Showing papers in "Journal of Non-crystalline Solids in 2010"

Influence of magnesia on the structure and properties of bioactive glasses

Abstract: Five bioactive glasses based on 49.5SiO2–1.1P2O5–(23.0(1 − x))CaO–xMgO–26.4Na2O mol.% were synthesised where CaO was replaced progressively on a molar basis by MgO (where 0 ⩽ x ⩽ 1). The glasses were characterised by 31P and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, dilatometry, differential thermal analysis (DTA) and density measurements with the aim of gaining a comprehensive understanding of the structure–property relationships. The 31P NMR spectra of the glasses exhibited well-defined resonances at ca. 9–14 ppm, corresponding to a Q0 orthophosphate environment. Full substitution of MgO caused a downfield shift of ca. 5 ppm, implying the preferential association of sodium with the orthophosphate species. The 29Si NMR spectra exhibited a peak at ca. −79 ppm, indicating a Q2 silicon species, along with a shoulder at ca. −90 ppm, corresponding to a Q3 species. On addition of MgO, the Q3 shoulder was seen to progressively increase in magnitude. These results suggest that MgO, rather than depolymerising the silicate network by acting as a network modifier, is behaving in part, as an intermediate oxide. Deconvolution of the 29Si NMR peaks suggest that, although 86% of the magnesium oxide is acting traditionally as a network modifying cation, up to 14% of the magnesium oxide is entering the silicate network as tetrahedral, MgO4, removing network modifying ions for charge compensation and resulting in the observed polymerisation of the silicate network. Correspondingly, both the glass transition temperature and dilatometric softening point values were seen to decrease whilst the thermal expansion coefficient values rose, with increasing MgO content. This was due to the significantly lower bond strength, of Mg–O compared to Si–O, weakening the glass network.

Aqueous corrosion of borosilicate glass under acidic conditions: A new corrosion mechanism

Abstract: One important application of borosilicate glass is its use as a nuclear waste form to immobilize high-level nuclear waste Understanding the corrosion mechanism of borosilicate glasses in aqueous solutions is essential to reliably predict their long-term behavior in the worst-case scenario of glass-groundwater contact in a geologic repository Traditional models evaluate the long-term corrosion process on the basis of diffusion-controlled hydration and ion exchange reactions that are followed by solid-state reconstruction of the hydrolyzed glass network Here we report textural, chemical, and 18 O and 26 Mg isotope tracer results from corrosion experiments with a borosilicate glass in an acidic aqueous solution (initial pH ≈ 0, T = 150 °C, 6 to 336 h) that contradict such a paradigm We propose a new mechanistic model for glass corrosion under acidic conditions that is based on congruent (stoichiometric) dissolution of the glass that is spatially and temporally coupled to the precipitation of amorphous silica at an inward moving reaction front The model potentially provides a novel framework to understand apparently contradictory observations made under more moderate conditions and to evaluate the long-term aqueous durability of silicate glasses

Structural, thermal and electrical properties of PVA–LiCF3SO3 polymer electrolyte

Abstract: The development of polymeric systems with high ionic conductivity is one of the main objectives in Li rechargeable battery. In the present study, the different composition of PVA–LiCF 3 SO 3 polymer electrolyte has been prepared by solution cast technique using DMSO as solvent. The FTIR study confirms the polymer–salt complex formation. The amorphous nature of the polymer has been confirmed by XRD analysis. DSC measurements show decrease in T g with increasing salt concentration. The temperature dependent conductivity obeys Arrhenius relationship. The maximum conductivity has been observed in the order of 7 × 10 − 4 S cm − 1 for 25 mol% of LiCF 3 SO 3 . The activation energy has been found to be 0.16 eV. The two peaks have been observed in the dielectric loss spectrum which shows two types of relaxation α and β.

