Band 97, Heft 5

Intermetallic/ceramic composites were fabricated by reactive infiltration of an Al melt into a porous TiO 2 preform which was prepared by indirect three-dimensional printing. At 1450°C the highly negative Gibbs energy of reaction (G r =–279.950kJ/mol) is the driving force for the redox reaction 13/2 Al+3/2 TiO 2 3/2 TiAl 3 +Al 2 O 3 to take place. Wetting of molten Al on the preform surface was improved by adding a carbon-bearing binder ((C 6 H 10 O 5 ) n , dextrin) to the powder blend, which decomposed to amorphous carbon (carbon yield 25wt.% of initial dextrin weight) after being heat-treated up to 800°C in an inert atmosphere. During annealing to 1400°C, carbon partially reduced TiO 2 at least on the surface of the grains to form Ti 3 O 5 and Ti 2 O 3 , which exhibit a significantly lower wetting angle compared to TiO 2 . In a low-oxygen-containing inert atmosphere and at 1200°C, Al melt spontaneously penetrated into the printed preform via inter-agglomerate pores (mean size 8lm) and intra-agglomerate pores (mean size 1lm). A fine-grained TiAl 3 /Al 2 O 3 composite microstructure was produced with a residual porosity of 9%.

The present study gives an overview on the potential, possibilities and perspectives that the synthesis of functional materials, such as BaTiO 3 , with microemulsions as the reaction medium offers. Well defined nanocrystalline particles in the form of powders for further ceramic processing, as well as stable colloidal dispersions for the preparation of coatings may be obtained. The paper describes the formation, phase behaviour and stabilization of microemulsions and related water/oil/surfactant systems that may be used as a nanoreactor during particle formation. The reaction principles of microemulsion mediated synthesis are outlined and some examples of structural aspects, revealing size effects in these systems are implemented using advanced characterization techniques, such as Raman spectroscopy. Finally, the preparation of mesoscopic layers, covering the thickness gap between thin film and thick film technology, is demonstrated.

Variable temperature 17 O NMR measurements, covering a temperature range from room temperature to 973K, were conducted on various samples from 17 O enriched yttria-stabilized zirconia for the first time. It is shown that spin-lattice (T 1 ) relaxation exhibits a strong temperature dependence originating from the motional displacements of the oxygen ions, and which is almost independent of the actual sample constitution. In addition, large temperature effects are found for the 17 O NMR line widths (i.e. spin-spin relaxation) which – as the spin-lattice relaxation data – reflect the onset of the oxygen ion mobility. It is anticipated that the NMR line width effects are caused by motions on a length-scale which is different from that examined by spin-lattice relaxation.

Galium nanodroplets induced the anisotropic growth of ultrathin (diameter (<5nm) silicon nanowires in accordance with the vapor-liquid-solid (VLS) mechanism. X-ray diffraction and dispersion spectroscopy, and scanning, and transmission electron microscopy (TEM) were used to characterize the product. High-resolution TEM analysis revealed the existence of twins in the nanowires. In contrast to the theoretically predicated 5-fold twinning in ultrathin Si nanowires (diameter (<6nm), no twinning on the {111} planes along the wire axial directions was observed. The possible reasons for the formation of novel nanostructures are discussed.

Non-existing inverted miscibility gaps in the liquid phase at high temperatures, reappearance of the liquid phase at low temperatures or reappearance of a solid phase at elevated temperatures, and other “approximation phantoms” sometimes result from thermodynamic assessments. A method for struggling against such artifacts is proposed. The method is based on introducing a sufficiently dense mesh of knots (not related to experimental points utilized in the optimization) and ensuring that specific inequality conditions (topological constraints) governing the appearance of the phase diagram are satisfied in these knots. It is shown that the syntax of the language describing conditions and experiments in the PARROT module of Thermo-Calc is suitable for performing such optimization. The workability of the approach proposed is exemplified by carrying out a re-optimization of the Mg–Sb system.

The phase equilibria of Mg-rich Mg–Mn–Zn alloys are scrutinized on the basis of computational thermochemistry. The experimentally well-established facts are very well reproduced by the present calculation. The important invariant reaction involving Mg-solid solution is calculated to be a transition type reaction L+(Mg)↔αMn+Mg 51 Mn 20 at 340.18°C. Some aspects that were incorrectly interpreted in early experimental works are pointed out and are explained by the present calculations also involving non-equilibrium effects.

