A mathematical model of heat flow through miniature wires could help develop thermoelectric devices that efficiently convert heat — even their own waste heat — into electricity. Developed at A*STAR, the model describes the movement of vibrations called phonons, which are responsible for carrying heat in insulating materials. Phonons typically move in straight lines in nanowires — threads barely a few atoms wide. Previous calculations suggested that if parts of a nanowire contained random arrangements of two different types of atoms, phonons would be stopped in their tracks. In actual alloy nanowires, though, atoms of the same element might cluster together to form short sections composed of the same elements. Now, Zhun-Yong Ong and Gang Zhang of the A*STAR Institute of High Performance Computing in Singapore have calculated the effects of such short-range order on the behavior of phonons ("Enhancement and reduction of one-dimensional heat conduction with correlated mass disorder"). Their results suggest that heat conduction in a nanowire does not just depend on the relative concentrations of the alloy atoms and the difference in their masses; it also depends on how the atoms are distributed. Their model simulated an 88-micrometer-long nanowire containing 160,000 atoms of two different elements. They found that when the nanowire was more ordered — containing clusters of the same elements — low-frequency phonons struggled to move. In contrast, high-frequency phonons could travel much further than the average length of the ordered regions in the alloy. “The high-frequency phonons were more mobile than we imagined,” says Ong. The researchers used their model to study the thermal resistance of a nanowire containing an equal mix of silicon and germanium atoms. Short-range ordering of the atoms allowed high-frequency phonons to travel freely through the wire, giving it a relatively low thermal resistance. In contrast, a random distribution of alloy atoms resulted in a higher resistance — over triple that of the ordered case for a 2.5-micrometer-long wire. “If this disorder can be realized in real composite materials then we could tailor the thermal conductivity of the system,” says Ong. Understanding the relative contribution of low- and high-frequency phonons to heat conduction could also help researchers tune the thermal properties of nanowires in the laboratory. “For instance, the surface roughening of nanowires is known to reduce the thermal conductivity contribution of high-frequency phonons,” says Ong. The researchers hope their model will help scientists design composite materials with low thermal conductivity. One attractive application is thermoelectric devices, explains Ong. “As these devices rely on a thermal differential, a low thermal conductivity is desirable for optimal performance.”
Thermal properties of nanowires - Follow the heat
read more "Thermal properties of nanowires - Follow the heat"
Promising thermo-magnetic data-storage technology with nanoislands
The mechanics and dynamics of heat-assisted magnetic recording (HAMR) are now better understood thanks to work by A*STAR and the National University of Singapore ("A study on dynamic heat assisted magnetization reversal mechanisms under insufficient reversal field conditions"). The experimental study will help scientists aiming to break the areal density barrier of current magnetic hard disk technology. 
A scanning electron micrograph of an array of 50-nm-diameter magnetic islands used to simulate a heat-assisted magnetic recording (HAMR) data storage medium. Today’s hard disk drives record and store data in minute magnetic domains on a spinning magnetic platter. As the magnetic domains become smaller, the thermal noise rises rapidly, making it increasingly difficult to record data reliably. HAMR is a promising future data storage scheme that uses a more stable magnetic medium, in combination with local heating, to achieve more reliable magnetization switching. “HAMR can be implemented with magnetic grains as small as 3 nanometers and higher magnetic anisotropy, which will make it possible to store magnetic information at recording densities beyond a terabyte per square inch,” says research leader Yunjie Chen from A*STAR’s Data Storage Institute. Theoretical simulations have demonstrated the potential of HAMR but also the possibility of density-limiting electrical noise and problematic non-reversal of magnetic domains in the recording process. Chen’s team designed some experiments to probe the dynamics of HAMR. “For practical application of HAMR, it is important to understand the thermo-magnetic reversal process and recording performance, including magnetization dynamics and effects that limit areal density,” says Chen. The experimental HAMR recording system devised by Chen’s team consisted of an array of magnetic islands of multilayer cobalt and palladium. Preparing this device involved sputtering multiple atomic layers of different combinations of elements, then patterning the device using electron-beam lithography to produce magnetic islands of about 50 nanometers in size (see image). The team then used far-field laser heating with a thermal spot size of about 1.5 micrometers in combination with a magnetic field to simulate the magnetization switching of the HAMR process. They observed the resultant magnetization patterns using a high-resolution magnetic force microscope. Chen’s team showed that when the material was laser heated to near its Curie temperature — the temperature at which the material’s permanent magnetism is overcome by an external magnetic field — the strength of the magnetic field required to induce complete magnetic switching was about 13 per cent of the intrinsic magnetic ‘coercivity’ of the islands. Surprisingly, however, the team also discovered that, due to thermal fluctuations, the optimal temperature for recording is slightly lower than the Curie temperature. Chen explains that the results provide critical operational parameters for the practical implementation of HAMR data storage technology.
