Thermal relaxation times and heat conduction in β-cristobalite and α-quartz silica structures

Several studies have shown the key role of the rigid unit modes, i.e. rotational motions between neighbouring SiO₄ tetrahedra, in glass and in β-cristobalite due to the disorder generated by the large atomic amplitudes. They do not however reach a conclusion concerning heat conduction. To determine the nature of the heat carriers in the amorphous phase, we carry out an ultrashort timescale analysis of heat transfer in β-cristobalite; the behaviour is comparable to that of amorphous silica, as shown by our density of states and heat flux autocorrelation function calculations. We prove that rigid unit modes can be identified as the heat carriers by showing that the thermal relaxation time is in very good agreement with the rigid unit mode relaxation time predicted in the literature. Finally, we provide thermal conductivity data for β-cristobalite which are not available in the literature.     

Polydimethylsiloxane at the interfaces of fumed silica and zirconia/fumed silica

Polydimethylsiloxane (PDMS)/fumed silica A-300 and PDMS/ZrO₂/A-300 were studied using adsorption, thermogravimetry, temperature-programmed desorption (TPD) mass-spectrometry, infrared spectroscopy, XRD, and broadband dielectric relaxation spectroscopy. ZrO₂ was synthesized on fumed silica with zirconium acetylacetonate in CCl₄ at 350 K for 1 h and calcinated at 773 K for 1 h (1-4 reaction cycles). PDMS (5-40 wt.%) was adsorbed onto silica and zirconia/silica from hexane solution and then dried. Grafted zirconia changes the chemistry of the surface (because of its catalytic capability) and the topology of secondary particles (because of occupation of voids in aggregates of primary silica particles by zirconia nanoparticles) responsible for the textural porosity of the powders. Therefore, many properties (such as structural characteristics of the composites, reactions on heating in air and vacuum, interfacial relaxation phenomena, hydrophobicity as a function of treatment temperature, etc.) of PDMS/zirconia/silica strongly differ from those of PDMS/A-300. Broadening of the α-relaxation of PDMS at the interfaces of disperse oxides suggests both weakening of the PDMS-PDMS interaction and strengthening of the PDMS-oxide interaction.     

Electronic transitions in α, θ and γ polymorphs of ultraporous monolithic alumina

Spectroscopy of α, θ, and γ phases of high‐purity ultraporous alumina has been studied at cryogenic temperatures of 7 K in the near‐IR–VUV range of spectra with synchrotron radiation excitation. The UV photoluminescence (PL) spectra are dominated by optical transitions of self‐trapped excitons, while the PL excitation spectra are assigned to free excitons and interband transitions. The analysis of PL excitation spectra indicates a tendency to fundamental bandgap narrowing in order of 9.36 eV (α) to 7.60 eV (θ) and 6.85 eV (γ). Structural defects related to oxygen vacancies are responsible for the visible F⁺/F transitions decrease in order γ > θ > α. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectroscopic and structural investigations of α‐, β‐, and γ‐AlH3 phases

With its reputation as a high‐energy density fuel, aluminum hydride (AlH₃) has received renewed attention as a material that is particularly suitable, not only for hydrogen storage but also for rocket propulsion. While the various phases of AlH₃ have been investigated theoretically, there is a shortage of experimental studies corroborating the theoretical findings. In response to this, we present here an investigation of these compounds based primarily on two research areas in which there is the greatest scarcity of information in the literature, namely Raman and infrared (IR) absorption analysis. To the authors' knowledge, this is the first report of experimental far‐IR absorption results on these compounds. Two different samples prepared by broadly similar ethereal reactions of AlCl₃ with LiAlH₄ were analyzed. Both Raman and IR absorption measurements indicate that one sample is purely γ‐AlH₃ and that the other is a mixture of α‐, β‐, and γ‐AlH₃ phases. X‐ray diffraction confirms the spectroscopic findings, most notably for the β‐AlH₃ phase, for which optical spectroscopic data are reported here for the first time. Copyright © 2010 John Wiley & Sons, Ltd.     

