Flexible, Sensitive Sensors from Silly Putty and Graphene

Everyone’s favorite viscoelastic non-Newtonian fluid has a new use, besides bouncing, stretching, and getting caught in your kid’s hair. Yes, it’s Silly Putty, and when mixed with graphene it turns out to make a dandy force sensor.

To be clear, [Jonathan Coleman] and his colleagues at Trinity College in Dublin aren’t buying the familiar plastic eggs from the local toy store for their experiments. They’re making they’re own silicone polymers, but their methods (listed in this paywalled article from the journal Science) are actually easy to replicate. They just mix silicone oil, or polydimethylsiloxane (PDMS), with boric acid, and apply a little heat. The boron compound cross-links the PDMS and makes a substance very similar to the bouncy putty. The lab also synthesizes its own graphene by sonicating graphite in a solvent and isolating the graphene with centrifugation and filtration; that might be a little hard for the home gamer to accomplish, but we’ve covered a DIY synthesis before, so it should be possible.

With the raw materials in hand, it’s a simple matter of mixing and kneading, and you’ve got a flexible, stretchable sensor. [Coleman] et al report using sensors fashioned from the mixture to detect the pulse in the carotid artery and even watch the footsteps of a spider. It looks like fun stuff to play with, and we can see tons of applications for flexible, inert strain sensors like these.

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Graphene Refractions

Researchers recently observed negative refraction of electrons in graphene PN junctions. The creation of PN junctions in graphene is quite interesting, itself. Negative refraction isn’t a new idea. It was first proposed in 1968 and occurs when a wave bends–or refracts–the opposite way at an interface compared to what you would usually expect. In optics, for example, this can allow for refocusing divergent waves and has been the basis for some proposed invisibility cloaking devices.

In theory, negative refraction for electrons should be easy to observe at PN junctions, but in practice, the band gap voltage causes most electrons to reflect at the junction instead of refract. However, a graphene PN junction has no band gap voltage, so it should be ideal. However, previous attempts to find negative refraction in graphene were not successful.

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Making Graphene More Practical

[James Tour] and others at Rice University announced an improved form of graphene that uses nanoscale rivets. The material incorporates carbon nanotubes along with carbon spheres that encase iron nanoparticles. The nanotubes provide strength and higher conductivity overall, while the spheres let the material transfer more easily.

Typically, placing graphene on something involves using chemical vapor deposition on a polymer layer before transferring to another site. The polymer tends to degrade the graphene’s properties. This new material doesn’t require this intermediate step. In addition, the spheres allow interfacing to the graphene more readily.

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Graphene Optical Boom Emits Light with No Diode

When a supersonic aircraft goes faster than the speed of sound, it produces a shockwave or sonic boom. MIT researchers have found a similar optical effect in graphene that causes an optical boom and could provide a new way to convert electricity into light.

The light emission occurs due to two odd properties of graphene: first, light gets trapped on the surface of graphene, effectively slowing it down. In addition, electrons pass through at very high speeds. Interestingly, the speeds are nearly the same–that is, electrons and trapped light travel at almost the same speed. The researchers found a way to make the electrons move faster than the speed of light (in the graphene) and thus created Cerenkov emissions. Because of the structure of graphene, the resulting light is intense and tightly focused.

The researchers speculate that this technique could be important in building graphene-based optical chips. We’ve talked about mixed graphene and semiconductor chips before. Graphene is pretty exotic stuff. It can even fold itself.

Graphene Batteries Appear, Results Questionable

If you listen to the zeitgeist, graphene is the next big thing. It’s the end of the oil industry, the solution to global warming, will feed and clothe millions, cure disease, is the foundation of a space elevator that will allow humanity to venture forth into the galaxy. Graphene makes you more attractive, feel younger, and allows you to win friends and influence people. Needless to say, there’s a little bit of hype surrounding graphene.

