全文获取类型
收费全文 | 90107篇 |
免费 | 1696篇 |
国内免费 | 924篇 |
专业分类
测绘学 | 2313篇 |
大气科学 | 6413篇 |
地球物理 | 18065篇 |
地质学 | 32261篇 |
海洋学 | 7856篇 |
天文学 | 20446篇 |
综合类 | 262篇 |
自然地理 | 5111篇 |
出版年
2022年 | 456篇 |
2021年 | 831篇 |
2020年 | 944篇 |
2019年 | 1029篇 |
2018年 | 2530篇 |
2017年 | 2370篇 |
2016年 | 2810篇 |
2015年 | 1659篇 |
2014年 | 2664篇 |
2013年 | 4642篇 |
2012年 | 2871篇 |
2011年 | 3837篇 |
2010年 | 3244篇 |
2009年 | 4294篇 |
2008年 | 3933篇 |
2007年 | 3754篇 |
2006年 | 3586篇 |
2005年 | 3230篇 |
2004年 | 3259篇 |
2003年 | 2888篇 |
2002年 | 2423篇 |
2001年 | 2185篇 |
2000年 | 2126篇 |
1999年 | 1743篇 |
1998年 | 1830篇 |
1997年 | 1733篇 |
1996年 | 1379篇 |
1995年 | 1420篇 |
1994年 | 1251篇 |
1993年 | 1096篇 |
1992年 | 1079篇 |
1991年 | 953篇 |
1990年 | 1091篇 |
1989年 | 917篇 |
1988年 | 852篇 |
1987年 | 1025篇 |
1986年 | 850篇 |
1985年 | 1103篇 |
1984年 | 1179篇 |
1983年 | 1114篇 |
1982年 | 1081篇 |
1981年 | 899篇 |
1980年 | 892篇 |
1979年 | 787篇 |
1978年 | 769篇 |
1977年 | 720篇 |
1976年 | 682篇 |
1975年 | 658篇 |
1974年 | 650篇 |
1973年 | 613篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
M. S. Kapdaşli 《Geo-Marine Letters》1990,10(1):45-49
Flume experiments were carried out to determine the threshold of movement of sand on rippled surface and flat beds under the
codirectional combined flows due to waves and currents. The results indicate that Shields's curve can be used to determine
the threshold condition under combined flows, provided that the maximum bed shear stress is used. 相似文献
992.
Regoli F Nigro M Chierici E Cerrano C Schiapparelli S Totti C Bavestrello G 《Marine environmental research》2004,58(2-5):637-640
The role of endosymbiotic diatoms as pro-oxidant stressors in porifera has been investigated in the Antarctic sponge Haliclona dancoi in which the presence of diatoms is influenced by marked seasonal variations during the austral summer. Both chlorophaeopigments and frustules were absent in sponge tissues sampled in early November at the beginning of the summer and increased from the mid of December with slightly shifted temporal trends. The efficiency of antioxidant defenses in the sponge showed a marked response to symbionts with clearly enhanced values corresponding to the peak of diatoms. 相似文献
993.
994.
M Salih Kirkgz 《Ocean Engineering》1991,18(1-2)
Laboratory tests are conducted to measure the impact pressures of breaking waves on vertical, 5° forward, and 5, 10, 20, 30, and 45° backward sloping walls. The base structure of the wall has a foreshore slope of
. Regular waves are used throughout the experiments for all wall angles. The maximum impact pressures on the wall are shown to satisfy the log-normal probability distribution. It is found from the present experiments that the impact pressures and resulting forces on sloping walls can be greater than those on a vertical wall. On the seven different walls tested, the maximum impact pressures occur most frequently slightly below the still-water level. The pattern of the impact pressure history does not change with the slope of the wall, and as the probability of maximum impact pressure decreases, the pressures around the peak pressure region of the impact pressure histories remain longer. 相似文献
995.
Undersampled records are susceptible to aliasing, in which a high frequency appears incorrectly as a lower one. We study the sampling requirements in a core taken from Rockall Trough using bulk density, P-wave velocity, and magnetic susceptibility as measured on an automated system. At 2-cm spacing (approximately 33 years in this core), all variables show a characteristic red-noise behavior, but with a spectral slope that is sufficiently weak so that significant aliasing appears to be present. P-wave velocity shows the largest potential corruption, probably owing to the weaker spatial averaging present in the sensor. Approximately 50% of the apparent low-frequency energy is aliased in all variables at some frequencies in both quiet and active regions of the record. In this core, a sampling interval of 0.2 cm appears to be "safe" in both active and quiet portions of the core, aliasing little or no energy, except in the P-wave record. For cores of different duration, sampling interval, and measurement type, the considerations will be identical, the importance of the problem depending directly upon the shape of the overall spectrum describing the entire frequency (wavenumber) range of physical variability. 相似文献
996.
997.
The muricid gastropod Bolinus brandaris exhibited imposex, a phenomenon linked to the use of organotin biocides. Several stages of imposex development (1-5) could be distinguished and were documented with SEM-photographs for the first time in this species. Additional alterations of the genital tract in females (blocked vulva preventing copulation and deposition of egg capsules) and males (excrecences on the penis and vas deferens) were shown. Imposex intensity on the Catalan coast was very high. The frequency of imposex off Barcelona and Vilanova i la Geltrú was 100%, with all females showing stage 4. Imposex frequency off Sant Carles de la Ràpita was 99.7% and the VDS index scored 3.9. Despite regulations of TBT-containing antifouling paints in Spain since 1990, the present study revealed that frequency and intensity of imposex have increased on the Catalan coast, at least since 1997. However, at present, imposex does not apparently affect the population dynamics of B. brandaris in the studied area, since no serious consequences on female reproduction (only one case of female sterilization) were detected. Gonadal activity and oogenesis were not suppressed or supplanted by spermatogenesis. Sex ratios were not male-biased. Possible sources of organotin biocides are examined. 相似文献
998.
Personal thermal insulation by means of neoprene from wet suits provides adequate temporary control of body heat loss only at the shallow end of the air diving depth range, but it constitutes by far the most popular approach to diver thermal support. Howewer, compression of the trapped gas phase in neoprene foam seriously reduces its insulation performance on exposure to high ambient pressure. With conventional wet suits equipped with heating, the necessary level of power required at depths greater than about 30 m is too high, and also unsafe without the back up that increased insulation would provide. One approach to the goal of depth-insensitive insulation is to use a wet suit with a continuous internal gas space pressurized nominally to ambient pressure, so that its thickness remains substantially constant at all depths. The composite material properties required are: outer skins that are tough, flexible and free of pin holes; an open foam internal structure capable of resisting, without significant dimensional change, the relatively small pressure changes that occur over the height of a man; and a high bond strength.Samples of a composite material that meets these requirements have been developed, and a prototype suit has been successfully fabricated by conventional techniques. 相似文献
999.
1000.
A theoretical and an experimental investigation was carried out, where a carbon fibre corrugated circular cylinder was tested to destuction under external hydrostatic pressure. The theoretical investigation was via the finite element method, where the structure was modelled with several orthotropic axisymmetric thin-walled shell elements. The experimental observations were aided with strategically placed strain gauges. Comparison between theory and experiment showed that the experimentally observed buckling pressure was a little lower than the theoretical prediction. This may have been due to the fact that the model had slight initial geometrical imperfections in the circumferenential direction. 相似文献