全文获取类型
收费全文 | 11091篇 |
免费 | 2344篇 |
国内免费 | 3084篇 |
专业分类
测绘学 | 1376篇 |
大气科学 | 1851篇 |
地球物理 | 2540篇 |
地质学 | 6017篇 |
海洋学 | 1908篇 |
天文学 | 478篇 |
综合类 | 976篇 |
自然地理 | 1373篇 |
出版年
2024年 | 41篇 |
2023年 | 189篇 |
2022年 | 572篇 |
2021年 | 621篇 |
2020年 | 497篇 |
2019年 | 574篇 |
2018年 | 639篇 |
2017年 | 632篇 |
2016年 | 669篇 |
2015年 | 676篇 |
2014年 | 680篇 |
2013年 | 827篇 |
2012年 | 864篇 |
2011年 | 877篇 |
2010年 | 897篇 |
2009年 | 815篇 |
2008年 | 857篇 |
2007年 | 711篇 |
2006年 | 643篇 |
2005年 | 579篇 |
2004年 | 414篇 |
2003年 | 392篇 |
2002年 | 403篇 |
2001年 | 377篇 |
2000年 | 369篇 |
1999年 | 303篇 |
1998年 | 227篇 |
1997年 | 196篇 |
1996年 | 173篇 |
1995年 | 176篇 |
1994年 | 119篇 |
1993年 | 121篇 |
1992年 | 78篇 |
1991年 | 63篇 |
1990年 | 46篇 |
1989年 | 42篇 |
1988年 | 39篇 |
1987年 | 16篇 |
1986年 | 22篇 |
1985年 | 15篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1958年 | 5篇 |
1957年 | 6篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
41.
Teruyuki Maruoka Christian Koeberl Jun‐ichi Matsuda Yasuhiko Syono 《Meteoritics & planetary science》2003,38(8):1255-1262
Abstract— Carbon isotopic compositions were measured for shock‐produced diamond and shocked graphite formed at peak pressures ranging from 37 to 52 GPa. The δ13C values of diamonds produced in a sealed container were generally lower than that of the initial graphite. The differences in the carbon isotopic composition between initial graphite and shocked graphite/diamond may reflect kinetic isotopic fractionation during the oxidation of the graphite/diamond and/or analytical artifacts possibly induced by impurities in the samples. The pressure effect on the isotopic fractionations between graphite and diamond can be estimated from the δ13C values of impurity‐free diamonds produced using a vented container from which gases, including oxygen, in pore spaces escaped during or after the diamond formation (e.g., 0.039 ± 0.085‰ at a peak pressure of 52 GPa). Any isotopic fractionation induced by shock conversion of graphite to diamond is too small to be detected in natural shock‐induced diamond‐graphite systems related to terrestrial impact cratering processes. 相似文献
42.
43.
In this paper, the equilibrium configurations of rapid rotating compact stars and some gravitational effects are studied within the general relativity by use of the Harrison-Wheeler equation of state and by the self-consistent field method. Numerical calculations show that the equilibrium configuration of a rotating star is a spheroid. For large spin velocities, say, ω > 3.0 × 102 sec−1 the eccentricity and mass increase very rapidly as the angular velocity increases, for the critical angular velocity of the rotating star, the eccentricity is about 0.7, the increase in mass is about 10–35%. The difference of the gravitational redshifts at the surface of the star caused by rotation, and the difference of the light bending when the beam moves in the direction of rotation or in the opposite direction are obvious. 相似文献
44.
Cao Shenglin 《Astrophysics and Space Science》1988,145(2):293-302
According to some local properties of Lorentz transformation, Einstein stated: Velocities greater than that of light have no possibility of existence. He neglected to point out the applicable range of the special theory of relativity. In fact, it could only be applied to the subluminal-speed. This paper shows that if we think of the possibility of the existence of the superluminal-speed and redescribe the special theory of relativity following Einstein's way, a new kinematic theory would be founded. The new theory would retain all kinematical meaning of the special theory of relativity when matters move with subluminal-speed and would give new content when matters move with superluminal-speed. The paper also discusses the observation principle for the motions with superluminal-speed. 相似文献
45.
Xie-Yan Song Hua-Wen Qi Paul T. Robinson Mei-Fu Zhou Zhi-Min Cao Lie-Meng Chen 《Lithos》2008,100(1-4):93-111
The Emeishan continental flood basalt (ECFB) sequence in Dongchuan, SW China comprises a basal tephrite unit overlain by an upper tholeiitic basalt unit. The upper basalts have high TiO2 contents (3.2–5.2 wt.%), relatively high rare-earth element (REE) concentrations (40 to 60 ppm La, 12.5 to 16.5 ppm Sm, and 3 to 4 ppm Yb), moderate Zr/Nb and Nb/La ratios (9.3–10.2 and 0.6–0.9, respectively) and relatively high Nd (t) values, ranging from − 0.94 to 2.3, and are comparable to the high-Ti ECFB elsewhere. The tephrites have relatively high P2O5 (1.3–2.0 wt.%), low REE concentrations (e.g., 17 to 23 ppm La, 4 to 5.3 ppm Sm, and 2 to 3 ppm Yb), high Nb/La (2.0–3.9) ratios, low Zr/Nb ratios (2.3–4.2), and extremely low Nd (t) values (mostly ranging from − 10.6 to − 11.1). The distinct compositional differences between the tephrites and the overlying tholeiitic basalts cannot be explained by either fractional crystallization or crustal contamination of a common parental magma. The tholeiitic basalts formed by partial melting of the Emeishan plume head at a depth where garnet was stable, perhaps > 80 km. We propose that the tephrites were derived from magmas formed when the base of the previously metasomatized, volatile-mineral bearing subcontinental lithospheric mantle was heated by the upwelling mantle plume. 相似文献
46.
