Photoluminescence and scintillation properties of Eu2O3–BaO–Nb2O5–TeO2 glass and glass ceramics

Journal of Materials Science: Materials in Electronics - 1Eu2O3–3BaO–20Nb2O5–76TeO2 glass and the corresponding glass-ceramics were synthesized with the aim to investigate the...     

Effects of glass components on crystallization and dielectric properties of BST glass–ceramics

Crystallization behavior was studied for glass powders in which some portions of AlF₃ in the net composition of 60(Ba_0.7Sr_0.3)TiO₃–25SiO₂–15AlF₃ were replaced with Ga₂O₃ or Bi₂O₃. The replacement with Ga₂O₃ resulted in a progressive increase in crystallization temperature, which effectively assisted the viscous sintering of glass powders to produce densified BST glass–ceramics at relatively lower temperatures. For the Bi₂O₃-replaced glass powders, an increasing amount of Bi₂O₃ replacement lowered the crystallization temperature and yielded less densified glass–ceramics containing a considerable amount of glassy phase. The temperature dependence of permittivity was estimated for the Ga₂O₃- and Bi₂O₃-replaced glass–ceramics as a function of sintering conditions and the amount of replacement, respectively.     

Effect of Bi4B2O9 addition on the sintering temperature and microwave dielectric properties of BaO–Nd2O3–4TiO2 ceramics

The effects of Bi₄B₂O₉ addition on the sintering temperature, phase transition and microwave dielectric properties of BaO–Nd₂O₃–4TiO₂ (BNT) ceramics have been investigated. With 10 wt% Bi₄B₂O₉ addition, the sintering temperature of the BNT ceramics can be lowered down to about 1,150 °C. The secondary phase was observed at the level of 15 wt% Bi₄B₂O₉ addition. The Bi₄B₂O₉ addition can significantly affects the microwave dielectric properties. The Q × f ₀ value is a function of the sintering temperature and the Bi₄B₂O₉ content. For the samples sintered at 1,150 °C, Q × f ₀ value varies from 6,300 to 3,300 GHz as the Bi₄B₂O₉ addition increases from 5 to 20 wt%. The addition of Bi₄B₂O₉ does not induce much degradation in ε_r but modified the τ_f value to near zero. Typically, When 10 wt% Bi₄B₂O₉ is added, the τ_f of the ceramics could be tuned to a near-zero value (~1.2 ppm/°C), a substantial ε_r (~86) and Q × f ₀ (~4,670 GHz) could also be achieved simultaneously. The Bi₄B₂O₉ is an efficient sintering additive to decrease the sintering temperature and tune the τ_f value of the microwave dielectric materials for the practical microwave applications.     

Effect of sintering atmosphere on the microstructure and dielectric properties of barium strontium titanate glass–ceramics

The sintering of barium strontium titanate glass–ceramics in nitrogen modified their dielectric properties significantly compared to the sintering in air. The experimental results demonstrate that the glass–ceramics sintered at low temperatures contain a major phase Ba₂TiSi₂O₈ (BTS), known as fresnoite. The fresnoite phase disappeared and the barium strontium titanate perovskite phase became the major phase when the sintering temperature was increased. In addition, the microstructure observation showed that both the proportion of crystal phase and the crystal size increase obviously with the increase of sintering temperature. Most importantly, impedance spectroscopy has been employed to study the electrical responses arising from the glass and the crystal phases in the glass–ceramics sintered at low temperatures and high temperatures. The magnitudes of impedance and modulus changed significantly for the glass–ceramics sintered at the two temperature ranges. The activation energy calculated from the complex impedance, complex modulus and dc conductivity suggests that the dielectric relaxation for the glass phase and the glass–crystal interface may be attributed to the motion of the dipole associated with oxygen vacancy. And for the barium strontium titanate perovskite glass–ceramics, the motion of the electrons from the second ionization of oxygen vacancies leads to dc electrical conduction. The mechanism for the giant dielectric properties of the glass–ceramics sintered at high temperatures in nitrogen is discussed.     

Effect of La2O3 additive on the dielectric properties of barium strontium titanate glass–ceramics

The barium strontium titanate (Ba_xSr_1–xTiO₃) glass–ceramics doped with different content of La were prepared via controlled crystallization. Phase compositions, microstructure and dielectric behaviors were investigated systematically. The results revealed that La₂O₃ additives had little influence on the dielectric constant but significantly changed the microstructure of the glass–ceramics, which led to improved breakdown strength (BDS). The optimized energy-storage density of 3.18 J/cm³ was achieved in the glass–ceramics with 1.0 wt% La₂O₃ content which is 2.56 times higher than pure BST glass–ceramics, suggesting glass–ceramics of this composition could be an attractive candidate for energy-storage applications.     

