Key Engineering Materials Vols. 512-515

Abstract: K0.5Na0.5NbO3-LiSbO3-BiMnO3 (KNN-LS-BM) lead-free piezoelectric ceramics were prepared by the traditional ceramic processing. The “soft” and “hard” doping characteristics of the ceramics were studied by the impedance-frequency spectrum method. Results show that the single 0.4at.% BM-doping KNN ceramics show “hard” characteristics because the top shape of profile θ-f the ceramics is almost rectangular, while the single LS-doping KNN ceramics shows “soft” characteristics because of the top shape of round. The LS and BM co-doping make KNN ceramics to show “soft” and “hard” doping characteristics at the same time. Because the soft doping of LS in KNN is more preponderant than that of BM, the co-doping KNN ceramics show “soft” doping characteristics as a whole.

Abstract: 0.92(Bi0.5Na0.5)TiO3-0.06BaTiO3-0.02(K0.5Na0.5)NbO3(92BNT-6BT-2KNN) is a high-stain material due to the volume change which come from antiferroelectric-ferroelectric (AFE-FE) phase transformation. Textured 92BNT-6BT-2KNN with a Lotgering factor of 67% was obtained by templated grain growth (TGG) method using plate-like (Bi0.5Na0.5)TiO3（BNT）particles as templates. The influences of sintering process on Lotgering factor and strain were investigated. It was found that the Lotgering factor increased as sintering temperature increase and then decreased with further temperature increase. The changing tendency of polar and unipolar strain did not keep the same accordance with that of Lotgering factor. The maximum of Lotgering factor (79%) and polar strain (0.36%) was obtained when the sintering temperature was 1240°C and 1220°C, respectively, due to the decrease of density when high sintering temperature was employed. Also it was found that the lotgering factor and polar strain reached its highest values when the soaking time was 4h. All these proved that the density and grain orientation codetermined the strain of 92BNT-6BT-2KNN ceramics.

Abstract: Lead-free piezoelectric ceramics, (Ba0.85-xSrxCa0.15)(Zr0.1Ti0.9)O3 (BSCZT, x=0.01-0.07), were prepared via a solid-state reaction route. The dielectric properties, ferroelectric properties, piezoelectric and strain properties of BSCZT ceramics were studied. The phase structure and microstructure were investigated by X-ray diffraction and scanning electron microscope, respectively. Results showed that dense ceramics with pure perovskite phase were obtained. At room temperature, the samples with x=0.03 exhibited excellent properties with large piezoelectric coefficient d33=534pC/N, planar mode electromechanical coupling coefficient kp=47.7%, thickness mode electromechanical coupling coefficient kt= 42% and high strain levels of 0.34%. In addition, the study of electrical properties suggested that the Curie temperature decreased linearly from 92oc to 73oc with the increasing doping content of strontium in BCZT ceramics. The remnant polarizations, piezoelectric coefficient and strain levels were all increased as the Sr content increased and then decreased with further increased Sr doping level, giving the maximum values at the Sr content of 3mol%. These results indicated that the BSCZT system is a promising lead-free material for applications in the future.

Abstract: In this paper, end members of tetragonal phase (Na_0.535K_0.485)_0.905Li_0.095(Nb_0.94Ta0.06)O₃ (abbreviated as NKLN) and rhombohedral phase (Na_0.5Bi_0.5)TiO₃ (abbreviated as NBT) were used to fabricate dense (1-x)NKLN-xNBT (x=0, 0.005, 0.01, 0.02, 0.03, 0.04) lead-free piezoelectric ceramics. By increasing the NBT content x value, the crystalline structure changes from tetragonal phase to rhombohedral one across a morphotropic phase boundary (MPB) composition of x=0.02. In this MPB composition, enhanced electrical properties were obtained as follows: piezoelectric constant d₃₃=202 pC/N, planar electromechanical coupling coefficient k_p=39.6%, relative dielectric constant ε_r=1020 and dielectric loss tgδ=2.67%, indicating that this ceramic was a promising lead-free piezoelectric material for practical applications.

