Structural characterization and photoluminescence behavior of pure and doped potassium strontium niobates ceramics with tetragonal tungsten–bronze structure

KSr₂Nb₅O₁₅ and K_1.2Sr_1.6Ln_0.2Nb₅O₁₅ (Ln=La, Pr, and Nd) were synthesized by the Pechini method and calcined at 1373 K for 10 h under air atmosphere. Rietveld refinements indicated the presence of structural defects in both KSN and KSN-Ln, especially distortions in O–Nb–O and Nb–O–Nb bonds. Modifications in bond distances of [NbO₆] clusters were also confirmed by Raman spectroscopy. Ultraviolet-visible absorption spectra and photoluminescence profiles were influenced by lanthanide atoms in the band structure of modified KSN materials, creating new defects and intermediary energy levels within the band gap.     

Highly-doped YAG:Sm3+ transparent ceramics: Effect of Sm3+ ions concentration

YAG:Sm³⁺ (3–15 at.%) transparent ceramics, a promising cladding material for suppressors of parasitic oscillations at 1064 nm of YAG:Nd³⁺ lasers, have been prepared by solid-state reactive sintering at 1725 °C. The effect of samarium ions concentration on the microstructure and optical properties of YAG:Sm³⁺ sintered ceramics was studied for the first time. The solubility limit of samarium ions in the garnet matrix was found to lie within the range of 9–11 at.%. The spectroscopic characterization of YAG:Sm³⁺ (3–15 at.%) ceramic samples showed that the absorption coefficients corresponding to Sm³⁺ ions transitions increased linearly with increasing Sm³⁺ doping. Also, the increase in the concentration of Sm³⁺ ions contributes to the increase in the intensities of the satellites, leading to the broadening of the main spectral lines and implicitly to the increase of the absorption coefficient around 1064 nm. It was shown that YAG:Sm³⁺ ceramics doped with 9 at.% Sm³⁺ ions possess optical losses of 0.07 cm^?1 at 808 nm and an optical absorption coefficient of 4.45 cm^?1 at 1064 nm. The concentration dependence of the ⁴G_5/2 level decay confirmed that the luminescence extinction is due to the energy transfer between the Sm³⁺ ions through cross-relaxation processes. All these results show that highly-doped YAG:Sm³⁺ (9 at.%) ceramics could be the best candidate for parasitic oscillation suppression in high-power YAG:Nd³⁺ lasers at 1064 nm.     

Tunable single-phase white-light-emitting Ba2Mg(BO3)2:Ce3+, Na+, Tb3+, Eu2+ phosphor based on energy transfer

A series of white-light-emitting phosphors of single-phase Ba₂Mg(BO₃)₂:Ce³⁺, Na⁺, Tb³⁺, Eu²⁺ were synthesized by conventional solid-state reaction. The crystal structure of the host was characterized by X-ray diffraction and investigated by Rietveld refinement. Photoluminescence properties were studied in detail. The energy transfer from Ce³⁺ to Tb³⁺ in Ba₂Mg(BO₃)₂ host was investigated and demonstrated to be a resonant type via a quadrupole–quadrupole mechanism. White light with wavelength tunable was realized by coupling the emission bands peaking at 417, 543 and 626 nm attributed to Ce³⁺, Tb³⁺ and Eu²⁺, respectively. By properly tuning the relative composition of Ce³⁺(Na⁺)/Tb³⁺/Eu²⁺, optimized Commission Internationale de l׳Eclairage (CIE) chromaticity coordinates (0.363, 0.295), high color rendering index (CRI) 90 and low correlated color temperature (CCT) 3793 K were obtained from the phosphor of Ba_1.90Ce_0.04Na_0.04Eu_0.02Mg_0.94Tb_0.06(BO₃)₂ upon the excitation of 296 nm UV radiation. These results indicate that Ba₂Mg(BO₃)₂:Ce³⁺, Na⁺, Tb³⁺, Eu²⁺ phosphor has a potential application as an UV radiation down-converting phosphor in white-light-emitting diodes.     

