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M M Fejer - One of the best experts on this subject based on the ideXlab platform.

  • quasi phase matched second harmonic generation of 532 nm radiation in 25 rotated x cut near stoichiometric lithium Tantalate fabricated by vapor transport equilibration
    Optics Letters, 2007
    Co-Authors: D S Hum, R K Route, M M Fejer
    Abstract:

    Quasi-phase-matched second-harmonic generation of 532 nm radiation in 25°-rotated, x-cut, near-stoichiometric lithium Tantalate has been performed. Using a face-normal topology for frequency conversion applications allows scalable surface area to avoid surface and volume damage in high-power interactions. First-order, quasi-phase-matched second-harmonic generation was achieved using near-stoichiometric lithium Tantalate fabricated by vapor transport equilibration. These crystals supported 1 J of 1064 nm radiation and generated 21 mJ of 532 nm radiation from a 7 ns, Q-switched Nd:YAG laser within a factor of 4.2 of expectation.

  • Optical properties and ferroelectric engineering of vapor-transport-equilibrated, near-stoichiometric lithium Tantalate for frequency conversion
    Journal of Applied Physics, 2007
    Co-Authors: D S Hum, R K Route, G D Miller, V Kondilenko, A Alexandrovski, Jie Huang, K Urbanek, R L Byer, M M Fejer
    Abstract:

    Near-stoichiometric lithium Tantalate (SLT) crystals were produced from congruent lithium Tantalate by vapor transport equilibration, and several important optical and ferroelectric properties were measured. The effect of vapor transport conditions and surface preparation on reproducible ferroelectric engineering of SLT has been studied. Control of these effects along with dramatic decreases in the sensitivity to photorefractive damage and 532 nm absorption has allowed near-room-temperature generation of 10 W of continuous wave 532 nm radiation by second harmonic generation from 29 W of 1064 nm radiation in a 4 cm long device.

  • vapor transport equilibrated near stoichiometric lithium Tantalate for frequency conversion applications
    Optics Letters, 2004
    Co-Authors: M Katz, D S Hum, R K Route, G D Miller, K R Parameswaran, M M Fejer
    Abstract:

    Near-stoichiometric lithium Tantalate (SLT) crystals were produced from congruent lithium Tantalate by a vapor-transport equilibration process. Because of the resultant increase in photoconductivity and reduction in photogalvanism, the crystals showed no observable photorefractive damage at 514.5 nm up to the highest intensity used, 2 MW/cm2. The crystals also exhibited low green-induced infrared absorption, a Curie temperature of 693 degrees C, and a coercive field of 80 V/mm. The SLT samples were periodically poled with an 8-microm-period grating, permitting first-order quasi-phase-matched second-harmonic generation of 532-nm radiation at 43 degrees C. A 17-mm-long sample generated 1.6 W of continuous-wave output power at 532 nm for 50 h. With 150-ns pulses at a 100-kHz repetition rate in the same sample, 5-W average-power, 532-nm radiation was generated for 1000 h. No damage to the crystal and no aging effects were observed during these experiments.

  • tunable ultraviolet radiation by second harmonic generation in periodically poled lithium Tantalate
    Optics Letters, 1997
    Co-Authors: J P Meyn, M M Fejer
    Abstract:

    We describe electric-field poling of fine-pitch ferroelectric domain gratings in lithium Tantalate and characterization of nonlinear-optical properties by single-pass quasi-phase-matched second-harmonic generation (QPM SHG). With a 7.5-µm-period grating, the observed effective nonlinear coefficient for first-order QPM SHG of 532-nm radiation is 9 pm/V, whereas for a grating with a 2.625-µm period, 2.6 pm/V was observed for second-order QPM SHG of 325-nm radiation. These values are 100% and 55% of the theoretically expected values, respectively. We derive a temperature-dependent Sellmeier equation for lithium Tantalate that is valid deeper into the UV than currently available results, based on temperature-tuning experiments at different QPM grating periods combined with refractive-index data in the literature.

