Showing papers in "Journal of Non-crystalline Solids in 2005"

Structural analysis of bioactive glasses

Abstract: In this study four series of single and mixed alkali glass systems were made and investigated using MAS NMR. Additionally the densities of the glasses were measured experimentally, as well as calculated theoretically using Doweidar’s model. MAS NMR was used to obtain a quantitative structural understanding of glasses by calculating the concentrations of bridging and non-bridging oxygens per silicon oxygen tetrahedron as a function of the alkali oxide concentration expressed as Qn. 29Si MAS NMR spectra exhibited a single resonance corresponding closely with Si in a Q2 state. The chemical shift of the 31P MAS NMR peak was attributed to phosphate in an orthophosphate environment. The 29Si NMR spectra are in agreement with the density data. Using Doweidar’s model the proportions of Q2 and Q3 were calculated, showing that all glasses studied are predominantly Q2 in structure, i.e. [ SiO 3 - ] n silica chains which readily dissolve. The changes in the chemical shifts of the Q2 and Q3 species with composition have been interpreted as resulting from the preferential association of Na+ with Q3 and Ca2+ with Q2.

Can glass stability parameters infer glass forming ability

Abstract: Glass-forming ability (GFA) is the easiness to vitrify a liquid on cooling, while glass stability (GS) is the glass resistance against devitrification on heating; but it is questionable if there is any direct relationship between these two parameters. Therefore, to test this possibility, we assess and compare GFA and several GS parameters through quantitative criteria. GFA and GS were calculated for six stoichiometric glass forming oxides that only present surface (heterogeneous) crystallization in laboratory time scales: GeO2 ,N a 2O AE 2SiO2, PbO AE SiO2, CaO AE Al2O3 AE 2SiO2, CaO AE MgO AE 2SiO2 and 2MgO AE 2Al2O3 AE 5SiO2; plus Li2O AE 2SiO2 and Li2O AE 2B2O3 that, in addition to surface nucleation, also present homogeneous (internal) crystallization. We gauge GFA by the critical cooling rate, qcr, which was calculated from an estimated number of heterogeneous nucleation sites per unit surface, Ns, and from experimental crystal growth rates, u(T), assuming a detectable surface crystallized fraction Xc = 0.001. We define GS parameters by fourteen different combinations of the following characteristic differential thermal analysis (DTA) or differential scanning calorimetry (DSC) temperatures: the glass transition temperature (Tg), the onset crystallization temperature on heating ðT h Þ, the peak crystallization temperature on heating ðT h Þ, and the melting point (Tm). To obtain the experimental GS parameters for each glass we carried out DSC runs using coarse and fine powders, and completed the necessary data with literature values for Tm. The results for fine and coarse particles were quite similar. Most of the GS parameters that consist of three characteristic DSC temperatures show excellent correlation with GFA, however, rather poor correlations were observed for parameters that use only two characteristic temperatures. We thus demonstrated that certain, but not all GS parameters can be used to infer GFA. � 2005 Elsevier B.V. All rights reserved.

Network connectivity in aluminoborosilicate glasses: A high-resolution 11B, 27Al and 17O NMR study

Abstract: O-17 enriched calcium and potassium aluminoborosilicate glasses with compositions similar (or analogous) to commercial E-glass were made and studied by 11B, 27Al and 17O NMR in order to explore the network speciation and mixing. Fractions of non-bridging oxygens associated with silicon and boron can be obtained simply from 17O magic angle spinning (MAS) NMR, while bridging oxygen populations result from complete peak assignment of high-resolution 17O triple quantum magic angle spinning (3QMAS) NMR spectra. Dramatic differences between the two samples in boron and oxygen speciation demonstrate a large effect of the charge of the modifier cation on mixing behavior and on the stabilization of non-bridging oxygens. The observed oxygen speciation is compared with that calculated from two models: random mixing of Si, B, and Al, and mixing with avoidance of linkages between tetrahedral aluminum and tetrahedral boron groups. Mixing of B and Al in the K-containing glass tends to follow the latter, while the network speciation in the Ca-containing glass is closer to the random model. The effects of Al and Si on boron speciation are discussed, and indicate that the maximum fractions of four-coordinated boron observed in a wide range of glass compositions are closely related to tetrahedral B and Al ‘avoidance’. A modified ‘Dell and Bray’ model is proposed which seems to accurately approximate boron speciation in many alkali aluminoborosilicate glasses. The higher field strength modifier cation (Ca2+ vs. K+) promotes the formation of non-bridging oxygen, trigonal over tetrahedral boron, and at least minor amounts of five-coordinated aluminum.

