Volume 92, Issue 2

Lantanum-modified lead zirconate titanate (Pb 0.88 La 0.08 Zr 0.65 Ti 0.35 O 3 , PLZT 8/65/35) thick films of final thickness 35 μτη were processed on alumina substrates by screen-printing and sintering at 900 °C. Bulk ceramics of the same composition were prepared under identical conditions. In order to identify the influence of the PbO-based liquid phase on the densification of the PLZT thick films, two compositions were chosen, the stoichiometric composition and a composition with an excess of 6 mol % PbO. The latter developed a liquid phase upon heating to 900 °C. The heat treatment was performed in a PbO-rich atmosphere determined by the equilibrium pressure of the PbO vapor phase above PbZrO 3 /PbO. Densification, microstructure, chemical homogeneity and electrical properties of the bulk ceramics and thick films are reported and discussed. There is a significant difference in the densification of bulk ceramics and thick films. While the former strongly depend on the presence of an excess of PbO at the onset of densification, the latter are only slightly dependent on the starting stoichiometry. It has been generally accepted that bulk densification depends on the amount of liquid phase whereas according to our results only a minimal amount of liquid phase is required for densification under constrained conditions. This also happens in samples with the stoichiometric composition due to the capillary condensation of supersaturated PbO vapor that further reacts with the alumina substrate.

This article reports experimental studies on fabricating of well-defined bicrystals of SrTiO 3 by diffusion bonding of single crystals in high vacuum. The bicrystals form a special tilt grain boundary with a fixed orientation. The atomistic structure and the composition of these grain boundaries in SrTiO 3 are determined by advanced techniques of transmission electron microscopy.

MnZn ferrites, sintered at various oxygen partial pressures, were subjected to a post-sintering thermal treatment below the sintering temperature where no additional grain growth can be expected. The ferrites were stabilised at 1100 °C and different oxygen partial pressures for various times were applied in order to improve the intrinsic and extrinsic properties. The most promising results were obtained after the ferrites were stabilised at 1100 °C and 0.7 vol.% of oxygen. During the stabilisation, the level of Fe 2+ ions in the bulk ferrite were substantially changed as were the intrinsic properties. In addition, an increase in the segregation of aliovalent ions at the grain boundaries was noted, which leads to a change in the extrinsic properties. All these changes have a strong effect on the final performances of the MnZn ferrites.

Far-infrared and infrared reflectivity measurements have been made on pure BaTiO 3 , BaTiO 3 doped with 5 mol% LiF and (Ba,Ca,Pb)TiO 3 at room temperature in the range between 50 and 1000 cm –1 . A fitting procedure, based on a four-parameter model, and Kramers-Krönig analysis of the spectra were performed and the optical parameters of the observed ionic oscillators determined. Group theory analysis has been done and the number of experimentally observed active modes was compared with theoretical predictions. It was shown that the high frequency dielectric permittivity increases from 1.88 to 5.13 for pure BaTiO 3 sintered at 1320 °C for 2 to 10 h, respectively. It is also shown that infrared optical reflectivity measurements could give useful results for boundary cases when it is difficult to distinguish if a sample has a cubic or tetragonal structure. Reflectivity data and its numerical analysis can also give information on the degree of the sintering process and density of the samples.

CuO is a very interesting material in the field of microwave sintering because it can act as an internal susceptor in BaTiO 3 ceramics. We examined powder tips of CuO in contact with BaTiO 3 green bodies by heating microscopy. The results show that the formed liquid phase cannot dissolve BaTiO 3 , at least in the examined temperature range up to 1140 °C. This melt is created at about 1130 °C. Additional differential thermo analysis/thermogravimetry investigations revealed that at first all the CuO is converted to Cu 2 O. Then only a part of the created Cu 2 O is retrans-formed into CuO. A liquid phase is formed from CuO and Cu 2 O, but not with the eutectic composition.

Modern classification of production schemes and current problems in the chemical engineering of bulk high-temperatures superconductors (HTSCs) of rare-earth-barium cuprate are discussed. The problems of manufacturing advanced largegrain melt-solidified ceramics and single crystals originate mostly from highly anisotropie crystal-lo-physical properties, small coherent length, necessity of artificial pinning center creation and chemical complexity of the HTSCs. The progress in engineering of HTSCs is sustained due to new synthetic techniques, allowing a precise control of the micro-, meso-, nano- and local structures of nonstoichiometric compounds.

