Structure and properties of NaPO3–ZnO–Nb2O5–Al2O3 glasses

In an effort to design low-melting, durable, transparent glasses, two series of glasses have been prepared in the NaPO₃–ZnO–Nb₂O₅–Al₂O₃ system with ZnO/Nb₂O₅ ratio of 2 and 1. The addition of ZnO and Nb₂O₅ to the sodium aluminophosphate matrix yields a linear increase of properties such as glass transition temperature, density, refractive index and elastic moduli. The chemical durability is also significantly, but nonlinearly, improved. The glass with the highest niobium concentration, 55NaPO₃–20ZnO–20Nb₂O₅–5Al₂O₃ was found to have a dissolution rate of 4.5 × 10^? 8 g cm^? 2 min^? 1, comparable to window glass. Structural models of the glasses were developed using Raman spectroscopy and nuclear magnetic resonance spectroscopy, and the models were correlated with the compositional dependence of the properties.     

Structural studies of AgPO3–MoO3 glasses using solid state NMR and vibrational spectroscopies

Vitreous samples (1-x) AgPO₃–x MoO₃ (0 ≤ x ≤ 0.5) were prepared by conventional melt-quenching and characterized by Differential Scanning Calorimetry (DSC). The structural evolution of the vitreous network was monitored by ³¹P solid state nuclear magnetic resonance and Raman scattering, and assignments were aided by corresponding studies on the model compound AgMoO₂PO₄. The ³¹P MAS-NMR data differentiate between species having two, one, and zero P―O―P linkages (Q⁽²⁾ Q⁽¹⁾, and Q⁽⁰⁾ species), respectively. Interatomic connectivities involving these units are revealed by two-dimensional INADEQUATE data, utilizing the formation of double quantum coherences mediated by indirect ³¹P–³¹P spin–spin interactions via P―O―P linkages. As this method discriminates against isolated P atoms, it also serves as an important spectral editing tool for constraining lineshape fits. ⁹⁵Mo NMR data and Raman spectra suggest that the Mo species are most likely six-coordinate, forming four P―O―Mo linkages and are otherwise invariant with composition, except at MoO₃ contents ≥ 40 mole %, where some Mo―O―Mo bonding and/or clustering is observed.     

Hardness and refractive index of Ca–Si–O–N glasses

Vickers hardness and refractive index was determined for Ca–Si–O–N glasses with 14.6–58 e/o N and 19–42 e/o Ca. By applying slow cooling rates, transparent glasses were obtained for compositions near Ca_9.94Si₁₀O_17.73N_8.14, while the majority of the glasses were opaque due to small inclusions of elemental Si and/or Ca-silicide. Determined glass densities varied between 2.80 and 3.25 g/cm³. Hardness was found to vary from 7.3 to 10.1 GPa at a load of 500 g and, respectively increase and decrease linearly with N and Ca content. The refractive index was found to increase linearly with N content from 1.62 to 1.95 and showed no significant dependence on Ca content.     

Corrosion behavior and in vitro biocompatibility of Zr–Al–Co–Ag bulk metallic glasses: An experimental case study

Ni- and Cu–free Zr–Al–Co–Ag bulk metallic glasses (BMGs) were synthesized by copper mold casting. The effects of Ag addition for partially replacing Co of Zr₅₃Al₁₆Co₃₁ BMG on the corrosion behavior, surface chemistry and in vitro biocompatibility of BMGs were investigated. The Zr–Al–Co–Ag BMGs are spontaneously passivated with low passive current densities in phosphate buffered saline (PBS) solution. Partial substitution of Co by Ag is effective in improving the corrosion resistance of the Zr–Al–Co BMG. X-ray photoelectron spectroscopy (XPS) measurements reveal that the Ag addition increases the concentration of Zr and decreases the concentration of Al in the surface passive film of BMGs, which is responsible for the enhanced corrosion resistance of Zr–Al–Co–Ag BMGs. Mouse MC3T3-E1 pre-osteoblast cell proliferation results and morphology observations show that the Zr–Al–Co–Ag BMGs exhibit comparable cell viability and proliferation activity with those of Ti–6Al–4V alloy, demonstrating their good biocompatibility. The high corrosion resistance in PBS and low in vitro cytotoxicity of Zr–Al–Co–Ag BMGs suggest an initial biocompatibility for biomedical applications.     

