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Zr
Terms modified by Zr Selected AbstractsSeparation of cytokinin isomers with a partial filling-micellar electrokinetic chromatography-mass spectrometry approachELECTROPHORESIS, Issue 10 2008Liya Ge Abstract A new method based on partial filling-MEKC (PF-MEKC) directly coupled to ESI-MS was developed for the simultaneous separation and determination of 13 structurally similar cytokinins, including various geometric and positional isomers of cytokinins. On the basis of the resolution of the neighboring isomer peaks, different parameters (i.e., pH and concentration of buffer, surfactant concentrations, length of the injected micellar plug, organic modifier, and applied separation voltage) were optimized to achieve a satisfactory PF-MEKC separation. Under optimum conditions, the separation of 13 cytokinin standards was accomplished within 25,min. MS/MS with multiple reaction monitoring detection was carried out to obtain sufficient selectivity. PF-MEKC-MS/MS allowed for the direct identification and confirmation of the cytokinins present in banana (Musa spp.) pulp sample after extraction and purification. Finally, trans- zeatin riboside (ZR) and trans- zeatin (Z) were unambiguously identified in banana pulp. It is anticipated that the current PF-MEKC-MS method can be applied to analyze cytokinins in a wide range of biological samples. [source] Effect of convective solar drying on colour, total phenols and radical scavenging activity of olive leaves (Olea europaea L.)INTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 12 2009Neila Bahloul Summary In this study, olive leaves, which are known for their therapeutic and antioxidant properties, were used to assess the effect of solar drying conditions (temperature and flow rate) on the drying time and quality parameters of olive leaves. Samples were dried at three drying temperatures (40, 50 and 60 °C) and at two drying air flow rates (1.62 and 3.3 m3 min,1) in a convective laboratory solar dryer. From the experimental results, it was noted that the drying time required to reduce the moisture content to 0.10 kg kg,1 d.b. depends mainly on the drying temperature. The quality attributes of the dehydrated olive leaf samples were investigated in term of colour, total phenols and radical scavenging activity. The effect of solar drying on L*, a* b* parameters was significant (P < 0.05) for all the studied olive leaves. Besides, the total phenols of olive leaves were significantly (P < 0.05) influenced by drying air conditions and tended to decrease with increased drying time. The DPPH radical scavenging activity was higher in fresh [EC50 39.40 (ZR)-39.95 (CH)] than in dried leaves. However, the radical scavenging activity was also high in leaves dried at 60 °C, 3.3 m3 min,1 [EC50 54.21 (ZR),68.79 (CL)]. [source] Optimal Response-Adaptive Designs for Normal ResponsesBIOMETRICAL JOURNAL, Issue 1 2009Atanu Biswas Abstract Most of the available response-adaptive designs in phase III clinical trial set up are not from any optimal consideration. An optimal design for binary responses is given by Rosenberger et al. (2001) and an optimal design for continuous responses is provided by Biswas and Mandal (2004). Recently, Zhang and Rosenberger (2006) [ZR] provided another design for normal responses. Biswas, Bhattacharya and Zhang (2007) pointed out that the design of ZR is not suitable for normally distributed responses, or any distribution having the possibility of negative mean, in general. But they only indicated the problem and bypassed the original problem and set up. In the present paper, we first start with the drawback of ZR. We then provide the appropriate optimal response-adaptive design for normal or continuous distributions which provides the necessary correction for the ZR problem. The proposed methods are illustrated using some real data (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Increased optical damage resistance of Zr:LiNbO3 crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 11 2007Liang Sun Abstract Zr: LiNbO3 crystals has been grown. The crystal composition and phase are analyzed by X-ray diffration. The optical damage resistance ability of Zr: LiNbO3 crystals is studied by the Sénarmont compensation method and the transmitted beam pattern distortion method. The saturated value of the birefringence change of 6 mol% Zr: LiNbO3 crystal is 1.01×10 -4, which is seven times smaller than that of congruent pure LiNbO3 crystal. The results of UV-Visible and IR absorption spectra of Zr: LiNbO3 crystals powerfully confirm that the optical damage resistance threshold concentration of the Zr4+ ions doped in LiNbO3 crystals is about 6 mol% in the melt. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Simultaneous determination of Zr(IV) and Hf(IV) by CE using precolumn complexation with a [PW11O39]7, ligandELECTROPHORESIS, Issue 10 2007Sadayuki Himeno Professor Abstract A CE method was developed for the simultaneous determination of Zr(IV) and Hf(IV) at trace levels. A lacunary Keggin-type [PW11O39]7, ligand reacted quantitatively with a mixture of trace amounts of Zr(IV) and Hf(IV) to form the so-called ternary Keggin-type anions [P(ZrIVW11)O40]5, and [P(HfIVW11)O40]5, in 0.010,M monochloroacetate buffer (pH,2.2). Since both ternary anions possessed different electrophoretic mobilities and high molar absorptivities in the UV region, Zr(IV) and Hf(IV) were determined simultaneously with direct UV detection at 258,nm. Each peak height was linearly dependent on the concentration of Zr(IV) or Hf(IV) in the range of 5.0×10,7,1.0×10,5,M; a detection limit of 2×10,7,M was achieved. The utility of the proposed CE method was demonstrated for the simultaneous determination of Zr(IV) and Hf(IV) in natural water samples with satisfactory results. [source] Mechanisms of H2, H2C=CH2, and O=CH2 Insertion into Cp2Zr(,2 -SiMe2=NtBu)(PMe3)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2007Siwei Bi Abstract In this paper, the mechanisms for the insertion of H2, H2C=CH2, and O=CH2 into the Zr,Si bond of Cp2Zr(,2 -SiMe2=NtBu)(PMe3) (R) are theoretically investigated with the aid of density functional theory (DFT) calculations. The structure of the H2 insertion product P is discussed on the basis of our calculations, and its bonding features are rationalized in terms of molecular orbital theory. The regiochemistry for insertion of O=CH2 has also been theoretically investigated. It is found that the relative stabilities of the three insertion products of R are in the order P < P, < P,. For the reactions of R with H2 and CH2=CH2, the rate-determining steps are the insertions of H2 and CH2=CH2 into the Zr,Si bond of Cp2Zr(,2 -SiMe2=NtBu) (Int1), whereas PMe3 dissociation is the rate-determining step for the reaction of R with O=CH2. Only the precursor Int2,, formed by the coordination of O=CH2 to the Zr atom, is located; those formed by the coordination of H2 and CH2=CH2 to Int1 are not found.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] High-Zirconium-Content Nano-Sized Bimodal Mesoporous SilicasEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2006David Ortiz de Zárate Abstract Silica-based nanoparticulated bimodal mesoporous materials with high Zr content (43 , Si/Zr , 4) have been synthesized by a one-pot surfactant-assisted procedure from a hydroalcoholic medium using a cationic surfactant (CTMABr = cetyltrimethylammonium bromide) as structure-directing agent, and starting from molecular atrane complexes of Zr and Si as hydrolytic inorganic precursors. This preparative technique allows optimization of the dispersion of the Zr guest species in the silica walls. The bimodal mesoporous nature of the final high surface area nano-sized materials is confirmed by XRD, TEM, and N2 adsorption,desorption isotherms. The small intraparticle mesopore system (with pore sizes around 2,3 nm) is due to the supramolecular templating effect of the surfactant, while the large mesopores (around 12,24 nm) have their origin in the packing voids generated by aggregation of the primary nanometric mesoporous particles. The basicity of the reaction medium seems to be a key parameter in the definition of this last pore system. The effects induced by the progressive incorporation of Zr atoms on the mesostructure have been examined, and the local environment of the Zr sites in the framework has been investigated by UV/Vis spectroscopy. Observations based on the consequences of post-treatments of the as-synthesized materials with HCl/ethanol mixtures corroborate that the atrane method leads to Zr-rich materials showing enhanced site accessibility and high chemical homogeneity throughout the pore walls. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Carbosilane Dendrons Functionalized at Their Focal PointEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 18 2005Román Andrés Abstract The Si,Ph bond of PhSi[(CH2)3SiMe2Bz]3 (5) is cleaved with triflic acid to give TfOSi[(CH2)3SiMe2Bz]3, which, in turn, reacts with triethylammonium chloride or potassium cyclopentadienide to give, respectively, ClSi[(CH2)3SiMe2Bz]3 (8) and (C5H5)Si[(CH2)3SiMe2Bz]3 (10). This strategy can be applied to the post-growth incorporation of nucleophiles to the focal point of carbosilane dendritic wedges. In this way, cyclopentadiene-functionalized dendritic wedges of second and third generation C5H5 -Gn -[(CH2)3SiMe2Bz]x (n = 2, x = 9, 11; n = 3, x = 27, 12) have been obtained starting from Ph-Gn -[(CH2)3SiMe2Bz]x (6, 7). The metallocenes [{(BzMe2SiCH2CH2CH2)3SiC5H4}2MCl2] (M = Ti, 14; Zr, 15) have also been obtained from 10 and their catalytic behavior in ethylene and propylene polymerization, using MAO as a cocatalyst, has been studied and compared to that of related non-dendritic complexes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Synthetic Explorations Towards Sterically Crowded 1,2,3-Substituted Bis(indenyl)zirconium(IV) DichloridesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2005Andreas C. Möller Abstract The systematic synthesis of 1,3-dialkyl-substituted 2-silylindenes and their suitability as zirconocene ligands is discussed. Unexpected reactivities rendered a number of substitution patterns unfeasible, especially for alkyl groups other than methyl in 2-(trimethylsilyl)indene derivatives, and essentially for all derivatives of 2-(dimethylsilyl)indene. The syntheses of rac/meso -bis[1-methyl-2-(trimethylsilyl)indenyl]zirconium(IV) dichloride (12) and bis[1,3-dimethyl-2-(trimethylsilyl)indenyl]zirconium(IV) dichloride (13b) are described. The solid-state structure of the latter displays strong deformations within the ligand framework and an unusually large Cpcentroid -Zr,Cpcentroid angle. Both, 12/MAO and 13b/MAO, displayed ethene and ethene-co-1-hexene polymerization activity. Curiously, 13b/MAO shows an extraordinary monomer selectivity, which can be rationalized by means of DFT calculations on the active site. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Preparation of Diamidochloro(cyclopentadienyl)titanium Derivatives as Pre-Catalysts for Olefin Polymerization , X-ray Molecular Structure of [Ti(,5 -C5H5){1,2-C6H4(NCH2CH2CH3)2}Cl] and [Ti{,5 -C5H4(SiMe3)}{1,2-C6H4(NCH2CH2CH3)2}Cl]EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2004Vanessa Tabernero Abstract The synthesis of N,N,-alkyl 1,2-phenylenediamines 1,2-C6H4(NHR)2 [R = CH2CH2CH3 (nPr), CH2tBu (Np)] was carried out in three steps by lithiation of the primary 1,2-phenylenediamine, reaction with the appropriate acyl chloride and reduction with LiAlH4. The addition of nBuLi to a stirred solution of N,N,-alkyl diamines in cold hexane resulted in the immediate deposition of the corresponding lithium salts, which react with [MCpR,Cl3] to give the diamidochloro(,5 -cyclopentadienyl)titanium and -zirconium complexes [MCpR,{1,2-C6H4(NR)2}Cl] (4,10) [M = Ti, Zr; CpR, = ,5 -C5H5, ,5 -C5(CH3)5, ,5 -C5H4(SiMe3); R = nPr, Np]. The compound [Ti(,5 -C5H5){1,2-C6H4(NnPr)2}Cl] (3) was obtained by treatment of [Ti(,5 -C5H5)Cl3] with 1,2-C6H4(NHnPr)2 in the presence of NEt3 in toluene. All the reported compounds were characterized by the usual analytical and NMR spectroscopic methods. The molecular structures of 3 and 7 were determined by single-crystal X-ray crystallography. The compounds described here were further investigated as potential olefin polymerization catalysts. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Iminohydroxamato Early and Late Transition Metal Halide Complexes , New Precatalysts for Aluminoxane-Cocatalyzed Olefin Insertion PolymerizationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2004Alexander Krajete Abstract We report on new families of non-metallocene metal precatalysts for olefin polymerization with titanium, zirconium, vanadium and nickel as the active metal sites. The novel ligand design concept is based on iminohydroxamic acids and their derivatives as the principal chelating units. Various anionic and neutral [N,O] and [N,N] ligand systems are easily accessible by a modular synthetic sequence of imidoyl chlorides with substituted hydroxylamines or hydrazines, respectively. Steric protection of the metal coordination site, a necessary requirement for suppression of chain termination pathways of non-metallocene