Effect of Filler Content on the Dielectric Properties of PTFE/ZnAl2O4–TiO2 Composites

ZnAl₂O₄–TiO₂ (ZAT)/polytetrafluoroethylene (PTFE) composites were prepared by powder processing method. The structure and microstructure of the composites were analyzed using X-ray diffraction and scanning electron microscopic techniques. The effect of different volume fraction of ZAT (0–0.6) on the dielectric properties of the composites was investigated at 1 MHz and at the microwave frequency of 7 GHz. The dielectric properties (relative permittivity and dielectric loss) were found to increase with the ZAT content. The samples with 60 vol.% of ZAT filler showed a relative permittivity of 3.9 and dielectric loss of 0.009, respectively, at 7 GHz. The variation of relative permittivity and dielectric loss showed only a very small variation with temperature in the range 25°–70°C for various ZAT/PTFE composites. The relative permittivity obtained experimentally were compared with that of the theoretical values predicted using modified Lichtenecker, Jayasundere—Smith, and Series mixing formulae. They were found to agree well with the predicted values for low filler content.     

Dielectric Constant and Optical Reflectance of Ceramic Filler-Added BaO–ZnO–B2O3–P2O5 Glass Composites

This study examined the influence of the addition of ceramic fillers (up to 20 wt% of TiO₂, Al₂O₃, and ZnO, respectively) to a BaO–ZnO–B₂O₃–P₂O₅ glass matrix on the dielectric and optical properties with the aim of using this material as the barrier ribs in plasma display panels. The modification of the dielectric constant by the fillers was related to the formation of secondary phases, the changes in the glass composition by the partial dissolution of the fillers, and the presence of pores. The reflectance of the composites ranged from 60% to 80% with the addition of 20 wt% filler.     

Origin of Microwave Dielectric Loss in ZnNb2O6-TiO2

(1− x )ZnNb₂O₆· x TiO₂ ceramics were prepared using both anatase and rutile forms of TiO₂. At a composition of x = 0.58, a mixture region of ixiolite (ZnTiNb₂O₈) and rutile was observed and the temperature coefficient of resonant frequency (τ_f) was ∼0 ppm/°C. We found that although ɛ_r and τ_f were comparable, the quality factor ( Q × f , Q ≈ 1/ tan δ, f = resonant frequency) of 0.42ZnNb₂O₆·0.58TiO₂ prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q × f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti⁴⁺. It is suggested that the increase of dielectric loss originates from an increase in Ti³⁺ and oxygen vacancies due to an anatase–rutile phase transition.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Microwave Dielectric Properties of Low-Fired ZnNb2O6 Ceramics with BiVO4 Addition

The BiVO₄ additive was found effective for low-temperature firing of ZnNb₂O₆ polycrystalline ceramics below 950°C in air without a serious degradation in their microwave dielectric properties. Dense BiVO₄-doped ZnNb₂O₆ samples of a relative sintered density over 95% could be prepared even at 925°C. An optimally processed specimen exhibited excellent microwave dielectric properties of Q · f = 55000 GHz, ɛ_r= 26, and τ_f=−57 ppm/°C. With increasing BiVO₄ addition up to 20 mol% relative to ZnNb₂O₆, while the quality factor Q · f was gradually decreased, the relative dielectric constant, ɛ_r, was linearly increased and the temperature coefficient of resonant frequency, τ_f, was slightly increased. The variations in Q · f and ɛ_r are surely attributable to the residual BiVO₄ in the ZnNb₂O₆ matrix. An unexpected slight increase in τ_f is probably due to the formation of the Bi(V,Nb)O₄-type solid solution.     

Dielectric Relaxations in Bi2O3–Nb2O5–NiO Cubic Pyrochlores

Dielectric properties of pyrochlores compositions from Bi₂O₃–Nb₂O₅–NiO system were analyzed. The dielectric properties are dominated with a low-temperature relaxation that is typical for Bi-pyrochlores. A vast pyrochlore homogeneity range that exists in this system allowed to correlate characteristics of the observed relaxations with a compositional variations within the A₂O'- and B₂O₆ pyrochlore sublattice. It was possible to make a distinction between different influences of the two sublattices, which can be satisfactorily described by the existing relaxation model for Bi_3/2ZnNb_3/2O₇. A new relaxor-like room temperature relaxation was found for Bi_1.6Ni_0.57Nb_1.43O_6.55.     

