02 April 2013

An aerogel "twice as heavy as hydrogen" (??)


Aerogels are truly amazing structures.  The one pictured above (reported in Nature) resting on an inflorescence of grass is described as being "lighter than helium, only twice as heavy as hydrogen."
Now, a new graphene aerogel created by scientists led by professor Gao Chao at the Zhejiang University has swept past, weighing in at just 0.16 milligrams per cubic centimetre.

For reference, the density of air is 1.2 milligrams per cubic centimetre — so the new material is 7.5 times lighter than air. It's twice as heavy as hydrogen — the lightest element there is — but beats out helium, which has a density of 0.1786 milligrams per cubic centimetre.
I have a hard time wrapping my mind around that description.  I thought helium was twice as heavy as hydrogen.  Geek says "the new aerogel has a lower density than helium and only twice as much as hydrogen."

I think some of the descriptions are conflating density and weight.

13 comments:

  1. I wonder if it's strong enough to support the force of the atmosphere? We could add a covering to a large chunk of it and evacuate the air. Then we would have a nice frame for a rigid airship, with non of the explosion risk of hydrogen, or the limited supply of helium. Probably wishful thinking on my part.

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  2. If it's lighter than air, why isn't it floating in the picture?

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    1. The descriptions are confusing.

      It is best to think of the material as a sponge, keeping in mind that both water and air are fluids. A sponge is a very low density object, but it is made slightly heavier by the air that fills the gaps in its structure. Submerse it in water instead of air, and it becomes much more "dense". Get it to the point of becoming waterlogged, and you can even cause it to sink, despite it's considerably very low density when measured in air.

      This material is like a lattice. Air fills the gaps in the lattice, making the weight of the object, plus the weight of the air it is failing to displace, greater than the weight of a volume of pure air of the same dimensions.

      Measured in a vacuum, the density of the material is indeed very low: an equal volume of helium would weigh more than this new aerogel.

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  4. The descriptions are confusing indeed. They talk about the density of air, hydrogen and helium as though those were constants of nature.

    The density of a gas depends on pressure and temperature, so the figure given for helium is at 0°C and standard atmospheric pressure (101,325 Pa). The figure of 1.225 mg/cm³ for air is as the same pressure, but at 15°C.

    The density of a solid like aerogel doesn't depend on anything, it just is 0.16 mg/cm³.
    So, if you take 1cm³ of aerogel and weigh it in a vacuum, the scale will say 0.16 mg. If you take 1cm³ of helium at 101,325 Pa and 0°C and weigh that in a vacuum (don't ask me how), it'll say 0.1786 mg.

    As for why it doesn't float, it's because it's porous, so it's filled with whatever fluid it's surrounded by (air in this case).

    If you wanted to compare the weights of two gases, you would look at their molecular (or atomic) masses. Helium is actually four times heavier than hydrogen, because the most common isotope of helium has 2 protons and 2 neutrons, while the most common isotope of hydrogen has only 1 proton (protons and neutrons have approximately the same mass).

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  5. Sebastian is right, that Helium is 2 protons and 2 neutrons, while Hydrogen is 1 proton, but Helium does not form molecules in the gaseous state, while Hydrogen does (in pairs, H2), so Helium gas is about 2x the mass of Hydrogen gas at the same temperature and pressure (not 4x).

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    1. That's true, my mistake ;-)

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  6. I though graphene was made out of carbon? If carbon is heavier than hydrogen and helium then how can the graphene aerogel be "lighter"?

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    1. Here's an analogue:

      Lead is much denser than aluminium. However, it would not be difficult to make a delicate lead lattice that is 1 metre by 1 metre, and far less dense than a solid block of aluminium of the same dimensions. Therefore the "lead" would be less dense than the aluminium. The gaps are what reduce the density.

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    2. erm, that should be 1 metre by 1 metre by 1 metre... We live in three dimensions =)

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  7. You can't say something is 'so-many-times' lighter than air, unless you give the reference point. It's really sloppy use of language.

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    1. I posted a long comment here before about whether you can say "three times less" or not, and if you could, what it would mean. I don't understand what reference point you're after, though. Surely the reference point is the density of air?

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  8. In physics terms, yes, this is all about density. Once you start talking about objects (which have a size) rather than materials (which don't) you can talk about mass. "Weight" only applies in a gravitational field such as a planet's, and isn't an intrinsic property of materials or objects.

    In fact, strictly speaking, since gravity depends on an observer, weight varies depending on your viewpoint. Consider two people in a plummeting lift: they're weightless relative to each other and the lift, and can float about in there, but when it hits the bottom they return to earth's less happy reference frame where they had weight the whole time...

    By this definition, iron probably weighs less than water, on average, because most of earth's iron is at its core, where it weighs almost nothing (because what would it fall toward?)

    But for most general discussion, "weight" refers to any property that makes a thing feel heavy and that's fine.

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