Tag Archives: heat

A different type of ‘smart’ window with a new solar cell technology

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I always like a ‘smart’ window story. Given my issues with summer (I don’t like the heat), anything which promises to help reduce the heat in my home at that time of year, has my vote. Unfortunately, solutions don’t seem to have made a serious impact on the marketplace. Nonetheless, there’s always hope and perhaps this development at Princeton University will be the one to break through the impasse. From a June 30, 2017 news item on ScienceDaily,

Smart windows equipped with controllable glazing can augment lighting, cooling and heating systems by varying their tint, saving up to 40 percent in an average building’s energy costs.

These smart windows require power for operation, so they are relatively complicated to install in existing buildings. But by applying a new solar cell technology, researchers at Princeton University have developed a different type of smart window: a self-powered version that promises to be inexpensive and easy to apply to existing windows. This system features solar cells that selectively absorb near-ultraviolet (near-UV) light, so the new windows are completely self-powered.

A June 30, 2017 Princeton University news release, which originated the news item, expands on the theme,

“Sunlight is a mixture of electromagnetic radiation made up of near-UV rays, visible light, and infrared energy, or heat,” said Yueh-Lin (Lynn) Loo, director of the Andlinger Center for Energy and the Environment, and the Theodora D. ’78 and William H. Walton III ’74 Professor in Engineering. “We wanted the smart window to dynamically control the amount of natural light and heat that can come inside, saving on energy cost and making the space more comfortable.”

The smart window controls the transmission of visible light and infrared heat into the building, while the new type of solar cell uses near-UV light to power the system.

“This new technology is actually smart management of the entire spectrum of sunlight,” said Loo, who is a professor of chemical and biological engineering. Loo is one of the authors of a paper, published June 30, that describes this technology, which was developed in her lab.

Because near-UV light is invisible to the human eye, the researchers set out to harness it for the electrical energy needed to activate the tinting technology.

“Using near-UV light to power these windows means that the solar cells can be transparent and occupy the same footprint of the window without competing for the same spectral range or imposing aesthetic and design constraints,” Loo added. “Typical solar cells made of silicon are black because they absorb all visible light and some infrared heat – so those would be unsuitable for this application.”

In the paper published in Nature Energy, the researchers described how they used organic semiconductors – contorted hexabenzocoronene (cHBC) derivatives – for constructing the solar cells. The researchers chose the material because its chemical structure could be modified to absorb a narrow range of wavelengths – in this case, near-UV light. To construct the solar cell, the semiconductor molecules are deposited as thin films on glass with the same production methods used by organic light-emitting diode manufacturers. When the solar cell is operational, sunlight excites the cHBC semiconductors to produce electricity.

At the same time, the researchers constructed a smart window consisting of electrochromic polymers, which control the tint, and can be operated solely using power produced by the solar cell. When near-UV light from the sun generates an electrical charge in the solar cell, the charge triggers a reaction in the electrochromic window, causing it to change from clear to dark blue. When darkened, the window can block more than 80 percent of light.

Nicholas Davy, a doctoral student in the chemical and biological engineering department and the paper’s lead author, said other researchers have already developed transparent solar cells, but those target infrared energy. However, infrared energy carries heat, so using it to generate electricity can conflict with a smart window’s function of controlling the flow of heat in or out of a building. Transparent near-UV solar cells, on the other hand, don’t generate as much power as the infrared version, but don’t impede the transmission of infrared radiation, so they complement the smart window’s task.

Davy said that the Princeton team’s aim is to create a flexible version of the solar-powered smart window system that can be applied to existing windows via lamination.

“Someone in their house or apartment could take these wireless smart window laminates – which could have a sticky backing that is peeled off – and install them on the interior of their windows,” said Davy. “Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort, and privacy.”

Joseph Berry, senior research scientist at the National Renewable Energy Laboratory, who studies solar cells but was not involved in the research, said the research project is interesting because the device scales well and targets a specific part of the solar spectrum.

“Integrating the solar cells into the smart windows makes them more attractive for retrofits and you don’t have to deal with wiring power,” said Berry. “And the voltage performance is quite good. The voltage they have been able to produce can drive electronic devices directly, which is technologically quite interesting.”

