全文获取类型
收费全文 | 1479篇 |
免费 | 56篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 16篇 |
综合类 | 3篇 |
化学工业 | 430篇 |
金属工艺 | 41篇 |
机械仪表 | 28篇 |
建筑科学 | 71篇 |
矿业工程 | 15篇 |
能源动力 | 22篇 |
轻工业 | 165篇 |
水利工程 | 1篇 |
石油天然气 | 11篇 |
无线电 | 82篇 |
一般工业技术 | 297篇 |
冶金工业 | 175篇 |
原子能技术 | 14篇 |
自动化技术 | 168篇 |
出版年
2021年 | 19篇 |
2020年 | 14篇 |
2019年 | 25篇 |
2018年 | 25篇 |
2017年 | 17篇 |
2016年 | 32篇 |
2015年 | 30篇 |
2014年 | 35篇 |
2013年 | 50篇 |
2012年 | 51篇 |
2011年 | 61篇 |
2010年 | 51篇 |
2009年 | 53篇 |
2008年 | 50篇 |
2007年 | 47篇 |
2006年 | 47篇 |
2005年 | 36篇 |
2004年 | 39篇 |
2003年 | 18篇 |
2002年 | 28篇 |
2001年 | 26篇 |
2000年 | 33篇 |
1999年 | 35篇 |
1998年 | 38篇 |
1997年 | 38篇 |
1996年 | 26篇 |
1995年 | 27篇 |
1994年 | 29篇 |
1993年 | 20篇 |
1992年 | 32篇 |
1991年 | 23篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 13篇 |
1985年 | 14篇 |
1984年 | 25篇 |
1983年 | 27篇 |
1982年 | 27篇 |
1980年 | 24篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1976年 | 27篇 |
1975年 | 36篇 |
1974年 | 20篇 |
1973年 | 24篇 |
1972年 | 14篇 |
1971年 | 15篇 |
1970年 | 19篇 |
排序方式: 共有1539条查询结果,搜索用时 234 毫秒
131.
The prospect of biofuels going ‘mainstream’ has drawn more attention to the social impacts of the production and use of transport biofuels. Since 2007, many media stories have appeared about alleged negative impacts of biofuels, notably the price of food going up or land-grab by plantation developers. These stories stand in stark contrast with the rosy picture painted by some academics involved in the technical development of bioethanol or biodiesel. This paper explores the questions when and why negative social impacts are likely to occur and under what circumstances more positive impacts might be expected. These impacts are discussed for three geographically defined biofuel supply chains; north-north, south-north and south-south. These three systems differ in the spatial scale of production and consumption and with that comes a different distribution of environmental, social and economic impacts. In the case of domestic production and consumption in developed countries, the social impacts are relatively minor and can be mitigated by social policies. Large scale, export-oriented production systems in developing countries could theoretically yield positive social impacts, but this would require on the one hand the tailored design of ‘pro-poor’ social innovations and interventions on the ground and on the other hand a certification of the supply chain feeding into consumer demand for ‘ethical’ fuel. The latent existence of this demand might be significant but recent NGO campaigns have severely undermined the ethical credentials of biofuels. It would require a persistent and collaborative effort to restore the brand value of ‘green’ fuel, an effort which will require better legislation and radically improved monitoring and enforcement practices in countries where the very absence of these has led to, and is still causing, the large scale destruction of habitats that are carbon sinks of global importance. The significant levels of government funding for biofuels stand in strong contrast with the problematic environmental and social governance of international biofuels supply chains. Notwithstanding the ‘must tackle climate change’ rhetoric by policy makers and in policy documents, this suggests that biofuels policy may be primarily driven by other concerns, especially regarding energy security. We argue that policies that are designed for a rather narrowly defined purpose of ‘security of supply’, cannot be realistically expected to yield high social or environmental benefits, and certainly not abroad. 相似文献
132.
Synthesis of novel tricalcium phosphate-bioactive glass composite and functionalization with rhBMP-2
Schickle K Zurlinden K Bergmann C Lindner M Kirsten A Laub M Telle R Jennissen H Fischer H 《Journal of materials science. Materials in medicine》2011,22(4):763-771
A functionalization is required for calcium phosphate-based bone substitute materials to achieve an entire bone remodeling.