C Drag - One of the best experts on this subject based on the ideXlab platform.

Akihiko Kudo - One of the best experts on this subject based on the ideXlab platform.

  • photocatalytic water splitting into h2 and o2 over various Tantalate photocatalysts
    Catalysis Today, 2003
    Co-Authors: Hideki Kato, Akihiko Kudo
    Abstract:

    Abstract Photocatalytic water splitting into H2 and O2 over various Tantalates was reviewed and factors affecting photocatalytic activities of Tantalates were discussed from a viewpoint of the crystal structure and the energy structure. Many Tantalates, K3Ta3Si2O13, alkali and alkaline earth Tantalates, were highly active photocatalysts for water splitting. The high activities were mainly due to the high conduction band level consisting of Ta5d orbitals. NiO-loaded NaTaO3 with the distorted perovskite structure showed the highest activity among Tantalates. Moreover, the activity of the NiO/NaTaO3 photocatalyst was remarkably improved by doping of lanthanoids. The apparent quantum yield of NiO/NaTaO3 doped with lanthanum was ca. 50% at 270 nm.

  • photocatalytic reduction of nitrate ions over Tantalate photocatalysts
    Physical Chemistry Chemical Physics, 2002
    Co-Authors: Hideki Kato, Akihiko Kudo
    Abstract:

    Tantalate photocatalysts reduced nitrate ions to nitrite ions, dinitrogen, and ammonia under UV irradiation even without co-catalysts or reducing reagents such as organic compounds. The photocatalytic activities of Tantalate photocatalysts were larger than those of TiO2 photocatalysts. The selectivity for the photocatalytic reduction of nitrate ions was affected by the addition of co-catalysts. In particular, the efficiencies of dinitrogen and ammonia formation increased by one to two orders of magnitude when a Ni co-catalyst was loaded. The selectivity was also affected by the amount of loaded Ni and the loading method. 44% and 39% of nitrate ions in an aqueous KNO3 solution (0.010 mol L−1, 400 mL) were reduced to dinitrogen and ammonia, respectively, over the Ni(1 wt.%)/KTaO3 photocatalyst after 25 h irradiation.

  • development of photocatalyst materials for water splitting with the aim at photon energy conversion
    Journal of the Ceramic Society of Japan, 2001
    Co-Authors: Akihiko Kudo
    Abstract:

    A photocatalyst which has extensively been studied so far is TiO2 with a 3.0-3.2eV band gap. Well-known photocatalysts with visible-light response are only Pt/CdS and WO3. Thus, photocatalyst materials mainly employed have been so limited. In such a background, new photocatalysts with high activity have recently been developed. Alkali and alkaline earth Tantalates have arisen as a new group of photocatalyst materials for water splitting into H2 and O2 under ultra-violet light irradiation. They showed the activities even without co-catalysts such as Pt, being different from titanate photocatalysts. When NiO co-catalysts were loaded on Tantalate photocatalysts, except for LiTaO3, the photocatalytic activities were drastically increased. Among the Tantalates, NiO/NaTaO3 showed the highest activity. Moreover, the activity of NiO/NaTaO3 was improved by La-doping. On the other hand, highly crystalline BiVO4 powders with scheelite (monoclinic) and zircon type (tetragonal) structure were selectively synthesized by an aqueous process. The BiVO4 powder with the scheelite structure showed a high photocatalytic activity for O2 evolution in the presence of sacrificial reagent (Ag+) under visible light irradiation (λ>420nm). The photocatalytic activity of the BiVO4 powder prepared by the aqueous process was much higher than that of BiVO4 prepared by a conventional solid state reaction. Zn0.957Cu0.043S (band gap: 2.5eV) and Zn0.999Ni0.001S (band gap: 2.3eV) photocatalysts showed high activities for H2 evolution from an aqueous K2SO3 and Na2S solution under visible light irradiation without co-catalysts such as Pt. ZnNb2O6, Bi2W2O9, Bi2WO6, and Bi3TiNbO9 consisting of ions with d10 and s2 configuration were also active for H2 or O2 evolution from aqueous solutions containing sacrificial reagents.

