Preparation and Optical Characterization of Organoeuropium-Doped Silica Gels

Gels have been investigated as potential matrices for rare earth luminescence. The use of complexing ligands in the sol-gel synthesis of the rare-earth doped glasses has been suggested to improve the rare earth ion luminescence properties in these matrices due to the changes in the local environment experienced by the rare earth ion. In this work, transparent Eu3+-doped gels were prepared from Si(OCH3)4 and fluorinated and/or complex-forming Eu3+ precursors (Eu(fod)3, (CF3 SO3)3Eu, (CF3CO2)Eu · 3H2O, Eu(thd)3 and Eu(acac)3). Results of emission, fluorescence line narrowing and lifetimes studies of Eu3+-doped gels derived from Si(OCH3)4 and fluorinated/chelate Eu3+ precursors are presented. The results were interpreted in terms of the evolution of the Eu3+ fluorescence in systems varying from solutions to the gels densified to 800°C. Analysis of the fluorescence decays of the 5D0 state suggests that the use of the fluorinated Eu3+ precursors reduces the hydrophobic content in the silica gels matrices. FLN studies indicate that Eu3+ clustering occurred in all densified samples.     

The slow molecular mobility in amorphous trehalose.

The molecular mobility in amorphous trehalose is studied by thermally stimulated depolarisation currents (TSDC). The effect of aging on the sub-T(g) motional processes was analysed during annealing at a given aging temperature, some degrees below the calorimetric glass transition temperature T(g)=115 degrees C. The features of different motional components of the secondary relaxation are monitored as a function of time as the glass structurally relaxes on aging. The faster components of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes show a significant dependence on aging consisting of some kind of local motions with some intermolecular nature. The dielectric strength of this relaxation decreases with increasing aging time, and there is no evidence for any modification with aging of the relaxation time of this local mobility. The TSDC study of the molecular mobility of amorphous trehalose in the temperature region of the glass transformation provides the unexpected result that no glass transition signal is observable in this temperature region.     

Color Centers in Alkali Borate Glasses Containing Cobalt,Nickel, or Copper

The absorption spectra and the magnetic moments of Co²⁺, Ni²⁺, and Cu²⁺ in alkali borate glasses of various compositions and in halide-containing glasses have been explained from the point of view of the ligand field theory. Spectra and magnetic moments are highly dependent on the composition of the base glass; this must be ascribed to the change in the coordination of the transition-metal ions. There are far-reaching analogies with corresponding complexes in solution or in the crystalline state.     

Geometric localization of the threshold in two-dimensional Ising ±J spin glasses forT=0

Following an approach of Toulouse, ground states in random 2D Ising ±J spin glasses (without external magnetic field), on square lattices, and with concentrations 0p0.5 of antiferromagnetic bonds are studied by means of minimal matchings of frustrated plaquettes. Lete(p) be the ground-state energy per spin in the thermodynamic limit. Then the well-known equatione(p)=–2+(p)f(p) holds, wheref(p) is the concentration of frustrated plaquettes and(p) is the average connection length between paired frustrated plaquettes in minimal matchings. Introducing (p) as the probability that a frustrated plaquette is matched to another frustrated plaquette by a connection of length (in a minimal matching), the average length(p) can be rewritten asgl(p)=(p). The study of(p) and its components (p) leads to an intervalp ^*pp ₂ (p ^*0.121±0.008,p ₂0.161±0.008) where the threshold between ferromagnet and paramagnet forT=0 lies. Analyzing a similar so-called adjoined average lengthl(p) admits further insight.     

Absence of self-averaging of the order parameter in the Sherrington-Kirkpatrick model

We prove that if _N is the Sherrington-Kirkpatrick (SK) Hamiltonian and the quantity converges in the variance to a nonrandom limit asN, then the mean free energy of the model converges to the expression obtained by SK. Since this expression is known not to be correct in the low-temperature region, our result implies the non-self-averaging of the order parameter of the SK model. This fact is an important ingredient of the Parisi theory, which is widely believed to be exact. We also prove that the variance of the free energy of the SK model converges to zero asN, i.e., the free energy has the self-averaging property.See the Remarks after the proof of Theorem 1 on the validity of our results for more general distributions ofJ _ij .     

