Tag Archives: #material

Researchers Propose A Method Of Magnetizing A Material Without Applying An External Magnetic Field (Physics)

Magnetizing a material without applying an external magnetic field is proposed by researchers at São Paulo State University (UNESP), Brazil, in an article published in the journal Scientific Reports, where they detail the experimental approach used to achieve this goal.

The study was part of the PhD research pursued by Lucas Squillante under the supervision of Mariano de Souza, a professor at UNESP’s Department of Physics in Rio Claro. Contributions were also made by Isys Mello, another PhD candidate supervised by Souza, and Antonio Seridonio, a professor at UNESP’s Department of Physics and Chemistry in Ilha Solteira. The group was supported by FAPESP

“Very briefly put, magnetization occurs when a salt is compressed adiabatically, without exchanging heat with the external environment,” Souza told Agência FAPESP. “Compression raises the temperature of the salt and at the same time rearranges its particles’ spins. As a result, the total entropy of the system remains constant and the system remains magnetized at the end of the process.”

To help understand the phenomenon, it is worth recalling the basics of spin and entropy.

Spin is a quantum property that makes elementary particles (quarks, electrons, photons, etc.), compound particles (protons, neutrons, mesons, etc.) and even atoms and molecules behave like tiny magnets, pointing north or south – up spin and down spin – when submitted to a magnetic field. 

“Paramagnetic materials like aluminum, which is a metal, are magnetized only when an external magnetic field is applied. Ferromagnetic materials, including iron, may display finite magnetization even in the absence of an applied magnetic field because they have magnetic domains,” Souza explained.

Entropy is basically a measure of accessible configurations or states of the system. The greater the number of accessible states, the greater the entropy. Austrian physicist Ludwig Boltzmann (1844-1906), using a statistical approach, associated the entropy of a system, which is a macroscopic magnitude, with the number of possible microscopic configurations that constitute its macrostate. “In the case of a paramagnetic material, entropy embodies a distribution of probabilities that describes the number of up spins or down spins in the particles it contains,” Souza said. 

In the recently published study, a paramagnetic salt was compressed in a single direction. “Application of uniaxial stress reduces the volume of the salt. Because the process is conducted without any exchange of heat with the environment, compression produces an adiabatic rise in the temperature of the material. A rise in temperature means a rise in entropy. To keep total entropy in the system constant, there must be a component of local reduction in entropy that offsets the rise in temperature. As a result, the spins tend to align, leading to magnetization of the system,” Souza said.

The total entropy of the system remains constant, and adiabatic compression results in magnetization. “Experimentally, adiabatic compression is achieved when the sample is compressed for less time than is required for thermal relaxation – the typical time taken by the system to exchange heat with the environment,” Souza said. 

The researchers also propose that the adiabatic rise in temperature could be used to investigate other interacting systems, such as Bose-Einstein condensates in magnetic insulators, and dipolar spin-ice systems.

The article “Elastocaloric-effect-induced adiabatic magnetization in paramagnetic salts due to the mutual interactions” is at: www.nature.com/articles/s41598-021-88778-4.

Featured image: The study shows that the phenomenon can be produced by means of adiabatic compression, without any exchange of heat with the environment. The process aligns the spins of the material’s particles and magnetizes the system (image: Wikimedia Commons)


Provided by FAPESP

Scientists Develop Tougher, Safer Bicycle Helmets Using New Plastic Material (Material Science)

As cities worldwide expand their networks of cycling paths and more cyclists take to the streets, the chances of cycling accidents and potential collisions increase as well, underscoring the need for proper cycling safety in dense urban areas.

According to a World Health Organisation report in 2020, more than 60 per cent of the reported bicycle-related deaths and long-term disabilities are a result of accidents with head injuries.

Researchers from Nanyang Technological University, Singapore (NTU Singapore), in collaboration with French specialty materials leader Arkema, have developed a tougher, safer bicycle helmet using a combination of materials. The new helmet prototype has higher energy absorption, reducing the amount of energy transferred to a cyclist’s head in the event of an accident and lowering the chances of serious injury.

Led by Associate Professor Leong Kah Fai from the School of Mechanical and Aerospace Engineering, the team, comprising research fellow Dr Bhudolia Somen Kumar, research associate Goram Gohel and MSc student Elisetty Shanmuga, created the composite helmet with an outer shell made primarily of a new type of acrylic thermoplastic resin, reinforced with carbon fibre.

The new thermoplastic resin, named Elium®, was developed by Arkema, one of NTU’s industry partners. The NTU team worked with Arkema engineers to develop a moulding process for Elium® to manufacture stronger bicycle helmets.

