under pressure

Perplexedjenluc_13Phase transitions are quite possibly one of the most fascinating areas of physics: different substances behave differently at various temperature and pressure points — sometimes in very remarkable ways. Yet it's easy to lose one's sense of wonder, because we see phase transitions around us every day. Water boils, ice melts (diluting the taste of one's otherwise killer cocktail) — no big deal, right? Wrong. Sure, you can produce a phase transition of sorts for just about any common material, but there's still a lot to learn about the underlying physics.

Case in point: Earlier this summer, an article appeared in Physical Review Letters (July 7, 2006), describing the results of a new computational study by scientists at Sandia National Laboratories indicating that a new conducting form of water — dubbed "metallic water" — could occur at a temperature of 4000 degrees Kelvin and a pressure of 100 gigapascals. Can we just say… yowza! Those are some high temperatures and pressures! And yet they are much lower than previous theoretical estimates calling for temperatures of 7000 degrees Kelvin and 250 gigapascals. (Confession: We do not actually know what a gigapascal entails, but we like the sound of it, and anything with "giga" as a prefix isn't exactly small.)

I meant to write a blog post about this whole metallic water business back in August, but as often happens, time got away from me. Also, the news broke while I was vacationing in Buenos Aires, so I was rather late in the game. My notes have been sitting in the blog fodder file ever since. Fortunately, I have a second chance to address the topic: the same researchers were on hand to present an update on their paper at the annual meeting of the APS Division of Plasma Physics, held this past week in Philadelphia. The authors — Thomas Mattson and Mike Desjarlais — summarized their earlier findings from their quantum simulations, which were sufficiently different from previous calculations to warrant some significant alterations in the standard theoretical phase diagram for water under such extreme conditions. (A phase diagram, for non-scientifically inclined readers, is just a graphical representation of the effects of pressure and temperature on the phase of a substance.Phasewater Go here to find out how to build your very own phase diagram from scratch. We make our own fun here at Cocktail Party Physics.)

Residents of Washington, DC, might assume "metallic water" refers to things like high lead levels in the drinking water, when in fact it describes a form of water that is electrically conductive. It wasn't immediately clear to me what's so special about this, since water is pretty conductive in its normal state, but Jen-Luc Piquant reminded me that very high temperatures and pressures can give rise to new and intriguing properties for even a common substance like water — in this case, ultra-high energy densities. How high? Far beyond those that would occur naturally anywhere on earth. In fact, it would take temperatures and pressures on a par with those believed to exist in the interiors of the gas giant planet Jupiter to produce equivalent energy densities in a lab like Sandia's Z-pinch machine.

We should have known this was coming. Back in 1996, William Nellis, a scientist at Lawrence Livermore National Laboratory, announced the successful achievement of metallic hydrogen, which, among other things, gave us some fascinating details about the compressed interior of Jupiter. (You can find out more than you probably ever wanted to know about metallic hydrogen here, along with some really nice graphics.) Hydrogen as we know it is a gas, but on Jupiter, it's believed to exist as a super-hot liquid metal because of the extreme pressures and temperatures that typify that planet. Eugene Wigner predicted back in 1935 that if you squeezed hydrogen gas hard enough, it would eventually metallicize, but the requisite pressure was so intense that physicists weren't able to achieve it for 60 years. Hydrogen being a primary component of water, it stands to reason that one day physicists would move on to exploring the possibility of metallic water. (Jen-Luc loves the fact that both these phases are known as "degenerate matter." Clearly the term means something completely different in physics.)

The usefulness of these new results isn't limited to the far reaches of our solar system. The whole point of doing the quantum simulations in the first place was to better understand the conditions deep in the heart of the Z machine, which emits incredibly powerful X-rays in very short (nanosecond) bursts — the equivalent of many times the electricity generation of the entire world. Sandia uses water both as an electrical insulator and as switches. Zap water with that much electricity, and you'll vaporize it, causing the surrounding water pressures to rise as the resulting shock wave spreads outward. The Z machine exploits this effect to achieve its impressive X-ray energies.

The operators send electrical pulses of about 20 million amps through the row of water switches. Initially, the water acts as an insulator, but eventually it's overcome as the incoming charge builds up. At that point, the water transmits the pulse, in the process shortening (or compressing) it from microseconds to nanoseconds. Voila! Extremely high temperatures and pressures ensue, in the form of a really, really big "spark." The Z machine is undergoing a major upgrade, and Sandia scientists want to understand how their water switches work at deeper, "first principle" levels, so they can attain the optimal transmission of electrical pulses in the future. In the same way that one can never be too rich, or too thin, physics experiments can never really be too high in energy — because higher energies invariably translate into discovering new and interesting physics.

