Thursday, September 28, 2017

Researchers Perfect New Technique for Making "Memory Crystals"

Scientists from Japan have found an easier way to make a special kind of crystal that can "remember" its shape if deformed. These materials—known as shape memory alloys—are valuable in a range of applications from medical devices to building construction, but manufacturing them can be difficult and costly. A recent paper published in Nature Communications is looking to change that.

Growing the size of crystals
You can bend and twist a rod made of shape memory alloy (within reason), and then restore it to its original shape simply by warming it up. These materials also often possess exceptional elasticity, making them ideal for applications that demand flexibility and durability such as flexible eyeglasses frames or prosthetic limbs. But just like with many miracle materials, there's a catch. Shape memory alloys work best when they are grown as single crystals. But single crystals are difficult to manufacture, so current applications use poly crystal forms of the materials, which have limitations.

A pair of shape memory alloy eyeglass frames revert to their original form as they warm up.
Image Credit: Youtuber Hero Khan

"These shape memory materials, when in the poly crystal form, tend to crack along grain boundaries," said Richard James, a materials scientist from the University of Minnesota in Minneapolis who was not involved in the study. Just like a particle board is weaker than a solid plank of wood, a poly crystal material has its weak points between the grains.

According to Toshihiro Omori, co-author of the Nature Communications paper and a materials scientist from Tohoku University in Sendai, Japan, current manufacturing methods can barely produce a single crystal grain of shape memory alloys much larger than a grain of rice. Looking to change that, Omori and his colleagues developed a new technique and grew a single grain of copper-based shape memory alloy more than 2 feet long. The technique they used, known as abnormal grain growth, sets off an uneven competition between the individual crystal grains and causes them to merge and form bigger and bigger grains.

"I think soap bubble is a good metaphor," said Omori, referring to the way a pile of smaller soap bubbles pop and merge together to form larger bubbles. Omori and his colleagues have discovered that if they repetitively heat and cool a bunch of tightly packed shape memory alloy crystals, the smaller crystals, just like the soap bubbles, can merge and form a larger, single grain crystal. The trick is to figure out the right temperature and speed to heat and cool the material, which is one of the main findings of their paper.

The researchers claim that the size of their 2-foot-long crystal was only limited by the size of their laboratory equipment, which is like saying that your wallet is too small to hold all your cash—not a bad problem to have. Even larger single crystals of this alloy can be made with larger industrial size equipment, Omori said.

Growing the number of applications
According to James, there's already a whole plethora of medical devices that make use of poly crystal shape memory alloys. As an example, he described a coronary stent that needs to be compressed down and fitted on the end of a guide wire, so that the doctor can feed it up through the patient's artery to place the stent.

"When you're paying $12,000 for a stent operation, the cost of the material could be less than a dollar—its contribution to the overall cost is not so great," said James. "But if you're making huge dampers for a building, the material cost would be a very significant factor."

Private and government agencies are trying to expand the range of applications for shape memory alloys, from making more durable spacecraft wings, to making more comfortable car seats. For these larger-scale applications, the use of shape memory alloys is often limited by the materials' cost-effectiveness relative to more conventional materials such as steel and aluminum. A cheaper and stronger shape memory alloy, perhaps in the form of a single crystal, can change that.

"We have been working on development of the copper-based shape memory alloys for anti-seismic applications," said Omori. Being from Japan, one of the most seismically active regions on the planet, Omori and his colleagues want to develop better construction materials for buildings and infrastructures.

With the new manufacturing technique, these materials may one day save lives not just as coronary stents, but also in earthquake-resistant buildings.

Yuen YiuInside Science News

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