Preparation And Luminescence Properties Of Ce/Mn Doped Garnet-based Ceramic Phosphors

Solid state lighting is recognized as a new type of green lighting source in the 21st century,it has the advantages of high efficiency,low energy consumption,good stability,long lifetime,non-toxic and environmental protection.Solid-state lighting includes white-light LEDs and laser lighting technology.They respectively use LED and LD chips to excite luminescent materials to obtain white light.The performance of luminescent materials determines the quality of white light to a great degree,thus more and more attentions have been paid to the research and development of luminescent materials.Nowadays,the high-power and high-brightness lighting sources are in urgent demand,which makes the luminescent materials facing sever challenges in heat dissipation.The traditional means of"phosphor+organic resin"which has low thermal-resistance is thus no longer applicable.Among several available forms of luminescent materials,ceramic phosphors have become a hot topic because of their higher thermal conductivity and free-encapsulation.However,compared with powder phosphors,the researches and applications of ceramic phosphors are still in the initial stage.There are some problems that need to be solved.First of all,for the common Ce³⁺doped garnet ceramic phosphors,their luminescence efficiency lags behind that of the products from Osram and Philips,which need to be further improved.Secondly,red-emitting ceramic phosphors,which can significantly improve the color rendering index of white light,are in the shortage and need to be developed.Finally,most of the ceramic phosphors reported in current researches match with blue LED/LD chips,the ceramic phosphors made for ultraviolet LED/LD chips are rarely seen.In order to solve these problems,the present research works on the preparation and luminescent properties of ceramic phosphors,using the classic garnets as host,and rare-earth ion of Ce³⁺and transition metal ion of Mn²⁺/Mn⁴⁺as luminescence centers.The main contents are as follows:?1?The structure designs and luminescence efficiency improvement of Lu₃Al₅O₁₂:Ce³⁺(Lu AG:Ce³⁺)ceramic phosphor.Three experimental designs were carried out:pores were introduced as scattering centers in the ceramics by controlling the sintering temperature,second phase of Al₂O₃ was added as scattering centers,and the micro-porous array was constructed on the surface of the ceramic via laser etching.As the pores were used as the scattering centers,the ceramic with a porosity of 2.88%reached the highest emission intensity.Adding 10 wt%of Al₂O₃ can significantly improve the emission intensity of the composite ceramic.On the other hand,the thermal conductivity of the composite ceramics increased with increasing the Al₂O₃ contents.As the addition amount of Al₂O₃ was increased to 60 wt%,the grains of Al₂O₃ were connected,the thermal conductivity of the sample was greatly improved to be 17.6 Wm^-1K^-1,however,the emission intensity reduced due to the decrease of luminescence component of Lu AG:Ce³⁺.Therefore,the luminescence efficiency and thermal conductivity of ceramics should be weighed in the selection of Al₂O₃ content.Micropore array was constructed on the surface of the ceramics without destroying the inner structure,with the decrease of the hole spacing,the emission intensity of the ceramic sample increased,the arrangement ways of the micropore array have little impact on the enhancement effects of emission intensity.Compared with the three methods,the order the emission intensity enhancement effect is‘pores>Al₂O₃ second phase>micropore array'.When excited by 450 nm laser,the luminous flux of the as-prepared Lu AG:Ce³⁺ceramic phosphor linearly increased with the the increase of incident powers,and the luminous efficiency exceeded 200 lm/W.In the laser activated remote phosphor test,the static and dynamic