J.L. Baptista

DiC12: Magnesium titanate microwave dielectric ceramics

Abstract Magnesium titanate ceramics have been prepared by a chemical route (Pechini method) and by the conventional mixed oxide route. Dielectric properties of sintered samples were measured at 8 GHz by the Hakki and Coleman technique. Microstructures were examined by optical microscopy and scanning electron microscopy. The use of the chemical method enabled the sintering temperature to be lowered and good quality, single phase, dense ceramics were obtained at temperatures down to 1150°C. Dielectric constants were approximately 17.5 and Q values up to 21200 were obtained at 8 GHz. The effect of powder preparation processes, sintering temperature and cooling rate on the densification, microstructure and microwave dielectric properties are discussed.

Dependence of the Structural and Dielectric Properties of Ba1-xSrxTiO3 Ceramic Solid Solutions on Raw Material Processing

Abstract The structures and dielectric properties of Ba1-xSrxTiO3 ceramic solid solutions in the whole x range and their dependence on the material processing are investigated. A short time pre-calcination milling–mixing of the precursor powders causes compositional inhomogeneity and thus a diffuse phase transition. For long milling–mixing time, the diffuse phase transition is observed only in the SrTiO3 rich compositions (x>0·8).

The effect of Cr and La on MgTiO3 and MgTiO3–CaTiO3 microwave dielectric ceramics

Magnesium titanate and MgTiO 3 –CaTiO 3 ceramics were prepared by a chemical (Pechini) route and classical mixed oxide route. Selected specimens were doped with Cr or La. Specimens were sintered at 1350 °C and 1400 °C. Microstructures were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Dielectric properties were determined at 8 GHz by the Hakki and Coleman method. The highest Q values were obtained for undoped, chemically prepared MgTiO 3 (20800); any dopants caused the Q value to be degraded. Additions of small amounts of Cr (≤1 mol %) to mixed oxide magnesium titanate increased the density to 97.1% theoretical, and increased the Q value (from 7000) to 13,000. Additions of La led to the formation of La 2 Ti 2 O 7 second phase and reduction in the Q value for both materials. Both Cr and La acted as effective sintering aids, increasing density (to a maximum of 99% theoretical for 1% mol La in chemically prepared samples) and relative permittivity (to 18.1 for the same specimens). The relative permittivity of MgTiO 3 –CaTiO 3 ceramics increased with calcium content, but the corresponding Q values decreased (∈ r = 19.9, Q = 8500 for Mg:Ca = 94: 6). Small additions of La to Mg–Ca titanates enhanced the dielectric Q values but decreased the relative permittivities.

Dielectric relaxation in Ba-based layered perovskites

Ferroelectric materials with Bi-layered structure such as SrBi2Ta2O9 and SrBi2Nb2O9 are now intensively investigated in view of their applications in nonvolatile computer memories and high-temperature piezoelectric transducers. When Sr2+ is substituted with Ba2+, a significant disorder is induced and the material exhibits broadening of the phase transition. Such broadening is essential for applications since it allows achieving smooth temperature characteristics while maintaining high dielectric and piezoelectric properties. In this work, stoichiometric dense BaBi2Nb2O9 (BBN) ceramics are sintered using a mixed oxide route. Dielectric and ferroelectric properties are investigated in a broad range of temperatures and frequencies. Strong dispersion of the complex relative dielectric permittivity is observed including typical relaxor features such as shift of the permittivity maximum with frequency and broadening of the relaxation time spectrum with decreasing temperature. The dielectric relaxation obeys the...

Relaxor properties of Ba-based layered perovskites

Abstract Ferroelectric materials with Bi-layered structure such as SrBi 2 Ta 2 O 9 (SBT) and SrBi 2 Nb 2 O 9 (SBN) are now intensively investigated in view of their applications in non-volatile computer memories and high-temperature piezoelectric transducers. When Sr is substituted with Ba, significant disorder is induced and material exhibits broadening of the phase transition. In this work, BaBi 2 Ta 2 O 9 (BBT) and Ba 2 Bi 2 Nb 2 O 9 (BBN) ceramics were sintered using mixed oxide route. Dielectric properties were investigated in the temperature range 20–600°C at frequencies 25 Hz to 1 MHz. Strong dispersion of the dielectric permittivity is found indicating relaxor nature of the phase transition. It is shown that the dielectric relaxation in BBN ceramics is different from that of conventional relaxors such as PbMg 1/3 Nb 2/3 O 3 (PMN). Fitting with Vogel–Fulcher relationship is used to evaluate parameters of the dielectric relaxation such as freezing temperature and attempt frequency.

