Super-Strong Graphene Has an Achilles' Heel | Hacker News Reader

Super-Strong Graphene Has an Achilles' Heel

livescience.com

62 pointsvelodrome27 comments

GAHHH. They messed up the units! The stated numbers (4 MPa for imperfect graphene) are wrong! They are meant to be fracture toughness (which is measured in Pa/m^0.5), not just Pa. The paper is at [0]

Table 1 is where you want to look at. It places the fracture stress of pre-cracked graphene in the 1-10 GPa range(which should be compard to the stated 100 GPa for perfect graphene).

I don't have the full materials properties of graphene with me, and I'm not a materials guy, so I may have done something wrong above (so please correct me), but I'm fairly certain that at a minimum, the article messed up the units.

[0] http://www-old.me.gatech.edu/zhugroup/paper/NC14.pdf

EDIT: replaced the link to a copy of the paper hosted by the author from the original nature (paywalled) link.

EDIT2: some changes after actually reading the paper instead of going off the abstract and supplied figures.

  > Perfect graphene can take ~100 Gigapascals (14 million pounds per square
  > inch) of force before it breaks...imperfect graphene...~4 Megapascals (580
  > pounds per square inch).

Can anyone put these numbers in perspective? I understand that 100 Gigapascals is a lot, AKA more than anything I personally own. What items does the common person interact-with/own that require > 4 megapascals? I have to imagine that my old titanium lacrosse shaft could handle more than 4 megapascals.

CONTEXT ADDENDUM: Graphene hammer handle versus Hickory Handle?

580 pounds per sq. inch seems a lot for household applications. Say a human were to stand on a Graphene TV, it would be ~250 pounds on ~100 sq. inches, i.e. 2.5 psi.

It seems like a lot until you take into account a woman of middling weight wearing high(ish) heels, which (when in good condition) often have a "footprint" of about 1/16 of a square inch (and much less if, as is often the case since the lift material doesn't last very long, the heal has been worn through to the pin).

Sure. 100 GPa is 14.5e6 psi. The pressure of an adult human standing on the ground is 1-2 psi (depending on weight, foot size). Pretty much all metals begin to yield at or less than 40e3-70e3 psi.

I love Google unit conversion. Just enter "100 psi to pascals" as a search phrase and you get an answer. Based on this, it's possible to note some typical things converted into Pascals:

Bike tire pressure 100 psi = 690 kPa

Tensile strength of 6061 aluminum (the most common alloy) = 45000 psi = 310 MPa

Whenever people say that they like google's unit conversion I like to mention GNU units. I would be surprised if google does a unit conversion that units can not handle.

After reading the aluminum statistic it seems that graphene is not going to replace my titanium shaft (that replaced the aluminum shaft).

Can GNU units or anything like it accept a string such as "100F to C" and understand what I'd like? If so I'd love to use that instead.

Temperature conversions are a little goofy because they are nonlinear. Google's 100F to C is:

  dfc@ronin:~$ units
  2866 units, 109 prefixes, 79 nonlinear units
  
  You have: tempF(100)
  You want: tempC
          37.777778

The full explanation is here: https://www.gnu.org/software/units/manual/units.html#Tempera...

I followed the like because I thought maybe i've been doimg temperature conversioms wrong all my life. Temperature conversions are so linear. They need to use a more correct term — e.g. simply proportional.

Having to add or substract a constant before or after scaling does not make a relationship non-linear.

This may be getting pedantic, but the relationship is affine not linear - linear would mean k times the original input would yield k times the original output.

It does not convert currencies like Google.

https://www.gnu.org/software/units/manual/units.html#Currenc...

http://www.wolframalpha.com/input/?i=4+megapascals

340 200-pound angels can dance on the head of an uncracked graphene pin (2mm dia). Only about 3 to 35 on cracked graphene.

So it is a bit like carbon fibre then? There is a surprise.

Anyone familiar with carbon fibre will know how some people are scared of it breaking in a catastrophic failure, as in it gets a scratch and then, under load, it breaks completely. In reality this is not much of a problem, however, this failure is very different to what you can expect from other materials such as steel and aluminium. I look forward to the 'high graphene content steel', which should be a perfect material for handy things like making tanks.

The ductility of metal has been a tremendous boon to fabrication of large machines. If parts don't quite fit, they deform to fit. This caused problems when carbon fiber started to be used to build large airplanes. Those parts don't bend and retained the internal stresses caused by mismatches. I read this happened to Boeing.

Glass and ceramics are harder than steel. The reason that glass is fragile, is that one can use its own strength against itself. The material itself acts as a lever to concentrate enormous stresses at the tips of fractures. This doesn't work nearly as well when materials are ductile.

Disclaimer: not a materials guy.

Phew. I thought the Achilles' Heel was going to be something more along the lines of: its great and everything, but:

    The Grey Goo Will Kill Us All - File Next To "Asbestos".

"Zhu, working with Jun Lou at Rice, found that graphene with cracks is 10 times more prone to breakage than steel, and closer in fracture toughness to aluminum oxide or silicon carbide-based ceramics."

How does this compare to fracture toughness in silicon wafers? Does this result mean that graphene-based ICs would be more or less difficult to manufacture than silicon-based ICs?

I'm not a materials (or IC) guy, but my guess is that fracture toughness isn't going to be huge issue. Once the wafer is made, they're all going to be treated pretty gently. The tricky step is actually making the wafer. My understanding is that silicon wafers are normally manufactured by growing a giant silicon crystal ingot, and then basically slicing them into wafers. My understanding of graphene is that since they more or less have to be grown in sheets, then there isn't much of a subtractive process to put high stress on the material.

Most of the stress comes from expansion and contraction of the graphene when it's heating up and cooling back down again, which can be more or less pronounced depending on the substrate (glass, SiO, etc.). With chemical vapor deposition for example, often the substrate needs to be heated and the graphene has to be transferred from one substrate to another so there is still plenty of room for stress during the process. At this point any poor alignments of graphene sheets or individual atoms (I think there's three classifications for the edge of the sheet alone) can cause cracks.

How much stress would radiation outside the Van Allen belts add?

(I could probably find this out myself, but it was quite a few years ago. I assume a sheet would be damaged by gamma?)

I don't think there is a known material that wouldn't be damaged by gamma rays but it's a statistical process and we just don't have that many gamma rays on our surface.

I was wondering about space applications.

A sheet might maybe self heal after gamma radiation, that is why I asked (depending on environment, there might not be much else to happen?). Most other radiation is charged and might be kept away with strong magnetic fields.

But, as I wrote, this was a long time ago.

Slightly off topic, but Professor Sir Andre Geim (the original inventor, I believe) was the very enjoyable guest on Radio 4's Desert Island Discs last month.

http://www.bbc.co.uk/radio4/features/desert-island-discs/cas...

http://www.extremetech.com/extreme/163997-carbyne-a-new-form...

Use carbyne!

Sorry for OT, but.. Jeremy Clarkson has ruined all carby.* names for me - I can't stop thinking about his joke.

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