Influence of Ti ions valence states on the optical and magnetic properties of GdCr1-xTixO3 (0≤x≤0.05)

The vacancy defects, optical and magnetic properties of GdCr_1-xTi_xO₃ (0≤x ≤ 0.05) polycrystalline samples were investigated in this research. The crystal structural analyses show that the orthorhombic perovskite structure exists in all the samples. The positron annihilation spectra reveal that the vacancy defects and local electron density vary with the change of Ti ions valence states. The magnetic measurements show that the antiferromagnetic transition is shifted to lower temperature with the replacement of Ti ions in Cr sites, and the value of spin reorientation caused by the exchange interaction between Gd³⁺ ion and Cr³⁺ ion can be influenced by the change of Ti ions valence states, which rises from x = 0 to 0.04 and then decreases at x = 0.05 sample. Simultaneously, the optical band gap decreases from 2.7 eV to 0.8 eV with the introduction of Ti ions into the lattice, the result can provide a reference for improving optical activity of rare-earth chromium oxides.     

The structure,vacancy characteristics,and magnetic properties of GdMn1-xCrxO3 ceramics

To investigate the evolution of the structural and enhanced magnetic properties of GdMnO₃ systems induced by the substitution of Mn with Cr, polycrystalline GdMn_1-xCr_xO₃ samples were synthesized via solid-state reactions. XRD characterization shows that all GdMn_1-xCr_xO₃ compounds with single-phase structures crystallize well and that Cr³⁺ ions entering the lattice sites of GdMnO₃ induce structural distortion. SEM results indicate that the grain size of the synthesized samples (a few microns) decreases as the Cr substitution concentration increases. Positron annihilation lifetime spectroscopy reveals that vacancy-type defects occur in GdMn_1-xCr_xO₃ ceramics and that the vacancy size and concentration clearly change with the Cr content. The temperature and field dependence of the magnetization curves show that Cr substitution significantly influences the magnetic ordering of the gadolinium sublattice, improving the weak ferromagnetic transition temperature and magnetization of GdMn_1-xCr_xO₃. The enhanced magnetization of GdMn_1-xCr_xO₃ is closely related to the vacancy defect concentration.     

Synthesis and optical properties of Ni/Co/Cr doped BaMg6Ti6O19

Novel green and blue chromophores based on Ni/Co/Cr doped BaMg₆Ti₆O₁₉ solid solutions are successfully synthesized through a solid-state reaction. The crystal structure of all the samples belongs to the magnetoplumbite structure with the space group of P6₃/mmc. The BaMg_6-x/2Ti_6-x/2Ni_xO₁₉ (0 ≤ x ≤ 0.3) compounds exhibit a light green (x = 0.1) to yellow-green (x = 0.3) color. The oxidation state of Ni is confirmed to be +2 valence and the d-d transition of Ni²⁺ in octahedral sites is responsible for color. BaMg_6-x/2Ti_6-x/2Co_xO₁₉ (0 ≤ x ≤ 0.3) series show a blue color and the intensity of blueness is increasing with the increase of Co content. The blue color is due to d-d transitions within Co²⁺ present in the tetrahedral sites. For BaMg_6-x/2Ti_6-x/2Cr_xO₁₉ (0 ≤ x ≤ 2) phases the color varies from light green (x = 0.2) to green (x = 2). Chromium exists in +3 and + 6 oxidation states and the observed color is due to charge transfer transition between Cr³⁺–Ti⁴⁺ and d-d transitions within octahedral Cr³⁺ sites resulting in strong absorption in the visible region. The synthesized colored oxides are mixed with PMMA to prepare novel green and blue PMMA polymer composites to evaluate their compatibility in plastics.     