Comments on the electric modulus function

Abstract: Arguments in favor of the electric modulus formalism are reviewed, and several misunderstandings and misrepresentations are corrected. It is argued that different representations of the same experimental data provide additional, rather then subtractive, insights into the difficult problem of understanding ionic conductivity in melts, glasses and crystals.

Relationship among glass-forming ability, fragility, and short-range bond ordering of liquids

Abstract: Glass-forming ability characterizes how easily we can avoid crystallization and thus make a glassy state of material from its liquid state upon cooling. Its quantitative measure is given by the critical cooling rate, which is the minimum cooling rate required for the formation of glass, or the avoidance of crystal nucleation. Here we consider what physical factors control the glass-forming ability of a liquid. We argue that short-range bond ordering, which is induced by a symmetry-selective part of the interaction potential, causes frustration against crystallization and helps vitrification if its local symmetry is not consistent with that of the equilibrium crystal. Based on this physical picture, we propose that the degree of short-range bond ordering in a liquid should be a new additional physical factor controlling the glass-forming ability. According to our model, it is also a controlling factor of the fragility, which characterizes how steeply viscosity increases upon cooling. This picture suggests a negative correlation between the glass-forming ability and the fragility. The close relationship between the glass-forming ability and the quasicrystal-forming ability in metallic glass formers is also discussed in the light of our model.

Excellent glass-forming ability in simple Cu50Zr50-based alloys

Abstract: On the basis of the novel binary CuZr bulk metallic glass (BMG), we report the excellent glass-forming ability (GFA) in the simple Cu50Zr50-based BMG-forming alloys with minor Al addition. When Al addition changes from 4 to 8 at.%, Cu50Zr50-based BMGs can be produced to cylindrical rods with a diameter of at least 5 mm. The alloys have a broad glass-forming composition range with the excellent GFA. The elastic moduli were derived from the ultrasonic and density measurements. The physical origin of this unique GFA in the alloys is investigated. Our results might provide an effective route for exploring new BMGs.

Structure of SiO2–Al2O3 glasses: Combined X-ray diffraction, IR and Raman studies

Abstract: The average structures of roller-quenched (SiO 2 ) 1− x (Al 2 O 3 ) x , x = 0.25, 0.38, 0.53 and 0.60, glasses have been analyzed by large-angle X-ray scattering, IR and Raman spectroscopies. The changes of the local structure around the Si and Al atoms with composition are investigated. The average T–O distances for the glasses with 25 and 38 mol% Al 2 O 3 are very similar to the average tetrahedral T–O distances calculated from ionic radii. The estimated coordination numbers of these glasses are about 4. On the other hand, the average T–O distances and coordination numbers of the glasses with 53 and 60 mol% Al 2 O 3 suggest the existence of AlO 5 and AlO 6 polyhedra. However, models of the first T–O peak in the radial distribution functions and the IR spectra indicate that the actual number of AlO 5 polyhedra should be very small in these glasses. The polarized Raman spectra indicate that a homogeneous SiO 2 –Al 2 O 3 network is formed in which the relative number of interconnected SiO 4 tetrahedra gradually decreases with increasing Al 2 O 3 content. Comparison with SiO 2 –Al 2 O 3 liquid structures, previously derived from molecular dynamics simulations, suggest that the AlO 5 species, which are dominant in the high-temperature liquids, transform to AlO 4 and AlO 6 species despite the relatively rapid quenching method used.

On the parameters to assess the glass forming ability of liquids

Abstract: A new parameter has been proposed to assess the glass forming ability of liquids and its effectiveness is compared with the existing parameters. The new parameter ( α = T x / T l ), which incorporates both the factors of stability of the liquid (a low T l ) and the thermal stability of glass (a high T x ), has good correlation with the critical cooling rate for glass formation in a wide variety of glass forming alloys. This parameter can even be used for those glass forming alloys, which do not show a clear glass transition temperature. It has also been shown that the heating rate used during differential scanning calorimetry has negligible effect on the different glass forming parameters.

