全文获取类型
收费全文 | 6045篇 |
免费 | 377篇 |
国内免费 | 181篇 |
出版年
2024年 | 5篇 |
2023年 | 45篇 |
2022年 | 60篇 |
2021年 | 83篇 |
2020年 | 140篇 |
2019年 | 179篇 |
2018年 | 194篇 |
2017年 | 143篇 |
2016年 | 147篇 |
2015年 | 184篇 |
2014年 | 289篇 |
2013年 | 504篇 |
2012年 | 217篇 |
2011年 | 356篇 |
2010年 | 166篇 |
2009年 | 314篇 |
2008年 | 376篇 |
2007年 | 399篇 |
2006年 | 368篇 |
2005年 | 326篇 |
2004年 | 338篇 |
2003年 | 275篇 |
2002年 | 226篇 |
2001年 | 146篇 |
2000年 | 142篇 |
1999年 | 131篇 |
1998年 | 151篇 |
1997年 | 107篇 |
1996年 | 109篇 |
1995年 | 107篇 |
1994年 | 68篇 |
1993年 | 42篇 |
1992年 | 42篇 |
1991年 | 24篇 |
1990年 | 23篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 26篇 |
1983年 | 14篇 |
1982年 | 18篇 |
1981年 | 10篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 8篇 |
排序方式: 共有6603条查询结果,搜索用时 468 毫秒
991.
Constituents from Scutellaria barbata Inhibiting Nitric Oxide Production in LPS‐Stimulated Microglial Cells
下载免费PDF全文
![点击此处可从《化学与生物多样性》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The arial parts of Scutellaria barbata D. Don (Lamiaceae) efficiently inhibited NO production in BV2 microglial cells, and the active constituents were further isolated based on activity‐guided isolation using silica‐gel column chromatography, RP‐C18 MPLC and prep‐HPLC. As the results, 2 flavonoids including 6‐methoxynaringenin ( 1 ) and 6‐O‐methylscutellarein ( 5 ), and 6 neo‐clerodane diterpenes such as scutebarbatine W ( 2 ), scutebatas B ( 3 ), scutebarbatine B ( 4 ), scutebarbatine A ( 6 ), 6‐O‐nicotinolylscutebarbatine G ( 7 ), and scutebarbatine X ( 8 ) were isolated. The structures of these compounds were elucidated based on NMR and MS data, and the comparison of literature values. All the compounds except compound 7 inhibited NO production efficiently with IC50 values of lower than 50 μm . Particularly, compounds 1 and 8 were the most efficient with IC50 values of 25.8 and 27.4 μm , respectively. This is the first report suggesting the potential of S. barbata on the reduction of neuroinflammation. 相似文献
992.
J. Zhao K. O. Brinkmann T. Hu N. Pourdavoud T. Becker T. Gahlmann R. Heiderhoff A. Polywka P. Görrn Y. Chen B. Cheng T. Riedl 《Liver Transplantation》2017,7(14)
Semitransparent perovskite solar cells (PSCs) are of interest for application in tandem solar cells and building‐integrated photovoltaics. Unfortunately, several perovskites decompose when exposed to moisture or elevated temperatures. Concomitantly, metal electrodes can be degraded by the corrosive decomposition products of the perovskite. This is even the more problematic for semitransparent PSCs, in which the semitransparent top electrode is based on ultrathin metal films. Here, we demonstrate outstandingly robust PSCs with semitransparent top electrodes, where an ultrathin Ag layer is sandwiched between SnOx grown by low‐temperature atomic layer deposition. The SnOx forms an electrically conductive permeation barrier, which protects both the perovskite and the ultrathin silver electrode against the detrimental impact of moisture. At the same time, the SnOx cladding layer underneath the ultra‐thin Ag layer shields the metal against corrosive halide compounds leaking out of the perovskite. Our semitransparent PSCs show an efficiency higher than 11% along with about 70% average transmittance in the near‐infrared region (λ > 800 nm) and an average transmittance of 29% for λ = 400–900 nm. The devices reveal an astonishing stability over more than 4500 hours regardless if they are exposed to ambient atmosphere or to elevated temperatures. 相似文献
993.
