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   26594篇
  免费   1807篇
  国内免费   1072篇
  2024年   34篇
  2023年   239篇
  2022年   544篇
  2021年   992篇
  2020年   697篇
  2019年   873篇
  2018年   1098篇
  2017年   793篇
  2016年   1174篇
  2015年   1404篇
  2014年   1654篇
  2013年   1990篇
  2012年   2179篇
  2011年   2103篇
  2010年   1286篇
  2009年   1069篇
  2008年   1456篇
  2007年   1362篇
  2006年   1284篇
  2005年   1139篇
  2004年   922篇
  2003年   927篇
  2002年   768篇
  2001年   545篇
  2000年   454篇
  1999年   398篇
  1998年   246篇
  1997年   160篇
  1996年   124篇
  1995年   133篇
  1994年   97篇
  1993年   84篇
  1992年   107篇
  1991年   115篇
  1990年   99篇
  1989年   77篇
  1988年   64篇
  1987年   69篇
  1986年   50篇
  1985年   69篇
  1984年   43篇
  1983年   39篇
  1982年   36篇
  1981年   27篇
  1980年   22篇
  1979年   38篇
  1978年   36篇
  1977年   34篇
  1972年   24篇
  1952年   40篇
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
971.
Abstract

Aquatic plant treatment system (APTS) is a widely used sewage purification technique; however, it requires a large area of land due to its long hydraulic retention time. In order to improve the economic value of APTS in the treatment of rural sewage, an aquatic vegetables (lettuce) purification system strengthened with a set of supplemented lighting was evaluated. The effect of supplemented lighting of blue and red light on lettuce growth and sewage purification was studied by batch experiments. The results showed that the lettuce growth and the removal rates of pollutants were enhanced by supplemented lighting, of which red light is superior to blue light, and the increase of red light intensity further promoted the growth of lettuce and the removal rate of pollutants. Supplementary light is a suitable method which could improve the purification effect of APTS in most weather conditions especially in countries where day-night light patterns change substantially between winter and summer. The results would be useful for the APTS design for treating rural domestic sewage.  相似文献   
972.
AimThe aim of the study was to assess the accuracy of radiological diagnosis of laryngeal cartilage infiltration by histopathological examination of laryngeal specimen after total laryngectomy.BackgroundDespite the development of new medical technologies and significant clinical advances allowing early diagnosis and treatment of laryngeal cancer, mortality is still on the rise. Neoplastic infiltration of the laryngeal cartilages is the most common source of error in the assessment of cancer staging. Furthermore, cartilage invasion is listed as a contraindication to partial surgical techniques as well as radiotherapy.Materials and methodsThe study was carried out on 21 larynges following total laryngectomy. Before taking the decision to perform surgery, high-resolution CT scans were performed in all cases. An extended histopathological examination was conducted using a unique vertical cross-section of the whole larynx.ResultsPathology reported 2 cases of arytenoid cartilage invasion, 5 cases of cricoid cartilage invasion, 12 cases of thyroid cartilage penetration, 1 case of internal cortex invasion and 9 cases of extra-laryngeal spread. CT imaging identified 8 of 13 cases (61.5%) of pathologically proven invasion of thyroid cartilage and only 2 cases (2/9, 22%) of extra-laryngeal spread. According to CT results, arytenoid cartilage invasion was correctly identified in 2 cases, cricoid cartilage invasion in 4 (4/5, 80%). The positive predictive values for thyroid, cricoid and arytenoid cartilage invasion and penetration were 80%, 66.7% and 50%, respectively. In case of pre-laryngeal spread the positive predictive value was 100%.ConclusionDespite increasingly advanced methods involved in the diagnosis of laryngeal cancer, many discrepancies may be observed between the radiological and histopathological assessments. CT imaging has limitations especially in thyroid cartilage penetration and extra-laryngeal spread assessment in advanced laryngeal cancer cases. An extended histopathological examination, involving vertical cross-sections of the whole larynx is a very precise study that allows a precise determination of local cancer staging (T).  相似文献   
973.
974.
The anaphase promoting complex/cyclosome (APC/C), a cell cycle-regulated E3 ubiquitin ligase, is responsible for the transition from metaphase to anaphase and the exit from mitosis. The anaphase promoting complex subunit 10 (APC10), a subunit of the APC/C, executes a vital function in substrate recognition. However, no research has reported the connection between APC10 and cancer until now. In this study, we uncovered a novel, unprecedented role of APC10 in tumor progression, which is independent of APC/C. First, aberrant increase of APC10 expression was validated in non-small cell lung cancer (NSCLC) cells and tissues, and the absence of APC10 repressed cell proliferation and migration. Of great interest, we found that APC10 inhibition induced cell cycle arrest at the G0/G1 phase and reduced the expression of the APC/C substrate, Cyclin B1; this finding is different from the conventional concept of the accumulation of Cyclin B1 and cell cycle arrest in metaphase. Further, APC10 was found to interact with glutaminase C (GAC), and the inhibition of APC10 weakened glutamine metabolism and induced excessive autophagy. Taken together, these findings identify a novel function of APC10 in the regulation of NSCLC tumorigenesis and point to the possibility of APC10 as a new target for cancer therapy.  相似文献   
975.
The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two‐step sequential deposited perovskite with the state‐of‐art composition (FAPbI3)1?x(MAPbBr3)x (x < 0.05) have achieved record power conversion efficiency (PCE), while their one‐step antisolvent dripping counterparts with typical composition Cs0.05FA0.81MA0.14Pb(I0.85Br0.15)3 with more bromine have exhibited much better long‐term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br‐rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI2 precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br‐rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1‐sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed‐cation‐halide perovskite film via sequential deposition method.  相似文献   
976.