Optical and electrical properties of pressureless sintered transparent (K0.37Na0.63)NbO3-based ceramics

Lead-free (1−x)(K_0.37Na_0.63)NbO₃-xCa(Sc_0.5Nb_0.5)O₃ (x=0.050, 0.070, 0.090, 0.095 and 0.100) transparent ferroelectric ceramics have been fabricated by pressureless sintering procedure. Transmittance of 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics sintered in sealed alumina crucible was 15% higher than those sintered unsealed in air. By increasing the content of Ca(Sc_0.5Nb_0.5)O₃, the phase structure of (K_0.37Na_0.63)NbO₃ ceramics transformed from orthorhombic to tetragonal symmetry first and then to pseudo cubic symmetry. The 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics exhibited high density (98%), high transmittance (60%) in the near-IR region and relatively good electrical properties (ε_r=1914, tanδ=0.037, T_c=147 °C, P_r=6.88 μC/cm², E_c=8.49 kV/cm). Meanwhile, the introduction of Ca(Sc_0.5Nb_0.5)O₃ induced a composition fluctuation in the (K_0.37Na_0.63)NbO₃ lattice and made the ceramics more relaxor-like, which would lead to a further reduction of light scattering. These results demonstrated that 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ could be promising lead-free transparent ferroelectric ceramics.     

Characterization and properties of lanthanide based titanium tantalate and aluminate electronic ceramic mixtures

The 0.9LnTiTaO₆ + 0.1Ln′AlO₃ (Ln = Ce, Pr and Nd and Ln′ = Pr, Nd and Sm) ceramic mixtures are prepared through the solid state ceramic route. The materials are characterized using X-ray diffraction analysis, scanning electron microscopy and energy dispersive spectrometry. The dielectric properties in the radio as well as in the microwave frequencies are measured. The photoluminescence property of a representative sample is also analyzed. The materials have dielectric constant in the range 35.5-28.6, the temperature coefficient of resonant frequency in the range +35 ppm/°C to +14.1 ppm/°C and high quality factor. The measured values of ?_r and τ_f are compared with the corresponding predicted values calculated using mixture rules. Most of the compositions are useful in the field of optoelectronics and microwave communication.     

Effect of Gd doping on structural,optical properties,photoluminescence and electrical characteristics of CdS nanoparticles for optoelectronics

Synthesis of pure and 0.1 to 5?wt.% Gd-doped CdS nanoparticles (NPs) was achieved through a modified domestic microwave-assisted route in a short timespan at 700?W power. The formation of hexagonal CdS NPs was verified via X-ray diffraction analysis, and no structural variation was observed except for lattice variation. The size of the crystallites (D), dislocation concentration, and lattice strain were calculated, and the D was in the range of 3–6?nm. Fourier transform-Raman analysis confirmed the presence of 1LO, 2LO, and 3LO modes at 294.76, 590, and 890?cm^?1, respectively, in all the synthesized nanostructures, with minute variations in their positions due to doping; however, no new mode was observed. The position of the vibration modes was red shifted compared to that of the bulk material, indicating a confinement effect. Scanning electron microscopy (SEM) mapping/energy-dispersive X-ray spectroscopy revealed homogeneous doping of Gd and the presence of all the constituents in the final products. The morphology of the synthesized materials was tested via field-emission SEM, which revealed spherical NPs with small dimensions. Additionally, high-resolution transmission electron microscopy was performed to visualize the shape and size of the prepared 0.1% Gd:CdS NPs. The energy gap was calculated using the Kubelka–Munk theory and found to be in the range of 2.31–2.41?eV. The photoluminescence emission spectra exhibited two green emission peaks at 516?±?2?nm and 555?±?2?nm and showed the reduction of defects with Gd doping in terms of intensity quenching. The dielectric constant (${\epsilon }^{\text{'}}$), loss, and alternating-current electrical properties were studied in the high-frequency range. The values of ${\epsilon }^{\text{'}}$ were in the range of 17–27. An enhancement of these values was observed for CdS when it was doped with Gd. The electrical conductivity exhibited frequency power law behavior.     

