72. Hafnium: The Isomer Bomb

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The science suggests hafnium can’t be used to create a gamma ray bomb, but that won’t stop the U.S. government from trying.

Featured above: Hafnium can fuel explosives in the universe of Elite Dangerous. It can also fetch tens of thousands of credits.

Show Notes

Whoops: At the end of the episode, I mistakenly said that tungsten is up next. We’ll actually be tackling tantalum. My apologies for the mistake, and thank you to those who pointed it out!

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For more on neon and excitation of electrons, see neon.

Hafnium can withstand some very, very high temperatures. It doesn’t have the highest melting point of all the elements, but it’s pretty close to the top of the list. It melts at 2,200 degrees kelvin. The refractory metals, like osmium, rhenium, and tungsten (next episode’s element!) tend to outclass it in this regard, but it’s actually carbon that takes the crown, with a melting point of 3,500 K.

But tantalum hafnium carbide has the highest melting point of any material humans have ever manufactured, somewhere north of 4,200 K. A hypothetical compound of hafnium, nitrogen, and carbon could push the limit even higher… if we can ever manage to actually create the stuff.

SciShow did a good video on all this (although it may be a little repetitive after learning what you just did):

As an atom that can really take the heat, it does the jobs that so many other elements can’t. For example, it tends to be found on rockets, both as a component of nozzles and heat shields. Hafnium is also used to make the electrodes in plasma torches, since it’s one of the few materials that will neither melt nor corrode under the extreme conditions demanded by the device.

Unsurprisingly, transistors have continued to shrink since 2007. Intel started producing 22 nanometer chips a mere three years later. Apple’s new M1 chip brings that size down to 5 nanometers, allowing them to fit 16 billion transistors on a single chip. The eggheads at Lawrence Berkeley National Labs have made them as tiny as one single nanometer.

For twenty years, tech pundits have cried that Moore’s Law is a relic of an earlier age, it was never meant to be taken so seriously, and it will fail to hold up over time. By every measure, those naysayers should be correct. Yet in defiance of scientists, journalists, and physics itself, engineers continue to barrel through every obstacle in their way to shrink transistors by half every couple of years.

Episode Script

As far back as the 1860s, scientists suspected that an element like hafnium must be out there somewhere — but they had a devil of a time pinning it down. Dmitri Mendeleev predicted the existence of something chemically similar to titanium and zirconium, but wasn’t sure what its atomic properties would be.

Henry Moseley, the scientist who realized the elements should be ordered by atomic number rather than atomic weight, realized that such an element would fit perfectly in slot number 72. This was purely theoretical, though. Moseley had no idea where element 72 might be found.

An American chemist named Edgar Smith came within an inch of discovering the element while investigating a sample of monazite sand. He told his colleagues that he was pretty sure there was a new element hiding out in there, but he’d take a closer look when he had more free time. For a chemist, I’m not sure what takes precedence over discovering a new building block of the universe, but he never got around to it.¹

Georges Urbain claimed to have made the discovery in 1907, but his fellow chemists weren’t so convinced by his argument. Urbain said that element 72, which he called celtium, was one of the rare earth elements. Scientists like Niels Bohr thought that whenever the element was finally found, it would behave more like a transition metal than a rare earth. Since today’s episode is about hafnium and not celtium, it should come as no surprise that Urbain was eventually found to be mistaken.

Ultimately, it took more than half a century for this game of hide-and-seek to come to a close, when Dirk Coster and Georg von Hevesy found the element tightly bound up in a sample of zirconium. They named their discovery “hafnium” after the Latin name for Copenhagen, where they lived at the time. The year was 1923, and the two men had finally found the last non-radioactive element on the periodic table.

You’re listening to The Episodic Table Of Elements, and I’m T. R. Appleton. Each episode, we take a look at the fascinating true stories behind one element on the periodic table.

Today, we’re investigating the rumors about hafnium.

Alfred Nobel. Werner von Braun. J. Robert Oppenheimer. For whatever else these scientists achieved, they earned great notoriety by devising new and elaborate ways to blow things up.

For a brief moment in the late ’90s, it looked like Carl Collins might join their inglorious ranks by unleashing the power of the isomer.

“Isomer” is a word that comes from Greek roots meaning, “having equal parts,” and it can refer to two different things. The first is when two different molecules have identical atomic composition, but arrange their atoms in different ways. For example, fructose and glucose both have the chemical formula C₆H₁₂O₆, but they arrange those carbons, hydrogens, and oxygens in different ways that make them distinct from one another.^{2 3}

That, however, is what we are not interested in today. Today we’re talking about atoms whose protons or neutrons are excited.

