materials

In what could be a major step forward for a new generation of solar cells called &ldquo;concentrator photovoltaics,&rdquo; a team of University of Michigan researchers has developed a new semiconductor alloy that can capture the near-infrared light located on the leading edge of the visible light spectrum.Easier to manufacture and at least 25 percent less costly than previous formulations, it&rsquo;s believed to be the world&rsquo;s most cost-effective material that can capture near-infrared light and is compatible with the gallium arsenide semiconductors often used in concentrator photovoltaics.Concentrator photovoltaics gather and focus sunlight onto small, high-efficiency solar cells made of gallium arsenide or germanium semiconductors. They&rsquo;re on track to achieve efficiency rates over 50 percent, while conventional flat-panel silicon solar cells top out in the mid 20s.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569904281278-jordan_ocena_in_story.jpg" style="width: 608px;" class="fr-fic fr-dib">Jordan Occena, a U-M graduate researcher and Sunyeol Jeon, a former U-M graduate student researcher, calibrate the molecular-beam epitaxy apparatus in the Carl A. Gerstacker Building on August 3, 2015. The apparatus is used for spray painting the &quot;magic&quot; chemical cocktail onto blank gallium arsenide wafers. PHOTO: Joseph Xu, Michigan Engineering&nbsp;&ldquo;Flat-panel silicon is basically maxed out in terms of efficiency,&rdquo; said Rachel S. Goldman, a U-M materials science and engineering professor whose lab developed the alloy. &ldquo;The cost of silicon isn&rsquo;t going down and efficiency isn&rsquo;t going up. Concentrator photovoltaics could power the next generation.&rdquo;Varieties of &nbsp;concentrator photovoltaics exist today. They are made of three different semiconductor alloys layered together. Sprayed onto a semiconductor wafer in a process called molecular-beam epitaxy&mdash;a bit like spray painting with individual elements&mdash;each layer is only a few microns thick. The layers capture different parts of the solar spectrum; light that gets through one layer is captured by the next.But near-infrared light slips through these cells unharnessed. For years, researchers have been working toward an elusive &ldquo;fourth layer&rdquo; alloy that could be sandwiched into cells to capture this light. It&rsquo;s a tall order; the alloy must be cost-effective, stable, durable and sensitive to infrared light, with an atomic structure that matches the other three layers in the solar cell.Getting all those variables right isn&rsquo;t easy, and until now, researchers have been stuck with prohibitively expensive formulas that use five elements or more.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569904354372-inside_machine.jpg" style="width: 699px;" class="fr-fic fr-dib">The inside of the main concourse of the molecular beam epitaxy apparatus in the Carl A. Gerstacker Building on August 3, 2015. A blank gallium arsenide wafer is placed in this concourse and moves down the tunnel to a growth chamber where the &quot;magic&quot; chemical cocktail is sprayed on. PHOTO: Joseph Xu, Michigan Engineering&nbsp;To find a simpler mix, Goldman&rsquo;s team devised a novel approach for keeping tabs on the many variables in the process. They combined on-the-ground measurement methods including X-ray diffraction done at U-M and ion beam analysis done at Los Alamos National Laboratory with custom-built computer modeling.Using this method, they discovered that a slightly different type of arsenic molecule would pair more effectively with the bismuth. They were able to tweak the amount of nitrogen and bismuth in the mix, enabling them to eliminate an additional manufacturing step that previous formulas required. And they found precisely the right temperature that would enable the elements to mix smoothly and stick to the substrate securely.&ldquo;&lsquo;Magic&rsquo; is not a word we use often as materials scientists,&rdquo; Goldman said. &ldquo;But that&rsquo;s what it felt like when we finally got it right.&rdquo;<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569904993498-results_in_story.jpg" style="width: 597px;" class="fr-fic fr-dib">A plate of semiconductors made by the molecular beam epitaxy apparatus in the Carl A. Gerstacker Building on August 3, 2015. PHOTO: Joseph Xu, Michigan Engineering&nbsp;The advance comes on the heels of another innovation from Goldman&rsquo;s lab that simplifies the &ldquo;doping&rdquo; process used to tweak the electrical properties of the chemical layers in gallium arsenide semiconductors. During doping, manufacturers apply a mix of chemicals called &ldquo;designer impurities&rdquo; to change how semiconductors conduct electricity and give them positive and negative polarity similar to the electrodes of a battery. The doping agents usually used for gallium arsenide semiconductors are silicon on the negative side and beryllium on the positive side.The beryllium is a problem&mdash;it&rsquo;s toxic and it costs about ten times more than silicon dopants. Beryllium is also sensitive to heat, which limits flexibility during the manufacturing process. But the U-M team discovered that by reducing the amount of arsenic below levels that were previously considered acceptable, they can &ldquo;flip&rdquo; the polarity of silicon dopants, enabling them to use the cheaper, safer element for both the&nbsp;positive and negative sides.&ldquo;Being able to change the polarity of the carrier is kind of like atomic &lsquo;ambidexterity&rsquo;,&rdquo; said Richard L. Field, a former U-M PhD student who worked on the project. &nbsp;&ldquo;Just like people with naturally born ambidexterity, it&rsquo;s fairly uncommon to find atomic impurities with this ability.&rdquo;Together, the improved doping process and the new alloy could make the semiconductors used in concentrator photovoltaics as much as 30 percent cheaper to produce, a big step toward making the high-efficiency cells practical for large-scale electricity generation.&ldquo;Essentially, this enables us to make these semiconductors with fewer atomic spray cans, and each can is significantly less expensive,&rdquo; Goldman said. &ldquo;In the manufacturing world, that kind of simplification is very significant. These new alloys and dopants are also more stable, which gives makers more flexibility as the semiconductors move through the manufacturing process.&rdquo;

Jordan Occena, a U-M graduate researcher and Sunyeol Jeon, a former U-M graduate student researcher, calibrate the molecular-beam epitaxy apparatus in the Carl A. Gerstacker Building on August 3, 2015. The apparatus is used for spray painting the "magic" chemical cocktail onto blank gallium arsenide wafers. PHOTO: Joseph Xu, Michigan Engineering

A new alloy could reduce the cost of high-efficiency solar cells called "concentrator photovoltaics.