The negative magnetization and dielectric relaxation in (La0.3Pr0.7)1-xCaxCrO3 ceramics

The (La_0.3Pr_0.7)_1-xCa_xCrO₃ (x = 0, 0.1, 0.3 and 0.5) ceramics have been synthesized by a sol-gel method. The compounds crystalize in orthorhombic structure with space group Pnma at room temperature. The magnetic measurements confirm that the materials form canted antiferromagnetic (AFM) ordering from 254.1, 231.2, 118.5, 49.3 K for the samples of x = 0, 0.1, 0.3 and 0.5, accompanied by a spin reorientation transition at 191.4, 145.1, 55.8, 38.9 K because of the Pr³⁺-Cr³⁺ interaction. Meanwhile, a positive exchange bias effect is observed due to the antiparallel coupling between the Pr³⁺ and the canted AFM structure of Cr³⁺. The Néel temperature, compensation temperature as well as the coupling strength of Pr³⁺-Cr³⁺ monotonously decrease with the increase of Ca²⁺ concentration. For the samples of x = 0.1 and 0.3, there exists a field induced Pr³⁺-Pr³⁺ AFM ordering. By means of dielectric measurements, large permittivity at room temperature and dielectric relaxation are observed in all the samples. The permittivity is decreased and dielectric relaxation moves to low temperature range with the increase of Ca²⁺ concentration. A relaxor-like ferroelectric transition is observed in the measuring temperature range in Ca²⁺ doped samples.     

Structural and thermoelectric properties of GdxSr2-xCoO4 layered perovskites

Gd_xSr_2-xCoO₄-layered perovskites with varying gadolinium content (x = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) were synthesised using a conventional solid-state reaction method in air. X-ray diffraction (XRD) analysis indicated that all samples were single phase with a tetragonal I4/mmm K₂NiF₄-type structure. Rietveld refinements showed that the lattice parameters and the in-plane Co–O bond length decreased with increasing Gd³⁺ content, suggesting stronger structural distortion. The X-ray photoelectron spectroscopy (XPS) analysis indicated that cobalt ions on the surface of Gd_xSr_2-xCoO₄ had Co³⁺ and Co⁴⁺mixed valence states and many oxygen vacancies. The thermoelectric properties of Gd_xSr_2-xCoO₄ (x = 0.7, 0.8) were also investigated. The temperature dependencies of the electrical conductivity (σ) and Seebeck coefficient (S) up to 750 °C for Gd_0.7Sr_1.3CoO₄ showed maximum and minimum values at approximately 550 °C. Below 550 °C with increasing Gd³⁺ content, S increased, whereas σ and the thermal conductivity (κ) decreased, leading to an increase in the figure of merit (ZT). A maximum ZT value of 0.0039 for Gd_0.8Sr_1.2CoO₄ was obtained at 400 °C. The variations of σ, S and κ with x and temperature are discussed.     

Linear correlation of crystal structure and spectral properties of Nd3+ in Ca1-xSrxF2 mixed crystals

Crystal structure and the optical spectral properties of 0.5 at.% Nd³⁺, 5 at.% Gd³⁺-codoped Ca_1-xSr_xF₂ (0 ≤ x≤1) crystals grown by the temperature gradient technique (TGT) were investigated. The linear variations of the crystal structure and spectral parameters as a function of Sr²⁺ proportion (x) were observed and correlated with each other. The results indicated that Sr²⁺ ions introduced as matrix-substituted ions could regulate the crystal structures and the spectroscopic properties.     

Structural,electrical, and magnetic transport properties of La0.72Ca0.28Mn1−xCrxO3 (0 ≤ x ≤ 0.06) ceramics

To obtain comprehensive materials with both high temperature coefficient of resistivity (TCR) and magnetoresistance (MR) at low magnetic fields, polycrystalline La_0.72Ca_0.28Mn_1?xCr_xO₃ (x = 0, 0.02, 0.04, 0.06) ceramics were prepared herein by sol–gel method. Electronic configuration of Cr³⁺ ions is similar to that of Mn⁴⁺ ions, therefore, successive substitution of Mn with Cr increases electrical resistivity and decreases metal–insulator transition temperature of ceramics, even yielding hump-like feature for Cr-rich (x = 0.06) samples. The best TCR (28.50%·K^?1) and MR (72.37%) values were obtained simultaneously at Cr dopant content of 0.02 (La_0.72Ca_0.28Mn_0.98Cr_0.02O₃). Strong response of the material to temperature and magnetic field was caused by minimal symmetry of orthorhombic structure and the most robust Jahn–Teller distortion. With increasing Cr content, Mn³⁺/Mn⁴⁺ or Mn³⁺/Cr³⁺ double exchange was diluted, and Cr³⁺/Cr³⁺ or Cr³⁺/Mn⁴⁺ superexchange was promoted. However, the internal competition effect was not conducive to the improvement of material properties.     