A scanning electron micrograph of an array of 50-nm-diameter magnetic islands used to simulate a heat-assisted magnetic recording (HAMR) data storage medium. Today’s hard disk drives record and store data in minute magnetic domains on a spinning magnetic platter. As the magnetic domains become smaller, the thermal noise rises rapidly, making it increasingly difficult to record data reliably. HAMR is a promising future data storage scheme that uses a more stable magnetic medium, in combination with local heating, to achieve more reliable magnetization switching. “HAMR can be implemented with magnetic grains as small as 3 nanometers and higher magnetic anisotropy, which will make it possible to store magnetic information at recording densities beyond a terabyte per square inch,” says research leader Yunjie Chen from A*STAR’s Data Storage Institute. Theoretical simulations have demonstrated the potential of HAMR but also the possibility of density-limiting electrical noise and problematic non-reversal of magnetic domains in the recording process. Chen’s team designed some experiments to probe the dynamics of HAMR. “For practical application of HAMR, it is important to understand the thermo-magnetic reversal process and recording performance, including magnetization dynamics and effects that limit areal density,” says Chen. The experimental HAMR recording system devised by Chen’s team consisted of an array of magnetic islands of multilayer cobalt and palladium. Preparing this device involved sputtering multiple atomic layers of different combinations of elements, then patterning the device using electron-beam lithography to produce magnetic islands of about 50 nanometers in size (see image). The team then used far-field laser heating with a thermal spot size of about 1.5 micrometers in combination with a magnetic field to simulate the magnetization switching of the HAMR process. They observed the resultant magnetization patterns using a high-resolution magnetic force microscope. Chen’s team showed that when the material was laser heated to near its Curie temperature — the temperature at which the material’s permanent magnetism is overcome by an external magnetic field — the strength of the magnetic field required to induce complete magnetic switching was about 13 per cent of the intrinsic magnetic ‘coercivity’ of the islands. Surprisingly, however, the team also discovered that, due to thermal fluctuations, the optimal temperature for recording is slightly lower than the Curie temperature. Chen explains that the results provide critical operational parameters for the practical implementation of HAMR data storage technology.Smart micelles for marine environments
‘Smart’ materials that alter their structure in response to specific, controllable stimuli have applications in various fields, from biomedical science to the oil industry. Now, A*STAR researchers have created a self-assembling polymeric material that changes its structure when moved from water to an electrolyte solution, such as salt water ("Dual hydrophilic and salt responsive schizophrenic block copolymers – synthesis and study of self-assembly behavior"). The material could help improve coatings used to protect surfaces from the build-up of biological contaminants, particularly surfaces under the sea. Materials composed of segments of two different monomers, each with different characteristics, are known as block copolymers. Vivek Vasantha at A*STAR Institute of Chemical and Engineering Sciences — together with scientists from across Singapore under the Innovative Marine Antifouling Solutions (IMAS) program — developed a new block copolymer that can self-assemble into spherical micelle structures in which one monomer forms the core and the other forms the outer shell. The monomers are the hydrophilic poly(ethylene glycol), or PEG, which mixes well with water, and the halophilic polysulfabetaine (PSB), which has a preference for salt solution. “We created salt-responsive block copolymers that self-assemble in water to form either ‘conventional’ or ‘inverse’ micelles,” states Vasantha. The conventional micelles form in deionized water and have a core of halophilic PSB with a hydrophilic PEG shell. However, the team showed that the micelles re-assemble themselves when immersed in salt solution; PEG formed the core, and PSB formed the shell to create an ‘inverse’ micelle. “The material is easily controlled by salt alone, rather than by a combination of several stimuli like pH, temperature or light, which some other smart materials require,” explains Vasantha. “It appears to be highly tolerant of fluctuations in pH and temperature