"Magic" alloy could spur the next generation of solar cells

To build more aerodynamic machines, researchers are drawing inspiration from an unlikely source: the ocean.A team of evolutionary biologists and engineers at Harvard University, in collaboration with colleagues from the University of South Carolina, have shed light on a decades-old mystery about sharkskin and, in the process, demonstrated a new, bioinspired structure that could improve the aerodynamic performance of planes, wind turbines, drones and cars.The research is published in the <a href="http://rsif.royalsocietypublishing.org/content/15/139/20170828" target="_blank">Journal of the Royal Society Interface</a>.Sharks and airplanes aren&rsquo;t actually all that different. Both are designed to efficiently move through fluid (water and air), using the shape of their bodies to generate lift and decrease drag. The difference is, sharks have about a 400 million-year head start on the design process.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569467990398-Figure_1+Sharkskin.jpg" style="width: 766px;" class="fr-fic fr-dib">Environmental scanning electron microscope image of denticles from the shortfin mako shark (a) and (its corresponding parametric 3D model (b). These denticles were arranged in a wide range of different configurations on an aerofoil, two examples of which are shown here (c,d ). (Image courtesy of Harvard University)&ldquo;The skin of sharks is covered by thousands and thousands of small scales, or denticles, which vary in shape and size around the body,&rdquo; said <a href="http://www.people.fas.harvard.edu/~glauder/lauder.htm" target="_blank">George Lauder</a>, the Henry Bryant Bigelow Professor of Ichthyology and Professor of Biology in the Department of Organismic and Evolutionary Biology, and co-author of the research. &nbsp;&ldquo;We know a lot about the structure of these denticles &mdash; which are very similar to human teeth &mdash; but the function has been debated.&rdquo;Most research has focused on the drag reducing properties of denticles but Lauder and his team wondered if there was more to the story.&ldquo;We asked, what if instead of mainly reducing drag, these particular shapes were actually better suited for increasing lift,&rdquo; said Mehdi Saadat, a postdoctoral fellow at Harvard and co-first author of the study. Saadat holds a joint appointment in Mechanical Engineering at the University of South Carolina.To help test that hypothesis, the researchers collaborated with a team of engineers from the <a href="http://seas.harvard.edu/" target="_blank">Harvard John A. Paulson School of Engineering and Applied Sciences</a> (SEAS). &nbsp;For inspiration, they turned to the shortfin mako, the fastest shark in the world. The mako&rsquo;s denticles have three raised ridges, like a trident. Using micro-CT scanning, the team imaged and modeled the denticles in three dimensions. Next, they 3-D printed the shapes on the surface of a wing with a curved aerodynamic cross-section, known as an airfoil.The skin of sharks is covered by thousands and thousands of small scales, or denticles, which vary in shape and size around the body.&nbsp;We know a lot about the structure of these denticles &mdash; which are very similar to human teeth &mdash; but the function has been debated.<a class="twitter-quote" href="https://twitter.com/intent/tweet?text=The%20skin%20of%20sharks%20is%20covered%20by%20thousands%20and%20thousands%20of%20small%20scales,%20or%20denticles,%20which%20vary%20in%20shape%20and%20size%20around%20the%20body.%C2%A0We%20know%20a%20lot%20about%20the%20structure%20of%20these%20denticles%20%E2%80%94%20which%20are%20very%20similar%20to%20human%20teeth%20%E2%80%94%20but%20the%20function%20has%20been%20debated." target="_blank">Tweet This</a>&ldquo;Airfoils are a primary component of all aerial devices,&rdquo; said August Domel, a Ph.D. student at Harvard and co-first author of the paper. &ldquo;We wanted to test these structures on airfoils as a way of measuring their effect on lift and drag for applications in the design of various aerial devices such as drones, airplanes, and wind turbines.&rdquo;The researchers tested 20 different configurations of denticle sizes, rows and row positions on airfoils inside a water flow tank. They found that in addition to reducing drag, the denticle-shaped structures significantly increased lift, acting as high-powered, low-profile vortex generators.Even if you don&rsquo;t know what a vortex generator is, you&rsquo;ve seen one in action. Cars and planes are equipped with these small, passive devices designed to alter the air flow over the surface of a moving object to make it more aerodynamic. Most vortex generators in the field today have a simple, blade-like design.&ldquo;These shark-inspired vortex generators achieve lift-to-drag ratio improvements of up to 323 percent compared to an airfoil without vortex generators,&rdquo; said Domel. &ldquo;With these proof of concept designs, we&rsquo;ve demonstrated that these bioinspired vortex generators have the potential to outperform traditional designs.&rdquo;&ldquo;You can imagine these vortex generators being used on wind turbines or drones to increase the efficiency of the blades,&rdquo; said <a href="https://www.seas.harvard.edu/directory/bertoldi" target="_blank">Katia Bertoldi</a>, William and Ami Kuan Danoff Professor of Applied Mechanics at SEAS and co-author of the study. &ldquo;The results open new avenues for improved, bioinspired aerodynamic designs.&rdquo;&ldquo;This research not only outlines a novel shape for vortex generators but also provides insight into the role of complex and potentially multifunctional shark denticles,&rdquo; said Lauder.

