Showing papers in "Journal of Non-crystalline Solids in 2004"
TL;DR: In this paper, the structure of x Na 2 O (100−− x )SiO 2 ( x ǫ = 0-50) glasses have been studied by molecular dynamics simulation.
Abstract: The structure of x Na 2 O (100 − x )SiO 2 ( x = 0–50) glasses have been studied by molecular dynamics simulation. Local structures around sodium and silicon agree well with neutron diffraction and EXAFS data. The medium range structures were characterized and compared with different structure models. The Q n distributions show a trend closer to, but more ordered than, random distribution and the compositional dependence agrees well with recent NMR results. A maximum in Q 3 species at the sodium disilicate composition is reproduced. The connectivities of Q n species are higher than those would be seen from a random distribution suggesting the existence of silica rich regions in these glasses. Ring size distributions show consistent decreases in the main peak at six-membered ring and increases in the number of larger member rings. The second moments due to sodium–sodium dipole coupling were calculated and compared with those derived from different theoretical models, crystalline and experimental data. The distribution of sodium ions was found to be slightly more clustered than a random distribution would predict. These results support the modified random network model.
300 citations
TL;DR: In this article, the feasibility of using silica aerogels as oral drug delivery systems was investigated and it was demonstrated that for all three drugs investigated here, high loading of the aerogel could be achieved.
Abstract: The feasibility of silica aerogels as oral drug delivery systems was investigated. Silica aerogels were loaded with several drugs by adsorption from their solutions in supercritical CO2. It was demonstrated that for all three drugs investigated here, high loading of the aerogel could be achieved. The loaded aerogels were characterized by IR- and UV spectroscopy and X-ray diffraction in order to show that no degradation of the drugs occurred during the loading procedure. The release profiles of two drugs (ketoprofen and griseofulvin) from loaded aerogels were measured. It was found that the drugs adsorbed on hydrophilic silica aerogels dissolve faster than the corresponding crystalline drugs. This fact can be explained by both an increase in the specific surface area of the drug adsorbed on the aerogel and its non-crystallinity in this state. The influence of density and hydrophobicity of aerogels on both the adsorption and release of drugs were studied.
289 citations
TL;DR: In this paper, the process used to grow hydrogenated microcrystalline silicon (μc-Si:H) from a H 2 /SiH 4 -glow discharge plasma is explained in comparison to those for hydrogenated amorphous silicon (a-Si-H).
Abstract: Processes used to grow hydrogenated microcrystalline silicon (μc-Si:H) from a H 2 /SiH 4 -glow discharge plasma are explained in comparison to those for hydrogenated amorphous silicon (a-Si:H). Differences and similarities between μc-Si:H and a-Si:H-growth reactions in the plasma and on the film-growing surface are discussed, and the nucleus-formation process followed by epitaxial-like crystal growth process is illustrated as unique processes for the formulation of μc-Si:H. Determination of the effect of dangling-bond defect density on the propagation of the resulting μc-Si:H films is also discussed in parallel with the effect on a-Si:H in order to obtain a clue to improve opto-electronic properties of those materials for device applications especially for thin-film-silicon solar cells. Material issues to produce low cost and high efficiency solar cells are described, and finally recent progress in those issues is demonstrated.
267 citations
TL;DR: In this article, the spectral analysis of biomolecules present in human lung cells by measuring their infrared signatures was performed using 77Se NMR spectroscopy, which is a useful tool for checking the local environment of the Se atoms.
Abstract: Compared to oxide-based glasses, vitreous materials involving chalcogens form a rather new family of glasses which have received attention, mainly because of their transmission in the mid-infrared. Indeed as low phonon compounds, these heavy-anion glasses allow the fabrication of molded optics for infrared cameras as well as infrared fibers operating in a large spectral range. These waveguides, when correctly tapered, allows the development of a new generation of sensitive evanescent-wave optical sensors which have been used for biomedical applications. Here we will focus on the spectral analysis of biomolecules present in human lung cells by measuring their infrared signatures. Because they contain heavy polarizable anions as well as lone-pair electrons, these glasses exhibit very large non-linear properties compared to silica and are candidates for fast optical switching and signal regeneration in telecom. Due to the technological interest in chalcogenide glasses, more information is needed on their structural organization and 77Se NMR spectroscopy appears to be a useful tool for checking the local environment of the Se atoms.
258 citations
TL;DR: In this article, the mesoporous surface of a preformed network of sol-gel-derived silica nanoparticles, resulting in a conformal "crosslinked" coating that renders the interparticle neck zone wider.
