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  • richardmitnick 8:59 pm on December 16, 2015 Permalink | Reply
    Tags: , Heat Studies,   

    From U Michigan: “Heat radiates 10,000 times faster at the nanoscale” 

    U Michigan bloc

    University of Michigan

    Nicole Casal Moore, Michigan Engineering

    No image credits

    When heat travels between two objects that aren’t touching, it flows differently at the smallest scales – distances on the order of the diameter of DNA, or 1/50,000 of a human hair.

    While researchers have been aware of this for decades, they haven’t understood the process. Heat flow often needs to be prevented or harnessed and the lack of an accurate way to predict it represents a bottleneck in nanotechnology development.

    Now, in a unique ultra-low vibration lab at the University of Michigan, engineers have measured how heat radiates from one surface to another in a vacuum at distances down to 2 nanometers.

    While the thermal energy still flows from the warmer place to the colder one, the researchers found it does so 10,000 times faster than it would at the scale of, say, a bonfire and a pair of chilly hands. “Faster” here refers to the speed at which the temperature of one sample changes the temperature of the other – and not the speed at which the heat itself travels. Heat is a form of electromagnetic radiation, so it moves at the speed of light. What’s different at the nanoscale is the efficiency of the process.

    “We’ve shown, for the first time, the dramatic enhancements of radiative heat fluxes in the extreme near-field,” said Pramod Reddy, an associate professor of mechanical engineering and materials science and engineering. “Our experiments and calculations imply that heat flows several orders of magnitude faster in these ultra small gaps.”

    Reddy and Edgar Meyhofer, a professor of mechanical engineering and biomedical engineering, led the work. A paper on the findings is newly published online in Nature.

    The findings have applications across nanotechnology. They could advance next-generation information storage such as heat-assisted magnetic recording. They could push forward devices that more directly convert heat into electricity, including heat generated in cars and spacecrafts that is now being wasted. Those are just a few potential uses.


    The phenomenon the researchers studied is “radiative heat” – the electromagnetic radiation, or light, that all matter above absolute zero emits. It is the emission of the internal energy of matter from movement of particles in matter – movement that only happens above absolute zero.

    Scientists can explain how this happens at macroscopic distances, dimensions we can readily perceive in the world around us, down to some we can’t see. More than 100 years ago, the German physicist Max Planck wrote the equations that make this possible. His model accurately describes heat transfer across large to relatively small voids, reaching to 10 micrometers at room temperature. But when the gap gets so tight it’s almost not there, the equations break down.

    In the middle of the last century, the Russian radio physicist Sergei Rytov proposed a new theory called “fluctuational electrodynamics” to describe heat transfer at smaller-than-10-micrometer distances. Since then, research hasn’t always resulted in supporting evidence.

    “There were experiments in the 1990s or early 2000s that tried to test these ideas further and they found large discrepancies between what theory would predict and what experiments revealed ,” Meyhofer said.

    Because of the sophistication of the U-M lab, the researchers say their findings close the case, and Rytov was right.

    “Our work, performed in collaboration with colleagues Professor Juan Carlos Cuevas and Professor Francisco García-Vidal at the Universidad Autónoma de Madrid, resolves an important controversy and represents a key contribution to the field of heat transfer,” Reddy said. “These results disprove current dogma in nanoscale heat transfer, which holds that radiative heat transfer in single digit nanometer-sized gaps cannot be explained by existing theory.”

    The facility the researchers used is an ultra-low vibration chamber in the G. G. Brown Laboratories, the university’s newly renovated mechanical engineering complex. The chamber – one of several – was custom designed for performing nanoscale experiments so precise that mere footsteps could disturb them if they were done somewhere else. The rooms can withstand vibration from outside, such as traffic, and inside, such as heating and cooling systems. They also limit acoustic noise, temperature and humidity variations, as well as radio frequency and magnetic interference.

    “Our facility represents the true state of the art,” Meyhofer said. “When creating nanoscale gaps such as those required for our nanoscale heat radiation experiments, the slightest perturbation can ruin an experiment.”

