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Showing papers in "Journal of Non-crystalline Solids in 1998"


Journal ArticleDOI
TL;DR: In this paper, the spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed.
Abstract: The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E′-centers are discussed. The effects of glassy disorder have a profound effect on defect properties and interconversion processes in silica.

1,157 citations


Journal ArticleDOI
TL;DR: In this paper, the synthesis, structure-property relationships, and performance of carbon aerogel electrodes used in electrochemical applications are examined for double layer capacitors, pseudocapacitors, and capacitive deionization units.
Abstract: A major advantage of highly crosslinked, organic aerogels is the ability to transform many of these materials into electrically conductive carbon aerogels. Carbon aerogels have been formed as monoliths, microspheres, irregularly-shaped powders, and thin film composites. In all cases, the carbon aerogels retain their high surface area (400–800 m2/g) and ultrafine cell/pore size (<100 nm). Carbon aerogels are being examined as electrodes for double layer capacitors, pseudocapacitors, and capacitive deionization units. This paper examines the synthesis, structure–property relationships, and performance of carbon aerogel electrodes used in electrochemical applications.

523 citations


Journal ArticleDOI
TL;DR: In this paper, the relative intensity of the IR bands responsible for the different phosphate units was analyzed to determine the relative spectrum intensity of different units, and it was shown that the substitution of P2O5 by Na2O influences the intermediate range order of the glass matrix.
Abstract: Infrared absorption spectra of sodium phosphate glasses, Na2O=30–50 mol%, have been studied in the region of 400–2000 cm−1. Spectra were analyzed to determine the relative intensity of the IR bands responsible for the different phosphate units. Spectral analyses has shown that the substitution of P2O5 by Na2O influences the intermediate range order of the glass matrix. Increasing Na2O content leads to a gradual degradation of the phosphate ring type structures, with consequent depolymerization causing compaction of the glass matrix. Analyses of IR spectra illustrates the reorganization effects of Na2O in the phosphate matrix.

419 citations


Journal ArticleDOI
TL;DR: Transparent glass-ceramics were made by casting crucible melted glass onto a steel plate and subsequently heating the resultant glass patty above the glass transition temperature to nucleate and grow LaF3 crystals as mentioned in this paper.
Abstract: New transparent oxyfluoride glass-ceramics can provide a low phonon energy fluoride environment for active rare earth ions while maintaining the formability and physical properties of an oxide glass. Transparent glass-ceramics were made by casting crucible melted glass onto a steel plate and subsequently heating the resultant glass patty above the glass transition temperature to nucleate and grow LaF3 crystals. Eu3+ doped glasses emit only red luminescence from the 5D0 level but after heat treatment emit blue, green, and red luminescence indicative of a low phonon energy rare earth environment. TEM micrographs and XRD show no features in the base glass, but do show extensive LaF3 crystallites after heat treatment that are responsible for the novel optical properties of these hybrid materials.

386 citations


Journal ArticleDOI
TL;DR: In this paper, a commercially attractive ambient pressure production process was developed which will allow broader commercialization of silica aerogel products for insulation applications and their preparation is described, and sound insulation properties including a remarkable absorption in the difficult low frequency range was found.
Abstract: Aerogels, nanoporous lightweight materials, were discovered more than 60 years ago. The supercritical manufacturing process and expensive raw materials typically used to produce aerogels prohibited commercialization on an industrial scale. Recently, a commercially attractive ambient pressure production process was developed which will allow broader commercialization of silica aerogel products. Some aerogel products for insulation applications and their preparation is described. Sound insulation properties including a remarkable absorption in the difficult low frequency range was found. In the case of insulation plates performance depends on binder and can be positively influenced by choosing the right binding material.

369 citations


Journal ArticleDOI
TL;DR: In this paper, the D peak in the Raman spectrum of polycrystalline graphite was studied as a function of excitation energy in the range from 1.16 to 4.3 eV.
Abstract: We have studied the D peak in the Raman spectrum of polycrystalline graphite as a function of excitation energy in the range from 1.16 to 4.3 eV. The D peak disperses with excitation energy (≈50 cm−1/eV) and its intensity decreases with increasing laser energy. The dispersion of the D peak is explained in terms of a novel k → -selective resonant Raman scattering process. The scattering cross-section is resonantly enhanced for phonons on one branch in the phonon dispersion curves which have the same k → -vector as the electronic transition responsible for the resonance.

