Viscosity properties of sodium borophosphate glasses

The viscosity behavior of (1 − x)NaPO₃−xNa₂B₄O₇ glasses (x = 0.05-0.20) have been measured as a function of temperature using beam-bending and parallel-plate viscometry. The viscosity was found to shift to higher temperatures with increasing sodium borate content. The kinetic fragility parameter, m, estimated from the viscosity curve, decreases from 52 to 33 when x increases from 0.05 to 0.20 indicating that the glass network transforms from fragile to strong with the addition of Na₂B₄O₇. The decrease in fragility with increasing x is due to the progressive depolymerization of the phosphate network by the preferred four-coordinated boron atoms present in the low alkali borate glasses. As confirmed by Raman spectroscopy increasing alkali borate leads to enhanced B-O-P linkages realized with the accompanying transition from solely four-coordinated boron (in BO₄ units) to mixed BO₄/BO₃ structures. The glass viscosity characteristics of the investigated glasses were compared to those of P-SF67 and N-FK5 commercial glasses from SCHOTT. We showed that the dependence of the viscosity of P-SF67 was similar to the investigated glasses due to similar phosphate network organization confirmed by Raman spectroscopy, whereas N-FK5 exhibited a very different viscosity curve and fragility parameter due to its highly coordinated silicate network.     

Raman spectroscopic investigation of sodium borosilicate glass structure

Raman spectra of sodium borosilicate glasses with a wide range of Na₂O/B₂O₃ ratios were systematically measured. Variations of the spectra with glass composition were studied to interpret the implied distribution of Na⁺ ions between silicate and borate units. When Na₂O/B₂O₃ is less than 1, all Na⁺ ions are associated with borate units as indicated by the absence of the 1100 cm⁻¹ band of Si-O⁻ non-bridging bond stretching. For the (1−x)Na₂O · SiO₂ ·xB₂O₃ glass withx≦0.4 the peak-height ratio of the 950 cm⁻¹ band to the 1080 cm⁻¹ band was used to analyse semiquantitatively the distribution of the Na⁺ ions between silicate and borate units. Sodium ions are divided between silicate and borate units approximately in proportion to the amount of SiO₂ and B₂O₃ present in these glasses. Some of the high sodium content glasses were crystallized and their spectra were compared with the bulk glass spectra. The distribution of Na⁺ ions in the glass was quite different from their distribution after crystallization. Spectra of high silica glasses that had been heat-treated for phase separation indicated exclusion of borate units from the silica network and the formation of borate groups. For high boron content glasses, no change was observed on heat treatment. Raman bands due to borate groups seem to be little affected by their environments. Also affiliated with the Department of Geosciences.     

Raman spectroscopic study of hexavalent chromium in some silicate and borate glasses

The polarized Raman spectra of chromium-doped silicate and borate glasses have been measured and interpreted. It is shown that hexavalent chromium exists in silicate glasses as isolated chromate anions (CrO₄⁼). In binary sodium borate glasses there is a change of species from chromate to dichromate as the B₂O₃ concentration is increased.     

Kinetics and mechanisms of the conversion of silicate (45S5), borate, and borosilicate glasses to hydroxyapatite in dilute phosphate solutions

Bioactive glasses with controllable conversion rates to hydroxyapatite (HA) may provide a novel class of scaffold materials for bone tissue engineering. The objective of the present work was to comprehensively characterize the conversion of a silicate bioactive glass (45S5), a borate glass, and two intermediate borosilicate glass compositions to HA in a dilute phosphate solution at 37°C. The borate glass and the borosilicate glasses were derived from the 45S5 glass by fully or partially replacing the SiO₂ with B₂O₃. Higher B₂O₃ content produced a more rapid conversion of the glass to HA and a lower pH value of the phosphate solution. Whereas the borate glass was fully converted to HA in less than 4 days, the silicate (45S5) and borosilicate compositions were only partially converted even after 70 days, and contained residual SiO₂ in a Na-depleted core. The concentration of Na⁺ in the phosphate solution increased with reaction time whereas the PO₄^3– concentration decreased, both reaching final limiting values at a rate that increased with the B₂O₃ content of the glass. However, the Ca²⁺ concentration in the solution remained low, below the detection limit of atomic absorption, throughout the reaction. Immersion of the glasses in a mixed solution of K₂HPO₄ and K₂CO₃ produced a carbonate-substituted HA but the presence of the K₂CO₃ had little effect on the kinetics of conversion to HA. The kinetics and mechanisms of the conversion process of the four glasses to HA are compared and used to develop a model for the process.     

