Sintering and compositional effects on the microwave dielectric characteristics of Mg(Ta1−x Nb x )2O6 ceramics with 0.25 ≦ x ≦ 0.35

1,500 °C−sintered MgTa₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 30.5, Q × f = 56,900 GHz, and τ _f = 28.3 ppm/°C, whereas 1,400 °C-sintered MgNb₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 21.7, Q × f = 89,900 GHz, and τ _f = −68.5 ppm/°C. In order to find the dielectric resonators with τ _f value close to 0 ppm/°C, the effects of sintering condition and composition on the microwave dielectric characteristics of Mg(Ta_1−x Nb _x )₂O₆ ceramics (0.25 ≦ x ≦ 0.35) prepared under sintering temperature of 1,300–1,450 °C are investigated. The results show that as the sintering temperature increases from 1,300 to 1,450 °C, the ɛ _r, Q × f and τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics all increase and saturate at 1,450 °C. On the other hand, as the Nb₂O₅ content decreases, the τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics will shift to near 0 ppm/°C. The optimized sintering conditions and composition to obtain the Mg(Ta_1−x Nb _x )₂O₆ dielectrics with τ _f close to 0 ppm/°C are sintering temperature of 1,450 °C, sintering duration of 4 h, and composition of x = 0.25, which exhibits the microwave dielectric characteristics of ɛ _r = 27.9, Q × f = 33,100 GHz, and τ _f = −0.7 ppm/°C.     

Dielectric properties and morphotropic phase boundaries in the xPb(Zn1/3Nb2/3)O3−(1−x)Pb(Zr0.5Ti0.5O3 pseudo-binary system

Ceramics in the xPb(Zn_1/3Nb_2/3)O₃−(1−x)Pb(Zr_0.5Ti_0.5)O₃ [xPZN–(1−x)PZT] solid solution system are expected to display excellent dielectric, piezoelectric, and ferroelectric properties in compositions close to the morphotropic phase boundary (MPB). The dielectric behavior of ceramics with x = 0.1−0.6 has been characterized in order to identify the MPB compositions in this system. Combined with X-ray diffraction results, ferroelectric hysteresis measurements, and Raman reflectivity analysis, it was consistently shown that an MPB exists between x = 0.2 and x = 0.3 in this binary system. When x ≤ 0.2, the tetragonal phase dominates at ambient temperatures. In the range of x ≥ 0.3, the rhombohedral phase dominates. For this rhombohedral phase, electrical measurements reveal a profound frequency dispersion in the dielectric response when x ≥ 0.6, suggesting a transition from normal ferroelectric to relaxor ferroelectric between 0.5 ≤ x ≤ 0.6. Excellent piezoelectric properties were found in 0.3PZN–0.7PZT, the composition closest to the MPB with a rhombohedral structure. The results are summarized in a PZN–PZT binary phase diagram.     

Structural evolution and microwave dielectric properties of (1 − x)BaZn1/3Nb2/3O3 + xBaMg1/2W1/2O3 ceramics

Structural evolution and microwave dielectric properties of (1 − x)BaZn_1/3Nb_2/3O₃ + xBaMg_1/2W_1/2O₃ (0 ≤ x ≤ 1) system have been investigated in this work. All samples exhibit single perovskite phase except for the samples with x = 0 and x ≥ 0.8 in which barium niobate and BaWO₄ second phase existed, respectively. 1:1 cations ordering existed in the samples with x ≥ 0.1, and the ordering degree increases with the increase of x. Liquid phase sintering was observed in the sample with x ≥ 0.8. Dielectric constant decreases almost linearly from 40.8 to 17.4 with increasing x. Q × f value monotonically increases from 26,162 GHz to 64,705 GHz with increasing x. The τ_f value changes from +30 ppm/°C to −27.8ppm/°C. Near zero τ_f value of −1.4 ppm/°C could be obtained at x = 0.4 composition.     