“Our partnership with Arkema is driven by the desire to develop a new type of helmet that is stronger and safer for cyclists,” said Assoc Prof Leong. “Helmets have been proven time and time again to play a critical role in reducing the severity of injuries and number of fatalities. Our prototype helmet has been subjected to a barrage of internationally benchmarked tests and has demonstrated the ability to provide greater protection for cyclists compared to conventional helmets.”

The findings by the research team were published in the peer-reviewed journal Composites Part B: Engineering in May.

Tougher, stiffer outer shell absorbs more energy

Bicycle helmets are made up of two components. The first is an outer shell, usually made from a mass-produced plastic like polycarbonate. Beneath it is a layer of expanded polystyrene foam – the same material used in product packaging and takeaway boxes.

The outer shell is designed to crack on impact in order to dissipate energy across the entire surface of the helmet. The foam layer then compresses and absorbs the bulk of impact energy so that less energy is transferred to the head.

The team’s composite helmet replaces the conventional polycarbonate outer shell with one using Elium® reinforced with carbon fibre.

This reinforcement makes the outer shell tougher, stiffer, and less brittle than a polycarbonate shell. It also increases the helmet’s contact time, which is the total time of impact in which the helmet experiences impact load.

These properties allow the outer shell to absorb more impact energy over a longer period, while also dissipating it evenly throughout the helmet. This results in less overall force reaching the head, thereby reducing the chances of critical injury.

“When the helmet hits a surface at high speed, we noticed that there is a deformation along with the spread failure of the composite shell, which means the outer shell is taking more load and absorbing more energy,” said Dr Somen. “This is what you really want – the more impact absorbed by the shell, the less of it that reaches the foam, and so there is less overall impact to the head. We found that in existing polycarbonate helmets, about 75 per cent of the energy is absorbed by the foam. This is not ideal as the foam is in direct contact with the human head.”

In contrast, the team’s composite helmet shell absorbed over 50 per cent of impact energy, leaving the foam to absorb much less energy at about 35 per cent.

Safety forged on the anvils of NTU

The researchers tested their helmets by driving them down at high speeds on three different types of anvils – flat, hemispherical (rounded), and curbstone (pyramid-shaped) – to simulate different road conditions.

These are the same tests used for the U.S. Consumer Product Safety Commission standard (CPSC 1203) certification, an internationally recognised safety standard for helmets. The team’s helmet prototype meets all CPSC 1203 guidelines.

The researchers paid particular attention to peak acceleration forces, which is a measure of how much force a helmet takes based on how fast it is moving at the point of impact. A helmet must have a peak acceleration of less than 300G (g-force) to be deemed fit for use under CPSC 1203, with lower g-force values being safer.

On two flat anvil tests, the researchers’ helmets performed on par with a control polycarbonate helmet, producing results of 194.7G and 197.2G to the control’s 195.4G and 198.2G.

However, tests on the hemispherical and curbstone anvils showed substantial improvements of the team’s composite helmet over the polycarbonate one. On two hemispherical anvil tests, the composite helmet recorded 100.9G and 103.1G, while the control helmet had a much higher peak acceleration of 173G and 178.7G.

On a single curbstone anvil test, the researchers’ helmets recorded 111.7G, a notable improvement over the reference helmet that produced a result of 128.7G.

The researchers referred to the most widely used injury metric called the Head Injury Criterion (HIC) to calculate the probability of serious injury and fatality while using the helmet. HIC values are derived from a combination of peak acceleration values and the duration of acceleration.

The team’s analysis of the flat anvil test results and the HIC showed that the composite helmet could potentially reduce critical and fatal injury rates from 28.7 per cent and 6 per cent to 16.7 per cent and 3 per cent respectively, compared to a polycarbonate helmet.

Even though peak acceleration was roughly equal between both types of helmets, the composite helmet’s tougher outer shell led to longer duration of acceleration during impact. This allows the outer shell to absorb more energy, therefore generating a lower HIC which means a lower chance of critical and fatal injuries.

More efficient manufacturing could lead to cheaper, tougher helmets

The prototype helmet is also easier to produce than a conventional helmet. Using Elium® instead of other conventional thermoplastics simplifies the composite helmet manufacturing process.

Elium® is liquid at ambient temperature, allowing it to be moulded at room temperature as opposed to other thermoplastic-based composite shells that require higher temperature processing.

The NTU researchers are working with Arkema to commercialise the helmet’s manufacturing process, which would allow interested manufacturers to produce them. Assoc Prof Leong says that helmets produced through their method would offer the same protection of current top-tier helmets, but potentially at the price of mid-tier helmets ($100-$150).

The researchers are currently working on developing composite helmets made from Elium® and polypropylene fabric, which is another type of thermoplastic. This is to overcome the composite helmet’s one current trade-off which is that they weigh about 20 per cent more than polycarbonate helmets.

