as a function of pH, was measured to obtain an estimate of the isoelectric point of the samples. All results indicated that the dopant was concentrated on the surface: the measurements of the particle size distribution show that the pH corresponding to maximum average particle size changes towards higher values when the Pr content increases. Measurements of the particle size distribution dependency on the application of high power ultrasound and the addition of the sodium dodecyl sulphate surfactant show that, under certain conditions, there is a better stabilisation of the nanopowders in a dispersion and undesirable agglomeration is hindered.

Abstract: The agglomeration of YAG (Yttrium-Aluminum Garnet Y3Al5O12) nanopowders doped with various rare earth ions obtained by the coprecipitation and calcination route is a major problem if it is wished to exploit the nano-size properties such as the transparency of dispersions of the powders or low temperature sintering. Investigations to optimize the preparation process of powders for High Pressure-Low temperature Sintering (HPLS) of semi-translucent pellets was undertaken. The appropriate milling time to decrease the agglomeration was evaluated with the help of ZETA- potential measurement. The dependence of the agglomerate size against the time of hand milling was used to optimize the process.

Abstract: Nanocomposites (nanocrystals) of KY(WO4)2 and KY(WO4)2+1% mol Yb were synthesized using a Complex Sol-Gel Process (CSGP). A chemical treatment with concentrated nitric acid and hydrogen peroxide was used to reduce the decarbonisation temperature. The expected monoclinic phase C2/c of the KYW of the nanocomposite powder was confirmed using XDR. From the X-ray diffraction measurements, the unit cell parameters and the size of nanoparticles was determined. Electron spin resonance studies in the X band were carried out on KYW and KYW:Yb nanocrystals. The sintered samples were made with using the high pressure technique at temperatures up to 600oC. In addition chemical analysis, X-ray diffraction measurements and ESR investigations were carried out on the sintered samples.