Thermally Controlled Cyclic Insertion/Ejection of Dopant Ions and Reversible Zinc Blende/Wurtzite Phase Changes in ZnS Nanostructures

We report a reversible phase transformation of platelet-shaped ZnS nanostructures between wurtzite (WZ) and zinc blende (ZB) phases by reversible insertion/ ejection of dopant Mn(II) ions induced by a thermocyclic process. In a reaction flask loaded with WZ ZnS platelets and Mn molecular precursors, during heating Mn ions are incorporated and change the phase of the host nanostructures to ZB; during cooling Mn ions are spontaneously ejected, returning the host nanoplatelets to the original WZ phase. These reversible changes are monitored for several cycles with PL, EPR, XRD, and HRTEM. Interestingly, the (0001) WZ platelets transform to (110) ZB following a nucleation and growth process triggered by a local increase/depletion of the Mn2+ concentration in the nanocrystals.

Relationship between nonlinear resistivity and the varistor forming mechanism in ZnO ceramics

The use of a number of perovskite phases M� M�O3-x, as the only forming additive in ZnO ceramics, produces a high nonlinearity index, ?(up to 45), where M� is a multivalent transition?metal ion and M� is an alkaline earth or a rare?earth ion. From this study, the formation parameters crucial to high nonlinearity, such as nonstoichiometry in the as?received ZnO powder, low x values of the additives and fast cooling rate after the sintering, are explainable on the basis of a depletion layer formation at the presintering stage. This is because of the surface states arising out of the chemisorbed oxygen. The depletion layer is retained during sintering as a result of the higher valence state of M� ions, preferentially present at the grain?boundary regions. The fast cooling freezes in the high?temperature concentration of donor?type defects, thereby decreasing the depletion layer width.

Structural and vibrational studies of the layered structure solid Fe1 ? xNixPS3 (1 greater-or-equal, slanted x greater-or-equal, slanted 0)

We report here the results of structural and vibrational studies on the solid solution Fe1 ? xNixPS3 (1 greater-or-equal, slanted x greater-or-equal, slanted 0) systems. From the structural analysis, we show that there is a lattice compaction as the composition x is varied from 0 to 1, the basic lattice symmetry being maintained. We find that the compaction is more in the basal plane. These subtle structural changes are also reflected in the vibrational bands. We observed splitting of certain bands due to these small changes in the lattice constants, which we explained as arising from a correlation splitting. These changes in the vibrational bands have also been seen on cooling where there is a preferential thermal compaction in the basal plane compared to that perpendicular to the plane.

Thermally Controlled Cyclic Insertion/Ejection of Dopant Ions and Reversible Zinc Blende/Wurtzite Phase Changes in ZnS Nanostructures

We report a reversible phase transformation of platelet-shaped ZnS nanostructures between wurtzite (WZ) and zinc blende (ZB) phases by reversible insertion/ ejection of dopant Mn(II) ions induced by a thermocyclic process. In a reaction flask loaded with WZ ZnS platelets and Mn molecular precursors, during heating Mn ions are incorporated and change the phase of the host nanostructures to ZB; during cooling Mn ions are spontaneously ejected, returning the host nanoplatelets to the original WZ phase. These reversible changes are monitored for several cycles with PL, EPR, XRD, and HRTEM. Interestingly, the (0001) WZ platelets transform to (110) ZB following a nucleation and growth process triggered by a local increase/depletion of the Mn2+ concentration in the nanocrystals.

On the decomposition of ? phase in some rapidly quenched titanium-eutectoid alloys

The decomposition of the beta phase in rapidly quenched Ti-2.8 at. pet Co, Ti-5.4 at. pet Ni, Ti-4.5 at. pet, and 5.5 at. pet Cu alloys has been investigated by electron microscopy. During rapid quenching, two competitive phase transformations, namely martensitic and eutectoid transformation, have occurred, and the region of eutectoid transformation is extended due to the high cooling rates involved. The beta phase decomposed into nonlamellar eutectoid product (bainite) having a globular morphology in Ti-2.8 pet Co and Ti-4.5 pet Cu (hypoeutectoid) alloys. In the near-eutectoid Ti-5.5 pet Cu alloy, the decomposition occurred by a lamellar (pearlite) type, whereas in Ti-5.4 pct Ni (hypereutectoid), both morphologies were observed. The interfaces between the proeutectoid alpha and the intermetallic compound in the nonlamellar type as well as between the proeutectoid alpha and the pearlite were often found to be partially coherent. These findings are in agreement with the Lee and Aaronson model proposed recently for the evolution of bainite and pearlite structures during the solid-state transformations of some titanium-eutectoid alloys. The evolution of the Ti2Cu phase during rapid quenching involved the formation of a metastable phase closely related to an ''omega-type'' phase before the equilibrium phase formed. Further, the lamellar intermetallic compound Ti2Cu was found to evolve by a sympathetic nucleation process. Evidence is established for the sympathetic nucleation of the proeutectoid alpha crystals formed during rapid quenching.

