全文获取类型
收费全文 | 45594篇 |
免费 | 8764篇 |
国内免费 | 12242篇 |
专业分类
测绘学 | 4644篇 |
大气科学 | 9292篇 |
地球物理 | 10449篇 |
地质学 | 23044篇 |
海洋学 | 7150篇 |
天文学 | 1992篇 |
综合类 | 4444篇 |
自然地理 | 5585篇 |
出版年
2024年 | 280篇 |
2023年 | 780篇 |
2022年 | 2202篇 |
2021年 | 2682篇 |
2020年 | 2208篇 |
2019年 | 2529篇 |
2018年 | 2593篇 |
2017年 | 2374篇 |
2016年 | 2650篇 |
2015年 | 2509篇 |
2014年 | 2786篇 |
2013年 | 2961篇 |
2012年 | 3002篇 |
2011年 | 3029篇 |
2010年 | 2944篇 |
2009年 | 2817篇 |
2008年 | 2665篇 |
2007年 | 2378篇 |
2006年 | 2081篇 |
2005年 | 1877篇 |
2004年 | 1435篇 |
2003年 | 1394篇 |
2002年 | 1517篇 |
2001年 | 1508篇 |
2000年 | 1365篇 |
1999年 | 1688篇 |
1998年 | 1381篇 |
1997年 | 1294篇 |
1996年 | 1234篇 |
1995年 | 1047篇 |
1994年 | 946篇 |
1993年 | 861篇 |
1992年 | 723篇 |
1991年 | 580篇 |
1990年 | 450篇 |
1989年 | 384篇 |
1988年 | 345篇 |
1987年 | 196篇 |
1986年 | 180篇 |
1985年 | 128篇 |
1984年 | 101篇 |
1983年 | 65篇 |
1982年 | 110篇 |
1981年 | 62篇 |
1980年 | 50篇 |
1979年 | 52篇 |
1978年 | 19篇 |
1977年 | 17篇 |
1976年 | 22篇 |
1958年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
Starfish oocytes with intact germinal vesicles (GVs) were cut along desired planes with glass needles or ligated using silk
thread loops into two parts and allowed to mature in vitro, and inseminated. The experimental results showed that (1) only
the parts with GVs or partial GV contents (PGVCs) cleaved, those without any GV materials did not; but nucleated and non-nucleated
fragments cut from mature eggs were able to divide; (2) the development of animal parts of oocytes containing GVs or PGVCs
was like that of animal fragments of matured oocytes with female pronuclei; most of them gave rise to permanent blastulae,
and just a few formed ectodermal vesicles with a little primary mesenchyme; (3) a large part of vegetal fragments with GVs
or PGVCs, and the vegetal parts of mature eggs without female pronuclei developed into small but normal embryos; (4) the fragments
containing GVs or PGVCs obtained from the oocytes along a plane parallel to the animal-vegetal (A-V) axis developed as normally
as the halves (with or without female pronuclei) severed from mature eggs along the same axis. Based on the data above, it
was concluded that (1) the non-chromatin materials in the oocyte GVs are indispensable for successful fertilization and cleavage
of starfish eggs; (2) some factor (s) located asymmetrically in the vegetal hemispheres of starfish oocytes is (are) responsible
for formation of the archenteron and primary mesenchyme. It is evident from the above findings that the oocyte cytoplasm of
the starfish had already regionalized before the GV break-down.
Contribution No. 1722 from the Institute of Oceanology, Academia Sinica 相似文献
23.
隧道工程地质评价的内容和方法 总被引:2,自引:0,他引:2
结合工程隧道实际,首先阐述了隧道工程的基本地质环境和工程地质条件,然后针对隧道工程可能出现的不良地质现象和可采取的工程措施,从大气降水、围岩稳定、围岩压力、洞口稳定、隧道比选等角度探讨隧道工程地质评价的主要方法和一般内容,进行隧道工程地质评价,为隧道施工、支护提供了依据. 相似文献
24.