Davy and Loo have started a new company, called Andluca Technologies, based on the technology described in the paper, and are already exploring other applications for the transparent solar cells. They explained that the near-UV solar cell technology can also power internet-of-things sensors and other low-power consumer products.

“It does not generate enough power for a car, but it can provide auxiliary power for smaller devices, for example, a fan to cool the car while it’s parked in the hot sun,” Loo said.

Here’s a link to and a citation for the paper,

Pairing of near-ultraviolet solar cells with electrochromic windows for smart management of the solar spectrum by Nicholas C. Davy, Melda Sezen-Edmonds, Jia Gao, Xin Lin, Amy Liu, Nan Yao, Antoine Kahn, & Yueh-Lin Loo. Nature Energy 2, Article number: 17104 (2017 doi:10.1038/nenergy.2017.104 Published online: 30 June 2017

This paper is behind a paywall.

Here’s what a sample of the special glass looks like,

Graduate student Nicholas Davy holds a sample of the special window glass. (Photos by David Kelly Crow)

Nano-chimneys to cut down heat

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Heat is always a problem with electronics—even nanoelectronics. Scientists at Rice University (US) believe they may have a solution for nanoelectronics heat problems, according to a Jan. 4, 2017 news item on ScienceDaily,

A few nanoscale adjustments may be all that is required to make graphene-nanotube junctions excel at transferring heat, according to Rice University scientists.

The Rice lab of theoretical physicist Boris Yakobson found that putting a cone-like “chimney” between the graphene and [carbon] nanotube all but eliminates a barrier that blocks heat from escaping.

Caption: Simulations by Rice University scientists show that placing cones between graphene and carbon nanotubes could enhance heat dissipation from nano-electronics. The nano-chimneys become better at conducting heat-carrying phonons by spreading out the number of heptagons required by the graphene-to-nanotube transition. Credit: Alex Kutana/Rice University

A Jan. 4, 2016 Rice University news release (also on EurekAlert), which originated the news item, describes the research in more detail,

Heat is transferred through phonons, quasiparticle waves that also transmit sound. The Rice theory offers a strategy to channel damaging heat away from next-generation nano-electronics.

Both graphene and carbon nanotubes consist of six-atom rings, which create a chicken-wire appearance, and both excel at the rapid transfer of electricity and phonons.

But when a nanotube grows from graphene, atoms facilitate the turn by forming heptagonal (seven-member) rings instead. Scientists have determined that forests of nanotubes grown from graphene are excellent for storing hydrogen for energy applications, but in electronics, the heptagons scatter phonons and hinder the escape of heat through the pillars.

The Rice researchers discovered through computer simulations that removing atoms here and there from the two-dimensional graphene base would force a cone to form between the graphene and the nanotube. The geometric properties (aka topology) of the graphene-to-cone and cone-to-nanotube transitions require the same total number of heptagons, but they are more sparsely spaced and leave a clear path of hexagons available for heat to race up the chimney.

“Our interest in advancing new applications for low-dimensional carbon — fullerenes, nanotubes and graphene — is broad,” Yakobson said. “One way is to use them as building blocks to fill three-dimensional spaces with different designs, creating anisotropic, nonuniform scaffolds with properties that none of the current bulk materials have. In this case, we studied a combination of nanotubes and graphene, connected by cones, motivated by seeing such shapes obtained in our colleagues’ experimental labs.”

The researchers tested phonon conduction through simulations of free-standing nanotubes, pillared graphene and nano-chimneys with a cone radius of either 20 or 40 angstroms. The pillared graphene was 20 percent less conductive than plain nanotubes. The 20-angstrom nano-chimneys were just as conductive as plain nanotubes, while 40-angstrom cones were 20 percent better than the nanotubes.

“The tunability of such structures is virtually limitless, stemming from the vast combinatorial possibilities of arranging the elementary modules,” said Alex Kutana, a Rice research scientist and co-author of the study. “The actual challenge is to find the most useful structures given a vast number of possibilities and then make them in the lab reliably.