Strontium containing bioactive glasses: Glass structure and physical properties

Abstract: The influence of substituting strontium for calcium in the following glass series 49.46 SiO 2 –1.07 P 2 O 5 –(23.08-X) CaO–X SrO–26.38 Na 2 O was studied on the physical properties. Solid state nuclear magnetic resonance and vibrational spectroscopy showed that the glasses were predominantly composed of Q 2 silicate chains. Addition of strontium did not result in any structural alteration of the glass network due to the similar role of SrO compared with that of CaO. The density increased with strontium content whilst the oxygen density decreased indicating a more expanded glass network. The glass transition temperature reduced with strontium substitution in a linear fashion and there was no evidence of a mixed alkaline earth effect with a lower than expected glass transition temperature. Dilatometric softening points also reduced with increasing strontium content, whilst the thermal expansion coefficients increased. The results are consistent with a weaker network as a result of the lower charge to size ratio of Sr 2+ compared to Ca 2+ .

Effect of densification on crack initiation under Vickers indentation test

Abstract: Crack initiation in various commercial glass compositions was investigated by measuring “crack resistance”, which is determined by a series of Vickers indentation and counting of cracks around the indentation. The crack resistance of glass does not have clear relationship with hardness, fracture toughness, nor “brittleness” which is a ratio of the hardness to the fracture toughness. However, the crack resistance has a strong relationship with densification. Glass experiencing larger densification around the indentation shows higher crack resistance. Densification is assumed to reduce residual stress around the indentation, resulting in an increase in the crack resistance.

Green synthesis of silver nanoparticles with sucrose and maltose: Morphological and structural characterization

Abstract: We present a totally green approach toward the rapid synthesis and stabilization of metal nanoparticles through the treatment of aqueous solutions of silver nitrate with two commonly available sugars, i.e., maltose and sucrose as reducing agents. The average size, size distribution, morphology and internal crystalline structure of the nanoparticles are studied through high resolution transmission electron microscopy, selected-area diffraction pattern and UV–Vis spectroscopic technique and are seen to be critically dependent on the used sugar. The great majority of sucrose-assisted synthesized nanoparticles is distributed in a size range less than 6.0 nm, with an arithmetic media of 5.2 nm and a statistical standard deviation of 1.3 nm. For the maltose synthesized sample, the size distribution plot reveals that nanoparticles are greater (with a mean size of 62.4 nm and a standard deviation of 9.5 nm) and exhibit a more anisotropic morphology.

The physics of chemical strengthening of glass: Room for a new view

Abstract: Glasses strengthen during an ion exchange experiment as a result of the high surface compression that develops from the stuffing of the invading alkali ion into the smaller host alkali ion site and the non-relaxation of this compression. The kinetics are described in terms of a Nernst–Planck interdiffusion coefficient of the exchanging ions in a stack of mixed-alkali glass layers with varying ratios of the alkali. The interdiffusion coefficient is suppressed by compression and enhanced by tension. However, the measured surface compression is usually 2–4 times lower than that calculated using thermal stress analogy incorporating an elastic suppression of the molar volume difference of the as-melted mixed-alkali glasses. A new view of the physics of deformations and failure of stress to continue to buildup is presented in an effort to seek technology improvements. Reduced stresses are ascribed to network yield in two separate modes: yielding of the shape-conserving hydrostatic stress component which prevents elastic expansion of molar volume and yielding of the volume-conserving deviatoric (pure shear) stress which leads to measurable shape change. An anomalous subsurface tension is also explained in this view. By drawing analogy to microindentation, it is concluded that optimum network topology glasses should undergo the least plastic deformation and, hence, develop a higher surface compression.

Decolorization of azo dye solution by Fe–Mo–Si–B amorphous alloy

Abstract: The authors recently find that (Fe(0.99)Mo(0.01))(78)Si(9)B(13) glassy ribbons have a prominent effect on the decolorization of azo dyes. Direct Blue 2B solution of 200 mg/L were found to be decolorized by the ribbons nearly completely within 30 min. Decolorization mechanism was discussed briefly, and kinetic analyses based on the experimental data elucidated that the decolorizaiton process could be described by a pseudo-first-order kinetic model. Recycled experiments proved that the ribbons could be reused at least four times without obvious decay of decolorization efficiency. Combined with low manufacturing cost of producing Fe-based glassy ribbons, this new finding seems attractive, valuable and promising for industrial applications in the future. (C) 2010 Elsevier B.V. All rights reserved.

