Structural Characteristics and Electrical Conductivity of Spark Plasma Sintered Ytterbia Co‐doped Scandia Stabilized Zirconia

The effect of replacing Sc₂O₃ with Yb₂O₃ on the structural and electrical properties of xYb₂O₃–(12–x)Sc₂O₃–88ZrO₂ has been investigated. Spark plasma sintering technique is employed to fabricate dense bulk samples from the nano‐sized powders. X‐ray diffraction and transmission electron microscopy performed on pellets indicate the existence of cubic and rhombohedral phases in 12ScSZ, and a single cubic phase in all the co‐doped compositions. However, Raman spectroscopic studies suggest the presence of a metastable tetragonal t″‐phase along with rhombohedral phases in 12ScSZ, whereas a single cubic phase in all the co‐doped compositions. Significant enhancement in the conductivity of grain and grain boundary is observed on replacing Sc₂O₃ with Yb₂O₃. In the intermediate temperature range, 1Yb11ScSZ exhibits the highest, while 12ScSZ shows the lowest conductivity values, which is attributed to corresponding phases present in that range. Through co‐doping with >1 mol% Yb₂O₃ leads to conductivity decrease, but the value remains higher than that of 12ScSZ. A sharp conductivity change is observed in 12ScSZ and 1Yb11ScSZ samples, which is attributed to partial phase transition as well to the formation of cation‐vacancy complexes. In this work, the beneficial effect of Yb₂O₃ co‐doping in 12ScSZ on the phase and conductivity has been highlighted.     

Microstructural evolution in infiltration‐growth processed MgB2 bulk superconductors

The study reports phase and microstructural evolution in MgB₂ bulk superconductors fabricated by an infiltration and growth (IG) process. Three distinct stages, (1) intermediate boride formation, (2) bulk liquid Mg infiltration, and (3) MgB₂ layer formation, were identified in IG process after detailed examination of series of samples prepared with varied heating conditions. The intermediate phase Mg₂B₂₅, isomorphous to β‐boron, was detected prior to MgB₂ phase formation in stage (1). Due to volume expansion involved in stage 1, cracks formed in the β‐boron particles and propagated radially inwards during stage 3. The growing MgB₂ particles sintered simultaneously with formation of grain boundaries during the process, as evidenced by the measured hardness and critical current density in these samples. From our observations, we estimate the total time needed for complete transformation to MgB₂.     

Understanding the structural origin of crystalline phase transformations in nepheline (NaAlSiO4)‐based glass‐ceramics

Nepheline (Na₆K₂Al₈Si₈O₃₂) is a rock‐forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline‐based glass‐ceramics proceeds through several polymorphic transformations — mainly orthorhombic, hexagonal, cubic — depending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline‐based glass‐ceramics. Accordingly, two different sets of glasses (meta‐aluminous and per‐alkaline) have been designed in the system Na₂O–CaO–Al₂O₃–SiO₂ in the crystallization field of nepheline and synthesized by the melt‐quench technique. The detailed structural analysis of glasses has been performed by ²⁹Si, ²⁷Al, and ²³Na magic‐angle spinning — nuclear magnetic resonance (MAS NMR), and multiple‐quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non‐isothermal conditions using differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass‐ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na₂O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO₂–poor glass‐ceramics with (Na₂O+CaO)/Al₂O₃ > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.     

Synthesis,characterization and demonstration of self-cleaning TiO2 coatings on glass and glazed ceramic tiles

An effort was made not only to demonstrate the performance of the self-cleaning coatings on building materials such as ceramic glazed tiles and glass windows, but also to understand the fundamental issues that are still alive in the field of self-cleaning surfaces based on photocatalysis. Nano TiO₂ transparent thin films were generated by dip, spray and flow coating method. The present results indicate that the inconsistent results in the self-cleaning studies may be due to the effect of aggregation of model pollutant (methylene blue) dye on TiO₂ surface. The effect of aliovalent metal ion (Ni²⁺, Fe³⁺, Nb⁵⁺) doping on phase formation, polymorphic transition, visible light absorbance and optical transparency of TiO₂ film were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV visible absorption spectroscopy. The improved visible light activity of doped TiO₂ thin film was correlated to the Ti(Ni/Fe)O₃ phase formation, UV and visible light absorbance, variation in the optical energy band gap and the probable light scattering associated with grain size.     

Catalytic oxidation of toluene and m-xylene by activated carbon fiber impregnated with transition metals

The catalytic oxidation of volatile organic compounds (VOCs) into mainly CO₂ and H₂O appears promising in the context of the abatement of atmospheric gaseous pollutants and is the subject of this paper. The catalytic oxidation of toluene and m-xylene was carried out in a tubular reactor on a phenolic resin based activated carbon fiber (ACF) impregnated with nitrates of Co, Cr, Ni, and Cu at the reaction temperatures below 300 °C. The extent of the removal (i.e. conversion) of toluene and m-xylene was examined through the breakthrough curves and was found to strongly depend on the types and loading of the metal precursors. The experimental results showed that the reaction rate was significantly affected by O₂ concentration below 3% (v/v). The performance of the ACFs impregnated with 5% (w/w) of Ni oxide was found to be superior to that of other metal oxides at reaction temperature between 170 and 290 °C. A surface kinetic mechanism for the catalytic oxidation of volatile organic compound was proposed and incorporated in a transport model developed to explain the experimental breakthrough data.     