too, which means it is potentially useful for dynamic marine environments.” The researchers mixed the block copolymers with primer to create a non-toxic coating to replace traditional antifouling paint. Current coatings to prevent fouling by marine organisms include toxic chemicals, and become ineffective after a short time because the additives in the coatings break down rapidly in sea water. Vasantha’s team applied the new coating to glass slides, which they then immersed in the sea for two weeks. “The antifouling behavior of coatings is normally tested in laboratory experiments,” says Vasantha. “Throughout our unique marine tests, the self-assembling micelles kept the coated surfaces intact and the coating displayed reasonable antifouling behavior by preventing settling by organisms such as barnacles.” The researchers anticipate that their smart material will have other potential applications in enhanced oil recovery and biomedical science.
Researchers achieve an unprecedented level of control over defects in liquid crystals (w/video)
Sitting with a joystick in the comfort of their chairs, scientists can play "rodeo" on a screen magnifying what is happening under their microscope. They rely on optical tweezers to manipulate an intangible ring created out of liquid crystal defects capable of attaching a microsphere to a long thin fibre. Maryam Nikkhou and colleagues from the Josef Stefan Institute, in Ljubljana, Slovenia, recently published in ("Topological binding and elastic interactions of microspheres and fibres in a nematic liquid crystal") the results of work performed under the supervision of Igor Musevic. They believe that their findings could ultimately open the door to controlling the flow of light using light of a specific frequency in the Gigahertz range in liquid crystal photonic microdevices. Watch the video from the article:
Liquid crystals are familiar to us from their application in LCD screens. What makes them so interesting is that they are rich in defects. Thanks to advances in manipulation tools such as optical tweezers, the authors were able to create an arbitrary number of defect pairs on a long thin fibre plunged into a nematic liquid crystal - an ordered fluid with long organic molecules all pointing in the same direction like sardines in a tin. Nikkhou and colleagues use very strong laser tweezers to locally melt the liquid crystal into a phase where the molecules are oriented in all directions, encircling one part of the fibre. They subsequently switch-off the laser light, resulting in the locally molten liquid crystal rapidly cooling down. Its molecules then revert back from being oriented in all directions to being parallel to each other, creating several pairs of defects - akin to localised disruptions of the crystal's ordering field - forming a ring. Because liquid crystals are made of soft materials, their defects can be moved and modified easily. The defect ring is used as a non-material "rope" to entangle and strongly bind a microsphere and long fibre of micrometric diameter. Because these defects are typically preserved when subjected to stretching and bending, they offer an ideal physical model of an abstract field of mathematics called topology.
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Liquid crystals are familiar to us from their application in LCD screens. What makes them so interesting is that they are rich in defects. Thanks to advances in manipulation tools such as optical tweezers, the authors were able to create an arbitrary number of defect pairs on a long thin fibre plunged into a nematic liquid crystal - an ordered fluid with long organic molecules all pointing in the same direction like sardines in a tin. Nikkhou and colleagues use very strong laser tweezers to locally melt the liquid crystal into a phase where the molecules are oriented in all directions, encircling one part of the fibre. They subsequently switch-off the laser light, resulting in the locally molten liquid crystal rapidly cooling down. Its molecules then revert back from being oriented in all directions to being parallel to each other, creating several pairs of defects - akin to localised disruptions of the crystal's ordering field - forming a ring. Because liquid crystals are made of soft materials, their defects can be moved and modified easily. The defect ring is used as a non-material "rope" to entangle and strongly bind a microsphere and long fibre of micrometric diameter. Because these defects are typically preserved when subjected to stretching and bending, they offer an ideal physical model of an abstract field of mathematics called topology.
