Eu3+ doped ferroelectric CaBi2Ta2O9 based glass-ceramic nanocomposites: Crystallization kinetics,optical and dielectric properties

We report Eu³⁺ doped transparent glass-ceramics (GCs) containing bismuth layer-structured ferroelectric (BLSF) CaBi₂Ta₂O₉ (CBT) as the major crystal phase. The CBT crystal phase was generated in a silica rich glass matrix of SiO₂-K₂O-CaO-Bi₂O₃-Ta₂O₅ glass system synthesized by melt quenching technique followed by controlled crystallization through ceramming heat-treatment. Non-isothermal DSC study was conducted to analyze crystallization kinetics of the glass in order to understand the crystallization mechanism. The optimum heat-treatment protocol for ceramization of precursor glass that has been determined through crystallization kinetics analysis was employed to fabricate transparent GCs containing CBT nanocrystals, which was otherwise difficult. Structural analysis of the GCs was carried out using XRD, TEM, FESEM and Raman spectroscopy and results confirmed the existence of CBT nanocrystals. The transmittance and optical band gap energies of the GCs were found to be less when compared to the precursor glass. The refractive indices of the GCs were increased monotonically with increase in heat-treatment time, signaling densification of samples upon heat-treatment. The dielectric constants (ε_r) of the GCs were progressively increased with increase in heat-treatment duration indicating evolution of ferroelectric CBT crystals phase upon heat-treatment.     

The red-emitting phosphors of Mn4+-activated A2MgWO6 (A = Ba,Sr, Ca) for light emitting diodes

A series of Mn⁴⁺ ions activated A₂MgWO₆ (A = Ba, Sr, Ca) phosphors, showing bright red emission peaks appeared around 700 nm under the excitation of 355 nm, were synthesized by the solid-state reaction. The crystal structures and photo-luminescent (PL) properties of these synthesized phosphors were deeply investigated with the aids of X-ray diffraction measurement (XRD), and the temperature dependent PL/decay curves in detail. The optimum doping concentration of Mn⁴⁺ ions in A₂MgWO₆ (A = Ba, Sr, Ca) lattices were studied through the relationship between the Mn⁴⁺ ions doping concentrations and the luminescent intensities. The thermal stability of the synthesized red-emitting phosphors was checked based on the temperature-dependent PL intensities ranging from 7 to 510 K. Comparative studies of the luminescent properties for Mn⁴⁺ ions in isostructural A₂MgWO₆ (A = Ba, Sr, Ca) lattice with double perovskite structure were studied. The results indicate that the synthesized red-emitting phosphors are the ideal choice for white light emitting diodes (W-LEDs).     

The doping concentration dependent tunable yellow luminescence of Sr5(PO4)2(SiO4):Eu

Color tunable yellow-emitting phosphors of Sr_5−5xEu_5x(PO₄)₂SiO₄ (x = 0.05-0.15) were prepared by conventional solid-state reaction method. The X-ray powder diffraction patterns, the photoluminescence excitation and emission spectra were measured. The main excitation bands of the phosphors locate at a broad band extending from 300 to 500 nm, which can match the emission of ultraviolet- and blue-emitting diode chips. The tunable luminescence color was realized by the changing Eu²⁺ doping in Sr₅(PO₄)₂SiO₄. The structure and luminescence properties were investigated. Sr_5−5x(PO₄)₂SiO₄:Eu_5x displays two typical luminescence centers, which originate from two different Sr²⁺ (Eu²⁺) sites in the host. The site-occupation, the luminescence intensity and energy transfer between the Eu²⁺ ions occupying two different crystallographic Sr²⁺ sites were discussed on the base of the luminescence spectra and crystal structure. This is helpful to improve this phosphor for a potential application as a white light emitting diode phosphor.     

Electrical properties of (Bi2O3)0.75(RE2O3)0.25 ceramics (RE=Dy, Y, Ho, Er and Yb)

Five kinds of rare earth stabilized bismuth oxide ceramics, (Bi₂O₃)_0.75(RE₂O₃)_0.25 (RE=Dy, Y, Ho, Er and Yb), were synthesized by sintering a mixture of Bi₂O₃ and RE₂O₃ at 900–1100 °C and their electrical properties were investigated. The bulk density and the lattice constant linearly increased with an increase in the atomic weight of RE and the ionic radius of RE³⁺, respectively. The electrical conductivity at 300 °C slightly increased with the increasing ionic radius of RE³⁺, while at 500 and 700 °C, it was constant regardless of the ionic radius of RE³⁺. The migration activation energy and the association activation energy showed a maximum value and a minimum value at RE=Er, respectively.     

