全文获取类型
收费全文 | 3294篇 |
免费 | 74篇 |
国内免费 | 18篇 |
专业分类
化学 | 1818篇 |
晶体学 | 8篇 |
力学 | 102篇 |
数学 | 550篇 |
物理学 | 649篇 |
无线电 | 259篇 |
出版年
2023年 | 18篇 |
2022年 | 31篇 |
2021年 | 65篇 |
2020年 | 53篇 |
2019年 | 35篇 |
2018年 | 43篇 |
2017年 | 34篇 |
2016年 | 98篇 |
2015年 | 52篇 |
2014年 | 71篇 |
2013年 | 179篇 |
2012年 | 150篇 |
2011年 | 228篇 |
2010年 | 122篇 |
2009年 | 100篇 |
2008年 | 209篇 |
2007年 | 199篇 |
2006年 | 166篇 |
2005年 | 142篇 |
2004年 | 107篇 |
2003年 | 90篇 |
2002年 | 85篇 |
2001年 | 55篇 |
2000年 | 51篇 |
1999年 | 48篇 |
1998年 | 48篇 |
1997年 | 57篇 |
1996年 | 50篇 |
1995年 | 46篇 |
1994年 | 61篇 |
1993年 | 55篇 |
1992年 | 48篇 |
1991年 | 27篇 |
1990年 | 28篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 20篇 |
1985年 | 42篇 |
1984年 | 28篇 |
1983年 | 29篇 |
1982年 | 36篇 |
1981年 | 40篇 |
1980年 | 26篇 |
1979年 | 28篇 |
1978年 | 16篇 |
1977年 | 17篇 |
1976年 | 20篇 |
1975年 | 20篇 |
1974年 | 17篇 |
1973年 | 20篇 |
排序方式: 共有3386条查询结果,搜索用时 62 毫秒
1.
2.
Daniel L. Feldheim Del R. Lawson Charles R. Martin 《Journal of Polymer Science.Polymer Physics》1993,31(8):953-957
We report a strong dependence of the thermal stability of Nafion® perfluorosulfonate ionomer on the nature of the counterion associated with the fixed sulfonate site. These results were obtained using thermal gravimetric analysis on a series of alkali metal and alkyl ammonium cation-exchanged Nafion films. We have found that the temperature of decomposition of Nafion is inversely dependent on the size of the exchanged cation; i.e., Nafion films show improved thermal stability as the size of the counter cation decreases. We attribute this inverse relationship of thermal stability with counterion size to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-coun-terion interaction. © 1993 John Wiley & Sons, Inc. 相似文献
3.
Daniele Cauzzi Roberto Giordano Enrico Sappa Antonio Tiripicchio Marisa Tiripicchio Camellini 《Journal of Cluster Science》1993,4(3):279-296
The title complex (complex1) was the first alkyne-substituted triruthenium dihydrido cluster to be reported and was characterized by spectroscopy as a triangular cluster with the alkyne parallel to a Ru-Ru edge. Recently, we have found that1 is a key intermediate in the homogeneous hydrogenation of diphenylacetylene catalyzed by tetrahedral Ru4 and FeRu3 clusters. Since the discovery of1, a great number of complexes with alkynes parallel to a cluster edge have been reported; at present this is the more common bonding mode for alkynes on trinuclear clusters. The structural features of1 allow a comparison with those of other ruthenium-containing derivatives and help to draw suggestions of the role of1 in hydrogenation catalysis. 相似文献
4.
This paper outlines studies relative to a research project on the development of a system that permits different people of an enterprise, with different roles and experience, to gain an adequate and personalized vision of the real status of the network and managed systems. What has been achieved is an easy to handle system, which permits quick and simple queries without utilizing complex man machine languages, and tailors personalized answers on the basis of the characteristics of the user. To achieve these results, User Modeling aspects have been considered and pursued. 相似文献
5.
4-Vinylphenols, useful compounds for industrial applications, were obtained by decarboxylation of 4-hydroxycinnamic acids under microwave irradiation in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base and basic aluminum oxide as solid support. The reactions were fast (15-30 min). The selective extraction of the final products with ethyl acetate avoids chromatographic purifications. The conversions are quantitative and the yields are satisfactory. Only the unstable 4-vinylcatechol was obtained in moderate yield. This procedure was successfully extended to a natural sample of ferulic acid extracted from wheat bran to get the corresponding 4-vinylguaiacol, a FEMA GRAS (Flavor and Extract Manufacturer's Association; General Regarded as Safe) approved flavoring agent. 相似文献
6.
Summary We study the integration theory for general integral metrics when restricted to upper integrals q, finding improvements in the relation between the classes of the q-integrable and the ql-integrable functions. We give new results and notions which lead to the desirable characterizations of q-integrable functions as ql-integrable f with q(|f|) < ∞, and of ql-integrable functions via the integrability of their upper truncations, under natural conditions which are fulfilled in most
finitely additive integration theories. 相似文献
7.
V Ravindran Pankaj Agrawal Rahul Basu Satyaki Bhattacharya J Blümlein V Del Duca R Harlander D Kosower Prakash Mathews Anurag Tripathi 《Pramana》2006,67(5):983-992
This is the report of the subgroup QCD of Working Group-4 at WHEPP-9. We present the activities that had taken place in the
subgroup and report some of the partial results arrived at following the discussion at the working group meetings. 相似文献
8.
Nicola Arcozzi Enrico Casadio Tarabusi Fausto Di Biase Massimo A. Picardello 《Transactions of the American Mathematical Society》2006,358(6):2781-2798
We establish a potential theoretic approach to the study of twist points in the boundary of simply connected planar domains.
9.
This paper proposes a low complexity joint space-time multiuser detection algorithm for asynchronous DS/CDMA antenna array systems. The proposed multiuser detector is composed of an adaptive antenna array, used as a linear beamformer, and a sliding window decorrelator. A QPSK modulation scheme is used in order to increase bandwidth efficiency. Numerical results are given in terms of Bit Error Rate (BER) under the assumption of a frequency-selective Rayleigh slow fading channel. In particular, the proposed receiver is shown to be near-far resistant, even in worst fading cases, and to exploit completely array introduction while maintaining acceptable computational complexity. The proposed architecture avoids linear filter realization of the decorrelator, which is impractical in the case of a large number of users, and operates with relatively short data frames instead of the complete information sequence. Finally, this receiver is very flexible to changes in timing configuration. 相似文献
10.
Gabor Schmera Adi Bulsara David Pierson Frank Moss Enrico Di Cera 《Journal of statistical physics》1993,71(5-6):1179-1190
We consider the class of experiments which can be characterized by a Fokker-PIanck dynamics corresponding to the overdamped motion of a state point in a suitable stochastic potential. We assume that the general form of the potential is known (or can be guessed with reasonable accuracy), but that its parameters are to be determined experimentally by measurements made with a noisy instrument. This possible method for determining the potential parameters, which exploits the system's own internal stochastic motion in order to explore rapidly its available parameter space, is substantially more efficient than traditional methods involving time averages of single point measurements, and yet does not appear to have been previously considered. The method could be important when, for example, the experiment must be completed in a limited time owing either to the expense of the experimental materials or to the temporary stationarity of the preparation, situations which are commonly encountered in experimental biochemistry and biology. 相似文献