Dielectric relaxation and variable-range-hopping conduction in BaSn1−x Cr x O3 system

The possibility of formation of a solid solution in the system BaSn_1−x Cr _x O₃ has been explored upto x ≤ 0.20. It has been confirmed that single phase solid solution forms upto x ≤ 0.10. Dielectric and conduction behaviour of single phase samples have been studied in the temperature range 400–610 K and frequency range 10 Hz–2 MHz. Two dielectric relaxation processes in two different frequency ranges have been observed. The temperature dependence of both dc and ac resistivity obey relation ρ = ρ _o exp(B/T^1/4), indicative of variable range hopping conduction mechanism. The activation energy for dc conduction is higher than that for relaxation time (τ) of low frequency dielectric relaxation process. It has been observed that activation energy for dielectric relaxation matches with activation energy for ac conductivity (at 100 kHz) for both the dielectric relaxation processes. Seebeck coefficient ‘α’ of the samples have been measured in the temperature range 350–650 K. Negative value of ‘α’ in the entire range of temperature measurement shows that conduction species are negatively charged. On the basis of value of activation energy for dc conduction and sign of Seebeck coefficient, conduction in the low temperature region (below 500 K) is attributed to hopping of weakly bonded electrons among Sn²⁺ ⇔ Sn⁴⁺ or Sn³⁺ ⇔ Sn⁴⁺ and that in the high temperature region (above 500 K) to hopping of doubly ionized oxygen vacancies .     

Low-temperature sintering and electrical properties of (1−x)Pb(Zr0.5Ti0.5)O3-xPb(Cu0.33Nb0.67)O3 ceramics

Electrical properties and sintering behaviors of (1 − x)Pb(Zr_0.5Ti_0.5)O₃-xPb(Cu_0.33Nb_0.67)O₃ ((1 − x)PZT-xPCN, 0.04 ≤ x ≤ 0.32) ceramics were investigated as a function of PCN content and sintering temperature. For the specimens sintered at 1050^∘C for 2 h, a single phase of perovskite structure was obtained up to x = 0.16, and the pyrochlore phase, Pb₂Nb₂O₇ was detected for further substitution. The dielectric constant (ε _r), electromechanical coupling factor (K_p) and the piezoelectric coefficient (d ₃₃) increased up to x = 0.08 and then decreased. These results were due to the coexistence of tetragonal and rhombohedral phases in the composition of x = 0.08. With an increasing of PCN content, Curie temperature (T_c) decreased and the dielectric loss (tanδ) increased. Typically, ε_r of 1636, K_p of 64% and d₃₃ of 473pC/N were obtained for the 0.92PZT-0.08PCN ceramics sintered at 950^∘C for 2 h.     

The electronic conductivity of Ba0.5Sr0.5CoxFe1?xO3?δ (BSCF: x?=?0?～?1.0) under different oxygen partial pressures

The electronic conductivity of sintered BSCF ceramics (Ba_0.5Sr_0.5Co _x Fe_1−x O_3−δ, 0 ≤  x  ≤ 1) was measured as a function of temperature up to 1273 K in air. The conductivity of BSC is thermally activated over 298–1273 K with an activation energy of 0.21 eV. The conductivity of BSF and BSCF (0.2 ≤  x  ≤ 0.8) is thermally activated below ∼673 K with activation energies of 0.21 eV–0.40 eV. Above 673 K, the formation of oxygen vacancies results in a decrease in p-type carrier concentration and a decrease in electronic conductivity. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF5582) was also measured under 10⁻⁵ atm ≤ pO₂ ≤ 1 atm. Below ∼673 K, the electronic conductivity of BSCF 5582 shows no dependence on pO₂. Above 673 K, the conductivity of BSCF5582 increases with increasing pO₂ for pO₂ ≥ 0.01 (p-type conduction) and decreases slightly with increasing pO₂ for pO₂  ≤ 0.01 atm. The activation energy for conduction above ∼673 K and at pO₂ ≥ 0.1 is ∼0.07 eV. Above ∼823K and at pO₂ ≥ 0.01 atm, the activation energy for conduction is ∼0.2 eV.     

