硅酸盐学报

Y_3Al_5O₁₂:Ce³⁺(YAG:Ce³⁺)发射谱中红光成分的不足导致了白光LED色温偏高且显色性较差，一种解决方案是引入能发射红光的激活离子与Ce³⁺共掺杂以使YAG:Ce³⁺发射谱红移。本工作报道了Ce³⁺–Eu³⁺共掺Y_3Al_5O₁₂(YAG:Ce³⁺/Eu³⁺)透明陶瓷的制备、结构和荧光性能。该材料体系在Eu含量高达25%时仍保持纯石榴石相结构，且Eu³⁺的荧光猝灭浓度高达9%左右，比Pr³⁺和Sm³⁺等离子更适合用作YAG的红光发射激活剂。YAG:Ce³⁺/Eu³⁺透明陶瓷在蓝光(如442和466 nm)激发下的Ce³⁺荧光发射随着Eu含量的增加而急剧下降，但在紫外光(363 nm)激发下Ce³⁺的荧光衰减在一定程度上可被Eu³⁺发射的增强所弥补，从而可以在保持较强荧光发射的同时使YAG:Ce³⁺的发射光谱逐步红移到橙红光区。YAG:Ce³⁺/Eu³⁺透明陶瓷能够强烈吸收380～405 nm的近紫外光并将之高效转化为红光，且荧光光谱热稳定性优异，适合用作近紫外LED芯片激发的红光发射荧光材料。同时，荧光发射强度及发射谱中的不同发射峰的强度比值均随温度线性变化，在荧光温度计领域具有潜在的应用前景。

Introduction Tremendous efforts are devoted to improving the luminous performance of white light-emitting diodes (WLEDs) as so-called fourth generation lighting source.The most common approach for manufacturing WLEDs is to cover a blue LED chip with a yellow-emitting Y_3Al_5O₁₂:Ce³⁺(YAG:Ce³⁺) phosphor.This construction has an advantage of high lumen efficiency,but an insufficient red light component in the spectrum leads to a low color rendering index and a high color temperature.An effective solution is to enhance the red emission of YAG:Ce³⁺via introducing red-emitting ions such as Pr³⁺,Eu³⁺,Sm³⁺,Mn²⁺,Mn⁴⁺and Cr³⁺.In Y_3Al_5O₁₂ and many other garnet-structured aluminates,Mn²⁺,Mn⁴⁺and Cr³⁺are suffered from serious luminescent thermal quenching,while Pr³⁺and Sm³⁺are severely limited by concentration quenching.In comparison,the luminescence of Eu³⁺in YAG is highly anticipated because it has an ion radius similar to Y³⁺,which is beneficial to increasing doping concentration and reducing luminescence quenching.In this paper,a series of Ce³⁺–Eu³⁺co-doped YAG transparent ceramics with different Eu concentrations were fabricated,and the effect of Eu³⁺-doping on the structure,optical transmittance and photoluminescence properties was also evaluated.Methods Ce_0.01Y_2.99–xEu_xAl_5O₁₂ (03,α-Al_2O₃,Eu_2O₃,and CeO₂ powders were used as starting materials.Oleic acid and tetraethoxysilane were used as a molding agent and a sintering aid,respectively.These materials were weighed in stoichiometric ratios,and ground in ethanol for 20 h.The obtained slurry was dried,ground and sieved through a 100 mesh sieve.The resulting powders were made into 20 mm discs under uniaxial pressure of 5–10 MPa,annealed in oxygen atmosphere at 800℃for10 h,and cold isostatically pressed under 200 MPa.Finally,the ceramics was mechanically thinned and polished to the thickness of1mm for the subsequent structural and performance characterizations.Results and discussion For YAG:Ce³⁺/Eu³⁺ceramics sintered at 1 600℃,a garnet phase appears when Eu content is in the range of 0–25%.However,for the ceramics sintered at 1700℃,the grains grow as Eu content (x) increases,and eventually become over-sintered and textured when x≥15%.YAG:Ce³⁺/Eu³⁺ceramics show some characteristic absorption/emission bands for both Ce³⁺and Eu³⁺.For Ce³⁺,the PLE spectrum involves two broad bands at 320–370 nm and 400–520 nm,and the PL spectrum spans from 440 nm to 650 nm and is dominated by yellow-green emission.Unlike a broad band absorption/emission of Ce³⁺,the PLE/PL spectra of Eu³⁺are composed of a series of lines originating form f–f transitions.There are some intense excitation lines around the maximum absorption locating at 395 nm,constructing a quasi-continuous broad band.It is therefore inferred that YAG:Ce³⁺/Eu³⁺ceramics can be efficiently pumped by NUV LED chips.The PL spectrum of Eu³⁺ions covers the spectral range from orange to deep red.The orange emission from ~5D₀→~7F₁ transition is more intense than the red emission from ~5D₀→~7F₂ transition because Eu³⁺ions occupy the lattice sites with an inversion symmetry.The PL spectra of YAG:Ce³⁺/Eu³⁺ceramics vary with excitation wavelength due to the difference in absorption spectra of Ce³⁺and Eu³⁺.Therefore,the effect of excitation wavelength on the PL spectra of YAG:Ce³⁺/Eu³⁺ceramics with various Eu³⁺concentrations