Abstract: Polymerization of di- and tri-isocyanates can be templated onto the mesoporous surface of a preformed network of sol–gel-derived silica nanoparticles, resulting in a conformal ‘crosslinked’ coating that renders the interparticle neck zone wider. Upon drying, these crosslinked networks yield aerogels which are up to ∼3× more dense than native aerogels based on the underlying silica framework, but also up to 10× less hygroscopic and they may take more than 300× the force to break. These results have been obtained with one-step based-catalyzed sol–gel silica networks, as well as with gels derived through a two-step process involving an acid-catalyzed sol and a based-catalyzed gel. Furthermore, it has been also found that crosslinking increases the dielectric constant only by ∼35% relative to values reported in the literature for native silica aerogels of about the same porosity. Chemical investigations into the polymerization reaction have shown that the process of crosslinking involves reaction of the isocyanate with: (a) OH groups at the surface of silica to form carbamate; and (b) adsorbed water, to form an amine and carbon dioxide. This amine then reacts with additional isocyanates resulting in polymer chain extension and bridging of particles with urethane-terminated polyurea.
228 citations
TL;DR: In this paper, the structure of glasses depends on the type of alkali earth cation; the modification of the phosphate network is higher for the Ca containing glasses with respect to the Mg ones, due to the well defined Ca properties as a modifying cation.
Abstract: Glasses of high CaO or MgO content in the systems x CaO (1 − x )P 2 O 5 (0.50 ⩽ x ⩽ 0.72) and x MgO (1 − x )P 2 O 5 (0.50 ⩽ x ⩽ 0.80) have been prepared for the first time using a twin roller device and studied by Raman spectroscopy. It has been shown that the structure of glasses depends on the type of alkali earth cation; the modification of the phosphate network is higher for the Ca containing glasses with respect to the Mg ones, at the same alkali earth content, due to the well defined Ca properties as a modifying cation. X-ray diffraction patterns of the thermally treated at 850 °C calcium containing glasses show precipitation of bioactive crystalline phases, such as β-Ca 3 (PO 4 ) 2 and β-Ca 2 P 2 O 7 . The molecular dynamics simulations have provided insight for the high Mg and Ca content phosphate glasses formation, where their average oxygen coordination number is calculated as five and six for all glass compositions studied, respectively.
207 citations
TL;DR: In this paper, the authors used X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge to determine iron oxidation state and first-neighbor coordination number.
Abstract: A series of 27 silicate glasses of various compositions containing 0.2–2 at.% iron were synthesized at various oxygen fugacity values. The glasses were examined using X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge in order to determine iron oxidation state and first-neighbor coordination number. Spectral information extracted from the pre-edge region and principal component analysis (PCA) of the XANES region, together with a spectral inversion, were used to derive the end-member spectral components for Fe(II) and Fe(III). Linear trends in the pre-edge features were observed for most compositional series of the glasses examined as a function of Fe(II)/Fe(III) content. These linear trends are believed to be due to the similarity of average coordination numbers for both Fe(II) and Fe(III) end-members in each series. This result is consistent with model simulations of the XANES region and molecular dynamics (MD) simulations for the two end-member compositions which also show that Fe(II) and Fe(III) have similar average coordination numbers. These simulations also suggest the presence of five-coordinated Fe(III) in the melt phase. Based on a bond valence analysis of these MD simulations, a simple model is proposed to help predict the speciation of iron in oxide and silicate glasses and melts.
184 citations
TL;DR: In this paper, the glass forming regions in the two systems have been determined and the dilatometric properties of the melted glasses (coefficient of thermal expansion, glass transition temperature and softening temperature) have been measured to identify the glasses that best fulfill the cell requirements, such as a thermal expansion coefficient in the range (8.5 −12) −10 −6 K −1.
Abstract: Glass-ceramics in the systems RO–BaO–SiO 2 (R = Mg, Zn) have good thermal properties for sealing materials on planar-type solid oxide fuel cells (SOFC) at high and intermediate temperatures. In this work, the glass forming regions in the two systems have been determined. Composition lines with different BaO/RO ratios have been defined varying the silica content. The dilatometric properties of the melted glasses (coefficient of thermal expansion, glass transition temperature and softening temperature) have been measured to identify the glasses that best fulfill the cell requirements, such as a thermal expansion coefficient in the range (8.5–12) × 10 −6 K −1 . Since the sealing process is carried out employing paste technology during the heating and working schedule of the cell, it is important to study the interaction between sintering and crystallization to make dense materials with an adequate microstructure. The sintering and crystallization kinetics have been characterized for some selected glass compositions by using hot-stage microscopy (HSM) and differential thermal analysis (DTA). A new parameter S C = T X − T MS which takes into account the onset crystallization temperature ( T X ), temperature of maximum shrinkage ( T MS ) is proposed as an empirical method to evaluate the ability of glass sintering of the studied compositions and indicate good materials for glass-ceramic SOFC products.
