FTM1000 said:
But graphite is made up of very small graphene sheets with weak interaction between each other while in my question I assumed that the graphene sheets are much larger, at least 5 x 50 square centimeters like in this article:
https://phys.org/news/2017-07-large-single-crystal-graphene.html
I've been putting off answering you because I'm honestly just tired of having the same argument over and over. But that's not your fault and you have a few valid scientific questions in there. For the feasibility of graphene armor, suffice it to say that it just won't work. As I said, you can use carbon nanomaterials to reinforce polymers in a composite (basically just mixing graphite and plastic together in a giant melt), and your polymer will be somewhat stronger as a result--maybe 50% higher tensile strength if you do it right. Not bad, but not revolutionary.
But let's go ahead and answer your scientific questions.
FTM1000 said:
How many large graphene sheets like this you can stack together?
You can stack as many graphene sheets together as you like. It's just a matter of how much time and money you have (it gets very time consuming and very expensive very quickly). The most I usually see in the literature is ~10 layers, and at that point graphene basically behaves like bulk graphite. In general, each layer needs to be polymer supported and the copper growth substrate needs to be etched away in order to stack the layers. This process takes a day or so from start to finish. Quicker methods have been reported, but they have drawbacks. The paper you linked in the OP (here's the original Science article:
https://science.sciencemag.org/content/346/6213/1092) uses HOPG with thicknesses on the order of 300 nm (meaning the microbullet experiments were done using the same "very samll graphene sheets with weak interaction" that you mention above--they don't stack individual graphene sheets either).
FTM1000 said:
will they react with each other?
Not unless a significant force is applied, and not really even then. At very low layer number (less than 4 or so layers), there are indications that graphene sheets can undergo interlayer bonding with one another under the right chemical conditions: see, e.g.,
https://pubs.acs.org/doi/10.1021/nl403938g,
https://arxiv.org/abs/1901.02131,
https://arxiv.org/ftp/arxiv/papers/1907/1907.09033.pdf,
Under high pressure, similar interlayer bonding has been observed (
https://www.nature.com/articles/s41565-017-0023-9?proof=trueMay.). Note that this effect only exists in very thin few-layer graphene material, and probably only at specific twist angles of graphene. For the pressure experiments, the "diamond" phase doesn't even seem to be metastable: as soon as the pressure is released, the diamond reconverts to few-layer graphene. Of course, graphite undergoes a well-known phase change to diamond at high pressures, but this is static isotropic pressure, not the kind of conditions you'd find in something like a bullet impact. Otherwise, you could just take a machine gun to a pile of graphite and pull out diamonds.
FTM1000 said:
You can still separate them with thin layers of some other material, may be even another 2-dimensional material.
Yes! and this is a booming and fascinating area of research right now (google "2D heterostructures" for more info). But it's still not going to lead to new armor anytime soon beyond the polymer composites already mentioned.
Here's why 2D materials are infeasible for armor: Let's say you're sitting in a tank and some jerk launches one of those nasty anti-tank rounds at you. What's the minimum thickness material you want between your head and an exploding anti-tank round? Keep in mind that current tank armor is several centimeters of steel/ceramic/polymer composite, and according to the Science article above, the graphene material is only about 10 times stronger than that under the best of conditions. Let's say you like to live dangerously, and you're comfortable being a hair's-breadth away from death. A hair's breadth is about 50 microns (already 200 times thinner than tank armor), so you want to make a 50 micron sheet of multilayer graphene (still at least 20 times too thin to protect you). How many graphene layers are there in a 50 micron sheet? At 0.35nm per layer, we get about 150,000 graphene layers. Assuming we could whittle the prep time for each layer, including growth and transfer, down to 1 minute (remember, it's about a day right now), we still get about 2 months prep time for the entire stack. This doesn't factor in the massive cost of copper that you're basically using and throwing away (again, it's possible to reuse, but things get tricky). It's just not going to happen any time soon.
