Is there anything nanotubes cannot do? Don’t answer. That was a rhetoric question. The answer is obviously a resounding ‘No!’. During a trawling of scientific literature, I came across an interesting article published in the journal ‘Advanced Materials’ titled ‘Transparent Film Heater Using Single-Walled Carbon Nanotubes‘ [1]. Intrigued, I downloaded the article but due to other pressing commitments such as writing my thesis, I carefully filed it away and ignored it for a few months. I accidentally happened upon this article recently but this time I gave it the attention it deserved.
The paper opens with the premise of exploring the extra-ordinarily high thermal conductivity of single walled nanotubes (SWNTs) that was successfully measured to be 3500 Wm-1K-1 [2]. Thermal conductivity is analogous to electrical conductivity in that it describes the ability of a material to transport heat. This analogy is so apt that even the terminology is the same. For example, high values indicate a good ‘conductor’ while a low value implies that the material is a heat ‘insulator’. To place this property in context, a good heat conductor such as copper has a thermal conductivity of 380 Wm-1K-1 [3] while air – a heat insulator – has a thermal conductivity of 0.025 Wm-1K-1 [3]. Topping the list for macroscopic materials is diamond with a whopping thermal conductivity between 900 – 2300 Wm-1K-1! [3].
Nevertheless, the crown for best thermal conductor sits firmly on the head of the SWNTs. One of the consequences of good thermal conductor is that the temperature of the material increases considerably even for a small heat input. Case and point: the copper bottom pan that becomes scorching hot even after a few seconds of sitting on the flame. The authors of the paper harnessed the thermal conductivity of carbon nanotubes by fabricating a mini heater from them. Here is where the properties of this material get interesting.
Random, entagled network of nanotubes
Those familiar with SWNTs know that these beauties are essentially a rolled-up tubule constructed from a single sheet of graphite. The tubes are capped at the end with half a fullerene. The aspect ratios of SWNTs are quite large. The raw length is usually in the order of microns while the diameter is around 1-2 nm. We can imagine them as pieces of exotic thread. Thread tangles. Everyone knows that. SWNTs tangle to form large, messy networks (as shown in the figure on left). Since the nanotubes are long and skinny in the nano-dimensions, the networks are not only thin but also transparent!
The authors in [1] found that 120 mW from a 12 V source removes frost within one minute (see photo above). They additionally found that entangled networks with a low sheet resistance reach a steady state temperature really quickly. This assertion relies on the intimate relations between electronic and thermal conduction. High electrical conductivity in random networks requires an abundance of low resistance connection paths. Electrons flow along these paths akin to a car cruising on a wide highway. A larger number of highly conducting paths means a lower sheet resistance. This also implies that heat flow along these good connections will be fast leading to a quicker saturation temperature.
So what is the point of having a heater that is also transparent? The title of this blog post should provide a clue…windscreen defrosters! It’s hard not to conjure up visions of an über-fancy car with magic windows that defrost within a minute. Being one of those car owners who have always struggled with a squeegee because the defroster would throw malfunctioning tantrums, dreams of super-quick window defrosters are only one step lower than dreams of chocolate cake slathered with chocolate sauce and a mountain of whipped cream.
References
[1] Yoon, Y.; Song, J.; Kim, D.; Kim, J.; Park, J.; Oh, S. & Han, C. (2007), ‘Transparent film heater using single-walled carbon nanotubes’, Advanced Materials 19(23), 4284–4284.
[2] Pop, E.; Mann, D.; Wang, Q.; Goodson, K. & Dai, H. (2006), ‘Thermal Conductance of an Individual Single-Wall Carbon Nanotube above Room Temperature’, Nano Letters 6(1), 96-100.
[3] http://en.wikipedia.org/wiki/Thermal_conductivity
I’ve decided to try something new. The name of the post title should give a hint. The reasons for this decision are some intricate and convoluted but basically they boil down to one thing: I want to be able to write about my current research in a way that provides me with insight. I have always found that when I am assimilating information to write something or re-ordering results for a paper, I usually end up with much more insight than I began with. So, doing the same process with my current research should have a similar effect. Another source of inspiration has been 
