Researchers from the Paul Scherrer Institute (PSI) teamed up with colleagues from the Swiss Materials Science Lab Empa to study a degenerative sign of ageing in concrete: the so-called alkali-aggregate reaction (AAR). In the course of AAR, a material forms that takes up more space than the original concrete and thus gradually cracks the concrete from within as the decades go by. The researchers have now explored the exact structure of this material. They managed to demonstrate that its atoms are arranged extremely regularly, making it a crystal. They also showed that the structure of this crystal is a so-called sheet-silicate structure. This specific structure had never been observed before. The researchers made their discovery thanks to measurements at the Swiss Light Source SLS at PSI. The research results could help towards the development of more durable concrete in future. AAR is a chemical reaction that affects outdoor concrete structures all over the world. It happens when concrete is exposed to water or moisture. For instance, numerous bridges and up to twenty per cent of the dam walls in Switzerland are affected by AAR. With AAR, the basic ingredients in the concrete are actually the problem: cement - the main component of concrete - contains alkali metals such as sodium and potassium. Any moisture infiltrating the concrete - stemming for example from rainwater - reacts with these alkali metals, leading to an alkaline solution. The second main ingredient in concrete is sand and gravel, which in turn are composed of minerals, such as quartz or feldspar. Chemically speaking, these minerals are so-called silicates. The alkaline water reacts with these silicates and forms a so-called alkali calcium silicate hydrate. This is itself able to absorb more moisture, which causes it to expand and gradually crack the concrete from within. This entire process is referred to as AAR. AAR takes place extremely slowly, so that the cracks are initially only tiny and invisible to the naked eye. Over the course of three or four decades, however, the cracks widen significantly and eventually jeopardise the durability of the entire concrete structure. Even if the chemical processes involved in AAR have long been known, nobody had identified the physical structure of the alkali calcium silicate hydrate formed in the course of AAR. The researchers at PSI and Empa have now managed to fill this knowledge gap. They studied the substance of a Swiss bridge constructed in 1969, which has been affected heavily by AAR. Researchers from Empa cut out a material sample from the bridge and ground down a small piece of it until they were left with a wafer-thin sample that was merely 0.02 millimetres thick. The sample was then taken to the Swiss Light Source SLS and irradiated with an extremely narrow x-ray beam, fifty times thinner than a human hair. Performing so-called diffraction measurements and a complex data analysis, the PSI researchers were eventually able to determine the crystal structure of the material with pinpoint precision. They found that the alkali calcium silicate hydrate has a previously undocumented sheet-silicate crystal structure. "Normally, discovering an uncatalogued crystal structure means you get to name it," explains Rainer Dähn, the first author of the study. "But it has to be a crystal found in nature, therefore we didn't get that honour," says the researcher with a smile. Andreas Leemann, Head of the Concrete Technology Group at Empa, had the idea for the current study. The researchers from PSI then brought their knowledge of the x-ray beam method to the table. "In principle, it's possible to add organic materials to the concrete that are able to reduce the build-up of tension," explains materials scientist Leemann. "Our new results provide a scientific basis for these considerations and could pave the way for the development of new materials." More information: R. Dähn et al. Application of micro X-ray diffraction to investigate the reaction products formed by the alkali–silica reaction in concrete structures, Cement and Concrete Research (2015). DOI: 10.1016/j.cemconres.2015.07.012
In the past year, researchers of the Paul Scherrer Institute PSI were among those who found experimental evidence for a particle whose existence had been predicted in the 1920s — the Weyl fermion. One of the particle’s peculiarities is that it can only exist in the interior of materials. Now the PSI researchers, together with colleagues at two Chinese research institutions as well as at ETH Zurich and EPF Lausanne, have made a subsequent discovery that opens the possibility of using the movement of Weyl fermions in future electronic devices. Such devices would be considerably smaller and more energy-efficient than their present-day counterparts. Today’s computer chips use the flow of electrons that move through the device’s conductive channels. Because, along the way, electrons are always colliding with each other or with other particles in the material, a relatively high amount of energy is needed to maintain the flow. That means not only that the device wastes a lot of energy, but also that it heats itself up enough to necessitate an elaborate cooling mechanism, which in turn requires additional space and energy. In contrast, Weyl fermions move virtually undisturbed through the material and thus encounter practically no resistance. You can compare it to driving on a highway where all of the cars are moving freely in the same direction, explains Ming Shi, a senior scientist at the PSI. The electron flow in present-day chips is more comparable to driving in congested city traffic, with cars coming from all directions and getting in each other’s way. While in the materials examined last year there were always several kinds of Weyl fermions, all moving in different ways, the PSI researchers and their colleagues have now produced a material in which only one kind of Weyl fermion occurs. This is important for applications in electronics, because here you must be able to precisely steer the particle flow, explains Nan Xu, a postdoctoral researcher at the PSI. Weyl fermions are named for the German mathematician Hermann Weyl, who predicted their existence in 1929. These particles have some striking characteristics, such as having no mass and moving at the speed of light. Weyl fermions were observed as quasiparticles in so-called Weyl semimetals. In contrast to real particles, quasiparticles can only exist inside materials. Weyl fermions are generated through the collective motion of electrons in suitable materials. In general, quasiparticles can be compared to waves on the surface of a body of water — without the water, the waves would not exist. At the same time, their movement is independent of the water’s motion. The material that the researchers have now investigated is a compound of the chemical elements tantalum and phosphorus, with the chemical formula TaP. The crucial experiments were carried out with X-rays at the Swiss Light Source SLS of the Paul Scherrer Institute. Studying novel materials with properties that could make them useful in future electronic devices is a central research area of the Paul Scherrer Institute. In the process, the researchers pursue a variety of approaches and use many different experimental methods.
