Preparation and optical properties of MgAl2O4-Ce:GdYAG composite ceramic phosphors for white LEDs

High-power light-emitting-diode (LED) lighting has attracted great attention in high-luminance applications. However, the color converters with high stability, high thermal conductivity and high efficiency are scarce. In this work, we developed promising MgAl₂O₄-Ce:GdYAG composite ceramic color converters in which a broadband yellow-emitting phosphor Ce:GdYAG is combined with a thermally stable spinel phase MgAl₂O₄. The microstructure, phase composition, photoluminescence and thermal stability were investigated in detail. After being packaged with blue LED, a high luminous efficacy value of 108.4 lm/W is achieved by the composite ceramic phosphor with the molar ratio (MgAl₂O₄ to Ce:GdYAG) of 1.0 and the thickness of 0.2 mm, as well as high CRI value of 70 and low CCT value of 4543. The light scattering nature of composite ceramic phosphors makes them an ideal and promising candidate for high-brightness white LEDs.     

Highly transparent yttrium titanate (Y2Ti2O7) ceramics from co-precipitated powders

Highly transparent yttrium titanate (Y₂Ti₂O₇) ceramics were fabricated by vacuum sintering using co-precipitated powders for the first time. The effects of the powder calcination temperature on the phase composition, morphology of the calcined powders, and on the microstructure and transmittance of the Y₂Ti₂O₇ ceramics were investigated. When the calcination temperature was above 850 °C, pure phase Y₂Ti₂O₇ nanopowders with high sintering activity were obtained. Transparent Y₂Ti₂O₇ ceramics were obtained after vacuum sintered at 1600 °C for 6 h and annealed at 1100 °C for 5 h in air. The highest transmittance reached 73% at 1000 nm when the calcination temperature was 1150 °C. The measured refractive index of Y₂Ti₂O₇ ceramics was higher than 2.24 at the wavelength range of 350–1000 nm, making it a promising candidate for optical devices.     

Gd3Al3Ga2O12:Ce,Mg2+ transparent ceramic phosphors for high-power white LEDs/LDs

Gd₃Al₃Ga₂O₁₂:1.5%Ce, xMg²⁺ (GAGG:1.5%Ce, xMg²⁺) transparent ceramic phosphors (TCPs) were prepared via a two-step sintering method. The effects of MgO on microstructures and luminescent properties of GAGG:Ce TCPs are investigated for the first time. For the optimized Mg²⁺ of x = 0.5%, the in-line transmittance of the obtained TCP reaches 78.6%. Performances of the titled TCPs in high-power light-emitting diodes (LEDs) and laser diodes (LDs) lighting are illustrated. The optimized TCP shows the luminous efficacy of 84.0 lm W^?1 in LD lighting. This work provides a strategy to modify TCPs for the next-generation LD lighting.     

Enhanced piezoelectric properties and temperature stability of Bi4Ti3O12-based Aurivillius ceramics via W/Nb substitution

Bi₄Ti₃O₁₂ (BIT), a typical Aurivillius ceramics with high Curie temperature (T_c ? 675 °C), has great potential for high temperature applications. This work provides an effective method of inducing structure distortion, relieving the tetragonal strain of the TiO₆ octahedron and decreasing the concentration of oxygen vacancies to improve the piezoelectricity and temperature stability of BIT ceramics. Bi₄Ti_2.98W_0.01Nb_0.01O₁₂ possesses an optimum piezoelectric coefficient (d₃₃) of 32 pC/N, a high T_c of 655 °C and a large resistivity of 3 × 10⁶ Ω·cm at 500 °C. The maximum d₃₃ reported here is approximately quadruple than that of pure BIT (?7 pC/N). Moreover, the d₃₃ of W/Nb co-doped BIT and the in-situ temperature stability of the compression-mode sensor present a highly stable characteristic in the range of 25–600 °C. These results imply that W/Nb-modified BIT ceramics is a promising candidate for application at high temperatures of up to 600 °C.     

Layered array Al2O3-LuAG: Ce composite ceramic phosphors for high-brightness display

Thermally robust and efficient composite ceramic phosphors (CCPs) combined both the merits of matrix and phosphor have received growing interests. However, high matrix content (e.g., Al₂O₃ >40 wt%) brings diluted activated ion concentration and dropped photoluminescence (PL) quantum yield (QY). Here, a novel layered array Al₂O₃-LuAG: Ce CCP, where Al₂O₃ and LuAG: Ce thin layers (10–250 µm) are alternately arranged, was presented. Owing to the special structure, thermal phonons and photons are respectively routed into Al₂O₃ layers and LuAG: Ce layers, which weakens the influence of Al₂O₃ and heat accumulation on PL properties. Consequently, it exhibits high PLQY (84.1%) and good thermal conductivity (17.1 W·m^?1·K^?1). When it is irradiated under high-power density (27.2 W·mm^?2) blue laser, the luminous efficiency and lumen-density are promoted to 220 lm·W^?1 and 5994 lm·mm^?2, respectively. This work provides a promising new microstructure in developing novel phosphor converters for high-brightness laser phosphor display.     

