New Material Shows Superior Lubricating Properties

English
REHOVOT, Israel -- June 19, 1997-- A material discovered at the Weizmann Institute of Science has shown superior properties as a machine lubricant in tests simulating industrial conditions. When compared with the best existing lubricants, the material reduced friction between moving metal parts to less than half, and also cut wear on parts by up to six times.
 
These findings are reported in the June 19 issue of Nature. Using the new material as a lubricant would significantly increase the lifespan and efficiency of machinery from power tools and motor vehicles to airplanes and satellites.
 
"Lubricants reduce friction, and reduced friction means that machinery parts work more efficiently, so that cars, for example, would use less gasoline," says Prof. Reshef Tenne of the Institute's Materials and Interfaces Department, who headed the research team."In fact, growing environmental awareness is increasing pressure to improve the lubricants used in all machinery as a way of raising energy efficiency."
 
Friction between moving metal parts is the main cause of reduced efficiency in all machines, and also causes wear-and-tear on parts. Lubricants aim to combat this, but even high-performance lubricants, which mix solid and liquid ingredients, tend to stick to metal parts and eventually rub off, slowing down the workings and causing wear.
 
Tenne and team have now produced a material made of molecules that adhere and rub off far less than the best existing lubricants."Existing lubricants contain crystallites, which are shaped like flat platelets and have chemically reactive edges. In working conditions, they stick to machinery parts and undergo chemical reactions that lead them to decompose and rub off," says Tenne. "In contrast, our molecules are round and inert, so they just roll against each other and against the machinery parts, and don't stick to anything, like Teflon."

Tenne's team was made up of Ph.D. students Yshai Feldmann and Moshe Homyonfer, Dr. Sidney Cohen of the Institute's Chemical Services Unit, and Dr. Lev Rapoport and other researchers from the Center for Technological Education in Holon.


Inorganic "Buckyballs"


In the mid-1980's, scientists made the revolutionary discovery that in certain conditions carbon atoms will cluster together to form a stable, hollow sphere that remarkably resembles a soccer ball.These round carbon molecules, which won their discoverers the 1996 Nobel Prize in Chemistry, were named fullerenes after R. Buckminster Fuller, the architect famous for designing domes with a soccer ball structure. Initially it was thought that fullerenes, or "buckyballs" as they are sometimes known, could be formed only by carbon, or possibly by other carbon-containing molecules.
 
But in 1992, Weizmann Institute scientists led by Tenne and colleagues discovered that a synthetic inorganic molecule, tungsten disulfide, also forms fullerene-like balls in certain conditions. This finding opened a new field of research in materials science.


Outperforming Existing Lubricants


Tenne soon realized that the tungsten disulfide fullerene-like molecule shows properties that make it particularly suitable for use as a lubricant.Its round shape means it does not adhere to other substances, and it is larger than the carbon fullerene, enabling it to maintain a significant distance between two moving metal parts. In addition, it is made up of many layers of balls, rather like an onion, so that if the top layer wears away, those underneath continue to maintain a lubricating action.
 
In the study now being published in Nature, Tenne's team synthesized tungsten disulfide fullerene-like molecules of a relatively uniform shape and size, each molecule measuring about one millionth of a centimeter across.The researchers showed that in conditions similar to those prevailing in industry, the molecule significantly outperformed all existing solid lubricants, including normal tungsten disulfide and molybdenum disulfide.
 
The Institute's technology transfer arm, Yeda Research and Development Co. Ltd., has already filed patent applications for the new material, and industrial companies worldwide are expressing interest.
 
Tenne's team is now developing ways to produce commercial quantities of the tungsten disulfide fullerene-like molecule. "The material works very well in the laboratory, and our challenge now is to synthesize it in large quantities for testing in the field," Tenne says.
 

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel. Its 2,400 scientists, students and support staff are engaged in more than 850 research projects across the spectrum of contemporary science.
Chemistry
English
  • Fullerene
  • Lubricant
  • Materials and Interfaces
  • Reshef Tenne
  • Yeda

Can Biology Teach Us to Design Better Materials?

English
a sea urchin spine undergoing repair after breakage
 
 

 

When Joyce Kilmer wrote “I think that I shall never see/ A poem lovely as a tree,” she might have been speaking for scientists. Nature still outshines us when it comes to elegance of design; and it still knows how to do things we don’t – including things that should, according to many of today’s theories, be impossible.

