全文获取类型
收费全文 | 43431篇 |
免费 | 4739篇 |
国内免费 | 4167篇 |
专业分类
化学 | 39108篇 |
晶体学 | 648篇 |
力学 | 1964篇 |
综合类 | 271篇 |
数学 | 1071篇 |
物理学 | 9275篇 |
出版年
2024年 | 35篇 |
2023年 | 465篇 |
2022年 | 876篇 |
2021年 | 1070篇 |
2020年 | 1421篇 |
2019年 | 1330篇 |
2018年 | 1370篇 |
2017年 | 1839篇 |
2016年 | 2223篇 |
2015年 | 1891篇 |
2014年 | 1989篇 |
2013年 | 4108篇 |
2012年 | 2994篇 |
2011年 | 2668篇 |
2010年 | 2267篇 |
2009年 | 2691篇 |
2008年 | 2326篇 |
2007年 | 2614篇 |
2006年 | 2271篇 |
2005年 | 1990篇 |
2004年 | 1790篇 |
2003年 | 1511篇 |
2002年 | 1325篇 |
2001年 | 1020篇 |
2000年 | 1028篇 |
1999年 | 908篇 |
1998年 | 820篇 |
1997年 | 715篇 |
1996年 | 625篇 |
1995年 | 651篇 |
1994年 | 542篇 |
1993年 | 462篇 |
1992年 | 399篇 |
1991年 | 378篇 |
1990年 | 277篇 |
1989年 | 235篇 |
1988年 | 256篇 |
1987年 | 228篇 |
1986年 | 156篇 |
1985年 | 120篇 |
1984年 | 129篇 |
1983年 | 42篇 |
1982年 | 89篇 |
1981年 | 54篇 |
1980年 | 31篇 |
1979年 | 35篇 |
1978年 | 13篇 |
1977年 | 9篇 |
1976年 | 8篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
31.
Triple-negative breast cancer (TNBC) is considered to be aggressive based on its low overall survival and disease-free rates. Currently, there is no molecular-targeted therapy. The identification of a suitable biomarker is still a research focus for TNBC at the present time. Amino acid metabolism fulfills multiple important physiological roles in humans. Their metabolic abnormalities have been reported in numerous cancer studies and amino acid pathways may also be chemotherapeutic targets. This study reports the profiling analysis of amino acids in TNBC and non-TNBC cell lines for detecting biomarkers based on the strategy of N-phosphorylation labeling with liquid chromatography–tandem mass spectrometry (LC–MS). Glutamine (Gln) was found to be significantly down-regulated in TNBC cells because it was largely absorbed and consumed in the TNBC cell lines. These results indicate faster proliferation of TNBC and higher consumption of glutamine to meet the material and energy demand, suggesting its potential role in TNBC progression. Hence, glutamine may be regarded as a biomarker and Gln-targeted approaches may become a new therapeutic strategies for TNBC. 相似文献
32.
近年来,基于透射电子显微技术、微纳加工技术和薄膜制造技术的发展,原位液相透射电子显微技术产生,为构建多种纳米级分辨率尺度下的微实验平台,发展新型纳米表征技术和众多领域的相关研究提供了途径.本文首先介绍了应用于原位液相透射电子显微技术的液体腔设计要求,然后介绍了液体腔的发展和典型的制备工艺,最后综述了近年来液体腔透射电子显微镜在纳米粒子成核和生长方面的应用研究,并探讨了该技术前沿发展面临的机遇和挑战.本文将为提高我国先进纳米表征技术和原子精准构筑技术提供相关讨论和支持. 相似文献
33.
Dr. Si-Yong Qin Yan Jiang Dr. Han Sun Han Liu Prof. Ai-Qing Zhang Prof. Xinxiang Lei 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(39):17245-17251
Residual dipolar coupling (RDC), a robust anisotropic NMR parameter for structural elucidation of organic molecules, is only accessible in an anisotropic environment. Herein, we introduce a novel alignment medium based on the molecular self-assembly of oligopeptide amphiphile (OPA). This medium is compatible with different intermediate and polar solvent systems, such as CD3OD, [D6]DMSO, and D2O. The preparation of the OPA-based medium is simple and rapid, while only very weak background signals were observed from OPAs. Furthermore, we show that the purity of OPA has only a minor influence on the quality of the RDC data. These advantages allow RDC measurements of organic molecules with different polarities and solubilities with high efficiency and accuracy. 相似文献
34.
Sensitivity analysis is a mathematical tool, first developed for optimization methods, which aim is to characterize a system response through the variations of its output parameters following modifications imposed on the input parameters of the system. Such an analysis may quickly become laborious when the thermal model under consideration is complex or the number of input parameters is high. In this paper, we develop a mathematical model to analyse the heat exchanges in four different types of solar air collectors. When building this thermal model we show that for each collector, at quasi-steady state, the energy balance equations of the components of the collector cascade into a single first-order non-linear differential equation that is able to predict the thermal behaviour of the collector. Our heat transfer model clearly demonstrates the existence of an important dimensionless parameter, referred to as the thermal performance factor of the collector, that compares the useful thermal energy which can be extracted from the heater to the overall thermal losses of that collector for a given set of input parameters. A sensitivity analysis of our thermal model has been performed for the most significant input parameters such as the incident solar irradiation, the inlet fluid temperature, the air mass flow rate, the depth of the fluid channel, the number and nature of the transparent covers in order to measure the impact of each of these parameters on our model. An important result which can be drawn from this study is that the heat transfer model developed is robust enough to be used for thermal design studies of most known flat plate solar air heaters, but also of flat plate solar water collectors and linear solar concentrators. 相似文献
35.
36.
37.
High index of refraction via quantum interference in a three-level system of Er^3+-doped yttrium aluminium garnet crystal 下载免费PDF全文
A simple three-level system is proposed to produce high index of refraction with zero absorption in an Er^3+-doped yttrium aluminium garnet (YAG) crystal, which is achieved for a probe field between the excited state 4I13/2 and ground state 4I15/2 by adjusting a strong coherent driving field between the upper excited state 4I11/2 and 4I15/2. It is found that the changes of the frequency of the coherent driving field and the concentration of Er^3+ ions in the YAG crystal can maximize the index of refraction accompanied by vanishing absorption. This result could be useful for the dispersion compensation in fibre communication, laser particle acceleration, high precision magnetometry and so on. 相似文献
38.
We report the results of experiments on electrically driven convection that occurs in a thin, freely suspended film of smectic A liquid crystal when an electric field is applied in the plane of the film. Convection in a vortex pattern is found above a well-defined critical voltage. The film behaves as a two-dimensional isotropic liquid: neither its thickness nor the director field are modified by the flow. We present measurements of the critical voltage at the onset of convection in two experimental configurations—one which allows the injection of charges into the film from the electrodes, and one which does not. When injection is present, the critical voltage for the onset of flow increases monotonically with increasing frequency of applied field. With no injection, there is no instability at DC and the critical voltage diverges there. The nature of the flow pattern observed at onset changes with frequency. Below a certain frequency the film flows in vortices that extend over the width of the film; above this frequency the flow is confined to two lines of smaller vortices localized along the electrodes. We present a simple discussion of the mechanisms which drive the convection. 相似文献
39.
40.