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  • richardmitnick 12:57 pm on May 15, 2014 Permalink | Reply
    Tags: , Heat Studies, ,   

    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.”

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

    ssrl
    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.

    lode
    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
    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
    Tags: , , Heat Studies,   

    From Cornell: “Tiny tool measures heat at the nanoscale” 

    Cornell Bloc

    Cornell University

    Feb. 26, 2014
    Story Contacts
    Cornell Chronicle

    Anne Ju

    607- 255-9735

    amj8@cornell.edu
    Media Contact

    Syl Kacapyr

    607-255-7701

    vpk6@cornell.edu

    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.

    heat
    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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