Viscosity in disordered media

Abstract: The paper addresses the properties of glassforming melts in an attempt to predict the dependence of viscosity on temperature and on pressure. A review on the main viscosity models (free volume model, the Adam and Gibbs model etc.) is given. It is demonstrated that the activation energy can be expressed through the glass-transition temperature Tg. The main attention is concentrated on the jump frequency model of Avramov and Milchev, the AM model. According to it, viscosity η depends on the entropy S of the system and through it on temperature and on pressure. The viscosity equation η ( P , T ) = η ∞ exp [ 2.3 ( 13.5 - lg η ∞ ) ( T g ( P ) T ) α ] (η measured in [dPa s]), derived in the framework of this approach, describes the plethora of experimental data, summarized in the article, with an accuracy, superior to that of the other models. It is demonstrated that the fragility power α depends on heat capacity. Therefore, the composition dependence of α is controlled by the Deby–Neumann law.

NMR and IR spectroscopy of silica aerogels with different hydrophobic characteristics

Abstract: Silica aerogels, prepared by the sol–gel method and dried by the CO 2 supercritical method, were produced from tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) in different proportions, in order to change the proportions of hydrophilic and hydrophobic surface groups. The aerogel hydrophobic or hydrophilic properties were related to their surface structural characteristics. The relative proportion of Si–OH, Si–OCH 3 or Si–CH 3 groups present on the aerogel pore surface and its silica network structure were determined by 1 H, 29 Si and 13 C nuclear magnetic resonance (NMR). The results obtained were correlated with the infrared transmission spectra. From these data, a possible description of the mixed hydrophilic–hydrophobic nature of the surface of these materials is proposed.

Elastic moduli and behaviors of metallic glasses

Abstract: We report apparent correlations among the elastic moduli, fracture strength, Vicker’s hardness and glass transition temperature for various and available metallic glasses with marked different elastic and mechanical properties. In particular, an attempt is made to link the observed correlations with glass transition, relaxation and glass-forming ability. The clear correlations imply that the physical properties of glasses would be better controlled by selection of elements with suitable elastic moduli as constituents.

Mesoporous bioactive glasses. I. Synthesis and structural characterization

Abstract: Highly ordered mesoporous bioactive glasses (MBGs) with different compositions have been synthesized by a combination of surfactant templating, sol-gel method and evaporation-induced self-assembly (EISA) processes. The texture properties and compositional homogeneity of MBGs have been characterized and compared with conventional bioactive glasses (BGs) synthesized in the absence of surfactants by evaporation method. The formation mechanism (pore - composition dependence) and compositional homogeneity in the case of MBG materials are different from those in conventional BGs. Unlike conventional sol-gel-derived BGs that shows a direct correlation between their composition and pore architecture, MBGs with different compositions may possess similar pore volume and uniformly distributed pore size when the same structure-directing agent is utilized. The framework of MBG is homogeneously distributed in composition at the nanoscale and the inorganic species generally exists in the form of amorphous phase. MBGs calcined at temperatures <= 1073 K exhibit ordered mesopores; at higher temperature such as 1173 K, the inorganic wall becomes crystalline and the mesostructure is collapsed. The leaching test of MBG in water indicates that MBGs may have excellent degradability in body fluid, which is important for prospective bio-applications. (c) 2005 Elsevier B.V. All rights reserved.

Complex primary crystallization kinetics of amorphous Finemet alloy

Abstract: The complex primary crystallization kinetics of the amorphous Finemet soft magnetic alloys has been analyzed by non-isothermal DSC measurements. The local activation energies E c ( α ) were determined by an isoconversional method without assuming the kinetic model function and its average value was about 383 kJ/mol. The nucleation activation energy E n and growth activation energy E g were 425 and 333 kJ/mol, respectively. And the apparent local activation energies E c can be expressed by E n and E g as follows: E c = aE n + bE g . The local Avrami exponents lies between 1 and 2 in a wide range of 0.2 α E c ( α ) and local Avrami exponents n ( α ) are applicable and correct in describing the primary crystallization process of the amorphous Finemet alloy according to the theoretical DSC curve simulation.

