Dielectric,ferroelectric and magnetoelectric properties of in-situ synthesized CoFe2O4/BaTiO3 composite ceramics

Magnetoelectric composite materials have attracted more and more attention because of their coupling of ferroelectricity and ferromagnetism. It is a hotspot to realize the combination of ferromagnetic phase and ferroelectric phase. In this work, we used a new strategy to prepare CoFe₂O₄/BaTiO₃ composite ceramics: firstly, porous ferromagnetic CoFe₂O₄ phase was prepared by annealing of MOFs (metal organic frameworks) precursor Fe₃[Co(CN)₆]₂. And then, the ferroelectric BaTiO₃ phase in-situ grew in the pores of CoFe₂O₄ by a hydrothermal method. In the end, the CoFe₂O₄/BaTiO₃ composite ceramics sintered at different temperatures have been synthesized. The effects of sintering temperature on the structure, dielectric and ferroelectric properties have also been studied. Because the crystallinity and density increase with the increase of sintering temperature, the composite ceramic sintered at 1200 °C shows the best dielectric properties. It is found that sintering temperature has little effect on the ferroelectric and magnetic properties of ceramics. Taking the CoFe₂O₄/BaTiO₃ composite ceramic sintered at 1200 °C as an example, derived from the interaction between the ferromagnetic CoFe₂O₄ phase and ferroelectric BaTiO₃ phase, the applied magnetic field lead to the reduction of P_r and E_c.     

MgO-Y2O3:Eu composite ceramics with high quantum yield and excellent thermal performance

MgO-Y₂O₃:Eu composite ceramics with high quantum yield and excellent thermal performance were successfully synthesized by vacuum sintering. All samples exhibited uniform composite microstructures and pure binary phase. Eu³⁺ ions were completely incorporated into Y₂O₃ phase, and the optimal Eu concentration is 15 at%. Sintered at 1800 °C, the fluorescent properties of MgO- z vol% Y₂O₃: Eu (z = 30, 40, 50, 60, 70, 100) composites proved to be independent on component proportion, including the similar fluorescence lifetimes (953–983 μs), quantum yield (70%−80%), and activation energy (ΔE) of thermal quenching (0.163 eV). Significantly, thermal conductivity of composites with 30 vol%, 50 vol% and 70 vol% MgO attained 11.58, 17.45, and 29.65 W/(m∙K) at room temperature, which are nearly 2, 3, and 5 times as high as that of 15 at% Eu:Y₂O₃ ceramic (5.90 W/(m∙K)), respectively, demonstrating their potential for application in high-power-density display and lighting technology.     

Effect of Li nonstoichiometry and TiO2 addition on the microwave dielectric properties of Li3PO4 ceramics

Li₃PO₄ ceramic is a promising microwave ceramic material with low dielectric constant. The effect of Li nonstoichiometry on phase compositions, microstructures, and microwave dielectric characteristics of Li₃PO₄ ceramics, on the other hand, has been examined infrequently. Therefore, in the first part of this study, the stoichiometry and Li nonstoichiometry compositions based on Li_3+xPO₄(x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared by conventional solid-phase method. The results show that a few nonstoichiometric lithium ions enter the lattice of Li_3+xPO₄. Compared with the chemical content of Li₃PO₄, the sintering characteristics, relative dielectric constants and quality factors of Li_3+xPO₄ ceramics can be improved by slightly excessive Li ions, while the properties of Li₃PO₄ ceramics can be deteriorated by excessive Li ions. Li_3.12PO₄ ceramics sintered at 975 °C for 2 h have good dielectric properties (ε_r = 5.89, Q×f = 44,000 GHz, τ_f = ?206 ppm/^°C). In order to improve its large negative temperature coefficient of resonant frequency, in the following study, rutile nano TiO₂ particles were added as τ_f compensator. Adding TiO₂ powders not only effectively improve the temperature stabilities of the multiphase ceramics, but also make the grain growth more uniform. With the increase of TiO₂ content from 0.40 to 0.60, τ_f increases from ?73.5 ppm/^°C to +42.3 ppm/^°C. The best dielectric property of 0.45Li_3.12PO₄-0.55TiO₂ composite ceramic is ε_r = 13.29, Q×f = 40,700 GHz, τ_f = +8.8 ppm/^°C.     