Influence of crystallization time on microstructures and dielectric properties of tungsten–bronze glass–ceramics

A glass with a composition of 22.5SrO–22.5BaO–15Nb₂O₅–40SiO₂ (mol %) was prepared by a melt-quenching method and then heat-treated at 950 °C for different crystallization time. Microstructure observations were carried out using scanning electron microscope and dielectric properties were measured by a LCR meter. The experimental results show that volume fraction of the crystalline phase increased, dielectric constant maximum enhanced, and Curie temperature shifted as the crystallization time is prolonged. The decrease in the Curie temperature for the sample crystallized at 950 °C for 1 h is considered to be caused by the clamping effect from the glass matrix or small compositional fluctuation. Impedance spectroscopy has been employed to study the polarization contributions arising from the glass and crystalline phases in the glass–ceramics for different crystallization time. With the increase in crystallization time, the magnitudes of impedance and modulus as well as the relaxation frequency changed significantly. The activation energy calculated from the relaxation frequency increased for the glass phase due to a denser network structure, while the crystalline phase showed a slight decrease implying there is no change in its polarization mechanism.     

Role of MgF2 on properties of glass–ceramics

Formation of machinable glass–ceramic in the system MgO–SiO₂–Al₂O₃–K₂O–B₂O₃–F with and without addition of MgF₂ has been investigated. Crystallization of glass sample was done by controlled thermal heat treatment at nucleation and crystallization temperatures. The results showed that MgF₂ in high concentration had a synergistic effect and enhanced the formation of interlocked mica crystals. Non-isothermal DTA experiments showed that the crystallization activation energies of base glasses were changed in the range of 235–405 kJ/mol, while the crystallization activation energies of samples with addition of MgF₂ were changed in the range of 548–752 kJ/mol.     

Effect of sintering process on electrical properties and ageing behavior of ZnO–V2O5–MnO2–Nb2O5 varistor ceramics

The effect of sintering process on microstructure, electrical properties, and ageing behavior of ZnO–V₂O₅–MnO₂–Nb₂O₅ (ZVMN) varistor ceramics was investigated at 875–950 °C. The sintered density decreased from 5.52 to 5.44 g/cm³ and the average grain size increased from 4.4 to 9.6 μm with the increase of sintering temperature. The breakdown field (E_{1 mA}) decreased from 6991 to 943 V/cm with the increase of sintering temperature. The ZVMN varistor ceramics sintered at 900 °C led to surprisingly high nonlinear coefficient (α = 50). The donor concentration (N_d) increased from 3.33 × 10¹⁷ cm⁻³ to 7.64 × 10¹⁷ cm⁻³ with the increase of sintering temperature and the barrier height (Φ_b) exhibited the maximum value (1.07 eV) at 900 °C. Concerning stability, the varistors sintered at 925 °C exhibited the most stable accelerated ageing characteristics, with %ΔE_{1 mA} = 1.5% and %Δα = 13.3% for DC accelerated ageing stress of 0.85 E_{1 mA}/85 °C/24 h.     

Effect of sintering temperature on electrical properties, dielectric characteristics, and aging behavior of ZnO–V2O5-based varistor ceramics modified with Bi2O3

The effect of sintering temperature on microstructure, electrical properties, dielectric characteristics, and aging behavior of ZnO–V₂O₅–MnO₂–Nb₂O₅–Bi₂O₃ varistor ceramics was systematically investigated at 875–950 °C. The sintered density decreased from 5.50 to 5.34 g/cm³ and the average grain size increased from 5.4 to 15.0 μm with an increase in the sintering temperature. The breakdown field (E_B) decreased from 5,785 to 1,181 V/cm with an increase in the sintering temperature. The varistor ceramics sintered at 900 °C exhibited a surprisingly high nonlinear coefficient (α = 61). The donor concentration (N_d) increased from 2.08 × 10¹⁷ to 4.64 × 10¹⁷ cm^?3 with an increase in the sintering temperature and the barrier height (Φ_b) exhibited 1.08 eV as the maximum value at 900 °C. Concerning stability, the varistors sintered at 950 °C exhibited the strongest accelerated aging characteristics, where %ΔE_B = ?1.4 % and %Δα = ?14.6 % for DC accelerated aging stress of 0.85 E_B/85 °C/24 h.     

Effects of MgO on properties of Li2O–Al2O3–SiO2 glass–ceramics for LTCC applications

Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramics for low temperature co-fired ceramics (LTCC) application were prepared by melting method, and the effects of MgO on the sinterability, microstructure, dielectric property, thermal expansion coefficient (CTE) and mechanical character of this glass–ceramics have been studied. The X-ray diffraction images represent that the main phase is β-spodumene solid solutions. And some ZrO₂ and CaMgSi₂O₆ phases in LAS glass–ceramics are detected. The LAS glass–ceramics without additive (MgO) sintered at 800° had the dielectric properties: dielectric constant (ε_r) of 5.3, dielectric loss (tanδ) of 2.97 × 10^?3 at 1 MHz, CTE value of 1.06 × 10^?6 K^?1, bulk density of 2.17 g/cm³, and flexural strength of 73 MPa. 5.5 wt% MgO-added LAS glass–ceramic achieves densification at 800° exhibited excellent properties: low dielectric constant and loss (ε_r = 7.1, tanδ = 2.02 × 10^?3 at 1 MHz), low CTE (2.89 × 10^?6 K^?1), bulk density = 2.65 g/cm³ as well as high flexural strength (145 MPa). The results indicate that the addition of MgO is helpful to improve the dielectric and mechanical properties. The formation of CaMgSi₂O₆ crystal phase with higher CTE leads to the increase of CTE value of LAS glass–ceramics due to the increasing MgO content, and the increase of CTE is favourable for matching with silicon (3.1 × 10^?6 K^?1). The prepared LAS glass–ceramics have the potential for LTCC application.     