Revealing Anisotropic Spinel Formation on Pristine Li‐ and Mn‐Rich Layered Oxide Surface and Its Impact on Cathode Performance
下载免费PDF全文
![点击此处可从《Liver Transplantation》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Saravanan Kuppan Alpesh Khushalchand Shukla Daniel Membreno Dennis Nordlund Guoying Chen 《Liver Transplantation》2017,7(11)
Surface properties of cathode particles play important roles in the transport of ions and electrons and they may ultimately dominate cathode's performance and stability in lithium‐ion batteries. Through the use of carefully prepared Li1.2Ni0.13Mn0.54Co0.13O2 crystal samples with six distinct morphologies, surface transition‐metal redox activities and crystal structural transformation are investigated as a function of surface area and surface crystalline orientation. Complementary depth‐profiled core‐level spectroscopy, namely, X‐ray absorption spectroscopy, electron energy loss spectroscopy, and atomic‐resolution scanning transmission electron microscopy, are applied in the study, presenting a fine example of combining advanced diagnostic techniques with a well‐defined model system of battery materials. The present study reports the following findings: (1) a thin layer of defective spinel with reduced transition metals, similar to what is reported on cycled conventional secondary particles in the literature, is found on pristine oxide surface even before cycling, and (2) surface crystal structure and chemical composition of both pristine and cycled particles are facet dependent. Oxide structural and cycling stabilities improve with maximum expression of surface facets stable against transition‐metal reduction. The intricate relationships among morphology, surface reactivity and structural transformation, electrochemical performance, and stability of the cathode materials are revealed. 相似文献
994.
Anion Doping: A New Strategy for Developing High‐Performance Perovskite‐Type Cathode Materials of Solid Oxide Fuel Cells
下载免费PDF全文
![点击此处可从《Liver Transplantation》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Overcoming the sluggish activity of cathode materials is critical to realizing the wide‐spread application of intermediate‐temperature solid oxide fuel cells. Herein, a new way is reported to tune the performance of perovskite‐type materials as oxygen reduction electrodes by embedding anions (F?) in oxygen sites. The obtained perovskite oxyfluorides SrFeO3?σ ?δ Fσ and SrFe0.9Ti0.1O3?σ ?δ Fσ (σ = 0.05 and 0.10) show improved electrocatalytic activity compared to their parent oxides, achieving area specific resistance values of 0.875, 0.393, and 0.491 Ω cm2 for SrFeO3?δ , SrFeO2.95?δ F0.05, and SrFeO2.90?δ F0.10, respectively, at 600 °C in air. Such improved performance is a result of the improved bulk diffusion and surface exchange properties due to anion doping. Moreover, favorable stability in performance is also demonstrated for the F? anion‐doped perovskites as oxygen reduction electrodes at 650 °C for a test period of ≈200 h. A combination of anion doping and cation doping may provide a highly attractive strategy for the future development of cathode materials. 相似文献
995.
Cerium Oxide Decorated Polymer Nanofibers as Effective Membrane Reinforcement for Durable,High‐Performance Fuel Cells
下载免费PDF全文
![点击此处可从《Liver Transplantation》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Matthias Breitwieser Carolin Klose Armin Hartmann Andreas Büchler Matthias Klingele Severin Vierrath Roland Zengerle Simon Thiele 《Liver Transplantation》2017,7(6)
High‐power, durable composite fuel cell membranes are fabricated here by direct membrane deposition (DMD). Poly(vinylidene fluoride‐co ‐hexafluoropropylene) (PVDF‐HFP) nanofibers, decorated with CeO2 nanoparticles are directly electrospun onto gas diffusion electrodes. The nanofiber mesh is impregnated by inkjet‐printed Nafion ionomer dispersion. This results in 12 µm thin multicomponent composite membranes. The nanofibers provide membrane reinforcement, whereas the attached CeO2 nanoparticles promote improved chemical membrane durability due to their radical scavenging properties. In a 100 h accelerated stress test under hot and dry conditions, the reinforced DMD fuel cell shows a more than three times lower voltage decay rate (0.39 mV h?1) compared to a comparably thin Gore membrane (1.36 mV h?1). The maximum power density of the DMD fuel cell drops by 9%, compared to 54% measured for the reference. Impedance spectroscopy reveals that ionic and mass transport resistance of the DMD fuel cell are unaffected by the accelerated stress test. This is in contrast to the reference, where a 90% increase of the mass transport resistance is measured. Energy dispersive X‐ray spectroscopy reveals that no significant migration of cerium into the catalyst layers occurs during degradation. This proves that the PVDF‐HFP backbone provides strong anchoring of CeO2 in the membrane. 相似文献
996.
Differences in field‐scale N2O flux linked to crop residue removal under two tillage systems in cold climates
下载免费PDF全文
![点击此处可从《Global Change Biology Bioenergy》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Katelyn A. Congreves Shannon E. Brown Deanna D. Németh Kari E. Dunfield Claudia Wagner‐Riddle 《Global Change Biology Bioenergy》2017,9(4):666-680
Residue removal for biofuel production may have unintended consequences for N2O emissions from soils, and it is not clear how N2O emissions are influenced by crop residue removal from different tillage systems. Thus, we measured field‐scale N2O flux over 5 years (2005–2007, 2010–2011) from an annual crop rotation to evaluate how N2O emissions are influenced by no‐till (NT) compared to conventional tillage (CV), and how crop residue removal (R?) rather than crop residue return to soil (R+) affects emissions from these two tillage systems. Data from all 5 years indicated no differences in N2O flux between tillage practices at the onset of the growing season, but CT had 1.4–6.3 times higher N2O flux than NT overwinter. Nitrous oxide emissions were higher due to R? compared to R+, but the effect was more marked under CT than NT and overwinter than during spring. Our results thus challenge the assumption based on IPCC methodology that crop residue removal will result in reduced N2O emissions. The potential for higher N2O emission with residue removal implies that the benefit of utilizing biomass as biofuels to mitigate greenhouse gas emission may be overestimated. Interestingly, prior to an overwinter thaw event, dissolved organic C (DOC) was negatively correlated to peak N2O flux (r = ?0.93). This suggests that lower N2O emissions with R+ vs. R? may reflect more complete stepwise denitrification to N2 during winter and possibly relate to the heterotrophic microbial capacity for processing crop residue into more soluble C compounds and a shift in the preferential C source utilized by the microbial community overwinter. 相似文献
997.