Abstract: In this study, single-crystalline plate-shaped KNN particles were successfully synthesized by a multi-step molten salt method (MMSM). First, precursor particles K₄Nb₆O₁₇ (K4N6) with layer-structure was synthesized at 1050 °C in molten KCl-salt (MMSM-1) by starting materials of Nb₂O₅ and K₂CO₃. Second, plate-shaped KNN was synthesized at 1000 °C in molten KCl-NaCl-salt (MMSM-2) from K4N6 particles and K₂CO₃ powders. X-ray diffraction (XRD) analysis revealed that the crystallographic {010} plane of K4N6 was converted into the pseudo-cubic {001} plane of KNN. SEM images showed that MMSM-synthesized KNN particles preserved the shape of K4N6 precursor particles, and had a thickness of about 3 ~ 5 μm, a width of 10 ~ 15 μm and a length of 15 ~ 20 μm. EDX spectrum proved that the K:Na mole ratio was 0.6:0.4. These plate-shaped KNN particles were very promising templates to fabricating textured KNN-based ceramics.

Abstract: In this study, the KNbO₃, NaNbO₃ and NaTaO₃ powders were synthesized by hydrothermal method and the KNN ceramics were prepared by conventional sintering technique. The physical phase constitution and morphology were analyzed by X-ray diffraction and SEM. The KNN ceramics sintering temperature and the influence of Ta⁵⁺ doping on ceramic properties were explored. The results indicate that the optimal sintering temperature of KNN ceramics is 950°C, and the main phase is orthorhombic structure. After the substitution of Ta⁵⁺, the optimal sintering temperature is increased to 975°C. As the increase of doping amount, the piezoelectric properties have been significantly enhanced. The specimen doping 0.08 mol% Ta⁵⁺ exhibits the enhanced electrical properties (d₃₃=125pC/N, Q_m=131, and k_p=0.24).

Abstract: Potassium sodium niobate ((K_0.5Na_0.5)_1-xLi_xNb_1-yTa_yO₃) powers were prepared by solid phase synthesis and (K_0.5Na_0.5)_1-xNb_1-yTa_yO₃ lead-free piezoelectric ceramics were fabricated by the pressuerless sintering. The relationship among the powder phase composition and electric properties was studied. The results show that when x=0.5, the sintering temperature is 1050°C,and the polarization voltage is 3 kV/mm, the (K_0.5Na_0.5)_1-xLi_xNb_1-yTa_yO₃ piezoelectric ceramics with the better piezoelectric properties are prepared. (The relative density is 94%, the piezoelectric constant d₃₃=105pC/N, the planar electro-mechanical coupling coefficient k_p=0.39, the medium loss tanδ=0.29, the mechanical quality factor Q_m=45, and the dielectric constant ε_r=720). As the increase of the polarization votage, the ceramic piezoelectric properties are increased. When the polarization voltage E=3kV/mm, the polarization tends to be saturation on the whole.

Abstract: Mn-Zn ferrites with different doping contents of W⁶⁺ ions were prepared by using standard ceramic technique. The microstructure and magnetic properties of the as-prepared Mn-Zn ferrites were investigated. It was found that all the samples with different contents of W⁶⁺ ions consisted of ferrite phase of typical spinel cubic structure. With increasing doping content of W⁶⁺ ions, the lattice constant of the ferrites decreased but the grain size increased. Through the measurement of magnetic properties, it was revealed that the saturation magnetization and initial permeability of the samples increased with small doping content of W⁶⁺ ions but decreased with additional doping, and the Curie temperature decreased monotonously with W6+ ion doping.

Abstract: MnZn ferrites have been widely used as magnetic core materials. It is well known that Ca addition is effective to obtain homogeneous microstructure of fine grains and highly resistive grain boundaries. However, the behaviors of calcium as one of the main additives at different temperature ranges during sintering process are not completely understood yet. In this study, the influence of CaCO3 content and sintering temperature on the microstructure was investigated in 1473-1623 K. It was found that there existed a critical temperature around 1550 K. The grain size decreased with the increase of Ca content when the sintering temperature was lower than the critical temperature, but completely opposite result was observed at higher temperatures range. Possible mechanisms were discussed. When the sintering temperature was lower than the critical temperature, Ca content greatly affected the grain boundary mobility and dominated the grain growth. At higher temperatures, however, formation of liquid phases might be the main cause for the grain growth.

Abstract: A series of Eu³⁺-substituted Mn _0.65 Zn _0.35 Fe _2-x Eu_xO₄ ( x = 0.00-0.04) nanoscaled ferrites were synthesized by a citrate sol-gel combustion processing. The effects of Eu ³⁺ substitution on the crystalline formation, microstructure and magnetic properties of Mn-Zn ferrite annealing at temperature from 500°C to 800°C were investigated. The results showed that the lattice parameter increased and it contributed to monophase spinel with the addition of Eu ³⁺ at x≤ 0.03. The remanence and coercive field increased with Eu ³⁺ substitution but it changed slightly in saturation magnetization when annealing ferrite powders at 700°C.