Scientists in the Weizmann Institute’s Faculty of Chemistry look to biology to learn how living organisms create the amazing variety of forms and substances found in nature, and to apply these lessons to completely new kinds of materials. These range from everyday materials with new qualities to exotic nanostructures the size of molecules.

Profs. Lia Addadi and Stephen Weiner have for many years been conducting collaborative research into nature’s strongest materials. Recent studies of theirs, which reveal how sea urchins build their spines and vertebrates build their bones, are leading to an upsurge of new research and possible applications. Other findings, on the formation of the colorless crystal structures that give fish scales their iridescence, might be used to design new paints or camouflage.

Prof. Michael Elbaum wants to learn the secrets of the nuclear pore – the opening in the membrane surrounding the cell nucleus that allows some molecules to pass into the nucleus and others to pass out. He recreates their functions in experimental, man-made membranes with active pores that might, one day, be able to filter poisons from blood or pollutants from water.

Prof. Roy Bar-Ziv incorporates such biological molecules as DNA into “smart” materials. Genetic sequences are fixed onto two-dimensional surfaces, creating dense compartments where biosynthesis can take place. Eventually he aims to create, from the bottom up, cell analogs that can sense their environment and respond; these could be designed for a number of different uses, including biosensors and diagnostics.

 
(l-r) Profs. Roy Bar-Ziv, Stephen Weiner, Lia Addadi and Michael Elbaum
 
 

 

Prof. Lia Addadi’s research is supported by the Clore Center for Biological Physics; the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research; the Aharon Katzir-Katchalsky Center; and the Carolito Stiftung. Prof. Lia Addadi is the incumbent of the Dorothy and Patrick Gorman Professorial Chair.

Prof. Roy Bar-Ziv’s research is supported by the Phyllis and Joseph Gurwin Fund for Scientific Advancement; the Helen and Martin Kimmel Center for Nanoscale Science; and the Carolito Stiftung.

Prof. Michael Elbaum’s research is supported by Mr. and Mrs. James Nathan, Beverly Hills, CA.

Prof. Stephen Weiner’s research is supported by the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research; the Helen and Martin Kimmel Center for Archaeological Science; the Maurice and Vivienne Wohl Charitable Foundation; and the estate of George Schwartzman. Prof. Weiner is the incumbent of the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Structural Biology.

 
 
Scanning electron microscope image of a sea urchin spine undergoing repair after breakage.
Chemistry
English
  • Lia Addadi
  • Materials science
  • Michael Elbaum
  • Roy Bar-Ziv
  • Sea urchin
  • Smart material
  • Stephen Weiner

Crowd Control

English

 

Dan Bracha, Prof. Roy Bar-Ziv, Dr. Shirley Daube and Dr. Amnon Buxboim

 

 

 

 

 

 

 

 

 

 

 

 

An observer in a busy train station will soon notice that the crowd is denser near ticket counters, kiosks and platforms. No walls separate the different activities, but each nevertheless occupies a distinct compartment within the space. Weizmann Institute scientists have now applied a similar principle to genes, packing them together in small areas on a chip. In the process, they are revealing the ways in which such “crowd management” might help to control the activities of genes in cells. Their method could not only provide new tools to realistically manipulate genes in lab experiments; it might prove to be an essential step in the quest to create artificial cells.
 
Gene activity in biological organisms takes place in the cramped quarters of the cell or, if the organism is not a bacterium, in the tight space of the cell nucleus. And recent evidence has suggested that within the jam-packed nucleus some of those activities are segregated into distinct areas. Yet lab experiments with genes typically use strands of DNA and various other molecules floating loosely in solution. Prof. Roy Bar-Ziv of the Institute’s Materials and Interfaces Department in the Faculty of Chemistry wanted to create a system that would combine the convenience of the test tube with the crowded conditions in a cell. Together with then doctoral student Amnon Buxboim, he developed a method for attaching fairly long strands of DNA perpendicularly onto a surface to create a thick, brush-like carpet. With the aid of a photo-lithography technique borrowed from microelectronics, they managed to control the density so precisely that they were able to create highly detailed, nanosized “photographs” printed in DNA.
 
