Partitioning of gadolinium and its induced phase separation in sodium-aluminoborosilicate glasses

Phase separation in sodium-aluminoborosilicate glasses was systematically studied as a function of Gd₂O₃ concentration with transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) methods. Gadolinium-induced phase separation in the glasses can be consistently explained by proposing that Gd cations partition to the borate-rich environments and subsequent agglomeration of the Gd-borate moieties, or short-range ordered structural groups, in the glass. Agglomeration of the Gd-borate rich environments is further discussed within the context of excess metal oxides, [Na₂O]_ex or [Al₂O₃]_ex=|Na₂O-Al₂O₃|, and excess B₂O₃, [B₂O₃]_ex, available for incorporating Gd cations. Results showed that agglomeration of the Gd-borate rich environments occurred at a much lower Gd₂O₃ concentration in the glass without [Na₂O]_ex or [Al₂O₃]_ex and at a significantly higher Gd₂O₃ concentration in the glass with either [Na₂O]_ex or [Al₂O₃]_ex. Assuming 1BO₄:1Gd:2BO₃ (based on literature-reported Gd-metaborate structure) as a local Gd-borate environment in glass, we introduced the saturation index of boron, SI_[B]=Gd₂O₃/(1/3[B₂O₃]_ex), to examine the glass susceptibility to Gd-induced phase separation for all three alkali-aluminoborosilicate systems. While our results have provided some insight to the glass structure, they also provide insight to the mechanism by which the metal oxide is dissolved into the melt. This appears to occur predominately through boron complexation of the metal oxide.     

The growth of carbon nanostructures on cobalt-doped carbon aerogels

By carbonizing cobalt-doped aerogel precursors directly at various temperatures, or by carbon monoxide decomposition of cobalt-doped carbon aerogels, different carbon nano-features such as carbon nano-filaments and graphitic nano-ribbons were grown on cobalt-doped carbon aerogel samples. Transmission electron spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction characterization results showed that metallic cobalt nano-particles form when heating the cobalt-doped aerogel samples over 500 °C. At low heating temperature, many highly oriented carbon thin films can be found on metallic cobalt nano-particles. When heating the samples at 850 °C, some carbon nano-filaments are obtained. While heating the samples at 1050 °C, many graphitic nano-ribbons are grown and the framework of the interconnected carbon particles of the sample is changed. Graphitic nano-ribbons can also be grown by CO decomposition of the cobalt-doped carbon aerogels. We can therefore control and modify the nanostructures of cobalt-doped carbon aerogels by heating them at different temperatures or by using CO decomposition.     

Characterisation of ALCVD Al2O3-ZrO2 nanolaminates, link between electrical and structural properties

Al₂O₃ and ZrO₂ mixtures for gate dielectrics have been investigated as replacements for silicon dioxide aiming to reduce the gate leakage current and reliability in future CMOS devices. Al₂O₃ and ZrO₂ films were deposited by atomic layer chemical vapor deposition (ALCVD) on HF dipped silicon wafers. The growth behavior has been characterized structurally and electrically. ALCVD growth of ZrO₂ on a hydrogen terminated silicon surface yields films with deteriorated electrical properties due to the uncontrolled formation of interfacial oxide while decent interfaces are obtained in the case of Al₂O₃. Another concern with respect to reliability aspects is the relatively low crystallization temperature of amorphous high-k materials deposited by ALCVD. In order to maintain the amorphous structure at high temperatures needed for dopant activation in the source drain regions of CMOS devices, binary Al/Zr compounds and laminated stacks of thin Al₂O₃ and ZrO₂ films were deposited. X-ray diffraction and transmission electron microscope analysis show that the crystallization temperature can be increased dramatically by using a mixed oxide approach. Electrical characterization shows orders of leakage current reduction at 1.1-1.7 nm of equivalent oxide thickness. The permittivity of the deposited films is determined by combining quantum mechanically corrected capacitance voltage measurements with structural analysis by transmission electron microscope, X-ray reflectivity, Rutherford backscattering, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectroscopy. The k-values are discussed with respect to formation of interfacial oxide and possible silicate formation.     