Helmets made from Elium® and polypropylene fabric will potentially make them just as light as polycarbonate ones but offer better protection.

The team’s research is supported by Singapore’s Agency for Science, Technology and Research (A*STAR) under the nation’s Research Innovation Enterprise 2020 Plan.

Notes to Editor:

Paper titled “Development and impact characterization of acrylic thermoplastic composite bicycle helmet shell with improved safety and performance”, published in Composites Part B: Engineering, 25 May 2021. DOI: 10.1016/j.compositesb.2021.109008

See NTU’s YouTube video of the research: https://www.youtube.com/watch?v=2hWTgrbDM3w

Featured image: (from L-R) Research associate Goram Gohel, Associate Professor Leong Kah Fai and research fellow Dr Bhudolia Somen Kumar, from NTU’s from the School of Mechanical and Aerospace Engineering, with their composite bicycle helmet prototype. © NTU Singapore


Provided by Nanyang Technological University

What Are The Effects Of Sm Doping On Cu-13.0Al-4.0Ni Alloy? (Material Science)

Zhang and colleagues investigated the effects of doping of rare earth element Sm on the structure, mechanical properties, and Shape Memory Effect (SME) of the Cu-13.0Al-4.0Ni alloy. They showed that it not only reduces the grain size but also it causes a 2H martensite to become a single phase martensite. In addition, it has been shown that, Sm doping greatly improved mechanical properties and the shape memory effect (SME) of the Cu-13.0Al-4.0Ni alloy. Their study recently appeared in the Journal Materials.

Shape memory alloys (SMA’s) are metals, which exhibit two very unique properties due to its reversible thermoelastic matensitic transformation: hyperelasticity, and the shape memory effect. These properties makes them suitable for sensing and drive applications. Some developed SMAs such as Ni-Ti have martensitic transformation temperature less than 100°C, which limits their further application. Thus, several studies are focusing on the application of SMAs at higher temperatures, and in particular, high temperature shape memory alloys (HTSMAs) have the potential to be used as solid-state actuators in high temperature fields of aerospace, nuclear power, fire, oil, and gas exploration.

At present, the Cu-Al-Ni alloy has become a potential HTSMA due to the low cost and outstanding properties of its single crystal. However, the severe brittleness of polycrystalline Cu-Al-Ni alloys limits its practical application, which is related to its large elastic anisotropy and large grain size. In recent years, powder metallurgy, rapid solidification, and alloying methods are all methods used for improving the mechanical properties and mechanical properties of alloys by reducing the grain size. Among them, the alloying method has the characteristic of simple equipment, and so the addition of the fourth element is considered to be an effective method to improve the mechanical properties of Cu-Al-Ni SMAs, with simple operation and convenient production.

Recent studies demonstrated that the addition of rare earth elements can change the microstructure and mechanical properties of Cu-Al-Ni SMAs. In addition, Zhang and colleagues previously conducted research on adding rare earth elements such as Gd and Nd to the Cu-13.0Al-4.0Ni alloy. Their results demonstrated that the addition of rare earth elements greatly improves the mechanical properties of the Cu-13.0Al-4.0Ni alloy. Now, they investigated the effects of doping of rare earth element Sm on the structure, mechanical properties, and SME of the Cu-13.0Al-4.0Ni alloy.

They first showed that the Sm addition reduces the grain size of the Cu-13.0Al-4.0Ni alloy from millimeters to hundreds of microns.

It also causes the 2H martensite in the Cu-13.0Al-4.0Ni alloy to disappear and become a single-phase 18R martensite, which is accompanied by the formation of a Sm-rich second phase (Cu-13.0Al-4.0Ni-xSm) in the process.

Compressive stress–strain curve of Cu-13.0Al-4.0Ni-xSm (x = 0.2, 0.5) alloys © authors

Additionally, they found that with the increase in Sm content to 0.5%, the compressive fracture strength and the compressive fracture strain of the sample also further increased to 1021 MPa and 14.8%, respectively.

“Compared with most non-rare earth elements, the rare earth element Sm can better improve the SME of the Cu-Al-Ni alloys, but its effect is not as good as that of the B element and the rare earth elements Gd and Nd.”

— they said

Finally, they found significant improvement in mechanical properties and the shape memory effect (SME) of the Cu-13.0Al-4.0Ni alloy, due to fine-grain strengthening, which was enhanced by Sm doping. However, the rare earth element Sm cannot be added to the Cu-Al-Ni alloys without limitation, because as the Sm content in the alloy increases, the number of the Sm-rich second phase will also increase.

“The influence of a Sm-rich second phase on the mechanical properties of the Cu-Al-Ni alloys is not clear, and thus an in-depth study is needed in the future, but a Sm-rich second phase can definitely lower the SME of the alloy.”