It’s kind of like when an electron jumps to a higher energy state, which is what happens to the neon in neon lights. Neutrons and protons can do the same thing — we just haven’t had much reason to talk about it before now. A subatomic particle in a heightened energy state is unstable — it can’t stay there forever. Eventually, the electron, neutron, or proton will fall back down to its resting state and release its excess energy in the process. Usually that energy gets released as visible light.

Hafnium-178 m2 is a nuclear isomer that’s a little more stable than most. In 1998, a University of Texas team led by Carl Collins conducted an experiment with that isomer at its very center. A small amount of hafnium-178 m2 was mounted atop an overturned styrofoam coffee cup, and a device made from a dental x-ray machine and an audio amplifier bombarded the hafnium with radiation for several days on end. Collins and his team pored over the data and declared success. They claimed to have found a way to make hafnium-178 m2 release far more energy than just a little blip of light. Rather, their method could trigger a gamma-ray explosion roughly one-tenth as strong as the atomic bomb detonated over Nagasaki.⁴

That kind of talk is like catnip for the U.S. Department of Defense, and sure enough, they became very interested very quickly. Plans were drawn up for all sorts of outlandish weapons, like a hafnium hand grenade. Any soldier unlucky enough to pull the pin on such a grenade was guaranteed to be its first casualty, but that didn’t matter to the men inside the Pentagon.

One of the reasons the DoD was so interested in an isomer bomb was the novelty of it. Not merely the sheer thrill of discovering some new kind of explosive, but the particular mechanism of action behind the hypothetical weapon. The hafnium-178 bomb would be one big boom, and it would release loads of radiation, but it involves neither fission nor fusion. That makes it distinct from nuclear weapons — scientifically speaking, but also legally. In other words, it wouldn’t be subject to the restrictions of the Nuclear Test Ban Treaties. Underground, underwater, open skies, even outer space — the military could test these bombs anywhere, without telling anyone, without breaking any rules.

DARPA, the Defense Advanced Research Projects Agency, spent $30 million and several years exploring the feasibility of hafnium weaponry in the early 2000s.⁵

There was just one problem: The results of Dr. Collins’ experiment were highly controversial. Other scientists in the community weren’t convinced by the conclusions he had drawn — and further experiments weren’t very successful, either. It’s fair to say the skepticism was overwhelming. A group of scientists at Lawrence Livermore National Labs published a thorough debunking of Collins’ work.

Meanwhile, supporters of the work were fervently devoted. Someone had printed up buttons that said “I Believe In Isomers” that became popular in Washington, D.C. for a while. At the University of Texas, the original styrofoam cup is kept in a glass case labeled, “Dr. C’s Memorial Target Holder.”

It’s been a long time since all that happened, and you might have noticed a conspicuous lack of public hysteria surrounding the hafnium bomb. You can probably guess why that is. While research in the field continues, it’s not the most active area of study, and the DoD is (supposedly) no closer to weaponizing element 72 than they were in the 1990s. Many scientists believe it will never happen. Perhaps some kind of battery technology or cancer treatment might blossom from the research, but that too remains hypothetical for now.

Has this all been an enormous waste of time, money, and talent? That depends on your perspective, I suppose. Plenty of people in the scientific and political communities believe so.⁶ James Carroll, a DARPA contractor and one of Collins’ former students, has a slightly more positive opinion on the matter. “Maybe you can never make anything practical out of it,” he once said, “but in the meantime, we will learn a lot about how the nucleus responds to people banging on it.”⁷

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While you’re not likely to find a sample of hafnium in any stockpile of weapons, it does tend to be found in a highly radioactive context.

Like its upstairs neighbor zirconium, hafnium is an essential material in the construction of nuclear power plants — albeit for the exact opposite reason.

Zirconium allows neutron radiation to pass right through it, almost as though it weren’t there at all. That makes it a good material for fuel rods, the containers that hold nuclear material as it undergoes fission. Hafnium, however, absorbs neutrons better than almost any other material. That’s exactly the kind of material that makes a good control rod.

Control rods in a nuclear power plant are kind of like the brakes on a car. They’re inserted into the reactor core to absorb some of the neutrons as they fly around, which halts them in their tracks and prevents them from splitting other atoms. This slows down the overall rate of fission and prevents the reaction from spiraling out of control, a situation that’s also called a “meltdown.”

