Category: Electronics

UT Dallas scientists have constructed novel fibers by wrapping sheets of tiny carbon nanotubes to form a sheath around a long rubber core. This illustration shows complex two-dimensional buckling, shown in yellow, of the carbon nanotube sheath/rubber-core fiber. The buckling results in a conductive fiber with super elasticity and novel electronic properties. (credit: UT Dallas Alan G. MacDiarmid Nanotech Institute)

An international research team based at The University of Texas at Dallas has made electrically conducting fibers that can be reversibly stretched to more than 14 times their initial length and whose electrical conductivity increases 200-fold when stretched.

The research team is using the new fibers to make artificial muscles, as well as capacitors with energy storage capacity that increases about tenfold when the fibers are stretched.

Fibers and cables derived from the invention might one day be used as interconnects for super-elastic electronic circuits, robots and exoskeletons having great reach, morphing aircraft, giant-range strain sensors, failure-free pacemaker leads, and super-stretchy charger cords for electronic devices.

Wrapping carbon nanotube sheets into fibers

In a study published in the July 24 issue of the journal Science, the scientists describe how they constructed the fibers by wrapping lighter-than-air, electrically conductive sheets of tiny carbon nanotubes to form a jelly-roll-like sheath around a long rubber core.

The new fibers differ from conventional materials in several ways. For example, when conventional fibers are stretched, the resulting increase in length and decrease in cross-sectional area restricts the flow of electrons through the material. But even a “giant” stretch of the new conducting sheath-core fibers causes little change in their electrical resistance, said Dr. Ray Baughman, senior author of the paper and director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas.

One key to the performance of the new conducting elastic fibers is the introduction of buckling into the carbon nanotube sheets. Because the rubber core is stretched along its length as the sheets are being wrapped around it, when the wrapped rubber relaxes, the carbon nanofibers form a complex buckled structure, which allows for repeated stretching of the fiber.

“Think of the buckling that occurs when an accordion is compressed, which makes the inelastic material of the accordion stretchable,” said Baughman, the Robert A. Welch Distinguished Chair in Chemistry at UT Dallas.

“We make the inelastic carbon nanotube sheaths of our sheath-core fibers super stretchable by modulating large buckles with small buckles, so that the elongation of both buckle types can contribute to elasticity. These amazing fibers maintain the same electrical resistance, even when stretched by giant amounts, because electrons can travel over such a hierarchically buckled sheath as easily as they can traverse a straight sheath.”

Radical electronic and mechanical devices possible

By adding a thin overcoat of rubber to the sheath-core fibers and then another carbon nanotube sheath, the researchers made strain sensors and artificial muscles in which the buckled nanotube sheaths serve as electrodes and the thin rubber layer is a dielectric, resulting in a fiber capacitor. These fiber capacitors exhibited the unrivaled capacitance change of 860 percent when the fiber was stretched 950 percent.

Adding twist to these double-sheath fibers resulted in fast, electrically powered torsional — or rotating — artificial muscles that could be used to rotate mirrors in optical circuits or pump liquids in miniature devices used for chemical analysis. The conducting elastomers can be fabricated in diameters ranging from the very small — about 150 microns, or twice the width of a human hair — to much larger sizes, depending on the size of the rubber core. Individual small fibers also can be combined into large bundles and plied together like yarn or rope,” according to the researchers.

“This technology could be well-suited for rapid commercialization,” said Dr. Raquel Ovalle-Robles MS’06 PhD’08, an author on the paper and chief research and intellectual properties strategist at Lintec of America’s Nano-Science & Technology Center.

“The rubber cores used for these sheath-core fibers are inexpensive and readily available,” she said. “The only exotic component is the carbon nanotube aerogel sheet used for the fiber sheath.”

UT Dallas | UT Dallas Nanotech CNT rubber fiber

In this video, two lab demonstrations show the near invariance of resistance during the stretching of carbon-nanotube-sheathed rubber fibers.

