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   6869篇
  免费   920篇
  国内免费   696篇
化学   2137篇
晶体学   114篇
力学   968篇
综合类   168篇
数学   2348篇
物理学   2750篇
  2024年   14篇
  2023年   66篇
  2022年   175篇
  2021年   195篇
  2020年   210篇
  2019年   169篇
  2018年   178篇
  2017年   262篇
  2016年   285篇
  2015年   237篇
  2014年   377篇
  2013年   567篇
  2012年   374篇
  2011年   431篇
  2010年   352篇
  2009年   451篇
  2008年   453篇
  2007年   420篇
  2006年   420篇
  2005年   366篇
  2004年   281篇
  2003年   294篇
  2002年   232篇
  2001年   221篇
  2000年   211篇
  1999年   177篇
  1998年   147篇
  1997年   145篇
  1996年   131篇
  1995年   132篇
  1994年   73篇
  1993年   69篇
  1992年   57篇
  1991年   40篇
  1990年   49篇
  1989年   38篇
  1988年   36篇
  1987年   20篇
  1986年   18篇
  1985年   25篇
  1984年   27篇
  1983年   5篇
  1982年   17篇
  1981年   13篇
  1979年   8篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1975年   2篇
  1957年   2篇
排序方式: 共有8485条查询结果,搜索用时 15 毫秒
41.
An input-output processZ = {Z(t), t 0} is said to be-rate stable ifZ(t) = o((t)) for some non-negative function(t). We prove that the processZ is -rate stable under weak conditions that include the assumption that input satisfies a linear burstiness condition and Z is asymptotically average stable. In many cases of interest, the conditions for-rate-stability can be verified from input data. For example, using input information, we establish-rate stability of the workload for multiserver queues, an ATM multiplexer, and-rate stability of queue-length processes for infinite server queues.  相似文献   
42.
We investigate a piecewise linear (area-preserving) mapT describing two coupled baker transformations on two squares, with coupling parameter 0c1. The resulting dynamical system is Kolmogorov for anyc0. For rational values ofc, we construct a generating partition on whichT induces a Markov chain. This Markov structure is used to discuss the decay of correlation functions: exponential decay is found for a class of functions related to the partition. Explicit results are given forc=2–n. The macroscopic analog of our model is a leaking process between two (badly) stirred containers: according to the Markov analysis, the corresponding progress variable decays exponentially, but the rate coefficients characterizing this decay are not those determined from the one-way flux across the cell boundary. The validity of the macroscopic rate law is discussed.  相似文献   
43.
根据Fe(Ⅲ)、Mo(Ⅵ)对H2O2氧化邻氨基酚的催化反应速率不同,结合流动注射停留技术,建立了同时测定Fe(Ⅲ)、Mo(Ⅵ)的速差动力学分光光度法.测定铁和钼的线性范围分别为0.1~2.5μg/mL和0.5~15μg/mL.除W(Ⅵ)的允许量较小外,其它离子不干扰测定.用本法测定了合成样液中铁和钼含量,结果满意.  相似文献   
44.
Rate constants for the reaction of carbonate radical with aniline and some parasubstituted anilines have been determined by the flash photolysis technique. Using σ+ para values the rate constants at pH 8.5 correlate very well with the Hammett equation yielding ρ= − 1. The carbonate radical oxidises aniline giving the anilino radical. The products so formed have been identified through studies under conditions of continuous irradiation.  相似文献   
45.
A new fertilizer of cocrystal type,known as durably efficacious ammonium bicarbonate (DEAB),has been developed by adding a certain amount of dicyandiamide (DCD) as an ammonia-stabilizing agent to ammonium bearbenate(AB)during the process of its production.As compared with AB,DEAB was found to have a reduction of direct volatilization loss by 53%,a fertilizer availability period prolonged from 35-45 to 90-110 d,and an increase in the rate of nitrogen in fertilizer being utilized by 5.9%-10.2%,and a saving of the amount of fertilizer to be ap-phed by 20%-30% for the same level of yield,or an increase of the crop yield by over 10% for the same level of ni-tregen fertilization;in addition,it was found to show usually a function of promoting the crop to early mature.It can be apptied as basal dressing all in one time to soil and thus also used as a labour-saving and crop yield-increasing fertil-izer for is non-mtertillage,plastics film covering and water-saving agriculture  相似文献   
46.
The kinetic curves at infinite temperature for the solid-state reactions of the interface shrinkage type were drawn theoretically by taking account the particle size distribution in the sample mixture. The CRTA curves for the reactions with the particle size distribution can be drawn by utilizing the universal kinetic curves at infinite temperature. The proper kinetic treatment for the CRTA curves with the particle size distribution is discussed in connection with the property of the kinetic equation with respect to the particle size distribution. The present kinetic consideration is taken as a simulation for the reactions with a certain distribution in among the reactant particles, produced preferably by the mass and heat transfer phenomena during the thermoanalytical measurements. The merit of the rate jump method by a single cyclic CRTA curve is also discussed on the basis of the present results.  相似文献   
47.
The kinetics of hydrolysis of 1,2-disubstituted imidazolines in aqueous media was studied (pH 2.0—12.5, T = 25—90 °C) by UV spectroscopy. The hydrolysis products were identified. The introduction of a branched substituent into position 2 of the imidazoline ring increases the hydrolytic stability of the compounds. The effect of the pH on the hydrolysis rate of imidazolines is discussed.  相似文献   
48.
超临界萃取装置的研制及其应用   总被引:5,自引:0,他引:5  
超临界流体萃取是近的上来兴起的一种新的分离技术,报道了将超临界萃取原理与回流技术相结合研制的二氧化碳回流中低压超临界流体萃取设备,解决了关键实验手段,通过应用证明设备可靠,费用低廉,有实用价值。  相似文献   
49.
This paper proposes a procedure for improving the rate of convergence of interior point methods for linear programming. If (x k ) is the sequence generated by an interior point method, the procedure derives an auxiliary sequence ( ). Under the suitable assumptions it is shown that the sequence ( ) converges superlinearly faster to the solution than (x k ). Application of the procedure to the projective and afflne scaling algorithms is discussed and some computational illustration is provided.  相似文献   
50.
We have analyzed decay kinetics of CF2 radicals in the afterglow of low-pressure, high-density C4F8 plasmas. The decay curve of CF2 density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10–4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF2 radicals (CF2+CF2C2F4). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10–14 cm3/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.  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