Environmental scanning electron microscope (ESEM) image of denticles from the shortfin mako shark (scale bar: 200 mm) (Image courtesy of Harvard University)

Bioinspired vortex generators increase airfoil lift, decrease drag

Using shark scales to design better drones, planes, and wind turbines

Harvard researchers have developed a platform for creating soft robots with embedded sensors that can sense movement, pressure, touch, and temperature. (Image courtesy of Ryan L. Truby/Harvard University)

Soft robots that can sense touch, pressure, movement and temperature

Novel 3D printing method embeds sensing capabilities within robotic actuators

CHAMPAIGN, Ill. &mdash; Batteries that charge faster and have greater capacity could boost portable electronic devices and electric cars. A new method to simultaneously test stress and strain in battery electrodes gives researchers a window into the mechanical, electrical and chemical forces within lithium-ion batteries. The method revealed an unexpected point of stress in the charging cycle, which could guide development of better batteries.University of Illinois&nbsp;<a href="http://www.chemistry.illinois.edu/" target="_blank">chemistry</a> professor&nbsp;<a href="http://www.chemistry.illinois.edu/faculty/agewirth.html" target="_blank">Andrew Gewirth</a>,&nbsp;<a href="http://www.matse.illinois.edu/" target="_blank">materials science and engineering</a> professor&nbsp;<a href="http://www.matse.illinois.edu/directory/profile/n-sottos" target="_blank">Nancy Sottos</a> and graduate students Elizabeth Jones and Hadi Tavassol published&nbsp;<a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4708.html" target="_blank">their work</a> in the journal Nature Materials.&ldquo;We are finding limitations that are preventing the material from operating properly,&rdquo; Gewirth said. &ldquo;We are trying to understand what those limitations are and then develop materials and ways of processing materials that overcome them.&rdquo;A lithium-ion battery works by transferring lithium ions between two electrodes: the cathode and anode. When the battery is charged, lithium goes into the anode, which causes the material to expand. When the battery is discharged, the lithium leaves the anode, causing it to contract. However, because the material is constrained within the battery cell, it cannot truly expand and contract as it would when unconstrained, which causes mechanical stress within the material, Sottos said. Most methods of studying battery operation focus on electrochemistry, overlooking this mechanical response, she said.&ldquo;The relationship between the electrochemistry and the development of stress and strain is not well understood,&rdquo; said Sottos. &ldquo;The stress and strain can actually change the way a battery functions, and usually in a negative way &ndash; like the capacity will fade, or it will cause small cracks that cause reactions you don&rsquo;t expect.&rdquo;The researchers combined two techniques to get a more holistic picture of the mechanical forces during battery charging and discharging. They measured the strain, or how much the material expands, by seeding the anode with fluorescent nanoparticles and precisely measuring their movement as the battery went through charging cycles. Meanwhile, they measured stress, or how much the material bends or buckles, using an advanced optical technique.&ldquo;There have been other ways of looking at batteries &ndash; electrochemical methods, circuit methods, techniques involving light &ndash; but no one has done a comprehensive mechanical probe the way that we have,&rdquo; Gewirth said. &ldquo;Stress and strain have been measured separately, but no one has even looked at them together before.&rdquo;The researchers have dubbed their holistic measurement &ldquo;electrochemical stiffness,&rdquo; defined as the ratio of stress to strain.The measurement has already revealed one surprising finding: Right before the lithium is taken up by the anode, there is a spike in stress.&ldquo;We didn&rsquo;t expect there would be a barrier or an impediment to the lithiation process,&rdquo; Gewirth said. &ldquo;First, it shows us where the impediment is for charging. Now that we know where it is, we can work on ways to fix it. Second, it gives us a better idea of how the battery actually works.&rdquo;The electrochemical stiffness measurement technique is not limited to lithium-ion batteries, the researchers say, but can be used on a wide variety of ion-battery materials. Now that they have studied the anode, they are using the technique to study the cathode, which has its own properties and behaviors, Sottos said.&ldquo;These experiments are giving insight that it&rsquo;s not just an electrochemical issue; it&rsquo;s a mechanical issue,&ldquo; Sottos said. &ldquo;We think we are beginning to understand some of the factors that are hindering faster battery charging. We may be able to identify things that let us charge faster and have higher capacity, with less capacity fade over time.&rdquo;The Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, supported this work. Sottos also is affiliated with the&nbsp;<a href="http://beckman.illinois.edu/" target="_blank">Beckman Institute for Advanced Science and Technology</a> at the U. of I.

Illinois professors Nancy Sottos and Andrew Gerwith developed a method to comprehensively measure the mechanical stress and strain in lithium-ion batteries. It revealed a point of stress in charging that, if addressed through new methods or materials, could lead to faster-charging batteries.   Photo by L. Brian Stauffer

CHAMPAIGN, Ill. — Batteries that charge faster and have greater capacity could boost portable electronic devices and electric cars.

