全文获取类型
收费全文 | 25246篇 |
免费 | 215篇 |
国内免费 | 943篇 |
专业分类
测绘学 | 1445篇 |
大气科学 | 2033篇 |
地球物理 | 4675篇 |
地质学 | 12077篇 |
海洋学 | 1039篇 |
天文学 | 1706篇 |
综合类 | 2169篇 |
自然地理 | 1260篇 |
出版年
2023年 | 6篇 |
2022年 | 36篇 |
2021年 | 43篇 |
2020年 | 39篇 |
2019年 | 29篇 |
2018年 | 4789篇 |
2017年 | 4076篇 |
2016年 | 2626篇 |
2015年 | 282篇 |
2014年 | 148篇 |
2013年 | 113篇 |
2012年 | 1025篇 |
2011年 | 2771篇 |
2010年 | 2041篇 |
2009年 | 2344篇 |
2008年 | 1918篇 |
2007年 | 2375篇 |
2006年 | 80篇 |
2005年 | 218篇 |
2004年 | 419篇 |
2003年 | 424篇 |
2002年 | 264篇 |
2001年 | 57篇 |
2000年 | 63篇 |
1999年 | 24篇 |
1998年 | 24篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 10篇 |
1981年 | 26篇 |
1980年 | 19篇 |
1978年 | 4篇 |
1976年 | 10篇 |
1975年 | 4篇 |
1973年 | 3篇 |
1970年 | 2篇 |
1969年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Naotaka Tomioka Takuo Okuchi Narangoo Purevjav Jun Abe Stefanus Harjo 《Physics and Chemistry of Minerals》2016,43(4):267-275
Hydrogen site positions and occupancy in the crystal structure of dense hydrous magnesium silicate (DHMS) phase E were determined for the first time by pulsed neutron powder diffraction. A fully deuterated pure phase E powder sample, which had space group \(R\overline{3} m\) and lattice parameters of a = 2.97065(8) Å and c = 13.9033(4) Å, was synthesized at 15 GPa and 1100 °C. Through quantitative evaluation of refined structure parameters obtained with sufficient spatial resolution and very high signal-to-background ratio, we conclude that the O–D dipoles in the refined phase E structure are tilted by 24° from the direction normal to the layers of edge-shared MgO6 octahedra (octahedral layers). The tilted dipole structure of phase E is in remarkable contrast to that of brucite, Mg(OH)2, which has dipoles exactly normal to the octahedral layer. This contrast exists because the O–Si–O bonding unique in the phase E structure connects two adjacent octahedral layers and thereby reduces the interlayer O···O distance. This shrinkage of the interlayer distance induces the tilting of the O–D dipole and also generates unique O–D···O hydrogen bonding connecting all the layers in the phase E structure. 相似文献
992.
Hamid El Maanaoui Fabian Wilangowski Aditya Maheshwari Hans-Dieter Wiemhöfer Rainer Abart Nicolaas A. Stolwijk 《Physics and Chemistry of Minerals》2016,43(5):327-340
We measured the ion conductivity of single-crystal alkali feldspar originating from two different locations in the Eifel/Germany, named Volkesfeld and Rockeskyller sanidine and having potassium site fractions \(C_\mathrm{K}\) of 0.83 and 0.71, respectively. The dc conductivities resulting from electrochemical impedance spectroscopy over the temperature range of 300–900 \(^{\circ }\hbox {C}\) show a weak composition dependence but pronounced differences between the b-direction [\(\perp (010)\)] and \(c^{*}\)-direction [\(\perp (001)\)] of the monoclinic feldspar structure. Conductivity activation energies obtained from the observed linear Arrhenius plots are close to 1.2 eV in all cases, which is closely similar to the activation energies of the \(^{22}\mathrm{Na}\) tracer diffusivity in the same crystals. Taking into account literature data on K tracer diffusion and diffusion correlation effects, the present results point to a predominance of the interstitialcy mechanism over the vacancy mechanism in mass and charge transport on the alkali sublattice in potassium-rich alkali feldspar. 相似文献
993.
