Personal Thermal Management by Metallic Nanowire-Coated Textile

Po-Chun Hsu, ^† Xiaoge Liu, ^‡ Chong Liu, ^† Xing Xie, ^§ Hye Ryoung Lee, ^∥ Alex J. Welch, ^† Tom Zhao, ^†
and Yi Cui^{* ,†,⊥
†} Department of Materials Science and Engineering, ‡ Department of Applied Physics, § Department of Civil and Environmental
Engineering, and
^∥ Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States
^⊥ Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park,
California 94025, United States

dx.doi.org/10.1021/nl5036572


Heat! Definitely something to conserve during the heavy Canadian winters. This paper exposes a new silver nanowires-based material with strong thermal insulation properties. Authors made a glove out of it, and the thermal images are impressive.

Globally, almost half of the energy produced is used for indoor heating purposes. Considering the commonly used materials, the heat retention efficiency is not ideal; we want to keep as much of the heat as possible indoors, or better, as close to our skin as possible. Another issue, which touches more than our comfort, is the greenhouse gases emission. Some studies show that about 1/3 of these emissions originate from our heating efforts. A large amount is wasted on regulating the temperature of empty spaces and inanimate objects. Some energy can be saved by implementing a "personal thermal management" system. Such devices are ideally worn with or around normal clothes, and they should be able to regulate the body's temperature actively and/or passively.

In order to properly understand the main phenomenon being manipulated here one must remember that heat can be emitted as Infra-Red (IR) radiation; electromagnetic waves of energy released or absorbed by molecules when they change their rotational-vibrational states (mostly vibrational, rotation is more in the micro-wave domain). Electromagnetic waves have specific wavelengths. They can pass physical barriers only if the holes in the material are larger than their wavelength. Smaller, and the waves tend to be reflected back. A simple analogy is a tennis ball as the wave, and a tennis racquet net as the material this radiation encounters. The holes in the net are smaller than the ball, this bounces back the ball. This is applied in the microwave ovens' design, allowing you to see your food as it's heated. Engineering that spacing size is the key in reflecting heat.

Now that their working principle is clear, let's see their work. In short, Silver NanoWires and Carbon Nano Tubes (AgNW, CNT) cloths were fabricated and their thermal properties and viability compared. The dimensions of the empty space between the fibres are smaller than the human body radiation wavelength spectrum (peak around 9 µm), controlled between 200 and 300 nm. This range allows for humidity to permeate as well (reduced breathability of 2% and 4.6% for AgNW and CNT coatings respectively). Moreover, both materials, silver and carbon nanotubes, are conductive. Electricity is another form of energy, and heat can be generated by passing a current through a conductor (Joule heating). It was shown that both materials have similar power consumption rates when used as heating modules.
The property of a material to emit thermal radiation is called emissivity. Silver's emissivity is significantly less than that of normal cloth and CNT, however, CNTs are chemically stable and flexible, reliable in personal thermal management systems. AgNW coating assures 21% more insulation than normal cotton cloth.

So far, they showed highly insulating, conductive materials being potentially and effectively used in clothing. The next step was to approximate how much energy could be saved by wearing a personal thermal management system of their design. In their testing conditions they arrived at savings of around 1000 kWh per year per person with a AgNW coating. Not bad! Moreover, the authors support the viability of other metal nanowires to be used for the same purpose: copper, nickel, aluminium, metal oxides, metal nitrides. Basically, any material with good reflective properties and low emissivity can be used in such an application.
Unfortunately for the fashionistas, this coating is not shiny in any way, it has a dark-grey tint, I hope it can be made in at least 50 shades.


At about 0.1 g of silver per square meter of coated surface this material appears to have an industrial future. I suppose they focused on silver due to the synthesis comfort and chemical stability. They did mention the oxidation of the metal's surface, but it can easily be avoided by surface passivation. A few years ago, graphene's viability as a corrosion-inhibiting film was shown, and one of my colleagues did her M.Sc. work on that subject. Ni and Cu are known catalysts for Gr growth. Nanowires from these materials coated with graphene can have the best of both worlds: low emissivity (just speculation from my part at this point), good conductivity, and high chemical stability. However, I know as a fact that Gr degrades with time, especially monolayers. This is due to its synthesis being polycrystalline. The defects present at the grain boundaries are not as stable as the bonds in the pristine Gr lattice. An ideal monocrystal graphene coating can indeed be a long-term surface protector (again, educated guess). I attempted the coating of Ag nanowires during my work, alas all the attempts were unsuccessful. However, I managed to see the result of interesting fluid dynamics in the reaction chamber which explained a phenomenon that takes place in CVD synthesis acting upon the catalyst's surface by encouraging heterogenous evaporation of the metal - my point is, I learned something from the efforts even if the desired results were lacking.

