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排序方式: 共有172条查询结果,搜索用时 15 毫秒
1.
2.
Qian Y Engel MH Macko SA Carpenter S Deming JW 《Geochimica et cosmochimica acta》1993,57(14):3281-3293
Dipeptide hydrolysis and amino acid decomposition appear to follow a first-order rate law. The hydrolysis rate increases exponentially with increasing temperature in aqueous solution at both 265 atm and water steam pressures over the temperature range of 100 to 220 degrees C. Dipeptide hydrolysis has a lower apparent activation energy at 265 atm (44.1 KJ/mol) than at water steam pressure (98.9 KJ/mol). At lower temperatures (<200-220 degrees C), the rate of peptide bond hydrolysis is faster at 265 atm than at water steam pressure. At higher temperatures (>200-220 degrees C), however, peptide bond hydrolysis is slower at 265 atm than at water steam pressure. In aqueous solution, amino acid decomposition rates also increase exponentially with increasing temperature. Amino acid decomposition rates are much higher at 265 atm than at water steam pressure over the entire temperature range investigated. 相似文献
3.
S. C. Tarantino M. Zema F. Maglia M. C. Domeneghetti M. A. Carpenter 《Physics and Chemistry of Minerals》2005,32(8-9):568-577
A suite of (Mn1-x
Fe
x
)Nb2O6 (x=0, 0.05, 0.25, 0.50, 0.75, 1) columbite samples has been prepared by solid-state reaction from oxides. X-ray diffraction
and spectroscopic investigations have been carried out in order to gain different perspectives on how the solid solution adapts
at different length scales to cation mixing. X-ray powder diffraction and powder absorption IR spectroscopy data are presented.
The powder diffraction data show that there is no significant excess volume of mixing on the Fe–Mn columbite join. All the
unit-cell parameters decrease linearly as a function of increasing Fe content. Substitution of Fe2+ for the larger Mn2+ cation causes a decrease in the volume of the A polyhedron, which also becomes more regular with respect to both bond-length
and edge-length distortion parameters. No significant variation of the B site has been observed. Wavenumber shifts of the
IR peaks nearly all vary linearly with composition, consistent with linear variations of the lattice parameters. Line broadening
has been quantified by autocorrelation analysis of the IR spectra. This is interpreted as suggesting that there is some element
of local strain or positional disorder at the length scale of second or third nearest neighbours around sites occupied by
Fe. 相似文献
4.
Origin of spongy textures in clinopyroxene and spinel from mantle xenoliths, Hessian Depression, Germany 总被引:1,自引:0,他引:1
Summary Spongy textures are observed in anhydrous Group 1 mantle xenoliths (harzburgite, lherzolite and wehrlite) hosted in Tertiary
alkali basaltic lavas from the Hessian Depression, Germany. These textures are developed only on clinopyroxene and spinel,
and occur as rims or cross-cutting veinlets and patches showing optical continuity with the host grain. They are often associated
with pools of amorphous glassy material. There is no preferential development of spongy domains against the xenolith-lava
contact suggesting that the host magma did not play any significant role in their formation. Spongy clinopyroxene and spinel
occur in all rock types, but, are more pervasive in wehrlite.
Chemically, spongy domains of clinopyroxene and spinel are more refractory than unaffected areas, which is consistent with
their formation through a partial melting event. The associated glassy material shows chemical characteristics which suggest
that the melt pools are genetically related to the development of the spongy textures. The partial melting event was probably
triggered by the infiltration of a low-density fluid. The fluid may have evolved from a silicate melt responsible for the
metasomatic Fe-enrichment recorded in wehrlite. In this context, the more pervasive development of spongy clinopyroxene in
wehrlite may be explained by a higher concentration of the evolved fluid phase at proximity to its silicate melt source.
Received March 15, 2000; revised version accepted September 6, 2001 相似文献
5.