  • water splitting into h2 and o2 on alkali Tantalate photocatalysts atao3 a li na and k
    Journal of Physical Chemistry B, 2001
    Co-Authors: Hideki Kato And, Akihiko Kudo
    Abstract:

    Alkali Tantalate ATaO3 (A = Li, Na, and K) photocatalysts with perovskite-like structure showed activities for water splitting into H2 and O2 under UV irradiation. When the alkali Tantalates were prepared in the presence of excess alkali, their activities were increased by 1 to 2 orders of magnitude. Scanning electron microscopy and photoluminescence measurements indicated that the excess alkali in the preparation increased the crystal size of NaTaO3 formed and prevented formation of alkali defects in NaTaO3 powder by volatilization, resulting in an increase in the photocatalytic activity. A LiTaO3 photocatalyst showed the highest activity among the naked alkali Tantalate photocatalysts prepared in the presence of excess alkali. On the other hand, the activity of a NaTaO3 photocatalyst was increased by 1 order of magnitude when a NiO cocatalyst was loaded. The NiO(0.05 wt %)/NaTaO3 photocatalyst produced H2 and O2 from pure water with rates of 3.39 and 1.58 mmol h-1, respectively. The apparent quantum yie...

  • new Tantalate photocatalysts for water decomposition into h2 and o2
    Chemical Physics Letters, 1998
    Co-Authors: Hideki Kato, Akihiko Kudo
    Abstract:

    Photocatalytic activities for the decomposition of distilled water into H2 and O2 were investigated on various Tantalates. In the alkali and alkaline earth Tantalates, LiTaO3, NaTaO3, KTaO3, MgTa2O6, and BaTa2O6 showed photocatalytic activities for water decomposition without co-catalysts. Among them, BaTa2O6 in the orthorhombic phase was the most active. The addition of a small amount of Ba(OH)2 into the water and supporting NiO drastically enhanced the photocatalytic reaction on the BaTa2O6 catalyst. On the other hand, in the transition metal Tantalates, NiTa2O6 produced both H2 and O2 without co-catalysts.

Rucheng Wang - One of the best experts on this subject based on the ideXlab platform.

  • U-Pb isotopic dating of columbite-tantalite minerals: Development of reference materials and in situ applications by ion microprobe
    Chemical Geology, 2019
    Co-Authors: Hélène Legros, Rolf L Romer, Julien Mercadier, Johan Villeneuve, Etienne Deloule, Marieke Van Lichtervelde, Stijn Dewaele, Philippe Lach, Xudong Che, Rucheng Wang
    Abstract:

    Columbite-tantalite group minerals are the most common Nb-Ta minerals. Columbite-tantalite is particularly suitable for U-Pb dating due to its high U and low common Pb contents. In situ isotopic dating of columbite-tantalite by LA-ICP-MS or SIMS requires certified reference material to properly account for potential matrix effects linked to substitutions between Nb and Ta and between Mn and Fe. Our study has two objectives: i) establish a database of reference materials for in situ U-Pb isotopic dating of columbite-tantalite minerals and ii) test the capability of SIMS to in situ U-Pb date columbite-tantalite minerals of different chemical composition. Tests of in situ U-Pb dating demonstrate that SIMS can easily be used to date columbite-tantalite minerals with errors and precisions overlapping the reference ID-TIMS age. There are, however, significant matrix effects for non-matching Nb-Ta-Fe-Mn compositions of sample and reference material. Matrix effects are highly correlated with the Ta/(Ta + Nb) ratio of columbite-tantalite, due to the significant difference in the atomic mass of Nb and Ta. The Mn/(Mn + Fe) ratio does not significantly contribute to the observed matrix effect as the two elements have similar atomic masses. The linear correlation between Ta/(Nb + Ta) and ((206Pb/238U)SIMS/(206Pb/238U)ID-TIMS) obtained for columbite-tantalite minerals of known ID-TIMS age demonstrates that the SIMS matrix-effect can be properly accounted for by using the chemical composition as determined by EMPA. The ability to measure 204Pb by SIMS also allows the use of reference materials with a small common lead contribution and to calculate accurate and precise ages for columbite-tantalite minerals with contributions of common lead.