(Ce,Gd):YAG-Al2O3 composite ceramics for high-brightness yellow light-emitting diode applications

(Ce,Gd):YAG-Al₂O₃ composite ceramics were prepared for high-brightness yellow LED (565?590 nm) applications. Phase fraction, microstructure, thermal quenching effect, and luminescent properties of composite ceramics with varying compositions were studied in detail. Collaborating composite ceramics with InGaN blue chips, the relationship between thermal conductivity, temperature rise during LED-driven phosphor conversion, and steady-state luminous efficiency were elucidated. As the proportion of Al₂O₃ increases from 0 to 40 wt%, the steady-state luminous efficiency of yellow LED is enhanced from 100.88–109.49 lm W^?1, benefiting from the increased thermal stability and reduced operating temperature of ceramics (from 141.1–132.2 °C). Additionally, scattering behavior and extraction efficiency of composite ceramics with different thicknesses were investigated.     

A novel redshift mechanism of Ce3+ emission in ZrO2-Ce: YAG composite phosphor ceramics

In this work, ZrO₂-Ce: YAG composite phosphor ceramic for the application of high brightness w-LEDs was fabricated for the first time. A novel redshift mechanism of Ce³⁺ emission is discovered in the composite ceramics that the oxygen vacancy introduced into YAG lattice by the high-temperature reduction of Zr⁴⁺ can cause a remarkable non-radiative transition of excited electrons and produce a large redshift from 540 nm to 570 ∼ 610 nm. This kind of redshift has provided a new approach to the supplement of the red component in Ce³⁺ emission spectra beyond the regular ways of larger rare-earth ions doping or M²⁺-N⁴⁺ double substitution.     

Shrinkage features,microstructure evolution and properties of Gd2O3-MgO optical composite ceramics with Zr as phase stabilizer

A novel composite ceramic, composed of equal-volumetric Zr-stabilized Gd₂O₃ and MgO phases, was prepared to be transparent in mid-wave infrared range. Zr stabilized Gd₂O₃ is proved to have a lower lattice parameter (10.7516 Å) using XRD refinement. Pressureless sintering behavior of Gd₂O₃-MgO with/without 2 at% Zr-doping (naming ZGM and GM) was studied via the real-time observation technique. The shrinkage of ZGM green body proceeds steadily up to 1400 °C while that of the undoped one shrinks sharply at 1250 °C due to Gd₂O₃ phase transition. The segregation of Zr element along the grain boundaries of Zr-Gd₂O₃ creates a synergized effect on the grain refinement with pinning effect. Dense ZGM ceramics exhibit superior transmittance of 78.3 %‐85.6 % at 3–5 µm, which show good consistency with the calculated values. The refractive index of Zr- Gd₂O₃ varies from 1.87 at 3 µm to 1.80 at 5 µm, which is smaller than those of monoclinic Gd₂O₃.     

Preparation,optical, and thermal stability of Tb3+-doped glass ceramics containing Al4Ti2SiO12

The Tb³⁺-doped transparent glass ceramics containing Al₄Ti₂SiO₁₂ crystal are synthesized by the melt-solid-crystallization method. The optimal heat treatment conditions for the glass ceramic samples are determined to be 750°C, 1.5 h based on differential scanning calorimeter, X-ray diffractometer, scanning electron microscope, and light transmittance curve. The luminescence properties of the glass ceramics samples are characterized by a fluorescence spectrometer. The emission spectrum revealed that the dominant emission is green attributed to Tb³⁺, the optimal doping concentration is 0.3%. And the thermal quenching activation energy of 0.3% Tb³⁺-doped glass ceramics is 0.2554 eV, while the relative sensitivity is 1.50% K⁻¹ at 298 K and the absolute sensitivity is 2.27×10⁻² K⁻¹ at 418 K through variable temperature fluorescence spectroscopy test and a series of calculations. These findings demonstrate the excellent stability and temperature sensitivity of glass ceramics, thereby highlighting their potential for use in green lighting and temperature sensing applications.     

