3D-Printed Ceramics Could Help Build Hypersonic Planes (livescience.com)

An anonymous reader writes: Researchers have used a 3-D printer to make specialized ceramic parts that have overcome one of the biggest problems with ceramic objects: their tendency to crack. This new method is 100 to 1,000 times faster than previous 3D-ceramic-printing techniques, the researchers said. Furthermore, electron microscopy of the end products detected none of the porosity or surface cracks that normally weaken ceramics; indeed, these silicon carbide materials were 10 times stronger than commercially available ceramic foams of similar density, the scientists noted. "If you go very fast, about 10 times speed of sound within the atmosphere, then any vehicle will heat up tremendously because of air friction," said Tobias Schaedler, senior scientist at HRL Laboratories in Malibu, Calif. "People want to build hypersonic vehicles and you need ceramics for the whole shell of the vehicle."

Comments

Compression, not friction

[Brett Buck]

The heating is largely from compression heating the air, not "friction" in the usual sense.

Re:Compression, not friction

[Rei]

It's not even that simple.... in the exosphere, it's more like individual particle collisions than like dealing with a bulk gas. And then when you get deeper you get into the atmosphere, it still doesn't behave like a normal gas - the dense compression shocks of air that you've built up in front of you that are so hot that you actually lose some of the heating energy to endothermic chemical reactions - there's a different equilibrium there than at lower temperatures. While there's enough time to reach the new equlibrium in the shocks in front of the spacecraft, in the sidestream the gas moves past so fast that it doesn't have time to reach its new equilibrium as it cools ("frozen reactions") - the reactions happen at a point well behind the spacecraft, releasing the energy there. So the spacecraft actually gets away with bypassing part of the energy it's losing to the atmosphere.

On the other side, these frozen reactions have downsides too - it's part of what makes scramjets so difficult (the desired combustion being "frozen" to past the end of the craft due to insufficient reaction time). In fact, if this didn't happen, you could potentially make spacecraft that propel themselves in the outer reaches of the atmoshere/low Earth orbit (anywhere over 100km really) without need for onboard propellant by recombining the free oxygen radicals that dominate there. (technically you still probably could, but it would require a long spacecraft indeed)

Re:Because ceramics don't get hot?

[Rei]

When was the last time your coffee mug melted?

The point isn't to stop them from getting hot - the point is to not melt or weaken when they do. Ceramics are the best materials in existence for this. For example, hafnium nitride carbide melts at 4126C. Iron boils at 2826C. And this is more meaningful than it sounds - because the only ways during reentry that one can get rid of heat are storage, ablation, and radiation. Depending on the Cp scaling factor, ablation and storage are proprortional to the temperature to the 1-2 power while radiation is proportional to the fourth power of the temperature. So being able to tolerate a given amount of higher temperatures translates to being able to dissipate far greater amounts of reentry heating.

The fact that their first material was silicon oxynitride I find interesting. I don't know how thick their layers are and whether they're able to get any transparency out of them, but thin films of silicon oxynitride are sometimes used for gradient-indexed optics - by changing the ratio of oxygen and nitrogen you can greatly change the refraction index, and thus make things like perfectly flat, thin transparent objects that function as lenses - like a fresnel lens but without roughness or distortion. And when you dope silicon oxynitride you can make phosphors of various colours. So depending on what blend of powder they lay down with the print head they may be able to use it as a rather nifty optics-printer. And since they're using UV to solidify the substance they're basically doing photolithography, aka they should be able to do very fine details. And it's a dielectric with good thermal conductivity. See where I'm going with this? Literally printing your own displays.