Influence of V2O5 Additions to 5Li2O–1Nb2O5–5TiO2 Ceramics on Sintering Temperature and Microwave Dielectric Properties

The effects of the addition of V₂O₅ on the sintering behavior, microstructure, and microwave dielectric properties of 5Li₂O–1Nb₂O₅–5TiO₂ (LNT) ceramics have been investigated. With low-level doping of V₂O₅ (≤3 wt%), the microstructure of the LNT ceramic changed from a special two-level intergrowth structure into a two-phase composite structure with separate grains. And the sintering temperature of the LNT ceramics could be lowered to around 900°C by adding a small amount of V₂O₅ without much degradation in microwave dielectric properties. Typically, better microwave dielectric properties of ɛ_r=41.7, Q × f =7820 GHz, and τ _f =45 ppm/°C could be obtained for the 1 wt% V₂O₅-doped ceramics sintered at 900°C.     

Microwave Dielectric Properties of 5Li2O–0.583Nb2O5–3.248TiO2 Ceramics with V2O5

The effects of V₂O₅ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O–0.583Nb₂O₅–3.248TiO₂ (LNT) ceramics have been investigated. With addition of low-level doping of V₂O₅ (≤2 wt%), the sintering temperature of the LNT ceramics could be lowered down to around 920°C due to the liquid phase effect. A secondary phase was observed at the level of 2 wt% V₂O₅ addition. The addition of V₂O₅ does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the excellent microwave dielectric properties of ɛ_r=21.5, Q × f =32 938 GHz, and τ_f=6.1 ppm/°C could be obtained for the 1 wt% V₂O₅-doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as an internal electrode.     

Low-Temperature Densification of Lead Zirconate-Titanate with Vanadium Pentoxide Additive

Additions of 0.1 to 6.0 wt% V₂O, to lead zirconate titanate (PZT) ceramics promoted rapid densification below 975°C, thereby eliminating the need for PbO atmosphere control The base PZT, Pb(Zr_0.53Ti_0.47)O₃, was prepared by coprecipitation from mixed oxides and butoxides. The V₂O₅ was incorporated as a batch addition during the PZT coprecipitation process, as mill additions to the calcined precipitated powder, and to a commercial PZT powder. Densification rates were enhanced by the addition of V₂O₅ (>98% of theoretical density was obtained in ∼15 min at 960°C by the addition of 0.1 to 1.0 wt% V₂O₅, compared to 4 h at 1280°C for the base PZT). Dielectric properties and piezoelectric coefficients varied slightly within the optimum range of 0.25 to 1.0 wt% V₂O₅ addition but were at least comparable to the base PZT. Indications are that V₂O₅ becomes incorporated into the surface layers of the oxide powders during mixing (or in the coprecipitation process) and that the accelerated densification is due to enhanced surface activation and liquid-phase sintering.     

Magnetic Behavior and Microstructure of Vanadium Phosphate Glasses

The magnetic properties and microstructures of the vanadium phosphate glass system over the composition range 60 to 90 mol% V₂O₅ were investigated to study magnetic ordering in the glass and the effect of microstructure on its magnetic properties. Direct antiferromagnetic coupling between V⁴⁺ ions in the glassy matrix exists, and a transition temperature near - 70°C was observed. As-cast glasses with high V₂O₅ concentrations separated into two glassy phases; this separation increased the ESR line width as a result of inhomogeneity broadening. The separation, which concentrated the vanadium ions in a vanadium-rich phase, caused a hysteresis in the plot of ESR line intensity vs temperature at the transition temperature. Reduction of the vanadium ions by dextrose added to the melt enhanced phase separation and resulted in weak antiferromagnetic transitions at +70° and -120°C, the Neel temperatures of VO₂ and V₂O₃, respectively.     

Electronic Conductivity and Related Properties of Amorphous and Crystallized V2O5-Based Glasses

Crystallization of V₂O₃ from V₂O₃P₂O₃, glasses containing 0 to 9 mol% B₂O₃, during heat treatment in the range 220° to 410°C, caused progressive micro structural changes which dramatically affected the electronic conductivity (γ), the activation energy for conduction ( W ), and the resistance to chemical attack. All compositions were ≊83% crystalline after heating to 410°C. As a result, the values of γ and W were almost identical to those observed for pure polycrystalline V₂O₅.     

Electrical Conductivity of PbO-P2O5-V2O5Glasses

The dc conductivities (α) of PbO-P₂O₅-V₂O₅ glasses containing up to 80 mol% V₂O₅ were measured at T = 100°C to T = 10°C below the glass transition temperature. Dielectric constants at 1 MHz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism of glasses containing >10 mol% V₂O₅ was considered to be small-polaron hopping, as previously reported for other vanadate glasses. The temperature dependence of α was exponential, with α= (αo/ T ) exp(− W/kT ). When the V₂O₅ content was ≥50 mol%, W decreased and α increased with increasing V₂O₅ content, and the adiabatic approximation could be applied. In the composition range between 10 and 50 mol% V₂O₅, α increased with increasing V₂O₅ content, but W varied little. In this region, the hopping conduction was characterized as nonadiabatic. The effect of dielectric constants and V ion spacing on W is discussed.     

