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   555334篇
  免费   8252篇
  国内免费   1808篇
电工技术   9989篇
综合类   2702篇
化学工业   80699篇
金属工艺   23535篇
机械仪表   20437篇
建筑科学   15166篇
矿业工程   2137篇
能源动力   13102篇
轻工业   48345篇
水利工程   5294篇
石油天然气   3700篇
武器工业   13篇
无线电   69759篇
一般工业技术   108411篇
冶金工业   83527篇
原子能技术   6939篇
自动化技术   71639篇
  2021年   3116篇
  2020年   2111篇
  2019年   2761篇
  2018年   23713篇
  2017年   23458篇
  2016年   16803篇
  2015年   4764篇
  2014年   6452篇
  2013年   20364篇
  2012年   15687篇
  2011年   30508篇
  2010年   25504篇
  2009年   24151篇
  2008年   25589篇
  2007年   28356篇
  2006年   13418篇
  2005年   16684篇
  2004年   14369篇
  2003年   13817篇
  2002年   12105篇
  2001年   11422篇
  2000年   10671篇
  1999年   10988篇
  1998年   24209篇
  1997年   17750篇
  1996年   14084篇
  1995年   11132篇
  1994年   9915篇
  1993年   9492篇
  1992年   7236篇
  1991年   6889篇
  1990年   6491篇
  1989年   6195篇
  1988年   5930篇
  1987年   4956篇
  1986年   4910篇
  1985年   6059篇
  1984年   5674篇
  1983年   4893篇
  1982年   4569篇
  1981年   4523篇
  1980年   4263篇
  1979年   4136篇
  1978年   3822篇
  1977年   4614篇
  1976年   6145篇
  1975年   3147篇
  1974年   3047篇
  1973年   2915篇
  1972年   2345篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Calmodulin (CaM) is an important intracellular protein that binds Ca2+ and functions as a critical second messenger involved in numerous biological activities through extensive interactions with proteins and peptides. CaM’s ability to adapt to binding targets with different structures is related to the flexible central helix separating the N- and C-terminal lobes, which allows for conformational changes between extended and collapsed forms of the protein. CaM-binding targets are most often identified using prediction algorithms that utilize sequence and structural data to predict regions of peptides and proteins that can interact with CaM. In this review, we provide an overview of different CaM-binding proteins, the motifs through which they interact with CaM, and shared properties that make them good binding partners for CaM. Additionally, we discuss the historical and current methods for predicting CaM binding, and the similarities and differences between these methods and their relative success at prediction. As new CaM-binding proteins are identified and classified, we will gain a broader understanding of the biological processes regulated through changes in Ca2+ concentration through interactions with CaM.  相似文献   
22.
Grass pea (Lathyrus sativus) is a leguminous plant of outstanding tolerance to abiotic stress. The aim of the presented study was to describe the mechanism of grass pea (Lathyrus sativus L.) photosynthetic apparatus acclimatisation strategies to salinity stress. The seedlings were cultivated in a hydroponic system in media containing various concentrations of NaCl (0, 50, and 100 mM), imitating none, moderate, and severe salinity, respectively, for three weeks. In order to characterise the function and structure of the photosynthetic apparatus, Chl a fluorescence, gas exchange measurements, proteome analysis, and Fourier-transform infrared spectroscopy (FT-IR) analysis were done inter alia. Significant differences in the response of the leaf and stem photosynthetic apparatus to severe salt stress were observed. Leaves became the place of harmful ion (Na+) accumulation, and the efficiency of their carboxylation decreased sharply. In turn, in stems, the reconstruction of the photosynthetic apparatus (antenna and photosystem complexes) activated alternative electron transport pathways, leading to effective ATP synthesis, which is required for the efficient translocation of Na+ to leaves. These changes enabled efficient stem carboxylation and made them the main source of assimilates. The observed changes indicate the high plasticity of grass pea photosynthetic apparatus, providing an effective mechanism of tolerance to salinity stress.  相似文献   
23.