  • chemical evolution and late stage re equilibration of zr hf u bearing columbite group minerals in the koktokay no 1 granitic pegmatite altai northwestern china
    Canadian Mineralogist, 2015
    Co-Authors: Rong Yin, Can Rao, Rucheng Wang, Aicheng Zhang, Jin Chu Zhu, Hui Zhang
    Abstract:

    The Koktokay No. 1 pegmatite in northwestern China is a highly fractionated Li-Cs-Ta (LCT) - type granitic pegmatite. As main Nb - Ta oxides in this pegmatite, columbite-group minerals (CGMs) not only display variations in Nb/Ta and Fe/Mn, but also are typified by noticeable Zr, Hf, and U contents. Here, we report on the chemical evolution and late-stage re-equilibration of CGMs in the processes of crystallization and evolution of this pegmatite. Columbite-group minerals record large variations in Ta/(Nb + Ta) values (0.06–0.98) and moderate variations in Mn/(Fe + Mn) values (0.58–0.99), which could be a result of fractional crystallization of pegmatite magma. The concentrations of ZrO 2 , HfO 2 , and UO 2 in CGMs from the contact zone inwards to zone II of this pegmatite are uniformly less than 0.35 wt.%. However, primary tantalite-(Mn) with Ta/(Nb + Ta) and Mn/(Fe + Mn) values of 0.63–0.98 and 0.86–0.99, respectively, show relatively high contents of ZrO 2 , HfO 2 , and UO 2 , up to 1.68, 0.73, and 1.25 wt.%, respectively. The contents of ZrO 2 , HfO 2 , and UO 2 are positively correlated with variations in Ta/(Nb + Ta) values within different grains and single grains of primary tantalite-(Mn). Crystal-chemical considerations suggest that Zr, Hf, and U are possibly present in the tantalite-(Mn) structure due to the substitution 3(Zr,Hf,U) 4+ = 2(Nb,Ta) 5+ + (Fe,Mn) 2+ . In some areas within the primary tantalite-(Mn) crystals in zone III, specific tantalite-(Mn) has been identified with sharp but irregular contacts with the host tantalite. Such tantalite-(Mn) is chemically depleted in Zr, Hf, and U, but contains many small inclusions of zircon, the MnZrTa 2 O 8 phase, uraninite, and U-rich member of the microlite group of minerals. In a similar way, the primary Zr-Hf-U-bearing tantalite-(Mn) experienced late-stage local re-equilibration, giving rise to the formation of secondary Zr-Hf-U-depleted tantalite-(Mn) and other Zr- or U-rich mineral inclusions in an open fluid-mediated system. The behaviors of Zr, Hf, and U in CGMs in the Koktokay No. 1 pegmatite are indicative of Zr, Hf, and U enrichment during fractional crystallization of a highly fractionated pegmatite magma on one hand and imply late-stage activities of fluids on the other. Considerable enrichment of U and Hf in CGMs indicates that this type of mineral may act as a target for in situ LA-ICP-MS U-Pb dating and Lu-Hf isotope measurements, which may provide key information for understanding the source of the magma and the evolution mechanism of the Nb-Ta mineralization.

