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Sintering and nanostability (defined as the stability against sintering) are critical phenomena present in the processing and application of nanoparticles. With important implications in obtaining high-quality dense ceramics with fine grains or in enabling high surface areas in nanoparticles for catalytic applications, the control of these interrelated phenomena has been the focuses of several studies. From a thermodynamic perspective, it is recognized that surface energy is a fundamental parameter in both cases, since it is the main driving force for sintering and also the reason that nanoparticles are thermodynamically unstable and have the tendency to coarsen at elevated temperatures. The role of grain-boundary energies is less recognized as relevant, but is also connected to densification, grain growth, and nanoparticle stability. In this paper, we review the critical aspects of the role of interfacial energies in the microstructure evolution, in particular addressing them as parameters to allow better control in addition to more conventional kinetic parameters. The concept is based on the nonsingularity of interfacial energies in a given system, which varies with temperature, atmosphere, and most importantly, chemical composition-this last offering a method to induce particular microstructural evolutions. While the model assumes isotropic grain boundaries but consequences to anisotropy are also discussed. The paper presents examples of the role of dopants on interfacial energies, how this is quantitatively related to their segregation at the interfaces, and the impact in sintering and nanostability. Given the importance of interface energetics to these phenomena, we also present a short review on the current methods used to obtain reliable interface thermodynamic data. [ABSTRACT FROM AUTHOR]

Monolithic 3 YTZP and 3 YTZP containing 2.5 vol% of single-walled carbon nanotubes ( SWCNT) were fabricated by Spark Plasma Sintering ( SPS) at 1250°C. Microstructural characterization of the as-fabricated 3 YTZP/ SWCNTs composite shows a homogeneous CNTs dispersion throughout the ceramic matrix. The specimens have been crept at temperatures between 1100°C and 1200°C in order to investigate the influence of the SWCNTs addition on high-temperature deformation mechanisms in zirconia. Slightly higher stress exponent values are found for 3 YTZP/ SWCNTs nanocomposites ( n~2.5) compared to monolithic 3 YTZP ( n~2.0). However, the activation energy in 3 YTZP ( Q = 715 ± 60 kJ/mol) experiences a reduction of about 25% by the addition of 2.5 vol% of SWCNTs ( Q = 540 ± 40 kJ/mol). Scanning electron microscopy studies indicate that there is no microstructural evolution in crept specimens, and Raman spectroscopy measurements show that SWCNTs preserved their integrity during the creep tests. All these results seem to indicate that the high-temperature deformation mechanism is grain-boundary sliding ( GBS) accommodated by grain-boundary diffusion, which is influenced by yttrium segregation and the presence of SWCNTs at the grain boundary. [ABSTRACT FROM AUTHOR]

This paper investigates the effective steady-state creep response of porous Ni-YSZ composites used in solid oxide fuel cell applications by numerical homogenization based on three-dimensional microstructural reconstructions and steady-state creep properties of the constituent phases. The Ni phase is found to carry insignificant stress in the composite and has a negligible role in the effective creep behavior. Thus, when determining effective creep, porous Ni-YSZ composites can be regarded as porous YSZ in which the Ni phase is counted as additional porosity. The stress exponents of porous YSZ are the same as that of dense YSZ, but the effective creep rate increases by a factor of 8-10 due to porosity. The relationship of creep rate and volume fraction of YSZ computed by numerical homogenization is underestimated by most existing analytical models. The Ramakrishnan-Arunchalam creep model provides the closest approximation among all analytical models. [ABSTRACT FROM AUTHOR]

Oxidation induced crack healing of Al2O3 composites loaded with a MAX phase based repair filler (Ti2Al0.5Sn0.5C) was examined. The fracture strength of 20 vol% repair filler loaded composites containing artificial indent cracks recovered fully to the level of the virgin material upon isothermal annealing in air atmosphere after 48 h at 700°C and 0.5 h at 900°C. SEM- EBSD analysis of crack microstructure indicates two different oxidation reaction regimes to govern the crack filling: near the surface SnO2, TiO2, and Al2O3 were formed whereas deeply inside the cracks Al2O3 and TiO2 and metallic Sn were detected. The presence of elemental Sn was attributed to partial oxidation of aluminum and titanium which lowered the local oxygen concentration below a threshold value required for Sn oxidation to SnO2. Thus, Ti2Al0.5Sn0.5C may represent an efficient repair filler system to trigger oxidation induced crack healing in ceramic composites at temperatures below 1000°C. [ABSTRACT FROM AUTHOR]

Growth behavior of Ti-based carbonitride ceramic grains in two high-Mo Ti(C,N)-based cermets with Ni and Ni-20Cr (wt %) binders was investigated during liquid-phase sintering under vacuum at 1410°C, using DSC, XRD, SEM, AEM, and EDS. Grain growth occurred primarily through two-dimensional nucleation and lateral growth. Most significantly, the grain growth kinetics followed the cubic law, which was controlled by the diffusion of dissolved mass through liquid Ni-based binder phase. However, when Ni-20Cr (wt%) was used as metallic binder, the inner rim of ceramic grains with the typical core-rim structure was seldom complete, and there were often some fine Ni-rich and Mo-rich speckles in their core. In Ni-rich and Mo-rich speckles, there were two kinds of microstructure: one consisted of Ni-based superlattice phase, and the other consisted of Ti-based carbonitride ceramic phase and unknown phases. The three-dimensional thermodynamic equilibrium shape of Ti-based carbonitride ceramic grains evolved from a {111}-faceted and round-edged octahedron to a {111}-faceted and sharp-edged octahedron. In addition, the grain growth rate increased, which was mainly attributed to that the decrease of solid/liquid transformation temperature of Ni-based binder phase led to the increase of the diffusion rate of dissolved mass through liquid Ni-based binder phase. [ABSTRACT FROM AUTHOR]

Compressive creep studies have been carried out on hot-pressed ZrB2- SiC (ZS) and ZrB2- SiC- Si3 N4 (ZSS) composites in air under stress and temperature ranges of 93-140 MPa and 1300°C-1425°C, respectively for time durations of ≈20-40 h. The results of these studies have shown the creep resistance of ZS composite to be greater than that of ZSS. As the temperature is increased from 1300°C to 1425°C, the stress exponent of ZS decreases from 1.7 to 1.1, whereas that of ZSS drops from 1.6 to 0.6. The activation energies for these composites have been found as ≈95 ± 32 kJ/mol at temperatures ≤1350°C, and as ≈470 ± 20 kJ/mol in the range of 1350°C-1425°C. Studies of the postcreep microstructures using scanning and transmission electron microscopy have shown the presence of glassy film with cracks at both ZrB2 grain boundaries and ZrB2- SiC interfaces. These results along with calculated values of activation volumes suggest grain-boundary sliding as the major damage mechanism, which is controlled by O2− diffusion through SiO2 at ≤1350°C, and by viscoplastic flow of the glassy interfacial film at temperatures ≥1350°C. Studies by transmission electron microscopy have shown formation of crystalline precipitates of Si2 N2 O near ZrB2- SiC interfaces in ZSS tested at ≥1400°C, which along with stress exponent values <1 suggests that grain-boundary sliding involving solution-precipitation-type mechanism is operative at these temperatures. [ABSTRACT FROM AUTHOR]