Once they had perfected the technique, Bar-Ziv, Buxboim, research student Dan Bracha and Dr. Shirley Daube, who directs the Institute’s Chemical and Biophysical NanoSciences labs of the Chemical Research Support Department, began to investigate what dense DNA “brushes” might reveal about gene activity at close quarters. Starting out with the simplest scenario, they created DNA brushes containing one type of gene and then observed the first stage of gene activity – the assembly of RNA strands from the genetic code – comparing this with the activities of DNA strands floating in solution.
 
The gene brushes created by the team did, indeed, act as tiny compartments, exhibiting properties that set them apart from their surroundings despite the lack of any physical barrier. The scientists tried out the gene brushes in different conditions – by changing the salt levels in the environment, for instance, or adding substances that affect DNA activity – and found that the crowded genes mimicked the actions of genes in cells much more closely than the free-floating control genes.
 
The team also experimented with variations on the brushes themselves: They increased or decreased the spacing between the genes; they added more or less “junk DNA” (non-coding DNA) between the genes and, finally, they flipped the genes over, so that one time the bit of DNA that gives the “begin copying here” signal was facing the free end and another time sat facing the attached end.
 
All of these changes affected the rate at which the genes worked – even the junk DNA. Greater crowding slowed things down, as did flipping the genes so the “start” codes were lower down. Dense placement appeared both to limit the access of the RNA copying machinery to the genes and to keep that machinery and the resulting RNA within the compartment area for longer. In addition, the researchers found that this arrangement enabled them to control the direction (and thus the protein output) in which the genetic information was copied – something that’s not possible in solution but happens as a matter of course in cells.
 
Although living cells don’t have their genes neatly lined up in rows, the scientists believe the dense brushes can provide some interesting insights into how the physical arrangement of DNA affects its workings. They think that junk DNA, for instance, which makes up around 90% of the DNA in our cells, might function as a sort of packing material that helps to maintain a certain level of density. The findings hint that some sort of open-plan compartmentalization may be a common, space-efficient strategy for keeping order in the busy cell. Such compartments, says Bar-Ziv, could even have preceded enclosed cells: “Rather than originating inside a membrane, the first cells might have started from a membrane-free compartment of complex molecules that clumped together, and this may have remained a basic organizing principle.”
 
Bar-Ziv: “The multidisciplinary skills of the team – Amnon and I come from a physics background, Shirley is a biochemist and Dan’s background is bioengineering – are what made this research possible.” He and his research team plan to keep refining their gene brushes, experimenting with more complex situations, including arrangements of multiple genes and gene recombination. “Eventually, we would like to build an artificial chromosome, and even an artificial cell. Unlike today’s gene chips, which are passive identification tools, the genes in our brushes are active, and we want to learn how to direct their actions.”

 
Prof. Roy Bar-Ziv’s research is supported by the Phyllis and Joseph Gurwin Fund for Scientific Advancement; the Helen and Martin Kimmel Center for Nanoscale Science; and the Carolito Stiftung.
 
(l-r) Dan Bracha, Prof. Roy Bar-Ziv, Dr. Shirley Daube and Dr. Amnon Buxboim. Close quarters Wall-free compartments might keep gene activities in order
Chemistry
English
  • DNA
  • Gene
  • Materials and Interfaces
  • Roy Bar-Ziv

Order Through Chaos

English

intriguing nanotube shapes

 
Thanks to the rising trend toward miniaturization, carbon nanotubes – being about 100,000 times thinner than a human hair and possessing unique mechanical, electronic, optical and thermal properties – have become the ideal candidates for use as building blocks for nanoelectronic and mechanical devices. But their minuteness and a tendency to clump together make it difficult for scientists to manipulate nanotubes for integration into such nanodevices.

Dr. Ernesto Joselevich, together with Ph.D. student Ariel Ismach and former M.Sc. student Noam Geblinger of the Materials and Interfaces Department, are developing techniques to coax the nanotubes to self-assemble into disentangled, orderly structures – essentially making the nanotubes do the hard work for them.

By applying the universal principle of “order through chaos,” the team has produced nanotubes that are strikingly more ordered and complex than those ever observed before. Their intriguing new type of nanotube structures, which they have termed “serpentines” due to their self-assembly into snake-like and looped configurations, has recently been featured on the cover of the journal Nature Nanotechnology.