Periodontitis is a chronic complex inflammatory disease associated with a destructive host immune response to microbial dysbiosis, leading to irreversible loss of tooth-supporting tissues. Regeneration of functional periodontal soft (periodontal ligament and gingiva) and hard tissue components (cementum and alveolar bone) to replace lost tissues is the ultimate goal of periodontal treatment, but clinically predictable treatments are lacking. Similarly, the identification of biomarkers that can be used to accurately diagnose periodontitis activity is lacking. A relatively novel category of molecules found in oral tissue, circular RNAs (circRNAs) are single-stranded endogenous, long, non-coding RNA molecules, with covalently circular-closed structures without a 5’ cap and a 3’ tail via non-classic backsplicing. Emerging research indicates that circRNAs are tissue and disease-specific expressed and have crucial regulatory functions in various diseases. CircRNAs can function as microRNA or RNA binding sites or can regulate mRNA. In this review, we explore the biogenesis and function of circRNAs in the context of the emerging role of circRNAs in periodontitis pathogenesis and the differentiation of periodontal cells. CircMAP3K11, circCDK8, circCDR1as, circ_0062491, and circ_0095812 are associated with pathological periodontitis tissues. Furthermore, circRNAs are expressed in periodontal cells in a cell-specific manner. They can function as microRNA sponges and can form circRNA–miRNA–mRNA networks during osteogenic differentiation for periodontal-tissue (or dental pulp)-derived progenitor cells.  相似文献   
24.
Wireless Networks - Orthogonal frequency-division multiplexing with random multiple access (OFDRMA) is discussed for down-link communications, whereby a single base-station transmits information...  相似文献   
25.
Cybernetics and Systems Analysis - The authors propose a method for solving the problem of approach of controlled objects in dynamic game problems with a terminal payoff function. The method is...  相似文献   
26.
Extracellular vesicles (EVs) are membranous structures, which are secreted by almost every cell type analyzed so far. In addition to their importance for cell-cell communication under physiological conditions, EVs are also released during pathogenesis and mechanistically contribute to this process. Here we summarize their functional relevance in asthma, one of the most common chronic non-communicable diseases. Asthma is a complex persistent inflammatory disorder of the airways characterized by reversible airflow obstruction and, from a long-term perspective, airway remodeling. Overall, mechanistic studies summarized here indicate the importance of different subtypes of EVs and their variable cargoes in the functioning of the pathways underlying asthma, and show some interesting potential for the development of future therapeutic interventions. Association studies in turn demonstrate a good diagnostic potential of EVs in asthma.  相似文献   
27.
In this study, the hydraulic reactivity and cement formation of baghdadite (Ca3ZrSi2O9) was investigated. The material was synthesized by sintering a mixture of CaCO3, SiO2, and ZrO2 and then mechanically activated using a planetary mill. This leads to a decrease in particle and crystallite size and a partial amorphization of baghdadite as shown by X-ray powder diffraction (XRD) and laser diffraction measurements. Baghdadite cements were formed by the addition of water at a powder to liquid ratio of 2.0 g/ml. Maximum compressive strengths were found to be ~2 MPa after 3-day setting for a 24-h ground material. Inductively coupled plasma mass spectrometry (ICP-MS) measurements showed an incongruent dissolution profile of set cements with a preferred dissolution of calcium and only marginal release of zirconium ions. Cement formation occurs under alkaline conditions, whereas the unground raw powder leads to a pH of 11.9 during setting, while prolonged grinding increased pH values to approximately 12.3.  相似文献   
28.
Multimedia Tools and Applications - The design of robots capable of operating autonomously in changing and unstructured environments, requires using complex software architectures in which,...  相似文献   
29.
Journal of Materials Science - For transformers and inductors to meet the world’s growing demand for electrical power, more efficient soft magnetic materials with high saturation magnetic...  相似文献   
30.
The main drawback of bioglasses is their restricted use in load bearing applications and the consequent need to develop stronger glassy materials. This has led to the consideration of oxynitride glasses for numerous biomedical applications. This paper investigated two different types of glasses at a constant cationic ratio, with and without nitrogen (a N containing and a N-free glass composition) to better understand the effect of N on the biological properties of glasses. The results revealed that the addition of N increased the glass transition temperature, isoelectric point (IEP) and slightly increased wettability. Moreover, compared to N including glass, N-free glass exhibited better anti-bacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), two key bacteria that infect implants. In summary, these in vitro results indicated that amine functional groups existing in N containing glasses which are missing in N-free glasses, caused a slight difference in wetting behavior and a more obvious change in isoelectric point and in bacterial response. N-free glasses exhibited better inhibitory results both against E. coli and S. aureus compared to N including glass suggesting that oxygen rich glasses should be further studied for their novel antibacterial properties.  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