An armorlike arapaima fish scale resists being fractured by a piranha tooth that is slowly pressed into it. In fact, it is the tooth that fails. Credit: Meyers Group: Credit: UCSD Jacobs Sch. of Eng.

If you ever watch cable TV’s River Monsters (and, honestly, who doesn’t!), you might be familiar with a large Amazonian “living fossil” fish that goes by the name arapaima.

The arapaima have a reputation for being one of the few animals in the Amazon that hungry piranha don’t bother. Why? Apparently it is because evolution has draped the arapaima in a flexible skin of “armor” that is effectively impenetrable to the piranhas’ teeth. The fish has “scales,” but there aren’t many other species among fish that come close to the defenses of the arapaima.

Researcher Marc Meyers, an expert on bio-inspirational design and a professor at the Jacobs School of Engineering at University of California, San Diego, knew of this reputation and speculated that if the arapaima are indeed protected from piranhas’ bites, maybe insights from the structure of the arapaima’s scales could provide ideas for engineering new materials, such as flexible ceramics.

(Meyers’ name may be familiar to some. He was one of the stars in one of the episodes of Nova’s “Making Stuff” TV series, in which he discussed the strength of mullosk shells.)

As can be seen in the above video, Meyers, along with his students and colleagues, set up a simple desktop test. They attach an arapaima scale to a soft rubber base (to simulate the fish’s soft muscle and tissue under the scale) and mounted a single piranha tooth in a press. The tooth is then pressed into the scale. In each case, the tooth can partially penetrate the scale, but the tooth cracks before the scale suffers a total fracture.

Credit: Meyers Group; UCSD.

Meyers says in a UC San Diego release that the structure of the scale is combination of a mineralized outer layer with a clever and tough internal design (see diagram from the press release). This tough inner layer has collagen fibers stacked in alternating directions “like a pile of plywood.” He says the mix of materials in the scale is similar to the hard enamel of a tooth deposited over softer dentin.

Implications? Meyers says that the arapaima’s design should serve as bioinspiration for lots of things that need to be both tough and flexible, for example body armor, fuel cells, insulation and aerospace designs.

Some lessons for engineers are from this work are:

• Combine hard and soft materials
• Stack the materials in the underlying layer with different orientations
• Texture is key, and a varying surface provides more capability.

On this last point, Meyers notes that each scale has an exterior that is “corrugated.” According to the story, “the corrugated surface keeps the scales’ thick mineralized surface intact while the fish flexes as it swims. Ceramic surfaces of constant thickness are strained when forced to follow a curved surface. The corrugations allow the scales to ‘be bent more easily without cracking,’ Meyers said.”

Meyers says he will also be studying the scales of another unusual fish, the alligator gar whose scales were reportedly used as arrow tips.

Corrugated texture of arapaima scales. Credit: Meyers Group; UCSD Jacobs Sch. of Eng.

Meyers et al. have written about their studies in the The Journal of the Mechanical Behavior of Biomedical Materials in the paper, “Biological materials: A materials science approach” (doi:10.1016/j.jmbbm.2010.08.005).

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Photographs of sample films at room temperature. Credit, Gao et al.; RSC Energy Environ. Sci.

The notion of making functional and flexible ceramic foils is fascinating, but a little counterintuitive, isn’t it?

Thus, I am always intrigued when new techniques and applications are discovered. A while back I wrote about a group from ETH Zurich that mastered ultrathin, transparent and flexible foils using yttria stabilized polycrystalline tetragonal zirconia.

This weeks brings news of another transparent and flexible film based on vanadium dioxide developed by Chinese researchers at the Shanghai Institute of Ceramics, including ACerS member Yanfeng Gao. The new VO₂ film is being eyed for use in thermochromic windows, which retain transparency in the visible range but dynamically regulate the passage of wavelengths that transfer solar heat and energy in the ultraviolet and infrared ranges.

Smart window researchers have had an interest in VO₂ because they’d like to exploit a particular property: At 68°C (in the case of VO₂ bulk single crystals), the material undergoes a reversible, thermally induced phase transition that shifts the optical properties in the near-infrared region from a low-temperature transparent state to a more reflective state.