The effect of A-site nonstoichiometry on the microstructure,electric properties,and phase stability of NaNbO3 polycrystalline ceramics

The impact of A-site nonstoichiometry on the microstructure, electric properties, and phase stability of sodium niobate ceramics (Na_1+xNbO₃, x = ?2 to 1 mol %) was investigated. All the components maintained an orthorhombic antiferroelectric (AFE) structure. The grain size increased from 3.9 to 14.3 μm with the variation in x from ?2 to 1. The AFE–FE phase transition electric field dramatically increased from 100 kV cm^?1 at x = 0 to 170 kV cm^?1 at x = ?2, confirming the enlarged energy barrier between AFE Pbma and FE Pmc2₁ phase under external field in A-site deficient components. This is attributed to the lattice compressive stress generated by introducing proper A-site vacancies. Combined results of transmission electron microscopy and Raman spectroscopy indicated that the AFE distortion of Pbma phase was significantly enhanced in A-site deficient components, which jointly contributed to the stability of AFE phase in A-site deficient NaNbO₃ material.     

Influence of LiBO2 addition on the microstructure and lithium-ion conductivity of Li1+xAlxTi2−x(PO4)3(x = 0.3) ceramic electrolyte

Concerning the safety problems of conventional Li-ion batteries with liquid electrolytes, it is crucial to develop reliable solid-state electrolytes with high ionic conductivity. Li_1+xAl_xTi_2?x(PO₄)₃ (LATP, x = 0.3) is regarded as one of the most promising solid electrolytes due to its high ionic conductivity and excellent chemical stability to humidity.Herein, a new strategy is proposed for improving the sintering behavior and enhancing the ionic conductivity of LATP by using LiBO₂ as the sintering aid via liquid phase sintering. The as-prepared sample LATP with homogeneous microstructure and high relative density of 97.1% was successfully synthesized, yielding high total ionic conductivity of 3.5 × 10^?4 S cm^?1 and low activation energy of 0.39 eV at room temperature. It was found that the addition of LiBO₂ could effectively enhance the densification and increase the ionic conductivity of LATP electrolyte, proving an effective way to synthesis LATP ceramics by a simple and reliable route.     

Effects of deformation rate on properties of Nd,Y-codoped CaF2 transparent ceramics

Different deformation rates of Nd,Y-codoped CaF₂ transparent ceramics were prepared by ceramization of single crystals. The deformation rate effects on the crystallization behaviors, microstructures, mechanical properties, and optical performances were investigated for the first time. The results indicate that the comprehensive performances of Nd,Y-codoped CaF₂ ceramic (△a?=?62%) are the most optimal compared with other ceramics having different deformation rates (△a?=?34%, 40%, 50%, and 75%). In further investigations of the optical properties, the Nd,Y-codoped CaF₂ ceramic (△a?=?62%) sample exhibited a high transparency (T_a?>?91%, 3-mm thick,250?～?1200?nm), low light scattering, superior fracture toughness (K_1c?～?0.71?MPa·m^1/2), strong fluorescence emission, long lifetime (τ?=?348.72?μs), and broad FWHM (29.2?nm), promising a good candidate for high-power laser material.     

Graphene nanoplatelets reinforced AgCuTi composite filler for brazing SiC ceramic

Graphene nanoplatelets (GNPs) were used as reinforcement in AgCuTi filler for brazing SiC ceramic. Ti from the filler reacted with SiC ceramic to form TiC and Ti₅Si₃ adjacent to the SiC ceramic. According to the TEM and HRTEM results, TiC layer exhibited good lattice matching with SiC substrate. TiC particles synthesized by the reaction between Ti and GNPs in situ promoted the heterogeneous nucleation of TiCu and Cu(s,s), and contributed to the refinement of microstructure. Shear tests results indicated that the adoption of GNPs affected the joint property significantly. The TiC particles and an appropriate TiC + Ti₅Si₃ layer thickness both relieved the residual stress of the brazed joint and thereby increased the joint strength. The shear strength of the joint reached the maximum value of 38 MPa when using AgCuTi/GNPs (GNPs reinforced AgCuTi) composite filler containing 1% GNPs, which was ～139% higher than that of the joint brazed with AgCuTi filler.     

