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Silver nanothreads keep you warm

09.01.2015

If we feel that we are cold, it means that the body is losing heat faster than our body can produce it. Therefore, at night we cover ourselves with a blanket, and in winter, in order not to freeze, we put on warm clothes. From the point of view of physics, a woolen sweater or a down jacket cannot warm - they only insulate the body from the external environment. As a result, the heat generated by the body heats the person himself, and not the environment.

It is estimated that on average the human body produces 187 watts of heat, of which approximately 24 watts is lost due to convection, and the remaining 163 watts are thermal radiation. The difference between convection and radiation is easy to understand with this example: when we breathe warm air on our frozen hands, convection heat transfer occurs, and if we extend the same hands closer to a burning fireplace, then infrared radiation warms them. Ordinary clothing prevents convection well, but protects against losses through radiation poorly. And this means that even in the warmest jacket we will still cool down, standing in the cold.

This order of things did not suit the researchers from Stanford, who, armed with knowledge of physics and nanotechnology, undertook to create the warmest clothes. The main challenge was to make a material that could effectively reflect the infrared rays emitted by the human body. Ordinary aluminum foil would do an excellent job of this - it effectively reflects heat radiation. But the material, in addition to keeping warm, must be permeable to moisture - clothes need to "breathe". Physically, it must delay infrared radiation, but at the same time let water vapor molecules through.

For these purposes, a layer of silver nanowires was deposited on an ordinary fabric. The threads form a mesh structure with a pore size of about 200-300 nm, which is about 250 times smaller than the diameter of a human hair. The wavelength of human thermal radiation is approximately 9 microns, so such rays are completely reflected from the nanogrid. At the same time, the pore diameter is sufficient for water molecules to freely pass through them - their size is about 0,2 nm. Another remarkable feature of this material is its conductivity for electricity. If current is applied to clothing coated with silver nanofilaments, it will heat up. To do this, you do not need to connect the sweater to the outlet and make it look like an electric chair, it is enough to use a voltage of less than one volt - absolutely safe for the body.

The natural question is how much silver will be used to make such a material and how strong will such a coating be? It takes about 0,1 gram of silver to make one square meter of silver nano-coated cotton fabric, which does not qualify the resulting clothing as jewelry. The creators of the material tested the stability of their development. It turned out that the fabric with silver nanothreads does not lose its properties after several washing cycles. In addition, silver has an antibacterial effect, which prolongs the life of the fabric.

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