Key Engineering Materials Vols. 602-603

Abstract: BaZr_xTi_1-xO₃-based (BZT) ceramics with Al₂O₃ and MgO addtives were prepared by the conventional solid state method with BaCO₃, ZrO₂, TiO₂, Al₂O₃ and MgO as raw materials and B₂O₃ and Li₂CO₃ as sintering additive. The morphologies were analysized by scanning electron microscopy (SEM). The dielectric constant and dielectric loss of ceramics were measured by LCR meter. The temperature dependences of dielectric constant were measured by high-low temperature incubator tank and LCR meter at 1 MHz and a temperature range-55 to 125 °C. The tunabilities were tested by C-T-V converter and LCR meter at 1 MHz at room temperature. The results show that with the increase of Zr/Ti, BZT ceramic dielectric constant increases, the loss increases, the Curie temperature moves to a lower temperature, and dielectric bias field coordination is relatively lower. The SEM images show that the grain size reaches about 1-2 μm when the sintering temperature is 1100 °C, and the addition of Al₂O₃ and MgO promote the grain growth and densification of the composite ceramics. The Curie peaks are broadened and depressed with the addition of Al₂O₃ and MgO. The tunability is improved to 9.59% under a DC electric field of 7.0 kV/cm after the addition of Al₂O₃. The dielectric constant and dielectric loss of BaZr_0.25Ti_0.75O₃-30wt%Al₂O₃ and BaZr_0.25Ti_0.75O₃-30wt%MgO are 586, 0.011 and 486, 0.003, respectively. The optimistic dielectric properties make it a promising candidate for the application of tunable capacitors and phase shifters.

Abstract: The La₂Ti₂O₇+xwt%ZnO(x=0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) ceramics were prepared by the conventional solid state reaction method, and effects of ZnO doped on the structure, dielectric and ferroelectric properties were investigated in detail. The X-ray diffraction patterns of La₂Ti₂O₇ ceramics doped by ZnO oxide showed that no second phase exists. The surface morphology is palte-like shape, the grains of La₂Ti₂O₇ ceramics increase with increasing the content of ZnO, and the result is to improve the density of La₂Ti₂O₇ ceramics. The dielectric constant and loss decreases with increasing frequency.When the content of ZnO is more than 0.05wt%, the dielectric constant and loss decreases with the amount of addition of ZnO oxide increasing. The ferroelectric hysteresis loop is not saturable, which is attributed to its large leakage current. The ferroelectric properties improve with the addition of ZnO oxide. .

Abstract: In the present study two-phase BiNbO₄//PVDF composites with 0-3 connectivity were studied by impedance spectroscopy within the frequency range Δν=100Hz-1MHz at room temperature. Polyvinylidene fluoride (PVDF) acted as a matrix whereas bismuth niobate (BiNbO₄) powder acted as a dispersed phase. The volume fraction of the ceramic phase was c_V=2, 4, 6, 8, 10, 16 and 20vol%. Analysis of the impedance data registered for composites as well as for BiNbO₄ ceramics and PVDF polymer was performed on the base of complex dielectric permittivity mathematical formalism. It was found by impedance spectroscopy that concentration of the dispersed phase had an effect on dielectric properties of ceramic-polymer composite. Experimental data of impedance spectroscopy were fitted to the corresponding equivalent circuit using the complex non-linear least squares method. An equivalent electric circuit consisting of a series combination of two parallel combinations of a resistance R and a constant phase element CPE was found to describe well the dynamic dielectric response of the objects under study. Parameters of the equivalent electric circuit were calculated and it was found that two distinct relaxation processes were present in the composite samples. It was also found that volume fraction of the dispersed phase c_V=8vol% corresponded to a local minimum of resistance whereas c_V=10vol% of the dispersed phase corresponded to a local minimum of capacitance.

Abstract: Ba_0.95Ca_0.05Ti_0.85Zr_0.15O₃ (BCTZ) dielectric ceramics were fabricated by traditional solid-state method, using barium carbonate, calcium carbonate, titanium dioxide and zirconium dioxide as the starting materials. Rare earth dopants have a significant influence on the dielectric properties of BCTZ ceramics. The doping effects of Y and Ho with different content on the dielectric properties and structure were studied respectively. The results reveal that the effects of Y and Ho are similar, they perfer to occpy B site, and they move the dielectric peak to lower temperature, widen the peak and increase the insulation resistivity, thus improve the dielectric properties obviously. The effects were investigated by X-ray diffraction, scanning electron microscope, and dielectric property measurements. After rare earth elements-doping, the BCTZ ceramics satify the Y5V EIA specifications, mainly due to the change of structure. The BCTZ dielectric ceramics with high permittivity over 10,000, high insulation resistivity over 10¹² Ωcm, low dielectric loss less than 1%, can be applied in multilayer ceramic capacitors (MLCCs).