A new method for functionalizing α,γ‐dinitro compounds at the β‐position: application to the cyclization of β‐alkoxy‐α,γ‐dinitro compounds

Treatment of 2,4‐dinitropentane with bromine and sodium methoxide in methanol, affords formation of an ether product, 2,4‐dibromo‐3‐methoxy‐2,4‐dinitropentane, in 59% yield as a mixture of three diastereomers. This observation has led to a general synthesis of 3‐alkoxy‐2,4‐dibromo‐2,4‐dinitropentanes, obtained in 75‐86% yield from 2,4‐dibromo‐2,4‐dinitropentane as the preferred reactant. 4‐Bromo‐2,4‐dinitro‐2‐pentene has been identified as an intermediate in these reactions. The nitroalkene has been isolated and undergoes conjugate addition with alkoxides to afford the same ether products after brominative work‐up. The nitroalkene undergoes conjugate addition with sodium azide to give 3‐azido‐2,4‐dibromo‐2,4‐dinitropentane in 38% yield as a mixture of two isomers in which the (R*,R*) isomer predominates. Sequential treatment of 2,4‐dibromo‐2,4‐dinitropentane with sodium methoxide followed by sodium iodide and acetic acid gives 3‐methoxy‐2,4‐dinitropentane in 63% yield, the overall product of simple methoxylation of 2,4‐dinitropentane. However, attempted complete debromination of 2,4‐dibromo‐3‐methoxy‐2,4‐dinitropentane with excess sodium iodide and acetic acid results only in monodebromination to give 2‐bromo‐3‐methoxy‐2,4‐dinitropentane in 86% yield. Likewise, 2‐bromo‐3‐ethoxy‐2,4‐dinitropentane is formed in 93% yield from the ethoxy analog. A mechanistic rationale is offered for condition‐specific removal of the second Br atom in these reactions. Treatment of 3‐methoxy‐2,4‐dinitropentane with potassium acetate/iodine in dimethyl sulfoxide affords formation of 4,5‐dihydro‐3,4‐dimethyl‐3‐methoxy‐4‐nitroisoxazole 2‐oxide in 30% yield as a single diastereomer. Conversion of 2‐bromo‐3‐methoxy‐2,4‐dinitropentane in 15% yield to 4,5‐dihydro‐3,4‐dimethyl‐3‐methoxy‐4‐nitroisoxazole 2‐oxide is also possible by using potassium acetate in dimethyl sulfoxide. The mechanistic pathways for formation of 4,5‐dihydro‐3,4‐dimethyl‐3‐methoxy‐4‐nitroisoxazole 2‐oxide apparently involve unstable 3‐methoxy‐1,2‐dimethyl‐1,2‐dinitrocyclopropane as the common intermediate. Similarly, 2‐bromo‐3‐ethoxy‐2,4‐dinitropentane affords 4,5‐dihydro‐3‐ethoxy‐3,4‐dimethyl‐4‐nitroisoxazole 2‐oxide in 13% yield. Copyright © 2013 John Wiley & Sons, Ltd.     

Ferromagnetic correlations in both the α and γ-phases of paramagnetic iron

Paramagnetic scattering experiments using polarised neutrons and polarisation analysis in the paramagnetic phase of iron are reported. The measurements have been performed at 1120 K in the α-bcc phase and 1320 K in the γ-fcc phase. Iron moments to respectively 1.3 (0.1)μ_B and 0.9 (0.1)μ_B have been shown to persist at these temperatures. In both phases, ferromagnetic correlations are present and are especially strong in the α-phase. The ferromagnetic correlations also determine the magnetic character of iron in the γ-phase, contrary to previous suggestions of antiferromagnetic behaviour postulated in the literature on the basis of indirect experimental arguments.     

Photoluminescence in γ-irradiated α-quartz investigated by synchrotron radiation

We report an experimental investigation of the photoluminescence, under excitation by synchrotron radiation within the absorption band at 7.6 eV, induced in γ-irradiated α-quartz. Two emissions centered at 4.9 and 2.7 eV are observed at low temperature: the former decreases above 40 K, whereas the second band exhibits an initial slight increase and its quenching is effective above 100 K. Furthermore, the decay kinetics of both emissions occur in a time scale of nanoseconds: at T=17.5 K we measured a lifetime τ1.0 ns for the photoluminescence at 4.9 eV and τ3.6 ns for that at 2.7 eV. These results give new insight on the optical properties associated with defects peculiar of crystalline matrix, also on the basis of their comparison with previous studies on silica.     