Some sort of hafnium failure was not to blame for the meltdowns at the Fukushima nuclear power plant in 2011 — that disaster happened after an earthquake and tsunami damaged the plant, flooded the buildings, and caused cooling systems to fail. Nonetheless, the accident did have consequences for people who, like you, are in the market for a pure supply of hafnium.

See, nobody’s really in the business of mining hafnium. There’s simply no need. What little demand there is for elemental hafnium is met in the course of mining zirconium. But after the disaster at Fukushima, Japan closed all of its nuclear power plants. Germany closed some of its nuclear plants, too. So did France. With so many sudden closures, demand for zirconium plummeted drastically. Production fell to meet this lower demand, and as a side effect, production of hafnium basically stopped, too. Practically immediately, the price per kilogram of hafnium doubled.^{8 9} It’s even higher now.

But where one door closes, another opens.

Right around the same time, the microchip industry was dealing with a problem. In prior episodes, we’ve learned how computer technology advanced from vacuum tubes to solid-state transistors to integrated microchips. In any case, the job of this bit of tech is to act as an on/off switch. An electrical current starts at one terminal and encounters a gate. The gate can either stop the current right there — off — or permit the current to travel to the next terminal — on.

Over time, engineers were able to design smaller and smaller transistors. That assists in the creation of smaller computers, from desktops to laptops to mobile phones to smartwatches, but it also permits the design of faster computers. By packing more transistors onto a microchip, the computer gains more raw processing power.

By 2007, the most advanced transistors in the world were 65 nanometers in size. That’s really small — smaller than most viruses!^{10 11} A single microchip could pack tens of millions of transistors onboard. But they couldn’t really get any smaller. The problem was with the gates, which were made of silicon. Any smaller than 65 nanometers and they started to leak. The electrical current could jump from one terminal to the other regardless of whether the gate was open or closed.^{12 13}

The solution laid with hafnium. Hafnium oxide makes an excellent insulator — much better than silicon. Gordon Moore, co-founder of Intel and the Moore of “Moore’s Law,” called the innovation “the biggest change in transistor technology since … the late 1960s.”¹⁴ Transistors shrank in size from 65 down to 45 nanometers — small enough that the period at the end of a sentence could contain two million of them.^{15 16}

That was long ago enough that hafnium has worked its way into most modern electronic devices. So even if a kilogram of raw hafnium falls rather outside of your budget, you can rest assured that you’ve probably already spent a fair bit of money to collect a little hafnium, whether you knew it or not.

Thanks for listening to The Episodic Table of Elements. Music is by Kai Engel. To learn about some other impressively extreme environments that hafnium can endure, visit episodic table dot com slash H f.

Next time, we’ll do some heavy lifting with tungsten.

Until then, this is T. R. Appleton, reminding you that a gamma bomb was what turned Bruce Banner into the Incredible Hulk. Your mileage may vary.

1. Nature’s Building Blocks, p. 174. John Emsley, 2003.
2. Cliff’s Notes Organic Chemistry I, Structural Isomers And Stereoisomers.
3. Biology Dictionary, Isomer. Last updated June 12, 2017.
4. Damn Interesting, Half Science And Hafnium Bombs. Alan Bellows, May 31, 2007.
5. Slate, Hafnium: Building The Doomsday Device Of Tomorrow. Sam Kean, 2010.
6. APS News, The Strange Tale Of The Hafnium Bomb: A Personal Narrative. Peter D. Zimmerman, June 2007.
7. The Washington Post, Scary Things Come In Small Packages. Sharon Weinberger, March 28, 2004.
8. Sophisticated Alloys, World Events Cause Hafnium Price To Soar.
9. Tech Metals Insider, Weak Zirconium Demand Depleting Hafnium Stockpiles. March 11, 2015.
10. Wolfram Alpha query for “65nm.”
11. Encyclopedia Britannica, Viruses – Size And Shape. Last updated November 12, 2020.
12. AnandTech, Intel Demonstrates New 45nm Transistors And Conroe’s Successor. Anand Lal Shimpi, January 27, 2007.
13. Chemistry World, Hafnium Oxide Helps Make Chips Smaller And Faster. Jon Evans, February 5, 2007.
14. Chemistry World, Hafnium Oxide Helps Make Chips Smaller And Faster. Jon Evans, February 5, 2007.
15. Intel White Paper, Introducing The 45nm Next-Generation Intel Core Microarchitecture.
16. ZDNet, Intel Rolls Out New Server, High-End PC Chip Lineup; Green Arms Race Heats Up. Larry Dignan, November 12, 2007.