Serpentines are common structures on the macroscale in such functional systems as antennae, radiators and cooling elements. Analogously, nanotube serpentines could find a wide range of nanodevice applications – for example, in cooling elements for electronic circuits, optoelectronic devices and power-generating single-molecule dynamos. “But the feature I find most intriguing about these serpentines,” says Joselevich, “is their beauty.”  

An animated movie explaining nanotube serpentine formation can be seen at :

 Dr. Ernesto Joselevich’s research is supported by the Helen and Martin Kimmel Center for Nanoscale Science; the Gerhardt Schmidt Minerva Center on Supramolecular Architectures; and the Wolfson Family Charitable Trust. Dr. Joselevich is the incumbent of the Dr. Victor L. Ehrlich Career Development Chair.
 
A nanotube serpentine observed by scanning electron microscopy
Chemistry
English
  • Carbon nanotubes
  • Ernesto Joselevich
  • Materials and Interfaces

Symphony in a Second

English
Prof. Lucio Frydman. fastest NMR
 
 
 
Each material plays its own unique "melody," if only we know how to listen in. When molecules are exposed to a magnetic field, the atoms in it begin to spin, and each spinning atomic nucleus emits waves of electromagnetic radiation in a distinctive pattern. For scientists, each electromagnetic symphony contains the secrets for understanding the chemical and physical properties of the material.
 
To decipher those secrets, scientists employ various methods of recording the traces of the radiation emitted by the nuclei. These include nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). Both of these techniques are non-invasive and energy-efficient, and they are favored for such chemical and physical studies as revealing the atomic structure of drug molecules. Because they don't harm living tissue, they can be used for such biological research as tracking the development of a fetal brain. There is, however, a drawback to these techniques: The low amount of energy emitted by most materials makes it hard to detect with precision; the measurements therefore may lack sensitivity compared with some other methods of analysis.
 
Prof. Lucio Frydman of the Weizmann Institute's Chemical Physics Department and Damir Blazina of Oxford Instruments Molecular Biotools Ltd. overcame this limitation by creating a method for obtaining multidimensional images of different materials at unprecedented levels of sensitivity and speed. The details were published in Nature Physics.
 
The method is based on a technique for amplifying the signal from the atomic nuclei. Dynamic nuclear hyperpolarization, as it's called, is a method for aligning the spin of the nuclei. It works something like exposing a bunch of compasses to a large magnet so that all of their needles point in the same direction. When the nuclei's spins are all attuned, their signal rises to a chorus – making it much easier to detect. Hyperpolarization can align about 20% (one in five) of the nuclear spins in a sample. That's an enormous improvement over existing NMR methods, which are capable of lining up a mere one in 50,000, at best. Hyperpolarization, however, has its own drawback: It's an exceptional state that takes a relatively long time to prepare, and it can only be sustained for a short time, basically permitting scientists to obtain no more than one "super scan" of a material.
 
To make the most of that one-time scan, the scientists combined it with a technique Frydman and his team had previously developed to speed up the process of obtaining multidimensional NMR images. Standard NMR techniques often take hours or days to complete, as successive images are recorded one at a time and then compiled. Frydman's method, called ultrafast NMR, "carves" the sample into thin slices and images them all at once.
 
The combined method should prove to be more sensitive than existing NMR techniques by several orders of magnitude, as well as many times faster. But the real excitement in this innovation, says Frydman, is in the new research possibilities that the method may open up: Many chemical and physical phenomena have so far remained beyond the reach of scientists because they take place too rapidly for existing techniques to measure them. "An ultrafast, highly sensitive technique will doubtlessly make interesting new scientific discoveries possible."
 
Prof. Lucio Frydman. Super-sensitive, ultra-fast
Chemistry
English
  • Chemical Physics
  • Lucio Frydman
  • NMR

Smart Ceramics

English
Prof. Igor Lubomirsky. Adjustable material
 
 
 
 
Smart. Flexible. Copes well with stress. Adjusts quickly to new situations. Sound like a candidate applying for a demanding job? Indeed, except that the "candidate" is an unusual ceramic material capable of adapting to changing and stressful conditions.
 