Relative stabilities and molecular structures of the isomeric enol ethers and carboxylic esters derived from α‐acetyl‐γ‐butyrolactone and α‐acetyl‐δ‐valerolactone

Recently recorded ¹⁷O NMR spectra of compounds studied in a previous work (Taskinen E. Acta Chem. Scand. 1985; B39 : 489–494) dealing with the thermodynamics of isomerization of the enol ethers of α‐acetyl‐γ‐butyrolactone reveal an error in compound identification, caused by an unexpected isomerization reaction during the synthetic procedure. Thus, acid‐catalyzed treatment of the lactone with HC(OR)₃ in the respective alcohol ROH is shown to lead initially to the desired enol ethers which, however, are gradually isomerized to a mixture of the enol ethers and an ester of 2‐methyl‐4,5‐dihydrofuran‐3‐carboxylic acid. As a result, only one of the two isomeric compounds detected in the previous equilibration study was the expected enol ether (the thermodynamically more stable E isomer) of α‐acetyl‐γ‐butyrolactone, while the other, dominating species was the respective carboxylic ester. In the present work, the evidence provided by the ¹⁷O NMR spectra is presented, and the relative stabilities of the isomeric compounds are discussed on the basis of computational enthalpy data. The treatment is also extended to the respective isomeric compounds derived from α‐acetyl‐δ‐valerolactone. Copyright © 2007 John Wiley & Sons, Ltd.     

Changes in electrical properties in α- and γ-exposed PADC track detector

The changes in the dielectric properties and temperature dependence of the d.c. conductivity of α-exposed poly allyl diglycol carbonate (PADC) have been studied. On α-irradiation the dielectric constant (′) as a function of frequency has been found to decrease significantly. The temperature dependence of resistivity in pristine and γ-irradiated samples is of the form ρ(T)=ρ_∝ exp(T₀/T) which can be attributed to conduction of thermally generated carriers. In case of (γ+α) irradiated samples the temperature dependence of resistivity is of the form ρ(T)=ρ_∝ exp(T₀/T)^1/2 which is due to one-dimensional hopping of carriers.     

High‐pressure Raman spectroscopy study of α and γ polymorphs of AlH3

For the first time, Raman spectroscopy of α and γ polymorphs of AlH₃ has been performed in the pressure range from ambient up to 16.9 and 32.7 GPa, respectively using the diamond anvil cell (DAC) technique. An analysis of pressure response wavenumbers (ν) for α‐AlH₃ showed a change of dν_i/dP at a pressure of about 8 GPa and may indicate a monoclinic distortion from the initially hexagonal α‐AlH₃. The distortion is stable at least up to 16.9 GPa. The γ form exhibited more complex behavior transforming to the α form at a pressure of about 12 GPa. The structural phase transition was shown to be an irreversible and kinetically slow process that required at least 5 h to complete. Copyright © 2008 John Wiley & Sons, Ltd.     

Chiral-loop and vector-meson contributions to η → ππγγ decays

The process η → π⁰π⁰γγ is discussed in Chiral Perturbation Theory (ChPT). Special attention is deveted to one-loop corrections, η-η′ mixing effects and vector-meson dominance of ChPT counter-terms. The less interesting η → π⁺π⁻γγ transition is briefly discussed too.     

Prediction of γ‐B28 ELNES with comparison to α‐B12

Using ab initio techniques we have calculated the electron energy loss near edge structure (ELNES) of a new high pressure phase of boron (γ‐B₂₈) and the structurally similar allotrope, α‐B₁₂. The total ELNES spectra are presented as weighted sums of the site specific spectra of the constituent non‐equivalent B atoms. The five different non‐equivalent B sites in γ‐B₂₈ all show rich ELNES spectra and their similarities and differences to the simpler α‐B₁₂ case are detailed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultraviolet radiation- and γ radiation-induced color centers in germanium-doped silica glass and fibers

Induced optical losses and paramagnetic Ge(n) centers were investigated in germanium-doped silica glass and optical fibers after γ and ultraviolet (UV) irradiation. It was found that both types of irradiation created similar effects. By means of selective UV irradiation, Ge (1 and 3) centers were identified in optical absorption spectra, presumably as induced bands centered at 4.4 eV and 6.2 eV, respectively. Moreover, photobleaching of Ge(1) centers took place under 266-nm wavelength excitation. In optical fibers no difference was observed between γ- and UV-induced loss spectra in the wavelength range from 480 nm to 1,900 nm. Partial reversibility of the photocoloration was observed. For comparison, the coloration effects were studied in glass prepared by means of modified chemical vapor deposition (MCVD) vapor-phase axial deposition (VAD), and plasma-activated chemical vapor deposition (PCVD) techniques.     

ηη′ threshold enhancement in p annihilations into πηη′ at rest

We report on the observation of an ηη′ threshold enhancement produced in p annihilations at rest into π⁰ηη′, which we identify with the recently discovered ƒ₀(1500).     

A measurement of the η radiative width Γη→γγ

The radiative width of the η meson has been measured at PETRA in photon-photon collisions. The resulting value is Γ_η→γγ = 0.53±0.04±0.04 keV.     

