Black phosphorus new material for improving optical communication

 

Courtesy : National Science Foundation This illustration shows a high performance photodetector, which uses few layers of black phosphorus (red atoms) to sense light in a waveguide (green material). Graphene (gray atoms) is also used to tune the performance. Phosphorus, a highly reactive element commonly found in match heads, tracer bullets, and fertilizers, can be turned into a stable crystalline form known as black phosphorus. In a study by the University of Minnesota (U of M), researchers used an ultrathin black phosphorus film--only 20 layers of atoms--to demonstrate high-speed data communication on nanoscale optical circuits. The devices showed vast improvement in efficiency over comparable devices using the earlier "wonder material" graphene. The existence of black phosphorus has been known for more than a century, but only in the past year has its potential as a semiconductor been realized. Because of unique properties, black phosphorus can be used to detect light very effectively, making it desirable for optical applications. The U of M team created intricate optical circuits in silicon, then laid thin flakes of black phosphorus over these structures using facilities at U of M's Nano Center. The team also showed that the devices could be used for real-world applications by sending high-speed optical data over fibers and recovering it using the black phosphorus photodetectors. The group demonstrated data speeds up to three billion bits per second, which is equivalent to downloading a typical HD movie in about 30 seconds. "Black phosphorus is an extremely versatile material," said Professor Steven Koester, who contributed to the project. "It makes great transistors and photodetectors, and has the potential for light emission and other novel devices, making it an ideal platform for a new type of adaptable electronics technology." Funded in part by the National Science Foundatioin (grant ECCS 13-51002). To learn more, see the U of M Discover story Black phosphorus is new wonder material for improving optical communication. (Date of Image: March 2015) Credit: University of Minnesota General Restrictions: Images and other media in the National Science Foundation Multimedia Gallery are available for use in print and electronic material by NSF employees, members of the media, university staff, teachers and the general public. All media in the gallery are intended for personal, educational and nonprofit/non-commercial use only. Images credited to the National Science Foundation, a federal agency, are in the public domain. The images were created by employees of the United States Government as part of their official duties or prepared by contractors as "works for hire" for NSF. You may freely use NSF-credited images and, at your discretion, credit NSF with a "Courtesy: National Science Foundation" notation. Additional information about general usage can be found in Conditions. source : nsf.gov

Ceramics don't have to be brittle: Incredibly light, strong materials recover original shape after being smashed

 

September 11, 2014

California Institute of Technology

Materials scientists have developed a method for creating new structural materials by taking advantage of the unusual properties that solids can have at the nanometer scale. They have used the method to produce a ceramic (e.g., a piece of chalk or a brick) that contains about 99.9 percent air yet is incredibly strong and can recover its original shape after being smashed by more than 50 percent.

---

This sequence shows how the Greer Lab's three-dimensional, ceramic nanolattices can recover after being compressed by more than 50 percent. Clockwise, from left to right, an alumina nanolattice before compression, during compression, fully compressed, and recovered following compression.

Imagine a balloon that could float without using any lighter-than-air gas. Instead, it could simply have all of its air sucked out while maintaining its filled shape. Such a vacuum balloon, which could help ease the world's current shortage of helium, can only be made if a new material existed that was strong enough to sustain the pressure generated by forcing out all that air while still being lightweight and flexible.

Caltech materials scientist Julia Greer and her colleagues are on the path to developing such a material and many others that possess unheard-of combinations of properties. For example, they might create a material that is thermally insulating but also extremely lightweight, or one that is simultaneously strong, lightweight, and nonbreakable -- properties that are generally thought to be mutually exclusive.

Greer's team has developed a method for constructing new structural materials by taking advantage of the unusual properties that solids can have at the nanometer scale, where features are measured in billionths of meters. In a paper published in the September 12 issue of the journal Science, the Caltech researchers explain how they used the method to produce a ceramic (e.g., a piece of chalk or a brick) that contains about 99.9 percent air yet is incredibly strong, and that can recover its original shape after being smashed by more than 50 percent.

"Ceramics have always been thought to be heavy and brittle," says Greer, a professor of materials science and mechanics in the Division of Engineering and Applied Science at Caltech. "We're showing that in fact, they don't have to be either. This very clearly demonstrates that if you use the concept of the nanoscale to create structures and then use those nanostructures like LEGO to construct larger materials, you can obtain nearly any set of properties you want. You can create materials by design."