Skin tough
When weighing the pluses and minuses of your skin add this to the plus column: Your skin - like that of all vertebrates - is remarkably resistant to tearing. Now, a collaboration of researchers at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) San Diego has shown why. Making good use of the X-ray beams at Berkeley Lab's Advanced Light Source (ALS), the collaboration made the first direct observations of the micro-scale mechanisms behind the ability of skin to resist tearing. They identified four specific mechanisms in collagen, the main structural protein in skin tissue, that act synergistically to diminish the effects of stress. 
(Left) Collagen fibrils in the dermis of the skin are normally curvy and highly disordered, but (right) in response to a tear align themselves with the tension axis (arrow) to resist further damage. (courtesy of Berkeley Lab) "Collagen fibrils and fibers rotate, straighten, stretch and slide to carry load and reduce the stresses at the tip of any tear in the skin," says Robert Ritchie of Berkeley Lab's Materials Sciences Division, co-leader of this study along with UC San Diego's Marc Meyers. "The movement of the collagen acts to effectively diminish stress concentrations associated with any hole, notch or tear." Ritchie and Meyers are the corresponding authors of a paper in ("On the tear resistance of skin") that describes this study. Who among us does not pay close attention to the condition of our skin? While our main concern might be appearance, skin serves a multitude of vital purposes including protection from the environment, temperature regulation and thermal energy collection. The skin also serves as a host for embedded sensors. Skin consists of three layers - the epidermis, dermis and endodermis. Mechanical properties are largely determined in the dermis, which is the thickest layer and is made up primarily of collagen and elastin proteins. Collagen provides for mechanical resistance to extension, while elastin allows for deformation in response to low strains. Studies of the skin's mechanical properties date back to 1831 when a physician investigated stabbing wounds that the victim claimed were self-inflicted. This eventually led to scientists characterizing skin as a nonlinear-elastic material with low strain-rate sensitivity. In recent years research has focused on collagen deformation but with little attention paid to tearing even though skin has superior tear-resistance to other natural materials. "Our study is the first to model and directly observe in real time the micro-scale behavior of the collagen fibrils associated with the skin's remarkable tear resistance," Ritchie says. Ritchie, Meyers and their colleagues performed mechanical and structural characterizations during in situ tension-loading in the dermis. They did this through a combination of images obtained via ultrahigh-resolution scanning and transmission electron microscopy, and small-angle X-ray scattering (SAXS) at ALS beamline 7.3.3. "The method of SAXS that can be done at ALS beamline 7.3.3 is ideal for identifying strains in the collagen fibrils and watching how they move," Ritchie says. The research team began their work by establishing that a tear in the skin does not propagate or induce fracture, as does damage to bone or tooth dentin, which are comprised of mineralized collagen fibrils. What they demonstrated instead is that the tearing or notching of skin triggers structural changes in the collagen fibrils of the dermis layer to reduce the stress concentration. Initially, these collagen fibrils are curvy and highly disordered. However, in response to a tear they rearrange themselves towards the tensile-loading direction, with rotation, straightening, stretching, sliding and delamination prior to fracturing. "The rotation mechanisms recruit collagen fibrils into alignment with the tension axis at which they are maximally strong or can accommodate shape change," says Meyers. "Straightening and stretching allow the uptake of strain without much stress increase, and sliding allows more energy dissipation during inelastic deformation. This reorganization of the fibrils is responsible for blunting the stress at the tips of tears and notches." This new study of the skin is part of an on-going collaboration between Ritchie and Meyers to examine how nature achieves specific properties through architecture and structure. "Natural inspiration is a powerful motivation to develop new synthetic materials with unique properties," Ritchie says. "For example, the mechanistic understanding we've identified in skin could be applied to the improvement of artificial skin, or to the development of thin film polymers for applications such as flexible electronics."
