Studies on adsorption of guar gum onto biotite mica

The interaction of guar gum with biotite mica has been investigated through adsorption, flotation and electrokinetic measurements. The adsorption densities of guar gum increase with increase of pH and the isotherms exhibit Langmuirian behaviour. Pretreatment of mica with a complexing agent such as EDTA results in a decrease in the adsorption density, highlighting the contribution of metal ions to the adsorption process. An increase in the surface face-to-edge ratio lends to an increase in the adsorption density. The flotation recoveries decrease as a function of pH, complementing the adsorption results. However, polymer depressant ability is reduced in the case of EDTA treated mica, consequent to reduction of metallic sites. Electrokinetic measurements portray conformational rearrangements of macromolecules with the loading, resulting in the shift of the shear plane, further away from the interface. Dissolution experiments indicate release of metal ions from mica, while co-precipitation tests confirm polymer-metal ion interaction in the bulk solution. The adsorption process is governed by hydrogen bonding as well as chemical interaction between guar gum and the surface metal hydroxide groups of mica. (C) 1997 Published by Elsevier Science Ltd.

Studies on age-hardening characteristics of ceramic particle/matrix interfaces in Al-Cu-SiCp composites using ultra low-load-dynamic microhardness measurements

Ultra low-load-dynamic microhardness testing facilitates the hardness measurements in a very low volume of the material and thus is suited for characterization of the interfaces in MMC's. This paper details the studies on age-hardening behavior of the interfaces in Al-Cu-5SiC(p) composites characterized using this technique. Results of hardness studies have been further substantiated by TEM observations. In the solution-treated condition, hardness is maximum at the particle/matrix interface and decreases with increasing distance from the interface. This could be attributed to the presence of maximum dislocation density at the interface which decreases with increasing distance from the interface. In the case of composites subjected to high temperature aging, hardening at the interface is found to be faster than the bulk matrix and the aging kinetics becomes progressively slower with increasing distance from the interface. This is attributed to the dislocation density gradient at the interface, leading to enhanced nucleation and growth of precipitates at the interface compared to the bulk matrix. TEM observations reveal that the sizes of the precipitates decrease with increasing distance from the interface and thus confirms the retardation in aging kinetics with increasing distance from the interface.

Studies on multi-metal ion tolerance of Thiobacillus ferrooxidans

The influence of different concentrations of base metal ions, such as CU2+, Zn2+ and Fe3+, when present either alone or in different possible binary and ternary combinations in a 9K medium, on the fel rous ion oxidation ability of Thiobacillus ferrooxidans was studied. Levels and degree of toxicity of these ions have been quantified in terms of toxicity index (TI). Copper and zinc tolerant strains of the bacteria were developed through serial subculturing and their activity tested in the presence of the above metal ions in comparison with the behavior of wild unadapted cells under similar conditions. Copper tolerant strains (25 g/L Cu2+) were found to be more efficient in the bioleaching of both copper and zinc concentrates than wild unadapted strains, while zinc tolerant strains (40 g/L Zn2+) exhibited better leaching efficiency only in the bioleaching of sphalerite concentrates. The significance and relevance of multi-metal ion tolerance in Thiobacillus ferrooxidans has been highlighted with respect to bioleaching of sulphide mineral concentrates. (C) 1997 Published by Elsevier Science Ltd.

Studies on structural and electrical properties of barium strontium titanate thin films developed by metallo-organic decomposition

Thin films of barium strontium titanate (BST) including BaTiO3 and SrTiO3 end members were deposited using the metallo-organic decomposition (MOD) technique. Processing parameters such as nonstoichiometry, annealing temperature and time, film thickness and doping concentration were correlated with the structural and electrical properties of the films. A random polycrystalline structure was observed for all MOD films under the processing conditions in this study. The microstructures of the films showed multi-grains structure through the film thickness. A dielectric constant of 563 was observed for (Ba0.7Sr0.3)TiO3 films rapid thermal annealed at 750 degrees C for 60 s. The dielectric constant increased with annealing temperature and film thickness, while the dielectric constant could reach the bulk values for thicknesses as thin as similar to 0.3 mu m. Nonstoichiometry and doping in the films resulted in a lowering of the dielectric constant. For near-stoichiometric films, a small dielectric dispersion obeying the Curie-von Schweidler type dielectric response was observed. This behavior may be attributed to the presence of the high density of disordered grain boundaries. All MOD processed films showed trap-distributed space-charge limited conduction (SCLC) behavior with slope of similar to 7.5-10 regardless of the chemistry and processing parameter due to the presence of main boundaries through the film thickness. The grain boundaries masked the effect of donor-doping, so that all films showed distributed-trap SCLC behavior without discrete-traps. Donor-doping could significantly improve the time-dependent dielectric breakdown behavior of BST thin films, mostly likely due to the lower oxygen vacancy concentration resulted from donor-doping. From the results of charge storage density, leakage current and time-dependent dielectric breakdown behavior, BST thin films are found to be promising candidates for 64 and 256Mb ULSI DRAM applications. (C) 1997 Elsevier Science S.A.

