全文获取类型
收费全文 | 2118篇 |
免费 | 168篇 |
国内免费 | 154篇 |
专业分类
化学 | 592篇 |
晶体学 | 1篇 |
力学 | 186篇 |
综合类 | 37篇 |
数学 | 1228篇 |
物理学 | 396篇 |
出版年
2023年 | 16篇 |
2022年 | 17篇 |
2021年 | 39篇 |
2020年 | 39篇 |
2019年 | 44篇 |
2018年 | 48篇 |
2017年 | 51篇 |
2016年 | 64篇 |
2015年 | 56篇 |
2014年 | 113篇 |
2013年 | 154篇 |
2012年 | 101篇 |
2011年 | 113篇 |
2010年 | 83篇 |
2009年 | 101篇 |
2008年 | 131篇 |
2007年 | 142篇 |
2006年 | 135篇 |
2005年 | 132篇 |
2004年 | 95篇 |
2003年 | 104篇 |
2002年 | 89篇 |
2001年 | 74篇 |
2000年 | 46篇 |
1999年 | 48篇 |
1998年 | 46篇 |
1997年 | 40篇 |
1996年 | 41篇 |
1995年 | 24篇 |
1994年 | 26篇 |
1993年 | 19篇 |
1992年 | 32篇 |
1991年 | 22篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 15篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有2440条查询结果,搜索用时 46 毫秒
41.
Zhengping Hao Lidun An Junling Zhou Hongli Wang 《Reaction Kinetics and Catalysis Letters》1996,59(2):295-300
Supported gold catalyst for eliminating hydrogen from CO2 feed gas in the production of urea is found to be superior to other industrial catalysts (e.g. Pt/Al2O3 and PdPt/Al2O3) in catalytic activity and resistance to sulfur poisoning. 相似文献
42.
Wansoo T Rhee 《Operations Research Letters》1985,4(3):121-123
Consider independent identically distributed random variables (Xi) valued in [0,1]. Let B(n) be the optimal (minimum) number of unit size bins needed to pack n items of size X1, X2,…,Xn. We prove that there exists a numerical constant C such that for t > 0, The constant C does not depend on the distribution of X. 相似文献
43.
S. Stupnikova E. Petushkova D. Muceniece V. Lūsis 《Chemistry of Heterocyclic Compounds》2007,43(1):41-49
The recyclization of 1,4-dihydropyridines in aqueous-alcoholic hydrochloric acid medium proceeds with cleavage of a C-N bond
and pyridine ring opening. Cyclohexenone derivatives are formed as a result of the subsequent intramolecular crotonic condensation
of the acyclic intermediate. The leaving carbonyl substituents depart simultaneously with recyclization, depending on the
acidity of the reaction medium.
Dedicated to Prof. Dr. E. Lukevics on the occasion of his 70th birthday.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 49–58, January, 2007. 相似文献
44.
Derrida's generalized random energy model is considered. Almost sure andL
p convergence of the free energy at any inverse temperature are proven for an arbitrary numbern of hierarchical levels. The explicit form of the free energy is given in the most general case and the limitn is discussed. 相似文献
45.
Ritei Shibata 《Annals of the Institute of Statistical Mathematics》1986,38(1):459-474
Summary A generalized Final Prediction Error (FPEα)_ criterion is considered. Based onn observations, the numberk of regression variables is selected from a given range 0≦k≦K, so as to minimize
. It is shown that if α tends to infinity withn, the selection is consistent but the maximum of the mean squared error of estimates of parameters diverges to infinity with
the same order of divergence as that of α. A meaningful minimax choice of α exists for a regret type mean squared error, while
for simple mean squared error it is trivially 0. The minimax regret choice of α converges to a constant, approximately 3.5
forK≧8 ifn−K increases simultaneously withn, otherwise it diverges to infinity withn. 相似文献
46.
Paolo Calvini Andrea Gorassini Giorgio Luciano Enrico Franceschi 《Vibrational Spectroscopy》2006,40(2):177-183
Cellulose paper (Whatman no.1, chromatographic grade) was oxidised with 0.1 M sodium metaperiodate at different oxidation levels (0–11 days at room temperature), and analysed with FTIR before and after chemical treatments (reduction, further oxidation, alkaline hydrolysis). The deconvolution of infrared data allowed us to verify that periodate oxidises cellulose in isolated domains, leading to the decrease of crystallinity, in agreement with the results of wide angle X-ray scattering (WAXS). In particular, the alkaline hydrolysis (β-alkoxy fragmentation) followed by mild acid treatment removed the oxidised groups and recovered most of the crystallinity of cellulose, as determined by the FTIR crystallinity index. 相似文献
47.
Yoshitaka Gotoh Yutaka Kohno Wakichi Fukuda Masao Tomoi 《Journal of polymer science. Part A, Polymer chemistry》1994,32(8):1543-1550
Soluble polystyrenes with crown ether structures and hydroxyl groups adjacent to the macrorings were prepared by the reaction of epoxide-containing polystyrenes with monoaza-15-crown-5 or monoaza-18-crown-6. Rate of formation of the polymer-bound alkali cation-crown alkoxide ion pair from the soluble polystyrenes and aqueous NaOH or KOH depended on the loading of crown ether and hydroxyl units and on the size of the macroring. The elimination of HCl from less reactive 2-chloroethylbenzene with aqueous NaOH or KOH in the presence of the soluble polystyrene catalysts under two-phase conditions was limited mainly by the intrinsic reaction rate. The elimination of HBr from more reactive 2-bromoethylbenzene in the presence of the less (11%) ring-substituted polymer catalyst with 18-crown unit was limited by the alkoxide formation rate. © 1994 John Wiley & Sons, Inc. 相似文献
48.
49.
A variety of diaryl acetylenes were obtained in good yields when lithium hexamethyldisilazide was added to a solution of arylmethyl sulfone, aryl aldehyde, and chlorodiethylphosphate in THF. In this one‐shot process, a number of transformations such as aldol reaction, phosphorylation of aldolate, and double elimination of the resulting β‐substituted sulfone proceeded successively to afford the desired acetylenes. The one‐shot process was accelerated by the substitution of halogen atoms on the phenyl groups, and unsymmetrically substituted diaryl acetylenes were obtained without contamination of the dehalogenated products. Diaryl acetylenes with other substituents such as CF3, ethoxycarbonyl, dimethylamino, TMS‐acetylene groups, as well as pyridinyl and thienyl moieties were also accessible with this method. However, methoxy‐substituted compounds were obtained in moderate yields under the same conditions, but the yields were increased when lithium diisopropylamide was used instead. 相似文献
50.
The reaction of cyclobutanone with an alkyne in the presence of a nickel(0) catalyst formally achieves intermolecular alkyne insertion between the carbonyl carbon and the α-carbon of a cyclobutanone, providing a six-membered carbocyclic skeleton. 相似文献