The ternary phase diagram In–Pt–Sb was studied in two isothermal sections at 400°C and 700°C, respectively, using X-ray powder diffraction (XRD), electron-probe microanalysis (EPMA), and metallographic techniques. The ternary phase reactions were determined by differential thermal analysis (DTA) in the composition range between 0 and 50at.% Pt. The obtained experimental phase diagram data were combined with literature data on the limiting binary systems to construct a ternary reaction scheme (Scheil diagram) in the investigated composition range between 0 and 50at.% Pt. The liquidus surface was derived from the results of DTA measurements.

The system Al–Cr–C contains one ternary phase, Cr 2 AlC. By combining a Calphad assessment with ab initio calculations and differential thermal analysis, the Gibbs energy of Cr 2 AlC could be determined and the complete phase diagram calculated. Cr 2 AlC melts incongruently at about 1773K to form Cr 7 C 3 (or Cr 3 C 2 ) and Al 4 C 3 in addition to liquid. According to the ab initio calculations the equilibrium composition of Cr 2 AlC is very nearly stoichiometric, but can dissolve some Al metastably by replacing Cr. The agreement between the thermodynamic calculation and the various sources of information is excellent.

A thermodynamic modeling of the Ni–Si–Ti system is conducted. All of the experimental phase diagram data available from the literature are critically reviewed and assessed using thermodynamic models for the Gibbs energies of individual phases. The thermodynamic parameters for five ternary compounds (τ 1 : Ti 1 Ni 1 Si 1 , τ 2 : Ti 4 Ni 4 Si 7 , τ 3 : Ti 13 Ni 40 Si 31 , τ 4 : Ti 6 Ni 16 Si 7 , and τ 5 : Ti 2 Ni 3 Si 1 ), the ternary solution phases (liquid, bcc_A2, bcc_B2, fcc_A1, and L1 2 ) as well as the binary phases (Ti 5 Si 3 , Ni 3 Si, Ni 5 Si 2 , and TiNi 3 ) showing noticeable solubilities for the third element are adjusted in the optimization. The order/disorder transitions between disordered bcc_A2 and ordered bcc_B2 phases as well as between disordered fcc_A1 and ordered L1 2 phases are treated using a two-sublattice model. The calculations indicate that the disordered and ordered phases can be described with a single equation. Comprehensive comparisons between the calculated and measured phase diagrams show that in the measured isothermal sections at 750°C, 900°C, 1000°C and 1100°C, most of the observed primary phase regions, as well as 16 out of the 26 experimentally observed invariant reaction temperatures are satisfactorily accounted for by the thermodynamic description. The liquidus projection and reaction scheme for the entire system are also presented. Further experimental work to solve a few discrepancies between calculation and experiment is emphasized.

The enthalpies of formation of β and γ alloys of the Ag–Cu–Zn system were determined by dissolution calorimetry. The melting and solid-state transformation temperatures as well as the enthalpies of the order/disorder and β/χ transformations were measured by differential scanning calorimetry. Thermodynamic descriptions are presented for the binary Ag–Zn system and for the ternary Ag–Cu–Zn system in the entire composition ranges. The thermodynamic model parameters of the constituent binaries Ag–Cu and Cu–Zn are taken from earlier assessments. Those for Ag–Zn and the Ag–Cu–Zn system are established based on relevant experimental data available in the literature completed with experimental data obtained in the present work. Several vertical and isothermal sections as well as the liquidus surface and thermodynamic properties are calculated using the evaluated parameters and show reasonably good agreement with experimental data available.

By application of the CALPHAD method, a consistent set of thermodynamic model parameters is optimized for the Cr–Mn–O system based on experimental data. Chromium manganese spinel Mn y Cr 3−y O 4 and its tetragonally distorted polymorph are described using the compound energy model, and the liquid is described using the two-sublattice model for ionic liquids. Also solid solutions of the phases (Cr 1–y Mn y ) 2+x O 3 , Mn 2–y Cr y O 3 , and (Mn 1–y Cr y ) 1–x O are considered. Relevance for solid oxide fuel cells is discussed.