— they concluded.

Featured image: Metallographic photo of Cu-13.0Al-4.0Ni-xSm (x = 0.2 (a), 0.5 (b)) alloys and SEM micrograph ofbCu-13.0Al-4.0Ni-xSm (x = 0.2 (c), 0.5 (d)) alloys, the illustration in (c,d) is a partial enlarged view of the box part © authors


Reference: Zhang, Q.; Cui, B.; Sun, B.; Zhang, X.; Dong, Z.; Liu, Q.; Cui, T. Effect of Sm Doping on the Microstructure, Mechanical Properties and Shape Memory Effect of Cu-13.0Al-4.0Ni Alloy. Materials 2021, 14, 4007. https://doi.org/10.3390/ma14144007


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The Origin of Bifurcated Current Sheets Explained (Material Science)

POSTECH-PAL research team expects a wide range of applications in space and fusion plasma research with the new finding.

A Korean research team has identified the origin of bifurcated current sheets, considered one of the most unsolved mysteries in the Earth’s magnetosphere and in magnetized plasma physics.

A POSTECH joint research team led by Professor Gunsu S. Yun of the Department of Physics and Division of Advanced Nuclear Engineering and Dr. Young Dae Yoon from the Pohang Accelerator Laboratory has theoretically established the process of collisionless equilibration of disequilibrated plasma current sheets.*1 In addition, by comparing this with particle simulations and satellite data from NASA, the origin of the bifurcated current sheets – which had remained largely unknown – has been revealed.

In the Earth’s magnetosphere, a sheet-shaped plasma is observed that is trapped between two regions of opposing magnetic fields. Because current flows inside it, it is also called a current sheet. According to the conventional theory, the current sheet exists as a single bulk in which the magnetic pressure due to the magnetic field generated by the current and the thermal pressure of the plasma balance one another, thereby forming an equilibrium. However, in 2003, the European Space Agency’s Cluster mission observed a bifurcated current sheet in Earth’s magnetosphere. Since then, similar phenomena have been
observed.

210718_기사내부13

On the other hand, extensive research has been accumulated on the condition in which the magnetic force and thermal pressure are perfectly balanced with each other in the current sheet. But the process through which a disequilibrated current sheet equilibrates remains largely unknown. Since plasma systems generally do not start from an equilibrium state, comprehension of the equilibration process is desired to better understand the current sheet plasma dynamics.

The joint research team thoroughly analyzed the process in which the disequilibrated sheet achieves equilibrium by considering the orbit classes and phase-space distributions of particles that constitute the current sheet and found that the current sheets can naturally bifurcate during the equilibration process. It was then confirmed that these theoretical predictions were consistent with the particle-in-cell simulation results performed by the KAIROS supercomputer*2 at the Korea Institute of Fusion Energy. In addition, the simulation data were compared and verified with NASA’s Magnetospheric Multiscale (MMS)*3 measurements.

This achievement has enhanced the comprehension of magnetized plasma dynamics by incorporating theoretical analyses, supercomputer simulations, and satellite observations. Since the Earth’s magnetospheric plasma has similar characteristics as other magnetized plasmas such as nuclear fusion plasmas in various ways, it is anticipated to contribute to a wide range of fields.

“This study has a significant academic value in that it simultaneously resolved two mysteries: the process through which disequilibrated current sheet equilibrates and the origin of bifurcated current sheets,” explained Professor Gunsu S. Yun of POSTECH who participated as a co-corresponding author in the study. “We are trying to extend the analysis framework for plasmas with strong guide fields and hope to understand similar phenomena that occur in fusion plasmas.”

Supported by the National Research Foundation of Korea, this study was published in Nature Communications on June 18, 2021.
 


1. Plasma current sheet
Plasma, the fourth state of matter in which atomic nuclei and electrons are separated and freely moving, occupies 99.9% of the universe. The material present in the Earth’s magnetosphere is also plasma. The state in which plasma exists in the form of a sheet where the current flows in a direction horizontal to the sheet is called a current sheet. In this current sheet, the magnetic resistance – caused by the magnetic field produced by the current – and the thermal intensity caused by the heat and density of the plasma act in opposite directions. When the magnitude of the force is perfectly equal to each other, it is called the equilibrium state. When it is not perfectly equal, it is called a disequilibrated state.

2. KAIROS at the Korea Fusion Energy Research Institute
KAIROS is a supercomputer built at the Korea Fusion Energy Research Institute in 2020 and has the performance of 1.56 petaflops, which is equivalent to the performance of 3,300 Intel i7-9700K desktop PCs. It is the largest supercomputer designated for a specific research area, and the third largest supercomputer housed by a public institution in Korea after KISTI and the Korea Meteorological Administration. It was built for fusion energy plasma simulations.

