Preparation And Properties Of Hydroxyapatite-based Composite Luminescent Ceramics

As a new generation of lighting technology,high-power light-emitting diodes（LEDs）have attracted extensive attention of researchers in the field of high-brightness lighting applications in recent years.Due to their excellent thermal stability,suitable scattering centers and machinability,luminescent ceramics have been widely studied as ideal light converters for high-power LEDs.At present,YAG:Ce ceramics are the main stream luminescent ceramics to be applied as light converters,while the lack of red emitting components leads to poor optical performance of white LEDs（WLEDs）assembled from them.Among rare earth elements,Eu³⁺presents a strong red band emission,which has been introduced into ceramics to be expected to solve the problems as low color rendering index caused by the lack of red light.However,the traditional luminescent composite ceramics with multi-component have problems such as low transmittance and serious re-absorption,so that the improvement of the color rendering index of the assembled LED often sacrifices the luminous efficiency.Meanwhile,the traditional solid-phase sintering preparation process of luminescent ceramics often requires a long period of high temperature and high pressure,and the preparation conditions are relatively harsh,which can easily cause thermal quenching of the luminescent components and decrease the luminescent properties.In this work,using mesoporous hydroxyapatite（HAP）powders as starting materials,Eu³⁺doped red light-emitting luminescent ceramics were rapidly prepared via spark plasma sintering（SPS）technology at low temperature,and the yellow phosphor（YAG:Ce）was further dispersed to construct single＆double-layer multi-component composite luminescent ceramics.Moreover,the as-prepared luminescent ceramics were assembled with a blue chip to obtain the LED devices,and a significant improvement was achieved in the color rendering index on the premise of ensuring the luminous efficiency.The main research contents are as follows:（1）Preparation and properties of HAP:Eu transparent luminescent ceramic:HAP:Eu mesoporous powders were prepared by a combination of co-precipitation method and hydrothermal method.Then,HAP:Eu luminescent ceramics with high transmittance were prepared quickly at low temperature by SPS technology.The phase structure and optical properties of HAP:Eu mesoporous powders and corresponding ceramics were studied,and the effects of ion doping on the phase structure and optical properties were also explored.The study found that the optimal excitation wavelength was 459 nm,and the fluorescent ceramics exhibited multiple characteristic luminescence peaks of rare earth ions Eu³⁺in the range of 560-660 nm under the excitation of this wavelength.What’s more,the luminescence intensity initially increases with the increase of Eu³⁺content,and reaches the maximum when the doping amount is 15 mol%,then decreases with the increase of doping concentration due to concentration quenching.The PL strength of HAP:Eu ceramics remained the same after being tested in a high temperature environment of 150°C for 480mins,and the samples could maintain 100%of the PL strength when stored under high temperature and high humidity（85°C/RH 85%）for 250 h.It shows that the obtained HAP:Eu ceramics possess excellent light stability and heat resistance.（2）Preparation and properties of HAP-based luminescent composite ceramics with multi-components:Using HAP:Eu powder and commercial YAG:Ce yellow phosphor as raw materials,HAP-based single-layered multi-components luminescent ceramics containing 3 wt%YAG:Ce and10 mol%Eu³⁺ions were prepared by SPS technology.Moreover,using HAP:Eu powders and the mixture of pure HAP with YAG:Ce powders as raw materials,the phase composition,microstructure and optical properties of the double-layered composite luminescent ceramics with multi-components prepared by the same process were studied.The results show that the adopted rapid preparation technology at low-temperature can effectively preserve the luminescence properties of phosphors and Eu³⁺ions.Under the excitation at 350 nm and 459 nm,the characteristic emission peaks of Ce³⁺ions appeared at～540 nm and several characteristic emission peaks of Eu³⁺appeared in the range of 359-550 nm,respectively,reflecting that the thermal migration of the luminescent centers Ce³⁺and Eu³⁺could not be significantly detected.In addition,the construction of the double-layered structure with the same substrate can not only avoid cracks caused by different thermal expansion coefficients of different substrates,but also can effectively suppress re-absorption,promoting the high luminescent performance.Similarly,the PL intensity of the double-layered composite fluorescent ceramics with multi-components remained the same after being placed at a high temperature of 150°C for 480 min or at a high temperature and high humidity（85°C/RH 85%）for 250 h,indication the excellent light stability and heat resistance.（3）Assembly and properties of WLEDs:The prepared double-layered composite luminescent ceramic with multi-components was assembled with a 460 nm blue chip to obtain a WLED device.By adjusting the thickness of the luminescent ceramic layers,the related optoelectronic properties of the WLEDs were adjusted,and the WLED device with best performance was obtained.The study shows that WLEDs perform best when thickness of both layers are 0.85 mm,whose CRI could be as high as 81 and the LE is 85.4 lm/W.At the same time,the comprehensive performance of the warm white LED device is among the best of WLED devices assembled with luminescent ceramics as light converters that have been reported.