Dielectric anomalies of Pb0.7La0.2TiO3-based perovskite

Dielectric transitions from normal ferroelectric to relaxor-like and then to quantum paraelectric-like behaviour were observed by substituting 10 to 60% of Ca²⁺ for Pb²⁺ into A-site of Pb_0.7La_0.2TiO₃ with the required stoichiometry of Pb_0.7(1−x)Ca_0.7x La_0.2TiO₃ (PCLT). The quantum ferroelectric relation that commonly applies to SrTiO₃-based perovskites, T _maxα (X − X _c )^1/2, fails to describe the observed dielectric anomalies in PCLT, whereby the transition temperature vanishes with a finite slope. Quantum ferroelectric behaviour that exhibits a sharp dielectric peak but violates the Curie-Weiss law was not observed throughout the composition range studied. Unlike the typical quantum paraelectric SrTiO₃, the quantum paraelectric-like behaviour observed for PCLT with x = 0.55 and 0.60 exhibit negative transition temperatures, as shown by the fittings to both the Curie-Weiss and Barrett’s relations. Thermal hysteresis was surprisingly observed in substituted PCLT with x = 0.30 and 0.40 that exhibit frequency dispersive relaxation. To establish a correlation between the dielectric anomalies and structural parameters, analyses on global and local perovskite lattices were carried out using X-ray diffraction technique and micro-Raman spectroscopy.     

Fabrication,property and application of novel pyroelectric single crystals—PMN–PT

The pyroelectric properties and temperature stability of Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃(PMN–xPT) single crystals (0.13 ≤ × ≤ 0.40) were investigated. The best choice for pyroelectric performance is [111]-oriented PMN–0.26PT single crystal whose figures of merit for voltage responsivity and detectivity are 0.11 m²/C and 15 × 10⁻⁵ Pa^−1/2, respectively. However, the [001]-oriented PMN–0.37PT single crystal has much better temperature stability, whose temperature coefficient of pyroelectric property is 0.5%/K in the range of 20 °C to 55 °C, and Curie temperature is high: 175 °C. We also found that PMN–xPT possessed low thermal diffusivity D ~ 4.4 × 10⁻⁷ m²/s, low volume specific heat C _v~ 2.5 × 10⁶ J/m³ K and tunable permittivity ε ~ (300–7000). The pyroelectric performances of PMN–xPT single crystals are superior to those of conventional pyroelectric materials and promising for IR device applications.     

Structural and ferroelectric characterization of BMZ–BF–PT ceramics

Bismuth containing crystalline solid solutions of [(BiMgZrO₃)_1 − y –(BiFeO₃) _y ] _x –(PbTiO₃)_1 − x , (BMZ–BF–PT) have been synthesized by high temperature solid-state reaction technique. The crystalline symmetry varied with the composition, indicating good solid state solubility of BMZ and BF with PT. X-ray diffraction (XRD) reveals that BMZ–PT and BMZ–BF–PT have a single-phase perovskite structure. The MPB of BMZ–PT lies in the region x = 0.55 to x = 0.6, which is supported by the transformation from rhombohedral to tetragonal phase. The SEM photographs exhibit the uniform distribution of grains in the matrix. Polarization–electric field hysteresis studies were carried out at Room Temperature for all the compositions up to an applied electric field of 3.5 kV/mm. It was found that with the increase of BiFeO₃ content in the composition the remnant polarization decreases and the ferroelectric loops get constricted. Variation of dielectric constant with temperature shows that there is an increase in Transition temperature (T _C) with the increase in applied frequency indicating a relaxor characteristic of the compound. Our results show that BMZ–BF–PT is a good low-lead (Pb) high-temperature ferroelectric ceramic.     

Acoustic properties of PLZT ceramics studied by Brillouin scattering

Brillouin spectra of lanthanum lead zirconate titanate (Pb_1−x La_x)(Zr_yTi_1−y )O₃ (PLZT-100(x/y/1−y)) relaxor ferroelectric ceramics with the famous composition of PLZT-10/65/35 have been investigated using a high-resolution Brillouin spectroscopy. The central peak of PLZT-10/65/35 appeared below ∼600 K, which is similar to the Burns temperature. The integrated intensity of the central peak grew on cooling and reached its maximum value at around 370 K, which is higher than the dielectric maximum temperatures. The overall shape and width of central peaks did not change in the electric field range of 0–7 kV/cm at room temperature. These observations were discussed in relation with the dynamics of nano-sized polar clusters, which have been suggested as the origin of the complex dynamical behaviors of ferroelectric relaxors.     