In this study it was hypothesized that a tailored composite of tricalcium phosphate and a bioactive glass can be loaded sufficiently
with rhBMP-2 for functionalization. A composite of 40 wt% tricalcium phosphate and 60 wt% bioactive glass resulted in two
crystalline phases, wollastonite and rhenanite after sintering. SEM analysis of the composite’s surface revealed a spongious
bone-like morphology after treatment with different acids. RhBMP-2 was immobilized non-covalently by treating with chrome
sulfuric acid (CSA) and 3-aminopropyltriethoxysilane (APS) and covalently by treating with CSA/APS, and additionally with
1,1′-carbonyldiimidazole. It was proved that samples containing non-covalently immobilized rhBMP-2 on the surface exhibit
significant biological activity in contrast to the samples with covalently bound protein on the surface. We conclude that
a tailored composite of tricalcium phosphate and bioactive glass can be loaded sufficiently with BMP-2. 相似文献
133.
134.
Andrew J. Allen Susan Krueger Ganesh Skandan Gabrielle G. Long Horst Hahn Helen M. Kerch John C. Parker Mohammad N. All 《Journal of the American Ceramic Society》1996,79(5):1201-1212
Using small-angle scattering, together with density measurements, X-ray diffraction, nitrogen adsorption, and both transmission and scanning electron microscopy, the microstructural evolution during sintering in nanostructured ceramic oxides has been studied. The availability of samples with sufficient size and uniformity has permitted absolute calibration of the scattering data and made possible determinations of microstructural parameters and surface areas that are statistically representative of the undisturbed nanophase morphologies. A fully quantitative microstructural model has been developed, and the effects of varying processing parameters during powder synthesis and sintering have been assessed. Predictions are made regarding the achievable sintered full-density microstructures. 相似文献
135.
A mesh-vertex finite volume scheme for solving the Euler equations on triangular unstructured meshes is implemented on a MIMD (multiple instruction/multiple data stream) parallel computer. Three partitioning strategies for distributing the work load onto the processors are discussed. Issues pertaining to the communication costs are also addressed. We find that the spectral bisection strategy yields the best performance. The performance of this unstructured computation on the Intel iPSC/860 compares very favorably with that on a one-processor CRAY Y-MP/1 and an earlier implementation on the Connection Machine.The authors are employees of Computer Sciences Corporation. This work was funded under contract NAS 2-12961 相似文献
136.
137.
Horst S. Bairavasubramanian R. Tentzeris M.M. Papapolymerou J. 《Microwave Theory and Techniques》2007,55(11):2439-2446
A modification of the Wilkinson power divider is presented that eases planar implementation while maintaining performance. By adding transmission lines between the resistor and the quarter-wave transformers of the traditional design, a range of valid solutions exists that meet the conditions of being reciprocal, isolated between the output ports, and matched at all ports. The proposed design is particularly useful at millimeter-wave frequencies where reduced physical dimensions make a circuit configuration suitable for low-cost package-level implementation difficult using traditional methods. Two frequency bands are demonstrated. At V-band, the circuit gives 0.3-dB excess insertion loss, 19-dB isolation, and 50% bandwidth. At the W-band, the circuit gives 0.75-dB excess insertion loss, 24-dB isolation, and 39% bandwidth. 相似文献
138.
139.
140.
CM van der Horst CA de Borgie JL Knopper PM Bossuyt 《Canadian Metallurgical Quarterly》1997,50(6):463-467
OBJECTIVE: To find out if lactulose can prevent the bacterial translocation that is induced by obstructive jaundice in rats. DESIGN: Laboratory experiment. SETTING: Teaching hospital, Turkey. MATERIAL: 50 male Wistar-albino rats. INTERVENTIONS: 10 rats were not operated on and used as controls; 20 rats underwent laparotomy and sham ligation of the common bile duct (CBD); 20 had the CBD ligated alone; and 20 had the CBD ligated and were given oral lactulose 2 ml/day until death. All rats were killed after 14 days. MAIN OUTCOME MEASURES: Presence of Escherichia coli in mesenteric lymph nodes (MLN), and bacterial overgrowth as indicated by counts of E coli in the caecum. RESULTS: There was significantly less bacterial translocation to MLN in the group that had been given lactulose compared with CBD-ligated and lactulose not given (2/20 compared with 8/20, p = 0.06). There was also a significant reduction in the number of Gram negative bacteria in that group (p = < 0.01). CONCLUSION: Lactulose seems to reduce the incidence of translocation from the gut to MLN in rats with obstructive jaundice. 相似文献