Low-Temperature High-Pressure Consolidation of Amorphous Al2O3–15 mol% Y2O3

This study examined pressure consolidation of amorphous Al₂O₃–15 mol% Y₂O₃ powders prepared by co-precipitation and spray pyrolysis. The two amorphous powders had similar true densities and crystallization sequences. Uniaxial hot pressing was carried out at 450°–600°C with a moderate pressure of 750 MPa. The co-precipitated powder could be hot pressed to a maximum relative density of 98% and remained amorphous. Pressure adversely affected the densification of the spray-pyrolyzed powder by favoring an early crystallization of γ-Al₂O₃ phase at 580°C. Plastic deformation of the amorphous phase is believed to be responsible for the large densification of the amorphous powders.     

Study of the adsorption of phenol by two soils based on kinetic and isotherm modeling analyses

Various natural adsorbents, which have been in used for removal of pollutants, in general, and phenol, in particular, are mostly directed towards improving the adsorption capacity of the adsorbents by various pretreatments (chemical, thermal or biological), which necessarily lead to increase in the cost as well as in the level of difficulties in regeneration/disposal of the adsorbent. The present studies, on the other hand, are aimed towards evaluating the feasibility of using two common soils as potential low-cost adsorbents for the removal of phenol from its aqueous solution, in their natural forms (i.e., without any pretreatment). Accordingly, experiments were carried out (in batch mode) for optimization of the adsorption parameters (such as pH, contact time, equilibrium time and adsorbent dosage), for varying initial phenol concentrations. The results showed that the maximum phenol adsorption capacity was found at pH ~6, under a constant temperature of 30 ± 2 °C (at 6-hour equilibrium period). Several kinetic models (viz. Lagergren first-order, pseudo-second-order, Elovich and intra-particle diffusion) as well as isotherm models (Langmuir, Freundlich, Redlich and Peterson and Sip) were applied to the experimental data. The pseudo-second-order model was found to be the most suitable model describing the adsorption of phenol by two soils (which indicated this adsorption as a chemisorption process). On analysis of equilibrium isotherms for the adsorption of phenol by two soils, Redlich-Peterson and Sip isotherms were found to be the best representative for phenol-sorption on two selected, soil adsorbents.     

Tomographic diagnosis of gas maldistribution in gas-solid fluidized beds

Phase distribution is one of the key hydrodynamic parameters useful for the design and performance assessment of fluidized bed dryers (FBDs). It has direct influence on the drying rate, thermal efficiency, residence time distribution and degree of mixing. The quality of fluidization strongly depends on the uniformity of distribution of the fluidizing gas and the physical properties of the material to be fluidized. In the present work, gamma ray tomography (GRT) study was carried out in the form of chordal solid hold-up, which was found to be greatly influenced by the gas distributor design. The performance of a gas distributor due to the prevalent practice of operating at lower values of distributor-to-bed pressure drop ratio was characterized in a 0.15 m diameter fluidized bed dryer over a broad range of superficial gas velocity. The effects of various parameters such as solids loading, particle size and particle density were analyzed with the help of the reconstructed solid hold-up profiles. The fluidization was studied in terms of maldistribution factor (χ), a value of 5% or less can be obtained by properly designing distributor for a given bed loading, particularly for batch fluidized bed dryers. An industrial size fluidized bed dryer of 1 m diameter was also examined tomographically to obtain quantitative information on the solid hold-up distribution within the bed.     

Antimicrobial properties of silver-doped hydroxyapatite nano-powders and thin films

Silver-doped hydroxyapatite nanopowders were prepared using a solution based sol-gel method and thoroughly characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Antibacterial tests showed silver-doped HAP powders prevented the growth and reproduction of bacteria. Silver-doped HAP powders were pressed into pellets and on these pellets a pulsed laser deposition (PLD) technique was employed to grow amorphous and crystalline thin films on sapphire substrates. Crystalline films had silver nano-particles present within the HAP matrix. Film stability tests showed crystalline films to be far more stable in prolonged solution submersion than their amorphous counterparts.     

Structural drivers controlling sulfur solubility in alkali aluminoborosilicate glasses

If the direct feed approach to vitrify the Hanford's tank waste is implemented, the low activity waste (LAW) will comprise higher concentrations of alkali/alkaline-earth sulfates than expected under the previously proposed vitrification scheme. To ensure a minimal impact of higher sulfate concentrations on the downstream operations and overall cost of vitrification, advanced glass formulations with enhanced sulfate loadings (solubility) are needed. While, the current sulfate solubility predictive models have been successful in designing LAW glasses with sulfate loadings <2 wt.%, it will be difficult for them to design glass compositions with enhanced loadings due to our limited understanding of the fundamental science governing these processes. In this pursuit, this article unearths the underlying compositional and structural drivers controlling the sulfate solubility in model LAW glasses. It has been shown that the preferentially removes non-framework cations from the modifier sites in the silicate network, thus, leading to the polymerization in the glass network via the formation of ring-structured borosilicate units. Furthermore, though the sulfate solubility slightly decreases with increasing Li⁺/Na⁺ in the glasses, the prefers to be charge compensated by Na⁺, as it is easier for to break Na–O bonds instead of Li–O bonds.