Bi1.5ZnNb1.5O7 cubic pyrochlore ceramics prepared by aqueous solution–gel method

Bi_1.5ZnNb_1.5O₇ cubic pyrochlore ceramic was successfully prepared by the aqueous solution method. The preparation, microstructure development and dielectric properties of ceramics were investigated. Homogeneous precalcined ceramics powders have a cubic pyrochlore phase after thermal treatment at the temperature as low as 450 °C. The aqueous solution–gel method, which Bi, Zn and Nb ions are chelated to form metal complexes, leading to the formation of cubic pyrochlore phase at low firing temperatures. No detectable intermediary phase such as BiNbO₄ or pseudo-orthorhombic pyrochlore is observed in the XRD patterns of ceramics at the sintering temperature range from 850 to 1,000 °C. The dielectric properties study revealed that the ceramics sintered at 900 °C show excellent performance with dielectric constant of 111 and dielectric loss of 2.3871 × 10⁻⁴ under 1 MHz at room temperature.     

AgNbO<Subscript>3</Subscript> ceramics synthesized by aqueous solution-gel method

AgNbO₃ powders and ceramics were prepared by aqueous solution-gel method. The phase evolution of the powders was investigated by TG/DSC and XRD. The results showed that the pure AgNbO₃ phase was obtained at 600 °C without special treatment. The sintering behavior and dielectric properties of the AgNbO₃ ceramics were also investigated. It showed the dense ceramics were obtained as lower as 925 °C, which had the excellent dielectric properties with the permittivity of 291 and dielectric loss of about 1.7% at 1 MHz. The coarse grains were observed for the sample sintered over 975 °C, and then they decreased with the sintering temperature further increasing to 1,050 °C.     

Dielectric behavior and impedance analysis of lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics

Pure (Na_0.50K_0.50)_0.95(Li_0.05Sb_0.05Nb_0.95)O₃ (NKNLS) and CuO doped NKNLS perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. x wt% of CuO (x = 0.2–0.8 wt%) was added in the NKNLS ceramics. X-ray diffraction patterns indicate that single phase was formed for pure NKNLS while a small amount of second phase (K₆Li₄Nb₁₀O₃₀ ∼ 3%) was present in Cu²⁺ doped NKNLS ceramics. Dielectric anomalies around the temperatures of 120 °C and 350 °C have been identified as the ferroelectric–paraelectric transition (orthorhombic to tetragonal and tetragonal to cubic) temperatures for pure NKNLS compound. The electrical behavior of the ceramics was studied by impedance study in the high temperature range. Impedance analysis has shown the grain and grain boundary contribution using an equivalent circuit model. The impedance response in pure and Cu²⁺ doped NKNLS ceramics could be resolved into two contributions, associated with the bulk (∼grains) and the grain boundaries. From the conductivity studies, it is found that activation energies are strongly frequency dependent. The activation energy obtained from dielectric relaxation data may be attributed to oxygen ion vacancies.     

Synthesis and microwave dielectric properties of Ca<Subscript>0.6</Subscript>La<Subscript>0.267</Subscript>TiO<Subscript>3</Subscript> nanocrystalline powders by sol–gel method

Ca_0.6La_0.267TiO₃ nanocrystalline powders were successfully synthesized by the sol–gel method using PEG1000 as a dispersant in this study. The sinterability of the powders and the microwave dielectric properties of the ceramics were also investigated. The XRD diffraction result showed that pure Ca_0.6La_0.267TiO₃ powder with orthorhombic perovskite structure could be synthesized at 600 °C for 2 h without any detectable intermediate phase. The average grain size of the as-synthesized powder was as low as 35 nm. Compared with Ca_0.6La_0.267TiO₃ ceramics fabricated by conventional solid-state process, the bulk materials prepared by sintering as-prepared nanopowders performed better in densification and microwave dielectric properties. The ceramics sintered at 1,300 °C exhibited a higher relative density of 98.3% combined with a dielectric constant (ε _r ) of 120.3, a quality factor (Q × f) of 23,550 GHz and a temperature coefficient of resonant frequency (τ _f ) of +220.7 ppm/°C, respectively.     