(Left) Collagen fibrils in the dermis of the skin are normally curvy and highly disordered, but (right) in response to a tear align themselves with the tension axis (arrow) to resist further damage. (courtesy of Berkeley Lab) "Collagen fibrils and fibers rotate, straighten, stretch and slide to carry load and reduce the stresses at the tip of any tear in the skin," says Robert Ritchie of Berkeley Lab's Materials Sciences Division, co-leader of this study along with UC San Diego's Marc Meyers. "The movement of the collagen acts to effectively diminish stress concentrations associated with any hole, notch or tear." Ritchie and Meyers are the corresponding authors of a paper in ("On the tear resistance of skin") that describes this study. Who among us does not pay close attention to the condition of our skin? While our main concern might be appearance, skin serves a multitude of vital purposes including protection from the environment, temperature regulation and thermal energy collection. The skin also serves as a host for embedded sensors. Skin consists of three layers - the epidermis, dermis and endodermis. Mechanical properties are largely determined in the dermis, which is the thickest layer and is made up primarily of collagen and elastin proteins. Collagen provides for mechanical resistance to extension, while elastin allows for deformation in response to low strains. Studies of the skin's mechanical properties date back to 1831 when a physician investigated stabbing wounds that the victim claimed were self-inflicted. This eventually led to scientists characterizing skin as a nonlinear-elastic material with low strain-rate sensitivity. In recent years research has focused on collagen deformation but with little attention paid to tearing even though skin has superior tear-resistance to other natural materials. "Our study is the first to model and directly observe in real time the micro-scale behavior of the collagen fibrils associated with the skin's remarkable tear resistance," Ritchie says. Ritchie, Meyers and their colleagues performed mechanical and structural characterizations during in situ tension-loading in the dermis. They did this through a combination of images obtained via ultrahigh-resolution scanning and transmission electron microscopy, and small-angle X-ray scattering (SAXS) at ALS beamline 7.3.3. "The method of SAXS that can be done at ALS beamline 7.3.3 is ideal for identifying strains in the collagen fibrils and watching how they move," Ritchie says. The research team began their work by establishing that a tear in the skin does not propagate or induce fracture, as does damage to bone or tooth dentin, which are comprised of mineralized collagen fibrils. What they demonstrated instead is that the tearing or notching of skin triggers structural changes in the collagen fibrils of the dermis layer to reduce the stress concentration. Initially, these collagen fibrils are curvy and highly disordered. However, in response to a tear they rearrange themselves towards the tensile-loading direction, with rotation, straightening, stretching, sliding and delamination prior to fracturing. "The rotation mechanisms recruit collagen fibrils into alignment with the tension axis at which they are maximally strong or can accommodate shape change," says Meyers. "Straightening and stretching allow the uptake of strain without much stress increase, and sliding allows more energy dissipation during inelastic deformation. This reorganization of the fibrils is responsible for blunting the stress at the tips of tears and notches." This new study of the skin is part of an on-going collaboration between Ritchie and Meyers to examine how nature achieves specific properties through architecture and structure. "Natural inspiration is a powerful motivation to develop new synthetic materials with unique properties," Ritchie says. "For example, the mechanistic understanding we've identified in skin could be applied to the improvement of artificial skin, or to the development of thin film polymers for applications such as flexible electronics."Natural nanocrystals shown to strengthen concrete
Cellulose nanocrystals derived from industrial byproducts have been shown to increase the strength of concrete, representing a potential renewable additive to improve the ubiquitous construction material. s The cellulose nanocrystals (CNCs) could be refined from byproducts generated in the paper, bioenergy, agriculture and pulp industries. They are extracted from structures called cellulose microfibrils, which help to give plants and trees their high strength, lightweight and resilience. Now, researchers at Purdue University have demonstrated that the cellulose nanocrystals can increase the tensile strength of concrete by 30 percent. "This is an abundant, renewable material that can be harvested from low-quality cellulose feedstocks already being produced in various industrial processes," said Pablo Zavattieri, an associate professor in the Lyles School of Civil Engineering. The cellulose nanocrystals might be used to create a new class of biomaterials with wide-ranging applications, such as strengthening construction materials and automotive components. 