Method opens a window on how stress and strain affect battery performance

New York, NY&mdash;September 27, 2018&mdash;With temperatures rising and heat-waves disrupting lives around the world, cooling solutions are becoming ever more essential. This is a critical issue especially in developing countries, where summer heat can be extreme and is projected to intensify. But common cooling methods such as air conditioners are expensive, consume significant amounts of energy, require ready access to electricity, and often require coolants that deplete ozone or have a strong greenhouse effect.An alternative to these energy-intensive cooling methods is passive daytime radiative cooling (PDRC), a phenomenon where a surface spontaneously cools by reflecting sunlight and radiating heat to the colder atmosphere. PDRC is most effective if a surface has a high solar reflectance (R) that minimizes solar heat gain, and a high, thermal emittance (Ɛ) that maximizes radiative heat loss to the sky. If R and Ɛ are sufficiently high, a net heat loss can occur, even under sunlight.Developing practical PDRC designs has been challenging: many recent design proposals are complex or costly, and cannot be widely implemented or applied on rooftops and buildings, which have different shapes and textures. Up to now, white paints, which are inexpensive and easy to apply, have been the benchmark for PDRC. White paints, however, usually have pigments that absorb UV light, and do not reflect longer solar wavelengths very well, so their performance is only modest at best.Researchers at <a href="https://engineering.columbia.edu/" target="_blank">Columbia Engineering</a> have invented a high-performance exterior PDRC polymer coating with nano-to-microscale air voids that acts as a spontaneous air cooler and can be fabricated, dyed, and applied like paint on rooftops, buildings, water tanks, vehicles, even spacecraft&mdash;anything that can be painted. They used a solution-based phase-inversion technique that gives the polymer a porous foam-like structure. The air voids in the porous polymer scatter and reflect sunlight, due to the difference in the refractive index between the air voids and the surrounding polymer. The polymer turns white and thus avoids solar heating, while its intrinsic emittance causes it to efficiently lose heat to the sky. The <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aat9513" target="_blank">study</a> is published online today in <a class="has-markup" href="http://www.sciencemag.org/" target="_blank">Science</a>.&nbsp;<iframe src="https://www.youtube.com/embed/8lTdZ-U0Qm8?&wmode=opaque" frameborder="0" allowfullscreen="" class="fr-draggable" style="width: 766px; height: 462px;"></iframe>Passive daytime radiative cooling (PDRC) involves simultaneously reflecting sunlight and radiating heat into the cold sky to achieve a net heat loss. The process, which is spontaneous, can cool down structures to sub-ambient temperatures.&nbsp;The team&mdash;<a href="https://engineering.columbia.edu/faculty/yuan-yang" target="_blank">Yuan Yang</a>, assistant professor of <a href="https://matsci.columbia.edu/" target="_blank">materials science and engineering</a>; <a href="https://engineering.columbia.edu/faculty/nanfang-yu" target="_blank">Nanfang Yu</a>, associate professor of <a href="http://apam.columbia.edu/" target="_blank">applied physics</a>; and Jyotirmoy Mandal, lead author of the study and a doctoral student in Yang&rsquo;s group (all department of applied physics and applied mathematics)&mdash;built upon <a href="http://innovation.columbia.edu/technologies/CU15297_versatile-bio-inspired-systems-for" target="_blank">earlier work</a> that demonstrated that simple plastics and polymers, including acrylic, silicone, and PET, are excellent heat radiators and could be used for PDRC. The challenges were how to get these normally transparent polymers to reflect sunlight without using silver mirrors as reflectors and how to make them easily deployable.They decided to use phase-inversion because it is a simple, solution-based method for making light-scattering air-voids in polymers. Polymers and solvents are already used in paints, and the Columbia Engineering method essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared.&ldquo;This simple but fundamental modification yields exceptional reflectance and emittance that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like,&rdquo; says Mandal.The researchers found their polymer coating&rsquo;s high solar reflectance (R &gt; 96%) and high thermal emittance (Ɛ ~ 97%) kept it significantly cooler than its environment under widely different skies, e.g. by 6˚C in the warm, arid desert in Arizona and 3˚C in the foggy, tropical environment of Bangladesh. &ldquo;The fact that cooling is achieved in both desert and tropical climates, without any thermal protection or shielding, demonstrates the utility of our design wherever cooling is required,&rdquo; Yang notes.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568901232607-ir-vis.jpg" style="width: 766px;" class="fr-fic fr-dib">When exposed to the sky, the porous polymer PDRC coating reflects sunlight and emits heat to attain significantly cooler temperatures than typical building materials or even the ambient air.&nbsp;<section class="col-lg-7 entity entity-paragraphs-item paragraphs-item-content">The team also created colored polymer coatings with cooling capabilities by adding dyes. &ldquo;Achieving a superior balance between color and cooling performance over current paints is one of the most important aspects of our work,&rdquo; Yu notes. &ldquo;For exterior coatings, the choice of color is often subjective, and paint manufacturers have been trying to make colored coatings, like those for roofs, for decades.&rdquo;The group took environmental and operational issues, such as recyclability, bio-compatibility, and high-temperature operability, into consideration, and showed that their technique can be generalized to a range of polymers to achieve these functionalities. &ldquo;Polymers are an amazingly diverse class of materials, and because this technique is generic, additional desirable properties can be conveniently integrated into our PDRC coatings, if suitable polymers are available,&rdquo; Mandal adds.&ldquo;Nature offers many ways for heating and cooling, some of which are extremely well known and widely studied and others that are poorly known. Radiative cooling&mdash;by using the sky as a heat sink&mdash;belongs to the latter group, and its potential has been strangely overlooked by materials scientists until a few years ago,&rdquo; says Uppsala University Physics Professor Claes-G&ouml;ran Granqvist, a pioneer in the field of radiative cooling, who was not involved with the study. &ldquo;The publication by Mandal et al. highlights the importance of radiative cooling and represents an important breakthrough by demonstrating that hierarchically porous polymer coatings, which can be prepared cheaply and conveniently, give excellent cooling even in full sunlight.&rdquo; <img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568901341598-ir-vis-process.jpg" style="width: 766px;" class="fr-fic fr-dib">Illustration showing how passive daytime radiative cooling (PDRC) involves simultaneously reflecting sunlight and radiating heat into the cold sky to achieve a net heat loss. The process, which is spontaneous, can cool down structures to sub-ambient temperatures.&nbsp;Yang, Yu, and Mandal are refining their design in terms of applicability, while exploring possibilities such as the use of completely biocompatible polymers and solvents. They are in talks with industry about next steps.&ldquo;Now is a critical time to develop promising solutions for sustainable humanity,&rdquo; Yang notes. &ldquo;This year, we witnessed heat waves and record-breaking temperatures in North America, Europe, Asia, and Australia. It is essential that we find solutions to this climate challenge, and we are very excited to be working on this new technology that addresses it.&rdquo;Yu adds that he used to think that white was the most unattainable color: &ldquo;When I studied watercolor painting years ago, white paints were the most expensive. Cremnitz white or lead white was the choice of great masters, including Rembrandt and Lucian Freud. We have now demonstrated that white is in fact the most achievable color. It can be made using nothing more than properly sized air voids embedded in a transparent medium. Air voids are what make snow white and <a href="https://engineering.columbia.edu/news/staying-cool-saharan-silver-ants" target="_blank">Saharan silver ants</a> silvery.&rdquo;###Columbia Engineering Columbia Engineering, based in New York City, is one of the top engineering schools in the U.S. and one of the oldest in the nation. Also known as The Fu Foundation School of Engineering and Applied Science, the School expands knowledge and advances technology through the pioneering research of its more than 250 faculty, while educating undergraduate and graduate students in a collaborative environment to become leaders informed by a firm foundation in engineering. The School&rsquo;s faculty are at the center of the University&rsquo;s cross-disciplinary research, contributing to the Data Science Institute, Earth Institute, Zuckerman Mind Brain Behavior Institute, Precision Medicine Initiative, and the Columbia Nano Initiative. Guided by its strategic vision, &ldquo;Columbia Engineering for Humanity,&rdquo; the School aims to translate ideas into innovations that foster a sustainable, healthy, secure, connected, and creative humanity.</section>

When exposed to the sky, the porous polymer PDRC coating reflects sunlight and emits heat to attain significantly cooler temperatures than typical building materials or even the ambient air.

