"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."
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, also known as 3D printing. 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 researchers also demonstrated that laser sintering could be used to create high quality HfC coatings of carbon-fiber reinforced carbon composites (C/C). Basically, the ceramic coating bonded to the underlying structure and didn't peel away.
"The HfC coatings on C/C substrates demonstrated strong adhesion, uniform coverage, and potential for use as thermal protection and an oxidation resistant layer," Xu says. "This is particularly useful because, in addition to hypersonic applications, carbon/carbon structures are used in rocket nozzles, brake discs and aerospace thermal protection systems such as nose cones and wing leading edges."
The new laser sintering technique is also significantly more efficient than conventional sintering in several ways.
"Our technique allows us to create ultra-high temperature ceramic structures and coatings in seconds or minutes, whereas conventional techniques take hours or days," Xu says. "And because laser sintering is faster and highly localized, it uses significantly less energy. What's more, our approach produces a higher yield. Specifically, laser sintering converts at least 50% of the precursor mass into ceramic. Conventional approaches typically convert only 20-40% of the precursor.
"Lastly, our technique is relatively portable," Xu says. "Yes, it has to be done in an inert environment, but transporting a vacuum chamber and additive manufacturing equipment is much easier than transporting a powerful, large-scale furnace.
"We are excited about this advance in ceramics and are open to working with public and private partners to transition this technology for use in practical applications," says Xu.
The paper, "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. Co-corresponding author of the paper is Tiegang Fang, a professor of mechanical and aerospace engineering at NC State. First author of the paper is Shalini Rajpoot, a postdoctoral researcher at NC State. The paper was co-authored by Kaushik Nonavinakere Vinod, a Ph.D. student at NC State.
The research was done with support from the Center for Additive Manufacture of Advanced Ceramics, which is based at the University of North Carolina at Charlotte.
Research Report:Synthesis of Hafnium Carbide (HfC) via One-Step Selective Laser Reaction Pyrolysis from Liquid Polymer Precursor
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