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   25284篇
  免费   2112篇
  国内免费   2061篇
  2024年   45篇
  2023年   310篇
  2022年   630篇
  2021年   1320篇
  2020年   864篇
  2019年   1053篇
  2018年   999篇
  2017年   740篇
  2016年   1055篇
  2015年   1555篇
  2014年   1830篇
  2013年   1934篇
  2012年   2364篇
  2011年   2024篇
  2010年   1283篇
  2009年   1024篇
  2008年   1392篇
  2007年   1166篇
  2006年   1055篇
  2005年   899篇
  2004年   745篇
  2003年   636篇
  2002年   580篇
  2001年   490篇
  2000年   378篇
  1999年   430篇
  1998年   255篇
  1997年   218篇
  1996年   255篇
  1995年   205篇
  1994年   252篇
  1993年   152篇
  1992年   220篇
  1991年   183篇
  1990年   174篇
  1989年   113篇
  1988年   83篇
  1987年   75篇
  1986年   49篇
  1985年   63篇
  1984年   48篇
  1983年   39篇
  1982年   37篇
  1981年   24篇
  1980年   16篇
  1979年   25篇
  1977年   15篇
  1976年   16篇
  1975年   15篇
  1973年   17篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
The derivatives of 1,4‐dimethoxybenzene are thus far the best performing redox shuttle additives for overcharge protection of Li‐ion batteries. The most durable molecules of this kind typically possess two in‐plane methoxy groups that are equivalent by inversion symmetry. However, such geometry leads to a vanishing average dipole moment that causes poor solubility of these molecules in carbonate‐based electrolytes. In this study, a novel redox shuttle additive, 1,2,3,4‐tetrahydro‐6,7‐dimethoxy‐1,1,4,4‐tetramethyl‐naphthalene (TDTN), is introduced. It has been demonstrated that reversible oxidation at 4.05 V versus Li+/Li, high polarity, high solubility (around 0.4 m ), and excellent electrochemical stability (150 overcharge cycles at C/2 rate with 100% overcharge) can all be achieved simultaneously by the imposition of axial symmetry in the corresponding radical cation that is generated by electrochemical oxidation of TDTN in the battery. The intricate interplay between the symmetry and the chemical stability of the radical cation is scrutinized using magnetic resonance spectroscopy and electron structure modeling.  相似文献   
995.
The ability of the white-rot fungus Ganoderma sp.En3 to decolorize different kinds of dyes widely applied in the textile and dyeing industry, including the anthraquinone dye Remazol Brilliant Blue R (RBBR), indigo dye indigo carmine and triphenylmethane dye methyl green, was evaluated in this study. Ganoderma sp.En3 had a strong capability of decolorizing high concentrations of RBBR, indigo carmine and methyl green. Obvious reduction of Chemical Oxygen Demand was observed after decolorization of different dyes. Ganoderma sp.En3 had a strong ability to tolerate RBBR, indigo carmine and methyl green with high concentrations. High concentrations of RBBR, indigo carmine and methyl green could also be efficiently decolorized by the crude enzyme of Ganoderma sp.En3. Different redox mediators such as syringaldehyde, acetosyringone and acetovanillone could enhance the decolorization capability for higher concentration of indigo carmine and methyl green. Different metal ions had little effect on the ability of the crude enzyme to decolorize indigo carmine and methyl green. Our study suggested that Ganoderma sp.En3 had a strong capability for decolorizing and tolerating high concentrations of different types of dyes such as RBBR, indigo carmine and methyl green.  相似文献   
996.
Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground‐state decay of the CT state is significantly slower than dissociation.  相似文献   
997.
A new concept of multiple redox semi‐solid‐liquid (MRSSL) flow battery that takes advantage of active materials in both liquid and solid phases, is proposed and demonstrated. Liquid lithium iodide (LiI) electrolyte and solid sulfur/carbon (S/C) composite, forming LiI‐S/C MRSSL catholyte, are employed to demonstrate this concept. Record volumetric capacity (550 Ah L?1catholyte) is achieved using highly concentrated and synergistic multiple redox reactions of LiI and sulfur. The liquid LiI electrolyte is found to increase the reversible volumetric capacity of the catholyte, improve the electrochemical utilization of the S/C composite, and reduce the viscosity of catholyte. A continuous flow test is demonstrated and the influence of the flow rate on the flow battery performance is discussed. The MRSSL flow battery concept transforms inactive component into bi‐functional active species and creates synergistic interactions between multiple redox couples, offering a new direction and wide‐open opportunities to develop high‐energy‐density flow batteries.  相似文献   
998.

Background

Monoacylglycerol lipase (MAGL), a critical lipolytic enzyme, has emerged as a key regulator of tumor progression, yet its biological function and clinical significance in hepatocellular carcinoma (HCC) is still unknown.

