全文获取类型
收费全文 | 50275篇 |
免费 | 8414篇 |
国内免费 | 11077篇 |
专业分类
测绘学 | 4021篇 |
大气科学 | 5955篇 |
地球物理 | 10201篇 |
地质学 | 28213篇 |
海洋学 | 6838篇 |
天文学 | 3409篇 |
综合类 | 3223篇 |
自然地理 | 7906篇 |
出版年
2024年 | 372篇 |
2023年 | 920篇 |
2022年 | 1760篇 |
2021年 | 2162篇 |
2020年 | 2039篇 |
2019年 | 2411篇 |
2018年 | 1823篇 |
2017年 | 2085篇 |
2016年 | 2086篇 |
2015年 | 2341篇 |
2014年 | 2901篇 |
2013年 | 3161篇 |
2012年 | 2984篇 |
2011年 | 3193篇 |
2010年 | 2782篇 |
2009年 | 3277篇 |
2008年 | 3245篇 |
2007年 | 3389篇 |
2006年 | 3318篇 |
2005年 | 3035篇 |
2004年 | 2697篇 |
2003年 | 2551篇 |
2002年 | 2236篇 |
2001年 | 1965篇 |
2000年 | 1906篇 |
1999年 | 1662篇 |
1998年 | 1438篇 |
1997年 | 1103篇 |
1996年 | 904篇 |
1995年 | 797篇 |
1994年 | 745篇 |
1993年 | 605篇 |
1992年 | 437篇 |
1991年 | 375篇 |
1990年 | 253篇 |
1989年 | 225篇 |
1988年 | 171篇 |
1987年 | 105篇 |
1986年 | 62篇 |
1985年 | 56篇 |
1984年 | 37篇 |
1983年 | 27篇 |
1982年 | 18篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 6篇 |
1978年 | 23篇 |
1977年 | 5篇 |
1971年 | 5篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
罗氏沼虾育苗用水中Mg2+与Ca2+含量及Mg2+/Ca2+对出苗率的影响 总被引:13,自引:0,他引:13
1991年3-6月和1992年3-6月在南汇县东海水产养殖公司对罗氏沼虾育苗用水调配原则与不同基础水的调配方法进行试验研究。结果表明,育苗用水中Mg^2+与Ca^2+含量及Mg^2+/Ca^2+必须达到一定的范围,这是调配的基本原则。以鱼塘水作基础水时,需添加6种化学药品,其中Mg^2+与Ca^2+含量可适范围分别为300.0-440.0mg/L与170.0-244.0mg/L,Mg^2+/Ca^ 相似文献
22.
Five sequences are defined in the Oligocene succession of the Danish North Sea sector. Two of the sequences, 4.1a and 4.3, have been identified onshore Denmark.Two types of prograding lowstand deposits are recognized. Sand-dominated deposits occur proximally, comprising sharp-based forced regressive deposits covered with prograding low-stand deposits. Clay-dominated prograding lowstand deposits occur distally in the sequences. The highstand deposits are proximally represented by thick prograding sandy deposits and distally by thin and condensed intervals.The main sediment input direction was from the north and the northeast. A succession oif lithofacies, from shallow marine facies dominated by sand to outer shelf facies dominated by clay, is mapped in each of the sequences. An overall southward progradation of the shoreline took place during the Oligocene, interrupted only by minor shoreline retreats. 相似文献
23.
公用微机硬盘信息保护的一种有效方法 总被引:1,自引:0,他引:1
张玉彪 《中国海洋大学学报(自然科学版)》1994,(Z2)
介绍一种保护公用微机硬盘信息的有效方法.不仅可以预防各种计算机病毒,更重要的是可以防止人为操作对硬盘信息的破坏。适用于学校等开放性微机实验室的信息安全与保护. 相似文献
24.