In an email, Gao says materials based on VO₂ nanoparticles, particularly VO₂ foils that easily can be used with glass panels, are attractive for applications in construction and automotive industries. Heretofore, making such foils from solutions of nanoparticles has been tricky and unreliable because of the instability of the nanoparticles. But, Gao and his colleagues report in a paper in the Royal Society of Chemistry’s journal, Energy and Environmental Science, that they have figured out a new process that solves previous shortcomings and may be scalable to large-area mass production.

Gao, who works in SIC’s State Key Laboratory of High Performance Ceramics and Superfine Microstructure, says, “In this paper, we report a novel all-solution process that can be used to prepare transparent, stable and flexible VO₂-based composite films. These films exhibit UV-shielding properties and an excellent temperature-responsive thermochromism in the near infrared region.”

The breakthrough? Gao says the answer came when the group coated the VO₂ nanoparticles with a thin SiO₂ shell. “The shell, Gao says, “significantly improved their anti-oxidation and anti-acid abilities.”

Essentially, the VO₂ nanoparticles are given a SiO₂ shell using tetraethyl orthosilicate (the thickness of the shell can be fine-tuned) and treated with a silane couple to increase dispersion. The last step is to cast the suspension on a PET substrate.

At 13.6% solar modulation efficiency, the researchers report in the paper that their VO₂-based film is able to match the efficiency levels of other thermochromic films. This is also considerably higher than VO₂ films produced by sputtering and other methods.

Gao says, “Traditional glass foils are usually based on thin notable metal layers for reflection of solar irradiation or organic dyes that can absorption solar heat. … The stability of these kinds of foils is still questionable. To our knowledge the current research reports on the first VO₂ ceramic foils, and more importantly, the foils show excellent optical properties (visible transmittance and solar modulation ability, maybe the best in the world.).

As far as “smart” performance goes, Gao et al. report in the paper that they observed while testing a typical sample of the VO₂ film, “… in a heating cycle from 35°C to 85°C, the transmittance at 1500 nm decreased from 57.7% (at 35°C) to 14.9% (at 81°C) gradually…  In a cooling cycle, the transmittance of film increased from 14.9% (at 75°C) to 57.7% (at 35°C).”

The promise of increased energy efficiency via thermochromic windows has drawn worldwide attention. Gao says it is a big concern for developing countries, such as China, which already has buildings occupying 52 billion square meters “waiting new techniques to improve their energy efficiency and to reduce greenhouse gas emissions. We are aiming to develop a new material along with a novel process that can be finally commercialized and used to for building glasses.”

Gao says scaling the group’s technique to large-area production is the next challenge, and says that a collaborative effort is worthwhile. “This method should be considered as a basis for mass production,” he says, “The method should combine with some techniques to efficiently fabricate and to improve performance-cost ratios. … We hope that colleagues working in related fields can join to consider innovations based on the current technology. As an important part of an eco-home, we hope that such kind of smart windows can be applied practically in the near future.”

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On relaxations and aging of various glasses

Slow relaxation occurs in many physical and biological systems. “Creep” is an example from everyday life. When stretching a rubber band, for example, the recovery to its equilibrium length is not, as one might think, exponential: The relaxation is slow, in many cases logarithmic, and can still be observed after many hours. The form of the relaxation also depends on the duration of the stretching, the “waiting time.” This ubiquitous phenomenon is called aging, and is abundant both in natural and technological applications. Here, we suggest a general mechanism for slow relaxations and aging, which predicts logarithmic relaxations, and a particular aging dependence on the waiting time. We demonstrate the generality of the approach by comparing our predictions to experimental data on a diverse range of physical phenomena, from conductance in granular metals to disordered insulators and dirty semiconductors, to the low temperature dielectric properties of glasses.

Built to withstand almost anything

Thanks to researchers at Department of Homeland Security S&T, communities can fortify today’s critical structures — and design tomorrow’s — to absorb blows and remain open if assaulted by extreme earth, wind, water, fire, or man. A new publication series, aimed at engineers, architects, building owners, city planners, and emergency managers, makes available years of government, industry, and academic research on designs and materials to make buildings and tunnels terror-resistant and terror-resilient. The Building and Infrastructure Protection Series provides architects and engineers a set of aids for designing critical infrastructure to withstand all kinds of hazards…at a cost that won’t break the budget.