catalysts, is brought about by bulky aryl substituents on the imino nitrogen atoms. Crystal structures of some of the hydroxamato ligands reveal interesting intermolecular hydrogen-bridged structures, whereas in the solid-state structure of one titanium precatalyst a five-membered chelate was observed, in line with the design principle of these systems. Preliminary ethylene polymerization studies with methylaluminoxane-activated metal complexes (M = Ti, Zr, V, Ni) show that the most active systems are [N,O]NiBr2 catalysts containing neutral O -alkyl iminohydroxamate ligands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Synthesis and Dynamic Features of (Chloro)zirconocene Cations Stabilised by Pendant (Diarylphosphanyl)alkyl and (Dimethylamino)alkyl Substituents at Their Cyclopentadienyl Ring SystemsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2003Steve Döring Abstract Treatment of the substituted (diarylphosphanyl)methyl group-4 metallocene complexes [(C5H4,CR1R2,PAr2)2ZrCl2] (2: R1/R2 = CH3/CH3, H/CH3, H/aryl) with Li[B(C6F5)4] in dichloromethane solution results in chloride ligand abstraction (with LiCl precipitation) to yield the complexes [(C5H4,CR1R2,PAr2)2Zr,Cl+] (5), with both phosphanyl groups internally coordinated to the metal centre. Three possible diastereoisomers are observed in the case of 5c (R1 = H; R2 = CH3), while bulkier R2 substituents give higher selectivities. The thermally induced (reversible) cleavage of the Zr,phosphane linkage results in dynamic NMR behaviour. Gibbs activation energies of ,G,(298 K) = 14.8 ± 0.5 and 14.5 ± 0.5 kcal/mol were obtained for these intramolecular equilibration processes in the complexes trans - 5d (R1 = H; R2 = Ph) and trans - 5e (R1 = H; R2 = ferrocenyl), respectively. Treatment of the substituted (dimethylamino)methyl metallocene complexes [(C5H4,CR1R2,NMe2)2ZrCl2] (6a, 6b) with Li[B(C6F5)4] proceeds analogously to yield the cation systems [{C5H4,C(CH3)2,NMe2}2ZrCl+] (12a) and [{C5H4,CH(CH3),NMe2}2ZrCl+] (12b, three possible diastereoisomers). Both complexes have their pairs of amino groups coordinated to the metal centre. The complexes exhibit dynamic NMR spectra. Selective equilibration of the diastereotopic N(CH3)A(CH3)B resonances of complex 12a is observed [,G,(233 K) = 11.5 ± 0.2 kcal/mol], whereas the adjacent C(CH3)A(CH3)B methyl groups remain diastereotopic. The dynamic equilibration of the latter was observed at a markedly higher temperature [,G,(333 K) = 17.3 ± 0.2 kcal/mol]. Treatment of [{C5H4,C(CH3)2,NMe2}CpZrCl2] (10) with Li[B(C6F5)4] resulted in the formation of complex [{C5H4,C(CH3)2,NMe2}CpZr,Cl+] (11), which shows the internal ,N(CH3)A(CH)B equilibration proceeding with a markedly higher activation barrier [,G,(333 K) = 17.6 ± 0.2 kcal/mol] than in 12a, and a stereochemical memory effect indicative of solvent coordination to the metal centre of the resulting highly electrophilic chlorozirconocene cation intermediate. Complex 11 was characterised by an X-ray crystal structure analysis, which shows the internal Zr,amine coordination. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Photochromic Properties of a Spirooxazine and a Spiropyran in Alcoholic Solutions of Zirconium and Aluminium Alkoxides: Influence of the Ethyl Acetoacetate Chelating Agent on the Optical PropertiesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 2 2003Aurélie Lafuma Abstract The photochromism of two spiro compounds, a spiropyran and a spirooxazine, dissolved in alcoholic solutions of metal alkoxides M(OR)n (M = Zr, Al) has been studied. In the case of the aluminium alkoxide Al(OsBu)3, special photochromic behaviour is observed characteristic of complexation between the coloured form of the dyes and the aluminium. This chemical interaction induces a considerable shift in the absorption spectra and a significant retardation rate in the dark. Addition of ethyl acetoacetate, a bidentate chelate of aluminium, leads to the disappearance of this particular photochromic behaviour. (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2003) [source] Phase Transformations During High-Pressure Torsion of Pure Zr and of a Zr-2.5%Nb Alloy,ADVANCED ENGINEERING MATERIALS, Issue 8 2010Alexander P. Zhilyaev Zirconium at normal conditions (room temperature and atmospheric pressure) has an HCP structure with lattice parameters a,=,3.2313,Ĺ and c,=,5.1477,Ĺ (,-phase). During loading under hydrostatic conditions in diamond