Elimination mechanism of micropores on bonding interface during solid-state crystal growth method

A pure yttrium aluminum garnet (YAG) ceramic/YAG single crystal composite material is prepared using the optimized solid-state crystal growth (SSCG) method. The micropores on the bonding interface of the composite sample are eliminated for the first time during the SSCG process and the transmittance is very close to the theoretical value, which reached 83.14% at 1064 nm. Meanwhile, the mechanism of elimination and migration of the pores under high temperature is studied. Additionally, the single crystal growth rate has significantly improved and the time of composite sample preparation has also significantly reduced proving that the SSCG method is an effective method for producing high quality composite material.     

New insights into linear electrical properties of pressureless sintered SiC-MoSi2-AlN composites

Highly conductive SiC-MoSi₂-AlN composites were fabricated by β-SiC, AlN and MoSi₂ powders with Y₂O₃ additive via pressureless sintering. The effect of MoSi₂ content on the microstructure, mechanical and electrical properties of SiC-MoSi₂-AlN composites was systematically investigated. A finer microstructure was obtained and electrical conductivity was enhanced with increasing MoSi₂ content. The impedance spectroscopy and potential-current measurements were implemented to figure out the electrical conduction mechanism. The introduction of MoSi₂ effectively reduced the Schottky barrier height at the grain boundary, and subsequently the U-I curves changed from nonlinear to linear electrical characteristics. The notable decrease in electrical resistivity was owing to the breakdown of grain boundaries and the formation of percolation paths. The percolation threshold was in the range of 0–5.44 vol% MoSi₂, much lower than the reference value. The composites with 10 wt% MoSi₂ exhibited an electrical resistivity of about 60 Ω cm, suitable for infrared source element applications.     

Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y₂O₃-5Al₂O₃ (mole ratio of 3:5) and 3Y₂O₃-5Al₂O₃-4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c-axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y₂O₃-5Al₂O₃-4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y₂O₃-5Al₂O₃ is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties.     

Structural,infrared reflectivity spectra and microwave dielectric properties of the Li7Ti3O9F ceramic

A cubic rock salt structured ceramic, Li₇Ti₃O₉F, was fabricated via the conventional solid-state reaction route. The synthesis conditions, sintering characteristics, and microwave dielectric properties of Li₇Ti₃O₉F ceramics were investigated by X-ray diffraction (XRD), thermal dilatometer, Scanning Electron Microscopy (SEM) accompanied with EDS mapping, and microwave resonant measurements. Rietveld refinement, selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) confirmed that Li₇Ti₃O₉F adopts a cubic rock-salt structure. The ceramic sintered at 950?°C presented the optimal microwave properties of ε_r?=?22.5, Q×f?=?88,200?GHz, and τ_f?=??24.2?ppm/^oC. Moreover, good chemical compatibility with Ag was verified through cofiring at 950?°C for 2?h. These results confirm a large potential for Li₇Ti₃O₉F ceramic to be utilized as substrates in the low temperature cofired ceramic (LTCC) technology. This work provides the possibility to exploit low-temperature-firing ceramics through solid solution between oxides and fluorides.     

A novel ultra-high-temperature oxidation protective MoSi2-TaSi2 ceramic coating for tantalum substrate

To protect refractory metal against oxidation at ultra-high temperatures, a MoSi₂-TaSi₂ ceramic coating was prepared on a pure tantalum (Ta) substrate using a novel three-step process, which included dip-coating with a molybdenum slurry, vacuum sintering, and halide-activated pack cementation (HAPC). The original coating had a MoSi₂-TaSi₂ double-layer structure from the surface to the substrate. After oxidation at 1700°C for 8 h in air, the coating exhibited a complex multi-layer structure composed of SiO₂-Mo₅Si₃-MoSi₂-(Mo,Ta)₅Si₃-TaSi₂-Ta₅Si₃ from the outer layer to the inner layer, due to the high-temperature phase transition and diffusion of Si and O. The coating effectively protected the Ta substrate at 1700°C for 12 h without failure, thereby demonstrating great improvement to its service life in an ultra-high-temperature aerobic environment. The protective effect was attributed to the integrity of the ceramic coating and the formation of a dense, uniform SiO₂ film that effectively lowered the inward oxygen diffusion rate.     

