共查询到20条相似文献，搜索用时 31 毫秒
1.
Carpenter MA 《Science (New York, N.Y.)》1979,206(4419):681-683
Combined annealing experiments and observations by transmission electron microscopy show that in natural pigeonite crystals antiphase domains coarsen approximately according to a rate law in which the tenth power of the average domain size is proportional to time. This result suggests that certain cations (possibly Ca(2+)) were segregated preferentially onto the antiphase boundaries. The domain size in samples quenched from above the high-low transformation temperature is large and apparently independent of annealing time and temperature. It appears that large domains can be generated either by very fast or by very slow cooling; thus the estimation of geological cooling rates from the sizes of antiphase domains in natural samples becomes rather difficult. 相似文献
2.
Goethite, known to be antiferromagnetic, acquires thermoremanent magnetization at its Neel temperature of 120 degrees C. This remanence, extremely stable, is due to the presence of unbalanced spins in the antiferromagnetic structure; the spins may result from grain size, imperfections, or impurities. 相似文献
3.
C(60) powders were shock-compressed quasi-isentropically and quenched from pressures in the range 10 to 110 GPa (0.1 to 1.1 Mbar). Recovered specimens were analyzed by Raman spectroscopy and optical microscopy. C(60) fullerenes are stable into the 13- to 17-GPa pressure range. The onset of a fast ( approximately 0.5 micros) reconstructive transformation to graphite occurs near 17 GPa. The graphite recovered from 27 GPa and about 600 degrees C is relatively well ordered with crystal planar domain size of about 100 A. Above 50 GPa a continuous transformation to an amorphous state is observed in recovered specimens. The fast transformation to graphite is proposed to occur by pi-electron rehybridization which initiates breakup of the ball structure and formation of the graphite structure at high density. 相似文献
4.
Marques L Mezouar M Hodeau J Nunez-Regueiro M Serebryanaya NR Ivdenko VA Blank VD Dubitsky GA 《Science (New York, N.Y.)》1999,283(5408):1720-1723
High-pressure studies on fullerenes have previously shown the existence of one- and two-dimensional (2D) polymerized C60 structures. Synchrotron radiation measurements, performed on C60 samples quenched from 13 gigapascals and 820 kelvin, yield unambiguous proof for the existence of a three-dimensional (3D) polymerized C60 derivative. Moreover, unusual ellipsoidal Debye-Scherrer diffraction patterns are observed, which shows that the giant anisotropic deformation induced by the nonhydrostatic compression is retained in the quenched samples. The multiple bonding possibilities of the highly symmetrical C60 allow the retention (down to ambient pressure) of the deformation, a phenomenon reported previously only under high pressure. 相似文献
5.
Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with (29)Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na(2)Si(2)O(5) glass quenched from 8 gigapascals and 1500 degrees C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure. 相似文献
6.
Natural pyrrhotites, when studied by high resolution electron microscopy, yield crystallographic information on a unit cell scale. Structural heterogeneity is prominent. The many reported superstructures are interpretable through an antiphase model. The 5C pyrrhotite superstructure results from an ordered sequence of antiphase domains while the higher temperature NC type results from a disordered sequence. 相似文献
7.
Ratcliff CD Geissman JW Perry FV Crowe BM Zeitler PK 《Science (New York, N.Y.)》1994,266(5184):412-416
Late Miocene (about 8.65 million years ago) mafic intrusions and lava flows along with remagnetized host rocks from Paiute Ridge, southern Nevada, provide a high-quality paleomagnetic record of a geomagnetic field reversal. These rocks yield thermoremanent magnetizations with declinations of 227 degrees to 310 degrees and inclinations of -7 degrees to 49 degrees , defining a reasonably continuous virtual geomagnetic pole path over west-central Pacific longitudes. Conductive cooling estimates for the intrusions suggest that this field transition, and mafic magmatism, lasted only a few hundred years. Because this record comes principally from intrusive rocks, rather than sediments or lavas, it is important in demonstrating the longitudinal confinement of the geomagnetic field during a reversal. 相似文献
8.
