全文获取类型
收费全文 | 32186篇 |
免费 | 5138篇 |
国内免费 | 7721篇 |
专业分类
测绘学 | 2145篇 |
大气科学 | 1490篇 |
地球物理 | 6677篇 |
地质学 | 22392篇 |
海洋学 | 2388篇 |
天文学 | 6502篇 |
综合类 | 1703篇 |
自然地理 | 1748篇 |
出版年
2024年 | 97篇 |
2023年 | 385篇 |
2022年 | 862篇 |
2021年 | 1128篇 |
2020年 | 1170篇 |
2019年 | 1383篇 |
2018年 | 1082篇 |
2017年 | 1147篇 |
2016年 | 1312篇 |
2015年 | 1444篇 |
2014年 | 2129篇 |
2013年 | 2196篇 |
2012年 | 2298篇 |
2011年 | 2138篇 |
2010年 | 1942篇 |
2009年 | 2390篇 |
2008年 | 2250篇 |
2007年 | 2422篇 |
2006年 | 2223篇 |
2005年 | 1936篇 |
2004年 | 1770篇 |
2003年 | 1592篇 |
2002年 | 1321篇 |
2001年 | 1156篇 |
2000年 | 1100篇 |
1999年 | 1074篇 |
1998年 | 936篇 |
1997年 | 731篇 |
1996年 | 663篇 |
1995年 | 523篇 |
1994年 | 412篇 |
1993年 | 416篇 |
1992年 | 295篇 |
1991年 | 216篇 |
1990年 | 192篇 |
1989年 | 183篇 |
1988年 | 146篇 |
1987年 | 82篇 |
1986年 | 70篇 |
1985年 | 57篇 |
1984年 | 45篇 |
1983年 | 22篇 |
1982年 | 20篇 |
1981年 | 11篇 |
1980年 | 19篇 |
1979年 | 5篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1973年 | 5篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
The kinetics of the reactions of C2H radical with ethane (k1), propane (k2), and n-butane (k3) are studied over the temperature range of T = 96-296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis-chemiluminescence technique. The C2H decay profiles in the presence of both the alkane reactant and O2 are monitored by the CH(A2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k1(T) = (0.51 ± 0.06) × 10−10 exp[(−76 ± 30)K/T] cm3 molecule−1 s−1 (T = 96-800 K), k2(T) = (0.98 ± 0.32) × 10−10exp[(−71 ± 60)K/T] cm3 molecule−1 s−1 (T = 96-361 K), and k3(T) = (1.23 ± 0.26) × 10−10 cm3 molecule−1 s−1 (T = 96-297 K). At T = 296 K, k1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed. 相似文献
12.
Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure. 相似文献
13.
14.
利用“地震预报计算机专家系统”的思想对大同 -阳高 Ms6 .1地震前每一前兆异常事件进行综合评估 ,以每一异常的最可能发震时间来计算发震概率 ,利用地震前兆综合加权信息熵研究了系统熵值与地震的关系。对华北地区的地震前兆综合加权信息熵研究表明 ,在大同 -阳高 Ms6 .1地震前 ,信息熵出现了明显的减熵有序变化 相似文献
15.
本文依据稳恒磁场中的高斯定理和安培环路定理,用迭代法推导出对称轴上的磁感应强度与空间任意一点磁感应强度的关系式,从而可得出轴对称稳恒磁场的无电流分布空间的解式。 相似文献
16.
17.
The structure, functioning and hydrodynamic properties of aquifers can be determined from an analysis of the spatial variability of baseflow in the streams with which they are associated. Such analyses are based on simple low‐cost measurements. Through interpreting the hydrological profiles (Q = f(A)) it is possible to locate the aquifer(s) linked to the stream network and to determine the type of interrelated flow, i.e. whether the stream drains or feeds the aquifer. Using an analytical solution developed for situations with a positive linear relationship, i.e. where the baseflow increases linearly with increasing catchment size, it is also possible to estimate the permeability of the aquifer(s) concerned at catchment scale. Applied to the hard‐rock aquifers of the Oman ophiolite, this method shows that the ‘gabbro’ aquifer is more permeable than the ‘peridotite’ aquifer. As a consequence the streams drain the peridotites and ‘leak’ into the gabbro. The hydrological profiles within the peridotite are linear and positive, and indicate homogeneity in the hydrodynamic properties of these formations at the kilometre scale. The permeability of the peridotite is estimated at 5 · 10?7 to 5 · 10?8 m/s. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
18.
We consider the evolution of certain low-mass binaries, incorporating models of (a) internal evolution, (b) tidal friction,
(c) dynamo activity driven by an elementary α,Ω dynamo, (d) stellar wind driven by the activity, and (e) magnetic braking
as a consequence of wind and poloidal dynamo-generated magnetic field. In some circumstances the stellar wind is found to
remove mass on a nuclear timescale, as is necessary to explain some observed systems.
We can hope that various uncertainties in the model may be clarified by a careful comparison of the models with such observed
quantities as rotation periods. These are modified by processes (a), (b) and (e). Assuming that stellar evolution is slow,
rotation rate should in some circumstances represent a balance between magnetic braking trying to slow the star down and tidal
friction trying to spin it up. Preliminary attempts are promising, but indicate that some fine tuning is necessary.
When there is a third body present, in an orbit which is inclined but not necessarily of short period, the eccentricity of
a close binary can be strongly modified by ‘Kozai cycles’. We show that this may complicate attempts to account for spin rates
of stars in close binaries. 相似文献
19.
20.
西秦岭温泉花岗岩体岩石学特征及岩浆混合标志 总被引:14,自引:5,他引:9
温泉花岗岩体由酸性端元的寄主岩石和暗色微细粒镁铁质包体群及基性岩墙群组成。无岩浆混合作用或岩浆混合作用较弱区段,寄主岩石以似斑状二长花岗岩为主.显示正常的花岗岩结构构造岩浆混合作用强烈区段。岩石的异常结构构造十分发育.矿物之间自形程度差异显著.常见包晶反应、包含结构、交代边、熔蚀边、交代蚕食的港湾状结构构造及交代缝合线、矿物镶边、斜长石异常环带和矿物残留等,多见指示岩浆混合的标志性矿物针状磷灰石。暗色微粒包体中多见寄主二长花岗岩中的捕掳晶。包体的形态、结构构造以及与寄主岩石强烈地成分交换等均是岩浆混合作用的标志。 相似文献