High energy storage performances of Bi1−xSmxFe0.95Sc0.05O3 lead-free ceramics synthesized by rapid hot press sintering

Lead-free Bi_1?xSm_xFe_0.95Sc_0.05O₃ (x = 0.15–0.19) ceramics were fabricated by rapid hot press sintering, and their structure, ferroelectric and energy storage properties were comprehensively investigated. All the samples are in the mixed phases with R3c rhombohedral and Pbnm orthorhombic structures. With increasing x, the ferroelectric polarization decreases gradually, while the polarization loop becomes gradually slimed too. An high recoverable energy density (?2.21 J/cm³) and a large efficiency (?76%) with good thermal stability (20 °C–120 °C) are obtained under electric field (230 kV/cm) for the optimized sample x = 0.17. Moreover, transmission electron microscopy and piezo-response force microscopy measurements reveal that the presence of two-phase coexistence favors the formation of polar nano-regions, leading to the linear-towards polarization behaviors and the enhanced dielectric breakdown field, which is responsible for the superior energy storage performance of Bi_1?xSm_xFe_0.95Sc_0.05O₃ ceramics. These results indicate a significant step to tailor lead-free BiFeO₃-based ceramics towards high dielectric energy storage applications.     

Advances of experimental study on gas production from synthetic hydrate reservoir in China

China has entered the area of new normal economy which requires the harmonious development of energy consumption, environmental protection and economic development. Natural gas hydrate is a potential clean energy with tremendous reserve in China. The successful field test of marine hydrate exploitation in South China Sea created a new record of the longest continuous gas production from natural gas hydrate. However, the corresponding fundamental research is still urgently needed in order to narrow the gap between field test and commercial production. This paper reviewed the latest advances of experimental study on gas production from hydrate reservoir in China. The experimental apparatus for investigating the performance of hydrate dissociation in China has developed from one dimensional to two dimensional and three dimensional. In addition, well configuration developed from one tube to complicated multi-well networks to satisfy the demand of different production models. Besides, diverse testing methods have been established. The reviewed papers preliminary discussed the mechanical properties and the sediment deformation situation during the process of hydrate dissociation. However, most reported articles only consider the physical factor, the coupled mechanism of physical and chemical factor for the mechanical properties of the sediment and the sand production problem should be studied further.     

Precipitating spinel into precursor glass and its assistance in crystallization

The crystallization phenomena of spinel in CaO-MgO-Al₂O₃-SiO₂-Fe₂O₃ glass have received much attention due to the particular role in preparation of glass-ceramic materials, which represent an effective option to manage hazardous waste. In this study, both preliminary spinel and secondary spinel were precipitated in the precursor glass. The formation of these spinel was meticulously assessed by a combination of X-ray diffractometry and scanning electron microscopy. The structure of the microenvironment in the precursor glass was characterized by Raman spectrums. These advanced techniques highlight the potential for one-step crystallization of the glass. The investigation, which focused on one-step crystallization, demonstrated the growth of pyroxene on spinel accompanying a migration of chromium. The results also show the microstructure of the obtained glass-ceramic was very dependent on the heat-treat temperature. This study not only unambiguously reveals the precipitation mechanisms of spinel but also provides more documentation for one-step crystallization in the glass-ceramics field.     

Effects of Sr2+ substitution on the crystal structure,Raman spectra,bond valence and microwave dielectric properties of Ba3-xSrx(VO4)2 solid solutions

The effects of Sr²⁺ substitution for Ba²⁺ on microwave dielectric properties and crystal structure of Ba_3-xSr_x(VO₄)₂ (0 ≤ x ≤ 3, BSVO) solid solution were investigated. Such Sr²⁺ substitution contributes to significant reduction in sintering temperature from 1400 °C to 1150 °C. Both permittivity (∑_r) and quality factor (Q × f) values decreased with increasing x value, which was determined to be related with the descending values of average polarizability and packing fraction, whereas the increase in τ_f value was explained by the decreased average VO bond length, A-site bond valence. BSVO ceramics possessed encouraging dielectric performances with ∑_r = 12.2–15.6 ± 0.1, Q × f = 44,340 - 62,000 ± 800 GHz, and τ_f = 24.5–64.5 ± 0.2 ppm/°C. Low-temperature sintering was manipulated by adding B₂O₃ as sintering additive for the representative Sr₃V₂O₈ (SVO) ceramic and only 1 wt.% B₂O₃ addition successfully contributed to a 21.7% decrease in sintering temperature to 900 °C, showing good chemical compatibility with silver electrodes, which render BSVO series and SVO ceramics potential candidates in multilayer electronic devices fabrication.