Improvement in the Microstructures and Dielectric Properties of Barium Strontium Titanate Glass–Ceramics by AlF3/MnO2 Addition

The microstructures and dielectric properties of barium strontium titanate glass–ceramics are closely related to the AlF₃ and MnO₂ additions. The grain morphology was changed by adding AlF₃, while the dielectric loss was decreased significantly by adding MnO₂. At the same time the breakdown strength (BDS) was improved by doping 4 mol% AlF₃ and 1 mol% MnO₂ with the glass–ceramics. The present investigation resulted in the development of glass–ceramic compositions with high dielectric BDS and low dielectric loss for high energy density capacitor applications.     

Phase Separation Inducing Controlled Crystallization of GeSe2–Ga2Se3–CsI Glasses for Fabricating Infrared Transmitting Glass–Ceramics

Infrared transmitting glass–ceramics based on the selected glass of 65GeSe₂–25Ga₂Se₃–10CsI were obtained by a two-stage heat-treatment method. Results of X-ray diffraction and scanning electronic microscopy indicated that droplet-like nanoparticles containing cubic Ga_2−δGe_δSe₃ crystals are homogeneously generated in the glass–ceramics and that the whole glass–ceramic process is composed of phase separation, nucleation, and crystal growth. Evolutions of the optical and mechanical properties of glass–ceramics versus annealing time at the first-stage heat treatment were also investigated. Compared with the parent glass, the fabricated glass–ceramics show considerably enhanced fracture toughness, practicable infrared transparence, and microhardness, which confer them with considerable competitive advantages over currently used infrared materials.     

Sintered Glass–Ceramics and Glass–Ceramic Matrix Composites from CRT Panel Glass

Sintering with simultaneous crystallization of powdered glass represents an interesting processing route for glass–ceramics, especially originating from wastes. Highly dense glass–ceramic samples may be obtained from a simple and short treatment at a relatively low temperature. In addition, glass–ceramic matrix composites may be obtained by mixing glass with suitable reinforcements. In this work sintered nepheline glass–ceramics, based on panel glass from cathode ray tubes, are illustrated. A limited addition of Al₂O₃ platelets caused a significant improvement in the mechanical properties (elastic modulus, bending strength, microhardness, fracture toughness), already remarkable for the un-reinforced glass–ceramic, compared with traditional nepheline glass–ceramics.     

Incorporation of Er3+ into BaTiO3

The incorporation of Er³⁺ into BaTiO₃ ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er³⁺ concentrations ≤1 at.%, dense and large-grained ceramics with low room-temperature resistivity (10²–10³Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er³⁺ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er₂Ti₂O₇ pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine-grained, and single phase. In this case, incorporation of Er³⁺ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er³⁺ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er³⁺ and the resulting properties are also strongly affected by sintering conditions.     

Effects of Samarium Oxide Addition on the Phase Composition, Microstructure, and Electrical Resistivity of Aluminum Nitride Ceramics

The phase composition, microstructure, and electrical resistivity of hot-pressed AlN ceramics with 0–4.8 wt% Sm₂O₃ additive were investigated. The phase composition was approximately consistent with that estimated from the Sm₂O₃–Al₂O₃ phase diagram using the amount of added Sm₂O₃ and oxygen content of the AlN raw material. When sintered at more than 1800°C, the AlN ceramics with 1.0–2.9 wt% Sm₂O₃ additive contained an Sm-β-alumina phase wetting the grain boundaries, and their electrical resistivity considerably decreased to 10¹⁰–10¹²Ω·cm. This resistivity decrease was caused by the continuity of the Sm-β-alumina phase with a resistivity lower than that of bulk AlN.     

