Crystallization of 2(Ca,Sr,Ba)O-TiO2-2SiO2 composition glasses

Crystallization was examined for glasses having chemical composition of 2(Ca,Sr,Ba)O-TiO₂-2SiO₂ in which the CaO/SrO/BaO molar ratio varied. Powdered glass samples were pelleted into disks and sintered at 950 °C for 2 h. The major phase precipitated in the sintered samples was (Ca,Sr,Ba)₂TiSi₂O₈ and minor phase of perovskite such as CaTiO₃ or SrTiO₃ increased with CaO content in the samples containing more than 40 mol% of CaO in total CaO+SrO+BaO. Three regions having different slopes were found in linear relationships between SrO mol% and exothermal peak temperature on DSC curves or d[0 0 2] values determined by powder XRD method. These facts suggested that the major phase precipitated in each region was a solid solution containing a different amount of CaO, SrO, BaO and that these compositions varied depending on SrO content in the sample. The micro-crystalline structure, which could be useful in fabricating a dielectric dense body, was observed for samples containing 30-70 mol% of SrO.     

Luminescence behavior of Er ions in glass-ceramics containing CaF2 nanocrystals

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.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Sintering process of glasses in the system Na2O-B2O3-SiO2

In this work, the sintering process of different glasses in the system Na₂O-B₂O₃-SiO₂ has been studied. The studied compositions are suitable for sealing the gas manifolds of molten carbonate fuel cells. Sealing glasses are usually applied on the surfaces to be sealed using powder glass mixed with an organic medium. The agglomerant elimination and the sintering of the glass powder take place during the thermal treatment. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and viscosity on sintering. The control of these parameters allows optimization of the sealing conditions of the glasses. Dense materials have been characterized as well as the sintering mechanism. Two processes take place during the thermal treatment: the sintering process and the quartz crystallisation. Both processes act in opposite directions on the glass densification. Crystallisation is the dominant process at long times and high temperatures. The viscous flow Scherer model has been adequately applied to the experimental data.     

Optical properties of the optical fiber containing Co doped ZnO-Al2O3-SiO2 glass-ceramics

Optical characteristics of silica glass optical fibers containing Co²⁺ doped ZnO-Al₂O₃-SiO₂ (ZAS) glass-ceramics prepared by slurry-doping method were investigated. The absorption and emission bands of the fibers were found to be originated from the tetrahedral Co²⁺ in ZnAl₂O₄ crystals in ZAS glass-ceramics particles embedded in the core of the fibers. The crystal field strength of the Co²⁺ ions in the optical fiber was found to be smaller than that of the Co²⁺ ions in the bulk ZAS glass-ceramics.     

Glass-forming ability and crystallization of bulk metallic glass (HfxZr1−x)52.5Cu17.9Ni14.6Al10Ti5

We have produced a series of bulk metallic glasses of composition (Hf_xZr_1−x)_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content from 6.65 g/cm³ (x=0) to 11.09 g/cm³ (x=1). Over the same composition range the glass-forming ability decreases, as demonstrated by the size of the largest amorphous ingots that can be cast without crystallization. Although both the glass transition temperature and the melting temperature increase linearly with increasing Hf content, the reduced glass transition temperature (T_g/T_m) decreases, from 0.64 (x=0) to 0.62 (x=1), which suggests that the `confusion principle' correlating increased glass-forming ability with increased number of components, does not apply in this case due to the chemical similarity between Zr and Hf. A different crystallization behavior is observed for Zr-based and Hf-based glasses. The final crystalline phases are CuZr₂ and Zr₂Ni for Zr-based alloys, and Al₁₆Hf₆Ni₇ and CuHf₂ for Hf-based alloys.     