Low temperature sintering and dielectric properties of Li2ZnTi3O8–TiO2 composite ceramics doped with CaO–B2O3–SiO2 glass

The effects of CaO–B₂O₃–SiO₂ (CBS) glass addition on the sintering temperature and dielectric properties of Li₂ZnTi₃O₈–TiO₂ (LZT) composite ceramics have been investigated. Due to the compensating effect of rutile TiO₂ (τ_f ≈ +450 ppm/ °C), the temperature coefficient of resonant frequency (τ_f) for Li₂ZnTi₃O₈ + 4 wt% TiO₂ with biphasic structure was adjusted to a value near zero. The pure LZT ceramics were usually sintered at high temperature of about 1,160 °C. It was found in our experiment that a small amount of CBS glass additives could effectively lower the sintering temperature of LZT ceramics to 900 °C. With increasing the content of CBS glass, both of dielectric constant (ε_r) and quality factor (Q × f) value decreased. Typically, the 1 wt% CBS glass added Li₂ZnTi₃O₈ + 4 wt% TiO₂ ceramic sintered at 900 °C for 4 h exhibited good microwave dielectric properties of ε_r = 26.9, Q × f = 23,563 GHz and τ_f = ?1.5 ppm/ °C, which made it promising for low temperature co-fired ceramics technology application.     

Dielectric behavior and energy storage properties in BaO–SrO–Nb2O5–B2O3 system glass–ceramics with Gd2O3 addition

A series of strontium barium niobate-based borate system glass–ceramics with Gd₂O₃ addition have been prepared by controlled crystallization method. The effect of Gd₂O₃ addition on the microstructure, phase evolution and dielectric properties has been investigated. The results show that the addition of Gd₂O₃ to the glass–ceramics changes the dielectric property and energy-storage density. Typically, the glass–ceramics with 0.5 mol% Gd₂O₃ heat treated at 630 °C/2 h + 800 °C/3 h possesses a dielectric constant of 136, a breakdown strength of 1,075 kV/mm and energy-storage density of 6.94 J/cm³, which is suitable for the application in high energy-storage capacitors.     

Varistor properties and microstructure of ZnO–BaO ceramics

Ceramics in the system ZnO–BaO have been investigated for possible use as varistors. Specimens were prepared by the mixed oxide route, and were sintered at temperatures in the range 1000–1400°C. The electrical properties were determined using d.c. A and impulse techniques. The ZnO–BaO ceramics have a two-phase microstructure comprising a ZnO phase and a Ba-rich grain boundary phase. Due to liquid phase sintering, the average grain sizes for the ZnO–BaO ceramics are large (typically 35–55 m for samples sintered at 1300°C). This results in low breakdown fields, (1000 V cm-1). The maximum non-linearity exponent obtained for ZnO–BaO ceramics (14) is higher than that for binary ZnO–Bi2O3 ceramics. However, the high water solubility of the Ba-rich phase may restrict the use of ZnO–BaO ceramics.     

Effects of nucleating agents on crystallization and microstructure of fluorophlogopite mica-containing glass–ceramics

The alteration of crystallization behavior, microstructure, and thermal properties of fluorophlogopite mica-containing glass–ceramics by nucleating agent is systematically studied. TiO₂, TiO₂ + ZrO₂, and ZrO₂ have been doped as the nucleating agents in the SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ (BMAPS) glass system and prepared by the melt-quench technique. The glass without nucleating agent is also prepared to ascertain the influence of nucleating agent. Addition of nucleating agents effectively increases the softening as well as glass transition temperatures. From the DSC study, it is found that the fluorophlogopite mica crystallization exotherm exhibited in the temperature range 800–850 °C and the activation energy varies in the range 167–182 kJ/mol. The opaque mica glass–ceramics are derived from these BMAPS glasses by a controlled heat treatment process and heat treatment at 1050 °C is found to be optimum. The mica crystals were identified as fluorophlogopite for all the four BMAPS glasses by X-ray powder diffraction (XRD) and subsequently confirmed by FTIR spectroscopy. Excellent matching with fluorophlogopite crystal was obtained in Zirconia-containing glass–ceramic as perceived from the XRD and FTIR studies. The microstructure of interlocked card-like mica flake crystals is found to form as seen from scanning electron microscopy, and such microstructure is obtained when ZrO₂ has been used as nucleating agent. Glass–ceramic without nucleating agent possesses Vickers hardness value 4.58 Gpa and it is increased with addition of the nucleating agent (5.67–6.56 GPa), ZrO₂-containing glass–ceramic possess lower hardness (5.67 GPa) and better machinability. Therefore, ZrO₂ is the most efficient nucleating agent to generate fluorophlogopite mica in these glass–ceramics useable for SOFC applications.