174 citations
TL;DR: In this paper, the use of monolithic silica aerogel as transparent insulation in windows was investigated and the results related to the window application were presented, where the tiles are evacuated rapidly and easily sealed between two glass panes and a specific rim seal.
Abstract: Within a finished and a current EU project, research and development are being carried out on the use of monolithic silica aerogel as transparent insulation in windows. Results related to the window application will be presented here. At the thermal envelope of buildings, the window area is the weakest part with respect to heat loss, but at the same time, this area also provides advantages, e.g., solar energy gain. Glazing prototypes have been made of aerogel tiles of about 55 × 55 cm2 (fabricated within the EU projects). The tiles are evacuated rapidly and easily sealed between two glass panes and a specific rim seal. A heat-treatment phase (after the supercritical CO2 drying) of the aerogel is currently being developed in order to improve its optical quality. This step increases the solar transmittance by ∼6 percentage points. For glazing prototypes with an aerogel thickness of ∼15 mm, a center heat-loss coefficient of
173 citations
TL;DR: In this article, the authors presented the first results of thin film transistors produced completely at room temperature using ZnO as the active channel and silicon oxynitride as the gate dielectric.
Abstract: In this paper we present the first results of thin film transistors produced completely at room temperature using ZnO as the active channel and silicon oxynitride as the gate dielectric. The ZnO-based thin film transistors (ZnO-TFT) present an average optical transmission (including the glass substrate) of 84% in the visible part of the spectrum. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 1.8 V. A field effect mobility of 70 cm 2 /V s, a gate voltage swing of 0.68 V/decade and an on-off ratio of 5 × 10 5 were obtained. The combination of transparency, high field-effect mobility and room temperature processing makes the ZnO-TFT very promising for the next generation of invisible and flexible electronics.
150 citations
TL;DR: In this article, the results of recent experimental and theoretical work on plastic deformation and electronic properties are reviewed and a characterization of these materials with respect to their salient structural features is presented.
Abstract: Structurally complex alloy phases are exceptional metallic systems based on crystal structures with giant unit cells comprising up to more than a thousand atoms per cell. They have since long been studied in crystallography. Nevertheless, they represent interesting new materials, since until recently hardly any studies have been carried out on their physical properties. An overview is presented offering first a characterization of these materials with respect to their salient structural features. Subsequently, the results of recent experimental and theoretical work on plastic deformation and electronic properties are reviewed.
TL;DR: In this article, an optimum process was presented for synthesis of nanoporous carbon aerogels via pyrolyzing resorcinol-formaldehyde (RF) organic aerogel, which could be costeffectively manufactured from RF wet gels by a modified ambient drying technique using acetone exchange/controlled evaporation instead of conventional supercritical drying.
Abstract: Carbon aerogels are promising materials as electrodes for capacitive deionization (CDI) units and electrical double layer capacitors (EDLCs). An optimum process is presented for synthesis of nanoporous carbon aerogels via pyrolyzing resorcinol–formaldehyde (RF) organic aerogels, which could be cost-effectively manufactured from RF wet gels by a modified ambient drying technique using acetone exchange/controlled evaporation instead of conventional supercritical drying. The linear shrinkage of RF aerogel during ambient-drying of wet gels was controlled to be less than 4%, almost same as that of supercritical drying. Carbon aerogels obtained by pyrolyzing of RF aerogels (specific surface area 300–400 m2/g) retains high specific surface area (400–700 m2/g), low density (0.40–1.16 g/cm3), and ultrafine pore size (<50 nm) depending on the pH 3.0–6.5 range of the starting RF solution. The specific capacitance of carbon aerogel electrodes activated at 450 °C in air environment for 1 h has been improved to 220 F/g.
TL;DR: In this article, the effect of the incorporation of cerium ions in silica sol-gel coatings on aluminum alloys as potential replacement of chromate treatments was studied. But, the main idea was to combine the barrier effect of silica coatings with the corrosion inhibitor effect of Cerium inside the coatings.
