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Minimizing thermal conductivity of crystalline material with optimal nanostructure

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Jun 12, 2020 (Nanowerk News) Professor Junichiro Shiomi et al. from The University of Tokyo aimed to reduce the thermal conductivity of semiconductor materials by reducing the internal nanostructure, and successfully minimized thermal conductivity by designing, fabricating, and evaluating the optimal nanostructure-multilayer materials through materials informatics (MI), which combines machine learning and molecular simulation (Physical Review X, “Machine-Learning-Optimized Aperiodic Superlattice Minimizes Coherent Phonon Heat Conduction”). optimum nanostructure designed through materials informatics The optimum nanostructure designed with MI (aperiodic superlattice structure) was actually fabricated, and the optimal performance was verified by assessing its thermal conductivity. Figure: the Actual Structure is the electron microscope image of the fabricated sample. In addition, by further analyzing the phonon transport in the optimal structure, the mechanism that reduces thermal conductivity was clarified. (Image: The University of Tokyo) In 2017, this research group developed a method to design an optimal structure that minimizes or maximizes thermal conductivity via MI based on computational science. However, it has not been experimentally demonstrated, and preparation of nano-scale structures and realization of an optimal structure based on property measurements were desired. Thus, the research group utilized a film deposition method able to regulate, at a molecular level, a superlattice structure wherein two materials were alternately layered at several nm thick, and a measurement method that could assess thermal conductivity of a film at nano-scale, and realized the optimal aperiodic superlattice structure that minimizes thermal conductivity. With the optimal structure, wave interference of the lattice vibration (phonon) that conducts heat was maximized, and thermal conductivity was strongly regulated. In the present study, using the semiconductor lattice structure as the model, the research group verified the utility of the MI method in design, fabrication, assessment, and mechanism elucidation toward regulation of thermal conductivity. In the future, application of the MI method to various material systems is anticipated. It was also shown that optimization of the aperiodic structure can regulate thermal conductivity by fully controlling the wave property of a phonon at near room temperature. This is expected to contribute to developments in phonon engineering for instance in thermoelectric conversion devices, optical sensors, and gas sensors, where low thermal conductivity is needed while maintaining electric conductivity and mechanical properties.

Source: https://feeds.nanowerk.com/~/627096422/0/nanowerk/agwb~Minimizing-thermal-conductivity-of-crystalline-material-with-optimal-nanostructure.php

Nano Technology

Charcoal a weapon to fight superoxide-induced disease, injury: Nanomaterials soak up radicals, could aid treatment of COVID-19

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Home > Press > Charcoal a weapon to fight superoxide-induced disease, injury: Nanomaterials soak up radicals, could aid treatment of COVID-19

Artificial enzymes made of treated charcoal, seen in this atomic force microscope image, could have the power to curtail damaging levels of superoxides, toxic radical oxygen ions that appear at high concentrations after an injury. (Credit: Tour Group/Rice University)
Artificial enzymes made of treated charcoal, seen in this atomic force microscope image, could have the power to curtail damaging levels of superoxides, toxic radical oxygen ions that appear at high concentrations after an injury. (Credit: Tour Group/Rice University)

Abstract:
Artificial enzymes made of treated charcoal could have the power to curtail damaging levels of superoxides, radical oxygen ions that are toxic at high concentrations.

Charcoal a weapon to fight superoxide-induced disease, injury: Nanomaterials soak up radicals, could aid treatment of COVID-19


Houston, TX | Posted on July 2nd, 2020

The nanozymes developed by a Texas Medical Center team are highly effective antioxidants that break down damaging reactive oxygen species (ROS) produced in abundance in response to an injury or stroke.

The researchers suggested the materials, described in the American Chemical Society journal ACS Applied Nano Materials, could aid treatment of COVID-19 patients.

The biocompatible, highly soluble charcoal is a superoxide dismutase, and was synthesized and tested by scientists at Rice University, the University of Texas Health Science Center’s McGovern Medical School and the Texas A&M Health Science Center.

Superoxide dismutases, or SODs, dismantle ROS into ordinary molecular oxygen and hydrogen peroxide. In the project co-led by Rice chemist James Tour, previous materials were successfully tested for their ability to activate the process, including graphene quantum dots drawn from coal and polyethylene glycol-hydrophilic carbon clusters made from carbon nanotubes.

