Investigation of Resistivity and Permittivity for (Ba,Pb)TiO3 PTCR Ceramics

A scheme to examine quantitatively the interdependence between the measured resistance and permittivity data for semiconducting (Ba,Pb)TiO₃ samples having Curie points above 300°C was developed based on the Schottky-type potential barrier model. Specifically, the nonvanishing spontaneous polarization in the ferroelectric state was considered in terms of a useful parameter to explain the low resistivity below the Curie point. The resistivity derived from the measured permittivity shows a consistent temperature dependence with the dc measured one, i.e., the positive temperature coefficient of resistance (PTCR) characteristics. It is also observed that more satisfactory agreement can be achieved when the acceptor states are assumed to be distributed over a certain energy interval. The values of acceptor-state densities thus obtained are found to be in excellent agreement with those extracted from the slope in the Arrhenius plots of resistivity vs l/( T ε_m), where T is the absolute temperature and ε_m the dielectric constant. The validity of the present methodology is justified by obedience to the Curie–Weiss law for the calculated bulk dielectric constant.     

Microstructure and Dielectric Properties of Textured SrTiO3 Thin Films

We investigate the relationship between microstructure and dielectric properties of textured SrTiO₃ thin films deposited by radio-frequency magnetron sputtering on epitaxial Pt electrodes on sapphire substrates. The microstructures of Pt electrodes and SrTiO₃ films are studied by transmission electron microscopy, atomic force microscopy, and X-ray diffraction. SrTiO₃ films grown on as-deposited and annealed Pt electrodes, respectively, consist of a mixture of (111)- and (110)-oriented grains. Temperature-dependent dielectric measurements show that differences in texture and microstructure are reflected in the Curie–Weiss behavior of the SrTiO₃ films. Phenomenological models that account for the effects of thermal mismatch strain on the dielectric behavior are developed for different film textures. The models predict that at a given temperature, paraelectric (111)-oriented films of SrTiO₃ on tensile substrates will have a higher Curie–Weiss temperature and a greater dielectric constant than (110)-oriented films or bulk SrTiO₃. The experimental dielectric behavior is compared with the predictions from theory, and different contributions, such as interfacial layers, film stress, and microstructure, to the Curie–Weiss behavior are discussed.     

Impedance Spectroscopy Study of Niobium-Doped Strontium Titanate Ceramics

Impedance spectroscopy was used to study the temperature dependence of the resistance and capacitance of niobium-doped strontium titanate ceramics and also to identify the main contributions determining potential applications. Nyquist plots ( Z " versus Z ') show a main contribution in the intermediate-frequency range, with a peak at ∼1 kHz at 500°C. The activation energy of this resistance contribution is close to 1.5 eV. Modulus representations (log M " versus log f ) show a second contribution in the high-frequency range, with a peak at ∼100 kHz at 500°C. This resistance contribution is much smaller, and its activation energy is also lower (close to 0.8 eV). The capacitance of the intermediate-frequency term increases from ∼0.1 μF/m at 700°C to ∼1 μF/m at 425°C. The capacitance values of the high-frequency contribution are also >0.1 μF/m, much higher than expected for the bulk. These contributions might be related to differences between the intergrain interfaces and outer grain shells, as suggested by SEM microstructures with core–shell formations. Representations of 1/ C against temperature suggest a Curie–Weiss law for the main contribution in the intermediate-frequency range, with a T _C of ∼400°C. However, the temperature dependence of the overall dielectric constant measurements (at constant frequency) fail to show a clear maximum, probably due to an additional low-frequency contribution ascribed to the Pt/SrTi_{1− x} Nb _x O_3−δ interface.     

