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351.
The general purpose finite element code DYNA2D is used to calculate the response of an unlined tunnel in jointed rock, with sliding interfaces used to represent the joints. The model problem selected for analysis closely follows a calculation using the discrete element code UDEC. Results for two cases [(1) jointed rock with lubricated joints (no joint friction) and (2) jointed rock with frictional resistance to joint motion] are presented and compared with each other and the UDEC results. 相似文献
352.
Ronald J. Kroll 《Solar physics》1994,152(1):105-110
In an effort to monitor solar limb-darkening variability, the continuum radiation intensity at 550 nm over the outermost 32 arcseconds of the limb is measured at various solar latitudes. Using the Finite Fourier Transform Definition, the edge location of the Sun is determined for a series of scan amplitudes at each of the observed positions. The differential radius is the difference between edge locations for a fixed pair of scan amplitudes, and is a quantity which characterizes the slope of the solar limb-darkening function. Utilizing the differential radius, such observations offer the possibility of revealing a latitudinal variation of the photospheric temperature gradient and could provide clues to the mechanisms and variability of energy transport out of the Sun. These observations began in 1988 with measurements at 24 separate limb positions and include observations since 1990 when 36 positions were observed. The daily differential radius measurements for each position that is free of contamination from solar active regions are weighted according to the corresponding daily variance and averaged to obtain an overall value at each position for the observing season. The results indicate that during the 1991 observing season, there were regions near 20°N latitude and 30°S latitude on the Sun where the differential radius values were significantly greater than surrounding regions. This suggests that perturbations to the temperature gradient occur in latitudinally localized regions and persist for at least several months. It is shown that this phenomenon could have the same origin as the observed latitudinal variations of surface temperature and could also speak to the question of a lag time between the cycles of irradiance and magnetic variation. 相似文献
353.
Simple evolutionary models of asteroids of various sizes and solar distances have been constructed assuming unipolar electrical induction heating due to passage of the Sun through a T Tauri phase with an increased magnetic field. Typical T Tauri conditions and an elementary solar wind model were used to calculate induced currents in modlels assuming electrical conductivities appropriate for carbonaceous material. Two restrictions with opposite dependence upon radius dominate the results. The electrical insulating tendency of a cold surface favors heating of larger bodies. The current-limiting backpressure of the induced magnetic field favors heating of smaller bodies. Thus it is found that maximum heating, in some cases sufficient for melting, occurs for model asteroids at the inner edge of the belt and with (model-dependent) radii from 25 to 250 km. This effect, if operant, would have produced a primordial distribution of metamorphosed asteroids primarily occurring at small solar distance and intermediate size. The observational evidence for such a distribution is unclear because the primordial distribution has likely been considerably modified by collisions, particularly at smaller sizes. There does seem to be some consistency with the model in the distribution of the largest asteroids, though data are sparse. In particular, this model seems relevant to the well-known dichotomy between Ceres and Vesta.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978Also Dept of Planetary Sciences 相似文献
354.
355.
356.
Julia Bosiö Christian Stiegler Margareta Johansson Herbert N. Mbufong Torben R. Christensen 《Climatic change》2014,127(2):321-334
This study was initiated to analyze the effect of increased snow cover on plant photosynthesis in subarctic mires underlain by permafrost. Snow fences were used to increase the accumulation of snow on a subarctic permafrost mire in northern Sweden. By measuring reflected photosynthetic active radiation (PAR) the effect of snow thickness and associated delay of the start of the growing season was assessed in terms of absorbed PAR and estimated gross primary production (GPP). Six plots experienced increased snow accumulation and six plots were untreated. Incoming and reflected PAR was logged hourly from August 2010 to October 2013. In 2010 PAR measurements were coupled with flux chamber measurements to assess GPP and light use efficiency of the plots. The increased snow thickness prolonged the duration of the snow cover in spring. The delay of the growing season start in the treated plots was 18 days in 2011, 3 days in 2012 and 22 days in 2013. Results show higher PAR absorption, together with almost 35 % higher light use efficiency, in treated plots compared to untreated plots. Estimations of GPP suggest that the loss in early season photosynthesis, due to the shortening of the growing season in the treatment plots, is well compensated for by the increased absorption of PAR and higher light use efficiency throughout the whole growing seasons. This compensation is likely to be explained by increased soil moisture and nutrients together with a shift in vegetation composition associated with the accelerated permafrost thaw in the treatment plots. 相似文献
357.
