Using light, researchers convert 2-D patterns into 3-D objects

“This is a novel application of existing materials, and has potential for rapid, high-volume manufacturing processes or packaging applications,” says Dr. Michael Dickey, an assistant professor of chemical and biomolecular engineering at NC State and co-author of a paper describing the research.

The process is remarkably simple. Researchers take a pre-stressed plastic sheet and run it through a conventional inkjet printer to print bold black lines on the material. The material is then cut into a desired pattern and placed under an infrared light, such as a heat lamp.

The bold black lines absorb more energy than the rest of the material, causing the plastic to contract – creating a hinge that folds the sheets into 3-D shapes. This technique can be used to create a variety of objects, such as cubes or pyramids, without ever having to physically touch the material. The technique is compatible with commercial printing techniques, such as screen printing, roll-to-roll printing, and inkjet printing, that are inexpensive and high-throughput but inherently 2-D.

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Researchers from North Carolina State University have developed a simple way to convert two-dimensional patterns into three-dimensional (3-D) objects using only light. Credit: Ying Liu, North Carolina State University

By varying the width of the black lines, or hinges, researchers are able to change how far each hinge folds. For example, they can create a hinge that folds 90 degrees for a cube, or a hinge that folds 120 degrees for a pyramid. The wider the hinge, the further it folds. Wider hinges also fold faster, because there is more surface area to absorb energy.

“You can also pattern the lines on either side of the material,” Dickey says, “which causes the hinges to fold in different directions. This allows you to create more complex structures.”

The researchers developed a computer-based model to explain how the process works. There were two key findings. First, the surface temperature of the hinge must exceed the glass transition temperature of the material, which is the point at which the material begins to soften. Second, the heat has to be localized to the hinge in order to have fast and effective folding. If all of the material is heated to the glass transition temperature, no folding will occur.

“This finding stems from work we were doing on shape memory polymers, in part to satisfy our own curiosity. As it turns out, it works incredibly well,” Dickey says.

More information: The paper, “Self-folding of polymer sheets using local light absorption,” was published Nov. 10 in the journal Soft Matter, and was co-authored by Dickey; NC State Celanese Professor of Chemical and Biomolecular Engineering Jan Genzer; NC State Ph.D. student Ying Liu; and NC State undergraduate Julie Boyles. The work was supported, in part, by the U.S. Department of Energy.

Abstract
This paper demonstrates experimentally and models computationally a novel and simple approach for self-folding of thin sheets of polymer using unfocused light. The sheets are made of optically transparent, pre-strained polystyrene (also known as Shrinky-Dinks) that shrink in-plane if heated uniformly. Black ink patterned on either side of the polymer sheet provides localized absorption of light, which heats the underlying polymer to temperatures above its glass transition. At these temperatures, the predefined inked regions (i.e., hinges) relax and shrink, and thereby cause the planar sheet to fold into a three-dimensional object. Self-folding is therefore achieved in a simple manner without the use of multiple fabrication steps and converts a uniform external stimulus (i.e., unfocused light) on an otherwise compositionally homogenous substrate into a hinging response. Modeling captures effectively the experimental folding trends as a function of the hinge width and support temperature and suggests that the hinged region must exceed the glass transition temperature of the sheet for folding to occur.

Provided by North Carolina State University (news : web)

UC chemistry research looks to turn food waste into fuel

The project, “Biodiesel Fuel from Food Waste,” focuses on the potential to produce biodiesel from oil extracted from garbage. With the depletion of oil reserves, research into alternative fuels has exploded, especially in the area of renewable resources such as garbage and food waste. One particular resource, brown grease from food waste, has yet to realize its full potential, but thanks to the efforts of University of Cincinnati chemistry researchers, the project is finding ways to optimize this resource.

One of the goals of the project was to determine the amount of usable oil that can be obtained from food waste. To do this, the researchers collected food waste—by hand— from an HCMC University of Science student canteen as well as a private residence. Samples of the food waste were then either sun-dried or dried in an oven—a process Nubel describes as “extremely dirty and smelly”—and then ground up and loaded into a homemade extraction thimble bag so that oils contained in the food could be extracted through a Soxhlet extraction.

It was during the extraction process that Nubel made an important discovery. “During this step I determined a much more efficient way of pulling remnants of oil from the bag by using the suction formed from the extractor.”

McCallister, who also helped with the food waste collection, sample preparation and oil extraction, adds that after the extraction, “we converted the oil into biofuel through an acid-based reaction.”

Part of the conversion process requires that the extracted oil be degummed to further purify it. “During the degumming process we realized that layer separation is best completed through centrifuge rather than other conventional methods because it’s both higher yielding and much quicker—by about 24 hours,” says Nubel.

From there, the oil was converted to biodiesel fuel by using a solvent, which required the researchers to determine the correct amount of solvent to add per time and heat in order to yield the highest amount of biodiesel fuel.