Microstructure and Current–Voltage Characteristics of Multicomponent Vanadium-Doped Zinc Oxide Varistors

The effects of the oxide additives MnO₂, Co₃O₄, and Sb₂O₃, commonly incorporated in commercial Bi₂O₃-doped ZnO varistors, on the current–voltage characteristics and microstructure of 0.25 mol% V₂O₅-doped ZnO varistors have been studied. MnO₂ is the most significant additive in terms of its effects on varistor performance. Varistor performance can also be improved by increasing the V₂O₅ content to 0.5 mol% in a ZnO ceramic containing 1 mol% MnO₂. Further increases in the V₂O₅ content of 1 mol% MnO₂-doped material cause a deterioration in varistor behavior. The microstructure of the samples consists mainly of ZnO grains with zinc vanadates as the minority secondary phases. Additional spinel phase is formed when Sb₂O₃ is incorporated.     

Preparation of Whisker β-Spodumene Glass–Ceramics

β-spodumene glass–ceramics with a whisker-like microstructure were prepared from the following materials (in wt%): 64.5 SiO₂, 18.0 Al₂O₃, 4.2 Li₂O, 4 ZrO₂, and 8 MgF₂. Scanning electron microscope (SEM) analysis showed that phase separation in the base glass leads to the formation of a primary crystal phase of MgF₂ that promotes the formation of spherical β-spodumene. Whisker spodumene crystals surrounded by spherical crystals are observed at 720°C after 1 h, and the whisker crystals grow at the cost of spherical crystals with increasing temperature and time. The flexural strengths of the glass–ceramics reach a maximum of 228 MPa after heat treatment at 850°C for 1 h.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

Barium/Lead-Rich High Permittivity Glass–Ceramics for Capacitor Applications

Dielectric properties of glass–ceramics containing barium/lead-based sodium niobates and barium titanate-based silicates were evaluated for capacitor applications. The glasses were formed by melt-rolling the respective constituents which were then crystallized by reheating them at higher temperatures. Crystallization schedules were formulated based on differential thermal analysis results. X-ray diffractometer patterns indicated that the samples were highly crystallized. Microstructure and microchemistry of samples were studied by transmission electron micropscopy. Perovskite, tungsten–bronze and fresnoite phases developed during crystallization have a strong effect on the resulted dielectric properties with permittivities ranging from 20 to 700. Resistivity measurements were done to study conduction mechanisms in samples and the resistivity values for glass–ceramics were found to be between 10¹¹ and 10¹³Ω·cm at 150°C. MnO₂ additions were made to improve the electrical resistivity of glass–ceramics.     

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.     

Structure and Mechanism of Conduction of Semiconductor Glasses

The area of glass formation in the system GeO₂-P₄O₁₀-V₂O₅ and the properties of the glasses in this area were determined. The glasses displayed electronic conduction at room temperature (25°C). Resistivity ranged from 500 ohm-cm to 10⁹ ohm-cm at 25°C. Some of the glasses had unusual negative temperature coefficients of resistance of the order of -760,000 ppm °C⁻¹ in the range 25° to -55°C. Volt-ampere characteristics indicated nonlinearity suitable for thermistor application. Other unusual properties included high refractive indices from 1.6 to >2.0 and dielectric constants from 6 to 33 at 1 Mc and 25°C. Values of loss-tangents, however, were high. Infrared spectra indicated that the V⁵⁺ ion existed in sixfold coordination in the glassy state as well as in the devitrified crystalline state. The normal vibrational frequency of the V–O bond at 1015 cm⁻¹ was observed for all glasses in the system. Property versus composition curves indicated that density, refractive index, and dielectric constant of ternary glasses in the system do not obey the additivity rule. The density versus mole % V₂O₅ curve goes through a minimum. Derived quantities from experimental data indicate pronounced influence of V₂O₅ on oxygen packing in the system. Addition of SiO₂, even in small quantities, destroys glass formation. The structure of these glasses, which differs from that of silicate glasses, is discussed. A mechanism of conduction is suggested, based on evidence from magnetic susceptibility, chemical analysis, activation energy, and infrared spectra.     

Electrical Conductivity of TiO2-V2O5-P205 Glasses

The dc conductivities (σ) of V₂O₅-P₂O₅ glasses containing up to 30 mol% TiO₂ were measured at T=100° to ∼10°C below the glass-transition temperature. Dielectric constants from 30 to 10⁶ Hz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism was considered to be small polaron hopping between V ions, as previously reported for V₂O₅-P₂O₅ glass. The temperature dependence of σ was exponential with σ = σ₀ exp(-W/kT ) in the high-temperature range. When part of the P₂O₅ was replaced by TiO_2,σ increased and W decreased. The hopping energy depended on the reciprocal dielectric constant which, in this case, increased with increasing TiO₂ content.     