anvil cells, a transition from the , -phase to an , -phase occurs at a pressure between 2 and 6 GPa and from , to , (bcc) at 30 GPa. It has been recently reported that the , to ,,+,, transformation might be induced by HPT processing. The resulting microstructures are stable at room temperature and atmospheric pressure. This paper explores the influence of previous processing steps and of composition in the feasibility of the HPT induced , to ,,+,, transformation. It will be shown that neither previous quenching nor high temperature HPT processing prevents the transformation from occurring during subsequent room temperature HPT. The addition of elements such as Nb also seems to favor the transformation. Understanding well the potential of HPT to stabilize high-pressure phases at normal conditions might be critical, as it will open a whole new range of applications for already existing materials. [source] A New Way to Improve Glass-Forming Ability by Controlling the Preparation Conditions of Bulk Metallic GlassADVANCED ENGINEERING MATERIALS, Issue 3 2010Jie Mao Abstract The effects of mold preheating temperature on the glass-forming ability of Zr64.9Al7.9Ni10.7Cu16.5 and Zr47Cu37.5Ag7.5Al8 alloys are investigated using traditional copper mold casting. It is found that there exists an optimum mold temperature for glass-forming alloy systems. For the two Zr-based alloy systems studied the optimum mold temperature is 353,K. This is explained from the perspectives of heat transfer efficiency and the wettability between liquid alloy and copper mold. [source] The Effects of Casting Temperature on the Glass Formation of Zr-Based Metallic Glasses,ADVANCED ENGINEERING MATERIALS, Issue 12 2009Jie Mao Abstract The glass1-forming ability of two alloys, Zr64.9Al7.9Ni10.7Cu16.5 and Zr47Cu37.5Ag7.5Al8, prepared by arc-melting a mixture of Zr, Cu, Al, Ni and Ag elements is studied as a function of casting temperature. Other processing parameters such as the alloy melt mass, and the vacuum and injection pressures during the copper-mold-casting process are kept constant so just the influence of the casting temperature is considered. The casting temperature determines the characteristics of the liquid melt and the cooling rate. The glass-forming ability is discussed in terms of dissipation of pre-exiting, metastable local-ordering clusters that act as nucleation sites promoting crystallization, the cooling rate at high casting temperatures, and the presence of oxygen in the alloys, which is increased at high casting temperatures. It is found that the glass-forming ranges of alloys shrink as the glass-forming size approaches a critical value. The optimum temperatures are around 1450,K and 1550,K for Zr64.9Al7.9Ni10.7Cu16.5 and Zr47Cu37.5Ag7.5Al8 alloys respectively. The alloys were studied by XRD, TEM, oxygen-level determination, and DSC. [source] Tribological Studies of a Zr-Based Glass-Forming Alloy with Different States,ADVANCED ENGINEERING MATERIALS, Issue 11 2009Feng Jiang Abstract The tribological characteristics of a glass-forming alloy, Zr52.5Cu17.9Ni14.6Al10.0Ti5.0, in atomic percent (at.%, Vit 105), with different microstructural states have been investigated. Friction and wear studies were conducted using a ball-on-flat reciprocating sliding apparatus against an AISI E52100 bearing steel under dry condition. The observed wear resistance in an ascending order is: the deformed, creep-tested, and as-cast states. Wear analyses suggested that the wear processes of glass-forming alloys involved abrasion, adhesion, and oxidation. The differences in hardness, free volume, and brittleness in different states significantly affected the friction and wear behaviors of the glass-forming alloys. [source] The Extraordinary Cocatalytic Action of Polymethylaluminoxane (MAO) in the Polymerization of Terminal Olefins by Metallocenes: Chemical Change in the Group 4 Metallocene Dimethyl Derivatives Induced by MAO,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2005John J. Eisch Abstract In the polymerization of olefins with Group 4 metallocene dichlorides or dimethyl derivatives as procatalysts the use of polymethylaluminoxane (MAO) as the cocatalyst, especially in extreme excess (102,103 times the metallocene equivalent), has been shown to have an extraordinary accelerating effect on the rate of olefin polymerization, when compared with the cocatalytic action of alkylaluminum halides. In attempts