“In the present case, the fine-tuning parameters could be cone shapes and radii, nanotube spacing, lengths and diameters. Interestingly, the nano-chimneys also act like thermal diodes, with heat flowing faster in one direction than the other,” he said.

Here’s a link to and a citation for the paper,

Nanochimneys: Topology and Thermal Conductance of 3D Nanotube–Graphene Cone Junctions by Ziang Zhang, Alex Kutana, Ajit Roy, and Boris I. Yakobson. J. Phys. Chem. C, Article ASAP DOI: 10.1021/acs.jpcc.6b11350 Publication Date (Web): December 21, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall.

Carbon nanotubes, acoustics, and heat

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I have a longstanding interest in carbon nanotubes and acoustics, which I first encountered in 2008. This latest work comes from the Michigan Technological University according to a July 28, 2015 news item on Nanowerk,

Troy Bouman reaches over, presses play, and the loudspeaker sitting on the desk starts playing the university fight song. But this is no ordinary loudspeaker. This is a carbon nanotube transducer—and it makes sound with heat.

Bouman and Mahsa Asgarisabet, both graduate students at Michigan Technological University, recently won a Best of Show Award at SAE International’s Noise and Vibration Conference and Exhibition 2015 for their acoustic research on carbon nanotube speakers. They work with Andrew Barnard, an assistant professor of mechanical engineering at Michigan Tech, to tease out the fundamental physics of these unusual loudspeakers.

While still a fledgling technology, the potential applications are nearly endless. Everything from de-icing helicopter blades to making lighter loudspeakers to doubling as a car speaker and heating filament for back windshield defrosters.

Here’s a few sound sound files featuring the students and their carbon nanotube speakers,


A July 28, 2015 Michigan Technological University news release, which originated the news item, goes on to describe how these carbon nanotubes are making sound,

The freestanding speaker itself is rather humble. In fact, it’s a bit flimsy. A teflon base props up two copper rods, and what seems like a see-through black cloth stretches between them.

“A little wind gust across them, and they would just blow away,” Barnard says. “But you could shake them as much as you want—since they have such low mass, there is virtually no inertia.”

The material is strong side to side, because what the naked eye can’t see is the collection of black nanotubes that make up that thin film.

The nanotubes are straw-like structures with walls only one carbon atom-thick and they can heat up and cool down up to 100,000 times each second. By comparison, a platinum sheet about 700 nanometers thick can only heat up and cool down about 16 times each second. The heating and cooling of the carbon nanotubes causes the adjacent air to expand and contract. That pushes air molecules around and creates sound waves.

“Traditional speakers use a moving coil, and that’s how they create sound waves,” Bouman says. “There are completely different physics behind carbon nanotube speakers.”

And because of these differences, the nearly weightless carbon nanotube speakers produce sound in a way that isn’t initially understood by our ears. Bouman’s research focuses on processing the sound waves to make them more intelligible. Take a listen.

Acoustics

To date, most research on carbon nanotubes has been on the materials side. Carbon nanotube speakers were discovered accidently in 2008, showing that the idea was viable. As mechanical engineers studying acoustics, Barnard, Bouman and Asgarisabet are refining the technology.

“They are very light weight and have no moving parts,” Asgarisabet says, which is ideal for her work in active noise control, where the carbon nanotube films could cancel out engine noise in airplanes or road noise in cars. But first, she says, “I want to focus first on getting a good thermal model of the speakers.”

Having an accurate model, Bouman adds, is a reflection of understanding the carbon nanotube loudspeakers themselves. The modeling work he and Asgarisabet are doing lays down the foundation to build up new applications for the technology.

While a lot of research remains on sorting out the underlying physics of carbon nanotube speakers, being able to use both the heat and sound properties makes them versatile. The thinness and weightlessness is also appealing.

“They’re basically conformable speakers,” Barnard says. The thin film could be draped over dashboards, windows, walls, seats and maybe even clothing. To get the speakers to that point, Barnard and his students will continue refining the technology’s efficiency and ruggedness, one carbon nanotube thin-film at a time.