347 citations


Journal ArticleDOI
TL;DR: In this article, a simulated high level waste, whose major components were 54.6 wt% Na2O, 14.9 wt%, P2O5 and 8.3 wt%), was successfully vitrified into iron phosphate wasteforms whose chemical durability was equivalent to that of borosilicate glass wasteforms.
Abstract: Up to 40 wt% of a simulated high level waste, whose major components were 54.6 wt% Na2O, 14.9 wt% P2O5 and 8.3 wt% Fe2O3, was successfully vitrified into iron phosphate wasteforms whose chemical durability was equivalent to that of borosilicate glass wasteforms. Because of their high fluidity, the iron phosphate wasteforms could be melted in as little as 30 min at temperatures between 1015°C and 1200°C. The addition of 3–7 wt% CaF2 to the batch decreased the melting time and temperature, by as much as 100°C, and improved the chemical durability, especially for crystallized iron phosphate wasteforms. Iron phosphate wasteforms are concluded to be a practical alternative for vitrifying those nuclear wastes not well suited for borosilicate glasses.

324 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of thermal and chemical activation of carbon aerogels was investigated as a means to increase accessible surface and double-layer capacities, and accessibility of pores and aerogel performance as electrodes in supercapacitors was electrochemically investigated.
Abstract: Applications of carbon aerogels as electrodes in supercapacitors require high surface area and simple production method. Carbon aerogels are derived via the pyrolysis of resorcinol-formaldehyde (RF) aerogels. In order to simplify production, subcritical drying of the RF gels is favoured, which is possible for gels prepared with high molar resorcinol to catalyst (R/C) ratios, i.e. very low catalyst concentrations. Resulting carbon aerogels consist of big microporous particles. The micropore surface area can account for more than 90% of the total surface area. The effect of thermal and chemical activation of such aerogels was investigated as a means to increase accessible surface and double-layer capacities. Aerogel structure was studied by small angle X-ray scattering (SAXS) and nitrogen – and CO2 – sorption. Accessibility of pores and aerogel performance as electrodes in supercapacitors was electrochemically investigated.

306 citations


Journal ArticleDOI
TL;DR: In this paper, anhydrous binary phosphate glasses containing from 0 to 50 mol% Li2O or Na2O have been prepared and examined by Raman scattering spectroscopy, and a new feature appears in the Raman spectra near 1160 cm−1 indicating the formation of Q2 phosphate tetrahedra with two bridging and two non-bridging oxygens.
Abstract: Anhydrous binary phosphate glasses containing from 0 to 50 mol% Li2O or Na2O have been prepared and examined by Raman scattering spectroscopy. The unpolarized Raman spectrum of vitreous P2O5 has intense bands near 640 cm−1, attributed to the symmetric stretching mode of POP bridging oxygens, (POP)sym, between Q3 phosphate tetrahedra, and at 1390 cm−1 due to the symmetric stretch of the PO terminal oxygens, (PO)sym. With the addition of alkali oxide to P2O5, a new feature appears in the Raman spectra near 1160 cm−1 indicating the formation of Q2 phosphate tetrahedra with two bridging and two non-bridging oxygens. The increase in relative amplitude of this new (PO2)sym band with increasing modifier content is consistent with a simple depolymerization of the phosphate network. From 20 to 50 mol% alkali oxide, the position of the (PO)sym Raman band decreases by ∼ 130 cm−1 whereas the frequency of the (POP)sym band increases by ∼ 60 cm−1. These frequency shifts are the result of π-bond delocalization on Q3 species that effectively lengthens the PO terminal oxygen bond and strengthens the POP linkages with increasing alkali oxide content. The compositional dependence of the π-bond delocalization on Q3 tetrahedra is described by considering the interconnections between neighboring Q3 and Q2 tetrahedra. The onset of π-bond delocalization on Q3 species corresponds with the anomalous Tg minimum at 20 mol% alkali oxide in alkali ultraphosphate glasses. The increase in Tg between 20 and 50 mol% alkali oxide is attributed to the increased ionic interconnection of what becomes a chain-like phosphate network at higher alkali contents. Finally, the Raman spectra of several alkali ultraphosphate glasses show high frequency shoulders on the Raman bands attributed to the (PO2)sym and (PO2)asym vibrational modes. These shoulders represent the presence of strained structural units, possibly three- or four-membered rings.