Structure of partially reduced xPbO (1 − x)SiO2 glasses: combined EXAFS and MD study

We have studied the structure of partially reduced lead-silicate glasses using combined EXAFS (extended X-ray absorption fine structure) and MD (molecular dynamics) methods. The analysis was performed for glasses of x[(1 − p)Pb pPbO] (1 − x)SiO₂ composition, x = 0.3, 0.5, 0.7, where parameter (1 − p) describes the degree of reduction, i.e. the content of the granular metallic phase, appearing as the result of the reduction process (e.g. annealing in hydrogen atmosphere). In the EXAFS experiment (1 − p) was expressed via the time of reduction realized at 400 °C (1.5 h, 24 h, 70 h), whereas in the MD simulations it was determined precisely by using proper numbers of particles (corresponding to (1 − p) = 0.0, 0.25, 0.5, 0.75 and 1.0). In the paper we describe in detail the local structure around lead atoms and its changes in the function of glass composition and reduction degree. The tendency for agglomeration of Pb⁰ into clusters, the formation of the granular metallic phase, and continuity of silica and lead oxide subnetworks are discussed. A good agreement between EXAFS-extractcd and MD-extracted parameters of the short-range structure encouraged us to preform a medium-range order analysis, based on the MD simulations only. Moreover, combining the EXAFS and MD methods we could correlate the reduction time (technological parameter) with the degree of reduction (1 − p) and the actual state of the granular structure. The latter relation may be useful for controlled production of reduced glasses of pre-requcstcd physical properties.     

Transport properties of copper-doped carbon aerogels

The magnetic susceptibility, conductivity, magnetoresistance (MR) and Hall effect of copper-doped carbon aerogels are measured and compared with corresponding results from the original carbon aerogels. The experimental results indicate that the temperature-dependent magnetic susceptibility of the copper-doped and of the original carbon aerogels is well fit by a Curie function at low temperatures. The copper-doped carbon aerogels show a higher susceptibility and spin concentration than the original carbon aerogel. After doping by copper, the materials exhibit a more linear current-voltage curve than the original carbon aerogel under the same measurement conditions. The electrical resistance of the copper-doped carbon aerogels is strikingly lower than that of the original carbon aerogels, and decreases with increasing copper content in the samples. The temperature-dependent resistivity ρ(T) of all of the copper-doped and original carbon aerogels can be fitted by an exp(T^−1/2) dependence for T<100 K. The copper-doped and pristine carbon aerogels follow a quadratic MR behavior Δρ/ρ=AB² in the magnetic field range B investigated (up to 5 T), except at very low temperatures (T<4 K).     

Mechanical properties of a TAS fiber: a preliminary study

The mechanical properties, including elastic moduli, hardness, fracture toughness and tensile strength of a glass fiber in the Te-As-Se system (TAS) were studied. The values for the hardness (1.4 GPa) and the fracture toughness (0.18 MPa √m) show that this glass is both soft and brittle in comparison to glasses from other systems. However, indentation measurements should be interpreted with caution due to an indentation creep phenomenon and to a delayed fracture process. In addition, the effect of treatments in air (relative humidity about 60%) at different temperatures below T_g were investigated. The main result of this study is that the studied TAS glass is sensitive to the presence of humidity, and aging treatments have a pronounced detrimental effect on the strength of the uncoated fibers.     

Theoretical studies on the solar cell parameters of n-C/p-Si heterojunction

Amorphous carbon (a-C) is a potential material for the development of low cost solar cells. The heterojunction n-C/p-Si solar cell has been recently developed by Krishna et al. It has been shown that the maximum quantum efficiency (25%) appears at wavelength λ (600 nm). In the present work, theoretical quantum efficiency has been calculated taking into account the contribution of hole photocurrent density, electron photocurrent density and the photocurrent within the depletion region. The variation of quantum efficiency with wavelength is found to be qualitatively similar to the experimentally observed variation. The solar cell parameters namely V_oc, I_sc, FF and efficiency have also been calculated and compared with the experimental values.     

High resolution patterning of silica aerogels

Three-dimensional metallic structures are fabricated with high spatial resolution in silica aerogels. In our method, silica hydrogels are prepared with a standard base-catalyzed route, and exchanged with an aqueous solution typically containing Ag⁺ ions (1 M) and 2-propanol (0.2 M). The metal ions are reduced photolytically with a table-top ultraviolet lamp, or radiolytically, with a focused X-ray beam. We fabricated dots and lines as small as 30 × 70 μm, protruding for several mm into the bulk of the materials. The hydrogels are eventually supercritically dried to yield aerogels, without any measurable change in the shape and spatial resolution of the lithographed structures. Transmission electron microscopy shows that illuminated regions are composed by Ag clusters with a size of several μm, separated by thin layers of silica.     

Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement

The crystal nucleation effect in lithium aluminosilicate glasses was investigated by the viscosity measurement with aid of the fiber elongation method. The abrupt increase of viscosity due to the crystallization of glass was observed in viscosity-temperature curve but the minimum viscosity temperature (T_η) related with crystallization showed a strong dependence on the nucleation state such as nucleation temperature, nucleation time and heating rate. The results by viscosity agreed well with those of DTA. The nucleation effect on the microstructure of glass-ceramics was also discussed. Finally, the nucleation effect on the crystallization kinetics was approached quantitatively by calculating the crystal volume from viscosity value.     

Effect of fluorspar and alumina on the viscous flow of calcium silicate melts containing MgO

For the investigation of the effect of fluorspar and alumina on the viscous flow and the crystallization behavior of the silicate melts, the viscosity of the CaO-SiO₂-10 wt%MgO-CaF₂ (or -Al₂O₃) system was measured. The addition of CaF₂ decreased the crystallization temperature (T_CR) of the CaO-SiO₂-MgO-CaF₂ system, while the T_CR of the CaO-SiO₂-MgO-Al₂O₃ (wt%CaO/wt%SiO₂=1.0) system exhibited a minimum value at 10 wt% Al₂O₃. The activation energy was decreased in the CSMF system and vice versa in the CSMA system by addition of CaF₂ or Al₂O₃ in each system. From the relationship between the activation energy and the composition of the glasses, the following equation could be obtained. , where the B^total is the ratio of (wt%CaO + wt%CaF₂ + wt%MgO)/(wt%SiO₂ + wt%Al₂O₃). This means that CaO, CaF₂, and MgO would behave as a network modifier, while SiO₂ and Al₂O₃ behave as a network former in the glasses investigated.     

Rotational probe relaxation and scaling in fragile glass formers

The rotational dynamics of a stiff paramagnetic tracer dissolved in supercooled SALOL is investigated via electron spin resonance spectroscopy. The study shows that the molecular rotation follows different dynamical regimes as the temperature is lowered. In particular, on cooling through the critical temperature T_C of the SALOL, the coupling between rotational relaxation and viscosity weakens and enhanced rotational diffusion is observed. In this temperature interval, the relationship between rotational correlation times and viscosity is fairly well described by a power law τ∝η^ξ (Fractional Debye-Stokes-Einstein law). Activated reorientation is observed in the temperature region around the glass transition of the SALOL. The rotational dynamics of the tracer dissolved in SALOL are compared with its rotation in ortotherphenyl (OTP) investigated in previous studies, and a scaling procedure is proposed.     

Local structure of LuBO3 prepared via sol-gel process and its transformation with temperature: B-NMR and L-edge XAS studies

The present work deals with the temperature effect on the stability range of LuBO₃ prepared by sol-gel synthesis. Structural modifications from vaterite to calcite form versus the thermal treatments have been measured by X-ray diffraction and infrared spectroscopy. Several samples derived from the xerogel annealed at 400, 800 and 1200 °C were studied by ¹¹B solid state NMR and at the lutetium L_I-L_III edges by X-ray absorption spectroscopy to determine the local atomic arrangement around the rare earth and the boron ions. The predominance of BO₄ groups and the eightfold coordination of lutetium atoms in the LuBO₃ xerogel in samples treated up to 800 °C indicate an evolution of the xerogel to vaterite form after thermal treatment instead of the expected calcite form as established for samples prepared by solid state reaction.     

An investigation of the local iron environment in iron phosphate glasses having different Fe(II) concentrations

The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe₂O₃-60P₂O₅ (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mössbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mössbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron-oxygen coordination is in the 4-5 range indicating that iron ions have mixed tetrahedral-octahedral coordination.     

