Microstructure and Nonlinear Properties of Microwave-Sintered ZnO-V2O5 Varistors: II, Effect of Mn3O4 Doping

The microstructure and nonlinear current-voltage characteristics of Mn₃O₄-doped ZnO-V₂O₅ ceramics, microwave-sintered at 800°-1200°C for 10 min, have been investigated. A high density (96% of the theoretical density) has been achieved. The incorporation of Mn₃O₄ additives does not significantly alter the densification behavior of the ZnO-V₂O₅ materials, but rather pronouncedly increases the nonlinear coefficient (α= 23.5) and markedly suppresses their leakage current density ( J _L= 2.4 10^-6 A/cm²). On the other hand, the intrinsic properties of the materials, including the Schottky barrier height (Phi_b) and the donor density ( N _d), are only moderately modified; that is, Phi_b= 1.16 eV and N _d= 5.4 10¹⁷/cm³. X-ray diffractometry analyses and energy-dispersive X-ray microanalyses (via scanning electron microscopy) indicate that the V₂O₅ species facilitate the densification and the development of microstructure via the formation of a liquid phase (Zn₃(VO₄)₂) along the grain boundaries, whereas the Mn₃O₄ species markedly enhance the nonohmic behavior of the ZnO-V₂O₅ materials by forming the surface states along the grain boundaries.     

Densification and Microstructural Development of SrTiO3 Sintered with V2O5

Precipitation of TiO₂ occurs during the sintering of SrTiO₃ with V₂O₅ added as a liquid-phase sintering agent. Satisfactory densification can be obtained at 1250°C when using a high content of V₂O₅ during sintering. However, a microstructure of fine grains and large pores results along with the precipitation of TiO₂. The precipitation of TiO₂ can be repressed by the addition of excess SrO. A well-sintered microstructure with superior densification can thus be obtained at 125O°C from specimens sintered with a low content of V₂O₅ and an appropriate amount of excess SrO.     

Synthesis of V2O3 Powder by Evaporative Decomposition of Solutions and H2 Reduction

The evaporative decomposition of solutions method was used to form V₂O₅. Spraying above the congruent melting temperature of V₂O₅ (690°C) resulted in dense spherical particles with a smooth surface. Spraying below the V₂O₅ melting temperature yielded porous V₂O₅ powder with a rough surface. Reduction of the V₂O₅ to V₂O₃ was done in a H₂ atmosphere. Spherical V₂O₃ powder was attained when the reduction temperature was low enough to reduce the V₂O₅ surface before partial sintering (necking) between V₂O₅ particles occurred. The resulting V₂O₃ particle size was smaller than the precursor V₂O₅ powder as expected by the differences in densities between V₂O₅ ( p = 3.36 g/cm³) and V₂O₃ ( p = 4.87 g/cm³).     

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.     

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.     

Reaction Sintering of ZnO-AI2O3

The reaction sintering of equimolar mixtures of ZnO and A1₂O₃ powders was investigated as a function of primary processing parameters such as the temperature, heating rate, green density, and particle size. The powder mixtures were prepared by two different methods. In one method, the ZnO and A1₂O₃ powders were ball-milled. In the other method, the ZnO powder was chemically precipitated onto the A1₂O₃ particles dispersed in a solution of zinc chloride. The sintering characteristics of the compacted powders prepared by each method were compared with those for a prereacted, single-phase powder of zinc aluminate, ZnAl₂O₄. The chemical reaction between ZnO and A1₂O₃ occurred prior to densification of the powder compact and was accompanied by fairly large expansion. The mixing procedure had a significant effect on the densification rate during reaction sintering. The densification rate of the compact formed from the ball-milled powder was strongly inhibited compared to that for the single-phase ZnAl₂O₄ powder. However, the densification rate of the compact formed from the chemically precipitated mixture was almost identical to that for the ZnAl₂O₄ powder. The difference in sintering between the ball-milled mixture and the chemically precipitated mixture is interpreted in terms of differences in the microstructural uniformity of the initial powder compacts resulting from the different preparation procedures.     

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.     

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.     

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.     