UT Dallas Comets |UTD Nanotech pacemaker lead demo

Abstract of Hierarchically buckled sheath-core fibers for superelastic electronics, sensors, and muscles

Superelastic conducting fibers with improved properties and functionalities are needed for diverse applications. Here we report the fabrication of highly stretchable (up to 1320%) sheath-core conducting fibers created by wrapping carbon nanotube sheets oriented in the fiber direction on stretched rubber fiber cores. The resulting structure exhibited distinct short- and long-period sheath buckling that occurred reversibly out of phase in the axial and belt directions, enabling a resistance change of less than 5% for a 1000% stretch. By including other rubber and carbon nanotube sheath layers, we demonstrated strain sensors generating an 860% capacitance change and electrically powered torsional muscles operating reversibly by a coupled tension-to-torsion actuation mechanism. Using theory, we quantitatively explain the complementary effects of an increase in muscle length and a large positive Poisson’s ratio on torsional actuation and electronic properties.

UC Berkeley engineers created a “smart cap” using 3-D-printed plastic with embedded electronics to wirelessly monitor the freshness of milk (credit: Photo and schematic by Sung-Yueh Wu)

UC Berkeley engineers, in collaboration with colleagues at Taiwan’s National Chiao Tung University, have developed a 3D printing process for creating basic electronic components, such as resistors, inductors, capacitors, and integrated wireless electrical sensing systems.

As a test, they printed a wireless “smart cap” for a milk carton that detected signs of spoilage using embedded sensors.

The findings were published Monday, July 20, in a new open-access journal in the Nature Publishing Group called Microsystems & Nanoengineering.

“Our paper describes the first demonstration of 3-D printing for working basic electrical components, as well as a working wireless sensor,” said senior author Liwei Lin, a professor of mechanical engineering and co-director of the Berkeley Sensor and Actuator Center.“One day, people may simply download 3D-printing files from the Internet with customized shapes and colors and print out useful devices at home.”

Engineers created a range of 3-D-printed electrical components, including an electrical resistor, inductor, capacitor and an LC tank (integrated inductor-capacitor system) (the penny is used for scale), shown here before removal of wax (credit: Photo by Sung-Yueh Wu)

The researchers started by printing polymers and wax. They then removed the wax, leaving hollow tubes into which liquid metal —- in their experiments they used silver — was injected and then cured.

The shape and design of the metal determined the function of different electrical components. For instance, thin wires acted as resistors, and flat plates were made into capacitors.

To demonstrate their use, the researchers integrated the electronic components into a plastic milk carton cap to monitor signs of spoilage. The “smart cap” was fitted with a capacitor and an inductor to form a resonant circuit. A quick flip of the carton allowed a bit of milk to get trapped in the cap’s capacitor gap, and the entire carton was then left unopened at room temperature (about 71.6 degrees Fahrenheit) for 36 hours.

3D-printed components (credit: Sung-Yueh Wu et al./Microsystems & Nanoengineering)

The circuit could detect the changes in electrical signals that accompany increased levels of bacteria. The researchers periodically monitored the changes with a wireless radio-frequency probe at the start of the experiment and every 12 hours thereafter, up to 36 hours. The property of milk changes gradually as it degrades, leading to variations in its electrical characteristics*.

Those changes were detected wirelessly using the smart cap, which found that the peak vibration frequency of the room-temperature milk dropped by 4.3 percent after 36 hours. In comparison, a carton of milk kept in refrigeration at 39.2 degrees Fahrenheit saw a relatively minor 0.12 percent shift in frequency over the same time period.

Cheap DIY electronic circuits 

“This 3D-printing technology could eventually make electronic circuits cheap enough to be added to packaging to provide food safety alerts for consumers,” said Lin. “You could imagine a scenario where you can use your cellphone to check the freshness of food while it’s still on the store shelves.”

