In-Situ Synthesis and Characterization of Nanocomposites in the Si-Ti-N and Si-Ti-C Systems

The pyrolysis (1000 °C) of a liquid poly(vinylmethyl-co-methyl)silazane modified by tetrakis(dimethylamido)titanium in flowing ammonia, nitrogen and argon followed by the annealing (1000–1800 °C) of as-pyrolyzed ceramic powders have been investigated in detail. We first provide a comprehensive mechanistic study of the polymer-to-ceramic conversion based on TG experiments coupled with in-situ mass spectrometry and ex-situ solid-state NMR and FTIR spectroscopies of both the chemically modified polymer and the pyrolysis intermediates. The pyrolysis leads to X-ray amorphous materials with chemical bonding and ceramic yields controlled by the nature of the atmosphere. Then, the structural evolution of the amorphous network of ammonia-, nitrogen- and argon-treated ceramics has been studied above 1000 °C under nitrogen and argon by X-ray diffraction and electron microscopy. HRTEM images coupled with XRD confirm the formation of nanocomposites after annealing at 1400 °C. Their unique nanostructural feature appears to be the result of both the molecular origin of the materials and the nature of the atmosphere used during pyrolysis. Samples are composed of an amorphous Si-based ceramic matrix in which TiN_xC_y nanocrystals (x + y = 1) are homogeneously formed “in situ” in the matrix during the process and evolve toward fully crystallized compounds as TiN/Si₃N₄, TiN_xC_y (x + y = 1)/SiC and TiC/SiC nanocomposites after annealing to 1800 °C as a function of the atmosphere.     

Functionalized heteropolyanions: high-valent metal nitrido fragments incorporated into a Keggin polyoxometalate structure

Three examples of nitrido-functionalized polyoxometalate species are reported, namely (n-Bu4N)4[PW11O39(OsN)] (1), (n-Bu4N)4[PW11O39(ReN)] (2), and (n-Bu4N)3[PW11O39(ReN)] (3), which feature the incorporation of [OsVI identical to N]3+, [ReVI identical to N]3+ and [ReVII identical to N]4+ fragments, respectively, into the framework of a Keggin-type heteropolyanion.     

Test of the Wiedemann-Franz law in an optimally doped cuprate

We present a study of heat and charge transport in Bi(2+x)Sr(2-x)CuO(6+delta) focused on the size of the low-temperature linear term of the thermal conductivity at optimal-doping level. In the superconducting state, the magnitude of this term implies a d-wave gap with an amplitude close to what has been reported. In the normal state, recovered by the application of a magnetic field, measurement of this term and residual resistivity yields a Lorenz number L=kappa(N)rho(0)/T=1.3+/-0.2L(0). The departure from the value expected by the Wiedemann-Franz law is thus slightly larger than our estimated experimental resolution.     

The EC networking on elemental speciation revisited

In Europe, the EC-funded Thematic Network Speciation 21 was instrumental in disseminating the existing knowledge about chemical speciation of trace elements. It created a forum in which analytical chemists working in method development were mixed with potential users from industry and representatives of legislative bodies. This endeavour is continuing in the currently established European Virtual Institute for Speciation Analysis (EVISA). Although there already exists substantial know-how in Europe for the speciation of a number of substances, such as organo-arsenic, organo-tin and organo-mercury compounds, to name just these three groups, there is an urgent need to enforce the analytical potential in speciation analysis on a much wider scope of compounds capable of providing data of a suitable quality. The ensuing need for quality assurance of the analytical procedures requires the analysis of representative reference materials, certified for the relevant species. Last but not least, where applicable, the European-wide legislation should be adapted to focus on toxic species of the elements, rather than the total element.Presented at BERM-9—Ninth International Symposium on Biological and Environmental Reference Materials, June 15–19, 2003, Berlin, Germany. Disclaimer The opinions expressed in the present article are entirely those of the authors. They do not represent the opinion of the European Community.     

Functionalization of polyoxometalates: from Lindqvist to Keggin derivatives. 1. synthesis, solution studies, and spectroscopic and ESI mass spectrometry characterization of the rhenium phenylimido tungstophosphate [PW11O39(ReNC6H5)]4-

Reaction of [Bu(4)N](4)[H(3)PW(11)O(39)] with [Re(NPh)Cl(3)(PPh(3))(2)], in acetonitrile and in the presence of NEt(3), provided the first Keggin-type organoimido derivative [Bu(4)N](4)[PW(11)O(39)(ReNPh)] (Ph = C(6)H(5)) (1). The functionalization was clearly demonstrated by various techniques including (1)H and (14)N NMR, electrochemistry, and ESI mass spectrometry. Conditions for the formation of 1 are also discussed.     

