Microstructure and dielectric properties of compositionally graded Ba1-xSrxTiO3 multilayer tunable capacitors

Li₂O/B₂O₃-added Ba_1-xSr_xTiO₃ (B_1-xS_xT) ceramics, where 0.2 ≤ x ≤ 0.35, were well densified at 920 °C with pure perovskite structure. The dielectric constant, tunability, and figure of merit (FOM) of B_1-xS_xT ceramics increased with x because of the decreasing Curie temperature (T_C). The specimen with x = 0.35, whose T_C was close to room temperature, exhibited a large tunability of 27.4 % and FOM of 110 at 10 kV/cm. A compositionally graded multilayer (CGML), which was sintered at 920 °C, was fabricated using B_1-xS_xT thick films to produce a temperature-stable tunable capacitor, and it evinced a dense microstructure and a continuous interface between the B_1-xS_xT thick film and the Ag electrode. This CGML capacitor showed a large tunability (51 %) and FOM (150) at 20 kV/cm. It also exhibited stable tunability (17–28 % at 10 kV/cm) at temperatures between 30–90 °C. Therefore, the B_1-xS_xT CGML capacitor is a suitable candidate for temperature-stable tunable capacitors.     

Bi-templated grain growth maximizing the effects of texture on piezoelectricity

Templated grain growth is beneficial for piezoelectric materials, the properties of which become the best in their single crystalline form. Nevertheless, a textured ceramic prepared by a templated grain growth technique often fails in exhibiting as good properties as expected in single crystals even with a high degree of orientation factor. Here, we propose a new strategy for maximizing texturing effect by suppressing the growth of untextured matrix grains. The textured ceramics made by our method, so-called bi-templated grain growth, are featured by ultrahigh piezoelectric properties (d₃₃ = ～1,031 pC/N, d?g = ～59,000, k_p = ～0.76). A special emphasis is on the achieved electric-field-induced strain of 0.13 % at 1 kV/mm, which is as high as that of single crystals. This work demonstrates that not only the degree of texture but also the coarsening of untextured matrix grains should be well-controlled to best exploit the templated grain growth technique.     

Structural and piezoelectric properties of textured NLKNS-CZ thick films and their application in planar piezoactuator

0.97(Na_0.5-xLi_xK_0.5)(Nb_0.89Sb_0.11)O₃-0.03CaZrO₃ [(N_0.5-xL_xK)(NS)-CZ] piezoceramic (x = 0.325) has a pseudocubic-tetragonal-orthorhombic (PC-T-O) multi-structure. The PC structure formed in this piezoceramic was identified as the R3m rhombohedral structure. This piezoceramic showed the large piezoelectric charge constant (d₃₃) of 515 pC/N due to the PC-T-O multi-structure. The NaNbO₃ (NN) templates were used to texture the (N_0.5-xL_xK)(NS)-CZ thick films along the (001) direction, and the textured thick film (x = 0.0375) had a large Lotgering factor of 95.6%. The PC-T-O multi-structure was observed in this thick film (x = 0.0375), but the thick film (x = 0.0325) showed a PC-O structure owing to the diffusion of the NN templates into the thick film. The textured thick film (x = 0.0375) exhibited an increased d₃₃ of 625 pC/N because of the PC-T-O multi-structure and the lineup of grains along the [001] direction. A textured thick film (x = 0.0375) was used to fabricate a planar-type actuator to confirm its applicability to electrical devices. This actuator exhibits large acceleration (580.3 G) and displacement (150 μm) at a low electric field of 0.2 kV/mm with a short response time of 3.0 ms. Therefore, the (N_0.5-xL_xK)(NS)-CZ thick films are excellent lead-free piezoceramics.     

Low‐Temperature Sintering and Piezoelectric Properties of CuO‐Added KNbO3 Ceramics

Dielectric and piezoelectric properties of CuO‐added KNbO₃ (KN) ceramics were investigated. The CuO reacted with the Nb₂O₅, formed a CuO–Nb₂O₅‐related liquid phase during the sintering, and assisted the densification of the KN ceramics at low temperatures. Moreover, some of the Cu²⁺ ions replaced the Nb⁵⁺ ions in the matrix and behaved as a hardener. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The 1.0 mol% CuO‐added KN ceramic sintered at 960°C for 1.0 h, which showed a maximum relative density, exhibited a high phase angle of 86.9°, P_r of 14.8 μC/cm², and E_c of 1.8 kV/mm. This specimen also exhibited good dielectric and piezoelectric properties: ε^T₃₃/ε_o of 364, d₃₃ of 122 pC/N, k_p of 0.29, and Q_m of 611.     

Nonlinear behavior of Tb4O7-modified ZnO-Pr6O11-based ceramics with high breakdown field

The effect of Tb4O7 on electrical behavior of the ZnO-Pr6O11-based varistor ceramics was investigated.Microstructural analysis indicated that the addition of Tb4O7 decreased average grain size from 3.6 to 3.2 μm and increased the sintered density from 5.58 to 5.68 g/cm3.As the amount of Tb4O7 increased,the breakdown field increased from 9393 to 12437 V/cm and the nonlinear coefficient increased from 50 to 65.The varistor ceramics added with 0.5 mol.% in the amount of Tb4O7 exhibited an excellent stability by exhibiting-0.1% in the variation rate of the breakdown field,0% in the variation rate of the nonlinear coefficient,and 8.8% in the variation rate of the leakage current density for DC-accelerated aging stress of 0.85 E1 mA/115℃/24 h.     