Bi precipitates in Na2O–B2O3 glasses

Bi particles of different sizes were produced in Na₂O–B₂O₃ glasses by melt quenching and heat treatment technique. Melting temperature of Bi particles was measured by differential scanning calorimetry and X-ray diffraction. Measured melting temperatures of Bi particles are lower than bulk Bi melting temperature. Results of transmission electron microscopy were analyzed for the dependence of melting temperature on particle radius. The pressure and surface energy effect on melting temperature is estimated. The melting behavior of Bi particles in Na₂O–B₂O₃ glasses depends on the difference in the interfacial energies between the solid particle/glass and liquid particle/glass, and liquid particle/glass, σ_sm−σ_lm, which is estimated to be 255×10⁻³ J m⁻².     

Mixed alkali effect in physical and optical properties of Li2O–Na2O–WO3–B2O3 glasses

Glasses with composition xLi₂O-(30 ? x)Na₂O–10WO₃–60B₂O₃ (where x = 0, 5, 10, 15, 20, 25 and 30 mol%) have been prepared using the melt quenching technique. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system through density and modulated DSC studies. The density and glass transition temperature of the present gasses varies non-linearly, the exhibiting the mixed alkali effect. From the optical absorption studies, the values of direct optical band gap, indirect optical band gap energy (E_o) and Urbach energy(ΔE) have been evaluated. The values of E_o and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The electronic polarizability of oxide ions, optical basicity and the Yamashita–Kurosawa's interaction parameter have been examined to check the correlation among them and bond character. Based on good correlation among electronic polarizability of oxide ions, optical basicity and the Yamashita–Kurosawa's interaction parameter, the present Li₂O–Na₂O–WO₃–B₂O₃ glasses were classified as normal ionic (basic) oxides.     

Raman and NMR study of bioactive Na2O–MgO–CaO–P2O5–SiO2 glasses

The results of a structural study combining NMR and Raman spectroscopy of several melt-derived glasses in the system Na₂O–MgO–CaO–P₂O₅–SiO₂ are presented. The Raman spectra show clear changes in the Si–O–Si vibrational modes (related to the bridging oxygen atoms, BO) and also verify the presence of non-bridging oxygen atoms (NBO), also named terminal oxygens. The intensity of the Si–O–NBO stretching mode depends on the cation concentration. It can be concluded from the NMR studies that the MgO-containing samples have orthophosphate units charge-compensated by Ca²⁺ and Mg²⁺. The silicate matrix also contains both types of two-valent cations and consists of Q² and Q¹ units. Similarly, the Na₂O-containing samples contain isolated orthophosphate units in a silicate matrix (Q² and Q³ units), both charge-compensated by mixed cations Ca²⁺ and Na⁺. These experimental data were compared with theoretical parameters given by the Stevels model, which is a suitable tool for understanding bioactive behavior of these glasses. Furthermore, results of the in vitro tests carried out in simulated body fluids are presented and compared with both Raman and NMR structural data.     

The effect of ionic and metallic silver on the crystalline phases developed in CaO–Al2O3–TiO2–P2O5 glasses

Development of crystallization in the CaO–Al₂O₃–TiO₂–P₂O₅ system glasses was investigated in the presence of ionic and metallic silver. Differential thermal analysis, X-ray diffractometry, ultra violet–visible spectrophotometry, atomic force microscopy and scanning electron microscopy were used to evaluate the resulted glasses and glass-ceramics. It was found that silver ions facilitated crystallization by decreasing the viscosity of the glasses. However, metallic silver, which was formed through heat treatment in hydrogen atmosphere, improved heterogeneous crystallization of the reduced glasses in the subsequent heat treatment. The preformed metallic silver led to effective crystallization of calcium titanium phosphate (CaTi₄(PO₄)₆), calcium metaphosphate (Ca(PO₃)₂) and calcium pyrophosphate (Ca₂P₂O₇) phases at significantly decreased temperatures. The two latter phases were partially dissolved out by leaching in acidic solution and left out a porous structure of calcium titanium phosphate glass-ceramic.     

Structural role and coordination environment of Fe in Fe2O3–PbO–SiO2–Na2O composite glasses

The structural role, coordination geometry and valence of Fe in a series of Fe₂O₃–PbO–SiO₂–Na₂O glasses are studied by means of Fe-K-NEXAFS and EXAFS spectroscopies. Parameters for the study are the concentration of the Fe and Pb-oxides, the SiO₂/Na₂O ratio and the cast temperature. The EXAFS and NEXAFS results reveal that the role of Fe³⁺ depends on the concentration of Fe₂O₃. More specifically, in most of the studied quaternary systems, the Fe³⁺ ion is a glass former, i.e. the Fe atoms belong to FeO₄ tetrahedra that participate in the formation of the glassy network. The role of Fe as an intermediate oxide is identified only in one sample with 20 wt% Fe₂O₃, where ～80 at.% of the Fe atoms are tetrahedrally coordinated with O atoms, while the remaining ～20 at.% of the Fe atoms occupy octahedral sites. It is also revealed that the tetrahedral coordination of Fe in the vitreous matrix is destroyed when a number of parameters is altered, such as the T_cast, the (Fe + Si)/O and the SiO₂/Na₂O ratio.     