Methods

In this study, we used a tissue microarray containing samples from 170 HCC patients to evaluate the expression of MAGL and its correlation with other clinicopathologic characteristics. In addition, we investigated the regulating effects of MAGL on various HCC lines. Finally, we identified the NF-κB signaling pathway participated in MAGL-mediated epithelial-mesenchymal transition (EMT) using HCC cell lines with different metastatic potentials.

Results

The expression of MAGL was significantly higher in HCC tumors than in matched peritumor tissues. Specifically, high MAGL expression was found in tumors with larger tumor size, microvascular invasion, poor differentiation, or advanced TNM stage. In addition, the clinical prognosis for the MAGLhigh group was markedly poorer than that for the MAGLlow group in the 1-, 3-, and 5-year overall survival times and recurrence rates of HCC patients. MAGL expression was an independent prognostic factor for both survival and recurrence after curative resection. Furthermore, the upregulation of MAGL in HCC cells promoted cell growth and invasiveness abilities, and accompanied by EMT. In contrast, downregulation of MAGL obviously inhibited these characteristics. Moreover, further investigations verified that MAGL facilitates HCC progression via NF-κB-mediated EMT process.

Conclusions

Our findings demonstrate MAGL could promote HCC progression by the induction of EMT and suggest a potential therapeutic target, as well as a biomarker for prognosis, in patients with HCC.
  相似文献   
999.

Background

Recent data have shown that γδ T cells can act as mediators for immune defense against tumors. Our previous study has demonstrated that persisting clonally expanded TRDV4 T cells might be relatively beneficial for the outcome of patients with T cell acute lymphoblastic leukemia after hematopoietic stem cell transplantation (HSCT). However, little is known about the distribution and clonality of the TRDV repertoire in T cell receptor (TCR) of γδ T cells and their effects on the clinical outcome of patients with acute myeloid leukemia (AML). The aim of this study was to assess whether the oligoclonal expansion of TCR Vδ T cells could be used as an immune biomarker for AML outcome.

Findings

γδ T cells were sorted from the peripheral blood of 30 patients with untreated AML and 12 healthy donors. The complementarity-determining region 3 (CDR3) sizes of eight TCR Vδ subfamily genes (TRDV1 to TRDV8) were analyzed in sorted γδ T cells using RT-PCR and GeneScan. The most frequently expressed TRDV subfamilies in the AML patients were TRDV8 (86.67 %) and TRDV2 (83.33 %), and the frequencies for TRDV1, TRDV3, TRDV4, and TRDV6 were significantly lower than those in healthy individuals. The most frequent clonally expanded TRDV subfamilies in the AML patients included TRDV8 (56.67 %) and TRDV4 (40 %). The clonal expansion frequencies of the TRDV2 and TRDV4 T cells were significantly higher than those in healthy individuals, whereas a significantly lower TRDV1 clonal expansion frequency was observed in those with AML. Moreover, the oligoclones of TRDV4 and TRDV8 were independent protective factors for complete remission. Furthermore, the oligoclonal expansion frequencies of TRDV5 and TRDV6 in patients with relapse were significantly higher than those in non-recurrent cases.

Conclusions

To the best of our knowledge, we characterized for the first time a significant alteration in the distribution and clonality of the TRDV subfamily members in γδ T cells sorted from AML patients. Clonally expanded TRDV4 and TRDV8 T cells might contribute to the immune response directed against AML, while oligoclonal TRDV5 and TRDV6 might occur in patients who undergo relapse. While the function of such γδ T cell clones requires further investigation, TRDV γδ T cell clones might be potential immune biomarkers for AML outcome.
  相似文献   
1000.
Existing mammographic screening solutions are generally associated with several major drawbacks, such as exposure to ionizing radiation or insufficient sensitivity in younger populations with radiographically‐dense breast. Even when combined with ultrasound or magnetic resonance imaging, X‐Ray mammography may still attain unspecific or false positive results. Thus, development of new breast imaging tools represents a timely medical challenge. We report on a new approach to high‐resolution functional and anatomical breast angiography using volumetric hand‐held optoacoustic tomography, which employs light intensities safe for human use. Experiments in young healthy volunteers with fibroglandular‐dominated dense breasts revealed the feasibility of rendering three‐dimensional images representing vascular anatomy and functional blood oxygenation parameters at video rate. Sufficient contrast was achieved at depths beyond 2 cm within dense breasts without compromising the real‐time imaging performance. The suggested solution may thus find applicability as a standalone or supplemental screening tool for early detection and follow‐up of carcinomas in radiographically‐dense breasts.

Volumetric handheld optoacoustic tomography scanner uses safe pulses of near‐infrared light to render three‐dimensional images of deep vascular anatomy, blood oxygenation and breast parenchyma at video rate.  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