Impedance analysis in Li2Pb2R2W2Ti4Nb4O30 (R = Y, Eu) ceramics

The polycrystalline materials [Li₂Pb₂R₂W₂Ti₄Nb₄O₃₀ (R = Y, Eu)] of tungsten bronze structural family have been synthesized using a high-temperature solid-state reaction (mixed-oxide) technique. Study of electrical properties (impedance, modulus, conductivity, etc.,) of the materials exhibits a strong correlation between their bulk, grain boundary and electrical parameters. The value and nature of temperature dependence of ac conductivity clearly exhibits that the materials have thermally activated transport properties of Arrhenius-type.     

Characterization and dielectric properties of Sr4La2Ti4M6O30 (M=Nb, Ta) ceramics

Sr₄La₂Ti₄M₆O₃₀ (M=Nb or Ta) compounds were synthesized by the conventional powder compaction and high temperature solid state reaction technique. The dielectric properties of the sintered samples were measured and they were further characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. Sr₄La₂Ti₄M₆O₃₀ compounds are paraelectric phases adopting filled tetragonal tungsten–bronze (TB) structure at room temperature. Sr₄La₂Ti₄Nb₆O₃₀ and Sr₄La₂Ti₄Ta₆O₃₀ show high dielectric constants of 387 and 166 together with low dielectric loss of 0.0084 and 0.0042 at 1 MHz, respectively.     

Double phase transitions in K2Pb2Sm2W2Ti4Nb4O30 ferroelectrics

This paper reports about the double phase transition (at 315 and 366 °C) in the polycrystalline sample of K₂Pb₂Sm₂W₂Ti₄Nb₄O₃₀ prepared by a high-temperature solid-state reaction technique. The calcination temperature was decided based on thermogravimetry analysis. Room temperature X-ray structural analysis confirms the formation of a single phase compound. The surface morphology recorded by scanning electron microscope exhibits a uniform grain distribution with high density. Detailed studies on the nature of variation (1) of dielectric parameters with temperature, and (2) polarization with temperature confirmed the existence of ferroelectricity in the material at room temperature. The temperature dependence of dc conductivity shows a typical Arrhenius behavior. The frequency dependence of ac conductivity suggests that the material obeys Jonscher’s universal power law. The variation of current with temperature shows that the material has high pyroelectric co-efficient and figure of merit, thus making it useful for pyroelectric sensors.     

Synthesis,structure and characterization of ceramic Ca4Bi2Ti4Nb6O30

The polycrystalline samples of Ca₄Bi₂Ti₄Nb₆O₃₀ (herein designated CBTN) were synthesized by the conventional ceramic method. Preliminary X-ray structural study of the compound showed the formation of a single phase solid solution having orthorhombic structure in the paraelectric phase. Measurements of the dielectric constant (ε) and dielectric loss (tan δ) as a function of temperature (−180–200°C) at 1 kHz and 10 kHz and also as a function of frequency (10² Hz to 10⁴ Hz) at five different temperatures [−180°C, −40°C, − 10°C 26°C (room temperature) and 75°C] have shown a dielectric anomaly and a phase transition at − 13 ±1°C in CBTN.     

Microwave dielectric properties of Ca4La2Ti5O17–LaAlO3 system ceramic materials

Compositions in the (1 − x) Ca₄La₂Ti₅O₁₇–xLaAlO₃ system were prepared in order to modify the positive temperature coefficient of the resonant frequency (τ _f ) of Ca₄La₂Ti₅O₁₇. The microwave dielectric properties and phase composition of this system ceramics were investigated. X-ray powder diffraction results showed that Ca₄La₂Ti₅O₁₇ and LaAlO₃ formed a solid solution only when x ≤ 0.2. The τ _f values showed a near linear decrease with increasing additions of LaAlO₃. It was observed that near zero τ _f value can be achieved as x = 0.64. The permittivity (ε_r) and the quality factor (Q × f) values exhibited a non-linear behavior with LaAlO₃ additions. The microwave dielectric properties were strongly correlated with composition, secondary phases and grain sizes. For practical application, a permittivity (ε_r) of 42, a quality factor (Q × f value) of 12,450 GHz and a temperature coefficient of resonant frequency (τ _f ) of ~0 ppm/°C for 0.36 Ca₄La₂Ti₅O₁₇−0.64 LaAlO₃ were proposed.     

