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SiBCN ceramics have a wide range of applications in the field of high temperature electromagnetic wave (EMW) absorption owing to their excellent EMW attenuation and oxidation resistance. However, excessive SiC and insufficient Si3N4 in the SiBCN ceramics deteriorate the impedance matching characteristics, and weaken the absorption ability for EMW. In this paper, the amount of SiC and Si3N4 is balanced by a competitive reaction strategy, in which HfZr atoms are introduced to modulate the dielectric phases in the SiBCN ceramics. The formation of a HfZrCN compound produces a cage-like structure around Si3N4, inhibiting the reaction of Si3N4 with carbon to form SiC. The appropriate amounts of SiC and Si3N4, and the abundant interfaces and defects jointly enhance the EMW absorption capability by simultaneously improving the impedance matching and attenuation capability. Thus, an effective absorption band of 4.96 GHz (11.20–16.16 GHz) is reached with a thickness of 1.7 mm for the HfZr-SiBCN ceramics. This work provides a new strategy for simultaneously improving the impedance matching and EMW attenuation capability of polymer-derived ceramics by introducing difficult-to-migrate atoms to regulate the content of the dielectric phase. [ABSTRACT FROM AUTHOR]

The appearance of a cyan cavity is unavoidable in the 480–520 nm wavebands when combining n-UV LED chips (380–420 nm) with trichromatic phosphors to obtain white-light-emitting devices. Here, a concentration perturbation effect is proposed to improve the properties of phosphors via trace substitution of Mn2+, Mg2+ or Zn2+ cations for Ba2+ and Y3+ in Ba0.97Y2Si3O10:Eu. Based on local structure distortion or electron cloud interaction, the emission band corresponding to Eu2+ is broadened with a full width at half maximum (FWHM) more than 125 nm due to redistribution of Eu2+ and the emission color can be tuned to the cyan region. The coexistence of the above two effects on the photoluminescence can be further evidenced by the shifts of emission energy for the two sites in these systems. The quenching temperature for all the samples is observed over 473 K and even higher than that for the non-substituted Ba0.97Y2Si3O10:0.03Eu and it is discussed combined with Pauling's principles, average Uiso and the distortion degree of polyhedra, which could provide some references for improvement of thermal stability. Moreover, the valence mismatch between Mn2+/Mg2+/Zn2+ and Y3+ ions leads to the coexistence of Eu2+–Eu3+, which thus realizes color selective manipulation and even white light emission in Ba0.98−nMg0.02Y2Si3O10:nEu. Our results demonstrate that the concentration perturbation effect-assisted synthesis strategy has potential in improving the properties of phosphors for WLEDs, and it can also be extended to adjusting the coordination environment of activators whose properties are sensitive to local lattice variations and electronic surroundings. [ABSTRACT FROM AUTHOR]

A series of Eu2+ and Tb3+ singly-doped and co-doped Sr3Y2(Si3O9)2 (SYSO) phosphors have been synthesized via a conventional high-temperature solid-state reaction. The crystal structures, photoluminescence properties, fluorescence lifetimes, thermal properties and energy transfer of SYSO:Eu2+,Tb3+ were systematically investigated in detail. Rietveld structure refinement of the obtained phosphors indicated that the SYSO host crystallized in a monoclinic system with the space group C2/c (15) and there are three kinds of cation sites for the doped ions to occupy forming emission centers. The photoluminescence (PL) emission bands of SYSO:xEu2+ show a red-shift tendency with increasing Eu2+ content which should be attributed to more Eu2+ ions entering into Sr2/Y2 and Sr3/Y3 sites from the Sr1/Y1 site. For the co-doped SYSO:Eu2+,Tb3+ samples, tunable colors from cyan to green can be realized by varying the doping concentration of the Tb3+ ions. The intense green emission was realized in the SYSO:Eu2+,Tb3+ phosphors on the basis of the highly efficient energy transfer from Eu2+ to Tb3+ with an efficiency of over 89%. As a result, the emission intensity of SYSO:0.01Eu2+,0.21Tb3+ is about 2.5 times higher than that of SYSO:0.21Tb3+ under 340 nm UV excitation. The energy transfer mechanism from Eu2+ to Tb3+ in the SYSO host was ascribed to the quadrupole–quadrupole interactions. These results indicated that the SYSO:Eu2+,Tb3+ phosphors can act as single-phase green emitting phosphors for possible applications in ultraviolet light-based white light-emitting diodes (w-LEDs). [ABSTRACT FROM AUTHOR]