Bismuth germanate and bismuth silicate glasses in cryogenic detectors

We present and discuss the results of measurements of conductivity and secondary electron emission coefficient of Bi_xGe_1−xO_2−0.5x (where x=0.13, 0.23, 0.33, 0.47) and Bi_xSi_1−xO_2−0.5x (where x=0.47, 0.57, 0.67) glasses reduced in hydrogen. The surface conductivity of reduced glasses strongly depends on reduction temperature, reduction time and bismuth content. The temperature dependence of the surface conductivity in a high temperature regime is well described by σ∼exp[−(T₀/T)ⁿ] law where n=1/4. The secondary electron emission coefficient of reduced glasses is practically independent of the degree of reduction but increases when binary glasses are modified by alkali ions. The research results confirmed that bismuth silicate and bismuth germanate glasses modified by alkali ions may be good materials for detectors working in cryogenic temperature.     

Inclusion of carbon nanotubes in a TiO2 sol-gel matrix

We report here the successful inclusion of carbon nanotubes (CNs) into a TiO₂ matrix prepared by a sol-gel method. The presence of CNs in the sol-gel matrix and the structure of the film were analyzed principally by transmission electron microscopy. Complementary information about the behavior of embedded carbon nanotubes versus heat treatment and ion irradiation were obtained by X-ray photoelectron spectroscopy. The elaboration of an inorganic matrix containing embedded carbon nanotubes leads to a new nanocomposite. The possible applications of this nanocomposite are discussed.     

X-ray absorption and Raman characterizations of TeO2-Ga2O3 glasses

The thermodynamic properties of transparent glasses prepared in the TeO₂-Ga₂O₃ system were investigated by differential scanning calorimetry. The change of the thermal parameters as a function of the chemical composition is discussed. Raman and both Te L_III and Ga K edge X-ray absorption spectroscopies at room temperature were used to examine the short range order. Analyses of the spectra suggest that the addition of Ga₂O₃ content to the TeO₂ glass matrix induces the transformation of trigonal bipyramids (TeO₄E, E=lone electronic pair 5s² of Te) to trigonal pyramids (TeO₃E) with formation of Te-O-Ga bridging bonds. Furthermore, Ga K edge XANES and EXAFS studies show that Ga atoms exhibit both tetrahedral (GaO₄) and octahedral (GaO₆) environments.     

Formation and characterization of tin oxide aerogel derived from sol-gel process based on Tetra(n-butoxy)tin(IV)

Transparent and translucent SnO₂ aerogels with high specific surface area (>300 m²/g) have been prepared by sol-gel process using tetra(n-butoxy)tin(IV) as a starting compound, and supercritical drying technique for solvent extraction. Light scattering measurements reveal that the polymeric cluster size distribution in sol system is gradually broadened during sol-gel transition. SEM images show that the aerogels are made up of the cottonlike oxide agglomerates with a large number of pores. TEM images show that these aerogels seem to be self-similar at different magnifications. Their pore size distribution is pretty wide ranging from mesopore to macropore especially for that of translucent aerogel.     

Electrical switching in As-Te-Ag system-composition and temperature dependence

The current-voltage characteristics related to switching phenomena in silver doped arsenic telluride glasses, As₂₀Te_80−xAg_x and As₄₀Te_60−xAg_x, have been investigated over a wide composition range (4?x?14). The samples are found to show threshold switching behavior with the number of switching cycles withstood by the samples depending on the ON state current. The switching voltages are found to decrease with increase in silver content and a sharp minimum is seen at the composition x=12 for the As₂₀Te_80−xAg_x glasses and x=11 for the As₄₀Te_60−xAg_x glasses. An effort has been made to understand the observed composition dependence on the basis of increase in the conductance of the samples with silver addition and local structural effects.     

Nanoindentation investigation of amorphous hydrogenated carbon thin films deposited by ECR-MPCVD

Nanomechanical properties of amorphous hydrogenated carbon thin films are performed by nanoindentation technique. The amorphous hydrogenated carbon films are produced on silicon substrate by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD). The effect of negative bias voltage on amorphous hydrogenated carbon films is examined by Raman spectroscopy and the results showed that the intensity ratio of D-peak to G-peak (I_D/I_G) of amorphous hydrogenated carbon films at various bias voltages, increased as the bias voltage increased. The results also showed that Young’s modulus and hardness also increased as the bias voltage increased. In addition, Young’s modulus and hardness both decreased as the indentation depth increased.