Variation of luminescence properties of Na2O-CaO-SiO2: Nd glass with crystallinity

The optical absorption and emission intensities of Nd³⁺-doped transparent glass-ceramics with high crystallinity in Na₂O-CaO-SiO₂ (NSC) system were studied. The transmittance of NCS decreases with increasing degree of crystallinity, however it still remains 65.5% at λ = 710 nm when the crystallization is almost completed. Judd-Ofelt theory is performed to evaluate the radiative transition probability as well as quality factor, branching ratio and emission cross section. The maximum values of Ω₂ and emission cross section (⁴F_3/2 → ⁴I_11/2) of NCS are obtained after nucleating at 630 °C for 10 h. The quality factor increases with increasing crystallinity, while branching ratio for ⁴F_3/2 → ⁴I_11/2 is opposite. The results show that transparent glass-ceramic with high crystallinity is a potential laser host for 1.06 μm emission.     

Phase separation and crystallization in the system SiO2-Al2O3-P2O5-B2O3-Na2O glasses

The phase separation and crystallization behavior in the system (80 − X)SiO₂ · X(Al₂O₃ + P₂O₅) · 5B₂O₃ · 15Na₂O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al₂O₃-P₂O₅-SiO₂ and forms a continuous phase. Glasses containing a larger amount of Al₂O₃-P₂O₅ (X = 40 and 50) readily crystallize and precipitates tridymite type AlPO₄ crystals. It is estimated that the phase separation occurs forming continuous Al₂O₃-P₂O₅-SiO₂ phase at first, and then tridymite type AlPO₄ crystals precipitate and grow in this phase. Highly transparent glass-ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass-ceramics are 20-30 nm and about 50%, respectively.     

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.     

The dielectric behavior of a thermoelectric treated B2O3-Li2O-Nb2O5 glass

A transparent glass with the composition 60B₂O₃-30Li₂O-10Nb₂O₅ (mol%) was prepared by the melt-quenching technique. Glass-ceramics, containing LiNbO₃ ferroelectric crystallites, were obtained by heat-treatment (HT) above 500 °C, with and without the presence of an external electric field. The dielectric properties of the glass and glass-ceramic were investigated, as a function of temperature (270-315 K), in the 10 mHz-32 MHz frequency range. The presence of an external electric field, during the heating process, improves the formation of LiNbO₃ crystallites. The rise of the treatment temperature and the applied field, during the heat-treatment, leads to a decrease in the dc electric conductivity (σ_dc), indicating a decrease of the charge carriers number. The dielectric permittivity (ε′) values (300 K;1 kHz) are between 16.25 and 18.83, with the exception of the 550 °C HT sample that presents a ε′ value of 11.25. An electric equivalent circuit composed by an R in parallel with a CPE element was used to adjust the dielectric data. The results reflect the important role carried out by the heat-treatment and the electric field during the HT in the electric properties of glass-ceramics.     

Crystallization processes of Li2O-Ga2O3-SiO2-NiO system glasses

Crystallization processes of Li₂O-Ga₂O₃-SiO₂-NiO system glasses have been studied by X-ray diffraction, differential calorimetry and optical absorption. Transparent glass-ceramic containing LiGa₅O₈:Ni²⁺ as the sole crystalline phase has been obtained from glass with the composition of 13Li₂O-23Ga₂O₃-64SiO₂-0.1NiO (in mol%) by the heat treatment in the temperature range from 923 to 953 K. It was revealed that the specific surface area of samples enhances crystallization of LiGaSi₂O₆ but obstructed that of LiGa₅O₈. LiGa₅O₈ grew to nano-sized crystallites dispersed in the glass matrices and did not affect the transparency seriously. In contrast, LiGaSi₂O₆ grew to crystallites with diameters more than 100 nm on the surface and made the glasses opaque. Optical absorption measurements revealed that doped Ni²⁺ occupied five-folded trigonal bipyramidal sites in the as-quenched glass matrices but six-folded octahedral sites of precipitated LiGa₅O₈ in the glass-ceramics. It was confirmed that transparent glass-ceramic containing Ni²⁺:LiGa₅O₈ was effectively obtained by the heat treatment at a temperature of 953 K for 10 h.     