The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite

The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite fired at 1550 °C for 2 h was investigated. The results indicated that, nano-silica improved in situ formed forsterite at the firing temperature, while zircon additions enhanced the sintering of the investigated samples. XRD analysis and SEM examination observed a good crystallinity of in situ formed forsterite with nano-silica and/or zircon additions. Densification parameter (BD ∼3.22 g/cm³ and AP ∼5.82%), cold crushing strength (CCS ∼285 MPa) and micro-hardness (Hv ∼660) were enhanced with zircon additions.     

Microstructural and optical properties of SnO2–ZnSnO3 ceramics

This work deals with some physical investigation on SnO₂–ZnSnO₃ ceramics grown on glass substrates at different temperatures (450 °C and 500 °C). Structural and optical properties were investigated using X-Ray diffraction (XRD), Raman, infrared (IR) absorption (FTIR), UV–visible spectroscopy and Photoluminescence (PL) techniques. XRD results revealed the existence of a mixture of SnO₂/ZnSnO₃ phases at different annealing temperatures. Structural analysis showed that both phases are polycrystalline. On the other hand, the optical constants (refractive index, extinction coefficient and the dielectric constants) have been obtained by the transmittance and the reflectance data. The optical band gap energy changed from 3.85 eV to 3.68 eV as substrate temperature increased from 450 °C to 500 °C. Raman, FTIR modes and PL reinforced this finding regarding the existence of biphasic (SnO₂ and ZnSnO₃) which is detected also by X-Ray diffraction analysis. Finally, the Lattice Compatibility Theory was evoked for explaining the unexpected incorporation of zinc ions in a rhombohedral structure within SnO₃ trigonal lattice, rather than the occupation of SnO₂ available free loci. All the results have been discussed in terms of annealing temperature.     

Stabilization of 6H-Hexagonal SrMnO3 Polymorph by Al2O3 insertion

We demonstrate the structural evolution of polymorphic phases in Al₂O₃-inserted SrMnO₃ ceramics synthesized by solid state reaction. While the 4H-hexagonal phase is predominant in pure SrMnO₃ ceramics, a small amount of 6H-hexagonal polymorph is identified in addition to the primary 4H-hexagonal SrMnO₃ and the secondary hexagonal SrAl₂O₄ phases in the as-sintered ceramics, evidenced by x-ray diffraction and subsequent Rietveld refinement analyses. The existence of the 6H-hexagonal SrMnO₃ phase is corroborated using Raman spectroscopy. The chemical compositions and electronic structures of the Al₂O₃-inserted SrMnO₃ compounds are also examined using energy dispersive spectroscopy and x-ray photoelectron spectroscopy, respectively. The first-principles calculations reveal that there is no clear difference between the total energies of 4H- and 6H-hexagonal polymorphs regardless of the presence/absence of Sr and oxygen vacancies. Possible origins are discussed with the estimation of actual strain based on the refined lattice parameter of 6H SrMnO₃.     

Relaxor behaviour and nonlinear dielectric properties of lead-free BZT–BZN composite ceramics

To improve the dielectric performance of Ba(Zr_0.2Ti_0.8)O₃-based ceramics, (1-x) Ba(Zr_0.2Ti_0.8)O₃-xBi(Zn_2/3Nb_1/3)O₃ composite ceramics (as short as (1-x) BZT20-xBZN) were synthesised using a solid-state reaction. The effects of Bi(Zn_2/3Nb_1/3)O₃ on the microstructure and dielectric properties were studied. Stable cubic symmetry and polar nanoregions (PNRs)-related first-order Raman modes were observed as the amount of BZN increased. The 0.94BZT20–0.06BZN sample exhibited improved dielectric constants with a higher degree of relaxor behaviour (γ = 1.91). BZN-dependent dielectric nonlinearity was observed under an applied electric field. The evolution of the relaxor behaviour and dielectric nonlinearity in the (1-x) BZT20-xBZN composite ceramics were related to PNRs variations. The largest figure of merit (~65) was obtained for the 0.94BZT20–0.06BZN ceramics, while the dielectric tunability was 75%.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