Field-induced thermal metal-to-insulator transition in underdoped La(2-x)Sr(x)CuO(4+delta)

The transport of heat and charge in cuprates was measured in single crystals of La(2-x)Sr(x)CuO(4+delta) (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T-->0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied magnetic field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.     

Direct functionalization of an optical fiber by a plasmonic nanosensor

We explore a rapid route for fabricating silver nanoparticles (NPs) at the end of an optical fiber. The size and number of silver NPs can be controlled by varying the exposure doses. The effect of the refractive index of different solvents on the extinction spectra have been studied as a proof of concept of a fiber integrated plasmon-based sensor.     

Enzymatic hydrolysis of poly(D, L-lactide) spread monolayers by cutinase

 The enzymatic hydrolysis of a model poly(D,L-lactide) by cutinase was studied by using a barostat surface balance. A theoretical approach based on the adaptation of the Michaelis–Menten scheme at the interface and a process of random fragmentation of the macromolecules was developed. The role of the interfacial organisation of the reaction products was discussed. Hydrolysis rate constant values and the specific activity were estimated and compared with those obtained for the hydrolysis of lipid monolayers. The process of fragmentation of the interfacial polymer structures was visualised by using AFM imaging. Received: 19 August 1996 Accepted: 13 December 1996     

Addition of N-heterocyclic carbenes to a ruthenium(VI) nitrido polyoxometalate: a new route to cyclic guanidines

The scope of N-atom transfer from the electrophilic ruthenium(VI) nitrido containing polyoxometalate [PW(11)O(39)Ru(VI)N](4-) has been extended to the N-heterocyclic carbene {CH(2)(Mes)N}(2)C and the coupling product {CH(2)(Mes)N}(2)CNH(2)(+) characterized by (1)H NMR and high-resolution mass spectrometry. Because guanidines display many fields of applications ranging from biology to supramolecular chemistry, this could afford an original route to the synthesis of cyclic guanidines. This also enlarges the potential of nitrido complexes in the synthesis of heterocycles, mainly illustrated in the literature through the formation of aziridines through N-atom transfer to alkenes. In the course of the reaction, the ruthenium(III)-containing polyoxometallic intermediate [PW(11)O(39)Ru(III){NC{N(Mes)CH(2)}(2)}](5-) has been thoroughly characterized by continuous-wave and pulsed electron paramagnetic resonance, which nicely confirms the presence of the organic moiety on the polyoxometallic framework, Ru K-edge X-ray absorption near-edge structure, and electrochemistry.     

Discrete covalent organic-inorganic hybrids: terpyridine functionalized polyoxometalates obtained by a modular strategy and their metal complexation

The rational design and synthesis of organic-inorganic hybrids as functional molecular materials relies on both the careful conception of building-blocks and the strategy for their assembly. Three families of trialkoxo polyoxometalates (Lindqvist 2, Anderson 3, Dawson 4) grafted with remote terpyridine coordination sites have been synthesized to extend the available building-blocks. These new units can be combined with metal complexes that play a role as (i) chromophores toward charge-separated systems in light-harvesting devices and (ii) coordination motifs for metal-directed self-assembly toward multifunctional molecular hybrid materials. The X-ray crystal structures of polyoxometalate-terpyridine hybrids indicate distances of 21 ? and 19 ? between the two terpyridyl coordination sites in 2 and 3, respectively, with angles between the coordination vectors of 180° and 177.4°, respectively. Lindqvist 2 displays a reduction at -0.52 V vs SCE while Anderson 3 exhibits one reversible oxidation attributed to Mn(III)/Mn(IV) (+0.75 V vs SCE) and a broad wave at -1.28 V vs SCE assigned to the Mn(III)/Mn(II) reduction. Dawson 4 displays several processes on a wide range of potentials (+0.5 to -2.0 V vs SCE) centered on V(V), W(VI) and the organic ligand in order of decreasing potentials. The grafted terpyridine ligands in Anderson 3 and Dawson 4 were successfully coordinated to {PdCl}(+) and {RuCl(3)} moieties, respectively. The polyoxometalates and transition metal complexes retain their intrinsic properties in the final assemblies.