Synthesis and characterization of Na2Pb2Pr2W2Ti4Ta4O30

A new complex oxide (Na₂Pb₂Pr₂W₂Ti₄Ta₄O₃₀) of tungsten bronze structural family has been synthesized by a high-temperature solid-state reaction (mixed-oxide) route at 1,050 °C. Room temperature structural analysis shows the formation of a single phase new compound. Study of microstructure of the pellet sample, recorded by scanning electron microscope, exhibits the uniform distribution of different size and shape of grains (with a few small voids) on the surface of the sample. Detailed studies of dielectric properties as a function of frequency and temperature show a dielectric anomaly above room temperature suggesting the existence of a ferroelectric phase transition in the material. Impedance spectroscopic analysis and electrical conductivity of the material exhibit a strong correlation between microstructure and electrical parameters. The temperature dependence of dc conductivity of the compound follows Arrhenius equation. The frequency and temperature dependence of ac conductivity (with fittings) shows the signature of Jonscher’s universal power law. The existence of non-exponential-type of conductivity relaxation in the compound was confirmed.     

Microstructure and dielectric properties of CaCu3Ti4O12 ceramic

CaCu₃Ti₄O₁₂ (CCTO) was prepared by the solid state technique. The sample was calcined at 900 °C/12 h and sintered at 1050 °C/24 h, then subjected to XRD to ensure CCTO formation. The microstructure was observed by SEM. XRD results identified both samples as single phase CCTO, whereas the microstructure shows abnormal grain growth and large pores. Sintering was studied in the temperature range of 950–1050 °C for 3–12 h. Increasing sintering temperature enhances the density and secondary formation of Cu₂O. A clear grain boundary and dense microstructure were observed. The results show that the sample sintered at 1040 °C/10 h yields a clearly uniform grain size with the highest ε_r (33,210).     

Phase formation and microwave dielectric properties of Pb2+ and Sr2+ doped La4Ti9O24 ceramics

Phase formation and microwave dielectric properties of the Pb²⁺ and Sr²⁺ doped La₄Ti₉O₂₄ ceramics were investigated. Using electron diffraction and Rietveld analysis of the X-ray powder diffraction patterns, we show that the increase in the concentration of Pb²⁺ and Sr²⁺ doping results in the structural transition from La₄Ti₉O₂₄ to a La_2/3TiO₃-type phase (Ibmm, No. 74). A change in the crystalline phase considerably affects the microwave dielectric properties, increasing the ɛ_r from 37 to 130, reducing Q × f from 25,000 to 5500, and increasing temperature coefficient of the resonant frequency (TCF) from 15 to 300 ppm/°C.     

Structural, dielectric and electrical properties of Pb(SnTi)O3 ferroelectric system

We have synthesized (PST) with x=0.45, 0.55, 0.65 ferroelectric ceramics by a solid-state reaction technique and performed preliminary X-ray diffraction (XRD) analysis and the temperature and frequency dependence dielectric measurements on them. The a.c. and d.c. conductivities have been investigated over a wide range of temperature and the activation energy have also been calculated. It is observed that: (i) the relative dielectric permittivity () and loss tangent (tan ) are dependent on frequency, (ii) the temperature of dielectric permittivity maximum shifts toward lower temperature side, (iii) permittivity maximum decreases with the increase of Sn content in the (PST) compounds and (iv) no frequency dispersion of the dielectric permittivity reveals the no-relaxor behavior of the materials.     

Thermoelectric properties of the ceramic system Pb2−x La x Li0·5 Nb1·5 O6+δ

The Seebeck voltage of the ceramic solid solutions of the system Pb_2−x La _x Li_0·5 Nb_1·5 O_6+δ has been studied as a function of temperature and concentration of lanthanum. The thermoelectric power was strongly dependent on the temperature and doping amount of lanthanum.     