Na2CaSi2O6–P2O5 based bioactive glasses. Part 1: Elasticity and structure

Abstract: The glass structure and elastic properties of two bioglasses having bulk compositions near Na2CaSi2O6 (45S5.2) and Na2CaSi3O8 (55S4.1) were studied using both Raman and Brillouin scattering techniques. The annealed 45S5.2 glass has more Q2 and Q0 but less Q3 species than 55S4.1 glass due to lower (Si4+ + P5+)/(Na+ + Ca2+) ratio. Brillouin scattering measurements of the as-annealed glasses indicated that 45S5.2 glass is ca. 2% and 9% higher in Young’s and bulk moduli than 55S4.1 glass due to more modifiers in the 45S5.2 glass. Nearly full crystallization of 45S5.2 glass was observed after treating it at 715 °C for ca. 30 min. Devitrification of the 45S5.2 glass caused an increase in the elastic moduli up to ca. 30% (fully crystallized) but a negligible change in density. This 45S5.2-derived crystalline phase displayed at least 17 Raman bands, and has the average elastic moduli of 72.4 (bulk), 41.6 (shear), and 104.7 (Young’s) GPa. The comparable elastic moduli with hydroxyapatite and the ability for developing a HCA layer in simulated body fluid indicate that the 45S5.2-derived phase may be better for using as a substitute of bone than its parent glass.

Is there geometrical/physical meaning of the fractional integral with complex exponent?

Abstract: The geometrical/physical meaning of the temporal fractional integral with complex fractional exponent has been found and discussed. It has been shown that the imaginary part of the fractional integral is related to discrete scale invariance (DSI) phenomenon and observed only for true regular (discrete) fractals. Numerical experiments show that the imaginary part of the complex fractional exponent can be well approximated by a simple and finite combination of the leading sine/cosine log-periodical functions with period ln ξ (ξ is a scaling parameter). In most cases analyzed, the leading Fourier components give a pair of complex conjugated exponents defining the imaginary part of the complex fractional integral. For random fractals, where invariant scaling properties are realized only in the statistical sense the imaginary part of the complex exponent is averaged and the result is expressed in the form of the conventional Riemann–Liouville integral. The conditions for realization of reind and recaps elements with complex power-law exponents have been found. Description of relaxation processes by kinetic equations containing complex fractional exponent and their possible recognition in the dielectric spectroscopy is discussed. New kinetics expressed in terms of non-integer operators with complex and real power-law exponents can be successfully applied for description of dielectric spectra of many non-crystalline solids.

Investigations on bismuth and aluminum co-doped germanium oxide glasses for ultra-broadband optical amplification

Abstract: The broadband luminescence covering 1.2-1.6 mu m was observed from bismuth and aluminum co-doped germanium oxide glasses pumped by 808 nm laser at room temperature. The spectroscopic properties of GeO2:Bi,Al glasses strongly depend on the glass compositions and the pumping sources. To a certain extent, the Al3+ ions play as dispersing reagent for the infrared-emission centers in the GeO2:Bi,Al glasses. The broad infrared luminescence with a full width at half maximum larger than 200 nm and a lifetime longer than 200 mu s possesses these glasses with the potential applications in broadly tunable laser sources and ultra-broadband fiber amplifiers in optical communication field. (c) 2005 Elsevier B.V. All rights reserved.

Characterization of rare earth aluminosilicate glasses

Abstract: Rare earth glasses are commonly used in lasers, sensors and radiation shield glasses. Rare earth aluminosilicate-based glasses have been successfully used as in vivo radiation delivery vehicles, in treatment of primary hepatocellular carcinoma, irradiation of diseased synovial membrane for the treatment of rheumatoid arthritis, and treatment of prostate tumors. These glasses have also been found useful to promote liquid-phase sintering of covalent ceramics and, specifically, silicon carbide-based ceramics. Usually, the SiO 2 –Al 2 O 3 –RE 2 O 3 system is used as sintering additives. In this work, several rare earth silicate glasses (RE = Y, La, Nd, Dy and Yb) were prepared based on 60%SiO 2 –20%Al 2 O 3 –20%RE 2 O 3 (mol%) compositions. These glasses were characterized by infrared spectroscopy and nuclear magnetic resonance. Moreover, the liquid phase sintering of silicon carbide ceramics was studied using dilatometric experiments with 10 vol.% additives of the 60%SiO 2 –20%Al 2 O 3 –20%RE 2 O 3 system.

