全文获取类型
收费全文 | 1622篇 |
免费 | 97篇 |
国内免费 | 95篇 |
出版年
2024年 | 2篇 |
2023年 | 12篇 |
2022年 | 15篇 |
2021年 | 33篇 |
2020年 | 46篇 |
2019年 | 36篇 |
2018年 | 43篇 |
2017年 | 45篇 |
2016年 | 66篇 |
2015年 | 54篇 |
2014年 | 67篇 |
2013年 | 106篇 |
2012年 | 44篇 |
2011年 | 84篇 |
2010年 | 52篇 |
2009年 | 65篇 |
2008年 | 93篇 |
2007年 | 91篇 |
2006年 | 59篇 |
2005年 | 51篇 |
2004年 | 71篇 |
2003年 | 52篇 |
2002年 | 52篇 |
2001年 | 57篇 |
2000年 | 52篇 |
1999年 | 42篇 |
1998年 | 37篇 |
1997年 | 38篇 |
1996年 | 33篇 |
1995年 | 25篇 |
1994年 | 21篇 |
1993年 | 39篇 |
1992年 | 26篇 |
1991年 | 21篇 |
1990年 | 26篇 |
1989年 | 21篇 |
1988年 | 17篇 |
1987年 | 9篇 |
1986年 | 15篇 |
1985年 | 16篇 |
1984年 | 14篇 |
1983年 | 10篇 |
1982年 | 17篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1976年 | 2篇 |
1975年 | 1篇 |
排序方式: 共有1814条查询结果,搜索用时 15 毫秒
61.
大尺度陆地生态系统动态变化与空间变异的过程模型及模拟系统 总被引:1,自引:0,他引:1
当代生态系统科学研究更加关注区域生态环境及生态系统状态变化的监测、评估、预测、预警及生态环境可持续管理。在深入理解陆地生态系统的要素、过程、功能、格局及其相互作用机理基础上,发展生态系统定量化描述方法和数值模拟技术,集成构建大陆尺度的“多过程耦合-多技术集成-多目标应用”的陆地生态系统数值模拟器已成为生态系统与全球变化及其资源、环境和灾害效应科学研究的重要科技任务。本研究围绕宏观生态系统模拟分析方法问题,在回顾陆地生态系统模型研究现状和发展趋势的基础上,深入讨论开发大尺度陆地生态系统动态变化和空间变异及其资源环境效应模拟系统的理念,以及模拟系统的功能定位、结构设计等基本问题,为构造中国陆地生态系统数值模拟器提供参考。 相似文献
62.
M. Skog A. Chandrapavan E. Hallberg T. Breithaupt 《Marine and Freshwater Behaviour and Physiology》2013,45(2):119-133
We studied the relevance of urine cues in Homarus gammarus dominance maintenance, hypothesising that urinary signals are necessary to mediate recognition of former opponents. Males in size-matched pairs interacted on two consecutive days with or without blocking urine release by adding catheters to both contestants on the second day. European lobsters established dominance in a first fight, and fight duration and aggression levels decreased strongly from first to second day in animals with free urine release, indicating the maintenance of this dominance relationship. If urine was blocked on the second day, fight durations were long in both first and second day interactions. Results demonstrate that urine signals contribute to the maintenance of dominance in H. gammarus males. 相似文献
63.
Competing group members tend to arrange in a social order that governs who will likely submit to whom. In many species the spatial distribution of individuals often reflects social status: dominants tend to occupy central locations while subordinates are often found along the group's periphery. This article explores the emergence of spatial consequences as a result of social rank differentiation. Rather than orienting centripetally, the movements of crayfish (Orconectes rusticus) primarily indicated a tendency to remain close to arena walls. Spatial locations were affected by the location of group members; but, rather than actively aggregating or clustering, individuals maintained a minimum distance. Previously established social rank did not affect spatial distributions. High population densities in the field are likely attributed to habitat constraints, rather than any social or centripetal tendencies of individual crayfish. 相似文献
64.