As I mentioned earlier I’m quite interested in carbon nanotubes speakers and, for that matter, all other nanomaterial speakers. For example, there was a November 18, 2013 posting titled: World’s* smallest FM radio transmitter made out of graphene which also featured the Zettl Group’s (University of California at Berkeley) carbon nanotube radio (unfortunately those sound files are no longer accessible).

Dexter Johnson in a July 30, 2015 posting (on his Nanoclast blog on the Institute of Electrical and Electronics Engineers [IEEE] website) provides some additional insights (Note: Links have been removed),

It’s been some time since we covered the use of nanomaterials in audio speakers. While not a hotly pursued research field, there is some tradition for it dating back to the first development of carbon nanotube-based speakers in 2008. While nanomaterial-based speakers are not going to win any audiophile prize anytime soon, they do offer some unusual characteristics that mainly stem from their magnet-less design.

Heat, evolution, and the shape of gold nanorods

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A Feb. 23, 2015 news item on Azonano features gold nanorods and their shapeshifting ways when releasing heat,

Researchers at the U.S. Department of Energy’s Argonne National Laboratory have revealed previously unobserved behaviors that show how details of the transfer of heat at the nanoscale cause nanoparticles to change shape in ensembles.

The new findings depict three distinct stages of evolution in groups of gold nanorods, from the initial rod shape to the intermediate shape to a sphere-shaped nanoparticle. The research suggests new rules for the behavior of nanorod ensembles, providing insights into how to increase heat transfer efficiency in a nanoscale system.

A Feb. 18, 2015 Argonne National Laboratory news release by Justin H. S. Breaux, which originated the news item, provides more details about the work,

At the nanoscale, individual gold nanorods have unique electronic, thermal and optical properties. Understanding these properties and managing how collections of these elongated nanoparticles absorb and release this energy as heat will drive new research towards next-generation technologies such as water purification systems, battery materials and cancer research.

A good deal is known about how single nanorods behave—but little is known about how nanorods behave in ensembles of millions. Understanding how the individual behavior of each nanorod, including how its orientation and rate of transition differ from those around it, impacts the collective kinetics of the ensemble and is critical to using nanorods in future technologies.

“We started with a lot of questions,” said Argonne physicist Yuelin Li, “like ‘How much power can the particles sustain before losing functionality? How do individual changes at the nanoscale affect the overall functionality? How much heat is released to the surrounding area?’ Each nanorod is continuously undergoing a change in shape when heated beyond melting temperature, which means a change in the surface area and thus a change in its thermal and hydrodynamic properties.”

The researchers used a laser to heat the nanoparticles and X-rays to analyze their changing shapes. Generally, nanorods transition into nanospheres more quickly when supplied with a higher intensity of laser power. In this case, completely different ensemble behaviors were observed when this intensity increased incrementally. The intensity of the heat applied changes not only the nanoparticles’ shape at various rates but also affects their ability to efficiently absorb and release heat.

“For us, the key was to understand just how efficient the nanorods were at transferring light into heat in many different scenarios,” said nanoscientist Subramanian Sankaranarayanan of Argonne’s Center for Nanoscale Materials. “Then we had to determine the physics behind how heat was transferred and all the different ways these nanorods could transition into nanospheres.”

To observe how the rod makes this transition, researchers first shine a laser pulse at the nanorod suspended in a water solution at Argonne’s Advanced Photon Source. The laser lasts for less than a hundred femtoseconds, nearly one trillion times faster than a blink of the eye. What follows is a series of focused and rapid X-ray bursts using a technique called small angle X-ray scattering. The resulting data is used to determine the average shape of the particle as it changes over time.

In this way, scientists can reconstruct the minute changes occurring in the shape of the nanorod. However, to understand the physics underlying this phenomenon, the researchers needed to look deeper at how individual atoms vibrate and move during the transition. For this, they turned to the field of molecular dynamics using the supercomputing power of the 10-petaflop Mira supercomputer at the Argonne Leadership Computing Facility.