3. NASA MMS satellite, USA
The Magnetospheric Multiscale (MMS) satellites are a group of satellites launched by NASA in 2015 to study the Earth’s magnetosphere. Four identical satellites orbit the Earth in a tetrahedral configuration, equipped with equipment to observe a variety of variables such as electromagnetic fields, currents, or particle distribution. Research is still actively conducted on magnetic field recombination, turbulence, and high-energy particles occurring in the Earth’s magnetosphere using data obtained from MMS.

All images credit: POSTECH


Reference: Yoon, Y.D., Yun, G.S., Wendel, D.E. et al. Collisionless relaxation of a disequilibrated current sheet and implications for bifurcated structures. Nat Commun 12, 3774 (2021). https://doi.org/10.1038/s41467-021-24006-x


Provided by POSTECH

New Material Could Mean Lightweight Armor, Protective Coatings (Material Science)

Army-funded research identified a new material that may lead to lightweight armor, protective coatings, blast shields and other impact-resistant structures.

Researchers at the U.S. Army’s Institute for Soldier Nanotechnologies at the Massachusetts Institute of TechnologyCaltech and ETH Zürich found that materials formed from precisely patterned nanoscale trusses are tougher than Kevlar and steel.

In experiments, the ultralight structures, called nanoarchitectured materials, absorbed the impact of microscopic projectiles accelerated to supersonic speeds.

“Increasing protection while simultaneously decreasing the weight that soldiers carry is an overreaching theme in our research,” said Dr. James Burgess, ISN program manager for the U.S. Army Combat Capabilities Development Command, known as DEVCOM, Army Research Laboratory. “This project is a really good example of such efforts where projectile energy absorption is nanostructured mechanism based.”

The research, published in Nature Materials, found that the material prevented the projectiles from tearing through it.

“The same amount of mass of our material would be much more efficient at stopping a projectile than the same amount of mass of Kevlar,” said Dr. Carlos Portela, assistant professor of mechanical engineering at MIT, the study’s lead author.

The researchers calculate that the new material absorbs impacts more efficiently than steel, Kevlar, aluminum and other impact-resistant materials of comparable weight.

“The knowledge from this work…could provide design principles for ultra-lightweight impact resistant materials [for use in] efficient armor materials, protective coatings, and blast-resistant shields desirable in defense and space applications,” said co-author Dr. Julia R. Greer, a professor of materials science, mechanics, and medical engineering at Caltech, whose lab fabricated the material.

Nanoarchitected materials are known to feature impressive properties like exceptional lightness and resilience; however, until now, the potential for additional applications has largely been untested.

“We only know about its response in a slow-deformation regime, whereas a lot of their practical use is hypothesized to be in real-world applications where nothing deforms slowly,” Portela said.

To help fill this vital knowledge gap, the research team set out to study nanoarchitected materials undergoing fast deformation, such as that caused by high-velocity impacts. At Caltech, researchers first fabricated a repeating pattern known as a tetrakaidecahedron—a lattice configuration composed of microscopic struts—using two-photo lithography, a technique that uses a high-powered laser to solidify microscopic structures in photosensitive resin.

To test the tetrakaidecahedron’s resilience to extreme, rapid deformation, the team performed experiments at MIT using the ISN-developed laser-induced particle impact array. This device aims an ultrafast laser through a glass slide.. As the laser passes through the slide, it generates a plasma, an immediate expansion of gas that launches the particles toward the target.

By adjusting the laser’s power to control the speed of the microparticle projectiles, the researchers tested microparticle velocities within the supersonic range.

“Some experiments achieved twice the speed of sound, easily,” Portela said.

Using a high-speed camera, the researchers captured videos of the microparticles impacting the nanoarchitected material. They had fabricated material of two different densities. A comparison of the two materials’ impact response, found the denser one to be more resilient, and microparticles tended to embed in the material rather than tear through it.

To get a closer look, the researchers carefully sliced through the embedded microparticles and nanarchitectured target. They found that the struts below the embedded particle had crumpled and compacted in response to the impact, but the surrounding struts remained intact.

“We show the material can absorb a lot of energy because of this shock compaction mechanism of struts at the nanoscale, versus something that’s fully dense and monolithic, not nanoarchitected,” Portela said.

Going forward, Portela plans to explore various nanostructured configurations other than carbon, and ways to scale up the production of these nanostructures, all with the goal of designing tougher, lighter materials.

“Nanoarchitected materials truly are promising as impact-mitigating materials,” Portela said. “There’s a lot we don’t know about them yet, and we’re starting this path to answering these questions and opening the door to their widespread applications.”

The U.S. Army established the MIT Institute for Nanotechnologies in 2002 as an interdisciplinary research center to dramatically improve the protection, survivability and mission capabilities of the Soldier and of Soldier-supporting platforms and systems.

