全文获取类型
收费全文 | 785112篇 |
免费 | 8866篇 |
国内免费 | 2443篇 |
专业分类
化学 | 411957篇 |
晶体学 | 11797篇 |
力学 | 35801篇 |
综合类 | 18篇 |
数学 | 103839篇 |
物理学 | 233009篇 |
出版年
2020年 | 5941篇 |
2019年 | 6491篇 |
2018年 | 13634篇 |
2017年 | 14025篇 |
2016年 | 15110篇 |
2015年 | 8830篇 |
2014年 | 12474篇 |
2013年 | 32674篇 |
2012年 | 26591篇 |
2011年 | 37441篇 |
2010年 | 25024篇 |
2009年 | 24787篇 |
2008年 | 33699篇 |
2007年 | 35776篇 |
2006年 | 25061篇 |
2005年 | 27257篇 |
2004年 | 23181篇 |
2003年 | 20756篇 |
2002年 | 19140篇 |
2001年 | 20665篇 |
2000年 | 15918篇 |
1999年 | 12548篇 |
1998年 | 10419篇 |
1997年 | 10180篇 |
1996年 | 10091篇 |
1995年 | 9071篇 |
1994年 | 8897篇 |
1993年 | 8710篇 |
1992年 | 9718篇 |
1991年 | 9679篇 |
1990年 | 9335篇 |
1989年 | 9185篇 |
1988年 | 9242篇 |
1987年 | 9079篇 |
1986年 | 8594篇 |
1985年 | 11402篇 |
1984年 | 11755篇 |
1983年 | 9590篇 |
1982年 | 10137篇 |
1981年 | 9902篇 |
1980年 | 9354篇 |
1979年 | 9901篇 |
1978年 | 10423篇 |
1977年 | 10085篇 |
1976年 | 10127篇 |
1975年 | 9390篇 |
1974年 | 9331篇 |
1973年 | 9657篇 |
1972年 | 6611篇 |
1968年 | 5414篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Synthetic routes to BEDT-TTF derivatives bearing side chain carboxylic ester and amide groups are reported. Methyl ET-ethanoate was prepared in five steps from vinylacetic acid; amide groups were installed early in the synthesis by mixed anhydride methods before the final coupling reaction. 相似文献
992.
In order to reduce infant mortality in the municipality of Rio de Janeiro it is desired to encourage all mothers-to-be to attend the appropriate health care facilities. A 3-level hierarchical prenatal–neonatal health care system is described together with a basic model for optimising accessibility to facilities. A genetic algorithm to solve the basic model is developed and some numerical experience reported. Extensions to the basic model and planned future research are briefly indicated. 相似文献
993.
M. Ohsaki 《Archive of Applied Mechanics (Ingenieur Archiv)》2003,73(3-4):241-251
Summary Characteristics of optimal solutions under nonlinear buckling constraints are investigated by using a bar-spring model. It is demonstrated that optimization under buckling constraints of a symmetric system often leads to a structure with hill-top branching, where a limit point and bifurcation points coincide. A general formulation is derived for imperfection sensitivity of the critical load factor corresponding to a hill-top branching point. It is shown that the critical load is not imperfection-sensitive even for the case where an asymmetric bifurcation point exists at the limit point. 相似文献
994.
E. Rebollar M. Oujja M. Castillejo S. Georgiou 《Applied Physics A: Materials Science & Processing》2004,79(4-6):1357-1360
Optical emission and laser-induced fluorescence spectroscopies (OES and LIF) are employed to examine the plume ejected into vacuum upon UV pulsed laser ablation (248 nm, 20 ns, and 266 nm, 5 ns) of poly(methyl methacrylate) PMMA films doped with photostable compounds naphthalene (NapH) and phenanthrene (PhenH), and their photolabile iodide derivatives iodonaphthalene (NapI) and iodophenanthrene (PhenI). Spontaneous emissions observed in the irradiation of NapI and PhenI sensitised films can be assigned to excited products resulting from photodissociation of the dopants and to excited C2, CH, and CN radicals. The presence in the plume of ground state products is revealed by LIF upon excitation at 266 nm. Measurements of the dependence of the spontaneous and LIF emissions on distance to the surface, ablation fluence, and flight velocities of species are discussed with reference to the mechanistic implications derived from the measurements. PACS 52.38.Mf; 61.82.Pv; 82.50 相似文献
995.