Effect of chemical composition on borosilicate glass behavior under irradiation

Abstract: Assessing the behavior under irradiation of oxide glasses used for nuclear waste immobilization is essential for qualifying the long-term behavior of the wasteform. This study focuses on a series of borosilicate glasses of increasing chemical complexity. The effects of irradiation in these materials were investigated through multi-energy external irradiation by gold ions to obtain a constant nuclear damage level to a depth of about 2 μm. The macroscopic behavior of the glass was estimated from Vickers hardness measurement. The mechanical properties of all the borosilicate glasses studied were observed to improve, their hardness decreasing with the dose down by 30–35%. This evolution is also associated to an increase of the fracture toughness of the glass under irradiation. Analysis of the structural changes in the sodium borosilicate glass common to all the compositions studied with respect to dose revealed a progressive shift to higher wavenumbers in the vibration Raman band near 495 cm −1 indicating a drop in the mean Si–O–Si angle. At the same time, modifications in the Q n band between 850 and 1200 cm −1 are observed and characterize increasing depolymerization of the silicate network.

Investigation of the physico-chemical reactivity of a mesoporous bioactive SiO2–CaO–P2O5 glass in simulated body fluid

Abstract: A new mesoporous bioactive glass (MpBaG) based on SiO 2 –CaO–P 2 O 5 by the sol–gel method has been successfully synthesized for biomedical applications. The obtained material was evaluated by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and N 2 adsorption analysis. The N 2 adsorption isotherm analysis showed that the pores were in mesopore size range and the MpBaG presented a specific surface area 137.9 m 2 g − 1 and a median pore size 15.4 nm. The surface reactivity of the MpBaG was studied in vitro in simulated body fluid (SBF). Inductively coupled plasma-atomic emission spectrometry (ICP-AES), SEM, EDS, XRD and FTIR were used to characterize the MpBaG surfaces and the SBF compositional changes. Immediately after immersion in SBF, reactions occurred on the surface of MpBaG. It is notable that the obtained results from FTIR and XRD analyses showed all typical characteristic peaks of bone-like hydroxyapatite (HAp). Also, it was obvious that the other new peaks after immersion in SBF were certainly related to the formation of hydroxycarbonate apatite (HCAp) and carbonate calcium (calcite). Finally, this research demonstrated that the synthesized MpBaG is a non-toxic and biocompatible material for ongoing osteogenic studies in segmental defects in the goat model in vivo .

Preparation and ionic conductivity of Li7P3S11 − z glass-ceramic electrolytes

Abstract: The 70Li 2 S·(30 − x )P 2 S 5 · x P 2 S 3 (mol%) glass and glass-ceramic electrolytes were prepared via mechanochemical route using a planetary ball mill apparatus. Ball-milling condition and glass composition for achieving high Li + ion conductivity in glass-ceramic electrolytes were investigated. The glass-ceramics prepared using ZrO 2 media (500 balls, 4 mm in diameter) exhibited higher conductivity than those prepared using Al 2 O 3 media (10 balls, 10 mm in diameter). The former experiment condition was appropriate for preparing homogeneous glasses. The substitution of 1 mol% of P 2 S 3 for P 2 S 5 enhanced the conductivity of the glass-ceramics and the maximum conductivity was 5.4 × 10 − 3 S cm − 1 at room temperature. The Li 7 P 3 S 11 analogous phase, Li 7 P 3 S 11 − z , was formed at the composition with 1 mol% P 2 S 3 and the Li 7 P 3 S 11 − z phase would have higher conductivity than the Li 7 P 3 S 11 crystal without substitution.