287 citations


Journal ArticleDOI
TL;DR: In this paper, a new cost-effective process was developed to avoid the costs of supercritical drying and expensive raw materials like tetraethoxysilane which prohibit a commercialization.
Abstract: The usual production of silica aerogels include supercritical drying and expensive raw materials like tetraethoxysilane which prohibit a commercialization. To avoid the costs of supercritical drying one can modify the internal surface with a silation agent. Hydrophobization of the lyogel makes it possible to dry at ambient pressure. The next step is to use waterglass as a cheaper silica source. This requires at least one solvent exchange from water for the surface modification. The diffusion time for solvent exchange and recycling of the water/solvent solutions by distillation cause considerable costs. A new cost-effective process has been developed to avoid these disadvantages. Silation is now carried out directly in the water phase of the hydrogel, which results in solvent exchange as well as phase separation of gelwater and solvent.

254 citations


Journal ArticleDOI
K. Hirao1, K Miura
TL;DR: In this article, the effects of 810 nm, femtosecond laser radiation on various glasses were investigated by focusing the laser beam via a microscope objective, transparent but visible, round-elliptical damage lines were successfully written inside high silica, borate, soda-lime-silicate, fluoride and chalcogenide glasses.
Abstract: With the goal of creating various optical glass devices for the telecommunications industry, the effects of 810 nm, femtosecond laser radiation on various glasses were investigated. By focusing the laser beam via a microscope objective, transparent but visible, round-elliptical damage lines were successfully written inside high silica, borate, soda-lime-silicate, fluoride and chalcogenide glasses. Microscopic ellipsometric measurements of the damaged region in pure and Ge-doped silica glasses showed refractive index increases of 0.01 to 0.035. The formation of several types of defects, including Si E′ or Ge E′ centers, non-bridging oxygen hole centers, and peroxy radicals, was also detected in addition to the identification. These results suggest that multi-photon interactions occurs in the glasses and that it is possible to write three-dimensional optical circuits in bulk glasses via such a focused laser beam technique.

Journal ArticleDOI
TL;DR: In this paper, the mechanical behavior of silica aerogels was investigated with hardness, compression, tension and shear tests, with particular attention paid to the effects of processing parameters, including fiber-reinforcement.
Abstract: Aerogels are extremely low density solids that are characterized by a high porosity and pore sizes in the order of nanometers The mechanical behavior of silica aerogel was investigated with hardness, compression, tension and shear tests The influences of testing conditions, storage environment and age were examined, with particular attention paid to the effects of processing parameters, including fiber-reinforcement Good correlation was found between hardness and compressive strength over a wide range of processing parameters Increasing fiber reinforcement generally retarded shrinkage during fabrication and yielded smaller matrix densities for a given target density For a given fiber content, hardness, compressive strength and elastic moduli increased and strain at fracture decreased with increasing matrix density In the lower ranges of matrix density, fiber reinforcement increased strain at fracture and elastic moduli The mechanical response was also sensitive to environment and storage history With age, the compressive strength and elastic moduli increased while the strain at fracture decreased Tension and shear results indicate that shear strength of aerogels exceeds tensile strength which is consistent with brittle materials response