As 3D printers become cheaper and better, the options for electronics will expand, said Lin, though he does not think people will be printing out their own smartphones or computers anytime soon.

“That would be very difficult because of the extremely small size of modern electronics,” he said. “It might also not be practical in terms of price since current integrated circuits are made by batch fabrication to keep costs low. Instead, I see 3D-printed microelectronic devices as very promising for systems that would benefit from customization.”

Lin said his lab is working on developing this technology for health applications, such as implantable devices with embedded transducers that can monitor blood pressure, muscle strain and drug concentrations.

* As the trapped milk hardened, the dielectric constant increased, increasing the capacitance and thus decreasing the frequency.

Abstract of 3D-printed microelectronics for integrated circuitry and passive wireless sensors

Three-dimensional (3D) additive manufacturing techniques have been utilized to make 3D electrical components, such as resistors, capacitors, and inductors, as well as circuits and passive wireless sensors. Using the fused deposition modeling technology and a multiple-nozzle system with a printing resolution of 30 μm, 3D structures with both supporting and sacrificial structures are constructed. After removing the sacrificial materials, suspensions with silver particles are injected subsequently solidified to form metallic elements/interconnects. The prototype results show good characteristics of fabricated 3D microelectronics components, including an inductor–capacitor-resonant tank circuitry with a resonance frequency at 0.53 GHz. A 3D “smart cap” with an embedded inductor–capacitor tank as the wireless passive sensor was demonstrated to monitor the quality of liquid food (e.g., milk and juice) wirelessly. The result shows a 4.3% resonance frequency shift from milk stored in the room temperature environment for 36 h. This work establishes an innovative approach to construct arbitrary 3D systems with embedded electrical structures as integrated circuitry for various applications, including the demonstrated passive wireless sensors.

A 3-D structure of hexagonal boron nitride sheets and boron nitride nanotubes could be a tunable material to control heat in electronics, according to researchers at Rice University (credit: Shahsavari Group/Rice University)

Three-dimensional structures of boron nitride are a viable candidate as a tunable material to keep electronics cool, according to scientists at Rice University researchers Rouzbeh Shahsavari and Navid Sakhavand.

Their work appears this month in the American Chemical Society journal Applied Materials and Interfaces.

In its two-dimensional form, hexagonal boron nitride (h-BN), aka white graphene, looks just like the atom-thick form of carbon known as graphene. One difference: h-BN is a natural insulator, where perfect graphene presents no barrier to electricity (is a natural electrical conductor).

But like graphene, h-BN is a also a good conductor of heat, which can be quantified in the form of phonons. (Technically, a phonon is a “quasiparticle” in a collective excitation of atoms.)

“Typically in all electronics, it is highly desired to get heat out of the system as quickly and efficiently as possible,” he said. “One of the drawbacks in electronics, especially when you have layered materials on a substrate, is that heat moves very quickly in one direction, along a conductive plane, but not so good from layer to layer. Multiple stacked graphene layers is a good example of this.”

Heat moves ballistically across flat planes of boron nitride, too, but the Rice simulations showed that 3-D structures of h-BN planes connected by boron nitride nanotubes would be able to move phonons in all directions, whether in-plane or across planes, Shahsavari said.

Phonon flows

The researchers calculated how phonons would flow across four such structures with nanotubes of various lengths and densities. They found the junctions of pillars and planes significantly slowed down the flow of phonons from layer to layer, Shahsavari said. Both the length and density of the pillars had an effect on the heat flow: more and/or shorter pillars slowed conduction, while longer pillars presented fewer barriers and thus sped things along.

Researchers have already made graphene/carbon nanotube junctions, but Shahsavari believes such junctions for boron nitride materials could be just as promising. “Given the insulating properties of boron nitride, they can enable and complement the creation of 3-D, graphene-based nanoelectronics,” Shahsavari said.