In addition to Army funding through the institute, the U.S. Office of Naval Research and the Vannevar Bush Faculty Fellowship supported the research.


Provided by US Army

A Transparent Phosphate Crystal Immobilizes Lead in Perovskite Solar Cells (Material Science)

Although a very promising solution for capturing solar energy, perovskite solar cells contain lead, which is toxic to the environment and a serious health hazard. EPFL scientists have now found a very elegant and efficient solution by adding a transparent phosphate salt that doesn’t interfere with light-conversion efficiency while preventing lead from seeping into the soil in cases of solar panel failure.

“The solar energy-to-electricity conversion of perovskite solar cells is unbelievably high, around 25%, which is now approaching the performance of the best silicon solar cells,” says Professor László Forró at EPFL’s School of Basic Sciences. “But their central element is lead, which is a poison; if the solar panel fails, it can wash out into the soil, get into the food chain, and cause serious diseases.”

The problem is that in most of the halide perovskites lead can dissolve in water. This water solubility and solubility in other solvents is actually a great advantage, as it makes building perovskite solar panels simpler and inexpensive – another perk along with their performance. But the water solubility of lead can become a real environmental and health hazard when the panel breaks or gets wet, e.g. when it rains.

So the lead must be captured before it gets to the soil, and it must be possible to recycle it. This issue has drawn much and intensive research because it is the main obstacle for regulatory authorities approving the production of perovskite solar cells on a large, commercial scale. However, attempts to synthesize non-water-soluble and lead-free perovskites have yielded poor performance.

Now, Forró’s group has come up with an elegant and efficient solution, which involves using a transparent phosphate salt, which does not block solar light, so it doesn’t affect performance. And if the solar panel fails, the phosphate salt immediately reacts with lead to produce a water-insoluble compound that cannot leach out to the soil, and which can be recycled. The work is published in ACS Applied Materials & Interfaces.

“A few years ago, we discovered that cheap and transparent phosphate salt crystals, like those in soil fertilizers, can be incorporated into various parts of the sandwich-like lead halide perovskite devices, like photodetectors, LEDs or solar cells,” says Endre Horváth, the study’s first author. “These salts instantaneously react with lead ions in the presence of water, and precipitate them into extremely non-water-soluble lead phosphates.”

“The ‘fail-safe’ chemistry keeps lead ions from leaching out and can render perovskite devices safer to use in the environment or close to humans,” says Márton Kollár, the chemist behind the growth of perovskite crystals.

“We show that this approach can be used to build functional photodetectors, and we suggest that the broad community of researchers and R&D centers working on various devices like solar cells and light-emitting diodes implements it in their respective prototypes,” adds Pavao Andričević, who characterized the sensitive photodetectors.

Forró concludes: “This is an extremely important study – I would say, a central one – for large-scale commercialization of perovskite-based solar cells.”

Funding

ERC Advanced Grant “Picoprop”

References

Endre Horváth, Márton Kollár, Pavao Andričević, Lidia Rossi, Xavier Mettan, László Forró. Fighting Health Hazards in Lead Halide Perovskite Optoelectronic Devices with Transparent Phosphate Salts.  ACS Applied Materials & Interfaces 15 July 2021. DOI: 10.1021/acsami.0c21137

Image and video: A transparent phosphate crystal, which incorporated into solar cells, can instantaneously immobilize the lead in case of failure and block its leaching out from the device. © EPFL


Provided by EPFL

New Discoveries and Insights into the Glass Transition (Material Science)

A collaborative group from Tohoku University and Johns Hopkins University have provided valuable insights into the glass transition.

When a liquid is cooled rapidly, it gains viscosity and eventually becomes a rigid solid glass. The point at which it does so is known as the glass transition.

But the exact physics behind the glass transition, and the nature of glass in general, still pose many questions for scientists.

Metallic Glasses (MGs) are highly sought after since they combine the flexibility of plastic with the strength of steel. They are amorphous materials with a disordered atomic structure and exhibit unique and divergent thermodynamic and dynamic characteristics, especially when approaching the glass-transition temperature.

The glass transition in MGs is usually determined by calorimetric and dynamical measurements. The calorimetric glass transition detects the temperature at which specific heat has an abrupt jump, whereas dynamical transition looks at the diverse relaxation responses that emerge with increasing temperature forms.

Generally, the calorimetric glass-transition temperature follows the same trend as the dynamic α-relaxation temperature.

However, the collaborative group discovered that high configuration entropy significantly influences the glass transition of MGs and leads to the decoupling between calorimetric and dynamical glass transitions of high entropy metallic glasses.