With hype comes marketing, and with marketing comes products making dubious claims. The latest of which is graphene batteries from HobbyKing. According to the literature, these lithium polymer battery packs for RC planes and quadcopters, ‘utilize carbon in the battery structure to form a single layer of graphene… The graphene particles for a highly dense compound allowing electrons to flow with less resistance compared to traditional Lipoly battery technologies” These batteries also come packaged in black shrink tubing and have a black battery connector, making them look much cooler than their non-graphene equivalent. That alone will add at least 5mph to the top speed of any RC airplane.

For the last several years, one of the most interesting potential applications for graphene is energy storage. Graphene ultracapacitors are on the horizon, promising incredible charge densities and fast recharge times. Hopefully, in a decade or two, we might see electric cars powered not by traditional lithium batteries, but graphene supercapacitors. They’ll be able to recharge in minutes and drive further, allowing the world to transition away from a fossil fuel-based economy. World peace commences about two weeks after that happens.

No one expected graphene batteries to show up now, though, and especially not from a company whose biggest market is selling parts to people who build their own quadcopters. How do these batteries hold up? According to the first independent review, it’s a good battery, but the graphene is mostly on the label.

[rampman] on the RCgroups forums did a few tests on the first production runs of the battery, and they’re actually quite good. You can pull a lot of amps out of them, they last through a lot of charging cycles, and the packaging – important for something that will be in a crash – is very good. Are these batteries actually using graphene in their chemistry? That’s the unanswered question, isn’t it?

To be fair, the graphene batteries shipped out to reviewers before HobbyKing’s official launch do perform remarkably well. In the interest of fairness, though, these are most certainly not stock ‘graphene’ battery packs. The reviewers probably aren’t shills, but these battery packs are the best HobbyKing can produce, and not necessarily representative of what we can buy.

It’s also doubtful these batteries use a significant amount of graphene in their construction. According to the available research, graphene increases the power and energy density of batteries. The new graphene batteries store about as much energy as the nano-tech batteries that have been around for years, but weigh significantly more. This might be due to the different construction of the battery pack itself, but the graphene battery should be lighter and smaller, not 20 grams heavier and 5 mm thicker.

In the RC world, HobbyKing is known as being ‘good enough’. It’s not the best stuff you can get, but it is cheap. It’s the Wal-Mart of the RC world, and Wal-Mart isn’t introducing bleeding edge technologies that will purportedly save the planet. Is there real graphene in these batteries? We await an in-depth teardown, preferably with an electron microscope, with baited breath.

Super Thin ICs are Coming

An ordinary integrated circuit is made of layers of material. Typically a layer is made from some material (like silicon dioxide, polysilicon, copper, or aluminum). Sometimes a process will modify parts of a layer (for example, using ion implantation to dope regions of silicon). Other times, some part of the layer will be cut away using a photolithography process.

Researchers at MIT have a new technique that allows super thin layers (1-3 atoms thick) and–even more importantly–enables you to use two materials in the same layer. They report that they have built all the basic components required to create a computer using the technique.

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Graphene Super Caps: Coming Soon?

If you read Hackaday regularly, you’ve probably heard that you can use a LASER to create graphene. There’s been a bit of research on how to make practical graphene supercapacitors using the technique (known as LIG or LASER-induced graphene). Researchers at Rice University have been working on this, and apparently they’ve had significant success inducing graphene capacitors on a Kapton substrate. The team has published a paper in Advanced Materials (which is behind a paywall) about their work.

In particular, Rice claims that they have easily produced supercapacitors with an energy density of 3.2 mW/cubic centimeter (that’s what the University’s website reports; they probably mean mW-hours/cubic centimeter) with capacitances near one millifarad per square centimeter. A key benefit of the construction method is that the capacitors continued to work after researchers bent them 10,000 times. A flexible capacitor is useful in wearable devices that would often flex, or in a device like a cell phone that could bend in your back pocket as you sit.

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