Space-selective precipitation of non-linear optical crystals inside silicate glasses using near-infrared femtosecond laser

Abstract: Tightly focused laser pulses with approximately 120 fs duration and with a wavelength of 800 nm have been used to induce space-selective precipitation of crystals with large second-order non-linear optical susceptibilities for BaO–TiO2–SiO2, Na2O–BaO–TiO2–SiO2, and Li2O–Nb2O5–SiO2 glass systems. During the laser irradiation, blue light due to the second harmonic generation is observed at 400 nm in response to the precipitation of the crystals in the vicinity of the focal point of the laser beam. X-ray diffraction analysis indicates that only a single phase of LiNbO3 is observed for Li2O–Nb2O5–SiO2 glass system, while BaTiO3 and Ba2TiSi2O8 phases are precipitated for Na2O–BaO–TiO2–SiO2 and BaO–TiO2–SiO2 glass systems. The elemental analysis around the irradiated area using an electron probe microanalyzer reveals that the laser irradiation allows glass-constituting cations to migrate to bring about the precipitation of a ring-shaped crystalline phase.

Luminescence behavior of Er3+ ions in glass–ceramics containing CaF2 nanocrystals

Abstract: The upconversion luminescence and near infrared luminescence of the Er3+ ions in transparent oxyfluoride glass–ceramics containing CaF2 nanocrystals have been investigated. The formation of CaF2 nanocrystals in the glass–ceramics was confirmed by XRD. The oscillator strengths for several transitions of the Er3+ ions in the glass and glass–ceramics have been obtained and then the Judd–Ofelt parameters were calculated. The split near infrared emission peaks of the Er3+ ions in the glass–ceramics can be observed because the Er3+ ions have been incorporated into crystalline environment of the CaF2 nanocrystals. The upconversion luminescence intensity of Er3+ ions in the glass–ceramics increased significantly with increasing heat treated time. The transition mechanism of the upconversion luminescence has been ascribed to a two-photon absorption process.

Molecular dynamics studies of brittle fracture in vitreous silica : Review and recent progress

Abstract: The dynamics of brittle fracture in vitreous silica has been a subject of many molecular dynamics (MD) simulations and experiments. A striking similarity between both simulations and experiments is the observation of nanoscale voids that eventually coalesce leading to failure. In this work, we review the above MD simulations and carry out further MD investigations using two variations of classical 2-body potentials. We study the effect of charge-transfer, an important aspect neglected by previous simulations. Further, we examine the growth of ‘critical’ voids and characterize regions surrounding the voids.

Polymers in nanoconfinement: What can be learned from relaxation and scattering experiments?

Abstract: Dielectric spectroscopy in combination with temperature modulated differential scanning calorimetry and quasielastic/inelastic neutron scattering are employed to investigate the molecular (glassy) dynamics of poly(dimethyl siloxane) (PDMS) and poly(methyl phenyl siloxane) (PMPS) confined to random nanoporous glasses with nominal pore sizes between 2.5 nm and 20 nm. Inside the pores PDMS and PMPS have faster molecular dynamics than in the bulk state. Down to a pore size of 7.5 nm the temperature dependence of the relaxation times (or rates) obeys the Vogel/Fulcher/Tammann (VFT) equation where the data obtained from dielectric and thermal spectroscopy agree quantitatively. At a pore size of 5 nm this VFT-like temperature dependence changes to an Arrhenius behavior. At the same confining length scale the increment of the specific heat capacity at Tg normalized to the weight of the confined polymer vanishes. The results indicate that a minimal length scale seems to be relevant for glassy dynamics in both polymers although the estimated length scale of about 5 nm seems to a bit too large in comparison to other experimental results and theoretical approaches. Neutron scattering is employed to investigate methyl group reorientation and the fast segmental dynamics of both polymers in confinement. Although the methyl group rotation is a localized process these experiments show that a part of the methyl groups is immobilized by the confinement whereas the effects for PDMS are much more pronounced than for PMPS. With regard to the segmental dynamics, neutron scattering reveals a big difference in the behavior of both polymers. Whereas the data obtained for PMPS are in accord with a boundary layer formed at the surfaces of the nanopores, for PDMS a considerable amount of elastic scattering is observed. To explain this result it is assumed that some structure formation of PDMS takes place in the nanopores, although the thermal data show no crystallization or melting effects.

