全文获取类型
收费全文 | 104341篇 |
免费 | 2101篇 |
国内免费 | 1719篇 |
专业分类
测绘学 | 2690篇 |
大气科学 | 7607篇 |
地球物理 | 21817篇 |
地质学 | 37564篇 |
海洋学 | 9112篇 |
天文学 | 22504篇 |
综合类 | 466篇 |
自然地理 | 6401篇 |
出版年
2022年 | 680篇 |
2021年 | 1173篇 |
2020年 | 1230篇 |
2019年 | 1282篇 |
2018年 | 2746篇 |
2017年 | 2575篇 |
2016年 | 3238篇 |
2015年 | 2021篇 |
2014年 | 3147篇 |
2013年 | 5528篇 |
2012年 | 3555篇 |
2011年 | 4706篇 |
2010年 | 4032篇 |
2009年 | 5215篇 |
2008年 | 4690篇 |
2007年 | 4493篇 |
2006年 | 4240篇 |
2005年 | 3387篇 |
2004年 | 3323篇 |
2003年 | 3041篇 |
2002年 | 2743篇 |
2001年 | 2508篇 |
2000年 | 2396篇 |
1999年 | 1933篇 |
1998年 | 2078篇 |
1997年 | 1900篇 |
1996年 | 1586篇 |
1995年 | 1579篇 |
1994年 | 1366篇 |
1993年 | 1243篇 |
1992年 | 1225篇 |
1991年 | 1114篇 |
1990年 | 1245篇 |
1989年 | 1039篇 |
1988年 | 945篇 |
1987年 | 1174篇 |
1986年 | 974篇 |
1985年 | 1243篇 |
1984年 | 1394篇 |
1983年 | 1261篇 |
1982年 | 1239篇 |
1981年 | 1070篇 |
1980年 | 1017篇 |
1979年 | 912篇 |
1978年 | 911篇 |
1977年 | 874篇 |
1976年 | 832篇 |
1975年 | 785篇 |
1974年 | 797篇 |
1973年 | 806篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
152.
An experimental study on reduction of U (Ⅵ) by anaerobic bacteria, Shewane//a putrefaciens, is first reported here in China. The experimental conditions were: 35℃ and pH =7.0-7.4, corresponding to a physicochemical environments in which the sandstone-hosted interlayer oxidation-zone type uranium deposit formed in Northwest China's Xinjiang. Bacteria adopted in the present experiment, Shewanella putrefaciens, occur extensively in natural environment. Our study shows that nano-crystal precipitates of uraninite quickly occurred on the surface of the cells within one week. It was found that the pitchblende was characterized by a random arrangement of uraninite nanocrystals (2-4 nm) in it, significantly different from natural pitchblende in which uraninite nanocrystals are arranged in order. Finally, a possible mechanism of uranium biomineralization by microorganisms in the deposits is discussed. Our investigation may supply a technical train of thoughts for bioremediation of nuclear-contaminated water environments and for underground dissolving extraction of the sandstone-hosted uranium ores. 相似文献
153.
Production of a molybdophore during metal-targeted dissolution of silicates by soil bacteria 总被引:1,自引:0,他引:1
Laura J. Liermann Robin L. Guynn Ariel Anbar Susan L. Brantley 《Chemical Geology》2005,220(3-4):285-302
Although many bioessential metals are scarce in natural water and rock systems, microbial secretion of high-affinity ligands for metal extraction from solid phases has only been documented for Fe. However, we have discovered that Mo is extracted from a silicate by a high-affinity ligand (a possible “molybdophore”) secreted by an N2-fixing soil bacterium. The putative molybdophore, aminochelin, is secreted as a siderophore under Fe-depleted conditions, but is also secreted under Fe-sufficient, Mo-depleted conditions. Presumably, molybdophore production facilitates uptake of Mo for use in Mo enzymes. In contrast, an Fe-requiring soil bacterium without a special Mo requirement only enhances the release of Fe from the silicate. Fractionation of Mo stable isotopes during uptake to cells may provide a “fingerprint” for the importance of chelating ligands in such systems. Many such metal-specific ligands secreted by prokaryotes for extraction of bioessential metals, their effects on Earth materials, and their possible utility in the recovery of economic metals remain to be discovered. 相似文献
154.