    In the chamber, the researchers used custom-built “scanning thermal microscopy probes” that allowed them to directly study how fast heat flows between two surfaces of silica, silicon nitride and gold. The researchers chose these materials because they’re commonly used in nanotechnology.

    For each material, they designated one sample that would be heated to 305 Fahrenheit, and they coated the tip of the probe with the same material, but kept it at a cooler 98 degrees. They slowly moved the sample and the probe together, beginning at 50 nanometers until they were touching, and they measured the temperature of the tip at regular intervals.

    The cause of the rapid heat transfer, the researchers discovered, is that in nanoscale gaps there can be an overlap of the two sides’ surface and evanescent waves, both of which carry heat.

    “These waves reach only a small distance into the gap between materials,” said Bai Song, a graduate student in mechanical engineering and one of the lead authors. “And their intensity at the extreme near-field is enormous compared to the electromagnetic waves at larger distances. When these waves from two different devices overlap, that’s when they allow tremendous heat flux.”

    The paper is titled Radiative heat transfer in the extreme near field. It also involved collaborators from Universidad Autónoma de Madrid, Massachusetts Institute of Technology and Donostia International Physics Center. The work was funded by the U.S. Department of Energy Basic Energy Sciences, the Army Research Office, the National Science Foundation, the Spanish Ministry of Economy and Competitiveness and other organizations.

    See the full article here .

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    U MIchigan Campus

    The University of Michigan (U-M, UM, UMich, or U of M), frequently referred to simply as Michigan, is a public research university located in Ann Arbor, Michigan, United States. Originally, founded in 1817 in Detroit as the Catholepistemiad, or University of Michigania, 20 years before the Michigan Territory officially became a state, the University of Michigan is the state’s oldest university. The university moved to Ann Arbor in 1837 onto 40 acres (16 ha) of what is now known as Central Campus. Since its establishment in Ann Arbor, the university campus has expanded to include more than 584 major buildings with a combined area of more than 34 million gross square feet (781 acres or 3.16 km²), and has two satellite campuses located in Flint and Dearborn. The University was one of the founding members of the Association of American Universities.

    Considered one of the foremost research universities in the United States,[7] the university has very high research activity and its comprehensive graduate program offers doctoral degrees in the humanities, social sciences, and STEM fields (Science, Technology, Engineering and Mathematics) as well as professional degrees in business, medicine, law, pharmacy, nursing, social work and dentistry. Michigan’s body of living alumni (as of 2012) comprises more than 500,000. Besides academic life, Michigan’s athletic teams compete in Division I of the NCAA and are collectively known as the Wolverines. They are members of the Big Ten Conference.

  • richardmitnick 12:43 pm on March 23, 2015 Permalink | Reply
    Tags: , Heat Studies,   

    From OSU: “Landmark study proves that magnets can control heat and sound” 


    Ohio State University

    March 23, 2015
    Pam Frost Gorder

    Researchers at The Ohio State University have discovered how to control heat with a magnetic field. An experiment proved that the phonon—the elementary particle that carries heat and sound—has magnetic properties. Here Joseph Heremans, Ohio Eminent Scholar in Nanotechnology, holds an artist’s rendering of a phonon heating solid material. Artist’s rendering by Renee Ripley. Photo by Kevin Fitzsimons, courtesy of The Ohio State University

    Researchers at The Ohio State University have discovered how to control heat with a magnetic field.

    In the March 23 issue of the journal Nature Materials, they describe how a magnetic field roughly the size of a medical MRI reduced the amount of heat flowing through a semiconductor by 12 percent.

    The study is the first ever to prove that acoustic phonons—the elemental particles that transmit both heat and sound—have magnetic properties.

    “This adds a new dimension to our understanding of acoustic waves,” said Joseph Heremans, Ohio Eminent Scholar in Nanotechnology and professor of mechanical engineering at Ohio State. “We’ve shown that we can steer heat magnetically. With a strong enough magnetic field, we should be able to steer sound waves, too.”

