全文获取类型
收费全文 | 2731篇 |
免费 | 108篇 |
国内免费 | 9篇 |
专业分类
化学 | 2464篇 |
晶体学 | 7篇 |
力学 | 17篇 |
数学 | 212篇 |
物理学 | 148篇 |
出版年
2023年 | 13篇 |
2022年 | 38篇 |
2021年 | 80篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 29篇 |
2017年 | 41篇 |
2016年 | 68篇 |
2015年 | 93篇 |
2014年 | 83篇 |
2013年 | 166篇 |
2012年 | 202篇 |
2011年 | 212篇 |
2010年 | 106篇 |
2009年 | 131篇 |
2008年 | 169篇 |
2007年 | 180篇 |
2006年 | 206篇 |
2005年 | 205篇 |
2004年 | 149篇 |
2003年 | 132篇 |
2002年 | 121篇 |
2001年 | 26篇 |
2000年 | 25篇 |
1999年 | 18篇 |
1998年 | 20篇 |
1997年 | 29篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1970年 | 2篇 |
1927年 | 2篇 |
排序方式: 共有2848条查询结果,搜索用时 31 毫秒
271.
Alessandra Russo Venera Cardile Adriana C.E. Graziano Carmen Formisano Marisa Canzoneri 《Natural product research》2015,29(17):1630-1640
The objectives of our research were to study the chemical composition and the in vitro anticancer effect of the essential oil of Salvia verbenaca growing in natural sites in comparison with those of cultivated (Sc) plants. The oil from wild (Sw) S. verbenaca presented hexadecanoic acid (23.1%) as the main constituent, while the oil from Sc plants contained high quantities of hexahydrofarnesyl acetone (9.7%), scarce in the natural oil (0.7%). The growth-inhibitory and proapoptotic effects of the essential oils from Sw and Sc S. verbenaca were evaluated in the human melanoma cell line M14, testing cell vitality, cell membrane integrity, genomic DNA fragmentation and caspase-3 activity. Both the essential oils were able to inhibit the growth of the cancer cells examined inducing also apoptotic cell death, but the essential oil from cultivated samples exhibited the major effects. 相似文献
272.
Study of acidified ignitable liquid residues in fire debris by solid‐phase microextraction with gas chromatography and mass spectrometry 下载免费PDF全文
Carlos Martín‐Alberca Carmen García‐Ruiz Olivier Delmont 《Journal of separation science》2015,38(18):3218-3227
The detection and identification of ignitable liquid residues in fire debris can be meaningful in fire investigations. However, background pyrolysis products and weathering hinder the identification and classification steps. In addition to those processes, the acidification of the ignitable liquids before the combustion process could make those tasks even more difficult. Nevertheless, there are no systematic studies assessing the extraction, analysis, and composition of acidified ignitable liquid residues obtained from fire debris. In this work, a method for the study of acidified ignitable liquid residues in fire debris by solid‐phase microextraction with gas chromatography and mass spectrometry is proposed. This methodology has been evaluated, first with simulated solutions (gasoline/sulfuric acid mixtures set on fire under controlled conditions), and then with analysis of samples from real fire debris obtained from 18 chemical ignition Molotov cocktails made with sulfuric acid and three different ignitable liquids (two types of gasoline and diesel fuel). In addition, the extensive modifications observed in chromatograms of acidified ignitable liquid residues regarding neat and weathered samples were studied. These alterations were produced by the combustion and acidification processes. As a consequence, tert‐butylated compounds are proposed as diagnostic indicators for the identification of acidified gasoline in fire debris, even in strongly weathered samples. 相似文献
273.
