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.