Welding Nanowires

February 10, 2012

One area in which I worked extensively was high temperature sensors. Portions of gas turbine engines are very hot, but these quite complicated machines still need to be monitored for safety, and to enhance performance.

A common technique is to infer the operation of a hot section by monitoring something in a cooler section, such as gas pressure and flow. These are indirect measurements that rely on computer models, so the data they provide can lag the actual values by a few seconds, and they're not as satisfying as having a real sensor in the right place.

Basic components of a gas turbine engine. Federal Aviation Administration diagram, via Wikimedia Commons.

One of my position sensors[1] operated at more than 500°C, and the biggest problem that I had in its development was making electrical connections of the various components to withstand those temperatures. Common solders work to about 300°C, but above that range you need to weld, and the usual option is electric resistance welding.

Resistance welders are used for such bonding tasks as assembling automobiles, but they are often used in assembly of small electric and electronic devices such as the filaments in (soon-to-be-outlawed) incandescent light bulbs. The resistance welding principle is very simple.

A current pulse is sent through copper electrodes that press from opposite sides of the metal components to be welded.[2] Because of contact resistance, the heating power (I²R) is concentrated at the junction between the components. Selecting the proper current and pulse time allows controlled melting of the metals into each other to make a junction.

My application was bonding tiny wires of just a few thousandths of an inch in diameter, but a team of materials scientists at Stanford University and their collaborators have achieved a welding record by using a novel process to weld wires more than three orders of magnitude smaller. The process uses light-induced plasmons to weld silver nanowires in mesh networks used as transparent conductors.[3-4]

Nanowire meshes are being researched as a means to make the transparent conductors used in displays and photovoltaic cells and replace Indium-tin oxide (ITO, typical composition 90% In₂O₃ and 10% SnO₂ by weight) for these applications. Indium is becoming scarce and expensive. The average price of indium in 2010 was about $560/kg.[5]

I reviewed a chemical approach to bonding silver nanowires in a recent article (Silver Nanowire Transparent Electrodes, December 19, 2011). A UCLA research team used a technique that brought nanowires into close contact. The conductive polymer, PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) coating was then used to establish the connections.[6-7]

The Stanford approach is an actual welding process. It's no wonder that silver is used as the wire material. Not only is silver highly conductive, but its surface plasmons are at ultraviolet wavelengths (3.9 eV). Light from a laser excites these surface plasmons, which are an oscillation of electrons at the optical frequency, in the silver wires. This electron excitation is enough to melt the silver in the regions where wires contact each other (see photograph).

A welded silver nanowire mesh. The top nanowires direct the plasmons into the bottom wires to create enough heat to weld the wires together. (Stanford University image by Mark Brongersma, used with permission.)

The plasmons are concentrated at the gaps between the wires, so the heating is localized to these areas. The process is nicely self-limiting, since the welding eliminates the gaps and terminates the heating. This allows formation of silver meshes on inexpensive, flexible, low temperature materials. [3-4] As a demonstration, the Stanford team made a conductive mesh on a sheet of Saran wrap. They were able to crumple the sheet and still maintain conductivity.[4]

This research was supported by the Center for Advanced Molecular Photovoltaics at Stanford University, which is funded by the King Abdullah University of Science and Technology.[4]

References:

1. Devlin M. Gualtieri, "Temperature compensated resonant transmission line sensor," US Patent No. 7,373,272, May 13, 2008.
2. A lesser used technique, which is good for welding wires to flat conductors, has closely spaced electrodes that press the wire against the conductor. The current passes through both contacts, creating two welds at the same time.
3. Erik C. Garnett, Wenshan Cai, Judy J. Cha, Fakhruddin Mahmood, Stephen T. Connor, M. Greyson Christoforo, Yi Cui, Michael D. McGehee and Mark L. Brongersma, "Self-limited plasmonic welding of silver nanowire junctions," Nature Materials, Document No. 3238, February 5, 2012.
4. Andrew Myers, "Stanford engineers weld nanowires with light," Stanford University Press Release, February 6, 2012.
5. US Geological Survey, Indium - USGS Mineral Resources Program, Mineral Commodity Summaries, 2011.
6. Jennifer Marcus, "UCLA team develops highly efficient method for creating flexible, transparent electrodes," UCLA Press Release, November 21, 2011.
7. Rui Zhu, Choong-Heui Chung, Kitty C. Cha, Wenbing Yang, Yue Bing Zheng, Huanping Zhou, Tze-Bin Song, Chun-Chao Chen, Paul S. Weiss, Gang Li and Yang Yang, "Fused Silver Nanowires with Metal Oxide Nanoparticles and Organic Polymers for Highly Transparent Conductors," ACS Nano (DOI: 10.1021/nn203576v), October 28, 2011.