Ultraviolet absorption of cerium(III) and cerium(IV) in some simple glasses

The optical absorption spectra of cerium (III) and cerium (IV) in Na₂O-B₂O₃, Na₂O-SiO₂, Na₂O-P₂O₅ glasses, and in H₂O-H₂SO₄ and H₂O-H₃PO₄ solutions have been studied. Individual molar extinction coefficients of cerium (III) and cerium (IV) at different wavelengths (350 to 200 nm) have been estimated. In all the glasses and solutions, cerium (IV) produces a very strong and broad charge transfer band around 250 nm; the intensity, half-width, and position of this band change appreciably with glass composition. Cerium (III) in glass and in aqueous solution produces a number of absorption bands in the ultraviolet region corresponding to the f → d transitions. The cerium (III) bands are sharp and well resolved in Na₂O-P₂O₅ and in low-alkali borate glasses; the sharpness of resolution deteriorates in Na₂O-SiO₂, and in high-alkali borate glasses. The intensity of cerium (III) absorption also changes with glass composition. In all the glasses the molar extinction coefficient of cerium (IV) is 5 to 10 times stronger than that of cerium (III).     

Development and mechanical properties of metal–particulate glass matrix composites from recycled glasses

The great number of glasses available from recycling activity and vitrification treatment of industrial wastes leads to the need for new applications, with the development of new materials, such as low-cost composite materials from a powder technology route. In the present work a variety of recycled glasses is investigated, in order to obtain aluminium reinforced glass matrix composites via cold-pressing and viscous flow sintering. A good compatibility between lead silicate glasses from cathode ray tubes dismantling and aluminium reinforcement is found to be effective. Composites exhibiting good mechanical properties were developed from these materials. A particular attention was due to fracture toughness (K_IC) determination. The absolute K_IC of glass matrix composites value remains low, but a notable increment in relation to unreinforced matrix is observed.     

Ce3+掺杂氧化物玻璃的发光特征

制备了掺铈硼酸盐、硅酸盐、磷酸盐和锗酸盐等玻璃样品，测试了激发光谱和发射光谱，结果表明，随着玻璃光碱度的增大，即从磷酸盐到锗酸盐，Ce^3 的激发峰值波长从290nm到380nm，发射峰值波长从349nm到480nm，表现出强烈的Nephelauxetic效应，Stokes位移从50nm到100nm。较长的激发波长和发射波长有利于闪烁光在玻璃中的透过。较大的Stokes位移可以减少发射光的自吸收。作为玻璃闪烁体的基质，建议硼硅酸盐、硅酸盐和锗酸盐玻璃是比较合适的，本文为研制高密度玻璃闪烁体打下基础。     

Matrix effect on local structure surrounding Yb3+and spontaneous emission probability in oxide glasses

Extended X-ray absorption fine structure measurements have been performed on Yb³⁺ in silicate, borate, phosphate, and gallate glasses in order to investigate the local structures surrounding Yb³⁺. The local structures of Yb³⁺ ions in silicate, borate, and phosphate glasses were similar to those of Er³⁺ ions. Yb³⁺ ions do not occupy the network structural sites, but sit around the terminal region of the network or the region between the networks in these glasses. On the other hand, Yb³⁺ ions substitute the Ga³⁺ sites in complex anion structural units of the K₂O-Ga₂O₃-Nb₂O₅ glasses. We classified the local structures surrounding Yb³⁺ ions in oxide glasses into two types: the former and the latter are interstitial and substitutive types, respectively. The relationship between the spontaneous emission probability for ²F_5/2 – ²F_7/2 transition of Yb³⁺ and the local structure of Yb³⁺ in oxide glasses are discussed in terms of these two types.     