Crystallization processes in amorphous Zr54Cu46 alloy

The crystallization of amorphous Zr₅₄Cu₄₆ alloy was investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The experimental results show that an endothermic peak in DSC traces for amorphous Zr₅₄Cu₄₆ alloy exists at about 1006 K, indicating following eutectoid reaction occurs, namely, Cu₁₀Zr₇+CuZr₂↔CuZr in amorphous Zr₅₄Cu₄₆ alloy during heating. With increasing the heating rate, the glass transition temperature T_g and onset crystallization temperature T_x of amorphous Zr₅₄Cu₄₆ alloy increase in parallel, and the supercooled liquid region ΔT_x (=T_x−T_g) holds almost constant with an average value of 44 K. Both XRD and TEM results prove that Cu₁₀Zr₇ and CuZr₂ are main crystallization products for amorphous Zr₅₄Cu₄₆ alloy under continuous heating conditions. No CuZr phase is identified because of its small precipitation amount. Finally, the crystallization processes of amorphous Zr₅₄Cu₄₆ alloy were summarized.     

Refractive index-structure correlations in Na2O-Al2O3-SiO2 glasses

The current structural models have been used to analyse the refractive index data of Na₂O-Al₂O₃-SiO₂ glasses (Al₂O₃/Na₂O?1). The SiO₂ content is the sole factor that controls the refractive index. Values could be obtained for the factors with which each structural unit contributes in the refractive index. The content of Al₂O₃ or Na₂O has no effect on the refractive index. The factors (differential refraction) are constant and do not change with composition. They have the same values for Na₂O-SiO₂ glasses. The differential refraction of a structural unit increases linearly with increasing the number of non-bridging oxygen ions. The difference of the contribution to the refractive index from a silicate unit to the next equals to a half of that for AlO₄ tetrahedron. The effect could be attributed to the change in both the concentration and differential refraction of structural units. The obtained factors for the structural units are useful in calculating the refractive index with a high degree of accuracy.     

Thermo-optic properties of B2O3 doped Li2O-Al2O3-SiO2 glass-ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li₂O-Al₂O₃-SiO₂ glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The dS/dT value of 2.6 mol% B₂O₃-doped glass-ceramic was 12.5  × 10⁻⁶/°C, which was 0.9 ×  10⁻⁶/°C smaller than that of B₂O₃-free glass-ceramic. On the other hand, reduction in dS/dT through B₂O₃ doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass-ceramic through B₂O₃ doping is caused by the reduction in ?. The reduction in ? from B₂O₃ doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B₂O₃-doped glass-ceramic crystallized under 196 MPa was 11.7 ×  10⁻⁶/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass.     

A molecular dynamics study on vibration spectra of a-SiO2 surface

The charge equilibration (QEq) method was used in the molecular dynamics study of the vibration spectra on the a-SiO₂ surface. The vibrational density of states (VDOS) of a silica glass surface and partial contribution of Si and O atoms were compared with those obtained from the fixed-charge (FQ) model. The VDOS in each of the samples has two groups of vibrational bands, i.e., wide-lower-frequency band (WB) between 0-25 THz and narrow-higher-frequency band (NB) between 25 and 40 THz, which is similar to the bulk VDOS obtained experimentally. The QEq surface exhibits a peak and two shoulders while two peaks are found for the FQ surface. At the surface, an excess peak (at 1-2 THz) was observed by substracting the Debye VDOS from the calculated VDOS, which is regarded as the experimentally observed the boson peak (BP). The position of the BP is shorter than that in the case of the bulk. In the FQ surface, the VDOS consists of narrower bands, while in the QEq sample, wider bands exist due to charge variation.     

Surface crystallization of β-BaB2O4 phase using a CO2 laser source

This letter describes decrystallization process of the β-BaB₂O₄ phase on the surface of 63BaO-26B₂O₃-11TiO₂ glassy sample when irradiated by a CO₂ laser beam for 30-40 s, using a laser power of ≈1.7 W. An analysis by optical microscopy of the irradiated region showed that the glass surface contained crystallites with the same morphology as those observed after traditional crystallization in an electric furnace. However, the crystal growth rate and the crystallite sizes appeared to be higher than those obtained by heating in an electric furnace. X-ray diffraction of the irradiated region showed a preferential orientation of the β-BaB₂O₄ crystallites on the glass surface.     

