09 December 2008

Mother-of-pearl inspires tough ceramics

The awesome properties of the beautiful pearly coating on marine snails is slowly being better understood.
Spider conch (Lambis lambis)
Called mother-of-pearl or nacre, it is made up of 95-97% argonite, a hard but brittle calcium carbonate mineral, but is 1,000-3,000 times more fracture-resistant than pure aragonite. The snail extracts this calcium carbonate from seawater. Nacre’s remarkable strength is derived from its structure based on soft organic layers and hard lime platelets. More astoundingly, scientists have discovered that 95% of the nacre is self-assembled, while only 5% is actively formed by the snail.

No human-synthesized composite outperforms its constituent materials by such a wide margin. The problem has been that nacre's structure varies over tiny lengths, from nanometers to micrometers.

A recent study has mimicked the structure of nacre to produce what may well be the toughest ceramic ever.

Bio-inspired Toughest Ceramic Mimics Mother Of Pearl
ScienceDaily 8 Dec 08;
Biomimicry – technological innovation inspired by nature – is one of the hottest ideas in science but has yet to yield many practical advances. Time for a change. Scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have mimicked the structure of mother of pearl to create what may well be the toughest ceramic ever produced.

Through the controlled freezing of suspensions in water of an aluminum oxide (alumina) and the addition of a well known polymer, polymethylmethacrylate (PMMA), a team of researchers has produced ceramics that are 300 times tougher than their constituent components. The team was led by Robert Ritchie, who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the Materials Science and Engineering Department at the University of California, Berkeley.

“We have emulated nature’s toughening mechanisms to make ice-templated alumina hybrids that are comparable in specific strength and toughness to aluminum alloys,” says Ritchie. “We believe these model materials can be used to identify key microstructural features that should guide the future synthesis of bio-inspired, yet non-biological, light-weight structural materials with unique strength and toughness.”

Naturally Tough

Mother of pearl, or nacre, the inner lining of the shells of abalone, mussels and certain other mollusks, is renowned for both its iridescent beauty and its amazing toughness. Nacre is 95-percent aragonite, a hard but brittle calcium carbonate mineral, with the rest of it made up of soft organic molecules. Yet nacre can be 3,000 times (in energy terms) more resistant to fracture than aragonite. No human-synthesized composite outperforms its constituent materials by such a wide margin. The problem has been that nacre’s remarkable strength is derived from a structural architecture that varies over lengths of scale ranging from nanometers to micrometers. Human engineering has not been able to replicate these length scale variances.

Two years ago, however, Berkeley Lab researchers Tomsia and Saiz found a way to improve the strength of bone substitutes through a processing technique that involved the freezing of seawater. This process yielded a ceramic that was four times stronger than artificial bone. When seawater freezes, ice crystals form a scaffolding of thin layers. These layers are pure ice because during their formation impurities, such as salt and microorganisms, are expelled and entrapped in the space between the layers. The resulting architecture roughly resembles that of nacre.

“Since seawater can freeze like a layered material, we allowed nature to guide the process by which we were able to freeze-cast ceramics that mimicked nacre,” said Tomsia when this research was reported.

Engineered to be Tough

In this latest research, Ritchie, working with Tomsia and Saiz, refined the freeze-casting technique and applied it to alumina/PMMA hybrid materials to create large porous ceramic scaffolds that much more closely mirrored the complex hierarchical microstructure of nacre. To do this, they first employed directional freezing to promote the formation of thin layers (lamellae) of ice that served as templates for the creation of the layered alumina scaffolds. After the ice was removed, spaces between the alumina lamellae were filled with polymer.

“The key to material toughness is the ability to dissipate strain energy,” says Ritchie. “Infiltrating the spaces between the alumina layers with polymer allows the hard alumina layers to slide (by a small amount) over one another when load is applied, thereby dissipating strain energy. The polymer acts as a lubricant, like the oil in an automobile engine.”

In addition to making the lamellar scaffolds, the team was also able to fabricate nacre-like “brick-and-mortar” structures with very high alumina content. They did this by collapsing the scaffolds in a perpendicular direction to the layers then sintering the resulting alumina “bricks” to promote brick densification and the formation of ceramic bridges between individual bricks.

Says Saiz, “Using such techniques, we have made complex hierarchical architectures where we can refine the lamellae thickness, control their macroscopic orientation, manipulate the chemistry and roughness of the inter-lamellae interfaces, and generate a given density of inorganic bridges, all over a range of size-scales.”

Next Step

For ceramic materials that are even tougher in the future, Ritchie says he and his colleagues need to improve the proportion of ceramic to polymer in their composites. The alumina/PMMA hybrid was only 85-percent alumina. They want to boost ceramic content and thin the layers even further. They also want to replace the PMMA with a better polymer and eventually replace the polymer content altogether with metal.

Says Ritchie, “The polymer is only capable of allowing things to slide past one another, not bear any load. Infiltrating the ceramic layers with metals would give us a lubricant that can also bear some of the load. This would improve strength as well as toughness of the composite.”

Such future composite materials would be lightweight and strong as well as tough, he says, and could find important applications in energy and transportation.

