首页 | 官方网站   微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   11767篇
  免费   4395篇
  国内免费   942篇
地球科学   17104篇
  2025年   97篇
  2024年   364篇
  2023年   433篇
  2022年   537篇
  2021年   640篇
  2020年   601篇
  2019年   556篇
  2018年   603篇
  2017年   549篇
  2016年   669篇
  2015年   524篇
  2014年   705篇
  2013年   645篇
  2012年   551篇
  2011年   640篇
  2010年   631篇
  2009年   625篇
  2008年   543篇
  2007年   527篇
  2006年   426篇
  2005年   432篇
  2004年   304篇
  2003年   308篇
  2002年   350篇
  2001年   363篇
  2000年   359篇
  1999年   529篇
  1998年   464篇
  1997年   438篇
  1996年   432篇
  1995年   333篇
  1994年   331篇
  1993年   295篇
  1992年   260篇
  1991年   211篇
  1990年   171篇
  1989年   144篇
  1988年   123篇
  1987年   66篇
  1986年   73篇
  1985年   50篇
  1984年   41篇
  1983年   29篇
  1982年   42篇
  1981年   13篇
  1980年   21篇
  1979年   14篇
  1978年   7篇
  1977年   6篇
  1958年   13篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
The aim of this study was to compare the effects of waterborne copper(Cu) and cadmium(Cd) on survival, anti-oxidative response, lipid peroxidation and metal accumulation in abalone Haliotis discus hannai. Experimental animals(initial weight: 7.49 g ± 0.01 g) were exposed to graded concentrations of waterborne Cu(0.02, 0.04, 0.06, 0.08 mg L-1) or Cd(0.025, 0.05, 0.25, 0.5mg L-1) for 28 days, respectively. Activities of the anti-oxidative enzymes(catalase, CAT; superoxide dismutase, SOD; glutathione peroxidases, GPx; glutathione S-transferase, GST), contents of the reduced glutathione(GSH) and malondiadehyde(MDA) in the hepatopancreas, and metal accumulation in hepatopancreas and muscles were analyzed after 0, 1, 3, 6, 10, 15, 21, 28 days of metal exposure, respectively. Results showed that 0.04 mg L-1, 0.06 mg L-1 and 0.08 mg L-1 Cu caused 100% death of abalone on the 21 st, 10 th and 6th day, respectively. However, no dead abalone was found during the 28-day waterborne Cd exposure at all experimental concentrations. Generally, activities of SOD and GST in hepatopancreas under all Cu concentrations followed a decrease trend as the exposure time prolonged. However, these activities were firstly increased and then decreased to the control level and increased again during Cd exposure. Activities of CAT in all Cu exposure treatments were higher than those in the control. These activities were firstly increased and then decreased to the control level and increased again during Cd exposure. Contents of MDA in hepatopancreas in all Cu treatments significantly increased first and then decreased to the control level. However, the MDA contents in hepatopancreas were not significantly changed during the 28-day Cd exposure. The metals accumulation in both hepatopancreas and muscles of abalone significantly increased with the increase of waterborne metals concentration and exposure time. These results indicated that H. discus hannai has a positive anti-oxidative defense against Cu or Cd. In conclusion, anti-oxidative mechanism in abalone to resist waterborne Cu did not follow the same pattern as that for waterborne Cd.  相似文献   
92.
    
Xu  Yu-ran  Xu  Yongfu 《Acta Geotechnica》2021,16(10):3133-3144
Acta Geotechnica - In the traditional Mohr–Coulomb criterion, the shear strength is determined by cohesion and friction angle. However, the particle breakage that occurs on the shear failure...  相似文献   
93.
许吉发 《吉林地质》1996,15(1):86-88
油管传佃式射孔是近几年来射孔新技术,应用于石油、天燃气勘探、开发井中。的使用,大大提高了油气井的射孔效率,增加油气产量,降低了作业成本,从而能获得较好的经济效益,解决了正压射孔存在的不足,是一项新的负压射孔方式。通过几年来在松辽地区的应用,效果很佳。大部分的高产油气流都是采用这种射孔方法进行的。  相似文献   
94.
We present a conceptual design study of external calibrators in the 21 cm experiment towards detecting the globally averaged radiation of the epoch of reionization(EoR). Employment of external calibrator instead of internal calibrator commonly used in current EoR experiments allows removing instrumental effects such as beam pattern, receiver gain and instability of the system if the conventional three-position switch measurements are implemented in a short time interval. Furthermore, in the new design the antenna system is placed in an underground anechoic chamber with an open/closing ceiling to maximally reduce the environmental effect such as RFI and ground radiation/reflection. It appears that three of the four external calibrators proposed in this paper, including two indoor artificial transmitters and one outdoor celestial radiation(the Galactic polarization), fail to meet our purpose. Diurnal motion of the Galactic diffuse emission turns out to be the most probable source as an external calibrator, for which we have discussed the observational strategy and the algorithm of extracting the EoR signal.  相似文献   
95.