Wentorf RH 《Science (New York, N.Y.)》1970,169(3941):175-176
The high density, 1.4 grams per cubic centimeter, reported for anomalous water suggests that high pressures should be conducive to the formation of anomalous water. Six attempts at 60 kilobars in which water was cooled from about 600 degrees C in nickel or platinum tubes, with or without the presence of silica, did not produce any detectable amounts of anomalous water. 相似文献
9.
Unblocking temperatures of natural remanent magnetization were found to extend well above the dominant Curie points in samples of oceanic basalts from the axis of the East Pacific Rise. This phenomenon is attributed to the natural presence in the basalts of three related magnetic phases: an abundant fine-grained and preferentially oxidized titanomagnetite that carries most of the natural remanent magnetism, a few coarser and less oxidized grains of titanomagnetite that account for most of the high-field magnetic properties, and a small contribution to both the natural remanent magnetism and high-field magnetic properties from magnetite that may be due to the disproportionation of the oxidized titanomagnetite under sea-floor conditions. This model is consistent with evidence from the Central Anomaly magnetic high that the original magnetization acquired by oceanic basalts upon cooling is rapidly altered and accounts for the lack of sensitivity of bulk rock magnetic parameters to the degree of alteration of the remanence carrier in oceanic basalts. 相似文献
10.
Poudel B Hao Q Ma Y Lan Y Minnich A Yu B Yan X Wang D Muto A Vashaee D Chen X Liu J Dresselhaus MS Chen G Ren Z 《Science (New York, N.Y.)》2008,320(5876):634-638
The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100 degrees C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250 degrees C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86 degrees , 106 degrees , and 119 degrees C with hot-side temperatures set at 50 degrees, 100 degrees, and 150 degrees C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials. 相似文献
11.
The solid electrolytes MAg(4)I(5), where M may be potassium, rubidium, or ammonium, have an ionic conductivity of 0.2 (ohm cm)(-1) at 20 degrees C. Although pure CsAg(4)I(5) does not form, partial substitution of cesium into the M position was obtained. The high-conducting phases of KAg(4)I(5) and RbAg(4)I(5) may be quenched to low temperatures, where they transform to resistive phases at -136 degrees and -155 degrees C, respectively. 相似文献
12.
Methods used to strengthen metals generally also cause a pronounced decrease in electrical conductivity, so that a tradeoff must be made between conductivity and mechanical strength. We synthesized pure copper samples with a high density of nanoscale growth twins. They showed a tensile strength about 10 times higher than that of conventional coarse-grained copper, while retaining an electrical conductivity comparable to that of pure copper. The ultrahigh strength originates from the effective blockage of dislocation motion by numerous coherent twin boundaries that possess an extremely low electrical resistivity, which is not the case for other types of grain boundaries. 相似文献
13.
Dunlop DJ 《Science (New York, N.Y.)》1972,176(4030):41-43
Maximum values for the single-domain threshold d(0) and superpara-magnetic threshold d(8) in pure magnetite are found to be 570+/-50 and 350+/-50 angstroms, respectively. Particles larger than do but smaller than about 0.25 micron have size-dependent saturation remanences and coercive forces like those of multidomain particles, but intense and stable thermoremanent magnetization like that of single-domain particles. The presence of magnetite grains in this size range could account for the essentially single-domain character of stable natural remanence in many volcanic and intrusive rocks. 相似文献
14.
The polymorphism of sulfur has been investigated by static and dynamic methods up to 500 degrees C at 35 kilobars and up to 350 degrees C at 100 kilobars. The melting curve of sulfur to 31 kilobars and phase boundaries of the so-called "4.04-angstrom phase" have been determined. Evidence has been obtained for phase fields of nine new high-pressure forms of sulfur. 相似文献
15.
The tetragonal garnet (Mg,Fe)SiO(3) is a high-pressure phase of pyroxene that is thought to be a major constituent of the earth's upper mantle. Its crystal structure is similar to that of cubic garnet, but it is slightly distorted to tetragonal symmetry so that its x-ray powder diffraction pattern shows a very small line splitting. A suite of tetragonal garnets with different compositions in the MgSiO(3)-rich portion of the MgSiO(3)-FeSiO(3) system was synthesized at about 20 gigapascals and 2000 degrees C. The lattice parameters a and c of quenched samples were determined by whole-powder-pattern decomposition analysis of Fe Kalpha x-ray powder diffraction data, which has the capacity to resolve to a high degree heavily overlapping reflections. It was found that the lattice parameters can be obtained from the following equations; a (in angstroms) = 11.516 + 0.088x and c (in angstroms) = 11.428 + 0.157x, where x, teh mole fraction of FeSiO(3), is 0.0 相似文献
16.
