Summary
This paper presents a school-student-friendly synthesis of cerium-doped yttrium aluminum garnet (YAG:Ce) in a laboratory microwave oven. YAG:Ce is the most widely applied luminescent material (“phosphor”) used in white LEDs. The initial blue light of a gallium nitride (GaN) based primary LED is partly down-converted by the YAG:Ce phosphor on top of the LED chip, resulting in yellow-green emission. Additive color mixing (blue + yellow-green) results in cold-white light generation. This experiment aims to demonstrate chemistry’s contribution to sustainable development in a comprehensible way. Based on such phosphor-converted LEDs, the interdependence of and cooperation between different scientific disciplines are outlined. The critical question of the related raw materials, in this case especially rare earth elements, their lifetime, and their possible recycling, represents another important issue for sustainable development and systems thinking and is shown in this example from everyday life.
The introductory work starts by using an infrared camera. Students are asked to justify the phasing out of light bulbs in the European Union by discussing power, temperature, color rendition (personal impression), and energy efficiency class. Power in watts is determined with a conventional electricity monitor for plug sockets and temperature with an infrared camera. The energy efficiency class can be easily read on the packaging of the lamps. Several different illuminants with a performance rating equal to that of a light bulb (1), a fluorescent lamp (2), a cold-white LED (3), and a warm-white LED (4) are used. The results are presented as part of an open discussion about illuminants in general, and problems are solved by newer generations of them. The target is to show the low degree of efficiency of light bulbs. Subsequently, the general setup of an LED is shown to convey the importance of luminophores for pc-LEDs. In the first experiment starting from the initial blue of the GaN-LED chip, all other colors of the visible spectrum can be obtained by down-conversion (luminescence) by placing color-converting materials on top of this chip, mostly embedded in silicone. By producing a mixture of this blue and yellow light, the result is cold-white light delivered by LED technology. The second experiment deals with the synthesis of YAG:Ce based on a combustion process in microwave oven. The result of this controlled combustion process at 500 W is YAG:Ce, which is verified by powder X-ray diffraction (XRD) and UV–vis spectroscopy during the third experiment. For students, YAG:Ce can be identified by yellow luminescence under UV excitation (e.g., with a UV torch) during the fourth experiment.
The last experiment gives the opportunity to students to show the relevance of chemistry to everyday life through luminophores. An example would be to have students analyze an LED flash for photography from a device (e.g. smartphone), which often contains YAG:Ce. This can be easily demonstrated under UV excitation by observing the fluorescence of this material. While explaining the general setup of pc-LEDs, the students identify the phosphor Y3Al5O12:Ce as light-converting material and make a hypothesis about how phosphors work. In most cases, the students immediately recognize the presence of rare earth elements, here yttrium and cerium.
The presence of rare earth elements in phosphors and semiconductors in LED technology leads to several challenges, for example, developing a sustainable disposal process, so that recycling systems must be created at national levels. Environmental damage and the ecological and social impacts on local villagers should be part of a holistic view taken on the topic of sustainable lighting with LEDs, especially when LED lighting is expected to be the technology with the brightest future perspectives and to have the potential to grow even further. All these aspects are part of the proposed experimental procedure. Because of the links between different systems and sciences, and the advantages and benefits of LED lighting, this experiment addresses systems thinking and makes it possible to integrate systems thinking into school chemistry and out-of-school learning opportunities such as science laboratories for school students.
Relevance for Complex Systems Knowledge
The paper deals with systems thinking and sustainable development.
Authors argue that this experiment demonstrates a systems thinking approach not only by focusing on various aspects of chemical expertise, involving doping, different phosphors, the function of light-converting materials, a redox reaction, and microwave synthesis. Systems thinking is also demonstrated by the fact that the whole context needs to be investigated, including environmental, social, and economic factors, and recycling problems with rare earth elements. Therefore, a spatial (local and global systems) and temporal view needs to be taken.
Systems thinking is also viewed as a critical and holistic approach that provides the necessary capacities for a critical consideration of all parts and their interactions to realize the goals of sustainable development and the three dimensions of sustainability: environmental, economic, and social.
Energy efficiency and a sustainable use of resources have become increasingly important in society and politics in order to ensure environmental sustainability (UN Millennium Development Goal 7). Authors claim that scarcer raw materials and climate change are scientific facts that make it necessary to enhance energy efficiency and to recycle raw material. Light-emitting diodes (LEDs) are considered as the most efficient and sustainable light sources of the future and chemistry is expected to play a significant role in the development of white, energy-efficient LEDs.