Sandia's on a roll these days when it comes to weird temperature and pressure phenomena. Also on hand in Philadelphia this week was Sandia scientist Marcus Knudson, who uses strong acoustic shock waves to melt diamond — more precisely, to determine its melting point. Diamond is one of the toughest substances known, standing its ground in defiance of all manner of onslaughts, but apparently it goes weak at the knees when confronted with the dulcet tones of Knudson's sound-speed technique. Okay, the tones are hardly dulcet: Knudson and his colleagues are producing acoustic shock waves with strengths between 6 and 10 Mbars, producing a mixture of molten carbon and solid diamond. I'm casting about for an appropriate analogy that a non-scientist can relate to, and coming up empty. Suffice to say that melting diamond completely requires shock waves stronger than 10 million times earth's atmospheric pressure.

Why does this happen? Just like with the Z machine, the shock wave transfers large amounts of energy to the diamond material when it strikes, increasing not just the pressure, but also the temperature. If the shock wave is powerful enough, the temperature will get so high, the diamond will begin to melt into liquid carbon. The Sandia scientists determined the "melting properties" of diamond by measuring the speed of sound in the shocked material. Basically, the strength of the sound wave correlates pretty neatly with whether the material in its shocked state is in solid or liquid form. They observed a steep drop in sound speed at the 6 and 7 Mbar levels — a sign the diamond was beginning to melt.

Knudson's work is  particularly relevant to continuing research on inertial confinement fusion (ICF), which requires a material like diamond or beryllium for the fuel capsules used to produce fusion. For ICF purposes, a low melting point is desirable. So diamond's high melting point is a bit of a disappointment in that respect: beryllium has similar properties, but a lower melting point, making it the preferred material for ICF applications.

And here's another fascinating temperature-related tidbit gleaned from the pages of an upcoming issue of PRL: slo-mo boiling. Vadim Nikolayev of the Ecole Superieure de Physique et de Chimie Industrielles in Paris thinks he can explain why certain industrial heat exchangers — such as those commonly used in power plants — occasionally experience a "boiling crisis." [Hat tip: AIP's Physics News Update] That's what happens when the steam gets so intense, the machines actually, um, melt. (Jen-Luc shudders to think what might happen were the heat exchangers subjected to the Sandia team's acoustic shock waves.) It's a bit like what happens when a water droplet hits a hot frying pan: it evaporates very slowly. Boiling is generally considered a form of accelerated evaporation by scientists who study the phenomenon, characterized by highly efficient energy transfer: energy moves from a heater to a liquid via the formation of vapor bubbles. Things get dangerous when the temperature gets too high: so many bubbles form that the entire heating element gets covered in a vaporous film,  preventing the liquid above from absorbing heat. So heat builds up, and the machine melts down. You can see the process in action here.

Scientists know that much, but they still don't understand the "boiling crisis" problem sufficiently well to devise effective counter-measures. Enter Nikolayev and other colleagues from the Commission of Atomic Energy in Grenoble and the University of Bordeaux. They simulated the boiling crisis to test a theory that the meltdown occurs as a result of vapor recoil. Just like the thrust from a rocket blast, a bubble will grow under intense heat and push aside any liquid near the heating element, giving rise to that dangerously insulating layer of vapor. Subsequent experiments confirmed the theory. The cool thing — literally — is that those experiments weren't performed at skin-blistering temperatures, but hear the critical temperature of liquid hydrogen: 33 degrees Kelvin. That's because boiling at normal thresholds is so rapid, it's difficult to make precise observations. At 33 degrees Kelvin, boiling happens much more slowly. And since the laws of fluid dynamics are universal, they should be able to extrapolate the same basic principles of behavior all the way up to fluids at 100 degrees Celsius.

Similarly, the lessons learned for large-scale industrial plants can be scaled down to the individual consumer level, perhaps to address the "boiling crisis"that occurs in, say, in exploding laptops. And you should definitely care: the smaller those electronic components get and the more densely they're packed, the higher the rate of heat dissipation will be. Heck, my spiffy new MacBook Pro is a great machine, but I've noticed that it does run a bit hotter than the older Power Book That Died (when the hard drive had a meltdown). I've invested in a "portable insulated laptop computer desk" (a.k.a., the Lapinator) accordingly, which mitigates the problem a little. At least it hasn't exploded or caught on fire. Yet.