Abstract: Al₂O₃-doped Ba_0.9Ca_0.1Ti_0.8Zr_0.2O₃ (short for BCTZ) based dielectric ceramics were fabricated by sintering samples at 1300°C for 2h with conventional ceramic processing method. The microstructure and electrical properties of the as-prepared samples were investigated. X-ray diffraction analysis showed that after the addition of Al₂O₃ with the amount designed in this study, no new phase was examined in the detection limit. Through scanning electron microscopy it was found that the doping of Al₂O₃ can help the growth of BCTZ grains, and the relative permittivities of the samples. The result of electrical properties indicated that the resistance and the break voltage of the samples could be improved to some extent with appropriate doping amount of Al₂O₃ resulting in the highest dielectric constant ~17000, low dielectric loss <10%, and highest break voltage 8.4 kV/mm, respectively.

Abstract: The microstructure and dielectric properties of the (Ca_0.3Sr_0.1)Sm_0.4TiO₃ ceramics with various sintering conditions have been investigated. The compounds were prepared by the conventional solid-state route with various sintering temperatures from 1340 to 1400°C and sintering duration from 3 h. The resultant microwave dielectric properties were analyzed based upon the densification, the X-ray diffraction patterns and the microstructures of the ceramics. The correlation between the microstructures, the microwave dielectric properties and sintering behaviors was also examined. The ε_r value of 96.7 and Q×f value of 11,600 GHz were obtained for (Ca_0.3Sr_0.1)Sm_0.4TiO₃ ceramics sintering at 1370°C for 3 h. The (Ca_0.3Sr_0.1)Sm_0.4TiO₃ ceramics had suitable dielectric properties can find applications in microwave devices such as resonator, filter and antenna.

Abstract: The Ni_0.5Zn_0.5Fe₂O₄/PVP composite micro/nanofibers with average diameters of around 300~500nm were prepared by sol-gel method combined with electrospinning technology. The influences of the calcination temperatures (800°C, 900°C, 1100°C) on the crystal structure, micromorphology and microwave absorbing property of the calcined products were characterized by means of XRD, FT-IR, SEM and vector network analyzer. The results showed that the pure spinel structure of Ni_0.5Zn_0.5Fe₂O₄ were all formed when the composite micro/nanofibers were calcined at above 800°C. With the increase of calcination temperature, the Ni_0.5Zn_0.5Fe₂O₄ grains gradually grown up which contained in the micro/nanofibers, and the micro/nanofiber morphology changed toward the bamboo-like structure, and eventually lost the fiber morphology and presented the irregular granlar forms. In the band of 2~18GHz, the minimum reflectivity of the products moved to high frequency gradually. The microwave absorbing property of the products after being calcinated at 800°Cwas better than being calcinated at 900°C and 1100°C.

Abstract: (Ca_0.9Mg_0.1)SiO₃ ceramics possess a low dielectric constant and a high Qf value, however, the densification temperature of (Ca_0.9Mg_0.1)SiO₃ ceramics is higher than 1280°C. In this paper, the effect of Li₂CO₃ addition on sinterability and dielectric properties of (Ca_0.9Mg_0.1)SiO₃ ceramics were studied. The phase presence and surface morphology were determined by XRD and SEM techniques, respectively. CaSiO₃ and Ca₂MgSi₂O₇ phases were observed. With the addition of >2.0 wt% Li₂CO₃, the sintering temperature of (Ca_0.9Mg_0.1)SiO₃ ceramic was significantly lowered, reaching to 1070°C. (Ca_0.9Mg_0.1)SiO₃ ceramics with 4wt% Li₂CO₃ sintered at 1070°C for 3 h shows excellent dielectric properties: ε_r=5.91, Qf = 15300GHz (at 10GHz).

Abstract: The low dielectric loss polymer-ceramic composites with high density polyethylene (HDPE) as matrix and BaO-Nd₂O₃-TiO₂ (BNT) as filter for wireless temperature sensation were prepared by an extrusion processing technique. For 30vol % ceramic loaded HDPE, as the ambient temperature increases from 30 to 100 °C, the relative permittivity and loss tangent of the composite vary from 6.25 to 6.09 and from 0.004 to 0.011 respectively, and the temperature coefficient of relative permittivity is -370ppm/°C. By using this composite as the patch microstrip antenna substrate, a simple UHF RFID temperature sensor antenna is fabricated and measured. The experimental results show that a 2MHz/10°C frequency increment can be obtained at 900 MHz in the temperature range from 25 to 100°C.

Abstract: Iron titanium oxides with different Ti:Fe ratios (molar ratio: 3:1, 2:1, 1:1, 1:2, 1:3) have been prepared by chemical coprecipitation process. The structures and properties of the samples have been characterized by X-ray diffraction, vibrating sample magnetometer and UV-vis spectrophotometer. Results show that the samples annealed at 460 °C have poor crystallinity. At 550 °C, the crystallinity of all the samples increased strikingly. The iron titanium oxides contained anatase TiO₂, rutile TiO₂, α-Fe₂O₃ and/or Fe₂TiO₅. Ti:Fe ratios have a great influence on the detailed phase compositions of each sample. The iron titanium oxides exhibited paramagnetism and the intensity of magnetization increased with the increasing content of Fe. Band gap energies of the oxides changed slightly with increasing content of Fe with an average value of 2.0 eV, obviously lower than that of TiO₂ .