Thiophene adsorption studies on clean and hydrogen-preadsorbed MoS2(100) surface

It is known from temperature-programmed desorption studies that the binding energy of thiophene over Mo/gamma-Al2O3 and Co-Mo/gamma-Al2O3, hydrodesulfurization catalysts, is lower in the presence of hydrogen. The adsorption of thiophene on clean and hydrogen-adsorbed MoS2 was modelled using extended Huckel tight binding band structure calculations. In the eta(1) adsorption configuration the calculations show a lower binding energy for adsorption on the hydrogen-preadsorbed surface similar to that observed experimentally. The lowering is due to an increased occupancy of the Mo density of states in the presence of hydrogen.

Time-resolved resonance Raman spectroscopic studies on the triplet excited state of fluoranil

Perfluoro substituted organic compounds have attracted attention owing to their unique structure and reactivity induced by the perfluoro effect. Fluoranil, a perfluoro derivative of p-benzoquinone, is the subject of this paper. Although the perfluoro effect in the ground state seems to have been well understood there is no information available about such effects on the excited state. Here, the time-resolved resonance Raman spectra of the triplet excited state of fluoranil are reported along with the Raman excitation profiles (REPs) of the various vibrational modes. The vibrational spectral analyses have been carried out by analogy with the fluoranil ground state, triplet benzoquinone, and triplet chloranil vibrational spectral assignments. Also, the assignments are further supported by the calculated frequencies using ab initio theoretical methods. It is observed that for fluoranil in the triplet excited state, due to the perfluoro effect, the structure is considerably less distorted than benzoquinone and also the electron delocalization in the pi* antibonding orbital is less than that of triplet excited state of benzoquinone.

Transient absorption studies on 3,6-dibromo polyvinylcarbazole and its model compounds

3,6-Dibromo-N-ethylcarbazole (DBNEC) and its polymeric analogue poly-3,6-dibromovinylcarbazole (PDBVCz) were studied by transient absorption spectroscopy. The transient absorption spectrum of the 3,6-dibromo-N-ethylcarbazole radical cation and decay rate constants of radical cations of 3,6-dibromo-N-ethylcarbazole and its polymeric analogue are presented. In the case of unsubstituted carbazole, the ratio of the yield of radical cation of monomer to polymer is 2.0, whereas in the case of PDBVCz, under the same experimental conditions, the yield of the radical cation is an order of magnitude less in comparison with the monomer model compound DBNEC. This drastic difference in yield has been correlated to the difference in the conformational structure of the polymer as evidenced by nuclear magnetic resonance spectroscopy. (C) 1997 Elsevier Science S.A.

UVPES and ab initio molecular orbital studies on the electron donor-acceptor complexes of bromine with methylamines

The He I photoelectron spectra of bromine, methylamine, and their complex have been obtained, and the spectra show that lone-pair orbital energy of nitrogen in methylamine is stabilized by 1.8 eV and the bromine orbital energies are destabilized by about 0.5 eV due to complexation. Ab initio calculations have been performed on the charge-transfer complexes of Br-2 with ammonia and methyl-, dimethyl-, and trimethylamines at the 3-21G*, 6-311G, and 6-311G* levels and also with effective core potentials. Calculations predict donor and acceptor orbital energy shifts upon complexation, and there is a reasonable agreement between the calculated and experimental results. Complexation energies have been corrected for BSSE. Frequency analysis has confirmed that ammonia and trimethylamine form complexes with C-3v symmetry and methylamine and dimethylamine with C-s symmetry. Calculations reveal that the lone-pair orbital of nitrogen in amine and the sigma* orbital of Br-2 are involved in the charge-transfer interaction. LANL1DZ basis seems to be consistent and give a reliable estimate of the complexation energy. The computed complexation energies, orbital energy shifts, and natural bond orbital analysis show that the strength of the complex gradually increases from ammonia to trimethylamine.