Ostendorp T.,University of Konstanz |
Diez J.,Swiss Light Source SLS |
Heizmann C.W.,University of Zurich |
Fritz G.,Albert Ludwigs University of Freiburg
Biochimica et Biophysica Acta - Molecular Cell Research | Year: 2011
S100B is a homodimeric zinc-, copper-, and calcium-binding protein of the family of EF-hand S100 proteins. Zn2+ binding to S100B increases its affinity towards Ca2+ as well as towards target peptides and proteins. Cu2+ and Zn2+ bind presumably to the same site in S100B. We determined the structures of human Zn2+- and Ca2+-loaded S100B at pH 6.5, pH 9, and pH 10 by X-ray crystallography at 1.5, 1.4, and 1.65Å resolution, respectively. Two Zn2+ ions are coordinated tetrahedrally at the dimer interface by His and Glu residues from both subunits. The crystal structures revealed that ligand swapping occurs for one of the four ligands in the Zn2+-binding sites. Whereas at pH 9, the Zn2+ ions are coordinated by His15, His25, His 85', and His 90', at pH 6.5 and pH 10, His90' is replaced by Glu89'. The results document that the Zn2+-binding sites are flexible to accommodate other metal ions such as Cu2+. Moreover, we characterized the structural changes upon Zn2+ binding, which might lead to increased affinity towards Ca2+ as well as towards target proteins. We observed that in Zn2+-Ca2+-loaded S100B the C-termini of helix IV adopt a distinct conformation. Zn2+ binding induces a repositioning of residues Phe87 and Phe88, which are involved in target protein binding. This article is part of a Special Issue entitled: 11th European Symposium on Calcium. © 2010.
Abstract: Bones are made up of tiny fibres that are roughly a thousand times finer than a human hair. One major feature of these so-called collagen fibrils is that they are ordered and aligned differently depending on the part of the bone they are found in. Although this ordering is decisive for the mechanical stability of the bone, traditional computer tomography (CT) can only be used to determine the density but not the local orientation of the underlying nanostructure. Researchers at the Paul Scherrer Institute PSI have now overcome this limitation thanks to an innovative computer-based algorithm. They applied the method to measurements of a piece of bone obtained using the Swiss Light Source SLS. Their approach enabled them to determine the localised order and alignment of the collagen fibrils inside the bone in three dimensions. Aside from bone, the method can be applied to a wide variety of biological and materials science specimens. The researchers published the result of their study in the renowned journal Nature. The arrangement of the nanostructure of a three-dimensional object can now be visualised thanks to a new method developed by researchers at the Paul Scherrer Institute PSI. The researchers demonstrated this new approach in collaboration with bone biomechanics experts at ETH Zurich and the University of Southampton, UK, using a small piece of a human vertebrae that was roughly two and a half millimetres long. Bone consists of tiny fibres that are referred to as collagen fibrils. Their local three-dimensional order and alignment, which plays a central role in determining a bone's mechanical properties, has now been visualised along the entire piece of bone. This novel imaging approach provides important information that could aid, for example, the study of degenerative bone disease such as osteoporosis. In general, the new method is suitable not only for examining biological objects but also for developing promising new materials. The data was obtained from PSI's Swiss Light Source SLS, where the piece of bone was screened with an extremely fine and intense X-ray beam. This beam is scanned across the sample, recording data point by point. The interaction of the X-rays with the sample provides information about the local nanostructure at each measurement point. The crucial step from 2D to 3D Until now, only two-dimensional samples could be scanned and examined in this way. Traditionally, the objects examined are thus cut into very thin slices. "But not every object can be cut as thinly as you'd want", explains project supervisor Manuel Guizar-Sicairos. "And sometimes when you cut it, you destroy or disturb the very nanostructure that you wanted to examine." Quite generally, a non-destructive method is preferable, leaving the object intact for subsequent investigations. In order to be able to image three-dimensional objects, the PSI researchers scanned their sample repeatedly, turning it by a small angle between each scan. This way, they obtained measurement data for all orientations that allowed them to subsequently reconstruct the three-dimensional object, including its nanostructure, on the computer. The new measurement method used by the PSI researchers draws on a basic principle from computer tomography (CT). CT also involves first taking many X-ray images of a patient or object from different angles and then combining them to form the desired images by means of a computer calculation. However, traditional computer tomography does not use a fine X-ray beam. Instead, the object is irradiated as a whole. While computer tomography can depict the varying density of the