Effect of ZnO and CuO on the Sintering Temperature and Piezoelectric Properties of a Hard Piezoelectric Ceramic

Hard piezoelectrics with high dielectric and piezoelectric constants are used for high-power applications. However, the sintering temperature of these ceramics is high, around 1200°C, restricting the usage of cheap base metal electrodes in fabrication of multi-layer components. This study investigates the effect of CuO and ZnO on the sintering temperature of a hard piezoelectric, APC 841, which is a MnO₂- and Nb₂O₅-modified PZT. The addition of CuO decreased the sintering temperature through the formation of a liquid phase. However, the piezoelectric properties of the CuO-added ceramics sintered at ≤950°C were lower than the desired values. The addition of ZnO resulted in a significant improvement in the piezoelectric properties. This enhancement was attributed to the formation of a homogeneous microstructure with large grains. The APC 841+0.2 wt% CuO+1.1 wt% ZnO ceramics sintered at 950°C showed excellent piezoelectric and dielectric properties with values of k _p=0.532, Q _m=750, d ₃₃=351 pC/N, ɛ₃₃/ɛ_o=1337, and T _c=280°C.     

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.     

Concentrations of Residual Carbonate Ions in the Bi-Sr-Ca-Cu-O Glasses

It was found that the IR absorption bands appearing at 600 to 1600 cm⁻¹, which had been previously assigned to the fundamental vibrations of [BiO₃] or [BiO₆] polyhedra, are due to residual carbonate ions (CO²⁻₃) dissolved in Bi-Sr-Ca-Cu-O glasses. The concentrations of the remaining CO²⁻₃ in the Bi_2.2Sr₂Ca₁Cu₂O _x glasses that melted at 1100° and 1400°C are 170 × 10⁻⁵ mol/cm³ (3.3 mol%) and 3.2 × 10⁻⁵ mol/cm³ (0.25 mol%), respectively. The apparent activation energy for the dissociation of the carbonates was approximately 220 kJ/mol. The CO²⁻₃ content in the precursor glasses did not significantly affect the superconducting properties of the resulting glass–ceramics.     

Electrochemical Studies in Glass: I, The System NiO-Na2Si2O5

A solid electrolyte electrochemical cell of the type Pt|Ni:NiO _{a =1}∥ZrO₂+7.5% CaO∥Ni:NiO _{a <1}+glass|Pt was used to measure the activities of NiO in sodium disilicate glass from 750° to 1100°C. The data indicate a solubility varying from 11 mol% (5.0 wt%) at 800° to 20 mol% (9.3 wt%) at 1100°C. From the variation in NiO activity, the activity of sodium disilicate in glass solution was estimated; from these combined data partial molar free energies and entropies of solution of NiO and Na₂Si₂O₅ and free energies and entropies of mixing were calculated. A partial phase diagram for the system NiO-Na₂Si₂O₅ proposed from solubility data indicates a eutectic at ∼12 mol% (5.3 wt%) NiO at 830°C.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Hot Corrosion Mechanism of Composite Alumina/Yttria-Stabilized Zirconia Coating in Molten Sulfate–Vanadate Salt

In order to improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al₂O₃ overlay has been deposited on the surface of YSZ by electron-beam physical vapor deposition. Hot corrosion tests have been performed on the YSZ coatings with and without an Al₂O₃ overlay in the molten salt mixture (Na₂SO₄+0–15 wt% V₂O₅) at 950°C. The presence of V₂O₅ in the molten salt exacerbates degradation of both the monolithic YSZ coating and the composite YSZ/Al₂O₃ system. The formation of a low-melting Na₂O–V₂O₅–Al₂O₃ liquid phase is responsible for degradation of the Al₂O₃ overlay. The Al₂O₃ overlay acts as a barrier against the infiltration of the molten salt into the YSZ coating during exposure to the molten salt mixture with <5 wt% vanadate.     

Preparation of Alkoxy-Derived Tantalum Vanadate

Compound formation in the system Ta₂O₅–V₂O₅ has been studied using amorphous materials prepared by the simultaneous hydrolysis of tantalum and vanadyl alkoxides. Three compounds exist in this system: 9Ta₂O₅· V₂O₅, 9Ta₂O₅·2V₂O₅, and Ta₂O₅·V₂O₅ (TaVO₅). Solid solutions of δ-Ta₂O₅ are formed at low temperatures up to 10 mol% V₂O₅. They transform to β-Ta₂O₅ solid solutions at higher temperatures; the transformation temperature falls with increasing V₂O₅ A new compound, 9Ta₂O₅·V₂O₅, 670° to 755°. It has an orthorhombic unit cell with a = 0.7859 nm, b = 1.733 nm, and c = 1.766 nm. Orthorhombic TaVO₅ crystallized at 535° to 560° decomposes into 9Ta₂O₅°V₂O₅ at 1010°.