    Joseph Heremans

    People might be surprised enough to learn that heat and sound have anything to do with each other, much less that either can be controlled by magnets, Heremans acknowledged. But both are expressions of the same form of energy, quantum mechanically speaking. So any force that controls one should control the other.

    “Essentially, heat is the vibration of atoms,” he explained. “Heat is conducted through materials by vibrations. The hotter a material is, the faster the atoms vibrate.

    “Sound is the vibration of atoms, too,” he continued. “It’s through vibrations that I talk to you, because my vocal chords compress the air and create vibrations that travel to you, and you pick them up in your ears as sound.”

    The name “phonon” sounds a lot like “photon.” That’s because researchers consider them to be cousins: Photons are particles of light, and phonons are particles of heat and sound. But researchers have studied photons intensely for a hundred years—ever since Einstein discovered the photoelectric effect. Phonons haven’t received as much attention, and so not as much is known about them beyond their properties of heat and sound.

    Hyungyu Jin

    This study shows that phonons have magnetic properties, too.

    “We believe that these general properties are present in any solid,” said Hyungyu Jin, Ohio State postdoctoral researcher and lead author of the study.

    The implication: In materials such as glass, stone, plastic—materials that are not conventionally magnetic—heat can be controlled magnetically, if you have a powerful enough magnet. The effect would go unnoticed in metals, which transmit so much heat via electrons that any heat carried by phonons is negligible by comparison.

    There won’t be any practical applications of this discovery any time soon: 7-tesla magnets like the one used in the study don’t exist outside of hospitals and laboratories, and the semiconductor had to be chilled to -450 degrees Fahrenheit (-268 degrees Celsius)—very close to absolute zero—to make the atoms in the material slow down enough for the phonons’ movements to be detectible.

    That’s why the experiment was so difficult, Jin said. Taking a thermal measurement at such a low temperature was tricky. His solution was to take a piece of the semiconductor indium antimonide and shape it into a lopsided tuning fork. One arm of the fork was 4 mm wide and the other 1 mm wide. He planted heaters at the base of the arms.

    The design worked because of a quirk in the behavior of the semiconductor at low temperatures. Normally, a material’s ability to transfer heat would depend solely on the kind of atoms of which it is made. But at very low temperatures, such as the ones used in this experiment, another factor comes into play: the size of the sample being tested. Under those conditions, a larger sample can transfer heat faster than a smaller sample of the same material. That means that the larger arm of the tuning fork could transfer more heat than the smaller arm.

    Heremans explained why.

    “Imagine that the tuning fork is a track, and the phonons flowing up from the base are runners on the track. The runners who take the narrow side of the fork barely have enough room to squeeze through, and they keep bumping into the walls of the track, which slows them down. The runners who take the wider track can run faster, because they have lots of room.

    “All of them end up passing through the material—the question is how fast,” he continued. “The more collisions they undergo, the slower they go.”

    In the experiment, Jin measured the temperature change in both arms of the tuning fork and subtracted one from the other, both with and without a 7-tesla magnetic field turned on.

    In the absence of the magnetic field, the larger arm on the tuning fork transferred more heat than the smaller arm, just as the researchers expected. But in the presence of the magnetic field, heat flow through the larger arm slowed down by 12 percent.

    So what changed? Heremans said that the magnetic field caused some of the phonons passing through the material to vibrate out of sync so that they bumped into one another, an effect identified and quantified through computer simulations performed by Nikolas Antolin, Oscar Restrepo and Wolfgang Windl, all of Ohio State’s Department of Materials Science and Engineering.

    In the larger arm, the freedom of movement worked against the phonons—they experienced more collisions. More phonons were knocked off course, and fewer—12 percent fewer—passed through the material unscathed.

    The phonons reacted to the magnetic field, so the particles must be sensitive to magnetism, the researchers concluded. Next, they plan to test whether they can deflect sound waves sideways with magnetic fields.