Effect of the Pt precursor on the morphology and catalytic performance of Pt-impregnated on Pd/C for the oxygen reduction reaction in polymer electrolyte fuel cells

We describe how the morphology and electrocatalytic activity of Pt-Pd with low levels of Pt are dependent on the type of Pt precursor that is used for the impregnation on to Pd/C. When a Pt precursor with a negative charge (H₂PtCl₆) is used in the preparation medium (Pt-Pd/C-H), its electrostatic interaction with the carbon surface results in some Pt nanoparticles being deposited on the carbon separately from the Pd surface. Due to the absence of such an electrostatic interaction with the Pt(NH₃)₄Cl₂ precursor, more selective deposition of Pt can be achieved on the Pd surface (Pt-Pd/C-N). Depending on the morphology, different electrocatalytic performance in oxygen reduction reaction would be observed. Compared to Pt-Pd/C-H, Pt-Pd/C-N shows 180% (half-cell at 0.9 V) and 160% (unit-cell at 0.8 V) enhanced performance, which is comparable to that on Pt/C. It is believed that the interaction between the Pt and the Pd substrate is more extensive in Pt-Pd/C-N than in Pt-Pd/C-H, and this is responsible for the large difference in the catalytic performances between these two catalysts.     

Investigation on the valence state of Te ions in the Bi6Ti5TeO22 thin film using X-ray photoelectron spectroscopy

Bi₆Ti₅TeO₂₂ (BTT) thin films were grown on a Pt/Ti/SiO₂/Si(1 0 0) substrate under various conditions and the valence state of the Te ion was investigated. For the BTT films grown at 300 °C, most of the Te ions existed as Te⁴⁺ ions. However, for the 10 mol% Mn-added BTT films grown at 300 °C, Te⁶⁺ ions were found even in the film grown under low oxygen partial pressure (OPP) and their number increased with increasing OPP. This increase was attributed to the presence of Mn²⁺ ions, which assisted the transition of Te⁴⁺ ions to Te⁶⁺ ions in order to maintain the charge balance of the Ti⁴⁺ sites. Furthermore, in the films grown at 300 °C under a high OPP of 80.0 Pa and subsequently annealed at 600 °C under a high oxygen pressure of 101 kPa, most of the Te ions existed as Te⁶⁺ ions. However, for the film grown at 300 °C under low OPP, even though the film was annealed under a high oxygen pressure of 101 kPa, only a few of Te⁶⁺ ions were formed, whereas most of Te ions remained as the Te⁴⁺ ions.     

An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2

The effects of surface passivation on the photoluminescence (PL) properties of CdS nanoparticles oxidized by straightforward H₂O₂ injection were examined. Compared to pristine cadmium sulfide nanocrystals (quantum efficiency ≅ 0.1%), the surface-passivated CdS nanoparticles showed significantly enhanced luminescence properties (quantum efficiency ≅ 20%). The surface passivation by H₂O₂ injection was characterized using X-ray photoelectron spectroscopy, X-ray diffraction, and time-resolved PL. The photoluminescence enhancement is due to the two-order increase in the radiative recombination rate by the sulfate passivation layer.     

Enhanced hydrogen storage capacity of Pt-loaded CNT@MOF-5 hybrid composites

We report on an easy synthesis method for the preparation of a hybrid composite of Pt-loaded MWCNTs@MOF-5 [Zn₄O(benzene-1,4-dicarboxylate)₃] that greatly enhanced hydrogen storage capacity at room temperature. To prepare the composite, we first prepared Pt-loaded MWCNTs, which were then incorporated in-situ into the MOF-5 crystals. The obtained composite was characterized by various techniques such as powder X-ray diffractometry, optical microscopy, porosimetry by nitrogen adsorption, and hydrogen adsorption. The analyses confirmed that the product has a highly crystalline structure with a Langmuir specific surface area of over 2000 m²/g. The hybrid composite was shown to have a hydrogen storage capacity of 1.25 wt% at room temperature and 100 bar, and 1.89 wt% at cryogenic temperature and 1 bar. These H₂ storage capacities represent significant increases over those of virgin MOF-5s and Pt-loaded MWCNTs.     

Microstructure, electrical and dielectric properties, and impulse clamping characteristics of ZnO–V2O5–Mn3O4 semiconducting ceramics modified with Er2O3

This study focuses on the effect of Er₂O₃ on microstructure, electrical and dielectric properties, and impulse clamping characteristics of the ZnO–V₂O₅–Mn₃O₄ varistor ceramics. Analysis of the microstructure indicated that the ZnO–V₂O₅–Mn₃O₄–Er₂O₃ ceramics consisted of major ZnO grain and minor secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, ErVO₄, and VO₂. As the amount of Er₂O₃ increased, the densities of sintered pellets increased from 5.46 to 5.52 g/cm³, whereas the average grain size decreased from 7.2 to 6.0 μm. The breakdown field increased from 1,016 to 3,185 V/cm with an increase in the amount of Er₂O₃. The highest nonlinear coefficient was obtained at the varistor modified with 0.1 mol%, reaching α = 30. The clamp voltage ratio (K), which indicates an impulse absorption capability, was improved with an increase in the amount of Er₂O₃ and the varistor modified with 0.25 mol% exhibited the best K = 2.41.