MultiScience - XXXIII. microCAD International Multidisciplinary Scientific Conference University of Miskolc, 23-24 May, 2019. ISBN 978-963-358-177-3
DOI: 10.26649/musci.2019.100
DEVELOPMENT OF INNOVATIVE, LIGHTING PAVINGS BY
RESEARCHING LUMINESCENT ADDITIVES AND DEVELOPING THEIR UNIQUE MANUFACTURING TECHNOLOGY
Viktor, Csókai; Ákos, Kiss
Borsod-Bos 2004 Kft. H-1211 Budapest, 10. Színesfém St.
ABSTRACT
The aim of our current R & D project is the productions of a new type of lighting pavings, which are colorful and attention-grabbing during the daytime, whereas an energy-free lighting pavings system after sunset. Other goal was the outdoor usability of the system and the developing of a unique manufacturing technology to produce it.
Key words: luminophore, solid phase chemistry, electromagnetic irradiation, aluminate, aluminosilicate, rare earth metal, crystal growing
INTRODUCTION
Materials emitting in the dark have been used for more than a hundred years, in particular for hour hands or clockfaces of pocket watches. The most commonly used and widespread material for this purpose is zinc-sulfide. There are new products since then, however, these materials do not complete the nowadays requirements. Our new innovative product is non-plastic based, i.e. resistant to environmental and mechanical impacts, has a sufficiently high energy coefficient and is able to emit light for a long time. The signboards have to operate in the dark due to workplace safety reasons, and also these signboards appear other services besides industrial users, where the ability to operate in dark is also essential. The outdoor usability of commercial boards are restricted because they are mostly placed in either a resin or in a PVC-based plastic film. Their UV and abrasion resistance is very weak, so they are not suitable for walking surfaces.
In the first step of the research, we have focused on the optimization of the luminescent light dust mixtures. Our primary goal was to find the composition that can absorb as much energy as possible from daylight and can emit this energy as long as possible after sunset while accomplishing today's requirements. The research project was extended later to the development of the appropriate activator, what is responsible for luminescence properties. Finding the right substance lies in the collective crystallization of the activator and co-activator rare earth minerals, but also very important factors are the particle size, the treatment temperature and the severe investigation of the proportion of the components.
Compounds with long persistent afterglow are moderately found in the literature. From these phosphors dysprosium, europium, cerium activated by manganese activity have outstanding properties. During our research project, we have faced many difficulties, a number of circumstances have been arisen that made it difficult, and sometimes even
hindered our solid-state chemical research. During the literature survey, it has quickly become clear that the production of phosphors is carried out in special quartz vessels at 1100°C to 1300°C, for several days, sometimes for a week, using nitrogen or argon atmosphere, supplied with hydrogen inlet. Activators usually active in their lower oxidation forms, so activators are not presented with their highest degree of oxidation in the compounds. For example, Europium, Cerium and Manganese are active in their Eu2+, Ce3+ and Mn2+ form. The protection of the activators against oxidation at such a high temperature is indispensable. If the selected base is an oxide, which are strontium, calcium, magnesium aluminate or aluminosilicate in most cases, and the solid-state chemical reaction of the activators is carried out under air atmosphere. In this case, the appropriate crystalline structure can also form, but the activator has to be reduced to the corresponding oxidation degree. Jia et al., worked with this method, but they needed 50 hour long reductive treatment at 1350°C to re-activate cesium to get Ce3+
and to achieve 8-10 hours of dark activity in magnesium aluminate.1
Experiments of Li at al. seems interesting in which they produced manganese-doped zinc borosilicate luminophores.2 The oxide compound was treated at exceptionally high temperatures (1500°C) and the oxidation of manganese could not be experienced without the use of the reductive atmosphere. Although Mn2 + ion tends to oxidation, it becomes brown in color in ambient atmosphere, indicating the formation of MnO2. Probably the production of manganese-doped zinc borosilicate luminophoreswith long persistent afterglow was successful because at the elevated temperature it was possible to form the appropriate crystalline structure applying only 1 hour reaction time and this time was not enough to the oxidation of Mn2+.
Among the family of luminophores with inorganic, long persistent afterglow, the most widely researched and used activators with aluminate matrix are the europium- dysprosium crystals. Katsumata's research team produced barium aluminate crystals doped with Eu2+/Dy3+ activator system and created a luminophore crystalline structure at 1150°C with a 24h treatment time. Due to the oxidation of europium to Eu3+, it had to be activated and thus 2.5% hydrogen gas was used for reducing Eu3+ to Eu2+.3 This way they managed to produce luminophores with a long afterglow up to 10-12 hours.