    Co-authors on the study included Stephen Boona, a postdoctoral researcher in mechanical and aerospace engineering; and Roberto Myers, an associate professor of materials science and engineering, electrical and computer engineering and physics.

    Funding for the study came from the U.S. Army Research Office, the U.S. Air Force Office of Scientific Research and the National Science Foundation (NSF), including funds from the NSF Materials Research Science and Engineering Center at Ohio State. Computing resources were provided by the Ohio Supercomputer Center.

    See the full article here.

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  • richardmitnick 8:53 am on March 7, 2015 Permalink | Reply
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    From EPFL Lausanne: “Graphene Meets Heat Waves” 

    EPFL bloc

    Ecole Polytechnique Federale Lausanne

    Laure-Anne Pessina


    EPFL researchers have shed new light on the fundamental mechanisms of heat dissipation in graphene and other two-dimensional materials. They have shown that heat can propagate as a wave over very long distances. This is key information for engineering the electronics of tomorrow.

    In the race to miniaturize electronic components, researchers are challenged with a major problem: the smaller or the faster your device, the more challenging it is to cool it down. One solution to improve the cooling is to use materials with very high thermal conductivity, such as graphene, to quickly dissipate heat and thereby cool down the circuits.

    At the moment, however, potential applications are facing a fundamental problem: how does heat propagate inside these sheets of materials that are no more than a few atoms thick?

    In a study published in Nature Communications, a team of EPFL researchers has shed new light on the mechanisms of thermal conductivity in graphene and other two-dimensional materials. They have demonstrated that heat propagates in the form of a wave, just like sound in air. This was up to now a very obscure phenomenon observed in few cases at temperatures close to the absolute zero.Their simulations provide a valuable tool for researchers studying graphene, whether to cool down circuits at the nanoscale, or to replace silicon in tomorrow’s electronics.

    Quasi-Lossless Propagation

    If it has been difficult so far to understand the propagation of heat in two-dimensional materials, it is because these sheets behave in unexpected ways compared to their three-dimensional cousins. In fact, they are capable of transferring heat with extremely limited losses, even at room temperature.

    Generally, heat propagates in a material through the vibration of atoms. These vibrations are are called “phonons“, and as heat propagates though a three-dimensional material, these phonons keep colliding with each other, merging together, or splitting. All these processes can limit the conductivity of heat along the way. Only under extreme conditions, when temperature goes close to the absolute zero ( -200 0C or lower), it is possible to observe quasi-lossless heat transfer.

    A wave of quantum heat

    The situation is very different in two dimensional materials, as shown by researchers at EPFL. Their work demonstrates that heat can propagate without significant losses in 2D even at room temperature, thanks to the phenomenon of wave-like diffusion, called “second sound“. In that case, all phonons march together in unison over very long distances. “Our simulations, based on first-principles physics, have shown that atomically thin sheets of materials behave, even at room temperature, in the same way as three-dimensional materials at extremely low temperatures” says Andrea Cepellotti, the first author of the study. “We can show that the thermal transport is described by waves, not only in graphene but also in other materials that have not been studied yet,” explains Cepellotti. “This is an extremely valuable information for engineers, who could adapt the design of future electronic components using some of these novel two-dimensional materials properties.”

    See the full article here.

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    EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

  • richardmitnick 12:57 pm on May 15, 2014 Permalink | Reply
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    From SLAC Lab: “Exploring Heat and Energy at the Smallest Scales” 

    SLAC Lab

    May 14, 2014
    Glenn Roberts Jr.

    Special low-alpha operating period enables precise measurement of changes in material

    In a recent experiment at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), scientists “tickled” atoms to explore the flow of heat and energy across materials at ultrasmall scales. The experiment, detailed in the May 6 edition of Structural Dynamics, enabled them to see subtle light-driven changes in the atomic structure of thin materials, relevant to thermoelectric and electronic devices.

