BiLaWO6: Er3+/Tm3+/Yb3+ phosphor: Study of multiple fluorescence intensity ratiometric thermometry at cryogenic temperatures

Highly thermally stable Er³⁺/Tm³⁺/Yb³⁺ tri-doped bismuth lanthanum tungstate phosphors were prepared by high temperature solid-state reaction method. The structural and morphological properties of the prepared phosphors were analysed by X-ray diffraction (XRD), Raman spectroscopy and Scanning electron microscopy (SEM) coupled with energy dispersion spectrum (EDS). Visible upconversion (UC) luminescence was measured by exciting the phosphors with 980 nm laser radiation. The dependence of the UC intensity of each emission band of Er³⁺ and Tm³⁺ ions as a function of temperature in the range from 30 to 300 K was monitored. Fluorescence intensity ratios (FIR) of thermally coupled levels (TCL) and non-thermally coupled levels (NTCL) were analysed and verified with appropriate theoretical validation. The absolute (S_A) and relative sensitivities (S_R) were estimated and compared with the reported systems. In the present case of BiLaWO₆: Er³⁺/Tm³⁺/Yb³⁺, S_R (0.43 % K^?1) related to TCL of Er³⁺ UC is found to have maximum sensitivity compared to any of the NTCL combinations at 300 K. From this study we inferred that the S_R values estimated from NTCL are smaller than that of TCL involved in BLW: Er³⁺/Tm³⁺/Yb³⁺ phosphor. The temperature dependent CIE color coordinates were also evaluated in the cryogenic temperature region.     

Room temperature multiferroic properties of Ni-doped Aurivillus phase Bi5Ti3FeO15

Single-phase Aurivillius Bi₅Ti₃Fe_0.5Ni_0.5O₁₅ (BTFN) ceramics were synthesized by the solid-state reaction method. The substitution of Ni for half Fe ions does not introduce magnetic impurity phase but increases magnetic moment more than two orders. The ferroelectric and magnetic Curie temperatures are determined to be 1100 K and 726 K. The room-temperature multiferroic behavior of the BTFN ceramics were demonstrated by the ferroelectric (2P_r=8.5 μC/cm², 2E_c=74 kV/cm) and ferromagnetic (2M_r=27.86 m emu/g, 2H_c=553 Oe) measurements. The ferromagnetism can be ascribed to the aggregation of magnetic ions at the inner octahedra by Ni doping and the spin canting of magnetic-ion-based sublattices via the Dzyaloshinskii-Moriya interaction. The present work suggests the possibility of doped Bi₅Ti₃FeO₁₅ as a potential room-temperature multiferroic.     

Structures and Dielectric Properties of La-Substituted SrBi8Ti7O27 Ceramics

SrBi₈Ti₇O₂₇ ferroelectric ceramics with mixed Aurivillius structure were modified by La-substitution for Bi, and the dielectric properties were investigated together with the microstructure characterization. Solid solution of Sr(Bi_{1 – x} La _x )₈Ti₇O₂₇ was formed in the present ceramics for x 0.1, and (Bi,La)₄Ti₃O₁₂ secondary phase appeared at x = 0.15. For x 0.25, another phase Sr(Bi,La)₄Ti₄O₁₅ appeared, and (Bi,La)₄Ti₃O₁₂ disappeared gradually with increasing x, and vanished entirely at x = 0.35. With increasing x, both the dielectric constant and dielectric loss of the present ceramics increased firstly and reached their maximums 291 and 0.023 at 1 MHz, then decreased after x > 0.25. The temperature stable high- dielectric ceramics with low dielectric loss were created at the composition x = 0.5: = 122, tan = 0.0003 and = –619 ppm/°C at 1 MHz.     

Structural transformation in Mn-substituted Sr2Bi2Ta2TiO12 Aurivillius phase synthesized by hydrothermal method: A comparative study and dielectric properties

In this study, a hydrothermal method was applied to synthesize the three-layer Aurivillius phase Sr₂Bi₂Ta₂TiO₁₂ (SBTTO) and Mn-substituted Sr_1·5Bi_2·5Ta₂Ti_0·5Mn_0·5O₁₂ (SBTTMO), with the use of NaOH as a mineralizer. The crystal structure, morphology, dielectric properties, and the correlation between the structural transformation and dielectric properties were investigated. The XRD data reveal that the SBTTO sample adopts a tetragonal crystal structure with the I4/mmm space group and is then transformed into an orthorhombic structure with the B2cb space group for SBTTMO. The morphology of both samples was observed by SEM, which showed anisotropic plate-like grains. With the Mn substitution, the ferroelectric transition temperature (T_c) significantly increases as the influence of the 6s² lone pair of Bi³⁺ increases, and this in turn further induces the relaxor-ferroelectric behavior. Consequently, the increase in T_c confirms the structural transformation from the paraelectric-tetragonal to the ferroelectric-orthorhombic phase.     

Enhancement of multiferroic properties of Aurivillius Bi5Ti3FeO15 ceramics by Co doping

Multiferroic Bi₅Ti₃Fe_1−xCo_xO₁₅ (BFCT-x, where x = 0, 0.1, 0.3, 0.5, 0.7) ceramics were synthesized via a conventional solid-state reaction process and their microstructural, ferroelectric, magnetic and magnetoelectric coupling properties were investigated in detail. All samples show layered perovskite Aurivillius phase with an orthorhombic structure. The highest remanent polarization (2P_r) (35 μC/cm²) has been observed in BFCT-0 ceramic while the BFCT-0.3 ceramic shows the highest remanent magnetization (M_r) (0.13 emu/g) and magnetoelectric coefficient (11.47 mV cm⁻¹ Oe⁻¹). The enhancement of magnetic properties and the magnetoelectric coupling of these ceramics are attributed to the structural distortion caused by Co substitution which subsequently led to ferromagnetic interactions via the Dzyaloshinskii-Moriya interaction.     