The development of light weight high temperature materials with high strength and high creep resistance is strongly driven by turbine engine manufacturers. Titanium aluminides represent an important class of alloys, which provide a unique set of physical and mechanical properties that can lead to substantial payoffs in future aircraft engines. The class of orthorhombic titanium aluminides based on the intermetallic phase Ti 2 AlNb represents the youngest material emerging out of the group of titanium aluminides. The monolithic Ti 2 AlNb base material exhibits an attractive combination of low temperature ductility and high temperature tensile strength and creep resistance. The mechanical property profile and the high chemical compatibility of orthorhombic alloys with SiC fibers in titanium matrix composites make them also very attractive for high temperature use. In this paper the phases, phase transformations, and microstructure evolution by thermomechanical processing of orthorhombic alloys is described based on investigations of the Ti 2 AlNb base alloy Ti–22A1–25Nb (at.%). Thermomechanical processing was successful in producing equiaxed as well as defined coarse and fine grained lamellar microstructures. Depending on heat treatment two and three phase microstructures were produced, i. e. beside the ordered orthorhombic O phase, the ordered β o phase (B2) and the α 2 phase (DO 19 ) were determined.

Additives out of the systems Al–C, Al–B–C and B–C have been investigated with respect to densification and microstructure development of SiC produced by pressureless sintering and subsequent post-hot isostatically pressing (HIP) treatment. Within the investigated compositional range, the Al–B–C and B–C systems provide sufficient densification to reach pore closure during pressureless sintering and nearly theoretical density during the HIP step of the process. Densification in the Al–B–C system takes place at significantly reduced temperatures, namely ~1900 °C vs. ~2050 °C in the Al- free product. Dilatometer experiments and microstructure development indicate the presence of a liquid phase in this system during sintering. Liquid phase sintering in the SiC/(Al–B–C) system vs. solid state sintering in the SiC/(B–C) system will be discussed on the basis of literature data and previous experimental studies of the authors.

The paper deals with the interrelations between preparation and microstructure of a Si/SiO 2 /Ti/Pt/TiO 2 /Pb(Zr,Ti)O 3 multilayer configuration. A wet chemical approach was used for Pb(Zr,Ti)O 3 (PZT) deposition on sputtered Ti/Pt thin films. The chosen sol-gel process allows heterogeneous nucleation of PZT on the underlying Pt electrode at a temperature of 400 °C without interference of Ti diffusion from the Ti interlayer. In this case the texture of the PZT film can be switched from (100) to (111) by the thin TiO 2 interlayer on top of the Pt (111). This knowledge is essential for the fabrication of ferroelectric thin film devices with tailored properties.

Effects of the hot pressing parameters on the designing of structures and properties of hydroxyapatite/poly-L-lactide biocomposite blocks have been studied. Interdependences of temperature, pressure and time of hot pressing and the density, compressive strength, elasticity modulus, and fracture type of biocomposite blocks were defined. The microstructures of the fracture surfaces were observed by scanning electron microscopy, while the compositions of the constituent phases after hot pressing were determined qualitatively by FT-infrared spectroscopy. A wide spectrum of block properties such as relative density of 86.2–99.6 %, compressive strength of 38–139 MPa and elasticity modulus of 2–10 GPa was obtained by varying the hot pressing parameters.

A novel ceramic near-net shaping method is presented which enables the production of complex-shaped green bodies directly from aqueous suspensions The hydrolysis assisted solidification (HAS) process relies on the time-delayed in situ hydrolysis of AlN powder added to concentrated aqueous slurries of the host powder(s), as a reactant. The incubation period before the hydrolysis starts can be controlled by the addition of anions, which adsorb onto the AlN powder’s surface, as well as by temperature. During the hydrolysis reactions part of the water is internally consumed, the aluminum-hydroxide reaction product gels to form a stiff network, whereas the formation of ammonia can be exploited for flocculation. As a result, the viscosity of the host ceramic slurry dramatically increases until its fluid character is lost and a saturated body is formed. Examples of using the HAS process in slurry forming alumina and zirconia bodies are given and the process potential and limitations are discussed.

A model has been developed describing the growth of pores in a dendritic network. It is assumed that the growing pores originate from pores (bubbles) that preexist in the melt and contain soluble and insoluble gases. The growth of an individual pore is tracked in the model as the cellular/dendritic solidification front passes. As long as the pore surface is not in contact with the solid phase, the pore maintains its spherical shape during cooling/solidification. When the pore size reaches the width of the interdendritic/intercellular channel, it assumes the shape of the channel, pushing the remaining liquid out. In a systematic study, the initial conditions for the numerical calculations are varied in order to investigate the influence of different processing parameters on final porosity and length of pores. Interfacial tension and initial pore size are shown to change the simulation results only sligthly. Solidification shrinkage is shown to have no effect on amount and shape of micropores. With decreasing temperature gradient, porosity increases drastically. Pore elongation is mainly a function of temperature gradient and density of pre-existing pores: pronounced temperature gradients and low initial pore density favour the formation of highly elongated pores.