Colossal dielectric constant in PrFeO3 semiconductor ceramics

The perovskite PrFeO₃ ceramics were synthesized via sol–gel method. The dielectric properties and impedance spectroscopy (IS) of these ceramics were studied in the frequency range from 100 Hz to 1000 kHz in the temperature range from 80 K to 300 K. These materials exhibited colossal dielectric constant value of ∼10⁴ at room temperature. The response is similar to that observed for relaxorferroelectrics. IS data analysis indicates the ceramics to be electrically heterogeneous semiconductor consisting of semiconducting grains with dielectric constant 30 and more resistive grain boundaries with effective dielectric constant ∼10⁴. We conclude, therefore that grain boundary effect is the primary source for the high effective permittivity in PrFeO₃ ceramics.     

Influence of sintering method on microstructure,electrical and magnetic properties of BiFeO3–BaTiO3 solid solution ceramics

The (1-x)BiFeO₃-xBaTiO₃ (short for (1-x)BFO-xBTO, x = 0.2–0.4) ceramics were prepared by solid-state reaction with microwave sintering (MWS) and conventional sintering (CS) methods. The crystal structure, microstructures, dielectric properties, ferroelectric properties, and magnetic properties of BFO-BTO ceramics sintered by MWS and CS were systematically investigated. It is found that the MWS can effectively decrease the grain size and enhance the compactness of BFO-BTO ceramics. The X-ray diffraction (XRD) results confirm that all ceramics exhibit a single perovskite structure, and the phase transforms from rhombohedral in BiFeO₃-rich compositions to pseudo-cubic phase gradually as x increases. Introducing BTO into BFO can strengthen its dielectric relaxation behavior. Compared with CS, the MWS samples have a lower remanent polarization (P_r) and a smaller coercive field (E_c) under the same electric field. Therefore, MWS contributes to the decrease of dielectric loss. Addition of BTO can contribute to the reduction of the coercive force (H_c) of BFO-based ceramic, and so decrease the hysteresisloss. At the same time, its remanent magnetization (M_r) value can be decreased by introducing BTO into BFO and using MWS method. The present research provides a route for decreasing the dielectric loss and hysteresis loss of BiFeO₃-based ceramics using the MWS method.     

Influence of sintering temperature on microstructure,critical current density and pinning potential of superconducting Bi1.6Pb0.5Sr1.8Dy0.2Ca1.1Cu2.1O8+δ ceramics

The influence of sintering temperature on the microstructure, critical current density (J_C), pinning potential values (U₀) and flux pinning properties of Bi_1.6Pb_0.5Sr_1.8Dy_0.2Ca_1.1Cu_2.1O_8+δ superconductor has been investigated. The samples are prepared by the solid-state route and sintered at temperatures ranging from 846 to 860 °C. A systematic correlation between the sintering temperature, Lotgering index, J_C, U₀ and flux pinning properties has been found. The samples sintered at lower sintering temperature (846 °C) have more grain boundaries with smaller grains while those sintered at a higher temperature (856 °C) contain larger grains with good texturing. The flux pinning force (F_P) calculated from the field dependent J_C values shows that the irreversibility lines (IL) of the Dy-doped samples shift towards higher fields to different extents depending on the sintering temperature. The maximum value of F_P = 1697 kN m^?3 is obtained for the sample sintered at 846 °C and the peak position of F_P is obtained at 0.96 T as against 616 kN m^?3 and 0.52 T for the sample sintered at 856 °C. The U₀ values calculated by Anderson's function is maximum for the sample sintered at 846 °C. But the self-field J_C value of this sample is lower than that of the samples sintered at 856 °C. The samples sintered at 856 °C show best self-field J_C due to the improved microstructure. The changes in microstructure followed by very high enhancement of self-field J_C, J_C(B) characteristics, F_P and U₀ values within a narrow temperature range, are of great scientific and technological significance and the results are explained on the basis of microstructural variation with respect to sintering temperature, hole optimization and formation of point defects due to the doping of Dy atoms in Bi_1.6Pb_0.5Sr_1.8Dy_0.2Ca_1.1Cu_2.1O_8+δ system.     

