(Left) A sapphire crystal before exposure to supercritical ammonium-sodium solution. (Middle) Sapphire crystal after exposure, showing corrosion. (Right) Silicon carbide was found to be stable when exposed to supercritical ammonium-chloride.Of the many ways for creating the gallium nitride (GaN) used in the production of light-emitting diodes (LEDs), one of the most promising is the ammonothermal method, which uses a reactor filled with liquid ammonia. It is identical to the hydrothermal method used to produce quartz, in which water is used instead of ammonia.
The downside to the ammonothermal method is that it requires high temperatures and a pressure 2500 times greater than atmospheric pressure, which together convert the ammonia into a supercritical fluid with properties of both a liquid and a gas. These high temperatures and pressures, together with the corrosive effects of the supercritical fluid, pose a challenge to the reactor chamber and thus to the manufacture of LED materials. In the ammonothermal method, around the same amount of energy is contained within the reactor as in a stick of dynamite, making the conditions fairly hostile, says Sami Suihkonen, a post-doctoral researcher at Aalto University in Finland.
So Suihkonen and a research group from the University of California, Santa Barbara, led by Nobel laureate Shuji Nakamura, set out to find the most suitable materials for constructing the reaction chamber. As they report in The Journal of Supercritical Fluids, this involved systematically analyzing the behaviors of 35 metals, two metalloids and 17 different ceramic materials exposed to three different supercritical fluids heated to 572°C.
“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,” explains Suihkonen. “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.”
According to Suihkonen, replacing the reactor's nickel-chromium alloy with other structural materials would require reshaping the manufacturing process. However, more robust reactors would allow the production of higher quality GaN containing fewer crystal defects, which in turn would lead to higher quality LEDs.
As more light can be obtained per surface-area unit from a high-quality LED and the price of an LED is governed by its surface area, better materials could reduce the price of LEDs to a fraction of their current price. Moreover, higher quality LEDs generate less heat and thus require smaller cooling elements, further reducing the price and leading to LED lighting fixtures that are more compact than current ones.
This story is adapted from material from Aalto University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.