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   252868篇
  免费   2787篇
  国内免费   872篇
电工技术   4815篇
综合类   194篇
化学工业   36117篇
金属工艺   9509篇
机械仪表   7548篇
建筑科学   6070篇
矿业工程   880篇
能源动力   7225篇
轻工业   21092篇
水利工程   2202篇
石油天然气   3230篇
武器工业   15篇
无线电   34280篇
一般工业技术   49975篇
冶金工业   48534篇
原子能技术   5085篇
自动化技术   19756篇
  2021年   2172篇
  2020年   1574篇
  2019年   1916篇
  2018年   3271篇
  2017年   3309篇
  2016年   3399篇
  2015年   2219篇
  2014年   3957篇
  2013年   11476篇
  2012年   6236篇
  2011年   8572篇
  2010年   6906篇
  2009年   7883篇
  2008年   8402篇
  2007年   8294篇
  2006年   7363篇
  2005年   6497篇
  2004年   6282篇
  2003年   6553篇
  2002年   6035篇
  2001年   6512篇
  2000年   5973篇
  1999年   6388篇
  1998年   16480篇
  1997年   11408篇
  1996年   8792篇
  1995年   6606篇
  1994年   5901篇
  1993年   5842篇
  1992年   4130篇
  1991年   4020篇
  1990年   3856篇
  1989年   3648篇
  1988年   3558篇
  1987年   3017篇
  1986年   2941篇
  1985年   3206篇
  1984年   2926篇
  1983年   2828篇
  1982年   2636篇
  1981年   2542篇
  1980年   2461篇
  1979年   2305篇
  1978年   2107篇
  1977年   2604篇
  1976年   3563篇
  1975年   1764篇
  1974年   1729篇
  1973年   1665篇
  1972年   1465篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Translated from Atomnaya Énergiya, Vol. 67, No. 2, pp. 97–100, August, 1989.  相似文献   
52.
53.
54.
Copolyesters containing poly(ethylene terephthalate) and poly(hexamethylene terephthalate) (PHT) were prepared by a melt condensation reaction. The copolymers were characterised by infrared spectroscopy and intrinsic viscosity measurements. The density of the copolyesters decreased with increasing percentage of PHT segments in the backbone. Glass transition temperatures (Tg). melting points (Tm) and crystallisation temperatures (Tc) were determined by differential scanning calorimetry. An increase in the percentage of PHT resulted in decrease in Tg, Tm and Tc. The as-prepared copolyesters were crystalline in nature and no exotherm indicative of cold crystallisation was observed. The relative thermal stability of the polymers was evaluated by dynamic thermogravimetry in a nitrogen atmosphere. An increase in percentage of PHT resulted in a decrease in initial decomposition temperature. The rate of crystallisation of the copolymers was studied by small angle light scattering. An increase in percentage of PHT resulted in an increase in the rate of crystallisation.  相似文献   
55.
This paper reports on the first demonstration of a half-bridge power inverter constructed from silicon carbide gate turn-off thyristors (GTOs) operated in the conventional GTO mode. This circuit was characterized with input bus voltages of up to 600 VDC and 2 A (peak current density of 540 A/cm2) with resistive loads using a pulse-width modulated switching frequency of 2 kHz. We discuss the implications of the thyristor's electrical characteristics and the circuit topology on the overall operation of the half-bridge circuit. This work has determined the conservative critical rate of rise value of the off-state voltage to be 200 V/μs in these devices  相似文献   
56.
A mixed K-value model allows existing computer programs for the simulation of vapor-liquid equilibrium stages to be used for three-phase vapor-liquid-liquid systems. The mixed K-value model has been implemented with a minimum effort into the Badger General Material Balance flow-sheeting program. Good convergence has been obtained with the mixed K-value model for a number of systems. A comparison is made with published literature [1, 2], showing excellent results with this generally applicable and easy-to-use method.  相似文献   
57.
58.
59.
This paper deals with accurate recipe implementation for abnormal condition management in a batch process using a case‐based reasoning (CBR) approach. A set of new problems can be solved by reusing proven process solutions. The proposed system integrates quantitative and qualitative parameters for adaptation of cases. A novel methodology to generate accurate recipes and to adapt to the processes is introduced during normal and abnormal conditions. In particular, the differences between current conditions and the references (recipes) should be managed to prevent any hazardous conditions arising. The processes are evaluated using their similarity to the past cases. This intelligent approach distinguishes plausible cases, generates accurate recipes, and adapts to new situations. The aim is to use the offline historical process data and safety related information in order to propose changes and adjustments in the processes.  相似文献   
60.
A new supported liquid membrane (SLM) system was prepared for the selective transport of bismuth ions from the aqueous feed into the aqueous permeate phase. The support of the SLM was a thin porous polypropylene or polyvinylidene fluoride membrane impregnated with diisooctyldithiophosphinic acid (Cyanex 301) as mobile carrier in 4‐chloroacetophenon as organic solvent. Cyanex 301 acts as a highly selective carrier for the uphill transport of bismuth ions through the SLM. In the presence of HNO3 as a metal ion acceptor in the strip solution, the transport of bismuth ions into the strip side reached 70 % of the initial feed concentration after 3.5 hours. The selectivity and efficiency of bismuth transport from aqueous solutions containing different mixtures of cations were investigated. In the presence of P2O72– ions as suitable masking agent in the feed solution, the interfering effects of other cations were completely eliminated. The selective transport of bismuth through SLM is superior to liquid‐liquid extraction or through bulk liquid membranes. This is due to the high efficiency. The SLM reduces the solvent requirements, combines extraction and stripping operations in a single process and allows the use of highly selective extractants. The system may be applied to samples containing very low bismuth concentrations.  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