Qiu Dahong 《中国海洋工程》1992,(2)
Based on the 1st order cnoidal wave theory, the nonlinear wave diffraction around a circular cylinder in shallow water is studied in this paper. The equation of the wave surface around the cylinder is formulated and by using this formula the wave surface elevation on the cylinder surface can be obtained. In this paper, the formula for calculating the cnoidal wave force on a circular cylinder is also derived. For the wave conditions which are often encountered in practical engineering designs, the ratios of the nonlinear wave forces to the linear wave forces are calculated, and the results are plotted in this paper for design purposes. In order to verify the theoretical results, model tests are conducted. After comparing the test results with the theoretical ones, it is concluded that, in shallow water, for the case of T g / d~(1/2) > 8-10 and H / d > 0.3, the cnoidal wave theory should be used to calculate the wave action on a cylindrical pier. 相似文献
25.
26.
The photochemical instability of several related pteridines in seawater was investigated by aseptic incubation of solutions at 20–22°C under illumination from cool-white light of intensity 6 kerg cm−2 sec−1, and the chemical changes were spectrophotometrically monitored. All the pteridines showed markedly accelerated degradation from this illumination relative to their behaviour in total darkness.Pterin and lumazine were degraded very slowly with zero-order reaction kinetics, while the other pteridines photolysed rapidly (according to first-order kinetics) with decomposition rates increasing in the order dioxylumazine (2,4,6,7-tetrahydroxypteridine) < leucopterin < isoxanthopterin < xanthopterin < oxylumazine (2,4,6-trihydroxypteridine). Excepting leucopterin and dioxylumazine, the photolysis rates were attributable to the pH of seawater and not its salt content; this was also the case with oxylumazine which required the salt content of seawater for decomposition in darkness. Leucopterin and dioxylumazine (both 6,7-dihydroxylated pteridines) gave evidence of complexation with the major divalent cations (Ca2+, Mg2+) of seawater, by virtue of which their photolytic degradation rates were enhanced to magnitudes obtained in pH-10 buffer without seawater. It is proposed that such complexation produces structural forms of these pteridines analogous to their normal ionic forms at pH 10–12.The photolysis of the 6-hydroxylated pteridines (xanthopterin, oxylumazine) proceeded via intermediate formation of their corresponding 7-hydroxylated derivatives (leucopterin, dioxylumazine). 相似文献
27.
28.
A preliminary optical classification of lakes in Estonia and south Finland which can also be used for small bays of the Baltic Sea is elaborated. The classification is based on the optical properties of water (diffuse attenuation coefficient, diffuse reflectance) and parameters that are routinely monitored in water bodies (Secchi depth, concentration of chlorophyll-a, total suspended matter and yellow substance). The data complex used for our classification covers different types of water ecosystems (ranging from oligotrophic to hypertrophic) and the variability of water constituent concentrations in the ice-free period in Estonia and south Finland. Using cluster analysis, we found 5 optical classes of waters: clear (C), moderate (M), turbid (T), very turbid (V) and brown (B). There is satisfactory correspondence between class of water, shape of diffuse attenuation coefficient and diffuse reflectance spectra and trophic state of the lakes. 相似文献
29.
30.
Incremental Differential Quadrature Method (IDQM) as a rapid and accurate method for numerical simulation of Nonlinear Shallow Water (NLSW) waves is employed. To the best of authors’ knowledge, this is the first endeavor to exploit DQM in coastal hydraulics. The one-dimensional NLSW equations and related boundary conditions are discretized in space and temporal directions by DQM rules and the resulting system of equations are used to compute the state variables in the entire computational domain. It was found that the splitting of total simulation time into a number of smaller time increments, could significantly enhance the performance of the proposed method. Furthermore, results of this study show two main advantages for IDQM compared with other conventional methods, namely; unconditional stability and minimal computational effort. Indeed, using IDQM, one can use a few grid points (in spatial or time direction) without imposing any stability condition on the time step to obtain an accurate convergent solution. 相似文献