The production temperature of luminophores is above 1100°C, but in some cases, the temperature can reach extremely high, up to 1600°C. Qiu et al. carried out aluminosilicate-based experiments at 1550°C with the above mentioned Eu2+/Dy3+
activator.4 The selected strontium aluminosilicate base transformed to a glass phase only in an hour at 1550°C. At the same temperature in nitrogen enriched hydrogen streams, it also can be activated in one hour. Therefore, the prerequisite of formation of luminophore crystals is a certain amount of energy transfer which can be carried out
1Jia, D. and Yen, W.M., Enhanced V3+ center afterglow in MgAl2O4 by doping with Ce3+, J.
Lumin., 101, 115, 2003.
2Li, C., et al. Photostimulated long lasting phosphorescence in Mn2+-doped zinc borosilicate glasses, J. Non-Cryst. Solids, 321, 191, 2003.
3Katsumata, T. et al., Growth and characteristics of long duration phosphor crystals, J. Cryst.
Growth., 198/199, 869, 1999.
4Qiu, J., et al., Phenomenon and mechanism of long-lasting phosphorescence in Eu2+-doped aluminosilicate glasses, J. Phys. Chem. Solids, 59, 1521, 1998.
either at a lower temperature, at 1000-1100°C for 3-5 days or at a higher temperature, at 1500 to 1600°C for up to an hour. To justify this theory, several other luminophore experiments were performed, including a simple calcium aluminate experiment, using Eu2+/Dy3+ activator to produce silica-free phosphor at elevated temperature.5 The calcium oxide, alumina oxide, europium oxide and dysprosium oxide system glassified for only one hour at 1450°C and then the previously applied atmosphere was replaced by an argon atmosphere containing hydrogen in order to form Eu2+/Dy3+ system by the reduction of glassyfied Eu3+.
Matsuzawa at al. has chosen a middle path for production of luminophores.6 The expensive but very effective europium-dysprosium system was chosen, and instead of a silicone matrix, the Eu2+/Dy3+ activator system was enclosed in strontium aluminate by glassification.
In our earlier solid-phase chemical experiments the production of aluminates, silicates and zirconium silicates had to be performed under similar high-temperature conditions, but we were not prepared with furnaces in which the temperature above 1100°C would have been sustained in a long term. Therefore, we have experimented with systems that enable to reach a very high temperature in a short time. Furthermore, we would have also liked to test besides the heating effect what other energy transfer solution can be utilized to create the right crystalline structures. Our experiments were carried out using silicon carbide heat transfer surfaces by electromagnetic wave excitation.
1. ábra: Time dependence of the temperature during the heating of a 1mm thick layer SiC (having 530cm2 surface area) with 900W electromagnetic wave irradiation.
5 Qiu, J. and Hirao, K., Long lasting phosphorescence in Eu2+-doped calcium aluminoborate glasses, Solid State Commun., 106, 795, 1998.
6 Matsuzawa, T. et al., A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+,Dy3+, J. Electrochem. Soc., 143, 2670, 1996.
We did not find an appropriate instrument –which can excite with electromagnetic wave- in laboratory scale for our chemical experiments, so we developed a system which is able to model our solid phase chemical reactions in laboratory scale. Our experiments were carried out in silicon carbide vessels and silicon carbide surface reactors. This is because silicon carbide can be excited well, completely absorbs electromagnetic radiation and transforms it into heat. As a result, the applied 2.4GHz magnetrons were able to reach 1000°C during 10 minutes of continuous irradiation by 900W electromagnetic radiation. The warming process can be seen in Figure 1. In this system, experiments can be performed between 20g and 70g. These solid-phase chemical reactions took place between 10 to 30 minutes.
In our previous experiments, we synthesized several, different, ceramic industrial’s pigments using this system and it was found that reaction mixtures containing both reductive and oxidative reactants simultaneously generate the formation of redox side reactions due to high energy irradiation. This was found in case of our experience, where the colorful reaction of chromium-oxide Cr2O3 and tin-dioxide SnO2, where we expected a bright pink color, but in contrast, we get deep green discoloration. When the reaction mixture was heated, it is strongly glassified and created a mostly water soluble foam, but what really surprised us is the metal thin separation under the
“product”. After repeating the reaction and shifting the amount of reactants, we were able to explain the unexpected phenomenon. Our system was not completely isolated from the electromagnetic irradiation. The electromagnetic radiation penetrated into the solid-phase chemical reaction space through one-quarter of the excited silicon carbide surface. In our previous work, we did not pay attention to this phenomena due to the dipole moment of the used metal oxides was not large enough to be excited by electromagnetic radiation. It has been found that cold reactions are not successful, i.e.