Preparation and dielectric properties of CaTaO2N and SrNbO2N ceramics

A new approach for the synthesis of perovskite oxynitride ceramics via ammonolysis of thin pellets of their corresponding oxide precursors is reported. Phase-pure CaTaO₂N- and SrNbO₂N-ceramics with relative densities up to 78% were obtained by ammonolysis of Ca₂Ta₂O₇- and Sr₂Nb₂O₇-pellets with ≈10% porosities at 950 °C (SrNbO₂N) and 1100 °C (CaTaO₂N). The oxynitride samples were investigated with respect to their optical, thermal and dielectric properties. Temperature- and frequency-dependent impedance measurements revealed that CaTaO₂N is an insulator at room temperature with a relative permittivity in the order of 100. In contrast, SrNbO₂N exhibits a high relative bulk permittivity of ε′ = 25000 and a comparatively high bulk conductivity of 1.6 × 10⁻⁴ S cm⁻¹ at room temperature. At elevated temperatures, the evolving small electrical conductivity of CaTaO₂N can be assigned to the formation of thermally activated electron-hole pairs. For SrNbO₂N two defect-dominated conduction paths can be distinguished and related to grain boundary and bulk conductivity, respectively.     

石墨层间化合物在压强下的稳定性

我们探究了一阶氯化铁石墨层间化合物(FeCl3-GICs)在单轴压力下的稳定性。通过分析一阶氯化铁石墨层间化合物在不同压强下X射线图谱的变化,我们发现在单向压强低于528MPa时,FeCl3-GIC会保持稳定状态。然而当进一步增大压强后,FeCl3-GIC将变得不稳定和脱插,在更大的单轴压强下甚至可以使一阶FeCl3-GIC改变到更高阶的石墨层间化合物,我们认为氯化铁石墨层间化合物在单轴压力下的脱插是在键的断裂和线缺陷的协助下完成的。     

Photoluminescence of zinc oxide nanopowder synthesized by a combustion method

Zinc oxide (ZnO) nanopowder was synthesized by a simple and quick combustion method using zinc nitrate as a precursor and glycine as a fuel material. The starting materials were mixed at room temperature and spontaneous ignition of which resulted into the ZnO nanopowder. The synthesized nanopowder was characterized by X-ray diffraction (XRD), scanning electronic microscope (SEM), Infrared (IR) spectrophotometer and spectroflurometer in order to study the structural, morphological, compositional and photoluminescence (PL) properties. The ZnO powder shows polycrystalline nature with preferential peak (101) having crystallite size 25 nm. A significant band at 532 cm⁻¹ in the IR spectrum corroborates the presence of characteristic band of ZnO. Room temperature photoluminescence spectrum of the synthesized nanopowder exhibits a dominant, sharp and strong ultraviolet (UV) emission with a suppressed deep-level emission indicating good crystal quality and optical properties.     

Phase reactions associated with the formation of Ti3SiC2 from TiC/Si powders

The objective of this paper was to investigate the high temperature phase reactions that take place in a 3TiC/2Si powder mixture during heating. Special attention was paid to the formation of Ti₃SiC₂ and to the evaporation of gases. Differential scanning calorimetry and thermogravimetry were used for in situ analysis of the phase reactions. Samples were heated at a rate of 5 K/min to various temperatures between 890 and 1450 °C and then cooled at a rate of 20 K/min. Heat treated samples were analysed by X-ray diffractometry. The first phase to form was TiSi₂, which was consumed in the Ti₃SiC₂ forming reactions. No decomposition of Ti₃SiC₂ was observed at temperatures below 1450 °C. Evaporation of CO(g) and small amounts of Si(g) were detected at 1430 °C.