Investigation on the Microstructure and Dielectric Properties of BaTiO3–Nb2O5–Fe2O3 Materials Doped with La2O3

The influence of La₂O₃ doped on the microstructure and dielectric properties, including the phase structure, temperature dependence of permittivity, and the hysteresis loop of BaTiO₃–Nb₂O₅–Fe₂O₃ (BTNF) materials has been investigated in X-ray diffraction, SEM, and LCR analyzer, respectively. Experiments revealed that incorporation of proper content of La₂O₃ basically soluted in the lattice of BaTiO₃ and can control the grain-growth, reduce the dielectric loss of the BTNF materials. The development of microstructure promoted by the additives can result in the improvement of the dielectric constant. When the doping concentration of La₂O₃ was 3.846 wt%, the relative dielectric constant of the sample sintered at 1280°C only for 2 h could reach 4308, and improve the dielectric-temperature characteristics markedly. As a result, a novel Y5P can be achieved in the BTNF ceramics, which is very promising for practical use in Y5P multilayer ceramic capacitors.     

Electroconductive Al2O3–NbN Composites

Electroconductive Al₂O₃–NbN ceramic composites were prepared by hot pressing. Dense sintered bodies of ball-milled Al₂O₃–NbN composite powders were obtained at 1550°C and 30 MPa for 1 h under a nitrogen atmosphere. The bending strength and fracture toughness of the composites were enhanced by incorporating niobium nitride (NbN) particles into the Al₂O₃ matrix. The electrical resistivity of the composites decreased with increasing amount of NbN phase. For a 25 vol% NbN–Al₂O₃ composite, the values of bending strength, fracture toughness, Vickers hardness, and electrical resistivity were 444.2 MPa, 4.59 MPa·m^1/2, 16.62 GPa, and 1.72 × 10⁻²Ω·cm, respectively, making the composite suitable for electrical discharge machining.     

Phase Relations and Dielectric Properties in the Bi2O3–ZnO–Ta2O5 System

Dielectric properties and phase formation of Bi-based pyrochlore ceramics were evaluated for the Bi₂O₃–ZnO–Ta₂O₅ system. The compositional range r Bi₂(Zn_1/3Ta_2/3)₂O₇· (1− r )(Bi_3/2Zn_1/2)(Zn_1/2Ta_3/2)O₇ (0 ≤ r ≤ 1) in Bi₂O₃–ZnO–Ta₂O₅ was investigated to determine the relative solubility of BZT cubic (α-BZT, r = 0) and the pseudo-orthorhombic (β-BZT, r = 1) end members. It was found that extrinsic factors, such as kinetically limited phase formation and bismuth loss, contribute to apparent phase boundaries in addition to thermodynamic stability of each phase. Considering this, the locations of true phase boundaries were r < 0.30 and r ≥ 0.74 for α and β phases, respectively. Dielectric constants between 58 and 80 and low dielectric loss (tan δ < 0.003) were measured for the complete compositional range. The temperature coefficient of capacitance was controlled by composition, which was found to be <30 ppm/°C at the edge of β-phase solid solution. In addition to the excellent dielectric properties these materials can be sintered at low temperatures, which make Bi-based pyrochlores promising candidates for high-frequency electronic applications.     

Properties of (Y)ZrO2–Al2O3 and (Y)ZrO2–Al2O3–(Ti or Si)C Composites

The effect of Al₂O₃ and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al₂O₃–(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al₂O₃ to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO₂ and TiC therefore must have inhibited the t - → m -ZrO₂ martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO₂ and Al₂O₃. In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton.     

Structure, Thermal Behavior, Chemical Durability, and Optical Properties of the Na2O–Al2O3–TiO2–Nb2O5–P2O5 Glass System

The FTIR, Raman, UV-Vis, ³¹P MAS-NMR, DTA, and refractive index measurements have been combined to investigate a series of glasses with the general formula 20Na₂O–5Al₂O₃− x TiO₂–(45− x )Nb₂O₅–30P₂O₅, 15≤ x ≤45. The glass structure, as well as thermal, optical, and chemical durability properties, were then described as functions of the f _Nb/ f _Ti ratio. An increase of the f _Nb/ f _Ti ratio correlates with a decrease in length of the average phosphate chains linked through Nb–O–P and Ti–O–P bonds, with an increase in the glass stability and with increase in the linear refractive indices at 632.8 nm from 1.79 to 1.89. Furthermore, niobium is more effective than titanium in improving chemical durability.     

Effect of Sb2O3 on Thermal Properties of Glasses in Bi2O3–B2O3–SiO2 System

Glasses with compositions 50Bi₂O₃– x Sb₂O₃–10B₂O₃–(40– x ) SiO₂ ( x =0, 1, 3, 5, 8, 10) have been prepared by conventional melt quench technique. Substitution of Sb₂O₃ for SiO₂ exerted an obvious effect on properties of glasses, especially, increased glass transition temperature ( T _g) and crystalline temperature ( T _c) greatly. Results of infrared transmission spectra attributed the effect to the formation of new bridging bonds of Sb–O–B and Sb–O–Si in glass network.     

