Tagget: Planetary Science
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- 23. august 2018 kl. 00:54 #318965
- Super Nova
Jeg blev opmærksom på denne artikel om metallisk deuterium, som blev omtalt som “TEKNOLOGI” på dr.dk. Artiklen indeholdt desuden nogle mystiske påstande.
Science 17 Aug 2018:
Vol. 361, Issue 6403, pp. 677-682
Dense fluid metallic hydrogen occupies the interiors of Jupiter, Saturn, and many extrasolar planets, where pressures reach millions of atmospheres. Planetary structure models must describe accurately the transition from the outer molecular envelopes to the interior metallic regions. We report optical measurements of dynamically compressed fluid deuterium to 600 gigapascals (GPa) that reveal an increasing refractive index, the onset of absorption of visible light near 150 GPa, and a transition to metal-like reflectivity (exceeding 30%) near 200 GPa, all at temperatures below 2000 kelvin. Our measurements and analysis address existing discrepancies between static and dynamic experiments for the insulator-metal transition in dense fluid hydrogen isotopes. They also provide new benchmarks for the theoretical calculations used to construct planetary models.
Jeg har medtaget lidt mere detaljeret tekst for at kunne bedømme, hvad artiklen egentlig omhandler.
The transformation of hydrogen from a molecular insulator to an atomic metal at high densities has been a longstanding focus in physics and planetary science. The unique quantum metallic properties of the low-temperature solid (i.e., below 300 K) have drawn sustained interest, and characterizing the transformation in the hot, dense fluid is crucial for understanding the internal structure and dynamics of giant planets, including the origin of their large magnetic fields. Numerous studies of the insulator-metal (IM) transition in dense fluid hydrogen, beginning with theoretical work five decades ago, predicted a first-order transition in the fluid with a critical point at very high temperatures (~13,000 to 15,000 K) and 60 to 90 GPa. However, the first experimental work on the IM transition in the fluid, carried out using dynamic compression techniques, provided evidence for a continuous transition with metallic states reached in the pressure range P = 50 to 140 GPa and temperatures T = 3000 to 8000 K . More recent predictions placed the critical point at a much lower temperature (~2000 K). This motivated several experimental studies using static diamond anvil cell (DAC) techniques and dynamic compression to probe the fluid properties below 2000 K and up to several hundred GPa.
Dynamic compression can explore a broad range of thermodynamic paths with time-varying manipulations of the applied pressure and controlled reverberation of pressure waves through the sample. This includes probing the dense fluid at temperatures below 2000 K, for example, with an initial jump in pressure delivered by a shock wave followed by shock reverberation or gradual ramp compression. The first demonstration of this strategy was carried out on deuterium with a magnetic compression technique at the Z facility. The results showed strong optical absorption beginning in the range 100 GPa < P < 130 GPa, followed by weak fluctuating reflectance in the range 130 GPa < P < 300 GPa, and culminated in abrupt jumps to high reflectance near 300 GPa. Knudson et al. attributed the absorption to band gap closure and determined that the reflectance jumps were associated with the first-order IM transition. The reflectance jumps occurred at higher pressures upon compression than upon decompression, plausibly as a result of thermal conduction. Meanwhile, improvements in static compression methods have allowed the exploration of the behavior of the fluid over part of this pressure-temperature (P-T) range (up to 170 GPa and >1800 K). Changes in optical properties from 120 to 170 GPa depending on temperature were attributed to the IM transition, whereas other experiments suggest the persistence of a finite (~1 eV) band gap at similar conditions.
The IM transition is the subject of a number of continuing theoretical studies that consistently predict a discontinuous transition below a critical point near ~2000 K, but over a broad range of pressures. Density functional theory (DFT)–based calculations show a spread in the transition pressure spanning 150 GPa, arising from the sensitivity of the boundary to the choice of exchange-correlation functional used and whether zero-point energy is accounted for. Quantum Monte Carlo (QMC) calculations should provide improved bounds on the transition pressures, although they disagree with a recent benchmarking experiment. Transition pressures for hydrogen and deuterium are expected to be different because of isotope effects, but with a small relative magnitude. The transition in deuterium from QMC simulations is 30 GPa higher than in hydrogen at 600 K, decreasing to 10 GPa higher at 1200 K. Despite experimental support for a first-order IM transition, the critical point has not been experimentally identified. Furthermore, the broad discrepancies in the measured transition pressure and character have made resolving the differences between the theoretical models challenging.
We completed a series of five dynamic compression experiments at the National Ignition Facility (NIF) to probe the IM transition up to 600 GPa at temperatures ranging from 900 K to 1600 K. The experiments were carried out using 168 laser beams to deliver up to 300 kJ of ultraviolet light that drove a near-isentropic reverberation compression of a cryogenic liquid deuterium sample. We adjusted the time dependence of the laser delivery (pulse shape) to control the compression sequence imposed on the sample as a function of time. Line-imaging Doppler velocimetry recorded both the compression history and the evolution of the optical properties of the D2 sample during the nanosecond compression process, using a probe laser operating at 660 nm.
Jeg blev ledt til denne artikel om “teknologi” på dr.dk:
“Et forskerhold har med verdens kraftigste laser skabt samme materiale, som findes i kernen af store stjerner.”
Dette lyder umiddelbart mystisk, men der er et billede af Jupiter med teksten:
I et laboratorie i USA har et forskerhold udsat gassen brint for samme forhold, som det bliver udsat for inderst inde i store planeter som Jupiter. Resultatet var metallisk brint – et stof som har et enormt potentiale for fremtidens teknologi.
Her er det “fremtidens teknologi”, som lyder mystisk. Længere nede læser man:
Det kan skabe revolutioner i alt fra magnet-tog, el-biler og alt anden elektronik. Derudover er metallisk brint det mest kraftfulde raketbrændstof, vi kender til.
Hvordan har man så tænkt sig, at den metalliske brint forbliver i denne tilstand, hvis man fjerner trykket på 200 milliarder Pascal?
Der står absolut intet herom i artiklen. Formålet med artiklen er forbedre modellerne for de store gasplaneter som Jupiter og Saturn. Temperaturen blev varieret mellem 900 K og 1600 K.
Bemærkningen om det “kraftfulde raketbrændstof” stammer fra dette spekulative fase-1 studie:
Atomic metallic hydrogen, if metastable at ambient pressure and temperature could be used as the most powerful chemical rocket fuel, as the atoms recombine to form molecular hydrogen. This light-weight high-energy density material would revolutionize rocketry, allowing single-stage rockets to enter orbit and chemically fueled rockets to explore our solar system. To transform solid molecular hydrogen to metallic hydrogen requires extreme high pressures, but has not yet been accomplished in the laboratory. In the proposed new approach electrons will be injected into solid hydrogen with the objective of lowering the critical pressure for transformation. This new approach may scale down the pressures needed to produce this potentially revolutionary rocket propellant.
Dette ville i sandhed være en sensation! Men bemærk formuleringerne: “if metastable”, “would revolutionize” og “This new approach may scale down the pressures needed to produce this potentially revolutionary rocket propellant”. Jeg har kun fundet denne artikel om fast metallisk hydrogen:
We have studied solid hydrogen under pressure at low temperatures. With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa. We have measured the reflectance as a function of wavelength in the visible spectrum finding values as high as 0.90 from the metallic hydrogen. We have fit the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T= 5.5 K, with a corresponding electron carrier density of 6.7×1023 particles/cm3, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.
Men dette er sket ved 495 milliarder Pascal, ikke ved et lavt tryk. Metastabilt fast metallisk brint ved lavt tryk forbliver fugle på taget.
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