全文获取类型
收费全文 | 40265篇 |
免费 | 3240篇 |
国内免费 | 2570篇 |
专业分类
化学 | 27700篇 |
晶体学 | 459篇 |
力学 | 1736篇 |
综合类 | 236篇 |
数学 | 4305篇 |
物理学 | 11639篇 |
出版年
2023年 | 300篇 |
2022年 | 366篇 |
2021年 | 838篇 |
2020年 | 860篇 |
2019年 | 928篇 |
2018年 | 772篇 |
2017年 | 756篇 |
2016年 | 1332篇 |
2015年 | 1296篇 |
2014年 | 1660篇 |
2013年 | 2552篇 |
2012年 | 3055篇 |
2011年 | 3382篇 |
2010年 | 2294篇 |
2009年 | 2200篇 |
2008年 | 2854篇 |
2007年 | 2651篇 |
2006年 | 2521篇 |
2005年 | 2284篇 |
2004年 | 1946篇 |
2003年 | 1536篇 |
2002年 | 1647篇 |
2001年 | 1171篇 |
2000年 | 981篇 |
1999年 | 622篇 |
1998年 | 444篇 |
1997年 | 424篇 |
1996年 | 451篇 |
1995年 | 342篇 |
1994年 | 347篇 |
1993年 | 331篇 |
1992年 | 312篇 |
1991年 | 250篇 |
1990年 | 183篇 |
1989年 | 173篇 |
1988年 | 174篇 |
1987年 | 138篇 |
1986年 | 108篇 |
1985年 | 185篇 |
1984年 | 118篇 |
1983年 | 101篇 |
1982年 | 123篇 |
1981年 | 90篇 |
1980年 | 87篇 |
1978年 | 93篇 |
1977年 | 88篇 |
1976年 | 97篇 |
1975年 | 102篇 |
1974年 | 80篇 |
1973年 | 103篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
Hejia Hu Linlin Wu Mei Li Cun Xue Guangcheng Wang Siying Chen Yong Huang Lin Zheng Aimin Wang Yueting Li Zipeng Gong 《Biomedical chromatography : BMC》2020,34(1):e4714
Eucommia ulmoides Oliv. (E. ulmoides) is a valuable and nourishing medicinal herb in China that has been used in the treatment of hypertension. Given the fact that most traditional Chinese medicine is mainly used to treat disease, investigating the pharmacokinetics of traditional Chinese medicines in the pathological state is more useful than that in the normal state. However, the differences in the absorption kinetics of active ingredients of E. ulmoides extract between pathological and physiological conditions have not been reported. Therefore, in this study, the rat intestinal in situ circulatory perfusion model was used to investigate the differences in absorption kinetics of seven active ingredients of E. ulmoides extract in normal and spontaneously hypertensive rats, namely, genipinic acid, protocatechuic acid, neochlorogenic acid, chlorogenic acid, cryptochlorogenic acid, (+)-pinoresinol di-O-β-D -glucopyranoside and (+)-pinoresinol 4′-O-β-D -glucopyranoside. Our results indicate that the pathological state of spontaneous hypertension may change the absorption of active components of E. ulmoides extracts, and these findings may provide a reference for improving the rational use of E. ulmoides in the clinic. 相似文献
22.
Quantitative analysis of polyhexamethylene guanidine (PHMG) oligomers via matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry with an ionic‐liquid matrix 下载免费PDF全文
23.
Elsye Agustina Jeungchoon Goak Suntae Lee Youngho Seo Jun-Young Park Naesung Lee 《ChemistryOpen》2015,4(5):613-619
Iron catalysts have been used widely for the mass production of carbon nanotubes (CNTs) with high yield. In this study, UV/visible spectroscopy was used to determine the Fe catalyst content in CNTs using a colorimetric technique. Fe ions in solution form red–orange complexes with 1,10-phenanthroline, producing an absorption peak at λ=510 nm, the intensity of which is proportional to the solution Fe concentration. A series of standard Fe solutions were formulated to establish the relationship between optical absorbance and Fe concentration. Many Fe catalysts were microscopically observed to be encased by graphitic layers, thus preventing their extraction. Fe catalyst dissolution from CNTs was investigated with various single and mixed acids, and Fe concentration was found to be highest with CNTs being held at reflux in HClO4/HNO3 and H2SO4/HNO3 mixtures. This novel colorimetric method to measure Fe concentrations by UV/Vis spectroscopy was validated by inductively coupled plasma optical emission spectroscopy, indicating its reliability and applicability to asses Fe content in CNTs. 相似文献
24.
25.
Guangrui Chen Yuxing Yan Dr. Jie Wang Prof. Yong Sik Ok Guiyuan Zhong Prof. Bu Yuan Guan Prof. Yusuke Yamauchi 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(44):19831-19836
Mesoporous core–shell nanostructures with controllable ultra-large open channels in their nanoshells are of great interest. However, soft template-directed cooperative assembly to mesoporous nanoshells with highly accessible pores larger than 30 nm, or even above 50 nm into macroporous range, remains a significant challenge. Herein we report a general approach for precisely tailored coating of hierarchically macro-/mesoporous polymer and carbon shells, possessing highly accessible radial channels with extremely wide pore size distribution from ca. 10 nm to ca. 200 nm, on diverse functional materials. This strategy creates opportunities to tailor the interfacial assembly of irregular mesostructured nanounits on core materials and generate various core–shell nanomaterials with controllable pore architectures. The obtained Fe,N-doped macro-/mesoporous carbon nanoshells show enhanced electrochemical performance for the oxygen reduction reaction in alkaline condition. 相似文献
26.
27.
28.
在1100℃制备了钙钛矿型非计量系列物Sr_(1-x)Bi_xFeO_(3-y),与文献相比,明显具有温度低、时间短、能耗少的特点。用化学分析方法测得其化学式及Fe离子的平均价态,用XRD、交流阻抗、IR、Mossbauer谱等方法研究了其结构与性质,该类化合物具有Pm3m对称性和半导体性质,铁离子会产生电荷歧化现象。 相似文献
29.
Uhm H.S. Lee P.H. Kim Y.I. Kim J.H. Chang H.Y. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1995,23(4):628-635
A theory is developed for the density profile of low temperature plasmas confined by applied magnetic field and an experiment of the electron-cyclotron-resonance (ECR) plasma is conducted to compare the theoretical prediction and experimental measurements. Due to a large electron mobility along the magnetic field, electrons move quickly out of the system, leaving ions behind and building a space charge potential, which leads to the ambipolar diffusion of ions. In a steady-state condition, the plasma generation by ionization of neutral molecules is in balance with plasma loss due to the diffusion, leading to the electron temperature equation, which is expressed in terms of the plasma size, chamber pressure, and the ionization energy and cross section of neutrals. The power balance condition leads to the plasma density equation, which is also expressed in terms of the electron temperature, the input microwave power and the chamber pressure. It is shown that the plasma density increases, reaches its peak and decreases, as the chamber pressure increases from a small value (0.1 mTorr). These simple expressions of electron temperature and density provide a scaling law of ECR plasma in terms of system parameters. After carrying out an experimental observation, it is concluded that the theoretical predictions of the electron temperature and plasma density agree remarkably well with experimental data 相似文献
30.
Using the continuous shape space formalism, we develop an immune system model involving both B lymphocytes and antibody molecules. The binding and cross-linking of receptors on B cells stimulates the cells to divide and, with a lag, to secrete antibody. Using the method of multiple scales, we show how to correctly formulate long-time-scale equations for the population dynamics of B cells, the total antibody concentration, and rate of antibody secretion. We compare our model with previous phenomenological formulations. 相似文献