Like discovering that your next-door neighbor, whom you supposed for years to possess average intelligence, is in fact a genius, a common ceramic material has been found to possess a hidden talent. Scientists at the Weizmann Institute, led by Prof. Igor Lubomirsky of the Materials and Interfaces Department, have discovered that under certain conditions, cerium gadolinium oxide behaves more like rubber than a regular ceramic: Much like a squeezed rubber ball, it adjusts to an externally imposed shape but regains its original shape once released from its constraints. The scientists designed a drum-like structure in which a "drumhead" – a thin film of the ceramic less than 1 micron thick – was tethered to a frame. At room temperature, the film was flat and perfectly fit the frame. When heated gradually, rather than buckling, as an average ceramic would, the "smart" ceramic remained steadfastly flat, even when the temperature was raised to 180°C. And when cooled back slowly, it still retained its original shape and showed no signs of cracks.
 
Instant heating and cooling, however, produced buckling and cracking – just like your run-of-the-mill ceramic. Apparently, the time factor played a crucial role. A rubber ball offers a perfect analogy for the two scenarios: When squeezed relatively slowly, it deforms to adjust to the stress and regains its shape when released. In contrast, upon fast impact – as, for example, when thrown against the floor – the ball bounces without altering its shape.
 
What mechanism makes the ceramic so "smart" and adjustable? The secret lies in the two types of so-called point defects the material contains: atoms of gadolinium that had been introduced into cerium oxide and vacant spots left where oxygen had been pushed out by the gadolinium atoms. These latter "vacancies" allowed the defects to move about in the material – something like movie viewers changing seats in a half-empty cinema. As the ceramic cools, the loss of energy drives the two types of defects closer together into a more "economical" state, and the material's volume shrinks gradually, without cracking.
 
Additional materials might possess a similar stress-coping ability, says Lubomirsky. Together with Anna Kossoy of the Materials and Interfaces Department, Dr. Yishay Feldman and Dr. Ellen Wachtel of Weizmann's Chemical Research Support, as well as Prof. Joachim Maier of the Max Planck Institute for Solid State Research in Stuttgart, Germany, he developed the theoretical framework for the ceramic's rubber-like behavior and supported it experimentally.
 
A material's ability to retain its original shape at all temperatures could be extremely useful, for example, in devices that undergo repetitive warming and cooling, such as fuel cells that convert chemical energy directly to electricity. The clever ceramic could also help in the manufacture of sophisticated microscopic devices that need to perform highly reproducible measurements, such as micro-sensors or miniature pumps.
 
Prof. Igor Lubomirsky's research is supported by Mr. and Mrs. Yossie Hollander, Israel.
 
Chemistry
English
  • Igor Lubomirsky
  • Materials and Interfaces
  • Smart material

Two-Way Stretch

English
cells react to stretch
 
 
Biomaterials combine living cells with an artificial gel-like medium. Such materials are being explored for use, among other things, in synthetic replacement tissues and organs. But some traits of biomaterials have been puzzling scientists for many years. Prof. Samuel Safran and Dr. Rumi De of the Institute's Materials and Interfaces Department, together with Dr. Assaf Zemel, formerly a postdoc in Safran's group and now at the University of California, Davis, have proposed a solution to one of those puzzles.
 
When the gel is stretched, say by pulling on either end, the cells respond by reducing the stress on their connections to the medium. But the timing of that stretch affects the way the cells orient themselves: When the gel is stretched slowly, the cells align parallel to the direction of the externally imposed stretch, but if it's stretched quickly, that alignment is nearly perpendicular.
 
What causes the difference? The scientists propose that in both cases a cell adjusts itself to maintain an optimal amount of stretch in the medium. In slow stretching, the cells, which tend to contract, have time to align themselves and take the steps needed to counteract the external stretch, as though steeling themselves for a tug-of-war. When the stretching is quick, however, they don't have enough time to develop this careful balancing of forces. Turning about 90 degrees effectively takes them out of the game of tug-of-war, since the medium cannot pull on the cells in this position.
 
This research, which appeared recently in Nature Physics, may aid in designing and processing biomaterials with specific properties, and may have implications for research in wound healing and muscle growth, as well as elucidating the behavior of cancer cells and more.
 
Prof. Samuel Safran's research is supported by the estate of David Turner. Prof. Safran is the incumbent of the Fern and Manfred Steinfeld Professorial Chair.
 
