Before we talk about the brief history of nitinol let me remind you Mark Twain once said that accident is the mother of invention. Take the light bulb for example, while Thomas Edison is famous for inventing the light bulb, he already knew that a wire glows when it conducts electricity. This was discovered much earlier, quite by accident by the scientist James Prescott Joule who quantified the amount of heat generated by electricity passing through a conductor. Well, nitinol is no different.
|Dr. Buehler discovered the shape memory properties of nitinol while working at Naval Ordinance Laboratories.|
It is important to note that nitinol was not the first shape memory alloy discovered. Researchers had been playing around with gold-cadmium since 1939, but the shape memory effect was minimal and the material was extremely expensive ($100/gram). Nitinol was discovered by a brilliant young scientist named William J. Buehler. Buehler was a metallurgist at the Naval Ordinance Labs (NOL), working on a project to develop a nose cone for the Polaris missile that was capable of withstanding the heat of re-entry into the Earth’s atmosphere.
Buehler described this project as ‘boring’ and was hoping that something ‘interesting’ would pop up. Well, it did, but not in the way that anyone would have expected. Buehler was looking at alloys with two solid states as possible materials for the nose cone. He had selected roughly 60 alloys for further examination from a book entitled Constitution of Binary Alloys–nitinol being one of them. When he made the ingots for testing, he intentionally dropped one of the cold ones on the floor. Hoping to hear a clear bell-like ring, indicating that the metal had the properties he was hoping for. Instead, it returned a dull thud–similar to dropping a sack of flour on the ground.
Worried that the ingot was filled with internal flaws, he dropped one of the ingots that hadn’t cooled yet. This returned a wonderful bell-like ring. However, after the ingot had been cooled in water, it returned a dull, leaden thud, just like the first one. This is the first indication that nitinol had a substantially different double state. Buehler named his alloy nitinol for Nickel-Titanium Naval Ordinance Laboratories. All of this occurred in 1959. However, the shape memory aspect of nitinol wasn’t discovered until a lab meeting in 1961.
Buehler had been performing tests to determine the fatigue life of nitinol by bending a strip into an accordion like shape over and over again. His project was brought under review and his technician was demonstrating the fatigue properties to senior officials. During this presentation, one of the officials present heated the nitinol with a lighter, at which point it rapidly straightened out.
This, of course, sent ripples throughout the scientific community. This material could take low grade heat and generate mechanical energy! Numerous scientists began experimenting with how to build engines with nitinol that would take low grade energy and transform it into very high grade energy that could be used to do work. This culminated in the Nitinol Heat Engine Conference, hosted by the Naval Surface Weapons Center (previously Naval Ordinance Labs) in 1974.
At the Nitinol Heat Engine Conference, the NSWC gathered together the top scientists who had been working on nitinol to discuss what had been done and what still needed to be done to make nitinol heat engines a reality. The presentations from this conference are available in the book Proceedings of the Nitinol Heat Engine Conference.
At this point, the activity surrounding nitinol seemed to all but dry up and disappear. Prior to the conference, nitinol researchers were featured on news channels like CNN and BBC. Afterwards, there was little to no attention given to nitinol by major television networks for over twenty years. This led to all kinds of conspiracy theories ranging from nitinol being kept for top secret government experiments to nitinol being an alien technology that was discovered in the Roswell accident.
However, the reality is very different. Behind the scenes, material scientists were working hard to figure out HOW nitinol worked. In order to fully optimize a nitinol heat engine, it must be understood what happens when nitinol undergoes the shape memory effect. This will be discussed in greater detail in chapter 2.
The next time nitinol appeared in public, it wasn’t referred to as nitinol, it was called just plain titanium. Of course, this is a misnomer since nitinol is slightly more nickel than titanium. This new public appearance didn’t even exhibit the shape memory effect as people were so excited about earlier. No, this was completely different and it was marketed under the trade name FlexonÒby the company Marchon Eyewear.