Fabrication and properties of highly transparent Tm3Al5O12 (TmAG) ceramics

Highly transparent Tm₃Al₅O₁₂ (TmAG) ceramics were fabricated by solid-state reaction and vacuum sintering. Densification, microstructure evolution, mechanical, thermal, and optical properties of the TmAG ceramics were investigated. Fully dense TmAG ceramic with average grain size of 15 μm was obtained by sintering at 1780 °C for 20 h. The in-line transmittance was 80.5% at 2000 nm. The absorption coefficients at 682 nm and 785 nm were 8.03 cm⁻¹ and 8.33 cm⁻¹, respectively. The Vickers hardness, the Young modulus, the bending strength, and the fracture toughness values were 15.14 GPa, 343 GPa, 230 MPa, and 2.35 MPa m^1/2, respectively. The thermal conductivity at room temperature was 3.3 W/m K and the average linear thermal expansion coefficient from 20 °C to 1000 °C was 8.915 × 10⁻⁶ K.     

Multiplicity of photoluminescence in Raman spectroscopy and defect chemistry of (Ba1−xRx)(Ti1−xHox)O3 (R = La,Pr, Nd,Sm) dielectric ceramics

(Ba_1?xR_x)(Ti_1?xHo_x)O₃ (R = La, Pr, Nd, Sm; x ≥ 0.04) (BRTH) ceramics were prepared using a mixed oxides method. The solubility limits in BRTH with R = La, Pr, Nd, Sm were determined by XRD to be x = 0.11, 0.12, 0.06, and 0.14, respectively. The ionic radius of R at Ti-site plays a decisive role in the solubility limit in BRTH. Only BRTH with R = La satisfied Vegard's law. The multiplicity of photoluminescence (PL) signals of Nd³⁺/Ho³⁺ and Sm³⁺/Ho³⁺ in Raman scattering under 532-nm excitation laser and the high-permittivity abnormality for the denser BRTH with R = Sm and at x = 0.07 were reported. The PL provided the evidence of a small number of Ho³⁺ at Ba-site in BRTH and it was determined that the number of Ba-site Ho³⁺ ions increased from 0.05 at% at R = La to 0.19 at% at R = Sm with increasing atomic number of light rare earth. BRTH exhibited a much broadened dielectric-temperature characteristics, marked by ×5 T, ×6 T, ×7 T, and ×8 S dielectric specifications for BRTH with R = La, Pr, Nd, Sm and at x = 0.06, respectively, and they exhibited lower dielectric loss (tan δ < 0.015) at room temperature. The dielectric-peak temperature (T_m) of BRTH decreased linearly at a rate of less than ?21 °C/%(R/Ho). The defect chemistry, solubility limit, lower dielectric loss, and dielectric abnormality are discussed.     

Photoluminescence and optical temperature sensing in Sm3+-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 lead-free ceramics

A series of orange-red emitting Sm³⁺ activated Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCZT: xSm³⁺, x?=?0.001–0.007) are synthesized by a conventional solid-state reaction method. The Sm³⁺ ions composition dependent photoluminescence properties are systematically investigated. Under the excitation of a 407?nm near-ultraviolet light, the ceramics exhibit strong characteristic emission of Sm³⁺ ions with dominant orange-red emission peak at around 595?nm, which is ascribed to the transition of ⁴G_5/2 → ⁶H_7/2. The BCZT: 0.004Sm³⁺ ceramic displays the optimal emission among these Sm³⁺-doped BCZT solid solutions. Moreover, the photoluminescence intensity exhibits extremely sensitive to temperature, suggesting that BCZT: 0.004Sm³⁺ could be applicable for temperature sensing. A maximum relative sensitivity of 1.89%?K^?1 at 453?K is obtained. Furthermore, the existence of ferroelectricity in the BCZT host combined with Sm³⁺ activated photoluminescence properties could be useful for developing optical-electro multifunctional materials and devices.     