Gd3+ doping induced microstructural evolution and enhanced visible luminescence of Pr3+ activated calcium fluoride transparent ceramics

Transparent Pr³⁺ doped Ca_1-xGd_xF_2+x (x = 0, 0.01, 0.03, 0.06, 0.10, 0.15) polycrystalline ceramics with fine-grained microstructures were prepared by the hot-pressing method. The dependence of microstructure, optical transmittance, luminescence performances and mechanical properties on the Gd³⁺ concentrations for Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics were investigated. The Gd³⁺ ions show positive effects on the microhardness of Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics as a result of the decrease in the grain sizes. Excited by the Xenon lamp of 444 nm, typical visible emissions located at 484 nm, 598 nm and 642 nm were observed. Furthermore, the incorporation of Gd³⁺ ions can greatly enhance the photoluminescence performance owing to the improvement in the concentration quenching effect. The quenching concentration of Pr³⁺ ions in CaF₂ transparent ceramics increased to 1 at.% as a result of the positive effect of Gd³⁺ codoping. The energy transfer mechanism of Pr³⁺ in the Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics has been investigated and discussed.     

Gadolinium ferrite nanoparticles: Synthesis and morphological,structural and magnetic properties

In this study we report on the successful synthesis of Gd_xFe_3−xO₄ nanoparticles with nominal Gd-content (x) in the range 0.00≤x≤0.50. The effect of the nominal Gd-content on morphological, structural and magnetic properties was investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and Mössbauer spectroscopy. We found the actual inclusion of Gd³⁺ ions into cubic ferrite structure lower than the nominal values, though no extra phase was observed in the whole range of our investigation. Moreover, from Mössbauer data we found evidences of Gd³⁺ ions replacing both Fe³⁺ and Fe²⁺ ions, the latter leading to iron vacancies in the cubic ferrite crystal structure. As the nominal Gd-content, the lattice parameter and the average crystallite size increases monotonically. We found that in the same range of nominal Gd-content the lattice parameter decreases with the increase of iron vacancy content.     

Structure evolution and tunable magneto-optical multifunctional property study of novel Ca18K3Sc1-xSmx(PO4)14 phosphors

The optical-magnetic multifunctional materials are of great importance owing to their potential applications in intersect discipline fields. However, the multifunctional materials with satisfied optical and magnetic property are limited, and the modulation mechanism of their magneto-optical property related to their structure feature is ambiguous. Here, a new multifunctional solid solution phosphors Ca₁₈K₃Sc_1-xSm_x(PO₄)₁₄ (0 ≤ x ≤ 1) are designed based on composition engineering and are successfully synthesized. The detailed structure feature including the structural evolution of the crystallographic parameters and local lattice environment as well as polyhedral distortion behavior are investigated through X-ray diffraction Rietveld structure refinement for Ca₁₈K₃Sc_1-xSm_x(PO₄)₁₄ in the full range (0 ≤ x ≤ 1). The photoluminescence spectra indicate that Ca₁₈K₃Sc_1-xSm_x(PO₄)₁₄ (0 ≤ x ≤ 1) can emit intense orange-red emission (IQE~42.6%) under ultraviolet light excitation ascribed to the⁴G_5/2 -⁶H_7/2 transition of Sm³⁺ ions. The structure modulated luminescence behavior, for example, the tunable emission ratio of I₆₄₈/I₅₆₅, high quenching content, and electric multipole interaction mechanism are studied. The versatile phosphors can exhibit tunable orange-red cathodoluminescence emission along with increasing the accelerating voltage. The electron transition mechanism of the photoluminescence and cathodoluminescence behavior is further discussed through a simple schematic diagram. Moreover, Ca₁₈K₃Sc_1-xSm_x(PO₄)₁₄ (0 ≤ x ≤ 1) shows a ferromagnetic behavior with controllable magnetization and coercive field. Finally, the impact of Fe ions on the photoluminescence and magnetic property of Ca₁₈K₃Sc_1-yFe_y(PO₄)₁₄: 0.4Sm³⁺ (0 ≤ y ≤ 0.05 and y = 1) phosphors are investigated. The current research can provide some ideas and mechanism to design and investigate more novel magneto-optical muntifunctional materials for versatile applications in intersect disciplines.     