The phase equilibria in the CeO 2–x –SiO 2 –ZrO 2 system were studied in the subsolidus region in argon atmosphere, under isothermal heating and hot pressing, at 1400°C, different ceria, silica, and zirconia mixtures. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were performed to identify phases and to establish phase equilibria. Based on the results of many preliminarily investigated ternary compositions, possible tie lines are predicted. Compositions from the proposed tie lines were successively fired and ground until the changes of the phase compositions were not detected or were low enough to be neglected. Phases identified in these compositions confirmed the existence of five tie lines: Ce 2 Si 2 O 7 –ZrSiO 4 , Ce 2 Si 2 O 7 –ZrO 2 , Ce 4.67 (SiO 4 ) 3 O–ZrO 2 , Ce 4.67 (SiO 4 ) 3 O–Ce 2 Zr 2 O 7 and Ce 2 SiO 5 –Ce 2 Zr 2 O 7 , which divide the CeO 2–x –SiO 2 –ZrO 2 composition triangle into six compatibility triangles. The formation of ternary compounds or ternary solid solutions in the CeO 2–x –SiO 2 –ZrO 2 system was not observed. The monoclinic high temperature polymorph of Ce 2 Si 2 O 7 was detected. The cubic pyrochlore phases had different lattice parameters, caused by incomplete reduction of the tetravalent cerium ions also observed in the non-stoichiometric cerium oxide.

We have recently developed a generalized JKR model for non-slipping adhesive contact between an elastic cylinder and a stretched substrate where both tangential and normal tractions are transmitted across the contact interface. Here we extend this model to a generalized Maugis–Dugdale model by adopting a Dugdale-type adhesive interaction law to eliminate the stress singularity near the edge of the contact zone. The non-slipping Maugis–Dugdale model is expected to have a broader range of validity in comparison with the non-slipping JKR model. The solution shares a number of common features with experimentally observed behaviors of cell reorientation on a cyclically stretched substrate.

Earlier results obtained by X-ray diffraction studies on tensile-deformed [001]-orientated copper single crystals have recently been re-assessed with respect to the evolution of the long-range internal stresses and the local flow stresses of the dislocation cell walls and the cell interiors. It was noted that the magnitudes of the long-range internal stresses and of the local flow stresses increase in good approximation linearly with the applied macroscopic flow stress. In the present work, the results of a similar analysis of earlier published X-ray data, which were obtained on tensile-deformed [001]-orientated Cu–Mn single crystals, are reported. When the friction stress of the alloy is taken into account, similar linear relationships to the case of deformed [001]-orientated copper single crystals were obtained. The implications of these results are analyzed in the framework of the composite model of crystal plasticity. It is shown that linear relationships of the type considered imply, in terms of the composite model, that the ratio between the local dislocation densities in the dislocation cell walls and the cell interiors must remain constant during deformation. This result is discussed with respect to the experimentally obtained data on local dislocation densities.

A novel process, “nitridation under kinetic control of the nitrogen activity”, has been developed for diffusing substantial amounts (≍10at.%) of interstitially dissolved nitrogen into the surface of Ti alloys (Ti–6Al–4V). By operating with a gas phase providing a very small, controlled nitrogen activity, this process generates a homogeneous Ti–N solid solution, free of detrimental titanium nitride precipitates, in which the nitrogen concentration smoothly decreases from the surface towards the interior. The process is conformal (applicable to workpieces of arbitrary shape) and provides a substantial (about twofold) increase in surface hardness. The hardened surface layer appears to possess adequate ductility for many structural applications. The concept of “surface alloying under kinetic control” is very general and may also serve to generate well-controlled surface concentration profiles of carbon or oxygen in Ti-base and other structural alloys.

Chemical bath deposition has been introduced as a means to produce films of inorganic substances with low engineering expenditure and close to ambient temperature. Organic additives have been used to control the growth of zinc oxide particles in solution and prevent the formation of the typical rod-like morphology. In the present work an attempt is made to combine these two approaches in order to produce thin films of zinc oxide. In the presence of β-cyclodextrin, spherical nano-crystalline deposits were obtained from solution; polyethylene glycol induced the formation of nano-structured thin films. With polyvinylpyrrolidone as an additive, uniform films were deposited. The structure of these films was analyzed by X-ray diffraction and transmission electron microscopy, which revealed the presence of domains of locally oriented nanocrystals. A mechanism of oriented attachment is proposed to explain the formation of domained ZnO films.