DSC traces of the La(Ce)NiAl system, the arrows indicate the calorimetric glass-transition temperature (Tg) (left). The Temperature dependence of the loss modulus of the La(Ce)NiAl system normalized by the maximum peak value. The arrows indicate the α-relaxation temperature (Tα) (right).


The results of their research were published in the journal Nature Communication on June 22, 2021.

Their study presents a new glass-forming system that uses high configurational entropy, named high entropy metallic glasses (HEMGs).

The group featured Specially Appointed Professor Jing Jiang and Professor Hidemi Kato from the Institute for Materials Research at Tohoku University and Professor Mingwei Chen from Johns Hopkins University.

“We are excited about this discovery and believe this work furthers our understanding of the fundamental mechanism behind the glass transition,” said members of the research group.


Publication Details:

  • Title: Decoupling between calorimetric and dynamical glass transitions in high-entropy metallic glasses
  • Authors: Jing Jiang, Zhen Lu, Jie Shen, Takeshi Wada, Hidemi Kato & Mingwei Chen
  • Journal: Nature Communications
  • DOI10.1038/s41467-021-24093-w

Provided by Tohoku University

Researchers Discover A New Inorganic Material With Lowest Thermal Conductivity Ever Reported (Material Science)

A collaborative research team, led by the University of Liverpool, has discovered a new inorganic material with the lowest thermal conductivity ever reported. This discovery paves the way for the development of new thermoelectric materials that will be critical for a sustainable society.

Reported in the journal Science, this discovery represents a breakthrough in the control of heat flow at the atomic scale, achieved by materials design. It offers fundamental new insights into the management of energy. The new understanding will accelerate the development of new materials for converting waste heat to power and for the efficient use of fuels.

The research team, led by Professor Matt Rosseinsky at the University’s Department of Chemistry and Materials Innovation Factory and Dr Jon Alaria at the University’s Department of Physics and Stephenson Institute for Renewable Energy, designed and synthesised the new material so that it combined two different arrangements of atoms that were each found to slow down the speed at which heat moves through the structure of a solid.

They identified the mechanisms responsible for the reduced heat transport in each of these two arrangements by measuring and modelling the thermal conductivities of two different structures, each of which contained one of the required arrangements.

Combining these mechanisms in a single material is difficult, because the researchers have to control exactly how the atoms are arranged within it. Intuitively, scientists would expect to get an average of the physical properties of the two components. By choosing favourable chemical interfaces between each of these different atomic arrangements, the team experimentally synthesized a material that combines them both (represented as the yellow and blue slabs in the image).

This new material, with two combined arrangements, has a much lower thermal conductivity than either of the parent materials with just one arrangement. This unexpected result shows the synergic effect of the chemical control of atomic locations in the structure, and is the reason why the properties of the whole structure are superior to those of the two individual parts.

If we take the thermal conductivity of steel as 1, then a titanium bar is 0.1, water and a construction brick is 0.01, the new material is 0.001 and air is 0.0005.

Approximately 70 percent of all the energy generated in the world is wasted as heat. Low thermal conductivity materials are essential to reduce and harness this waste. The development of new and more efficient thermoelectric materials, which can convert heat into electricity, is considered a key source of clean energy.

Professor Matt Rosseinsky said: “The material we have discovered has the lowest thermal conductivity of any inorganic solid and is nearly as poor a conductor of heat as air itself.

“The implications of this discovery are significant, both for fundamental scientific understanding and for practical applications in thermoelectric devices that harvest waste heat and as thermal barrier coatings for more efficient gas turbines.”

Dr Jon Alaria said: “The exciting finding of this study is that it is possible to enhance the property of a material using complementary physics concepts and appropriate atomistic interfacing. Beyond heat transport, this strategy could be applied to other important fundamental physical properties such as magnetism and superconductivity, leading to lower energy computing and more efficient transport of electricity.”

The paper`Low thermal conductivity in a modular inorganic material with bonding anisotropy and mismatch’ (DOI: https://doi.org/10.1126/science.abh1619) is published in the journal Science.

The research team includes researchers from the University of Liverpool’s Leverhulme Research Centre for Functional Materials Design, University College London, ISIS Rutherford Appleton Laboratory and Laboratoire CRISMAT.

This project has received funding from the Engineering and Physical Science Research Council (EPSRC grant EP/N004884), the Leverhulme Trust and the Royal Society.

Featured image: Using the right chemistry, it is possible to combine two different atomic arrangement (yellow and blue slabs) that provide mechanisms to slow down the motion of heat through a solid. This strategy gives the lowest thermal conductivity reported in an inorganic material. © University of Liverpool


Provided by University of Liverpool

Scientists From NTU and Rice University Uncover Secret Behind One of the World’s Toughest Materials (Material Science)

A team of scientists led by Nanyang Technological University (NTU Singapore) and Rice University in the US, has uncovered the key to the outstanding toughness of hexagonal boron nitride (h-BN). h-BN can withstand ten times the amount of force that graphene can, which is known as one of the toughest materials on Earth.