Structural, dielectric and electrical properties of Sm-modified Pb(SnTi)O3 ferroelectric system

We have synthesized (Pb_1-xSm_x)(Sn_yTi_1-y)_1-x/4O₃ (PSmST) polycrystalline ferroelectric ceramics with x = 005, 007, 01 and y = 0.45 by a solid-state reaction technique and performed preliminary X-ray diffraction (XRD) analysis, detailed temperature and frequency dependence dielectric measurements on them. The a.c. conductivity has been investigated over a wide range of temperature and the activation energy (E _a.c) has also been calculated. It is observed that (i) the dielectric permittivity (e) and loss tangent (tan °) are dependent on frequency, (ii) the temperature of dielectric permittivity maximum shifts toward lower temperature side with the increase of samarium ion (Sm₃) concentration at the Pb sites, and (iii) observed and calculated d-values of XRD patterns show that the compounds have been formed in orthorhombic single phase.     

Preparation, characterization and dielectric properties of Ba3Ln3Ti5Nb5O30 (Ln=La, Nd) ceramics

Ba₃Ln₃Ti₅Nb₅O₃₀ (Ln=La, Nd) ceramics were prepared by the conventional solid-state ceramic route. The sintered samples were characterized by X-ray diffraction and scanning electron microscopy (SEM) methods. They have high dielectric constants, 141 and 203 at 13 MHz for La-based and Nd-based ceramics, respectively. The loss factor of the ceramics increases with frequency.     

Preparation,phase structure and microwave dielectric properties of CoLi2/3Ti4/3O4 ceramic

A new low loss microwave dielectric ceramic with composition of CoLi_2/3Ti_4/3O₄ was prepared by a conventional solid-state reaction method. The compound has a cubic spinel structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.3939 Å, V = 591.42 Å³, Z = 8 and ρ = 4.30 g/cm³. This ceramic has a low sintering temperature (～1050 °C) and good microwave dielectric properties with relative permittivity of 21.4, Q × f value of 35,000 GHz and τ_f value of ?22 ppm/°C. Furthermore, the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1050 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added CoLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Structural and dielectric properties of Pb0.91(La,K)0.09(Zr0.65Ti0.35)0.9775O3 ceramics

Potassium-modified polycrystalline samples of PLZT, Pb_0.91(La_{1 – z/3}K_z)_0.09(Zr_0.65Ti_0.35)_0.9775O₃ [z = 0.0, 0.1, 0.3, 0.5, 0.7], were prepared using sol-gel technique. X-ray diffraction (XRD) of these compounds show that they can be formed in single phase at 800 °C. Pellets prepared from the above powders were sintered at 1100 °C. Detailed studies of the dielectric constant () and loss tangent (tan ) of the compounds at different frequencies (400 Hz to 10 kHz) at room temperature (RT) and temperature (RT to 350 °C) suggest that the compounds undergo ferroelectric phase transition of diffuse-type on increasing K-concentration. Analysis of diffuseness of these compounds gives a value close to 2, which indicates the presence of higher degree of disorder in PLZT substituted by K. The dielectric constant () is found to increase initially and then to decrease with increasing K concentration.     

Ferroelectric and optical properties of Ba5Li2Ti2Nb8O30 ceramics potential for memory applications

Giant grained (42 μm) translucent Ba₅Li₂Ti₂Nb₈O₃₀ ceramic was fabricated by conventional sintering technique using the powders obtained via solid state reaction route. These samples were confirmed to possess tetragonal tungsten bronze structure (P4bm) at room temperature. The scanning electron microscopy established the average grain size to be close to 20 μm. The photoluminescence studies carried out on these ceramics indicated sharp emission bands around 433 and 578 nm at an excitation wavelength of 350 nm which were attributed to band-edge emission as the band gap was 2.76 eV determined by Kubelka–Munk function. The dielectric properties of these ceramics were studied over wide frequency range (100–1 MHz) at room temperature. The decrease in dielectric constant with frequency could be explained on the basis of Koops theory. The dielectric constant and the loss were found to decrease with increasing frequency. The Curie temperature was confirmed to be ~370 °C based on the dielectric anomaly observed when these measurements were carried out over a temperature range of 30–500 °C. This shows a deviation from Curie–Weiss behaviour and hence an indicator of the occurrence of disordering in the system, the γ = 1.23 which confirms the diffuse ferroelectric transition. These ceramics at room temperature exhibited P–E hysteresis loops, though not well saturated akin to that of their single crystalline counterparts. These are the suitable properties for ferroelectric random access memory applications.