The microstructures of three hot-pressed SiC ceramic materials with 5vol.%, 10vol.%, and 15vol.% AlN were investigated by analytical electron microscopy, especially electron energy-loss spectroscopic mapping (EELS mapping). AlN additives dissolve into SiC to form a core/rim structure and to refine the SiC grains. The crystallographic configurations for cores revealed by EELS mapping elucidate the sintering mechanism as liquid-phase sintering in the SiC–AlN composites. The additional 0.5wt.% Y 2 O 3 additives that distribute in the intergranular regions facilitate the formation of liquids to improve the sintering property. With the increase of AlN, the microstructure evolution experienced three stages characterized as solid solution with core/rim structure for a few elongated grains, core/rim for the general equiaxed grains, and the emergence of individual AlN grains with SiC-rich precipitations, respectively. The refined grains and a variety of microstructures contribute to an excellent combination of mechanical properties, with the flexural strength as high as 1000MPa while the fracture toughness remains moderately high.

We report on the crystallization behavior of initially amorphous precursor-derived Si 3 B 1 C 4.3 N 2 ceramics, making use of macroscopic and atomic scale investigation techniques. As derived from our kinetic studies upon heating, the following temperature sequence of solid state processes is observed with approximately the same kinetic time constants: after an initial densification in the amorphous state (1673K), crystallization occurs (1873K) with subsequent strain relaxation and disappearance of nanovoids (2073K). This behavior indicates an increasing activation enthalpy of the atomic mechanisms giving rise to these processes when the annealing temperature is increased. The nanovoids may be located in the turbostratic BNC x layers as derived from positron annihilation studies. For grain growth at high temperatures an activation energy of H G =3.9eV is derived.

In precursor-derived nanocrystalline Si–B–C–N materials, grain or interphase boundaries constitute a major part of the material. They control various properties of nanostructured solids, e.g., by forming fast diffusion pathways. In such highly covalent materials, where the interfaces between the individual crystallites or domains tend to be broadened and structurally disordered, high-temperature plastic deformation (i.e., the creep behavior) can be used as a probe which is particularly sensitive to processes involving internal interfaces and glass-like features of the microstructure. The focus of the present study is an investigation of the deformation behavior of precursor-derived nanocrystalline Si–B–C–N ceramics in the phase-separated amorphous and nanocrystalline states under inert atmospheres. A concise analysis of creep mechanisms free of any oxidation effects was carried out. Isothermal compression creep experiments at 1400°C and at various loads (50MPa–150MPa) have indicated a stress exponent near to unity hinting diffusion creep, and an activation energy of 0.16 ± 0.03 MJ/mol was derived from temperature change experiments under an iso-stress condition. The mechanisms of deformation are elucidated using a free volume model.

The work reported in this paper was performed in order to develop and to optimize the dispersion of Cr 2 O 3 nanopowders by a ball-milling method and to produce spherical micrometer-sized granules by spray-drying. The targeted application for such granules is the development of wear resistant nanostructured Cr 2 O 3 coatings by plasma spraying. Cr 2 O 3 nanopowders were dispersed in deionized water. The suitable dispersant (DarvanC) was determined by zeta potential measurements and the dispersant quantity was optimized by rheological tests. The influence of milling time, diameter of milling balls and weight ratio of milling balls to powder was studied by granulometric measurements. A well-dispersed and stable suspension was then obtained and spray-dried. Dense and spherical micrometer-sized granules, with a monodispersed distribution centered about 50μm, have been achieved and Cr 2 O 3 plasma-sprayed coatings have been realized.

An electroless, room temperature solution growth method of depositing 50 to 150 μm thick, well-crystallized brushite (CaHPO 4 · 2H 2 O, DCPD, dicalcium phosphate dihydrate) layers on the surface of an implantable medical device, such as a titanium alloy (Ti–6Al–4V) substrate, was developed. High ionic strength (189 to 609 mM) solutions with a Ca/P molar ratio of 2.50 and a pH value between 4.70 and 6.10 were prepared by dissolving appropriate quantities of CaCl 2 · 2H 2 O, NaH 2 PO 4 , NaHCO 3 , and NaCl in deionized water. Surface-etched Ti–6Al–4V substrates were simply soaked in the solutions from 24 to 72 hours at room temperature. Elongated CaHPO 4 · 2H 2 O crystals were in-situ- grown on the entire surfaces of the etched Ti–6Al–4V substrates.