Tetragonal zirconium oxide, stable at room temperature, was synthesized at 15 to 20 kb and 12000 to 17000C. Rapid quenching produced a mixture of the monoclinic and tetragonal phases. The high-pressure specimens, when heated to 1200 degrees C in air and quenched, reverted to the all-monoclinic form. 相似文献
17.
Ramesh R Hwang DM Venkatesan T Ravi TS Nazar L Inam A Wu XD Dutta B Thomas G Marshall AF Geballe TH 《Science (New York, N.Y.)》1990,247(4938):57-59
The defect structure of in situ pulsed, laser-deposited, thin films of the high-transition temperature superconductor Y-Ba-Cu-O has been observed directly by atomic resolution electron microscopy. In a thin film with the nominal composition YBa(2)Cu(3)O(7) (123), stacking defects corresponding to the cationic stoichiometry of the 248, 247, and 224 compounds have been observed. Other defects observed include edge dislocations and antiphase boundaries. These defects, which are related to the nonequilibrium processing conditions, are likely to be responsible for the higher critical currents observed in these films as compared to single crystals. 相似文献
18.
Dynamics of a Second-Order Phase Transition: P macr; to I1 Phase Transition in Anorthite, CaAl2Si2O8
Electron microscopic study of the reversible P1 to I1 phase transition in anorthite (transition temperature T(c) = 516 Kelvin) shows that the antiphase boundaries (APBs) with the displacement vector R = 1/2[111] become unstable at T(c), and numerous small APB loops are formed. These interfaces are highly mobile, and their vibration frequency increases strongly with temperature. These observations suggest that close to T(c), breathing-motion-type lattice vibrations of the framework cause the two different configurations around the calcium atoms, which are related by a translation of R approximately 1/2[111], to interchange dynamically through an intermediate I1 configuration. The high-temperature I1 structure is interpreted as a statistical-dynamic average of highly mobile antiphase domains of primitive anorthite. 相似文献
19.
Limits of microbial existence: temperature and pH 总被引：10，自引：0，他引：10
A microscopic survey made to detect the presence of bacteria in hot springs of varying temperature and pH characteristics revealed that in neutral and alkaline hot springs bacteria are found at temperatures up to the boiling point of water (92 degrees to 100 degrees C, depending on the altitude). In hot springs of increasing acidity the upper temperature limit at which bacteria are found decreases; at pH 2 to 3 the upper temperature limit is 75 degrees to 80 degrees C. Bacteria have thus been able to evolve with the ability to grow at either high temperature or high acidity, but not at both high temperature and high acidity. These results suggest that there are physicochemical limitations of the environment beyond which life is impossible. 相似文献
20.
AE Pashitski A Gurevich AA Polyanskii DC Larbalestier A Goyal ED Specht DM Kroeger JA DeLuca JE Tkaczyk 《Science (New York, N.Y.)》1997,275(5298):367-369
Magneto-optical imaging was used to visualize the inhomogeneous penetration of magnetic flux into polycrystalline TlBa2Ca2Cu3Ox films with high critical current densities, to reconstruct the local two-dimensional supercurrent flow patterns and to correlate inhomogeneities in this flow with the local crystallographic misorientation. The films have almost perfect c-axis alignment and considerable local a- and b-axis texture because the grains tend to form colonies with only slightly misaligned a and b axes. Current flows freely over these low-angle grain boundaries but is strongly reduced at intermittent colony boundaries of high misorientation. The local (<10-micrometer scale) critical current density Jc varies widely, being up to 10 times as great as the transport Jc (scale of approximately 1 millimeter), which itself varies by a factor of about 5 in different sections of the film. The combined experiments show that the magnitude of the transport Jc is largely determined by a few high-angle boundaries. 相似文献