全文获取类型
收费全文 | 114986篇 |
免费 | 18606篇 |
国内免费 | 12125篇 |
专业分类
航空 | 81213篇 |
航天技术 | 18109篇 |
综合类 | 12709篇 |
航天 | 33686篇 |
出版年
2024年 | 1148篇 |
2023年 | 4576篇 |
2022年 | 5554篇 |
2021年 | 4739篇 |
2020年 | 5329篇 |
2019年 | 5798篇 |
2018年 | 5507篇 |
2017年 | 4128篇 |
2016年 | 4716篇 |
2015年 | 4430篇 |
2014年 | 5898篇 |
2013年 | 5588篇 |
2012年 | 6427篇 |
2011年 | 7225篇 |
2010年 | 7053篇 |
2009年 | 7135篇 |
2008年 | 6679篇 |
2007年 | 6116篇 |
2006年 | 5720篇 |
2005年 | 4752篇 |
2004年 | 4275篇 |
2003年 | 4074篇 |
2002年 | 3404篇 |
2001年 | 3189篇 |
2000年 | 2619篇 |
1999年 | 2244篇 |
1998年 | 1514篇 |
1997年 | 1437篇 |
1996年 | 1425篇 |
1995年 | 1440篇 |
1994年 | 1527篇 |
1993年 | 1025篇 |
1992年 | 996篇 |
1991年 | 951篇 |
1990年 | 849篇 |
1989年 | 1060篇 |
1988年 | 845篇 |
1987年 | 789篇 |
1986年 | 478篇 |
1985年 | 571篇 |
1984年 | 483篇 |
1983年 | 515篇 |
1982年 | 396篇 |
1981年 | 460篇 |
1980年 | 216篇 |
1979年 | 153篇 |
1978年 | 80篇 |
1977年 | 47篇 |
1976年 | 24篇 |
1975年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
882.
883.
884.
介绍了航天器太阳阵等大型附件柔性多体系充物理模型和结构有限元单元模型,利用二体撞击动力学模型阐述了撞击处理的撞击-释放条件,并给出了多体撞击动力学模型。 相似文献
885.
将文献[1]中的局部模型跟踪变结构理论用于导弹控制系统设计,探索了一条设计控制系统的新路。某型号导弹控制系统设计中采用了本文提出的参数选择法,获得了良好的仿真结果。 相似文献
886.
尺寸参数对气动液阀启动特性的影响 总被引:1,自引:0,他引:1
为气动液阀的启动过程建立了数学模型,分析了尺寸参数对该阀启动特性的影响。计算结果表明:在保证必要的工作寿命的前提下,适当地增大控制腔气孔的直径或增大靠近控制腔的活塞端面直径有利于提高该阀的响应能力。本文所得的结论有利于此类阀门的设计。 相似文献
887.
Green AR Andrews HR Bennett LG Clifford ET Ing H Jonkmans G Lewis BJ Noulty RA Ough EA 《Acta Astronautica》2005,56(9-12):949-960
In light of the importance of the neutron contribution to the dose equivalent received by space workers in the near-Earth radiation environment, there is an increasing need for a personal dosimeter that is passive in nature and able to respond to this neutron field in real time. Recent Canadian technology has led to the development of a bubble detector, which is sensitive to neutrons, but insensitive to low linear energy transfer (LET) radiation. By changing the composition of the bubble detector fluid (or “superheat”), the detectors can be fabricated to respond to different types of radiation. This paper describes a preliminary ground-based research effort to better characterize the bubble detectors of different compositions at various charged-particle accelerator facilities, which are capable of simulating the space radiation field. 相似文献
888.
Laser-Raman imagery is a non-intrusive, non-destructive analytical technique, recently introduced to Precambrian paleobiology, that can be used to demonstrate a one-to-one spatial correlation between the optically discernible morphology and kerogenous composition of permineralized fossil microorganisms. Made possible by the submicron-scale resolution of the technique and its high sensitivity to the Raman signal of carbonaceous matter, such analyses can be used to determine the chemical-structural characteristics of organic-walled microfossils and associated sapropelic carbonaceous matter in acid-resistant residues and petrographic thin sections. Here we use this technique to analyze kerogenous microscopic fossils and associated carbonaceous sapropel permineralized in 22 unmetamorphosed or little-metamorphosed fine-grained chert units ranging from approximately 400 to approximately 2,100 Ma old. The lineshapes of the Raman spectra acquired vary systematically with five indices of organic geochemical maturation: (1) the mineral-based metamorphic grade of the fossil-bearing units; (2) the fidelity of preservation of the fossils studied; (3) the color of the organic matter analyzed; and both the (4) H/C and (5) N/C ratios measured in particulate kerogens isolated from bulk samples of the fossil-bearing cherts. Deconvolution of relevant spectra shows that those of relatively well-preserved permineralized kerogens analyzed in situ exhibit a distinctive set of Raman bands that are identifiable also in hydrated organic-walled microfossils and particulate carbonaceous matter freed from the cherts by acid maceration. These distinctive Raman bands, however, become indeterminate upon dehydration of such specimens. To compare quantitatively the variations observed among the spectra measured, we introduce the Raman Index of Preservation, an approximate measure of the geochemical maturity of the kerogens studied that is consistent both with the five indices of organic geochemical alteration and with spectra acquired from fossils experimentally heated under controlled laboratory conditions. The results reported provide new insight into the chemical-structural characteristics of ancient carbonaceous matter, the physicochemical changes that accompany organic geochemical maturation, and a new criterion to be added to the suite of evidence by which to evaluate the origin of minute fossil-like objects of possible but uncertain biogenicity. 相似文献
889.
890.
考虑各向异性散射介质内的辐射传递方向,本文考察了辐射能的重新分配过程。利用射线踪迹法结合Hottel和Sarofim区域法推导了辐射传递系数(RTCs)。考虑一维各向异性参与性半透明灰介质。两边界表面为镜反射,一个表面半透明,另一个表面不透明。辐射传递系数用于计算辐射热源项。瞬态能量方程由全隐式控制容积法求解。外界辐射加热与对流冷却同时作用于边界表面。考虑四个线性散射相函数。以各向同性散射情况比较,研究不透明边界两侧发射特性和线性散射相函数对瞬态温度分布的影响。结果表明,各向异性散射与各向同性散射介质间的相对温度差异与散射不对称参数近似成正比。 相似文献