全文获取类型
收费全文 | 10943篇 |
免费 | 794篇 |
国内免费 | 291篇 |
专业分类
系统科学 | 1283篇 |
丛书文集 | 171篇 |
教育与普及 | 128篇 |
理论与方法论 | 291篇 |
现状及发展 | 883篇 |
研究方法 | 15篇 |
综合类 | 9255篇 |
自然研究 | 2篇 |
出版年
2024年 | 28篇 |
2023年 | 58篇 |
2022年 | 104篇 |
2021年 | 108篇 |
2020年 | 66篇 |
2019年 | 27篇 |
2018年 | 762篇 |
2017年 | 789篇 |
2016年 | 481篇 |
2015年 | 172篇 |
2014年 | 145篇 |
2013年 | 144篇 |
2012年 | 414篇 |
2011年 | 1144篇 |
2010年 | 965篇 |
2009年 | 654篇 |
2008年 | 749篇 |
2007年 | 1001篇 |
2006年 | 207篇 |
2005年 | 269篇 |
2004年 | 272篇 |
2003年 | 297篇 |
2002年 | 202篇 |
2001年 | 177篇 |
2000年 | 158篇 |
1999年 | 313篇 |
1998年 | 325篇 |
1997年 | 330篇 |
1996年 | 271篇 |
1995年 | 237篇 |
1994年 | 207篇 |
1993年 | 188篇 |
1992年 | 163篇 |
1991年 | 154篇 |
1990年 | 126篇 |
1989年 | 107篇 |
1988年 | 92篇 |
1987年 | 70篇 |
1986年 | 26篇 |
1985年 | 17篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1967年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
LUO YunLi SUN XiangJun 《科学通报(英文版)》2007,52(15):2115-2122
Based on deep-sea pollen results (512-76 m) from ODP Site 1143 in the southern South China Sea (SCS),the climate and vegetation evolution sequence on the surrounding islands and the exposed continental shelf are discussed. The pollen records show that the pollen influx was quite low before 8.15 Ma and increased dramatically afterwards. The influx changes can be ascribed,on one side,to tectonics deformations around the southern SCS resulting in rapid uplift of islands and subsequent increase of the sediment rates and pollen influx and on the other side to climate cooling and monsoon enhancement. Around 2.63 Ma was another obvious boundary,the increasing of pollen and spores influx since this time was mainly related to global climate cooling. Spectrum analysis of pollen influx values shows that 2 Ma,0.67 Ma,and 0.19-0.17 Ma cycles existed during 12-3.0 Ma,while 0.1 Ma and 46.9 ka cycles existed during 3.0-2.0 Ma. 相似文献
992.
This paper presents a methodology which determines the allocation of power demand among the committed generating units while minimizes number of objectives as well as meets physical and technological system constraints. The procedure considers two decoupled problems based upon the dependency of their goals on either active power or reactive power generation. Both the problems have been solved sequentially to achieve optimal allocation of active and reactive power generation while minimizes operating cost, gaseous pollutants emission objectives and active power transmission loss with consideration of system operating constraints along with generators prohibited operating zones and transmission line flow limits. The active and reactive power line flows are obtained with the help of generalized generation shift distribution factors (GGDF) and generalized Z-bus distribution factors (GZBDF), respectively. First problem is solved in multi-objective framework in which the best weights assigned to objectives are determined while employing weighting method and in second problem, active power loss of the system is minimized subject to system constraints. The validity of the proposed method is demonstrated on 30-bus IEEE power system. 相似文献
993.
994.
During the first leg of a Chinese research cruise to the Southeast Indian Ridge (SEIR) in 2007, we revisited the K-segment of the SEIR and observed a water column turbidity anomaly at a CTD cast station. This station was located ~400 m from a dredge site on a plume anomaly detected in 1996 by US scientists. The characteristics of the turbidity anomaly detected in our survey are different from those observed 11 years previously, implying that either the detected anomalies have distinct and different plume sources or the robustness of the plume has changed in the intervening time. In addition, turbidity anomalies were observed at two localities on the K-segment in a deep tow-yo profile. If these two anomalies are not connected, the plume incidence calculated would be about 0.28, larger than the predicted value of 0.25 for a 65 mm/a spreading rate. This increase in plume incidence on the K-segment seems to be related to the influence of the St. Paul-Amsterdam hotspot, located about 300 km to the northwest. 相似文献
995.
Coordination polymers (or metal-organic frameworks, MOFs), as one of the essences of crystal engineering and coordination chemistry, have attracted considerable atten-tion from chemists, 相似文献
996.
A number of recently evolved animals possess poison glands for feeding and/or defense.However,examples of such animals are rare in the fossil record.We report a fossil arthropod Isoxys curvirostratus from the Early Cambrian Chengjiang biota of China.This species is regarded as the oldest known venomous arthropod based on the presence of venomous glands in its head region.The adult animal is 2-5 cm long and the body is covered entirely with a carapace.The presence of large stalk eyes and a pair of stout grasping appendages with a terminal spine suggest it was raptorial.Interestingly,the two pear-shaped,three-dimensionally preserved objects that are present in the head region and at the base of the grasping appendages closely resemble the venom glands of some living arthropods in size,shape,and position.These features indicate that the presence of venomous predators could date back 520 million years.Furthermore,our observations suggest that the feeding strategies and organs adapted for this purpose had already reached a high level of diversity and anatomical sophistication in the Early Cambrian ecosystems. 相似文献
997.
ShangJian Zhang ChangYu Lin Xuan Li Zhitai Zou Yong Liu YongZhi Liu 《科学通报(英文版)》2011,56(15):1633-1636
A novel method for characterizing the nonlinearity of all-optical sampling is proposed based on the normalized transfer function.Simulation results demonstrate the effectiveness of our method.Furthermore,an all-optical sampling experiment is performed to verify our method.Both simulation and experiment show consistency between our method and the measurements.The method only requires normalization and polynomial fitting of the transfer curve,and enables direct expression of the nonlinearity with the coefficients of the normalized transfer function. 相似文献
998.
林向阳 《长沙理工大学学报(自然科学版)》2000,(1)
研究选用氧化剂溴酸钾、维生素C ,以改善面粉面筋强度 ,从而改善面包的纹理结构 ,通过对实验结果的评判 ,来判断它们对面包焙烤的影响 ,确立它们在面包制作过程中的添加量。对溴酸钾存在的安全性问题也进行了分析讨论 相似文献
999.
1000.
用X-射线衍射(XRD)和透射电子显微镜(TEM)等研究了不同水/表面活性剂([W]/[S])比的微乳液制备纳米Ni-Fe复合微粒的组成和粒径分布,结果表明,([W]/[S])比值不同,得到的Ni-Fe复合物微粒不但粒径不同,而产物的组成也有差异,当([W]/[S])>240时,微粒粒径最小,但组成复杂;当([W]/[S])<198时,微粒粒径呈寺大,但组成较单一,且可得到Ni-Fe合金相,各样品磁参数的测量表明,当将Ni-Fe组分总量规一化时,发现随Ni/Fe比增加,磁参数减小。 相似文献