They have now found oxidized charcoal nanoparticles are not only effective antioxidants but can also be made from an activated carbon source that is inexpensive, good manufacturing practice (GMP)-certified and already being used in humans to treat acute poisoning.

“That these nanozymes are made from a GMP source opens the door for drug manufacturers,” said Tour, who led the project with A&M neurologist Thomas Kent and UTHealth biochemist Ah-Lim Tsai. “While coal was effective, an issue is that it can have a variety of toxic metallic elements and impurities that are not consistent across samples. And the clusters made from carbon nanotubes are very expensive.”

The disclike nanozymes are prepared from powdered, medical-grade charcoal oxidized by treatment with highly concentrated nitric acid. The nanozymes teem with oxygen-containing functional groups that bust up superoxides in solution.

Tour noted the nanozymes are able to pass through the membranes of cells’ mitochondria to quench a major source of free radicals without killing the cells themselves. “We published a paper on this recently,” he said. “This seems to be really important to why these work so well in traumatic brain injury and stroke.”

The researchers noted it may be worthwhile to study the application of their nanozymes to treat the cytokine storms — an excessive immune system response to infection — suspected of contributing to tissue and organ damage in COVID-19 patients.

“While speculative that these particles will be helpful in COVID-19, if administration is timed correctly, they could reduce the damaging radicals that accompany the cytokine storm and could be further chemically modified to reduce other injury-causing features of this disease,” Kent said.

Gang Wu, an assistant professor of hematology at McGovern, and Rice graduate student Emily McHugh are co-lead authors of the study. Co-authors are Vladimir Berka, a senior research scientist at McGovern; Rice graduate students Weiyin Chen, Zhe Wang and Jacob Beckham; Rice undergraduate Trenton Roy; and Paul Derry, an assistant professor at Texas A&M’s Institute of Biosciences and Technology.

Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice. Kent is the Robert A. Welch Chair Professor in the Institute of Biosciences and Technology at Texas A&M-Houston Campus and an adjunct chemistry professor at Rice and at Houston Methodist Hospital. Tsai is a professor of hematology at UTHealth.

The National Institutes of Health and the Welch Foundation supported the research.

####

About Rice University
Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation’s top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,962 undergraduates and 3,027 graduate students, Rice’s undergraduate student-to-faculty ratio is just under 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for lots of race/class interaction and No. 4 for quality of life by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger’s Personal Finance.

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Nano Technology

Microscopic structures could further improve perovskite solar cells

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Jul 03, 2020 (Nanowerk News) Solar cells based on perovskite compounds could soon make electricity generation from sunlight even more efficient and cheaper. The laboratory efficiency of these perovskite solar cells already exceeds that of the well-known silicon solar cells (Energy & Environmental Science, “Anisotropic carrier diffusion in single MAPbI3 grains correlates to their twin domains”). An international team led by Stefan Weber from the Max Planck Institute for Polymer Research in Mainz has found microscopic structures in perovskite crystals that can guide the charge transport in the solar cell. Clever alignment of these electron highways could make perovskite solar cells even more powerful. Along microscopic structures in perovskite solar cells electrons can move faster Along microscopic structures in perovskite solar cells electrons can move faster. (Image: MPI for Polymer Research) When solar cells convert sunlight into electricity, the electrons of the material inside the cell absorb the energy of the light. Traditionally, this light-absorbing material is silicon, but perovskites could prove to be a cheaper alternative. The electrons excited by the sunlight are collected by special contacts on the top and bottom of the cell. However, if the electrons remain in the material for too long, they can lose their energy again. To minimize losses, they should therefore reach the contacts as quickly as possible. Microscopically small structures in the perovskites – so-called ferroelastic twin domains – could be helpful in this respect: They can influence how fast the electrons move. An international research group led by Stefan Weber at the Max Planck Institute for Polymer Research in Mainz discovered this phenomenon. The stripe-shaped structures that the scientists investigated form spontaneously during the fabrication of the perovskite by mechanical stress in the material. By combining two microscopy methods, the researchers were able to show that electrons move much faster parallel to the stripes than perpendicular to them. “The domains act as tiny highways for electrons,” compares Stefan Weber.