Properties of Modified Lead Zirconate Titanate Ceramics Prepared at Low Temperature (800°C) by Hot Isostatic Pressing

High-density lead zirconate titanate (PZT) ceramics were fabricated for the first time at a temperature as low as 800°C via the hot isostatic pressing (HIP) of a PZT powder with a modified composition of 0.92Pb(Zr_0.53Ti_0.47)O₃—0.05BiFeO₃—0.03Ba(Cu_0.5W_0.5)O₃ that contained 0.5 mass% MnO₂. The resultant PZT ceramics exhibited a microstructure that was denser and finer than that of PZT sintered at 935°C, which is the lowest temperature for the densification of the same composition via normal sintering. The relevant dielectric and piezoelectric properties of the HIPed PZT ceramics were as follows: coefficient of electromechanical coupling ( K ₃₁), 31.8%; mechanical quality factor ( Q _m), 1364; piezoelectric constant ( d ₃₁), −73.7 × 10⁻¹² C/N; relative dielectric constant (ɛ₃₃^T/ɛ₀), 633; dielectric loss factor (tan δ), 0.5%; Curie temperature ( T _c), 285°C; and density (ρ), 8.06 g/cm³. In addition to these reasonably good piezoelectric properties, the HIPed PZT exhibited better mechanical properties—particularly, higher fracture strength—than the normally sintered PZT.     

Sintering and Electrical Properties of Lead-Free Na0.5K0.5NbO3 Piezoelectric Ceramics

Lead-free Na_0.5K_0.5NbO₃ (NKN) piezoelectric ceramics were fairly well densified at a relatively low temperature under atmospheric conditions. A relative density of 96%–99% can be achieved by either using high-energy attrition milling or adding 1 mol% oxide additives. It is suggested that ultra-fine starting powders by active milling or oxygen vacancies and even liquid phases from B-site oxide additives mainly lead to improved sintering. Not only were dielectric properties influenced by oxide additives, such as the Curie temperature ( T _c) and dielectric loss ( D ), but also the ferroelectricity was modified. A relatively large remanent polarization was produced, ranging from 16 μC/cm² for pure NKN to 23 μC/cm² for ZnO-added NKN samples. The following dielectric and piezoelectric properties were obtained: relative permittivity ɛ ^T ₃₃/ɛ ₀ =570–650, planar mode electromechanical coupling factor, k _p=32%–44%, and piezoelectric strain constant, d ₃₃=92–117 pC/N.     

Dielectric Properties and Microstructure of Nb-Co Codoped BaTiO3–(Bi0.5Na0.5)TiO3 Ceramics

X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and an impedance analyzer were used to examine the Nb–Co codoping effects on the densification, crystalline phase, microstructure development, and dielectric–temperature characteristics of BaTiO₃–(Bi_0.5Na_0.5)TiO₃ ceramics. The results indicate that the Curie temperature shifted to a higher temperature (above 140°C) by adding BNT. The dielectric constant–temperature (ɛ– T ) curve broadened at the Curie temperature due to the small grain size (0.3–0.4 μm). A core-shell structure was developed, which is helpful to flatten the ɛ– T curve of BaTiO₃ ceramics at high temperatures.     

Effect of CuO on the Sintering Temperature and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

When a small amount of CuO was added to (Na_0.5K_0.5)NbO₃ (NKN) ceramics sintered at 960°C for 2 h, a dense microstructure with increased grains was developed, probably due to liquid-phase sintering. The Curie temperature slightly increased when CuO exceeded 1.5 mol%. The Cu²⁺ ion was considered to have replaced the Nb⁵⁺ ion and acted as a hardener, which increased the E _c and Q _m values of the NKN ceramics. High piezoelectric properties of k _p=0.37, Q _m=844, and ɛ₃ ^T /ɛ₀=229 were obtained from the specimen containing 1.5 mol% of CuO sintered at 960°C for 2 h.     