Monocrystalline GaN and Al x Ga 1– x N films have been grown via the pendeo-epitaxy (PE) [1] technique with and without Si 3 N 4 masks on GaN/AlN/6H–SiC(0001) and GaN(0001)/AlN(0001)/3C–SiC(111)/Si(111) substrates using organometallic vapor phase deposition. Scanning and transmission electron microscopies were used to evaluate the external microstructures and the distribution of dislocations, respectively. The dislocation densities in the PE grown films were reduced at least five orders of magnitude relative to the initial GaN seed layers. Tilting to 0.2° in the portion of the coalesced GaN epilayers grown over the silicon nitride masks was observed via X-ray diffraction. Neither tilting nor low angle boundaries were observed within areas of coalescence in the material grown on substrates without the masks.

Nitrided Sm 2 Fe 17 -based materials possess excellent intrinsic magnetic properties, and in this study we have investigated two compositions: Sm 13.8 Fe 82.2 Ta 4.0 and Sm 13.7 Fe 86.3 . The stoichiometry of each phase was determined and the SmFeTa material was found to include Ta 3 Fe 7 , in addition to the Sm 2 Fe 17 , SmFe 2 and SmFe 3 phases observed in the SmFe alloy, but without the αFe dendrites which are characteristic of the binary material. Hydrogen absorption-desorption studies carried out on both materials using thermopiezic and gas-flow analyses demonstrated the different hydrogenation disproportionation desorption recombination (HDDR) behavior of the SmFeTa alloy. Scanning and transmission electron microscopy studies revealed that in the cast structure some 2.0 at.% Ta is initially dissolved in the Sm 2 (FeTa) 17 phase, however, a single HDDR cycle causes the formation of Ta-based precipitates leaving the Sm 2 (FeTa) 17 phase with much less dissolved Ta.

The phase equilibria in the ternary system Fe–Nd–C were established by metallography, X-ray diffraction and differential thermal analysis and are described with the help of a liquidus projection, an isothermal section at 850 °C, two vertical sections, and a reaction scheme. The formation of the magnetically hard phase Fe 14 Nd 2 C in cast alloys is accompanied by Nd-rich carbides that cause severe corrosion. The equilibria involving Fe 14 Nd 2 C are extremely slow to set in, but they can be accelerated substantially by adding small amounts of boron and copper. Doping with copper provides the added benefit of eliminating the corrosive carbides.

The structures and the mechanical properties of industrially electromagnetic centrifugal cast HK40 reforming tubes were investigated and compared with the traditional commercial HK40 tubes. It is found that the structure changes caused by the electromagnetic field primarily consist of grain refining and increase of the eutectic carbides. Both the tensile properties at room temperature and the stress rupture strength at elevated temperature are noticeably improved by the introduction of the electromagnetic field. A further increase of the exciting current has no evident influence on the tensile properties, but markedly improves the stress rupture strength of the cast tubes. The relationships between the mechanical properties and the structure changes of the cast tubes induced by the electromagnetic field are discussed in detail.

This work describes the development of texture during hot rolling and annealing of two binary Ti –Mn alloys. The alloys Ti –0.4Mn (α-type) and Ti –1.8Mn (near α-type) have been unidirectionally rolled up to 70% reduction at 750 °C, and air-cooled. The samples taken from these hot rolled plates were subjected to isothermal annealing at 700 °C for different lengths of time. After hot rolling, Ti –0.4Mn shows nearly a c-type texture, and Ti –1.8Mn an R-type texture. Annealing of the Ti –0.4Mn alloy generates a T-type texture, whilst annealing of the Ti –1.8Mn alloy leads to a continuous change in the hot rolling R-type texture. These results are compared with hot rolling and annealing textures of titanium and its alloys.

Das Faserverhalten von unidirektional, metallischen Verbundwerkstoffen wurde im Low-cycle-fatigue-Bereich experimentell untersucht, wobei das sukzessive Faserknicken in Druckphasen bei Wechselverformung und bei schwellender Druckbelastung von Bedeutung ist. Der Verbundwerkstoff besteht aus einer Kupfermatrix, die mit parallelen kontinuierlichen Fasern aus austenitischem Stahl verstärkt ist. Die experimentellen Ergebnisse werden kurz dargestellt und das Faserverhalten (die Fasern knicken und bilden ein Scherband aus) mit Hilfe der Finiten-Elemente-Methode simuliert. Die Entstehung und der Fortschritt des entstehenden Scherbandes sind anhand der Finiten-Elemente-Simulationen erklärbar.