There is a point to this typically lengthy and rambling post, if you recall: Phase transitions are cool. (I didn't say the point was especially profound.) They are still, centuries later, giving rise to cutting-edge science — like telling us more about the composition of Jupiter, or the potential for achieving a practical fusion energy source — and yet also helping human beings stay cool as they obsessively blog and surf the Web. So thanks, Sandia scientists, Dr. Nikolayev (and cohorts)!

18 thoughts on “under pressure”

  1. Nice blog. You are probably the most attractive woman I have ever seen write about physics. But then, this is a lot like thermodynamics, the only class in engineering I didn’t “get”. Your blog title is also clever. Have a visit to my blog and see if you like it.
    Take care,

  2. “…water is pretty conductive in its normal state”
    Actually, pure water is not a conductor in its normal state. It’s the ions (like Na+ and Cl- from dissolved salt) that carry the current. Sea water is approximately 1,000,000 more conductive than ultra-pure water. (See http://www.lenntech.com/water-conductivity.htm for example.) Excellent article though!

  3. Ah, count me as one of those obsessive surfers – one can learn so much if they hang around the right places. “Phase transitions” reminds me of an article I just read:
    “Researchers have now used x-rays to dissociate water at high pressure to form a solid mixture–an alloy–of molecular oxygen and molecular hydrogen. …
    The researchers subjected a sample of water to extremely high pressures–about 170,000 times the pressure at sea level (17 Gigapascals)–using a diamond anvil, and zapped it with high-energy x-rays.”
    Gigapascals is a great word, but I’m trying to wrap my mind around the diamond anvil part of this. Can anyone explain?

  4. Atmospheric pressure at sea level is 101 and a bit kilopascals, so 250 gigapascals is not quite 2.5 million atmospheres. (That’s one of the easier conversion factors to remember: it sure beats the heck out of converting electron volts to kilocalories per mole. . . .)

  5. I did my thesis work using a diamond anvil cell! Finally something I am qualified to comment on!
    The basic premise is quite simple. Mount two diamonds on opposing metal plates. They (the diamonds) are often in a modified brilliant (standard) cut, but have their points polished off to produce a flat tip. Place a small metal gasket between the diamonds and drill a hole in it smaller than the diamond flats. Place sample of interest in the hole, with some pressure medium (I used liquid argon), and screw the two plates together. Because the backs of the diamonds are usually much larger than their tips, you can convert a moderate force into a high pressure (p=F/A). This squeezes the incompressible fluid pressure medium, which transmits the pressure to your sample. I usually worked in the 40-50 kilobar pressure range (40-50,000 atmospheres = 4-5GPa), which is on the low side for most diamond anvil cell work.
    By the way, I can assure you that diamonds are NOT, in fact, “forever.” If you don’t tighten the corners of your cell properly, it is not hard to shatter one or both diamonds!

  6. According to SteveT, “If you don’t tighten the corners of your cell properly, it is not hard to shatter one or both diamonds!”
    There’s a great metaphor for romance in there, but I’m not bitter enough this morning to drag it out.

  7. Years ago (so many it hurts me to recall)I was laid to waste when I first came across the ice phase diagram – at the time sporting just 8 phases. Since then we’ve had poly water that threatened to jellify the world and turbo/cerebro rocker Joe Satriani treating us to ice 9. Water never ceases to amaze: a nice post Jennifer (et Jen Luc aussi)

  8. SteveT, thank you, that’s the kind of nuts and bolts explanation I was looking for. It must be fun “playing” in a lab (or similar), even when your diamonds shatter. I guess nothing lasts forever – in one state or another. (Entropy?) I guess we can safely assume your thesis went well!
    Blake, we definitely want only shiny happy metaphors on Fridays and Saturdays – the bitter mishaps can be dragged out on Mondays, when most of us are dragging ourselves into work. 🙂