旅游地图是人们旅行和导购导游的常用工具,对各地旅游业的发展具有十分重要的推动作用。本文论述了网络化旅游地图发行方式上所具有的优势,分析了当前旅游地图网络化的发展现状及存在问题,探讨了加快旅游地图网络化发展的策略与措施。  相似文献   
96.
Tienstra’s method was developed to solve parameter adjustment with linear equality constraints, which has been otherwise often carried out by directly applying the conventional (or standard) method of Lagrange multipliers to quadratic optimization problems with a positive definite matrix. We analyze the computational complexity of the celebrated Tienstra’s method and compare it with that of the method of Lagrange multipliers. We show that Tienstra’s method is not only statistically elegant but also interestingly of significant computational advantage over the method of Lagrange multipliers to solve weighted least squares adjustment with linear equality constraints, with the saving of computational cost by a minimum of about 38% to a maximum of 100%. Tienstra’s method can be very important for large scale problems of adjustment and inversion with linear equality constraints.  相似文献   
97.
本文给出一种用积分方程法配合有限差分法计算位于导电介质中三维导体的电磁响应的新途径,在二层大地条件下给出了具体计算方法和结果。详细讨论了围岩介质的导电性对导体异常的影响。这对于开展低阻覆盖层地区的电磁法工作具有一定的指导意义。  相似文献   
98.
The strata-bound Cu−Pb−Zn polymetallic sulfide deposits occur in metamorphic rocks of greenschist phase of the middle-upper Proterozoic Langshan Group in central Inner Mongolia. δ34S values for sulfides range from −3.1‰ to +37.3‰, and an apparent difference is noticed between vein sulfides and those in bedded rocks. For example, δ34S values for bedded pyrite range from +10.6‰ to +20.0‰, while those for vein pyrite vary from −3.1‰ to +14.1‰. δ34S of bedded pyrrhotite is in the range +7.9‰–+23.5‰ in comparison with +6.5‰–+17.1‰ for vein pyrrhotite. The wide scatter of δ34S and the enrichment of heavier sulfur indicate that sulfur may have been derived from H2S as a result of bacterial reduction of sulfates in the sea water. Sulfur isotopic composition also differs from deposit to deposit in this area because of the difference in environment in which they were formed. The mobilization of bedded sulfides in response to regional metamorphism and magmatic intrusion led to the formation of vein sulfides. δ18O and δ13C of ore-bearing rocks and wall rocks are within the range typical of ordinary marine facies, with the exception of lower values for ore-bearing marble at Huogeqi probably due to diopsidization and tremalitization of carbonate rocks. Pb isotopic composition is relatively stable and characterized by lower radio-genetic lead. The age of basement rocks was calculated to be about 23.9 Ma and ore-forming age 7.8 Ma.207Pb/204Pb−206Pb/204Pb and208Pb/204Pb−206Pb/204Pb plots indicate that Pb may probably be derived from the lower crust or upper mantle. It is believed that the deposits in this region are related to submarine volcanic exhalation superimposed by later regional metamorphism and magmatic intrusion.  相似文献   
99.
We use the rotation curves of 13 galaxies to test the nonsymmetric gravitaional theory (NGT). If we follow Moffat's assumption of a constant mass-to-light for all galaxies then we shall find that the two supposedly universal constants r0, L0 to show a large scatter. By regarding the mass-to-light ratio as a free parameter for each galaxy, and adjusting the values of r0, L0, we find that NGT can well account for the observed rotation curves. Further, the mass-to-light ratios so found show the well-known trend along the Hubble morphological sequence.  相似文献   
100.
某铅锌尾矿库对地下水环境影响及防治措施   总被引:1,自引:0,他引:1  
铅锌尾矿的浸出液中含有重金属等对地下水环境有重要影响的污染因子。利用地下水数值模拟专业软件GMS,结合研究区水文地质条件,对不同工况下地下水环境的影响程度及范围进行模拟计算。结果表明,达到Ⅱ类工业固体废弃物处置场规定的防渗要求条件下,尾矿库运行浸出液下渗量有限(2.8 m3/d),并不会使区域地下水超出《地下水质量标准》Ⅲ类标准,而防渗层仅出现10%破损时,浸出液下渗量为(97.22 m3/d),使地下水下游方向50 m内地下水中Pb超出Ⅲ类标准,超标范围约0.0375 km2。反映出该类处置场中的防渗措施是对地下水环境保护的重要保证。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司    京ICP备09084417号

京公网安备 11010802026262号