Glass preparation and temperature-induced crystallization in multicomponent B2O3-PbX2-PbO-Al2O3-WO3-Dy2O3 (X = F, Cl, Br) system

Multicomponent lead borate glasses modified by PbX₂ (X = F, Cl or Br) were examined. For the first time, lead tungstate PbWO₄ crystallites dispersed into glass matrices were successfully obtained from controlled crystallization. Excitation by 310 nm line leads to broad blue luminescence related to the radiative transition which occurred in the PbWO₄ crystallites. It was found that halogen X ions (X = F, Cl or Br) were also incorporated in the distorted crystal system of PbWO₄. It was proved by results obtained from X-ray diffraction as well as luminescence measurements.     

Vitrification and crystallization in the system of K2O-B2O3-TiO2

The vitrification and crystallization behavior of melts produced at 1400 °C in the ternary system of K₂O-B₂O₃-TiO₂ is investigated. It is shown that there are two fields of compositions (indicated in mol%) which allow obtaining the glass-ceramic materials with continuous glassy matrix after the cooling of molten compositions. In the first field [TiO₂] = 25-57, [K₂O] = 30-50 and [B₂O₃] = 0-25, the glass-ceramics consisted of the potassium-titanium-borate glassy phase and different crystalline potassium titanates (K₄Ti₃O₈, K₂Ti₂O₅, K₂Ti₄O₉, K₂Ti₆O₁₃). The ratio of TiO₂:K₂O in the obtained titanates increases with [TiO₂] and [B₂O₃]. In the second field, [TiO₂] = 7-37, [K₂O] = 0-25 and [B₂O₃] = 52-93, the obtained glass-ceramics consisted of a similar vitreous phase, as mentioned above, and TiO₂ crystals. During the cooling of the melts, short whiskers-like crystals of anatase formed in the compositions with relatively low [TiO₂] and relatively high [K₂O], whereas long fiber-shaped crystals of rutile appeared with the compositions characterized with relatively high [TiO₂] and relatively low [K₂O]. The possible application of the obtained glass-ceramic materials as a source of fibrous TiO₂, for composite reinforcement, and as solid lubricants is discussed.     

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.     

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.     

Optical properties of Cr ion in lithium metasilicate Li2O · SiO2 transparent glass-ceramics

The optical properties of Cr³⁺ ions in lithium metasilicate (Li₂O · SiO₂) transparent glass-ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li₂O · SiO₂) crystal. The percent crystallinity and crystalline size were ranging 65-75% and 20-35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr³⁺ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr³⁺ ions substitute for three Li⁺ ions and occupy the distorted octahedral site between single [SiO₄]_n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq = 13 088 cm⁻¹, B = 453 cm⁻¹, Dq/B = 2.89 and C = 2036 cm⁻¹. The near-infrared luminescence centered at 1250 nm was not detected in the deep pink glass-ceramics unlike emerald green glass.     

Glasses and glass-ceramics in the oxyfluoride ternary system Pb(PO3)2-WO3-PbF2

Glasses were prepared by the melt-quenching method in the ternary system Pb(PO₃)₂-WO₃-PbF₂ and doped with Er³⁺ in order to prepare luminescent transparent glass-ceramics. This work focused on thermal and structural characterization of tungsten lead-phosphate glasses and crystallization study for preparing transparent glass-ceramics. Thermal properties such as thermal stability and crystallization behavior upon heating were investigated by DSC in function of PbF₂ content. For low PbF₂ concentrations, only one crystallization peak due to Pb₃(PO₄)₂ is observed whereas samples containing more than 15% of PbF₂ present another exothermic event at lower temperatures related with precipitation of PbF₂, Pb₂P₂O₇ and Pb₂OF₂. Structural investigations by Raman spectroscopy suggest that PbF₂ modifies the tungsten-phosphate network through the formation of P―F and P―O―Pb bonds but the average network connectivity remains almost constant. A crystallization study has been performed by DSC to investigate the dominant crystallization mechanisms in these glasses and it has been established that Pb₃(PO₄)₂ is nucleated on the surface whereas PbF₂, Pb₂P₂O₇ and Pb₂OF₂ crystallize dominantly from the glassy bulk. Transparent glass-ceramics containing nanosized PbF₂ crystallites were also prepared by suitable heat-treatment on the glass sample containing 20% of PbF₂ and Raman microscopy of these glass-ceramics supports the crystallization mechanisms determined by DSC.     

