ADSORPTION OF WATER VAPOR ON MOLECULAR SIEVE 13X

Adsorption isotherms of water vapor on molecular sieve 13X were obtained at 288, 298, and 308 K. The isotherms were of Type 1 and exhibited hysteresis upon desorption. The heat of adsorption data indicated that water was held on the surface by physical forces and the molecular sieve 13X provided a heterogeneous surface for the water vapor. The equilibrium data were correlated by the Polanyi potential theory and the theory of the pore volume filling proposed by Dubinin and co-workers.     

Detection and recovery techniques for database corruption

Increasingly, for extensibility and performance, special purpose application code is being integrated with database system code. Such application code has direct access to database system buffers, and as a result, the danger of data being corrupted due to inadvertent application writes is increased. Previously proposed hardware techniques to protect from corruption require system calls, and their performance depends on details of the hardware architecture. We investigate an alternative approach which uses codewords associated with regions of data to detect corruption and to prevent corrupted data from being used by subsequent transactions. We develop several such techniques which vary in the level of protection, space overhead, performance, and impact on concurrency. These techniques are implemented in the Dali main-memory storage manager, and the performance impact of each on normal processing is evaluated. Novel techniques are developed to recover when a transaction has read corrupted data caused by a bad write and gone on to write other data in the database. These techniques use limited and relatively low-cost logging of transaction reads to trace the corruption and may also prove useful when resolving problems caused by incorrect data entry and other logical errors.     

Effect of crystallographic dislocations on the reverse performance of 4H-SiC p-n diodes

A quantitative study was performed to investigate the impact of crystallographic dislocation defects, including screw dislocation, basal plane dislocation, and threading edge dislocation, and their locations in the active and JTE region, on the reverse performance of 4H-SiC p-n diodes. It was found that higher leakage current in diodes is associated with basal plane dislocations, while lower breakdown voltage is attributed to screw dislocations. The above influence increases in severity when the dislocation is in the active region than in the JTE region. Furthermore, due to the closed-core nature, the impact of threading edge dislocation on the reverse performance of the p-n diodes is less severe than that of other dislocations although its density is much higher.     

Positron Annihilation in Off‐Stoichiometric and Ta‐Doped Zn2TiO4

While Zn vacancies are regarded as the charge compensators in Ti‐rich Zn₂TiO₄, no evidence of cation vacancies in essentially single‐phase samples containing up to 13 wt% excess TiO₂ was evident from positron annihilation lifetime spectroscopy (PALS). Nor was there strong evidence of cation vacancies in Ta‐doped samples targeted to contain either tetrahedral or octahedral vacancies, although there was a small increase in the lifetime as the doping level increased and the structure changed from cubic to tetragonal.     

Generalized model development for a cryo-adsorber and 1-D results for the isobaric refueling period

We have developed 3-D model equations for a cryo-adsorption hydrogen storage tank, where the energy balance accommodates the temperature and pressure variation of all the thermodynamic properties. We then reduce the 3-D model to the 1-D isobaric system and study the isobaric refueling period, for simplified geometry and charging conditions. The hydrogen capacity evolution predicted by the 1-D axial bed model is significantly different than that predicted by the lumped-parameter model because of the presence of sharp temperature gradients during refueling. The 1-D model predicts a higher hydrogen capacity than the lumped-parameter model. This observation can be rationalized by the fact that a bed with temperature gradients on equilibration should desorb gas, whenever the adsorbed phase entropy is lower than the gas phase entropy. The 1-D analysis of the isobaric refueling period does not show any significant difference in hydrogen capacity evolution among the axial, single and multicartridge annular bed designs. Hence, a multicartridge annular design, though giving a slightly lower pressure drop, does not offer any heat and mass transfer enhancement over the single cartridge design. And, the single cartridge annular design appears to be optimal.     

