全文获取类型
收费全文 | 15617篇 |
免费 | 644篇 |
国内免费 | 37篇 |
专业分类
系统科学 | 1098篇 |
丛书文集 | 48篇 |
教育与普及 | 31篇 |
理论与方法论 | 296篇 |
现状及发展 | 4806篇 |
研究方法 | 548篇 |
综合类 | 9303篇 |
自然研究 | 168篇 |
出版年
2018年 | 733篇 |
2017年 | 741篇 |
2016年 | 465篇 |
2012年 | 412篇 |
2011年 | 1278篇 |
2010年 | 871篇 |
2009年 | 505篇 |
2008年 | 677篇 |
2007年 | 967篇 |
2006年 | 199篇 |
2005年 | 260篇 |
2004年 | 333篇 |
2003年 | 342篇 |
2002年 | 264篇 |
2001年 | 429篇 |
2000年 | 438篇 |
1999年 | 295篇 |
1994年 | 270篇 |
1992年 | 252篇 |
1991年 | 204篇 |
1990年 | 230篇 |
1989年 | 192篇 |
1988年 | 185篇 |
1987年 | 190篇 |
1986年 | 201篇 |
1985年 | 264篇 |
1984年 | 201篇 |
1983年 | 163篇 |
1982年 | 135篇 |
1981年 | 140篇 |
1980年 | 146篇 |
1979年 | 325篇 |
1978年 | 264篇 |
1977年 | 215篇 |
1976年 | 191篇 |
1975年 | 196篇 |
1974年 | 206篇 |
1973年 | 176篇 |
1972年 | 206篇 |
1971年 | 245篇 |
1970年 | 303篇 |
1969年 | 229篇 |
1968年 | 226篇 |
1967年 | 194篇 |
1966年 | 223篇 |
1965年 | 149篇 |
1959年 | 75篇 |
1958年 | 123篇 |
1957年 | 81篇 |
1954年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
41.
史济群 《华中科技大学学报(自然科学版)》1994,(3)
报道一种新颖的激光开槽、埋槽电极硅太阳电池的结构、工艺流程及其研制结果。在AM1.5,25℃,100mW/cm2的条件下,以面积为45cm3的36片硅片研制的硅太阳电池的输出参数的平均值为JSC=36.1mA/cm2,V∞=633mV,F.F.=0.798,η=18.23%,最后分析了种高性能硅太阳电池的设计特点。 相似文献
42.
FROM MANUFACTURING SCHEDULING TO SUPPLY CHAIN COORDINATION:THE CONTROL OF COMPLEXITY AND UNCERTAINTY
Peter B.LUH 《系统科学与系统工程学报(英文版)》2003,12(3):279-297
With time-based competition and rapid technology advancements, effective manufacturingscheduling and supply chain coordination are critical to quickly respond to changing marketconditions. These problems, however, are difficult in view of inherent complexity and variousuncertainties involved. Based on a series of results by the authors, decomposition and coordination byusing Lagrangian relaxation is identified in this paper as an effective way to control complexity anduncertainty.A manufacturing scheduling problem is first formulated within the job shop context withuncertain order arrivals, processing times, due dates, and part priorities as a separable optimizationproblem. A solution methodology that combines Lagrangian relaxation, stochastic dynamicprogramming, and heuristics is developed. Method improvements to effectively solve large problemsare also highlighted. To extend manufacturing scheduling within a factory to coordinate autonomicmembers across chains of suppliers, a decentralized supply chai 相似文献
43.
Development and function of T cells in mice rendered interleukin-2 deficient by gene targeting. 总被引:69,自引:0,他引:69
Interleukin-2 (IL-2) is a lymphocytotropic hormone which is thought to have a key role in the immune response of mammalian cells. It is produced by a subpopulation of activated T-lymphocytes and acts in vitro as the principal auto- and paracrine T-cell growth factor (for reviews see refs 1-3). IL-2 is, however, not the sole T-cell growth factor, nor does it act exclusively on T cells, also promoting growth of NK cells and differentiation of B cells. A role for IL-2 in T-cell development has been postulated but remains controversial. Here we test the requirement for IL-2 in vivo using IL-2-deficient mice generated by targeted recombination. We find that mice homozygous for the IL-2 gene mutation are normal with regard to thymocyte and peripheral T-cell subset composition, but that a dysregulation of the immune system is manifested by reduced polyclonal in vitro T-cell responses and by dramatic changes in the isotype levels of serum immunoglobulins. 相似文献
44.
M. Ishibashi M. Tsuda Y. Ohizumi T. Sasaki J. Kobayashi 《Cellular and molecular life sciences : CMLS》1991,47(3):299-300
Summary A new bromotyrosine-derived alkaloid with antileukemic activity, purealidin A (5), has been isolated from the Okinawan marine spongePsammaplysilla purea and its chemical structure elucidated on the basis of the spectroscopic data. 相似文献
45.