Aging of nickel manganite NTC ceramics

Effect of thermal history and chemical composition on aging of Ni _x Mn_3 − x O_{4 +  δ} (0.56 ≤ x ≤ 1.0) ceramics was investigated. It was found that all the Ni _x Mn_3 − x O_{4 +  δ} ceramic samples metallized by co-firing at 1050°C showed significant electrical stability with an aging coefficient less than 1.0%, while aging of those metallized by annealing at 850°C was increasingly serious with a rise in Ni content x, the aging coefficient ranging from 0.2% to 3.8%. However, the ceramic samples with Ni content x ≤ 0.70, whether metallized by co-firing or by annealing, exhibited extraordinarily high electrical stability with an aging coefficient less than 0.5%. The composition dependence of aging of the ceramic samples was explained qualitatively, based on the electrical conduction mechanism of small polaron hopping and on the aging mechanism of the cationic vacancy-assisted migration of cations to their thermodynamically preferable sites under thermal stress.     

Electrical properties of BaPb1+x−y Sb y O3 ceramics derived from a modified sol-gel method

BaPbO₃ ceramics has attracted considerable interest due to the promising electrical applications. For the best control of the microstructure and phase, Sb-doped BaPbO₃ ceramics were prepared by a sol-gel route. Inorganic compounds were used as starting chemicals, citric acid and Ethylene diamine tetra-acetic acid (EDTA) as complex chelate agent, and distilled water as solvent. Experimental results demonstrate that the Sb-doped BaPbO₃ ceramics with homogeneous composition could be prepared by the method mentioned above. Influence of Pb/Ba ratio and Sb concentration on the room-temperature resistivity and the PTCR behavior of BaPb_1+x−y Sb _y O₃ (x = 0.0, 0.1, 0.2 and 0 ≤ y ≤ 0.2) compositions was investigated. The PTCR behavior was related to the Pb/Ba ratio and major compensating defect in BaPbO₃. The lowest electrical resistivity of Sb-doped BaPbO₃ was calculated to be 2.69 × 10⁻⁴ Ω·cm when the Sb concentration y = 12–13 mol%. The 0.5 mol% Sb-doped BaPbO₃ showed the best PTCR behavior, and its Curie temperature is about 850 °C.     

Synthesis of (Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>)(Ti<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> ceramics and effect of Sr content on room temperature dielectric properties

The compositions in the system (Ba_1−x Sr _x )(Ti_0.5Zr_0.5)O₃ with different Sr (x) content, were synthesized through solid oxide reaction route. The phase formation behaviors in the system were investigated by XRD. The room temperature dielectric properties of the compositions were investigated in the frequency range 10 Hz to 13 MHz. The solid solution system Ba_1−x Sr _x Ti_0.5Zr_0.5O₃ remains as cubic perovskite up to x < 0.6 and transforms into the tetragonal structure above x > 0.6. Composition with x = 0.6 contains a mixture of cubic and tetragonal phases with broadened diffraction pattern. It is observed that the increasing of Sr substitution results in the decreasing of bulk density, average grain size and dielectric constants etc. in the composition system. The AC dielectric conductivity of the ceramics also decreases with the increase in Sr-substitution due to decrease in loss as well as grain size with that substitution.     

Structure and microwave dielectric properties of Sm(2−x)/3Li x TiO3

The structural evolution, and microwave dielectric properties of ceramics (x = 0.0 ≤ x ≤ 0.5) were investigated in this work. X-ray diffraction (XRD) results show that samples with x > 0.3 exhibit a single perovskite phase. Impurity phases of Sm₂Ti₂O₇ and TiO₂ appear and their amount increases with the decrease of x when x ≤ 0.3. TEM observation indicates that the A-site is ordered in x = 0.5, but not in x = 0.3). The dielectric constant decreases with the increase of x for 0.1 ≤ x ≤ 0.4 and then increases with further increase in x up to x = 0.5. The Q×f value decreases with the decrease of x due to the increased occurrence of Sm₂T₂O₇ secondary phase, defects and twinning boundaries. The temperature coefficient of resonant frequency is negative and its absolute value decreases greatly with the decrease of x value.     