Dielectric, Piezoelectric, and Pyroelectric Properties of Lead Zirconate—Lead Zinc Niobate Ceramics

New piezoelectric and pyroelectric ceramics consisting of antiferroelectric lead zirconate (PZ) and relaxor ferroelectric lead zinc niobate (PZN) are studied from an application view-point of the field-induced antiferroelectric-to-ferroelectric phase transition. An antiferroelectric-ferroelectric phase boundary exists in PbZr_x(Zn_1/3Nb_2/3)_1−xO₃ (PZZN-1000x) close to x = 0.93 to 0.94 at room temperature. A new ferroelectric rhombohedral phase change, F_α–F'_α, at low temperature is found and studied by the temperature dependence of the pyroelectric coefficient. Electrical poling in these ceramics is easy, and the coercive field E_c∼8 to kV/cm is rather low. Samples with compositions in the range PZZN-86 to PZZN-92 have a large electromechanical coupling constant, k (k_t and k₁₅∼50% to 60%), and a low dielectric constant, ɛ_s (ɛ^T₃₃/ɛ₀= 260 to 320, ɛ^T₁₁/ɛ₀= 380). PZZN ceramics appear to be potential candidates for high-frequency ultrasonic transducers used in the thickness shear mode. The pyroelectric figure of merit (F_v) of these ceramics is comparable to the values published for the PZT-based or PbTiO₃-based materials.     

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.     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

High Piezoelectric and Dielectric Properties of La-Doped 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb(ZrxTi1−x)O3 Ceramics Near Morphotropic Phase Boundary

La-doped 0.3Pb(Zn_1/3Nb_2/3)O₃–0.7Pb(Zr _x Ti_{1− x} )O₃ ( x =0.5–0.53) piezoelectric ceramics with pure perovskite phase were synthesized by a two-step hot-pressing route. The piezoelectric properties of various compositions near the morphotropic phase boundary (MPB) were systematically investigated. Not only was the exact MPB of this system determined via X-ray diffractometry analysis, but also the peak of piezoelectric properties was found near the MPB. The optimum piezoelectric properties of this series were observed in the specimen with Zr/Ti=51/49. The piezoelectric coefficient ( d ₃₃) and electromechanical coupling factor ( k _p) were 845 pC/N and 0.70, respectively, which have not been reported in this system so far. Large permittivity (ɛ_r=4088) and permittivity maximum (ɛ_m=29 500) were also obtained for the poled specimens. The temperatures ( T _max) of the permittivity maxima ranged from 206° to 213°C with various Zr/Ti ratios.     

Low-Temperature Sintering and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

(Na_0.5K_0.5)NbO₃ (NKN) ceramic with 1.5 mol% CuO added (NKNC) was well sintered even at a low temperature of 900°C with the addition of ZnO. Most of the ZnO reacted with the CuO and formed the liquid phase that assisted the densification of the specimens at 900°C. A few Zn²⁺ ions entered the matrix of the specimens and increased the coercive field ( E _c) and Q _m values of the specimens. High-piezoelectric properties of k _p=0.37, Q _m=755, and ɛ₃ ^T /ɛ₀=327 were obtained from the NKNC ceramics containing 1.0 mol% ZnO sintered at 900°C for 2 h.     

Effect of Acceptor (Mg) Concentration on the Resistance Degradation Behavior in Acceptor (Mg)-Doped BaTiO3 Bulk Ceramics: II. Thermally Stimulated Depolarization Current Analysis

Thermally stimulated depolarization current (TSDC) of acceptor (Mg)-doped BaTiO₃ ceramics was analyzed with different acceptor concentrations for coarse-grained specimens with uniform grain sizes. In all specimens, the increase of the polarization temperature ( T _P) for a fixed condition of polarization field ( E _P) and polarization time ( t _P) increased TSDC peak associated with an oxygen vacancy ( V _O^••) relaxation. When the acceptor concentration is increased, both T _P to generate same magnitude of TSDC and the relaxation temperature ( T _m) of the TSDC peak systematically decreased. On the other hand, the activation energy of the oxygen vacancy relaxation showed roughly constant values of ∼0.9 eV, irrespective of acceptor concentration. Such behavior can be explained by a decrease in the relaxation time constant (τ₀), which is in turn associated with the shape of oxygen vacancy profile in the specimen after polarization. The decrease of T _P, T _m, τ₀, and the little change of activation energy from the TSDC data with the increase of acceptor concentration implies an increase in the oxygen vacancy concentration C ( V _O^••). The experimentally observed behavior of C ( V _O^••) vs acceptor concentration could be explained by the defect chemical model, and from these results, the acceptor ionization energy E _A was estimated to be about 1.0 eV.     

