Characteristics investigation on heat transfer growth of sonochemically synthesized ZnO-DW based nanofluids inside square heat exchanger

Journal of Thermal Analysis and Calorimetry - In recent decades, the growth of heat transfer using nanomaterials in the conventional base fluid has caught the attention of researchers...     

The effect of rapid rotation on a vertical bridgman furnace at large Rayleigh number

In quasi-steady operation, convection currents in a Bridgmandevice, used for producing a semi-conductor crystal, createinhomogeneities that may make the crystal unusable. It has oftenbeen suggested that additional forces due to rotation or magnetismmight be efficacious in reducing the segregation of the elementsof the alloy. It has been found that, over a wide range of rotationrates, there is no improvement in performance due to rotationabout the vertical axis. However, numerical results that havebeen obtained previously (Lee & Pearlstein, J. Crys. Growth240, 2002) indicate that, when effects of centrifugal buoyancyare introduced, a substantial reduction in segregation is achieved.In the work reported here, by contrast, in which we extend previouslarge-Rayleigh-number asymptotic analysis to include centrifugalbuoyancy, we find no improvement in radial segregation, butrather increasing segregation with increasing rotation rate.     

Δ4,4' -4-chalcogenapyranyl-4h-chalcogenapyrans : Synthesis,electrochemical oxidation,and esr investigations of radical-cation states

The O-, S-, Se- and Te-containing Δ^4,4' -2,2?,6,6'-tetramethyl-, -tetra-t-butyl- and -tetraphenyl-4-(chalcogenapyranyl)-4H-chalcogenapyrans were prepared from the corresponding chalcogenapyran-4-ones. The thia-, selena- and tellurapyran-4-ones were prepared by sodium borohydride reduction of the appropriate chalcogen and addition of the disodium chalcogenide to a 1,4-diyn-3-one. The comparative electrochemistry of all three series was examined by cyclic voltammetry. Within each scries the first oxidation (E₁) became more difficult with increasing chalcogen size. The radical-cation states of the tetra-t-butyl analogues were examined by ESR. The O and S analogues gave well-defined five-line spectra, but the Se and Te analogues gave broad single lines. The g value increased with increasing size of the heteroatom. A linear correlation exists between g value and spin-orbit coupling constants λ for the chalcogens, suggesting that the electron densities on the heteroatoms are similar in each heteroatom analogue.     

Dendrimeric organochalcogen catalysts for the activation of hydrogen peroxide: origins of the "dendrimer effect" with catalysts terminating in phenylseleno groups

Several scenarios were evaluated to explain the large "dendrimer effect" observed in the bromination of cyclohexene with H(2)O(2) and NaBr catalyzed by the addition of Frechét-type dendrimers terminating in -O(CH(2))(3)SePh groups. Although phenylseleninic acid was an efficient catalyst for the oxidation of NaBr with H(2)O(2), first-order rate constants for the selenoxide elimination were too small to produce PhSeO(2)H at a rate sufficient to explain the rates of catalysis and no dendrimer effect was observed in the rates of selenoxide elimination. An induction period was observed using 1-SePh as a catalyst for the oxidation of Br(-) with H(2)O(2). The addition of preformed selenoxide 1-Se(=O)Ph gave immediate catalysis with no induction period. However, rates of oxidation of the selenides with H(2)O(2) under homogeneous or biphasic conditions or with t-BuOOH under homogeneous conditions were too slow to account for the rates of catalysis, and no dendrimer effect was observed in the rates of oxidation. The primary oxidant for converting selenides to selenoxides was "Br(+)" produced initially by the uncatalyzed background reaction of H(2)O(2) with NaBr and then produced catalytically following formation of selenoxide groups. Autocatalysis is observed, and the rate of oxidation increases with the number of SePh groups. Autocatalysis is the source of the large dendrimer effect observed with the SePh series of catalysts.     