Boise State researchers create new way to study ground fractures

Boise State geophysics researchers have created a new way to study fractures by producing elastic waves, or vibrations, through high-intensity light focused directly on the fracture itself. The new technique developed in the Physical Acoustics Lab may help determine if there is a fluid, such as magma or water, or natural gas inside fractures in the Earth. Typically, scientists create sound waves at the surface to listen for echoes from fractures in the ground, but this new technique could provide more accurate information about the cracks because sound does not have to travel to the fracture and back again. The new technique aims to enhance scientists’ abilities to image faults in the Earth, including those man-made through the process of hydraulic fracturing, or fracking.

Antennaless RFID tags developed at NDSU solve problem of tracking metal and liquids

Tracking and identifying metal objects can prove difficult for some radio frequency identification systems. A patent-pending technology developed by a research team at the Center for Nanoscale Science and Engineering at North Dakota State University, Fargo, could solve these RFID tracking problems. The antennaless RFID tag developed at CNSE could help companies track products as varied as barrels of oil to metal cargo contaRFID tag bottleiners. A typical RFID tag is made up of an integrated circuit and an antenna. While there are different types of tags available, many don’t work well on metal objects or on containers filled with liquid. Previous attempts to solve this problem have resulted in bulky tags that are easily destroyed by routine handling. Researchers at the center have developed a patent-pending novel approach, with an antennaless RFID tag, allowing for an inexpensive and manufacturable product tracking solution that meets EPCglobal Standards.

MIT envisions DIY solar cells made from grass clippings

Research scientist Andreas Mershin has a dream to bring inexpensive solar power to the masses, especially those in developing countries. After years of research, he and his team at MIT’s Center for Bits and Atoms, along with University of Tennessee biochemist Barry Bruce, have worked out a process that extracts functional photosynthetic molecules from common yard and agricultural waste. If all goes well, in a few years it should be possible to gather up a pile of grass clippings, mix it with a blend of cheap chemicals, paint it on your roof and begin producing electricity. Talk about redefining green power plants!

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Jim O’Neil, a fellow graduate student a good while ago, liked to say, “Ceramic engineering is all about making big rocks into little rocks, and then making little rocks into big rocks.”

I’ve lost track of Jim, but a new Rapid Communication in the Feb. 2012 Journal of the American Ceramic Society reminded me of his take on our branch of materials science.

The short paper by a Korean team, Son and Jung, describes a novel method of making discrete ferroelectric particles by a method that amounts to making “little rocks out of big rocks” — but on a nanoscale!

The investigators were interested in fabricating PbTiO₃ nanodots for ferroelectric random access memory, which is a promising material for a nonvolatile memory applications.

Demand for smaller devices is driving the development of high density, high performance memories, and further downsizing is starting to run into physical limitations imposed by materials properties and processing.

For example, some types of RAM materials, are susceptible to a surface effect on the memory switching mechanism as they are scaled down. In ferroelectric RAM materials, there is a critical size that is determined by the maximum size of the nonferroelectric component.

Processing, too, imposes size limitations. Several methods have been used to fabricate ferroelectric nanostructures, such as self-assembly, an anodizing aluminum oxide template process, e-beam lithography and dip-pen lithography. According to the paper, PTO nanodots have been fabricated in the 22 nm to 60 nm range by these methods. The smallest PTO nanodots (22 nm) were made by self-assembly, however, the authors note that “these nanodots did not exhibit a canonical piezoelectric hysteresis loop,” even though the critical size for PTO nanodots to exhibit ferroelectricity is a few nanometers. Dip-pen lithography can make nanodots of about 40 nm with good ferroelectric properties. The anodizing aluminum oxide process is not conducive to making dots less than 60 nm.

Son and Jung’s idea was simple: Make a nanodot, whack it with a hammer, anneal to crystallize the shattered pieces and, finally, test for ferroelectricity. Their goal was to fabricate PTO nanodots less than 10 nm diameter with ferroelectric properties.

So, using dip-pen lithography, they made a PTO nanodot that was 40 nm diameter and 25 nm thick. Using an atomic force microscope tip as a hammer, the dot was shattered after “repeated collisions,” that is, they had to beat on it. The resulting nanoparticles ranged in size from several nanometers to a few tens of nanometers, and were of “diverse sizes in both diameter and thickness.”

After crystallization, tests showed that ferroelectric properties were present in a 25 nm dia. x 11 nm thick nanodot and in one that was 10 nm dia. x 8 nm thick. The 10-nm-diameter nanodot, according to the article, is “closer to the theoretical critical size” than has been achieved in other studies. PTO nanodots that were less than about 3 nm thick proved difficult to test because of high leakage currents.