Optimization of nanostructured titania photoanodes for dye-sensitized solar cells: Study and experimentation of TiCl4 treatment

Abstract: Titanium tetrachloride (TiCl 4 ) treatment processed by chemical bath deposition is usually adopted as pre- and post-treatment for nanocrystalline titanium dioxide (TiO 2 ) film deposition in the dye-sensitized solar cell (DSC) technology. Pre-treatment influences positively the bonding strength between the fluorinated tin oxide (FTO) substrate and the porous TiO 2 layer, blocking the charge recombination at the interface between the conduction glass FTO and the I 3 − ions present in the I − /I 3 − red-ox couple. Additionally, TiCl 4 post-treatment is a widely known method capable of improving the performance of dye-sensitized solar cells, in particular, the photocurrent collected from the device. In this study, the influence and effect of TiCl 4 pre- and post-treatment on the TiO 2 layer is proposed and compared to the untreated film. The relative DSC devices are characterized in terms of short circuit current density, open circuit voltage, fill factor, conversion efficiency and IPCE. The dark current characteristics of cells with a treated and untreated TiO 2 layer are also shown in order to evaluate the effect of TiCl 4 pre-treatment as a blocking layer.

Crystallization behavior of glass ceramics prepared from the mixture of nickel slag, blast furnace slag and quartz sand

Abstract: The purpose of this paper was to investigate the crystallization process of glass ceramics prepared from a mixture which was composed of nickel slag, blast furnace slag and a small amount of quartz sand. First, the parent glass of the mixture was prepared. Then, the glass ceramics were obtained by heat-treating for the parent glass. The crystallization behavior was studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). A large number of tiny spherical crystals were observed when the glass was heated up to 700 °C. Subsequently, the radial crystals were observed when the glass was heated up to 820 °C. By XRD analyzing, the spherical crystals and radial crystals were likely to be the crystals of diopside (CaMg(Si,Al,Fe)2O6) and hedenbergite (CaFe(Si,Al,Fe)2O6). The crystallization temperature was 860 °C and the final crystalline phases were identified to be diopside and hedenbergite. In addition, the activation energy of crystallization was evaluated by Kissinger method.

JSC-1A lunar soil simulant: Characterization, glass formation, and selected glass properties

Abstract: The chemical composition of a volcanic ash deposited near Flagstaff, Arizona, USA closely resembles that of the soil from the Maria geological terrain of the Moon. After mining and processing, this volcanic ash was designated as JSC-1A lunar simulant, and made available by NASA to the scientific research community in support of its future exploration programs on the lunar surface. The present paper describes characterization of the JSC-1A lunar simulant using DTA, TGA, XRD, chemical analysis and Mossbauer spectroscopy and the feasibility of developing glass and ceramic materials using in-situ resources on the surface of the Moon. The overall chemical composition of the JSC-1A lunar simulant is close to that of the actual lunar soil collected by Apollo 17 mission, and the total iron content in the simulant and the lunar soil is nearly the same. The JSC-1A lunar simulant contains both Fe 2+ (∼ 76%) and Fe 3+ (∼ 24%) ions as opposed to the actual lunar soil which contains only Fe 2+ ions, as expected. The glass forming characteristics of the melt of this simulant as determined by measuring its critical cooling rate for glass formation suggests that the simulant easily forms glass when melted and cooled at nominal rates between 50 and 55 °C/min. The coefficient of thermal expansion of the glass measured by dilatometry is in close agreement with that of alumina or YSZ, which makes the glass suitable for use as a coating and sealing material on these ceramics. Potential applications envisaged up to this time of these glass/ceramics on the surface of the Moon are also discussed.

Preparation of nano-silica materials: The concept from wheat straw

Abstract: In this paper, amorphous hydrated silica in Gramineae plant, named as phytoliths, was extracted and investigated from wheat straw. Porous nano-structured silica was prepared from agricultural waste materials through combustion and acid leaching. The results show that: i) the phytoliths in epidermal cells of wheat straw are round with a diameter of 14–22 μm, while those in trachea are oblong with a length of 18–40 μm and a width of 12–18 μm. These different phytoliths are all core–shell structures with the silica shell and the organism core of the plant cells; ii) The distribution of particle size, surface area, pore diameter and pore volume of nano-structured silica samples decreases with the increase of calcining temperature, and at a higher temperature, some agglomerates are formed. The results of this work are useful for scientists pursuing new synthetic route for valuable and widely applicable nanoscale silica materials, also helping to solve disposal and pollution problems.

