Sintering and Microstructure of Mullite Aerogels

Mullite (3Al₂O₃·2SiO₂) was prepared by a sol-gel process and dried by supercritical extraction with CO₂. The aerogel (<0.05 of the theoretical density of mullite) experienced a shrinkage of up to 0.6 and reached a density of only ∼0.5 of theoretical after 1 h at 1350°C Mechanically compacted aerogels, however, sintered to nearly theoretical density below 1200°C. This density is somewhat higher than those of gels prepared by conventional drying (i.e., exposure to the atmosphere) and is considerably higher than mullite prepared from mixed powders at higher temperatures. Although the X-ray diffraction pattern of the sintered gels was almost identical to that of mullite, transmission electron microscopy and energy-dispersive microanalysis showed two types of grain structure. Elongated grains with an Al/Si atomic ratio corresponding to that of stoichiometric mullite were surrounded by equiaxed grains with a lower Al/Si ratio.     

Liquid-Phase Sintering of MgO–Bi2O3

Liquid-phase sintering of MgO-5 wt% Bi₂O₃ was studied by loading dilatometry. The ratio of the creep viscosity to the densification viscosity (∼1.8) and the sintering stress remained nearly constant in a wide density interval. These results, together with results on several other systems, indicate that the constancy of the sintering stress during densification may be a general phenomenon, regardless of densification mechanism.     

Ambient to high-temperature fracture toughness and cyclic fatigue behavior in Al-containing silicon carbide ceramics

A series of in situ toughened, Al, B and C containing, silicon carbide ceramics (ABC-SiC) has been examined with Al contents varying from 3 to 7 wt.%. With increasing Al additions, the grain morphology in the as-processed microstructures varied from elongated to bimodal to equiaxed, with a change in the nature of the grain-boundary film from amorphous to partially crystalline to fully crystalline. Fracture toughness and cyclic fatigue tests on these microstructures revealed that although the 7 wt.% Al containing material (7ABC) was extremely brittle, the 3 and particularly 5 wt.% Al materials (3ABC and 5ABC, respectively) displayed excellent crack-growth resistance at both ambient (25 °C) and elevated (1300 °C) temperatures. Indeed, no evidence of creep damage, in the form of grain-boundary cavitation, was seen at temperatures at 1300 °C or below. The enhanced toughness of the higher Al-containing materials was associated with extensive crack bridging from both interlocking grains (in 3ABC) and uncracked ligaments (in 5ABC); in contrast, the 7ABC SiC showed no such bridging, concomitant with a marked reduction in the volume fraction of elongated grains. Mechanistically, cyclic fatigue-crack growth in 3ABC and 5ABC SiC involved the progressive degradation of such bridging ligaments in the crack wake, with the difference in the degree of elastic vs. frictional bridging affecting the slope, i.e. Paris law exponent, of the crack-growth curve.     

Initial Coarsening and Microstructural Evolution of Fast-Fired and MgO-Doped Al2O3

The effect of an initial coarsening step (50-200 h at 800°C) on the subsequent densification and microstructural evolution of high–quality compacts of undoped and MgO–doped Al₂O₃ has been investigated during fast–firing (5 min at 1750°C) and during constant–heating–rate sintering (4°C/min to 1450°C). In constant–heating–rate sintering of both the undoped and MgO–doped Al₂O₃, a refinement of the microstructure has been achieved for the compact subjected to the coarsening step. A combination of the coarsening step and MgO doping produces the most significant refinement of the microstructure. In fast–firing of the MgO–doped Al₂O₃, the coarsening step produces a measurable increase in the density and a small refinement of the grain size, when compared with similar compacts fast–fired conventionally (i.e., without the coarsening step). This result indicates that the accepted view of the deleterious role of coarsening in the sintering of real powder compacts must be reexamined. Although extensive coarsening after the onset of densification must be reduced for the achievement of high density, limited coarsening prior to densification is beneficial for subsequent sintering.     

Atomic-scale imaging and the effect of yttrium on the fracture toughness of silicon carbide ceramics

In SiC sintered with Al, B and C additions (ABC–SiC), the presence of Y in the Al–Si–O–C grain-boundary phase leads to less frequent crack deflection and lower toughness. When Y is absent from the grain-boundary phase and remains in the triple pockets, crack deflection is restored, and higher toughness results from grain-bridging mechanisms. The observations are consistent with elastic modulus changes in the intergranular phase, which depend on their yttria and silica content, and indicate that these can play an important role in determining crack deflection. While high-toughness ceramics such as ABC–SiC and Si₃N₄ rely on sintering additives forming crack-deflecting intergranular films, the present case is a striking example where the presence of a segregant in the grain boundary promotes transgranular fracture by raising the modulus of the nanoscale intergranular grain-boundary film.     

