Preparation And Optical Properties Of Rare-earth-doped Optical Functional Ceramics

Rare-earth-doped optical functional ceramics have the advantages of stable physical and chemical properties,high thermal conductivity,and super resistance against laser radiation.They are often used in solid-state laser gain media and phosphor conversion materials for laser lighting.2.0 μm laser is widely used in biomedicine,organic material processing,photoelectric countermeasures,pump source for 3～5 μm mid-infrared laser and other fields.Tm:（Lu Sc）₂O₃ transparent ceramics can be pumped by commercial 808 nm semiconductor laser to yield 2.0 μm laser,which has become a research hotspot at home and abroad.Some researchers reported the preparation of Tm:（Lu Sc）₂O₃ transparent ceramics and achieved 2.0 μm laser.However,all these ceramics were prepared by solid state reaction.It is difficult to control the morphology of the powders and the resulting powders have low sintering activity.Large amounts of sintering aids are required to promote the densification of the ceramics,which do harm to the laser performance of the ceramics.Furthermore,this traditional technique generally requires repeated mechanical mixing and grinding,which leads to possible contaminations.Therefore,we prepared the Tm:(Lu_0.8Sc_0.2)₂O₃ ceramics by co-precipitation method.However,it is difficult to obtain the nanopowders with pure phase,spherical size and high dispersity.The effect of the preparation process on the performance of the powder is not clear.The shaping process plays important roles in determining the microstructure of the green body.The green bodies with high-density and uniform microstructure can promote the densification of the ceramic,reduce the sintering temperature of the ceramic,and avoid the abnormal growth of crystal grains.But the strong interaction forces between these nanopowders promote agglomerations,which make it difficult to get green compacts with high microstructural homogeneity by the commonly used dry pressing.Another important application of rare-earth-doped photofunctional ceramics is phosphor conversion materials for laser lighting.Laser lighting has the advantages of high brightness,high power density,small size,and high efficiency.Therefore,white light laser lighting based on laser diodes is expected to become a new generation of green solid-state lighting.Blue laser active remote phosphors（LARP）is the most common way to generate white laser lighting due to its advantages of low cost and simple structure.The remote phosphor materials are the key device of LARP technology.Its main function is to convert blue laser light into white light,which directly determines the efficiency,color rendering index,and lighting uniformity of white laser lighting.However,the preparation of the inorganic remote phosphors requires high-temperature sintering,and this would cause the degradation of the most common red nitride phosphors.The color rendering index of laser lighting is low（～60）due to the lack red light component in the spectra,which make it difficult meet the needs of demand market for laser lighting applications.In this paper,we systematically investigated the effects of the precipitant,titration method,reaction temperature,salt solution concentration,（NH₄）₂SO₄ and calcinated temperature on the morphology of powders.Well dispersed,uniform and spherical nanopowders were achieved by co-precipitation method.To make Lu³⁺and Sc³⁺ precipitate simultaneously,we prepared the precursors by strike titration,which is able to obtain pure phase powder at low calcinated temperature.When ammonium hydrogen carbonate was used as the precipitant,spherical precursors were obtained at room temperature.It was found that the dispersity of the precursors was effectively improved by the presence of ethanol.The alcohol can plays the same role as the disperser and contribute to the well-dispersed precursor.Hydrogen bonding interaction of alcohol is weaker than that of water,and alcohol is introduced as OHscavenging reagent to alleviate the agglomeration of the precursors.After calcination,well dispersed,uniform and spherical nanopowders were prepared by modified co-precipitation method.Its particle size distribution is about 25-500 nm and the average particle size is 90 nm.By optimizing the preparation process of the ceramic slurry for gelcasting,the green bodies with high density（its relative density is 52%）,uniform microstructure and high sintering activity were obtained.After calcination,high-quality Tm:(Lu_0.8Sc_0.2)₂O₃ ceramics were obtained with optimized sintering aid.The relative density of ceramics is up to 99.96% and there are almost no pores among or within the grains.The grain size is evenly distributed between 2 and 5 μm,and the average grain size is 3.2 μm.The optical transmittance of the mirror-polished 2at% Tm:(Lu_0.8Sc_0.2)₂O₃ ceramics（～12 mm in length）was about 80.8% at 2090 nm.Using the obtained ceramics as a laser gain medium,when pumped with 796 nm LD,the maximum laser power at 2090 nm reached 1.88 W with a slope efficiency of 24.6%.In this paper,commercial colloidal silica is used as an inorganic binder to attach the phosphor ceramic film to a sapphire substrate at low temperature.Ceramic films combining Lu₃Al₅O₁₂:Ce³⁺ and CaAlSiN₃:Eu²⁺ phosphors were prepared and tested for improved color rendering.When excited by blue diodes laser,the optimal film exhibits excellent thermal stability（it maintains 89.1% of the room-temperature intensity at 200 oC）,super resistance against laser irradiation（12.9 W/mm2）,a broad emission spectra with a full width at half maximum of 180 nm,a high luminous efficiency（152 lm/W）,and a high color rendering index（CRI）（85）.These results indicate that the developed phosphor ceramics films offer great potential for use in high-power laser-driven white lighting with excellent CRI.