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   271183篇
  免费   5648篇
  国内免费   1805篇
电工技术   5476篇
技术理论   5篇
综合类   1152篇
化学工业   46268篇
金属工艺   11122篇
机械仪表   8013篇
建筑科学   7016篇
矿业工程   1672篇
能源动力   5886篇
轻工业   28624篇
水利工程   2981篇
石油天然气   6017篇
武器工业   243篇
无线电   28188篇
一般工业技术   50683篇
冶金工业   47621篇
原子能技术   5642篇
自动化技术   22027篇
  2021年   3416篇
  2020年   2036篇
  2019年   2489篇
  2018年   3807篇
  2017年   3796篇
  2016年   4164篇
  2015年   3199篇
  2014年   5096篇
  2013年   12128篇
  2012年   8435篇
  2011年   11038篇
  2010年   8764篇
  2009年   9268篇
  2008年   10020篇
  2007年   10114篇
  2006年   8837篇
  2005年   7689篇
  2004年   6909篇
  2003年   6435篇
  2002年   6422篇
  2001年   6449篇
  2000年   5991篇
  1999年   5986篇
  1998年   12950篇
  1997年   9631篇
  1996年   7370篇
  1995年   5592篇
  1994年   5128篇
  1993年   5001篇
  1992年   4001篇
  1991年   3793篇
  1990年   3858篇
  1989年   3799篇
  1988年   3561篇
  1987年   3047篇
  1986年   3085篇
  1985年   3437篇
  1984年   3345篇
  1983年   3092篇
  1982年   2711篇
  1981年   2917篇
  1980年   2661篇
  1979年   2857篇
  1978年   2754篇
  1977年   2854篇
  1976年   3699篇
  1975年   2466篇
  1974年   2291篇
  1973年   2323篇
  1972年   1982篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
The objective of this study was to characterise the nutritional potential of leaves and identify a diversity centre with low cyanide and high nutrient content among 178 Latin American cassava genotypes. This field-based collection represents the seven diversity centres, held at The International Center for Tropical Agriculture (CIAT Palmira, Colombia) by the Cassava Program. The cyanide, all-trans-β-carotene and lutein concentrations in cassava leaves ranged from 346 to 7484 ppm dry basis (db), from 174–547 μg g−1 db and 15–181 μg g−1 db, respectively. Cassava leaves also showed significant levels of essential amino acids leucine, lysine, phenylalanine, valine and threonine, and average total protein content of 26.24 g 100 g−1 db. Among seven diversity centres, South American rainforest group showed low cyanide and high carotene content in leaves. In addition, VEN77 and PAN51 genotypes stood out for having low cyanide in leaves and roots and high carotene in leaves. This genetic diversity can be used to select high potential progenitors for breeding purposes.  相似文献   
92.
The phase shift characteristics reflect the state change of electromagnetic wave in plasma sheath and can be used to reveal deeply the action mechanism between electromagnetic wave and plasma sheath. In this paper, the phase shift characteristics of electromagnetic wave propagation in plasma were investigated. Firstly, the impact factors of phase shift including electron density,collision frequency and incident frequency were discussed. Then, the plasma with different electron density distribution profiles were employed to investigate the influence on the phase shift characteristics. In a real case, the plasma sheath around the hypersonic vehicle will affect and even break down the communication. Based on the hypersonic vehicle model, we studied the electromagnetic wave phase shift under different flight altitude, speed, and attack angle. The results indicate that the phase shift is inversely proportional to the flight altitude and positively proportional to the flight speed and attack angle. Our work provides a theoretical guidance for the further research of phase shift characteristics and parameters inversion in plasma.  相似文献   
93.
One-third of all proteins are estimated to require metals for structural stability and/or catalytic activity. Desthiobiotin probes containing metal binding groups can be used to capture metalloproteins with exposed active-site metals under mild conditions so as to prevent changes in metallation state. The proof-of-concept was demonstrated with carbonic anhydrase (CA), an open active site, Zn2+-containing protein. CA was targeted by using sulfonamide derivatives. Linkers of various lengths and structures were screened to determine the optimal structure for capture of the native protein. The optimized probes could selectively pull down CA from red blood cell lysate and other protein mixtures. Pull-down of differently metallated CAs was also investigated.  相似文献   
94.
