首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5958篇
  免费   507篇
  国内免费   398篇
化学   1189篇
晶体学   61篇
力学   3504篇
综合类   74篇
数学   594篇
物理学   1441篇
  2024年   6篇
  2023年   36篇
  2022年   112篇
  2021年   234篇
  2020年   162篇
  2019年   98篇
  2018年   118篇
  2017年   162篇
  2016年   217篇
  2015年   170篇
  2014年   231篇
  2013年   353篇
  2012年   216篇
  2011年   304篇
  2010年   241篇
  2009年   300篇
  2008年   299篇
  2007年   361篇
  2006年   372篇
  2005年   303篇
  2004年   316篇
  2003年   259篇
  2002年   188篇
  2001年   208篇
  2000年   218篇
  1999年   178篇
  1998年   147篇
  1997年   140篇
  1996年   135篇
  1995年   114篇
  1994年   93篇
  1993年   96篇
  1992年   83篇
  1991年   75篇
  1990年   51篇
  1989年   41篇
  1988年   46篇
  1987年   44篇
  1986年   31篇
  1985年   23篇
  1984年   19篇
  1983年   11篇
  1982年   20篇
  1981年   18篇
  1980年   3篇
  1979年   4篇
  1977年   2篇
  1973年   1篇
  1971年   1篇
  1957年   2篇
排序方式: 共有6863条查询结果,搜索用时 140 毫秒
121.
ATP-binding cassette (ABC) transporter is a large gene superfamily that utilizes the energy released from ATP hydrolysis for transporting myriad of substrates across the biological membranes. Although many investigations have been done on the structural and functional analysis of the ABC transporters in Oryza sativa, much less is known about molecular phylogenetic and global expression pattern of the complete ABC family in rice. In this study, we have carried out a comprehensive phylogenetic analysis constructing neighbor-joining and maximum-likelihood trees based on various statistical methods of different ABC protein subfamily of five plant lineages including Chlamydomonas reinhardtii (green algae), Physcomitrella patens (moss), Selaginella moellendorffii (lycophyte), Arabidopsis thaliana (dicot) and O. sativa (monocot) to explore the origin and evolutionary patterns of these ABC genes. We have identified several conserved motifs in nucleotide binding domain (NBD) of ABC proteins among all plant lineages during evolution. Amongst the different ABC protein subfamilies, ‘ABCE’ has not yet been identified in lower plant genomes (algae, moss and lycophytes). The result indicated that gene duplication and diversification process acted upon these genes as a major operative force creating new groups and subgroups and functional divergence during evolution. We have demonstrated that rice ABCI subfamily consists of only half size transporters that represented highly dynamic members showing maximum sequence variations among the other rice ABC subfamilies. The evolutionary and the expression analysis contribute to a deep insight into the evolution and diversity of rice ABC proteins and their roles in response to salt stress that facilitate our further understanding on rice ABC transporters.  相似文献   
122.
Acetaminophen (N-acetyl-p-aminophenol, APAP) overdosage can produce fatal centrilobular hepatic necrosis in humans. The present study attempted to investigate the protective effect of an anthocyanin-rich extract from bilberries and blackcurrants (AE) against APAP-induced acute hepatic damage in rats. Treatment with AE normalised blood activities of glutamate oxaloacetate and glutamate pyruvate transaminase and prevented APAP-induced plasmatic and tissutal alterations in biomarkers of oxidative stress, probably due to various bioproperties of the components of the extract.  相似文献   
123.
The quasi-ferrite model is proposed and an appropriate PBE exchange functional with the spin density functional theory(SDFT) is selected for the calculation of the relation between magnetic moment and residual stress in ferrite using a quantum mechanics code. The relationship between ferrite magnetism and the carbon content is determined,and then a ferrite interstitial solid solution(ISS) model in a low carbon concentration state is replaced with an α- Fe model in the case of majority magnetic calculation. The band structure of the loaded-Fe is compared with that of the unloaded α-Fe. The comparison shows that the energy of Fe atomic 3d orbital changes a little,while the energy of electron orbital of iron core below 3d almost keeps unchanged. The relationship between the magnetic moment and the stress appears intermittent due to the Bragg total reflection. The change in the magnetic moment due to lattice mismatch is much larger than that caused by mechanical loading.  相似文献   
124.
对吊装机械中使用的几种吊钩在吊装重物时的受力情况进行分析和比较,说明了各自的特点及其应用.  相似文献   
125.