Self organized nano crystallinity of magnetite precipitated from a 4.9Na2O · 33.3CaO · 17.1Fe2O3 · 44.7B2O3 glass

Glasses with the mol% composition 4.9Na₂O · 33.3CaO · 17.1Fe₂O₃ · 44.7B₂O₃ were melted, rapidly quenched using a twin roller technique, and subsequently tempered in the range from 550 to 620 °C. This led to the crystallization of magnetite with mean crystallite sizes in the 10-20 nm range. Using higher temperatures resulted in a larger quantity of formed crystallites and slightly larger mean crystallite sizes. Larger tempering times did not lead to substantial crystal growth. The time law of Ostwald ripening was not followed. This is explained by an increase in viscosity of the residual glassy phase during nucleation and crystal growth. Here, the smaller iron concentration near the crystals leads to higher viscosities and to the formation of a diffusional barrier around the crystals, which reduces further crystal growth. The crystallization stops, if T_g of the residual glassy phase is equal to the tempering temperature. Magnetite nano crystals with sizes in the 10-20 nm range offer a wide range of applications, such as the preparation of ferrofluids or of materials for medical diagnostics and therapy.     

Development and characterizations of BaO-CaO-Al2O3-SiO2 glass-ceramic sealants for intermediate temperature solid oxide fuel cell application

Several compositions based on BaO-CaO-Al₂O₃-SiO₂ (BCAS) glass system have been studied in this investigation to see their applicability as sealant for solid oxide fuel cell (SOFC). The glasses as well as the corresponding glass-ceramics have been systematically characterized by differential thermal analysis, dilatometry, X-ray diffractometry, electron microscopy and impedance analysis to examine their suitability as sealant. While the glass transition temperature (T_g) determined from DTA are within 600-665 °C, the coefficient of thermal expansion (CTE) can be tailored between 9.5 and 13.0 × 10⁻⁶ K⁻¹. These glasses are found to be well adhered with metallic interconnects, such as commercial ferritic steel (Crofer22APU), at an optimum sealing temperature of 850 °C. The shrinkage behavior of the developed glasses in their pellet form has also been investigated. The resistivities of the glass-ceramics, as obtained from impedance analysis, are found to be within 10⁴-10⁶ Ω cm at 800 °C. Under sandwiched condition between two metals, some of the developed compositions are found to maintain this high resistivity even after 100 h of operation. One of the glass compositions has shown a low leak-rate of the order of ∼10⁻⁷ Pa m² s⁻¹.     

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.     

Synthesis and physical properties of glasses in the Sb2O3-PbCl2-MoO3 system

Lead chloroantimonite glasses form stable binary glasses that may accommodate numerous oxides or halides as a third component. Molybdenum trioxide is a glass progenitor leading to molybdate glasses. Ternary glasses have been synthesized and studied in the Sb₂O₃-PbCl₂-MoO₃ system. Compositional limits of glass formation are reported and two series of glass samples have been prepared corresponding to the general formulas: (90 − x)Sb₂O₃-xPbCl₂-10MoO₃ and (90 − x)Sb₂O₃-xMoO₃-10PbCl₂. Glass transition temperature is close to 290 °C at high Sb₂O₃ content and decreases as antimony oxide is substituted by MoO₃ or PbCl₂. Position, width and intensity of crystallization peak suggest that devitrification rate is small in some composition ranges. The evolution of density, thermal expansion, refractive index and microhardness has been studied as a function of composition parameter x. Deviations from linearity are observed. They suggest structural changes in the case of the MoO₃/Sb₂O₃ substitution while it appears that molar volume increases linearly versus lead content in the other series of glasses. Refractive index is close to 2.04. Optical transmission ranges from 550 nm in the visible spectrum to 5.5 μm in the infrared. It is limited by extrinsic absorption bands arising from hydroxyls and silicon impurities. Young's, bulk and shear moduli have been measured for the two series of samples.     

Effect of P2O5 on the early stage crystallization of K-fluorrichterite glass-ceramics

The addition of 2 mol% P₂O₅ to stoichiometric K-fluorrichterite (KNaCaMg₅Si₈O₂₂F₂, KFR) has been reported to enhance the mechanical properties and improve the in vitro biocompatibility of this glass-ceramic by promoting the formation of enstatite and fluorapatite (FA). Here, the effect of further increasing the P₂O₅ concentration on phase evolution of KFR has been investigated. XRD data showed that mica crystallized in samples with 4 and 5 mol% P₂O₅ (GP4 and GP5, respectively) at 650 °C, but no diopside was detected at higher temperatures, in contrast with the general phase evolution in KFR based glass-ceramics. More importantly, however, the addition of ?4 mol% P₂O₅ induced phase separation of the glass into a silica glass matrix and phosphate rich droplets prior to crystallization. EDS traces taken from samples heat-treated at 600 °C, revealed that the silica glass matrix was deficient in Mg and unlikely to be the host for crystallization of mica. Conversely, the P₂O₅ rich regions contained excess Mg and were considered to be the host for the formation of mica and FA.     