Top: The cell pulls to maintain a fixed stretch in the gel. Middle: If the gel is externally stretched, the cell can reduce the force it exerts. Bottom: If the gel is alternately stretched and relaxed, the frustrated cell cannot "decide" how much force to exert. This results in the cell orienting perpendicular to the stretch direction
Chemistry
English
  • Biophysics
  • Cell
  • Materials and Interfaces
  • Samuel Safran

A Story with a Twist

English

Joselevich and research team. Twisting tubes

 

 

Doing the twist may have faded away along with other fads of the 1960s, but the twist promises to make a comeback in the world of tiny molecular structures. As described recently in the first issue of the new journal Nature Nanotechnology, Dr. Ernesto Joselevich and his team in the Weizmann Institute’s Materials and Interfaces Department have managed to manipulate the properties of carbon nanotubes by twisting.
 
Carbon nanotubes are strong, flexible molecular wires that are harder than diamond and conduct electricity better than copper. Thanks to their unusual characteristics, they are ideal candidates to be components of tiny electronic and mechanical devices. Since their discovery more than a decade ago, scientists have been exerting great efforts toward understanding their properties, mainly in order to use them for building all sorts of specialized, highly structured instruments.
 
One of the traits of carbon nanotubes is their “split personality” when it comes to electrical conductivity. They can behave either like a metal, which is characterized by excellent conductivity, or like a semiconductor – such as the silicon in electronic chips – which, under different conditions, can be  either a conductor or an insulator. Whether carbon nanotubes behave like conductors or semiconductors depends on their diameter and the way they form a spiral – a property known as chirality – the direction in which the carbon surface “rolls up” to create the tube. The chirality phenomenon generates molecules that have identical chemical compositions but differ from one another in their spatial structure, so that one molecule is a mirror image of the other – just as the right and left hand both resemble and differ from each other (hence the name: chiros means “hand” in Greek). Therefore, despite their identical chemical makeup, these molecules cannot overlap with one another, just as the right hand cannot overlap with the left.
 
Chirality had always been considered a basic property that could not be changed, but Joselevich asked himself whether it could be altered by twisting the nanotube – thereby turning a conductor into a semiconductor, or vice versa. The team, in addition to Joselevich, included research students Tzahi Cohen-Karni, Lior Segev and Onit Srur-Lavi, as well as Dr. Sidney Cohen of Chemical Research Support.
 
To twist nanotubes, the scientists created a unique device: a nanotube connected to two electrical contacts, with a pedal in the middle. Pressing the pedal with the tiny tip of an atomic force microscope caused the tube to twist, and the change in conductivity produced by the twisting was measured by the electrical contacts.
They found that gradual twisting of the nanotube led to periodic increases and decreases in conductivity. A check of the electron distribution in the nanotube during the twisting showed that what was taking place was indeed a periodic transition from a conductor to a semiconductor. The scientists then proposed a mathematical model that makes it possible to calculate and predict the oscillations in conductivity as a function of twisting.
 
These findings might in the future help in the design and production of tiny smart springs capable of measuring their own twisting by monitoring the changes in the electric current passing through them. Such springs could form the basis for a variety of nano-electromechanical devices, such as chemical or biological sensors, or gyroscopes for guiding miniature aircraft.  
 
Dr. Ernesto Joselevich’s research is supported by the Helen and Martin Kimmel Center for Nanoscale Science; and the Asher and Jeannette Alhadeff Research Award. Dr. Joselevich is the incumbent of the Dr. Victor L. Ehrlich Career Development Chair.
Chemistry
English
  • Carbon nanotubes
  • Ernesto Joselevich
  • Materials and Interfaces
  • Nanoelectronics

Breaking News

English
 
Could engineers have known ahead of time exactly how much pressure the levees protecting New Orleans would be able to withstand before giving way? Is it possible to predict when and under what conditions material wear and tear will become critical, causing planes to crash or bridges to collapse? Weizmann Institute scientists have taken a new and original approach to the study of how materials fracture and crack.
 
Physicists attempting to find a formula for the dynamics of cracking have faced a serious obstacle. The difficulty lies in pinning down, objectively, the fundamental directionality of the cracking process: From any given angle of observation or starting point of measurement, the crack will look different and yield different results. Until now, no one has successfully managed to come up with a method for analyzing the progression of a forming crack.
 
To address this problem, Prof. Itamar Procaccia and research students Eran Bouchbinder and Shani Sela of the Chemical Physics Department first divided up the cracks' ridged surfaces into mathematically determined sectors. For each sector they were able to measure and evaluate different aspects of the crack's formation and assign it simple directional properties. After some complex data analysis of the combined information from all the sectors, the team found their method allowed them to gain a deeper understanding of the process of cracking, no matter which direction the measurements started from. They then successfully applied the method to a variety of materials - plastic, glass and metal.
 
