Materials Science Archives - Science Rocks My World

Novel Perovskite Solar Cell Could Rival Silicon [Video]

A new design for solar cells that uses inexpensive, commonly available materials could rival and even outperform conventional cells made of silicon.

Scientists have used tin and other abundant elements to create novel forms of perovskite—a photovoltaic crystalline material that’s thinner, more flexible, and easier to manufacture than silicon crystals. “Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost,” says study coauthor Michael McGehee, professor of materials science and engineering at Stanford University. “We have designed a robust, all-perovskite device that converts sunlight into electricity with an efficiency of 20.3 percent, a rate comparable to silicon solar cells on the market today.”

Cross-section of a new tandem solar cell. The brown upper layer of perovskite captures low-energy lightwaves, and the red perovskite layer captures high-energy waves. (Credit: Scanning electron microscopy image by Rebecca Belisle, Giles Eperon)

Double perovskite stack

The new device consists of two perovskite solar cells stacked in tandem. Each cell is printed on glass, but the same technology could be used to print the cells on plastic.

“The all-perovskite tandem cells we have demonstrated clearly outline a roadmap for thin-film solar cells to deliver over 30 percent efficiency,” says coauthor Henry Snaith, professor of physics at Oxford University. “This is just the beginning.”

Previous studies showed that adding a layer of perovskite can improve the efficiency of silicon solar cells. But a tandem device consisting of two all-perovskite cells would be cheaper and less energy-intensive to build, scientists say.

“A silicon solar panel begins by converting silica rock into silicon crystals through a process that involves temperatures above 3,000 degrees Fahrenheit (1,600 degrees Celsius),” says colead author Tomas Leijtens, a postdoctoral scholar at Stanford. “Perovskite cells can be processed in a laboratory from common materials like lead, tin, and bromine, then printed on glass at room temperature.”

A difficult challenge

But building an all-perovskite tandem device has been a difficult challenge. The main problem is creating stable perovskite materials capable of capturing enough energy from the sun to produce a decent voltage.

A typical perovskite cell harvests photons from the visible part of the solar spectrum. Higher-energy photons can cause electrons in the perovskite crystal to jump across an “energy gap” and create an electric current.

A solar cell with a small energy gap can absorb most photons but produces a very low voltage. A cell with a larger energy gap generates a higher voltage, but lower-energy photons pass right through it.

An efficient tandem device would consist of two ideally matched cells, says co-lead author Giles Eperon, an Oxford postdoctoral scholar currently at the University of Washington.

“The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage,” Eperon says. “The cell with the smaller energy gap can harvest photons that aren’t collected by the first cell and still produce a voltage.”

Stability problem

The smaller gap has proven to be the bigger challenge for scientists. Working together, Eperon and Leijtens used a unique combination of tin, lead, cesium, iodine, and organic materials to create an efficient cell with a small energy gap.

“We developed a novel perovskite that absorbs lower-energy infrared light and delivers a 14.8 percent conversion efficiency,” Eperon says. “We then combined it with a perovskite cell composed of similar materials but with a larger energy gap.”

The result: A tandem device consisting of two perovskite cells with a combined efficiency of 20.3 percent.

“There are thousands of possible compounds for perovskites,” Leijtens says, “but this one works very well, quite a bit better than anything before it.”

One concern with perovskites is stability. Rooftop solar panels made of silicon typically last 25 years or more. But some perovskites degrade quickly when exposed to moisture or light. In previous experiments, perovskites made with tin were found to be particularly unstable.

To assess stability, the research team subjected both experimental cells to temperatures of 212 degrees Fahrenheit (100 degrees Celsius) for four days.

“Crucially, we found that our cells exhibit excellent thermal and atmospheric stability, unprecedented for tin-based perovskites,” the authors write.

“The efficiency of our tandem device is already far in excess of the best tandem solar cells made with other low-cost semiconductors, such as organic small molecules and microcrystalline silicon,” McGehee says. “Those who see the potential realize that these results are amazing.”

The next step is to optimize the composition of the materials to absorb more light and generate an even higher current, Snaith says.

“The versatility of perovskites, the low cost of materials and manufacturing, now coupled with the potential to achieve very high efficiencies, will be transformative to the photovoltaic industry once manufacturability and acceptable stability are also proven,” he says.

Other researchers from Stanford, Oxford, Hasselt University in Belgium, and SunPreme Inc. are coauthors of the study. They report their research in the journal Science.

Source: Republished from Futurity.org as a derivative work under the Attribution 4.0 International license. Original article posted to Futurity by Mark Shwartz-Stanford.

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New Breakthrough Crystal Heals Itself After Being Broken in Half

For the first time in the field of solid-state chemistry scientists at NYU Abu Dhabi have developed a smart crystal that can heal itself after breakage without any chemical or biological intervention. The crystal relies on its own molecular structure and physical contact to heal — similar to cuts on skin.

Self-healing polymers has been researched by chemists extensively for more than a decade and until now has only been observed in softer materials like rubber and plastic.

Pale yellow crystals the size of a baby’s fingernail — and about 0.5 mm thick — were grown in a lab by Dr. Patrick Commins, postdoctoral associate researcher at NYU Abu Dhabi and lead author of the research paper recently published in leading peer-reviewed scientific journal Angwandte Chemie International Edition. The two-year study was conducted on dipyrazolethiuram disulfide crystals.
Commin said his research was inspired by the relationship between sulphur atoms in soft polymers, which tend to flow toward neighbouring sulphur atoms and bond with them easily resulting in self-repair. He decided to test the same bond in crystals.