    “These results show that we can really follow the flow of energy across nanoscale devices, and resolve the dynamics in a way that hasn’t been possible before. It opens the door to new, more efficient types of devices,” said research team member Aaron Lindenberg, an assistant professor at SLAC and Stanford affiliated with the Stanford PULSE Institute and the Stanford Institute for Materials and Energy Sciences [SIMES].

    Striking superthin materials with specially timed X-ray and laser pulses fired at a rate of more than one million times per second, scientists caused atoms to vibrate and measured their movement with accuracy down to a fraction of a femtometer, which is a billion-billionth of a meter.

    “We were able to see remarkably small structural changes that we had never envisioned we could,” said Michael Kozina, a graduate student with the Stanford PULSE Institute, a joint institute of SLAC and Stanford, who led the research.

    Researchers observed a longer-than-expected time delay, measured at about a billionth of a second, in the transfer of heat from the thin films to the surface below.

    The cause of this delay has important implications for materials research, Kozina said. “In electronic devices, you want to dissipate the heat as fast as you can, and in thermoelectric devices you want to maintain that delay as long as you can and prevent heat from flowing rapidly,” he added. “Now we have a way to directly look at this.”

    An optical laser casts a green glow during a low-alpha-mode experiment at SSRL. (Aaron Lindenberg/SLAC)

    A view of a materials science experimental setup at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL). The circular instrument that frames this photo is part of a diffractometer that was used to align samples and a detector with X-rays. The metallic cylinders are motors used to align the samples. The blue box is one of the X-ray detectors used in the experiment. (Mike Kozina/SLAC)

    The response of the materials to the rapid-fire laser pulses, which was too fast to be measured for each individual pulse, was averaged out over time.

    The experiment was performed during a series of special operating periods at SSRL known as low-alpha mode, in which the accelerator ring that feeds X-rays to SSRL experiments is tuned to produce shorter-than-usual pulses, measured in trillionths of a second, and its electric current is dialed down. SSRL is one of just a few synchrotrons in the world to run in low-alpha mode.

    An optical laser interacts with a thin-film material in an experiment at SLAC’s Stanford Synchrotron Radiation Lightsource. The circular instrument is part of an X-ray diffractometer, and the bright light toward the middle of the photo is a view of the laser light striking the sample. The other bright spot in this image, at upper left, is produced by laser light glaring on an X-ray detector. In this experiment, laser pulses were synchronized with rapid-fire X-ray pulses to study very slight atomic-scale changes in samples. (Mike Kozina/SLAC)

    “Short-pulse research is an important component in SSRL’s science strategy and provides capabilities that are complementary to the Linac Coherent Light Source,” SLAC’s X-ray laser, said Piero Pianetta, acting director of SSRL.

    Green laser light is visible in an experimental setup at SLAC’s SSRL. Infared laser light was “frequency-doubled” to produce this green laser light. The large apparatus on the left is an X-ray diffractometer that was used to align the sample and detector with X-rays. (Mike Kozina/SLAC)

    Kozina said low-alpha-mode experiments are complementary to other research the group has conducted at LCLS and using other tools, because they allow researchers to probe very slight processes in materials and don’t require jarring the material with higher-energy pulses to get a measurable response. “It’s like the difference between tickling the atomic structure in the samples versus hitting it with a hammer,” he said.

    The findings from this experiment, which explored films of bismuth, bismuth ferrite and PZT (a blend containing lead, zirconium and titanium) measuring just billionths of an inch thick, mark the first journal-published scientific results obtained during low-alpha-mode operations at SSRL.

    A next step in the research is to study different alignments of the samples with respect to the surface they rest on to measure whether those changes slow or speed the transfer of heat and charge, Kozina said.

    SSRL has three scheduled periods each year, each spanning a few days, for low-alpha mode, and Kozina said that the latest research is the culmination of a handful of experimental runs over the course of several years. “Incremental successes have finally reached the threshold of experimental success,” he said, “The goal is to make this operating mode more turn-key and open it up to visiting researchers.”

    See the full article here.