Calcium sulfate (CaSO4), one of the major sulfate minerals in the Earth’s crust, is expected to play a major role in sulfur recycling into the deep mantle. Here, we investigated the crystal structure and phase relation of CaSO4 up to ~90 GPa and 2300 K through a series of high-pressure experiments combined with in situ X-ray diffraction. CaSO4 forms three thermodynamically stable polymorphs: anhydrite (stable below 3 GPa), monazite-type phase (stable between 3 and ~13 GPa) and barite-type phase (stable up to at least 93 GPa). Anhydrite to monazite-type phase transition is induced by pressure even at room temperature, while monazite- to barite-type transition requires heating at least to 1500 K at ~20 GPa. The barite-type phase cannot always be quenched from high temperature and is distorted to metastable AgMnO4-type structure or another modified barite structure depending on pressure. We obtained the pressure–volume data and density of anhydrite, monazite- and barite-type phases and found that their densities are lower than those calculated from the PREM model in the studied P–T conditions. This suggests that CaSO4 is gravitationally unstable in the mantle and fluid/melt phase into which sulfur dissolves and/or sulfate–sulfide speciation may play a major role in the sulfur recycling into the deep Earth. 相似文献
994.
D. Szymanowski B. S. Ellis J. F. Wotzlaw Y. Buret A. von Quadt I. Peytcheva I. N. Bindeman O. Bachmann 《Contributions to Mineralogy and Petrology》2016,171(12):106
Understanding the processes of differentiation of the Yellowstone–Snake River Plain (YSRP) rhyolites is typically impeded by the apparent lack of erupted intermediate compositions as well as the complex nature of their shallow interaction with the surrounding crust responsible for their typically low O isotopic ratios. A pair of normal-δ18O rhyolitic eruptions from the Heise eruptive centre in eastern Idaho, the Wolverine Creek Tuff and the Conant Creek Tuff, represent unique magmatic products of the Yellowstone hotspot preserving abundant vestiges of the intermediate differentiation steps leading to rhyolite generation. We address both shallow and deep processes of magma generation and storage in the two units by combining high-precision ID–TIMS U–Pb zircon geochronology, trace element, O and Hf isotopic studies of zircon, and Sr isotopic analyses of individual high-Mg# pyroxenes inherited from lower- to mid-crustal differentiation stages. The zircon geochronology confirms the derivation of both tuffs from the same rhyolitic magma reservoir erupted at 5.5941 ± 0.0097 Ma, preceded by at least 92 ± 14 ky of continuous or intermittent zircon saturation approximating the length of pre-eruptive magma accumulation in the upper crust. Some low-Mg# pyroxenes enclosing zircons predate the eruption by at least 45 ± 27 ky, illustrating the co-crystallisation of major and accessory phases in the near-liquidus rhyolitic melts of the YSRP over a significant period of time. Coeval zircon crystals are isotopically heterogeneous (two populations at εHf ~?5 and ?13), requiring the assembly of isotopically distinct melt pockets directly prior to, or during, the eruption. The primitive Mg# 60–90 pyroxenes are out of isotopic equilibrium with the host rhyolitic melt (87Sr/86Sri = 0.70889), covering a range of 87Sr/86Sri = 0.70705–0.70883 corresponding to ratios typical of the most radiogenic YSRP basalts to the least radiogenic YSRP rhyolites. Together with the low εHf in zircon, the Sr isotopic ratios illustrate limited assimilation dominated by radiogenic Archean crustal source materials incorporated into variably evolved YSRP melts as they progress towards rhyolitic compositions by assimilation–fractional crystallisation. 相似文献
995.