Studies on magnetic properties of MnTi1-xNbxO3 system

Synthesis and characterization of electrical and magnetic properties of ilmenite phases of the type MnTi1-xNbxO3 have been carried out. Single phase materials could be obtained for 0.0 less than or equal to x less than or equal to 0.25. The electrical conductivity increases with increasing Nb content. Magnetic susceptibility studies show that the phases exhibit 2D antiferromagnetic behavior. The magnetic susceptibility data has been analyzed using Fisher's specific heat to determine the long range ordering temperature, (C) 1998 Academic Press.

Photoluminescence studies on gray tracked KTiOPO4 single crystals

The photochromic, electrochromic and x-ray irradiation damages (commonly called the gray tracks) produced in KTiOPO4 single crystals have been studied using photoluminescence (PL) spectroscopy. Gray tracks were produced in this material by exposure to high laser powers (similar to MW/cm(2)), application of electric fields (similar to kV), and exposure to x rays (30 kV). The PL spectra recorded for such gray tracked samples at 4.2 K, exhibited a luminescence band in the 1-1.8 eV range with a peak at 1.41 eV. Temperature and excitation intensity dependence of PL peaks were carried out to probe the exact nature of the broad emission band in the gray tracked samples. The observed photoluminescence is attributed to transitions in the Ti3+ levels, created on irradiation. The microscopic effects produced in the crystal by electric field, optical field, and x rays are similar, as can be concluded from the similarity of PL spectra as well as their intensity and temperature dependences. (C) 1999 American Institute of Physics. [S0021-8979(99)04512-0].

Studies on process induced deep levels in silicon

An attempt was made to study the deep level impurities and defects introduced into thyristor grade silicon under different processing conditions. DLTS, C-V and I-V measurements were carried out. The ideality factors of the diodes is around 1-7. Activation energy, trap density and minority carrier lifetime were measured.

Thermal analysis studies on hydroxobridged homo and hetero trinuclear cobalt(III) complexes

The synthesis and thermal analysis studies of several hydroxobridged homo and hetero trinuclear cobalt(III) complexes are reported. The complexes are of the type [M(H2O)(x) {(OH)(2)Co(en)(2)}(2)](SO4)(2). nH(2)O and [M(H2O)(x){(OH)(2)Co(NH3)(4)}(2)] (SO4)(2). nH(2)O where en denotes ethylenediamine and M =Co(II), Ni(II), Cu(II) and Zn(II) with x=0 for Cu(II), and 2 for other metal ions, and n =3, 4 or 5. The TG and DTA studies of these compounds show that one or more intermediate compounds are formed in each case before the metal oxides are produced.

Studies on Cu/CeO2: A new NO reduction catalyst

Fine particle and large surface area Cu/CeO2 catalysts of crystallite sizes in the range of 100-200 Angstrom synthesized by the solution combustion method have been investigated for NO reduction. Five percent Cu/CeO2 catalyst shows nearly 100% conversion of NO by NH3 below 300 degrees C, whereas pure ceria and Zr, Y, and Ca doped ceria show 85-95% NO conversion above 600 degrees C. Similarly NO reduction by CO has been observed over 5% Cu/CeO2 with nearly 100% conversion below 300 degrees C. Hydrocarbon (n-butane) oxidation by NO to CO2, N-2, and H2O has also been demonstrated over this catalyst below 350 degrees C making Cu/CeO2 a new NO reduction catalyst in the low temperature window of 150-350 degrees C. Kinetics of NO reduction over 5% Cu/CeO2 have also been investigated. The rate constants are in the range of 1.4 x 10(4) to 2.3 x 10(4) cm(3) g(-1) s(-1) between 170 and 300 degrees C. Cu/CeO2 catalysts are characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy where Cu2+ ions are shown to be dispersed on the CeO2 surface. (C) 1999 Academic Press.

Studies on grinding media wear and its effect on flotation of ferrugenous phosphate ore

Marked ball grinding rests were carried out in the laboratory with a low grade phosphate ore under different experimental conditions. Two types of balls were used, namely high carbon low alloy (HCLA) cast steel and high chrome cast iron. Results of marked ball grinding tests indicated that ball wear increased with time and showed a sharp increase for wet grinding over dry grinding. Ball wear under wet grinding conditions was also influenced by the gaseous atmosphere in the mill. The grinding ball materials could be arranged in the following order with respect to their overall wear resistance: High chrome cast iron > HCLA cast steel balls Methods to minimize ball wear through control of mill atmosphere and addition of flotation reagents are discussed. Effect of grinding media and additions of flotation reagents during grinding on phosphate ore flotation are also discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.