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   16621篇
  免费   448篇
  国内免费   135篇
化学   11432篇
晶体学   134篇
力学   518篇
数学   2566篇
物理学   2554篇
  2022年   99篇
  2021年   152篇
  2020年   222篇
  2019年   205篇
  2018年   184篇
  2017年   148篇
  2016年   320篇
  2015年   274篇
  2014年   319篇
  2013年   785篇
  2012年   775篇
  2011年   1056篇
  2010年   490篇
  2009年   441篇
  2008年   851篇
  2007年   891篇
  2006年   939篇
  2005年   861篇
  2004年   791篇
  2003年   722篇
  2002年   688篇
  2001年   237篇
  2000年   205篇
  1999年   161篇
  1998年   195篇
  1997年   203篇
  1996年   238篇
  1995年   181篇
  1994年   187篇
  1993年   150篇
  1992年   154篇
  1991年   160篇
  1990年   136篇
  1989年   114篇
  1988年   152篇
  1987年   148篇
  1986年   111篇
  1985年   217篇
  1984年   237篇
  1983年   172篇
  1982年   248篇
  1981年   226篇
  1980年   230篇
  1979年   210篇
  1978年   203篇
  1977年   186篇
  1976年   185篇
  1975年   153篇
  1974年   163篇
  1973年   158篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Applied Biochemistry and Biotechnology - Both oxidative and reductive enzymes can be utilized to enhance coal solubilization in aqueous and organic media. Aerobic solubilization was carried out...  相似文献   
122.
Lithiation of 8-chlorodibenz[b,f][1,4]oxazepine-10-tert-butylcarbamate ( 1 ) is described. Electrophilic substitution of the resulting N-Boc dibenzoxazepine α- lithioamine 2 with ketones, aldehydes, nitriles, iso-cyanates and imines, followed by an in-situ cyclization, gave fused carbamates 5–26 , fused 2H-imidazol-2-ones 27–29 , fused hydantoins 30–32 , and fused ureas 33–35 , respectively, in 11–66% yield.  相似文献   
123.
Goal programming, and in particular lexicographic goal programming (i.e. goal programming within a so-called ‘pre-emptive priority’ structure or having non-Archimedean weights), has become one of the most widely used of the approaches for multi-objective mathematical programming. While also applicable to non-linear or integer models, most of the literature has considered the lexicographic linear goal-programming model and its solution via primal simplex-based methods. However, in many cases, enhanced efficiency (and significant additional flexibility) may be gained via an investigation of the dual of this problem. In this paper we consider an algorithm for solving such a dual and also indicate how it may be implemented on conventional (i.e. single objective) simplex software.  相似文献   
124.
THE ROLE OF SOLAR ULTRAVIOLET RADIATION IN 'NATURAL' WATER PURIFICATION   总被引:2,自引:0,他引:2  
Abstract— The concentration of Escherichia coli in the input and output of a tertiary wastewater system (4 lagoons) has been monitored over an 11 month period. The integrated flux of biologically active solar ultraviolet (UV) radiation was measured during this period. By also determining (1) the effective temperature in the system, (2) the growth rate of E. coli at the effective temperature, (3) the penetration of the solar UV into the lagoons, (4) the dose-response relation for killing of E. coli by UV and (5) the retention time of water in the system, it is possible to compare the 'die off' expected from solar UV exposure to the actual 'die off' observed for different batches of water.
The observed killing of E. coli was quite close to the values calculated, considering the numerous factors involved. Solar UV light would thus seem to be a very important factor in the natural purification of water. Because each successful species must possess characteristics (physiological or behavioral) which provide adequate resistance to solar UV, the ecological role of solar UV radiation has not been widely appreciated.  相似文献   
125.
126.
Racemic isopropylidene (1-η5-cyclopentadienyl)(1-η5-indenyl) dichlorozirconium and the 3-methylindenyl derivative have been synthesized and characterized. These precursors activated with methylaluminoxane produce poly(propylene) with hemiisotactic microstructures. © 1994 John Wiley & Sons, Inc.  相似文献   
127.
128.
High density (98% of theoretical) was achieved at 900°C for an all-alkoxide derived bulk celsian gel prior to crystallisation. TEM indicated that crystallisation was dominated by volume nucleation and growth of hexagonal disc shaped hexacelsian. Kinetic studies using SEM indicate a maximum nucleation rate at 980°C and an activation energy for crystal growth of 566 KJ/mol. The high rates of densification and of nucleation are attributed to the high OH content in the gel-derived glass.  相似文献   
129.
The cucurbit[n]uril (CB[n]) family of macrocycles occupies a prominent role in molecular recognition and self-assembly studies despite the current inability to access specific cucurbit[n]uril homologues, derivatives, and analogues by straightforward tailor-made synthetic procedures. In this paper, we explore an approach that circumvents the challenges posed by the tailor-made synthesis of macrocyclic CB[n] by preparing 1, which functions as an acyclic CB[6] congener. The o-xylylene connections to the glycoluril rings preorganize 1 into the (a,a,a,a)-1 conformation required for binding and reduce its tendency to undergo self-association. We surveyed the binding properties of 1 toward 16 amines (K(a) 相似文献   
130.
Redox proteins attached to surfaces designed for biocatalysis hold promise for future clean synthetic routes. It is advantageous for these biocatalysts to operate in low-toxicity fluids with a high capacity to dissolve reactants. Here we report cross-linked films of myoglobin (Mb) and poly(L-lysine) (PLL) chemically attached to oxidized carbon cloth cathodes that in microemulsions feature the protein in a water-rich film environment with reactant in an oil-rich environment. These cross-linked Mb/PLL films were the most stable in microemuslions and had the largest turnover rates for epoxidation of styrene compared to lightly cross-linked or uncross-linked Mb/poly(styrene sulfonate) films. Up to 40-fold larger turnover rates were found in bicontinuous microemulsions compared to oil-in-water microemulsions and micelles. Enhanced turnover rates are correlated with up to 10-fold faster mass transport of solutes in the oil phases of the bicontinuous fluids.  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