Abstract: Chromates are among the most common substances used as corrosion inhibitors. However, these compounds are highly toxic, and an intense effort is being undertaken to replace them. Cerium compounds seem to fulfil the basic requirements for consideration as alternative corrosion inhibitors. The aim of this work was to study the effect of the incorporation of cerium ions in silica sol–gel coatings on aluminum alloys as potential replacement of chromate treatments. The main idea was to combine the ‘barrier’ effect of silica coatings with the ‘corrosion inhibitor’ effect of the cerium inside the coatings. Thin (below 1 μm for a single layer) and transparent cerium doped silica sol–gel coatings were prepared by dipping 3005 aluminum alloys in sol–gel solutions. Ultra-violet–visible spectra (UV–vis) show that cerium ions, Ce 3+ and Ce 4+ , always are present in the coatings, independently of the cerium salt or firing atmosphere used. Active protection with single and two layer coatings prepared with Ce (IV) salt seems to improve corrosion protection of the coated aluminum while coatings prepared with Ce(III) salt only entails a protection when applied as a two layer, possibly due to sealing of pre-existent defects in the first layer. The improvement of active protection with immersion time would imply that corrosion is inhibited by cerium ions that migrate through the coating to the site of the attack (a defect in the coatings) and then react to passivate the site.
TL;DR: In this paper, the authors describe the development and characterization of coatings obtained by the sol-gel technique, applied on stainless steel used in orthopaedic surgery, which are applied to reduce metal corrosion and adverse reactions when implanted.
Abstract: This work describes the development and characterization of coatings obtained by the sol–gel technique, applied on stainless steel used in orthopaedic surgery. These coatings are applied to reduce metal corrosion and adverse reactions when implanted. Hybrid coatings of silica containing hydroxyapatite, bioactive glass and glass–ceramic particles were prepared and applied on metal substrates. The coated samples were further tested in vitro to study their electrochemical properties and bioactive response. The electrochemical properties were evaluated by means of potentiodynamic polarization assays using simulated body fluid (SBF) as electrolyte. In vitro bioactivity tests were performed by soaking the coated samples in SBF at 37 °C with a ratio sample area/fluid volume of 0.35 cm/ml for different periods. The coatings improve corrosion resistance of the steel substrate and in vitro tests revealed that all the films show signs of bioactivity.
TL;DR: Pt-doped carbon aerogels have been prepared following two methods as mentioned in this paper : sol-gel reaction precursors 2,4-dihydroxybenzoic acid and formaldehyde.
Abstract: Pt-doped carbon aerogels have been prepared following two methods. In the first, the electrocatalytic material was synthesized from the sol–gel reaction precursors 2,4-dihydroxybenzoic acid and formaldehyde. The platinum precursor was Pt(NH3)4Cl2, ion-exchanged onto the gel surface. The doped gel was dried supercritically and pyrolyzed at high temperature. This material was characterized with transmission electron microscopy (TEM) and by X-ray diffraction (XRD). The size distribution of the Pt nanoparticles changed with the pyrolysis temperature but the range of the size distribution did not. Platinum accessibility and electroactivity were evaluated with voltammetry. The second kind of Pt-doped carbon aerogels was synthesized from resorcinol and formaldehyde, supercritically dried, and pyrolyzed. The ground carbon aerogel was impregnated with a platinum precursor H2PtCl6 aqueous solution. This platinum salt was chemically reduced with NaBH4. Two carbon blacks, Vulcan XC-72 and BP2000, were doped following the same technique so that the influence of the carbon type on the properties of the deposited platinum nanoparticles could be investigated. The Pt-doped carbons so obtained were characterized with TEM, XRD, hydrogen-adsorption/desorption cyclic voltammetry, CO-stripping voltammetry, and oxygen reduction reaction (ORR) activity. The Pt-doped carbon aerogel exhibited high ORR specific activity but comparable mass activity as compared with Pt-doped carbon blacks.
TL;DR: In this article, a complete set of Gibbs energy equations was developed for the phase SiO 2 (L), comprising glass, supercooled liquid and stable liquid silica, the phase siO(am) and the solid and liquid silicon solution phases, and a consistent thermodynamic description of the entire Si-O system was constructed, covering the complete composition range.
