Elemental Metamorphosis
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PHYSICS & ASTRONOMY_High-Pressure Research Elemental Metamorphosis The inside of planets, stellar shells and numerous other uncomfortable spots in space have one thing in common: matter there is under extreme pressure of several million atmospheres. Mikhail Eremets and his colleagues produce such cosmic pressures in their lab at the Max Planck Institute for Chemistry in Mainz – and they do so in surprisingly simple experiments. They are researching which unique transformations gases, but also metals, undergo under these conditions. TEXT ROLAND WENGENMAYR he cosmos is full of places Since this world-record-setting mea- where the extreme is nor- surement, many chemists, materials mal. “In the core of Jupiter, scientists and physicists are contem- for example, the pressure is plating new, fascinating questions: more than 30 million atmo- Does hydrogen even become super- T spheres,” explains Mikhail Eremets at conducting at room temperature at the Max Planck Institute for Chemistry even higher pressure – in other words, Ich weiß nicht genau, was zusammengehört, in Mainz: “And planetary researchers does it conduct electricity entirely have long suspected that hydrogen, without electrical resistance? That is which is the primary component of precisely what theoreticians predict. these gas planets, is metallic there.” Only residents of Jupiter – if there American physicists Eugene Wigner were any – would be able to use super- and Hillard Bell Huntington predicted conductive extreme-pressure hydro- this strange, solid state of hydrogen as gen in electrical cables. However, the far back as 1935. Until recently, though, discovery of room-temperature super- it remained just speculation. It wasn’t conductivity in hydrogen could help until 2011 that the small Max Planck in solving the mystery of high-tem- team led by Mikhail Eremets, a Russian, perature superconductors and in tai- provided certainty with its experiments. loring these materials for technical In fact, the Mainz-based scientists were applications. Ceramics lose their resis- the first to observe how hydrogen be- tance at higher temperatures than comes a metal – at an enormous pres- conventional superconductors, but sure of more than 2.7 million bar, or still too far below zero to really be 2.7 million atmospheres. technologically useful. > 46 MaxPlanckResearch 4 | 12 The researchers in Mainz store their diamonds, not in a pocket-type safe, but in an anvil cell. They produce up to 4.4 million atmospheres of pressure between the flattened tips of the precious stones. Photo: Thomas Hartmann Creating pressure through a gentle turn: An Allen wrench and four screws are sufficient to produce pressures of several million atmospheres inside the anvil cell. Only diamonds can withstand this load. However, the stones turn brown where they crash into each other, as can be seen by looking through one of the diamonds in the cell (right). Regardless of potential new findings for lions of kilometers of cosmic travel sep- small, explains Eremets, “the largest energy technology, the experiments arating us from the answer to our ques- one is just 0.1 carat.” He picks up a lit- with hydrogen are of interest to physi- tion. The solution is located right in tle box with small, glittering stones, all cists and chemists for a very basic rea- Eremets’ cramped study, where an old of which shattered in experiments. son: hydrogen is the simplest atom and clock placidly ticks away the time. It’s Even the hardest material can’t with- the most common of all chemical ele- lying on a workbench with a stereomi- stand every pressure, and that’s what ments in the cosmos. This makes it the croscope, and at first glance, it looks al- makes this field of research so difficult. model organism, the Drosophila, of most disappointingly unspectacular. Eremets and his colleague Ivan quantum physics. In fact, it was the at- Troyan, who is also Russian, even had tempt at a physically precise descrip- EXPERIMENTS AT A PRESSURE to become experts in precision grind- tion of the hydrogen atom that first LIKE THAT INSIDE THE EARTH ing of diamonds. The shape they re- gave rise to modern quantum mechan- quire has nothing in common with ics. At the provisional end of this quan- Smiling, Eremets hands his visitor a the grinding that gives diamonds their tum revolution stands the semiconduc- compact metal tube. The brass-colored fire. The Max Planck scientists first tor electronics that catapulted our object has the diameter of a large coin give the small diamonds a basic coni- culture into the Internet age. and vaguely resembles a specialty part cal shape. Then they grind a tiny piece As a matter of fact, at more than 2.2 of a water fitting, but the massive caps of the tip off. This creates a flat surface million atmospheres, hydrogen, too, that