Yttrium oxysulfide – Y2O2S is knows an excellent host material for the Eu(III) activated red emission phosphor, which has been used for decades in the CRT displays and conventional TV due to its excellent brightness, color purity, life time and compatibility with the efficient manufacturing technology. After the development of the field emission displays (FED), this material again became in the focus of researchers attention, and it became essential to develop synthesis methods, which can provide round shape submicron particles with high luminescence intensity. Solution based methods became obvious candidates for the fabrication of such materials.
This paper describes synthesis of Y2O2S phase and the corresponding Eu(III) activated phosphor by sulfurization of the fine round shape particles obtained from the yttrium chloride solution by slow hydrolysis of the salt during decomposition of the urea. Synthesis is not described in details in this article and it is referred to the earlier paper D. Sordelet, M. Akinc “Preparation of spherical, monosized Y2O3 precursor particles” Journal of Colloid and Interface Science Volume 122, Issue 1, March 1988, Pages 47-59 (the References list provides incorrect publication year). The phase composition of the prepared powders was confirmed by powder X-ray diffraction, particles morphology was characterized by SEM. FTIR spectroscopy results showed absence of carbonate or sulfate groups, indicating formation of oxysulfide as the only product. Luminescence spectra showed typical spectra of Y2O2S:Eu emission without Y2O3:Eu contribution.
The synthesis of the submicron particles of Y2O2S:Eu in this work can be described as follows.
First, spherical particles of YOHCO3 composition were prepared adopting the procedures from the earlier works by the reaction between 0.05M solution of yttrium chloride and urea at 105OC. It was found that increase of the yttrium concentration above 0.05M resulted in the deviation of the particles shape from spherical, and that it is important to use excess of urea to achieve reasonable reaction yield. In my experiments, good results were obtained when cation/urea ratio was higher than 8. It may be also convenient to follow the reaction by measuring pH, if pH meter can operate at elevated temperatures. Heating of the precipitate above 180OC results in the formation of Y2O2CO3, which decomposes after annealing above 610OC.
Sulfurisation of the YOHCO3 precipitate was carried out by annealing in sulfur vapor. The precursor powder was placed into the alumina crucible and sealed in the furnace. Sulfur was heated by the auxiliary furnace at 220OC and the vapor was transported by argon flow at the rate of 116 ml/min. The sulfurization reaction was carried out by gradual heating of the furnace to 770OC at 2 deg/min rate. After the desired reaction time (which was not specified in the paper) the furnace was shut off and the temperature was further increased to 800OC and the sample was kept at least for 2h.
Based on our experience, better reproducibility of sulfurization reaction can be achieved by annealing of the precursor powders in the 10mol%H2S/Ar gas flow with the flow rate of 20 ml/min. This reaction can be carried out in the wide temperatures range but remarkable particles growth and agglomeration was observed only above 1000OC. Sulfurization reaction is rather rapid process, and it takes place at the significant rate above decomposition temperature of Y2O2CO3. The holding time depends mainly on the kinetics of sulfur or H2S diffusion in the powder placed into the crucible or alumina boat. In our case, when precursor was placed into the alumina boat with the thickness of 3-4 mm, the typical reaction time was 2h at 800OC without the need for post annealing.
We also used a “double crucible method” for sulfurization of the precursor. This method is applied for the industrial production of Y2O2S:Eu phosphors. In this technique, the sulfur powder is mixed with sodium carbonate in the 1:1 weight ratio to make a flux for sulfurization of oxide precursor. The flux is placed into the crucible, and another smaller crucible with the precursor is placed inside the crucible with flux. Then, larger crucible is covered and the system is heated up in the furnace with Ar atmosphere. In our system, the reaction kinetics was more sluggish due to the bigger amount of precursor powders and typical sulfurization time was 2h and furnace heating rate 10 deg/min.
The key point in controlling the morphology of the submicron powders in this synthesis is connected to the interplay of nucleation and particles growth process during the decomposition of urea. We could control the precursors morphology by changing the viscosity of the reaction media e.g. by adding a glycol or carrying out the reaction at elevated temperatures in the sealed autoclave.