Abstract: A comprehensive thermodynamic assessment has been carried out on the phases SiO 2 (L), including SiO 2 (glass), the amorphous monoxide SiO(am) and on the solubility of oxygen in solid and liquid silicon, jointly with the equilibrium partition coefficient k . Contradictions in the sparse thermodynamic data of SiO(am) are pointed out. Our key experiments were performed on the decomposition and condensation of SiO(am). X-ray diffraction on samples heated in evacuated silica capsules was used. The condensation experiment was performed in local equilibrium on a silica rod in a commercial CZ-Si apparatus. With the Calphad method a complete set of Gibbs energy equations was developed for the phase SiO 2 (L), comprising glass, supercooled liquid and stable liquid silica, the phase SiO(am) and the solid and liquid silicon solution phases. Combining this with the data for crystalline silica and the gas phase, a consistent thermodynamic description of the entire Si–O system is constructed, covering the complete composition range. Based on that, a number of stable and metastable phase diagrams at various pressures are calculated and applied to better understand important process steps in the production of semiconductor silicon.
TL;DR: In this paper, the luminescence spectra of Eu 3+ -doped sol-gel glasses have been analyzed during the densification process and compared according to the presence or not of aluminum as a codoping ion.
Abstract: Luminescent spectra of Eu 3+ -doped sol–gel glasses have been analyzed during the densification process and compared according to the presence or not of aluminum as a codoping ion. A transition temperature from hydrated to dehydroxyled environments has been found different for doped and codoped samples. However, only slight modifications have been displayed from luminescence measurements beyond this transition. To support the experimental analysis, molecular dynamics simulations have been performed to model the doped and codoped glass structures. Despite no evidence of rare earth clustering reduction due to aluminum has been found, the modeled structures have shown that the luminescent ions are mainly located in aluminum-rich domains. The synthesis of both experimental and numerical analyses has lead us to interpret the aluminum effect as responsible for differences in structure of the luminescent sites rather than for an effective dispersion of the rare earth ions. 2004 Elsevier B.V. All rights reserved.
TL;DR: In this paper, two calcium aluminosilicate glasses examined were based on the composition 2SiO2·Al2O3·2CaO (ART1) and the mineral anorthite (ART2), and the observed chemical shifts for 29Si and 27Al agreed with previous studies.
Abstract: Calcium aluminosilicate and calcium fluoro-aluminosilicate glasses have been characterized by 29Si, 27Al and 19F MAS-NMR. The two calcium aluminosilicate glasses examined were based on the composition 2SiO2 · Al2O3 · 2CaO (ART1) and the mineral anorthite 2SiO2 · Al2O3 · CaO (ART2). The observed chemical shifts for 29Si and 27Al agreed with previous studies. The fluorine containing glasses were based on 2SiO2 · Al2O3 · (2−X)CaO · XCaF2. The 29Si chemical shift moved in a negative direction with increase fluorine content indicating a progressive reduction in the average number of non-bridging oxygens, NBO, attached to a silicon. The 27Al spectra indicated the presence of four coordinate aluminium in the glasses with X=0.0–0.75, but aluminium was present in Al(IV), Al(V) and Al(VI) coordination states in the highest fluorine content glass with X=1.0. The 19F spectra indicated the presence of F–Ca(n) in low fluorine content glasses and both F–Ca(n) and Al–F–Ca(n) in high fluorine content glasses. We speculate here that the Al–F–Ca(n) species are oxyfluorides [AlOxFy]n−, where x=1–6, y=1–6 and n is the charge on the total complex when aluminium is in Al(IV), Al(V) and Al(VI) coordinate states. The reduction in the average number of NBO per silicon with increasing fluorine content is explained by fluorine converting Ca2+ to F–Ca(n).
TL;DR: In this paper, the epoxide addition method was used to obtain a monolithic pure nickel (II) oxide aerogel material using powder X-ray diffraction, electron microscopy and elemental analysis.
Abstract: The synthesis and characterization of nickel (II) oxide aerogel materials prepared using the epoxide addition method is described. The addition of the organic epoxide propylene oxide to an ethanolic solution of NiCl{sub 2} 6H{sub 2}O resulted in the formation of an opaque light green monolithic gel and subsequent drying with supercritical CO{sub 2} gave a monolithic aerogel material of the same color. This material has been characterized using powder X-ray diffraction, electron microscopy, elemental analysis, and nitrogen adsorption/desorption analysis. The results indicate that the nickel (II) oxide aerogel has very low bulk density (98 kg/m{sup 3} ({approx}98 %porous)), high surface area (413 m{sup 2}/g), and has a particulate-type aerogel microstructure made up of very fine spherical particles with an open porous network. By comparison, a precipitate of Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} is obtained when the same preparation is attempted with the common Ni(NO{sub 3}){sub 2} 6H{sub 2}O salt as the precursor. The implications of the difference of reactivity of the two different precursors are discussed in the context of the mechanism of gel formation via the epoxide addition method. The synthesis of nickel (II) oxide aerogel, using the epoxide addition method, is especially unique in our experience. It ismore » our first example of the successful preparation of a metal oxide aerogel using a metal divalent metal ion and may have implications for the application of this method to the preparation of aerogels or nanoparticles of other divalent metal oxides. To our knowledge this is the first report of a monolithic pure nickel (II) oxide aerogel materials.« less
TL;DR: In this paper, the authors investigated the nonlinear optical performance and structure of TeO 2 -Nb 2 O 5 -ZnO glasses as a function of ZnO content.