Mira used mathematical equations to pinpoint the individual movements of nearly two million of the nanorods’ atoms in the water. Using factors such as the shape, temperature and rate of change, the researchers built simulations of the nanorod in many different scenarios to see how the structure changes over time.

“In the end,” said Sankaranarayanan, “we discovered the heat transfer rates for shorter but wider nanospheres are lower than for their rod-shaped predecessors. This decrease in heat transfer efficiency at the nanoscale plays a key role in accelerating the transition from rod to sphere when heated beyond the melting temperature.”

Here’s a link to and a citation for the paper,

Femtosecond Laser Pulse Driven Melting in Gold Nanorod Aqueous Colloidal Suspension: Identification of a Transition from Stretched to Exponential Kinetics by Yuelin Li, Zhang Jiang, Xiao-Min Lin, Haidan Wen, Donald A. Walko, Sanket A. Deshmukh, Ram Subbaraman, Subramanian K. R. S. Sankaranarayanan, Stephen K. Gray, & Phay Ho. Scientific Reports 5, Article number: 8146 doi:10.1038/srep08146 Published 30 January 2015

This article is open access.

Cooling it—an application using carbon nanotubes and a theory that hotter leads to cooler

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The only thing these two news items have in common is their focus on cooling down electronic devices. Well, there’s also the fact that the work is being done at the nanoscale.

First, there’s a Jan. 23, 2014 news item on Azonano about a technique using carbon nanotubes to cool down microprocessors,

“Cool it!” That’s a prime directive for microprocessor chips and a promising new solution to meeting this imperative is in the offing. Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a “process friendly” technique that would enable the cooling of microprocessor chips through carbon nanotubes.

Frank Ogletree, a physicist with Berkeley Lab’s Materials Sciences Division, led a study in which organic molecules were used to form strong covalent bonds between carbon nanotubes and metal surfaces. This improved by six-fold the flow of heat from the metal to the carbon nanotubes, paving the way for faster, more efficient cooling of computer chips. The technique is done through gas vapor or liquid chemistry at low temperatures, making it suitable for the manufacturing of computer chips.

The Jan. 22, 2014 Berkeley Lab news release (also on EurekAlert), which originated the news item, describes the nature  of the problem in more detail,

Overheating is the bane of microprocessors. As transistors heat up, their performance can deteriorate to the point where they no longer function as transistors. With microprocessor chips becoming more densely packed and processing speeds continuing to increase, the overheating problem looms ever larger. The first challenge is to conduct heat out of the chip and onto the circuit board where fans and other techniques can be used for cooling. Carbon nanotubes have demonstrated exceptionally high thermal conductivity but their use for cooling microprocessor chips and other devices has been hampered by high thermal interface resistances in nanostructured systems.

“The thermal conductivity of carbon nanotubes exceeds that of diamond or any other natural material but because carbon nanotubes are so chemically stable, their chemical interactions with most other materials are relatively weak, which makes for  high thermal interface resistance,” Ogletree says. “Intel came to the Molecular Foundry wanting to improve the performance of carbon nanotubes in devices. Working with Nachiket Raravikar and Ravi Prasher, who were both Intel engineers when the project was initiated, we were able to increase and strengthen the contact between carbon nanotubes and the surfaces of other materials. This reduces thermal resistance and substantially improves heat transport efficiency.”

The news release then describes the proposed solution,

Sumanjeet Kaur, lead author of the Nature Communications paper and an expert on carbon nanotubes, with assistance from co-author and Molecular Foundry chemist Brett Helms, used reactive molecules to bridge the carbon nanotube/metal interface – aminopropyl-trialkoxy-silane (APS) for oxide-forming metals, and cysteamine for noble metals. First vertically aligned carbon nanotube arrays were grown on silicon wafers, and thin films of aluminum or gold were evaporated on glass microscope cover slips. The metal films were then “functionalized” and allowed to bond with the carbon nanotube arrays. Enhanced heat flow was confirmed using a characterization technique developed by Ogletree that allows for interface-specific measurements of heat transport.