Raman spectroscopy and characterisation of α‐gallium oxyhydroxide and β‐gallium oxide nanorods

Raman spectroscopy complemented by infrared spectroscopy was used to characterise both gallium oxyhydroxide (α‐GaO(OH)) and gallium oxide (β‐Ga₂O₃) nanorods synthesised with and without the surfactants using a soft chemical methodology at low temperatures. Nano‐ to micro‐sized gallium oxyhydroxide and gallium oxide materials were characterised and analysed by both X‐ray diffraction and Raman spectroscopy. Rod‐like GaO(OH) crystals with average length of ∼2.5 µm and width of 1.5 µm were obtained. Upon thermally treating gallium oxyhydroxide GaO(OH) to 900 °C, β‐Ga₂O₃ was synthesised retaining the initial GaO(OH) morphology. Raman spectroscopy has been used to study the structure of nanorods of GaO(OH) and Ga₂O₃ crystals. Raman spectroscopy shows bands characteristic of GaO(OH) at 950 and ∼1000 cm⁻¹ attributed to Ga OH deformation modes. Bands at 261, 275, 433 and 522 cm⁻¹ are assigned to vibrational modes involving Ga OH units. Bands observed at 320, 346, 418 and 472 cm⁻¹ are assigned to the deformation modes of Ga₂O₆ octahedra. Two sharp infrared bands at 2948 and 2916 cm⁻¹ are attributed to the GaO(OH) symmetric stretching vibrations. Raman spectroscopy of Ga₂O₃ provides bands at 630, 656 and 767 cm⁻¹ which are assigned to the bending and stretching of GaO₄ units. Raman bands at 417 and 475 cm⁻¹ are attributed to the symmetric stretching modes of GaO₂ units. The Raman bands at 319 and 347 cm⁻¹ are assigned to the bending modes of GaO₂ units. Copyright © 2008 John Wiley & Sons, Ltd.     

Observation of χb production in the exclusive reaction ′→γγχb→γγ→γγ(eeorμμ−)

We report on the observation of 1 ³P_J (χ_b) production in the reaction ′→γχ_b→γγ→γγ(e⁺e⁻ or μ⁺μ⁻). The data were recorded with the nonmagnetic CUSB detector at the Cornell Electron Storage Ring, CESR. We observe 124 γγ events with either an electron or muon pair in the final state. In the γγ correlation plot about 40% of the events cluster around (120, 430) MeV.     

Density functional study of α–β phase transition of polyvinylidene difluoride

We perform density functional calculations to investigate structural and dynamical properties of crystalline polyvinylidene difluoride (PVDF) associated with the transition from α to β phase. We examine the change of the conformational energy and the corresponding structure of each phase depending on the lattice parameters of the orthorhombic crystalline structure. From this information, we construct the path that connects the point where the α phase is most stable to the point where the β phase is most stable, and identify the sub‐ region in the lattice parameter space where α and β phases have the same energy. In this sub‐region, we locate the point which gives the lowest conformation energy for both α and β phases, and examine the behaviour of the lowest energy profile and corresponding change of intermediate structures as the conformation of the PVDF chain transforms from α phase to β phase. Finally we perform ab‐initio molecular dynamics simulations and analyse the characteristic dynamics associated with transition from α to β phase. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Complexation of niflumic acid with native and hydroxypropylated α‐ and β‐cyclodextrins in aqueous solution

Interactions between niflumic acid and native and hydroxypropylated α‐ and β‐cyclodextrins (CDs) were investigated by ¹H NMR, UV‐vis spectroscopy, densimetry, and calorimetry at pH = 7.4 (phosphate buffer) and T = 298.15 K. Thermodynamic parameters of 1:1 complex formation were calculated and discussed in terms of influence of cavity size and availability of hydroxypropyl substituents on the complex stability. The ¹H NMR data indicated the inclusion of niflumic acid into macrocyclic cavity of all CDs under study. It was found that both phenyl and pyridine rings of niflumic acid molecule can be included in the cyclodextrin cavity. The co‐existence of two different kinds of 1:1 inclusion complexes in the solution was suggested. In spite of the fact that binding of niflumic acid with α‐cyclodextrin is more enthalpically favorable, stability of the inclusion complexes is very low due to the enthalpy–entropy compensation effect. Complex formation of β‐CDs with niflumic acid is characterized by the higher enthalpy and entropy changes caused by more intense dehydration. Introduction of hydroxypropyl groups in the cyclodextrin molecule was found to promote the binding with niflumic acid. Copyright © 2008 John Wiley & Sons, Ltd.