Manufacturing Bits: Feb. 3

Robotic plants; better nanowires; controlling band gaps.

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Robotic plants
In 2012, the European Commission launched the so-called Plantoid project. In the project, researchers hope to devise synthetic robotic plants. Inspired by plant roots, the robots could be used for soil monitoring and other applications.

The group is devising so-called artifacts. These components resemble plants and plant roots. The new technologies expected to result from the project include energy-efficient actuation systems, sensors and other systems.

The Plantoid project aims at taking inspiration from plant roots to develop a new generation of robots. (Source: Plantoid Project)

The Plantoid project aims at taking inspiration from plant roots to develop a new generation of robots. (Source: Plantoid Project)

In the journal of Advanced Materials, released in January, researchers from the program presented a paper, entitled “Hygromorphic Soft Actuators.” Researchers have devised a new class of polymers, which can respond to heat, electric voltage or light. These polymers can be used in several applications, such as soft robotics, active sensing and actuation.

One type of material, conjugated polymers (CP), have shown great potential with respect to actuation. More specifically, researchers have found that electrochemically synthesized polypyrrole (PPy) exhibit a reversible volume expansion in air resulting from the absorption and desorption of water vapor present in ambient air.

PPy could pave the way for robotic plants. “The ubiquitous presence of humidity in ambient air and its variation makes the development of humidity-responsive actuators both appealing and of importance,’’ according to researchers in Advanced Materials. “By chance, the capability to convert simple environmental stimuli, such as humidity, into mechanical reversible motion is regularly observed in living systems, particularly plants.”

Better nanowires
Sandia National Laboratories has found a way to improve the efficiency in thermoelectric nanowires. Nanowires could allow carmakers to harvest power from the heat wasted by exhaust systems. In microelectronics, researchers are developing next-generation transistors based on nanowire technologies, such as nanowire FETs, gate-all-around nanowire FETs, among others.

Sandia devised a method called room-temperature electroforming, which is a metal forming process based on electro-deposition. Researchers used the technology to allow the nanowires to grow at a steady rate, enabling 70nm to 75nm structures.

Sandia National Laboratories has developed a single electroforming technique for nanowire applications. (Source: Sandia)

Sandia National Laboratories has developed a single electroforming technique for nanowire applications. (Source: Sandia)

Using galvanostatic pulse deposition, researchers studied electroformed nanowires based on a bismuth-antimony (Bi1–x Sbx) compound. Researchers studied the nanowires with respect to composition, crystallinity and orientation.

Bi-Sb alloys have a high thermoelectric performance, according to researchers. Bi-Sb also acts as a conductor of electricity and an insulator against heat, but existing materials don’t produce effective solid-state cooling, according to Sandia.

In the lab, researchers devised two non-aqueous baths. Different Sb salts were investigated. In the first bath, nanowire arrays were electroformed using an SbI3-based chemistry. The arrays were polycrystalline with no preferred orientation, according to Sandia.

Meanwhile, in another solution, arrays electroformed from an SbCl3-based chemistry were crystallographically textured with the desired trigonal orientation, according to Sandia.

“From the SbCl3 bath, the electroformed nanowire arrays were optimized to have nanocompositional uniformity, with a nearly constant composition along the nanowire length,” according to researcher from Sandia. “Nanowires harvested from the center of the array had an average composition of Bi0.75Sb0.25. However, the nanowire compositions were slightly enriched in Sb in a small region near the edges of the array, with the composition approaching Bi0.70Sb0.30.”

On Sandia’s Web site, researcher Graham Yelton said: “The chemistry allowed us to go from poly nano-crystalline structure to near single crystals of 2-5 micrometers.”

The next step is to make an electrical contact. “Thermoelectric materials readily form oxides or intermetallics, leading to poor contact connections or higher electrical contact resistance. That reduces the gains achieved in developing the materials,” Yelton said.

Controlling band gaps
Northwestern University has devised a way to control the electronic band gap in oxide materials without changing the composition. This, in turn, could lead to better lasers and energy-generation materials.

The electronic band structure describes the ranges of energy that an electron within a solid may have energy bands. The traditional methods to tune band gaps rely on chemical alloying, quantum size effects, lattice mismatch formation, according to Northwestern, which added that the spectral variation is often limited to

In a breakthrough, researchers reported on large band gap changes of up to 200% or ~2 eV without modifying the chemical composition or use of epitaxial strain in the LaSrAlO4 Ruddlesden-Popper oxide. Ruddlesden-Popper phases are a form of layered perovskite structures. They consist of 2D perovskite slabs interleaved with cations

By tuning the arrangement of the cations, researchers demonstrated a band gap variation of more than two electronvolts. “There really aren’t any perfect materials to collect the sun’s light,” said James Rondinelli, assistant professor of materials science and engineering in the McCormick School of Engineering at Northwestern, on the university’s Web site. “So, as materials scientists, we’re trying to engineer one from the bottom up. We try to understand the structure of a material, the manner in which the atoms are arranged, and how that ‘genome’ supports a material’s properties and functionality.”

Atomic scale structure of 'designer' layered oxides: Band-gap engineering is enabled by varying the sequence of the well-defined layers, seen as planes of similarly colored (green and purple) atoms, in transition metal oxides without changing the materials overall chemical composition. (Source: Northwestern)

Band-gap engineering is enabled by varying the sequence of the well-defined layers, seen as planes of similarly colored (green and purple) atoms, in transition metal oxides without changing the materials overall chemical composition. (Source: Northwestern)