Addition of GeO2 to Reduce the Viscosity of Parent Glasses for Low-Expansion, Transparent Glass–Ceramics Containing High-Quartz Solid Solutions

Parent glasses for fabricating glass–ceramics with nanometer-sized crystals usually have high viscosities, resulting in high processing temperatures. In this study, GeO₂ was added to a transparent, near-zero thermal-expansion Li₂O–Al₂O₃–SiO₂ glass–ceramic to reduce the viscosity of the parent glass. The effects of this compositional modification on the viscosity and crystal-nucleation rate of the parent glasses, and on the crystal size, thermal expansion, and optical transparency of the resulting glass–ceramics were investigated. It was found that addition of GeO₂ was useful in reducing the glass viscosity. Owing to the reduced nucleating rate with the increase in the GeO₂ content, the nucleating times required for reaching the smallest crystal size, the lowest coefficient of thermal expansion, and the highest transparency were all increased. With increasing GeO₂ content, the lowest coefficient of thermal expansion that can be reached for glass–ceramics increased (0.14–2.9 × 10⁻⁶ K⁻¹). The highest transparency of the GeO₂-containing glass–ceramics is almost as good as that of the GeO₂-free glass–ceramic and is almost independent of GeO₂ content when the crystal size is smaller than about 65 nm.     

Effect of Al Doping on the Electric and Dielectric Properties of CaCu3Ti4O12

Al-doped CaCu₃Ti_{4− x} Al _x O_{12− x /2} (CCTO, x =0–0.1) ceramics were prepared by the solid-state reaction, and their electric and dielectric properties were investigated. Al doping has been shown to reduce the dielectric loss remarkably while maintaining a high dielectric constant. At x =0.06, the loss tangent (tan δ) was below 0.06 over the frequency range of 10²–10⁴ Hz, and the dielectric constant was 41 000 at 10 kHz. Impedance spectra indicated that Al doping increased the resistivity of the grain boundary by an order of magnitude. The improvement of the dielectric loss in Al-doped CCTO was attributed to the enhanced grain boundary resistivity.     

Temperature–Time–Mechanical Properties of Glass–Ceramics Produced from Coal Fly Ash

Physical and mechanical properties of glass–ceramics fabricated from thermal power plant fly ash were analyzed and compared with suggest a temperature–time–mechanical (T–T–M) diagram. Coal fly ash with SiO₂–Al₂O₃–MgO–CaO as major components and TiO₂ as a nuclear agent were used to develop glass–ceramic materials which were heat treated at 900°–1050°C for 0.5–4 h for crystallization. It was verified that the high aspect ratio of unknown crystallines in the microstructure contributed high hardness, strength, fracture toughness, and wear resistance. These results are correlated with heat treatment conditions and microstructure and a T–T–M properties (hardness, strength, elastic constant, toughness, and wear rate) diagram on glass–ceramics produced from coal fly ash is proposed.     

Effects of V and Mn Colorants on the Crystallization Behavior and Optical Properties of Ce-Doped Li-Disilicate Glass–Ceramics

The critical cooling rate and fluorescence properties of lithium (Li) disilicate glasses and glass–ceramics, doped with 2.0 wt% CeO₂ and with up to 0.7 wt% V₂O₅ and 0.3 wt% MnO₂ added as colorants, were investigated. The critical cooling rates, R _c, of glass melts were determined using differential thermal analysis and were found to be dependent on the relative concentrations of V₂O₅ and MnO₂, decreasing from 25±3° to 16±3°C/min. Annealed glasses were heat treated first to 670°C, and then to 850°C to form Li metasilicate and Li disilicate glass–ceramics, respectively. The fluorescence intensities of the Ce-doped glasses and glass–ceramics decrease by a factor of 100 with the addition of the transition metal oxides. This optical quenching effect is explained by the association of the Ce³⁺ ions with the transition metal ions in the residual glassy phase of the glass–ceramics.     

Lithium Ion-Conducting Glass–Ceramics of Li1.5Al0.5Ge1.5(PO4)3–xLi2O (x=0.0–0.20) with Good Electrical and Electrochemical Properties

NASICON-type structured Li_1.5Al_0.5Ge_1.5(PO₄)₃– x Li₂O Li-ion-conducting glass–ceramics were successfully prepared from as-prepared glasses. The differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, and field emission scanning electron microscope results reveal that the excess Li₂O is not only incorporated into the crystal lattice of the NASICON-type structure but also exists as a secondary phase and acts as a nucleating agent to considerably promote the crystallization of the as-prepared glasses during heat treatment, leading to an improvement in the connection between the glass–ceramic grains and hence a dense microstructure with a uniform grain size. These beneficial effects enhance both the bulk and total ionic conductivities at room temperature, which reach 1.18 × 10⁻³ and 7.25 × 10⁻⁴ S/cm, respectively. In addition, the Li_1.5Al_0.5Ge_1.5(PO₄)₃–0.05Li₂O glass–ceramics display favorable electrochemical stability against lithium metal with an electrochemical window of about 6 V. The high ionic conductivity, good electrochemical stability, and wide electrochemical window of LAGP–0.05LO glass–ceramics suggest that they are promising solid-state electrolytes for all solid-state lithium batteries with high power density.     