seal the two ends of the metal there measuring just around 20 mi- first becomes a semiconductor before tube dispel this impression. Through crometers in diameter. Then the re- it mutates into a metal. This is anoth- small, thick windows, they permit a searchers put their diamonds – flat- er thing the group in Mainz observed glimpse of the inside. And precisely tened tips pointed toward each other for the first time – even imaginative there, in a volume of about five mi- – into a tiny, ring-shaped mount that theoreticians didn’t expect this behav- crometers in diameter – one microme- is likewise highly stable. ior. It is precisely this transformability ter is one-thousandth of a millimeter Before an experiment, the Mainz- under increasing pressure that makes – pressure conditions like those inside based researchers fill the tiny volume hydrogen so fascinating for science, the earth prevail. At least when the de- between the diamond tips in a closed which aims to use such experiments to vice is in operation. box with ultra-pure hydrogen – or with draw basic conclusions about the mat- Of course this raises the question of another material: sodium and nitrogen ter in our cosmos. But how do the Max which materials can withstand such have also been used in the cells in Planck scientists in Mainz manage to enormous pressure. In answer, one Mainz. Now the enormous pressure is study their samples under such ex- might borrow Marilyn Monroe’s im- needed. Mikhail Eremets demonstrates treme conditions? mortal song “... but diamonds are a re- how amazingly easily this is done. He In any case, they don’t need, for ex- searcher’s best friend.” Diamonds are, doesn’t need a gigantic press, as a lay- ample, expensive space probes that in fact, the high-pressure researcher’s person might expect. All the scientist crash into the bottomless abyss of the best friends. The diamond anvil cell, as needs is a screwdriver and the ring of gas planet Jupiter. So, at the institute in the small tube is called, contains two precision screws that enclose each of Mainz, there aren’t millions and mil- diamonds. However, they are quite the two windows in the metal caps. Photos: Thomas Hartmann (left, 2); MPI for Chemistry (right) 48 MaxPlanckResearch 4 | 12 PHYSICS & ASTRONOMY_High-Pressure Research » At a pressure above 2.2 million atmospheres, hydrogen initially becomes a semiconductor, before mutating into a metal. The researchers in Mainz were the first to observe this phenomenon. The rest is pure and simple mechan- a couple of delicate turns of the screws mittedly yet more extreme – states of ics. By slowly tightening the screws to ultimately result in one, two, three matter. And still there are only a few around the coin-sized surface of the or more million atmospheres of pres- research groups worldwide that can cap, the researcher applies growing sure in the core of the cell. The abso- play in the top extreme-pressure league pressure to the inside of the cell. This lute record for the group in Mainz is with the Max Planck scientists in pressure, which can reach up to one currently 4.4 million atmospheres. Mainz. This is by no means due to a ton, focuses the conical internal work- lack of interest in such experiments. ings on the tiny diamond tip. Its sur- THE PRINCIPLE IS SIMPLE – The reason is the many difficulties in face is just a few square micrometers THE DEVIL IS IN THE DETAIL the details, which cause even experi- in size, while the cap surface, on the enced researchers to fail. The story of other hand, measuring a few square So the principle is surprisingly simple, this years-long battle is also told in centimeters, is huge in comparison. especially considering how much tech- precisely those shattered diamonds in This extreme area ratio acts like an nical effort other disciplines, such as Eremets’ little box. Persistence is what enormous transmission gear, allowing particle physics, expend to reach – ad- helped his group succeed with its ex- At the trigger: Ivan Troyan (left) and Mikhail Eremets prepare an experiment in which they compress hydrogen to an extreme extent. In doing so, they discovered the previously unknown semiconducting state of the gas, and turned the lightest of all the elements into a metal for the first time. Photo: Thomas Hartmann PHYSICS & ASTRONOMY_High-Pressure Research » The European Research Council has been supporting the group since 2011, to the tune of 1.9 million euros – a great acknowledgement of the research field and the team’s discoveries. periments. “We also owe it to the good polished cutting surface always exhib- chamber. Despite this, it protects the conditions at the Max Planck Society, its microcracks. Under pressure, the cracked diamonds against the aggres- which allows us to work on long-term small hydrogen molecules penetrate sive hydrogen. This trick is what ulti- projects,” stresses Mikhail Eremets.