Mesut Aslan

Microstructural characterization of Al2O3-SiC nanocomposites

Al2O3 composite ceramics containing 10 vol% SiC nanoparticles were prepared by pressureless sintering. SiC particles < 150 nm detach from the matrix grain boundaries during grain growth and are predominantly observed in intragranular positions, whereas larger SiC particles retain intergranular positions. Thermal expansion mismatch causes local residual tensile stresses in the matrix grains, giving rise to strain contrasts in TEM imaging; occasionally microcracking around intragranular inclusions ≥ 100 nm is observed in thin TEM foils. It appears that the volume fraction of intragranular SiC particles in the size range 100 < d < 150 nm should not exceed ≈ 0.5% in high strength/high toughness Al2O3-SiC nanocomposites. The interaction between propagating cracks and internal stress fields around intragranular inclusions forces a transgranular fracture mode. Grain boundary pinning by large intergranular SiC particles, in combination with solute drag inhibits matrix grain growth, while the presence of an amorphous phase at Al2O3 grain boundaries and Al2O3-SiC phase boundaries assists in densification. Excess liquid phase is exuded from the interfaces during consolidation and accumulates in large pores, stabilized by the accidental agglomeration of large SiC particles.

Characterization of polycrystalline YBa2Cu3O7-x by bulk and microdistribution analysis

When analysing various YBa2Cu3O7-x powders (some commercially available) for main, subsidiary and trace components by emission spectrometry (OES-ICP), significant deviations in some from the nominal stoichiometric composition were found, as well as considerable concentrations of carbon and water. Optical micrographs of sintered and post-annealed samples prepared from an off-stochiometric powder show second phases at grain boundaries and inclusions within the grains. In addition, a strongly reduced twin formation in the grain occurs, compared with that obtained from a sample with a stoichiometric composition, which can be traced to reduced oxygen diffusion. In the grain boundary phase a higher concentration of Cu (CuO segregation) along with a lower concentration of Ba and, in inclusions, higher Ba and relatively high C concentrations (BaCO3 segregation) as opposed to the 1-2-3 phase were found by SEM-EDX/WDX analysis. The results of the analytical investigations are correlated with superconducting properties (Tc,Jc).

Elastic twins in YBa2Cu3O7 crystals

Abstract On cooling from sintering temperatures YBa2Cu3O7 undergoes a tetragonal-to-orthorhombic phase transformation induced by oxygen ordering. The resulting lattice strain is relieved by twin formation. Besides residual twin lamellae also elastic twins are formed. Using optical microscopy observations of twin geometries, we show that elastic twins can be described by dislocation theory. This allows us to express twin geometries as a function of constants of plastic deformation, such as shear modulus, critical stress, and twin plane energy. From this expression we obtain γ≈1 J/m2 for the twin plane energy. This value is compared with the twin plane energy of other ceramic and metallic materials.

Influence of microstructure on the superconducting properties of polycrystalline YBa2Cu3O7−x

Abstract Microstructural development and shrinkage during sintering of YBa 2 Cu 3 O 7−x is determined not only by the sintering parameters but also by the starting components and calcination procedure. During sintering the residual carbonate content in calcined powders causes gas porosity and inhomogeneous microstructural development. Small amounts of second phases lead to reduced sintering rate. The critical current density of sintered samples is related to the average grain size. A fine-grained microstructure enhances the current transport capability remarkably. After identical post-annealing treatment, fine-grained samples show comparatively higher values of oxygen concentration. The enhancement of critical current density by a fine-grained microstructure is traced back to the favorable distribution of the non-superconducting phases present, fewer cracks, and an increased and more homogeneous oxygen concentration.

Processing and superconducting properties of Bi-Sr-Ca-Cu-O compounds

Abstract Samples of various compositions in the system Bi-Sr-Ca-Cu-O were prepared and screened for signs of the 110 K superconductive phase. Samples having nominal Bi-Sr-Ca-Cu composition 1112 show the steepest drop in resistivity near 110 K when prepared near the partial melting point of 870°C in air as determined by DTA. Optical and scanning electron microscopy as well as EDX were used to characterize and to identify the phases, two of which do not contain Bi. Separate high-temperature processing of carbonate mixtures at 1400°C proved to be of advantage for a low residual carbon content of the samples.