Journal ArticleDOI
TL;DR: In this paper, an interferometric method was used to determine the refractive index of silica glass in the infrared wavelength range by means of IR spectroscopy, and the wavelength-dependent refractive indices were measured for wavelengths ranging from 3 to 6.7 μm.
Abstract: An interferometric method was used to determine the refractive index of silica glass in the infrared wavelength range by means of IR spectroscopy. The wavelength-dependent refractive indices were measured for wavelengths ranging from 3 to 6.7 μm. The refractive index in the investigated wavelength region can be well described with a three-term Sellmeier equation.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the non-exponential relaxation results from a superposition of dynamically distinguishable entities, and that the intrinsic response is compatible with single exponential relaxation.
Abstract: Several experimental methods feature the potential to distinguish between slow and fast contributions to the non-exponential, ensemble averaged primary response in glass-forming materials. Some of these techniques are based on the selection of subensembles using multi-dimensional nuclear magnetic resonance, optical bleaching, and non-resonant spectral hole burning. Others, such as the time-dependent solvation spectroscopy, measure microscopic responses induced by local perturbations. Using several of these methods it could be demonstrated for various glass-forming materials that the non-exponential relaxation results from a superposition of dynamically distinguishable entities. The experimental observation that subensembles can be selected efficiently indicates a large degree of heterogeneity. The intrinsic response is compatible with single exponential relaxation.

Journal ArticleDOI
TL;DR: In this paper, the electron donation from the 2p orbitals of the oxygen ions, as well as π-electron donation from resonating PÀ-bond were determined from the absorption spectra by the Judd-Ofelt treatment.
Abstract: For fluoride phosphate and phosphate glasses, Judd–Ofelt Ωt parameters, spontaneous emission probabilities, radiative lifetimes and branching ratios of several Er3+ transitions were determined from the absorption spectra by the Judd–Ofelt treatment. The compositional changes of Ω2 and Ω6 are attributed to changes in the bonding between Er3+ and surrounding ligand groups. The phosphate groups play a special role. They show electron donation from the 2p orbitals of the oxygen ions, as well π-electron donation from the resonating P–O π-bond. Larger electron donation from the ligand anions leads to larger Ω2, but smaller Ω6. By contrast, larger π-electron donation from the phosphate groups increases Ω6. The changes of Ω2 with increasing phosphate content and with variation of modifier ions are ascribed to changes in the content and polarizability of the oxygen ions, leading to changes in the electron donation from the ligands at the rare earth (RE) sites. In the case of Pb2+, changes in the degree of asymmetry at the RE sites could also be important. The smaller Ω6 parameter of phosphate glasses compared with fluoride phosphate glasses is caused by larger σ-electron donation due to larger oxygen content and by smaller π-electron donation due to smaller delocalization of the π-bonds. The changes of Ω6 with phosphate content, in the case of fluoride phosphate glasses and with variation of modifier ions, are dominated by changes in the π-electron donation.

Journal ArticleDOI
TL;DR: In this paper, the transient behavior of the second-harmonic generation signal and the poling current for type-II fused silica samples under a variety of poling histories are reported.
Abstract: Measurements of the transient behavior of both the second-harmonic generation signal and the poling current for type-II fused silica samples under a variety of poling histories are reported. The applied voltage was switched between +5 kV, 0 V, and −5 kV with the sample maintained at 275°C. Observations include: multiple time scales (seconds to minutes) for development of the non-linearity depending on the poling history; a transient second-harmonic signal on the new cathode side of the sample following voltage reversal; and hysteretic incubation intervals before growth of the non-linearity. These observations are incompatible with the usual single mobile ion (e.g. Na+) model for establishing the strong local electric field that leads to the non-linearity. An expanded model including ion-exchange between a high mobility ion (as Na+) and a much lower mobility ion (related to H+) provides a good qualitative fit to the experiments.

Journal ArticleDOI
TL;DR: In this paper, high-resolution 11B, 29Si and 27Al nuclear magnetic resonance spectroscopy was used to quantitatively determine temperature dependent structural changes in Na-borate, Na-borsilicate and Li-boroaluminate liquids.
Abstract: High-resolution 11B, 29Si and 27Al nuclear magnetic resonance spectroscopy is used to quantitatively determine temperature dependent structural changes in Na-borate, Na-borosilicate and Li-boroaluminate liquids. Such changes are found to involve transformation of (a) BO3 units from boroxyl ring to non-ring configuration in the borate and of (b) BO4 units into asymmetric BO3 units in the borosilicate liquid with increasing temperature. No temperature dependent speciation of the silicate units was observed in the borosilicate liquid. In boroaluminate liquids, only the peraluminous composition shows transformation of higher-coordinated AlV and AlVI species into AlIV with increasing temperature. No temperature dependent borate speciation is detectable in these compositions. It is shown that although such structural changes may be the principal source of configurational entropy in the lower-alkali borate liquids, other sources must be sought for the higher-alkali borate, borosilicate and boroaluminate liquids.