O nuclear magnetic resonance study of Na2O-P2O5 glasses

The preparation and structural investigation of ¹⁷O-enriched xNa₂O-(100−x)P₂O₅ glasses (46.5?x?62.8) by nuclear magnetic resonance (NMR) is described. Enriched phosphoric acid was prepared by hydrolysis of PCl₅ with ¹⁷O-enriched water and neutralized with sodium carbonate. The sodium metaphosphate was then melted at 800 °C for 15 h and quenched. Polyphosphate and ultraphosphate glass compositions were prepared by remelting the metaphosphate with sodium carbonate and phosphorus pentoxide, respectively. ³¹P magic angle sample spinning (MAS) NMR was used to determine the Na₂O/P₂O₅ content in the glasses. ¹⁷O NMR spectra (quadrupole echo for non-rotating samples and multiple-quantum excitation for rotating samples (MQMAS)) show two oxygen sites in the samples with large quadrupolar coupling constants (4.7 and 7.7 MHz), in accordance with the high phosphorus electronegativity. According to the correlation of ¹⁷O quadrupolar constants with bond ionicity, these two components are attributed to bridging P-O-P and non-bridging P-O?Na oxygens. The average P-O-P bond angle is estimated with the quadrupolar asymmetry derived from the fit of the static echo spectra. The MQMAS spectrum shows a distribution of non-bridging oxygen chemical shifts, attributed to a variation of bond length and angle.     

Spectroscopic and magnetic behavior of xNd2O3(1−x)(3Bi2O3 · PbO) glasses

Glasses of the xNd₂O₃(1−x)(3Bi₂O₃ · PbO) system with 0?x?0.30 were obtained and studied by IR spectroscopy, X-ray photoelectron spectroscopy (XPS), density and magnetic susceptibility measurements. IR and density measurements show that the addition of neodymium ions produces structural changes and the neodymium ions play a network modifier role in the host glass matrix. XPS investigation permitted following the evolution of the structural disorder, of the degree of polymerization of bismuthate chains and of the fraction of bridging oxygens with respect to the neodymium ion concentration of the studied glasses. Magnetic susceptibility data show that the Nd³⁺ ions are present as isolated species for x?0.05 and as both isolated and exchange coupled species for higher x values.     

Structural investigation of SnO-B2O3 glasses by solid-state NMR and X-ray photoelectron spectroscopy

Local structure of the SnO-B₂O₃ glasses was investigated using several spectroscopic techniques. ¹¹B MAS-NMR spectra suggested that BO₄ tetrahedral units maximized at around the composition with 50 mol% SnO. The BO₄ units were still present at compositions with high SnO content (67 mol% SnO), suggesting that SnO acted not only as a network modifier but also as a network former. O1s photoelectron spectra revealed that the addition of small amounts of SnO formed non-bridging oxygens (NBO) (B-O?Sn) and the amounts of NBO increased with an increase in SnO content. ¹¹⁹Sn Mössbauer spectra indicated that Sn was present only as Sn(II) in the glasses. The structure of the SnO-B₂O₃ glasses was compared with that of conventional alkali borate glasses and lead borate glasses. The thermal and viscous properties of these glasses were discussed on the basis of the glass structure revealed in the present study.     

Effect of heat-treatment condition on crystallization behavior and thermal expansion coefficient of Li2O-ZnO-Al2O3-SiO2-P2O5 glass-ceramics

Utilizing P₂O₅ as nucleation agent, a Li₂O-ZnO-Al₂O₃-SiO₂ glass was prepared by conventional melt quenching technique and subsequently converted to glass-ceramics with different crystal phases. During the processing, two-step heat-treatments including nucleation and crystallization were adopted. The effects of heat-treatment on the crystal type, the microstructure and the thermal expansion behavior of the glass-ceramics were studied by means of differential scanning calorimetry, X-ray powder diffraction analysis, scanning electron microscopy and thermal expansion coefficient tests. It was shown that the crystallization of occurred after the glass was treated at 580 °C. As the temperature increased from 580 °C to 630 °C, cristobalite and were identified as main and second crystal phases, respectively, in the glass-ceramic. An increase in the temperature to 700 °C, the β-quartz solid solution in the glass-ceramic accompanied by a decrease in cristobalite content. The transformation from to γ₀-Li₂ZnSiO₄ took place from 700 °C to 750 °C. The resulting crystallization phases in the glass-ceramics obtained at the temperature higher than 750 °C were β-quartz solid solution and γ₀-Li₂ZnSiO₄. The glass-ceramics containing or β-quartz solid solution crystal phase possessed a microstructure formed by the development of dendritic crystals. The thermal expansion coefficient of the glass-ceramics varied from 36.7 to 123.8 × 10⁻⁷ °C⁻¹ in the temperature range of 20-400 °C, this precise value is dependent on the type and the proportion of the crystalline phases presented.     