Effect of hydrocarbon addition on tip opening of hydrogen-air bunsen flames

The effect of hydrocarbon addition on tip opening of lean and stoichiometric hydrogen-air flames is studied computationally by performing two-dimensional numerical simulations. The numerical study reveals that the flame tip of the H₂-air burner stabilized flame is open at lean and stoichiometric mixture conditions. The flame tip closes upon hydrocarbon addition. The tip closing is mainly affected by preferential diffusion of the multi-component mixture and the stretch effects. In the addition of light hydrocarbon (CH₄), the tip closing starts at a higher percentage of hydrocarbon addition in H₂-air flames. Whereas, upon the addition of heavy hydrocarbons such as propane and butane in H₂-air flames, tip closing starts with a lesser amount of hydrocarbon addition. Temperature, OH mole fraction and heat release rate have been investigated, focusing on the flame structure at the tip. The tip opening regime diagram for H₂–CH₄-air, H₂–C₃H₈-air and H₂–C₄H₁₀-air mixtures are presented.     

Defect profiling in organic semiconductor multilayers

Defect depth profile study has been carried out in organic semiconductor (OSC) multilayers to characterize the buried interfaces and layers using beam based positron annihilation spectroscopy. The bilayer and trilayer heterostructures (p–n, p–p and n–p–n) comprise of organic–organic and organic–inorganic (substrate) interfaces. Our study reveals the presence of defects at the interfaces whose concentration is seen to vary with the layer thickness. The S–W correlation has been used to examine the effect of organic materials as well as thickness of the layers on the defect microstructure in multilayers. The nature and type of defects in p–p bilayer are seen to be different as compared to p–n and n–p–n multilayers. Positron mobility in OSC layers has been calculated from the fitted diffusion length which is seen to be of the same order as the effective mobility of charge carrier obtained from the measured current density–voltage (J–V) characteristics. The role of structural defects and the intrinsic electric field at the interfaces on positron systematics is also examined. Positron diffusion modeling together with experimental data suggests that the defect at the interfaces has a stronger influence on the positron systematics than the intrinsic electric field across organic–organic interfaces.     

Molybdenum silicide based materials and their properties

Molybdenum disilicide (MoSi₂) is a promising candidate material for high temperature structural applications. It is a high melting point (2030 °C) material with excellent oxidation resistance and a moderate density (6.24 g/cm³). However, low toughness at low temperatures and high creep rates at elevated temperatures have hindered its commercialization in structural applications. Much effort has been invested in MoSi₂ composites as alternatives to pure molybdenum disilicide for oxidizing and aggressive environments. Molybdenum disilicide-based heating elements have been used extensively in high-temperature furnaces. The low electrical resistance of silicides in combination with high thermal stability, electronmigration resistance, and excellent diffusion-barrier characteristics is important for microelectronic applications. Projected applications of MoSi₂-based materials include turbine airfoils, combustion chamber components in oxidizing environments, missile nozzles, molten metal lances, industrial gas burners, diesel engine glow plugs, and materials for glass processing. In this paper, synthesis, fabrication, and properties of the monolithic and composite molybdenum silicides are reviewed.     

Dry sliding wear of fly ash particle reinforced A356 Al composites

In the present study aluminium alloy (A356) composites containing 6 and 12 vol. % of fly ash particles have been fabricated. The dry sliding wear behaviour of unreinforced alloy and composites are studied using Pin-On-Disc machine at a load of 10, 20, 50, 65 and 80 N at a constant sliding velocity of 1 m/s. Results show that the dry sliding wear resistance of Al-fly ash composite is almost similar to that of Al₂O₃ and SiC reinforced Al-alloy. Composites exhibit better wear resistance compared to unreinforced alloy up to a load of 80 N. Fly ash particle size and its volume fraction significantly affect the wear and friction properties of composites. Microscopic examination of the worn surfaces, subsurfaces and debris has been done. At high loads (>50 N), where fly ash particles act as load bearing constituents, the wear resistance of A356 Al alloy reinforced with narrow size range (53–106 μm) fly ash particles were superior to that of the composite having the same volume fraction of particles in the wide size range (0.5–400 μm).