Journal ArticleDOI
TL;DR: In this article, the thermal performance of granule compacts, at ambient pressure and vacuum, is similar to that of aerogel monoliths without the cost and processing problems of monolithics.
Abstract: Commercial development of aerogel granules, produced via ambient pressure processing, for advanced thermal insulation is discussed. By employing modeling and experimental verification, the thermal performance of granule compacts was studied. The application of a novel approach for surface modifying carbon black to the problem of adding opacifiers to silica aerogels is shown and improved thermal and mechanical properties were obtained. When opacified granules are produced with the correct density/strength, the application of a 1 bar load (as is naturally applied in vacuum panels) results in deformation/rearrangement such that a packing fraction approaching one is achieved. The thermal performance of these granule compacts, at ambient pressure and vacuum, is similar to that of aerogel monoliths without the cost and processing problems of monoliths.

Journal ArticleDOI
TL;DR: In this paper, the effect of changing concentrations of Er 3+ ions (0.1-1.5 ǫ)-sensitizers and sensitizers Cr 3+ ion and Yb 3 + ion (2.35-2.3 ) on laser performance were investigated.
Abstract: Phosphate glass samples with various Cr 2 O 3 , Yb 2 O 3 , and Er 2 O 3 contents based upon 67P 2 O 5 ·14Al 2 O 3 ·14Li 2 O·1K 2 O·4(Yb 2 O 3 + Er 2 O 3 ) were prepared. The effect of changing concentrations of Er 3+ ions (0.1–1.5 × 10 19 ions cm −3 ) and sensitizers Cr 3+ ion and Yb 3+ ion (2–16 × 10 18 ions cm −3 and 1.35–2.3 × 10 21 ions cm −3 respectively) on laser performance were investigated. The thermal shock resistance of this glass was doubled after an ion-exchange chemical strengthening process in a KNO 3 /NaNO 3 molten salt bath. Lasers with repetition rates of 20 Hz at free-running and 15 Hz at Q-switched single mode were demonstrated by utilizing chemically strengthened laser glass rods.

Journal ArticleDOI
TL;DR: In this article, non-linear refractive indices and nonlinear absorptions of chalcogenide glasses have been measured by the z-scan method at 1.06 μm.
Abstract: Non-linear refractive indices and non-linear absorptions of several chalcogenide glasses have been measured by the z-scan method at 1.06 μm, to evaluate their potential use in telecommunication as all optical ultra-fast switching devices. The index obtained for the reference glass As 2 S 3 (2.5 × 10 −18 m 2 /W) agrees with that found in the literature. Glasses in Ge–Se and Ge–Se–As systems have been measured. Non-linear refractive indices four times larger (10.2 × 10 −18 m 2 /W) than that of As 2 S 3 have been obtained for a Ge–Se–As glass.

Journal ArticleDOI
TL;DR: In this article, the relationship between structural and porous features and the mechanical properties of aerogels is discussed in terms of percolation theory and the fractal approach and it is shown that the exponent of the power law does not depend on fractal features.
Abstract: The density and the structure (fractal and non-fractal) of aerogels are modified either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process or by an isostatic pressure deformation. These aerogels have porosities ranging from 98% to 0%. The mechanical properties of the different aerogels (elastic modulus and strength) measured by 3 point bending, are dependent on their structure; they vary by five orders of magnitude as a function of density and follow power law evolution. However for the same relative density the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. These structural differences have been observed by SAXS experiments. The effects of the sintering process compared to that of the plastic transformation on the mechanical properties are explained by the associated structural changes. Sintering increases the network connectivity and the densification by compression leads to a new spatial arrangement of the clusters but their internal structure is not affected. In addition, relationships between structural and porous features and the mechanical properties are discussed in terms of percolation theory and the fractal approach. We show that the exponent of the power law does not depend on the fractal feature and percolation is only an approximation which cannot describe results.