Effects of structural characteristics on microwave dielectric properties of (1−x)CaWO4 –xLaNbO4 ceramics

Dependences of microwave dielectric properties on the structural characteristics of (1−x)CaWO₄–xLaNbO₄ ceramics were investigated as a function of LaNbO₄ content (0.0 ≤ x ≤ 0.5). A single phase with tetragonal scheelite structure was obtained up to x = 0.35, and then the mixture phases with scheelite and fergusonite structure were detected. With the increase of LaNbO₄, the deviation of the observed dielectric polarizabilities calculated by the Clausius-Mosotti equation from the theoretical values calculated by the additivity rule of dielectric polarizability, was decreased due to the decrease of oxygen bond valence in ABO₄ scheelite structure. Dielectric constant (K) and temperature coefficient of resonant frequency (TCF) were increased with LaNbO₄ content due to the decrease of oxygen bond valence. Q ⋅ fvalue was dependent on the atomic packing fraction of unit cell as well as the grain size. Typically, K = 13.3, Qf = 50,000 GHz and TCF = −8.7 ppm/^oC were obtained for the specimens with 0.3 mol of LaNbO₄ sintered at 1150^oC for 3 h.     

Sintering behavior, structures and microwave dielectric properties of a rutile solid solution system: (AxNb2x)Ti1–3xO2 (A=Cu, Ni)

The sintering behavior, structures and microwave dielectric properties in a rutile solid solution system—(A_xNb_2x)Ti_1–3xO₂ (A=Cu, Ni)—were investigated and the samples were prepared by conventional solid state reaction method. Single phase of tetragonal rutile structure has been obtained through the entire range of compositions (0.02 ≤ x ≤ 0.20). The sintering temperature was lowered to 900°C by (Cu _{x /3}Nb_2x/3)⁴⁺ substituting for Ti⁴⁺ in the solid solution. Comparing with that of rutile TiO₂ (465 ppm/°C), the temperature coefficient of resonant frequency (TCF) of the rutile solid solution is much lower (about 250 ppm/°C), and the dielectric constant and the quality factor (Qf value) of the solid solution are about 70~80 and 7,000G Hz. The substitution of (Cu _{x /3}Nb_2x/3)⁴⁺or (Ni _{x /3}Nb_2x/3)⁴⁺ for Ti⁴⁺ in the solid solution improved the microwave dielectric properties of the rutile TiO₂ ceramics.     

Phase transition and electrical studies of wolframium doped SrBi2Ta2O9 ferroelectric ceramics

In this study, crystalline structure, dielectric and impedance properties of SrBi₂Ta₂O₉ (SBT) - based ferroelectric ceramics have been investigated with the substitution of wolframium/tungsten (W) onto the tantalum site. Wolframium doped SrBi₂(W _x Ta_{1 − x} )₂O₉ (0.0 ≤ x ≤ 0.20) ceramics were synthesized by solid state reaction method. The X-ray diffractogram analysis revealed that the substitution formed a single phase layered perovskite structure for the doping content up to x ≤ 0.05. The dielectric measurements as a function of temperature show an increase in Curie temperature (T _c ) over the composition range of x = 0.05 to 0.20. The W^{6 +} substitution in perovskite-like units results in a sharp dielectric anomaly at the ferroelectric phase transition. Furthermore, the dielectric constant at their respective Curie temperature increases with wolframium doping. Both enhanced Curie temperatures and dielectric constants at the Curie points indicate an increase in polarizability, which could be attributed to the increased “rattling space” due to the incorporation of the smaller tungsten cations. The dielectric loss reduces significantly with tungsten addition. AC impedance properties vis-à-vis wolframium content has also been studied.     