Enhanced magnetodielectric properties of Co2Z ferrite ceramics for ultrahigh frequency applications achieved via Cr3+ ion doping

Phase formation, microstructure, magnetic properties, and dielectric properties of Ba_1.5Sr_1.5Co₂Fe₍₂₃−_x)Cr_xO₄₁ (0.0 ≤ x ≤ 1.0) ceramics, in which Fe³⁺ ions were substituted by Cr³⁺ ions, were systematically investigated. X-ray diffraction results reveal that Z-type hexagonal ferrite was formed by sintering at 1250 °C, and Cr³⁺ ions successfully enter lattice without destroying crystal structure. Analysis of the microstructure reveals that Cr³⁺ ion doping has significant effect on crystal micromorphology. Samples with x = 0.4 have the most homogeneous micromorphology and the highest sintering density of 5.12 g/cm³. In addition, under the influence of external magnetic field, all samples exhibit typical soft magnetic character and hysteresis characteristics, with saturation magnetization up to 63.86 emu/g (x = 0.6). Particularly, compared with undoped sample, Cr-doped samples have outstanding magnetic–dielectric properties. Firstly, with increasing Cr³⁺ amount, real part of the permeability (μ′) reaches the maximum value of 10.70 at x = 0.4, while cutoff frequency exceeds 2 GHz, and Snoek constant reaches ∼19.50 GHz. Furthermore, due to more homogeneous microstructure, samples with x = 0.4 have low magnetic loss and can maintain high quality factor (Q) over a broad frequency range. Moreover, real part of the permittivity (ε′) reaches the maximum value of 16.90 at x = 0.6, and dielectric loss remains lower than 0.013 for frequencies below 0.7 GHz. Consequently, magnetic–dielectric materials prepared in this work are expected to have extensive application prospects for ultrahigh-frequency devices.     

High capacity immobilization of U3O8 in Gd2Zr2O7 ceramics via appropriate occupation designs

An optimal occupation of U⁴⁺ and U⁶⁺ in Gd₂Zr₂O₇ is necessary for a really high immobilization capacity of U₃O₈ in Gd₂Zr₂O₇ based waste forms. Based on four kinds of occupation methods, a series of U₃O₈-doped Gd₂Zr₂O₇ compositions have been synthesized. The effects of U₃O₈ content on the phase and microstructure evolution of Gd₂Zr₂O₇ pyrochlore waste forms were investigated. Detailed XRD analysis show that the four sets of samples exhibit a single defect fluorite structure within the range of 0<x≤0.4, 0<x≤0.66, 0<x≤0.6 and 0<x≤1, respectively. The highest solubility of U₃O₈ is about 82.29 wt% when the occupation design (U⁴⁺ and U⁶⁺ substitute for Gd and Zr, respectively) was employed. It was found that the cell parameters of compounds in Set A (Gd_2–3x(U⁴⁺_xU⁶⁺_2x)Zr₂O_7+7x/2) decrease with increasing _x, while those of the other compositions increase. Moreover, the uranium are almost homogeneously distributed in all samples.     