EPR and optical absorption studies of Cr3+ ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cr³⁺ ions in Calcium alumino borate (CaAB) glasses have been studied. The EPR spectra exhibit weak resonance signal at g ≈ 4.50 and intense resonance signal at g ≈ 1.98. A sharp resonance signal at g ≈ 1.97 was also observed at lower concentrations of chromium. The concentration dependence of the linewidth of the resonance signal at g ≈ 1.98 suggests the formation of Cr³⁺ ion clusters by magnetic superexchange interactions. The temperature dependence of the peak to peak intensity and the linewidth of the resonance signal at g ≈ 1.98 suggests that the exchange interactions between Cr³⁺ ions in the present sample were antiferromagnetic in nature with Néel temperature, T _N = 233 K. From the number of spins participating in the resonance at g ≈ 1.98, the paramagnetic susceptibility (χ) was calculated at different temperatures (233–295 K). A plot of 1/χ and T was found to obey Curie-Weiss law with negative Curie temperature. By measuring the relative intensities of the resonance signal at g ≈ 1.98, at different temperatures, the value of antiferromagnetic coupling constant (J) has been estimated. The optical absorption spectrum of chromium doped CaAB glass exhibits four bands, characteristic of Cr³⁺ ions, in nearly octahedral symmetry. From the band positions, the crystal field splitting parameter, Dq and the Racah interelectronic repulsion parameters, B and C were evaluated. The optical band gap (E_opt) and the Urbach energy (ΔE) were calculated from the ultraviolet absorption edges.     

Transformation of Na2O-CeO2-B2O3 glass into a material with interconnected pores

Substitution of SiO₂ with CeO₂ in the ternary sodium borosilicate system was found to produce phase-separable glasses. Heat treatment of these glasses resulted in separation into two different phases. The one phase enriched in sodium borate was then leached out leaving a CeO₂-rich framework. The structure of the leached material was crystalline (Pt/Rh crucible melt) which changed to a rather net-like appearance if Al₂O₃ resulting from erosion of alumina crucibles was added. B₂O₃ remained partially in the insoluble CeO₂-skeleton. X-ray diffraction analysis of leached material proved the presence of crystalline cubic CeO₂ and cerium borate (metaborate of the aragonite type) in Pt/Rh crucible melts, whereas cubic CeO₂, 2Al₂O₃ · B₂O₃ and traces of sodium borate were detected in Al₂O₃ containing melts. The specific surface areas of the leached materials ranged between 25 and 120m²g^–1 while the main radii of interconnected pores were calculated to be between 0.5 and 17nm. A sintering temperature of about 1500° C was estimated from void volume and bulk density measurements.     

Influence of mixed alkali on the spectral properties of vanadyl ions doped xNa2O-(30 − x)K2O-60B2O3 glasses—an EPR and optical study

Electron paramagnetic resonance (EPR) and optical investigations have been performed in the mixed alkali borate xNa₂O-(30 − x)K₂O-60B₂O₃ (5 ≤ x ≤ 25) glasses doped with 10 mol% of vanadyl ions in order to look for the effect of ‘mixed alkalis’ on the spectral properties of the glasses. The observed EPR spectra have structures for x > 5 mol% which are characteristic of a hyperfine interaction arising from an unpaired electron with the ⁵¹V nucleus and it builds up in intensity as x increases. It is observed that the mixed alkali play a significant role in accommodating the vanadyl ions in these mixed alkali glasses and for x > 5 mol%, shows a well resolved hyperfine structure typical for isolated VO²⁺ ions. The spin-Hamiltonian parameters (g and A), the dipolar hyperfine coupling parameter (P) and Fermi contact interaction parameter (k) have been evaluated. It is observed that the spin-Hamiltonian parameters do not vary much with the change in composition. It is observed that with increase of x, an increase occurs in tetragonal distortion for VO²⁺. The number of spins (N) participating in resonance and the paramagnetic susceptibility (χ) have been calculated. It is observed that N and χ increase with x. The optical bandgap energies evaluated for these glasses slightly increase with x and reach a maximum around x = 20 and thereafter decrease showing the mixed alkali effect. Optical band gap energies obtained in the present work vary from 2.73 to 3.10 eV for both the direct and indirect transitions. The physical parameters of the glasses are also determined with respect to the composition.     