Quench rate and temperature effects on boron coordination in aluminoborosilicate melts

Abstract: The effect of temperature on the structure of aluminoborosilicate liquids has been studied by 11 B MAS NMR, 11 B 3QMAS NMR, 23 Na and 27 Al MAS NMR spectroscopies using glass samples prepared with different cooling rates and thus different fictive temperatures. The abundance of BO 3 group and non-bridging oxygen (NBO) increases with increasing fictive temperature, indicating that the reaction BO 4 ↔ BO 3 + NBO shifts to the right with increasing temperature. The observed temperature dependence of BO 4 species abundance allows us to estimate the ∆ H of this reaction to be 24 to 50 kJ mol −1 in different glass compositions, and closely related with the amount of NBOs in the glass. Configurational heat capacities have been measured by differential scanning calorimetry (DSC). We found that the redistribution of boron species (BO 4 /BO 3 ) contributes a varying range of configurational heat capacity, from 7% to 30%, with different glass compositions. 11 B 3QMAS NMR indicates two different BO 3 sites, the populations of which are also affected by fictive temperature. The exchange between two different BO 3 sites should also contribute to configurational heat capacity.

Effect of TiO2 addition on structure, solubility and crystallisation of phosphate invert glasses for biomedical applications

Abstract: Phosphate invert glasses in the system P2O5–CaO–MgO–Na2O are completely soluble and exhibit a neutral pH in aqueous media, and they are therefore of interest for use as degradable implant materials. Their structure consists of small phosphate units such as pyrophosphate (P2O74−), and hence they are prone to crystallisation. Addition of TiO2 is known to improve processing of the melt and also to control glass solubility. The glass structure of phosphate glasses with 37 and 35 mol% P2O5 and addition of 1 to 10 mol% TiO2 was analysed using 31P MAS NMR, and the influence of structural changes on solubility, thermal properties, processing window and crystallisation behaviour was investigated. Glasses showed an increase in activation energy for crystallisation with increasing TiO2 content, resulting in an increased processing window, thereby allowing for fibre drawing and sintering of porous scaffolds. Deconvolution of 31P MAS NMR and calculation of network connectivity and average chain lengths suggest that Ti is acting as a network modifier with Ti4+ units acting as ionic crosslinks between phosphate units thereby impeding crystallisation as well as chain hydration and subsequent chain dissolution.

Amorphous niobium oxide thin films

Abstract: Amorphous niobium oxide thin films were deposited by unbalanced reactive magnetron sputtering under different conditions of pressure (2 to 4 Pa) and oxygen percentage (9, 17, and 23%). The films were characterized to obtain the relationships between the deposition parameters and the most relevant physical properties (structural, optical, mechanical, surface morphology and optical). The composition was determined by Rutherford backscattering spectroscopy and energy dispersive X-ray spectroscopy. From the results of X-ray diffraction and the composition, we can conclude that for all of the deposition pressures and flow ratios used, the films were stoichiometric Nb2O5 and amorphous. Similarly, the mechanical and optical properties did not show significant variation between the different deposition conditions; all the films were transparent with a bandgap of about 3.4 eV and a hardness around 5 GPa. Concerning the electrochemical properties, the response of the films to DC polarization in a 0.89% NaCl solution was significantly different. The parameters used to compare the electrochemical response were the polarization resistance and the corrosion resistance obtained from the Tafel analysis.

Spectroscopic properties and simulation of white-light in Dy3+-doped silicate glass

Abstract: Spectroscopic properties of various concentrations Dy3+-doped silicate glasses were characterized by excitation and emission spectra. The optimal doping concentration of Dy3+ ions was found to be 3.0 wt%, and the nature of resonance energy transfer was confirmed to be electric dipole–dipole interaction according to Huang’s rule. Simulation of white-light for these glasses was also performed by varying the excitation wavelength. The results show that the white-light luminescence color could be tuned to various wavelength excitations, and the present silicate glass is more suitable for generation of white-light for blue LED chips.