Phase evolution,microstructure and chemical stability of Ca1-xZr1-xGd2xTi2O7 (0.0 ≤ x ≤ 1.0) system for immobilizing nuclear waste

In order to ascertain the structural relationship of zirconolite and pyrochlore for their potential application in HLW immobilization, the Gd-doped zirconolite-pyrochlore composite ceramics (Ca_1-xZr_1-xGd_2xTi₂O₇) were systematically synthesized with x?=?0.0–1.0 by traditional solid-phase reaction method. The phase evolution and microstructure of the as-prepared samples have been elucidated by XRD and Rietveld refinement, Raman spectroscopy, BSE-EDS and HRTEM analysis. The results showed that zirconolite-2M, zirconolite-4M, perovskite and pyrochlore, four phases were identified in Ca_1-xZr_1-xGd_2xTi₂O₇ system and could be coexisted at x?=?0.4 composition. With the increase of Gd³⁺ substitution, the phase evolution was followed by zirconolite-2M→zirconolite-4M→pyrochlore. It is illustrated that the phase transformation from zirconolite-2M to zirconolite-4M was promoted by the preferential substitution of Gd³⁺ for Ca²⁺. And the solubility of Gd³⁺ in zirconolite-2M, zirconolite-4M and pyrochlore increased in sequence. The chemical stability test was also measured by the PCT leaching method. The normalized elemental release rates of Ca, Zr, Ti and Gd in Ca_1-xZr_1-xGd_2xTi₂O₇ system were fairly low and in the range of 10^?6?10^?8 g?m^?2 d^?1, which indicated a potential ceramics composite ensemble of CaZrTi₂O₇-Gd₂Ti₂O₇ system for nuclear HLW immobilization.     

Superior hybrid improper ferroelectricity and enhanced ferromagnetism of Ca3(Ti1-xCox)2O7 ceramics through the superexchange interaction

Ca₃(Ti_1-xCo_x)₂O₇ ceramics were prepared by a tartaric acid sol-gel method and sintered in an oxygen atmosphere. The introduction of Co²⁺/Co³⁺ as acceptor dopants leads to the formation of more oxygen vacancies and defect dipoles in Ca₃(Ti_1-xCo_x)₂O₇ ceramics. Oxygen vacancy and defect dipoles lead to the transition of dielectric, leakage, and ferroelectric behaviors of Ca₃(Ti_1-xCo_x)₂O₇ ceramics. The coexistence of hybrid improper ferroelectricity and ferromagnetism at room temperature in Ca₃(Ti_1-xCo_x)₂O₇ ceramics has been successfully realized through the superexchange interaction of Co–O–Co. Ca₃(Ti_1-xCo_x)₂O₇ ceramics exhibit superior ferroelectricity (the remnant polarization is 3.29 μC/cm²) and enhanced ferromagnetism (the remnant magnetization reaches 6.4×10^?3 emu/g). This strategy based on the introduction of transition metal ions with unfilled 3d shells at B sites is an important approach to realize novel room-temperature single-phase multiferroic materials for Ca₃Ti₂O₇-based materials.     

Oxygen ion conductivity studies of bismuth and bismuth-calcium co-doped ThO2

A series of trivalent (Bi³⁺) doped and divalent (Ca²⁺) co-doped ThO₂ samples i.e., Th_0.50-xCa_xBi_0.50O_2-δ (x = 0.00, 0.05, 0.10, 0.15 and 0.20) have been synthesized by citrate-nitrate solution combustion route and investigated in the context of oxygen ion conductivity and dielectric relaxation phenomena. The Rietveld refinement of the Powder X-ray diffraction data confirmed monophasic fluorite structures (S.G. Fm$\stackrel{￣}{3}$m) for calcium concentrations up to 20 mol %. The optical band gap decreased with the increase in Ca²⁺ concentration up to 10 mol %. In contrast, the defect band's intensity in the Raman spectra increased due to oxygen vacancies on divalent addition. The quantitative aspect of oxygen vacancy and defect concentration was derived from Raman spectra. The crystallographic index application was further employed to interpret the optimum doping concentration to maximize oxide ion conductivity. Remarkably high oxide ion conductivity (~10^?3 S/cm) was observed for Bi³⁺ doped (50 mol %), and Bi³⁺ (50 mol %)-Ca²⁺ (10 mol %) co-doped ThO₂ samples at 773 K. The Nyquist plot exhibited grain contribution for low dopant levels. Both grain and grain boundary contribution were present in the higher dopant concentrations. Conductivity, dielectric, and modulus properties of doped and co-doped samples have been compared, from which 10 mol % of Ca²⁺-doping was identified to be the optimum concentration.     

