In Situ Toughened Silicon Carbide with Al-B-C Additions

" In situ toughened" silicon carbides, containing Al, B, and C additives, were prepared by hot pressing. Densification, phase transformations, and microstructural development were described. The microstructures, secondary phases, and grain boundaries were characterized using a range of analytical techniques including TEM, SEM, AES, and XRD. The modulus of rupture was determined from fourpoint bend tests, while the fracture toughness was derived either from bend tests of beam-shaped samples with a controlled surface flaw, or from standard disk-shaped compact-tension specimens precracked in cyclic fatigue. The R -curve behavior of an in situ toughened SiC was also examined. A steady-state toughness over 9 MPa·m^1/2 was recorded for the silicon carbide prepared with minimal additives under optimum processing conditions. This increase in fracture toughness, more than a factor of three compared to that of a commercial SiC, was achieved while maintaining a bend strength of 650 MPa. The mechanical properties were found to be related to a microstructure in which platelike grain development had been promoted and where crack bridging by intact grains was a principal source of toughening.     

Sintering of Spherical Glass Powder under a Uniaxial Stress

The sintering of spherical borosilicate glass powder (particle size 5 to 10 μm) under a uniaxial stress was studied at 800°C. The experiments allowed the measurement of the kinetics of densification and creep, the viscosities for creep and bulk deformation, and the sintering stress which was found to increase with density. The data show excellent qualitative agreement with Scherer's theory of viscous sintering. In addition, the quantitative comparison between theory and experiment shows good agreement; the measured viscosity of the bulk glass was ∽1×10⁹ P (∽1×10⁸ Pa·s) compared to ∽3×10⁹ P (∽3 Pa·s) obtained by fitting the data with Scherer's theory.     

Sintering of Nanophase γ-Al2O3 Powder

The sintering of ultrafine γ-Al₂O₃ powder (particle size ∼10–20 nm) prepared by an inert gas condensation technique was investigated in air at a constant heating rate of 10°C/min. Qualitatively, the kinetics followed those of transition aluminas prepared by other methods. Measurable shrinkage commenced at ∼ 1000°C and showed a region of rapid sintering between ∼1125° and 1175°C followed by a transition to a much reduced sintering rate at higher temperatures. Starting from an initial density of ∼0.60 relative to the theoretical value, the powder compact reached a relative density of 0.82 after sintering to 1350°C. Compared to compacts prepared from the as-received powder, dispersion of the powder in water prior to compaction produced a drastic change in the microstructural evolution and a significant reduction in the densification rate during sintering. The incorporation of a step involving the rapid heating of the loose powder to ∼1300°C prior to compaction (which resulted in the transformation to α-Al₂O₃) provided a method for significantly increasing the density during sintering.     

Toughness and fatigue of encapsulation-processed silicon carbide-polymer matrix particulate composites

A new process for composite fabrication was developed which improves distribution of the particulate reinforcing phase by polymer encapsulation of the particulate prior to consolidation. The effect of such processing on the fatigue-crack propagation and fracture toughness behaviour of particulate thermoplastic composites was investigated. Composites of several particulate size ranges were fabricated into disc-shaped, compact tension specimens and tested under cyclic and monotonie loading conditions. For comparison, a composite was also fabricated using a standard casting technique. The observed fatigue-crack growth rates spanned three orders of magnitude (10^–11 to 10^–9 m per cycle) over an applied stress intensity range, K, of 0.3 to 1.1 MPa m^1/2. The measured fracture toughness values ranged from 0.69 to 2.95 MPa m^1/2. Comparison of the two processing techniques indicated that encapsulation processing increased the fracture toughness of the composite by approximately 33%; however, the fatigue-crack growth behaviour was unaffected. In addition, a trend of increasing crack growth resistance (toughness) with increasing reinforcement particle size was observed. These results are discussed in the light of crack shielding and bridging models for composite toughening.     

Nanocomposite Ag–LSM solid oxide fuel cell electrodes

Advances in infiltration technology have enabled the creation of innovative electrode architectures that are key to highly effective SOFC anodes and cathodes. In this work, an Ag-infiltrated electrode has been created using a pre-sintered porous scandia-stabilized zirconia (SSZ) electrode backbone. The well-sintered SSZ provides a highly connected ion-conducting pathway throughout the electrode, while the nanometer thickness of the Ag particle layer minimizes the oxygen transport resistance that otherwise limits reaction rates in typical Ag composite electrodes. The new Ag composite electrode had minimal activation polarization by 750 °C.     

