全文获取类型
收费全文 | 2132篇 |
免费 | 516篇 |
国内免费 | 308篇 |
专业分类
测绘学 | 288篇 |
大气科学 | 227篇 |
地球物理 | 1009篇 |
地质学 | 801篇 |
海洋学 | 244篇 |
天文学 | 69篇 |
综合类 | 221篇 |
自然地理 | 97篇 |
出版年
2024年 | 14篇 |
2023年 | 34篇 |
2022年 | 68篇 |
2021年 | 93篇 |
2020年 | 111篇 |
2019年 | 115篇 |
2018年 | 89篇 |
2017年 | 88篇 |
2016年 | 97篇 |
2015年 | 97篇 |
2014年 | 121篇 |
2013年 | 135篇 |
2012年 | 135篇 |
2011年 | 130篇 |
2010年 | 130篇 |
2009年 | 220篇 |
2008年 | 235篇 |
2007年 | 129篇 |
2006年 | 169篇 |
2005年 | 119篇 |
2004年 | 94篇 |
2003年 | 80篇 |
2002年 | 75篇 |
2001年 | 67篇 |
2000年 | 48篇 |
1999年 | 43篇 |
1998年 | 39篇 |
1997年 | 38篇 |
1996年 | 26篇 |
1995年 | 28篇 |
1994年 | 20篇 |
1993年 | 18篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有2956条查询结果,搜索用时 15 毫秒
991.
The seismic capacity of beam‐to‐column connections in steel high‐rise frames is a matter of concern, particularly when they are subjected to long‐period ground motions. A previous full‐scale shaking table test conducted at the E‐Defense National Research Institute for Earth Science and Disaster Prevention in Japan disclosed cracks and fractures in such beam‐to‐column connections. This paper examines the effects of three types of beam‐to‐column connection retrofit: supplemental welds, wing plates, and a haunch. Quasi‐static member tests and a series of shaking table tests applied to a full‐scale specimen are conducted to quantify the respective performances of the retrofit schemes. The performance of a total of 28 connections tested by the member and shaking table tests is evaluated together with that of an additional 12 unretrofitted connections tested in the previous test. When the supplemental welds are applied only to the shear tab to the web, the connection fractures at the same instant as the connection without retrofit. The corresponding cumulative plastic rotation is not improved. When the supplement welds are further applied to the web‐to‐column connection, strain concentration at the bottom flange, primarily promoted by the presence of the RC floor slab, is significantly reduced, and the cumulative plastic rotation capacity is increased to eight times that of the connection without retrofit. For the wing plate connection and haunch connection, the critical section is moved from the beam end to the beam cross‐section corresponding to the tip of the wing plates or haunch, resulting in an improvement of ductility by eight times that of the unretrofitted connection. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
992.
?????????HHT????????????????????????????÷?????С??????????????о?????ν?HHT????????????????????????????????С? 相似文献
993.
?????????????Kalman??????????????????????????????????÷??????????????????????????????????????????????????????ò???????????Kalman?????????ο?????????????????????ù???????????????????????????????????и?????????????????y???CORS????????????÷????????????????????÷??????????????????????????????????????????? 相似文献
994.
995.
996.
V. I. Shishov S. A. Tyul’bashev I. A. Subaev I. V. Chashei 《Solar System Research》2008,42(4):341-350
The results of a series of 24-hour observations of radio-source interplanetary and ionospheric scintillation performed on April 4–10, 2006, at the Pushchino Radio Astronomy Observatory are presented. The observations were carried out with the Large Phased Array radio telescope of the Lebedev Institute of Physics, Russian Academy of Sciences, at a frequency of 110 MHz. The scintillating fluxes of all radio sources that fall within a field of sky between declinations +28° and +31° were automatically recorded applying eight beams of the reception pattern operating simultaneously. All of the sources with flux densities of 0.2 Jy or higher were detected. The structure functions of the flux fluctuations were measured for time shifts 1 and 10 s, which characterize the interplanetary (1 s) and ionospheric (10 s) scintillation, respectively. The mean scintillation index m IPP (on a characteristic time scale of 1 s) of an ensemble of radio sources located within a sky band 4° wide in declination and 1 h wide in right ascension was measured as the parameter that characterizes the interplanetary plasma. Diurnal variations of the interplanetary scintillation index were determined. The maximum m IPP value at daytime equals 0.3, and the minimum value at nighttime equals 0.10. Weak interday variations of the mean daytime and nighttime scintillation indices were detected. The ionospheric scintillation indices m Ion are small compared to m IPP at daytime, but m Ion ? m IPP at nighttime. On the whole, both the interplanetary plasma and ionosphere were quiet during the observations. 相似文献
997.