Beneficial effect of minor Al substitution on the magnetocaloric effect of Mn1 − xAlxAs

Magnetocaloric effect near room temperature is enhanced by minor Al substitution in Mn_1 ? xAl_xAs compounds (x = 0, 0.015, 0.03). The Curie temperature and magnetic entropy change can be tuned by Al concentration. Under a field change of 2 T, which is below the critical field of metamagnetic transition and easy to reach in applications, a large reversible magnetic entropy change 28.5 J kg^? 1 K^? 1, and a huge magnetic entropy change 65.7 J kg^? 1 K^? 1 under a field change of 5 T is obtained at x = 0.015 near room temperature. Compared with that of MnAs mother alloy, it is concluded that minor Al substitution is beneficial for enhancing the magnetic entropy in Mn_1 ? xAl_xAs alloys.     

Microinhomogeneity of high-alkali aluminosilicate glasses studied by nuclear magnetic resonance spectroscopy

The structure of quenched aluminosilicate glasses with Na₂O/Al₂O₃ > 1 has been studied by high-resolution nuclear magnetic resonance spectroscopy. The aluminum in the structure of the glasses is shown to be in fourfold coordination. Increasing the sodium oxide content of the glasses reduces the degree of polymerization in their structure and leads to a nonuniform nonbridging oxygen distribution over the aluminosilicate glass network. The glasses have a locally microinhomogeneous structure due to the presence of both highly polymerized aluminosilicate anion groups and relatively depolymerized silicate anions.     

Intermediate valence behavior of ternary cerium and uranium transition metal borides

Low-temperature specific heat, electrical resistivity, ac magnetic susceptibility, and dc magnetization measurements were made on ternary cerium and uranium transition metal borides with the general formula CeT₃B₂ (T=Co, Ru, Rh, and Ir) and UT₃B₂ (T=Co, Ru, and Ir). The compound CeRu₃B₂ was found to exhibit superconductivity below 0.68 K. The values of the electronic specific heat coefficient range from 9.7 mJ/mole-K² for CeCo₃B₂ to 64 mJ/mole-K² for UIr₃B₂. The electrical resistivity versus temperature curves of all of the compounds exhibit negative curvature and are reminiscent of valence fluctuation behavior. In the case of CeIr₃B₂, the electrical resistivity attains a maximum value at 180 K, while the dc magnetic susceptibility has a temperature dependence that is typical of intermediate valence Ce compounds, approaching a finite value at zero temperature. The electrical resistivity of the ferromagnetic compound CeRh₃B₂ reveals a rapid decrease in spin disorder resistivity below 120 K. The dc magnetic susceptibility of this material can be described as the sum of a constant term and a Curie-Weiss contribution with an effective magnetic moment of 1.01 µ_B per formula unit and a Curie-Weiss temperature of 119 K. Magnetization measurements on CeRh₃B₂ yield a saturation magnetic moment of 0.37 µ_B per formula unit and a Curie temperature of 113 K.     

