Papers by Author: Jan Kulawik

Abstract: The paper reports on processing and characterization of multilayer NTC (negative temperature coefficient) perovskite thermistors. Three materials La_0.8Sr_0.2Ti_0.4Fe_0.6O₃, CaTi_.0.8Co_0.2O₃ and CaTi_0.9Y_0.1O₃ with the stable perovskite structure were synthesized by solid state reactions and used for preparation of slurries for tape casting. Green sheets with screen printed internal Pt electrodes were stacked, laminated isostatically and cofired at 1250-1350°C. SEM observations revealed dense, fine-grained microstructure of ceramic layers, lack of delaminations and cracks and a good cooperation between ceramic and Pt electrode layers. Resistance-temperature characteristics of the fabricated multilayer thermistors were measured in the temperature range 20-820°C. The temperature coefficients of resistance were high, ranging from-9.7 to-1%/°C. Two developed compositions CaTi_.0.8Co_0.2O₃ and CaTi_0.9Y_0.1O₃ were found to be suitable for use at a higher temperature range of 150-500°C. The endurance tests showed small resistance changes (below 1%) after long term ageing at 300-400°C.

Abstract: We present the technology of obtaining and the main properties of multilayered ceramic capacitors (MLCC) based on the PMN-PT-PFN solid solution. PMN-PT-PFN is the abbreviation of the material with general formula (1-y)[(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3]-yPb(Fe1/2Nb1/2)O3. In our work the investigated material was PMN-PT-PFN with x=0.25, y=0.1 i.e. 0.9(0.75PMN-0.25PT)-0.1PFN. The powder of 0.9(0.75PMN-0.25PT)-0.1PFN has been obtained in three steps. In first step we obtained MgNb2O6. In second step FeNbO4 was obtained. In final third step the 0.9(0.75PMN-0.25PT)-0.1PFN was obtained from mixed powders MgNb2O6, FeNbO4, PbO and TiO2. Thick film pastes for obtaining MLCC were prepared by mixing of PMN-PT-PFN powder with organic vehicle and firing in furnace in the temperature range up to 1050°C. Platinum paste has been used as electrodes. The thickness of single layer was about 45 µm (including electrodes). For obtained MLCC XRD investigations were performed as well as investigations of microstructure, EDS and main dielectric properties.

Abstract: In this work perovskite materials A2/3Cu3Ti4O12 (where A=Nd, Sm, Gd, Dy) were synthesized by conventional solid state reaction and sintered at 1000-1100°C. X-ray diffraction analysis confirmed single-phase composition of the investigated ceramics. Dielectric properties of the samples were investigated in the temperature range from -55 to 300°C at frequencies 10 Hz – 2 MHz. Dielectric permittivities of the ceramics are very high, exceeding 105 at low frequencies and/or elevated temperatures (above 50°C) and 103 at higher frequencies and/or low temperatures. Two observed contributions to the dielectric response are attributed to semiconducting grains and more resistive grain boundaries. For Nd2/3Cu3Ti4O12 and Sm2/3Cu3Ti4O12 ceramics, the observed low- and high frequency plateaus are lower than those for Gd2/3Cu3Ti4O12 and Dy2/3Cu3Ti4O12. The relaxation times determined on the basis of impedance data were found to decrease with increasing atomic number of lanthanide. The analysis of the impedance spectroscopic data and the microstructure implies spontaneous formation of internal barrier layer capacitors in the investigated materials.

Abstract: The paper reports on bulk and layered multiferroic composites based on cobalt ferrite and lead iron tantalate-lead titanate solid solution. Syntheses of CoFe2O4 and Pb(Fe1/2Ta1/2)O3 were performed by conventional solid state reactions. 0.5CoFe2O4–0.425Pb(Fe1/2Ta1/2)O3-0.075PbTiO3 bulk composites were prepared by sintering at 900°C. Multilayer composites were fabricated by tape casting, stacking and lamination of alternate ferrite and relaxor layers, followed by cosintering at 950°C. X-ray diffraction analysis and scanning electron microscopic observations confirm that the obtained ceramic samples are composed of ferrite spinel and relaxor perovskite phases. Impedance spectroscopic studies carried out in the temperature range 218-623 K at frequencies 10 Hz–2 MHz show high and broad maxima of dielectric permittivity. On the basis of investigations of magnetization versus magnetic field (up to 85 kOe) and temperature (4-400 K), the behavior typical of hard magnetic materials was found for CF-PFT-PT composites. The measurements carried out at room temperature as a function of the external dc magnetic field and frequency of the sinusoidal ac modulation field, reveal a distinct magnetoelectric effect of the investigated bulk and layered composites.