Columbia Engineers make white paint whiter—and cooler—by removing white pigment and invent a polymer coating, with nano-to-microscale air voids, that acts as a spontaneous air cooler and can be fabricated, dyed, and applied
like paint.

Polymer Coating Cools Down Buildings

<h2 data-element-id="headingsMap-4-0">Introduction</h2>3D printing technologies based on <a href="https://www.wevolver.com/article/digital.light.processing.3d.printing.explained" rel="noopener noreferrer" target="_blank">DLP printing (Digital Light Processing)</a> are frequently used for fabricating complex objects without tooling and machining. DLP printing involves the light-mediated conversion of a liquid resin containing monomer or oligomer photopolymers to a solid object. This process leverages the versatility of polymer chemistry to make complex objects with programmable optical, chemical or mechanical properties. Versatility underpins the popularity of polymer materials within the 3D printing community, encouraging research in photopolymer chemistry and printing applications. <iframe src="https://www.youtube.com/embed/97ARLiTHjX0?&amp;wmode=opaque&amp;enablejsapi=1&amp;origin=https://www.wevolver.com" allowfullscreen="" class="fr-draggable" style="width: 788px; height: 500px;" frameborder="0" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true"></iframe>This video explains the process and working of DLP printers in detail - by Professor Bill Hammack, Department of Chemical Engineering, University of Illinois Despite the growing enthusiasm for 3D printing in rapid prototyping applications, implementation in manufacturing environments to produce end-use parts has lagged. Common problems include setting and maintaining minimum specifications, specifically those related to material durability and weathering. Additive manufacturing methods rely on tight synchronicity between hardware, software and chemistry to achieve reproducibility at scale. This article highlights the role of polymerization in the co-development and simultaneous tuning of each component in the DLP process. By understanding the fundamentals of polymerization, developers can simultaneously tune process components, avoiding common pitfalls contributing to part defects, detachment, post-processing and weathering problems.This article is sponsored by polySpectra, which makes functional materials for advanced additive manufacturing. PolySpectra CEO Raymond Weitekamp explained to Wevolver that they've spent thousands of hours and hundreds of thousands of dollars characterizing the thermomechanical properties of dozens of additive manufacturing polymers. For Wevolver readers they are providing a free specification sheet of their newest materials at&nbsp;<a href="http://www.polyspectra.com/wevolver" rel="noopener noreferrer" target="_blank">www.polyspectra.com/wevolver</a> <h2 data-element-id="headingsMap-6-0">Photopolymerization: A multi-stage, dynamic process</h2>A photopolymer is a polymer whose properties alter upon exposure to light. Photopolymers are a liquid mixture containing monomers and oligomers, which can be manipulated to achieve specified physical properties. The process of photopolymerization (also known as photocuring or photo crosslinking) describes the crosslinking of monomers / oligomers within the liquid resin to form a network polymer. This process typically occurs via cationic or radical mechanisms and is mediated by the addition of photoinitiators. Photoinitiators are molecules that convert photolytic energy into reactive species to activate polymerization through specific functional groups.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4Mjk2NjE4NDExLVBob3RvLXBvbHltZXJfc2NoZW1lMSAwMi5wbmciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" style="width: 550px;" class="fr-fic fr-dib">Photopolymerization is an attractive method of curing polymers for 3D printing applications because it can be performed selectively. However, the process of photopolymerization is complex and sensitive to a range of parameters including wavelength, temperature and moisture. The extent and homogeneity of the polymerized cross-linked network is an important determinant in the properties of the final hardened material. Photopolymerization is a multi-stage and dynamic process. <a href="http://rawwerks.com/sla-utility/" rel="noopener noreferrer" target="_blank">Photopolymerization kinetics</a> is described as a “working curve”, mapping cure depth (depth to which light induces polymer crosslinking) as a function of light exposure at different wavelengths. Critical exposure (Y-intercept on the working curve) is defined as the minimum light exposure required for the onset of curing. The behaviour of the photopolymer under specified light exposure conditions (wavelength, power, exposure time and velocity) affect the printing process. There are a wide range of photopolymers available, responding to wavelengths in the UV (190-400 nm), visible (400-700 nm) and IR ranges (700-1000 nm). Light of different wavelengths penetrate materials to different cure depths. <img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4Mjk4MzAwNTYzLVBSLTQ4LVdDLnBuZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=">An example of a Working Curved that shows the relation between UV light dose and curing depth of the resin. Image courtesy Polyspectra. <iframe src="https://www.youtube.com/embed/Lw3BY2_yGVU?&amp;wmode=opaque&amp;enablejsapi=1&amp;origin=https://www.wevolver.com" allowfullscreen="" class="fr-draggable" style="width: 788px; height: 470px;" frameborder="0" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true"></iframe>This video explains how the working curve impacts the 3D print and how to find the correct settings - by Autodesk Ember <h2 data-element-id="headingsMap-8-0">Photopolymerization: A fundamental process in DLP printing</h2>The dynamic nature of photopolymerization suggests that it is a non-binary process. In DLP printing, light is projected into the polymer resin forming a 2D image. This simultaneously exposes all the designated regions within a polymer layer. The overall printing time is reduced through compared to rastered stereolithography, due to the parallelization of the photocuring process. The projected 2D image is composed of pixels, which become voxels when translated into three dimensions. This has led to the misconception that DLP is an essentially digital binary processtechnology and that print resolution is purely a function of printer hardwarethe DLP optics. In reality, voxels contain photopolymers that are fluxing between stages of the photopolymerization process: going from liquid to their gel point and then on towards their full cross-linking density.<iframe src="https://www.youtube.com/embed/5qTAmPrHLow?