Sandia’s Hongyou Fan
A Slick, Cheap Way to Grow Coatings
August / September 2010
Volume 8 Number 4
Say you were starting a company to grow the film-like coatings used to protect or help transmit optical information from silicon and other materials. Such coatings are widely used in consumer electronics, semiconductor devices and high-performance glass and ceramics. The two main commercial methods of achieving these coatings—sputtering and chemical vapor deposition (CVD)—require expensive equipment, unforgiving protocols, highly trained operators and a certain amount of risk because lethal chemicals must be maintained at high temperatures and in high vacuum.
But now you learn that another, recently developed technique to accomplish the same end requires no vacuum, is achievable at room temperatures and requires a relatively small budget for equipment. Further, its nanoscopic components self-assemble in a process flexible enough to meet the requirements of a variety of commercial outputs. Finally, safety concerns are minimized because the process takes place in a basically benign environment.
The range of relatively easily achievable applications include optical coatings (e.g., anti-reflective coatings that cover the spectrum from visible to far infrared), three-dimensional nanoscale capacitors, sensors and photovoltaics.
Interested?
Sandia National Laboratories’ Hongyou Fan currently leads the development of this method, called Multifunctional Optical Coatings by Rapid Self-Assembly. Fan says the self-assembly coating process is, for many thin-film applications, competitive with current technologies and it surpasses those processes in manufacturing freedom, reduced equipment and personnel costs and environmental friendliness.
In addition to possessing tunable physical and optical properties, the hollow particles that constitute these thin films may be easily altered to provide new capabilities or may be filled with active particles, such as quantum dots, dye molecules and conducting nanoparticles.
The control over properties opens the door for engineered thin films that may be repaired or adapted in the field.
“We think this breakthrough will certainly impact the thin-film deposition industry,” Fan says.
The technique was recently selected a winner of an R&D 100 Award. Technical papers were published in Science magazine and featured on the cover and frontispiece of technical journals including the Journal of the American Chemical Society, Chemistry and Chemical Communications. Two patents applications already have been filed.
Rather than sputtering tiny bits of material across a small distance to coat a surface, or forming it by depositing material by means of a chemical vapor, the Sandia method disperses commercially available polymers merely by inserting them in common solvents under ambient conditions. It then uses simple spin, dip or spray techniques to coat surfaces. Evaporation of the solvents induces the polymers to self-assemble into multifunctional nanoparticles, as well as into films with tailored optical properties and a nanostructured surface. Because the process is compatible with conventional spray processing, it can be applied directly to the coating of large or complex parts, which current commercial methods are less able to do.
The simple, safe and economical coating process, with equipment costs in the thousands instead of millions of dollars (as is the case for the competing methods) enables the development of multifunctional nanomaterials and optical coatings with architectures and properties regions not attainable by current processing methods.
Because this coating technology is applicable to a wide range of products, costs would vary depending on the application, materials and performance requirements. The material costs are estimated to range from pennies per square foot for simple dielectric films to $15 per square foot for complex high-performance coatings.
The formation of hybrid coatings through the incorporation of organic and inorganic functional elements brings new chemical and physical properties to bear that are not available in conventional coating processes. Other advantages are that the intrinsic hydrophobic nature of the polymer eliminates surface tension and drying stress that can cause cracking in conventional film deposition processes.
Additionally, the low surface energy and nanostructured character of the coating cause the surface to be superhydrophobic, which prevents moisture from deteriorating optical performance.
Final film thicknesses ranging from 100 nanometers to tens of micrometers can be easily controlled by modifications to the precursor concentration and coating process (e.g., coating speed) without the use of expensive equipment.
The work was done in partnership with Lockheed Martin Corp. to identify significant applications for industry, the departments of Defense and Energy and the National Aeronautics and Space Administration; and to integrate this technology into product lines.
For example, applying an optimized anti-reflective coating can inexpensively add an additional 8 to 9 percent transmission through the canopy of aircraft, thereby providing an additional margin of safety to the pilot. Furthermore, the coating is adaptable to field repairs, unlike conventional anti-reflective coatings, which must be repaired in a manufacturing facility after disassembly and shipping.
The low cost lends itself to application on all forms of architectural windows—both glass and plastic, Fan says. Greater optical transmission reduces glare and the need for artificial light. Furthermore, the hydrophobic properties add a self-cleaning feature that reduces maintenance and time spent cleaning windows in skyscrapers, houses, greenhouses and aquariums, among other possibilities.
Neal Singer is a science writer at Sandia National Laboratories.
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