Xiao Qiang Liu

Complex-permittivity measurement on high-Q materials via combined numerical approaches

A modified indirect algorithm for extracting real permittivities from measured resonant frequencies has been developed. It is based on the perturbation theory along with the Newton iteration method, and improves the speed of the calculation with a factor of 2-4. It is superior, especially when system matrices under calculation are large. Based on the complementary advantages of the modified algorithm and a direct algorithm, a mixed algorithm of high speed and accuracy has been described. A real measurement system incorporating the linear fractional curve-fitting method and the mixed algorithm has been built to measure complex permittivities of high-Q materials.

Giant dielectric response in two-dimensional charge-ordered nickelate ceramics

Dielectric relaxations of charge-ordered Ln1.5Sr0.5NiO4 (Ln=La and Nd) ceramics were investigated over a broad temperature range. The giant dielectric constant (over 70 000) with a low dielectric loss of ∼0.1 was determined at high frequencies (up to 5 MHz) over a broad temperature range. There are two dielectric relaxations in the vicinity of charge ordering temperatures. The thermal activated small polaronic hopping between two charge ordering temperatures should contribute to the giant dielectric response in the present ceramics. Compared to other giant dielectric constant materials, the present materials have the notable advantage for high frequency applications.

Re-entrant relaxor behavior of Ba5RTi3Nb7O30 (R = La, Nd, Sm) tungsten bronze ceramics

Ba5RTi3Nb7O30 (R = La, Nd, Sm) tungsten bronze ceramics were prepared, and the dielectric and ferroelectric properties were investigated over a broad temperature range. The relaxor nature was determined for all compositions in their permittivity curves, and a second anomaly of the dielectric loss (tan δ) was observed around 250 K in Ba5NdTi3Nb7O30 and around 275 K in Ba5SmTi3Nb7O30. Both the maximum and remanent polarization tended to decrease and vanish at low temperatures in the ferroelectric phase for all compositions, which was referred to as the low temperature re-entrant relaxor behavior. The remanent polarization increased with decreasing temperature first and then reached the maximum value at the re-entrant temperature (Tr). For Ba5RTi3Nb7O30 (R = La, Nd, Sm), Tr decreased with the radius of R3+ cations and the applied field amplitude.

Temperature-stable giant dielectric response in orthorhombic samarium strontium nickelate ceramics

The crystalline structure and dielectric properties of Sm1.5Sr0.5NiO4 ceramics are presented. The present ceramics is refined as orthorhombic Bmab phase and the orthorhombic strain may change the statue of charge ordering. The temperature-stable giant dielectric constant (∼100 000) with low dielectric loss of ∼0.1 is observed at frequency up to 5 MHz over a broad range of temperature (150–500 K) and frequency (100 kHz–5 MHz). The grain interior should be the dominative factor which contributes the giant dielectric response in the present ceramics after the equivalent circuit fitting, and the thermal activated small polaronic hopping related to the charge ordering is that factor. Compared to other giant dielectric materials, the present materials have a great potential in the practical application, especially for the high frequency application.

Relaxor ferroelectric characteristics of Ba5LaTi3Nb7O30 tungsten bronze ceramics

Ba5LaTi3Nb7O30 tungsten-bronze ferroelectric ceramics were synthesized and characterized. The tetragonal tungsten bronze structure in space group P4/mbm was confirmed, and a broad permittivity peak with strong frequency dispersion was observed around 250 K where the peak points well fitted the Vogel-Fulcher relationship [H. Vogel, Phys. Zeit. 22, 645 (1921); G. S. Fulcher, J. Am. Ceram. Soc. 8, 339 (1925)]. The temperature dependence of the ferroelectric hysteresis indicated the paraelectric to ferroelectric phase transition in the temperature range between 153 and 273 K. The high temperature permittivity curve deviated from the Curie-Weiss law in quite a narrow temperature region above Tmax, which reflected the weak correlations between the polar nanoregions. The Curie-Weiss constant (C) was 1.218 × 105 K, which was consistent with that for the displacive type ferroelectric. No DSC peak was detected over the temperature region investigated here. Moreover, the Curie-Weiss constant below Tmax (C′) was just...

Effect of BaTiO3 addition on structures and mechanical properties of 3Y-TZP ceramics

Abstract Effect of BaTiO 3 addition on grain size, phase constitution and mechanical properties of 3Y-TZP ceramics was investigated for discussing the possible application of piezoelectric secondary phase toughening process. Under certain conditions, high content of tetragonal zirconia could be obtained with modest amount of BaTiO 3 addition, and the hardness of the composite ceramics could be increased with the addition of BaTiO 3 . However, the increased stability of t-ZrO 2 appears as a new problem which obstructs the stress-induced t–m transformation and subsequently suppresses the effect of transformation toughening.