material, it does not capture details like the order and alignment of the underlying nanostructure. The latter only becomes possible through accurate measurement of the interaction between sample and X-rays which is enabled by the narrow, intense X-ray beam of the SLS in conjunction with state of the art detectors. Images emerge thanks to mathematical algorithms The most complex step was to compile a computer image of the three-dimensional sample from the vast amount of data. To do this, the researchers developed their own sophisticated mathematical algorithm. "The X-ray beam always penetrates the entire depth of the sample and we only see the end result", explains Marianne Liebi, lead author of the study. "What the three-dimensional structure actually looks like is something we have to find out afterwards." For each point on the inside of the sample, Liebi's algorithm searches for the structure that best corresponds to all the data measured. In the algorithm, the researchers took advantage of the fact that they could assume a certain symmetry in the arrangement of the collagen fibrils in the bone, thus reducing their data to a manageable level. Nevertheless, there still remained 2.2 million parameters to be found. These were optimised using a computer program that tests better and better solutions until it finds one that can best explain all measurements. "I was amazed that after so much pure mathematics, an image emerged that really looked like a bone," said Liebi. "The details in it were plausible right away." Like a map of the vegetation zones While classic computer tomography generates greyscale images, the new method provides coloured images with considerably more information: The multi-coloured cylinders show the orientation on the nanoscale and even provide information on the degree of the orientation, which is high if adjacent collagen fibrils all have the same orientation and low if they are randomly oriented. "We can't image each individual collagen fibril directly, but that's not necessary anyway", explains Guizar-Sicairos. "Our imaging technique is akin to a map of vegetation zones. There too, one averages over certain areas, stating that one region is dominated by coniferous trees, another by deciduous trees and yet another by mixed woodland." In this way, it is possible to map the vegetation of entire continents without having to classify every single tree. By analogy it can be said that with traditional microscopic and nanoscopic methods this depiction of individual trees was necessary. That's why until now, the smaller the structure of an object was the smaller the imaged section also had to be. Their new method allowed the PSI researchers to circumvent this limitation: From a piece of bone visible to the naked eye, they recorded the arrangement of the nanostructure in one single image. At the same time as their publication, Nature will feature a second publication with research led by another researcher team with Liebi and Guizar-Sicairos as co-authors. That publication introduces an alternative algorithm that leads to a similar result: The researchers were able to determine the three-dimensional internal nanostructure of a human tooth. Full bibliographic information Nanostructure surveys on macroscopic specimens by small-angle scattering tensor tomography M. Liebi, M. Georgiadis, A. Menzel, P. Schneider, J. Kohlbrecher, O. Bunk and M. Guizar-Sicairos, Nature 19 November 2015 Six-dimensional real and reciprocal space small-angle X-ray scattering tomography F. Schaff, M. Bech, P. Zaslansky, C. Jud, M. Liebi, M. Guizar-Sicairos and F. Pfeiffer, Nature 19 November 2015 About Paul Scherrer Institut (PSI) The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 1900 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 380 million. For more information, please click Contacts: Paul Piwnicki Dr. Marianne Liebi Coherent X-ray Scattering group Laboratory for Macromolecules and Bioimaging Paul Scherrer Institute telephone: +41 56 310 54 53 [German, English] Dr. Manuel Guizar-Sicairos Coherent X-ray Scattering group Laboratory for Macromolecules and Bioimaging Paul Scherrer Institute telephone: +41 56 310 34 09 [English, Spanish] If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.
News Article | November 19, 2015
Bones are made up of tiny fibres that are roughly a thousand times finer than a human hair. One major feature of these so-called collagen fibrils is that they are ordered and aligned differently depending on the part of the bone they are found in. Although this ordering is decisive for the mechanical stability of the bone, traditional computer tomography (CT) can only be used to determine the density but not the local orientation of the underlying nanostructure. Researchers at the Paul Scherrer Institute PSI have now overcome this limitation thanks to an innovative computer-based algorithm. They applied the method to measurements of a piece of bone obtained using the Swiss Light Source SLS. Their approach enabled them to determine the localised order and alignment of the collagen fibrils inside the bone in three dimensions. Aside from bone, the method can be applied to a wide variety of biological and materials science specimens.