“What happens when we break the crystal is that we have all these sulphurs moving around and when we press them together they reform their bonds and they heal,” Commins explained.

Commins said the crystal was broken using a machine built specifically to hold tiny objects and that has the capability to break the object cleanly. The two halves of the broken crystal were mechanically brought into contact with each other at room temperature. 24 hours later the crystal was whole again healed. The only defect was a superficial mark left behind by the crack in the middle. The percentage of healing was 6.7 percent and was calculated by comparing the amount of force required to break the crystal before and after the healing process.

“This is actually a small breakthrough because it kind of shows a concept that was not considered possible before,” said Dr. Pance Naumov, NYU Abu Dhabi associate professor of chemistry and study co-author. “It is the first time we’ve observed that rigid entities like crystals can self-repair. This was not expected. It’s certainly a shift in our understanding of crystals.”

“Crystals found in nature are made from minerals, like calcium and silicates but this crystal is different,” Commins added. “The crystal has been made specifically to have many close sulfur-sulfur bonds and they grow in rather small sizes.”

Commins believes this research is just the beginning.

“We think other crystals can do it (self-heal) but no one has actually explored it. We’ve only investigated one aspect of this field. We are expanding upon the subject and are trying to find other self-healing crystals,” he said.

Dr. Hideyuki Hara, research scientist at Bruker in Japan, is also a co-author on the paper.

Source: Press Release from NYU Abu Dhabi

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Finding better ways to get hydrogen fuel from water

By Peter Byrley, University of California, Riverside.

With hydrogen power stations in California, a new Japanese consumer car and portable hydrogen fuel cells for electronics, hydrogen as a zero emission fuel source is now finally becoming a reality for the average consumer. When combined with oxygen in the presence of a catalyst, hydrogen releases energy and bonds with the oxygen to form water.

The two main difficulties preventing us from having hydrogen power everything we have are storage and production. At the moment, hydrogen production is energy-intensive and expensive. Normally, industrial production of hydrogen requires high temperatures, large facilities and an enormous amount of energy. In fact, it usually comes from fossil fuels like natural gas – and therefore isn’t actually a zero-emission fuel source. Making the process cheaper, efficient and sustainable would go a long way toward making hydrogen a more commonly used fuel.

An excellent – and abundant – source of hydrogen is water. But chemically, that requires reversing the reaction in which hydrogen releases energy when combining with other chemicals. That means we have to put energy into a compound, to get the hydrogen out. Maximizing the efficiency of this process would be significant progress toward a clean-energy future.

One method involves mixing water with a helpful chemical, a catalyst, to reduce the amount of energy needed to break the connections between hydrogen and oxygen atoms. There are several promising catalysts for hydrogen generation, including molybdenum sulfide, graphene and cadmium sulfate. My research focuses on modifying the molecular properties of molybdenum sulfide to make the reaction even more effective and more efficient.

Making hydrogen

Hydrogen is the most abundant element in the universe, but it’s rarely available as pure hydrogen. Rather, it combines with other elements to form a great many chemicals and compounds, such as organic solvents like methanol, and proteins in the human body. Its pure form, H₂, can used as a transportable and efficient fuel.

There are several ways to produce hydrogen to be usable as fuel. Electrolysis uses electricity to split water into hydrogen and oxygen. Steam methane reforming starts with methane (four hydrogen atoms bound to a carbon atom) and heats it, separating the hydrogen from the carbon. This energy-intensive method is usually how industries produce hydrogen that is used in things like producing ammonia or the refining of oil.

The method I’m focusing on is photocatalytic water splitting. With a catalyst’s help, the amount of energy needed to “split” water into hydrogen and oxygen can be provided by another abundant resource – light. When exposed to light, a proper mixture of water and a catalyst produces both oxygen and hydrogen. This is very attractive to industry because it then allows us to use water as the source of hydrogen instead of dirty fossil fuels.

Understanding catalysts

Just as not every two people start up a conversation if they’re in the same elevator, some chemical interactions don’t occur just because the two materials are introduced. Water molecules can be split into hydrogen and oxygen with the addition of energy, but the amount of energy needed would be more than would be generated as a result of the reaction.

Sometimes it takes a third party to get things going. In chemistry, that’s called a catalyst. Chemically speaking, a catalyst lowers the amount of energy needed for two compounds to react. Some catalysts function only when exposed to light. These compounds, like titanium dioxide, are called photocatalysts.

With a photocatalyst in the mix, the energy needed to split water drops significantly, so that the effort nets an energy gain at the end of the process. We can make the splitting even more efficient by adding another substance, in a role called co-catalyst. Co-catalysts in hydrogen generation alter the electronic structure of the reaction, making it more effective at producing hydrogen.

So far, there aren’t any commercialized systems for producing hydrogen this way. This is in part because of cost. The best catalysts and co-catalysts we’ve found are efficient at helping with the chemical reaction, but are very expensive. For example, the first promising combination, titanium dioxide and platinum, was discovered in 1972. Platinum, however, is a very expensive metal (well over US$1,000 per ounce). Even rhenium, another useful catalyst, costs around $70 an ounce. Metals like these are so rare in the Earth’s crust that this makes them not suitable for large-scale applications even though there are processes being developed to recycle these materials.

