Papers by Author: Jerzy Lis

Abstract: Bismuth layer-structured compounds in the Bi-Ti-Fe-O system known as Aurivillius phases are single phase multiferroics. It was stated that substitution of some rare earth elements for bismuth ions in such structure can modified its magnetic properties. Powders of Bi7Fe3Ti3O21 and Bi6.3Sm0.7Fe3Ti3O21 were prepared by co-precipitation – calcination method and then were sintered to dense polycrystalline materials. Low field DC susceptibility was measured in the zero field cooled (ZFC) and field cooled (FC) modes at 10÷350 K. For selected temperatures magnetisation curves and hysteresis loops were also measured. The FC and ZFC curves of both samples diverge at temperatures below 250 K indicating a spin glass-like behaviour. The compound with samarium exhibits magnetic hysteresis already at room temperature with the coercive field increasing to 870 Oe at 10 K. The low temperature hysteresis loops of the samarium containing compound are shift with respect to zero field which can be attributed to a magneto-electrical coupling of the samarium sublattice "exchange biased" by the iron one, which orders anti-ferromagnetically at a higher temperature than the samarium sublattice.

Abstract: Double yttrium-aluminium oxide with perovskite structure (YAP) doped with chromium(III) ions seems to be a promising material for red pigments for new ceramic bodies and glazes. The aim of the present paper was to examine the influence of the precursors morphology on colour properties of the YAP pigments. Two kinds of aluminium hydroxide powders were used, fine- and coarse-grained. The fine Al(OH)3 powder was amorphous with D50=4.58 µm and the coarse one was crystalline gibbsite with D50=42.4 µm. It was stated that using of aluminium hydroxide powders of different morphology led to substantial differences in phase composition of the final materials. Using coarse-grained aluminium hydroxide powder caused formation of over 95 wt% of YAP phase whereas the pigments prepared with amorphous aluminium hydroxide contain significant amounts of other phases in the Y2O3-Al2O3 system. The differences in phase compositions resulted in differences in colour properties of the pigments.

Abstract: Some of ternary materials in the Ti-Al-C system are called MAX-phases and are characterised by heterodesmic layer structure. Their specific structure consisting of covalent and metallic chemical bonds influence its semi-ductile features locating them on the boundary between metals and ceramics, which may lead to many potential applications, for example as a part of a ceramic armour. Ti2AlC is one of this nanolaminate materials. Self-propagating High-temperature Synthesis (SHS) was applied to obtain sinterable powders of Ti2AlC Utilization of heat produced in exothermal reaction in adiabatic conditions to sustain process until all substrates are transformed into product is one of the advantages of the method that result in low energy consumption and low cost combined with high efficiency. Different substrates were used to produce fine powders of ternary material. Phase compositions of obtained powder were examined by XRD method. Than selected powders were used for sintering in various temperature both in a presureless sintering and hot-pressing in argon atmosphere. Properties and phase composition of obtained products were examined.

Abstract: Basic stages of progress in composite materials, prepared by SHS method, from a scientific approach to a promising and rapidly developing applications are discussed in this paper. The systematic review of different forms of composites prepared directly by SHS or by SHS-origin precursors is presented. Powders are usually the starting material for manufacturing of ceramic and a lot of attention has been paid to find new routes for synthesis powders in form of nano or micro particles. The present work is aimed at efficient and convenient powder processing by SHS as an important target for future composites technology. The use of SHS may bring a considerable development in ceramic technology, by enabling a manufacturing of sinterable, high-purity nano or micro powders. It can be demonstrated in different ceramic systems explored by the authors and coworkers using SHS e.g. (a) Si-C-N, (b) Al-O-N as well as (c) Ti-Si-C-N. Rapid combustion conditions were successfully used to manufacturing composite powders and nanopowders suitable for preparing multiphase composite materials having controlled properties.

Abstract: Dense polycrystalline aluminium oxynitride with spinel structure, -alon, is noted for its excellent thermal properties, high-temperature mechanical properties, low dielectric constant, thermal expansion coefficients and intrinsic transparency extending from ultraviolet to mid-infrared wavelengths. The dense materials were made by reactive pressureless sintering or hot-pressing of the SHS derived powders. Powders were synthesized from mixtures of aluminium and corundum powders of different proportions. The products of the SHS synthesis were composed mostly of -alon and in lesser extend of aluminium nitride. Ground powders were pressureless sintered at temperatures of 1800-2100°C for 2-6hs as well as hot-pressed at 1750-1950°C for 1 h under 25 MPa in nitrogen flow. The present work is focused on phase evolution of -alon materials during pressureless sintering and hot-pressing. The structural changes of -alon, a Al2O3 and AlN were also examined.