Oxidation behavior of hot-pressed Si3N4 with Re2O3 (Re=Y, Yb, Er, La)

Four different β-Si₃N₄ ceramics with silicon oxynitrides [Y₁₀(SiO₄)₆N₂, Yb₄Si₂N₂O₇, Er₂Si₃N₄O₃, and La₁₀(SiO₄)₆N₂, respectively] as secondary phases have been fabricated by hot-pressing the Si₃N₄–Re₄Si₂N₂O₇ (Re=Y, Yb, Er, and La) compositions at 1820°C for 2 h under a pressure of 25 MPa. The oxidation behavior of the hot-pressed ceramics was characterized and compared with that of the ceramics fabricated from Si₃N₄–Re₂Si₂O₇ compositions. All Si₃N₄ ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C and the oxidation products of the ceramics were SiO₂ and Re₂Si₂O₇. The Si₃N₄–Re₄Si₂N₂O₇ compositions showed inferior oxidation resistance to those from Si₃N₄–Re₂Si₂O₇ compositions, owing to the incompatibility of the secondary phases of those ceramics with SiO₂, the oxidation product of Si₃N₄. Si₃N₄ ceramics from a Si₃N₄–Er₄Si ₂N₂O₇ composition showed the best oxidation resistance of 0·198 mg cm⁻² after oxidation at 1400°C for 192 h in air among the compositions investigated herein.     

Evolution of microstructure and electrical properties of Aurivillius phase (CaBi4Ti4O15)1-x(Bi4Ti3O12)x ceramics

(CaBi₄Ti₄O₁₅)_1-x(Bi₄Ti₃O₁₂)_x (CBT-xBIT) Aurivillius phase ceramics were synthesized by the conventional solid reaction method. The evolution of the structure and the electrical properties of CBT-xBIT ceramics were systematically investigated. Due to the enhanced spontaneous polarization induced by internal stresses on the Bi₂O₂ layers in the CBT-xBIT structure, the optimal piezoelectric coefficient (d₃₃ ~ 13?pC/N) was obtained in the ceramics with x?=?0.3 while exhibiting a relatively good thermal stability in the temperature range of 20–700?°C. The dc resistivity (ρ_dc) of the CBT-xBIT ceramics exhibited a higher value (≥?10⁹ Ω?cm) at room temperature, and the tan δ value of CBT-xBIT (x= 0, 0.1 and 0.3) within the temperature range of 20–500?°C maintained stability as a result of the domain structure and point defect concentration in the ceramics. In addition, a distinctive double dielectric peak anomaly was observed in the ε_r-T curves of the CBT-xBIT (x= 0.3, 0.5 and 0.7) ceramics, and it plays a remarkable role in the thermal stability of the piezoelectricity of CBT-xBIT ceramics. As a result, such research can benefit high temperature practical piezoelectric devices.     

Characterization of Ln4Al2O9 (Ln=Y,Sm, Eu,Gd, Tb) rare-earth aluminates as novel high-temperature barrier materials

A family of cuspidine-type rare-earth (RE) aluminates with the general formula Ln₄Al₂O₉ (Ln= Y, Sm, Eu, Gd, Tb) was prepared for potential use as thermal-barrier coating (TBC) materials with appropriate properties. Various trivalent lanthanides were applied to tailor the properties of the oxides for use as ceramic top coat (TC) materials intended for high-temperature applications. Following various heat treatments, the X-ray diffraction (XRD) results obtained demonstrated that Eu₄Al₂O₉ (EuAM) possessed the greatest structural stability of all the samples at 1200 and 1300?°C. Moreover, Y₄Al₂O₉ (YAM) had a long lifetime at 1000?°C, and was stable at 1100?°C. At 1200?°C, Sm₄Al₂O₉ (SmAM) and Gd₄Al₂O₉ (GdAM) were more stable than Tb₄Al₂O₉ (TbAM). However, at 300–1000?°C, the TbAM exhibited the highest thermal expansion coefficient (TEC) of all the samples. At 600?°C, the thermal diffusivity values of the five compositions were favourable, and were lower than that of yttria-stabilised zirconia (YSZ) oxides.     

Synthesis of nano-sized Yb3Al5O12 powders by the urea co-precipitation method

Nano-sized polycrystalline ytterbium aluminum garnet (YbAG, Yb₃Al₅O₁₂) powders were successfully synthesized by a simple urea co-precipitation method. The thermal behavior of the YbAG precursor was investigated. The calcined amorphous YbAG precursor was directly converted to 20–30 nm monophase YbAG at as low as temperature 900 °C, without any other intermediate phases. The nano-sized YbAG powders calcined at 900 °C distributed evenly with a slight aggregation, and the specific surface area of the powders reached 29.7 m² g⁻¹. The absorption spectrum of the YbAG powders was measured, and there were two strong absorption peaks centered at 931 and 965 nm. The results did not correspond to the absorption peaks of Yb:YAG crystal exactly, as the lattice parameters were different between YbAG and YAG.     