Many of the zinc-lead deposits of NE Washington State are poorly known examples of Mississippi Valley Type (MVT) mineralization. This study compares inclusion fluids from the Josephine Breccia ores with the later cross-cutting sulfide-bearing quartz veins. The breccia ores are cemented mainly by open space fillings of dolomite, sphalerite, quartz, galena, jasperoid and calcite. Replacement is of minor importance. Ore and gangue deposition occurred over the range 150–250 °C with most of the temperatures less than 200 °C. The aqueous brines typically contain 17–23 equivalent weight percent NaCl with often substantial amounts of Ca and/or Mg chlorides. Homogenization temperatures do not delineate any cooling or paragenetic sequence. The cross-cutting vein quartz contains CO2-rich inclusions with overall densities usually less than 0.7 g/cc and homogenization temperatures from 250–325 °C. Sulfur isotope analyses yield two populations with the quartz vein ores being lighter (<13 permil CDT) than the average for the conformable ores. The later veins are not remobilized MVT sulfides but represent a separate, high-silver period of mineralization. 相似文献
996.
Rehman Sufia Sahana Mehebub Hong Haoyuan Sajjad Haroon Ahmed Baharin Bin 《Natural Hazards》2019,96(2):975-998
Natural Hazards - Floods have always been associated with widespread devastation and destruction since the emergence of human civilization. The intensity of this disaster has been increasing due to... 相似文献
997.
Kai Liu Xiaojuan Qiao Baoling Li Ying Sun Zhiping Li Conglin Pu 《Environmental Earth Sciences》2016,75(23):1485
This study investigates the characteristics of geothermal water in 10 geothermal fields in Beijing. The relationships between the deuterium excess parameter (d) and temperature, depth, age of geothermal groundwater, groundwater flow field, and Eh were investigated using geothermal groundwater samples. Results showed that (1) the average d value of geothermal water is 5.4, whereas that of the groundwater in normal temperature is 6.04. The differences are induced by the oxygen isotope exchange during the water–rock interaction, which may be more easily completed in geothermal water than in cold groundwater. (2) The d value increases remarkably with the age of the geothermal groundwater. The d value increases from 11.2 to 14.6 when the age of the geothermal water is 12,760 ± 130 a and 38,960 ± 630 a, respectively. Moreover, the isotope heat exchange for composition of the hydrogen and oxygen isotopes in the geothermal groundwater proceeds sufficiently with time. (3) The d value decreases from 5.72 to 3.03 when the depth increases from 125.13 to 3221 m. Generally, in the same area, the d value decreases with depth because the temperature is increasing. (4) The d value of the groundwater gradually reduces from the northern recharge area to the southern discharge area. The average d value is 7.31 in the northern recharge area and 5.68 in the middle Beijing Depression, whereas the d value in the southern area of Fengheying is ?9.20. The larger difference in d values between the recharge and discharge areas is due to the slower velocity of underwater flow, which induces longer time for oxygen exchange. (5) The relationship between the d and Eh is complex. When Eh is <200 mV, the d value of the geothermal water decreases with the decrease in Eh. When Eh is higher than 200 mV, the d value increases slightly with the decrease in Eh. The study of the characteristics of deuterium excess parameters for geothermal water could provide a scientific isotopic evidence for assessment and exploitation measures in geothermal groundwater systems. 相似文献
998.
Many hydrological, environmental, or engineering exploration tasks require predicting spatially continuous scenarios of sparsely measured borehole logging data. We present a methodology to probabilistically predict such scenarios constrained by ill-posed geophysical tomography. Our approach allows for transducing tomographic reconstruction ambiguity into the probabilistic prediction of spatially continuous target parameter scenarios. It is even applicable to data sets where petrophysical relations in the survey area are non-unique, i.e., different facies related petrophysical relations may be present. We employ static two-layer artificial neural networks (ANNs) for prediction and additionally evaluate, whether the training performance of the ANNs can be used to rank geophysical tomograms, which are mathematically equal reconstructions of physical parameter distributions in the ground. We illustrate our methodology using a realistic synthetic database for maximal control about the prediction performance and ranking potential of the approach. For doing so, we try to link geophysical radar and seismic tomography as input parameters to porosity of the ground as target parameter of ANN. However, the approach is flexible and can cope with any combination of geophysical tomograms and hydrologic, environmental or engineering target parameters. Ranking of equivalent geophysical tomograms based on additional borehole logging data is found to be generally possible, but risks remain that the ranking based on the ANN training performance does not fully coincide with the closeness of geophysical tomograms to ground truth. Since geophysical field data sets do usually not offer control options similar to those used in our synthetic database, we do not recommend the utilization of recurrent ANNs to learn weights for the individual geophysical tomograms used in the prediction procedure. 相似文献
999.