Thermal,structural, and impedance analysis of nanocrystalline magnesium chromite spinel synthesized via hydrothermal process

Nanocrystalline magnesium chromite spinel was synthesized through hydrothermal reaction of metal nitrate solutions in stoichiometric amount at different pH, temperature and time intervals. The synthesized products were characterized for crystallinity, phase identification, and surface morphology by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD patterns showed that as-synthesized product remained amorphous up to 250 °C. However, well-crystallized magnesium chromite spinel structure is formed after calcination at 850 °C. Rietveld refinement study confirms the formation of single-phase cubic structure MgCr₂O₄ with lattice parameter a = 8.3347 Å, and Fd3m space group. The as-processed MgCr₂O₄ products showed extensive XRD line broadening, and the mean crystallite size of such crystals was found to be mainly in size range of 85–124 nm. Surface SEM images of calcined specimens revealed that the matrix is uniform, and no separation of secondary phase was detected. Thermal stability was examined by thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry. TG/DTA reveals that MgCr₂O₄ is thermally stable above 700 °C. Fourier transform infrared (FTIR) spectra studies shows two strong bands, one around 600 cm^?1 which is attributed to the intrinsic vibrations of tetrahedral and other at 400 cm^?1 is due to octahedral one. FTIR confirms the formation of metal oxides. The bandgap energy was estimated by absorption spectroscopy in ultraviolet–visible range and was found to be 0.693 eV for MgCr₂O₄ specimen sintered at 1,000 °C. Isothermal shrinkage characteristic and coefficient of thermal expansion were determined by dilatometry. The powder specimens showed excellent densification at 1,250 °C temperature and uniformly fine grain sintered ceramics (>90 % relative density) with submicron grain size (2–5 μm) were obtained after sintering at 1,000–1,250 °C. Impedance studies were carried out at room temperature and equivalent circuit model (R ₁ Q ₁) (R ₂ Q ₂) (R ₃ Q ₃) is used to explain different relaxation processes. We report largest impedance values i.e., 6.74 × 10⁸ Ω, reduced dielectric constant (≈1.0), and low tangent loss (0.8) for MgCr₂O₄ sintered at 1,250 °C.     

Influence of microstructure and crystalline phases on impedance spectra of sodium conducting glass ceramics produced from glass powder

Crystallization of highly ionic conductive N5 (Na₅YSi₄O₁₂) phase from melted Na_3+3x-1Y_1-xP_ySi_3-yO₉ parent glass provides an attractive pathway for cost-effective manufacturing of Na-ion conducting thin electrolyte substrates. The temperature-dependent crystallization of parent glass results in several crystalline phases in the microstructure (N3 (Na₃YSi₂O₇), N5 and N8 (Na8.1Y Si6O18) phases) as well as in rest glass phase with temperature dependent viscosity. The electrical properties of dense parent glass and of compositions densified and crystallized at 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C are investigated by impedance spectroscopy and linked to their microstructure and crystalline phase content determined by Rietveld refinement. The parent glass has high isolation resistance and predominantly electrons as charge carriers. For sintering at ≥ 900 °C, sufficient N5 phase content is formed to exceed the percolation limit and form ion-conducting pathways. At the same time, the highest content of crystalline phase and the lowest grain boundary resistance are observed. Further increase of the sintering temperature leads to a decrease of the grain resistance and an increase of grain boundary resistance. The grain boundary resistance increases remarkably for samples sintered at 1100 °C due to softening of the residual glass phase and wetting of the grain boundaries. The conductivity of fully crystallized N5 phase (grain conductivity) is calculated from thorough impedance spectra analysis using its volume content estimated from Rietveld analysis, density measurements and assuming reasonable tortuosity to 2.8 10⁻³ S cm⁻¹ at room temperature. The excellent conductivity and easy processing demonstrate the great potential for the use of this phase in the preparation of solid-state sodium electrolytes.