&amp;wmode=opaque&amp;enablejsapi=1&amp;origin=https://www.wevolver.com" allowfullscreen="" class="fr-draggable" style="width: 788px; height: 470px;" frameborder="0" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true" data-headingsmap-frameid="1370"></iframe>This video shows the impact of voxels on the 3D print and how printers can actually achieve resolutions smaller than the nominal resolution of the DLP projector - by Autodesk Ember. This can influence the 3D printing process in several ways. Firstly, printing speed should be calibrated to account for photopolymerization kinetics. If not, voxels may expand beyond their digitally defined physical space, reducing feature resolution of the finished part. Further, photopolymerization may cause shrinkage in the hardened polymer. Shrinkage coupled with cycles of thermal expansion and contraction experienced during the printing process may induce stressinduces stresses on the part, . These stresseswhich can manifest in warping or layer splitting or separation in the finished object. Strategies to mitigate these effects include tuning both software and hardware to select appropriate exposure protocols (velocity and speed) and control environmental influences (temperature and humidity). Finally, the cure depth is an important parameter to consider when implementing a DLP printing protocol. Cure depth directly impacts the z- axis settings of the printer. It may also influence the selection of printing parameters such as print velocity.Recommended reading:&nbsp;<a href="https://www.wevolver.com/article/digital.light.processing.3d.printing.explained" rel="noopener noreferrer" target="_blank">Digital Light Processing 3D printing explained</a><h2 data-element-id="headingsMap-10-0">Detachment: Building and breaking interactions between materials and hardware</h2>3D objects are rendered through the sequential hardening of photopolymer layers onto a substrate (known as a build plate). An essential step in any 3D printing process is the detachment of the finished part from the substrate. This step can prove problematic, as the printing process can induce suction forces, securing the part to the substrate. Numerous separation mechanics have been described to overcome these suction forces. Examples include tilting the resin tray and peeling the part off or using a sliding mechanism. Building the part on a flexible window, to which pressure may be applied to break suction forces is also a common method.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4MzU3NDgwMzU4LUNhcmJvbiBCZWQuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==">A print (with support structure) attached to a print bed after printing. Source: Carbon 3DBuilding an oxygen permeable window within the substrate allows oxygen to access the photopolymer directly in contact with the substrate. This creates a depletion zone of inactive resin which does not adhere to the substrate, facilitating part removal. However, this method is only suitable for photopolymers which are quenched by oxygen, such as acrylates and methacrylates. An alternative strategy is to modify the viscosity of the liquid photopolymer resin. Low viscosity solutions enable higher printing speeds and concomitant reduction in the suction forces adhering the part to the substrate. This highlights an important aspect of 3D printing; the relationship between the photopolymer materials and the design of the printer hardware. It is widely recognised that surface adhesion is important to provide a foundation for the object to build upon. Consequently, the materials within the hardware and the finished printed object should be compatible.<h2 data-element-id="headingsMap-11-0">Post-processing: The significance of photopolymerization in scaling production</h2>DLP printing found initial application as an instrument for rapid prototyping in research settings. Introducing DLP printing into manufacturing settings has been slower due to problems with scaling production, reproducibility and part quality. The co-development of printing processes and materials is not only critical during the initial build process. Photopolymerization kinetics also affect post-processing of end-use parts. The acceptance of 3D printing into manufacturing settings has been restricted by extensive post-processing requirements including:<ul><li>Removal of the support structure (onto which the part is fabricated).</li><li>Part cleaning to remove debris and residual materials.</li><li>Post-curing (usually thermal or optical) to completely harden certain materials.</li><li>Part finishing (such as sanding, painting or varnishing) to remove the “boxy” surface finish characteristic of DLP printing.</li></ul><img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4MzU3NTQ3NzY2LXBvc3QgcHJvY2Vzc2luZy5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" style="width: 766px;" class="fr-fic fr-dib">Rinsing off resing, removing support, and curing with UV light. Source: FormlabsPost-processing steps are often performed in small batches under carefully controlled conditions. These processes can be difficult to automate, adding substantial time and cost to the production process. Advances in automation are ongoing, with some companies utilising robotics to automate assembly lines integrating 3D printers with post-processing equipment. Advances in printing hardware and software enabling automated monitoring could enable DLP printing at scale. However, the widespread implementation of automated processing lines has yet to be realised. Post-processing requirements are heavily dependent on photopolymer characteristics, with some materials requiring more extensive modification than others. Strategies to streamline operations should include selecting materials with limited post-processing requirements yet withstand weathering processes to maintain durability.<h2 data-element-id="headingsMap-12-0">Conclusion</h2>The combination of pivotal developments in 3D printing hardware, software and materials has paved the way towards the implementation in prototyping and manufacturing. With an increased awareness of the importance of co-developing each component simultaneously, the development of new applications for 3D printing of end-use parts is slowly becoming a reality. The relevance of DLP printing in the manufacturing sector will depend heavily on the ability to cost-effectively produce end-parts that are durable. The dynamic nature of photopolymerization continues throughout the lifetime of a part. The weatherability of hardened polymers is an important determinant of part durability, specifically the ability to maintain toughness and impact strength over time. By understanding the kinetics of photopolymerization and thermodynamic properties of the resulting polymer networks, developers can select materials to mitigate common problems, yet maintain desired properties throughout the lifetime of a part. <hr>This article was sponsored by&nbsp;<a href="http://polyspectra.com/wevolver" rel="noopener noreferrer" target="_blank">polySpectra</a>. polySpectra makes functional materials for advanced additive manufacturing in Berkeley, California, USA. They use light-activated catalysts to 3D print advanced functional materials. Polyspectra's platform enables them to deliver materials with a broad spectrum of tailored properties from a single chemical system.For Wevolver readers polySpectra is providing a free specification sheet of their newest materials at&nbsp;<a href="http://www.polyspectra.com/wevolver" rel="noopener noreferrer" target="_blank">www.polyspectra.com/wevolver</a>

DLP (Digital Light Processing) 3D printing uses photopolymerization to create 3D objects. Understanding the photopolymerization process can help you mitigate common problems that make your prints fail or that reduce print quality.