Controlled hydrothermal growth of ZnO nanostructures by sequestering the Zn metal ions with the chelating agent EDTA

In the present work, a controlled growth of ZnO nanostructures by manipulating Zn metal ion concentration by the chelating action of ethylene diaminetetra acetic acid in hydrothermal method is studied. EDTA produces metal–chelate complex by the formation of bidentate ligand with Zn²⁺ in the solution and diminishes the reactivity of Zn metal cations. Concentration of EDTA in the mother solution was varied in different ranges like 3, 5 and 10 mM while retaining the zinc metal salt and the NaOH concentration the same. Three different morphologies of wurtzite structured ZnO nanostructures such as nanorods-bunch, separate/discrete uniformly sized hexagonal nanorods and tapered flower petals like shapes are achieved by 3, 5 and 10 mM strengths of EDTA, respectively. The medium concentration 5 mM of EDTA is found to have moderate control over producing ZnO nanostructures of uniform diameter and a high aspect (length to diameter) ratio. An array of vertically aligned free standing ZnO nanorods with uniform spacing is successfully achieved by the addition of 5 mM of EDTA in the mother solution and the same is studied for its fluorescence property at an excitation of 325 nm and it has exhibited a characteristic UV emission of ZnO around 383 nm.     

Extreme red shifted SERS nanotags

Surfaced enhanced Raman scattering (SERS) nanotags operating with 1280 nm excitation were constructed from reporter molecules selected from a library of 14 chalcogenopyrylium dyes containing phenyl, 2-thienyl, and 2-selenophenyl substituents and a surface of hollow gold nanoshells (HGNs). These 1280 SERS nanotags are unique as they have multiple chalcogen atoms available which allow them to adsorb strongly onto the gold surface of the HGN thus producing exceptional SERS signals at this long excitation wavelength. Picomolar limits of detection (LOD) were observed and individual reporters of the library were identified by principal component analysis and classified according to their unique structure and SERS spectra.     

Dithiaporphyrin derivatives as photosensitizers in membranes and cells

We synthesized a series of analogues of 5,20-diphenyl-10,15-bis(4-carboxylatomethoxy)phenyl-21,23-dithiaporphyrin (I) as potential photosensitizers for photodynamic therapy (PDT). The photosensitizers differ in the length of the side chains that bind the carboxyl to the phenol at positions 10 and 15 of the thiaporphyrin. The spectroscopic, photophysical, and biophysical properties of these photosensitizers are reported. The structural changes have almost no effect on the excitation/emission spectra with respect to I's spectra or on singlet oxygen generation in MeOH. All of the photosensitizers have a very high, close to 1.00, singlet oxygen quantum yield in MeOH. On the contrary, singlet oxygen generation in liposomes was considerably affected by the structural change in the photosensitizers. The photosensitizers possessing short side chains (one and three carbons) showed high quantum yields of around 0.7, whereas the photosensitizers possessing longer side chains showed smaller quantum yield, down to 0.14 for compound X (possessing side-chain length of 10 carbons), all at 1 microM. Moreover a self-quenching process of singlet oxygen was observed, and the quantum yield decreased as the photosensitizer's concentration increased. We measured the binding constant of I to liposomes and found Kb = 23.3 +/- 1.6 (mg/mL)-1. All the other photosensitizers with longer side chains exhibited very slow binding to liposomes, which prevented us from assessing their Kb's. We carried out fluorescence resonance energy transfer (FRET) measurements to determine the relative depth in which each photosensitizer is intercalated in the liposome bilayer. We found that the longer the side chain the deeper the photosensitizer core is embedded in the bilayer. This finding suggests that the photosensitizers are bound to the bilayer with their acid ends close to the aqueous medium interface and their core inside the bilayer. We performed PDT with the dithiaporphyrins on U937 cells and R3230AC cells. We found that the dark toxicity of the photosensitizers with the longer side chain (X, VI, V) is significantly higher than the dark toxicity of sensitizers with shorter side chains (I, III, IV). Phototoxicity measurements showed the opposite direction; the photosensitizers with shorter side chains were found to be more phototoxic than those with longer side chains. These differences are attributed to the relationship between diffusion and endocytosis in each photosensitizer, which determines the location of the photosensitizer in the cell and hence its phototoxicity.     

A microwave-assisted synthesis of julolidine-9-carboxamide derivatives and their conversion to chalcogenoxanthones via directed metalation

9-formyljulolidine was oxidized via a microwave-assisted Willgerodt-Kindler reaction to the N-piperidine or N-morpholine julolidine-9-thioamide. 9-formyl-1,1,7,7-tetramethyljulolidine gave the corresponding N-piperidine tetramethyljulolidine-9-thioamide. The thioamides were converted to the corresponding carboxamides with trifluoroacetic anhydride. The amide group directed ortho-metalation in the julolidine system, but not in the tetramethyljulolidine system. The resulting anion was captured by dichalcogenide electrophiles. The resulting products were converted to chalcogenoxanthones with phosphorus oxychloride and triethylamine (POCl3/Et3N).