Mechanical properties of silicate glasses as a function of composition

Abstract: Bulk production of silicate glasses is based around a narrow range of soda–lime–silica compositions and as a result any variation in mechanical properties with composition is normally considered to be of little interest. However, studies by a range of authors over the last 50 years or so have demonstrated some intriguing variations of properties such as toughness, brittleness and, more controversially, strength with composition. We have produced several series of alkali-mixed alkaline earth-silicate glasses and are assessing the variation of their mechanical properties with composition using both conventional indentation and nanoindentation. The nanoindentation studies are also being used to assess the effects of surface hydration on the near surface mechanical properties of these glasses. This presentation reviews the results we have obtained to date and attempts to draw out some wider conclusions concerning the compositional dependences of the mechanical properties of silicate glasses.

Preparation and characterization of lead and zinc tellurite glasses

Abstract: Glasses of two systems: x PbO–(100 − x )TeO 2 ( x = 13, 15, 17, 19 and 21 mol%) and y ZnO–(100 − y)TeO 2 ( y = 18, 20, 22, 25, 30, 33 and 35 mol%) were prepared at two melt-cooling rates and characterized by density, UV–visible and Raman spectroscopy, DSC and XRD measurements. ZnO produces a larger compaction in the tellurite network than PbO. Density decreases with melt-cooling rate in both glass series. DSC studies found that the glass transition temperature decreases with increase in PbO concentration in lead tellurite glasses, but increases with increase in ZnO concentration in zinc tellurite glasses. Lead tellurites prepared at higher cooling rates were mostly amorphous while samples containing 19 and 21 mol% of PbO, prepared at slower cooling rates were translucent glass–ceramics containing crystals of PbTeO 3 , Pb 2 Te 3 O 8 and TeO 2 . These slowly cooled samples exhibit an absorption shoulder in the UV–visible absorption spectra, just below the absorption edge. This is attributed to excitonic transitions in nanocrystals that coexist with glassy phase in slowly cooled samples. Raman scattering studies found that the addition of PbO and ZnO produces an increase in the intensity of the absorption band between 720 and 745 cm −1 relative to the intensity of band between 645 and 670 cm −1 . This is due to the continuous distortion of symmetry of TeO 4 units, leading to the creation of TeO 3+1 polyhedron and/or TeO 3 trigonal units. Raman studies found a very sharp peak at 76 cm −1 in both lead and zinc tellurite glasses. The position of this peak was independent of the glass composition.

Specific absorption spectra of cerium in multicomponent silicate glasses

Abstract: Photoionization of glass requires photon energies larger than the bandgap of the glass matrix. To shift the photosensitivity of a glass to longer wavelengths, it is common to add a sensitizer, e.g. cerium. Cerium has two valence states but only the Ce 3+ is UV-photoactive. Hence, we present a spectral method that allows determining specific absorption spectra of both valent states of cerium in two different silicate glasses. Two overlapped absorption spectra, one for Ce 3+ and one for Ce 4+ , were observed in soda lime silicate glass. It was found that both spectra consist from two Gaussian bands. Specific absorption of each ion was calculated on the basis of balance equations written for glasses melted under different redox conditions and used to estimate the absolute concentration of each ion. An identical analysis was performed on a photosensitive multicomponent silicate glass (photo-thermo-refractive (PTR) glass). It was found that absorption band of Ce 3+ in PTR glass is shifted to short wavelength region due to the presence of fluorine while absorption spectra of Ce 4+ are identical. The effect of UV-exposure on absorption spectra of cerium was studied. An absorption spectrum of photo-ionized Ce 3+ is not identical to that of Ce 4+ : UV-exposure induces a new band resulting from Ce 3++ and electron color centers.

