Phase analysis and dielectric properties of ceramics in PbO-MgO-ZnO-Nb<SUB>2</SUB>O<SUB>5</SUB> system: a comparative study of materials obtained by ceramic and molten salt synthesis routes

Compositions of the type 3PbO-MgO/ZnO-Nb2O5 were synthesized by the ceramic route at 1000&#176;C and sintered at 1200&#176;C. Powder X-ray diffraction studies of the 1000&#176;C heated products show the presence of the cubic pyrochlore and the columbite (Mg/ZnNb2O6) type phase in the ratio of 3 : 1 for all possible combinations of MgO and ZnO. Further heating at 1200&#176;C led to a decrease in the cubic pyrochlore phase and an increase in the columbite phase by around 10%. Compacted pellets sintered further showed the appearance of the perovskite phase. Similar compositions synthesized using the KCl-NaCl molten salt method at 900&#176;C for 6 h gave a significant amount of the cubic perovskite related phase of the type Pb(Mg/Zn)1/3Nb2/3O3 for all compositions containing MgO. The amount of the perovskite phase was nearly 55% for the Mg rich compositions and decreased with increase in Zn content, the pure Zn composition yielding mainly the cubic pyrochlore phase. On sintering these phases at 1000&#176;C the perovskite phase content decreased. The dielectric constant of the composite materials formed by the ceramic route was in the region of 14 to 20 and varied little with frequency. The composites obtained by the molten salt method, however, showed much larger dielectric constants in the region 40-150 at 500 kHz for various compositions. The dielectric loss tangent of these composites were lower by an order (0.005-0.03 at 500 kHz) compared to the ceramic route

Self-assembling behaviour of Pt nanoparticles onto surface of TiO2 and their resulting photocatalytic activity

In the present study, self-assembling behaviour of guest nanoparticles (platinum) onto the surface of host support (titanium dioxide) during photodeposition process as a function of solution pH has been explored in detail by means of transmission electron microscope (TEM). The photocatalytic activity of the resulting bimetallic nanoassembly (Pt/TiO2) was evaluated by studying the degradation of two organic pollutants viz. triclopyr and methyl orange. Microscopic studies revealed that the deposition and/or distribution of Pt nanoparticles onto the surface of TiO2 were strongly guided by the ionization state of support which in turn was regulated by the solution pH of photodeposition process. A direct relationship between the solution pH of deposition process and the photocatalytic activity of resulting bimetallic catalyst has been observed. A mechanism based on the interparticle interaction between TiO2 and hydrolytic products of metal ions has been proposed for the differences in the photocatalytic activity of the resulting nanocomposite

Magnetic properties of Mn-doped Ge46 and Ba8Ge46 clathrates

We present a detailed study of the magnetic properties of unique cluster assembled solids namely Mn doped Ge46 and Ba8Ge46 clathrates using density functional theory. We find that ferromagnetic (FM) ground states may be realized in both the compounds when doped with Mn. In Mn2Ge44, ferromagnetism is driven by hybridization induced negative exchange splitting, a generic mechanism operating in many diluted magnetic semiconductors. However, for Mn-doped Ba8Ge46 clathrates incorporation of conduction electrons via Ba encapsulation results in RKKY-like magnetic interactions between the Mn ions. We show that our results are consistent with the major experimental observations for this system.Comment: 6 pages, 4 figure

Influence of strontium on the cubic to ordered hexagonal phase transformation in barium magnesium niobate

Oxides of the type Ba3−xSrxMgNb2O9 were synthesized by the solid state route. The x=0 composition (Ba3MgNb2O9) was found to crystallize in a disordered (cubic) perovskite structure when sintered at 1000C. For higher Sr doping (x ≥ 0.5), there was clearly the presence of an ordered hexagonal phase indicated by the growth of superstructure reflections in the powder X-ray diffraction patterns. In all the compositions there was the presence of a minor amount of Ba5−xSrxNb4O15 phase which increased with Sr substitution up to x =1 and then it remained nearly constant at about 5%. Samples sintered at 1300C showed the hexagonally ordered phase for the entire range of composition (0 ≤x ≤ 3). The degree of ordering being considerably greater than in the 1000C heated samples as evidenced by several superstructure reflections

Anisotropic silica mesostructures for DNA encapsulation

The encapsulation of biomolecules in inert meso or nanostructures is an important step towards controlling drug delivery agents. Mesoporous silica nanoparticles (MSN) are of immense importance owing to their high surface area, large pore size, uniform particle size and chemical inertness. Reverse micellar method with CTAB as the surfactant has been used to synthesize anisotropic mesoporous silica materials. We have used the anisotropic silica nanostructures for DNA encapsulation studies and observed a loading capacity of ~ 8 μ g mg−1 of the sample. On functionalizing the pores of silica with amine group, the amount of DNA loaded on the rods decreases which is due to a reduction in the pore size upon grafting of amine groups

Synthesis of Ba<SUB>3</SUB>ZnNb<SUB>2</SUB>O<SUB>9</SUB>-Sr<SUB>3</SUB>ZnNb<SUB>2</SUB>O<SUB>9</SUB> solid solution and their dielectric properties

Oxides of the type, Ba3−xSrxZnNb2O9 (0 ≤ x ≤ 3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4·0938(2) forx = 0 to 4·0067(2) for x = 3. Scanning electron micrographs show maximum grain size for thex = 1 composition (~ 2 μm) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of x with the maximum dielectric constant at x = 1

Formation of calcium oxalate nanoparticles in leaves: significant role of water content and age of leaves

Calcium oxalate monohydrate (COM) was synthesized in the presence of leaves which act as a source of oxalate ions. Three different leaves, viz. Ficus religiosa (peepal in India), Heterophragma adenophyllum (Katsagon) and Spinacia oleracea Linn. (spinach) were used for the synthesis, which led to nanoparticles of COM of size 100–300 nm, 40–70 nm and 150–200 nm respectively. The particles are highly monodisperse, spherical and uniform in size as shown by TEM studies. The amount of COM formed varied significantly (35–80%) with the type of leaf used. The yield could be inversely correlated with the water content of the leaves; though water is essential for the formation of COM (dry leaves do not yield COM). Our studies show that the secretion of oxalate ions (to form COM) from the cells occurs even in leaves which have been removed from the living plants and the ability to secrete oxalate ions increases with the age of the leaf. The young leaves of F. religiosa do not show COM formation. In the presence of chelating agents (like citric acid) which can bind to Ca2+ ions, COM is absent, suggesting that the secretion of oxalate ions is prevented