Apatite formation on TiO2 anatase microspheres

Titanium dioxide (TiO₂) coatings have been long considered as biocompatible interfaces to promote the physico-chemical bonding between the bone tissues and implant material (e.g., titanium and stainless steel). Monodispersed TiO₂ (anatase, the low temperature polymorph of TiO₂) microspheres, produced in the form of colloidal precipitates, were deposited on different substrates and apatite formation was induced on the resulted surface by immersing the coated substrates in simulated body fluid solution. Analytical and microstructural investigations, conducted by X-ray diffraction, energy depressive X-ray spectroscopy and scanning electron microscopy techniques, showed considerable higher rates of apatite formation, in vitro, on the anatase microspheres compared to the sol-gel-derived thin films of the same oxide. We concluded that the particular surface morphology of the packed TiO₂ microspheres, promotes a faster apatite formation in vitro.     

Crystallization behavior of glasses in the system of Na2O-CaO-MgO-Fe2O3-Al2O3-SiO2 with high contents of nickel oxide

The structural behavior of nickel oxide in glassy and glass-ceramic materials, obtained in the system of Na₂O-CaO-MgO-Fe₂O₃-Al₂O₃-SiO₂, was investigated. The influence of the NiO content on the vitrification, crystallization, structure and exploitation properties of two model compositions, with different ratios [CaO]/[MgO] was analyzed. On the basis of DSC and XRD data, it is shown that NiO promoted the formation of bunsenite crystals, as nuclei for crystallization. On the other hand, NiO promoted formation of pyroxenes even for compositions with low MgO contents, which formed gehlenite without NiO admixtures. It is shown that in the composition with relatively high MgO contents, NiO could participate in the formation of two types of pyroxenes with the structure and chemical composition similar to (MgO_0.4NiO_0.6)(CaO_0.9NiO_0.1)Si₂O₆ and diopside-hedenbergite solid solutions. The optimal contents of NiO in both model compositions was about of 7 wt%, since higher contents reduced the exploitation properties. The glass-ceramics with optimal contents of NiO were also produced using Ni bearing galvanic slurry and coal ash; the resulting materials showed similar exploitation properties to those mentioned above.     

The coordination state and valence state of selected transition metal ions in SiO2-B2O3 xerogels

A series of SiO₂-B₂O₃ xerogels with changing SiO₂/B₂O₃ mol% and doped with selected transition metal ions was prepared. These mixed oxide materials contained copper, nickel, cobalt, manganese, chromium and vanadium ions coordinated to oxygen donor atoms in water and OH groups. Extensive studies of the transition metal complexes in the xerogels by such spectral techniques as diffuse reflectance (UV-vis), electron paramagnetic resonance and fluorescence spectroscopies show that there exist Cu(II) in the coordination environment of D_4h symmetry, Ni(II) in octahedral coordination sphere, Co(II) in both tetrahedral and octahedral environments, Mn(II) preferably in the O_h coordination and Mn(III) in pseudo-octahedral sphere; then octahedrally coordinated Cr(III) ions occur in coupled pairs or clusters and V(IV) as VO²⁺ ions exist in distorted (C_4v) octahedral surrounding.     

Study of the structural insertion of Al in the Al2O3-SiO2 and Nd2O3-Al2O3-SiO2 glass systems

Glasses in the Al₂O₃-SiO₂ and Nd₂O₃-Al₂O₃-SiO₂ systems were prepared by the sol-gel method. The gel-glass evolution was studied using near-infrared (NIR) and ultraviolet-visible (UV-VIS) spectrophotometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR). The results obtained indicate a relationship between the sample compositions, the treatment temperature and the Al coordination. In the samples of the Nd₂O₃-Al₂O₃-SiO₂ system the densification of the structure, when the treatment temperature increases, leads to the segregation of neodymium oxide particles.