DLP 3D Printing: Understanding process and materials enables you to prevent production problems

A nanoscale coating that&rsquo;s at least 95 percent air repels the broadest range of liquids of any material in its class, causing them to bounce off the treated surface, according to the University of Michigan engineering researchers who developed it.In addition to super stain-resistant clothes, the coating could lead to breathable garments to protect soldiers and scientists from chemicals and advanced waterproof paints that dramatically reduce drag on ships.Droplets of solutions that would normally damage either your shirt or your skin recoil when they touch the new &ldquo;superomniphobic surface.&rdquo;&ldquo;Virtually any liquid you throw on it bounces right off without wetting it. For many of the other similar coatings, very low surface tension liquids such as oils, alcohols, organic acids, organic bases and solvents stick to them and they could start to diffuse through and that&rsquo;s not what you want,&rdquo; said Anish Tuteja, assistant professor of materials science and engineering, chemical engineering and macromolecular science and engineering. Tuteja is the corresponding author of a paper on the coating published in the current issue of the Journal of the American Chemical Society.<iframe src="https://www.youtube.com/embed/ICw5k1tDZSk?&wmode=opaque" allowfullscreen="" class="fr-draggable" style="width: 767px; height: 432px;" frameborder="0"></iframe>Anish Tuteja, assistant professor of materials science and engineering, chemical engineering and macromolecular science and engineering, discusses and demonstrates his work with the &quot;superomniphobic&quot; coating.The researchers tested more than 100 liquids and found only two that were able to penetrate the coating. They were chlorofluorocarbons &ndash; chemicals used in refrigerators and air conditioners. In Tuteja&rsquo;s lab, in a demonstration, the surface repelled coffee, soy sauce and vegetable oil, as well as toxic hydrochloric and sulfuric acids that could burn skin. Tuteja says it&rsquo;s also resistant to gasoline and various alcohols.To apply the coating, the researchers use a technique called electrospinning that uses an electric charge to create fine particles of solid from a liquid solution. So far, they&rsquo;ve coated small tiles of screen and postage-stamp sized swaths of fabric.The coating is a mixture of rubbery plastic particles of &ldquo;polydimethylsiloxane,&rdquo; or PDMS, and liquid-resisting nanoscale cubes developed by the Air Force that contain carbon, fluorine, silicon and oxygen. The material&rsquo;s chemistry is important, but so is its texture. It hugs the pore structure of whatever surface it&rsquo;s being applied to, and it also creates a finer web within those pores. This structure means that between 95 and 99 percent of the coating is actually air pockets, so any liquid that comes in contact with the coating is barely touching a solid surface.<div class="wow fadeIn" data-wow-delay=".09s">Because the liquid touches mere filaments of the solid surface, as opposed to a greater area, the developed coating can dramatically reduce the intermolecular forces that normally draw the two states of matter together. These Van der Waals interaction forces are kept at a minimum.&ldquo;Normally, when the two materials get close, they imbue a small positive or negative charge on each other, and as soon as the liquid comes in contact with the solid surface it will start to spread,&rdquo; Tuteja said. &ldquo;We&rsquo;ve drastically reduced the interaction between the surface and the droplet.&rdquo;With almost no incentive to spread, the droplets stay intact, interacting only with molecules of themselves, maintaining a spherical shape, and literally bouncing off the coating.One classification of liquid that this coating repels is the so-called non-Newtonian category, which includes shampoos, custards, blood, paints, clays and printer inks, for example. These are liquids that change their viscosity depending on the forces applied to them. They differ from the Newtonians, such as water and most other liquids, whose viscosity stays the same no matter the force applied. Viscosity is a measure of a liquid&rsquo;s resistance to flow on the application of force,and it&rsquo;s sometimes thought of as its thickness.&ldquo;No one&rsquo;s ever demonstrated the bouncing of low surface tension non-Newtonian liquids,&rdquo; Tuteja said.<hr>The paper is titled &ldquo;<a href="https://www.researchgate.net/publication/233983140_Superomniphobic_Surfaces_for_Effective_Chemical_Shielding" rel="noopener noreferrer" target="_blank">Superomniphobic Surfaces for Effective Chemical Shielding</a>.&rdquo; Doctoral student Shuaijun Pan and postdoctoral researcher Arun Kota, both in materials science and engineering, are the first authors of the paper. Also contributing is Joseph M. Mabry, in the rocket propulsion division of the Air Force Research Laboratory. The work is funded by the Air Force Office of Scientific Research.</div>

IMAGE:  Researchers in MSE Professor Anish Tuteja's Research Group, Polymers, Surfaces, and Interfaces (PSI-ψ); demonstrate use of their new development -- surfaces that repel every known liquid known as "superomniphobic surfaces." Photo: Joseph Xu, Michigan Engineering Communications & Marketing

A nanoscale coating that’s at least 95 percent air repels the broadest range of liquids of any material in its class, causing them to bounce off the treated surface, according to the engineering researchers who developed it.