Finding a new catalyst

There are many requirements for a good catalyst, such as being able to be recycled and being able to withstand the heat and pressure involved in the reaction. But just as crucial is how common the material is, because the most abundant catalysts are the cheapest.

*Properties of a good photocatalyst.*
*Peter Byrley*

One of the newest and most promising materials is molybdenum sulfide, MoS₂. Because it is made up of the elements molybdenum and sulfur – both relatively common on Earth – it is far cheaper than more traditional catalysts, well under a dollar per ounce. It also has the correct electronic properties and other attributes.

Chips of molybdenum sulfide. Materialscientist, CC BY-SA

Single layers of molybdenum sulfide (MoS₂) on glass (SiO₂). Scale bar is 10 micrometers (μm). Peter Byrley

Before the late 1990s, researchers had found that molybdenum sulfide was not particularly effective at turning water into hydrogen. But that was because researchers were using thick chunks of the mineral, essentially the form it’s in when mined from the ground. Today, however, we can use processes like chemical vapor deposition or solution-based processes to create much thinner crystals of MoS₂ – even down to the thickness of a single molecule – which are vastly more efficient at extracting hydrogen from water.

Making the process even better

Molybdenum sulfide can be made even more effective by manipulating its physical and electrical properties. A process known as “phase change” makes more of the substance available to participate in the hydrogen-producing reaction.

When molybdenum sulfide forms crystals, the atoms and molecules on the outside of the solid mass are ready to accept or donate electrons to water when excited by light to drive the creation of hydrogen. Normally, the MoS₂ molecules on the inside of the structure will not donate or accept electrons as efficiently as the edge sites, and so can’t help as much with the reaction.

But adding energy to the MoS₂ by bombarding it with electrons, or increasing the surrounding pressure, causes what is called “phase change” to occur. This phase change is not what you learn in basic chemistry (involving one substance taking forms of gas, liquid or solid) but rather a slight structural change in the molecular arrangement that changes the MoS₂ from a semiconductor to a metal.

As a result, the electrical properties of the molecules on the inside become available to the reaction as well. This makes the same amount of catalyst potentially 600 times more effective in the hydrogen evolution reaction.

If the methods behind this sort of breakthrough can be perfected, then we may be a big step closer to making hydrogen production cheaper and more efficient, which in turn will move us toward a future powered by truly clean, renewable energy.

Peter Byrley, Ph.D. Candidate in Chemical Engineering, University of California, Riverside

This article was originally published on The Conversation. Read the original article.

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Surprising New Formulation Makes Ultra-Hard Replacement Joints

Titanium is the leading material for artificial knee and hip joints because it’s strong, wear-resistant, and nontoxic, but adding gold might make implants even better.

“It is about 3-4 times harder than most steels,” says Emilia Morosan, the lead scientist of a new study in Science Advances that describes the properties of a 3-to-1 mixture of titanium and gold with a specific atomic structure that imparts hardness. “It’s four times harder than pure titanium, which is what’s currently being used in most dental implants and replacement joints.”

TiAu_rotation_smaller-2koly18 — Crystal structure of beta titanium-3 gold. (Credit: E. Morosan/Rice University)

The new study is “a first for me in a number of ways,” says Morosan, professor of physics and astronomy, of chemistry, and of materials science and nanoengineering at Rice University. “This compound is not difficult to make, and it’s not a new material.”

In fact, the atomic structure of the material—its atoms are tightly packed in a “cubic” crystalline structure that’s often associated with hardness—was previously known.

It’s not even clear that Morosan and former graduate student and coauthor Eteri Svanidze were the first to make a pure sample of the ultrahard “beta” form of the compound. But due to a couple of lucky breaks, they are the first to document the material’s remarkable properties.

“This began from my core research,” says Morosan, a physicist who specializes in the design and synthesis of compounds with exotic electronic and magnetic properties. “We published a study not long ago on titanium-gold, a 1-to-1 ratio compound that was a magnetic material made from nonmagnetic elements.

“One of the things that we do when we make a new compound is try to grind it into powder for X-ray purposes. This helps with identifying the composition, the purity, the crystal structure, and other structural properties. When we tried to grind up titanium-gold, we couldn’t. I even bought a diamond (coated) mortar and pestle, and we still couldn’t grind it up.”

4x harder than titanium

The researchers decided to do follow-up tests to determine exactly how hard the compound was, and while they were at it, they also decided to measure the hardness of the other compositions of titanium and gold that they had used as comparisons in the original study.

One of the extra compounds was a mixture of three parts titanium and one part gold that had been prepared at high temperature.

What they didn’t know at the time was that making titanium-3-gold at relatively high temperature produces an almost pure crystalline form of the beta version of the alloy—the crystal structure that’s four times harder than titanium.

At lower temperatures, the atoms tend to arrange in another cubic structure—the alpha form of titanium-3-gold. The alpha structure is about as hard as regular titanium. It appears that labs that had previously measured the hardness of titanium-3-gold had measured samples that largely consisted of the alpha arrangement of atoms.