Effect of surface functionalization of silica aerogel on their adsorptive and release properties

Abstract: Due to their high surface area, open pore structure and biocompatibility silica aerogels were shown to be a promising candidate for drug delivery systems. This work aims to show the possibility of functionalizing silica aerogel with functional groups to tailor their surface properties for specific applications. Amino groups were taken as a model functional group. Ketoprofen loading on aerogels was taken as a drug delivery application. Two different functionalization approaches were followed: pretreatment of the gel before the drying step, and a post treatment of the dried aerogels. The amino-functionalized aerogels obtained by both methods were characterized by CHN and BET analysis, as well as UV-spectroscopy. It has been shown that the functionalized silica aerogels obtained by these methods maintain similar structural properties as the original ones. For the first time transparent amino-modified aerogels with a surface area up to 1000 m 2 /g were obtained. These aerogels were loaded with the drug ketoprofen by adsorption from supercritical CO 2 . The loading was shown to be a function of the concentration of the amino groups on the aerogel surface, due to their strong adsorptive interactions with the acidic group of ketoprofen. The release rate of ketoprofen from the loaded amino-modified aerogels was close to that of original ones. Thus, aerogels functionalization opens the possibility to design a high capacity carrier for a specific active substance, as well as to control the release of that substance in the corresponding media.

Design of oxy-fluoride glass-ceramics containing NaLaF4 nano-crystals

Abstract: Fluoride glass-ceramics doped with lanthanide ions are very promising materials for optical devices. In this study, transparent fluoride glass-ceramics have been obtained from oxy-fluoride glasses in the Na2O–Al2O3–SiO2–LaF3 system by heat treatment slightly above Tg, upon which NaLaF4 nano-crystals with a size of up to 20 nm precipitate. The crystallisation mechanism is suggested to involve the formation of a SiO2-enriched shell around the nano-crystals which hampers their further growth, limits Ostwald ripening, and promotes uniformity in size. This desired enrichment of lanthanide ions inside nano-crystals with an adjustable and uniform size distribution is essential for future photonic applications. To validate the crystallisation mechanism of this glass system, the structure and composition of the crystalline phase, glass matrix and the developing interface have been studied by viscosity, dilatometry, X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques.

Hierarchical nanostructured semiconductor porous materials for gas sensors

Abstract: Gas sensitive layers with a fractal structure based on tin oxide were prepared by the sol–gel-method. Investigations of the film structure were carried out by atomic force microscopy. The basic evolution steps of this fractal system were found to be diffusion-limited aggregation, cluster–cluster aggregation, formation of percolating transition and netting nanostructures. Gas-sensitivity measurements of synthesized samples were taken. The emergence of a new relaxation in the presence of reducing gases was found in the Cole–Cole plots. Sensor nanostructures were investigated by impedance spectroscopy in different gas environments and gas reagent detection temperatures. AC measurements made it possible to develop selective gas sensors and multisensor systems.

Structural influence of PbO by means of FTIR and acoustics on calcium alumino-borosilicate glass system

Abstract: Glasses of the system x PbO– R Na 2 B 4 O 7 –(100 − R − x )CAS with 0 ⩽ x ⩽ 50, and 50 ⩽ R ⩽ 75 mol% have been synthesized in two compositional series and characterized by FTIR, and acoustics to explore the role of PbO on the structure of the glasses. The elastic properties of glasses have been investigated using sound velocity measurements at 4 MHz. According to the IR analysis, there is a depolymerization of the silicate network and a conversion of BO 3 into BO 4 structural units with the formation of non-bridging oxygens. PbO is incorporated into borosilicate network via B–O–Pb, and Si–O–Pb bonds. It is assumed that PbO plays the role of a modifier by replacing the high strength B–O bond with a lower strength Pb–O bond which decreases the glass-transition temperature T g . The increase of the density was attributed to the replacement of less dense borosilicate structural units by the more dense lead structural units. The observed compositional dependence of the ultrasonic velocity and the elastic moduli was interpreted in terms of the effect of PbO on the coordination of the borate structural units with oxygens and the depolymerization of the silicate network.