Journal ArticleDOI
TL;DR: The Raman spectra of GeS2-Ga2-S3 glasses contain two bands at 265 and 385 cm−1 which increased in intensity with the addition of Ga2S3.
Abstract: In comparison to other chalcogenide glasses, Ge–Ga–S glasses have the largest rare-earth solubility. Raman spectra of GeS2–Ga2S3 glasses contain two bands at 265 and 385 cm−1 which increased in intensity with the addition of Ga2S3. These are associated with the vibration of Ge–Ge bonds and edge-shared GaS4 tetrahedra, respectively. Upon the addition of La2S3 into GeS2–Ga2S3 glasses, intensities of these two bands decreased while a new band appeared at 375 cm−1 due to the vibration of tetrahedra with non-bridging sulfur(s). Dissociation of Ge–Ge bonds and edge-shared GaS4 tetrahedra through the addition of La2S3 are responsible for the non-bridging sulfur formation. La3+ can be dissolved into the glass network as charge compensators for these non-bridging sulfurs. Since no such structural unit is present in glasses without Ga2S3, this connection scheme and the structural modification in Ga-containing chalcogenide glasses seem to be crucial for an increase of La3+ solubility.

Journal ArticleDOI
TL;DR: In this article, variable coherence microscopy shows that amorphous germanium and silicon contain medium-range order, which can be explained by a fine-grained paracrystallite material in which intergranular stress has been relaxed by deformation.
Abstract: Variable coherence microscopy shows that as-deposited amorphous germanium and silicon contain medium-range order. The ordering can be explained by a fine-grained paracrystallite material in which intergranular stress has been relaxed by deformation. On annealing, the paracrystallite structure transforms towards the lower-energy continuous random network (CRN). We present data on a variety of vacuum-evaporated samples, and on molecular dynamics simulations of candidate structures.

Journal ArticleDOI
TL;DR: In this article, the Fourier transform Raman spectra with laser excitation in the near infrared were recorded during the base-catalyzed sol-gel reaction of the system 3-glycidoxypropyl-trimethoxysilane (GPTS)/3-aminopropyl-triethoxylynne (APTS).
Abstract: Fourier transform Raman spectra with laser excitation in the near infrared were recorded during the base-catalyzed sol–gel reaction of the system 3-glycidoxypropyl-trimethoxysilane (GPTS)/3-aminopropyl-triethoxysilane (APTS). This combination of organo-(alkoxy)silanes is used for the synthesis of hybrid polymers. Raman bands of the trimethoxysilyl- and the epoxy-moieties were used to monitor the kinetic behaviour of the hydrolysis reaction and the ring cleavage as a function of temperature. A decrease of the rate of hydrolysis of GPTS at elevated temperatures is attributed to a decrease of the pH caused by temporarily accumulating silanol groups. After the start of the condensation reactions, the pH increases and forces hydrolysis to accelerate. The results indicate the existence of a pentacoordinated transition state with a negative charge at the silicon atom during base catalyzed hydrolysis.

Journal ArticleDOI
TL;DR: In this paper, the authors review the classical phenomenological models for non-Arrhenius behavior of glass-forming liquids and discuss a recently proposed model according to which the activation energy of the average relaxation time is determined by the work done in shoving aside the surrounding liquid to create space needed for a flow event.
Abstract: A major mystery of glass-forming liquids is the non-Arrhenius temperature-dependence of the average relaxation time. This paper briefly reviews the classical phenomenological models for non-Arrhenius behavior – the free volume model and the entropy model – and critiques against these models. We then discuss a recently proposed model according to which the activation energy of the average relaxation time is determined by the work done in shoving aside the surrounding liquid to create space needed for a “flow event”. In this model, which is based on the fact that intermolecular interactions are anharmonic, the non-Arrhenius temperature-dependence of the average relaxation time is a consequence of the fact that the instantaneous shear modulus increases upon cooling.

Journal ArticleDOI
TL;DR: In this paper, the authors performed dielectric spectroscopy on supercooled glycerol for temperatures between 184 and 413 K. The measurements cover 18 decades of frequency and extend into the far infrared regime.
Abstract: Broadband dielectric spectroscopy has been performed on supercooled glycerol for temperatures between 184 and 413 K. The measurements cover 18 decades of frequency and extend into the far infrared regime. The timescale of the α-process is observed over 14 decades of frequency as a function of temperature. In the THz range an additional loss peak was observed which is identified as the boson peak known from neutron and light scattering experiments. Between the α and boson peak the loss minimum has been observed as a function of temperature down to the glass temperature. It is very broad and cannot be ascribed to a simple superposition of α and boson peak. The data are compared to the susceptibilities obtained from neutron and light scattering.