The energy balance in the Ramdas layer

On calm clear nights, air at a height of a few decimetres above bare soil can be cooler than the surface by several degrees in what we shall call the Ramdas layer (Ramdas and Atmanathan, 1932). The authors have recently offered a logical explanation for such a lifted temperature minimum, together with a detailed numerical model. In this paper, we provide physical insight into the phenomenon by a detailed discussion of the energy budget in four typical cases, including one with a lifted minimum. It is shown that the net cooling rate near ground is the small difference between two dominant terms, representing respectively radiative upflux from the ground and from the air layers just above ground. The delicate energy balance that leads to the lifted minimum is upset by turbulent transport, by surface emissivity approaching unity, or by high ground cooling rates. The rapid variation of the flux emissivity of humid air is shown to dominate radiative transport near the ground.

Unsteady laminar compressible swirling flow with massive blowing

THE study of swirling boundary layers is of considerable importance in many rotodynamic machines such as rockets, jet engines, swirl generators, swirl atomizers, arc heaters, etc. For example, the introduction of swirl in a flow acceleration device such as a nozzle in a rocket engine promises efficient mass flow control. In nuclear rockets, swirl is used to retain the uranium atoms in the rocket chamber. With these applications in mind, Back1 and Muthanna and Nath2 have obtained the similarity solutions for a low-speed three-dimensional steady laminar compressible boundary layer with swirl inside an axisymmetric surface of variable cross section. The aim of the present analysis is to study the effect of massive blowing rates on the unsteady laminar swirling compressible boundary-layer flow of an axisymmetric body of arbitrary cross section when the freestream velocity and blowing rate vary with time. The type of swirl considered here is that of a free vortex superimposed on the longitudinal flow of a compressible fluid with variable properties. The analysis is applicable to external flow over a body as well as internal flow along a surface. For the case of external flow, strong blowing can have significant use in cooling the surface of hypervelocity vehicles, particularly when ablation occurs under large aerodynamic or radiative heating, but there may not be such an important application of strong blowing in the case of internal flow. The governing partial differential equations have been solved numerically using an implicit finite difference scheme with a quasilinearization technique.3 High temperature gas effects, such as radiation, dissociation, and ionization, etc., are not investigated. The nomenclature is usually that of Ref. 4 and is listed in the full paper.

Charge-ordering in manganates

Ordering of Mn3+ and Mn4+ ions occurs in the rare earth manganates of the general composition Ln(1-x)A(x)MnO(3) (Ln rare earth, A = Ca, Sr). Such charge-ordering is associated with antiferromagnetic and insulating properties. This phenomenon is to be contrasted with the ferromagnetic metallic behavior that occurs when double-exchange between the Mn3+ and Mn4+ ions predominates. Two distinct types of charge-ordering can be delineated. In one, a ferromagnetic metallic (FMM) state transforms to the charge-ordered (CO) state on cooling. In the other scenario, the CO state is found in the paramagnetic ground stale and there is no ferromagnetism down to the lowest temperatures. Magnetic fields transform the CO state to the FMM state, when the average radius of the A-site cations is sufficiently large ([r(A)] > 1.17 Angstrom). Chemical melting of the CO state by Cr3+ substitution in the Mn site is also found only when [r(A)] greater than or similar to 1.17 Angstrom. The effect of the size of the A-cations on the Mn-O-Mn angle is not enough to explain the observed variations of the charge-ordering temperature as well as the ferromagnetic Curie temperature T-c. An explanation based on a competition between the Mn and A-cation orbitals for sigma-bonding with the oxygen rho(sigma) orbitals is considered to account for the large changes in T-c and hence the true bandwidth, with [r(A]). Effects of radiation, electric field, and other factors on the CO state are discussed along with charge-ordering in other manganate systems. Complex phase transitions, accompanied by changes in electronic and magnetic properties, occur in manganates with critical values of(rA) Or bandwidth. Charge-ordering is found in layered manganates, BixCa1-xMnO3 and CaMnO3-delta.