977.
The dendritic structure is a disastrous problem of lithium metal batteries as well as other metal rechargeable batteries. The dendritic structures are usually caused by diffusion limitation. Here, a novel strategy is reported to inhibit lithium dendrites based on the understanding of their formation mechanism. An alternating current field perpendicular to the anode is set up, which promotes Li+ movement along the anode surface and prevents ions' deposition on the tips from forming dendrites. Furthermore, an external direct current field parallel to the current is employed, which accelerates the transport of Li+ in electrolytes to mitigate the concentration gradient nearby the anode and thus inhibits the formation of dendritic structures. A simultaneous employment of these two fields gains five times increase of the lifespan of batteries at the high charging current density of 2 mA cm?2, confirming the effectiveness of this strategy in protecting the metal anode and inhibiting lithium dendrites. This strategy may have a wide feasibility since it does not change the materials and structures of batteries.  相似文献   
978.
Dual‐ion batteries (DIBs) with high operation voltage offer promising candidates for low‐cost clean energy chemistries. However, there still exist tough issues, including structural collapse of the graphite cathode due to solvent co‐intercalation and electrolyte decomposition on the electrode/electrolyte interface, which results in unsatisfactory cyclability and fast battery failure. Herein, Li4Ti5O12 (LTO) modified mesocarbon microbeads (MCMBs) are proposed as a cathode material. The LTO layer functions as a skeleton and offers electrocatalytic active sites for in situ generation of a favorable and compatible cathode electrolyte interface (CEI) layer. The synergetic LTO‐CEI network can change the thermodynamic behavior of the PF6? intercalation process and maintain the structural integrity of the graphite cathode, as a “Great Wall” to protect the cathode from structural collapse and electrolyte decomposition. Such LTO‐CEI reinforced cathode exhibits a prolonged cyclability with 85.1% capacity retention after 2000 cycles even at cut‐off potential of 5.4 V versus Li+/Li. Moreover, the LTO‐modified MCMB (+)//prelithiated MCMB (?) full cell exhibits a high energy density of ≈200 Wh kg?1, remarkably enhanced cyclability with 93.5% capacity retention after 1000 cycles. Undoubtedly, this work offers in‐depth insight into interface chemistry, which can arouse new originality to boost the development of DIBs.  相似文献   
979.
The aim of this study was to determine the effect of a polyphenol product (PP) (Proviox) and vitamin E on the antioxidant status and meat quality of broiler chickens exposed to high temperature. The experimental materials comprised 120 ROSS 308 broilers (6 treatments, 10 replications, 2 birds per replication). Dietary supplementation with vitamin E and PP was applied in the following experimental design: group I (negative control) – without supplementation; group II (positive control) – without supplementation; group III – supplementation with 100 mg vitamin E/kg; group IV – 200 mg vitamin E/kg; group V – 100 mg vitamin E/kg and 100 mg PP/kg; group VI – 200 mg PP/kg. In groups II–VI, broiler chickens aged 21–35 d were exposed to increased temperature (34°C for 10 h daily). In chickens exposed to high temperature, dietary supplementation with antioxidants, mostly PP, improved growth performance parameters, including body weight, body weight gain and feed intake until 28 d of age. Vitamin E added to broiler chicken diets at 200 mg/kg and vitamin E combined with PP was most effective in improving the total antioxidant status of birds, enhancing blood antioxidant enzyme activities and increasing vitamin E concentrations in the liver and breast muscles. Broilers fed diets supplemented with 200 mg/kg of vitamin E alone and vitamin E in combination with PP were characterised by a higher percentage content of breast muscles in the carcass. Dietary supplementation with antioxidants improved the water-holding capacity of meat, reduced natural drip loss and increased the crude ash content of meat. The breast muscles of chickens fed diets supplemented with PP had a lower contribution of yellowness. The breast muscles of chickens receiving diets with 100 mg vitamin E/kg(group III) and diets supplemented with PP (groups V and VI) were characterised by the highest concentrations of polyunsaturated fatty acids. The PP can be a valuable component of diets for broiler chickens exposed to high temperature.  相似文献   
980.
Marine mammals are important models for studying convergent evolution and aquatic adaption, and thus reference genomes of marine mammals can provide evolutionary insights. Here, we present the first chromosome‐level marine mammal genome assembly based on the data generated by the BGISEQ‐500 platform, for a stranded female sperm whale (Physeter macrocephalus). Using this reference genome, we performed chromosome evolution analysis of the sperm whale, including constructing ancestral chromosomes, identifying chromosome rearrangement events and comparing with cattle chromosomes, which provides a resource for exploring marine mammal adaptation and speciation. We detected a high proportion of long interspersed nuclear elements and expanded gene families, and contraction of major histocompatibility complex region genes which were specific to sperm whale. Using comparisons with sheep and cattle, we analysed positively selected genes to identify gene pathways that may be related to adaptation to the marine environment. Further, we identified possible convergent evolution in aquatic mammals by testing for positively selected genes across three orders of marine mammals. In addition, we used publicly available resequencing data to confirm a rapid decline in global population size in the Pliocene to Pleistocene transition. This study sheds light on the chromosome evolution and genetic mechanisms underpinning sperm whale adaptations, providing valuable resources for future comparative genomics.  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