Journal ArticleDOI
TL;DR: In this paper, lead free sodium-copper-phosphate glasses in the range (mol%) 30-80P2O5, 0-70Na2O and 0 -70CuO were investigated.
Abstract: Lead free sodium-copper-phosphate glasses in the range (mol%) 30-80P2O5, 0-70Na2O and 0-70CuO were investigated. The glass formation region is P2O5≥40, CuO≤50 and Na2O≤60 mol%. Glasses with a glass transition temperature (Tg) below 420°C, a thermal expansion coefficient from 258 × 10−7 to 99 × 10−7/°C and a dissolution rate in 30°C deionized water at the order of 10−7 g/cm2 min can be prepared. Substitution of CuO for Na2O increases Tg, the softening temperature (Td), hardness and chemical durability and decreases the thermal expansion coefficient. The structure of 50P2O5-(50 - x)Na2O-xCuO glass was examined by XPS and FTIR spectroscopies, which reveal the formation of POCu bonds in these glasses. The formation of POCu bonds, which replace PωO− ⋯ Na+ bonds while keeping the same fraction of POP bonds, increases the crosslink density in the glass network and, therefore, enhances the chemical durability and increases Tg, Td and the hardness of the glasses.

Journal ArticleDOI
TL;DR: In this paper, Nicolaon and Teichner reported that in a basic medium, the aerogels so obtained were totally transparent even through a thickness of 1 cm, and this remark attracted the attention of scientists working in high energy physics with charged particles and looking for the construction of a new type of a solid-state Cerenkov counter.
Abstract: This paper begins with the story of monolithic, transparent silica aerogels when in 1968 a paper was published by Nicolaon and Teichner where at the end of it two lines quoted that “⋯in a basic medium, the aerogels so obtained were totally transparent even through a thickness of 1 cm”. This remark attracted the attention of scientists working in high energy physics with charged particles and looking for the construction of a new type of a solid-state Cerenkov counter. Due to their very high thermal insulation properties and their optical transparency in the visible region, they soon were proposed for double pane windows. Transparent silica is also interesting in the production of hollow spheres for inertial confinement fusion as well as for daylight thermal insulation applications. Parameters influencing optical transparency both at the sol–gel stage (monomer precursors and partially polymerized precursors of silica, catalysts, solvents) and at the drying step: supercritical drying at high or low temperature, were described.

Journal ArticleDOI
Abstract: We describe the spectroscopic properties of the trivalent lanthanide ions Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+ and Yb3+ in the fluorophosphate glasses 75NaPO3–24CaF2–1LnF3 and 75NaPO3–20CaF2–5LnF3 (where Ln=lanthanide ion) Absorption and luminescence spectra are discussed The dipole strengths of the transitions in the absorption spectrum are parametrized in terms of three Ωλ (λ=2, 4 and 6) Judd–Ofelt intensity parameters These phenomenological parameters are used to predict the luminescence properties of the lanthanide ions in the fluorophosphate glasses Attention is paid to the trend of the intensity parameters over the lanthanide series

Journal ArticleDOI
TL;DR: In this article, the influence of different catalysts on the crystalline phases obtained by thermal annealing is studied, including phonon confinement and pressure effects, and the analysis of the Raman spectra after thermal treatments in air and N 2, indicates the importance of the non-stoichiometry due to oxygen vacancies.
Abstract: TiO 2 polycrystalline powders have been prepared by different sol–gel routes and have been investigated by Raman spectroscopy and transmission electron microscopy. The influence of different catalysts on the crystalline phases obtained by thermal annealing is studied. The mechanisms that determine the Raman line shapes are discussed, including phonon confinement and pressure effects. The analysis of the Raman spectra, after thermal treatments in air and N 2 , indicates the importance of the non-stoichiometry due to oxygen vacancies.