at explaining the greatly superior catalytic activity of MAO in olefin polymerization (the MAO conundrum), hypotheses have generally paralleled the steps involved in the cocatalytic action of RnAlCl3,n, namely the alkylation of Cp2MtCl2, ionization of Cp2Mt(R)Cl into the metallocenium cation, [Cp2Mt,R]+, and anion, [Rn,1AlCl4,n], and subsequent ion-pair separation. In order to understand any differences in catalytic action between such cocatalysts, we have studied the individual action of MAO (100 equiv.) and of MeAlCl2 (1,2 equiv.) on each of the Group 4 metallocene derivatives, Cp2TiCl2, Cp2ZrCl2, Cp2Ti(CH3)2 and Cp2Zr(CH3)2. With MeAlCl2 each of the metallocene derivatives appeared to form the cation, [Cp2Mt,CH3]+, with greater (Ti) or lesser (Zr) ease, because an alkyne such as diphenylacetylene was then found to insert into the Mt,CH3 bond stereoselectively. In striking contrast, treatment of each metallocene with MAO gave two reactions very different from MeAlCl2, namely a steady evolution of methane gas upon mixing and a finding upon hydrolytic workup that the diphenylacetylene present had undergone no insertion into the Mt,CH3 bond but instead had been reductively dimerized completely to (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene. To account for this astonishing difference in chemical behavior between MAO and MeAlCl2 in their cocatalytic activation of Group 4 metallocenes to olefin polymerization, it is necessary to postulate a novel, unique sequence of reaction steps occurring between MAO and the metallocene. If one starts with the metallocene dichloride, then the free TMA present in the MAO would generate the Cp2Mt(CH3)2. This metallocene dimethyl derivative, complexed with an oligomeric MAO unit, would undergo a transfer-epimetallation with added olefin or acetylene to form a metallacyclopropane or metallacyclopropene, respectively. With added diphenylacetylene the resulting 2,3-diphenylmetallacyclopropene would be expected rapidly to insert a second alkyne to form the 2,3,4,5-tetraphenyl-1-metallacyclopentadiene. Simple hydrolysis of the latter intermediate would generate (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene while alternative workup with D2O would give the 1,4-dideuterio derivative of this butadiene. Both such expectations were confirmed by experiment. In the case of added olefin, similar metallacyclopropane and metallacyclopentane intermediates should be produced until ring opening of the latter five-membered ring leads to an open-chain zwitterion, a process having ample precedent in the research of Gerhard Erker. The solution to the MAO conundrum then, namely the extraordinary cocatalytic activity of MAO in olefin polymerization by metallocenes, lies in the unique catalytic activation of the Group 4 metallocene dimethyl derivative, which occurs by transfer-epimetallation of the olefin monomer by the Cp2Mt(CH3)2,MAO complex. The most advantageous Lewis acidic sites in the MAO,oligomeric mixture for such metallocene,MAO complexation are suggested to be terminal Me2Al,O,AlMe, segments of an open-chain oligomer. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] The Synthesis and Reactivity of Group 4 Zwitterionic Complexes of the Type Mt+CH2AlCl3,: One-Component Stereoselective Polymerization and Oligomerization Catalysts for Olefins and AcetylenesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 15 2004John J. Eisch Abstract A reinvestigation of the interaction of TiCl4 with 2 equiv. of Me3Al in toluene between ,78 °C and 25 °C over 24 h has now established that the ultimate black product obtained is an associated zwitterion of the type [Ti+,CH2,AlCl3,]n, supported by multinuclear NMR spectroscopy and mass spectrometric and gasometric analyses of the gases evolved (CH4, H2) upon its protolysis. Chemical reactions of the zwitterion have corroborated specific aspects of its proposed structure: 1) its methylene character, by its transformation of benzophenone into 1,1-diphenylethylene; 2) its divalent titanium content, by the substantial reductive dimerization of benzophenone to tetraphenylethylene, and 3) its Lewis acidic Ti center, by its catalytic isomerization of trans -stilbene oxide to 1,1-diphenylacetaldehyde. Similar individual reactions of ZrCl4 or HfCl4 with Me3Al have led to the analogous zwitterions [Zr+,CH2,AlCl3,]n and [Hf+,CH2,AlCl3,]n, respectively. These zwitterions of Ti, Zr and Hf have been proven