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Polycrystalline Sm3Fe5O12 ceramics are synthesized by a sol–gel method. High temperature dielectric properties in the frequency range from 100 Hz to 10 MHz are investigated at different temperatures from 300 to 800 K. This material exhibits high dielectric permittivity and low dielectric loss in a wide range of temperature and frequency. The contribution of electrode polarization to the dielectric permittivity is ruled out by the dc bias voltage measurement at room temperature. Two types of dielectric relaxations are clearly observed, and both of them are thermal activated relaxations following the Arrhenius law. The high frequency relaxation is shown to originate from the carrier hopping between Fe2+ and Fe3+, while the low frequency relaxation most likely arises from the conduction. The concentration of Fe2+ is proved to influence the low frequency dielectric relaxation.

Ba 0.5 Sr 0.5 TiO 3 –MgO–Mg 3 B 2 O 6 composite ceramics were fabricated by a solid state reaction method and their dielectric tunable characteristics were investigated. The addition of MgO–Mg 3 B 2 O 6 into Ba 0.5 Sr 0.5 TiO 3 forms ferroelectric (Ba 0.5 Sr 0.5 TiO 3 )–dielectric (MgO–Mg 3 B 2 O 6 ) composites. Increasing MgO content causes a decrease of Curie temperature T C and an increase of dielectric constant peak e max . With the increase of the MgO content and the decrease of Mg 3 B 2 O 6 , the dielectric constant of composite ceramics increases slightly; meanwhile, the loss tangent decreases and the tunability increases. When MgO content exceeds 10 wt%, the electric field dependence of the dielectric constant of composite ceramics is well described by an additional “extrinsic” contribution due to the polar clusters, instead of Johnson׳s approximation. The dielectric constant and Qf 0 values of composite ceramics at microwave frequency tend to increase with the increase of MgO content. The improved tunability and Qf 0 values are beneficial for tunable microwave applications.

La 2 NiMnO 6 powders were prepared by using a polyvinyl alcohol (PVA) sol–gel route. Differential scanning calorimetry (DSC), thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. Single-phase La 2 NiMnO 6 of about 40–50 nm diameters was synthesized at 600 °C for 2 h, without the formation of any intermediate phase. La 2 NiMnO 6 ceramics sintered at 1000 °C had tunability of 28% and 20% at 8 V/mm at 100 kHz and 1 MHz, respectively. The electric field dependence of dielectric constant of the ceramics was well described by using the “multi-polarization mechanism” model.

LaMn 0.5 Fe 0.5 O 3 powders were synthesized via a sol–gel route based on polyvinyl alcohol (PVA). Differential thermal (DTA) and thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Field emission scanning electron microscopy (FESEM) techniques were used to characterize precursors and derived oxide powders. The effect of the molar ratios of hydroxyl groups of PVA to metal ions (OH/M) on the formation of LaMn 0.5 Fe 0.5 O 3 was investigated. XRD analysis showed single-phase and well-crystallized LaMn 0.5 Fe 0.5 O 3 of around 25 nm diameters was synthesized from the OH/M=1.5 precursor at 600 °C. For the precursor with OH/M=0.75, phase pure LaMn 0.5 Fe 0.5 O 3 was not obtained even at 1000 °C and La(OH) 3 was observed as impurity phase. LaMn 0.5 Fe 0.5 O 3 ceramics sintered at 1300 °C showed a dielectric constant of 4300 at room temperature and 100 kHz.

(Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics were prepared by a solid-state reaction method. The microstructures, microwave dielectric characteristics, and tunability of composite ceramics were investigated. An anomalous correlation between tunability and dielectric constant was observed: with the increase in Mg2SiO4 content and the decrease in MgO content, the dielectric constant of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics decreases, but the tunability increases. The anomalous increased tunability is beneficial for tunable microwave applications and can be attributed to the redistribution of the electric field. For 50Ba0.5Sr0.5TiO3–(50−x)MgO–xMg2SiO4, the dielectric constant was decreased from 164.2 to 126.5 by increasing Mg2SiO4 content from 5 to 45 wt% and the tunability at 3.9 kV/mm increased from 11.5% to 15.2%.