were investigated.Under the excitation at 363 nm and 466 nm,the emission bands/peaks of both Ce³⁺and Eu³⁺appear.The PL spectrum excited at 442 nm contains almost only a broadband emission of Ce³⁺ions,as Eu³⁺ions hardly absorb photons near this wavelength.In contrast,the PL spectrum under the excitation at 395 nm is dominated by Eu³⁺emission due to a weak absorption of Ce³⁺.Clearly,the PL spectrum and color coordinates of YAG:Ce³⁺/Eu³⁺exhibit an excitation wavelength dependence,which has a potential application.In addition,under excitation at 442 or 466 nm,the emission of Eu³⁺is weak,while that of Ce³⁺rapidly decays with increasing Eu content.As a result,the emission intensity of YAG:Ce³⁺/Eu³⁺ceramics decreases rapidly with the increase of Eu content when excited by blue light.However,when excited by UV light (at 363 nm),the attenuation of Ce³⁺emission is compensated by the enhancement of Eu³⁺emission to some extent,and the overall photoluminescence intensity can be maintained at a high level.Especially,the samples with Eu contents of 1%–3%can emit an intense orange-yellow light,thus increasing the red component of YAG:Ce³⁺effectively.This kind of phosphor has a certain practicability in WLEDs.Also,under the excitation at 395 nm,the PL spectrum originates mainly from the f–f transitions of Eu³⁺,and the quenching concentration of Eu³⁺in YAG:Ce³⁺ceramics is 9%,which is much higher than that of Pr³⁺(i.e.,0.8%) and Sm³⁺(i.e.,3%).For rare-earth ions with forbidden f–f transition,a high quenching concentration is crucial for achieving a high fluorescence emission intensity and meeting the inevitable requirements of LED applications.Thermal quenching is a key factor in evaluating luminescent materials.For the YAG:Ce³⁺/Eu³⁺ceramics with Eu contents of 1%and 9%,the profile of PL spectrum does not change with temperature,leading to a good temperature stability of color coordinates.The relative PL intensity at 575 K remains 83%and 61%of that at room temperature (i.e.,300 K) for 1%and 9%Eu³⁺-doped samples,respectively,indicating that the photoluminescence thermal quenching in YAG:Ce³⁺/Eu³⁺ceramics is rather weak.Furthermore,the relative PL intensity (I_T/I₀) and the ratio of emission arising from ~5D₀ to different ~7F_J levels,including(~5D₀→~7F₁)/(~5D₀→~7F₂)、(~5D₀→~7F₄)/(~5D₀→~7F₂) and (~5D₀→~7F_1,2)/(~5D₀→~7F₄),vary linearly with temperature.This feature can be used in the field of fluorescent thermometers,which is one of the potential applications of YAG:Ce³⁺/Eu³⁺transparent ceramics.Conclusions In YAG:Ce³⁺/Eu³⁺transparent ceramics,Eu³⁺could be excited by UV (at 363 nm),NUV (at 380–405 nm),and blue(at 466 nm) light,emitting a series of lines in the spectra range from 580 nm to 750 nm,i.e.,from orange to deep red.These PL lines had an important application in lighting and display.Eu³⁺showed the maximum emission when its concentration was between 5%and 9%.The high quenching concentration had a advantage in enhancing the red component in the emission spectrum of YAG:Ce³⁺.Furthermore,the red component in the spectrum could be regulated via adjusting the doping concentration of Eu³⁺.However,the increase in Eu content led to a decrease in Ce³⁺emission intensity.The similar phenomena appeared in various dual/multi activators co-doped systems.The solution of this common problem was crucial for the application of this kind of luminescent materials in WLEDs.A highlight of YAG:Ce³⁺/Eu³⁺transparent ceramics was that they could absorb NUV light from 380 nm to 405 nm and emit an orange-to-red light with a high quantum efficiency and a high thermal stability.Therefore,they could be used as red-emitting phosphors excited by NUV LED chips.Besides,the ratios of emission arising from ~5D₀ to different ~7F_J levels varied linearly with temperature,having a promising potential application in the field of fluorescent thermometers.