The researchers use a direct laser writing method called two-photon lithography to "write" a three-dimensional pattern in a polymer by allowing a laser beam to crosslink and harden the polymer wherever it is focused. The parts of the polymer that were exposed to the laser remain intact while the rest is dissolved away, revealing a three-dimensional scaffold. That structure can then be coated with a thin layer of just about any kind of material -- a metal, an alloy, a glass, a semiconductor, etc. Then the researchers use another method to etch out the polymer from within the structure, leaving a hollow architecture.

The applications of this technique are practically limitless, Greer says. Since pretty much any material can be deposited on the scaffolds, the method could be particularly useful for applications in optics, energy efficiency, and biomedicine. For example, it could be used to reproduce complex structures such as bone, producing a scaffold out of biocompatible materials on which cells could proliferate.

In the latest work, Greer and her students used the technique to produce what they call three-dimensional nanolattices that are formed by a repeating nanoscale pattern. After the patterning step, they coated the polymer scaffold with a ceramic called alumina (i.e., aluminum oxide), producing hollow-tube alumina structures with walls ranging in thickness from 5 to 60 nanometers and tubes from 450 to 1,380 nanometers in diameter.

Greer's team next wanted to test the mechanical properties of the various nanolattices they created. Using two different devices for poking and prodding materials on the nanoscale, they squished, stretched, and otherwise tried to deform the samples to see how they held up.

They found that the alumina structures with a wall thickness of 50 nanometers and a tube diameter of about 1 micron shattered when compressed. That was not surprising given that ceramics, especially those that are porous, are brittle. However, compressing lattices with a lower ratio of wall thickness to tube diameter -- where the wall thickness was only 10 nanometers -- produced a very different result.

"You deform it, and all of a sudden, it springs back," Greer says. "In some cases, we were able to deform these samples by as much as 85 percent, and they could still recover."

To understand why, consider that most brittle materials such as ceramics, silicon, and glass shatter because they are filled with flaws -- imperfections such as small voids and inclusions. The more perfect the material, the less likely you are to find a weak spot where it will fail. Therefore, the researchers hypothesize, when you reduce these structures down to the point where individual walls are only 10 nanometers thick, both the number of flaws and the size of any flaws are kept to a minimum, making the whole structure much less likely to fail.

"One of the benefits of using nanolattices is that you significantly improve the quality of the material because you're using such small dimensions," Greer says. "It's basically as close to an ideal material as you can get, and you get the added benefit of needing only a very small amount of material in making them."

The Greer lab is now aggressively pursuing various ways of scaling up the production of these so-called meta-materials.

 

California Institute of Technology. "Ceramics don't have to be brittle: Incredibly light, strong materials recover original shape after being smashed." ScienceDaily. ScienceDaily, 11 September 2014. <www.sciencedaily.com/releases/2014/09/140911135450.htm>.

 

 

---

Story Source:

The above story is based on materials provided by California Institute of Technology. The original article was written by Kimm Fesenmaier. Note: Materials may be edited for content and length.

---

Journal Reference:

1. L. R. Meza, S. Das, J. R. Greer. Strong, lightweight, and recoverable three-dimensional ceramic nanolattices. Science, 2014; 345 (6202): 1322 DOI: 10.1126/science.1255908

Materials for the building industry: A shape-conscious alloy

 

June 20, 2014

Empa Swiss Federal Laboratories for Materials Science and Technology

When the frame of a pair of glasses is bent out of shape, it's not that easy to return it to its original form. If, however, your spectacles are made of a shape memory alloy then you don't have a problem. Just place the frame in hot water and bingo! – they're as good as new again. Empa researchers have now shown that these materials can also find applications in the building industry. For example in the reinforcement of bridges.

---

Forging an iron-based shape memory alloy (SMA) cast block: To shape the block, it is heated to around 1,150 degrees Celsius.

Shape memory alloys, or SMAs, possess the ability to return to their original shape after being severely deformed, either spontaneously or following the application of heat. This makes them useful materials, not just for making spectacle frames but also for technical applications such as thermostats, stents and micro-actuators. Other applications in the construction industry are conceivable too, for example in the reinforcement of bridges.

If a concrete beam is cast with reinforcing rods made of an SMA material, these can then be "activated" through the application of heat. They attempt to return to their original shape, but because of their concrete sheath they cannot do so, thus exerting a pre-stressing force on the beam. This effect can be used, for example, to pre-stress a complete bridge span. In order to generate the necessary force the SMA rods must simply be heated by passing an electric current through them. This obviates the need for using elaborate tensioning systems and jacket tubes, as used in conventional pre-stressing techniques.