  • complex internal textures in oxide minerals from the nanping no 31 dyke of granitic pegmatite fujian province southeastern china
    Canadian Mineralogist, 2009
    Co-Authors: Can Rao, Rucheng Wang, Wen Lan Zhang
    Abstract:

    The Nanping No. 31 granitic pegmatite, a highly evolved spodumene-type pegmatite in the Nanping pegmatite district, southeastern China, can be divided into five internal zones from the border inward: zones I–III as the outer zones and zones IV–V as the inner zones. It is strongly mineralized with Nb–Ta–Sn oxide minerals. We investigated these minerals by using back-scattered electron (BSE) imaging and quantitative electron-microprobe analyses (EMPA) to characterize their complex internal textures. The primary Nb–Ta oxide minerals show a sequence from columbite-(Fe) (zone I) through tantalite-(Fe) (zone II) to tantalite-(Mn) (zone III), but distinctly become columbite + wodginite in zone IV and, rarely, columbite in the zone V. Microscopic tapiolite-(Fe) is identified as a product of exsolution in cassiterite from the zones II–III, whereas complex associations of wodginite, columbite–tantalite and tapiolite-(Fe) are characteristic of cassiterite in zone IV. The classic variation of the columbite-group minerals from the zones I to III seems to result from the greater stability of tantalite than columbite components during fractional crystallization of the melt. However, decreasing values of Ta/(Nb + Ta) in columbite-group minerals in the inner zones are more probably compensated by crystallization of abundant primary wodginite-group minerals, controlled by the f (O2) of the magma. Columbite-group minerals commonly exhibit zoning, but complex and multiple-generation phenomena are notable in columbite–tantalite crystals from zones IV–V. In the latter case, later generations of columbite–tantalite overgrow or cross-cut the earlier ones. Irregular patterns of zoning are linked to the activity of hydrothermal fluids at the late stages of pegmatite evolution. The high mobility of Ta in such fluids is supported by the development of veinlets of tantalite. They cross-cut first the earlier-formed columbite + wodginite aggregates, and later fill brittle-deformation-induced fissures in the columbite, forming a network texture. The complex internal textures of Nb–Ta–Sn oxide minerals from the zones IV–V indicate a distinctly fluid-rich environment at the late stage of crystallization of the Nanping No. 31 pegmatite.

  • chemical evolution of nb ta oxides and zircon from the koktokay no 3 granitic pegmatite altai northwestern china
    Mineralogical Magazine, 2004
    Co-Authors: Aicheng Zhang, Rucheng Wang, Jin Chu Zhu, Hui Zhang, Xiaoming Chen
    Abstract:

    The Koktokay No. 3 granitic pegmatite, Altai, northwestern China, is a strongly zoned rare-element granitic pegmatite, where the petrographic zones were distinguished into two groups: outer zones (I to IV) and inner zones (V to IX). Nb-Ta oxides and zircon are investigated in this paper by using quantitative electron-microprobe analyses (EMPA) and backscattered-electron (BSE) imaging. Columbite-tantalite and zircon occur in most textural zones, whereas tapiolite and uranmicrolite are mainly restricted to zone VII. Manganocolumbite and zircon from the outer zones (zones II and IV) are homogeneous except for a few exceptions, whereas manganotantalite and hafnian zircon from the inner zones (V–VII) are obviously heterogeneous and strongly zoned. Chemically, Ta/(Nb+Ta) in columbite-tantalite and Hf/(Zr+Hf) in zircon increase from the outer to the inner zones on one hand, and from core to rim in single zoned crystals on the other hand. Observations of intra-zonal variations of the chemical composition of Nb-Ta oxides and zircon in the Koktokay No. 3 granitic pegmatite may suggest that the outer zones crystallize under magmatic conditions, whereas the inner zones crystallize under fluid-rich magmatic conditions, and locally under hydrothermal conditions. The extreme enrichments of columbite-tantalite in Ta, and of zircon in Hf, as well as the occurrence of uranmicrolite and tapiolite, indicate an elevated evolution of the Koktokay No. 3 granitic pegmatite.

