Platinum metal was shock compressed to 660 GPa using a two-stage light-gas gun to qualify this material as an ultrahigh-pressure standard for both dynamic and static experiments. The shock velocity data are consistent with most of the previously measured low-pressure data, and an overall linear us−up relationship is found over the range 32–660 GPa. As a part of this work, we have also extended the Hugoniot of the tantalum standard we use to 560 GPa; we have included these data into a new linear fit of the tantalum Hugoniot between 55–560 GPa. We also present the results of a first-principles theoretical treatment of compressed platinum. The fcc phase is predicted to remain stable to beyond 550 GPa. In addition, we have calculated the 300-K pressure-volume isotherm and the Hugoniot. The latter is in excellent agreement with experimental results and qualifies the former to at least 10% accuracy.
Seismological data can yield physical properties of the Earth's core, such as its size and seismic anisotropy. A well-constrained iron phase diagram, however, is essential to determine the temperatures at core boundaries and the crystal structure of the solid inner core. To date, the iron phase diagram at high pressure has been investigated experimentally through both laser-heated diamond-anvil cell and shock-compression techniques, as well as through theoretical calculations. Despite these contributions, a consensus on the melt line or the high-pressure, high-temperature phase of iron is lacking. Here we report new and re-analysed sound velocity measurements of shock-compressed iron at Earth-core conditions. We show that melting starts at 225 +/- 3 GPa (5,100 +/- 500 K) and is complete at 260 +/- 3 GPa (6,100 +/- 500 K), both on the Hugoniot curve-the locus of shock-compressed states. This new melting pressure is lower than previously reported, and we find no evidence for a previously reported solid-solid phase transition on the Hugoniot curve near 200 GPa (ref. 16).
The temperature of shock compressed iron has been measured to 340 GPa, using well characterized iron films sputtered on transparent diamond substrates and a 1 ns time-resolved optical method. We find a knee on the (P, T) iron Hugoniot indicating melting at 6350 K and 235 GPa and at 6720 K and 300GPa. An extrapolation yields an iron melting temperature of 6830 (+ 500) K at 330 GPa, the pressure of the Earth inner-outer core boundary. Implication of the melting data for the iron phase diagram is also discussed. PACS numbers: 62.50.+p, 64.70.Dv The phase diagram of iron, in addition to being of intrinsic scientific interest, provides a critical constraint for modeling the chemical composition and energy balance of the Earth's core. The Earth's core contains mostly iron distributed in two layers: the solid inner layer of nearly pure iron and the liquid outer layer of iron alloys with lighter elements like S, 0, H, Si, Mg, etc. Thus, the iron melting temperature at the pressure of the Earth innercore and outer-core boundary (IOB), 330 GPa, may provide an upper bound for the temperature. The recrystallization of iron occurring at the boundary releases latent heat and gravitational energy which provide the heat necessary for convection in the outer core and produce Earth's magnetic field [1]. Current Earth core models rely strongly on extrapolations of the melting data of iron from below 100 GPa.Ho~ever, these extrapolations not only give a large uncertainty in the IOB temperature ranging from 4000 to 9000 K [1], but also yield phase diagrams that are qualitatively diA'erent from one another at the IOB conditions [2,3]. For example, the melting temperature reported by Williams and co-workers [2] increases rapidly with pressure, the extrapolation of which results in a e-y-liquid iron triple point at the IOB pressure 330 GPa and 7600 K. On the other hand, Boehler, von Bargen, and Chopelas [3] present the e-y-liquid triple point at the substantially lower pressure of 100 GPa and 2800 K and suggest an IOB temperature near 4200 K. Recently, the situation has become confused even further by the findings of Boehler [4] and Saxena, Shen, and Lazor [5] of a new solid phase of unknown structure in what has been believed to be the stability field of the e phase. Brown and McQueen [6] have observed two discontinuous changes of the iron sound velocity at shock pressures of 200 and 240 GPa, which are attributed to phase transitions of iron and the latter to melting. However, the temperatures were not measured, but were estimated from the shock energy to be 5800~500 K at the IOB pressure.A direct method for obtaining melting temperatures above a megabar and several thousand degrees is by measuring shock temperatures [7]. This is typically done by optical pyrometry, which measures the thermal radiation of shocked materials at several discrete wavelengths. However, di%culties are introduced in the case of nontransparent materials like metals, because of a thin optical penetration depth -20 nm and a short shock wave transit time over ...
