全文获取类型
收费全文 | 16899篇 |
免费 | 4010篇 |
国内免费 | 5436篇 |
专业分类
测绘学 | 2768篇 |
大气科学 | 2682篇 |
地球物理 | 3402篇 |
地质学 | 10158篇 |
海洋学 | 3025篇 |
天文学 | 413篇 |
综合类 | 1413篇 |
自然地理 | 2484篇 |
出版年
2024年 | 113篇 |
2023年 | 371篇 |
2022年 | 984篇 |
2021年 | 1087篇 |
2020年 | 918篇 |
2019年 | 1127篇 |
2018年 | 1020篇 |
2017年 | 994篇 |
2016年 | 995篇 |
2015年 | 1118篇 |
2014年 | 1086篇 |
2013年 | 1355篇 |
2012年 | 1490篇 |
2011年 | 1520篇 |
2010年 | 1472篇 |
2009年 | 1375篇 |
2008年 | 1466篇 |
2007年 | 1311篇 |
2006年 | 1253篇 |
2005年 | 1015篇 |
2004年 | 797篇 |
2003年 | 599篇 |
2002年 | 659篇 |
2001年 | 569篇 |
2000年 | 509篇 |
1999年 | 274篇 |
1998年 | 161篇 |
1997年 | 130篇 |
1996年 | 95篇 |
1995年 | 90篇 |
1994年 | 60篇 |
1993年 | 63篇 |
1992年 | 47篇 |
1991年 | 38篇 |
1990年 | 37篇 |
1989年 | 21篇 |
1988年 | 20篇 |
1987年 | 12篇 |
1986年 | 17篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 12篇 |
1978年 | 3篇 |
1974年 | 2篇 |
1958年 | 3篇 |
1957年 | 12篇 |
1954年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
活动星系核(AGN)是宇宙中最奇特的天体之一。它是真正意义上的全波天体,其中X射线波段的发射功率占到全波段功率的50%左右。AGN的X射线辐射研究涉及天体物理中的最基本问题,例如能量产生、辐射机制和宇宙论等,而Chandra X射线卫星的高分辨率图像和光谱对这一研究有着重要作用。以Chandra卫星的部分观测结果为例,简要介绍了几类不同类型AGN的X射线辐射研究进展:(1)宽吸收线类星体APM08279+5255(z=3.91)的X射线谱分析,以及高红移类星体的观测概况;(2)Seyfert星系NGC 4151延展X射线发射问题的解决,及NGC 1068X射线辐射与光学波段的高激发态发射线([OⅢ]λ5007)有很强相关性的发现;(3)6个BLLac天体样本的X射线环境分析;(4)射电星系X射线喷流的观测等。 相似文献
22.
低频扩频中伪随机序列产生方法的研究 总被引:2,自引:0,他引:2
伪随机序列的产生是实现BPC低频时码发播台扩展频谱技术升级的一个重要环节。针对BPC低频时码发播台的现有系统,提出了伪随机序列的产生方案,并对其性能作了分析。 相似文献
23.
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
24.
25.
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
26.
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
27.
圆盘固相萃取富集-气相色谱法测定地表水中有机氯和有机磷农药 总被引:4,自引:2,他引:2
采用环境友好的圆盘固相萃取新技术富集水体中有机氯农药和有机磷农药,分别用微池电子捕获检测器(μECD)和火焰光度检测器(FPD)气相色谱法检测,实现了水中有机氯和有机磷农药残留物的测定。结果表明,16种有机氯农药的平均回收率为64.7%~102%,精密度(RSD,n=6)为2.9%~15%;13种有机磷农药的平均回收率为65.9%~104%,精密度(RSD,n=6)为1.7%~17%。方法快速、灵敏、低污染,可用于水体中多种有机氯农药和有机磷农药的残留分析。 相似文献
28.
29.
30.