Ammonium sulfate and PEG composite surfactant to promote dispersibility of precursors and Y2O3 powders for transparent ceramics

Well-dispersed Y₂O₃ powders were synthesized by a precipitation method from yttrium nitrate solution using (NH₄)₂SO₄ and PEG4000 as the composite surfactant, and the dispersion mechanism of the composite surfactant was investigated in detail. Furthermore, the grain size, densification, mechanical and optical quality of as-prepared ceramics were also discussed. The dispersion state of both precursors and Y₂O₃ powders could be remarkably effected by the mass ratio of (NH₄)₂SO₄ and PEG4000 during the liquid phase reaction process. When the function of electrostatic repulsion and the steric hindrance were combined properly, the aggregation state of powders could be effectively inhibited. With the addition of the mass ratio of 3:2 ((NH₄)₂SO₄ to PEG4000), fine powders (130?nm) with high dispersion state were obtained. Meanwhile the Y₂O₃ ceramics with an average grain size of 3.65?µm were fabricated by sintering at 1750?°C for 8?h.     

0-3 type magnetoelectric 0.94Na0.5Bi0.5TiO3-0.06BaTiO3:CoFe2O4 composite ceramics with a deferred thermal depolarization

Developing Na_0.5Bi_0.5TiO₃-based magnetoelectric (ME) coupling composites with higher depolarization temperature is highly valuable for the environment-friendly smart electronic devices. We have developed a new kind of 0-3 type 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃:xCoFe₂O₄ (NBTBT:xCFO, x = 0, 0.1, 0.2, 0.3) composite ceramics with a deferred depolarization temperature, together with an additional strong ME coupling of 9.2 mV/cm·Oe for the NBTBT:0.2CFO. The basic structure, ferroelectric/ferromagnetic properties, and the depolarization temperature of the NBTBT:xCFO composite ceramics were investigated. It was found that an enhancement of depolarization temperature (>25 °C) was obtained in these 0-3 type composites relative to the pure NBTBT ones (115 °C vs 90 °C). The mechanism of the enhanced depolarization temperature of the composites is discussed. The present results demonstrate that NBTBT:xCFO composites have great potential for ME devices.     

Electrosteric colloidal stabilization for obtaining SrTiO3/TiO2 heterojunction: Microstructural evolution in the interface and photonics properties

Researches on solids heterojunctions has proven to be an important field for technological applications due to synergism achieved by interface phenomena. In this work, we present an understanding of SrTiO₃/TiO₂ interface and its effects on structural and microstructural characteristics, and relate them to photoluminescence properties of heterojunctions obtained by a simple method. It was observed that SrTiO₃ proportion directly influences the structural order-disorder, as observed by X-Ray diffraction, Raman spectroscopy and Transmission electron microscopy. Photoluminescence behavior of heterojunction with 1% of SrTiO₃ showed a shift to blue emission region compared to TiO₂, and enhanced of emission intensity compared to SrTiO₃, resulted from defects generated by interface effects, attracting possible applications on selective color emitter. Therefore, we conclude that in same materials phases, nature and concentration of structural defects has strong dependence of SrTiO₃ concentration, which leads to different photoluminescence properties.     