Luminescence and energy transfer in Dy3+/Tb3+ co-doped CaO–Al2O3–B2O3–RE2O3 glass

The new calcium aluminoborate glasses with the composition of CaO–Al₂O₃–B₂O₃–RE₂O₃ (RE = Dy and Tb) were synthesized and the luminescence of Dy³⁺ and Tb³⁺ was investigated. The results show that the emission intensity of Tb³⁺ ion was enhanced when introducing Dy³⁺ ion into CaO–Al₂O₃–B₂O₃–Tb₂O₃ glass due to the energy transfer processes between Dy³⁺ and Tb³⁺. The energy transfer efficiencies, transfer probabilities as well as donor–acceptor critical distances were also calculated. The energy transfer mechanism between Dy³⁺ and Tb³⁺ ions is electric dipole–dipole interaction, which can be concluded by both fluorescence decay and emission intensity ratio varieties.     

Performance investigation of Li2O–Al2O3–4SiO2 based glass–ceramics with B2O3, Na3AlF6 and Na2O fluxes

Influence of single fluxes (10 wt.% B₂O₃), bi-component fluxes (4 wt.% B₂O₃ + 6 wt.% Na₃AlF₆), and complex fluxes (4 wt.% B₂O₃ + 4 wt.% Na₃AlF₆ + 2 wt.% Na₂O) on the thermal kinetic parameters, microstructure, flexural strength and coefficient of thermal expansion (CTE) of Li₂O–Al₂O₃–4SiO₂ (LAS) glass–ceramics was investigated through differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscope (SEM). The results showed that complex fluxes could efficiently decrease transition temperature (T_g) and crystallization temperature (T_p), and accelerate the formation of needle-like β-spodumene crystals which benefit high flexural strength. The homogeneous LAS glass–ceramic (sample C₃) which has a high strength of 132.4 MPa and low CTE (100–650 °C) of 2.74 × 10^? 6/°C is obtained by doping of the initial LAS glass by complex fluxes of 4 wt.% B₂O₃, 4 wt.% Na₃AlF₆, and 2 wt.% Na₂O, nucleating at 630 °C/120 min and then crystallized at 780 °C/120 min. It is worthy of further investigation as a bonder of diamond composite material due to its outstanding prosperities.     

Influence of various alkali and divalent metal oxides on phase transformations in NiO-doped glasses of the Li2O–Al2O3–SiO2–TiO2 system

Regularities of phase transformations in glasses of the Li₂O–Al₂O₃–SiO₂–TiO₂ system doped with up to 2.5 mol% of alkali- and divalent metal oxides were studied by X-ray diffraction analysis, Raman scattering and optical spectroscopy. Ni(II) ions were used as spectral probes of phase transformations because Ni(II)-ions enter the inhomogeneous regions formed during the phase separation and crystallization, and their absorption spectra change with heat-treatment temperature reflecting formation of aluminotitanate amorphous regions, spinel nanosized crystals and β-quartz solid solutions, consequently.It was demonstrated that the technological additives do not change the sequence of the phases' formation but accelerate the liquid phase separation and crystallization. Addition of MgO and ZnO leads to increasing the temperature range of spinel precipitation. Addition of CaO, BaO and PbO results in increasing the light scattering of prepared glass-ceramics.In selection of the technological additives for decreasing the melting temperature of glass-ceramics for optical and photonic applications the influence of the additives on the structure and optical properties of the prepared material should be considered.     

Hf–Cu–Ni–Al bulk metallic glasses: Optimization of glass-forming ability and plasticity

In the Hf–Cu–Ni–Al quaternary system, the Hf₅₁Cu_27.75Ni_9.25Al₁₂ bulk metallic glass (BMG) exhibits the best combination of the large glass-forming ability (GFA) and compressive plasticity. Minor substitution of Nb for Hf in Hf–Cu–Ni–Al BMGs degrades not only the GFA but also plasticity, while the substitution of Ta does not have an appreciable effect on both properties. For the investigated Hf-based BMGs, the shear modulus G is a more sensitive indicator to correlate with their plasticity than the Poisson′s ratio. Meanwhile, the correlation between the G and the glass transition temperature T_g for the Hf-based BMGs can be expressed as G = 9.9 + 576 (T_g/V_m)[1 ? 4/9(T/T_g)^2/3].     

