Effects of A-site ionic size on the phase transition behavior of lead-free niobate ceramics

Understanding the phase structure evolution is important for developing high performance lead-free piezoelectric materials. In this work, the effects of A-site ionic size of monovalent ions on the phase transition behaviors for the lead-free niobate ceramics ANbO₃ (A = Li, Na, Ag, and K) are investigated using XRD analysis and dielectric measurement. The iso-valent ionic doping restrains the relaxation behavior that usually appears in the hetero-valent ionic-doped niobate ceramics. The A-site average ionic size of R_A and its ionic radius differences of ΔR_A are found to be crucial influence factors on the phase transition behaviors of the ANbO₃ ceramics. Small Li⁺ doping stabilize tetragonal phase of the ANbO₃ ceramics with R_A > 1.47 Å, but stabilize rhombohedral phase of the ones with R_A < 1.47 Å. On the other hand, The ANbO₃ ceramics without Li⁺ doping prefer to orthorhombic phase due to indistinctive ionic size differences (ΔR_A < 0.25 Å). Our results suggest that a certain phase and phase transition boundary could be designed by appropriate ionic doping for developing the niobate-based lead free piezoelectric ceramics.     

Subsolidus solution and electrical properties of Sr-substituted bismuth magnesium niobate pyrochlores

Here we investigated the structural and dielectric properties of (Bi_3.36Mg_0.64-xSr_x)(Mg_1.28Nb_2.72)O_13.76 (0≤ x≤0.5) subsolidus solution. Sr-substituted bismuth magnesium niobate (BMSN) pyrochlores were prepared by solid-state reaction at 1025 °C over 1–2 days. X-ray diffraction (XRD) analysis confirmed that the BMSN pyrochlores crystallise in cubic symmetry, space group Fd3m with lattice parameters in the range 10.5968 (4)-10.5671 (17) Å. The surface morphologies of these samples, as confirmed by Scanning Electron Microscopy (SEM), were composed of irregular shaped grains. Both Scherrer and Williamson-Hall methods revealed that the crystallite sizes were in the range 46–75 nm. No thermal event was discernible over the temperature range 30–1000 °C, thus confirming the thermal stability of these materials. On the other hand, Arrhenius conductivity plots showed the BMSN pyrochlores to be highly insulating with activation energies of ~1.20–1.49 eV. At ~30 °C and 1 MHz, BMSN pyrochlores exhibited moderate high bulk dielectric constants, ɛ’, 90–186 and low dielectric losses, tan δ in the order of 10⁻²–10⁻¹, respectively. Both the ɛ’ and tan δ values of the BMSN pyrochlores showed a nearly two-fold decrease with increasing Sr concentration. Negative temperature coefficient of capacitances, TCC, −408 to −713 ppm/°C were recorded over ~30–300 °C at 1 MHz.     

Facile layer-by-layer self-assembly of 2D perovskite niobate and layered double hydroxide nanosheets for enhanced photocatalytic oxygen generation

Photocatalytic O₂-generation reaction is recognized as a crucial step in water splitting and has drawn great attention of researchers. In this work, a hetero-layered composite photocatalyst was successfully prepared by a facile self-assembly method based on electrostatic interaction between oppositely charged Zn/Cr-layered double hydroxide (Zn/Cr-LDH) and lead niobate nanosheets. The layer-by-layer stacking of Zn/Cr-LDH and HPb₂Nb₃O₁₀ nanosheets was beneficial for rapid migration of photo-induced charge carriers inside the photocatalyst because of large contact area. In the meantime, Zn/Cr-LDH and HPb₂Nb₃O₁₀ components exhibited suitable energy-band alignment, which led to efficient separation of photo-induced charge carriers. The composite photocatalyst showed enhanced photocatalytic O₂-generation activity under visible-light irradiation without loading cocatalyst. Briefly, this work expanded the applications of AB₂Nb₃O₁₀-based materials in photocatalytic energy conversion and proved that constructing composites based on electrostatic self-assembly of complementary 2D materials is a promising strategy for development of more efficient photocatalysts.     

Multi-Step Growth of Oriented LiNbO 3 Thin Films

Highly-oriented LiNbO 3 thin films have been successfully synthesized through a refined 2-step growth process consisting of the successive use of a physical technique (r.f. sputtering) and a chemical one (pyrosol). Taking the strict control of processing parameters, a <001>-preferential out-of-plane orientation could be promoted on both <001>-Al 2 O 3 and <111>-Si substrates while the in-plane texture of the deposited layers was found to depend strongly upon the nature of the underlying templates.     

