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Tech Front: Absorbing Research for Cheaper, Better Solar Cells

The idea that solar cells might someday viably compete with carbon-based fuels has driven researchers for more than a century. Rice University researchers may have now found a path to produce cheaper, more efficient metal-based solar cells, making the US goal of reducing the cost of solar electricity to $0.06/kW-hr seem reachable.

In a study published in Nature Communications in July, scientists from Rice’s Laboratory for Nanophotonics (LANP) describe a new method that solar-panel designers could use to incorporate light-capturing nanomaterials into future designs. Experimentation led LANP graduate student Bob Zheng and postdoctoral research associate Alejandro Manjavacas to create a methodology that solar engineers can use to determine the electricity-producing potential for any arrangement of metallic nanoparticles.

LANP researchers study light-capturing nanomaterials, including metallic nanoparticles that convert light into plasmons—waves of electrons that flow like a fluid across the particles’ surface. “When you shine light on a metallic nanoparticle you can excite some subset of electrons in the metal to a much higher energy level,” said Zheng. Rice researchers selectively filtered high-energy hot electrons from their less-energetic counterparts using a Schottky barrier (left) created with a gold nanowire on a titanium dioxide semiconductor.

LANP Director and study co-author Naomi Halas said these “hot electrons” are particularly interesting for solar-energy applications because they can be used to create devices that produce direct current or to drive chemical reactions on otherwise inert metal surfaces.

Photovoltaic cells usually use semiconductors made from expensive elements like gallium and indium. Halas said one way to lower manufacturing costs would be to incorporate high-efficiency light-gathering plasmonic nanostructures with low-cost semiconductors like metal oxides. In addition to being less expensive to make, their optical properties can be precisely controlled by modifying their shape.

Zheng’s experimental setup selectively filtered hot electrons from their less-energetic counterparts. To accomplish this, Zheng created two types of plasmonic devices (as shown below). The first setup created a Schottky barrier and allowed only hot electrons to pass to the semiconductor. The second setup allowed all electrons to pass.

“The experiment clearly showed that some electrons are hotter than others, and it allowed us to correlate those with certain properties of the system,” Manjavacas said. Zheng and Manjavacas are conducting further tests to optimize the output of hot electrons.

 

Soft-Bodied Robots Leap Forward

Traditional robots are made of components and rigid materials like you might see on an automotive assembly line—metal and hydraulic parts, harshly rigid, and extremely strong. But for robots to harmoniously assist humans in close-range tasks—on or away from assembly lines—scientists are designing new classes of soft–bodied robots. One of the challenges is integrating soft materials with requisite rigid components that power and control the robot’s body. At the interface of these materials, stresses concentrate and structural integrity can be compromised, which often results in mechanical failure.

But now, by understanding how organisms solve this problem by self–assembling their bodies in a way that produces a gradual transitioning from hard to soft parts, a team of researchers at Wyss Institute for Biologically Inspired Engineering at Harvard University have been able to use a novel 3D printing strategy to construct entire robots in a single build that incorporate this biodesign principle. The strategy permits construction of highly complex and robust structures that can’t be achieved using conventional nuts-and-bolts manufacturing. A proof-of-concept prototype—a soft-bodied autonomous jumping robot reported in the July 10 issue of Science—was 3D-printed layer upon layer to ease the transition from its rigid core components to a soft outer exterior using a series of nine sequential material gradients.

“By employing a gradient material strategy, we have greatly reduced stress concentrations typically found at the interfaces of soft and rigid components which has resulted in an extremely durable robot,” said the study’s co–senior author Robert Wood, who is co-leader of the Bioinspired Robotics Platform at the Wyss Institute.