to be capable of the cyclotrimerization and/or polymerization of acetylenes with varying facility, as evidenced by their catalytic action on 1-hexyne, phenylacetylene, di- n -butylacetylene, and diphenylacetylene. Furthermore, all three zwitterions were able to polymerize ethylene, without any added cocatalyst, with an activity following the order Zr > Ti > Hf. The Ti and Zr zwitterions effected the stereoselective polymerization of propylene to yield 50% of isotactic polymer, and all three catalysts induced the polymerization of 1-hexene to yield 85% (Zr, Hf) or 100% (Ti) of isotactic polymer. These oligomerizations and stereoselective polymerizations of acetylenes and olefins can be rationalized through a model for the active site resembling a three-membered metallacyclopropa(e)nium ion intermediate formed from the attack of the Group 4 metal zwitterion on the unsaturated hydrocarbon. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Ring-Expansion of MCPs in the Presence of DIAD or DEAD and Lewis AcidsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 2 2004Li-Xiong Shao Abstract Treatment of methylenecyclopropanes (MCPs) with DIAD or DEAD in MeCN under mild conditions in the presence of Lewis acid Zr(OTf)4 gave the cyclobutanone ring-expansion products in good to high yields based on the employed DIAD or DEAD. From a deuterium labeling experiment, the oxygen atom is derived from ambient water. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Air-Oxidation of a Cu45Zr45Al5Ag5 Bulk Metallic Glass,ADVANCED ENGINEERING MATERIALS, Issue 5 2009Wu Kai This work studied the oxidation behavior of a Ag-containing Cu,Zr,Al-based amorphous alloy (CZA4-BMG) in dry air at 375,500,°C. The oxidation kinetics of CZA4-BMG obeyed a multi-stage parabolic-rate law, and its steady-state oxidation-rate constants were slightly lower than those of Ag-free amorphous alloy (CZA3-BMG) at the temperature below 450,°C, indicating that the addition of Ag played a partly blocking effect in reducing the available cross-sections of the substrate, which in turn reduced inward diffusion of oxygen, thereby leading to the reduction of oxidation rates for CZA4-BMG. [source] Designing Ductile Zr-Based Bulk Metallic Glasses with Phase Separated Microstructure,ADVANCED ENGINEERING MATERIALS, Issue 5 2009Xinghao Du Using the thermodynamic computation, the phase-separated Zr-based bulk metallic glasses with a enhanced plasticity up to 20% are developed. The as-cast microstructure is characterized by the macroscopic heterogeneities consisting of the phase-separated regions and glassy matrix regions. The microscaled phase-separated feature is the cause of the remarkable plasticity, and the homogeneous and concurrent formation of multiple shear bands is crucial for the plasticity improvement in metallic glasses. [source] Atomic Layer Deposition of High- k Oxides of the Group 4 Metals for Memory Applications,ADVANCED ENGINEERING MATERIALS, Issue 4 2009Jaakko Niinistö Abstract This paper reviews several high-k ALD processes potentially applicable to the production of capacitors, concentrating on very recent developments. A list of the dielectric materials under investigation consists of the oxides of several metals, including the Group 4 (Ti, Zr, Hf) elements. The binary oxides of Group 4 metals, as well as their mixtures with other oxides, doped hosts, or multi-layers in the form of nano-laminates are of interest.Several examples of our recent results are shown, including possible ALD routes to materials not previously grown, as well as advances in process development. [source] Structure, reactivity and spectroscopic properties of minerals from lateritic soils: insights from ab initio calculationsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2007E. Balan Summary We review here some recent applications of ab initio calculations to the modelling of spectroscopic and energetic properties of minerals, which are key components of lateritic soils or govern their geochemical properties. Quantum mechanical ab initio calculations are based on density functional theory and density functional perturbation theory. Among the minerals investigated, zircon is a typical resistant primary mineral. Its resistance to weathering is at the origin of the peculiar geochemical behaviour of Zr, an element often used in mass balance calculations of continental weathering. Numerical modelling gives a unique picture of