Dielectric Characteristics of Nd(Zn1/2Ti1/2)O3 Ceramics at Microwave Frequencies

The microwave dielectric properties and the microstructures of Nd(Zn_1/2Ti_1/2)O₃ (NZT) ceramics prepared by the conventional solid-state route have been studied. The prepared NZT exhibited a mixture of Zn and Ti showing 1:1 order in the B-site. The dielectric constant values (ɛ_r) saturated at 29.1–31.6. The quality factor ( Q × f ) values of 56 700–170 000 (at 8.5 GHz) can be obtained when the sintering temperatures are in the range of 1300°–1420°C. The temperature coefficient of resonant frequency τ_f was not sensitive to the sintering temperature. The ɛ _r value of 31.6, the Q × f value of 170 000 (at 8.5 GHz), and the τ_f value of −42 ppm/°C were obtained for NZT ceramics sintering at 1330°C for 4 h. For applications of high selective microwave ceramic resonators, filters, and antennas, NZT is proposed as a suitable material candidate.     

Characteristics of High-Q Microwave Dielectric Ceramics Nd(Co1/2Ti1/2)O3 With CuO Addition

We report the microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by the conventional solid-state route. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibits a mixture of Co and Ti showing a 1:1 order in the B site. Lowering the sintering temperature (as low as 1260°C) and promoting the densification of Nd(Co_1/2Ti_1/2)O₃ ceramics could be effectively achieved by adding CuO (up to 0.75 wt%). At 1350°C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% CuO addition possess a dielectric constant (ɛ_r) of 27.6, a Q × f value of 165 000 GHz (at 9 GHz), and a temperature coefficient of resonant frequency (τ_f) of −20 ppm/°C. By comparing with pure Nd(Co_1/2Ti_1/2)O₃ ceramics, incorporating additional CuO helps to render a dielectric material with a higher dielectric constant, a smaller τ_f value, and a 20% dielectric loss reduction, which makes it a very promising candidate for applications requiring low microwave dielectric loss.     

Effects of Pore Morphology and Grain Size on the Dielectric Properties and Tetragonal–Cubic Phase Transition of High-Purity Barium Titanate

The effects of pore morphology and grain size on the dielectric behavior of high-purity stoichiometric BaTiO₃ have been intensively investigated. It was found that the dielectric constant was influenced not only by grain size but also by pore morphology. Dielectric constants below the Curie temperature could be evaluated by the Maxwell relationship for specimens with fractional density >90%ρ_t and be estimated by the modified Niesel's equation, but depolarization might be involved for specimens with fractional density <90%ρ_t. Dielectric Behavior above the Curie temperature followed the Curie–Weiss low. The Curie constants could be separated into two regions depending on the pore morphology, decreasing linearly with increasing porosity at different rates. The results suggest that the tetragonal–cubic phase transition temperature of specimens with fractional density <90%ρ_t is affected by depolarization due to the presence of continous channel pores. The dissipation factor was increased with increasing porosity due to the adsorption of water. In this study, a high-density (<99%ρ_t), uniform, and fine-grained (∼1.2 μm) microstructure of high-purity stoichimetric barium titanate has been produced by using wet processing ad pressureless sintering, in which a high dielectric constant (>6100 at 25°C and 1 kHz) and a low dissipation factor (<0.025) could be achieved.     

Preparation and Ferroelectric Properties of SBN:50 Ceramics

Stoichiometric quantities of AR-grade chemicals, e.g., Ba(NO₃)₂, Sr(NO₃)₂, and Nb₂O₅, were used to synthesize Sr_0.5Ba_0.5Nb₂O₆ (SBN:50) by the solid-state reaction route. The reaction mixture was characterized by DTA/TG/DTG to locate the formation temperature of SBN:50. X-ray diffrction was used to identify the major and any additional phases formed in the calcined as well as sintered materials. The materials were further characterized by electrical measurements, the temperature dependence of the dielectric constant (ɛ) and the loss (tan δ), FE hysteresis-loop parameters ( P _s and E _c), and room-temperature dc resistivity. SBN:50 exhibited a broad maximum of ɛ in the temperature range 65° to 95°C. Moreover, the FE hysteresis loop persisted even at 130°C, which is much higher than the Curie range determined from an ɛ vs T plot. Microstructural photographs revealed varied grain-size distribution between 5 and 20 μm. The presence of residual porosity (∼ 95% dense) resulted in opaque compacts.     