Penta-graphene: A new carbon allotrope

Shunhong Zhang ^a,b,c , Jian Zhou ^c , Qian Wang ^a,b,c,1 , Xiaoshuang Chen ^d,e , Yoshiyuki Kawazoe ^f , and Puru Jena ^c
a Center for Applied Physics and Technology, College of Engineering, Peking University, Beijing 100871, China; ^b Collaborative Innovation Center of Inertial Fusion Sciences and Applications, Ministry of Education, Beijing 100871, China;
^c Department of Physics, Virginia Commonwealth University, Richmond, VA 23284;
^d National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China;
^e Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China;
^f Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved January 5, 2015  (received for review August 28, 2014)

www.pnas.org/cgi/doi/10.1073/pnas.1416591112
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1416591112/-/DCSupplemental.



I have little knowledge on theoretical chemistry laboratories, I should try to explore that section a bit more. I am not familiar with the authors, however I enjoyed their work exposed here.

Graphite, diamond, graphene, carbon nanocones, nanochains, graphdyine, and several other predicted allotropes are composed of carbon atoms arranged in specific structures. Graphene, the 2D "miracle" material, has received a significant amount of attention since its isolation in 2004 by Novoselov and Geim, and not discovery as many reporters mistakenly state.
It has been studied since the late 1800s under the graphite oxide form. My first contact with it was the 1962 paper from Boehm, Clauss, Fischer, and Hofmann. Among the first TEM images were taken by Ruess and Vogt in 1948. From 1970s to late 1990s the studies continued, but no few-layer specimens could be easily obtained, until 2004. Note, SiC epitaxial growth of Gr has been around since the 1970s, but the quality of the obtained "monolayer graphite" (as it was initially called) was unsatisfactory for further studies; the substrate-graphite interactions were studied by Oshima and Nagashima in 1996.

There are a few predicted carbon allotropes with various designs showing more impressive properties than Gr, such as anisotropic Dirac cones (from what I understand, these allow a better control over the charge carrier mobility, in other words how fast the charges are transported through the material), inherent ferromagnetism (atom-thick magnetic sheets, probably only imagination can limit their applications), superconductivity (if found at room temperature it becomes an instant gold mine), and high catalytic reactivity (atom-thick catalytic sheets with a significant active surface/volume ratio). Impressive, but there is still work to be done until those structures become easy to obtain.

In most cases the popular carbon allotropes are arranged in hexagons, with pentagons and heptagons (sometimes even higher order polygons) being considered as structure defects, true for graphene and carbon nanotubes. However, the 5C rings are important in bucky balls and at the carbon nanotube termini. Graphene has its properties from the hexagonal structure, and in this paper the properties of a pentagonal lattice (think Cairo pentagonal tiling) are theoretically studied. An immediate observation which came to mind was the fact that there are sp² and sp³ hybridized carbons in the structure, I was intrigued to learn how the charges migrate in it.

Interesting detail, but predictable simply from the design, is that this sheet, if stretched on one axis, expands on the one perpendicular to it as well. Imagine expanding an image from a side in a software, but maintaining its aspect ratio. The image expands on both axis. This property is described by a negative Poisson's ratio, and labels penta-graphene (pGr) as an auxetic material. This is usually seen in some polymer structures (Gore-Tex), minerals, and even paper.

From an energetic point of view, this structure is preferred to the smallest C20 fullerene, but is metastable compared to graphene. This means that its synthesis is highly plausible. This stability is in part due to the presence of sp³ bonds which release a bit of the strain. Thermal studies showed that it can handle temperature up to 1000K, meaning that this structure is positioned in a reasonably deep stability well.

Electronically, pGr is a semi-conductor with a 3.25 eV band gap [calculated through the Heyd–Scuseria–Ernzerhof (HSE06) functional]. They indicate that the electronic states near its Fermi level originate from the sp² hybridized C atoms, and that the electron delocalization is a consequence of the sp²  p_z orbitals' spatial separation from the presence of the sp³ hybridizations. I was slightly disappointed to not have seen any theoretical charge-transfer mobility calculations, then again there might be a reason for that which I am missing.

Moreover, they even considered the penta-Gr nanotubes, or "penta-tubes". Graphene nanotubes are known to have different properties (from metallic to semiconducting) depending on their zone folding [defined by the chiral vector (n, m)]. In the penta-Gr case, the authors predict "chirality-independent semiconducting carbon nanotubes". No matter the zone folding, they will more or less behave in a similar fashion. I am still trying to figure out the implications.

As soon as I finished reading the paper I considered the challenge of synthesising the material. Their proposal remains theoretical (chemical exfoliation from a T12-carbon phase). Stability domains in which both sp² and sp³  hybridizations can form bonds in an ordered manner unfortunately surpass my knowledge base. I see this comparable to a composer who wants to harmoniously join two musical keys in one composition. Due to the metastable nature of pGr and to its symmetry, I hardly support its atomic catalytic construction route (similar to CVD for Gr) from a precursor. A step by step chemical synthesis route could be imagined. Probably a variation and succession of McMurry, Michael, and/or Suzuki reactions, allowing to make nanosheets of pGr (disclaimer: my organic chemistry is rusty). Diels-Alder could work as well. I also believe that the chemical frame of interest should be around the cyclopentadiene moiety in these cases. I hope that this material will be made a reality as soon as possible. There are reasons to believe that multilayered variations of pGr and Gr can produce intriguing semiconducting structures.