Despite the complexity of the process—and the sometimes unseemly conditions—the results were worth the effort. “This job may have easily qualified for Mike Rowe’s show, ‘Dirty Jobs,’ but the experiment was a success and the results are awaiting publication,” says Nubel.

Herrmann also worked on the project, though in a slightly different area. He worked on taking vegetation found in Vietnam and extracting compounds from them. In a process similar to the Soxhlet extraction, Herrmann used column chromatography, which meant he also had to grind up the plants so that compounds could be extracted. “My job was to run these columns and attempt to purify one compound from thousands. We did manage to separate one compound from the plant we worked on and it was the first time that the compound had been extracted from that plant,” he says.

Not only was the research project a success, but so were the researchers themselves. Pinhas says, “What I was told in Vietnam is that the group of students from UC was the best group of students that ever participated in this program with chemists in Vietnam.”

Provided by University of Cincinnati (news : web)

New technology improves both energy capacity and charge rate in rechargeable batteries

A team of engineers has created an electrode for lithium-ion batteries -- rechargeable batteries such as those found in cellphones and iPods -- that allows the batteries to hold a charge up to 10 times greater than current technology. Batteries with the new electrode also can charge 10 times faster than current batteries.

The researchers combined two chemical engineering approaches to address two major battery limitations -- energy capacity and charge rate -- in one fell swoop. In addition to better batteries for cellphones and iPods, the technology could pave the way for more efficient, smaller batteries for electric cars.

The technology could be seen in the marketplace in the next three to five years, the researchers said.

A paper describing the research is published by the journal Advanced Energy Materials.

"We have found a way to extend a new lithium-ion battery's charge life by 10 times," said Harold H. Kung, lead author of the paper. "Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium-ion batteries on the market today."

Kung is professor of chemical and biological engineering in the McCormick School of Engineering and Applied Science. He also is a Dorothy Ann and Clarence L. Ver Steeg Distinguished Research Fellow.

Lithium-ion batteries charge through a chemical reaction in which lithium ions are sent between two ends of the battery, the anode and the cathode. As energy in the battery is used, the lithium ions travel from the anode, through the electrolyte, and to the cathode; as the battery is recharged, they travel in the reverse direction.

With current technology, the performance of a lithium-ion battery is limited in two ways. Its energy capacity -- how long a battery can maintain its charge -- is limited by the charge density, or how many lithium ions can be packed into the anode or cathode. Meanwhile, a battery's charge rate -- the speed at which it recharges -- is limited by another factor: the speed at which the lithium ions can make their way from the electrolyte into the anode.

In current rechargeable batteries, the anode -- made of layer upon layer of carbon-based graphene sheets -- can only accommodate one lithium atom for every six carbon atoms. To increase energy capacity, scientists have previously experimented with replacing the carbon with silicon, as silicon can accommodate much more lithium: four lithium atoms for every silicon atom. However, silicon expands and contracts dramatically in the charging process, causing fragmentation and losing its charge capacity rapidly.

Currently, the speed of a battery's charge rate is hindered by the shape of the graphene sheets: they are extremely thin -- just one carbon atom thick -- but by comparison, very long. During the charging process, a lithium ion must travel all the way to the outer edges of the graphene sheet before entering and coming to rest between the sheets. And because it takes so long for lithium to travel to the middle of the graphene sheet, a sort of ionic traffic jam occurs around the edges of the material.

Now, Kung's research team has combined two techniques to combat both these problems. First, to stabilize the silicon in order to maintain maximum charge capacity, they sandwiched clusters of silicon between the graphene sheets. This allowed for a greater number of lithium atoms in the electrode while utilizing the flexibility of graphene sheets to accommodate the volume changes of silicon during use.

"Now we almost have the best of both worlds," Kung said. "We have much higher energy density because of the silicon, and the sandwiching reduces the capacity loss caused by the silicon expanding and contracting. Even if the silicon clusters break up, the silicon won't be lost."

Kung's team also used a chemical oxidation process to create miniscule holes (10 to 20 nanometers) in the graphene sheets -- termed "in-plane defects" -- so the lithium ions would have a "shortcut" into the anode and be stored there by reaction with silicon. This reduced the time it takes the battery to recharge by up to 10 times.

This research was all focused on the anode; next, the researchers will begin studying changes in the cathode that could further increase effectiveness of the batteries. They also will look into developing an electrolyte system that will allow the battery to automatically and reversibly shut off at high temperatures -- a safety mechanism that could prove vital in electric car applications.

More information: The paper is titled "In-Plane Vacancy-Enabled High-Power Si-Graphene Composite Electrode for Lithium-Ion Batteries."

Provided by Northwestern University (news : web)

Converting waste heat into electricity

More than half of today's energy consumption is squandered in useless waste heat, such as the heat from refrigerators and all sorts of gadgets and the heat from factories and power plants. The energy losses are even greater in cars. Automobile motors only manage to utilise 30 per cent of the energy they generate. The rest of it is lost. Part of the heat loss ends up as warm brakes and a hot exhaust pipe.