Effect of Y3+-O2- partial substitution with Ca2+-F- on the luminescence enhancement of Y2Mo3O12:Sm3+ red-emitting phosphors

To improve the luminescence property of Sm³⁺ in Y₂Mo₃O₁₂, partial Ca²⁺-F^- co-substituted Y₂Mo₃O₁₂:Sm³⁺ phosphor, namely Y_2-xCa_xMo₃O_12-xF_x:Sm³⁺, was prepared using a solid-state method. The effect of introducing Ca²⁺-F^- ion pairs on structure and luminescence properties of Y₂Mo₃O₁₂:Sm³⁺ was studied in depth. XRD patterns not only manifested that all as-prepared Y_2-xCa_xMo₃O_12-xF_x:Sm³⁺ samples had standard Y₂Mo₃O₁₂ structure, but also indicated the introduction of Ca²⁺-F^- ion pairs did not cause the change of crystal structure. Under the near ultraviolet excitation of 404 nm, the emission peaks of Y₂Mo₃O₁₂:Sm³⁺ were located at 567 nm, 605 nm and 652 nm, respectively, resulting from the 4f→4f electron transitions of Sm³⁺ ions. Furthermore, the luminescence intensity of Sm³⁺ was obviously enhanced through the co-substitution of Y³⁺-O^2- ions with Ca²⁺-F^- ions in Y₂Mo₃O₁₂ structure, and the chromaticity coordinates moved towards red region, which due to the environmental effect of crystal field around Sm³⁺. Besides, the red LED device was manufactured for suitable chromaticity parameters. All results indicated that the as-prepared Y_1.84Ca_0.06Mo₃O_11.94F_0.06:0.10Sm³⁺ red-emitting phosphor could become a promising candidate for application of white light-emitting diodes and plant illumination.     

Luminescence properties of Ca2GdNbO6: Sm3+, Eu3+ red phosphors with high quantum yield and excellent thermal stability for WLEDs

A series of Ca₂GdNbO₆: xSm³⁺ (0.01 ≤ x ≤ 0.15) and Ca₂GdNbO₆: 0.03Sm³⁺, yEu³⁺ (0.05 ≤ y ≤ 0.3) phosphors were synthesized by the traditional solid-state sintering process. XRD and the corresponding refinement results indicate that both Sm³⁺ and Eu³⁺ ions are doped successfully into the lattice of Ca₂GdNbO₆. The micro-morphology shows that the elements of Ca₂GdNbO₆: 0.03Sm³⁺, 0.2Eu³⁺ phosphor are evenly distributed in the sample, and the particle size is about 2 μm. The optical properties and fluorescence lifetime of Ca₂GdNbO₆: 0.03Sm³⁺, Eu³⁺ phosphors were detailedly studied. The emission peak at ⁵D₀→⁷F₂ (614 nm) is the strongest and emits red light under 406 nm excitation. The increase of Eu³⁺ concentration causes the energy transfers from Sm³⁺ to Eu³⁺ ions, and the transfer efficiency reaches 28.6%. Ca₂GdNbO₆: 0.03Sm³⁺, 0.2Eu³⁺ phosphor has a quantum yield of about 82.7%, and thermal quenching activation energy is of 0.312 eV. The color coordinate (0.646, 0.352) of Ca₂GdNbO₆: 0.03Sm³⁺, 0.2Eu³⁺ phosphors is located in the red area. The LED device fabricated based on the above phosphor emit bright white light, and CCT = 5400 K, Ra = 92.8. The results present that Ca₂GdNbO₆: 0.03Sm³⁺, Eu³⁺ phosphors potentially find use in the future.     