A Modified Barium Titanate for Capacitors

The modified barium titanate ceramic Ba_0.80Pb_0.20 (Ti_0.88Zr_0.12) O₃ doped with 0.8 mol% CaO and 1 mol%ZrO₂ was prepared. Investigation of the electrical and dielectric properties shows that the material has a room-temperature dielectric constant of 3550 with a variation of 5.7% in the temperature range 10° to 60°C, a loss factor of 0.005, a dc resistivity 1.3 × 10¹²Ω-cm, and a linear variation of resistivity in the log (resistivity) vs temperature plot. This material is a good choice for the fabrication of ceramic capacitors.     

Formation of Core-Shell Structure in Ultrafine-Grained BaTiO3-Based Ceramics Through Nanodopant Method

The formation of core-shell structure is the key issue of temperature stable barium titanate-based dielectric ceramic for multilayer ceramic capacitor application. It is difficult to obtain and observe the core-shell structure in ceramic grains when the grain size becomes smaller, especially to sizes <200 nm. A nanodopant method has been developed for the preparation of ultrafine-grained barium titanate-based ceramics with grain size below 200 nm. The existence of core-shell structure was proved by transmission electron microscopy observation and energy-dispersive spectroscopy analysis. High-performance X7R dielectric ceramics were produced in a reducing atmosphere by normal sintering and two-step sintering method. The dielectric constant at room temperature could reach over 2000, with a low dielectric loss at about 1.0% and high insulation resistivity ∼10¹²Ω·cm.     

Glass–Ceramic Composites with Strontium Hexaferrite Ultrafine Particles in the SrO–Fe2O3–B2O3–SiO2 System

Glass samples with nominal compositions SrFe₁₂O₁₉+(12− n )SrB₂O₄+nSrSiO₃, n =3, 6, 9 were prepared by rapid quenching of the melt. Processes of glass devitrification were studied. The samples were annealed at temperatures of 600–900°C, and the resulting glass–ceramics was characterized by XRD, SEM, EDX, and magnetic measurements. SrFe₁₂O₁₉ crystallizes above 700°C and forms nano- and submicron platelet particles with the aspect ratio depending on the thermal treatment conditions. The glass–ceramic samples annealed at 900°C show coercive force values in the range of 422–455 kA/m.     

Bi4Ti3O12 Ceramics from Powders Prepared by Alternative Routes: Wet No-Coprecipitation Chemistry and Mechanochemical Activation

The processing of ferroelectric Bi₄Ti₃O₁₂ ceramics from powders prepared by wet no-coprecipitation chemistry (WNCC) and mechanochemical activation (MCA) has been investigated. Dense ceramics were obtained at sintering temperatures as low as 900°C. Exaggerated grain growth was observed for samples from WNCC, but not for those from MCA. Dielectric properties are discussed in relation to the type and concentration of defects, which is smaller for ceramic samples from WNCC. The activation energy of the dielectric relaxation for ceramics from MCA suggests that additional V _O^•• are present at the pseudoperovskite [Bi₂Ti₃O₁₀]²⁻ block in this case.     

Thermal Stability of Barium Ferrite (BaFe12O19)

Compositions in the system Fe₂O3-FeO-BaO in the vicinity of the compound BaFe₁₂O₁₉ were studied at temperatures from 1300° to 1550°C and oxygen pressures from 10⁻² to 10² atm. Equilibrium relations involving several barium ferrous ferrites are described. Barium ferrite can be crystallized congruently from the melt at 40 atm oxygen pressure and 15400°C.     