Scientists at the Centre for Materials Science and Nanotechnology at the University of Oslo in Norway (UiO) are now collaborating with SINTEF (the Foundation for Scientific and Industrial Research at the Norwegian Institute of Technology) to develop a new environmentally friendly technology called thermoelectricity, which can convert waste heat into electricity. To put it briefly, the technology involves making use of temperature differences.

Today: Toxic and expensive

Thermoelectric materials are put to many uses in space flight. When a space probe travels far enough away from the sun, its solar cells cease to work. Batteries have much too short a lifetime. Nuclear power cannot be used. However, a lump of Plutonium will do the trick.

With a temperature of a thousand degrees, it is hot. Outer space is cold. Thanks to the temperature difference, the space probe gets enough electricity.

Plutonium is a good solution for space probes that will not return to earth, but it is not a practical solution for cars and other earthly objects.

Thermoelectric materials are also currently used in the type of cooler bags that keep things cold without making use of their own cooling elements. These cooler bags are full of the elements Lead and Tellurium. Both of these substances are also toxic.

"We want to replace them with inexpensive and readily available substances. Moreover, there is not enough Tellurium to equip all of the cars in the world," says Ole Martin Lovvik, who is both an associate professor in the Department of Physics at the University of Oslo and a senior scientist at SINTEF.

Tomorrow: Environmentally friendly and inexpensive

With the current technology, it is possible to recover scarcely ten per cent of the lost energy. Together with the team of scientists led by Professor Johan Tafto, Lovvik is now searching for pollution-free, inexpensive materials that can recover fifteen per cent of all energy losses. That is an improvement of fully fifty per cent.

"I think we will manage to solve this problem with nanotechnology. The technology is simple and flexible and is almost too good to be true. In the long run, the technology can utilise all heat sources, such as solar energy and geothermal energy. The only limits are in our imagination," states Lovvik to the research magazine Apollon at University of Oslo

The new technology will initially be put to use in thermoelectric generators in cars. Several major automobile manufacturers are already interested. Lovvik and his colleagues are currently discussing the situation with General Motors.

"Modern cars need a lot of electricity. By covering the exhaust system with thermoelectric plates, the heat from the exhaust system can increase the car's efficiency by almost ten per cent at a single stroke. If we succeed, this will be a revolution in the modern automotive industry."

The new technology can also replace the hum of today's refrigerator.

"In the future, refrigerators can be soundless and built into cabinets without any movable parts and with the possibility of maintaining different temperatures in each compartment.

In order to extract as much energy as possible, the temperature difference should be as large as possible.

"Initially then, we want to utilise high-temperature waste heat, but there is also an upper limit."

If it becomes too hot, some materials will break down either by melting or by being transformed into other materials. That would mean that they wouldn't work any more.

Apparently self-contradictory.

In order to create thermoelectric materials, physicists have to resolve an apparent paradox. A metal conducts both electricity and heat. An insulator conducts neither electricity nor heat.

A good thermoelectric material ought to be a semi-conductor with very special properties: Its thermal resistance must be as high as possible at the same time as current must flow through it easily.

"This is not a simple combination, and it may even sound like a self-contradiction. The best solution is to create small structures that reflect the heat waves at the same time as the current is not reflected."

In order to understand why this is so, you must first understand how heat is dissipated. When a material becomes hot, the atoms vibrate. The hotter it becomes, the greater the vibrations, and when an atom vibrates, it will also affect the vibration of the adjacent atom.

When these vibrations spread through the material, they can be called heat waves. If we create barriers in the material so that some atoms vibrate at different frequencies from their adjacent atoms, the heat will not be so easily dissipated.

"Moreover, the atomic barrier must be created in such a way that it does not prevent the electric current from flowing through it."

Grinding nano-cavities at minus 196 degrees.

The scientists have found a method of creating these atomic barriers. The barriers are introduced densely in the special semi-conductors.

"We have achieved this by using a completely new "mill." Just as the miller grinds grain, the scientists will grind down semi-conductors to nano-sized grains. They will do that by cooling them down with liquid Nitrogen to minus 196 degrees. That makes the material more brittle, less sticky and easier to crush. It is important to grind down the grains as small as possible. Afterwards the grains are glued back together again, and in this way the barriers are created."

"The small irregularities in the barriers reflect the heat waves," says Lovvik.

The team of scientists uses an electron microscope to examine the micro-structures in the material.

"We have now discovered new nano-cavities in the materials and learned more about how they reflect heat waves."

The thermal resistance is measured in the Norwegian Micro and Nano Laboratories that are jointly operated by UiO and SINTEF. Lovvik's specialised field is mathematical models. With these models, he can predict how the atoms should be arranged in the materials.

Renaissance for cobalt

The scientists are now searching for the next generation of thermoelectric materials. They have just tested the cobalt arsenide mineral, skutterudite, which may be found at Skutterud at Blafarvevarket in Modum, Norway.

"It was just recently discovered that skutterudite may have atoms located in small nano-cavities. These cavities act as barriers to heat dissipation," concludes Lovvik.

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