A material that most liquids wont wet

Additive manufacturing, otherwise known as 3D printing, can be used to manufacture porous electrodes for lithium-ion batteries&mdash;but because of the nature of the manufacturing process, the design of these 3D printed electrodes is limited to just a few possible architectures. Until now, the internal geometry that produced the best porous electrodes through additive manufacturing was what&rsquo;s known as an interdigitated geometry&mdash;metal prongs interlocked like the fingers of two clasped hands, with the lithium shuttling between the two sides.Lithium-ion battery capacity can be vastly improved if, on the microscale, their electrodes have pores and channels. An interdigitated geometry, though it does allow lithium to transport through the battery efficiently during charging and discharging, is not optimal.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214668191-0730-em-rahul-battery.png" style="width: 766px;" class="fr-fic fr-dib">Lattice architecture can provide channels for effective transportation of electrolyte inside the volume of material, while for the cube electrode, most of the material will not be exposed to the electrolyte.Rahul Panat, an associate professor of mechanical engineering at Carnegie Mellon University, and a team of researchers from Carnegie Mellon in collaboration with Jonghyun Park, assistant professor of mechanical and aerospace engineering at Missouri S&amp;T have developed a revolutionary new method of 3D printing battery electrodes that creates a 3D microlattice structure with controlled porosity. 3D printing this microlattice structure, the researchers show in a paper <a href="https://www.sciencedirect.com/science/article/pii/S2214860418302379#bib0095" rel="noopener" target="_blank">published in the journal Additive Manufacturing</a>, vastly improves the capacity and charge-discharge rates for lithium-ion batteries.&ldquo;In the case of lithium-ion batteries, the electrodes with porous architectures can lead to higher charge capacities,&rdquo; says Panat. &ldquo;This is because such architectures allow the lithium to penetrate through the electrode volume leading to very high electrode utilization, and thereby higher energy storage capacity. In normal batteries, 30-50% of the total electrode volume is unutilized. Our method overcomes this issue by using 3D printing where we create a microlattice electrode architecture that allows the efficient transport of lithium through the entire electrode, which also increases the battery charging rates.&rdquo;<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214704238-Screenshot_2019-09-11+3D+printing+the+next+generation+of+batteries.png" style="width: 766px;" class="fr-fic fr-dib"> The additive manufacturing method presented in Panat and Park&rsquo;s paper represents a major advance in printing complex geometries for 3D battery architectures, as well as an important step toward geometrically optimizing 3D configurations for electrochemical energy storage. The researchers estimate that this technology will be ready to translate to industrial applications in about two to three years.The microlattice structure (Ag) used as lithium-ion batteries&rsquo; electrodes was shown to improve battery performance in several ways such as a fourfold increase in specific capacity and a twofold increase in areal capacity when compared to a solid block (Ag) electrode. Furthermore, the electrodes retained their complex 3D lattice structures after forty electrochemical cycles demonstrating their mechanical robustness. The batteries can thus have high capacity for the same weight or alternately, for the same capacity, a vastly reduced weight, which is an important attribute for transportation applications.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214728313-0730-em-rahul-battery-microscope.png" style="width: 766px;" class="fr-fic fr-dib">SEM images of 3D printed electrodes for Li-ion batteries used for electrochemical cycling in the researchers&rsquo; study. Image taken from top of microlattice electrodes with height of about 250mm.The Carnegie Mellon researchers developed their own 3D printing method to create the porous microlattice architectures while leveraging the existing capabilities of an Aerosol Jet 3D printing system. The Aerosol Jet system also allows the researchers to print planar sensors and other electronics on a micro-scale, which was deployed at Carnegie Mellon University&rsquo;s College of Engineering earlier this year. This novel 3-D printing method could be applied to battery technology with a collaboration with the Missouri university, where researchers developed 3-D battery design and analysis, cell fabrication technology, and electrochemical analysis via simulation and modeling.Until now, 3D printed battery efforts were limited to extrusion-based printing, where a wire of material is extruded from a nozzle, creating continuous structures. Interdigitated structures were possible using this method. With the method developed in Panat&rsquo;s lab, the researchers are able to 3D print the battery electrodes by rapidly assembling individual droplets one-by-one into three-dimensional structures. The resulting structures have complex geometries impossible to fabricate using typical extrusion methods.&ldquo;Because these droplets are separated from each other, we can create these new complex geometries,&rdquo; says Panat. &ldquo;If this was a single stream of material, as in the case of extrusion printing, we wouldn&rsquo;t be able to make them. This is a new thing. I don&rsquo;t believe anybody until now has used 3D printing to create these kinds of complex structures.&rdquo;This revolutionary method will be very important for consumer electronics, the medical devices industry, as well as aerospace applications. This research will integrate well with the biomedical electronic devices, where miniaturized batteries are required. Non-biological electronic micro-devices will also benefit from this work. And on a bigger scale, electronic devices, small drones, and aerospace applications themselves can use this technology as well, due to the low weight and high capacity of the batteries printed using this method.The team, which also includes mechanical engineering Ph.D. student Mohammad Sadeq Saleh and postdoctoral researcher Jie Li at Missouri S&amp;T, is also working on creating more complex three-dimensional structures, which can simultaneously be used as structural materials and as functional materials. For example, a part of a drone can act as a wing, a structural material, while simultaneously acting as a functional material such as a battery.

A new method of fabricating battery electrodes using Aerosol Jet 3D printing.

3D printing the next generation of batteries

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Warping occurs when 3D printed parts curl away from the build plate

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LGA uses flat pads for connections, while BGA relies on solder balls for permanent mounting. This guide compares their thermal performance, electrical traits, reliability, and assembly pros and cons to help you choose the best IC package for your needs.

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Is soldering paste the same as flux? This technical article clarifies the differences for electronics engineers, covering their functionalities and practical implementations in both manual soldering (repairs, prototyping) and automated soldering (SMT reflow assembly)

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Here’s what you need to know about recycling PLA 3D printer filament and cutting back on 3D printing plastic waste.


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ABS can endure moderate temperatures before it melts

ABS is a widely used 3D printing filament with good impact resistance and flexural strength. It also exhibits moderate heat resistance, making it a go-to material for mid-temperature 3D printing applications.

Materials

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