Preparation and characterization of corundum ceramics doped with Fe2O3 and TiO2 for high temperature thermal storage

High-temperature thermal storage materials have received urgent attention for efficient thermal transfer in solar thermal power generation. Corundum ceramics doped with Fe₂O₃ and TiO₂ were prepared via a pressureless sintering. A Fe₂O₃–TiO₂ system with different Fe₂O₃/TiO₂ ratios was applied to corundum ceramics. Phase composition, microstructural evolution, sintering properties, high temperature resistance and thermophysical properties were evaluated. The results indicated that Fe₂O₃ and TiO₂ rendered the grains highly active and enhanced the bonding between grains due to existing stably in the lattice of corundum. In addition, decrease in the Fe₂O₃/TiO₂ ratio led to a new phase of FeAlTiO₅, which refined the grains. These effects gave the samples good sintering properties and thermal shock resistance, but the thermal expansion coefficient mismatch between FeAlTiO₅ and corundum deteriorated the high-temperature (1300 °C) stability. Formula C1 (Fe₂O₃/TiO₂ ratio of 9:1) sintered at 1600 °C had the optimum comprehensive properties, possessing a bending strength loss rate of 1.54% after 30 cycles of thermal shock (1100 °C-room temperature, air cooling) and a constant strength retention rate of approximately 71.34% after 90 h high-temperature cycle. The corresponding thermal conductivity and specific heat capacity were 18.81 W/(m·K) and 1.02 J/(g·K) at 25 °C, which was suitable as a high-temperature thermal storage material.     

Microstructure tailoring of red-emitting AlN-CaAlSiN3:Eu2+ composite phosphor ceramics with higher optical properties for laser lighting

Laser lighting is superior to light-emitting diodes (LEDs) in brightness, beam aperture and efficiency, which largely depends on the light extraction efficiency and thermal properties of color converters. In this paper, we proposed to improve the light extraction efficiency of red-emitting AlN-CaAlSiN₃:Eu composite phosphor ceramics by controlling the light scattering, and to improve the thermal conductivity of the phosphor ceramics by increasing grain size. The composite phosphor ceramics with moderate light scattering and high thermal conductivity can be obtained by gas pressure sintering (GPS) and the followed hot isostatic pressing (HIP). The saturation threshold of the sample is increased from 7.0 to 10.8 W, and the luminous flux is enhanced from 32.3 to 51.0 lm after the HIP post-treatment, which is 55.5% higher than the previously reported dense AlN-CaAlSiN₃:Eu composite phosphor ceramics (32.8 lm). This work emphasizes the role of pore size distribution in light extraction efficiency of phosphor ceramics.     

Effect of Sm2O3 on microstructure,thermal shock resistance and thermal conductivity of cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline

Cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline were fabricated via pressureless sintering at a low sintering temperature with added Sm₂O₃. The effects of Sm₂O₃ on sintering behaviors, mechanical property, phase transformation, microstructure, thermal shock resistance and thermal conductivity of the composite ceramics were investigated. TEM analysis results demonstrated that Sm³⁺ located in glass and grain boundaries to facilitate the densification via the liquid-phase sintering mechanism and improve bending strength by grain refinement, respectively. Proper addition (3 wt%) of Sm₂O₃ could promote the crystallization of cordierite, and improve thermal shock resistance of the composite ceramics with an increasing rate of 16.70% for bending strength after 30 thermal shock cycles (air cooling from 1100 °C to RT). The composite ceramics possessed a superior thermal shock resistance, where a large amount of particles were formed to suppress crack initiation and propagation during thermal shock. Cordierite-mullite-corundum composite ceramics with proper Sm₂O₃ addition (3 wt%) had a lower thermal conductivity than that of composite ceramics without Sm₂O₃ addition by strengthening the scattering of phonon, which could reduce the heat loss during solar heat transmission process.     