Uranium-series dating of oxygen and carbon isotope records for stalagmite SJ3 collected in Songjia Cave, central China, shows significant variation in past climate and environment during the period 20-10 ka. Stalagmite SJ3 is located more than 1000 km inland of the coastal Hulu Cave in East China and more than 700 km north of the Dongge Cave in Southwest China and, despite minor differences, displays a clear first-order similarity with the Hulu and Dongge records. The coldest climatic phase since the Last Glacial Maximum, which is associated with the Heinrich Event 1 in the North Atlantic region, was clearly recorded in SJ3 between 17.6 and 14.5 ka, in good agreement in timing, duration and extent with the records from Hulu and Dongge caves and the Greenland ice core. The results indicate that there have been synchronous and significant climatic changes across monsoonal China and strong teleconnections between the North Atlantic and East Asia regions during the period 20-10 ka. This is much different from the Holocene Optimum which shows a time shift of more than several thousands years from southeast coastal to inland China. It is likely that temperature change at northern high latitudes during glacial periods exerts stronger influence on the Asian summer monsoon relative to insolation and appears to be capable of perturbing large-scale atmospheric/oceanic circulation patterns in the Northern Hemisphere and thus monsoonal rainfall and paleovegetation in East Asia. Climatic signals in the North Atlantic region propagate rapidly to East Asia during glacial periods by influencing the winter land-sea temperature contrast in the East Asian monsoon region. 相似文献
47.
Impact of land use covers upon karst processes in a typical Fengcong depression system of Nongla, Guangxi, China 总被引:2,自引:0,他引:2
Cheng Zhang Jianguo Pei Yunqiu Xie Jianhua Cao Lanling Wang 《Environmental Geology》2008,55(8):1621-1626
The direction and intensity of karst processes can be deeply affected by soil physical and chemical variations which were
resulted from land use. Taking Nongla Fengcong depression area, Mashan County, Guangxi as an example, authors discussed the
impact of land use on karst processes based on the data of field limestone tablet. The results showed that the corrosional
rates at varied soil depth are quite different. Corrosional rate in woodland and orchard is mostly bigger than 20 mg/a, which
is much higher than that in tilled land and shrub. Generally, corrosional rate decreased from orchard, woodland, tilled land,
fallow land and shrub successively, in which soil organic matter (OM) and soil pH are two major controlling factors: corrosion
process is controlled remarkably by soil OM in woodland and orchard. The higher the organic matter content is and the less
the pH value is, the higher the corrosional rate is. Owing to lower organic matter content, the corrosional rate is mainly
affected by soil CO2 in tilled land and shrub. 相似文献
48.
49.
土壤酶对碘离子在土壤中吸附行为的影响 总被引:1,自引:0,他引:1
以批式试验法研究了微生物、葡萄糖氧化酶、脲酶、纤维素酶及其催化底物葡萄糖、尿素和纤维素对Ⅰ-在土壤中吸附的影响,结果表明微生物能显著提高Ⅰ-吸附量,但葡萄糖氧化酶、脲酶、纤维素酶及葡萄糖、尿素和纤维素对Ⅰ-吸附并无促进作用.葡萄糖氧化酶及葡萄糖含量在小于100 mg/L时对Ⅰ-吸附无影响,1000 mg/L时与Ⅰ-呈竞争吸附;脲酶及尿素随浓度增大,对Ⅰ-吸附的抑制作用变得明显;1~1000 mg/L范围内,纤维素酶和纤维素对Ⅰ-吸附没有影响;但葡萄糖氧化酶和葡萄糖生成的中间产物可促进土壤对Ⅰ-的吸附.平衡液中溶解性有机碳(DOC)含量随时间增加而逐渐降低,有机质浓度越大,降低趋势越明显. 相似文献
50.
贵州普定陈旗堡地区土体主要理化性质与持水量相关性研究 总被引:2,自引:0,他引:2
根据贵州普定陈旗堡典型喀斯特地区的生态特性,从水文地质与工程地质角度,借助土壤学对土壤水分特性的研究理论,对表层土壤20cm以下土层的土体物理化学性质与土体水分含量,即土体的持水量(waterholding capacity),进行了相关性研究。分析结果表明:土体颗粒成分、孔隙比、矿物成分含量等对土体水分含量均有影响。土体中粉粒含量较高而粘粒含量较少,且粒度分配不均匀时,土体水分含量相对较高;孔隙率高的土体水分含量较高;伊蒙混层矿物、伊利石和绿泥石等粘土矿物含量较高时,土体水分含量相对较高,而长石、碳酸盐等原生矿物含量较高时,土体水分含量相对较小。石英对于土体持水量影响不明显。 相似文献