Carbon containing castables: current status and future prospects

Abstract

Measures to overcome the main technical di fficulties hindering the development and application of carbon containing castables are discussed. The aqueous wettability and dispersion properties of graphite can be improved by coating with materials such as carbides (SiC) and oxides (Al₂O₃, TiO₂, SiO₂, MgO, ZrO₂) or by forming micropellets / briquettes. Thick and dense crack free coatings are needed not only to improve the aqueous wettability and dispersion of graphite, but also its oxidation resistance. Small and dense micropellets or briquettes enable a homogeneous distribution of graphite in the matrix to be achieved, along with acceptable mechanical strength and corrosion resistance. Coating techniques have also been used to improve the hydration resistance of aluminium based antioxidants, but detailed studies in this area are still needed. The main binder systems are currently based on superfine silica fume and hydratable alumina as these do not form low melting phases in castables. As well as developing the existing coating and micropellet/briquette fabrication techniques, future work will continue to seek novel methods of incorporating graphite, and will begin to investigate installation methods and drying and heating schedules.     

Dielectric and pyroelectric anisotropy in the melt-quenched barium bismuth niobate ceramics

BaBi₂Nb₂O₉ textured ceramics were fabricated via melt-quenching followed by high temperature (800–1000 °C) sintering process. The resulting ceramics possessed a {00l} texture with the a–b plane of the grains oriented parallel to the major face of the quenched plate. The influence of the sintering temperature on the orientation factor (f) and microstructure was evaluated via X-ray powder diffraction (XRD) and Scanning electron microscopy (SEM). The orientation factor was found to increase with increase in the sintering temperature and reached a maximum value of 0.47 for the samples sintered at 1000 °C for 10 h. Relative density and the grain-size of the ceramics were found to increase with increase in the sintering temperatures. The effect of texture on the dielectric and pyroelectric properties was evaluated. The measurements performed along the direction perpendicular to the pressing axis exhibited superior values than that of the direction parallel to the pressing axis. The observed anisotropy in the physical properties was attributed to the increased contribution from the highly polarizable a–b planes which are oriented in the direction perpendicular the pressing axis.     

Competing Instabilities in Ferroelectric Aurivillius Compounds

The stability of SrBi₂Ta₂O₉ has been studied by means of ab-initio calculations. The main instability of the tetragonal configuration concerns a non-polar mode at the Brillouin zone border. It can be related through Brillouin zone folding to the well-known soft rigid-unit mode R₂₅ in perovskites. This implies a complex scenario for the whole Aurivillius family with two competing order parameters, which can result in a sequence of two continuous phase transitions or a single discontinuous one. A third hard-mode is shown to play an essential role in the stabilization of the ferroelectric phase.     

Photocatalytic behaviour of Bi2MO6 polymetalates for rhodamine B degradation

Bi₂MO₆ (M = W or Mo) have been successfully synthesized using the citrate complex method. The samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area and UV–vis spectroscopy. The solids crystallized in the orthorhombic Aurivillius-type structure and display optimum light absorption properties to be used as photocatalysts. The photoactivity power of the samples was investigated systematically using the rhodamine B degradation under different irradiation wavelengths. The behaviour of the solids and the photodegradation mechanism was studied as a function of the irradiation light (UV or visible) employed. Direct solar light was also employed as irradiation source showing that this type of structures could drive to a plausible strategy for developing finest photocatalyst to degrade wastewaters by using solar light.     

Ferroelectric and photocatalytic properties of Aurivillius phase Ca2Bi4Ti5O18

Aurivillius phase Ca₂Bi₄Ti₅O₁₈ powders with micrometer size were produced by solid-state reaction. X-ray diffraction revealed that the powders had polar orthorhombic structure with space group of B2cb. Ca₂Bi₄Ti₅O₁₈ ceramic exhibited frequency independent dielectric anomaly at 774°C. The piezoelectric coefficient d₃₃ value of poled Ca₂Bi₄Ti₅O₁₈ pellets was 0.7 ± 0.2 pC/N. Both frequency independent dielectric anomaly and detectable d₃₃ value clearly indicated that Ca₂Bi₄Ti₅O₁₈ is a ferroelectric material with Curie point of 774 ℃. UV–vis absorption spectra revealed that Ca₂Bi₄Ti₅O₁₈ had a direct band gap of 3.2 eV. Photocatalytic activity of the Ca₂Bi₄Ti₅O₁₈ powders was examined by degradation of rhodamine B (RhB) under simulated solar light. 16% of RhB solution was degraded by Ca₂Bi₄Ti₅O₁₈ powders after 4 hours UV-vis irradiation. With Ag nanoparticles deposited on the Ca₂Bi₄Ti₅O₁₈ powders surface, 50% of RhB was degraded under the same irradiation condition. The fitted degradation rate constant of Ag decorated Ca₂Bi₄Ti₅O₁₈ was 4 times higher than that of bare Ca₂Bi₄Ti₅O₁₈. This work suggested that the Aurivillius ferroelectric Ca₂Bi₄Ti₅O₁₈ is a promising candidate for photocatalytic applications.