electromagnetic experiments without silicon carbide are inefficient. The dipole moment of each material varies according to the temperature, so heating the reaction mixture generates a more sensitive condition at which electromagnetic radiation is able to exert its effect, and this exaggregated energy intake provides an opportunity to the formation of unprecedented side reactions such as redox reactions. During the research, we have investigated the conversion of the reaction mixture and found that the possible Cr3+-Cr6+ transition and the Sn4+-Sn0 transition only occur when high reaction time and high temperature are ensured. Based on this experience, we launched our experiments. We were also able to synthesize luminophores with dark activity based on solid-phase chemical reactions by electromagnetic wave excitation.
The selected luminophores were synthesized from the following metal oxides composition:
1. Barium aluminate with dysprosium-europium activator3 BaO-Al2O3-Dy3+/Eu2+
Unlike solid phase chemistry conditions published by Katsuma et al. (at a temperature of 1150°C for 24h, then argon atmosphere with 2.5% hydrogen content) we did not use a hydrogen atmosphere for re-reduction of the Europium, instead of it we used activated carbon as a reducing agent in the high-energy electromagnetic system.
Materials:
BaO 153 g/mol 145 g 0,95 mol Al2O3 102 g/mol 102 g 1 mol Dy2O3 373 g/mol 18,65 g 50 mmol Eu2O3 352 g/mol 8,8 g 25 mmol activated carbon 12 g/mol 24 g 2 mol
B2O3 70 g/mol 21 g 0,3 mol
The gray powder mixture was placed in a SiC vessel after a very intensive homogenization. The reducing agent was also homogenized with the reaction mixture.
Boroxide was present as a flux. The sample was exposed with electromagnetic wave irradiation of 900W for 50 minutes, causing it to become completely, bright red glowing. The mixed components were certainly reacted, the reaction mixture was yellow powder, completely lost the black coloring of the carbon. The desired product shows a dark activity, it has a suitable high-intensity emission, it emits 500nm for 10 hours. The final product mix does not contain carbon, which is probably due to the fact that the boron acts as a fluidizing agent, and provides a continuously renewable, larger contact surface.
2. Strontium aluminosilicate with dysprosium europium activator5 SrO-Al2O3- SiO2-Dy3+/Eu2+
Qui at al. have synthesized the luminophore - that we have intended to produce - in 1 hour at 1550°C using an air atmosphere and then for another hour at 1550°C under nitrogen atmosphere enriched with hydrogen. The wholly white powder mixture after a very intensive homogenization was re-homogenized with the activated carbon added to the reaction mixture and finally placed in a SiC vessel.
Materials:
SrO 104 g/mol 104 g 1 mol
Al2O3 102 g/mol 76,5 g 0,75 mol SiO2 60 g/mol 45 g 0,75 mol B2O3 70 g/mol 21 g 0,3 mol Dy2O3 373 g/mol 18,65 g 50 mmol Eu2O3 352 g/mol 17,6 g 50 mmol activated carbon 12 g/mol 12 g 1 mol
Similarly to our previous experiments, the activated carbon was the reducing agent in the system. We assumed that carbon with high specific surface area distracts oxygen from the europium, so it can reduce the luminescent material, so it can be produced in only one step. The applied electromagnetic radiation was 900W, but in this case, 30
minutes exposure time was not enough, on average an hour of continuous excitation was required for the desired luminophore product. The total amount of powder was heated up to the required transformation temperature. The carbide surface was glowing white, while the powder was glowing red. The system became white in color, which means that the active carbon burnt out of it completely. The final product was pale yellow. It shows dark activity at 510nm for 10-12 hours, but then the efficiency of emission is reduced very rapidly. In some cases, it has been found that on the surface, where the reaction mixture is exposed with direct electromagnetic radiation, the components are burnt in a 5-6mm thick layer resulting in a gray cementitious material.
This phenomenon comes dominantly when the high energy radiation exceeds one-hour continuous irradiation.
3. Strontium aluminate with dysprosium europium activator6 SrO-Al2O3- Dy3+/Eu2+
Matsuzawa et al. produced this kind of luminophore under atmosphere enriched with hydrogen in an hour treatment at 1300°C.