“This type of 3-D thermal-management system can open up opportunities for thermal switches, or thermal rectifiers, where the heat flowing in one direction can be different than the reverse direction. This can be done by changing the shape of the material, or changing its mass — say one side is heavier than the other — to create a switch. The heat would always prefer to go one way, but in the reverse direction it would be slower.”

Abstract of Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride Nanostructures

Although Boron Nitride nanotubes (BNNT) and hexagonal-BN (h-BN) are superb one-dimensional (1D) and 2D thermal conductors respectively, bringing this quality into 3D remain elusive. Here, we focus on Pillared Boron Nitride (PBN) as a class of 3D BN allotropes and demonstrate how the junctions, pillar length and pillar distance control phonon scattering in PBN and impart tailorable thermal conductivity in 3D. Using reverse non equilibrium molecular dynamics simulations, our results indicate that while a clear phonon scattering at the junctions accounts for the lower thermal conductivity of PBN compared to its parent BNNT and h-BN allotropes, it acts as an effective design tool and provides 3D thermo-mutable features that are absent in the parent structures. Propelled by the junction spacing, while one geometrical parameter, e.g., pillar length, controls the thermal transport along the out-of-plane direction of PBN, the other parameter, e.g., pillar distance, dictates the gross cross-sectional area, which is key for design of 3D thermal management systems. Furthermore, the junctions have a more pronounced effect in creating a Kapitza effect in the out-of-plane direction, due to the change in dimensionality of the phonon transport. This work is the first report on thermo-mutable properties of hybrid BN allotropes and can potentially impact thermal management of other hybrid 3D BN architectures.

Graphene-based film on a hot electronic component (credit: Johan Liu)

A method for efficiently cooling electronics using graphene-based film — with a thermal conductivity capacity four times higher than copper — has been developed by researchers at Chalmers University of Technology. The film can be attached to computer chips and other silicon-based electronic components.

Electronic systems available today accumulate a great deal of heat, mostly due to the ever-increasing demand on functionality. Getting rid of excess heat in efficient ways is needed for chip lifespan and reduction in energy usage.

A research team led by Johan Liu, a professor at Chalmers University of Technology, originally found that graphene can have a cooling effect on silicon-based electronics, but that it’s not efficient because it’s limited to a few layers of graphene atoms. “When you try to add more layers of graphene, the graphene will no longer adhere to the surface, since the adhesion is [due to] weak van der Waals bonds,” he said.

Silane coupling between graphene and silicon. After heating and hydrolysis of (3-Aminopropyl) triethoxysilane (APTES) molecules (top right), silane coupling (bottom right) is created, providing mechanical strength and good thermal pathways (credit: Johan Liu)

The researchers solved that by creating strong covalent bonds between the graphene film and the surface. The stronger bonds result from adding (3-Aminopropyl) triethoxysilane (APTES) molecules to the film. Heating and hydrolysis then creates silane bonds between the graphene and the electronic component, doubling thermal conductivity.

“Increased thermal capacity could lead to several new [cooling] applications for graphene,” says Liu, including LEDs, lasers, and radio frequency components.

Abstract of Improved Heat Spreading Performance of Functionalized Graphene in Microelectronic Device Application

It is demonstrated that a graphene-based film (GBF) functionalized with silane molecules strongly enhances thermal performance. The resistance temperature detector results show that the inclusion of silane molecules doubles the heat spreading ability. Furthermore, molecular dynamics simulations show that the thermal conductivity (κ) of the GBF increased by 15%–56% with respect to the number density of molecules compared to that with the nonfunctionalized graphene substrate. This increase in κ is attributed to the enhanced in-plane heat conduction of the GBF, resulting from the simultaneous increase of the thermal resistance between the GBF and the functionalized substrate limiting cross-plane phonon scattering. Enhancement of the thermal performance by inserting silane-functionalized molecules is important for the development of next-generation electronic devices and proposed application of GBFs for thermal management.