    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

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  • richardmitnick 2:35 pm on April 17, 2014 Permalink | Reply
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    From Cornell: “Tiny tool measures heat at the nanoscale” 

    Cornell Bloc

    Cornell University

    Feb. 26, 2014
    Story Contacts
    Cornell Chronicle

    Anne Ju

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    Syl Kacapyr



    How heat flows at the nanoscale can be very different than at larger scales. Understanding how surfaces affect the transport of the fundamental units of heat, called phonons, could impact everything from thermoelectric materials to microelectronic cooling devices.

    Design of the spectrometer to probe phonon transmission through silicon nanosheet arrays.

    Cornell researchers have developed a new way to precisely measure the extremely subtle movement of heat in nanostructures. Recently published online in Nano Letters and highlighted in Physics Today, the study features the researchers’ phonon spectrometer, whose measurements are 10 times sharper than standard methods. This boosted sensitivity has uncovered never-before-seen effects of phonon transport.

    The scientists used the new instrument to directly measure the surface scattering of phonons in silicon nanosheets. They made nanosheets only 100 nanometers wide, which is 1,000 times thinner than a human hair, using special tools at the Cornell NanoScale Science and Technology Facility (CNF) – a key component in the success of their project, said senior author Richard Robinson, assistant professor of materials science and engineering.

    The scattering of phonons on surfaces influences how well heat can flow through a structure. Similar to how light bounces off a lake, if a surface is smooth, phonons reflect off it, but when surfaces are rough phonons scatter in random directions, called diffuse scattering.

    “If waters are calm you see a reflection, but in choppy waters you see diffuse scattering,” said Jared Hertzberg, the paper’s first author, a former postdoctoral associate. “This diffuse scattering slows down the transmission of phonons. This decrease in phonon transport becomes particularly important in nanoscale materials where surfaces play a larger role in the heat flow.”

    Precise experimental techniques for probing phonon surface interactions – which depend on surface roughness and phonon wavelength – are lacking, Robinson said.

    “The fundamental science of heat flow is not as well understood in nanostructures as it is in bulk materials,” Robinson said. “If we can precisely understand how this process works, then we can begin to engineer heat flow at the nanoscale, which can lead to more efficient alternate energy applications, such as thermoelectrics, or advanced phononic heat-logic circuits. We’ve just scratched the surface, so to speak, of how heat behaves at the nanoscale. There’s so much more to learn, and so much more that can be done with these phonons now that we know how to spectroscopically measure them.”

    The researchers fabricated silicon nanosheets and measured phonon transmission rates with their spectrometer, and gauged the nanosheets’ surface roughness using atomic force microscopy. By comparing transmission rates with those predicted by theory, they could assess the validity of a 50-year-old theory called the Casimir-Ziman theory, which determines the probability of phonon scattering based on surface roughness and the phonon’s wavelength. While a perfectly smooth surface will reflect phonons perfectly, and a perfectly rough surface randomly scatters phonons in all directions, real surfaces fall somewhere in between.

    Yet the scientists found, in fact, that the total diffusive scattering occurred at much lower frequencies than had been previously predicted by the Casmimir-Ziman theory.

    Since diffusive scattering effectively lowers phonon transmission, high phonon scattering rates have implications for thermal conductivity in nanostructures: The actual thermal conductance will be much lower than predicted using the standard Casimir-Ziman theory.

    The paper, Direct Measurements of Surface Scattering in Si Nanosheets using a Microscale Phonon Spectrometer: Implications for Casimir-Limit Predicted by Ziman Theory, also co-authored by graduate students Mahmut Aksit and Obafemi Otelaja and Derek Stewart, a CNF senior research associate, was supported by the National Science Foundation and the Department of Energy, Office of Basic Energy Science.

    See the full article here.

    Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.

    Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.

    On the Ithaca campus alone nearly 20,000 students representing every state and 120 countries choose from among 4,000 courses in 11 undergraduate, graduate, and professional schools. Many undergraduates participate in a wide range of interdisciplinary programs, play meaningful roles in original research, and study in Cornell programs in Washington, New York City, and the world over.

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