Effect of PbO-SiO2 and PbO-B2O3 flux systems on the crystalline and magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite prepared from the mixed powders

Lead borate and lead silicate were added to lower the sintering temperature of a Ni_0.5Zn_0.5Fe₂O₄ ferrite prepared from the blend of two types of powders and to homogenize the grain size. 5PbO·SiO₂ and 5PbO·B₂O₃ flux systems were added to lower the sintering temperature and diminish the magnetic loss at high frequencies. The ferrites were studied by bulk density, scanning electron microscopy and impedance analysis. It was found that the addition of PbO markedly accelerated the grain growth, while SiO₂ and B₂O₃ were found to be effective to obstruct the movement of grain boundaries and to minimize the grain size. Doping with PbO in the mixed powders appropriately increased the densification and initial permeability. The ferrite doped with 1% of 5PbO·SiO₂ possessed the lowest loss tangent (tgδ) in the range of 5 M-40 MHz and the highest threshold frequency.     

Energy storage behaviour of some binary alkali borate glasses by thermoluminescence studies

Thermoluminescence behaviour of a series of binary alkali borate glasses has been investigated to study their energy storage mechanism. Sodium borate glasses of varying composition have been prepared and their glow curves recorded after exposing them to X-rays (CuK radiation, 30 kV, 10 mA) of different dosages at room temperature. The effect of the nature and concentration of alkali oxide and the dose of irradiation on the nature of thermoluminescent glow curves were also studied. Borate glasses containing different concentrations of Na₂O exhibit significantly different glow curves. These glow curves have been analysed and the nature of traps responsible for TL emission are tentatively identified. The broad and complex nature of the glow pattern is attributed to distribution of trap depths in these materials. The viability of borate glasses in the construction of TL dosimeters are discussed.     

Preparation, X-ray analysis and magnetic investigation of nanostructured Co:Yb2O3

Cubic nanocrystalline Co:Yb₂O₃ in the size range of 8-38 nm were synthesized by sol-gel method. Annealing temperature should be higher then 600 °C. The morphology and the particle size were controlled using the electronic microscopy (SEM). Magnetic susceptibility measurements of all samples were done in a temperature range 2-50 K. In this region only the lowest Kramer's doublet is populated, and for that level the effective magnetic quantum number is deduced. The Curie-Weiss paramagnetic temperature θ(x) obtained for the low-temperature region depend linearly on the Co³⁺ concentration. Above 30 K the inverse magnetic susceptibility deviates from linearity due to the crystal field influence.     

Sodium silicate bonded borate glass scaffolds for tissue engineering

Borate and silicate glass particles and microspheres with size distributions in the range of approximately 100–400 micron were loosely compacted and bonded by sodium silicate solution to prepare resorbable, porous glass constructs with porosity 30–50%. Conversion of the binding borate glass to hydroxyapatite was investigated by measuring the weight loss of the constructs in a solution of 0.25 M K₂HPO₄ with a pH value of 9.0 at 37 °C, as a function of time. Almost full conversion of the borate glass to hydroxyapatite was achieved in less than 6 days. X-ray diffraction revealed an initially amorphous product that subsequently crystallized to hydroxyapatite.     

Relationship between the local structure and spontaneous emission probability of Er3+ in silicate,borate, and phosphate glasses

Extended X-ray absorption fine structure (EXAFS) measurements have been performed on Er³¹ in silicate, borate, and phosphate glasses in order to investigate the local structure surrounding the Er³¹.Er³¹ ions coordinate to non-bridging oxygen ion sites, where alkali or alkaline earth ions terminate the network structure of silicate glasses. In borate glasses, the local structure surrounding Er³¹ ions is altered by the structural change ofthe borate anion. Er³¹ ions coordinate to non-bridging oxygen ion sites and BO₄ structural units in the cases with and without the formation of non-bridging oxygen, respectively. The former is similar to the case in silicate glasses. Er³¹ ions selectively coordinate to the PyO site regardless of the glass composition variation. A correlation was observed between the spontaneous emission probability for ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ and the average Er–O distance calculated by EXAFS analysis. It shows the maximum value near 2.32 Å, and we conclude that the overlapping radial integral of the 4f and 5d orbitals of Er³⁺ would be the largest at the optimum Er³¹– O²² distance 2.3 Å.