“You can think of interface resistance in steady-state heat flow as being an extra amount of distance the heat has to flow through the material,” Kaur says. “With carbon nanotubes, thermal interface resistance adds something like 40 microns of distance on each side of the actual carbon nanotube layer. With our technique, we’re able to decrease the interface resistance so that the extra distance is around seven microns at each interface.”

Although the approach used by Ogletree, Kaur and their colleagues substantially strengthened the contact between a metal and individual carbon nanotubes within an array, a majority of the nanotubes within the array may still fail to connect with the metal. The Berkeley team is now developing a way to improve the density of carbon nanotube/metal contacts. Their technique should also be applicable to single and multi-layer graphene devices, which face the same cooling issues.

For anyone who’s never heard of the Molecular Foundry before (from the news release),

The Molecular Foundry is one of five DOE [Department of Energy] Nanoscale Science Research Centers (NSRCs), national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize, and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos national laboratories.

My second item comes from the University of Buffalo (UB), located in the US. From a Jan. 21, 2014 University of Buffalo news release by Cory Nealon (also on EurekAlert),

Heat in electronic devices is generated by the movement of electrons through transistors, resistors and other elements of an electrical network. Depending on the network, there are a variety of ways, such as cooling fans and heat sinks, to prevent the circuits from overheating.

But as more integrated circuits and transistors are added to devices to boost their computing power, it’s becoming more difficult to keep those elements cool. Most nanoelectrics research centers are working to develop advanced materials that are capable of withstanding the extreme environment inside smartphones, laptops and other devices.

While advanced materials show tremendous potential, the UB research suggests there may still be room within the existing paradigm of electronic devices to continue developing more powerful computers.

To support their findings, the researchers fabricated nanoscale semiconductor devices in a state-of-the-art gallium arsenide crystal provided to UB by Sandia’s Reno [John L. Reno, Center for Integrated Nanotechnologies at Sandia National Laboratories]. The researchers then subjected the chip to a large voltage, squeezing an electrical current through the nanoconductors. This, in turn, increased the amount of heat circulating through the chip’s nanotransistor.

But instead of degrading the device, the nanotransistor spontaneously transformed itself into a quantum state that was protected from the effect of heating and provided a robust channel of electric current. To help explain, Bird [Jonathan Bird, UB professor of electrical engineering] offered an analogy to Niagara Falls.

“The water, or energy, comes from a source; in this case, the Great Lakes. It’s channeled into a narrow point (the Niagara River) and ultimately flows over Niagara Falls. At the bottom of waterfall is dissipated energy. But unlike the waterfall, this dissipated energy recirculates throughout the chip and changes how heat affects, or in this case doesn’t affect, the network’s operation.”

While this behavior may seem unusual, especially conceptualizing it in terms of water flowing over a waterfall, it is the direct result of the quantum mechanical nature of electronics when viewed on the nanoscale. The current is made up of electrons which spontaneously organize to form a narrow conducting filament through the nanoconductor. It is this filament that is so robust against the effects of heating.

“We’re not actually eliminating the heat, but we’ve managed to stop it from affecting the electrical network. In a way, this is an optimization of the current paradigm,” said Han [J. E. Han, UB Dept. of Physics], who developed the theoretical models which explain the findings.

What an interesting and counter-intuitive approach to managing the heat in our devices.

For those who want more, here’s a link to and citation for the carbon nanotube paper,

Enhanced thermal transport at covalently functionalized carbon nanotube array interfaces by Sumanjeet Kaur, Nachiket Raravikar, Brett A. Helms, Ravi Prasher, & D. Frank Ogletree. Nature Communications 5, Article number: 3082 doi:10.1038/ncomms4082 Published 22 January 2014

This paper is behind a paywall.

Now here’s a link to and a citation for the ‘making it hotter to make it cooler’ paper,

Formation of a protected sub-band for conduction in quantum point contacts under extreme biasing by J. Lee, J. E. Han, S. Xiao, J. Song, J. L. Reno, & J. P. Bird. Nature Nanotechnology (2014) doi:10.1038/nnano.2013.297 Published online 19 January 2014

This paper is behind a paywall although there is an option to preview it for free via ReadCube Access.