In the production of ceramics containing AlN it is necessary to prevent the AlN powder from reacting with water. To do that, non-aqueous powder processing is required or water-resistant AlN powders must be used. Alternatively, as in the Hydrolysis Assisted Solidification process, which exploits the hydrolysis of AlN for the solidification of the ceramic suspensions, the hydrolysis has to be prevented at room temperature but initiated at elevated temperatures. In this study the role of the chemisorbed anions in the hydrolysis of the AlN was investigated in order to make the control of the hydrolysis reactions possible. The method for preparing water-resistant, hydrophilic AlN powder using the dispersion of the powder in a solution of aluminium dihydrogen phosphate is also presented. The mechanism leading to the formation of the protective surface layer is proposed and explained. The method was also successfully tested on nanosized AlN powder. This powder was processed in water and sintered to high density.

The structure and the electrical properties of porous and dense liquid-phase sintered silicon carbide ceramics (LPS-SiC), containing yttria and alumina additives, have been studied. The electrical resistance of LPS-SiC-materials varies in a wide range. This paper is focused on the influence of porosity on the electrical resistance of the sintered SiC. The porosity was controlled by the SiC grain size. Porous LPS-SiC materials were found to have substantially lower electrical resistance in comparison to dense materials of the same type. The structure of the materials was investigated by XRD and FESEM. The porous materials consist of large grains due to the coarser starting powders in comparison to the dense materials. This results in a reduction of the dissolved and re-precipitated fraction of the SiC during sintering. Using the in-lens SEM mode, the high conductivity of the formed rims of the SiC grains could be shown. These observations reveal that the rim volume of dense LPS-SiC is much more extended than the rims of porous materials showing the higher conductivity.

For improving the performance of ceramic thermoelectric materials an increase in the effective mass is required because it is the main factor determining the Seebeck coefficient, in the case of a semiconductor with sufficiently high carrier concentration. The Ruddlesden–Popper phase Sr 3 Ti 2 O 7 , with its conducting perovskites layer in between the phonon absorbing SrO layers, is an example for advanced material design by nano-block integration. Using ab-initio simulations, the electronic bandstructure was calculated, and from the curvature of the bands the effective mass was deduced. While Sr 3 Ti 2 O 7 has a smaller effective mass than the Nb-doped SrTiO 3 , the layered perovskites with substitution of Ta instead of Ti possess very large effective masses (m*/m 0 =9 in average) and a large thermopower is expected. In the experiments, however, the sufficient charge carrier concentration could not yet be reached.

Al 2 O 3 /TiN nanocomposites were fabricated by hot pressing of powders prepared through ball milling (BM) and in-situ nitridation (PN) methods, respectively. And the mechanical and electrical properties as well as microstructure of the nanocomposites were studied. It was found that TiN showed a strong reinforcing effect in both materials. Due to the difference in the powder preparation method, the Al 2 O 3 /TiN nanocomposite from PN powder showed a finer TiN particle size and a more uniform microstructure than the nanocomposite from BM powder. As a result, the former material showed a superior electrical conducting behavior and better mechanical properties.

Far-infrared reflection spectra of sintered α-MnSe were measured at room temperature. The measured spectrum had one pronounced peak in accordance with literature data and group theory estimation and one smaller peak resulting from the applied sintering procedure. The measured reflection spectrum was analyzed using Kramers–Kronig analysis and a three-component effective medium model based on Bruggeman's theory assuming the presence of a small amount of pores, dominant crystalline grains and intergranular material. In the model the crystalline grains were modeled as a single oscillator appropriate to the single crystal and the intergranular material was also defined as a single ionic oscillator.

The transformation of spherical particle assemblies into dense materials by sintering is an idealized micromechanical model for studying the densification. The equilibrium configurations of particle clusters containing canonical Bernal holes, which are typical structural units in random packing of spheres, were calculated by using the Surface Evolver program. The quasi-equilibrium sintering of particle clusters, in which pores shrink keeping their equilibrium shapes, were analyzed as a response to the sintering stress and the macroscopic viscosity. Both sintering stress and macroscopic viscosity were determined as a function of the pore volume, the dihedral angle, and the pore coordination number. The densification curves in sintering simulation agreed with the predicted shrinkage rate.

The reinforcement of light metal alloys with ceramic preforms offers a wide range of possibilities to improve mechanical and thermal properties and makes them usable for technical application. Starting with the Lokasil ® composite material, different metal ceramic materials were created under serial-like conditions with the aim of improving the tensile strength, the bending strength, the thermal expansion, the damage tolerance, and the tribological properties.