This research was supported by DOE’s Office of Science, through the Division of Materials Sciences and Engineering in the Basic Energy Sciences office.
New Composite Material Is Almost Better Than Mother-of-pearl
ScienceDaily 10 Mar 08;
Researchers in ETH Zurich’s Department of Materials (D-MATL) have developed a new nacre-like composite that is twice the strength of naturally-occuring mother-of-pearl. Stronger ceramic platelets combined with ductile biopolymer Chitosan have created composites capable of withstanding a deformation of 25% before rupturing.

Nacre, or mother-of-pearl, is one of nature’s outstanding examples of a durable brick and mortar structure. Made of stiff, inorganic aragonite platelets and ductile biopolymers, the material combines toughness with a surprisingly high degree of strength. The researchers, led by Ludwig Gauckler, Professor of Non-mettalic Inorganic Materials have shown that ceramic alumina platelets and biopolymer Chitosan can be assembled layer-by-layer to form thin foils of a composite material exhibiting a nacre-like structure.

Nearly better than the original

In comparison to the stronger composite material developed, natural nacre deforms only one to two percent before reaching breaking point. Because it is not yet possible to obtain defect-free structures of such high platelet content as nacre, the stiffness of the new composite is five to seven times less than that of its natural counterpart. However, the new composite retains most of the ductility of polymer matrix composites, materials which can be used at high temperatures and are stronger, lighter and more resistant to corrosion.

Conventional thin foils of other materials such as metals, polymers or fiber-reinforced composites may be up to one order of magnitude stronger and stiffer, but few materials reach the same combination of strength and ductibility per unit weight as the new nacre-like foils developed by the ETH Zurich team.

Development of the new nacre-like composite has opened the door to further research, such as manufacturing the foils at high speed. The ETH Zurich team is also exploring the use of different “glues” and platelets of different geometry in order to improve the composite’s mechanical properties. As well under study is the optimization of the platelet-glue interface. This research is currently being carried out in collaboration with Professor J. Woltersdorf and Dr. E. Pippel at the Max Planck Institute for Microstructure Physics in Halle, Germany and the polymer groups at ETH Zurich.

Further research underway

ETH Zurich’s research establishes concepts for tailoring the mechanical properties of composite materials. The combination of nature’s smart structural design with the enhanced properties of artificial building blocks should make possibe the creation of even more composites with similar combinations of mechanical properties.

Future research will address achieving ever-thinner polymer layers and ceramic platelets while maintaining the integrity of the mechanical concept of nacre, as well as researching whether the polymer layer can approach atomic thickness yet keep the nacre-like behaviour of the composite.

Journal reference: Bonderer, Lorenz J., André R. Studart & Ludwig J. Gauckler (2008): Bio-inspired Design and Assembly of Platelet Reinforced Polymer Films, Science Vol. 319, 1069 (2008); DOI: 10.1126/science. 1148726
Mother-of-pearl: Classic Beauty And Remarkable Strength
ScienceDaily 6 Jul 07;
While the shiny material of pearls and abalone shells has long been prized for its iridescence and aesthetic value in jewelry and decorations, scientists admire mother-of-pearl for other physical properties as well.

Also called nacre ("NAY-ker"), mother-of-pearl is 3,000 times more fracture-resistant than the mineral it is made of, aragonite, says Pupa Gilbert, a physicist at the University of Wisconsin-Madison. "You can go over it with a truck and not break it - you will crumble the outside [of the shell] but not the [nacre] inside. And we don't understand how it forms - that's why it's so fun to study."

Understanding the mechanism by which nacre forms would be the first step toward harnessing its strength and simplicity, she says. "We don't know how to synthesize materials that are better than the sum of their parts."

Writing in the June 29 issue of Physical Review Letters, Gilbert and her colleagues in the UW-Madison department of physics and School of Veterinary Medicine, the Institute for the Physics of Complex Matter in Switzerland and the UW-Madison Synchrotron Radiation Center, now describe unexpected elements of nacre architecture that may underlie its strength and offer clues into how this remarkable material forms.

Like our bones and teeth, nacre is a biomineral, a combination of organic molecules - made by living organisms - and mineral components that organisms ingest or collect from their environment. The aragonite mineral in nacre is made of calcium carbonate, which marine animals form from elements abundant in seawater.

Though a mere 5 percent of abalone nacre is organic, this small fraction somehow lays enough foundation for the mineral components to assemble spontaneously, Gilbert says.

"Ninety-five percent of the mass of this biomineral is self-assembled, while only 5 percent is actively formed by the organism," she says. "It is one of the most efficient mechanisms you can think of."

To gain insight into this self-assembly process, Gilbert and graduate student Rebecca Metzler examined the structure of abalone nacre using synchrotron radiation - light emitted by electrons speeding around a curved track.

When used to examine a cross-section of an abalone shell, previously seen to resemble a brick wall with layers of organic "mortar" separating individual crystalline "bricks," the polarized light from the synchrotron revealed that the nacre wall was not uniform.

Instead, the wall contained distinct clumps of bricks, each an irregular column of crystals with identical composition but a crystal orientation different than neighboring columns.