We use the following numerical model for the collapse stage of a Type II supernova of 15 M⊙. Our electron capture rate includes the effects of the inverse reaction and the neutron-proton mass difference. This decreases the electron density at the collapse stage and led to rather large values of the maximum inward velocity and of the corresponding mass (Umax = 3.06 × 109cm/s, Mmax=0.76 M⊙). These larger values are more favourable for the propagation of shock after the rebounce and the triggering-off of a Type-II supernova explosion. For neutrino transport, we use a leakage model and an equilibrium diffusion model, respectively, for the thin and thick stages and a grey atmosphere model to assess the effect of neutrino precipitation on the collapse. We found this effect to be small, the energy precipitation to be not more than 10?5 the neutrino energy loss and the momentum precipitation not more than 10?6 the gravitational acceleration. 相似文献
25.
26.
Xie Rui-xiang Wang Min Shi Shuo-biao Xu Chun Li Wei-hua Yan Yi-hua 《Chinese Astronomy and Astrophysics》2003,27(4):426-434
Using the decimetric (700–1500 MHz) radio spectrometer and the synchronous observational system with high temporal resolution at four frequencies (1420, 2130, 2840 and 4260 MHz) of Yunnan Observatory, two rare events were observed on 2001 June 24 and 1990 July 30. The former was a small radio burst exhibiting pulsations with short periods (about 29, 40 and 100 ms) in the impulsive phase. The latter was a large radio burst, which at 2840 MHz produced radio pulsations with period of about 30 ms. This paper focuses on pulsations with very short periods in the range of 29–40 ms. The mechanism of generation of such pulsations may be modulation of radio radiation by the periodic trains of whistler packets originating in unstable regions of the corona. Alternatively, these pulsations can be attributed to wave-wave non-linear interactions of electrostatic upper hybrid waves driven by beams of precipitating electrons in flaring loops. 相似文献
27.
28.
HUXue-lian CHENGCheng-qi WUDe-wen MAAi-nai 《中国地理科学(英文版)》2003,13(1):84-90
Based on the analysis of the development of GIS technology and application,this paper brought forward the concept of GoGIS,namely Cooperative GIS ,CoGIS is GIS facing group-users and supporting human-human interaction,which makes it differ from the former GISs,Then,the characteristics of general Computer Spport Cooperative Work (CSCW)applications and the complexity of Geographic Information Science were analyzed,and the conclusion the CoGIS was not a simple GIS layer on CSCW was reached,Further,this paper gaver the hierarchical architecture of CoGIS,and analyzed the coperative platform in detail from the following:1)basic elements;2) collaboration patterns;3) cooperation control mechanism;4) synchronization mechanism;5) security and 6) group communication and so on.With those,the problems about GIS applications are discussed,such as 1)distributed multi-source GIS information and knowledge sharing platform;2)the fusion and visualization of GIS information;3)virtual reality cooperative modeling;4) dymamic simulation;5)expert system and 6) decision-making.Finally,this paper analyzed CoGIS application mode in brief. 相似文献
29.
同时采用4个台站的国际超导重力仪长期连续重力观测资料和国际地球自转服务中心提供的同步地球自转参数,研究了极移引起的地球重力场变化特征。利用自回归模型估计了各序列的功率谱密度和积谱密度,结果表明极移导致的重力效应的主要能量集中在Chandler摆动和周年项附近,叠积后实际重力观测与极移重力信号理论值之间的差异分别为0.4%和3.9%,说明超导重力仪可有效监测极移导致的重力变化。 相似文献
30.
An elastoplastic model for sands is presented in this paper, which can describe stress–strain behaviour dependent on mean effective stress level and void ratio. The main features of the proposed model are: (a) a new state parameter, which is dependent on the initial void ratio and initial mean stress, is proposed and applied to the yield function in order to predict the plastic deformation for very loose sands; and (b) another new state parameter, which is used to determine the peak strength and describe the critical state behaviour of sands during shearing, is proposed in order to predict simply negative/positive dilatancy and the hardening/softening behaviour of medium or dense sands. In addition, the proposed model can also predict the stress–strain behaviour of sands under three-dimensional stress conditions by using a transformed stress tensor instead of ordinary stress tensor. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献