Abstract: The non-linear optical performance and structure of TeO 2 –Nb 2 O 5 –ZnO glasses was investigated as a function of ZnO content. The third-order non-linear optical susceptibility ( χ (3) ) as measured by a Degenerate Four Wave Mixing (DFWM) method, initially increased with increasing ZnO content to about 8.2 × 10 −13 esu for a glass containing 2.5 wt% ZnO, and then decreased to 5.9 × 10 −13 esu as the ZnO content increased to 10 wt%. There was no noticeable change as the ZnO content increased from 10 to 15 wt%. The non-linear optical response time, which caused electron cloud deformation, was from 450 to 500 fs. The structure of these glasses as analyzed by Raman spectroscopy and FT-IR spectra, was affected by the addition of ZnO up to 5 wt%, when, it is believed, the Zn 2+ ions occupied the interstitial positions in the glass network by replacing the Nb 5+ ions. The replaced Nb 5+ ions occupied the network forming positions as the Te 4+ ions. Increasing ZnO > 5 wt% did not have any further effect on the glass structure.
TL;DR: In this paper, the dissolution kinetics of sodium borosilicate glasses xNa2O−yB2O3−(1−−x−−y)SiO2 are studied at constant temperature and pH as a function of the glass composition.
Abstract: The corrosion of sodium borosilicate glasses xNa2O–yB2O3–(1 − x − y)SiO2 is studied at constant temperature and pH as a function of the glass composition for 0.10 < x ≈ y < 0.25. Two series of experiments are reported with different glass surface area to solution volume ratios (S/V = 100 and 2000 m−1). The dissolution kinetics are monitored by following the concentrations of the three cations (Si, B and Na) in solution as a function of time. The altered glasses are characterized by NMR and SAXS. For all glasses, the concentrations of released cations reach a stationary limit. The time required to reach the stationary state decreases from 40 days to a few hours when the content in boron and sodium oxides is increased from 10 to 25 mol.%. The final leached fraction of silicon, LF(Si), is controlled by an apparent silica saturation concentration, which is nearly independent of the initial glass composition. On the contrary, the final leached fraction of boron and sodium, LF(B) and LF(Na), which are always close to each other, show a strong compositional dependence. The LF(B) and LF(Na) are low and nearly equal the LF(Si) for the glasses with boron (and sodium) oxide content less than 15%. This is the mark of a nearly congruent dissolution for these glasses. When the boron (and sodium) oxide content increases from 15% to 20%, there is a sharp increase of LF(B) and LF(Na) in comparison to LF(Si). Above 20%, the LF(B) and LF(Na) saturate to unity, which means that all the soluble cations have been removed, whereas the dissolved fraction of silicon may be less than 10% (depending on the S/V ratio). 29Si NMR shows a considerable reconstruction of the altered glasses. SAXS puts in evidence the formation of a porous network with a typical pore diameter of 5 nm. The results are qualitatively explained through the competition between (i) the extraction of the soluble boron and sodium, which initiates the formation of a porous network at the nanometer scale, and (ii) the recondensation of silica, which makes the surface layer more polymerized and more resistant than the original glass at the atomic scale.
TL;DR: In this paper, the authors synthesized and characterized nanostructured polyurethane aerogel-like materials processed via subcritical drying routes, and the influence of the composition of the reaction media is discussed.
Abstract: Previous work has demonstrated the feasibility of synthesizing low-density polyurethane- and polyisocyanurate-based aerogels that exhibit low effective thermal conductivity. On the basis of this literature, the present study synthesized and characterized nanostructured polyurethane aerogel-like materials processed via subcritical drying routes. Two families of polyurethane gels were studied. Wet gels were synthesized with two polyols of different functionality. The influence of the composition of the reaction media is discussed. Depending on the solubility of the precursors, macroporous foam-like or mesoporous aerogel-like materials can be obtained as observed by scanning electron microscopy coupled with mercury porosimetry. Prior to drying, specific washing steps were performed. Preliminary results obtained by evaporative and freeze-drying were then compared to reference aerogel materials dried through a direct supercritical route. Only a slight density increase was observed. Effective thermal conductivities were also measured and discussed.
