全文获取类型
收费全文 | 16936篇 |
免费 | 34篇 |
国内免费 | 41篇 |
专业分类
系统科学 | 92篇 |
丛书文集 | 263篇 |
教育与普及 | 30篇 |
理论与方法论 | 72篇 |
现状及发展 | 8015篇 |
研究方法 | 710篇 |
综合类 | 7649篇 |
自然研究 | 180篇 |
出版年
2013年 | 82篇 |
2012年 | 199篇 |
2011年 | 392篇 |
2010年 | 76篇 |
2008年 | 230篇 |
2007年 | 249篇 |
2006年 | 293篇 |
2005年 | 287篇 |
2004年 | 385篇 |
2003年 | 265篇 |
2002年 | 262篇 |
2001年 | 482篇 |
2000年 | 443篇 |
1999年 | 327篇 |
1992年 | 288篇 |
1991年 | 222篇 |
1990年 | 238篇 |
1989年 | 235篇 |
1988年 | 203篇 |
1987年 | 234篇 |
1986年 | 279篇 |
1985年 | 340篇 |
1984年 | 231篇 |
1983年 | 226篇 |
1982年 | 206篇 |
1981年 | 217篇 |
1980年 | 224篇 |
1979年 | 563篇 |
1978年 | 476篇 |
1977年 | 437篇 |
1976年 | 342篇 |
1975年 | 404篇 |
1974年 | 564篇 |
1973年 | 452篇 |
1972年 | 473篇 |
1971年 | 556篇 |
1970年 | 696篇 |
1969年 | 538篇 |
1968年 | 539篇 |
1967年 | 536篇 |
1966年 | 430篇 |
1965年 | 371篇 |
1964年 | 121篇 |
1959年 | 193篇 |
1958年 | 330篇 |
1957年 | 247篇 |
1956年 | 190篇 |
1955年 | 192篇 |
1954年 | 190篇 |
1948年 | 127篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
Selective functionalization of mesoporous silica nanospheres (MSNs) is crucial for nanoengineering of MSNs.
Herein, we have combined “surface-protected etching strategy” and “cationic surfactant assisted etching
strategy” to prepare functionalized MSNs with externally attached amino groups. The externally attached NH2
groups endow the catalysts with excellent catalytic performance for nitroaldol reaction between nitromethane
and benzaldehyde. In addition, those NH2-MSNs can also be used to support gold nanoparticles, which display
very good catalytic performance for reduction of 4-nitrophenol. It can be envisioned that the synthesis protocol
developed in this work could also be extended to nanoengineered MSNs, which provides opportunities for nanoreactors
design. 相似文献
172.
Rodney G.Downey ;Judith Egan ;Michael R.Fellows ;Frances A.Rosamond ;Peter Shaw 《清华大学学报》2014,(4):329-337
The main purpose of this paper is to exposit two very different, but very general, motivational schemes in the art of parameterization and a concrete example connecting them. We introduce a dynamic version of the DOMINATING SET problem and prove that it is fixed-parameter tractable(FPT). The problem is motivated by settings where problem instances evolve. It also arises in the quest to improve a natural greedy heuristic for the DOMINATING SET problem. 相似文献
173.
174.
175.
176.
Zhitao Xu X. G. Ming Wenyan Song Lina He Miao Li 《Systemic Practice and Action Research》2014,27(2):141-164
Quite often over the last decade, heavy equipment manufacturers are trying to improve their project management performance with the use of effective project management methods. However, the challenges encountered aren’t isolated problems and a systemic approach is required. This paper exemplifies how a systemic approach can be applied to heavy equipment manufacturing project (HEMP) management, through of a multifaceted vision. The findings present in the paper are based on a two-year case study in a metallurgic equipment manufacturing enterprise in China. Based on the analysis of HEMP management problems and the contributions on systemic approach application in project management, the paper enriches existing theory by: (1) interpreting the HEMP management system with the top-down disassembly method; (2) developing the collaborative project management approach with a systems view highlighting the project characteristics management and inter-departmental collaboration; (3) making suggestions for systemic approach application in project management. For practitioners and researchers, the findings are particularly meaningful for manufacturing project management and systemic approach application. 相似文献
177.
The positive impacts of managing projects as a portfolio are quantified by comparing the value of the integrated risk of a project portfolio and the aggregation of single project risks implemented separately. Firstly, the integrated risk is defined by proposing risky events based on set theory. Secondly, as projects interact with each other in a project portfolio, the integrated risk is evaluated by using a Bayesian network structure learning algorithm to construct an interdependent network of risks. Finally, the integrated risk of a practical case is assessed using this method, and the results show that the proposed method is an effective tool for calculating the extent of risk reduction of implementing a project portfolio and identifying the most risky project, so as to assist companies in making comprehensive decisions in the phase of portfolio selection and portfolio controlling. 相似文献
178.
Carol D. Curtis Reema B. Davis Kyle G. Ingram Courtney T. Griffin 《Cellular and molecular life sciences : CMLS》2012,69(23):3921-3931
Vascular development is a dynamic process that relies on the coordinated expression of numerous genes, but the factors that regulate gene expression during blood vessel development are not well defined. ATP-dependent chromatin-remodeling complexes are gaining attention for their specific temporal and spatial effects on gene expression during vascular development. Genetic mutations in chromatin-remodeling complex subunits are revealing roles for the complexes in vascular signaling pathways at discrete developmental time points. Phenotypic analysis of these models at various stages of vascular development will continue to expand our understanding of how chromatin remodeling impacts new blood vessel growth. Such research could also provide novel therapeutic targets for the treatment of vascular pathologies. 相似文献
179.
Studies in several areas in Arizona and New Mexico show that dwarf mistletoe ( Arceuthobium douglasii ) is rare in young Douglas-firs growing under infected overstories. Less than 5% of the Douglas-firs under 26 years old and less than 6% of those under 1.4m tall were infected in 77 mistletoe-infested stands. Both percent infection and mean dwarf mistletoe rating of young Douglas-firs increased as tree age, height, and stand dwarf mistletoe ratings increased. 相似文献
180.
Keith R. Shaw 《西北部美国博物学家》2011,39(3)
The floristic composition of the Lee Creek valley in southwestern Alberta, Canada, is presented. The valley flora consists of 299 species of vascular plants representing 173 genera and 46 families. The flora is dominated by forbs (73 percent), followed by grasses (24 percent), shrubs (12 percent), and trees (11 percent). The valley of Lee Creek is divided into upper and lower sections on the basis of elevational, climatic, and biotic differences. Floristic richness of the Lee Creek valley is quite evenly divided between the upper and lower valleys: 105 species limited to the upper valley, 95 to the lower valley, and 99 species shared by both upper and lower valleys. 相似文献