Released for public sale in 1995, Flexonâwas unusual in that you could bend it through incredible distortions and it would just snap back to its original shape once you released it. Once Nike began to use it in their Vision line of glasses, athletes everywhere began buying it up. Gone were the days of breaking your glasses on a regular basis just because you lived an active lifestyle. You could sit on them, you could intentionally bend them, you could tackle someone in football, and they would just bounce back. These glasses would forgive those bumps and bangs over and over again–seemingly forever. People who broke their glasses every six months now could go several years on a single frame. The frames were termed ‘superelastic’.
Once superelastic nitinol was discovered, it wasn’t long before surgeons began using it as they worked on people. Vascular stents was one of the first applications because the stent could be folded so flat that it could be inserted through the tiniest of holes into the patient’s bloodstream–minimizing recovery time. Once in place, the superelastic nitinol wire could withstand severe deformation and outlast stainless steel by an order of magnitude.
To put this in perspective, superelastic nitinol stents were capable of undergoing a 30% deformation with a cycles to failure life expectancy greater than 10,000,000. Stainless steel, on the other hand, could withstand a deformation of just 0.5% with fatigue life of around 1 million cycles. This made intravascular stents something that could be considered permanent, never needing replacement.
The only question that the medical community had revolved around the issue that nitinol is more than 50% nickel and people are allergic to nickel. Yes, strange as it may seem, our five cent coin is made from a metal that we are allergic to. Of course, this could potentially cause huge problems for patients if enough nickel were to dissolve out of the nitinol and into the bloodstream. Fortunately, researchers quickly discovered that nitinol’s biocompatibility is unsurpassed and that doctors should feel free to implant it into patients in any way they see fit.
Since then, nitinol has replaced other alloys in just about every kind of implant in the human body. Researchers have found that it makes a great hip replacement material because the superelastic phenomenon damps out the vibrations caused by walking–greatly extending the useful life of a joint replacement. Because of the broad spectrum use of nitinol in surgical implants, the medical field is the largest consumer of nitinol worldwide. The second largest consumer of nitinol being the eyeglasses industry.
But whatever happened to the interest in the shape memory effect that had once lit up the scientific community? Well, while all of this was going on, the National Science Foundation (NSF) was sponsoring materials science research at universities across the United States. These small, independent research groups turned up all sorts of interesting, useful knowledge about nitinol.
To organize all of this research and to share knowledge gained, the American Society of Materials (ASM) created the Shape Memory and Superelastic Technologies (SMST) group that would meet every eighteen months to discuss the latest improvements in the material science of nitinol. First meeting in 1997 with a gathering of experts from all over the world, SMST began corroborating findings about all sorts of things. New alloys of nitinol were discovered. Termed 55-Nitinol, 60-Nitinol, and 65-Nitinol for the approximate percent nickel content, these alloys had slightly different properties than regular nitinol. It wasn’t long before scientists were discussing adding third and fourth elements into nitinol alloys to alter the properties to give something more desirable. All of this will be covered in greater detail in Chapter 4.
With all of this research into the material science of nitinol and the discoveries that followed, this sparked renewed interest from mechanical engineering scientists. In 2012, General Motors announced that they were working on a nitinol heat engine that would capture the waste heat from the exhaust from the engine and generate electricity. The hope here was to replace the alternator and use less fuel.
Then in 2013, Kellogg’s Research Labs published that they had built a generator capable of harnessing the energy from the daily change in air temperature. In their report, KRL stated that their thermal efficiency was between 1 and 4 percent. This is absolutely horrible when comparing to cars, which run at roughly 25% efficiency, or the power plants that generate electricity to power the nation, which run at 43% efficiency. However, unlike those systems, the amount of heat energy discharged by the atmosphere on a daily basis is several orders of magnitude greater than all of the power generated by all power plants on earth, and it’s free. So, 1% of a very large number is still a very large number.