Manufacturing Bits: March 5

WAAM process; hard coatings; titanium solar cells.


WAAM process
Thales Alenia Space, Cranfield University and Glenalmond Technologies have produced a prototype of a titanium pressure vessel for use in future space missions.

The vessel is 1 meter in height and weighs 8.5kg. The titanium alloy is made using Cranfield’s additive technology, dubbed the Wire + Arc Additive Manufacturing (WAAM) process.

Related to 3D printing technology, WAAM makes use of an electric arc as the heat source and wire as the feedstock. WAAM hardware makes use of standard welding equipment. It is used for the manufacturing of medium to large scale components with lower manufacturing costs and lead times.

The vessel is one example. “If manufactured traditionally, the component would have required about 30 times more raw material than its final mass. By using the WAAM process, more than 200 kg of Ti-6Al-4V has been saved for each item,” according to researchers.

Hard coatings
Using atomic layer deposition (ALD), Lehigh University has developed what it says are among the world’s hardest and thinnest coatings.

The coatings, which approach the wear resistance of diamonds, are based on titanium and vanadium nitride films.

Both materials are hard and wear resistant. Typically, titanium and vanadium nitride are produced using sputtering, chemical vapor deposition (CVD) or other methods, according to researchers from Lehigh.

To boost the properties of the materials, researchers from Lehigh used a plasma-enhanced ALD system from Veeco. ALD is a deposition technique that deposits materials one layer at a time. In traditional ALD tools, wafers are placed in a chamber. A chemistry enters the chamber and they process the wafers. Then, the chemistries are purged.

In the case of Lehigh’s technology, researchers first pumped a titanium precursor into the ALD system, forming a monolayer on a substrate. The titanium is removed. Then, nitrogen plasma is pumped into the chamber, forming a second monolayer. This process is repeated, eventually forming hard coatings.

“For growing nitrides, you need a lot of thermal energy, like 800 degrees Celsius,” said Nicholas Strandwitz, a professor at Lehigh. “Or, you need a plasma to make the nitrogen more reactive. Generating plasma means we’re knocking electrons off the nitrogen molecules as they’re flying around in the gas, making the nitrogen more reactive so it will bond to the surface and become part of the film. If you just float nitrogen gas through there, nothing would happen because the N2 molecule is super stable. So with plasma, we can grow these films at 50 degrees Celsius, just slightly above room temperature.”

Commenting on the results, Brandon Krick, a professor at Lehigh, added: “These films are approaching the wear resistance of diamonds. They’re 100 times better than the commercial nitride coatings.”

Titanium solar cells
Using different mineral forms of titanium oxide, Kanazawa University has improved the efficiency of a new type of solar cell.

Kanazawa University has devised a solar cell using a double layer of pure anatase and brookite, which are two different forms of titanium oxide. A brookite layer is fabricated on top of anatase, thereby increasing the solar cell efficiency by up to 16.82%.

Today’s solar cells are made of silicon. To boost the efficiencies, researchers are exploring different cell types. Among those are based on metal halide perovskites. A perovskite, which is a calcium titanium oxide mineral, turns light into electricity.

“They have to be sandwiched between a negative and positive electrode,” according to Kanazawa University. “One of these electrodes has to be transparent, however, to allow the sun’s light to reach the perovskites. Not only that, any other materials used to help charges flow from the perovskites to the electrode must also be transparent.”

To develop these types of cells, researchers from Kanazawa used anatase and brookite layers. The anatase layers were sprayed on a glass coated with a transparent electrode at 450 °C. Then, brookite nanoparticles formed the brookite layers.

“Using different mineral phases and combinations of these phases allows for better control of the electron transport out of the perovskite layer and also stops charges from recombining at the border between the perovskite material and the electron transport layer,” said Md. Shahiduzzaman of Kanazawa University. “Together, both these effects allow us to achieve higher solar cell efficiencies.”

Koji Tomita of Kanazawa University added: “By layering brookite on top of anatase we were able to improve solar cell efficiency by up to 16.82%.

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