Phase Transition and Physical Characteristics of Nanograined BaTiO3 Ceramics Synthesized from Surface-Coated Nanopowders

Nanograined BaTiO₃ ceramics prepared from 40-nm-size BaTiO₃ nanopowders exhibited the cubic as well as the tetragonal phase, while nanograined BaTiO₃ ceramics prepared from BaTiO₃ nanopowders coated with Mn had only the tetragonal phase. The dielectric constant of the latter was 10 times larger than that of the former; the latter exhibited PTCR behavior with a resistivity jump ratio of about 5.0 × 10⁴. These physical properties of the BaTiO₃ ceramics appeared to be significantly affected by the strain near grain boundaries; such strain resulted in a phase transition from the cubic to the tetragonal phase in the nanograined BaTiO₃ ceramics, even though the grain size was about 40 nm.     

Effect of Reoxidation on the Grain-Boundary Acceptor-State Density of Reduced BaTiO3 Ceramics

To investigate the effect of reoxidation on the grain-boundary acceptor-state density of reduced barium titanate, n -doped BaTiO₃ ceramics are sintered in a reducing atmosphere (2% H₂+ 98% N₂) and then annealed in oxygen. After annealing at 1150°C for different times, the experimental results show a relationship between temperature-averaged acceptor-state density and annealing time as N _s= N _so Bt ^1/n with n between 2 and 3. An inherent acceptorstate density N _so= 4.25 × 10¹² cm⁻² is obtained with an increase rate B = 4.8 × 10¹² cm⁻². min^−1/3, when n reaches 3. The inherent grain-boundary acceptor states in the reduced n -doped BaTiO₃ ceramics are believed not due to adsorbed oxygen ions.     

Viscous Flow as the Driving Force for the Densification of Low-Temperature Co-Fired Ceramics

Filled glass–ceramic composites, like low-temperature co-fired ceramics (LTCC), must densify at temperatures <900°C. The densification mechanism of LTCC is often described by liquid-phase sintering. The results of this paper clearly show that densification of ceramic-filled glass–composites with a glass content above 60 wt% can be attributed to viscous sintering, which is decisively controlled by the viscosity of the glass during the heat treatment. This is demonstrated by the experimental determination of the viscosity of a MgO–Al₂O₃–B₂O₃–SiO₂ glass dependent on temperature, by investigation of the wetting behavior of the glass on the ceramic filler mullite, and of the microstructural development. It was found that the glass does not wet the filler material in a temperature range up to 1000°C. Therefore, liquid-phase sintering can be excluded. Independent of any wetting effect and therefore in the absence of capillary forces, densification starts at a temperature of 750°C, which corresponds to a viscosity of 10^9.5 dPa·s. This densification can be attributed to viscous flow of the glass matrix composite.     

Fabrication and Tunable Dielectric Properties of (Ba0.7Sr0.3)TiO3-Glass-Based Thick-Film Capacitors

Ferroelectric glass–ceramics of composition 0.90 (Ba_0.7Sr_0.3) TiO₃–0.10(B₂O₃:SiO₂) (0.90 BST:0.10 BS) synthesized by sol–gel method have been used for the preparation of dielectric thick-film inks. The particle dispersion of the glass–ceramic powders in the thick-film ink formulations have been studied through rheological measurements for fabricating thick-film capacitors by screen printing technique. The thick films derived from such glass–ceramics are found to sinter at considerably lower temperatures than the pure ceramic, and exhibit good dielectric characteristics with a tunability of 32% at 1 MHz under a dc bias field of 35 kV/cm.     

Chemical Characterization of Si-Al-C-O Precursor and Its Pyrolysis

Polycarbosilane was modified by reaction with an aluminum alkoxide to get a precursor for Si-Al-C-O ceramics. The precursor was essentially characterized by magic-angle spinning nuclear magnetic resonance (²⁹Si, ²⁷Al, and ¹³C) and appeared as a dispersion of Al(OH)₃-based particles in a polycarbosilane chain matrix. After the material was heat-treated under argon at 1500°C, X-ray diffraction showed that it crystallized mainly as SiC 2H. The presence of this rather unusual polytype for polycarbosilane-derived ceramics seemed to be related to the presence of aluminum atoms.