Impacts of natural factors and farming practices on greenhouse gas emissions in the North China Plain: A meta‐analysis
下载免费PDF全文
![点击此处可从《Ecology and evolution》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Requirements for mitigation of the continued increase in greenhouse gas (GHG ) emissions are much needed for the North China Plain (NCP ). We conducted a meta‐analysis of 76 published studies of 24 sites in the NCP to examine the effects of natural conditions and farming practices on GHG emissions in that region. We found that N2O was the main component of the area‐scaled total GHG balance, and the CH 4 contribution was <5%. Precipitation, temperature, soil pH , and texture had no significant impacts on annual GHG emissions, because of limited variation of these factors in the NCP . The N2O emissions increased exponentially with mineral fertilizer N application rate, with y = 0.2389e0.0058x for wheat season and y = 0.365e0.0071x for maize season. Emission factors were estimated at 0.37% for wheat and 0.90% for maize at conventional fertilizer N application rates. The agronomic optimal N rates (241 and 185 kg N ha?1 for wheat and maize, respectively) exhibited great potential for reducing N2O emissions, by 0.39 (29%) and 1.71 (56%) kg N2O‐N ha?1 season?1 for the wheat and maize seasons, respectively. Mixed application of organic manure with reduced mineral fertilizer N could reduce annual N2O emissions by 16% relative to mineral N application alone while maintaining a high crop yield. Compared with conventional tillage, no‐tillage significantly reduced N2O emissions by ~30% in the wheat season, whereas it increased those emissions by ~10% in the maize season. This may have resulted from the lower soil temperature in winter and increased soil moisture in summer under no‐tillage practice. Straw incorporation significantly increased annual N2O emissions, by 26% relative to straw removal. Our analysis indicates that these farming practices could be further tested to mitigate GHG emission and maintain high crop yields in the NCP . 相似文献
998.
目的:探讨激活多巴胺Ⅰ类受体(DR1)对氧化型低密度脂蛋白(ox-LDL)诱导的人单核细胞(THP-1)分泌一氧化氮/一氧化氮合酶(NO/NOS)的影响及可能机制。方法:THP-1细胞经佛波酯PMA诱导分化,分为正常对照组(control),氧化型低密度脂蛋白处理组(ox-LDL),DR1激动剂干预组(SKF),DR1阻断剂干预组(SCH),ERK阻断剂干预组(PD98059);应用油红O染色法鉴定泡沫细胞;硝酸还原法检测NO、NOS的变化情况;免疫荧光和Western blot检测各组细胞蛋白表达情况。结果:ox-LDL刺激48 h可形成泡沫细胞;DR1在THP1细胞上表达,ox-LDL刺激后,DR1蛋白表达降低(P<0.01);激活DR1受体能够明显抑制由ox-LDL引起的NO、iNOS增多(P<0.01);在MAPK阻断剂PD98059存在的情况下,SKF的作用部分丧失。结论:激活DR1受体可抑制ox-LDL引起的THP-1细胞NO的大量产生,此过程可能由ERK信号通路所介导。 相似文献
999.
1000.
CHEN Ruijuan JIANG Jiangang XIAO Xiao & WANG Daowen . The Institute of Hypertension Department of Internal Medicine Tongji Hospital Tongji Medical College Huazhong University of Science & Technology Wuhan China . Departments of Molecular Genetics Biochemistry & Gene Therapy Center University of Pittsburgh Pittsburgh PA USA 《中国科学:生命科学英文版》2005,48(5):495-505
Vascular endothelial cells play crucial roles in regulating cardiovascular function, maintaining car-diovascular homeostasis and preventing the occur-rence of cardiac and cerebral vascular diseases. All these protective effects are fulfilled through various vasoactive products secreted by endothelium including nitric oxide (NO), prostacyclin (PGI2) and endothe-lium-derived hyperpolarizing factor (EDHF). NO, pro-duced from L-arginine by endothelial nitric-oxide synthase (eNOS), is an impor… 相似文献