     

Bi0·5K0·5TiO3-based lead-free relaxor ferroelectric with high energy storage performances via the grain size and bandgap engineering

Dielectric capacitors with high energy storage performance show a huge competitive advantage for use in vehicles and power electronics. Here, the (1-x) (0.75Bi_0·5K_0·5TiO₃?0.25BiFeO₃) – xSr_0.7Bi_0·2TiO₃ (BKT-BF-xSBT) materials were designed based on the dual optimization strategy of grain size and bandgap. The optimal performance was obtained in the BKT-BF-0.4SBT sample, achieving high energy storage density W_rec of 5.21 J/cm³ and high energy storage efficiency η of 90.87% at 360 kV/cm. The addition of SBT optimized the microstructure and relaxation behavior of BKT-BF ceramic, giving rise to the formation of polar nanoregions (PNRs), small grain size (G), compact grain boundaries, and wide bandgap (E_g) synchronously. Encouragingly, prominent stabilities against temperature, frequency, and cycle numbers, and favorable charge/discharge performance (power density P_D ≈ 64.5 MW/cm³, τ_0.9 ≈ 90 ns) are also accomplished in designed BKT-based ceramics. These findings indicate that the BKT-BF-xSBT ceramic system has excellent potential in the dielectric energy storage field and also provides a viable prospect for engineering design of high-efficiency lead-free dielectrics via bandgap and grain size.     

Dielectric properties of Ni-doped Ba0.5Sr0.5TiO3 ceramics prepared with hydrothermal synthesized nanopowders

Ba_0.5Sr_0.5Ti_1?xNi_xO₃ (BSTN) ceramics were prepared from BSTN nanopowders synthesized by a hydrothermal method. The phase and microstructure of samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy. XRD results indicate a cubic structure of the pure BST nanopowders. The cubic structure can be converted to the orthorhombic phase with increasing of Ni content to x = 0.01 and returned to the cubic structure with the presence of Ni(OH)₂ impurity phase for x = 0.03 and 0.05. However, the BSTN ceramics sintered at 1,200 °C for 3 h revealed the orthorhombic phase structure with NiO impurity phase for all Ni content. The doping of Ni in Ba_0.5Sr_0.5TiO₃ structure can increase the grain size of samples from 1.47 to 3.26 μm. The dielectric constant, loss tangent (tanδ) and phase transition temperature of BSTN ceramics were reduced with increasing Ni content.     

Dielectric properties of FeNbO4 ceramics prepared by the sol-gel method

In this work, FeNbO₄ powders were prepared using the sol-gel method. The fine powder particles were pressed into pellets and sintered at temperatures between 500 and 1200 °C. The powder was studied by X-ray diffraction and Raman spectroscopy. The morphology of the grains was investigated by scanning electron microscopy. Heat-treatment of the particles results in higher crystallinity, larger grains, and a decrease in the porosity of the material.The dielectric properties were measured in the frequency range of 10²–10⁶ Hz, in function of temperature (200–370 K). In all samples the real (ε′) and imaginary (ε″) parts of the complex permittivity increase with increasing annealing temperature. The sample heat treated at 1200 °C shows the highest ε′, > 10⁴ at 300 K. All samples show a dielectric relaxation phenomenon, analysed using the modulus formalism. The evolution of the ac conduction activation energy and of the activation energy associated with the relaxation mechanism, is directly related with the changes promoted by the heat treatment in the structure and in the morphology of the base powders.     

Synthesis of nanopowders of the aluminum-substituted lanthanum gallate solid electrolyte by mechanochemical route

The room temperature mechanosynthesis of La_1−xSr_xGa_1−y−zMg_yAl_zO_3−δ nanopowders is successfully demonstrated for a broad compositional range (x ≤ 0.1; y ≤ 0.2, z ≤ 0.4) by resorting to a nearly amorphous alumina precursor with enhanced reactivity. It is shown that ceramics with one single phase and free from open porosity can be obtained by sintering these nanopowders at 1350–1450 °C. Microstructural data show that the substitution of Ga by Al hinders densification and decreases the grain size of ceramics. This is explained assuming the segregation of aluminum cations to the grain boundaries as a result of the decrease of the cationic diffusion coefficients.     