Terahertz time-domain spectroscopy: A new tool for the study of glasses in the far infrared

Abstract: Terahertz time-domain transmission spectroscopy was used to obtain the absorption coefficients and refractive indices of polycrystalline quartz, amorphous silica, Pyrex and BK7 glasses The results were analyzed in terms of the power-law model of far-infrared absorption Evidence of the Boson peak was seen in the absorption spectra Relationships were observed between THz absorption and refractive indices on the one hand, and glass structure and properties on the other THz TDS is demonstrated to be a useful tool in the study of far-infrared transmission properties of glasses, producing low-noise, high resolution measurements of absorption coefficients and refractive indices

Initiation and propagation of shear bands in Zr-based bulk metallic glass under quasi-static and dynamic shear loadings

Abstract: The effect of strain rate on the initiation and propagation of shear bands in the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass under shear loading was investigated. The quasi-static (at a strain rate of 1.5 × 10−3 s−1) and the dynamic shear tests (at a strain rate of 1.4 × 103 s−1) were conducted at room temperature using a GATAN Microtest-2000 instrument and a split Hopkinson pressure bar (SHPB) with a specially designed ‘Plate-shear’ specimen, respectively. The complete process of shear band initiation, propagation, and shear band unstable propagation-induced fracture was revealed. The experimental results demonstrated that the macroscopic shear strength is relatively insensitive to the strain rate, whereas shear band initiation and fracture are significantly dependent on strain rate. A dimensionless Deborah number was introduced to characterize the effect of the strain rate on the formation of shear bands. Additionally, the observed numerous liquid droplets and melted belts on the fracture surface at high-strain rates demonstrate that the adiabatic heating exerts a significant effect on fracture behavior of the material.

Dielectric behavior and impedance spectroscopy of bismuth iron phosphate glasses

Abstract: The electrical and dielectrical properties of Bi2O3–Fe2O3–P2O5 glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 4 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe2O3 content and Fe(II)/Fetot ratio. With increasing Fe(II) ion content from 17% to 34% in the bismuth-free 39.4Fe2O3–59.6P2O5 and 9.8Bi2O3–31.7Fe2O3–58.5P2O5 glasses, the dc conductivity increases. On the other hand, the decrease in dc conductivity for the glasses with 18.9 mol% Bi2O3 is attributed to the decrease in Fe2O3 content from 31.7 to 23.5 mol%, which indicates that the conductivity for these glasses depends on Fe2O3 content. The conductivity for these glasses is independent of the Bi2O3 content and arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electronic conduction. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions. The Raman spectra show that the addition of up to 18.9 mol% of Bi2O3 does not produce any changes in the glass structure which consists predominantly of pyrophosphate units.

Enhancement of glass forming ability and plasticity by addition of Nb in Cu–Ti–Zr–Ni–Si bulk metallic glasses

Abstract: The effect of Nb substitution for Zr on the glass forming ability (GFA) and mechanical properties of Cu47Ti33Zr11−xNbxNi8Si1 (x = 0, 2, 4, 6, 8, 11) alloys have been investigated using differential scanning calorimetry (DSC), X-ray diffractometry (XRD), high resolution transmission electron microscope (HRTEM), and atom probe field ion microscope (APFIM). Partial substitution of Zr by Nb in Cu47Ti33Zr11Ni8Si1 promotes the GFA. Cu47Ti33Zr11−xNbxNi8Si1 (x = 2, 4) bulk metallic glasses (BMGs) with diameter of 5 mm can be fabricated by Cu-mold injection casting method. The plastic elongation of ∼4.2% in Cu47Ti33Zr7Nb4Ni8Si1 BMG, when compared with ∼1.5% in Cu47Ti33Zr11Ni8Si1 BMG, demonstrates that small amount of Nb addition can have a dramatic effect on plasticity enhancement in Cu–Ti–Zr–Ni–Si BMG. HRTEM and APFIM results show that the as-cast Cu47Ti33Zr7Nb4Ni8Si1 BMG exhibits a fully amorphous structure without nm-scale compositional fluctuation, indicating that the plasticity of the BMGs can be improved even though no microstructural obstacles hindering or branching the propagation of the shear bands are observed by HRTEM and APFIM.

Low-density organic and carbon aerogels from the sol–gel polymerization of phenol with formaldehyde

Abstract: Low-density organic and carbon aerogels are prepared from the sol–gel polymerization of cheap phenol with formaldehyde using NaOH as the base catalyst, followed by ethanol supercritical drying. The effects of the preparation conditions, including the mole ratio of phenol to NaOH (P/C), the mole ratio of phenol to formaldehyde (P/F), phenol–formaldehyde (PF) concentration and gelation temperature, on the gelation time, the bulk density as well as the physical and chemical structures of the resultant organic and carbon aerogels were studied. The experimental results of TEM and nitrogen adsorption showed that the phenol derived organic and carbon aerogels have a three-dimensional network that consists of interconnected bead-like particles with diameters of approximately 10–15 nm, which define numerous mesopores less than 50 nm. The highest measured BET surface area and mesopore volume of the carbon aerogels obtained reached 714 m 2 g −1 and 1.84 cm 3 g −1 , respectively.

