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   964篇
  免费   89篇
  国内免费   297篇
化学   748篇
晶体学   6篇
力学   53篇
综合类   3篇
数学   279篇
物理学   261篇
  2024年   5篇
  2023年   28篇
  2022年   50篇
  2021年   41篇
  2020年   66篇
  2019年   39篇
  2018年   42篇
  2017年   27篇
  2016年   27篇
  2015年   41篇
  2014年   77篇
  2013年   149篇
  2012年   138篇
  2011年   116篇
  2010年   77篇
  2009年   76篇
  2008年   63篇
  2007年   67篇
  2006年   50篇
  2005年   33篇
  2004年   14篇
  2003年   12篇
  2002年   7篇
  2001年   16篇
  2000年   8篇
  1999年   13篇
  1998年   6篇
  1997年   5篇
  1996年   3篇
  1993年   2篇
  1992年   1篇
  1989年   3篇
  1988年   8篇
  1987年   20篇
  1986年   14篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
  1957年   1篇
排序方式: 共有1350条查询结果,搜索用时 390 毫秒
991.
最优分配采购策略和应急采购策略被广泛应用于供应中断风险管理中,基于最优分配采购策略和应急采购策略,研究了由两个制造商与两个零售商组成的两级供应链在供应中断下的订购和生产决策问题,给出了最优订购和最优生产策略.通过数值例子就可靠性如何影响制造商的最优价格和利润进行了比较.结果表明:随着可靠性的增加,两个制造商的最优价格呈递减态势,且制造商A的递减幅度小于制造商B的递减幅度;当可靠性低于0.3时,制造商A的最优价格低于制造商B的最优价格,当可靠性高于0.3时,制造商A的最优价格高于制造商B的最优价格.随着可靠性的增加,制造商A的利润呈递减态势,制造商B的利润呈递增态势;当可靠性低于0.8时,制造商A的利润高于制造商B的利润,当可靠性低高于0.8时,制造商A利润低于制造商B的利润.  相似文献   
992.
梁腾  马堃  武中文  张登红  董晨钟  师应龙 《物理学报》2016,65(14):143401-143401
基于多组态Dirac-Fock方法和密度矩阵理论,系统地研究了在197 Me V/u的碰撞能量下,Xe~(53+)离子与Xe原子的辐射电子俘获过程(REC)以及电子被俘获到激发态后辐射退激发产生的特征谱线.计算了炮弹Xe~(53+)离子俘获电子到不同壳层np_(1/2,3/2)(n=2—5)的总截面与相应的REC光子能量和角分布,以及由激发组态1snp_(1/2,3/2)(n=2—5)J_f=1向基态1s~2Jd=0辐射退激发的跃迁能量、跃迁概率和特征光子的角分布和线性极化度.计算结果表明,辐射光子具有显著的角各向异性特征.此外,1snp_(3/2)J_f=1→1s~2J_d=0退激发特征光子也显示出很强的线性极化和角各向异性特征,而1snp_(1/2)J_f=1→1s~2J_d=0退激发特征光子的线性极化度趋于零并且角分布也趋于各向同性.  相似文献   
993.
In this paper, an atmospheric pressure dual‐frequency (50 kHz/33 MHz) micro‐plasma jet was used to deposit organosilicon film. The discharge generated in atmospheric environment. Plasma composition was characterized by optical emission spectroscopy. With introduction of tetraethyl orthosilicate, we observed various spectra, for example Si(251.6 nm), OH(308.9 nm), C(247.8 nm), O(777.5 nm). Abundant reactive radical species which are benefit to film deposition were generated in plasma. The deposited film was characterized by scanning electron microscopy, X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The film is mostly composed of Si and O. The film has Si‐O‐Si backbone with a small number of organic component (‐CHx). (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
994.
A novel method to determine isoniazid with high sensitivity and good selectivity has been established by using potassium ferricyanide as spectroscopic probe reagent. In the presence of potassium ferricyanide, it has been demonstrated that iron(III) is reduced to iron(II) by isoniazid at pH 4.0. In addition, the in situ formed iron(II) reacts with potassium ferricyanide to give soluble prussian blue. The absorbance of soluble prussian blue is measured at the absorption maximum of 735 nm, and the amount of isoniazid can be calculated based on this absorbance. A good linear relationship of the concentration of isoniazid versus absorbance is observed with a linear range of 0.040-8.00 μg mL−1. The linear regression equation is A = −0.00286 + 0.28588C (μg mL−1) with a correlation coefficient of 0.9992. The detection limit (3σ/k) is 0.037 μg mL−1, and its relative standard deviation (R.S.D.) is 0.35% (n = 11). Moreover, the apparent molar absorption coefficient of indirect determination of isoniazid is 3.92 × 104 L mol−1 cm−1. The parameters with regard to determination are optimized, and the reaction mechanism is discussed. This method has been successfully applied to the determination of isoniazid in pharmaceutical samples and saliva samples of patients.  相似文献   
995.
