Synthesis of Inorganic Luminescent Materials

Luminescent materials are materials that can absorb energy in some way and convert them into light radiation (unbalanced radiation). The process of absorbing energy inside a substance in some way and converting it into light radiation (unbalanced radiation) is called luminescence. In practical applications, solids that emit light when excited by the outside world are called luminescent materials. They can be used in the form of powder, single crystal, thin film or amorphous. The main components are rare earth metal compounds and semiconductor materials, which are closely related to non-ferrous metals.

 

Representatives of inorganic fluorescent materials are rare earth ion luminescence and rare earth fluorescent materials, which have the advantages of strong absorption capacity and high conversion rate. The narrow-band emission of the central ion of the rare earth complex is conducive to full-color display, and the physicochemical properties are stable. Due to the rich energy levels and 4f electronic transition characteristics of rare earth ions, rare earth ions have become a treasure trove of luminescence, providing high performance light emitting materials for high-tech fields, especially information and communication fields. Until the beginning of the 21st century, common inorganic fluorescent materials used alkaline earth metal sulfides (such as ZnS, CaS) aluminates (SrAl2O4, CaAl2O4, BaAl2O4) as light-emitting substrates, and rare earth lanthanides.

 

The traditional method for preparing inorganic phosphors is a high-temperature solid-phase method. However, with the rapid update of new technologies, the improvement of the performance index of luminescent materials needs to overcome the inherent defects of classic synthesis methods. Some new methods have emerged as the times require, such as the combustion method, sol-gel method, hydrothermal precipitation method, and microwave method and so on.

 

The application of photoluminescence materials in safety is its most common. In terms of safety, the photoluminescent material can be used as a safety exit indication mark, an evacuation mark, and the like. When used as these markers, photo luminescent materials must undergo rigorous testing to ensure they meet safety standards. The application of photo luminescent materials is different from that of decorations or other small items in terms of safety, and it is required that the luminescent materials maintain the brightest illumination and long-lasting illumination.

 

 

The synthesis method of the phosphor is as follows

 

1. Sol-gel method

 

One or several salts are uniformly dispersed in a solvent, so that they become transparent colloids, that is, sols. The sol is subjected to aging treatment under a certain condition (temperature, pH, etc.) to obtain a transparent frozen substance, which is called a gel. The sol-gel method can precisely control the content of each component, so that different components can be uniformly mixed at the molecular / atomic level, and the whole process is simple and the process conditions are easy to control. The gel formation mechanism usually has to go through three necessary processes: the monomers are polymerized into primary particles; the particles grow up; the particles are crosslinked into a chain and form a three-dimensional network structure.

 

The disadvantages of the sol-gel method are as follows: higher raw material costs, the presence of small residual holes, the presence of residual carbon, the long reaction time, and the organic solvents are harmful to the human body.

 

2. Hydrothermal method

 

The hydrothermal method uses water as a reaction medium in a high-temperature, high-pressure reaction environment in an autoclave, so that generally insoluble or insoluble substances are dissolved, and the reaction can be recrystallized. Hydrothermal technology has two characteristics, one is its relatively low temperature, and the other is carried out in a closed container to avoid volatilization of the components. Nano-powder was prepared by hydrothermal reaction in 1982.

 

3. Solvothermal Synthesis

 

Organic solvents (such as benzene and ether) were used instead of water as the medium, and nano-powders were prepared by a similar principle to hydrothermal synthesis. The replacement of water by non-aqueous solvents not only expands the application range of hydrothermal technology, but also enables reactions that cannot be achieved under ordinary conditions, including the preparation of materials with metastable structures.

 

The characteristics of the solvothermal method are as follows: the reaction conditions are very mild, which can stabilize the pressure-stabilized phase, prepare new substances, and develop new preparation routes. The process is relatively simple and easy to control, and it can effectively prevent the volatilization of toxic substances and prepare air-sensitive precursors in a closed system. In addition, the formation of the phase, the size of the particle size, and the morphology can also be controlled, and the dispersibility of the product is good. Under solvothermal conditions, the properties of the solvents (density, viscosity, dispersion) interact with each other and vary widely, and their properties differ greatly from those under ordinary conditions. Correspondingly, the dissolution and dispersion of the reactants (usually solids) and the chemical reaction activity are greatly improved or enhanced.

 

In short, this is a conventional dehydration process using hydroxide as a precursor. The reactant solid is dissolved in the solvent, and the product is then crystallized from the solvent. This method can prepare many single or composite oxides.

 

4. High temperature solid phase reaction method

 

Precursors are formed from solid compounds or through solid-phase reactions, and nano-sized powders are obtained by pyrolysis. Two important factors that determine the solid-phase reaction are the nucleation and diffusion rate. If there is a structural similarity between the product and the reactants, nucleation is easy to proceed. When the nucleation rate is faster than the growth rate, it is beneficial to generate nanoparticles. If the growth rate is faster than the nucleation rate, massive crystals are formed. In the solid phase method, a metal salt or a metal oxide can be sufficiently mixed in a certain proportion, and then calcined after grinding. Nano-sized particles can be directly prepared by solid-phase reaction, or pulverized again to obtain nano-sized powder. It is generally believed that the solid-phase reaction process goes through four stages: (1) reactant diffusion; (2) chemical reaction; (3) product nucleation; (4) crystal growth.

 

The luminescent powder produced by the solid-phase reaction method usually needs to be post-processed, including pulverization, powder selection, powder washing, coating, screening and other processes. The main advantages of the luminescent powder prepared by the solid-phase reaction method are: excellent crystal quality of the crystallites, few surface defects, large luminous brightness, long afterglow time, which is conducive to industrial production.

 

The disadvantages of the solid-phase reaction method are: high calcination temperature, long holding time, and high requirements on equipment. The uneven particle size distribution makes it difficult to obtain spherical particles, and the particles are easy to agglomerate. The particle size needs to be reduced to reduce the crystal shape of the light emitter, reduce the crystallinity of the phosphor, and reduce the light emitting performance.

 

5. High temperature combustion synthesis method

 

By using the external energy to provide the necessary energy to induce a high exothermic chemical reaction, the system locally reacts to form a chemical reaction front (combustion wave). The chemical reaction proceeds quickly with the support of its own heat, and the combustion wave spreads throughout the system. The reaction heat causes the precursors to decompose quickly, leading to a large amount of gas evolution, avoiding the precursors from sticking due to melting, and reducing the particle size of the product. The system reaches a high temperature of several thousand degrees in an instant, and can evaporate and remove volatile impurities.