全文获取类型
收费全文 | 9854篇 |
免费 | 178篇 |
国内免费 | 38篇 |
专业分类
化学 | 5581篇 |
晶体学 | 71篇 |
力学 | 349篇 |
数学 | 1616篇 |
物理学 | 2453篇 |
出版年
2021年 | 99篇 |
2020年 | 122篇 |
2019年 | 123篇 |
2018年 | 117篇 |
2017年 | 102篇 |
2016年 | 178篇 |
2015年 | 160篇 |
2014年 | 165篇 |
2013年 | 566篇 |
2012年 | 353篇 |
2011年 | 438篇 |
2010年 | 317篇 |
2009年 | 272篇 |
2008年 | 397篇 |
2007年 | 356篇 |
2006年 | 348篇 |
2005年 | 316篇 |
2004年 | 335篇 |
2003年 | 268篇 |
2002年 | 252篇 |
2001年 | 182篇 |
2000年 | 163篇 |
1999年 | 128篇 |
1998年 | 96篇 |
1997年 | 113篇 |
1996年 | 142篇 |
1995年 | 121篇 |
1994年 | 122篇 |
1993年 | 148篇 |
1992年 | 154篇 |
1991年 | 156篇 |
1990年 | 115篇 |
1989年 | 118篇 |
1988年 | 131篇 |
1987年 | 179篇 |
1986年 | 146篇 |
1985年 | 195篇 |
1984年 | 184篇 |
1983年 | 141篇 |
1982年 | 161篇 |
1981年 | 176篇 |
1980年 | 171篇 |
1979年 | 170篇 |
1978年 | 164篇 |
1977年 | 162篇 |
1976年 | 141篇 |
1975年 | 158篇 |
1974年 | 126篇 |
1973年 | 123篇 |
1972年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Cover Picture: A Versatile Approach to CF3‐Containing 2‐Pyrrolidones by Tandem Michael Addition–Cyclization: Exemplification in the Synthesis of Amidine Class BACE1 Inhibitors (Chem. Eur. J. 33/2015) 下载免费PDF全文
92.
93.
94.
Metabolism of methylstenbolone studied with human liver microsomes and the uPA+/+‐SCID chimeric mouse model 下载免费PDF全文
Lore Geldof Leen Lootens Michael Polet Daniel Eichner Thane Campbell Vinod Nair Francesco Botrè Philip Meuleman Geert Leroux‐Roels Koen Deventer Peter Van Eenoo 《Biomedical chromatography : BMC》2014,28(7):974-985
Anti‐doping laboratories need to be aware of evolutions on the steroid market and elucidate steroid metabolism to identify markers of misuse. Owing to ethical considerations, in vivo and in vitro models are preferred to human excretion for nonpharmaceutical grade substances. In this study the chimeric mouse model and human liver microsomes (HLM) were used to elucidate the phase I metabolism of a new steroid product containing, according to the label, methylstenbolone. Analysis revealed the presence of both methylstenbolone and methasterone, a structurally closely related steroid. Via HPLC fraction collection, methylstenbolone was isolated and studied with both models. Using HLM, 10 mono‐hydroxylated derivatives (U1–U10) and a still unidentified derivative of methylstenbolone (U13) were detected. In chimeric mouse urine only di‐hydroxylated metabolites (U11–U12) were identified. Although closely related, neither methasterone nor its metabolites were detected after administration of isolated methylstenbolone. Administration of the steroid product resulted mainly in the detection of methasterone metabolites, which were similar to those already described in the literature. Methylstenbolone metabolites previously described were not detected. A GC‐MS/MS multiple reaction monitoring method was developed to detect methylstenbolone misuse. In one out of three samples, previously tested positive for methasterone, methylstenbolone and U13 were additionally detected, indicating the applicability of the method. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
95.
In this paper, a computational study using the moving element method (MEM) is carried out to investigate the dynamic response of a high-speed rail (HSR) traveling at non-uniform speeds. A new and exact formulation for calculating the generalized mass, damping and stiffness matrices of the moving element is proposed. Two wheel–rail contact models are examined. One is linear and the other nonlinear. A parametric study is carried out to understand the effects of various factors on the dynamic amplification factor (DAF) in contact force between the wheel and rail such as the amplitude of acceleration/deceleration of the train, the severity of railhead roughness and the wheel load. Resonance in the vibration response can possibly occur at various stages of the journey of the HSR when the speed of the train matches the resonance speed. As to be expected, the DAF in contact force peaks when resonance occurs. The effects of the severity of railhead roughness and the wheel load on the occurrence of the jumping wheel phenomenon, which occurs when there is a momentary loss of contact between the wheel and track, are investigated. 相似文献
96.