TL;DR: In this article, the surface chemical modification of silica aerogels using various precursors and co-precursors based on mono-, di-, tri-and tetrafunctional organosilane compounds of the type RnSiX4−n (where R = alkyl or aryl groups, X = cl or alkoxy groups, n = 0-4).
Abstract: We report the experimental results dealing with the surface chemical modification of silica aerogels using various precursors and co-precursors based on mono-, di-, tri- and tetrafunctional organosilane compounds of the type RnSiX4−n (where R = alkyl or aryl groups, X = Cl or alkoxy groups, n = 0–4). The precursors exhibit either tetrafunctional or trifunctional chemical functions while the co-precursors have a number of functional groups varying from 1 to 3. The organosilane based on the methyltrimethoxysilane (MTMS) can be used as a precursor as well as co-precursor. The chemically modified aerogels have been produced by (i) co-precursor, and (ii) derivatization methods. The co-precursor method results in aerogels with higher contact angle (θ ≈ 136°) but the aerogels are opaque, whereas transparent (>80% optical transmission in the visible range) aerogels with lower contact angle (θ ≈ 120°) are obtained using the derivatization method. The hydrophobicity of the phenyl-modified aerogels has been found to be thermally stable up to a temperature of 520 °C. Using the MTMS precursor, aerogels with contact angles as high as 175° have been obtained, but the aerogels are opaque. The aerogels obtained using TEOS precursor along with the trimethylethoxysilane (TMES) co-precursor, show negligible volume shrinkage. Water intrusion into the MTMS-modified aerogels at pressures greater than the Laplace pressure exhibits hysteresis, as shown in the pressure-volume curves. Water droplets placed on surfaces coated with superhydrophobic aerogel powder with 8° of inclination showed velocities as high as 0.4 ms−1. The results are discussed with respect to the ratios of organic and inorganic components of the organosilane compounds.
TL;DR: In this article, the authors report that the addition of rare earth elements can significantly improve the glass-forming ability and manufacturability of Mg-based alloys, which can withstand very low vacuum in preparation process.
Abstract: One of the unsolved problems for the manufacturability and the applications of bulk metallic glasses is that their glass-forming ability is very sensitive to the preparation vacuum and impurity of components because oxygen in the environments would markedly deteriorate the glass-forming ability. Here we report that the addition of rare earth elements can significantly improve the glass-forming ability and manufacturability of Mg-based alloys. The Mg-based glass-forming alloys can withstand very low vacuum in preparation process. The beneficial effects of the Gd addition on the glass-forming ability and oxygen resistance during the Mg-based glass formation are explored.
TL;DR: In this paper, the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO23Al2O31.5P2O53CaO2CaF2, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca5(PO4)3F).
Abstract: Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO23Al2O31.5P2O53CaO2CaF2, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca5(PO4)3F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The 29 Si, 27 Al and 31 P NMR spectra for the glasses with different strontium contents were identical. The 19 F spectra indicated the presence of F–Ca(n) and Al–F–Ca(n) species in the calcium glasses and in the strontium glasses F–Sr(n) and Al–F–Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The lowstrontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium–strontium apatite and the fully strontium substituted glass to strontium fluorapatite. 2004 Elsevier B.V. All rights reserved.
TL;DR: In this paper, the crystallization kinetics of a lithium zinc silicate glass have been determined using isothermal and non-isothermal DSC and NISTA, and the values for the kinetic parameters for as-quenched and nucleated samples are reported and discussed.
Abstract: The crystallization kinetics of a lithium zinc silicate glass have been determined using isothermal and non-isothermal DSC and non-isothermal DTA. Values for the kinetic parameters for as-quenched and nucleated samples are reported and discussed. Particular reference is given to the similarities and differences noted between isothermally and non-isothermally derived data.
TL;DR: In this paper, the structure of xCuO·(1−−x)P2O5 glasses (0,⩽ ǫx,x,ǫ 0.50) prepared in vacuum sealed silica ampoules were investigated using vibrational spectroscopies, and the effects of glass structure on the glass properties, including density, refractive index, and glass transition temperature, were discussed.