Fabrication and characterization of highly transparent Y2O3 ceramics by hybrid sintering: A combination of hot pressing and a subsequent HIP treatment

We succeeded in the optimization of highly transparent Y₂O₃ ceramics with a submicrometer grain size approximately 0.6?μm by hot pressing (1300–1550?°C) and a subsequent HIP (1450?°C) treatment using commercial Y₂O₃ powders as starting powders and ZrO₂ as a sintering additive. The optimum microstructure for the HIP treatment was prepared by hot pressing at a temperature as low as 1400?°C for 3?h with a relative density of 99.3%. The thus HIP-treated specimen showed the best transmittance (2?mm thick) ever reported of 83.4% and 78.3% at 1100 and 400?nm, respectively. Specifically, the transmittance using this hybrid sintering method improved substantially in the visible range compared to that of the counterpart using hot pressing only. A simulation of the transmittance based on the Beer-Lambert law and Mie scattering theory has proved that this improvement is mainly due to the elimination of nanopores below 15?nm in size.     

Effect of NaF doping on the transparency,microstructure and spectral properties of Yb3+: CaF2 transparent ceramics

Yb³⁺:CaF₂ transparent ceramics are promising laser gain media with outstanding performance. However, low transmittance in the visible range is the main challenge that restricts the application of Yb³⁺:CaF₂ ceramics in the laser system. In this paper, a new scheme to eliminate the residual pores in the Yb³⁺:CaF₂ transparent ceramics based on doping of NaF as a sintering aid is proposed. Microstructural characterization indicated that NaF could inhibit the grain growth and increase the transmittance in the visible range significantly. The corresponding transmittance was measured to be 85% at the wavelength of 400 nm. The spectra results showed that co-doped with Na⁺ ions could break the clusters of Yb³⁺ ions and modulate the spectroscopy properties of Yb³⁺: CaF₂ lattice efficiently. This paper proved that doping with NaF is an efficient strategy to improve the transmittance and fluorescence quantum efficiency of Yb³⁺:CaF₂ transparent ceramics.     

Effect of glass additions on the sintering and microwave properties of composite dielectric ceramics based on BaO–Ln2O3–TiO2 (Ln = Nd, La)

Ten weight percent BBZS (Bi₂O₃, B₂O₃, ZnO and SiO₂) glass was added to x(Ba₄Nd_9.333Ti₁₈O₅₄) − (1 − x)(BaLa₄Ti₄O₁₅) (BNLT, 0 ≤ x ≤ 1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10 wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140 °C, depending on composition (x), an average reduction of sintering temperature by 350 °C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on _r and τ_f, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT − (1 − x)BLT)/0.1BBZS (x = 0.75) had _r  61, τ_f  38 ppm/°C and Qf  2305 GHz.     

Low loss and good chemical compatibility with silver electrodes of Cu2+-substituted Ba3Ti4Nb4O21 ceramics realizing low temperature co-fired ceramics technology: Crystal structure,vibration modes and chemical bonds studies

New low loss and low-sintering temperature co-fired Ba_3-xCu_xTi₄Nb₄O₂₁ (BCTN, 0 ≤ x ≤ 0.12) ceramics with 0.60 wt% Li₂O-B₂O₃-SiO₂-CaO-Al₂O₃ (LBSCA) glass were prepared by solid-state reaction methodology. This work showed that CuO and LBSCA were effective sintering aid, which improved the densification and decreased sintering temperature. Thus, the excellent microwave dielectric properties of BCTN ceramics (x = 0.08) were obtained after sintering at 925 ℃ with ε_r ~ 44.18, Q×f ~ 17,860 GHz (@ 5.6 GHz) and τ_f ~ 94.76 ppm/℃. Q×f value was increased nearly 3-fold compared to pure BTN ceramics (~ 6090 GHz). Based on the P-V-L bond theory, the Ti-O and Nb-O bonds together contributed greatly to ε_r. The Nb-O bonds was the main factor affecting the internal loss on Q×f. The τ_f closely related to the oxygen octahedron [Ti1/Nb1O₆]. The BCTN ceramics would not react with Ag electrodes, and had great potential to be used in LTCC microwave devices.     