In this paper, we propose a compensated pixelwise calibration that integrates the effects of the camera housing temperature. The results of this calibration are compared on black body images to classic two points Non Uniformity Correction based calibrations, compensated or not. It is shown that the proposed approach leads to a significant improvement in the thermal resolution with a reduction in the mean error as well as the standard deviation. The approach is finally challenged on a real case measurement focusing on thermoelasticity. The gain in terms of accuracy measurement is highlighted by comparing the proposed calibration to classic calibrations and the scope of interest of this new calibration is discussed.  相似文献   
126.
In geomagnetic field, a series of tensile experiments on the low-carbon steel sticks were carried out. A special homemade detector was used to measure the magnetic gradient on the material surface. The results showed that the relationship between the magnetic gradient and the stress varied with different conditions of measurement. There was no obvious correlation between the magnetic gradient and the tensile stress if the sample remained on the material test machine. If the sample was taken off from the machine, the measured magnetic gradient was linear with the prior maximum stress. In Nanjing, PR China, a place of 32°N latitude, the slope of the linear relationship was about 67 (uT/m)/MPa. This offered a new method of non-destructive stress testing by measuring the magnetic gradient on the ferromagnetic component surface. The prior maximum applied stress of the sample could be tested by measuring the present surface magnetic gradient. Actually this phenomenon was the metal magnetic memory (MMM). The magnetic gradient near the stress concentration zone of the sample, the necking point, was much larger than other area. Thus, the hidden damage in the ferromagnetic component could be detected early by measuring the magnetic gradient distribution on its surface. In addition, the magnetic memory signal gradually weakened as the sample was taken off and laid aside. Therefore, it was effective for a given period of time to detect the stress or stress concentration based on the MMM testing.  相似文献   
127.
The stress–strain dependences of segmented elastomers based on polyetherurethanes, polyesterurethane ureas, and polybutadieneurethane ureas were investigated. These dependences were adequately described using the generalized approach proposed earlier by the authors for amorphous elastomers. The experimental data obtained for four series of samples with chemical cross-linking confirmed an applicability of this approach for segmented elastomers. Unlike amorphous elastomers, the effective network density value was affected not only by the density of the chemical network, but the contribution of the network formed by the hard domains also needs be taken into account.  相似文献   
128.
129.

Environmental stress cracking resistance (ESCR) is an important indicator of performance for high density polyethylene (HDPE) in structural and polymer pipe applications. The commonly used test for determining ESCR of HDPE can be time consuming and rather imprecise. A tensile strain hardening test was recently proposed to offer a faster way to characterize ESCR of polyethylene. In this paper, a practical approach is adopted whereby the test is extended to room temperature and shown to relate reliably to the ESCR of HDPE. Several HDPE resins (including pipe‐grade resins) are analyzed at strain rates of 0.5 mm/min and 7 mm/min to compare the effect of strain rate. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of HDPE in a reliable fashion. In our study the more direct measure of “hardening stiffness” is used to compare resins instead of strain hardening modulus. Because no true stress‐strain measurement is needed, this is a much simpler test method than other methods previously suggested. In addition, the use of the natural drawing ratio (NDR) as ESCR ranking indicator is examined. Results show that NDR can also be employed as a strain rate‐independent indicator of ESCR of HDPE. The test proposed herein is practical, simple and precise, and hence a more reliable indicator of ESCR performance of HDPE.  相似文献   
130.
Apoptosis has an important role in maintaining tissue homeostasis in cellular stress responses such as inflammation, endoplasmic reticulum stress, and oxidative stress. T-cell death-associated gene 51 (TDAG51) is a member of the pleckstrin homology-like domain family and was first identified as a pro-apoptotic gene in T-cell receptor-mediated cell death. However, its pro-apoptotic function remains controversial. In this study, we investigated the role of TDAG51 in oxidative stress-induced apoptotic cell death in mouse embryonic fibroblasts (MEFs). TDAG51 expression was highly increased by oxidative stress responses. In response to oxidative stress, the production of intracellular reactive oxygen species was significantly enhanced in TDAG51-deficient MEFs, resulting in the activation of caspase-3. Thus, TDAG51 deficiency promotes apoptotic cell death in MEFs, and these results indicate that TDAG51 has a protective role in oxidative stress-induced cell death in MEFs.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号