Solution‐Processable Epitaxial Metallic Delafossite Oxide Films

Metallic delafossites ABO₂, as the new benchmark of ultra‐high conductive oxides, have recently attracted great interest. The harsh processing conditions and dimensional sizes obviously limit the fundamental sciences and technological applications. Here, a low‐cost and facile solution approach is realized to synthesize epitaxial metallic ABO₂ thin films including PtCoO₂, PdCoO₂, and PdCrO₂ with a dimension up to two‐inches in diameter, showing ultra‐high room temperature conductivity. The electrical properties, growth mechanisms, and potential technical applications including transparent conducting films and hydrogen evolution reaction activity are unveiled. The achievements of epitaxial metallic delafossites ABO₂ thin films through solution methods will pave the route to producing wafer‐scaled ultra‐high conductive metallic delafossites.     

Reaction of 1-chloro-2-phenylethane-2,2-dithiol with divalent metal (Cu, Hg, Pb, Fe, Co, and Ni) salts

Reactions of 1-chloro-2-phenylethane-2,2-dithiol with lead(II) acetate and mercury(II) chloride lead to 1-chloro-2-phenylethane-2,2-dithiolates of lead and mercury. The reactions with copper(II) chloride, iron(II) sulfate hydrate, cobalt (II) chloride hydrate, and nickel(II) acetate give metal-containing compounds C₁₆H₁₈S₄M. The¹H NMR, IR, and ESR spectra of the obtained compounds were recorded; their specific dark electroconductivities and activation energies of dark conductivity were determined, and their film-forming ability was studied. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1224–1226, June, 1997.     

Ultra-small-angle scattering studies of complex fluids

Recent experimental results from ultra-small-angle neutron and X-ray scattering (USANS and USAXS) studies of complex fluids, including colloidal dispersions, colloidal glasses, polymer blends, and biopolymer gels, are reviewed. We focus on data analysis and interpretation in the low q regime. New notable results include the apparent existence of large-scale structure in attractive colloidal glasses, the discovery of new morphological transitions in polymer blends via USANS, and the detection of micron-scale water channels in biopolymer gels.     

Na-Gd phosphate glasses

Summary The present paper deals with preparation, thermal properties and radioluminescence of Ce-doped Na-Gd phosphate glasses. Thanks to their high radioluminescence intensity, three times greater than that of BGO, these glasses are promising materials for the detection of neutrons, - and X-rays. The glasses with a Gd concentration up to 89 mol% were prepared by a rapid quenching technique in air. Their thermal properties were determined using DSC and temperatures of glass transition were measured in addition using TMA. Temperatures of glass transition, crystallization and melting depend on Gd concentration and they follow the liquidus line in a phase diagram of binary system in which two eutectics and a congruently melting compound exist. High glass-forming ability of this glass system was found evidenced. The glasses containing at least 30 mol% Gd were moisture resistant.     

Structure and Electron Spin Resonance of Annealed Sol-Gel Glasses Containing Ag

In this work, SiO₂ samples with silver, prepared using the sol-gel method, were analyzed after being thermally treated in air in the range of 100 to 800°C. The sol-gel starting solutions were prepared by mixing tetra-ethyl-orthosilicate (TEOS), water and ethanol. Samples with 4 different H₂O/TEOS molar ratios (3.3, 5, 7.5 and 11.7 respectively) and with different nominal Ag concentrations were prepared (1, 2 and 4%vol. of Ag). It was found that upon annealing, different silver spices were formed, such as Ag₂ ⁺, Ag⁺, Ag°, and metallic silver aggregates. The identification of these spices was carried out by means of X-ray diffraction, Electron Paramagnetic Resonance (EPR), optical emission and optical absorption. It was also found that the specific type of silver spices observed depends on the structure of the SiO₂ matrix and on the annealing temperatures. It was found that samples prepared from precursor solutions with a low H₂O/TEOS ratio have a more open structure, and therefore silver diffuses faster and forms agglomerates at lower temperatures. Samples prepared from solutions with larger H₂O/TEOS ratios have a more dense structure, which allows the formation of atomic or molecular spices in addition to silver particles. A systematic study of this system was carried out using EPR on samples prepared from solutions having different H₂O/TEOS molar ratios, various Ag concentrations and subjected to different thermal treatments.