In the present work blends of polystyrene (PS) with sepiolites have been produced using a melt extrusion process. The dispersion degree of the sepiolites in the PS has been analyzed by dynamic shear rheology and X-ray micro-computed tomography. Sepiolites treated with quaternary ammonium salts (O-QASEP) are better dispersed in the PS matrix than natural sepiolites (N-SEP) or sepiolites organo-modified with silane groups (O-SGSEP). A percolated network is obtained when using 6.0 wt% of O-QASEP, 8.0 wt% of N-SEP and 10.0 wt% of O-SGSEP. It has been shown that multiple extrusion processes have a negative effect on the polymer architecture. They produce a reduction in the length of the polymeric chains, and they do not lead to a better dispersion of the particles in the polymer matrix. Foams have been produced using a gas dissolution foaming process, where a strong effect of the dispersion degree on the cellular structure of the different foams was found. The effects on the cellular structure obtained by using different types of sepiolites, different contents of sepiolites and different extrusion conditions have been analyzed. The foams produced with the formulations containing O-QASEP present the lowest cell size and the most homogeneous cellular structures.  相似文献   
95.
Russian Microelectronics - The recently developed nanomaterials and their production technologies as intellectual property objects (IPOs) are considered. The role of the informational-analytical...  相似文献   
96.
Protection of Metals and Physical Chemistry of Surfaces - In this work, a universal sorbent has been obtained that works according to the mechanisms of chemisorption and adsorption by impregnating...  相似文献   
97.
The autonomic nervous system (ANS) plays a crucial role both in acute and chronic psychological stress eliciting changes in many local and systemic physiological and biochemical processes. Salivary secretion is also regulated by ANS. In this study, we explored salivary proteome changes produced in thirty-eight University students by a test stress, which simulated an oral exam. Students underwent a relaxation phase followed by the stress test during which an electrocardiogram was recorded. To evaluate the effect of an olfactory stimulus, half of the students were exposed to a pleasant odor diffused in the room throughout the whole session. Saliva samples were collected after the relaxation phase (T0) and the stress test (T1). State anxiety was also evaluated at T0 and T1. Salivary proteins were separated by two-dimensional electrophoresis, and patterns at different times were compared. Spots differentially expressed were trypsin digested and identified by mass spectrometry. Western blot analysis was used to validate proteomic results. Anxiety scores and heart rate changes indicated that the fake exam induced anxiety. Significant changes of α-amylase, polymeric immunoglobulin receptor (PIGR), and immunoglobulin α chain (IGHA) secretion were observed after the stress test was performed in the two conditions. Moreover, the presence of pleasant odor reduced the acute social stress affecting salivary proteome changes. Therefore, saliva proteomic analysis was a useful approach to evaluate the rapid responses associated to an acute stress test also highlighting known biomarkers.  相似文献   
98.
Water Resources Management - Public awareness of water quality concerns has increased since the Flint, MI crisis, but knowledge about water resources management remains low. Consequently,...  相似文献   
99.
Microsystem Technologies - The strategy for analysis of noise generated in the analog circuit is presented here. Further, methodology for optimization of noise to improve the performance of the...  相似文献   
100.
Sarcopenia is the loss of skeletal muscle mass and function with advancing age. It involves both complex genetic and modifiable risk factors, such as lack of exercise, malnutrition and reduced neurological drive. Cognitive decline refers to diminished or impaired mental and/or intellectual functioning. Contracting skeletal muscle is a major source of neurotrophic factors, including brain-derived neurotrophic factor, which regulate synapses in the brain. Furthermore, skeletal muscle activity has important immune and redox effects that modify brain function and reduce muscle catabolism. The identification of common risk factors and underlying mechanisms for sarcopenia and cognition may allow the development of targeted interventions that slow or reverse sarcopenia and also certain forms of cognitive decline. However, the links between cognition and skeletal muscle have not been elucidated fully. This review provides a critical appraisal of the literature on the relationship between skeletal muscle health and cognition. The literature suggests that sarcopenia and cognitive decline share pathophysiological pathways. Ageing plays a role in both skeletal muscle deterioration and cognitive decline. Furthermore, lifestyle risk factors, such as physical inactivity, poor diet and smoking, are common to both disorders, so their potential role in the muscle–brain relationship warrants investigation.  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