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Thu Jun 21, 2018, 08:53 PM

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:

Dirac Semimetals
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:

Graphene, a two-dimensional carbon material, has garnered attention because of its excellent electronic, mechanical, optical, and thermal properties1−3 and potential application in numerous fields.4−6 Various methods have been proposed for the preparation of graphenes, such as micromechanical exfoliation, 1−3 chemical vapor deposition,7,8 reduction of graphene oxide,9,10 and liquid-phase exfoliation.11−24 Liquid exfoliation was considered to be a scale-up and low-cost method in which ultrasound probe and high-shear mixer were often applied. Liquid exfoliation via the fluid shear stress induced by a high-shear mixer can produce large quantities of defect-free graphene.18−24 A four-blade impeller with high shear rate causing strong turbulence was applied to create graphene.18 A kitchen blender was reported to exfoliate graphite into graphene too.19 Coleman et al. and Liu et al. reported the large-scale production of the graphene in N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) solvent, respectively, using a high-shear rotor− stator mixer,20,21 and the minimum shear rate required was 104 s−1. The exfoliation reason was attributed to the high-shear force induced by the large velocity gradient generated by the high-speed fluid when the high-speed blades expelled the solvents to flow in the narrow gap between the stator and the rotor. The cavitation and collision effects caused by the mixer were also factors to exfoliate graphite into graphene.16 Similarly, Xu et al. used a conical tube as a stator to prepare the graphene in NMP solvent.22 Most recently, supercritical CO2 as a green solvent was used to assist in exfoliating graphene.23. Our group developed a scalable approach to exfoliate graphite into graphene via fluid dynamic force in supercritical CO2 using a rotor−stator mixer.27...

...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:

Figure 1. Computational domains of modeling.


Rotor design and (low, if higher than usual where graphene is concerned) yields:



The caption:

Figure 2. Structure of rotors and the production of graphene. Digital photos of the six-tooth rotor (A) and the cross rotor (B). The yield of graphene made by the cross rotor and six-tooth rotor in different shearing speed (C)


A little discussion of the mathematical physics of the situation:

2.2. Numerical Simulations. Five 3D physical models of the reactor were built. A Eulerian−Eulerian two-fluid model which contains the kinetic theory of a granular flow was used to describe liquid−solid two-phase flow in the reactor.

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. Numerical Simulations. Five 3D physical models of the reactor were built. A Eulerian−Eulerian two-fluid model which contains the kinetic theory of a granular flow was used to describe liquid−solid two-phase flow in the reactor.

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:

Figure 5. Contours of velocity at 3000 rpm distribution of horizontal fluid flow pattern induced by an 8-tooth stator (A) and a 10-tooth stator (B).


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:

Figure 10. SEM images of (A) graphite powder, (B) graphene sheets prepared in supercritical CO2, (C) graphene sheets made in water, and (D) graphene sheets prepared in NMP.


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:

Figure 11. AFM and TEM images of graphene sheets. (A, B) AFM images of graphene and the height profile along the line shown in the panel; (C−F) TEM images of graphene in low-resolution and in high-resolution; (G) distribution of the number of graphene layers based on TEM.


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:

Figure 9. Yield of graphene obtained in supercritical CO2, water, and NMP under different shearing speeds.


Some concluding remarks:

In this work, we explored the exfoliation mechanism of graphite into graphene by the rotor and stator geometry in supercritical CO2 and optimized the rotor−stator structure by combining CFD simulation and experiments. The fluid flow patterns corresponding to the rotor and stator with different structures were analyzed by FLUENT 6.3. The experiment and simulation results show that the graphene yield was influenced by the volume of the active region, which is the gap between the stator and the rotor (including the high-speed fluid), and the effective exfoliation time. These two primary factors are more influenced by the geometry of the stator rather than that of the rotor. The multiwall and the extended-wall stator were demonstrated to enable the yield to be nearly doubled and increased by 40%, respectively...


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.













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