Critical assessment of DTA-DSC methods for the study of nucleation kinetics in glasses

Abstract: Microscopy methods are usually employed to estimate the number density of super critical nuclei and the resulting crystal nucleation rates, I(T). These traditional techniques rely on a double-stage treatment, i.e. the development of the nuclei at a temperature higher than the previous nucleation temperature up to a size large enough to be visible with optical or electron microscopy. These methods can give reliable results for I(T), but are rather laborious and time-consuming. On the other hand, non-isothermal (DTA/ DSC) methods are, in principle, much faster. In this paper, we experimentally test two non-isothermal methods by comparison with a traditional optical microscopy method. We found that, if they are properly employed, non-isothermal methods can give useful kinetic information, which includes the crystal number density and nucleation rates, but to get accurate quantitative data they need some preliminary information about nucleation and growth rates in the studied glass and, in addition, are as laborious as the traditional microscopy methods!

Temperature dependent negative capacitance behavior in (Ni/Au)/AlGaN/AlN/GaN heterostructures

Abstract: The temperature dependent capacitance–voltage (C–V) and conductance–voltage (G/x–V) characteristics of (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures were investigated by considering the series resistance (Rs) effect in the temperature range of 80–390 K. The experimental results show that the values of C and G/x are strongly functioning of temperature and bias voltage. The values of C cross at a certain forward bias voltage point (�2.8 V) and then change to negative values for each temperature, which is known as negative capacitance (NC) behavior. In order to explain the NC behavior, we drawn the C vs I and G/x vs I plots for various temperatures at the same bias voltage. The negativity of the C decreases with increasing temperature at the forward bias voltage, and this decrement in the NC corresponds to the increment of the conductance. When the temperature was increased, the value of C decreased and the intersection point shifted towards the zero bias direction. This behavior of the C and G/x values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. Rs values increase with increasing temperature. Such temperature dependence is in obvious disagreement with the negative temperature coefficient of R or G reported in the literature. The intersection behavior of C–V curves and the increase in Rs with temperature can be explained by the lack of free charge carriers, especially at low temperatures.

Spectroscopic studies of Er3+ doped Ge-Ga-S glass containing silver nanoparticles

Abstract: The spectroscopic characteristics of an erbium-doped Ga 10 Ge 25 S 65 (GGS) glass containing silver were studied and compared with that of an erbium-doped GGS glass without silver. The optical absorption spectra revealed the surface plasmon resonance (SPR) related characteristic of silver nanoparticles (NPs) around 490 nm. Enhancement of Er 3+ luminescence was observed for a broad excitation wavelength region, and the largest enhancement was observed for the excitation at 488 nm, which is near the peak of SPR of silver NPs in GGS glass and in resonance with Er 3+ ions. For the excitation at 405 nm, enhancement of luminescence from both GGS host and Er 3+ ions was observed. A smaller enhancement was observed for a two-step upconversion luminescence excited at 801 nm. The observed enhancement of Er 3+ luminescence is mainly attributed to the local field effects: the SPR of Ag NPs causes an intensified electromagnetic field around NPs, resulting in enhanced optical transitions of Er 3+ ions in the vicinity.

Luminescence and defect centres in MgSrAl10O17:Sm3+ phosphor

Abstract: An efficient reddish orange emission MgSrAl 10 O 17 :Sm 3+ phosphor was prepared by the combustion method. The phosphor has been characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis measurements. Photoluminescence spectrum revealed that samarium ions are present in trivalent oxidation states. The phosphor exhibits two thermally stimulated luminescence (TSL) peaks at 210 °C and 450 °C. Electron spin resonance studies were carried out to identify the defect centres responsible for the TSL process in MgSrAl 10 O 17 :Sm 3+ phosphor. Three defect centres have been identified in irradiated phosphor and these centres are tentatively assigned to an O − ion and F + centres. O − ion (hole centre) correlates with the 210 °C TSL peak while one of the F + centres (electron centre) appears to relate to the 450 °C TSL peak.