conversion efficiencies of the ceramic phosphor were 44.4%and 50.2%,respectively,which are equilent to that of the prouct from Philips.?2?Lu AG:Mn⁴⁺red-emitting ceramic phosphors were prepared via reactive sintering in air atmosphere,the crystal structure and luminescent properties were investigated in detail.Lu AG:Mn⁴⁺ceramic phosphors with a density of 99.6%were prepared via directly reactive sintering at 1650?in air atmosphere,using Mg O as sintering aids.Mg²⁺replaced octahedral Al³⁺site,thus Mn⁴⁺-Mg²⁺replacing Al³⁺-Al³⁺reached the charge balance.In addition,Mg²⁺with larger ion radius leaded to the increase of the average bond length of Al-O bond,which changed the crystal field environment of Mn⁴⁺and had a greater impact on the photoluminescence spectra.When Mg²⁺doping content was above 4 at%,Lu AG:Mn⁴⁺ceramic phosphors exhibited strong excitation peaks in both ultraviolet and blue light regions,and gave bright red emission located at 668 nm.The optimal doping amount of Mn⁴⁺was 0.3 at%,and it had a high quantum efficiency of 47.8%under blue light excitation.?3?Transparent Lu AG:Mn⁴⁺red-emitting ceramic phosphors were prepared through vacuum sintering followed by annealing oxidation,the valence states of Mn ions and the optical properties of ceramics were elaborately studied.Mg source was still used as sintering aids and valence states compensator.The Mn ions in the vacuum-sintered sample presented as Mn²⁺,the common annealing treatment in air atmosphere can only convert part of Mn²⁺to Mn⁴⁺.The strong oxidation atmosphere of O₂ was more favorable for converting Mn²⁺to Mn⁴⁺,and the conversion degree of Mn²⁺to Mn⁴⁺increased with the increase of oxidation temperature.Only after oxidizing at 1600?in the O₂ atmosphere,Mn²⁺was almost completely converted to Mn⁴⁺.The transmittance of the as-prepared ceramics increased with the increase of Mn doping contents.When Mn doping content was 3 at%,the transmittance reached the highest of?81%.After high-temperature vacuum sintering,a large number of Mn were evaporated in the ceramics,it's beneficial for the migration and discharge of the pores in the ceramics,so as to promote the densification process.Finally,the thermal stability and thermal quenching mechanism of Lu AG:Mn⁴⁺red ceramic phosphors were analyzed in detail.?4?Ca₂Lu Hf₂Al₃O₁₂:Ce³⁺(CLHA:Ce³⁺)and Ca₂Lu Hf₂Al₃O₁₂:Ce³⁺,Mn²⁺(CLHA:Ce³⁺,Mn²⁺)near-ultraviolet?n-UV?light excitation ceramic phosphors were prepared via a solid-state reactive sintering method under vacuum.The phase formation process,microstructure and luminescence properties were elaborately investigated.It's found that the phase forming process of CLHA ceramics was more complicated than that of Lu AG.In the sintering process,multiple intermediate phases were generated and then transformed into CLHA.The finally prepared densified CLHA ceramics had good crystallinity,no pores were existed,but it contained a small amount of second phase,thus it appeared to be trasluscent.Both CLHA:Ce³⁺and CLHA:Ce³⁺,Mn²⁺ceramic phosphors showed two broad excitation bands peaked at 330 nm and 400 nm,which perfectly matched with the?400 nm near-ultraviolet chip.The emission band of CLHA:Ce³⁺was located at 478 nm?495 nm.The optimal doping concentration of Ce³⁺was 1at%,the highest quantum efficiency was 53.9%.The CLHA:Ce³⁺showed satisfied thermal stability with emission intensity maintained 71%at 450 K compared to that at room temperature.CLHA:Ce³⁺,Mn²⁺ceramic phosphors not only had the cyan emission band of Ce³⁺,but also exhibited orange emission band of Mn²⁺.The highest quantum efficiency of CLHA:Ce³⁺,Mn²⁺was 36.0%.At 450 K,the emission intensity of CLHA:Ce³⁺,Mn²⁺maintained 67%.Both kinds of the CLHA ceramic phosphors can be excited by an n-UV LD,and the luminous flux increased linearly with the increase of incident power.Thus the as-prepared ceramic phosphors are promising to be applied in ultraviolet laser lighting.