Crystallization behavior during cooling and glass-forming ability of Al2O3-poor Li2O–Al2O3–SiO2 melts

The influence of K₂O/(MgO + K₂O) on some melt properties, including crystallization during cooling of melts and glass-forming ability, was investigated in the Li₂O–Al₂O₃–SiO₂ system with low Al₂O₃ content. The dependence of viscosity on K₂O/(MgO + K₂O) above 1000 °C showed a monotonic decrease due to the reduction of [MgO₄] concentration and the conductivity also decreased due to the larger ion radius of K. The temperature dependence of conductivity for all melts showed an abrupt change at one temperature due to crystallization in which temperature of crystallization decreases with increase of K₂O. The crystallization behavior near liquidus temperature was studied quantitatively by calculating the crystal volume fraction from apparent viscosity value. The glass-forming ability of the melts was discussed by using data related with viscosity and crystallization. Finally, it was suggested that the melts with K₂O/(MgO + K₂O) ? 0.75 have a good glass-forming ability.     

Optical spectroscopy and local structure of Er3+ luminescence centres in СaO–Ga2O3–GeO2 glasses

Optical absorption, luminescence excitation and emission spectra of Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂:Er glass with Er content of 1.46 wt% are presented and analysed. Luminescence kinetics for the main Er³⁺ transitions was satisfactorily described by single exponential decays with characteristic lifetimes. Oscillator strengths, phenomenological Judd–Ofelt intensity parameters, radiative decay rates (emission probabilities of transitions), branching ratios and radiative lifetimes for Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂:Er glass are calculated and compared with the corresponding parameters of the Ca₃Sc₂Ge₃O₁₂:Er³⁺ garnet and other crystals and glasses. Quantum efficiency, η, of the ⁴I_13/2 → ⁴I_15/2 Er³⁺ transition is determined. Incorporation peculiarities and local structure of Er³⁺ luminescence centres in Ca₃Ga₂Ge₃O₁₂:Er³⁺ glass are discussed in comparison with garnet crystals and oxide glasses. On the basis of the presented results and referenced EXAFS data for Er, Eu and Ho impurities (L₃-edge) it has been shown that Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂ glass occupy network sites with the coordination number to oxygen of N = 6.     

NMR studies on rapidly solidified SiO2Al2O3 and SiO2Al2O3Na2O-glasses

²⁷Al and ²⁹Si MAS NMR studies were performed on roller-quenched SiO₂Al₂O₃-glasses with Al₂O₃ contents ranging from 10 to 60 mol% and on SiO₂Al₂O₃Na₂O glasses containing 10 mol% Al₂O₃ and 2.5 to 10 mol% Na₂O. Pure aluminium silicate glasses show NMR peaks at 0, 30 and 60 ppm. The frequency distribution of the different Al-sites is not affected by the glass composition. In glasses of the system SiO₂Al₂O₃Na₂O the 30 ppm peak decreases to zero as the Na₂O content increases. The 30 ppm peak is assigned to distorted triclustered AlO-tetrahedra, rather than to fivefold coordinated Al. Triclustering of tetrahedra may provide for charge neutrality in glasses with molar excess of Al₂O₃ over Na₂O. As charge balance is increasingly achieved by addition of alkali ions, the tendency of tetrahedral triclustering is reduced, reflected by the disappearance of the 30 ppm peak in glasses containing ≥ 7.5 mol% Na₂O.     

Effect of alumina on the structure and properties of Li2O–B2O3–P2O5 glasses

The structure of glasses within the system Li₂O–Al₂O₃–B₂O₃–P₂O₅ has been studied through ³¹P, ¹¹B and ²⁷Al Nuclear Magnetic Resonance, and the effect of Al₂O₃ substitution by B₂O₃ and P₂O₅ network formers on the structure and properties investigated for a constant Li₂O content. Multinuclear NMR results reveal that substitution of Al₂O₃ for B₂O₃ and P₂O₅ network formers in a glass with composition 50Li₂O·15B₂O₃·35P₂O₅ produces a change in boron environment from four-fold to three-fold coordination. Meanwhile aluminum can be present in four-, five- and six-fold coordinations a higher amount of Al(IV) groups is found for increasing alumina contents. The behavior of the glass transition temperature and electrical conductivity of the glasses has been interpreted as a function of the structural changes induced in the glass network when alumina is substituted for B₂O₃, P₂O₅ or both. Small additions of alumina produce a drastic increase in glass transition temperature, while it does not change for [Al₂O₃] greater than 3 mol.%. However, the electrical conductivity shows very different behavior depending on the type of substitution; it can remain constant when B₂O₃ content decreases or sharply decrease when P₂O₅ is substituted by Al₂O₃, which is attributed to a higher amount of BO₃ and phase separation.