The paper is “Ferroelectric PbTiO₃ Nanodots Shattered Using Atomic Force Microscopy,” Jong Yeog Son, Inhwa Jung, JACerS, Feb. 2012. DOI: 10.1111/j.1551-2916.2011.05026.x.

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Demonstration of the flexibility of cellulose–silica composite aerogel. Credit: J. Cai et al.; Angewandte Chemie.

This sounds like the type of breakthrough aerogel fans have yearning for.

A newly published paper in Angewandte Chemie reports on an Asian group’s success at using cellulose fibers as a scaffold/template for a resultant silica aerogel that delivers a product that has great mechanical strength and flexibility, while retaining a large surface area and semitransparency.

Aerogel has been something of a tease for many years. It has incredible insulating abilities, but the one enormous problem for silica aerogel is that it is frustratingly brittle and difficult to work into practical applications. Some developers have found limited success via hybridization techniques with support materials such as polyurethane, polystyrene or even nanofibrillar bacterial cellulose and microfibrillated cellulose gel.

However, with support from the Japan Society for the Promotion of Science’s Foreign Researcher Fund of Japan and the National Basic Research Program of China, researchers at Wuhan University, China, and University of Tokyo, took a different cellulose-based route. They already knew that they could exploit “cellulose II” crystallinity  (dissolution and then regeneration/reassembly of fibrils) to form aerogels with good mechanical strength, light transmittance and high porosity — characteristics that they suspected would make it an effective substrate for silica aerogel.

In brief, the group, led by Lina Zhang, impregnated a sample of nanoporous cellulose gel (with its interconnected nanofibrillar network) with a silica precursor, tetraethyl orthosilicate. According to the paper, “The resulting composite gels were dried with supercritical CO₂ to give cellulose–silica aerogels with low density, moderate light transmittance, a large surface area, high mechanical integrity and excellent heat insulation.”

They then went one step farther and used calcination to remove the cellulose matrix, leaving a silica-only aerogel. The key point here is that this silica aerogel’s structure is much different than pure silica aerogel. In the latter, primary silica nanoparticles form and then randomly coagulate resulting in an isotropic 3D network. “In contrast,” again quoting from the paper, the authors say, “the formation of silica nanoparticles in the cellulose gel seems to cause their deposition onto the cellulose fibrils. As a result, removal of cellulose by calcination results in the nanofibrillar silica network.”

The group compared a variety of aerogels, including silica-only and cellulose-only aerogels; cellulose-silica composites, with varying levels of silica; and cellulose-templated silica aerogel.

What they found at the macroscopic level is that the composite aerogels didn’t inherit the fragility of the silica, but instead seem to inherit the flexibility and strength of the cellulose network (see knotted sample of one of the composites, above).

While the tensile modulus and strength of the cellulose–silica aerogel were less than pure cellulose aerogel, “the compression modulus of the composite (7.9MPa) is more than two orders of magnitude higher than that of silica aerogel, and about 50 times higher than that of the aerogel prepared from bacterial cellulose.”

Because of the cellulose content, the composite aerogels break down when used above 300°C. However, below that temperature, the cellulose-silica aerogel retained strong heat insulating properties. Thermal conductivity of the prepared samples ranged from 0.025 W m^-1 K^-1 to 0.045 W m^-1 K^-1.

These numbers compare favorably with polystyrene foam (0.030 W m^-1 K^-1), however, the researchers note that the ability of the cellulose–silica aerogels to perform up to 300°C give it a leg up on insulation materials made of polymer that soften and breakdown at similar temperatures.

“Thus,” according to the authors,”the cellulose–silica composite is potentially useful as heat insulating material with high mechanical stability, together with processability to form sheets, fibers, or beads. … [They] retained the mechanical strength and flexibility, large surface area, semitransparency, and low thermal conductivity of the cellulose aerogels. The ease of preparation and wide tuneability of composition/properties with this method are expected to form the basis for the development of various advanced nano-porous materials.”

The paper, ”Cellulose-silica nanocomposite aerogels by in situ formation of silica in cellulose gel,” (doi:10.1002/ange.201105730) is written by Jie Cai, Shilin Liu, Jiao Feng, Satoshi Kimura, Masahisa Wada, Shigenori Kuga, and Lina Zhang.

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