Temperatures of shock compressed liquid deuterium and hydrogen up to 5200 K were measured at pressures up to 83 GPa (830 kbar). The measurements are in excellent agreement with earlier calculations to about 20 GPa and show evidence for dissociation above 20 GPa. At the highest measured temperatures and pressures current theories break down and a revised theory is proposed.
Diversity in 39 HLA-A, -B, and -C molecules is derived from 20 amino acid positions of high variability and 71 positions of low variability. Variation in the structurally homologous a, and a2 domains is distinct and may correlate with partial segregation of peptide and T-cell receptor binding functions. Comparison of 15 HLA-A with 20 HLA-B molecules reveals considerable locus-specific character, due primarily to differences at polymorphic residues. The results indicate that genetic exchange between alleles of the same locus has been a more important mechanism in the generation of HLA-A, -B, and -C diversity than genetic exchange events between alleles of different loci.Class I major histocompatibility complex glycoproteins are peptide-binding proteins that present processed antigens to cytotoxic T lymphocytes. The genes coding for these molecules are the most polymorphic loci known in higher vertebrates and for humans a total of 19 HLA-A, 37 HLA-B, and 8 HLA-C molecules have been defined (1). Although the basic features of class I molecules are well defined (2), accumulation of allelic sequences has been slow. The paucity of sequences has limited our understanding of the scope of the polymorphism, its function, and its generation. We present here a comparison of 39 HLA-A, -B, and -C sequences and a general assessment of their patterns of diversity. MATERIALS AND METHODSGenomic clones encoding HLA-A1, -B8, -B14, -B18, -Bw4l, -Bw42, -B44.2, -Bw65, and -Cw2.2 were isolated from the following cell lines: S. Gar (HLA-Aw24,3;B18,w41;Cw6), BB (HLA-Aw68.2,30;Bw42,w6S), FMB (HLA-AJ,32;$44,w57; Cw5,w6), MRWC (HLA-A2,32$JA,27;Cw2), MVL (HLAAw32;B27;Cw2), and LCL721 (HLA-Ai,2$.i,5). The cloned genes are underlined. Construction of libraries, isolation and identification of genes, and sequencing of genes with exonspecific oligonucleotide primers were as described (3). Genomic clones encoding HLA-A1 and HLA-B8 were kindly provided by H. T. Orr (University of Minnesota) (4).RESULTS AND DISCUSSION Variability in HLA-A, -B, and -C Molecules. Genes encoding HLA-A1, -B8, -B14, -B18, -B44, -Bw4l, -Bw42, -Bw65, and -Cw2 were isolated, and the exons were sequenced. The sequences of the HLA-B44 and HLA-Cw2 proteins each differ by 3 amino acids from identically typed molecules isolated from other cell lines (5, 6). These represent distinct subtypes, and we have designated them as HLA-B44.2 and HLA-Cw2.2 compared to HLA-B44.1 and HLA-Cw2.1 for the published sequences (5, 6). (a,, a2, and a3) that interact with antigenic peptides, the T-cell receptor, and the CD8 molecule. As single residues predominate at most positions, a consensus sequence can be made (Fig. 1). Individual molecules differ from the consensus by [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] residues, showing all have considerably diverged from a common ancestor.Without knowing the total number of alleles, it is difficult to predict how many sequences are required to gain an accurate description of HLA-A, -B, and -C polymorphism...
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