Papers by Author: Stanislaw Gierlotka

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.

Abstract: Nano-composites consisting of a primary matrix phase of hard nanocrystalline SiC and a secondary nanocrystalline GaAs semiconductor phase were obtained by high-pressure zone infiltration. The synthesis occurs in three stages: (i) at room- temperature the SiC nanopowder is compacted under high pressure to 8 GPa, (ii) the temperature is increased to 1240°C, above the melting point of GaAs, and the pores were filled with liquid, (iii) on cooling GaAs nanocrystallites grow in the pores. The synthesis was performed using a toroid-type high-pressure apparatus (IHPP PAS, Warsaw) and a six anvil cubic press (MAX80 at HASYLAB, Hamburg). X-ray diffraction studies were performed with a laboratory D5000 Siemens diffractometer. The phase compositionn, grain size and macrostrains in the synthesized materials were examined. The microstructure of the composites was characterized using a Scanning Electron Microscopy (SEM), and High Resolution Transmission Electron Microscopy (TEM). Far-infrared reflectivity and Raman spectroscopy measurements were used to trace built-in strains.

Abstract: Two-nanophase SiC-Zn composites were synthesized under pressure up to 8 GPa at up to 1000oC using an high-pressure infiltration method. The advantage of this technique is that in a single, continuous process the ceramic nanopowder is compressed to form the matrix with nanopores; the nanopores are filled with a liquid secondary phase, (here Zn), which crystallizes as nano-scale grains. The key limitation is that the pores in the infiltrated preform have to stay open during the entire process. For this reason only powders of very hard ceramic materials can be used as a matrix. Two types of SiC nanopowders with average crystallite size of 10 nm and 60 nm and average particle size of 30 nm and 100 nm, respectively were used. The measurements of porosity of the green compacts prepared from these powders, pressed at 2.5 GPa and 8 GPa at room temperature, indicated that open porosity was maintained. The nanocomposites obtained show a “nano-nano” type microstructure with a uniform mixture of SiC and Zn phases. The volume fraction of Zn is 20 % independent of the process conditions and initial powder morphology. The process parameters and powder granularity influenced the crystal size of the secondary phase. The average grain size of Zn varied from 20 to 85 nm and was smaller in the composites obtained with the finer matrix, under higher pressure and at lower temperature. The microhardness HV02 of SiCZn nanocomposites varied from 6 to 22 GPa and increased with an increase of pressure and temperature of the infiltration process, and was significantly larger for the finer grained composites.

Abstract: The fabrication of 3D interpenetrating phases in ceramic-metal composites by high pressure sintering of ceramic powder coated with Ni-P nanoparticles produced by electro-less chemical plating is reported. Electro-less nickel plating resulted in a nano-metric layer of spherical Ni-P nanoparticles of approximately 20-50 nm diameter over the entire surface of the ceramic powder. The coated powders were consolidated by hot pressing (HP) process followed by pressureless sintering after cold isostatic pressing (CIP). SEM, TEM and XRD methods were used to investigate the influence of the consolidation temperature and pressure on the microstructure of composites and particularly the morphology of metallic Ni-P phase. The homogeneity of the interpenetrating network structure was measured by computer image analysis and compared with the results of electric resistance measurements. The results indicate that the use of electroless nickel plating and high pressure consolidation process enables the fabrication of uniform 3D interpenetrating continuous metal-ceramic composites and controlled-density composites possessing a metallic phase of nano- or micro-metre size.

Abstract: The paper examines the phase evolution in blends consisting of different proportions of stainless steel (SS316) and Al (0, 25, 65 and 85 wt. %) powders during high-energy ball milling by x-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy. An attempt has also been made to study the mechanical property of the bulk samples obtained by hot pressing the ball milled powder blend at suitable a temperature and pressure. The results of microstructural changes and mechanical property and the ability of consolidation of the amorphous/nanocrystalline powders by high-pressure techniques to develop engineering components has been discussed and highlighted.

Abstract: An Al2O3/Ni-P composite was formed by hot pressing of alumina powder, coated with chemically plated nano Ni-P. The powders were consolidated at room temperature, 600 and 1000oC. The consolidated specimens were studied by SEM, TEM/HRTM, MFM (magnetic force microscopy) and tested or hardness. It was found that the fabrication method results in a structure of interpenetrating phases of high electric conductivity and that samples consolidated at room temperature and sintered at 600oC retain the nanometric grain size of the metallic phase. Hardness measurements are discussed in terms of the fabrication temperature and structure of the composites.

Abstract: The preparation of transparent nanoceramics from nanocrystalline Y3Al5O12 (YAG) powders doped with rare-earth ions has been described and the results of investigation of the structure and morphology have been presented. Decomposition of YAG nanocrystals into YAlO3 (YAP) was observed. The temperature and pressure for the decomposition was much lower than that reported for larger crystals. The transformation was connected with grain coarsening. The influence of the method of preparation of the YAG nanopowders on the final transparency of the nanoceramic produced was determined. Preliminary results of the dependence of luminescence properties on the structural transformation of the nanograins are presented.