The origin of nanostructuring in potassium niobiosilicate glasses by Raman and FTIR spectroscopy

Abstract: The structure of potassium niobium silicate glasses in a wide compositional range has been studied by means of Raman and FTIR spectroscopy. The glasses spectra were compared to those of the KNbSi 2 O 7 and K 3 Nb 3 O 6 Si 2 O 7 polycrystalline samples, obtained by crystallization of glasses of the same composition. It was found that the structure of such glasses is formed by SiO 4 tetrahedra and distorted NbO 6 octahedra. The amount of highly distorted (edge-sharing, non-bridging oxygens) octahedra results essentially unchanged from the glass composition. By contrast, the fraction of octahedra with a lower distortion degree (corner-sharing, bridging oxygens) increases with the Nb 2 O 5 content. Raman and FTIR investigations indicate that during long heat treatments at temperatures near T g , in the 23K 2 O · 27Nb 2 O 5 · 50SiO 2 glass, a structural change occurs regarding the amorphous matrix with a decrease of the niobium octahedra distortion. This can be related to a segregation process producing niobium rich regions nanometric in size. In the first heat treatment (2 h) the glass remains amorphous while for more prolonged heat treatments, nanocrystals of an unidentified phase are formed. In the same time the changes of the amorphous matrix hinder further crystallization.

The glass transition temperature of silicate and borate glasses

Abstract: The dependence of the glass transition temperature on silicate composition as well as on borate composition is discussed. Some new experimental results on borosilicate glass-enamels are put together with the existing literature data. It is demonstrated that in the glass transition interval the activation energy E(Tg) for viscous flow is related to the glass transition temperature. The activation energy for viscous flow is given by E(Tg) = 260Tg ± 10% [J/mol]. In the case of silicates, the glass transition temperature decreases almost linearly with the molar fraction x of network modifier, NM. In the interval 0.01 < x < 0.6 the glass transition temperature is:

Two-order-parameter model of the liquid-glass transition. I. Relation between glass transition and crystallization

Abstract: Recently we proposed a two-order-parameter model of liquid to understand the liquid–glass transition, liquid–liquid transition, and anomalies of water-type liquids in a unified manner. Here we discuss the relation between our model and previous models of the liquid–glass transition, focusing on the difference in the basic standpoint among the models. We argue that (i) actual liquids universally have a tendency of spontaneous formation of locally favored structures and (ii) liquid–glass transition is controlled by the competition between long-range density ordering toward crystallization and short-range bond ordering toward the formation of locally favored structures due to the incompatibility in their symmetry. Thus, we regard vitrification as phenomena that are intrinsically related to crystallization. Previous models, on the other hand, regard vitrification as a result of (a) an homogeneous increase in the density and the resulting cooperativity in molecular motion or (b) the frustration intrinsic to a liquid state itself. Accordingly, they presuppose the kinetic avoidance of crystallization and thus do not put focus on ‘crystallization’. This leads to the essential difference in the physical picture between our model and previous models. By comparing models, we aim at gaining further insight into what is the physical origin of the liquid–glass transition and its possible connection with crystallization.

Molar volume, glass-transition temperature, and ionic conductivity of Na- and Rb-borate glasses in comparison with mixed Na–Rb borate glasses

Abstract: Mass densities, molar volumes, glass-transition temperatures, and ionic conductivities are measured in series of YNa2O · (1 − Y)B2O3 glasses, with Y = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20, 0.25, 0.30 and YRb2O · (1 − Y)B2O3 glasses, with Y = 0.00, 0.12, 0.16, 0.20, 0.25, 0.30. Measurements of the molar volumes indicate that the incorporation of rubidium ions leads to a considerable expansion of the network, which is not observed for sodium ions. The glass-transition temperature increases with increasing alkali content and reaches a maximum near Y = 0.25 for both glass systems. These trends are attributed to changes in the glass network. For each glass composition an Arrhenius-activated increase of the product of dc conductivity and temperature is observed. The activation enthalpy decreases with increasing number density of ions. A comparison between the binary sodium- and rubidium-borate glasses from this work, with the ternary sodium–rubidium borate glasses studied earlier in our laboratory, provides interesting insights in the influence of the glass structure on ionic transport processes and the mixed-alkali effect.