Materials:
SrO 104 g/mol 104 g 1 mol
Al2O3 102 g/mol 102 g 1 mol B2O3 70 g/mol 21 g 0,3 mol Dy2O3 373 g/mol 17,9 g 48 mmol Eu2O3 352 g/mol 8,45 g 24 mmol activated carbon 12 g/mol 18 g 1,5 mol
B2O3 was used as a flux in order to the homogenous heat transfer and redox reaction and to the lower temperature of the glassification. The gray powder was exposed to continuous electromagnetic irradiation of 900W and after 40 minutes it became yellow, after cooling step its yellow color still remains. Its dark activity at 520nm is 10 hours.
4. Zinc boro-silicate with manganese activation2 ZnO-B2O3-SiO2-Mn2+
Li et al. treated zinc borosilicate system activated with manganese in a closed system at 1500°C but under air atmosphere. They did not experience the oxidation of Mn2+, the discoloration of the system. According to the literature, in this case, it was not necessary to ensure the reduction conditions because manganese was introduced in Mn2+ form into the system and after calcination, the system was left cool down.
Materials:
ZnO 81 g/mol 89,1 g 1,1 mol
SiO2 60 g/mol 24 g 0,4 mol
MnCO3 115 g/mol 0,138 g 1,2 mmol B2O3 70 g/mol 35 g 0,5 mol activated carbon 12 g/mol 18 1,5 mol
This reaction has experimented with glucose, as reductive agent. Since glucose is more expensive and more difficult to handle and the thermal transformations, morphology changes and carbonization of the organic molecule modify relatively many unexamined parameters in the reaction mixture, as well as it contains production additives. Chemically determine its main effect is harder compared to pure carbon powder. Therefore, we have shifted to the previously used reducing agent.
During the solid-phase chemical reactions, we had to ensure that the activator does not oxidize to Mn4+ during the reaction, despite the fact that the literature in the case, if the initial activator is in the right oxidation state does not offer a reducing atmosphere.
However, since the previous experiments did not produce a good result without the reductive agent, so first glucose and then later activated carbon was added to the system, which is a very effective oxygen scavenger. The slightly pink, almost completely white powder mixture was placed in a SiC vessel after a very intense homogenization. It was exposed with the previously used electromagnetic radiation excitation for 60 minutes, the whole sample started to glow, and become red in color while the silicon carbide vessel glowed in white. The components were reacted and after the cooling step, the reaction mixture did not turn brown which would indicate the presence of Mn4+ ions. The product mixture is white, it shows dark activity at 590nm with a very intense red color emitting.
5. Magnesium aluminate with cerium activator1 MgO-Al2O3-Ce3+
Jia et al. achieved luminescence with the desired dark activity of 2 hours glowing at 900°C and then it was glowing at 1350°C for 50 hours in nitrogen streams enriched with hydrogen.
Materials:
MgCO3 84 g/mol 84 g 1 mol Al2O3 102 g/mol 102 g 1 mol Ce2(CO3)3 H2O 460 g/mol 8,28 g 18 mmol activated carbon 12 g/mol 18 g 1,5 mol
B2O3 70 g/mol 21 g 0,3 mol
In this case, according to the literature, the reductive atmosphere is still needed, but only in the second, crystal growing step. It was not necessary to grow large crystals
because the luminophore properties require a maximum 100μm particle size. If the particle size is more than 100μm, the luminophore particle can more easily break away from the binding bed that binds it to the surface. It seemed to be more important to perform a solid-phase chemical experiment at a lower temperature but with high energy irradiation. The very intense homogenization of Ce3+ in the system is indispensable. It is also very important to protect Ce3+ from oxidation. In this case, we also used glucose, which is a more expensive but much more efficient oxygen scavenger agent, and is a better indicator, because it also draws oxygen through its carbonization and by the burning of carbon the system is lost its gray color what is added to the white powder mixture. At the same time, the activated carbon is easier to handle, so in this case, we have also replaced to this reducing agent. The gray powder mixture was placed in a SiC vessel after very intensive homogenization. As in the previous experiments, the samples were exposed to a continuous electromagnetic excitation (900W) for 60 minutes, the whole sample was glowing. After the yellow glow, the cooled system is not yellow, it became completely white. It also shows dark activity, light emission was observed at 520nm for 10 hours.
In summary, our research and development activities have been able to produce phosphors at high activation energy, which in our case do not require high temperature for long periods of time, and the activation energy required for solid phase chemical reactions was provided by electromagnetic irradiation. In some cases, the reductive atmosphere required was achieved using reducing agents instead of the hydrogen atmosphere of today's science. All of the high yield phosphors have 10-12 hours dark activity effect.