Strain correlated effect on structural,magnetic, and dielectric properties in Ti4+ substituted Bi0.8Ba0.2Fe1−xTixO3

Ti⁴⁺ substituted Bi_0.8Ba_0.2Fe_1−xTi_xO₃ for x = 0.0, 0.1 and 0.2 are prepared by modified solid state reaction method. The prepared samples sintered at 850 °C for 1 h show a single phase nature. A structural change was observed on Ti⁴⁺ substitutions are confirmed through X-ray Diffraction, Fourier Transform Infrared spectroscopy and Raman spectra. An anomalous phase transition is observed in Bi_0.8Ba_0.2FeO₃ at 1173 K. The absence of ferroelectric transition and enhancement of decomposition temperature is observed in the substituted samples from the thermal analysis. A dielectric spectroscopic measurement shows that on Ti⁴⁺ substitutions, the magnitude of dielectric constant and loss tangent (tan δ) value is decreased. Vibrating Sample Magnetometer (VSM) study shows both antiferromagnetic and ferromagnetic phases coexist in the M−H curve. On Ti⁴⁺ substitutions in Bi_0.8Ba_0.2FeO₃, the antiferromagnetism dominates over the ferromagnetic phase. In corroboration to magnetisation process, ZFC–FC measurement confirms it that on Ti⁴⁺ substitution, the antiferromagnetic behaviour gets dominated. The report suggests that the interplay of strain upon Ti⁴⁺ substitution causes the structural and magnetic phase transition.     

Phase evolution,dielectric, ferroelectric,and piezoelectric properties of Bi(Mg0.5Hf0.5)O3–modified BiFeO3–BaTiO3

Bi(Mg_0.5Hf_0.5)O₃–modified BiFeO₃–BaTiO₃ ternary solid solutions of (0.725-x)BiFeO₃-0.275BaTiO₃-xBi(Mg_0.5Hf_0.5)O₃ (0 < x ≤ 0.05, abbreviated as BFO-BTO-xBMHO) were prepared for lead-free piezoelectrics. The addition of BMHO delivers a rhombohedral (R3c, denoted as R-phase) to tetragonal (P4mm, denoted as T-phase) phase transition at x = 0.05, giving the coexistence of R- and T-phase in intermediate compositions: R-phase dominated in x = 0.01–0.02 and T-phase dominated in x = 0.03–0.04. The increment of BMHO tunes the grain size, lowers the ferroelectric transition temperature (T_C) and dielectric loss (tanδ), and drives a gradually ferroelectric to relaxor transition. The morphotropic phase boundary between the R-and T-phases, together with the homogeneous morphology, results in the best performance for x = 0.04 case with piezoelectric d₃₃ of 130 pC/N, K_p of 0.286, Q_m of 58.993, electrostrain S_max of 0.18%, and T_C of 428 °C, showing potential applications for lead-free piezoelectric ceramics at considerably high temperature.     

Annealing effects on the morphology and luminescence of cubic boron nitride based ceramics

Cubic boron nitride based ceramics with silicon were sintered at 1350 °C under a pressure of 5.0 GPa. The effects of post-annealing on grain morphology, surface morphology, and photoluminescence of Si–cBN ceramics were investigated by scanning electron microscope and room temperature photoluminescence measurements. The results showed that the annealing treatment had great influence on cBN grain morphology, rather than the surface morphology. The luminescence intensity increased with annealing temperature and annealing time. The void-net structure formed by continuous distribution of SiO_x particulate on the ceramic surface resulted in the emission band peaking at about 701.2 nm, and the tense passivation of Si by SiO_x led to the peak's low intensity. The near ultraviolet emission band peaking at about 317 nm was attributed to the oxygen vacancies formed in cBN grain surface, caused by the scavenging of oxygen from the cBN grain surface by the added Si.     

Synthesis and characterization of SmAlO<Subscript>3</Subscript> dielectric material by citrate precursor method

SmAlO₃ nanopowder synthesized by a citrate precursor method using citric acid as a chelating agent and ethylene glycol as an esterifying agent was reported in this paper. The phase purity of the as-prepared powder was examined using thermogravimetry (TG) analysis and differential scanning calorimetry (DSC) analysis, Fourier transform infrared spectroscopy (FTIR). The X-ray diffraction (XRD) studies showed that pure SmAlO₃ phase with orthorhombic perovskite structure could be synthesized at 800 °C for 2 h without any detectable intermediate phase. The average particle size calculated from transmission electron microscopy (TEM) investigation for the powder synthesized at 900 °C was as low as 45 nm. The nanopowder was sintered to a density of 97% of the theoretical density at 1,550 °C for 2 h and the bulk ceramics exhibited excellent microwave dielectric properties as follows: a dielectric constant of 20.54, a quality factor of 75,380 GHz and a temperature coefficient of resonate frequency of −69.2 ppm/K.     

