全文获取类型
收费全文 | 245493篇 |
免费 | 4539篇 |
国内免费 | 2206篇 |
专业分类
化学 | 125742篇 |
晶体学 | 4109篇 |
力学 | 11615篇 |
综合类 | 89篇 |
数学 | 27103篇 |
物理学 | 83580篇 |
出版年
2021年 | 2069篇 |
2020年 | 2318篇 |
2019年 | 2513篇 |
2018年 | 3172篇 |
2017年 | 3050篇 |
2016年 | 4484篇 |
2015年 | 3022篇 |
2014年 | 4697篇 |
2013年 | 11029篇 |
2012年 | 8629篇 |
2011年 | 10513篇 |
2010年 | 7462篇 |
2009年 | 7350篇 |
2008年 | 9498篇 |
2007年 | 9478篇 |
2006年 | 8931篇 |
2005年 | 8035篇 |
2004年 | 7312篇 |
2003年 | 6441篇 |
2002年 | 6364篇 |
2001年 | 7346篇 |
2000年 | 5598篇 |
1999年 | 4508篇 |
1998年 | 3812篇 |
1997年 | 3660篇 |
1996年 | 3504篇 |
1995年 | 3217篇 |
1994年 | 3132篇 |
1993年 | 3193篇 |
1992年 | 3272篇 |
1991年 | 3472篇 |
1990年 | 3224篇 |
1989年 | 3135篇 |
1988年 | 3157篇 |
1987年 | 3033篇 |
1986年 | 2857篇 |
1985年 | 3870篇 |
1984年 | 4033篇 |
1983年 | 3230篇 |
1982年 | 3433篇 |
1981年 | 3355篇 |
1980年 | 3243篇 |
1979年 | 3345篇 |
1978年 | 3429篇 |
1977年 | 3360篇 |
1976年 | 3454篇 |
1975年 | 3144篇 |
1974年 | 3154篇 |
1973年 | 3201篇 |
1972年 | 2238篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
K.V. Stepurska О.О. Soldatkin I.S. Kucherenko V.M. Arkhypova S.V. Dzyadevych A.P. Soldatkin 《Analytica chimica acta》2015
This study was aimed at the development of a conductometric biosensor based on acetylcholinesterase considering the feasibility of its application for the inhibitory analysis of various toxicants. In this paper, the optimum conditions for enzyme immobilization on the transducer surface are selected as well as the optimum concentration of substrate for inhibitory analysis. Sensitivity of the developed biosensor to different classes of toxic compounds (organophosphorus pesticides, heavy metal ions, surfactants, aflatoxin, glycoalkaloids) was tested. It is shown that the developed biosensor can be successfully used for the analysis of pesticides and mycotoxins, as well as for determination of total toxicity of the samples. A new method of biosensor analysis of toxic substances of different classes in complex multicomponent aqueous samples is proposed. 相似文献
43.
44.
Iodine(III)‐Catalyzed Rearrangements of Imides: A Versatile Route to α,α‐Dialkylated α‐Hydroxy Carboxylamides
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Anna Ulmer Dr. Maciej Stodulski Stefanie V. Kohlhepp Christoph Patzelt Dr. Alexander Pöthig Dr. Wolfgang Bettray Dr. Tanja Gulder 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(4):1444-1448
A tertiary hydroxy group α to a carboxyl moiety comprises a key structural motif in many bioactive substances. With the herein presented metal‐free rearrangement of imides triggered by hypervalent λ3‐iodane, an easy and selective way to gain access to such a compound class, namely α,α‐disubstituted‐α‐hydroxy carboxylamides, was established. Their additional methylene bromide side chain constitutes a useful handle for rapid diversification, as demonstrated by a series of further functionalizations. Moreover, the in situ formation of an iodine(III) species under the reaction conditions was proven. Our findings clearly corroborate that hypervalent λ3‐benziodoxolones are involved in these organocatalytic reactions. 相似文献
45.
Mass concentration and isotopic values δ13C and 14C are presented for the water-insoluble refractory carbon(WIRC) component of total suspended particulates(TSP),collected weekly during 2003,as well as from October 2005 to May 2006 at the WMO-GAW Mt.Waliguan(WLG) site.The overall average WIRC mass concentration was(1183±120)ng/m3(n = 79),while seasonal averages were 2081 ±1707(spring),454±205(summer),650±411(autumn),and 1019±703(winter) ng/m3.Seasonal variations in WIRC mass concentrations were consistent with black carbon measurements from an aethalometer,although WIRC concentrations were typically higher,especially in winter and spring.The δ13C PDB value(-25.3 ± 0.8)%.determined for WIRC suggests that its sources are C3 biomass or fossil fuel combustion.No seasonal change in δ13C PDB was evident.The average percent Modern Carbon(pMC) for 14C in WIRC for winter and spring was(67.2 ± 7.7)%(n = 29).Lower pMC values were associated with air masses transported from the area east of WLG,while higher pMC values were associated with air masses from the Tibetan Plateau,southwest of WLG.Elevated pMC values with abnormally high mass concentrations of TSP and WIRC were measured during a dust storm event. 相似文献
46.
Applied Magnetic Resonance - The suggested approach allowed us to derive analytical expressions for modeling the shape of solid-echo signal and its time evolution for a system of three-spin groups... 相似文献
47.
48.
49.
50.
M. Kroon E. Andreasson E. Persson Jutemar V. Petersson L. Persson M. Dorn P.A.T. Olsson 《Experimental Mechanics》2018,58(1):75-86
Injection-moulding is one of the most common manufacturing processes used for polymers. In many applications, the mechanical properties of the product is of great importance. Injection-moulding of thin-walled polymer products tends to leave the polymer structure in a state where the mechanical properties are anisotropic, due to alignment of polymer chains along the melt flow direction. The anisotropic elastic-viscoplastic properties of low-density polyethylene, that has undergone an injection-moulding process, are therefore examined in the present work. Test specimens were punched out from injection-moulded plates and tested in uniaxial tension. Three in-plane material directions were investigated. Because of the small thickness of the plates, only the in-plane properties could be determined. Tensile tests with both monotonic and cyclic loading were performed, and the local strains on the surface of the test specimens were measured using image analysis. True stress vs. true strain diagrams were constructed, and the material response was evaluated using an elastic-viscoplasticity law. The components of the anisotropic compliance matrix were determined together with the direction-specific plastic hardening parameters. 相似文献