Pictures of the Future    Fall 2007

Promising Particles

Unless climate change can be slowed, the consequences will be dramatic: drought, floods, storms, famine, species extinction, and mass migration. Yet there is still time to prevent the worst from happening—if a substantial reduction in global emissions of greenhouse gases such as carbon dioxide (CO₂) can be achieved (Pictures of the Future, Spring 2007, Technology for the Environment).

"Thanks to the use of new materials we can improve efficiency in the generation, transmission and consumption of energy, both on the part of utilities and consumers," says Dr. Thomas Grandke, head of the Materials & Microsystems department at Siemens Corporate Technology (CT). The use of innovative new coatings, for example, can protect gas and steam turbine blades against the effects of heat and corrosion, which in turn enables higher operating temperatures and thus increases in efficiency (Optimizing Turbine Blades). Ceramic heat shields fulfill the same function in the annular combustion chambers of gas turbines (Ceramic Heat Shields).

Additional examples of climate-friendly technologies include light-emitting diodes (LEDs), which are destined to become one of the most environmentally compatible forms of lighting around. They consume around 80 % less electricity than conventional incandescent lamps and also last as much as 50 times longer (Light-Emitting Developments). Siemens is likewise helping to enhance the world’s subways, express trains, aircraft, and ships with the use of lightweight engineering, enhanced drive systems and, in many cases, new materials (Transportation). For example, the new lightweight aluminum railcars of the Oslo subway are now more environmentally compatible thanks to the use of new materials. They use a third less power than their predecessors, are free of harmful materials, and are more than 94 % recyclable. The use of plastics produced by bacteria should also help make electronic products more environmentally friendly in the future. In the BioFun research alliance, for instance, Siemens scientists are currently investigating the material properties of these biopolymers (Renewable Materials).

At present, materials research is undergoing a veritable revolution. However, the revolutionaries themselves are often invisible to the naked eye. Many of them are smaller than 100 nm—1 nm is one billionth of a meter. Five years ago research institutes were proud if they could produce a few grams of these so-called nanoparticles; today more and more producers are marketing such substances on a commercial level. The stage has therefore been set for the advent of industrial applications on a large scale. Yet this will require more than just the nanoparticles of metal or metal oxides that are currently available on the market.

Their special properties do not fully develop until the nanoparticles have been endowed with certain functions and embedded in a stable medium. It’s only then that they genuinely open the door to enhanced or completely new material properties—and therefore also to materials that can further reduce the burden on the environment. "No matter whether you take a massive block or a small particle of a specific substance, its physical and chemical characteristics such as its electrical conductivity, hardness, magnetism and chemical reactivity remain the same. But as soon as we enter the nanoworld, these properties change dramatically," explains Grandke. "Nanoscale particles have a huge surface area in proportion to their volume, and they experience quantum-mechanical effects."

The result of this basic difference is a range of completely new materials. Below 150 nm, for example, the white pigment titanium dioxide becomes an effective absorber of UV light, which is why nanotechnology is even impacting products such as cosmetics (suntan lotions). Another example is gold. Although known for being extremely inert and therefore a favored anticorrosion agent for high-grade components, gold as a nanoparticle is in fact extremely reactive—a new material property which is now being exploited in the development of new catalysts.

Once again, the reason for this is the difference between a nanoparticle’s surface area and its volume. Whereas a solid cube of 1 cm³ has a surface area of 6 cm², the same-sized cube filled with particles each 10 nm in diameter has a surface area of around 450 m²—some 740,000 times as much. "The great thing is that each element and each structure can in principle be reduced to the nanoscale, where it will then exhibit completely different properties," says Grandke.

Dr. Jens Dahl Jensen has a striking comparison to explain the size of the nanoworld: "Imagine the earth next to a soccer ball, and the soccer ball next to a nanoparticle—that’s the scale of magnitude we’re talking about." Jensen heads the nanoparticle competence field at Siemens CT in Berlin and leads NanoBase, a project sponsored by the German Ministry of Education and Research (BMBF), which involves Siemens as well as other companies and research establishments. The aim of the project is to develop new types of coatings on the basis of functionalized nanoparticles, which will enhance existing technologies and also enable completely new applications.

At Nanolab Jensen and his colleagues are currently investigating how they will have to modify nanoparticles in order to give them special properties. Work safety and environmental protection are paramount considerations here. Strict regulations apply in the lab. Researchers conduct experiments in a fume hood and wear protective clothing. Likewise, the lab’s air conditioning is separate from the system used for the rest of the building. Both incoming and outgoing air is specially filtered in order to prevent any nanoparticles from escaping into the atmosphere.

"Future products containing nanoparticles will have to fix these substances in a protective paint or surface coating. We must ensure that these substances cannot escape into the environment," explains Jensen. "Any potential health risks from this source are also a subject of discussion in the current debate on diesel particulates."

Better Cabin Air. Siemens’ research for the NanoBase project is also focusing on highly active catalytic coatings, which—when incorporated in an appropriate catalytic converter—will be able, for example, to decompose ozone in surrounding air. "These ozone converters could be used in aircraft air conditioning units," Jensen explains.