The team's method will give engineers and materials scientists new tools to understand how basic materials act under different stresses, to predict how and when microscopic or internal, unseen fractures might turn life-threatening, or to improve these materials to make them more resistant to the formation or spread of cracks.
 
Prof. Itamar Procaccia's research is supported by the Minerva Center for Nonlinear Physics of Complex Systems; and the Naftali and Anna Backenroth-Bronicki Fund for Research in Chaos and Complexity. Prof. Procaccia is the incumbent of the Barbara and Morris L. Levinson Professorial Chair in Chemical Physics.
Chemistry
English
  • Chemical Physics
  • Cracking
  • Itamar Procaccia
  • Materials science

Making a Switch

English

Prof. David Cahen and Adi Salomon. Negative resistance

 

From simple light fixtures to the latest in cell phone technology or medical equipment, electrical switches are wired into the circuit. Whether made of metal contacts or engraved in silicon, their basic function is to stop and start the flow of electrons. But as scientists and inventors attempt to shrink new devices into the realm of nano-technology, the limitations of switches made of these materials are becoming apparent. What will replace them?
 
Prof. David Cahen of the Materials and Interfaces Department and his Ph.D. student and Clore Fellow Adi Salomon think that organic (that is, carbon-based) molecules may hold the answer. They have demonstrated a new kind of electrical switch created from organic molecules that could be used in future nanoscale electronic components.
 
Their approach involved rethinking a phenomenon that drives many of today’s high-speed semiconductors. Negative differential resistance (NDR) – for which its discoverer, Leo Esaki, won the 1973 Nobel Prize – works contrary to the standard laws of electricity. Normally, an increase in voltage translates into an increase in current. In NDR, as the voltage steadily increases, the current peaks and then drops off, essentially constituting a switch with no moving parts. Until now, how-ever, attempts to recreate NDR at the molecular scale were achieved only sporadically, mostly at extremely low temperatures or as an unstable, hard-to-reproduce phenomenon. “In hindsight, most efforts were probably aimed too squarely at trying to force molecules to behave like conventional materials, and too little at exploring the chemistry of the molecules,” say the researchers.
 
Some clues to practical nanoscale NDR emerged from earlier work at the Weizmann Institute conducted by Dr. Yoram Selzer (now at Tel Aviv University) and Salomon, under Cahen’s guidance, on connecting organic molecules to metal wires. They found that molecules and metals, like people, need chemistry between them for the juice to really flow. For a given voltage, if the molecules are held to the wire by chemical bonds (in which the two are linked by shared electrons), the current flowing through them will be many times higher than if they are only touching - a mere physical bond.
 
Using this insight, the team de-signed organic molecules that pass electricity through chemical bonds at a lower voltage, but through physical bonds at a higher voltage. As the voltage approaches the higher level, sulfur atoms at one end of the molecule loosen their chemical bonds to the wire and, as the switchover occurs, the current drops off.
 
But the scientists still didn’t have a functional switch. Once the chemical bond to the wire was broken, the molecules tended to move apart, preventing them from switching back to the chemically bonded state. Prof. Abraham Shanzer of the Organic Chemistry Department, who worked with the team on the original molecular design, helped them create long add-on tails to hold the molecules in place. With this modification, the NDR then became stable, rever-sible and reproducible at room temperature.
 
Cahen and Salomon believe their work supports the notion that the future of miniaturized electronics may lie in methods that combine chemistry with nanoengineering. “We don’t take human-sized objects and try to scale them down; rather, from a different universe of possibilities, we create new things specifically designed to function in the nanoworld.”
 
Prof. David Cahen’s research is supported by Minerva Stiftung Gesellschaft fuer die Forschung m.b.H; the Wolfson Advanced Research Center; the Philip M. Klutznick Fund for Research; and the Delores and the Eugene M. Zemsky Weizmann-Johns Hopkins Research Program. Prof. Cahen is the incumbent of the Rowland Schaefer Professorial Chair in Energy Research.
Prof. David Cahen and Ph.D. student Adi Salomon. Switching bonds
Chemistry
English
  • Abraham Shanzer
  • David Cahen
  • Materials and Interfaces
  • Nanoelectronics
  • Organic Chemistry

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