Microstructural characterization of electron beam-physical vapor deposition thermal barrier coatings through high-resolution computed microtomography

Thermal barrier coatings (TBCs), deposited using the electron beam-physical vapor deposition (EB-PVD) process, comprise a unique architecture of porosity capable of bridging the technological gap between insulation/life extension and prime reliance. The TBC microstructures consist of columnar structure, nucleated via vapor condensation, along with a high degree of intercolumnar porosity, thus providing enhanced stress relief on thermomechanical loading and also accommodating misfit stresses resulting from CTE mismatch. In this article, we report the characterization of these coatings using high-resolution synchrotron-based X-ray computed microtomography (XMT) at 1.3-μm resolution. Experiments focused on quantitative characterization/visualization of imperfections in these coatings and on the relative changes in microstructural features upon isothermal annealing. The influence of time/temperature of exposure was investigated and the results were correlated with elastic modulus. This article is based on a presentation made at the symposium “Characterization and Representation of Material Microstructures in 3-D” held October 8–10, 2002, in Columbus, OH, under the auspices of ASM International’s Phase Transformations committee.     

Fabrication of cyclodextrin nanosponges for quercetin delivery: physicochemical characterization,photostability, and antioxidant effects

Quercetin is a flavonoid widely distributed in vegetables and fruits and exhibits strong antioxidant activity, but the poor solubility and stability of quercetin limit its function and application. The purpose of this study was to enhance the dissolution rate and stability of a poorly water-soluble drug quercetin by complexation with cyclodextrin-based nanosponges. Nanosponges are recently developed sponge-like structures and have the capacity to interact with small molecules in its matrix. In this study, five types of nanosponges were purposely designed by varying the molar ratio of β-cyclodextrin and diphenyl carbonate. Quercetin was loaded into nanosponges by freeze-drying method. The particle sizes of plain and quercetin-loaded nanosponges are in between 40 and 100 nm with low polydispersity indices. Zeta potential is sufficiently high to obtain a stable colloidal nanosuspension. Fourier transformed infrared, Raman spectroscopy, differential scanning calorimetry, and X-ray powder diffraction studies confirmed the interaction of quercetin with nanosponges. Particle sizes measured from TEM images were in agreement with DLS results. The dissolution of the quercetin nanosponges was significantly higher compared with the pure drug. The stability of encapsulated quercetin nanosponge was tracked in a simulated intestinal fluid. A marked improvement in the photostability was also observed. In addition, the antioxidant activity of the quercetin nanosponges was more effective than pure quercetin on DPPH scavenging, anti-superoxide formation, and superoxide anion scavenging. These results signify that nanosponge formulations can be used as effective nanocarriers for the delivery of quercetin.     

Effects of Electromigration on the Creep and Thermal Fatigue Behavior of Sn58Bi Solder Joints

Electromigration (EM), creep, and thermal fatigue (TF) are the most important aspects of the reliability of electronic solder joints, the failure mechanisms of which used to be investigated separately. However, current, mechanical loading, and temperature fluctuation usually co-exist under real service conditions, especially as the magnitude of current density is increasing with joint miniaturization. The importance of EM can no longer be simply ignored when analyzing the creep and TF behavior of a solder joint. The published literature reports that current density substantially changes creep rate, but the intrinsic mechanism is still unclear. Hence, the purpose of this study was to investigate the effects of EM on the creep and TF behavior of Sn58Bi solder joints by analyzing the evolution of electrical resistance and microstructure. The results indicated that EM shortens the lifetime of creep or TF of Sn58Bi solder joints. During creep, EM delays or suppresses the cracking and deforming process, so fracture occurs at the cathode interface. During TF, EM suppresses the cracking process and changes the interfacial structure.     

Recent Progress on Reduced Graphene Oxide-Based Counter Electrodes for Cost-Effective Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are one type of highly efficient low-cost solar cells among third-generation photovoltaic devices. Replacing the expensive components of DSSCs with alternative inexpensive and earth-abundant materials would further reduce their price in the solar cell market. Recently, graphene-based low-cost counter electrodes (CEs) have been developed, which could serve as a potential alternative to the expensive platinum-based CEs. In this review article, we have summarized recent research on various reduced graphene oxide (rGO)-based composite CE materials, methods for their synthesis, their catalytic activity, and the effective utilization of such CEs in DSSCs. The photovoltaic performance of DSSCs made of rGO-based composite CEs were compared with the reference Pt-based cells, and the photovoltaic parameters are summarized in tables.     

Pd Nanoparticles and Thin Films for Room Temperature Hydrogen Sensor

We report the application of palladium nanoparticles and thin films for hydrogen sensor. Electrochemically grown palladium particles with spherical shapes deposited on Si substrate and sputter deposited Pd thin films were used to detect hydrogen at room temperature. Grain size dependence of H₂ sensing behavior has been discussed for both types of Pd films. The electrochemically grown Pd nanoparticles were observed to show better hydrogen sensing response than the sputtered palladium thin films. The demonstration of size dependent room temperature H₂ sensing paves the ways to fabricate the room temperature metallic and metal–metal oxide semiconductor sensor by tuning the size of metal catalyst in mixed systems. H₂ sensing by the Pd nanostructures is attributed to the chemical and electronic sensitization mechanisms.     

Hot-corrosion resistance of dissimilar AISI 4340 and AISI 304L weldments in the molten salt environment at 600°C

High temperature corrosion resistance of welded components in power plant industry, hot sections of gas turbines, boilers, industrial waste incinerators, metallurgical furnaces and petrochemical installations is one of the serious necessity. The failure of hot sections of welded component is due to the deposition of molten salt which accelerates the hot corrosion reactions in the weld joint. In this research work, the oxy-chlorination of dissimilar weldments between AISI 304L austenitic stainless steel and AISI 4340 alloy steel at 600°C in eutectic mixture of K₂SO₄?+?60% NaCl under molten salt conditions was investigated. Corrosion kinetics for this dissimilar joints calculated from the thermogravimetric charts were represented. Corrosion on the dissimilar joint employing E309L filler in pulsed current gas tungsten arc welding process is found to be very aggressive, due to the fragmented delta ferrites at numerous locations resulting in the formation of the micro electrochemical cells, which act as sites for corrosion initiation and thus leading to higher corrosion rate. However, in the case of continuous current, the joint is able to protect itself better in this environment due to lower amounts of delta ferrites in weld zone, which is attributed to the higher temperature available in continuous mode leading to austenisation of delta ferrites in the multipass welding.