At an altitude of 10,000 m the air contains up to 550 ppb of ozone per cubic meter, which means it must be treated before being fed into the cabin. That's because ozone is an aggressive and noxious gas. Regulations stipulate a maximum permissible volume of 100 ppb over a three-hour period. Current aircraft air conditioning systems transform ozone into oxygen, but only at temperatures of between 150 and 200 °C before cooling it to cabin temperature. At these high temperatures, catalytic converters using precious metals can efficiently decompose ozone into oxygen.

The goal of Germany’s NanoBase project is to develop materials that will support the transformation of ozone into oxygen without the use of precious metals and at temperatures well under 100 °C. This would give more flexibility to aircraft air conditioning designers since converters would no longer be dependent on the use of high temperatures. This will be particularly important for planes that, for example, use electric compressors to achieve cabin pressure using external air. Such planes will no longer need to use air that has been heated by the engines in order to reach catalytic temperatures.

Although this goal is still a long way from being fully achieved, an initial demonstration model should be ready within two years. This will be able to convert ozone at well under 100 °C. "We’re now developing this prototype for the NanoBase project in cooperation with EADS and other partners," says Jensen. "We’re combining a method introduced in the late 1960s—the so-called chemical nickel process—with nanotechnology." As a rule, such chemical nickel coatings consist of nickel-phosphorus alloys that are deposited on a base material—mainly metallic materials but increasingly plastics and glass as well—to protect them against wear and corrosion. This process involves immersing the base material in a dip tank. On its own, however, the nickel alloy is a poor catalyst. "But if we evenly embed nanoparticles of metal or metal oxide in the topmost layer of the alloy, this creates so-called nanocomposite coatings with highly catalytic properties," explains Jensen.

These modified coatings decompose ozone at much lower temperatures and also work much faster than is the case with conventional converters. Siemens researchers are currently refining the deposition process and testing a wide range of nanoparticles, which is a very time-consuming task. "Just to keep the nanoparticles stable and make sure they don’t clump together in the dip tank and sink to the bottom is a science in itself," Jensen says. "Another major challenge is to ensure that they are evenly embedded in the nickel alloy. It takes all our know-how, and we still learn something new every day."

But it’s not just the aerospace industry that is interested in these high-tech catalysts. "In just a few years we could well see our nanocomposite coatings in high-speed trains and in cars. It’s a huge market," says Jensen. "In railcars, for example, they could be used not only for air conditioning but also to keep vehicle bodies clean. That’s because catalytically active, self-cleaning surfaces would also be impervious to graffiti."

This would represent a major benefit for rail operators, who today spend a huge amount of time and money on removing spray paint. It takes two to three employees a whole working day to clean a suburban train, for example. Often the graffiti can only be removed with the help of powerful chemicals that get rid of not only the scribbles and scrawls but also the paint and coatings underneath.

"Deutsche Bahn alone could save tens of millions of euros in this area every year," says Jensen. "Alternatively, nanocomposite coatings can also be used in filter elements for water treatment systems. Furthermore, they can increase the sensitivity of the chemical sensors used for quick and easy detection of drugs or explosives."

Withstanding the Elements. Aside from the development of highly active catalytic coatings, the NanoBase project is also looking at improved protective coatings for products and systems used for electrical engineering and transportation. Today, plastic sheathing is normally used to protect electronic components and systems against the elements. Yet this is not always sufficient, especially when components are exposed to rough conditions, such as those in vehicle engine compartments and industrial machinery.

Molecules of water, air, or harmful gases can penetrate the plastic and cause electronic component inside to fail. "This can even knock out complete industrial plants or traffic guidance systems, sometimes with serious consequences for human safety and the environment, not to mention the financial impact," says Dr. Peter Gröppel, a chemist at Siemens CT in Erlangen.

Likewise, the service life of organic LEDs decreases markedly when they are exposed to dampness and oxygen. Gröppel is therefore working on new nanopaints and adhesives that offer a radically improved barrier effect. "In our labs here in Erlangen we’re synthesizing nanocomposites on the basis of modified sheet silicates. These consist of nanoparticles with a thickness of one nanometer and a length and breadth of 500 nm. These dimensions generate the desired barrier effect. Just to give you an example, it takes water molecules about ten times as long to penetrate this coating compared with conventional protective paints," explains Gröppel.

What’s more, conventional protective paints have an additional disadvantage. In many cases they contain organic solvents that are harmful to the environment. "In the NanoBase project, our target for 2009 is to develop a solvent-free, water-based protective nanopaint that also possesses greatly enhanced product properties," states Gröppel.

Visionaries in the nanotechnology field are already dreaming of developing a self-repairing paint. People would never have to worry again about getting minor scratches on their cars. Instead, nanocapsules in the paint would open at the edge of a scratch, releasing a catalyst that would react with other components in the paint. Such components might contain tiny drops of a smaller functionalized polymer. These would fill and seal the scratch before the metal underneath could begin to corrode, with the result that the vehicle would once again look as good as new.
                                                                                                           Ulrike Zechbauer