C. Berbecaru

Ba(Zn1/3Ta2/3)O3 Ceramics for Microwave and Millimeter-wave Applications

The Ba(Zn1/3Ta2/3)O3 (BZT) ceramic samples were prepared by solid‐state reaction and sintered in the range 1550–1650°C for 2 h. Several methods—X‐ray diffraction (XRD) and scanning electron microscopy (SEM)—were used for structural and morphological characterization. The unit cell distortion and the presence of the secondary phase content were studied by XRD. A long‐range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was noticed with the increase of the sintering temperature. SEM investigations revealed polyhedral well‐faceted grains and large grain size distribution. The dielectric properties in the microwave range were measured at room temperature and at 1 kHz on a large temperature interval (±150°C). The dielectric parameters were correlated with morphological and structural properties. Ceramic samples were annealed at 1410°C for 30 h to improve the microwave properties. The dielectric constant of BZT samples measured at 6 GHz and at 1 kHz was between 27 and 28 on the whole temperature range, that is, typical values for BZT material. The temperature coefficient of the resonance frequency at 6 GHz exhibits positive values less than 6 ppm/°C.

Pyroelectric Properties of Alanine Doped TGS Single Crystalline Thick Films under Constant Electric Stress

Pyroelectric properties of triglycine sulfate (TGS) thick films, separately doped with L and D alanine were investigated. Internal bias field of about 1 kV/cm, induced by the two dopants, stabilize the polarization in the opposite direction on the ferroelectric axis. Pyroelectric current (under constant stress) was recorded with a computer controlled Keithley 6517 electrometer, crossing up and down the Curie point. A reverse external electric field was applied on doped materials during heating, crossing up the Curie point. It is shown that the pyroelectric coefficient can be increased about four times at room temperature under un optimized DC electric field applied on the pyroelectric wafer.

Ferroelectric properties of pure and doped triglycine sulphate crystal

Abstract Permitivity and losses of pure TGS crystal show a peculiar behaviour crossing up and down the Curie point between room temperature and 60 °C in two successive runs. The TGS:LD alanine doped crystal show peculiar hysteresis loops, composed of three elementary cycles due to a “free switching” component P′ and two other non-equivalent components PA and PB of opposite directions pinned by dopants. They change their shares in the hysteresis loop versus AC electric field and the pinned components retain up to 20% of the polarisation for pure TGS at saturation. The coercive and the bias fields of the two pinned components show different behaviour both for increasing AC field and temperature, suggesting a non-equivalent alanine substitution in the crystal lattice.

Low-frequency dielectric spectroscopy of BaTiO3:Ce3+ ceramics

Dielectric spectroscopy of BaTiO3 (BT) doped with Ce3+ (BT:Ce3+) ceramics was performed at extended temperatures and in the low-frequency ranges. BT:Ce3+ displays decreasing values of all phase transition temperatures with increasing Ce3+ content. Permittivity versus temperature plots showed decreasing values with increasing frequency with low dispersion and shifting values of the Curie temperatures (Tc) towards high frequencies. Comparable values of losses for BT and BT:Ce3+ ceramics suggest good charge compensation. At low frequencies and higher temperatures, thermally activated mechanisms of conduction increase the loss values. With increasing frequency, the BT:Ce3+ system displays increased and shifted loss peaks up to Tc and decreasing values above Tc. The characteristic exponent of the modified Curie–Weiss law is higher than one. These figures suggest a relaxor-like behaviour of BT:Ce3+ ceramics for low Ce3+ content. Phase diagrams of BT:Ce3+ ceramics show a strong decrease of Tc for low Ce3+ content.

Ba/sub 1-x/Sr/sub x/TiO/sub 3/ ceramics for tunable microwave devices

The paper describes the investigations of Ba_1-xSr_xTiO₃ ceramic materials prepared by solid-state reaction techniques. The addition of MgO and MnO₂ improves sintering process and decreases the dielectric loss. The materials exhibit a dielectric constant epsiv_r~1000 and low loss tandelta~1O ^-3 at microwave frequencies

BNT ceramics in paraelectric phase: dielectric properties

The paper describes the investigations of Nd doped barium titanate (BNT) ferroelectric ceramic materials prepared by solid state reaction techniques. The decrease of the Curie temperature with the increase of the Nd/sub 2/O/sub 3/ concentration from 0 to 28 wt % was observed. High density values of BNT ceramics were obtained by adding 0.5 mol % PbO substitutional for BaO. The samples were sintered at temperatures T/sub s/=1200+1260/spl deg/C. At room temperature, the materials are in paraelectric phase, suitable for high frequency applications.