Preparation and characterization of CaCu3Ti4O12 powders by non-hydrolytic sol–gel method

CaCu₃Ti₄O₁₂ (CCTO) powders were prepared via a non-hydrolytic sol–gel (NHSG) method by using acetylacetone as chelating agent and ethylene glycol as solvent. The samples were characterized by TG–DSC, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscope. The dielectric properties of ceramics were also measured. The pure perovskite-like CCTO powders were obtained by heat treatment at 800 °C for 2 h. The average particle sizes of CCTO powders calcined at 800 °C were approximately 350–450 nm. The samples sintered at 1,000 °C showed the mean grain size of 2.5–4 μm. Specially, the ceramics exhibited high dielectric constant (1.19 × 10⁵–1.40 × 10⁵) and low dielectric loss (0.051–0.1) in the temperature range of 30–110 °C. Moreover, with the NHSG method the period of synthesis process was greatly shortened.     

Influence of grain size on the oxygen permeation performance of perovskite-type (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3−δ membranes

The effect of grain size distribution in perovskite-type (Ba_0.5Sr_0.5)(Fe_0.8Zn_0.2)O_3−δ (BSFZ) ceramics on their oxygen permeation behaviour has been investigated by variation of calcination temperature in powder production and sintering time for the ceramics. The membranes were examined via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. We found that the dwell time during sintering has an important influence on the microstructure of the ceramic. The longer the dwell time, the further proceeds the grain coarsening, which affects the oxygen permeation in a positive way and leads to an enhanced permeation. Supplementary, decreasing calcination temperature in perovskite powder synthesis delivers fine powders with grain sizes less than one micrometer and thus smaller grains in the ceramic. Unfortunately, the grain size distribution in sintered membranes is not constant through membrane cross-sections since grains in the bulk are smaller compared to those at the surface which is not favorable for the oxygen permeation of the ceramics. The activation energy was determined to be in the range of 51–53 kJ/mol and its variation does not exhibit a dependence of grain size changes. High-resolution transmission electron microscopy proved that grain boundaries are atomically thin without any interfacial phases. We come to the conclusion that the transport rate of the oxygen permeation is limited predominantly by bulk diffusion and due to the fact that grain boundaries in BSFZ act as barriers for bulk diffusion, this material is a high mobility material.     

Microstructural Evolution and High-Performance Giant Dielectric Properties of Lu3+/Nb5+ Co-Doped TiO2 Ceramics

Giant dielectric (GD) oxides exhibiting extremely large dielectric permittivities (ε’ > 10⁴) have been extensively studied because of their potential for use in passive electronic devices. However, the unacceptable loss tangents (tanδ) and temperature instability with respect to ε’ continue to be a significant hindrance to their development. In this study, a novel GD oxide, exhibiting an extremely large ε’ value of approximately 7.55 × 10⁴ and an extremely low tanδ value of approximately 0.007 at 10³ Hz, has been reported. These remarkable properties were attributed to the synthesis of a Lu³⁺/Nb⁵⁺ co-doped TiO₂ (LuNTO) ceramic containing an appropriate co-dopant concentration. Furthermore, the variation in the ε’ values between the temperatures of −60 °C and 210 °C did not exceed ±15% of the reference value obtained at 25 °C. The effects of the grains, grain boundaries, and second phase particles on the dielectric properties were evaluated to determine the dielectric properties exhibited by LuNTO ceramics. A highly dense microstructure was obtained in the as-sintered ceramics. The existence of a LuNbTiO₆ microwave-dielectric phase was confirmed when the co-dopant concentration was increased to 1%, thereby affecting the dielectric behavior of the LuNTO ceramics. The excellent dielectric properties exhibited by the LuNTO ceramics were attributed to their inhomogeneous microstructure. The microstructure was composed of semiconducting grains, consisting of Ti³⁺ ions formed by Nb⁵⁺ dopant ions, alongside ultra-high-resistance grain boundaries. The effects of the semiconducting grains, insulating grain boundaries (GBs), and secondary microwave phase particles on the dielectric relaxations are explained based on their interfacial polarizations. The results suggest that a significant enhancement of the GB properties is the key toward improvement of the GD properties, while the presence of second phase particles may not always be effective.