Adhesive-Bonded Ca(Mg1/3Nb2/3)O3/Ba(Zn1/3Nb2/3)O3 Layered Dielectric Resonators with Tunable Temperature Coefficient of Resonant Frequency

Ca(Mg_1/3Nb_2/3)O₃ and Ba(Zn_1/3Nb_2/3)O₃ ceramic cylinders with the same diameter were bonded by adhesive with low dielectric loss to yield the layered dielectric resonators, and the microwave dielectric characteristics were evaluated with TE_01δ mode. With increasing the Ba(Zn_1/3Nb_2/3)O₃ thickness fraction, the resonant frequency ( f ₀) decreased, while the effective dielectric constant (ɛ _{r ,eff}) and temperature coefficient of resonant frequency (τ _f ) increased. Good microwave dielectric characteristics were attained for the samples with the Ba(Zn_1/3Nb_2/3)O₃ thickness fraction of 0.5: ɛ _{r ,eff}=34.33, Q × f =57 930 GHz and τ _f =2.6 ppm/°C. Finite-element method was used to predict the microwave dielectric characteristics of the layered resonators and good agreements were attained between the experimental results and predicted ones. Also, both experiment and finite-element analysis indicated that the effects of the adhesive on f ₀, ɛ _{r ,eff}, and τ _f were slight, while that on Q × f value was significant.     

Structure–Property Relations in xBaTiO3–(1−x)La(Mg1/2Ti1/2)O3 Solid Solutions

The microwave dielectric properties and microstructures of compounds in the solid solution series x BaTiO₃–(1− x )La(Mg_1/2Ti_1/2)O₃ (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where x ≤ 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x ≤ 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < x < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (τ_f) vs the relative permittivity (ɛ_r) was linear until x = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. τ_f values close to zero (−2 ppm/°C) were achieved at x = 0.5 (ɛ_r∼ 60), which had a quality factor ( Q · f _o) of 9600 GHz.     

A- and B Site Cosubstituted Ba6−3xSm8+2xTi18O54Microwave Dielectric Ceramics

Tin (Sn) substitution into the B-site and Nd/Sn cosubstitution into the A- and B-sites were investigated in a Ba _{6−3 x} Sm_{8+2 x} Ti₁₈O₅₄solid solution ( x = 2/3). A small amount of tin substitution for titanium improved the temperature coefficient of resonant frequency (τ_f) but led to a decrease of the relative dielectric constant (ɛ) and the quality factor ( Qf ). The Ba_{6−3 x} Sm_{8+2 x} (Ti_{1− z} Sn_z)₁₈O₅₄-based tungsten-bronze phase became unstable for compositions with a tin content of ≥10 mol%, where BaSm₂O₄and Sm₂(Sn,Ti)₂O₇appeared, and finally, these phases became the major phases. On the other hand, Nd/Sn cosubstitution led to a good combination of high ɛ, high Qf , and near-zero τ_f. Excellent microwave dielectric properties were achieved in Ba_{6−3 x} (Sm_{1− y} Nd _y )_{8+2 x} (Ti_{1− z} Sn _z )₁₈O₅₄ceramics with y = 0.8 and z = 0.05 sintered at 1360°C for 3 h: ɛ= 82, Qf = 10 000 GHz, and calculated τ_f=+17 ppm/°C. The tolerance factor and electronegativity difference exhibited important effects on the microwave dielectric properties, especially the Qf value. A large tolerance factor and high electronegativity difference generally led to a higher Qf value.