Pyroelectric and dielectric properties of lead lanthanum zirconate titanate (Pb0.92La0.08)(Zr0.65Ti0.35)O3-P(VDF/TFE)(0.98/0.02) nanocomposites

PLZT-P(VDF/TFE) 0–3 composites with nanosized lead lanthanum zirconate titanate (Pb_0.92La_0.08)(Zr_0.65Ti_0.35)O₃ (PLZT 8/65/35) ceramic powders of volume fraction Φ up to 0.2 were fabricated using PLZT powders imbedded in a copolymer P(VDF/TFE)(0.98/0.02) matrix. The PLZT nanopowders were prepared by the sol-gel technique. The PLZT-P(VDF/TFE) composite samples were prepared from ceramic and polymer powders by the hot-pressing method. Dielectric response was studied in the frequency range from 100 Hz to 1 MHz and at temperatures from 100 to 450 K. The pyroelectric properties were studied by dynamic method with modulation frequency from 1 to 100 Hz. The dielectric response of the ceramics-polymer composite was found to be a combination of the responses of the pure polymer and the ceramics: (1) the addition of the PLZT ceramics increases the value of the dielectric permittivity ɛ′, (2) the composite shows the maximum of the permittivity coming from the PLZT ceramics, (3) the temperature dependences of the dielectric loss tgδ are characterized by the maximum attributed to the α-relaxation (glass transition) in the pure polymer. The pyroelectric coefficient of the composite increases from ∼20 μC/m²K in pure P(VDF/TFE) to ∼140 μC/m²K in the composites of Φ = 0.15.     

Preparation of sol-gel-derived Ca1−x Sr x ZrO3 perovskite dielectric thin films

The sol-gel wet chemical synthesis of dielectric thin films of perovskite Ca_1−x Sr _x ZrO₃ has been studied in detail using different techniques. To the best of our knowledge, it is the first time in the literature to systemically study the properties of Ca_1−x Sr _x ZrO₃ dielectric thin films in the whole solid solution composition range. Based on X-ray diffraction (XRD), Flourier transform infrared (FT-IR) reflectivity spectroscopy and atomic force microscope (AFM) data, the mechanisms of Ca_1−x Sr _x ZrO₃ phase transformation and crystallinity have been investigated. The results show that the film annealed at 550 °C is amorphous with existing of carbonates, while the carbonates and other organics are decomposed at 600 °C and above, and the film is crystallized into the perovskite phase with increasing annealing temperature. The values of dielectric constant in a range of 16–30 for Ca_1−x Sr _x ZrO₃ thin films have been obtained. Measured dielectric properties show that those films exhibit stable dielectric properties nearly independent on the applied electrical field and frequency at room temperature. Based on above data, it can be concluded that Ca_1−x Sr _x ZrO₃ thin films are a promising candidate system for the high-k microelectronic devices application.     

Synthesis of (Ba0.5Sr0.5)(Ti1 − x Zr x )O3 ceramics: Effect of Zr content on room temperature electrical properties

The phase formation behavior and room temperature dielectric properties of bulk perovskite solid solution composition (Ba_0.5Sr_0.5)(Ti_1 − x Zr _x )O₃ have been investigated. The samples with different Zr-content were prepared through solid state reaction. The XRD investigation showed that Zr⁺⁴ is systematically dissolved in Ba_0.5Sr_0.5TiO₃ lattice up to about 60 atm.% substitution, having cubic Pm3m structure. Eighty atom percent Zr substituted composition showed to contain a cubic phase similar to that of x = 0.6 composition and a tetragonal (I4/mcm) phase. That is the solid solution breaks around at 80 atm.% Zr substitutions. Ba_0.5Sr_0.5ZrO₃ was having orthorhombic Imma structure. Decrease in grain sizes were observed with increase in Zr content. The permittivity of the ceramics decreased with the increase in Zr substitution. The frequency dependency of dielectric loss in the frequency range 10 Hz to 10 MHz, were improved with Zr substitution in the ceramics. The room temperature ac and dc conductivity also decreased significantly with the increase in Zr-content.