Researchers measured the hardness of the beta form of the crystal and also performed other comparisons with titanium. For biomedical implants, for example, two key measures are biocompatibility and wear resistance. Because titanium and gold by themselves are among the most biocompatible metals and are often used in medical implants, the team believed titanium-3-gold would be comparable.

In fact, tests determined that the new alloy was even more biocompatible than pure titanium. The story proved much the same for wear resistance: Titanium-3-gold also outperformed pure titanium.

Other scientists from Rice and from Texas A&M University and Florida State University are coauthors of the study that was supported by the National Science Foundation, the Department of Energy, Texas A&M’s Turbomachinery Laboratory, and the Florida State University Research Foundation. Their research is publisehed in Science Advances.

Source: Republished from Futurity.org as a derivative work under the Attribution 4.0 International license. Original article posted to Futurity by Jade Boyd-Rice.

Featured Image Credit: BruceBlaus, via Wikimedia Commons

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Graphene isn’t the only Lego in the materials-science toy box

By Peter Byrley, University of California, Riverside.

You may have heard of graphene, a sheet of pure carbon, one atom thick, that’s all the rage in materials-science circles, and getting plenty of media hype as well. Reports have trumpeted graphene as an ultra-thin, super-strong, super-conductive, super-flexible material. You could be excused for thinking it might even save all of humanity from certain doom.

Not exactly. In the current world of nano-electronics, there is a lot more going on than just graphene. One of the materials I work with, molybdenum disulphide (MoS₂), is a one-layer material with interesting properties beyond those of graphene. MoS₂ can absorb five times as much visible light as graphene, making it useful in light detectors and solar cells. In addition, even newer materials like borophene (a one-layer material made of boron atoms projected to be mechanically stronger than graphene) are being proposed and synthesized every day.

Layering two-dimensional materials. Peter Byrley, Author provided

These and other materials yet to be discovered will be used like Lego pieces to build the electronics of the future. By stacking multiple materials in different ways, we can take advantage of different properties in each of them. The new electronics built with these combined structures will be faster, smaller, more environmentally resistant and cheaper than what we have now.

Looking for an energy gap

There is a key reason that graphene will not be the versatile cure-all material that the hype might suggest. You can’t just stack graphene repeatedly to get what you want. The electronic property preventing this is the lack of what is called an “energy gap.” (The more technical term is “band gap.”)

What the energy gap looks like. Peter Byrley

Metals will conduct electricity through them regardless of the environment. However, any other material that is not a metal needs a little boost of energy from the outside to get electrons to move through the band gap and into the conducting state. How much of a boost the material needs is called the energy gap. The energy gap is one of the factors that determines how much total energy needs to be put into your entire electrical device, from either heat or applied electrical voltage, to get it to conduct electricity. You essentially have to put in enough starting energy if you want your device to work.

Some materials have a gap so large that almost no amount of energy can get electrons flowing through them. These materials are called insulators (think glass). Other materials have either an extremely small gap or no gap at all. These materials are called metals (think copper). This is why we use copper (a metal with instant conductivity) for wiring, while we use plastics (an insulator that blocks electricity) as the protective outer coating.

Everything else, with gaps in between these two extremes, is called a semiconductor (think silicon). Semiconductors, at the theoretical temperature of absolute zero, behave as insulators because they have no heat energy to get their electrons into the conducting state. At room temperature, however, heat from the surrounding environment provides just enough energy to get some electrons (hence the term, “semi”-conducting) over the small band gap and into the conducting state ready to conduct electricity.

Comparing the band gap in metals (left), semiconductors (center) and insulators (right). Peter Byrley

Graphene’s energy gap

Graphene is in fact a semi-metal. It has no energy gap, which means it will always conduct electricity – you can’t turn off its conductivity.

This is a problem because electronic devices use electrical current to communicate. At their most fundamental level, computers communicate by sending 1’s and 0’s – on and off signals. If a computer’s components were made from graphene, the system would always be on, everywhere. It would be unable to perform tasks because its lack of energy gap prevents graphene from ever becoming a zero; the computer would keep reading 1’s all the time. Semiconductors, by contrast, have an energy gap that is small enough to let some electrons conduct electricity but is large enough to have a clear distinction between on and off states.

*Imagining using a computer based on graphene.*
*Woman with computer via shutterstock.com*

Finding the right materials

Not all hope is lost, however. Researchers are looking at three main ways to tackle this:

Using new materials similar to graphene that actually have a sufficient energy gap and finding ways to further improve their conductivity.
Altering graphene itself to create this energy gap.
Combining graphene with other materials to optimize their combined properties.

There are many one-layer materials currently being looked at that actually have a sufficient energy gap. One such material, MoS₂, has been studied in recent years as a potential replacement for traditional silicon and also as a light detector and gas sensor.

The only drawback with these other materials is that so far, we have not found one that matches the excellent though always-on conductivity of graphene. The other materials can be turned off, but when on, they are not as good as graphene. MoS₂ itself is estimated to have 1/15th to 1/10th the conductivity of graphene in small devices. Researchers, including me, are now looking at ways to alter these materials to increase their conductivity.

Using graphene as an ingredient

Strangely, an energy gap in graphene can actually be induced through modifications like bending it, turning it into a nanoribbon, inserting foreign chemicals into it or using two layers of graphene. But each of these modifications can reduce the graphene’s conductivity or limit how it can be used.