An index of biotic integrity based on the summer polyhaline zooplankton community of the Chesapeake Bay 总被引:1,自引:0,他引:1
A zooplankton index of biotic integrity was developed for the polyhaline waters of the Chesapeake Bay using data from a long-term environmental assessment program in which both zooplankton and water quality were regularly monitored. Summer (July to September) sampling events were classified as either coming from impaired or reference (least-impaired) conditions based on water quality conditions. Seventeen zooplankton community metrics were evaluated under these criteria and nine were chosen for a composite index. These were the Simpson diversity index, and abundance of barnacle larvae, rotifers, cladocerans, copepods, total mesozooplankton, and predators. The composite index of biotic integrity correctly classified about 94% of the impaired samples and about 82% of the reference samples. Average classification efficiency was 88%. This index appears to be an effective measure of eutrophication for the summer polyhaline waters of the Chesapeake Bay ecosystem. 相似文献
6.
7.
M. A. Carpenter R. J. Angel L. W. Finger 《Contributions to Mineralogy and Petrology》1990,104(4):471-480
New single crystal diffraction data for natural and heat-treated anorthite crystals (Angel et al. 1990) allow the determination of their states of Al/Si order in terms of a macroscopic order parameter,Q
OD
, for the transition. Numerical values ofQ
OD
obtained from estimates of site occupancies are shown to vary with the scalar spontaneous strain,
s
, as
s
Q
OD
2
, and with the ratio of the sums of typeb (superlattice) reflections and typea (sublattice) reflections asI
b/I
a
Q
OD
2
. An empirical calibration for pure anorthite is obtained giving
varies between 0.92 and 0.87 in samples equilibrated at T1300° C, but then falls off relatively rapidly with increasing temperature, reaching 0.7 near the melting point ( 1557° C). The observed temperature dependence does not conform to the predictions of the simplest single order parameter models; coupling ofQ
OD
withQ of the transition is suspeeted. 相似文献
8.
9.
临界转换的早期预警信号 总被引:1,自引:0,他引:1
Marten Scheffer George Sugihara Jordi Bascompte Victor Brovkin Vasilis Dakos Hermann Held Max Rietkerk 李迎春 闫伟 蒋长胜 《国际地震动态》2009,(9):1-14
从生态系统到金融市场和气候在内的很多复杂动力系统,都会有临界点,在这样的点上系统可能会发生突变,从而演变到一个对立的动力模式上。在这样的临界点到达之前对其进行预测极为困难,但是现在,不同科学领域的研究工作表明,普遍性的早期预警信号有迹可循。对一系列不同类型的系统来说,这些信号会指示是否正在趋近一个临界阈值。 相似文献
10.
Axel Wittmann Randy L. Korotev Bradley L. Jolliff Paul K. Carpenter 《Meteoritics & planetary science》2019,54(2):379-394
Magnesium‐rich spinel assemblages occur in the two lunar vitric breccia meteorites—Dhofar (Dho) 1528 and Graves Nunataks (GRA) 06157. Dho 1528 contains up to ~0.7 mm cumulate Mg‐rich spinel crystals associated with Mg‐rich olivine, Mg‐ and Al‐rich pyroxene, plagioclase, and rare cordierite. Using thermodynamic calculations of these mineral assemblages, we constrain equilibration depths and discuss an origin of these lithologies in the upper mantle of the Moon. In contrast, small, 10 to 20 μm spinel phenocryst assemblages in glassy melt rock clasts in Dho 1528 and GRA 06157 formed from the impact melting of Mg‐rich rocks. Some of these spinel phenocrysts match compositional constraints for spinel associated with “pink spinel anorthosites” inferred from remote sensing data. However, such spinel phenocrysts in meteorites and Apollo samples are typically associated with significant amounts of olivine ± pyroxene that exceed the compositional constraints for pink spinel anorthosites. We conclude that the remotely sensed “pink spinel anorthosites” have not been observed in the collections of lunar rocks. Moreover, we discuss impact‐excavation scenarios for the spinel‐bearing assemblages in Dhofar 1528 and compare the bulk rock composition of Dho 1528 to strikingly similar compositions of Luna 20 samples that contain ejecta from the Crisium impact basin. 相似文献