Abstract The often pragmatic division of studies of function (physiology), and the regulation of distribution and abundance of organisms (ecology), as laboratory and field studies respectively, can create an unhelpful intellectual division that runs the risk of ignoring the interaction of physiology, behaviour and environment that regulates the lives of animals in the wild. This review examines the historical and current contribution of ecophysiological research conducted from the University of Auckland's Leigh Laboratory in bridging these paradigms, and generating new insights into animal function and community organisation. The assessment focuses on endocrine control processes, and metabolic and behavioural responses of fish to artificial and natural stressors, and examines tracks of future research needed to underpin understanding of likely effects of predicted environmental change on individuals and populations. 相似文献
358.
Knut METZLER Addi BISCHOFF Richard C. GREENWOOD Herbert PALME Marko GELLISSEN Jens HOPP Ian A. FRANCHI Mario TRIELOFF 《Meteoritics & planetary science》2011,46(5):652-680
Abstract– Northwest Africa (NWA) 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons. It is an L3–6 chondrite and probably represents the largest sample of the rare regolith breccias from the L–chondrite asteroid. It contains unequilibrated and equilibrated chondrite clasts, some of which display shock‐darkening. Impact melt rocks (IMRs), both clast‐free and clast‐poor, are strongly depleted in Fe,Ni metal, and sulfides. An unequilibrated microbreccia, two different light inclusions and two different SiO2‐bearing objects were found. Although the matrix of this breccia appears partly clastic, it is not a simple mixture of fine‐grained debris formed from the above lithologies, but mainly represents an additional specific lithology of low petrologic type. We speculate that this material stems from a region of the parent body that was only weakly consolidated. One IMR clast and one SiO2‐bearing object show Δ17O values similar to bulk NWA 869, suggesting that both are related to the host rock. In contrast, one light inclusion and one IMR clast appear to be unrelated to NWA 869, suggesting that the IMR clast is contaminated with impactor material. 40Ar‐39Ar analyses of a type 4 chondrite clast yield a plateau age of 4402 ± 7 Ma, which is interpreted to be the result of impact heating. Other impact events are recorded by an IMR clast at 1790 ± 36 Ma and a shock‐darkened clast at 2216 ± 40 Ma, demonstrating that NWA 869 escaped major reset in the course of the event at approximately 470 Ma that affected many L–chondrites. 相似文献
359.
Low- and high-temperature heat capacities were measured for a series of synthetic high-structural state (K,Ca)-feldspars (Or–An)
using both a relaxation and a differential scanning calorimeter. The data were collected at temperatures between 5 and 800 K
on polycrystalline samples that had been synthesised and characterised in a previous study. Below T = 300 K, Or90An10, and Or80An20 showed excess heat capacities of mixing with maximum values of ~3 J mol−1 K−1. The other members of this binary (An > 20 mol%) had lower excess heat capacity values of up to ~1 J mol−1 K−1. Above T = 300 K, some compositions exhibited negative excess heat capacities of mixing (with maximum values of −2 J mol−1 K−1). The vibrational entropy at 298.15 K for Or90An10 and Or80An20 deviated strongly from the behaviour of a mechanical mixture, with excess entropy values of ~3.5 J mol−1 K−1. More An-rich members had only small excess vibrational entropies at T = 298.15 K. The difference in behaviour between members with An > 20 mol% and those with An ≤ 20 mol% is probably a consequence
of the structural state of the (K,Ca)-feldspars, i.e., (K,Ca)-feldspars with An ≤ 20 mol% have monoclinic symmetry, whereas
those with An > 20 mol% are triclinic. At T = 800 K, the vibrational entropy values were found to scatter around the values expected for a mechanical mixture and, thus,
correspond to a quasi-ideal behaviour. The solvus for the (K,Ca)-feldspar binary was calculated based on the entropy data
from this study in combination with enthalpy and volume of mixing data from a previous study. 相似文献
360.
A ternary feldspar-mixing model based on calorimetric data: development and application 总被引:8,自引:0,他引:8
Artur Benisek Edgar Dachs Herbert Kroll 《Contributions to Mineralogy and Petrology》2010,160(3):327-337
A mixing model for high structural state ternary feldspars in the NaAlSi3O8–KAlSi3O8–CaAl2Si2O8 system is presented based exclusively on calorimetric and volumetric measurements. Comparisons with existing mixing models,
which are based on phase-equilibrium experiments, reveal distinct differences. The incorporation of K into Ca-rich plagioclase
and of Ca into K-rich alkali feldspar is more strongly limited by our calorimetry-based model, whereas the stability field
of Na-rich feldspars is broadened. Natural feldspar assemblages from well-studied magmatic and high-grade metamorphic rocks
(i.e. a teschenite sill in Scotland, the Klokken syenogabbro in Greenland, and a granulite-facies metapelite in Sri Lanka)
were used to test the mixing models. It was found that the new model largely eliminates discrepancies between observed and
predicted feldspar compositions that were present in earlier attempts. The reasons for the problems associated with phase-equilibrium
based mixing models are discussed. 相似文献