The novel biodegradable polylactic acid-β-cyclodextrin (PLA-β-CD) cross-linked copolymer microgels were prepared by the radical copolymerization of PLA macromonomer and polymerizable β-CD derivatives. The β-CD derivatives with various numbers of polymerizable vinyl groups were synthesized from 1-allyloxy-2,3-epoxy propane (also called allylglycidyl ether, AGE) and β-CD. The chemical structures of polymerizable monomers were determined by NMR. The thermal properties, size, morphology, in vitro degradation and swelling behavior of the microgels were investigated. The results indicated that the microgels were stable under thermal conditions up to 200 °C. The microgels were spherical in aqueous solution. The hydrophilicity of the microgels increased with increasing β-CD contents, while the swelling ratios and degradation rate decreased. The more vinyl groups on β-CD, the higher cross-linked density, which resulted in a decrease of the swelling ratios and the rate of degradation.  相似文献   
996.
(Un)substituted benzaldehyde (5‐aryl‐1,3,4‐thiadiazol‐2‐yl)hydrazones were efficiently synthesized by reactions of (un)substituted benzaldehyde thiocarbohydrazones with aromatic carboxylic acids by using silica‐supported dichlorophosphate as a recoverable dehydrant under microwave irradiation. The protocol has advantages of short reaction time, high yield, easy work‐up procedure and no environmental pollution.  相似文献   
997.
《印度化学会志》2022,99(12):100787
Pt-based metals are very effective catalysts widely adopted in many fields. But the high cost prevents its further industrial application. One of the effective ways to solve the problem is to replace platinum with relatively cheap palladium and its alloy with copper. However, producing Pd/Cu bimetallic catalysts efficiently and economically with controllable particle size and uniform distribution is still challenging, especially when trying to reduce the consumption of precious metals. In this paper, ultrasmall palladium/copper (Pd/Cu) bimetallic catalysts with even dispersion were prepared on multi-walled carbon nanotubes (MWCNTs) by adding polyethylene glycol 400 (PEG 400) as a reducing agent and stabilizer under visible light irradiation at room temperature. The catalytic performance was studied in the catalysis of p-nitrophenol (p-NP) reduction. Of all the bimetallic catalysts produced in different conditions, the best one was obtained under the reaction condition of pH = 7 and violet light irradiation (wavelength 380–435 nm). The average particle size of 0.85 nm, and the apparent rate constant in the catalysis is 1.47 min?1. This research probes the role of visible light as a key kinetic controlling method in the formation of ultrasmall particles (UPs). It proves the effectiveness of using visible light irradiation as an effective and more “green chemistry” approach to get precious metal UPs as catalysts beyond the traditional ultraviolet or laser photochemistry methods.  相似文献   
998.
Li  P.  La  Y.-T.  Feng  L.-C.  Dong  W.-K. 《Russian Journal of General Chemistry》2021,91(7):1409-1419
Russian Journal of General Chemistry - Two multi-nuclear Co(II) and Ni(II) complexes, [{Co3(L)2(EtOH)2(μ-OAc)2}2]·2CH3CH2OH· CH2Cl2·CHCl3·CH3CN (1) and...  相似文献   
999.
Sorptive pre-concentration of pesticide residues in food and environmental samples is increasingly gaining momentum. This can be attributed to fact that most sorptive extraction techniques are solvent-free as well as the availability of emerging sorbents with relatively high adsorption capacities. Metal organic frameworks (MOFs) are among the emerging sorbents that have captured the interests of many researchers during pesticide analysis. There are many types of MOFs that have been used by researchers to pre-concentrate pesticides in food and environmental samples. Some researchers have successfully used MIL-based MOFs during sorptive extraction of pesticides in complex matrices. This review gives a detailed discussion of their application during pesticide pre-concentration. Other researchers have used the ZIF series of MOFs for the sorptive pre-concentration of pesticides in food and environmental samples. The utilisation of the ZIF series of MOFs during pesticide pre-concentration is well-articulated in this review. The review also devoted to the application of UiO and HKUST series of MOFs during the pre-concentration of pesticides in food and environmental samples. In addition, the challenges associated with the use of MOFs during sorptive pre-concentration of pesticides are also discussed in this review.  相似文献   
1000.
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