Microstructural development and mechanical properties of pressureless sintered Al2O3/SiC composites

Al2O3 green bodies containing 5 vol% SiC were prepared by slip coating. In order to obtain stable slips, the zeta potential of the SiC was adapted by chemical modificaiton of the surface using functional silanes (e.g. amino ethyl amino propyltrimethoxysilane). Another route for the preparation of homogeneous slips was developed by heterocoagulation of Al2O3 and SiC under well controlled pH conditions and by stabilizing of the heterocoagulated system with a non-ionic surfactant. Both routes led to homogeneous aqueous slips which could be slip-casted to green compacts with densities around 60% of theory showing an excellent distribution of the SiC within the Al2O3 matrix. Green compacts were pressureless sintered at 1800°C to densities above 98.5% showing a homogeneous microstructure with average grain sizes between 2 and 2.5 µm, a fracture strength of about 650 MPa and fracture toughness around 3.0 MPa m0.5.

Microstructural development and electrical properties of sintered and annealed BiSrCaCu2Ox

Abstract Samples of nominal composition BiSrCaCu 2 O x were sintered and subsequently annealed. Differential thermal analysis (DTA) runs on calcined powders were used to determine the sintering temperature. Following sintering at 875°C the samples exhibit a multiphase microstructure, the matrix phase consisting of Br 2 Sr 2 CaCu 2 O x and Bi 2 Sr 2 Ca 2 Cu 3 O x with a volume fraction of about 80%. The resistivity vs. temperature of the sintered samples shows a small drop at 110 K and a non-vanishing residual resistivity below 70 K. Additional annealing of these samples at 860 °C results in a resistivity drop at 110 K and vanishing resistivity below 100 K. The critical current density initially increased with increasing annealing time and then became smaller for longer annealing times. This behaviour is explained by changes of internal interfaces caused by the postsinter annealing treatment.

Colloidal processing and sintering of nano-scale TiN

Surface modification of nano-scale TiN powder with guanidine propionic acid was used for the preparation of well dispersed slurries. The starting powder could be completely deagglomerated for modifier concentrations exeeding 7.5 wt.-% at pH = 9. Such slurries ({le} 40 wt.-% solid content) behave like an ideal Netwonian liquid. Green compacts with relative densities around 50 % were prepared by pressure filtration. The specimens are characterized by a homogeneous green microstructure with pore sizes in the range of 3 to 10 mn, which could be sintered to high density (> 96 %) below 1400{degrees}C. Up to densities of 96 % grain coarsening can be neglected, but further densification was accompanied by an exaggregated grain growth.

Processing and sintering of nanosized TiN

The concept of surface modification of nano-scale particles with short-chained organic molecules has been used for the preparation of deagglomerated slurries of n-TiN using guanidine propionic acid as surface modifier, the strongly agglomerated powder could be redispersed to its primary particle size of 30 to 40 nm. From such suspensions green specimens with densities of 50% could be prepared by pressure filtration, which could be sintered to high densities (>96%) at temperatures below 1400°C without significant grain growth.

Preparation of Al2O3/SiC nanocomposites with in-situ formed SiC

Powder mixtures containing Al 2 O 3 , SiO 2 and carbon black were calcined at 1625°C to form nanoscaled SiC within Al 2 O 3 by means of the carbothermal reduction of SiO 2 . The prepared powders contained 5 and 10 vol. % SiC. These powders were densified by hot pressing, and composite ceramics with densities up to 99 % obtained. Microstructural analyses of the hot-pressed composites showed a homogeneous distribution of SiC particles with a particle size mainly under 100 nm. The fracture mode was intragranular. The fracture toughness determined by the ICL method using the Niihara approach for Palmqvist cracks was in the range of 5 MPa·m 0,5 . The fracture strength determined in a four-point bending test was around 540 MPa for the hot-pressed samples and around 630 MPa for the hot-pressed and post-annealed samples.