Low Temperature Sintering of Zn3Nb2O8 Ceramics from Fine Powders

A possibility to produce microwave (MW) dielectric materials by liquid-phase sintering of fine particles was investigated. Zn₃Nb₂O₈ powders with a grain size 50–300 nm were obtained by the thermal decomposition of freeze-dried Zn–Nb hydroxides or frozen oxalate solutions. The crystallization of Zn₃Nb₂O₈ from amorphous decomposition products was often accompanied by the simultaneous formation of ZnNb₂O₆. Maximum sintering activity was observed for single-phase crystalline Zn₃Nb₂O₈ powders obtained at the lowest temperature. The sintering of as-obtained powders with CuO–V₂O₅ sintering aids results in producing MW dielectric ceramics with a density 93%–97% of the theoretical, and a Q × f product up to 36 000 GHz at sintering temperature ( T _s)≥680°C. The high level of MW dielectric properties of ceramics was ensured by intensive grain growth during the densification and the thermal processing of ceramics.     

Microstructural Development of ZnO Pellets Doped with Different Vanadium Oxides (V2O5 and V2O3)

This work presents the effect of zinc oxide (ZnO) ceramics, doped with different percentages of vanadium trioxide (V₂O₃) and vanadium pentoxide (V₂O₅). Samples were analyzed by X-ray diffraction and scanning electron microscopy. The addition of V₂O₃ or V₂O₅ produced changes in the composition and morphology of the pellets. In both cases, different zinc vanadates were detected as secondary phases: α-Zn₃(VO₄)₂ and Zn₄V₂O₉. Furthermore, when the vanadium concentration was equal to or higher than 3 wt%, the presence of filaments was detected on the surface of the pellets. These filaments were produced due to vanadium segregation. However, this effect was only observed at the surface of the pellets. On the bulk, the filaments were not observed. Instead, vanadium was found at the interfaces between the ZnO grains and at triple points, as it could be expected.     

Effects of La2O3/ZnO Additives on Microstructure And Microwave Dielectric Properties of Zr0.8Sn0.2TiO4 Ceramics

Dielectric ceramics of Zr_0.8Sn_0.2TiO₄ containing La₂O₃ and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low-level doping with La₂O₃ and ZnO (up to 0.30 wt%) is good for densification and dielectric properties. These additives do not affect the dielectric constant and the temperature coefficient. Dielectric losses increase significantly at additive levels higher than 0.15 wt%. The combined additives La₂O₃ and ZnO act as grain growth enhancers. With 0.15 wt% additives, a ceramic having a dielectric constant, a quality factor, and a temperature coefficient of frequency at 4.2 GHz of 37.6, 12 800, and –2.9 ppm/°C, respectively, was obtained. The quality factor was considerably improved by prolonged sintering.     

Effect of Nb2O5/ZnO Addition on Microwave Properties of (Zr0.8Sn0.2)TiO4 Ceramics

The effects of Nb₂O₅ and ZnO addition on the dielectric properties, especially the quality factor, of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics were investigated in terms of the sintered density acquired by the zinc. For ZST ceramics with 2 mol% added ZnO, the relative density of the samples decreased with >0.5 mol% addition of Nb₂O₅. On the other hand, for samples with 6 mol% added ZnO, the relative density remained >97%, even when the amount of Nb₂O₅ was increased to 2.0 mol%. When >0.5 mol% Nb₂O₅ was added, both the quality factor and the dielectric constant exhibited similar trends with sintered density. The ZST ceramics with 6 mol% added ZnO, especially, still manifested a quality factor >40 000 and a dielectric constant of 37, even when the amount of Nb₂O₅ was increased, values that are not explainable by the previously suggested electronic defect model.     

Phase Equilibria in the System SrO-CdO-V2O5

Phase equilibria in the system SrO-CdO-V₂O₅ in air were established from data obtained by DTA, quenching, and high-temperature solid-state reaction experiments. The SrO-V₂O₅ boundary system contains 4 compounds at SrO to V₂O₅ molar ratios of 4:1, 3:1, 2:1, and 1:1. A fifth compound with a molar composition of ∼10:3 with the apatite crystal structure was also found; it may, however, be a hydroxyapatite phase. The CdO-V₂O₅ system contains the compounds 3CdO·V₂O₅, 2CdO·V₂O₅, and CdO·V₂O₅. The latter compound exhibits a rapid reversible polymorphic transition at 180°C. Complete solid solubility exists in the SrO-CdO system. The most probable compatibility relations were determined from the data available for the SrO-CdO-V₂O₅ ternary system. Limited solid solubility exists between SrO·V₂O₅ and CdO·V₂O₅, and the high-temperature CdO·V₂O₅ polymorph is stabilized to room temperature by solid solution of SrO·V₂O₅. Evidence for the existence of 2 ternary compounds with limited local solid solubility is also presented.     