  9. TBB, You’re most welcome! Yes working/playing in a lab is fun most of the time. There was a lot of stress (no pun intended!) associated with the diamond anvil cell work, though. I can still vividly remember (after 10 years) the nervous, clenched feeling I got in my stomach whenever I had to tighten the allen bolts on the cell to increase the pressure. I was always holding my breath, waiting to hear the snapping sound that signified that I had just reduced a ~1/2 carat, gem-quality diamond to a pile of white powder! Meeting with my advisor on those weeks was never a pleasant experience. To be fair though, he was a remarkably forgiving man. And yes, the thesis went well enough.
    One of my favorite parts of the work was watching exactly the kind of phase transition that Jennifer refers to here. Like water, my argon pressure medium would undergo a phase transition as I changed the cell pressure. At room temp, it was a molecular solid (like ice), and I could see the grain boundaries under a microscope. As the cell pressure increased, the argon would melt and the grain boundaries would disappear. You could go back and forth across the phase line pretty repeatably, watching the argon freeze and melt, all at room temp. I always thought this was WAY cool! I guess that fact that I can derive pleasure out of observing first order phase transitions might suggest that I am just a tad bit geeky.

  10. Great post on phase transitions. Really interesting.
    But as a charter member of the gotcha gang, niggling division, I have to note:
    “The cool thing — literally — is that those experiments weren’t performed at skin-blistering temperatures, but hear the critical temperature of liquid hydrogen: 33 degrees Kelvin.” Tsk tsk, Jennifer, 33K IS skin-blistering – and how! anything below about -60F causes blisters.

  11. Hi since I agree with the happy weekends, especially now that Halloween & bonfire night are over, …..
    Just to add to Steve T and the kind of pressures that make ‘real’ diamonds (and synthetic diamonds) or even shatter diamonds
    Diamonds are formed by prolonged exposure of carbon bearing materials to high pressure and temperature. On Earth, the formation of diamonds is possible because there are regions deep within the Earth that are at a high enough pressure and temperature that the formation of diamonds is thermodynamically favorable.
    Under continental crust, diamonds form starting at depths of about 150 kilometers (90 miles), where pressure is roughly 5 gigapascals and the temperature is around 1200 degrees Celsius (2200 degrees Fahrenheit).
    Diamond formation under oceanic crust takes place at greater depths because of higher temperatures, which require higher pressure for diamond formation. Long periods of exposure to these high pressures and temperatures allow diamond crystals to grow larger.
    250 gigapascals??? whoooa!
    Take it easy if checking tire pressures in Britain
    And just because it is Sun Day

  12. Lol, I kid you not, I fell asleep on the couch and discovered I am missing a synthetic diamond stud – I think one of you snuck in and stole it! 😉
    Seriously, though, it’s interesting to hear real live people talk about their lab experiences and their own trials and tribulations in the physics world, rather than just reading articles. Some of us non-scientists tend to put physicists on a pedestal, so we love it when you communicate on a level we understand. (Steve, now I’m curious about the real-world application of your thesis!)
    I see “geekiness” as a term that has now evolved to simply mean that someone likes and knows a subject that someone else doesn’t know or care to learn about. Even I have my own little private geeky moments (though my sister will use the more perjorative ‘nerd’ when she thinks I’m being boring). Physicists who embrace their geekiness have a lot of power at their disposable when they can communicate well.
    I give credit to Jennifer for causing me to revisit my Teaching Company catalogue; unfortunately the course I wanted was not on sale (if I’m going to spend nearly $300 I might as well go back to school and get credit for it!). I decided I should go back to basics and “revisit” my college “Physics for Non-Physics Majors” type of course. Elementary to you all, but I also want to see how this professor (http://www.teach12.com/store/professor.asp?id=251&d=Steven+Pollock) teaches it, besides eating humble pie and starting from the basics. The class I had years ago was theater-style seating, 100 or so students, dry lectures with no fun lab work, and multiple-choice tests. I didn’t study for the first test, got a D+, was horrified at that, but ended up with an A- for a final grade, so I know I can somewhat get it. The unfortunate thing about core college courses is that much of the information trickles out of the brain over time while one is pursuing their major, or just living. Professor Pollock appears to have a stellar resume and looks to be a friendly, polytech-looking sort of guy – hopefully he’ll be as good they say. (He also sells credit courses through his university.)
    Argh, I just looked and the Einstein Relativity course I wanted is now part of a set with what I just bought! There should be some mathematic principle about the chances of when you pay full price for something that it will then go on sale the next day. Time is *not* on my side.
    Thanks, Quasar9, for that interesting addition about diamonds and the link, which I’ll read when the caffeine gets pumping. I guess we can also thank Blaise Pascal for the barometer, syringe, and hydraulic press.
    Off topic: A retrospective of the late Edward Gorey is in town, and I’ve always been a big fan of his work. I read that besides his avid consumption and contributions to books, he admitted enjoying the X-Files and Buffy the Vampire Slayer – a factoid about him that I did not know. Of course this made me think of Jennifer. 🙂
    For those not familiar with Gorey: http://www.goreyography.com/west/west.htm
    OK, back to lurking…

  13. I said: Physicists who embrace their geekiness have a lot of power at their disposable when they can communicate well.
    …umm, that should be “disposal.” Sorry, brain –> fingers –> not awake yet.