Possible applications in light-emitting diodes and radiation detectors

For their experiments, the researchers first had to visualize the stripe-shaped domains. They succeeded in doing this with a piezo force microscope (PFM). Five years ago, Weber and his colleagues discovered the domains for the first time in a perovskite crystal using this method. “Back then, we already wondered whether the structures had an influence on the operation of a perovskite solar cell,” Weber explains. “Our latest results now show that this is the case.” The breakthrough came when the researchers compared their PFM images with data obtained from another method called photoluminescence microscopy. “Our photoluminescence detector works like a speed trap,” explains Ilka Hermes, researcher in Weber’s group and first author of the study. “We use it to measure the speed of electrons in different directions at the microscopic level.” Hermes discovered that along the stripes the electrons moved about 50 to 60 percent faster than perpendicular to them. “If we were able to make the stripes point directly to the electrodes, a perovskite solar cell could become much more efficient”, concludes Hermes. With the new results, not only solar cells could be improved. Other optoelectronic applications such as light-emitting diodes or radiation detectors could also benefit from directed charge transport. “In general, it is an advantage if we can direct the electrons in the right direction,” explains Stefan Weber. The researchers’ idea: to put perovskite crystals under mechanical stress during their production. This so-called strain engineering would enable an optimized orientation of the electron highways.

Source: https://feeds.nanowerk.com/~/629438054/0/nanowerk/agwb~Microscopic-structures-could-further-improve-perovskite-solar-cells.php

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How to get rid of the coffee-stain effect

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Jul 03, 2020 (Nanowerk News) Previously, researchers of the University of Twente discovered that the well-known coffee-stain effect is caused by a remarkable mechanism, showing avalanche-like behavior of particles in a fluid. In their latest paper, they now show how to prevent the ring-shaped coffee-stain and get a uniform distribution of the particles instead. The ‘coffee-stain’ effect is a well-known effect in physics and daily life: a dark-coloured edge remains when a fluid, containing particles, evaporates. This is caused by an ‘avalanche’ of particles moving to the outer edge, UT scientists showed in an earlier publication. In inkjet and 3D printing, this is an undesired effect. The effect can be suppressed by modifying the surface using an oily layer, researchers now show in the Proceedings of the National Academy of Sciences of the USA (“Evaporating droplets on oil-wetted surfaces: suppression of the coffee-stain effect”). coffee stain effect In the coffee stain effect, the particles are concentrated in one or more rings. (Image: University of Twente) Earlier work of the UT research group shows that if a particle-laden droplet evaporates, the particles start moving to the edge of the droplet. At first, this is a slow and regular movement, but as soon as the droplet loses height by evaporation, the particles rush to the edge disorderly, like in an avalanche. After full evaporation of the liquid, a dark ring remains. In fact, the diameter of the droplet already dictates this processs very early. How can we prevent this, is the question, because in many cases a homogeneous distribution is required and not a ring-shaped dark area. An oil-wetted surface is the answer, the new results show.

From homogeneous distribution to ‘coffee-eye’

In this case as well, the droplet has an edge, but this is limited by a layer of oil that does not evaporate. It also prevents the water at the droplet’s edge from evaporating fast. This, in turn, prevents particles from all moving to the edge. They even move the other way: from the edge inside the droplet. Once all water is evaporated, the particles are all over the surface and not just in a ring. The researcher also saw another effect, when the oily layer entirely covers the droptlet. In that case, a concentration of particles is formed, a so-callend nano-eye or ‘coffee eye’. This could even be a desired effect as well, in some applications, like in nanoparticle assembly. Through adding some surfactant to the droplet, the final particle deposition can be manipulated, from the concentrated ‘coffee-eye’ to a homogeneous distribution. The Physics of Fluids group has a long-term research relationship with printer manufacturer Océ, now calledCanon Production Printing. This company already adds a special layer to the substrate (like paper) before the droplets of the printer drop on it: the new research results are very valuable for improving the processes. For other application areas, the new insights and improved control are valuable too, like 3D printing and surface patterning. Better insights in the way liquids evaporate on a surface, may also provide more knowledge about the way viruses like corona are transferred from one person to another. The research was done in the Physics of Fluids group and the Max Planck Center for Complex Fluid Dynamics. A week before publication, the main author of the paper, Yaxing Li (Anhui 1991) successfully defended his PhD thesis ‘Evaporating multicomponent droplets.’

Source: https://feeds.nanowerk.com/~/629437586/0/nanowerk/agwb~How-to-get-rid-of-the-coffeestain-effect.php

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