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   4021篇
  免费   4篇
化学工业   102篇
金属工艺   3篇
机械仪表   5篇
建筑科学   1篇
能源动力   1篇
轻工业   14篇
石油天然气   2篇
无线电   13篇
一般工业技术   30篇
冶金工业   3833篇
原子能技术   3篇
自动化技术   18篇
  2024年   4篇
  2023年   5篇
  2022年   2篇
  2021年   3篇
  2020年   2篇
  2019年   6篇
  2017年   5篇
  2016年   2篇
  2014年   5篇
  2013年   8篇
  2012年   8篇
  2011年   3篇
  2010年   4篇
  2009年   6篇
  2008年   8篇
  2007年   8篇
  2006年   10篇
  2005年   11篇
  2004年   7篇
  2003年   12篇
  2002年   14篇
  2001年   5篇
  2000年   3篇
  1999年   119篇
  1998年   1264篇
  1997年   733篇
  1996年   481篇
  1995年   245篇
  1994年   173篇
  1993年   224篇
  1992年   21篇
  1991年   46篇
  1990年   45篇
  1989年   42篇
  1988年   50篇
  1987年   33篇
  1986年   36篇
  1985年   32篇
  1983年   6篇
  1982年   11篇
  1981年   13篇
  1980年   29篇
  1979年   4篇
  1978年   6篇
  1977年   76篇
  1976年   180篇
  1975年   3篇
  1974年   2篇
  1964年   1篇
  1955年   4篇
排序方式: 共有4025条查询结果,搜索用时 15 毫秒
81.
    
Cellulose modification often employs chemical processes to tailor its properties and functionalities to fit the demands of a wide range of applications, maximizing its potential as a versatile and sustainable material. From both synthetic and environmental standpoints, one of the ultimate goals is to achieve significant modifications to enhance the end properties of the cellulose while minimizing the number of modified building blocks. The current study demonstrates that a synthetic glucose derivative, 6-deoxy-6-fluoro-glucose (6F-Glc), fed into the fertilized cotton ovules, resulted in the accumulation of fluorine inside the cotton fibers with no apparent alteration to their morphology or development. These fibers exhibited a degree of substitution of 0.006, which is 170 times lower than that reported for chemical methods for cellulose modification. However, the physical characterization of the modified fibers showed a surprisingly large impact of this low-level modification on the cellulose structure (e.g., hydrogen bonding network rearrangement) and a modest increase in the mechanical properties of the fibers. The obtained results exemplify the use of biological systems to introduce low quantities of new functionalities while maximizing the impact on fiber properties.  相似文献   
82.
The electrochemical characterization related to the capacity fading of Li1.1V3O8 nano grains has been investigated. It reveals that capacity loss is associated to the use of liquid electrolyte and that it mainly stems from the active material irrespective of the nature of the liquid electrolyte, the water content, synthesis routes and the positive electrode chemistry. SEM and in operando XRD experiments allow elucidating undergoing physical processes. A film forms from electrolyte decomposition on the surface of the electrode and progressively propagates on cycling through the electrode thickness. Embedded grains become either less or none electrochemically reactive. In the former case, embedded grains give rise to strongly polarized and energetically less favored processes. In the latter case the capacity is not accessible anymore within the potential range resulting in capacity fading.  相似文献   
83.
The effect of spherulitic crystallization on the elastic moduli and fracture toughness of a barium aluminum silicate glass was investigated. The crystallization process results in Ba2Si3O8 phase and is initiated from Ba rich nuclei. Nucleation is optimal in the 690-720 °C interval. Young’s modulus is increased by 12.5% when the glass-ceramic conversion is nearly complete. Nevertheless, as the size and the volume fraction of crystals are increased, some microcracking shows up upon cooling from the crystallization temperature. An optimal improvement of the fracture toughness (SEPB method) by 27 % is observed for a 49 % volume fraction of 5 to 10 μm large spherulites.  相似文献   
84.
The hydrodynamics generated by the straight version of the Maxblend® impeller with Newtonian fluids in a baffled stirred vessel under the transitional and turbulent regime has been experimentally characterized by means of the particle image velocimetry technique. The flow fields obtained with the Maxblend were compared with those obtained with a double stage classical pitched blade turbine (PBT) and a double Ekato Intermig® impellers under the same specific power draw. It is shown that these open impellers induce complex local flows in the radial and axial direction, with an intensity decreasing away from the blades. By contrast, the Maxblend impeller generates a more regular circulation pattern, with efficient top-to-bottom pumping.  相似文献   
85.