The electrical properties of (Ba,Ca, Ce,Ti, Zr)O3 thermistor for wide-temperature sensor architecture

A series of new negative temperature coefficient (NTC) thermal materials based on (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ (0.00 ≤ x ≤ 0.20) ceramics were synthesized by a solid-state method. X-ray diffraction, scanning electron microscope and X-ray photoelectron spectroscopy were used to demonstrate the crystal structure, morphology, and composition of the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics, which were composed of solid solution based on the BaTiO₃ phase. The average grain size of doped ceramic samples experienced the process of first decreasing and then increasing. The doping of Ce has reduced the sintering temperature. The temperature-dependent resistance analysis revealed that with the change of doping amount x, the thermal constant B_300/1200 (1.21 × 10⁴–1.13 × 10⁴ K) and the activation energy E_a300/1200 (0.9777–1.0471eV) was initially increased to maximum values at x = 0.05, followed by the decreasing when x > 0.05. It has been established that the concentration of oxygen vacancies is affected by the transition between Ce⁴⁺ and Ce³⁺ provided by high levels of Ce doping. (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics exhibited excellent negative temperature characteristics in the range of 300–1200 °C. Moreover, the temperature resistance linearity was improved after samples were aged. Hence, the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics were regarded as a promising material for high-temperature NTC thermistors in a wide temperature range.     

The co-optimization of efficiency and emission bandwidth in GSAGG:Cr3+ NIR ceramic phosphors

The NIR phosphor-converted light-emitting diode (NIR pc-LED) is a new near-infrared light source that has been widely studied. Among various NIR phosphors, Cr³⁺ doped gadolinium aluminum gallium garnet (GAGG:Cr³⁺) ceramic phosphor has shown great potential due to its ultra-high efficiency and thermal stability. Despite its capabilities, its detection range may be limited due to a relatively narrow emission bandwidth. To make the GAGG:Cr³⁺ ceramic phosphors achieve both high efficiency and broadband emission, a series of Gd₃Al_2-x-ySc_xGa₃O₁₂:yCr³⁺ (GASGG:Cr³⁺) ceramic phosphors were prepared. Thanks to the decrease of crystal field strength with the doping of Sc³⁺, the full width at half maximum (FWHM) of GASGG:Cr³⁺ ceramic phosphors were extended from 84 nm to 117 nm, and the emission peak exhibited a red-shift of 46 nm. Meanwhile, it still retained extremely high external quantum efficiency (EQE = 47%) and excellent thermal stability (90.7%@150 °C). Then, a NIR pc-LED prototype device was fabricated by combining GASGG:Cr³⁺ ceramic phosphor with a blue LED chip. The NIR light output power and the photoelectric conversion efficiency of this device achieved 646 mW and 19.2%, respectively. Finally, the application effect in night vision and venography of this prototype device was demonstrated.     

Effects of (Cr0.5Ta0.5)4+ on structure and microwave dielectric properties of Ca0.61Nd0.26TiO3 ceramics

The Ca_0.61Nd_0.26Ti_1-x(Cr_0.5Ta_0.5)_xO₃ (CNT-CTx) ceramics with orthorhombic perovskite structure were prepared using the conventional solid-state method. The X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectra (XPS) were employed to investigate the correlations between crystal structure and microwave dielectric properties of CNT-CTx ceramics. The XRD results showed that all CNT-CTx samples were crystallized into the orthorhombic perovskite structure. The SEM micrographs indicated that the average grain size of samples depended on the sintering temperature. As (Cr_0.5Ta_0.5)⁴⁺ concentration increased, there was a significant decrease in the average grain size of samples. The short range order (SRO) structure and structural distortion of oxygen octahedra proved to exist in CNT-CTx crystals according to the analysis of Raman spectra results. The microwave dielectric properties highly depended on the full width at half maximum (FWHM) of Raman spectra, oxygen octahedra distortion, reduction of Ti⁴⁺ to Ti³⁺ and bond valence. At last, the CNT-CT0.05 ceramic sintered at 1420?°C for 4?h exhibited the good and stable comprehensive microwave dielectric properties: relative permittivity of 96.5, quality factor of 14,360?GHz, and temperature coefficient of resonant frequency of +153.3?ppm/°C.