Low-molecular-weight GTP-binding proteins are strong candidates for regulators of membrane traffic. In yeast, mutations in the sec4 or ypt1 genes encoding small GTP-binding proteins inhibit constitutive membrane flow at the plasma membrane or Golgi complex, respectively. It has been suggested that membrane fusion-fission events are regulated by cycling of small GTP-binding proteins between a membrane-bound and free state, but although most of these small proteins are found in both soluble and tightly membrane-bound forms, there is no direct evidence to support such cycling. In rat brain a small GTP-binding protein, rab3A, is exclusively associated with synaptic vesicles, the secretory organelles of nerve terminals. Here we use isolated nerve terminals to study the fate of rab3A during synaptic vesicle exocytosis. We find that rab3A dissociates quantitatively from the vesicle membrane after Ca2(+)-dependent exocytosis and that this dissociation is partially reversible during recovery after stimulation. These results are direct evidence for an association-dissociation cycle of a small GTP-binding protein during traffic of its host membrane. 相似文献
46.
对目前的UBBE模型进行了适当的推广,使在解决估计问题时可以考虑那些可能是误差上界的数值,并在此基础上提出了一种方法,能够通过对估计精度和可靠性进行合理的权衡确定所需估计值,实际案例研究结果表明所提方法能够较好地解决估计精度和可靠性之间的矛盾。 相似文献
47.
Structure of the detoxification catalyst mercuric ion reductase from Bacillus sp. strain RC607 总被引:10,自引:0,他引:10
Several hundred million tons of toxic mercurials are dispersed in the biosphere. Microbes can detoxify organo-mercurials and mercury salts through sequential action of two enzymes, organomercury lyase and mercuric ion reductase (MerA). The latter, a homodimer with homology to the FAD-dependent disulphide oxidoreductases, catalyses the reaction NADPH + Hg(II)----NADP+ + H+ + Hg(0), one of the very rare enzymic reactions with metal substrates. Human glutathione reductase serves as a reference molecule for FAD-dependent disulphide reductases and between its primary structure and that of MerA from Tn501 (Pseudomonas), Tn21 (Shigella), p1258 (Staphylococcus) and Bacillus, 25-30% of the residues have been conserved. All MerAs have a C-terminal extension about 15 residues long but have very varied N termini. Although the enzyme from Streptomyces lividans has no addition, from Pseudomonas aeruginosa Tn501 and Bacillus sp. strain RC607 it has one and two copies respectively of a domain of 80-85 residues, highly homologous to MerP, the periplasmic component of proteins encoded by the mer operon. These domains can be proteolytically cleaved off without changing the catalytic efficiency. We report here the crystal structure of MerA from the Gram-positive bacterium Bacillus sp. strain RC607. Analysis of its complexes with nicotinamide dinucleotide substrates and the inhibitor Cd(II) reveals how limited structural changes enable an enzyme to accept as substrate what used to be a dangerous inhibitor. Knowledge of the mode of mercury ligation is a prerequisite for understanding this unique detoxification mechanism. 相似文献
48.
49.
Shape skeletonization (i.e., medial axis extraction) is powerful in many visual computing applications, such as pattern recognition, object segmentation, registration, and animation. In this paper, the authors expand the use of diffusion equations combined with distance field information to approximate medial axes of arbitrary 3D differential properties. It offers an alternative solids represented by polygonal meshes based on their but natural way for medial axis extraction for commonly used 3D polygonal models. By solving the PDE along time axis, this system can not only quickly extract diffusion-based medial axes of input meshes, but also allow users to visualize the extraction process at each time step. In addition, the proposed model provides users a set of manipulation toolkits to sculpt extracted medial axes, then use diffusion-based techniques to recover corresponding deformed shapes according to the original input datasets. This skeleton-based shape manipulation offers a fast and easy way for animation and deformation of complicated mesh objects. 相似文献
50.
Systemic Intervention in a University Department: Reflections on Arrested Action Research 总被引:1,自引:1,他引:0
Don Houston 《Systemic Practice and Action Research》2008,21(2):133-152
This paper reflects on my experience as an insider researcher attempting to use critical systems ideas and practices to promote
quality improvement in a university engineering department. Reflection is a key part of learning. This paper is intended to
contribute to critically, self-reflective learning for the community of systems practitioners. These reflections on my questions
about participation, ethics, politics of process, and the choices and actions resulting from them may help others to formulate
their own. The complexity of systems practice places substantial demands on the researcher, particularly in the case of insider,
practitioner research. Nevertheless, the exploration of critical systems approaches to critique boundaries and structure ‘problems’
in the core aspects of higher education in locally meaningful ways should continue. While demanding, it still can work to
promote learning about authentic quality. 相似文献