F. Allaeys G. Luyckx C. Sarrazin W. Van Paepegem L. Jovanov W. Philips 《Experimental Mechanics》2014,54(6):999-1009
The in-house validation of finite element models for bird strike events is currently carried out by means of experimental tests on flat plates. High displacement speeds in these experiments require a low exposure time of the high-speed camera (up to 1/50,000 s). In order to acquire images of sufficient quality, a special, high-intensity light source has to be used, which does not always turn out to be possible. Therefore, the regions with high displacement speeds often result in blurry images. In such cases, a printed line pattern to estimate the shape of the plate during the test, offers major advantages over a speckle pattern in terms of the reconstruction and optimization of the blurry regions. In this article, a stereo vision technique is presented that was developed to reconstructs 3D shape maps using images of impacted plates with printed line patterns. It is shown that two cameras are necessary to calculate accurate shape maps in case of large deflections. The resulting shapes can be used for the validation of numerical simulations. 相似文献
97.
Hoo Van Nguyen Naing Min Tun A. Yu. Kryukov I. M. Izvol’skii E. G. Rakov 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(9):1559-1563
The “solubility” of oxidized carbon nanotubes depends on the acidity of aqueous solutions and increases drastically when pH increases from 1 (when it is almost zero) to 2. The same behavior is inherent in graphite and graphene oxides, the only difference lying in smoother increase in the solubility and in a wider range of pH. The reason for the change in solubility was shown to be cleavage of hydrogen bonds between the carboxyl groups of different particles and the behavior of the oxidized fragments (oxidation debris). It was concluded that the character of the dependences was dictated by the different geometries of nanoparticles and different distributions of carboxyl groups on their surface. 相似文献
98.
Maribel Rodríguez‐Aguilar Sofía Ramírez‐García Cesar Ilizaliturri‐Hernndez Alejandro Gmez‐Gmez Evelyn Van‐Brussel Fernando Díaz‐Barriga Susanna Medellín‐Garibay Rogelio Flores‐Ramírez 《Biomedical chromatography : BMC》2019,33(12)
An analytical method to identify volatile organic compounds (VOCs) in the exhaled breath from patients with a diagnosis of chronic obstructive pulmonary disease (COPD) using a ultrafast gas chromatography system equipped with an electronic nose detector (FGC eNose) has been developed. A prospective study was performed in 23 COPD patients and 33 healthy volunteers; exhalation breathing tests were performed with Tedlar bags. Each sample was analyzed by FCG eNose and the identification of VOCs was based on the Kovats index. Raw data were reduced by principal component analysis (PCA) and canonical discriminant analysis [canonical analysis of principal coordinates (CAP)]. The FCG eNose technology was able to identify 17 VOCs that distinguish COPD patients from healthy volunteers. At all stages of PCA and CAP the discrimination between groups was obvious. Chemical prints were correctly classified up to 82.2%, and were matched with 78.9% of the VOCs detected in the exhaled breath samples. Receiver operating characteristic curve analysis indicated the sensitivity and specificity to be 96% and 91%, respectively. This pilot study demonstrates that FGC eNose is a useful tool to identify VOCs as biomarkers in exhaled breath from COPD patients. Further studies should be performed to enhance the clinical relevance of this quick and ease methodology for COPD diagnosis. 相似文献
99.
Seyyed Javad Sabounchei Khadijeh Badpa Mehdi Bayat Roya Karamian Fatemeh Ghasemlou Robert W. Gable Parviz Gohari Derakhshandeh Kristof Van Hecke 《应用有机金属化学》2019,33(3)
The reaction of α‐keto‐stabilized diphosphine ylides [Ph2P(CH2)nPPh2═C(H)C(O)C6H4‐p‐CN] (n = 1 (Y1); n = 2 (Y2)) with dibromo(1,5‐cyclooctadiene) palladium(II)/platinum(II) complexes, [Pd/PtBr2(cod)], in equimolar ratio gave the new cyclometalated Pd(II) and Pt(II) complexes [Br2Pd(κ2‐Y1)] ( 1 ), [Br2Pt(κ2‐Y1)] ( 2 ), [Br2Pd(κ2‐Y2)] ( 3 ) and [Br2Pt(κ2‐Y2)] ( 4 ). These compounds were screened in a search for novel antibacterial agents and characterized successfully using Fourier transfer infrared and NMR (1H, 13C and 31P) spectroscopic methods. Also, the structures of complexes 1 and 2 were characterized using X‐ray crystallography. The results showed that the P,C‐chelated complexes 1 and 2 have structures consisting of five‐membered rings, while 3 and 4 have six‐membered rings, formed by coordination of the ligand through the phosphine group and the ylidic carbon atom to the metal centre. Also, a theoretical study of the structures of complexes 1 – 4 was conducted at the BP86/def2‐SVP level of theory. The nature of metal–ligand bonds in the complexes was investigated using energy decomposition analyses (EDA) and extended transition state combined with natural orbitals for chemical valence analyses. The results of EDA confirmed that the main portions of ΔEint, about 57–58%, in the complexes are allocated to ΔEelstat. 相似文献
100.
Physics of Atomic Nuclei - Chiral spinors and self dual tensors of the Lie superalgebra osp(m|n) are infinite-dimensional representations belonging to the class of representations with Dynkin... 相似文献