  14. Wow, some terrific comments and discussion here, all happening the weekend I was out of town with no time for blog reading! Sorry for being silent for so long…
    Thanks to Urijah for reminding me about the salty aspect of water conductivity — it’s an error I make AGAIN and AGAIN, for some reason. One of those physics facts that just won’t stay in my brain.
    I am so glad Blake and Steve T. hashed out the diamond anvil bit, as I was wondering about that myself, yet had no time to nose around for more information. (I was pretty much relying on the meeting abstract and a lay language paper posted online.)
    Robinki, I’ve got an entire post pending about “The Gotcha Game” — awfully close to your “Gotcha Gang,” but I swear, I came up with it independently (although it’s likely I picked it up subconsciously from the Cyber-ether at some point). Should I ever catch up with my workload this week, I’ll get around to finishing and posting it. I will also check out Atul’s blog once time permits… I promise. And for the record, I never specifically said cold temperatures WEREN’T skin-blistering. 🙂
    TBB, I don’t think I’d pay $300 for a Teaching Company course; chances are it’ll go on sale eventually. I just noticed a course on statistics that seemed interesting… that’s an area where it’s extremely easy to get misled, or misunderstand something in an especially crucial way, so it wouldn’t hurt to bone up on that area. Although most journalism classes do cover, tangentially, the basics of how to read and use statistics in news stories, a refresher course is often useful from time to time.]
    Finally, I am a diehard Gorey fan, as everyone here probably knows. I own the Dracula Toy Theater and everything! So nice to hear he was a fellot X-Files and Buffy fan. The man was eccentric, but he certainly had taste. 🙂

  15. Hey, I just found your site. I love it!
    Water is a truly fascinating substance. It has many phase transitions (I lose track of how many different forms of “ice” there are, and how many phase transitions between the different forms that you can have). All that, and it is one of the more common chemical compounds in the universe. Truly remarkable.

  16. Hi, another diamond anvil cell user here. I tend to work in the 1-10 Gigapascal (or GPa as we like call it) range. A lot of very interesting things happen if you cool things down as well as squeezing them between diamonds. It turns out that a good 50-odd elements become superconducting (another phase transition) if you cool them to close to absolute zero, 23 of those needing at least some pressure. Even oxygen becomes metallic, and superconducting at high enough pressure.
    The lab I work in also does really really high pressure stuff, in the 100-200GPa range, and in that case you can sometimes only use the diamonds once! I did hear though that to get the highest pressures, you should select the diamond that didn’t break from a pair which didn’t survive a high pressure run. A bit like a high-stakes game of conkers. (Do you have that in the US? Its a schoolboy (mostly) game where you drill a hole in a horsechestnut (conker), dangle it from some string, and try and smash your opponent’s conker, with the winner staying on to face allcomers.)
    The experiments we do are really tricky in fact, since we often want to measure the electrical resistance of something inside a diamond anvil cell, which means spot-welding four tiny gold wires to a sample a few tenths of a mm across (about the size of a large grain of dust). Then we have to make sure the wires don’t short-circuit on the metal gasket, or get broken by the sharp edges of the diamond. It’s quite difficult!

  17. I have a new appreciation for diamond anvils – they’re not merely thingamajigs anymore. You guys rock!

  18. One atmosphere is about 100,00 pascals = 15 pounds/inch^2, for all you Americans with tires. A gigapascal is 10^9 pascals = 10,000 atmospheres, as previously mentioned. But how much is that *really*? Well, 10,000 atm = 150,000 pounds/inch^2, so you’d get a gigapascal of pressure if you took a 150,000 pound weight and balanced it (very carefully of course) on a post with the area of a postage stamp. A diamond anvil cell works in much this fashion except that scientists use a screw instead of a weight to apply force and they use diamonds for the posts. To get 250 GPa, you’d have to stack 250 of the 150,000 pound weights on top of the post. Don’t try this at home.

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