The nickel titanium alloys used to make spectacle frames or stents are not very suitable for use in the construction industry. Iron-based SMA products are much more attractive, since both the raw materials and the processing costs are far cheaper. However, to date one problem has remained a stumbling block: to activate the memory effect the materials currently used must be heated up to 400° C, which for applications involving concrete or mortar, or other heat sensitive materials, is too high. Empa researchers led by Christian Leinenbach of the Joining Technology and Corrosion Laboratory have now succeeded in developing a novel iron-manganese-silicon SMA alloy which is activated at just 160° C, a temperature much more suitable for use with concrete. The material science researchers "designed" a range of virtual alloys using thermodynamic simulations, and then selected the most promising combinations. These were then manufactured in the laboratory and their shape memory characteristics tested, with great success. Several of the new materials met the construction engineers' requirements, an important milestone on the path to providing economic shape memory steel alloys for industrial applications -- in other words, manufacturing them by the ton.

The long road from laboratory to finished product

Christoph Czaderski, of Empa's Engineering Structures Laboratory, believes that iron-based SMA materials have a promising future in the building industry since the process of pre-stressing is simpler and therefore cheaper than in conventional techniques. In addition they may allow engineers to create pre-stressed structures which are impossible or very difficult to achieve using conventional techniques. These include the use of short fibre concrete, near surface mounted laminates, column wrapping and ribbed armouring steel. A feasibility study financed by the Commission for Technology and Innovation (CTI) recently showed that it is possible to produce the new alloys on an industrial scale, not just a few kilos for laboratory use. The manufacturing process has been developed in collaboration with Leoben University (Austria), the Technical University Bergakademie Freiberg (Germany), and the German company G. Rau GmbH.

The working of cast ingots, each about 100 kg in weight, into thin strips around 2 mm thick or ribbed armouring steel rods at temperatures over 1000° calls for high degree of technical knowledge, and the appropriate infrastructure. The working process also needs to be adapted for use with the novel alloys. The metal strips produced in this way demonstrated their capabilities in the tests which followed, during which they were inserted into slits cut in the surface of concrete beams and fixed with adhesive. To carry forward the developments made at Empa, a start-up company, re-Fer AG, has been set up. This will in future produce and distribute iron-based SMA for the construction industry. The cost of the new products is expected to be about the same order of magnitude as that for stainless steel based materials.

Engineers develop new materials for hydrogen storage

 

From left, Scanning electron microscope image of the hexaboride structures at 5 microns and 500 nanometers.

Engineers at the University of California, San Diego, have created new ceramic materials that could be used to store hydrogen safely and efficiently.

The researchers have created for the first time compounds made from mixtures of calcium hexaboride, strontium and barium hexaboride. They also have demonstrated that the compounds could be manufactured using a simple, low-cost manufacturing method known as combustion synthesis.

The work is at the proof of concept stage and is part of a $1.2 million project funded by the National Science Foundation, a collaboration between UC San Diego, Alfred University in upstate New York and the University of Nevada, Reno. The manufacturing process for the ceramics is faster and simpler than traditional methods used to manufacture these types of materials. The researchers presented their work in March 2014 at the third International Symposium on Nanoscience and Nanomaterials in Mexico.

“We are looking for solid materials that can store and release hydrogen easily,” said Olivia Graeve, a professor at the Jacobs School of Engineering at UC San Diego, who has gained international recognition as a nanomaterials manufacturing expert.

Storing hydrogen has become increasingly important as hydrogen fuel cells become more popular power sources in industry and elsewhere. But hydrogen, the lightest element on the periodic table, is difficult to store. It tends to diffuse through the walls of pressurized tanks. It also needs to be compressed in order to occupy manageable amounts of space when stored.

The resulting ceramics are essentially crystalline structures in a cage of boron. To store hydrogen, the researchers would swap the calcium, strontium and boron with hydrogen atoms within the cage.

Engineers mixed boron with metal nitrates and organic fuels, such as urea, in a box furnace at temperatures below 400 degrees Celsius—roughly 750 degrees Fahrenheit—cooler than a commercial pizza oven. The nitrates and organic fuels ignite, generating heat that then drives the reaction without the need for an external source of power. This method is known as combustion synthesis.

“It’s a very simple, nice process,” Graeve explained.

Graeve earned a bachelor’s degree in structural engineering from UC San Diego in 1995 before earning a Ph.D. in materials science and engineering from the University of California, Davis in 2001. She was on faculty at the University of Nevada at Reno and Alfred University before joining UC San Diego in 2013.

Graduate student James Cahill will present the work at Research Expo, April 17, at the Jacobs School.