Abstract: In this article an experiment is described which is included into a course in materials science and allows for better understanding of a nature of sintering. It is a fusion of a computer simulation and a physical experiment – both performed in model systems. Part I of the experiment concerns microstructure of powders. Packing density and coordination numbers of the grains in the sintered powder are analysed with application of computer simulation. In part II of the exercise, the kinetics of the sintering of glass grains is measured via microstructure-evolution observations. The results may be used to discuss the mass transport mechanism.

Abstract: A series of amorphous hydrogenated carbon layers doped with nitrogen (a-C:N:H) was deposited on Si (001). The synthesis was performed from gaseous N2/CH4 mixture using PE CVD (RF CVD technique; 13,56 MHz). An influence of the processing conditions on layer-growth rate was analysed. Thickness of the layers deposited during 1 hour at various temperatures, pressures and RF powers were taken as a basis. It has been proved that the substrate temperature is a key parameter for the layer formation. Temperature rise results in the deposition rate decrease. This unfavourable effect may be reduced by application of increased gas pressure and/or higher plasma RF generator power. At optimal conditions (46 oC; 0,8 Tr; 60 W) the deposition rate reaches up to 600 nm/hour. FT-IR spectra of the layers were measured within 1250 - 4000 cm-1 and discussed with regard to the atomic structure. The intensities of the characteristic absorption bands were compared. The results show that the layers have various N/C ratios according to the applied processing conditions.

Abstract: As the result of oxidation of Cr-Mn steels in SO2 the three layer scale is formed. The intermediate layer of this scale is composed mainly of MnCr2O4 spinel whereas FeCr2O4 spinel is present in small amount. MnO dominates in the outer layer. The inner, very thin scale layer contains oxides/sulfides mixture. The aim of this study was to examine self-diffusion processes in both spinels by multitracer method of diffusion measurements to know which of the transport processes during oxidation is the smallest one and deciding on the corrosion rate. In diffusion experiments the radioisotopes 54Mn, 51Cr and 59Fe were used. The serial sectioning method was applied to simultaneous evaluation of diffusion rates of chromium, manganese and iron in both spinels at 1073 K and 1173 K under the pressure of 105 Pa in SO2 containing 10 Pa O2. These spinels were obtained by modified sol-gel method from nitrates. Structures of the spinels were examined by X-ray spectrometry. It was found, that the diffusion rates of metals are higher in MnCr2O4 spinel. Moreover the dominant mechanism of manganese transport (the highest one) in studied samples is the volume diffusion while chromium and iron are transported mainly through the high diffusivity paths.

Abstract: Dense polycrystalline aluminium oxynitride with spinel structure, γ-alon, is noted for its excellent thermal properties, high-temperature mechanical properties, low dielectric constant, thermal expansion coefficients and intrinsic transparency extending from ultraviolet to mid-infrared wavelengths. The conventional way for synthesis of γ-alon powder is high-temperature reaction of aluminium nitride and corundum in pure nitrogen or a vacuum. The dense materials are made by reactive pressureless sintering or hot-pressing of a powder compact. This work is focused on preparation of γ-alon materials derived from SHS synthesized powders. The powders for sintering were synthesized from mixtures of aluminium and corundum powders of different proportions. The products of the SHS synthesis were composed mostly of γ-alon and aluminium nitride with small amount of non-reacted substrates. Ground powders were hot-pressed at 1750, 1850 and 1950°C for 1 h under 25 MPa in nitrogen flow. Such procedure allowed dense material composed of pure γ-alon with good mechanical properties to obtain.

Abstract: Materials in the Ti-Al-C-N system due to their specific heterodesmic structure show pseudo-plastic properties. Direct synthesis of these compounds from respective elements requires high-temperature and long-lasting reaction. Presented work shows attempts to prepare some ternary compounds using self-propagating high-temperature synthesis (SHS). Intermetalic precursors, TiAl and Ti3Al, were used to synthesize fine and sinterable powders of 211 and 312 complex structure materials.