I. V. Arkhangelskaja A. I. Arkhangelsky Yu. D. Kotov S. N. Kuznetsov A. S. Glyanenko 《Solar System Research》2008,42(4):351-358
Six solar flares were detected by the AVS-F apparatus onboard the CORONAS-F satellite in January 2005. We discuss the temporal profiles and energy spectra of the solar flares of January 20, 17, and 15, 2005 (class X7.1, X3.8, and X2.6, respectively) on the AVS-F data. The active region NOAA 10720 was the source of these flares. The spectra of the flares of January 17 and 20, 2005 contain nuclear lines, a positron line, and a line due to neutron capture line, while only the positron and neutron capture lines can be identified in the spectrum of the flare of January 15, 2005. The spectral features corresponding to these lines were observed during the whole duration of the flares. Analysis of the temporal profile of the flare of January 20, 2005 with a 1-ms temporal resolution in the energy range 0.1–20 MeV reveals the presence of a thin structure (at the 99% confidence level) with typical timescales of 7 to 35 ms. 相似文献
998.
V. A. Shishov 《Solar System Research》2008,42(4):319-328
For the needs of the upcoming flight to Phobos within the framework of the project Phobos-Grunt, a dynamical numerical theory of the motion of Phobos is developed on the basis of Earth-based and spacecraft (SC) optical measurements and Earth-based radio observations of the SC motion. The theory uses the most reliable astronomical constants. Within the framework of a unified dynamical model, an algorithm is proposed for the joint correction of Phobos and the spacecraft parameters of motion, among which are the secular acceleration of Phobos and its gravitational constant. The accuracy of the obtained ephemeris is estimated for the time of the SC arrival to Phobos. 相似文献
999.
Frank I. Dromgoole 《新西兰海洋与淡水研究杂志》2013,47(4):617-619
The CO2 exchange characteristics and chlorophyll content of the pneumatophores of Avicennia marina (Forsk.) Vierh. var. resinifera (Forst.) Bakh. were examined. The results indicate that, in addition to their primary role in ventilation of the buried roots, the aerial roots have the ability to fix exogenous CO2 via photosynthesis. Consequently, during the day these organs are almost auto trophic in terms of carbon balance. When corrected for the respiration of non‐chlorophyllous tissue, they show photosynthetic rates comparable to those of many other woody plants. 相似文献
1000.
利用来自英国气象局哈德莱中心(Met Office Hadley Centre)的海表面温度(Sea Surface Temperature, SST)资料, 对全球海域近140余年来SST的长期变化进行分析。研究发现:(1)1870—2011年期间, 全球大部分海域的SST表现出显著的逐年线性递增趋势, 在两极大部分海域、格陵兰南部海域呈显著的递减趋势, 仅在部分小范围海域及一些零星海域的SST无显著变化。近海的递增趋势强于大洋, 大洋西岸的递增趋势强于大洋东岸。(2)近140余年来, 整体上全球的SST以0.0038℃/a的速度显著增加, 其中1870—1910年表现出较为显著的递减趋势, 1910年至今表现出较强的递增趋势。(3)1870—1910年期间, 全球大范围海域的SST无显著变化; 呈显著递减的海域主要分布于东海、阿拉斯加半岛南部海域、北大西洋30°N附近海域、冰岛南部海域、南印度洋西风带海域、新西兰附近海域; 呈显著递增的区域较为分散。1910—2011年期间, 全球大范围海域的SST逐年显著递增, 在两极大部分海域呈显著的递减趋势。(4)1870—1960年期间, 两极的SST走势较为平缓; 1960—2011年期间, 南极尤其是威德尔海的SST表现出显著的递减趋势, 其中逐8月的递减趋势尤为强劲; 北极逐2月、逐5月的SST呈显著性递减, 逐8月呈显著性递增且趋势较为强劲, 达到0.0402℃/a, 逐11月的SST无显著变化。 相似文献