Mechanical Properties and Oxidation Behavior of Al2O3–AlN–TiN Composites

The study examines the effect which the composition of hot-pressed electroconductive ceramics has on their structure, mechanical properties, and oxidation behavior, for ceramics of the type AIN–Al₂O₃–42 wt% TiN, differing in the AIN/Al₂O₃ ratio. The results are physico-mechanical property data, including density, hardness, strength, fracture toughness, and wear resistance. A correlation was found between the wear resistance and fracture toughness. The analysis of oxidation products revealed the formation of α-Al₂O₃ and rutile in the temperature range from 600° to 1100°C and aluminum titanate above 1200°C. The spallation of the oxide layer caused low oxidation resistance of Al₂O₃-rich composites above 1250°C. The oxidation of composites was compared with the oxidation of pure TiN. The relationship is discussed between material properties, composition, phases, and processing parameters.     

Phase Relations in the System CoNb2O6–CoTa2O6

The pseudobinary system CoNb₂O₅–CoTa₂O₆ was investigated. CoNb₂O₆ crystallizes in either the columbite or rutile structure, whereas CoTa₂O₆ assumes only the trirutile structure. In an argon atmosphere at about 1400°C, CoNb₂O₆ undergoes a phase transition from columbite or rutile. Between 1000° and 1400°C the solubility of CoTa₂O₆ in CoNb₂O₆ is about 10 mole %; in the same temperature region the solubility of CoNb₂O₆ in CoTa₂O₆ varies from about 40 to 70 mole %. The extensive solubility of CoNb₂O₆ in CoTa₂O₆ is explained by the ability of niobium to induce some disorder in the trirutile phase. The columbite to rutile transformation is also discussed on this basis.     

Preparation of Al2O3–AlN–Ni Composites

A method is proposed to prepare Al₂O₃-AlN-Ni composites. The composites are prepared by sintering Al₂O₃/NiAl powder mixtures at 1600°C in a mixture of nitrogen and carbon monoxide. The presence of NiAl particles raises the green density of Al₂O₃/NiAl powder compacts. During sintering, NiAl reacts with nitrogen to form AlN and Ni inclusions. A volume expansion accompanies the reaction. Because of the high green density and the reaction, the volume shrinkage of the Al₂O₃-AlN-Ni composite decreases with the increase of added NiAl content.     

Crystallization and Dielectric Properties of SrO–BaO–Nb2O5–SiO2 Tungsten-Bronze Glass-Ceramics

The crystallization and dielectric properties of SrO–BaO–Nb₂O₅–SiO₂ glass-ceramics have been investigated. Glass-ceramics that contain strontium barium niobate (SBN) as a primary crystalline phase, which has a tungsten bronze structure, are produced. The formation of crystalline secondary phases also has been studied. The SBN phase shows evidence of both surface nucleation and bulk nucleation, and the crystals have an average composition of Sr_0.47Ba_0.53Nb₂O₆. The dendritic morphology of the SBN crystals has been examined. The SBN content and composite dielectric constant each has been studied as a function of heating temperature/time. The highest SBN content and dielectric constant obtained in the present study are 42 vol% and 180, respectively. The dielectric constant of the glass-ceramics is determined primarily by the SBN content and the residual glass phase. The dielectric constant of the randomly oriented SBN crystal in the glass-ceramics is calculated, using dielectric mixture rules, to be ∼400.     

The Bi2O3–Nb2O5–NiO Phase Diagram

The Bi₂O₃–Nb₂O₅–NiO phase diagram at 1100°C was determined by means of solid-state synthesis, X-ray diffraction, and scanning electron microscopy. A ternary eutectic with a melting point below 1100°C was found to exist in the field between NiO, Bi₂O₃, and the end-member of the δBi₂O₃–Nb₂O₅ solid solution. The existence of the previously reported Bi₃Ni₂NbO₉ phase was disproved. A pyrochlore homogeneity range around Bi_1.5Ni_0.67Nb_1.33O_6.25 was determined together with all the phase relations in this phase diagram.     

Dielectric Properties of Glasses in the System Nb2O5–Na2O–SiO2

The effect of niobia on the dielectric properties of glasses in the system Nb₂O₅–Na₂O–SiO₂ has been studied from 100 to 10¹⁰ cps. The dielectric constant is high even at frequencies up to 10¹⁰ cps. The Nb⁵⁺ ion, with its small ionic radius and high charge, reinforces the network and raises the dielectric constant.