Scaling Graphene.
There's a lot being written about graphene these days. Graphene, for those who don't know, is a carbon allotrope that has the carbons bonded an a series of almost infinite series of fused hexagonal aromatic rings that make it planar. The neat thing about this allotrope is that it is exactly one atom thick. If it's thinker than one atom, it's graphite, most commonly experience by most people as pencil lead.
There are thousands of pictures on the internet. Here's an electron micrograph, out of the Los Alamos National Laboratory, of the stuff with resolution on an atomic scale:
Source Page of the Image.
Graphene is proposed to have many uses and if I actually read all the papers I've seen in which it appears in the title, I'd be able to discuss some of them intelligently, but frankly, I skip over a lot of these papers, quite possibly all of them in fact because I'm too interested in other stuff. Mostly I've just mused to myself about the stuff, particularly its oxide, which I imagined might be functionalized as an interesting carbon capture material, but well, there's lots and lots and lots of those. The problem is not discovering new carbon capture materials; the problem is utilizing them without creating carbon dioxide waste dumps that don't exist and, were they to exist, would be unacceptably dangerous to future generations, not that we care about future generations.
When my son was touring Materials Science Departments at various universities in both "informational" sessions and in "accepted students" forums the word graphene came up a lot. At one such session, we were introduced to a professor who was described as having developed a way to prepare "kg quantities" of graphene, and I meekly raised my hand and asked, "What does a 'kilogram of graphene actually mean?" If I was as rude as I sometimes am around here, I might have asked the question as "Isn't a kilogram of stacked graphene just graphite?"
But I wasn't. I didn't want to screw things up for my kid if he decided to go there. (He didn't.)
At another university, during an informational session for students who might apply, a graduate student, who was writing his thesis at the time, took an interest in my son and decided to give us a full tour of the department. Somehow I used (or muttered) the word "graphene" during the tour and he, a somewhat jaded guy with a decidedly sarcastic edge - my kind of guy - said, "Well, I'm sure it would be useful if they knew how to make it in useful quantities, but they don't."
My son did apply there, by the way, was accepted there, and is, in fact, going there, a wonderful university.
To my surprise I suddenly find myself interested in graphene though because of a recent lecture on a subject about which I know nothing but about which I am interested in finding about more, as I discussed last night in a post in this space: Topological Semimetals.
The paper I linked in that post has the following remark:
The prototype of a DSM is graphene. The “perfect” DSM has the same electronic structure of graphene; i.e., it should consist of two sets of linearly dispersed bands that cross each other at a single point. Ideally, no other states should interfere at the Fermi level.
Graphene is a "perfect DSM," a "perfect Dirac Semimetal."
And today in my library hour, what should happen but that I was to come across a paper that reports an approach to scaling up graphene.
The paper is here: Exfoliation of Graphite into Graphene by a Rotor–Stator in Supercritical CO2: Experiment and Simulation (Zhao et al, Ind. Eng. Chem. Res., 2018, 57 (24), pp 8220–8229)
I have been and am very interested in supercritical CO2, by the way. "Supercritical" refers to a substance that is neither a liquid nor a gas but exists in a state that has properties of both and can only exist above certain temperatures and pressures called respectively the "critical temperature" and, of course, "the critical pressure." The critical temperature of carbon dioxide is only a little above room temperature, which makes it a readily accessible material.
As my wont lately in this space when discussing scientific papers, I'll do some brief excerpts and invite you to look at the pictures, since whenever I decide whether or not to actually read a paper upon which I stumble (as opposed to a paper that I've sought for some reason), that's what I do, look at the pictures.
From the intro:
...The purpose of this work is to investigate the exfoliation mechanism of a rotor?stator mixer in supercritical CO2 by a combination of the experiment and CFD simulation and to make the optimal design of the rotor?stator mixer in terms of exfoliation efficiency for the potential industrial application.
CFD is computational fluid dynamics if you didn't know.
Now some pictures...
Here's a schematic of things they evaluate by computer simulation:
The caption:
Rotor design and (low, if higher than usual where graphene is concerned) yields:
The caption:
A little discussion of the mathematical physics of the situation:
2.2.1. Eulerian?Eulerian Two-Fluid Equations. Different phases were treated as interpenetration continuum. The conservation equations were solved simultaneously for each phase in the Eulerian framework. Then, the continuity equations for phase n (n = l for the liquid phase, s for the solid phase) can be expressed by
...
Some more cool math:
2.2.1. Eulerian?Eulerian Two-Fluid Equations. Different phases were treated as interpenetration continuum. The conservation equations were solved simultaneously for each phase in the Eulerian framework. Then, the continuity equations for phase n (n = l for the liquid phase, s for the solid phase) can be expressed by
At the same time, a granular temperature was introduced into the model:
...
Some simulation results:
The caption:
More simulation showing vessels and rotors:
The caption:
Figure 6. Stator and the contours of velocity and volume fraction in multiwall stator at 3000 rpm (A). The lateral view and the vertical view of the multiwall stator, (B) horizontal, and (C) perpendicular fluid flow pattern induced by multiwall stator; the graphite of volume fraction in (D) eighttooth stator and (E) multiwall stator.
Then they set about making themselves some graphene. It, along with graphene by other processes is pictured here:
The caption:
NMP is N-methylpyrollidine. I've used it, I'm still alive but know nothing of its toxicology. If it turns out to be toxic, we can use it to make solar cells, whereupon it will be declared "green," no matter what it's effect on living things.
Some more electron micrographs:
The caption:
Nevertheless the yields are not spectacular enough to make industrial application straight forward, although if it turns out that graphene solar cells are "great" we can bet the planetary atmosphere on the expectation that they'll be available "by 2050" when I - happily for many people who don't find me amusing - will be dead.
The caption:
Some concluding remarks:
Love that percent talk!
Interesting, I think, although I think that graduate student had a point.
I hope your Friday will be pleasant and productive.
