全文获取类型
收费全文 | 13955篇 |
免费 | 122篇 |
国内免费 | 157篇 |
专业分类
系统科学 | 165篇 |
丛书文集 | 103篇 |
教育与普及 | 110篇 |
理论与方法论 | 87篇 |
现状及发展 | 4348篇 |
研究方法 | 542篇 |
综合类 | 8607篇 |
自然研究 | 272篇 |
出版年
2013年 | 152篇 |
2012年 | 253篇 |
2011年 | 573篇 |
2010年 | 163篇 |
2009年 | 168篇 |
2008年 | 280篇 |
2007年 | 317篇 |
2006年 | 308篇 |
2005年 | 340篇 |
2004年 | 341篇 |
2003年 | 297篇 |
2002年 | 282篇 |
2001年 | 385篇 |
2000年 | 413篇 |
1999年 | 381篇 |
1998年 | 149篇 |
1997年 | 163篇 |
1996年 | 169篇 |
1995年 | 143篇 |
1994年 | 146篇 |
1992年 | 285篇 |
1991年 | 234篇 |
1990年 | 239篇 |
1989年 | 225篇 |
1988年 | 218篇 |
1987年 | 225篇 |
1986年 | 185篇 |
1985年 | 253篇 |
1984年 | 180篇 |
1983年 | 140篇 |
1980年 | 137篇 |
1979年 | 332篇 |
1978年 | 256篇 |
1977年 | 244篇 |
1976年 | 225篇 |
1975年 | 242篇 |
1974年 | 308篇 |
1973年 | 280篇 |
1972年 | 237篇 |
1971年 | 339篇 |
1970年 | 474篇 |
1969年 | 328篇 |
1968年 | 313篇 |
1967年 | 320篇 |
1966年 | 349篇 |
1965年 | 223篇 |
1958年 | 209篇 |
1957年 | 118篇 |
1956年 | 130篇 |
1948年 | 127篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
211.
三车道元胞自动机交通流模型研究 总被引:12,自引:0,他引:12
在Nagel-Schrekenberg(NS)元胞自动机交通流模型的基础上,提出了一种高速公路三车道元胞自动机交通流模型.该模型中所有的车辆均遵循相同的演化规则,并允许车辆以一定概率换道.根据计算机数值模拟.研究了车辆减速概率、换道概率对车流量、车道利用率影响关系,结果表明,在相同情况下,此模型所得到的基本图显示出交通流量比单车道NS模型大,并且发现减少不必要的车辆换道行为可以使驾驶更安全.这些研究为管理三车道的道路系统提供了一定的参考依据. 相似文献
212.
213.
This paper built a combined heat and power (CHP) dispatch model for wind-CHP system with solid heat storage device (SHS) aiming at minimizing system coal consumption, and set system demand-supply balance and units’ operation conditions as the operation constraints. Furthermore, robust stochastic optimization theory was used to describe wind power output uncertainty. The simulation result showed that SHS increased CHP peak-valley shifting capability and reduced abandoned wind rate from 12% to 6%, and reduced 5% coal consumption, compared with the original system operation by flexible charging electric power and heating. The payback period of employing SHS in wind-CHP system is far shorter than SHS expected service life. 相似文献
214.
Fabrication of homogeneously dispersed graphene/Al composites by solution mixing and powder metallurgy
下载免费PDF全文
![点击此处可从《矿物冶金与材料学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Graphene-reinforced aluminum (Al) matrix composites were successfully prepared via solution mixing and powder metallurgy in this study. The mechanical properties of the composites were studied using microhardness and tensile tests. Compared to the pure Al alloy, the graphene/Al composites showed increased strength and hardness. A tensile strength of 255 MPa was achieved for the graphene/Al composite with only 0.3wt% graphene, which has a 25% increase over the tensile strength of the pure Al matrix. Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to investigate the morphologies, chemical compositions, and microstructures of the graphene and the graphene/Al composites. On the basis of fractographic evidence, a relevant fracture mechanism is proposed. 相似文献
215.
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
216.
Inferentialists about scientific representation hold that an apparatus's representing a target system consists in the apparatus allowing “surrogative inferences” about the target. I argue that a serious problem for inferentialism arises from the fact that many scientific theories and models contain internal inconsistencies. Inferentialism, left unamended, implies that inconsistent scientific models have unlimited representational power, since an inconsistency permits any conclusion to be inferred. I consider a number of ways that inferentialists can respond to this challenge before suggesting my own solution. I develop an analogy to exploitable glitches in a game. Even though inconsistent representational apparatuses may in some sense allow for contradictions to be generated within them, doing so violates the intended function of the apparatus's parts and hence violates representational “gameplay”. 相似文献
217.
218.
219.
220.
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