This transmission electron microscope image shows cellulose nanocrystals, tiny structures derived from renewable sources that might be used to create a new class of biomaterials with many potential applications. The structures have been shown to increase the strength of concrete. (Image: Purdue Life Sciences Microscopy Center) Research findings were published in February in the journal ("The influence of cellulose nanocrystal additions on the performance of cement paste"). The work was conducted by Jason Weiss, Purdue's Jack and Kay Hockema Professor of Civil Engineering and director of the Pankow Materials Laboratory; Robert J. Moon, a researcher from the U.S. Forest Service's Forest Products Laboratory; Jeffrey Youngblood, an associate professor of materials engineering; doctoral student Yizheng Cao; and Zavattieri. One factor limiting the strength and durability of today's concrete is that not all of the cement particles are hydrated after being mixed, leaving pores and defects that hamper strength and durability. "So, in essence, we are not using 100 percent of the cement," Zavattieri said. However, the researchers have discovered that the cellulose nanocrystals increase the hydration of the concrete mixture, allowing more of it to cure and potentially altering the structure of concrete and strengthening it. As a result, less concrete needs to be used. The cellulose nanocrystals are about 3 to 20 nanometers wide by 50-500 nanometers long - or about 1/1,000th the width of a grain of sand - making them too small to study with light microscopes and difficult to measure with laboratory instruments. They come from a variety of biological sources, primarily trees and plants. The concrete was studied using several analytical and imaging techniques. Because chemical reactions in concrete hardening are exothermic, some of the tests measured the amount of heat released, indicating an increase in hydration of the concrete. The researchers also hypothesized the precise location of the nanocrystals in the cement matrix and learned how they interact with cement particles in both fresh and hardened concrete. The nanocrystals were shown to form little inlets for water to better penetrate the concrete. The research dovetails with the goals of P3Nano, a public-private partnership supporting development and use of wood-based nanomaterial for a wide-range of commercial products. "The idea is to support and help Purdue further advance the CNC-Cement technology for full-scale field trials and the potential for commercialization," Zavattieri said.
This transmission electron microscope image shows cellulose nanocrystals, tiny structures derived from renewable sources that might be used to create a new class of biomaterials with many potential applications. The structures have been shown to increase the strength of concrete. (Image: Purdue Life Sciences Microscopy Center) Research findings were published in February in the journal ("The influence of cellulose nanocrystal additions on the performance of cement paste"). The work was conducted by Jason Weiss, Purdue's Jack and Kay Hockema Professor of Civil Engineering and director of the Pankow Materials Laboratory; Robert J. Moon, a researcher from the U.S. Forest Service's Forest Products Laboratory; Jeffrey Youngblood, an associate professor of materials engineering; doctoral student Yizheng Cao; and Zavattieri. One factor limiting the strength and durability of today's concrete is that not all of the cement particles are hydrated after being mixed, leaving pores and defects that hamper strength and durability. "So, in essence, we are not using 100 percent of the cement," Zavattieri said. However, the researchers have discovered that the cellulose nanocrystals increase the hydration of the concrete mixture, allowing more of it to cure and potentially altering the structure of concrete and strengthening it. As a result, less concrete needs to be used. The cellulose nanocrystals are about 3 to 20 nanometers wide by 50-500 nanometers long - or about 1/1,000th the width of a grain of sand - making them too small to study with light microscopes and difficult to measure with laboratory instruments. They come from a variety of biological sources, primarily trees and plants. The concrete was studied using several analytical and imaging techniques. Because chemical reactions in concrete hardening are exothermic, some of the tests measured the amount of heat released, indicating an increase in hydration of the concrete. The researchers also hypothesized the precise location of the nanocrystals in the cement matrix and learned how they interact with cement particles in both fresh and hardened concrete. The nanocrystals were shown to form little inlets for water to better penetrate the concrete. The research dovetails with the goals of P3Nano, a public-private partnership supporting development and use of wood-based nanomaterial for a wide-range of commercial products. "The idea is to support and help Purdue further advance the CNC-Cement technology for full-scale field trials and the potential for commercialization," Zavattieri said.
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