A two-dimensional (2D) material, h-BN has a thickness of just one atom. First used in cosmetics in the 1940s, it was soon abandoned due to its high price, making a resurgence in the late 1990s after technology made its production cheaper.

Today, it is used by nearly all leading producers of cosmetic products because of its ability to absorb excess facial sebum and disperse pigment evenly, and as a protective layer in 2D electronics, as it insulates against electricity and withstands temperatures of up to 1000 °C.

The NTU and Rice scientists said their new understanding of the compound’s unique properties could pave the way to designing new flexible materials for electronics.

When scientists examined h-BN that had been exposed to stress, they saw that any breakages in the material branched like forks in a road, instead of travelling straight through the material (see Image 3), and meaning that fractures in h-BN are less likely to grow when further stress is applied.

Elaborating on the significance of their findings, Professor Gao Huajian, a Distinguished University Profesor in NTU’s School of Mechanical and Aerospace Engineering, who led the study, said: “Our experiments show that h-BN is the toughest nanomaterial measured to date. What makes this work so exciting is that it unveils an intrinsic toughening mechanism in this material – which should be brittle as it is only one atom thick. This is unexpected as there is often a trade-off between the strength and brittleness of nanomaterials.”

Fig 2: A computational simulation at NTU showing how h-BN fractures. The material’s intrinsic toughness arises from slight asymmetries in its atomic structure (left), which produce a permanent tendency for moving cracks to follow branched paths (right). © NTU Singapore

This latest research breakthrough is another of Prof Gao’s achievements in the field of applied mechanics. He was recently awarded the prestigious 2021 Timoshenko Medal by the American Society of Mechanical Engineers (ASME)[1] in recognition of his pioneering contributions to nanomechanics of engineering and biological systems, a new research field at the interface of solid mechanics, materials science and biophysics.

Professor Lou Jun, from Rice University’s Department of Materials Science and NanoEngineering, who also led the study, said: “In the real world, no material is free from defects, which is why understanding fracture toughness – or resistance to crack growth – is so important in engineering. It describes how much punishment a real-world material can withstand before failing.”

The research was published in the top scientific journal Nature in June.

Unveiling the secret behind h-BN’s toughness

After 1,000 hours of lab experiments and the use of computer simulations, the scientists traced the vastly different fracture toughness of graphene and h-BN to their chemical compositions.

Like a honeycomb, both h-BN and graphene are arranged in interconnecting hexagons (see Image 3). However, the hexagons in graphene consist solely of carbon atoms, while each hexagon structure in h-BN consists of three nitrogen and three boron atoms.

This difference in composition is what causes a moving crack in h-BN to branch off its path, and this tendency to branch or turn means it takes more energy for a crack to be driven further into it. By contrast, graphene breaks more easily, as fractures travel straight through the material like a zipper.

In both graphene and hexagonal boron nitride (h-BN) atoms, atoms are arranged in a flat lattice of interconnecting hexagons. In graphene, all the atoms are carbon. In h-BN, each hexagon contains three nitrogen and three boron atoms. © NTU Singapore

The researchers say that h-BN’s surprising toughness could make it the ideal option for making tear-resistant flexible electronics, such as wearable medical devices and foldable smartphones. It could also be added to strengthen electronics made from two-dimensional (2D) materials, which tend to be brittle.

Besides its flexibility, h-BN’s heat resistance and chemical stability would allow it to serve as both a supporting base and an insulating layer between electronic components, setting it apart from other traditional materials used in electronics.

Elaborating on the future applications of their study, Prof Gao said: “Our findings also point to a new route to produce tough materials by adding structural asymmetry into their designs. This would reduce the likelihood of materials fracturing under extreme stress, which may cause the devices to fail and lead to catastrophic effects.”

Prof Lou added: “The niche area for 2D material-based electronics like h-BN are in flexible electronic devices. In addition to applications like electronic textiles, 2D electronic devices are thin enough for more exotic applications like electronic tattoos and implants that could be attached directly to the brain.”

The scientists are now using their findings to explore new methods to produce tougher materials for mechanical and electronic manufacturing.

Featured image: Professor Gao Huajian, a Distinguished University Profesor from NTU’s School of Mechanical and Aerospace Engineering, presenting a computational simulation of h-BN. © NTU Singapore


Reference: Yang, Y., Song, Z., Lu, G. et al. Intrinsic toughening and stable crack propagation in hexagonal boron nitride. Nature 594, 57–61 (2021). https://doi.org/10.1038/s41586-021-03488-1


Provided by Nanyang Technological University