Pore size distribution,grain growth,and the sintering stress

The effects of a pore size distribution and of the pore separation on the sintering stress is examined using a simple model. The sintering stress is found to be proportional to the mean of the pore sizes weighted according to the Voronoi cell pertaining to each pore, rather than to the simple pore size average. Large heteropores are shown to have little effect on the mean effective sintering stress. Decreases in pore coordination number of such pores, resulting from grain growth can significantly increase the stress intensification factor. The near-constancy of the sintering stress, observed experimentally for many powders over a wide range of sintered densities, does not directly follow from the simple model. It is argued that this constancy results from pore shrinkage, due to densification, which is compensated by pore growth due to coarsening.     

Nano-precipitation in hot-pressed silicon carbide

Heat treatments at 1300°C, 1400°C, 1500°C, and 1600°C in Ar were found to produce nanoscale precipitates in hot-pressed silicon carbide containing aluminum, boron, and carbon sintering additives (ABC-SiC). The precipitates were studied by transmission electron microscopy (TEM) and nano-probe energy-dispersive X-ray spectroscopy (nEDS). The precipitates were plate-like in shape, with a thickness, length and separation of only a few nanometers, and their size coarsened with increasing annealing temperature, accompanied by reduced number density. The distribution of the precipitates was uniform inside the SiC grains, but depleted zones were observed in the vicinity of the SiC grain boundaries. A coherent orientation relationship between the precipitates and the SiC matrix was found. Combined high-resolution electron microscopy, computer simulation, and nEDS identified an Al₄C₃-based structure and composition for the nano-precipitates. Most Al ions in SiC lattice exsolved as precipitates during the annealing at 1400 to 1500°C. Formation mechanism and possible influences of the nanoscale precipitates on mechanical properties are discussed.     

Grain Boundary Evolution in Hot-Pressed ABC-SiC

Silicon carbide with aluminum, boron, and carbon additions (ABC-SiC) was hot-pressed to full density. The samples were examined by transmission electron microscopy (TEM), with an emphasis on high-resolution electron microscopy (HREM). Amorphous grain boundary interlayers, typically less than 2 nm wide, were formed between SiC grains. Heat-treating the ABC-SiC at temperatures as low as 1100°C in Ar crystallized the grain boundary interlayers completely without significantly changing the dominant chemical constituents. Chemical microanalyses demonstrated Al and O enrichment for all examined grain boundaries in both as-prepared and annealed samples. Quantitative EDS analyses revealed Al₂OC- and Al₂O₃-related species (with Si, C, B, or S substitutions) as two of the most likely grain boundary interlayer materials, both before and after heat treatment. Al₂O₃, and (Al_{1− x} Si _x )₂OC with a 2H-type wurtzite structure, were identified as grain boundary films by HREM images. The structural evolution in the grain boundary phases during the hot pressing and postannealing is discussed.     

Dynamics and mechanisms of interfacial photoinduced electron transfer processes of third generation photovoltaics and photocatalysis

Photoinduced electron transfer (PET) across molecular/bulk interfaces has gained attention only recently and is still poorly understood. These interfaces offer an excellent case study, pertinent to a variety of photovoltaic systems, photo- and electrochemistry, molecular electronics, analytical detection, photography, and quantum confinement devices. They play in particular a key role in the emerging fields of third-generation photovoltaic energy converters and artificial photosynthetic systems aimed at the production of solar fuels, creating a need for a better understanding and theoretical treatment of the dynamics and mechanisms of interfacial PET processes. We aim to achieve a fundamental understanding of these phenomena by designing experiments that can be used to test and alter modern theory and computational modeling. One example illustrating recent investigations into the details of the ultrafast processes that form the basis for photoinduced charge separation at a molecular/bulk interface relevant to dye-sensitized solar cells is briefly presented here: Kinetics of interfacial PET and charge recombination processes were measured by fs and ns transient spectroscopy in a heterogeneous donor-bridge-acceptor (D-B-A) system, where D is a Ru(II)(terpyridyl-PO3)(NCS)3 complex, B an oligo-p-phenylene bridge, and A nanocrystalline TiO2. The forward ET reaction was found to be faster than vibrational relaxation of the vibronic excited state of the donor. Instead, the back ET occurred on the micros time scale and involved fully thermalized species. The D-A distance dependence of the electron transfer rate was studied by varying the number of p-phenylene units contained in the bridge moiety. The remarkably low damping factor beta = 0.16 angstroms(-1) observed for the ultrafast charge injection from the dye excited state into the conduction band of TiO2 is attributed to the coupling of electron tunneling with nonequilibrium vibrations redistributed on the bridge, giving rise to polaronic transport of charges from the donor ligand to the acceptor solid oxide surface.