To avoid specialized setups, we could just combine graphene with other materials. By doing this, we are also combining the properties of the materials in order to reap the best benefits. We could, for example, invent new electronic components that have a material allowing them to be shut off or on (like MoS₂) but have graphene’s great conductivity when turned on. New solar cells will work on this concept.

A combined structure could, for example, be a solar panel made for harsh environments: We could layer a thin, transparent protective material over the top of a very efficient solar-collecting material, which in turn could be on top of a material that is excellent at conducting electricity to a nearby battery. Other middle layers could include materials that are good at selectively detecting gases such as methane or carbon dioxide.

Researchers are now racing to figure out what the best combination is for different applications. Whoever finds the best combination will eventually win numerous rights to patents for improved electronic products.

The truth is, though, we don’t know what our future electronics will look like. New Lego pieces are being invented all the time; the ways we stack or rearrange them are changing constantly, too. All that’s certain is that the insides of electronic devices will look drastically different in the future than they do today.

Peter Byrley, Ph.D. Candidate in Chemical Engineering, University of California, Riverside

This article was originally published on The Conversation. Read the original article.

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This Electronic Material Heals After Being Cut in Two [Video]

A new electronic material can heal all its functions automatically—even after breaking multiple times. This material could improve the durability of wearable electronics.

“Wearable and bendable electronics are subject to mechanical deformation over time, which could destroy or break them,” says Qing Wang, professor of materials science and engineering at Penn State. “We wanted to find an electronic material that would repair itself to restore all of its functionality, and do so after multiple breaks.”

Self-healable materials are those that, after withstanding physical deformation such as being cut in half, naturally repair themselves with little to no external influence.

In the past, researchers have been able to create self-healable materials that can restore one function after breaking, but restoring a suite of functions is critical for creating effective wearable electronics. For example, if a dielectric material retains its electrical resistivity after self-healing but not its thermal conductivity, that could put electronics at risk of overheating.

The material that Wang and his team created restores all properties needed for use as a dielectric in wearable electronics—mechanical strength, breakdown strength to protect against surges, electrical resistivity, thermal conductivity, and dielectric, or insulating, properties. They report their findings in Advanced Functional Materials.

Most self-healable materials are soft or “gum-like,” says Wang, but the material he and his colleagues created is very tough in comparison. His team added boron nitride nanosheets to a base material of plastic polymer. Like graphene, boron nitride nanosheets are two dimensional, but instead of conducting electricity like graphene they resist and insulate against it.

“Most research into self-healable electronic materials has focused on electrical conductivity but dielectrics have been overlooked,” says Wang. “We need conducting elements in circuits but we also need insulation and protection for microelectronics.”

The material is able to self-heal because boron nitride nanosheets connect to one another with hydrogen bonding groups functionalized onto their surface. When two pieces are placed in close proximity, the electrostatic attraction naturally occurring between both bonding elements draws them close together. When the hydrogen bond is restored, the two pieces are “healed.” Depending on the percentage of boron nitride nanosheets added to the polymer, this self-healing may require additional heat or pressure, but some forms of the new material can self-heal at room temperature when placed next to each other.

Unlike other healable materials that use hydrogen bonds, boron nitride nanosheets are impermeable to moisture. This means that devices using this dielectric material can operate effectively within high humidity contexts such as in a shower or at a beach.

“This is the first time that a self-healable material has been created that can restore multiple properties over multiple breaks, and we see this being useful across many applications,” says Wang.

Additional researchers contributed from Penn State and Harbin Institute of Technology. The China Scholarship Council supported the work, published in in Advanced Functional Materials.

Source: Republished from Futurity.org as a derivative work under the Attribution 4.0 International license. Original article posted to Futurity by Patricia L. Craig-Penn State.

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Vibrating Grains May Lead to New Designer Coatings

On the microscale, granular materials interact in remarkably complex ways. That complexity makes them one of the least understood forms of matter.

Now scientists want to figure out how to take advantage of those interactions to design impact-absorbing materials. For example, these new materials might minimize vibrations in vehicles, better protect military convoys, or potentially make buildings safer during an earthquake.

As a first step, researchers have analyzed particle vibrations in very small 2D granular crystals. The results could ultimately help predict how these tiny arrays of particles behave as forces are applied.

‘BUILD BETTER SANDBAGS’

One of the more interesting characteristics of granular materials is that they are dynamically responsive—when you hit them harder, they react differently.

“You can take a pencil and push it through a sandbag, but at the same time it can stop a bullet,” says Nicholas Boechler, assistant professor of mechanical engineering at the University of Washington and senior author of the paper published in Physical Review Letters. “So in some ways what we’re trying to do is build better sandbags in an informed way.

Boechler and colleagues discovered that microscale granular crystals—made of spheres that are smaller than a human blood cell—exhibit significantly different physical phenomena than granular materials with larger particles. Adhesive forces play a more important role, for instance. The array of tiny particles also resonates in complex patterns as forces are applied, and they knock into each other, including combinations of up-and-down, horizontal, and rotational motion.

Tiny granular particles resonates in complex patterns as forces are applied, and they knock into each other, including combinations of up-and-down, horizontal, and rotational motion. (Credit: Samuel Wallen/University of Washington)

“This material has properties that we wouldn’t normally see in a solid material like glass or metal,” says Morgan Hiraiwa, lead author and mechanical engineering doctoral student. “You can think of it as all these different knobs we can turn to get the material to do what we want.”