Effects of compressive stress on the dielectric properties of PIN–PT ceramics

Lead indium niobate, Pb(In_1/2Nb_1/2)O₃ (PIN), is an interesting ferroelectric due to a transition from a disordered to an ordered state by long-time thermal annealing. However, the temperature related to the maximum dielectric constant (T_max) of PIN in relaxor phase is low (at 1 kHz, T_max = 66 °C). In this study, lead titanate PbTiO₃ (PT) was added to PIN with compositions (1 − x)PIN–xPT (for x = 0.1–0.5) to increase their T_max. The influence of stress on dielectric properties of (1 − x)PIN–PT ceramics was then investigated. The dielectric properties were measured under various uniaxial compressive stresses up to 400 MPa. The results showed the reduction of dielectric constant in 0.9PIN–0.1PT with superimposed compression load. For other compositions, dielectric constants first increased with compressive stress, then decreased when the stress was further increased up to 400 MPa. The loss tangent of all composition was found to decrease with increasing compressive stress.     

Preparation, Characterization and Dielectric Properties of Ba_6Mg_(0.67)Nb_(9.33)O_(30) Ceramic

1IntroductionSince the discovery of ferroelectricity andrelated prop-ertiesin BaTiO3,alarge amount of research workonoxides,insearch of novel materials for industrial applications,hasbeen done.High performance dielectric ceramics is appliedas key material…     

Co-precipitation synthesis,band modulation and improved visible-light-driven photocatalysis of Te4+/Ti4+-codoped Bi3Nb17O47

Bismuth niobate semiconductors are of considerable interest in both contaminant degradation and H₂-generation. However, the wide band gap strictly limits the optical absorption in visible-light wavelength. In this work, a new niobate semiconductor Bi₃Nb₁₇O₄₇ was prepared with co-precipitation synthesis. To modify the band structure, Te⁴⁺-, Ti⁴⁺-, and Te⁴⁺/Ti⁴⁺-doping were conducted in Bi₃Nb₁₇O₄₇ lattices. Rietveld refinements were used to investigate the crystal phase and structure. The UV–vis absorption measurements concluded that Te⁴⁺-, Ti⁴⁺- doping could greatly modify the band energy of Bi₃Nb₁₇O₄₇. The Te⁴⁺/Ti⁴⁺-doped sample could harvest more visible light in the longer-wavelength region being favorable for photocatalysis performances. This was verified by RhB photodegradation tests under the visible-light irradiation (λ > 420 nm). To discuss the photocatalytic mechanisms, XPS and impedance spectra were measured. The improved photocatalysis was related to the microstructure changes, charge carrier dynamics, oxygen vacancies, and redox couples of multivalent ions. The present work provides a valid route to modify the band structure and to improve the photocatalysis abilities via impurity ions Te⁴⁺/Ti⁴⁺-doping in bismuth niobate semiconductors.     

Excitation dynamics of cerium in niobate crystals

Transient gratings have been observed in Ce³⁺-doped SBN(Sr_xBa_1−xNb₂O₆) and KNSBN (potassium–sodium SBN) niobate crystals by picosecond-pulse, two-photon excitation using degenerate four-wave-mixing (DFWM) in the case where the C-axis is perpendicular to the grating vector. The charge-carriers produced in the conduction band are responsible for the observed coherent signal associated with the third-order nonlinearity, while a post-coherent, slow signal is due to the temporal lattice distortion associated with formation of Nb⁴⁺-hole polarons. Following excitation of the niobate host, energy transfer between host and rare-earth ions occurred, resulting in the excited state of cerium ions being populated and cross-luminescence from host to Ce³⁺ being observed. This excitation mechanism was also verified by a fluorescence study.     

铌酸锶钡铁电薄膜的微结构与薄膜厚度的关系

叙述了使用溶胶—凝胶法在 Si( 0 0 1 )基片上制备不同厚度的铁电铌酸锶钡薄膜的过程 ,使用 X射线衍射 ,扫描电子显微镜 ,拉曼散射光谱等方法研究薄膜的微结构与薄膜厚度之间的关系 ,薄膜的厚度一直能够达到 5 μm。实验发现 ,随着厚度的增加 ,SBN60薄膜在 ( 0 0 1 )方向的优先取向性越来越好。在逐层生长的过程中 ,处于底层的膜层能够起到缓冲层的作用 ,以逐渐改善薄膜与基片之间的晶格失配 ,从而使得晶体的结晶取向性越来越好