  • tantalite in suzhou granite mineralogy and geological implications
    Chinese Journal of Geochemistry, 1997
    Co-Authors: Rucheng Wang, Fontan Francois, Chengyi Lin
    Abstract:

    Tantalite, occurring as intergranular tabular crystals, was reported for the first time in the Suzhou granite. Electron microprobe analyses show that it is rich in W and Ti, with a Ta/(Ta + Nb) ratio ranging from 0.5 to 0.73 and a Mn/(Mn + Fe) ratio between 0.20 and 0.40. It is structurally distinct from isomorphic tapiolite by a remarked Ag Raman peak at 880 cm−1. The associated zircon is striking by significant enrichment of Hf, with the HfO2 content amounting up to 35% – 40%. The discovery of tantalite suggests that the Suzhou granite should be classified as a S-type granite instead of A-type as considered previously.

Michael Wark - One of the best experts on this subject based on the ideXlab platform.

  • improved overall water splitting with barium Tantalate mixed oxide composites
    Chemical Science, 2014
    Co-Authors: Julia Soldat, Roland Marschall, Michael Wark
    Abstract:

    The combination of effective charge carrier separation and improved electron transfer in highly crystalline barium Tantalate composites modified with Rh–Cr2O3 core–shell co-catalyst systems induces enhanced activity for overall water splitting (OWS) with stoichiometric amounts of H2 and O2 (2 : 1). A sol–gel route employing complexing reagents was investigated to prepare selectively defined mixed oxide materials with improved surface areas and smaller particle sizes compared to the conventional solid state reaction (SSR). The catalytic activities of the materials are investigated in photocatalytic test reactions for hydrogen production and overall water splitting. The formation of Rh–Cr2O3 core–shell co-catalyst systems for water splitting is evidenced by transmission electron microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). Moreover, we developed new and highly active barium Tantalate composites for hydrogen generation from aqueous methanol solutions under UV-light, which show the highest hydrogen evolution rate for a three-component composite consisting of Ba5Ta4O15/Ba3Ta5O15/BaTa2O6. Hydrogen rates of more than 6 mmol h−1 can be achieved without any co-catalyst. Using Rh–Cr2O3 core–shell co-catalysts on these three-component composites simultaneous generation of H2 and O2 from pure water splitting reaches rates up to 70% higher than for the pure Ba5Ta4O15.

  • control of phase coexistence in calcium Tantalate composite photocatalysts for highly efficient hydrogen production
    ChemInform, 2014
    Co-Authors: Ping Wang, Roland Marschall, Peirong Chen, Aleksander Kostka, Michael Wark
    Abstract:

    A series of phase-controlled and composition-tuned Ca-Tantalate composite photocatalysts is prepared by adjusting the initial atomic ratio of Ta/Ca precursors from 0.9 to 2.0 using Ca2CO3, Ta2O5, and a 45:55 wt% NaCl/KCl melt flux (corundum crucible, 1123 K, 2 h).

  • control of phase coexistence in calcium Tantalate composite photocatalysts for highly efficient hydrogen production
    Chemistry of Materials, 2013
    Co-Authors: Ping Wang, Roland Marschall, Peirong Chen, Aleksander Kostka, Michael Wark
    Abstract:

    Design and fabrication of semiconductor based composite photocatalysts with matching band structure is an important strategy to improve charge separation of photogenerated electron–hole pairs for photocatalytic hydrogen production. In our study, by aid of the simple and cost-effective molten salts method, a series of phase-controlled and composition-tuned calcium Tantalate composite photocatalysts has been prepared by adjusting the initial atomic ratio of Ta/Ca precursors. We demonstrate the strong correlation between the photocatalytic activities of calcium Tantalate composite photocatalysts for hydrogen evolution and respective phase compositions. Without any cocatalysts, these composites with the optimized phase composition of cubic α-CaTa2O6/hexagonal Ca2Ta2O7, cubic CaTa2O6/hexagonal Ca2Ta2O7/orthorhombic β-CaTa2O6, or cubic α-CaTa2O6/orthorhombic β-CaTa2O6 showed very high photocatalytic H2 production activities in the presence of methanol. It is attributed mainly to a significantly improved photoex...