Since orientation affects how crystals emit electrons, "some of the columns of bricks appear white and others appear black and more appear gray, depending on their crystal orientation," Gilbert explains.

The overall effect resembles a camouflage pattern, each roughly columnar cluster a slightly different shade.

She suggests that this mosaic architecture of nacre, with numerous non-aligned crystals, could lead to a stronger material by preventing the formation of natural cleavage planes - like those that form the facets of a cut diamond - where a single crystal can easily break. "It is intuitive that a poly-crystal is mechanically stronger than a single crystal, so perhaps that is an advantage for the animal," Gilbert says.

With this new information about nacre structure and the help of UW-Madison theoretical physicist Susan Coppersmith, the group turned to modeling to try to understand how such a structure could form.

"By looking at the final result and comparing it to the result of different growth models, you get insight into what the actual mechanism of the growth is," Coppersmith says.

The group developed a model that suggests that the animal creates the organic "mortar" layers first, peppered with randomly distributed crystal nucleation, or seeding, sites.

From their observations, they predict that mineral crystals start growing inside the shell and extend horizontally until they contact another growing crystal and vertically until they hit the overlying mortar. If that crystal contacts another of the scattered crystal formation sites on the next tier up, it should trigger growth of a new crystal with the same crystal orientation, gradually building a rough column of irregular width.

With further experiments, the researchers hope to test and refine their model as well as examine other biominerals, such as human teeth and the nacre of other species such as pearl oysters, mussels, or nautiluses, to improve their understanding of biomineral formation and assembly.

"If you understand how it forms, you could think of reproducing it, producing a synthetic material that's inspired by nature - a so-called 'biomimetic' material," Gilbert explains. "If we learn how to harness the mechanism of formation, then we could, for example, produce cars that absorb all the energy at the impact point but do not fracture.

"But from my point of view, it's most interesting because of the fundamental mechanisms of how it forms - these natural self-assembly mechanisms we are only just beginning to understand."

This work is funded by grants from the UW-Madison Graduate School and the National Science Foundation.
Mother-of-pearl In Highest Resolution
ScienceDaily 6 Oct 05;
Mother-of-pearl, also known as nacre, is not just an iridescent substance whose optical characteristics impress the observer and which is often used for jewellery. It is also an excellent material for working with. Nacre consists of 97 percent lime, but has a thousand times higher breaking strength. The reason has to do with the layer composition of mother-of-pearl. Now, Max Planck and BAM scientists have discovered that the surface of the lime platelets in mother-of-pearl is not at all ordered in layers, as had been previously assumed. Because of this fact, it can be ruled out that the crystals are controlled through ordered layers on the organic matrix. This understanding of nacre, and the mechanism by which it is built, is essential for emulating the same refined principle in building new materials.

For a long time, mother-of-pearl has been considered as an interesting biogenous material. During this time, researchers have been trying to understand its astounding characteristics. Its unusual breaking strength is due to a structure based on soft organic layers and hard lime platelets.

If we could only begin to copy this building principle, it would lead to a revolution in the construction industry. Possible goals for this kind of biomimetic materials research could be firmer gypsum plasterboard, or pieces of concrete with a lower weight but with the same strength. The lime platelets in nacre crystallise into aragonite - a crystal form which is normally not stable under ambient conditions. Until now, researchers had assumed that this crystallization of the lime platelets was determined by ordered layers of protein which lie on a pre-formed layer of chitin. Chitin can be found in nature, as for example a scaffolding material in the shells of insects.

But the latest findings of the Max Planck scientists have found these assumptions to be false. Instead of an ordered crystalline layer, which would be in contact with the organic matrix, the scientists found tiny - five nanometres thick - layers of amorphous - that, is disordered - calcium carbonate on the surface of the monocrystalline platelets in nacre.

This disordered and wavy surface provides evidence against the postulated specific interaction between the mineral material and the organic matrix. The finding could be clearly supported by 13C and 1H solid state nuclear magnetic resonance (NMR) spectroscopy. Furthermore, in NMR experiments the researchers detected the amorphous character of the surface layer and ruled out any interaction between it and the organic scaffolding.

The reason for the existence and development of the disordered upper layer on the crystal could be based upon the fact that impurities accumulate in the surface layer. In crystallization, these are not built into the ordered crystal lattice - similar to what happens in the process of zone melting in metallurgy.

The amorphous layer (ACC) could indeed have another function. It replaces the previously assumed direct interaction of the high energy (001) aragonite layer through a gradient layer made of aragonite, ACC, and organic matrix. The energies of the boundary layer could be significantly lower here, and thus a thermodynamic force could exist for the development of an amorphous upper layer. It is still not clear in which direction the crystallographic orientation of the platelet eventually moves. In the current study, the scientists have acted on the assumption that an electrostatic attraction exists between the inorganic platelets and the organic matrix.

Original work:

Nadine Nassif, Nicola Pinna, Nicole Gehrke, Markus Antonietti, Christian Jäger, and Helmut Cölfen
Amorphous layer around aragonite platelets in nacre
PNAS 2005 102: 12653-12655; published online before print: August 29 2005, print: September 6, 2005, Vol. 102, No. 36

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