Low temperature cofiring and compatibility with silver electrode of ZnO-SnO2-TiO2-Nb2O5 ceramics with BaCu(B2O5) addition

The effect of BaCu(B₂O₅) (BCB) addition on the sintering temperature and microwave dielectric properties of 2.5ZnO-0.2SnO₂-4.8TiO₂-2.5Nb₂O₅ (ZSTN) has been investigated by the solid-state ceramic route. X-ray diffraction and scanning electron microscopy techniques were used to analysis the structure and microstructure. The microwave dielectric properties were measured by the resonance method. It was found that the addition of BCB can effectively lower the sintering temperature from 1100 °C to 900 °C, and improves the microwave dielectric properties of ZSTN ceramics. The BCB doped ZSTN ceramics can be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application.     

Structure,dielectric and optical properties of Bi1.5+xZnNb1.5O7+1.5x pyrochlores

Non-stoichiometric pyrochlore ceramics with formula Bi_1.5+xZnNb_1.5O_7+1.5x were systematically investigated. Crystal structures of the compounds were studied by X-ray diffraction (XRD) technique. The structures were identified as pure cubic pyrochlores when |x| < 0.1. Dielectric and optical properties of the compositions when x = −0.1, 0 and 0.1 were studied. All samples have high resistivities and low dielectric loss. With increasing x in Bi_1.5+xZnNb_1.5O_7+1.5x, the lattice constant, permittivity, temperature coefficient of permittivity and thermal expansion coefficient increased, while dielectric loss decreased. Raman spectra indicated that the intensity of Bi–O stretching become stronger with increasing x. A vibration mode emerging at 861 cm⁻¹ when x = −0.1 means that the B–O coordination environment is significantly more disordered. Absorption spectra suggested that the bandgap energy become lower from 2.86 to 2.70 eV as lattice constants increased. Strong absorption occurs at wavelengths from 433 to 459 nm, shows that samples have the ability to respond to wavelengths in the visible light region.     

Evaluation on the optical properties of Ga2O3−x thin films co-doped with Tb and transition metals (Mn,Cr) prepared by a photochemical route

Gallium β-diketonate complexes were studied as precursors for the photochemical deposition of amorphous thin films of gallium oxide doped with terbium and co-doped with chromium or manganese. Solutions of the inorganic complexes were spin coated on Si(100) and quartz substrates and photolyzed at room temperature using 254 nm UV light. The photolysis of these films induces the fragmentation of the complexes and the partial reduction of the metal ion together with the release of volatile organic compounds as sub-products. When the metallic complexes are irradiated under air, the products of the reactions are metal oxide thin films. The photochemical reactivity of these films was monitored by UV–vis spectroscopy, followed by a post-annealing treatment. The obtained films were characterized by X-ray photoelectron spectroscopy and X-ray diffraction. The optical properties of the films showed that these are highly transparent in the visible spectrum but decrease significantly in doped and co-doped films. Under UV light excitation (254 nm) the doped films (Ga₂O_3−x/Tb) show the characteristic emissions at 486, 530, 542 and 610 nm associated to ⁵D₄→⁷F_J (J=6,5,4,3) transitions of Tb⁺³ ion. However, these emissions decrease and deteriorate in the co-doped films (Ga₂O_3−x/Tb/M, where M=Mn or Cr). A possible emission mechanism and energy transfer have been proposed.     

Sintering and electrical properties of La-modified (Na0.52K0.45Li0.03)1−3xLax(Nb0.88Sb0.09Ta0.03)O3 lead-free ceramics

(Na_0.52K_0.45Li_0.03)_1−3xLa_x(Nb_0.88Sb_0.09Ta_0.03)O₃ (NKLL_xNST) lead-free ceramics were prepared by normal sintering and their dielectric and piezoelectric properties were investigated. The X-ray methods indicate that the NKLL_xNST ceramics with x≤0.003 present a pure perovskite phase at room temperature. The bulk density of NKLL_xNST ceramics increases with proper amount of La₂O₃ contents, and reaches its highest value of 4.544 g/cm³ with the addition of 0.3 mol% La₂O₃. At x=0.003, remnant polarization P_r, piezoelectric constant d₃₃ and planar mode electromechanical coupling factor k_p of NKLL_xNST ceramics reach the highest values of 37.80 μC/cm², 346 pC/N and 40%, respectively, exhibiting excellent “soft” piezoelectric characteristics, demonstrating a tremendous potential of the compositions studied for device applications.     