LASER-GENERATED WAVES

The team manufactured the 2D ordered layer of micron-sized glass spheres through self-assembly—meaning the millions of particles assemble themselves into a larger functional unit under the right conditions.

Building large amounts of material composed of microscopic particles, such as a panel for a vehicle, is impractical using conventional manufacturing techniques because of the amount of time it would take, Boechler says. Self-assembly offers a scalable, faster, and less expensive way to manufacture microstructured materials.

The team then used laser ultrasonic techniques to observe the dynamics between microscale granular particles as they interact. That involves sending a laser-generated acoustic wave through the crystal and using a separate laser to pick up very small vibrations of the microscopic particles.

Researchers have studied the dynamics of granular crystals with large particles, but this is the first time such complex dynamics have been observed and analyzed in microscale crystals, which have advantages over their larger counterparts. Their small size makes it easier to integrate them into coatings or other materials, and they also resonate at higher frequencies, making them potentially useful for signal processing and other applications.

“The larger systems are really nice for modeling, but can be difficult to integrate into many potential products,” Boechler says.

SPACECRAFT COATING

So far, the team has conducted its experiments using low-amplitude waves. Next steps include exploring high-amplitude, nonlinear regimes in 3D crystals—in which the granular particles are moving more vigorously and even more interesting dynamics may occur.

“Ultimately, the goal is to use this knowledge to start designing materials with new properties,” Boechler says. “For instance, if you could design a coating that has unique impact-absorbing capabilities, it could have applications ranging from spacecraft micro-meteorite shielding to improved bulletproof vests.”

The National Science Foundation, the US Army Research Office, and the University of Washington Royalty Research Foundation funded the work. Additional researchers from the University of Washington and the Massachusetts Institute of Technology took part in the study.

Source: Republished from Futurity.org as a derivative work under the Attribution 4.0 International license. Original article posted to Futurity by Jennifer Langston-Washington.

Featured Image Credit: University of Washington

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How Gecko Feet Inspired New Dust Cleaning Polymer

Micrometric and sub-micrometric contaminant particles—what most of us call “dust”—are everywhere.

Dust makes it tough to keep the house clean, but it also causes problems for art conservators, the electronics industry, and aerospace engineers. Now, scientists want to use static cling and the science behind gecko feet to develop a tool that could help fight dust.

T. Kyle Vanderlick, dean of the School of Engineering and Applied Science at Yale University, took on the dust problem shortly after the university established art conservation labs at its Institute for the Preservation of Cultural Heritage (IPCH).

Dust is a particular problem when it comes to modern paintings that feature acrylic paint, says Cindy Schwarz, assistant conservator of painting at the Yale University Art Gallery.

“Acrylic paints are incredibly porous, so anything you’re putting on the surface could get into the pores, and then work from the insides of the pores to soften the paints,” she says.

New technology, described in the journal ACS Applied Materials and Interfaces, has the potential to solve the long-standing problem.

If dust particles are bigger than 10 micrometers, you can remove them with minimal fuss, usually with an air jet or nitrogen jet. It’s a whole other world of trouble for particles less than 10 micrometers. There are plenty of removal methods, but each has its drawbacks. Wet cleaning is limited in its ability to remove particles, and can possibly damage the object being cleaned.

In recent years, the electronics industry and art conservators have turned to dry cleaning techniques, such as lasers, micro-abrasive particles, and carbon dioxide snow jets. They remove dust well, but can be just as damaging to artwork as wet cleaning methods.

The solution is deceptively simple. In the lab, Hadi Izadi, a postdoctoral associate and the paper’s lead author, says the secret is what looks like an ordinary plastic sheet, but is actually an elastic and non-sticky polymer called polydimethylsiloxane (PDMS).

Microscopic image of silica dust particles lifted by micropillars, 50 micrometers in diameter. (Credit: Vanderlick Lab)

Put it under a microscope, and you can see millions of tiny columns. Depending on the size of dust particles you’re removing, the pillars range from 2 to 50 micrometers in diameter—bigger particles require bigger pillars.

Izadi is familiar with fibrillar structures and micropillars. His previous research explored the mystery of how geckos effortlessly stick to walls. It turns out that a lot of it has to do with electrostatic charges and the microscopic pillars on the pads on their feet. Applying some of this science to cleaning microparticles makes sense, he says. “When you’re talking about dust, you’re talking about electrostatic charges.”

The micropillar structures used for dust cleaning, however, differ from those of geckos in that they’re designed specifically not to stick. The PDMS polymer has minimal interaction with the substrate—whether it’s an iPhone or a sculpture—but it produces enough electrostatic charge to detach the dust particles.

Once you match up a sheet with the appropriately sized pillars, cleaning is simply a matter of tapping the polymer on the surface. Particles absorbed by the polymer go around the pillars. Tests on various surfaces in the lab have shown total cleaning of silica dust particles and no damage to the surface.

“Dust is something at the nanometer level,” Vanderlick says. “And there’s a lot of interesting thin film, surface, and interfacial physics associated with the preservation of art.”

The research is published the journal ACS Applied Materials and Interfaces.

Source: Republished from Futurity.org as a derivative work under the Attribution 4.0 International license. Original article posted to Futurity by William Weir-Yale.