CaAlSiN3:Eu/glass composite film in reflective configuration: A thermally robust and efficient red-emitting color converter with high saturation threshold for high-power high color rendering laser lighting

To achieve high color rendering and proper color temperature, a red color converter is essential for phosphor-converted white lighting devices. CaAlSiN₃:Eu²⁺ (CASN) is a highly suitable red phosphor for white light-emitting diodes. However, it can be hardly used in high-power laser lighting due to poor thermal/chemical performance of the phosphor/silicone resin mixture. A series of all-inorganic CASN-based phosphors (e.g., composite ceramic and phosphor-in-glass) were developed to avoid the use of resin. However, new challenges emerged: none of them showed sufficient luminous efficacy (i.e., >50 lm/W) and adequate saturation-threshold (i.e., >30 W or 10 W/mm²). Here, we report a facile fabrication of CASN/glass composite films using a simple and efficient blade-coating method. Upon 450 nm excitation, the resultant composite film presents a high internal quantum efficiency of ~83%, comparable to that of pristine CASN powder (~90%). When irradiated with a blue laser, the composite film shows a record high luminous efficacy of 82 lm/W. Furthermore, its saturation threshold was investigated in high power and high power density mode, respectively. When measured in high power mode, it shows a high saturation threshold over 29.7 W (1.75 W/mm²), thus achieving a high luminous flux of 1576 lm; when measured in high power density mode, it shows a saturation threshold of ~10.2 W/mm² (1.13 W). With abovementioned excellent properties, the CASN/glass composite film has great potential for use in high-power and high color rendering laser lighting.     

Synthesis and strengthening mechanism for (Ti,Nb)3SiC2/Al2O3 ceramics: A combined experimental and first-principles investigation

(Ti,Nb)₃SiC₂/60 vol%-Al₂O₃ composite ceramic samples with different Ti/Nb atomic ratios were prepared by hot-pressing sintering using Ti/Si/Al/TiC/NbC/Al₂O₃ powders as starting materials. X-ray diffraction analysis showed that the solid solubility of Nb is approximately 7.5 at%. Excessive NbC powder addition can lead to the generation of an impurity phase. Scanning transmission electron microscopy and X-ray photoelectron spectroscopy images reveal that most of the Nb-doped atoms aggregate at 2a-sites, as this configuration exhibits the lowest system energy. The optimized hardness, flexural strength and fracture toughness of (Ti,Nb)₃SiC₂/Al₂O₃ were determined to be 16.0 GPa, 672 MPa and 6.9 MPa m^1/2, which corresponds to an increase of 15.9%, 18.7% and 25.4% respectively, compared with Ti₃SiC₂/Al₂O₃ composite ceramics. By means of first-principle calculations, the strengthening mechanism is derived from both intragranular improvement in (Ti,Nb)₃SiC₂ solid solutions and intergranular enhancement of (Ti,Nb)₃SiC₂/Al₂O₃ grains interfaces.     

Biodegradable magnesium alloy coated with TiO2/MgO two-layer composite via magnetic sputtering for orthopedic applications: A study on the surface characterization,corrosion, and biocompatibility

In this study, a coating of thin TiO₂ layer and a TiO₂/MgO double layer were created on the surface of AZ91D alloy by magnetic sputtering method in order to improve the corrosion and biocompatibility properties of this alloy. The microstructural studies by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) showed that coatings were formed continuously and homogeneously on the alloy surface. In the double-layer coating, MgTiO₃ and Mg₂TiO₄ compounds were formed at the coating/substrate interface in addition to TiO₂ and MgO phases as the main phases in the coating structure. The results of corrosion test showed that in general, coating improves the corrosion of AZ91D alloy in simulated-body fluid (SBF). The double-layer coating showed the best corrosion resistance at a corrosion current of 5.743 × 10^?7 μA/cm² and a corrosion potential of ?1.513 V due to its cathodic protection of the substrate and blockage of the path of the corrosive solution towards the substrate. In vitro tests showed that considering the good match between the used materials as the coating and body, no toxic material exits which results in improvement in biocompatibility, adhesion, and bone-cell multiplication.