Nickel complexes 1–4 ligated with 2,9-disubstituted-1,10-phenanthroline were synthesized and characterized by FT-IR spectra and elemental analysis.
The molecular structure of complex 2 was confirmed by X-ray crystal diffraction analysis. Activated with methylaluminoxane (MAO), those complexes showed moderate
activities for ethylene oligomerization.
Published in Kinetika i Kataliz, 2007, Vol. 48, No. 5, pp. 710–714.
This article was submitted by the authors in English. 相似文献
996.
Azizollah Habibi Leyla Mousavifar Issa Yavari Mohammad R. Yazdanbakhsh 《Monatshefte für Chemie / Chemical Monthly》2007,4(8):603-606
Alkyl isocyanides undergo a smooth reaction with alkylidene Meldrum’s acids in the presence of arylhydroxylamines to produce N
1-alkyl-2-(3,5-dioxo-2-aryltetrahydro-4-isoxazolyl)alkanamides in high yields. 相似文献
997.
Monika Goikoetxea María J. Barandiaran José M. Asua 《Journal of polymer science. Part A, Polymer chemistry》2007,45(24):5838-5846
The mechanisms involved in the formation of n‐butanol during the synthesis of butyl acrylate containing latices were investigated. The experimental results showed that neither the hydrolysis of butyl acrylate nor of the ester bond in the butyl acrylate segments of the polymer played a major role in the formation of n‐butanol, which was mainly generated from the polymer backbone, by transfer reactions to polymer chain followed by cyclization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5838–5846, 2007 相似文献
998.
Fabrina R S Bentlin Fábio A Duarte Valderi L Dressler Dirce Pozebon 《Analytical sciences》2007,23(9):1097-1101
This work deals with As determination in marine sediment using ultrasound for sample preparation. It is shown that As can be quantitatively extracted from marine sediment using 20% (v/v) HCl and sonication. The slurry is centrifuged and the analyte is determined in the supernatant by hydride generation atomic absorption spectrometry (HG AAS). A flow injection (FI) system is employed for hydride generation, with 0.5% (m/v) NaBH(4) used as reducdant and a 20% (v/v) HCl used as sample carrier. The limit of quantification is 1.6 microg g(-1) of As, which is based on 800 microl of sample solution and 0.200 g of sample mass in a volume of 50 mL. Certified and non certified marine sediment samples were analyzed; the results were in accordance with the certified or reference values. Speciation analysis by HPLC-ICP-MS showed that As(V) is the only detectable As species present in the supernatant of the centrifuged sample. 相似文献
999.
A. Khalafi-Nezhad A. Parhami A. Zare A. R. Moosavi Zare 《Journal of the Iranian Chemical Society》2008,5(3):413-419
A simple, clean and efficient solvent-free procedure for the preparation of aryl carboxylates is described from the direct reaction of carboxylic acids and phenols, in the presence of 1-methylimidazole as base and tosyl chloride (TsCl) as coupling agent. This method can be easily applied for different substituted phenols and carboxylic acids. It can also be applied for the selective acylation when other functional group such as hydroxyl is present on phenol ring. 相似文献
1000.
R. Bairava Ganesh Hitoshi Matsuo Takahiro Kawamura Yoshihiro Kangawa Koji Arafune Yoshio Ohshita Masafumi Yamaguchi Koichi Kakimoto 《Journal of Crystal Growth》2008,310(11):2697-2701
Multicrystalline silicon was grown by unidirectional solidification method using the accelerated crucible rotation technique. The application of the accelerated crucible rotation technique in unidirectional solidification method induced growth striations across the axial direction of the grown crystal. This striation pattern was observed from carbon concentration distribution, obtained by using Fourier transform infrared spectroscopy. The generated striation pattern was found to be weak and discontinuous. Some striations were absent, probably due to back melting, caused during each crucible rotation. From the growth striations and applied time period in crucible rotation, the growth rate was estimated by using Fourier transformation analysis. 相似文献