Crystallization behavior and microstructure of powdered and bulk ZnO-Al2O3-SiO2 glass-ceramics

The crystallization behavior of glass with the composition: 55.6 mol% SiO₂, 22.8 mol% Al₂O₃, 17.7 mol% ZnO and 3.84 mol% of TiO₂ as nucleating agent and with different particle sizes has been studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and tranmission electron microscopy (TEM). In glass powders two crystalline phases: zinc-aluminosilicate s.s. with high-quartz structure, Zn_x/2Al_xSi_3−xO₆, (x varies dependent on heat-treatment temperature) and gahnite are formed. The ratio of these phases depends on particle sizes. In bulk glass, however, gahnite is the sole crystalline phase. The composition of initially formed zinc-aluminosilicate s.s. was determined by Rietveld refinement of XRD patterns to be Zn_0.69Al_1.38Si_1.62O₆. With temperature increase, the amount of zinc-aluminosilicate s.s decreased with simultaneous reduce of zinc and aluminum incorporated in the structure. Eventually at 1423 K almost pure high-quartz structure was formed. The activation energies of zinc-aluminosilicate s.s. and gahnite crystallization were determined by non-isothermal method to be 510 ± 18 and 344 ± 17 kJ mol⁻¹, respectively. The latter value matches well with those cited in literature for crystal growth of gahnite in similar glasses. That is attributed to the fact that the high-quartz structure acts as a precursor for gahnite crystallization.     

Effect of La doping on microstructure of SnO2 nanopowders prepared by co-precipitation method

A series of La-doped SnO₂ nanopowders with various dopant concentrations were prepared by chemical co-precipitation technique, and the nanopowders prepared were characterized by differential scanning calorimeter (DSC), thermo-gravimetric (TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results show that La doping can obviously prevent the growth of nanosized SnO₂ crystallites. When the La concentration reaches and surpasses 5 at.%, SnO₂ crystallite size reaches a minimum value and remains almost constant. With the increase of La concentration, La tends to dissolve in the bulk phase of SnO₂ to form solid solution below 10 at.% addition and then starts to disperse onto the surface of the solid solution as a monolayer above 10 at.%. The effect of La doping on hindering crystallite growth can be attributed to the solute drag and lattice distortion resulting from La dissolving in the bulk phase of SnO₂ to form solid solution, rather than the monolayer of La on the surfaces of the SnO₂ powders.     

Non-isothermal crystallization kinetics of La2CaB10O19 from glass

The non-isothermal crystallization kinetics of La₂CaB₁₀O₁₉ (LCB) from a La₂O₃-CaO-B₂O₃ glass was studied. Differential thermal analysis methods were performed on three glass powders to obtain the kinetic parameters of LCB crystallization mechanism. The activation energies for overall crystallization (E), obtained by the methods of Kissinger and Ozawa, were in the range of 479-569 kJ/mol. Multiple (five) analysis methods were used to estimate the Avrami exponent (n), which could consequently be reduced into the single value of n = 3.1 ± 0.3. The growth morphology index (m) of LCB was corroborated by microscopy (optical and electron) images, which revealed a three dimensional growth. Energy dispersive spectroscopy confirmed that LCB is the crystallizing phase from the glass by an interface controlled mechanism. The parameters of the Johnson-Mehl-Avrami kinetic model for the analysis of LCB crystallization from glass were found to be n = m = 3.     

Glass formation in the Sb2O3-CdCl2-SrCl2 ternary system

Vitreous systems based on antimony oxide Sb₂O₃ have been investigated. The limits for glass formation are reported in the Sb₂O₃-CdCl₂-SrCl₂ ternary system, and in the Sb₂O₃-SrCl₂-(0.5CdCl₂ + 0.5ZnCl₂) pseudo ternary system. Amorphous state is confirmed by XRD patterns. Thermal measurements implemented by differential scanning calorimetry show that Tg is above 300 °C. As a general trend, Tg increases with chloride content. These glasses have an extended transmission in the mid-infrared spectrum with a refractive index larger than 2. The influence of the CdCl₂/Sb₂O₃ substitution on the physical properties has been studied in the (90 − x)Sb₂O₃-xCdCl₂-10SrCl₂ system: thermal expansion, Vickers microhardness, Young and bulk modulus decrease as cadmium concentration increases. An inverse dependence is observed for shear modulus. This behaviour is discussed in relation to structural hypothesis.