Highly uniform Gd(OH)3 and Gd2O3:Eu hexagram-like microcrystals: glucose-assisted hydrothermal synthesis,growth mechanism and luminescence property

Monodispersed and uniform Gd(OH)₃ microcrystals with novel hexagram-like morphology have been successfully prepared through a simple and green hydrothermal process with the assistance of glucose. The results of control experiments revealed that the glucose concentration in the reaction system was an important factor, which affected the morphology of the Gd(OH)₃ microcrystals significantly. On the basis of time-dependent experiments, the growth mechanism of the Gd(OH)₃ hexagrams was discussed. Furthermore, the Gd₂O₃ and Eu³⁺-doped Gd₂O₃ hexagram-like microcrystals, inheriting the Gd(OH)₃ precursors' morphology, were obtained during a direct annealing process in air. The corresponding Gd₂O₃:Eu³⁺ hexagram-like microcrystals exhibit strong red emission pertaining to the ⁵D₀→⁷F₂ transition of the Eu³⁺ ions under UV light, which have potential applications in novel optoelectronic devices.     

Fabrication, microstructure and optical properties of polycrystalline Er:Y3Al5O12 ceramics

Highly transparent polycrystalline Er³⁺:Y₃Al₅O₁₂ (Er:YAG) ceramics with different Er³⁺ ions content from 1% to 90% were prepared by the solid-state reaction and the vacuum-sintering technique. The grain boundary is clean and narrow with a width of about 1 nm. The best sintering temperature of the ceramics is about 1800 °C. The relationships between fabrication, microstructure and transparency of the ceramics were discussed. Grain size distributions in axial direction of cylinder samples were characterized by electron probe micro-analyzer (EPMA). The luminescence spectra were measured and discussed.     

Enhanced multiferroicity and narrow band gap in B-site Co-doped Aurivillius Bi5FeTi3O15

The effects of Co ions substitution on the structural, multiferroic, dielectric, and optical properties of Aurivillius Bi₅FeTi₃O₁₅ are investigated. High resolution transmission electron microscope measurements imply that Bi₅FeTi₂Nb_0.5Co_0.5O₁₅ (BFTNCO) exhibits a typical four-layered Aurivillius structure. The co-doping of Nb/Co ions at Ti sites is found to induce the lattice distortion. In comparison with Bi₅FeTi₃O₁₅ and Bi₅Fe_0.5Co_0.5Ti₃O₁₅, the sample BFTNCO shows a more superior multiferroicity with the remanent magnetization M_r and polarization P_r of 76.4?memu/g and 10.2?μC/cm², respectively. The improved multiferroicity can be ascribed to reduced oxygen vacancy, large lattice distortion and large grain size. In addition, a relatively narrow optical band gap E_g can be obtained as 2.30?eV for BFTNCO. The decreased E_g in BFTNCO can be attributed to large lattice distortion and the overlap of Co 3d state with Ti t_2g state. These results reveal that Ti-site doped Aurivillius phase may be the potential candidates for exploring excellent room-temperature multiferroics with narrow band gap.     

Structural and optical properties of MoO3 and V2O5 thin films prepared by Spray Pyrolysis

MoO₃ and V₂O₅ thin films were prepared on glass substrates by Spray Pyrolysis technique at a substrate temperature of 423 K. The precursor solutions were obtained by varying the concentrations of MoCl₅ and VCl₃ in bi-distilled water. The structural investigation conducted by X-ray diffraction showed that MoO₃ and V₂O₅ thin films were polycrystalline with orthorhombic structure. The optical properties studied in the 300–2500 nm range suggest that the thin film behaviours are related to bound electronic states. The optical band gaps have been estimated from slopes of ln(hν) versus hν plots of MoO₃ and V₂O₅ films were 3.35 and 2.44 eV, respectively. The electrical conductivity was measured using four probes method.