Elena Sanna Eduardo C. Escudero-Adán Antonio Bauzá Pablo Ballester Antonio Frontera Carmen Rotger Antonio Costa 《Chemical science》2015,6(10):5466-5472
A crystalline porous material showing one-dimensional (1-D) rectangular micropores (12 × 9 Å2) has been assembled from a semirigid macrocyclic tetraimine and EtOAc as the templating agent. The 1-D nature of the material is intrinsic to the conformationally rigid structure of a macrocyclic sub-unit bearing four cyclohexylidene residues. The multiple dispersive forces established among the aliphatic residues glutted the 1-D channels and provided thermal stability to the material at temperatures below 160 °C. Upon removal of the template, the structure of the empty solid exhibited permanent microporosity (S
BET = 342 m2 g–1). Being a true molecular sponge, the channel framework of this material allowed the inclusion of a variety of molecular sample guests without compromising its crystalline nature. Remarkably, this crystalline material enabled the structure determination by X-ray diffraction of the included molecules. Theoretical studies demonstrated the vital role played by the dispersive forces in the overall stabilization of the crystal packing. 相似文献
274.
Effects of Ultraviolet Radiation (UVA+UVB) and Copper on the Morphology,Ultrastructural Organization and Physiological Responses of the Red Alga Pterocladiella capillacea 下载免费PDF全文
Éder C. Schmidt Marianne Kreusch Marthiellen R. de L. Felix Debora T. Pereira Giulia B. Costa Carmen Simioni Luciane C. Ouriques Francine L. Farias‐Soares Neusa Steiner Fungyi Chow Fernanda Ramlov Marcelo Maraschin Zenilda L. Bouzon 《Photochemistry and photobiology》2015,91(2):359-370
The effect of ultraviolet (UV) radiation and copper (Cu) on apical segments of Pterocladiella capillacea was examined under two different conditions of radiation, PAR (control) and PAR+UVA+UVB (PAR+UVAB), and three copper concentrations, ranging from 0 (control) to 0.62, 1.25 and 2.50 μm . Algae were exposed in vitro to photosynthetically active radiation (PAR) at 70 μmol photons m?2 s?1, PAR + UVB at 0.35 W m?2 and PAR +UVA at 0.70 W m?2 during a 12‐h photocycle for 3 h each day for 7 days. The effects of radiation and copper on growth rates, content of photosynthetic pigments and photosynthetic performance were analyzed. In addition, samples were processed for light and transmission electron microscopy. The content of photosynthetic pigments decreased after exposure to radiation and Cu. Compared with PAR radiation and copper treatments modified the kinetics patterns of the photosynthesis/irradiance curve. The treatments also caused changes in the ultrastructure of cortical and subcortical cells, including increased cell wall thickness and accumulation of plastoglobuli, as well as changes in the organization of chloroplasts. The results indicate that the synergistic interaction between UV radiation and Cu in P. capillacea, led to the failure of protective mechanisms and causing more drastic changes and cellular imbalances. 相似文献
275.
276.
277.
Inside Cover: A Boron Dipyrromethene (BODIPY)‐Based CuII–Bipyridine Complex for Highly Selective NO Detection (Chem. Eur. J. 44/2015) 下载免费PDF全文
278.
279.
Paolo Boggiatto Carmen Fernández Antonio Galbis 《Applied and Computational Harmonic Analysis》2017,42(1):65-87
Inspired by results of Kim and Ron, given a Gabor frame in , we determine a non-countable generalized frame for the non-separable space of the Besicovic almost periodic functions. Gabor type frames for suitable separable subspaces of are constructed. We show furthermore that Bessel-type estimates hold for the AP norm with respect to a countable Gabor system using suitable almost periodic norms of sequences. 相似文献
280.
Carmen Cortázar Manuel Elgueta Patricio Felmer 《Archive for Rational Mechanics and Analysis》1998,142(2):127-141
We study the uniqueness of radial ground states for the semilinear elliptic partial differential equation in ℝ
N
. We assume that the function f has two zeros, the origin and u
0>0. Above u
0 the function f is positive, is locally Lipschitz continuous and satisfies convexity and growth conditions of a superlinear nature. Below
u
0, f is assumed to be non-positive, non-identically zero and merely continuous. Our results are obtained through a careful analysis
of the solutions of an associated initial‐value problem, and the use of a monotone separation theorem. It is known that, for
a large class of functions f, the ground states of (*) are radially symmetric. In these cases our result implies that (*) possesses at most one ground
state.
(Accepted July 3, 1996) 相似文献