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We don’t talk much about nanotechnology risks anymore, but that doesn’t mean they’re gone

By Andrew Maynard, Arizona State University.

Back in 2008, carbon nanotubes – exceptionally fine tubes made up of carbon atoms – were making headlines. A new study from the U.K. had just shown that, under some conditions, these long, slender fiber-like tubes could cause harm in mice in the same way that some asbestos fibers do.

As a collaborator in that study, I was at the time heavily involved in exploring the risks and benefits of novel nanoscale materials. Back then, there was intense interest in understanding how materials like this could be dangerous, and how they might be made safer.

Fast forward to a few weeks ago, when carbon nanotubes were in the news again, but for a very different reason. This time, there was outrage not over potential risks, but because the artist Anish Kapoor had been given exclusive rights to a carbon nanotube-based pigment – claimed to be one of the blackest pigments ever made.

The worries that even nanotech proponents had in the early 2000s about possible health and environmental risks – and their impact on investor and consumer confidence – seem to have evaporated.

So what’s changed?

Artist Anish Kapoor is known for the rich pigments he uses in his work. Andrew Winning/Reuters

Carbon nanotube concerns, or lack thereof

The pigment at the center of the Kapoor story is a material called Vantablack S-VIS, developed by the British company Surrey NanoSystems. It’s a carbon nanotube-based spray paint so black that surfaces coated with it reflect next to no light.

The original Vantablack was a specialty carbon nanotube coating designed for use in space, to reduce the amount of stray light entering space-based optical instruments. It was this far remove from any people that made Vantablack seem pretty safe. Whatever its toxicity, the chances of it getting into someone’s body were vanishingly small. It wasn’t nontoxic, but the risk of exposure was minuscule.

In contrast, Vantablack S-VIS is designed to be used where people might touch it, inhale it, or even (unintentionally) ingest it.

To be clear, Vantablack S-VIS is not comparable to asbestos – the carbon nanotubes it relies on are too short, and too tightly bound together to behave like needle-like asbestos fibers. Yet its combination of novelty, low density and high surface area, together with the possibility of human exposure, still raise serious risk questions.

For instance, as an expert in nanomaterial safety, I would want to know how readily the spray – or bits of material dislodged from surfaces – can be inhaled or otherwise get into the body; what these particles look like; what is known about how their size, shape, surface area, porosity and chemistry affect their ability to damage cells; whether they can act as “Trojan horses” and carry more toxic materials into the body; and what is known about what happens when they get out into the environment.

These are all questions that are highly relevant to understanding whether a new material might be harmful if used inappropriately. And yet they’re notable in their absence in media coverage around the Vantablack S-VIS. The original use was seemingly safe and got people wondering about impacts. The new use appears more risky and yet hasn’t started conversations around safety. What happened to public interest in possible nanotech risks?

Federal funding around nanotech safety

By 2008, the U.S. federal government was plowing nearly US$60 million a year into researching the health and environmental impacts of nanotechnology. This year, U.S. federal agencies are proposing to invest $105.4 million in research to understand and address potential health and environmental risks of nanotechnology. This is a massive 80 percent increase compared to eight years ago, and reflects ongoing concerns that there’s still a lot we don’t know about the potential risks of purposely designed and engineered nanoscale materials.

It could be argued that maybe investment in nanotechnology safety research has achieved one of its original intentions, by boosting public confidence in the safety of the technology. Yet ongoing research suggests that, even if public concerns have been allayed, privately they are still very much alive.

I suspect the reason for lack of public interest is simple. It’s more likely that nanotechnology safety isn’t hitting the public radar because journalists and other commentators just don’t realize they should shining a spotlight on it.

Responsibility around risk

With the U.S.’s current level of investment, it seems reasonable to assume there are many scientists across the country who know a thing or two about nanotechnology safety. And who, if confronted with an application designed to spray carbon nanotubes onto surfaces that might subsequently be touched, rubbed or scraped, might hesitate to give it an unqualified thumbs up.

Let’s hear what the researchers know and are concerned about.
Surrey NanoSystems, CC BY-ND

Yet in the case of Vantablack S-VIS, there’s been a conspicuous absence of such nanotechnology safety experts in media coverage.

This lack of engagement isn’t too surprising – publicly commenting on emerging topics is something we rarely train, or even encourage, our scientists to do.

And yet, where technologies are being commercialized at the same time their safety is being researched, there’s a need for clear lines of communication between scientists, users, journalists and other influencers. Otherwise, how else are people to know what questions they should be asking, and where the answers might lie?

In 2008, initiatives existed such as those at the Center for Biological and Environmental Nanotechnology (CBEN) at Rice University and the Project on Emerging Nanotechnologies (PEN) at the Woodrow Wilson International Center for Scholars (where I served as science advisor) that took this role seriously. These and similar programs worked closely with journalists and others to ensure an informed public dialogue around the safe, responsible and beneficial uses of nanotechnology.

In 2016, there are no comparable programs, to my knowledge – both CBEN and PEN came to the end of their funding some years ago.

This, I would argue, needs to change. Developers and consumers alike have a greater need than ever to know what they should be asking to ensure responsible nanotech products, and to avoid unanticipated harm to health and the environment.

