Improved accuracy and convergence of discretized population balance for aggregation: The cell average technique

A new discretization method for aggregation equations is developed. It is compared to the fixed pivot technique proposed by Kumar and Ramkrishna (1996a. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51, 1311-1332). The numerical results for aggregation problems by discretized population balances are consistently overpredicting and diverge before the gelling point in the case of a gelling kernel. The present work establishes a new technique which assigns the particles within the cells more precisely. This is achieved by taking first the average of the newborn particles within the cell and then assigning them to the neighboring nodes such that pre-chosen properties are exactly preserved. The new technique preserves all the advantages of the conventional discretized methods and provides a significant improvement in predicting the particle size distribution (PSD). In addition, it is found that the technique is a powerful tool for the computation of gelling problems. The effectiveness of the technique is illustrated by application to several aggregation problems for suitably selected aggregation kernels including physically relevant kernels.     

ShOC rectifier: a new metal-semiconductor device with excellent forward and reverse characteristics

We report a new structure, called the shielded ohmic contact (ShOC) rectifier which utilizes trenches filled with a high-barrier metal to shield an Ohmic contact during the reverse bias. When the device is forward biased, the ohmic contact conducts with a low forward drop. However, when reverse biased, the Ohmic contact is completely shielded by the high-barrier Schottky contact resulting in a low reverse leakage current. Two dimensional numerical simulation is used to evaluate and explain the superior performance of the proposed ShOC rectifier.     

Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated. Paper presented at the poster session of MRSI AGM VI. Kharagpur. 1995     

Estimation of wood char size at the end of devolatilization in a bubbling fluidized bed combustor

Two processes, namely shrinkage and primary fragmentation are known to be the major causes of size reduction of wood during its devolatilization in a fluidized bed combustor. A simple phenomenological model incorporating these effects to compute the average char size at the end of devolatilization is proposed. Experiments are conducted in a bubbling fluidized bed combustor using wood having three different shapes namely, cylinder, cuboid and sphere, to measure the average char size at the end of devolatilization. The model prediction of average char size agrees with the measured values within a deviation of 15%. An experimental correlation is derived to determine the number of fragments and is used to estimate the mean char size.     

Modelling and experimental investigation of devolatilizing wood in a fluidized bed combustor

A two-dimensional model is developed for the determination of devolatilization time and char yield of cylindrical wood particles in a bubbling fluidized bed combustor. By using the concept of shape factor, the model is extended to particles of cuboid shape. The model prediction of the devolatilization time agrees with the measured data (present and those reported in the literature) for cylindrical and cuboidal shaped particles within ±20% while the char yield is predicted within ±17%. Influence of some important parameters namely, thermal diffusivity, external heat transfer coefficient and shrinkage, on the devolatilization time and char yield are studied. Thermal diffusivity shows noticeable influence on devolatilization time. The external heat transfer coefficient shows little influence beyond a value of 300 W/(m² K). However particle shrinkage shows negligible effect on the devolatilization time but has a significant influence on the char yield.     

Oxidation of some aliphatic polyols on anodically deposited MnO2

Galvanostatic steady state current potential measurements were carried out for oxidation of a series of aliphatic alcohols having varying number of hydroxyl groups. The anodically deposited layer of MnO₂ on platinum was used as the electrode material. The deposit was characterised by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and electrode potential measurements. The catalytic role of MnO₂ in the electro-oxidation of alcohols was indicated by the chronopotentiograms and the cyclic voltammograms. An analysis of the electrochemical data indicated a catalytic EC mechanism in which Mn (V) is generated electrochemically and consumed chemically in succession. Based on this and the hydrogen bonding interaction between alcoholic hydroxyl groups and MnO₂ layer, a mechanism was proposed which accounts for the variation in the observed electrochemical reaction orders. Tafel behaviour was found to be followed only approximately. Current efficiency of the electrochemical oxidation of polyols was studied. Replacement of platinum by carbon as current collector was found to leave the electrocatalytic activity of the MnO₂ deposit practically unaltered.     

NMR studies of poly(2-hydroxy ethyl methacrylate-co-2-vinyl pyridine)

Copolymers of 2-hydroxy ethyl methacrylate-2-vinyl pyridine (H/V) of different composition were synthesized by free radical bulk polymerization using azobisisobutyronitrile (AIBN) as an initiator under nitrogen atmosphere. The copolymer compositions were calculated from ¹H NMR spectra. The reactivity ratios for H/V copolymers obtained from a linear Kelen-Tudos method (KT) and nonlinear error-in-variables method (EVM) are r_H = 0.50 ± 0.10, r_V = 1.04 ± 0.08 and r_H = 0.55, r_V = 1.06 respectively. The complete spectral assignment of methine, methylene, methyl, carbonyl, and aromatic carbon regions in term of compositional and configurational sequences of H/V copolymers were done with the help of ¹³C{¹H} NMR, distortionless enhancement by polarization transfer (DEPT), two-dimensional heteronuclear single quantum coherence (HSQC) along with total correlated spectroscopy (TOCSY). © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

On the microhardness testing of electrically stressed poly(phenylene oxide): Polystyrene blends

Microhardness measurements have been performed on untreated (virgin) and electrically stressed, solvent‐cast laboratory‐prepared samples of pure poly(phenylene oxide) (PPO), pure polystyrene (PS), and PPO : PS polyblends with different weight proportions. Results of such measurement on untreated polyblend sample show that microhardness (H_v) increases with increase in the content of PS up to 10 wt %, which attributed to the existence of homogeneous phase morphology. However, this feature is not observable in samples containing higher content of PS. Electrical stress is found to modify considerably the mechanical property of polymer. The effect of electric field on the microhardness of such samples (PPO : PS :: 90 : 10) has been characterized by the existence of a peak. Trapping of charge carriers in electrically stressed samples imparts hardening to the polyblend up to an applied step field of 190 kV/cm. However, the excessive charging beyond this step field value destroys this characteristic. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Semiflexible random thermotropic copolymers from 8‐(3‐hydroxy phenyl) octanoic acid and 3‐chloro‐4‐hydroxy benzoic acid/3,5‐dibromo‐4‐hydroxy benzoic acid

Two series of semiflexible random thermotropic copolymers containing 8‐(3‐hydroxy phenyl) octanoic acid (HPOA) with either 3‐chloro‐4‐hydroxy benzoic acid or 3,5‐dibromo‐4‐hydroxy benzoic acid were prepared by melt polycondensation techniques. The copolyesters were characterized with Fourier transform infrared spectroscopy, dilute solution viscometry, hot‐stage polarized light microscopy, differential scanning calorimetry, thermogravimetric analysis, and wide‐angle X‐ray diffraction. Studies revealed that the amount of HPOA as a disruptor incorporated into the backbone of substituted 4‐hydroxy benzoic acids had a detrimental effect on the liquid‐crystalline behavior. Mesophase‐transition temperatures were observed between 210 and 250°C, and the optical textures were of typical nematic phases. The degree of crystallinity decreased with an increase in the HPOA content. The thermal stability of the copolymers was in the range of 310–370°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008