With the expertise of study co-author James Weaver—also a scientist at Wyss and a leader in high-resolution, multimaterial 3D printing—the team was able to 3D-print the jumping robot’s body in one single 3D printing session. Usually, 3D printing is only used to fabricate parts of robots, and is only very recently being used to print entire functional robots. This jumping robot is the first entire robot to ever be 3D printed using a gradient rigid-to-soft layering strategy.The layers of material gradient in this soft-bodied jumping robot make it durable and squishy to the touch—perfect attributes for a robot designed to jump across rough terrain and be safe for use in c

The autonomous robot is powered—without the use of wires or tethers—by an explosive actuator on its body that harnesses the combustion energy of butane and oxygen. It uses three tilting pneumatic legs to control the direction of its jumps, and its soft, squishy exterior reduces the risk of damage upon landings, makes it safer for humans to operate in close proximity, and increases the robot’s overall lifespan. It was developed based on previous combustion-based robots designed by co-senior author George Whitesides of Wyss Institute.

“Traditional molding-based manufacturing would be impractical to achieve this—you would need a new mold every time you changed designs. 3D printing is ideal for fabricating the complex and layered body of our jumping robot,” said Nicholas Bartlett, a study co–first author and a graduate researcher in bioinspired robotics at Wyss. 

 

Mechanical Tensioning, Laser
Processing Modify Steel Welds

Welding has long been the method of choice for joining ASI Type 304L austenitic stainless steels. In a multipass weld, the development of residual stress largely depends on the response of the weld metal, heat-affected zone and parent material to complex thermo-mechanical cycles during welding. Mechanical tensioning and heat treatment have been used to modify the residual stress distribution.

A different method is explored in the April 2015 edition of SME’s Journal of Manufacturing Processes. Jibrin Sule and Supriyo Ganguly (Cranfield University, Bedford, UK), Harry Coules (University of Bristol, Bristol, UK) and Thilo Pirling (Institut Max von Laue-Paul Langevin, Grenoble, France) describe their strategy in “Application of local mechanical tensioning and laser processing to refine microstructure and modify residual stress state of a multipass 304L austenitic steels welds.”

In their study, modification of the residual stress state was attempted by using high-pressure cold rolling followed by laser processing in 12-mm thick 304L austenitic stainless steels—a novel technique. Read their paper at no cost at http://tinyurl.com/JMP-Weld.

 

What Your Clothes May

Say about You—Literally

Moving closer to the possibility of “materials that compute” and wearing your computer on your sleeve, researchers at the University of Pittsburgh Swanson School of Engineering have designed a responsive hybrid material that is fueled by an oscillatory chemical reaction and can perform computations based on changes in the environment or movement, and potentially even respond to human vital signs. The material system is sufficiently small and flexible that it could ultimately be integrated into a fabric or introduced as an inset into a shoe.

Anna Balazs and Steven Levitan integrated models for self-oscillating polymer gels and piezoelectric micro-electric-mechanical systems to devise a new reactive material system capable of performing computations without external energy inputs, amplification or computer mediation.

Their research, “Achieving synchronization with active hybrid materials: Coupling self-oscillating gels and piezoelectric [PZ] films,” appeared online this June in the journal Scientific Reports, published by Nature. The studies combine Balazs’ research in Belousov-Zhabotinsky (BZ) gels, a substance that oscillates in the absence of external stimuli, and Levitan’s expertise in computational modeling and oscillator-based computing systems. By working with Victor Yashin, lead author on the paper, the researchers developed design rules for creating a hybrid “BZ-PZ” material.

“The BZ reaction drives the periodic oxidation and reduction of a metal catalyst that is anchored to the gel; this, in turn, makes the gel swell and shrink. We put a thin piezoelectric [PZ] cantilever over the gel so that when the PZ is bent by the oscillating gel, it generates an electric potential [voltage]. Conversely, an electric potential applied to the PZ cantilever causes it to bend,” said Balazs. “So, when a single BZ-PZ unit is wired to another such unit, the expansion of the oscillating BZ gel in the first unit deflects the piezoelectric cantilever.” A resulting ‘seesaw-like’ oscillation permits communication and an exchange of information between the units.

Multiple BZ-PZ units can be connected, allowing complicated patterns of oscillation to be generated and stored. These different oscillatory patterns form a type of ‘memory,’ allowing the material to be used for computation.

 

This article was first published in the September 2015 edition of Manufacturing Engineering magazine. Click here for PDF.


Published Date : 9/1/2015

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