Combustion synthesis and properties of nanostructured ceria-zirconia solid solutions

Nanostructured ceria-zirconia solid solutions (Ce1 − xZrxO2, X = 0 to 0.9) have been synthesized by a single step solution combustion process using cerous nitrate, zirconyl nitrate and oxalyl dihydrazide (ODH) / carbohydrazide (CH). The as-synthesized powders show extensive XRD line broadening and the crystallite sizes calculated from the XRD line broadening are in the nanometer range (6–11 nm). The combustion derived ceria zirconia solid solutions have high surface area in the range of 36–120 m2/g. Calcination of Ce1 −xZrxO2 at 1350 °C showed three distinct solid solution regions: single phase cubic (x ≤ 0.2), biphasic cubic-tetragonal (0.2 < x Image .8) and tetragonal (x > 0.8). When x ≥ 0.9, the metastable tetragonal phase formed transforms to monoclinic phase on cooling after calcination above 1100 °C. The homogeneity of Ce1 − xZrxO2 has been confirmed by EDAX analysis. The Temperature Programmed Reduction (TPR) measurement of Ce0.5Zr0.5O2 was carried out with H2 and the TPR profile showed two water formation peaks corresponding to the utilization of surface and bulk oxygen.

Some microbiological aspects of bauxite mineralization and beneficiation

A microbial survey of Jamnagar bauxite mines in Gujarat, India, revealed the indigenous presence of a variety of autotrophic and heterotrophic bacteria and fungi associated with the ore body and water ponds in the vicinity. Among these, bacteria belonging to the genera Thiobacillus, Bacillus and Pseudomonas are implicated in the weathering of aluminosilicates; the precipitation of iron oxyhydroxides; the dissolution and conversion of alkaline metal species; and the formation of alumina, silica and calcite minerals. Fungi belonging to the genus Cladosporium can reduce ferric iron and dissolve alumina silicates. Biogenesis thus plays a significant role in bauxite mineralization. Various types of bacteria and fungi, such as Bacillus polymyxa, Bacillus coagulans and Aspergillus niger, were found to be efficient in significant calcium solubilization and partial iron removal from bauxite ore. Probable mechanisms in the biobeneficiation process are analyzed. Biobeneficiation is shown to be an effective technique for the removal of iron and calcium from bauxite ores for use in refractories and ceramics.

Synthesis of nanodispersed phases during rapid solidification and crystallization of glasses in Ti75Ni25 alloys

The formation of the metallic glass and crystalline phases and related microstructures and the decomposition behavior of rapidly solidified Ti75Ni25 alloys obtained under different processing conditions have been investigated in detail. The competition between glass transition and nucleation of beta-Ti during rapid solidification leads to the possibility of synthesizing the nanocomposites of beta-Ti and glass. Additionally, it is shown that the presence of a small amount of Si also promotes simultaneous nucleation of fine Ti2Ni intermetallic compound. Thermodynamic calculation of the metastable phase diagram indicates the presence of a metastable eutectic reaction between alpha-Ti and Ti2Ni. Evidence of this reaction at lower cooling rates has been presented. On heating, the glass decomposes through this reaction. Finally, on the basis of understanding of the microstructural evolution during decomposition, a new approach has been adopted to synthesize a nanodispersed composite of alpha-Ti in the crystalline Ti2Ni matrix with a narrow size distribution by controlling the devitrification heat treatment of the metallic glass.

The nature of charge ordering in rare earth manganates and its strong dependence on the size of the A-site cations

Charge ordering in rare earth manganates of the type Ln(0.5)A(0.5)MnO(3) (Ln = rare earth, A = alkaline earth) is highly sensitive to the average radius of the A-site cations, [r(A)]. Tn the small [r(A)] regime (e.g., Y0.5Ca0.5MnO3), charge ordering occurs in the paramagnetic state, the transformation to an antiferromagnetic state occurring at still lower temperatures. At moderate [r(A)] values (e.g., Nd0.5Sr0.5MnO3), a ferromagnetic metallic state transforms to a charge-ordered antiferromagnetic state with cooling. These two distinct types of charge ordering and associated properties are explained in terms of the variation of the exchange couplings J(FM) and J(AFM) with [r(A)] and the invariance of the single-ion Jahn-Teller energy with [r(A)]. A qualitative temperature-[r(A)] phase diagram, consistent with the experimental observations, has been constructed to describe the properties of the manganates in the different [r(A)] regimes. (C) 1997 Academic Press.

Modelling electricity demand with representative load curves

Models for electricity planning require inclusion of demand. Depending on the type of planning, the demand is usually represented as an annual demand for electricity (GWh), a peak demand (MW) or in the form of annual load-duration curves. The demand for electricity varies with the seasons, economic activities, etc. Existing schemes do not capture the dynamics of demand variations that are important for planning. For this purpose, we introduce the concept of representative load curves (RLCs). Advantages of RLCs are demonstrated in a case study for the state of Karnataka in India. Multiple discriminant analysis is used to cluster the 365 daily load curves for 1993-94 into nine RLCs. Further analyses of these RLCs help to identify important factors, namely, seasonal, industrial, agricultural, and residential (water heating and air-cooling) demand variations besides rationing by the utility. (C) 1999 Elsevier Science Ltd. All rights reserved.