Dielectric properties of Ba(Ti1−yYy)O3 ceramics

Temperature and frequency dependence of the real (e′) and imaginary (e″) parts of the dielectric permittivity of Ba(Ti1−yYy)O3 (0.028⩽y⩽0.258) ceramic samples were studied in the temperature range 12–500 K at 102, 103, and 104, and 105 Hz. The permittivity peaks are rounded and frequency dispersive when y>0.028. The maximum of the rounded peak (em′) decreases with an increase in the yttrium concentration. The temperature corresponding to the permittivity maximum is linearly shifted to lower temperatures at a rate of 29 K per molar percent of yttrium when y⩽0.122, but it remains constant when y⩾0.122. The solid solubility limit of 0.122 is confirmed. Typical relaxor behavior was observed and characterized with empirical parameters in Ba(Ti1−yYy)O3 solid solutions.

Dielectric spectroscopy of MgTiO3-based ceramics in the 109–1014Hz region

Magnesium titanate (MgTiO3) powder was prepared by a chemical route (Pechini method) and different dopants were added to prepare several compositions. These pure and doped compositions were sintered in air and dense ceramics were obtained. The pure MgTiO3 samples were also subjected to different heat treatments during sintering. Complex permittivity spectra of ceramic samples were determined by various techniques in the 109–1014 Hz range. These techniques included infrared spectroscopy in transmission and reflectivity modes and microwave dielectric measurements. Extrapolation to microwave frequencies from infrared data, according to the proportionality ɛ″(v) ∝ v, agrees quite well with the microwave data measured at 8 GHz and it is a useful procedure to estimate intrinsic microwave losses. Fast cooling from high temperatures of MgTiO3 samples increased dielectric loss, probably due to a structural disorder. Dopants have two types of effect depending on whether they form a distinct second phase or a solid solution with MgTiO3. In this last case intrinsic losses are strongly affected.

Dielectric properties of bismuth doped Ba1-xSrxTiO3 ceramics

Abstract The dielectric properties of bismuth (5 at.%) doped Ba 1− x Sr x TiO 3 ( x =0, 0.2, 0.4, 0.6 and 0.8) ceramics are investigated. Bi doping significantly decreases the dielectric permittivity maximum of ferroelectric–paraelectric phase transition of Ba 1− x Sr x TiO 3 solid solutions and shifts the ferroelectric–paraelectric phase transition temperature to lower temperatures for the x =0, 0.2, 0.4 and 0.6 compositions but to higher temperature for the x =0.8 composition. Bi doped BaTiO 3 still exhibits normal ferroelectric characteristic while a relaxor behavior was observed in Bi doped Ba 1− x Sr x TiO 3 and the degree of the diffuseness and the relaxation increases as x increases. A random electric field is suggested to be responsible for the relaxor behavior observations.

Dielectric properties of Bi doped SrTiO3 ceramics in the temperature range 500–800 K

The (Sr1−1.5xBix)TiO3 (0.0133⩽x⩽0.133) ceramic system reveals several sets of dielectric permittivity peaks in different temperature ranges. Dielectric permittivity and dielectric loss peaks were detected in the temperature range 500–800 K and in the present article the dielectric polarization behavior is presented and discussed. The activation energy of the dielectric relaxation is in the range of 0.99–1.12 eV. It is suggested that the permittivity peaks are related to the movement of oxygen ions or oxygen vacancies.

Synthesis and characterization of Ba(Ti1−xCex)O3 ceramics

Abstract A series of Ba(Ti 1− x Ce x )O 3 dielectric ceramic materials was synthesized by the mixed oxide method with x = 0.1, 0.2, 0.3, 0.33, 0.4 and 0.5. The phase-forming process, the crystal structure of the main phase, the ceramic microstructure and the dielectric properties were studied. The results show that the compositions with x ≤ 1 3 are mainly single-phase solid solutions of BaTiO 3 and BaCeO 3 . The solubility limit is exceeded for x = 0.4. Permittivity values ranging from 800 to 3700 with dissipation factors between 0.01 and 0.1 were measured at 1 MHz for the solid solutions, and a dielectric relaxation behaviour was observed.