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METALLIC HYDROGEN

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What surprises lie in hydrogen - the most common element of the universe? It would seem that everything has been studied for a long time. But still this topic is not yet closed today.

Back in 1935, the classic work of E. Wigner and H. Huntington appeared, in which they first assumed that hydrogen at high pressures from a dielectric gas would become a conductive metal. According to their calculations, solid metallic hydrogen should have a body-centered lattice (at 0 K and zero pressure), and its density under the same conditions should be much higher than the density of solid molecular hydrogen (0.59 g / cm ³ instead of 0.089 g / cm 3 ³ ).

The transformation, according to the authors, was to occur at a pressure of about 250,000 atm., And in addition, they believed that the transition requires the embryos of a new phase. In 1968, N. Ashcroft predicted that metallic hydrogen would have completely unusual properties, for example, superconductivity at high temperatures (more than 200K). Moreover, scientists have suggested that metallic hydrogen will be in the form of a liquid. This further deepened the curiosity of the researchers. The problem
Compressed hydrogen were included in the list of the most important problems of solid state physics.

The simplest molecule turned out to be quite difficult, almost seventy years, and scientists not only did not receive metallic hydrogen, but do not even have the exact theoretical methods for constructing a model of this process.

Peak studies of metallic hydrogen fell in the 60-70s of the last century. This problem was interesting, in particular, to astrophysicists. The sun and heavy planets (Jupiter, Saturn) are more than 90% hydrogen. In addition, scientists suggest that since Jupiter has a rather low temperature (100-200 K) and a strong magnetic field, then if hydrogen is in the metallic phase and exhibits its superconducting properties, this should lead to a variety of interesting phenomena. But the most interesting thing is that the problem of superconducting metallic hydrogen, perhaps, is not at all theoretical, but quite applied.

In 1971, the work of our theoreticians (Yu. Kagan's group) appeared, which proved that metallic hydrogen could prove to be metastable. This means that after removing the high pressure, hydrogen does not turn back into a dielectric gas, but remains a metal. The question is, will the time of existence of such a metastable phase be sufficient to measure its properties and have time to apply it.

A well-known example is an artificial diamond (the metastable phase of carbon, into which a stable graphite phase transforms). The lifetime of a metastable diamond is so great that mankind has been using it for more than a decade. Well, about what superconducting hydrogen at almost normal temperatures is useful for, it is possible to build assumptions for a long time.

While this is all fantasy. As it will in fact, it is unknown, since no one has ever been able to "hold in hand" metallic hydrogen.
Although as soon as it was not squeezed!

To obtain ultra-high pressures, either diamond anvils (static compression) or explosive methods (dynamic compression) are usually used.

Diamond anvil device is quite simple and small (though it costs 10 000 dollars). Two diamonds are cut in a special way (and this is very difficult) and a sample is placed between their central flat surfaces inside the cavity. In the cavity, there must be a metal gasket. After the stones are squeezed, a pressure acting on the sample is inversely proportional to the area of ​​the lower flat part of the diamond, whose diameter is 20 to 600 μm.

It is very difficult to work with hydrogen. He not only physically penetrates the metal of the gasket and makes it brittle, but also enters into chemical reactions with it, forming hydrides. Compressed to a certain pressure, hydrogen passes into a molecular crystalline state, turning into a rather unusual substance. Probably, this is due to the properties of the hydrogen molecule, it is so light that even in a solid crystalline state at small pressures the molecules continue to rotate.

In the last quarter century after the invention of diamond anvils, researchers have systematically studied the properties of solid hydrogen up to
At the present time, scientists know that even at these pressures, there are at least three phases of metallic hydrogen, each of which makes a dielectric-metal transition at its pressure. One at 1.6 million atm. , When other phases still remain dielectrics. The latest theoretical data allow us to hope that all hydrogen will pass into the metallic phase at 4 million atm. (At 0 ° K)

Again, the question remains whether hydrogen is decayed into atoms or remains in the molecular state. It is already known that the "colleagues" of hydrogen in the properties of bromine and iodine become conductors at high pressure precisely in the process of melting, that is, in atomic form. On the other hand, there is evidence that in static experiments at the pressures reached, hydrogen is mainly in the form of molecules.

A much more productive method of obtaining high pressures is the explosive method, when the experimenters hit the cell with the sample with metal plates or a gas jet accelerated to hypersonic speeds. Now there are installations for single shock compression, in which hydrogen can be compressed to 10 million atm.

At the time of impact, when the pressure reaches millions of atmospheres, hydrogen inevitably heats up to thousands of degrees Kelvin and goes into a liquid state. Scientists are trying to figure out how to reduce temperatures in the experiment, but so far it's still thousands of degrees. Moreover, in microseconds, when the shock wave ends, hydrogen again becomes a gas, so it's very difficult to measure something.

But, solving the problem of the atomic bomb, scientists have learned to cope with this. In dynamic experiments, the density of hydrogen is measured by radiating a sample by X-ray radiation, or judged by what happens, by signals from optical and electrical sensors. Thus, the pressure in such experiments is a calculated value.

The last record is 15 million atm. Great pressures were achieved by scientists from the Livermore National Laboratory (USA), and in Russia by researchers from the All-Union Scientific Research Institute of Experimental Physics (Sarov) and the Institute of Problems of Chemical Physics of the Russian Academy of Sciences (Chernogolovka).

Measuring the resistance in dynamic experiments, the researchers saw that hydrogen becomes a conductor, with conductivity almost like that of liquid metals. But this conductivity still weakly depended on temperature, which indicates that hydrogen is not yet a metal. Scientists characterize the state of hydrogen, which they observe in dynamic experiments, as a "disordered conducting medium" (disordered as temperatures are too high) or "dense low-temperature non-ideal plasma," and the emerging "ionization by pressure" conduction effect

So what are we waiting for ...

print version
PS The material is protected.
Date of publication 15.01.2004гг

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