Effects of Ca-substitution on structural, dielectric, and ferroelectric properties of Ba5SmTi3Nb7O30 tungsten bronze ceramics

Effects of Ca-substitution on the crystal structure and dielectric properties were investigated for Ba5SmTi3Nb7O30 tungsten bronze ceramics. The tetragonal tungsten bronze structure in space group P4bm was determined in Ba4CaSmTi3Nb7O30 ceramics, which was the same for Ba5SmTi3Nb7O30. The c-axis off center displacement in both B1 and B2-sites for Ba5SmTi3Nb7O30 was slightly larger than that in BaCaSmTi3Nb7O30, while the distortion of Ti/Nb(2)O6 octahedra in Ba4CaSmTi3Nb7O30 was obviously suppressed compared with that in Ba5SmTi3Nb7O30. Though there were also two dielectric anomalies observed in Ba4CaSmTi3Nb7O30, the relaxor ferroelectric nature was changed to the normal ferroelectric one, and both the endothermal peak in differential scanning calorimetry curve and the significant thermal hysteresis of the phase transition confirmed the first order ferroelectric transition. These results indicated that the ferroelectric nature of Ba4CaSmTi3Nb7O30 was more close to that in Ba4Sm2Ti4Nb6O30, and it could be c...

Structural evolution of SrLaAl1−x(Zn0.5Ti0.5)xO4 ceramics and effects on their microwave dielectric properties

The structure and microwave dielectric properties of SrLaAl1−x(Zn0.5Ti0.5)xO4 ceramics were determined in the composition range of x = 0–0.9. The single phase solid solutions with the I4/mmm (No. 139) space group were revealed by the Rietveld analysis of the X-ray diffraction (XRD) patterns. Abnormal variations of Sr/La–O2b and Al/(Zn,Ti)–O2 bonds along the c-axis were observed when x > 0.5, which were further confirmed by the evolutions of corresponding modes in Raman spectra. The Q × f value could be improved at the compositions around x = 0.5 and then turned to decrease rapidly with further increasing x, while the dielectric constant er and the temperature coefficient of resonant frequency τf ascended linearly with increasing x. The drop of the Q × f value could be ascribed to the more unstable structure owing to the abnormal variations of the axial bonds, where lattice distortions and inhomogeneous regions were also confirmed by the HRTEM observation. Moreover, the infrared reflectivity (IR) spectra were recorded and fitted to further analyze the variation of the theoretical dielectric loss. The best combination of microwave dielectric properties was achieved at x = 0.5: er = 23.5, Q × f = 102000 GHz, τf = −3.4 ppm per °C.

Sr2LaAlTiO7: a new Ruddlesden–Popper compound with excellent microwave dielectric properties

A new n = 2 type Ruddlesden–Popper compound Sr2LaAlTiO7 was synthesized and characterized, and the microwave dielectric properties were investigated together with their microstructures. X-ray diffraction (XRD) Rietveld analysis and selected area electron diffraction (SAED) patterns confirmed that these compounds belong to the I4/mmm space group with tetragonal crystal symmetry. The lattice parameters were calculated as a = b = 3.840 A and c = 20.270 A. The interlayer polarization was considerably lower than that of the previously reported SrLa2Al2O7 ceramics, and a well ordered distribution of ions along the c axis was confirmed from the high resolution transmission electron microscopy (HRTEM) images. Excellent microwave dielectric properties were obtained and an optimal combination (er = 26.5, Q × f = 110 850 GHz, τf = 2.95 ppm °C−1) was obtained for the sample sintered at 1600 °C in air for 3 h. Considering the merits of a simple preparation, less process sensitivity and, most importantly, the comparatively cheap raw materials, Sr2LaAlTiO7 ceramics could be expected as promising new candidate for ultra-low loss microwave dielectric ceramics.

Dielectric relaxations, ultrasonic attenuation, and their structure dependence in Sr4(LaxNd1-x)2Ti4Nb6O30 tungsten bronze ceramics

The dielectric anomalies and their structure dependence were evaluated and discussed in Sr 4 (La x Nd 1-x ) 2 Ti 4 Nb 6 O 30 ceramics, together with the analysis of ultrasonic velocity shift and attenuation spectra in the low-temperature range. The room-temperature structure was confirmed as the tetragonal in space group P4bm for all compositions. One diffuse ferroelectric peak and three relaxor ferroelectric peaks corresponding to the commensurate/incommensurate modulation of oxygen octahedra, polar clusters of A-site ion ordering, and B-site ion ordering, respectively, were observed in the composition with x = 0.25. With decreasing the radius difference between A1- and A2-ions (increasing x), the dielectric relaxations, especially the one originating from the polar clusters of A-site ion ordering, tended to increase significantly and overlap the diffuse ferroelectric peak, which was completely overlapped for x ⩾ 0.75. This process just reflected the increased disordering degree of both A- and B-site ions, and the analysis of ultrasonic attenuation strongly supported the above conclusions on dielectric relaxations and their structural origins. The ultrasonic attenuation peak at approximately 100 K corresponded to the freezing process of the dielectric relaxations, and the fluctuation with composition of the ultrasonic attenuation peaks between 150 and 260 K suggested the possible structure variation.