Application of Tungsten as a Carbon Sink for Synthesis of Large-domain Uniform Monolayer Graphene Free of Bilayers/Multilayers

Wenjing Fang† , Allen Hsu† , Yong Cheol Shin§ , Albert Liao† , Shengxi Huang† , Yi Song† , Xi Ling† , Mildred S. Dresselhaus†,∥ , Tomas Palacios† and Jing Kong†,*

†Department of Electrical Engineering and Computer Sciences, §Department of Material Science and Engineering and ∥Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States  

Keywords: single crystal, monolayer graphene, Cu enclosure, carbon diffusion, carbon sink

DOI: 10.1039/C4NR07418A

  

As a student who has worked extensively on graphene (Gr) synthesis, I have received this paper with open arms for several reasons. First, Mildred Dresselhaus is one of my favourite characters in Carbon Science. She is now an Emerita Institute Professor at MIT with numerous awards and papers, proof of an exemplary career. Second, the growth process used is Chemical Vapour Deposition (CVD) with which I am pretty familiar. As far as I know, it is Pr. Ruoff's team which (I believe) first studied graphene synthesis on copper through a CVD process. A carbon precursor, popularly methane (CH₄), is exposed to copper at a high temperature and low pressure. A catalytic reaction then takes place, decomposing the methane and forming honeycomb-like organized crystal structures of sp² C=C bonds. Usually several crystals of this material are grown, and ideally a single crystal is desired, with as large a surface as possible.

What happened here? In short, it was found that tungsten metal can be used to trap carbon:

2W + C --> W₂C 

in their copper enclosures, which in turn allows for the synthesis of monolayer graphene.

Carbon leaks through the small gaps of the enclosures which then diffuses inside and acts as a C source. In their previous work they obtain mono and bilayer Gr inside the enclosure using this method. So far so good. The catch is that in most applications monolayer Gr is more valuable than multilayer. They found that by adding a tungsten foil in their enclosure there was no inside Gr growth, and that the outside presented only monolayer Gr!
The process is a bit more interesting than that, though. Their results expose the inverse behaviour of Gr growth on the enclosures with and without W foil. Without the foil the Gr layers grow and grow, making multilayer islands, a typical behaviour (often observed by other labs, and I still deal with it as well). However, with the foil, Gr grows outside for about 20 minutes, hinting at the formation of multilayered islands, which eventually start to disappear. Only monolayer graphene remains by the end of the 2h reaction time! But, after 5h, the multi-layer islands begin to appear again, and Gr grows inside as well. This points to some sort of saturation IMO. Some images are given in their Supporting Information.

They further investigated the effect of W on the monolayer using sequential flowing of ¹³CH₄ for 15 minutes, and ¹²CH₄ for the rest of the 2h. This isotope usage can give interesting data when using spectroscopy techniques which are dependant on the atom's mass, such as Raman, IR. Raman spectroscopy is probably the most useful carbon materials characterisation tool, which is what they used. The Raman peaks for the isotopes are easily distinguishable and they allow for ratio calculations. They discovered that only ¹³C is present in the monolayer, showing that the tungsten does not affect it, no carbon substitutions for the rest of the reaction time.
By the way, this isotope substitution technique has also been nicely used by Xuesong Li et al. from the Ruoff group on their study of graphene growth.

More data follows! Curiosity is key in science, thus they looked further into tungsten's role in this with an X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) study of the chemical changes. It was discovered that W does not evaporate on the copper, nor on the Gr monolayer. However, they have found copper on the tungsten foil. Interesting, but to be expected, as it is known that copper exhibits a small degree of evaporation in thee conditions (1045⁰C being close to copper's melting point). Does this copper layer affect tungsten's behaviour? Apparently not. They took the analysis further by constructing an enclosure with one more level. A tungsten foil trapped in a copper enclosure (A) which is closed in another copper enclosure (B). This has several implications. By using a double enclosure geometry, the carbon quantity migrating towards the core of the enclosure gets limited at the inside surface of B. There, graphene starts to form, eventually completely passivating both the inside and outside surfaces. Any more carbon trying to go in will be blocked by a monolayer of the touchest material on earth. This eventually limits the amount of C arriving on the outside surface of A. At this level we have the behaviour previously presented with a single layer of Gr obtained outside, and nothing growing on the inside surface, due to carbon being consumed towards the formation of W₂C.


I find this multi enclosure design enthralling, makes me think of a sponge which filters as it absorbs deeper and deeper towards its core. Imagine having such enclosures with different entities (I'm thinking metals and their oxides, ceramics, and other inorganics) having specific interactions at each surface level. Growth of nano-rods/wires joined with graphene, and other novel nano structures seem plausible with this design. Tuning the surfaces with nucleation sites is one of the pathways I see.
The Dresselhaus group is one to be closely followed as far as carbon science goes. I am looking forward to more of their work.