Shengzhang Wang Yisen Zhang Xinjian Yang 《Computer methods in biomechanics and biomedical engineering》2013,16(11):927-937
Endovascular stents are being commonly used to treat cerebral wide-necked aneurysms recently. The effect of a stent placed in the parent artery is not only to protect the parent artery from occlusion, due to extension of coils and thrombosis, but also to act as flow diverter to vary the haemodynamics in the aneurysm. In this article, two idealised cerebral wide-necked aneurysms were created, one was sidewall aneurysm with curved parent vessel and the other was terminal aneurysm with the bifurcated parent vessel. The plexiglass models of the two aneurysms were ‘treated’ with commercial porous intravascular stents. The stented physical models were scanned by Micro-CT and the numerical models of the two idealised cerebral wide-necked aneurysms after stent placement were constructed from the scanned image files. The pulsatile flow of non-Newtonian fluid inside the models was simulated by using computational fluid dynamics package. From the simulated flow dynamics, various haemodynamic characteristics such as velocity contours, wall shear stress and oscillatory shear index (OSI) were computed. The velocity of the jet entering the sacs reduced after stent was deployed across the necks of both sidewall and terminal aneurysms; the wall shear stress on the distal neck of sidewall aneurysm reduced, the wall shear stress on the dome of the terminal aneurysm increased and the OSI on the dome of the terminal aneurysm reduced. Therefore, stent placement not only promotes thrombus formation in both aneurysm models but also reduces the regrowth risk of the sidewall aneurysm and the rupture risk of the terminal aneurysm. 相似文献
65.
A. Borghi E. Foa R. Balossino F. Migliavacca 《Computer methods in biomechanics and biomedical engineering》2013,16(4):367-377
Today the most popular approach for the prevention of the restenosis consists in the use of the drug eluting stents. The stent acts as a source of drug, from a coating or from a reservoir, which is transported into and through the artery wall. In this study, the behaviour of a model of a hydrophilic drug (heparin) released from a coronary stent into the arterial wall is investigated. The presence of the specific binding site action is modelled using a reversible chemical reaction that explains the prolonged presence of drug in the vascular tissue. An axi-symmetric model of a single stent strut is considered. First an advection–diffusion problem is solved using the finite element method. Then a simplified model with diffusion only in the arterial wall is compared with: (i) a model including the presence of reversible binding sites in the vascular wall and (ii) a model featuring a drug reservoir made of a degradable polymeric matrix. The results show that the inclusion of a reversible binding for the drug leads to delayed release curves and that the polymer erosion affects the drug release showing a quicker elution of the drug from the stent. 相似文献
66.
N. Amor L. Geris J. Vander Sloten 《Computer methods in biomechanics and biomedical engineering》2013,16(4):459-468
The objective of this study was to see whether a mathematical model of fracture healing was able to mimic bone formation around an unloaded screw-shaped titanium implant as it is well-believed that both processes exhibit many biological similarities. This model describes the spatio-temporal evolution of cellular activities, ranging from mesenchymal stem cell migration, proliferation, differentiation to bone formation, which are initiated and regulated by the growth factors present at the peri-implant site. For the simulations, a finite volume code was used and adequate initial and boundary conditions were applied. Two sets of analyses have been performed, in which either initial and boundary condition or model parameter values were changed with respect to the fracture healing model parameter values. For a number of combinations, the spatio-temporal evolution of bone density was well-predicted. However reducing cell proliferation rate and increasing osteoblast differentiation and osteogenic growth factor synthesis rates, the simulation results were in agreement with the experimental data. 相似文献
67.
68.
Yubing Shi Yuhui Shi 《Computer methods in biomechanics and biomedical engineering》2013,16(11):995-1007
This paper investigates ventricular assist device (VAD)-assisted cardiovascular dynamics under proportion–integration–differentiation (PID) feedback control. Previously, we have studied the cardiovascular responses under the support of an in-series connected reciprocating-valve VAD through numerical simulation, and no feedback control was applied in the VAD. In this research, we explore the contribution of the VAD control on the circulatory dynamics assisted by the reciprocating-valve VAD, in response to the changing physiological conditions. The classical PID control algorithm is implemented to regulate the VAD stroke beat-to-beat, based on the error signal between the expected and the realistic mean aortic pressures. Simulation results show that under the PID VAD control, physiological variables such as left atrial, ventricular and systemic arterial pressures, cardiac output and ventricular volumes are satisfactorily maintained in the physiological ranges. With the online PID feedback control, operation of the reciprocating-valve VAD can be satisfactorily regulated to accommodate metabolic requirements under various physiological conditions including normal resting and exercise situations. 相似文献
69.
Field studies of community assembly patterns increasingly use phylogenetic relatedness as a surrogate for traits. Recent experiments appear to validate this approach by showing effects of correlated trait and phylogenetic distances on coexistence. However, traits governing resource use in animals are often labile. To test whether feeding trait or phylogenetic diversity can predict competition and production in communities of grazing amphipods, we manipulated both types of diversity independently in mesocosms. We found that increasing the feeding trait diversity of the community increased the number of species coexisting, reduced dominance and changed food availability. In contrast, phylogenetic diversity had no effect, suggesting that whatever additional ecological information it represents was not relevant in this context. Although community phylogenetic structure in the field may result from multiple traits with potential for phylogenetic signal, phylogenetic effects on species interactions in controlled experiments may depend on the lability of fewer key traits. 相似文献
70.