Researchers from NC State University have explored the use of lasers to create ceramics that can withstand ultra-high temperatures. The technique can be used to create ceramic coatings, tiles, or complex three-dimensional structures, which allows for increased versatility when engineering new devices and technologies.
Applications achieved with this technique would range from nuclear power technologies to spacecraft and jet exhaust systems.
“Sintering is the process by which raw materials – either powders or liquids – are converted into a ceramic material,” says Cheryl Xu, co-corresponding author of a paper on this research and a professor of mechanical and aerospace engineering at North Carolina State University. “For this work, we focused on an ultra-high temperature ceramic called hafnium carbide (HfC). Traditionally, sintering HfC requires placing the raw materials in a furnace that can reach temperatures of at least 2,200 degrees Celsius – a process that is time-consuming and energy intensive. Our technique is faster, easier and requires less energy.”
How does it work?
The new technique works by applying a 120-watt laser to the surface of a liquid polymer precursor in an inert environment, such as a vacuum chamber or a chamber filled with argon. The laser sinters the liquid, turning it into a solid ceramic. This can be used in two different ways.
First, the liquid precursor can be applied as a coating to an underlying structure, such as carbon composites used in hypersonic technologies like missiles and space exploration vehicles. The precursor can be applied to the surface of the structure and then sintered with the laser.
“Because the sintering process does not require exposing the entire structure to the heat of the furnace, the new technique holds promise for allowing us to apply ultra-high temperature ceramic coatings to materials that may be damaged by sintering in a furnace,” Xu says.
The second way that engineers can make use of the new sintering technique involves additive manufacturing.
Combining laser sintering with a technique similar to SLA
Specifically, the laser sintering method can be used in conjunction with a technique that is similar to stereolithography.
In this technique, a laser is mounted on a table that sits in a bath of the liquid precursor. To create a three-dimensional structure, researchers create a digital design of the structure and then “slice” that structure into layers. To begin, the laser draws the profile of the first layer of the structure in the polymer, filling the profile in as if coloring in a picture. As the laser “fills in” this area, thermal energy converts the liquid polymer into ceramic. The table then lowers a little bit further into the polymer bath, and a blade sweeps across the top to even out the surface. The laser then sinters the second layer of the structure, and this process repeats itself until you have a finished product made of the sintered ceramic.
“It’s actually a bit of an oversimplification to say that the laser is only sintering the liquid precursor,” Xu says. “It is more accurate to say that the laser first converts the liquid polymer into a solid polymer and then converts the solid polymer into a ceramic. However, all of this happens very quickly – it’s essentially a one-step process.”
In proof-of-concept testing, the researchers demonstrated that the laser sintering technique produced crystalline, phase-pure HfC from a liquid polymer precursor.
“This is the first time we know of where someone was able to create HfC of this quality from a liquid polymer precursor,” Xu says. “And ultra-high temperature ceramics, as the name suggests, are useful for a wide range of applications where technologies must withstand extreme temperatures, such as nuclear energy production.”
The new laser sintering technique is also significantly more efficient than conventional sintering in several ways.
The full research, “Synthesis of Hafnium Carbide (HfC) via One-Step Selective Laser Reaction Pyrolysis from Liquid Polymer Precursor,” is published in the Journal of the American Ceramic Society.
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