Rapid Rate Sintering of Nanocrystalline ZrO2−3 mol% Y2O3

Conventional ramp-and-hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO₂–3 mol% Y₂O₃ powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO₂–3 mol% Y₂O₃ produced mixed results. In the case of submicrometer ZrO₂–3 mol% Y₂O₃, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO₂–3 mol% Y₂O₃, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO₂–3 mol% Y₂O₃ specimens.     

Sintering, Crystallization, and Properties of B2O3/P2O5-Doped Li2O·Al2O3·4SiO2 Glass-Ceramics

Sintering, crystallization, microstructure, and thermal expansion of Li₂O·Al₂O₃·4SiO₂ glass-ceramics doped with B₂O₃, P₂O₅, or (B₂O₃+ P₂O₅) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10^-6 to −0.1 × 10^-6 K^-1. Codoping of B₂O₃ and P₂O₅ resulted in the highest densification and an extremely low coefficient of thermal expansion.     

Field-Assisted Densification of Superhard B6O Materials with Y2O3/Al2O3 Addition

B₆O is a possible candidate of superhard materials with a hardness of 45 GPa measured on single crystals. Up to now, densification of these materials was only possible at high pressure. However, recently it was found that Al₂O₃ can be utilized as an effective sintering additive, similar to the addition of Y₂O₃/Al₂O₃ that was used in this work. The densification behavior of the material as a function of applied pressure, its microstructure evolution, and the resulting mechanical properties were investigated. A strong dependence of the densification with increasing pressure was found. The material revealed characteristic triple junctions filled with amorphous residue composed of B₂O₃, Al₂O₃, and Y₂O₃, while no amorphous grain-boundary films were observed along internal interfaces. Mechanical testing revealed on average a hardness of 33 GPa, a fracture toughness of 4 MPa·m^1/2, and a strength value of 520 MPa.     

Activated Sintering of ThO2 and ThO2-Y2O3 with NiO

The effect of additives on the sintering of ThO₂ and ThO₂-Y₂O₃ compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high-density bodies at temperatures below 1500°C. The presence of Y₂O₃ as a separate phase increases the rate of sintering of ThO₂, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid-state mechanism may be responsible.     

Effect of Europium Concentration on Densification of Transparent Eu:Y2O3 Scintillator Ceramics Using Hot Pressing

Europium (Eu) was found to act as a solid-state sintering aid in Y₂O₃ optical ceramics by controlling ionic diffusivity, which in turn leads to enhanced optical transparency. Transparent ceramic samples of Eu-doped Y₂O₃, with no additional additives, were sintered by uniaxial vacuum hot pressing under 40 MPa and maximum temperature of 1580°C. Optical attenuation was found to decrease with increasing Eu concentrations between 0 and 5 at% for ceramics processed under the same sintering conditions. In order to study the effect of Eu concentration on ceramic densification, the strain rate and grain size during sintering at constant temperature and varied pressure were measured. A diffusional flow densification model was used to derive instantaneous effective diffusion constants for the densification process. Diffusion constants were found to increase with increasing Eu concentration according to a log–linear relationship. Eu²⁺ was detected in samples after hot pressing through fluorescence spectroscopy, and the extrinsic defect chemistry was found to be dominated by the reduced Eu in solid solution with Y₂O₃. A sintering model with diffusion rate limited by yttrium interstitial transport and controlled by the incorporation of Eu²⁺ onto the cation sublattice was found to be in good agreement with experimental diffusivity data.     

Low-Temperature Sintering, Densification, and Properties of Z-type Hexaferrite with Bi2O3 Additives

The effect of the addition of Bi₂O₃ on the densification, low-temperature sintering, and electromagnetic properties of Z-type planar hexaferrite was investigated. The results show that Bi₂O₃ additives can improve the densification and promote low-temperature sintering of Z-type hexaferrite prepared by a solid-state reaction method. The presence of Bi₂O₃ in the grain boundaries and the generation of Fe²⁺ degrade the initial permeability of the samples but make the quality factor and cut-off frequency increase. Various possible mechanisms involved in generating these effects were also discussed.