    
Due to an increasing demand for oxide glasses with a better mechanical performance, there is a need to improve our understanding of the composition-structure-mechanical property relations in these brittle materials. At present, some properties such as Young's modulus can to a large extent be predicted based on the chemical composition, while others—in particular fracture-related properties—are typically optimized based on a trial-and-error approach. In this work, we study the mechanical properties of a series of 20 glasses in the quartenary Na2O–Al2O3–B2O3–SiO2 system with fixed soda content, thus accessing different structural domains. Ultrasonic echography is used to determine the elastic moduli and Poisson's ratio, while Vickers indentation is used to determine hardness. Furthermore, the single-edge precracked beam method is used to estimate the fracture toughness (KIc) for some compositions of interest. The compositional evolutions of Vickers hardness and Young's modulus are in good agreement with those predicted from models based on bond constraint density and strength. Although there is a larger deviation, the overall compositional trend in KIc can also be predicted by a model based on the strength of the bonds assumed to be involved in the fracture process.  相似文献   
86.
Bacterial strains of Aeromonas salmonicida included in the recognized subsp. acromogenes, subsp. masoucida, and subsp. smithia in addition to the large number of strains not included in any of the described subspecies are referred to as atypical A. salmonicida. The atypical strains form a very heterogeneous group with respect to biochemical characteristics, growth conditions, and production of extracellular proteasess. Consequently, the present taxonomy of the species A. salmonicida is rather ambiguous. Atypical A. salmonicida has been isolated from a wide range of cultivated and wild fish species, non-salmonids as well as salmonids, inhabiting fresh water, brackish water and marine environments in northern and central Europe, South Africa, North America, Japan and Australia. In non-salmonid fish species, infections with atypical strains often manifest themselves as superficial skin ulcerations. The best known diseases associated with atypical A. salmonicida are carp Cyprinus carpio erythrodermatitis, goldfish Carassius auratus ulcer disease, and ulcer disease of flounder Platichthys flesus, but atypical strains are apparently involved in more disease outbreaks than previously suspected. Macroscopical and microscopical studies of ulcerated fish indicate internal organs are infrequently invaded by atypical A. salmonicida. This view is supported by the fact that atypical strains are irregularly isolated from visceral organs of ulcerated fish. High mortality caused by atypical A. salmonicida has been observed in populations of wild non-salmonids and farmed salmonids, although the association between the mortality in the wild fish stocks and atypical A. salmonicida has not always been properly assessed. In injection experiments the pathogenicity of the atypical strains examined showed large variation. An extacellular A-layer has been detected in different atypical strains, but virulence mechanisms different from those described for (typical) A. salmonicida subsp. salmonicida, for example an extracellular metallo-protease and a different iron utilization mechanism, have been described. Limited information is available about the ecology, spread and survival of atypical strains in water. The commonly used therapeutic methods for the control of diseases in farmed fish caused by atypical A. salmonicida are generally effective against the atypical strains. Resistance to different antibiotics and transferable plasmid encoding multiple drug resistance have been observed in atpical A. salmonicida. Studies aimed at producing a vaccine against atypical strains are in progress.  相似文献   
87.
DK Rajan  DL Croteau  SG Sturza  ML Harvill  CJ Mehall 《Canadian Metallurgical Quarterly》1998,18(5):1155-67; discussion 1167-70
Access to the central venous circulation for hemodialysis has traditionally been achieved via the subclavian or jugular venous routes. With ongoing improvements in medical management, many hemodialysis recipients develop exhaustion of these routes and require alternative means of central venous access. Inferior vena caval (IVC) catheters have been placed with a percutaneous translumbar approach to allow central venous access for chemotherapy, harvesting of stem cells, and total parenteral nutrition. Translumbar placement of IVC catheters has become accepted by some as a useful and reliable alternative in patients who require long-term hemodialysis but have exhausted traditional access sites. IVC catheters have been placed in patients with IVC filters, and IVC filters have been placed in patients with IVC catheters. Complications include those associated with central venous catheters, for example, sepsis, fibrin sheaths, and thrombosis. A complication specific to placement of IVC hemodialysis catheters is migration of the catheter into the subcutaneous soft tissues, retroperitoneum, or iliac veins. Translumbar placement of IVC catheters is performed only in patients considered to have few or no other medical options and is not intended as a primary means of central venous access.  相似文献   
88.
89.
A wide range of clinical requirements exists among PEM fellowship programs. Programs are equally split concerning the question of whether fellows should work with supervision or independently in the first year; a significant number of fellowship programs require continued supervision of fellows in subsequent years. Orientation for first year fellows and requirements for completion of PALS, advanced pediatric life support (APLS), ACLS, or ATLS courses prior to their first independent shift varied greatly. In particular, a minority of programs required ATLS completion even though a majority of overall fellowship programs operate in a hospital designated as a Level 1 Trauma Center. Programs in which first-year fellows worked independently had fewer attendings and were less likely to provide 24-hour coverage. Fellows appear to work a similar or less demanding schedule than PEM attendings in most fellowship programs, and most fellowship directors feel that their fellows should continue with their current schedule.  相似文献   
90.
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