全文获取类型
收费全文 | 36907篇 |
免费 | 102篇 |
国内免费 | 177篇 |
专业分类
系统科学 | 206篇 |
丛书文集 | 523篇 |
教育与普及 | 77篇 |
理论与方法论 | 170篇 |
现状及发展 | 16719篇 |
研究方法 | 1464篇 |
综合类 | 17418篇 |
自然研究 | 609篇 |
出版年
2013年 | 314篇 |
2012年 | 508篇 |
2011年 | 1140篇 |
2010年 | 217篇 |
2008年 | 638篇 |
2007年 | 698篇 |
2006年 | 707篇 |
2005年 | 673篇 |
2004年 | 727篇 |
2003年 | 653篇 |
2002年 | 639篇 |
2001年 | 1081篇 |
2000年 | 1046篇 |
1999年 | 712篇 |
1992年 | 715篇 |
1991年 | 514篇 |
1990年 | 593篇 |
1989年 | 598篇 |
1988年 | 559篇 |
1987年 | 656篇 |
1986年 | 586篇 |
1985年 | 748篇 |
1984年 | 598篇 |
1983年 | 464篇 |
1982年 | 431篇 |
1981年 | 446篇 |
1980年 | 550篇 |
1979年 | 1134篇 |
1978年 | 953篇 |
1977年 | 921篇 |
1976年 | 770篇 |
1975年 | 816篇 |
1974年 | 1072篇 |
1973年 | 964篇 |
1972年 | 962篇 |
1971年 | 1109篇 |
1970年 | 1441篇 |
1969年 | 1066篇 |
1968年 | 1063篇 |
1967年 | 1045篇 |
1966年 | 956篇 |
1965年 | 662篇 |
1964年 | 235篇 |
1959年 | 347篇 |
1958年 | 608篇 |
1957年 | 388篇 |
1956年 | 369篇 |
1955年 | 335篇 |
1954年 | 313篇 |
1948年 | 266篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Surface Preparation is very important in adhesive b on ding of spray coatings to the surface of a work piece. The common practice is gr it-blasting of the surface before subjecting it to the spray coating process. In this study, grit-blasting of an AISI 4130 steel (of different heat treatmen ts) with Al 2O 3 particles was studied. Various grit-blasting parameters such as blasting particle size, the distance between blasting nozzle and the work pi ece (25, 30 and 40 cm.), blasting pressure (3,4,5,6 and ... 相似文献
22.
Jacob W. Ulvila 《Journal of forecasting》1985,4(4):377-385
Recent years have seen an increasing cross-fertilization between the fields of decision analysis and forecasting. Decision-analytic models often require forecasts as inputs, and aspects of the Bayesian decision-theoretic framework underlying decision analysis have proved useful to forecasting, particularly in contexts where subjective judgemental inputs are required. This paper describes the use of decision tree analysis for forecasting and illustrates its use for corporate divisional forecasting and planning. A specialized decision-analytic technique, acts as events, is also described and illustrated to forecast a new product's earnings. Conclusions are drawn about the applicability of decision analysis for forecasting. 相似文献
23.
J. W. Faigle H. Stierlin H. Mory T. Winkler H. -P. Kriemler 《Cellular and molecular life sciences : CMLS》1985,41(4):476-478
Summary Indoxyl derivatives were detected as minor products among the urinary metabolites of two trial drugs, a benzodiazepine (GP 55 129) and a benzophenone (CGP 11 952). Their structures were elucidated by NMR and mass spectroscopy. Presumably, metabolites containing potential aldehyde functions react spontaneously with endogenous indoxyl. Such derivatives have not hitherto been encountered in drug metabolism. 相似文献
24.
25.
We investigated the sound communication associated with mating inNezara viridula (L.) to clarify conflicting reports on the medium through which acoustic signals are borne. Recordings made from virgin pairs ofN. viridula during precopulatory and compulatory behaviour have revealed that signals transmitted as vibrations through the substrate are a more likely means of communication than signals transmitted by a radiation of sound pressure waves through the air (at least in Australian populations). We present evidence (additional to that already in the literature) that air-borne acoustics are an unlikely mechanism of sexual communication in this species. Specifically, the recorded pulse train patterns showed no consistency or repeatability in relation to different stages of mating behaviour. In contrast, frequency spectrograms of substrate-borne signals reveal repeatable patterns that do correspond with the stage of mating behaviour. 相似文献
26.
卢武星 《北京师范大学学报(自然科学版)》1994,(4)
1.0MeV208Pb离子在非晶Si中的投影射程RP和射程偏差ΔRP作为注量和温度二者的函数用背散射法进行测定.注量的变化范围为5×1013~7×1014cm-2.注入是在室温和t=-120℃下完成的.由由实验所确定的投影射程,射程偏差与注量或温度无关,并且分别等于295和72.2nm.与TRIM86的计算结果相比较,发现RP的偏离为18%,而ΔRP的偏离为36%.RP和ΔRP二者与注量及温度的无关性,排除了所观察到的与TRIM的矛盾是由于注入期间辐射增强扩散或离子束混合效应而引起的解释。 相似文献
27.
Aspirin-like drugs may block pain independently of prostaglandin synthesis inhibition 总被引:2,自引:0,他引:2
K. Brune W. S. Beck G. Geisslinger S. Menzel-Soglowek B. M. Peskar B. A. Peskar 《Cellular and molecular life sciences : CMLS》1991,47(3):257-261
Summary Using flurbiprofen, a chiral anti-inflammatory and analgesic 2-arylpropionic acid derivative, the enantiomers of which are not converted to each other (less than 5%) in rats or man, we obtained evidence that prostaglandin synthesis inhibition is primarily mediating the anti-inflammatory activity but prostaglandin synthesis independent mechanisms contribute to the analgesic effects. Thus, the S-form inhibited prostaglandin synthesis, inflammation and nociception in rats. The R-form had much less effect on prostaglandin synthesis and did not affect inflammation. It did, however, block nociception in rats almost as potently as the S-form. S-flurbiprofen, in contrast to the R-form, was clearly ulcerogenic in the gastrointestinal mucosa. These results indicate additional molecular mechanisms of analgesia and suggest the use of R-arylpropionic acids as analgesics. 相似文献
28.
29.
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
30.
We develop in this paper an efficient way to select the best subset threshold autoregressive model. The proposed method uses a stochastic search idea. Differing from most conventional approaches, our method does not require us to fix the delay or the threshold parameters in advance. By adopting the Markov chain Monte Carlo techniques, we can identify the best subset model from a very large of number of possible models, and at the same time estimate the unknown parameters. A simulation experiment shows that the method is very effective. In its application to the US unemployment rate, the stochastic search method successfully selects lag one as the time delay and five best models from more than 4000 choices. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献