Ceramic grain boundaries, nature

The variety of mixed conductors is also very high and is associated with their applications in solid oxide fuel cells as electrode materials. The addition of the transition metal oxides to these conductors limits the decrease of the total conductivity and maintenance of its ionic nature. Further doping leads to a separation of a transition metal oxide phase inducing a correlative decrease of the ionic conductivity and an increase of the electronic conductivity, especially at low temperatures. Mixed conductivity was observed only in transition metal oxide phases and electronic conductivity in the zirconia-rich parts of the binary and ternary systems. The phase separation of a transition metal oxide starts at the ceramic grain boundaries. Then, the second phase forms grains and cross-linked structures, and the conductivity approaches to that of the pure transition metal oxide. The enhancement of the solid solubility of the transition metal cations with the increase of the temperature is characteristic of most studied ternary systems. When the concentration of the stabilizing dopant is changed, the... 

There apparently exists a critical amount of liquid phase for the optimization of grain/interface boundary sliding during superplastic deformation. The optimum amount of liquid phase may depend upon the precise material composition and the precise nature of a grain boundary or interface, such as local chemistry (which determines the chemical interactions between atoms in the liquid phase and atoms in its neighboring grains) and misorientation. The existence of an equilibrium thickness of intergranular liquid phase in ceramics has been discussed [14]. This area of detailed study in metal alloys has not been addressed. 

Y NMR has also been reported extensively from yttrium sialon ceramics. Yttria is an important sintering aid for densifying silicon nitride. The properties of the sialon produced are determined by the nature of the grain boundary phase, which is usually a crystalline or glassy yttrium sialon. The often disordered nature of such phases can mean that broader 89Y resonances are... 

The electron microprobe provides a wealih of information about the physical and chemical nature ofsur laces. It has had imix>rtani applications to phase siud ies in metallurgy and ceramics, the investigation of grain boundaries in alloys, the measurement of dilTu-sion rales of impurities in semiconductors, the deier-mination of occluded species in crystals, and the study of ihc active silos ol heterogeneous catalysts, lii all of Ihcsc applications, both qualitative and quaniiiaiive information about surfaces is obtained. 

TADB-derived ceramic fibers, with the idealized composition SiBNsC, do not reach the E-modulus of the most advanced SiC fibers at room temperature. However, they are clearly superior to the latter in a crucial point, namely the drop of the E-modulus and of the creep resistance at high temperatures. SiC fibers already lose a large part of their mechanical strength below 1400 °C, as can be measured by creep resistance. These limitations are fundamental in nature, since they are related to grain boundary sliding and thus to the crystallinity of SiC. In contrast, amorphous SiBNsC fibers do not show any grain boundaries and, moreover, the concentration of microstructural flaws is extremely low. 

The thermal conductivity depends on the mean free path of the phonons, which is the distance between collisions of the phonons in the stmcmre. A short mean free path correlates with a low thermal conductivity. Defects in a stmcmre drastically shorten the mean free path and reduce thermal conductivity significantly. An inherent problem with ceramic materials is that they are usually formed by sintering. This process naturally leads to the formation of many internal defects such as grain boundaries, pores and voids. Because of this, the thermal conductivity of sintered bodies is usually much lower than the intrinsic thermal conductivity. Fired-clay ceramics have very high porosity and a very low thermal conductivity. At... 

The wavelike nature of electrons enables electron diffraction studies of materials. Most electron diffraction patterns are obtained in a transmission electron microscope, which allows us to obtain structural information from very small regions. This is of particular importance in many new ceramics where we are often dealing with thin interface layers (such as at grain boundaries) and very small grains (nanopowders). 

Fig. 1.1 Schematic microstructure of a ceramic with various light-scattering sources (1) a grain boundary, (2) residual pores, (3) secondary phase, (4) double refraction, (5) inclusions and (6) surface roughness in ceramics prohibits applications in optics. Reproduced with permission from [3]. Copyright 2008, Nature Publishing Group...

The accommodation processes are responsible for the rate-control of superplasticity, and no single mechanism exists to accommodate GBS, even with regards to a particular ceramic system. As noted above, several factors can affect the different mechanisms, among which should be included the nature of the impurities present in the grain boundaries, the secondary phases, and the testing conditions. The different mechanisms for accommodation will be analyzed in the following sections. 

Direct-bonded ceramics. Ceramics in which there is no change in chemical composition at the grain boundaries, but only of particle orientation, so that the bonds across the grain-boundries are of the same nature as those in the bulk of the ceramic. Such ceramics have high mechanical strength. 

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