A satellite rainfall retrieval technique is proposed here. The relationships of rain rate with each of cloud water path (CWP) and cloud top temperature (CTT) are investigated. The CWP and CTT are retrieved from SEVIRI data (spinning enhanced visible and infrared imager), and corresponding rain rates are measured by weather radar. The rain rates are compared to corresponding CWP and then to corresponding CTT. The investigation demonstrates an exponential functional dependency between rain rates and CWP for low and moderate rain rates (stratiform rainfall). Conversely, the rain rates are more closely related to CTT for high rain rates (convective rainfall). Therefore, two separate relationships are established for rain rate retrievals. The results show rain rates estimated by the developed scheme are in good correlation with those observed by weather radar. 相似文献
1000.
Brajesh Singh Santosh Kumar Masao Ban Kazuo Nakashima 《Journal of Earth System Science》2016,125(7):1329-1352
Felsic magmatism in the southern part of Himachal Higher Himalaya is constituted by Neoproterozoic granite gneiss (GGn), Early Palaeozoic granitoids (EPG) and Tertiary tourmaline-bearing leucogranite (TLg). Magnetic susceptibility values (<3 ×10?3 SI), molar Al2 O 3/(CaO + Na2 O + K 2O) (≥1.1), mineral assemblage (bt–ms–pl–kf–qtz ± tur ± ap), and the presence of normative corundum relate these granitoids to peraluminous S-type, ilmenite series (reduced type) granites formed in a syncollisional tectonic setting. Plagioclase from GGn (An10–An31) and EPG (An15–An33) represents oligoclase to andesine and TLg (An2–An15) represents albite to oligoclase, whereas compositional ranges of K-feldspar are more-or-less similar (Or88 to Or95 in GGn, Or86 to Or97 in EPG and Or87 to Or94 in TLg). Biotites in GGn (Mg/Mg + Fet= 0.34–0.45), EPG (Mg/Mg + Fet= 0.27–0.47), and TLg (Mg/Mg + Fet= 0.25–0.30) are ferribiotites enriched in siderophyllite, which stabilised between FMQ and HM buffers and are characterised by dominant 3Fe\(\rightleftharpoons \)2Al, 3Mg\(\rightleftharpoons \)2Al substitutions typical of peraluminous (S-type), reducing felsic melts. Muscovite in GGn (Mg/Mg + Fet=0.58–0.66), EPG (Mg/Mg + Fet=0.31?0.59), and TLg (Mg/Mg + Fet=0.29–0.42) represent celadonite and paragonite solid solutions, and the tourmaline from EPG and TLg belongs to the schorl-elbaite series, which are characteristics of peraluminous, Li-poor, biotite-tourmaline granites. Geochemical features reveal that the GGn and EPG precursor melts were most likely derived from melting of biotite-rich metapelite and metagraywacke sources, whereas TLg melt appears to have formed from biotite-muscovite rich metapelite and metagraywacke sources. Major and trace elements modelling suggest that the GGn, EPG and TLg parental melts have experienced low degrees (~13, ~17 and ~13%, respectively) of kf–pl–bt fractionation, respectively, subsequent to partial melting. The GGn and EPG melts are the results of a pre-Himalayan, syn-collisional Pan-African felsic magmatic event, whereas the TLg is a magmatic product of Himalayan collision tectonics. 相似文献