155.
156.
157.
G. M. Richter 《Astronomische Nachrichten》1971,293(3):111-117
On the basis of a radio index-surface brightness diagram recently published, the luminosity function and the luminosity diameter function are obtained. The uncertainties due to the incompleteness of the sample are of the same order as the statistical uncertainties. The luminosity function differs considerably from a simple power law and supports the distinction of two populations. The density of the weak population (P<1035 W Hz−1 ster−1 at 1400 MHz) follows nearly a power law in P and increases towards small diameters at least down to I kpc. The density of the strong population (the high luminosity and small diameter part of which is occupied by the quasars) has a maximum between 1025 and 1028 W Hz−1 ster−1 and around 100 kpc. A strong evolution effect is clearly present and is in a good agreement with the models obtained from the log N-log S counts. 相似文献
158.
M. Narayanasamy D. Dhanasekaran G. Vinothini N. Thajuddin 《International Journal of Environmental Science and Technology》2018,15(1):119-132
Printed circuit boards contain precious metals. They are produced in large volumes, rendering them an important component of the electronic waste. In view of the heterogeneity of the metals present, reprocessing of electronic waste is a heinous task. The present study focused on leaching of valuable metals from electronic waste printed circuit boards using Aspergillus niger DDNS1. The adaptation phases began at 0.1, 0.5 and 1.0% of fine powder of printed circuit boards with 10% inoculum and were optimized with three effective factors, viz. initial pH, particle size and pulp density, to achieve the maximum simultaneous recovery of the valuable metals. The interactions of these metals were also deciphered using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectrum and atomic absorption spectroscopy. The results indicated that extraction of the precious metals was accomplished mainly through the unique organic acids originating from A. niger DDNS1. The initial pH played an important role in the extraction of the precious metals and the metals precipitate formation. The leaching rate of the metals was generally higher at low powder dosage of printed circuit boards. The toxicity of the printed circuit boards had little effect on two-step bioleaching at the pulp density of 0.1% compared to one-step bioleaching. The two-step bioleaching process was followed under organic acid-forming conditions for the maximum mobilization of metals. Thus, the precious metals from printed circuit boards could be mobilized through fungal bioleaching which promises an important industrial application in recycling of electronic wastes. 相似文献
159.
The study of REE distribution in the clay fraction of sedimentary rocks from two coalfields made it possible to distinguish
several types of REE distribution, which correlate with their mineral composition. It is shown that the REE fractionation
was related to the mineral reconstruction of the primary clay fraction and some detrital minerals in the course of postsedimentary
transformations of rocks during diagenesis, early catagenesis, and beginning of late catagenesis. These transformations were
governed by several factors, such as the composition of sediments; hydrochemical features of accumulation environment; the
chemical composition, dynamics, and feeding sources of pore solutions; the porosity and permeability of sediments and rocks;
and the content of organic matter and its reaction ability. 相似文献
160.
An investigation of the influence of humate on the mobility of copper(II) ions in a kaolinite soil using leaching tests and electrokinetic experiments is reported. The data are interpreted in terms of humate–copper–clay interactions and humate electrical charge. Humate is mostly immobile below pH8 but is more mobile in alkaline conditions (sorption to kaolinite reduces its mobility in neutral conditions). Copper humate complexes are mobile in both acidic and alkaline conditions, but not in neutral conditions where they are sorbed. The dissolved copper humate complexes that form in acidic conditions are positively charged. The net effect of humate is to increase cupric ion mobility in kaolinite soil, especially in alkaline conditions. 相似文献