Synthesis and Structure–Activity Relationship Studies of Benzo[b][1,4]oxazin-3(4H)-one Analogues as Inhibitors of Mycobacterial Thymidylate Synthase X

Since the discovery of a flavin-dependent thymidylate synthase (ThyX or FDTS) that is absent in humans but crucial for DNA biosynthesis in a diverse group of pathogens, the enzyme has been pursued for the development of new antibacterial agents against Mycobacterium tuberculosis, the causative agent of the widespread infectious disease tuberculosis (TB). In response to a growing need for more effective anti-TB drugs, we have built upon our previous screening efforts and report herein an optimization campaign of a novel series of inhibitors with a unique inhibition profile. The inhibitors display competitive inhibition toward the methylene tetrahydrofolate cofactor of ThyX, enabling us to generate a model of the compounds bound to their target, thus offering insight into their structure–activity relationships.     

A braze system for sealing metal-supported solid oxide fuel cells

A composite braze, consisting of Ag–Cu–Ti braze alloy and particulate Al₂TiO₅ filler, was used to produce metal/braze/metal and metal/braze/YSZ joints to seal and interconnect metal-supported SOFC membranes. The addition of Al₂TiO₅ to the braze alloy lowers the coefficient of thermal expansion (CTE) of the resulting composite sufficiently so as to produce joints in which the YSZ does not crack due to CTE mismatch. Optimization of the reactive element (Ti) loading is discussed with regard to its effect on electrolyte conductivity. Electronic conductivity, sealing ability, and strength of the braze alloy remain acceptable after complete oxidation at 700 °C in air. Joints were also tested in air/fuel dual atmosphere environment at 700 °C. After this exposure, the joint remains hermetically sealed, and no significant degradation of the joint was observed. This is in contrast to a free-standing foil of the braze alloy, which failed upon dual atmosphere exposure. The composite braze material was used to seal a metal-supported thin-film YSZ cell. The sealed cell was thermally cycled 30 times very rapidly without any deterioration of the open circuit voltage.     

Alumina-coated hollow glass spheres/alumina composites

Coating of alumina onto the surface of hollow glass spheres was accomplished by controlled heterogeneous precipitation from aqueous solutions. The processing conditions were optimized to yield thin and uniform precursor coatings. After calcination, converting the precursor to alumina, the alumina-coated hollow glass spheres formed free-flowing powders that were used to produce glass/alumina composites with up to 35 vol% of controlled and well dispersed closed porosity. The dielectric constants and the flexural strengths of such porous composites were determined as a function of porosity. This revised version was published online in November 2006 with corrections to the Cover Date.     

Presintering Heat Treatment, Densification, and Mechanical Properties of Silicon Carbide

Silicon carbides were subjected to extended heat treatments at 1400° or 1600°C prior to a 1-h isochronal sintering at 2000°C in argon, and the resulting microstructures, final densities, and room-temperature four-point bend strengths were compared to those of samples prepared without the pretreatments. The samples heat-treated at 1600°C exhibited higher relative densities and flexure strengths. Microstructural observation revealed that the heat treatment at 1600°C modified the initial powder compact microstructure, decreasing the development of large, dense domains in the early and intermediate sintering stages. This evolution was considered to promote a more uniform subsequent densification. The final size and distribution of the grains as well as of the pores were found to be affected favorably by the pretreatment.     

Photoinduced interfacial electron transfer and lateral charge transport in molecular donor-acceptor photovoltaic systems

Nanostructured liquid/solid and solid/solid bulk heterojunctions designed for the conversion of solar energy offer ideal models for the investigation of light-induced ET dynamics at surfaces. Despite significant study of processes leading to charge generation in third-generation solar cells, a conclusive picture of the photophysics of these photovoltaic converters is still missing. More specifically searched is the link between the molecular structure of the interface and the kinetics of surface photoredox reactions. Fundamental scientific issues in this field are addressed by the research project undertaken in the frame of the NCCR MUST endeavor, an outline of which is given here.