Materials discovery aims to produce lower cost LEDs
There are many new possibilities for the development of gallium nitride (GaN) used in the production LEDs. One of the most promising production methods of gallium
nitride is the ammonothermal method which uses a reactor filled up with liquid ammonia. The method is identical with the hydrothermal method utilised in the
production of quartz, in which water is used instead of ammonia.
The high temperature inside the ammonothermal reactor combined with a pressure 2,500 times the atmospheric pressure and the corrosive effects of the so-called
supercritical fluid pose a challenge to the reactor chamber and thus to the manufacture of LED materials. To find a solution to the problem, Aalto University Post-Doctoral Researcher Sami Suihkonen and a research group from the University of California, Santa Barbara led by Nobelist in Physics Shuji Nakamura and Post-Doctoral Researcher Siddha Pimputkar systematically analysed the behaviours of 35 metals, two metalloids and 17 different ceramic materials with three different supercritical fluid chemistries heated to a temperature of 572 degrees Celsius.
– In the ammonothermal method, the energy contained in the reactor corresponds roughly to a stick of dynamite, making the conditions fairly hostile pointed out Sami Suihkonen.
– A nickel-chromium alloy commonly used in the reactors tolerates ordinary supercritical ammonia quite well but poorly withstands the effects of the mixtures used in the production of GaN which include the addition of ammonium chloride or sodium. Our research indicated that vanadium, niobium and tungsten carbide are stable in all three supercritical fluids. For practical applications, however, it is more important to find a material best suited for a certain type of chemistry. For ammonium-sodium this was silver; with ammonium-chloride, silicon nitride and noble metals appear the most promising.
The corrosion resistance of various materials was analyzed in super critical ammonia solutions. Left picture shows a sapphire crystal before tests. Corrosion caused by supercritical ammonium-sodium solution to sapphire is shown in the middle picture. Silicon carbide was found to be stable in ammonium-chloride, as shown in the picture at the right.
To replace the reactor’s nickel-chromium alloy with other structural materials would require the reshaping of the manufacturing process according to Suihkonen. More
robust reactors would nevertheless enable the production of higher quality GaN containing fewer crystal defects which in turn leads to higher quality LEDs. Better LED quality translates to cheaper price.
– From a high-quality LED more light can be obtained per surface-area unit. As the price of an LED is governed by its surface area, better materials could reduce the
price of LEDs to even a fraction of the current price Suihkonen calculates. Higher quality LEDs generate less heat and require smaller cooling elements, which could further reduce the price and enable LED lighting fixtures that are more compact than the current ones.
Apart from their use in more economical and efficient illumination, the better materials could be useful also in power electronics to power control of electric vehicles as well as for use in power supplies and converters.
Reference
The Stability of Materials in Supercritical Ammonia Solutions study was recently published in The Journal of Supercritical Fluids.
Related articles and links:
www.sciencedirect.com/science/article/pii/S0896844615301686
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