Liquid Phase Synthesis And Luminescence Properties Of Manganese (Ⅳ) Activated Red-Emitting Crystal Materials

Due to the high efficiency,long lifetime,and mercury-free feature,white light-emitting diodes（WLED）have replaced incandescent lamps and fluorescent lamps,becoming the fourth generation of lighting source.The combination of blue LED chip with garnet-structural yellow phosphor is the mainstream commercial solution for the spectrum-converting WLED.Introduction of red phosphor to improve the red spectral component can significantly increase its color rendering index and obtain high-quality warm white light.Mn⁴⁺-activated red phosphor has become one of the research hotspots in the past decade.It is reported that the Mn⁴⁺-activated crystal phosphors with single crystal nature exhibit better performance on aspects of moisture-resistance,and quantum efficiency,compared with its polycrystalline powdery form.However,the synthesis methods for these materials suffer from problems such as time-consuming,high cost,high risk（HF volatilization）,and complicated procedures.In this thesis,with regards to the different physical and chemical properties of the host compound,several typical Mn⁴⁺-activated phosphors with soluble,sparingly soluble,and insoluble matrix were selected as the objects.Accordingly,liquid phase-assisted synthesis methods such as cooling crystallization,phase transformation by fluorination,and molten salt method were used to synthesize a series of Mn⁴⁺-activated crystal phosphors.The doping behavior and valence-evolution of Mn⁴⁺,emission spectrum broadening,hydrolysis degradation and quantum efficiency,etc.,were analyzed and discussed,and ascertain the internal mechanism of the influence on luminescence performance.It provided novel routes for the low-cost and simple synthesis for white LEDs.The main content were as follows:（1）By the cooling crystallization,Na₂SiF₆:Mn⁴⁺,K₂Ti F₆:Mn⁴⁺,（NH₄）₂SiF₆:Mn⁴⁺,（NH₄）₂Ti F₆:Mn⁴⁺millimeter/centimeter-sized crystal fluorescent materials with soluble fluoride matrix were prepared.Taking advantage of the feature that the solubility of solutes decreases with the decrease of temperature,firstly,Na₂SiF₆:Mn⁴⁺crystals were grown by the method of cooling the saturated solution from high temperature（90℃）to room temperature（20℃）,but the high temperature led to serious deterioration of Mn⁴⁺in the solution.Furthermore,the undoped Na₂SiF₆crystal was prepared through the above cooling step,and then was doped with Mn⁴⁺through ion exchange reaction at room temperature.The product inherited the size and columnar morphology of the original crystal,and own strong zero phonon line which intensity reached 58.8%of Stokesν₆.Moreover,using the process of cooling from room temperature to-30℃,the in-situ doping of Mn⁴⁺was conducted during the crystal growth and it was found that Mn⁴⁺in the solution did not exhibit obviously degradation under low temperature.This method was extended to prepare other phosphors with similar matrix.The as-prepared K₂Ti F₆:Mn⁴⁺crystal was yellow,regular hexagonal flakes,and the size can reach～1 cm.The as-prepared（NH₄）₂SiF₆:Mn⁴⁺crystal was yellow,hexagonal columnar,and the length reached～2mm.With the increase of Mn⁴⁺doping concentration,the intensity of the zero-phonon line gradually increased and the emission peaks blue shifted.The as-prepared（NH₄）Ti F₆:Mn⁴⁺crystal was yellow and the size reached～1 cm.（2）BaTiF₆:Mn⁴⁺ crystal phosphor with sparingly soluble matrix was prepared by phase transformation method at room temperature with length reaching hundreds of micrometers.Firstly,Ba Ti F₆:Mn⁴⁺crystal phosphor was prepared by co-precipitation at room temperature and the influence of Ba sources（Ba Cl₂,（CH₃COO）₂Ba,Ba F₂,or Ba（NO₃）₂）on the morphology and luminescence of the product were explored.Chloride ions can cause the deterioration of Mn⁴⁺in the solution and the product obtained by this method had small particle size and irregular morphology.Secondly,by using a newly developed room-temperature liquid phase synthesis route,i.e.,the phase transformation method,we immersed the Ba Ti OF₄precursor into the K₂Mn F₆/HF solution which finally converted into Ba Ti F₆:Mn⁴⁺after fluorination.It was found that the crystallinity of Ba Ti OF₄had an obvious influence on the phase transformation rate and the distribution of Mn⁴⁺dopant.It was further optimized that by adding highly crystalline Ba Ti OF₄and K₂Mn F₆into the HF solution at a frequency of once per hour,the Ba Ti F₆:Mn⁴⁺crystals can gradually grow reaching a length of200～300μm.The procedure of continuous feeding of the raw materials significantly improved the doping uniformity of Mn⁴⁺.The as-prepared Ba Ti F₆:Mn⁴⁺crystals exhibited excellent moisture resistance,which can maintain 92.9%of the initial luminous intensity after immersing in water for an hour.The Ba Ti F₆:Mn⁴⁺crystals were mixed with commercial yellow phosphor and coated on blue chip to obtain WLED.The obtained color rendering index was 83.4,correlated color temperature was 3978 K,and lumen efficiency was 128.2 lm·W^-1.（3）The micro-sized MgAl₂O₄:Mn⁴⁺ crystal phosphor with insoluble oxide matrix was prepared by molten salt method.Mg Al₂O₄is the aluminate matrix in which the Mn⁴⁺²E_g→⁴A_2gtransition exhibits the highest emission energy,with a peak at 651nm.Manganese ions can occupy multiple sites in Mg Al₂O₄and exhibit multiple valence states.Firstly,by controlling the crystal form of raw Al₂O₃,the Mg Al₂O₄:Mn⁴⁺phosphor free from Mn²⁺/Mn³⁺impure valence state was prepared.Then,the molten salt was introduced as medium for the crystal growth and the types of molten salts were optimized.The influences of the properties of the precursors containing Mg²⁺/Al³⁺,the proportion of molten salt and the reaction time on the reaction kinetics,and the dissolution and diffusion process were discussed.Using nano-γ-Al₂O₃as Al³⁺-precursor and micro-Mg O as Mg²⁺-precursor,we successfully synthesized Mg Al₂O₄:Mn⁴⁺crystal with size of 1～3μm and tetragon-shaped morphology.The formation process was jointly determined by the“dissolution-diffusion-precipitation”mechanism and the“template formation”mechanism.The quenching temperature(T_50%)of the synthesized Mg Al₂O₄:Mn⁴⁺crystal was about 400 K,and the thermal activation energy（ΔEa）was～0.23 e V.It was mixed with commercial yellow phosphor and coated on the blue chip to form the packaged WLED.The obtained color rendering index was 82.4 and the correlated color temperature was 4488 K.