the origin of the chemical durability and radiation-induced amorphization of zircon. We also present several applications of ab initio calculations to the description of properties of secondary minerals, such as kaolinite-group minerals and gibbsite. Special attention is given to the calculation of infrared and Raman spectra. Surface properties and particle shape are major properties of finely-divided materials such as clay minerals. We show how theoretical modelling of infrared spectroscopic data provides information on natural samples at both the microscopic (atomic structure) and macroscopic (particle shape) length-scale. The systematic comparison of experimental and theoretical data significantly improves our understanding of mineral transformations during soil formation and evolution in lateritic environments. [source] Effects of Zirconium Additions on the Microstructure of As-Cast and Aged AZ91 Magnesium AlloyADVANCED ENGINEERING MATERIALS, Issue 3 2009Farhoud Kabirian The effects of Zr addition on the microstructure of AZ91 alloy were investigated under as-cast and isothermally aged conditions. The microstructures contained a eutectic , -Mg17Al12 phase together with fine Al,Zr intermetallic compounds. These intermetallic compounds inhibited grain growth during the 420,°C isothermal aging of Zr-containing alloys. Microstructural changes caused by Zr additions were most probably due to the consumption of Al by Zr, and redistribution of Al during solidification of dendrites. [source] Back Cover: TMS Bulk-Metallic Glasses Symposium V (Adv. Eng.ADVANCED ENGINEERING MATERIALS, Issue 11 2008Mater. The backcover shows the nanoindentation of a Zr-based bulk metallic glass with an Al-concentration of 13 at%. Around the indent shear bands are observed indicating some plasticity of that glass. More about the glass-forming ability and ductility of Zr-based and Al-rich bulk metallic glasses can be found in the paper by R. Wunderlich et al. on page 1020. [source] Formation of a Multiphase Gradient Structure in a Zr,Cu,Ni,Al,O Alloy,ADVANCED ENGINEERING MATERIALS, Issue 4 2008F. Qiu The phase formation upon rapid cooling by copper mould casting of a Zr-Cu-Al-Ni alloy with two different oxygen contents is reported. The nominally oxygen-free alloy produces a glassy phase, whereas the presence of 0.56 at.% oxygen results in crystallization. The crystalline phase selection is governed by the cooling rate. The microstructure and mechanical properties as well as the correlation between them in the oxygen-containing alloy were investigated. [source] Atom Probe Tomography II.ADVANCED ENGINEERING MATERIALS, Issue 12 2006The Precipitation in Al Base Alloys Abstract This paper presents two illustrations of the study of phase separation in Al-based alloys by means of tomographic atom probe. In an AlMgSi alloy, calculations of pair correlation functions have revealed that the nature of solute short range order differs from a pre-aged specimen to a non-pre-aged specimen. This results in different responses to aged hardening treatments. Concerning Al3(Zr,Sc)-type dispersoids observed in another Al-based alloy, they are found to display a core-shell structure wherein Zr is mainly present in the dispersoids periphery. This duplex structure is likely to explain the relative stability of Al3(Zr,Sc) dispersoids. [source] Cluster Dynamics Modelling of Precipitation Kinetics in Al(Zr,Sc) AlloysADVANCED ENGINEERING MATERIALS, Issue 12 2006P. Guyot Cluster dynamics simulations in multicomponents alloys emphasize the complexity of the respective roles of atomic mobility and thermodynamics in the precipitation path. Some heuristic assumptions, like the maximum driving force criterion, used in conventional treatments, may fail and need to be carefully revisited at the light of these results. [source] Thermoplastic Extrusion to Highly-Loaded Thin Green Fibres Containing Pb(Zr,Ti)O3,ADVANCED ENGINEERING MATERIALS, Issue 5 2005J. Heiber Because of their piezoelectric properties lead zirconate titanate (PZT) fibres are used in several electronic applications. Such fibres can be produced by extrusion. Therefore the extrusion behaviour of highly loaded thermoplastic feedstocks (PZT-polymer compound) has been investigated depending on the surfactant concentration and the powder preparation method. After sintering butterfly curves were measured for single fibres. [source] | ||||||||||||||||||||||||||||||||||||||||