Abstract: The structures of xCuO · (1 − x)P2O5 glasses (0 ⩽ x ⩽ 0.50) prepared in vacuum sealed silica ampoules were investigated using vibrational spectroscopies. With the addition of CuO, both infrared and Raman spectra indicate a systematic transformation from a three-dimensional ultraphosphate network dominated by Q3 tetrahedra into a chain-like metaphosphate structure dominated by Q2 tetrahedra. IR spectra clearly show two distinct Q3 sites with bands at 1378 and 1306 cm−1, assigned to P O bonds on isolated Q3 tetrahedra and P O bonds on Q Cu 3 tetrahedra that participate in the coordination environments of the Cu-octahedra, respectively. As CuO content increases, the intensity of the P O band associated with the Q Cu 3 tetrahedra increases to a maximum x ∼ 0.33, then decreases with a concomitant increase of the intensity of the band at 1265 cm−1, due to the asymmetric vibration of the PO2 groups on Q2 tetrahedra. When x > 0.33 the isolated Cu-octahedra begin to share common oxygens to form a sub-network in the phosphate matrix. The effects of glass structure on the glass properties, including density, refractive index, and glass transition temperature, are discussed.
TL;DR: In this article, limits for JSC, Voc, FF and efficiency of single-junction solar cells and tandems are derived, as a function of E g, showing that doublejunction'micromorph' solar cells constitute an optimum combination of bandgap values; the main future gain for thin-film silicon solar cells will be increasing Jsc by light trapping.
Abstract: Theoretical: Limits for Jsc, Voc, FF and efficiency of single-junction solar cells and tandems are derived, as a function of E g, showing that: (1) double-junction 'micromorph' solar cells constitute an optimum combination of bandgap values; (2) main future gain for thin-film silicon solar cells will be increasing Jsc, by light trapping. Spectral ranges where light trapping has to act are presented, separately for a:Si:H and μc-Si:H. Experimental: For glass-pin type cells, light trapping is obtained at the front end, by use of rough TCO layers. For plastic-nip type solar cells, light trapping is obtained by texturing of the back reflector, which acts as basic layer for further growth. Both random texturing and periodic gratings have been used, but results presented are limited to single-junction a-Si:H and μc-Si:H solar cells. So far, the highest Jsc-enhancement obtained is approximately 20%. © 2004 Elsevier B.V. All rights reserved.
TL;DR: A comprehensive study of a new TeO2-BaO-SrO-Nb2O5 glass system is reported in this paper, where compositional dependence of glass transition (Tg) and crystallization (Tx) temperatures are presented for all the compositions under study.
Abstract: A comprehensive study of a new TeO2–BaO–SrO–Nb2O5 glass system is reported in this paper. Compositional dependence of glass transition (Tg) and crystallization (Tx) temperatures are presented for all the compositions under study. The glass stability is discussed based on two parameters ΔT (Tx−Tg) and Kgl (Hruby parameter). This glass system is found to have a high thermal stability among tellurite glasses. The non-linear optical susceptibility χ(3) of these glasses was predicted based on the refractive index and linear optical polarizability. The optical properties, viz., optical energy band gap and Urbach energy were determined based on the optical absorption spectra. The Raman spectrum of a typical glass was also analyzed. All the above studies predicted that this glass system could be a potential candidate for fiber device applications.
TL;DR: In this article, the structural characteristics of phosphate glasses containing different amounts of Nb were investigated by X-ray diffraction, thermal analysis, Raman, and infrared spectroscopy.
Abstract: The structural characteristics of phosphate glasses containing different amounts of Nb were investigated by X-ray diffraction, thermal analysis, Raman, and infrared spectroscopy. Phosphate glasses can be produced with relatively high amount of niobium by increasing the cooling rate without crystallization. Niobium is a glass former, is located in octahedral sites, and replaces tetrahedral phosphate groups by P, Nb, and O linked mixed chains. The density increases from 3.32 g/cm3 to 3.73 g/cm3, the elastic modulus increases from 56 GPa to 78 GPa, and the linear thermal expansion coefficient decreases from 24.7 × 10−6 °C−1 to 7.9 × 10−6 °C−1 as the amount of Nb increases. The glass transition temperature (Tg) increases from 480 °C to 653 °C as the amount of Nb increases from 5 to 37 mol % because the Nb–O–P and Nb–O–Nb linkages are stronger than the O–P bond, requiring higher temperatures for relaxation. Electrical measurements showed that the activation energy for ionic conduction increases from 0.46 eV to 0.59 eV for glasses containing 19–32 mol % of Nb2O5.