Some of the onus here lies with scientists themselves to make appropriate connections with developers, consumers and others. But to do this, they need the support of the institutions they work in, as well as the organizations who fund them. This is not a new idea – there is of course a long and ongoing debate about how to ensure academic research can benefit ordinary people.

Yet the fact remains that new technologies all too easily slip under the radar of critical public evaluation, simply because few people know what questions they should be asking about risks and benefits.

Talking publicly about what’s known and what isn’t about potential risks – and the questions that people might want to ask – goes beyond maintaining investor and consumer confidence which, to be honest, depends more on a perception of safety rather than actual dealing with risk. Rather, it gets to the very heart of what it means to engage in socially responsible research and innovation.

Andrew Maynard, Director, Risk Innovation Lab, Arizona State University

This article was originally published on The Conversation. Read the original article.

Featured Photo Credit: Surrey NanoSystems, CC BY-ND

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This Stretchy Material Heals Itself and Twitches Like Muscle Tissue

Jolting a super-stretchy, self-healing material with an electrical field causes it to twitch or pulse in a muscle-like fashion. The polymer can also stretch to 100 times its original length, and even repair itself if punctured.

Cheng-Hui Li, working in the Stanford University lab of chemical engineering professor Zhenan Bao, wanted to test the stretchiness of a rubberlike type of plastic known as an elastomer that he had just synthesized. Such materials can normally be stretched two or three times their original length and spring back to original size. One common stress test involves stretching an elastomer beyond this point until it snaps.

But Li, a visiting scholar from China, hit a snag: The clamping machine typically used to measure elasticity could only stretch about 45 inches. To find the breaking point of their one-inch sample, Li and another lab member had to hold opposing ends in their hands, standing further and further apart, eventually stretching a 1-inch polymer film to more than 100 inches.

Bao was stunned.

“I said, ‘How can that be possible? Are you sure?’” she recalls.

In the journal Nature Chemistry, the researchers explain how they made this super-stretchy substance. They also showed that they could make this new elastomer twitch by exposing it to an electric field, causing it to expand and contract, making it potentially useful as an artificial muscle.

SELF-HEALING IN THE COLD

Artificial muscles currently have applications in some consumer technology and robotics, but they have shortcomings compared to a real bicep, Bao says. Small holes or defects in the materials currently used to make artificial muscle can rob them of their resilience. Nor are they able to self-repair if punctured or scratched.

But this new material, in addition to being extraordinarily stretchy, has remarkable self-healing characteristics. Damaged polymers typically require a solvent or heat treatment to restore their properties, but the new material showed a remarkable ability to heal itself at room temperature, even if the damaged pieces are aged for days. Indeed, researchers found that it could self-repair at temperatures as low as negative 4 degrees Fahrenheit (-20 C), or about as cold as a commercial walk-in freezer.

The team attributes the extreme stretching and self-healing ability of their new material to some critical improvements to a type of chemical bonding process known as crosslinking. This process, which involves connecting linear chains of linked molecules in a sort of fishnet pattern, has previously yielded a tenfold stretch in polymers.

First they designed special organic molecules to attach to the short polymer strands in their crosslink to create a series of structure called ligands. These ligands joined together to form longer polymer chains—spring-like coils with inherent stretchiness.

Then they added to the material metal ions, which have a chemical affinity for the ligands. When this combined material is strained, the knots loosen and allow the ligands to separate. But when relaxed, the affinity between the metal ions and the ligands pulls the fishnet taut. The result is a strong, stretchable and self-repairing elastomer.

“Basically the polymers become linked together like a big net through the metal ions and the ligands,” Bao explains. “Each metal ion binds to at least two ligands, so if one ligand breaks away on one side, the metal ion may still be connected to a ligand on the other side. And when the stress is released, the ion can readily reconnect with another ligand if it is close enough.”

ARTIFICIAL MUSCLE AND SKIN?

The team found that they could tune the polymer to be stretchier or heal faster by varying the amount or type of metal ion included. The version that exceeded the measuring machine’s limits, for example, was created by decreasing the ratio of iron atoms to the polymers and organic molecules in the material.

The researchers also showed that this new polymer with the metal additives would twitch in response to an electric field. They have to do more work to increase the degree to which the material expands and contracts and control it more precisely. But this observation opens the door to promising applications.

In addition to its long-term potential for use as artificial muscle, this research dovetails with Bao’s efforts to create artificial skin that might be used to restore some sensory capabilities to people with prosthetic limbs. In previous studies her team has created flexible but fragile polymers, studded with pressure sensors to detect the difference between a handshake and a butterfly landing. This new, durable material could form part of the physical structure of a fully developed artificial skin.

“Artificial skin is not just made of one material,” says Franziska Lissel, a postdoctoral fellow in Bao’s lab and member of the research team. “We want to create a very complex system.”

Even before artificial muscle and artificial skin become practical, this work in the development of strong, flexible, electronically active polymers could spawn a new generation of wearable electronics, or medical implants that would last a long time without being repaired or replaced.

The Air Force Office of Scientific Research, Samsung Electronics, and the Major State Basic Research Development Program of China supported the work at Stanford. Other members of the research team are from University of California, Riverside and University of Colorado, Boulder.

Source: Republished from Futurity.org as a derivative work under the Attribution 4.0 International license. Original article published to Futurity by Tom Abate-Stanford.

Featured Image Credit: Bao Research Group/Stanford