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   152333篇
  免费   11210篇
  国内免费   7625篇
化学   75015篇
晶体学   1512篇
力学   11339篇
综合类   562篇
数学   40343篇
物理学   42397篇
  2024年   249篇
  2023年   1538篇
  2022年   2503篇
  2021年   2666篇
  2020年   2793篇
  2019年   2681篇
  2018年   12211篇
  2017年   11807篇
  2016年   9043篇
  2015年   4170篇
  2014年   4314篇
  2013年   5687篇
  2012年   10192篇
  2011年   16729篇
  2010年   9851篇
  2009年   10157篇
  2008年   11012篇
  2007年   12713篇
  2006年   4191篇
  2005年   4721篇
  2004年   4206篇
  2003年   3979篇
  2002年   2864篇
  2001年   1931篇
  2000年   1863篇
  1999年   1848篇
  1998年   1643篇
  1997年   1413篇
  1996年   1515篇
  1995年   1309篇
  1994年   1189篇
  1993年   1041篇
  1992年   894篇
  1991年   789篇
  1990年   666篇
  1989年   568篇
  1988年   465篇
  1987年   435篇
  1986年   386篇
  1985年   376篇
  1984年   267篇
  1983年   220篇
  1982年   205篇
  1981年   162篇
  1980年   133篇
  1979年   100篇
  1978年   100篇
  1977年   78篇
  1975年   80篇
  1973年   85篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A copper microparticle-modified carbon fiber microdisk array electrode was fabricated and employed in capillary electrophoresis for the simultaneous determination of the five aminoglycoside antibiotics (AGs) including netilmicin, tobramycin, lincomycin, kanamycin and amikacin. The array electrode exhibited high catalytic activity for AGs, good reproducibility and stability. Under the optimum separation conditions (separation voltage of 6.2 kV, electrophoretic medium of 125 mM NaOH), the five AGs above were baseline separated within 20 min. At a working electrode potential of 0.7 V (versus saturated calomel electrode), the calibration curves were linear over two orders of magnitude of concentration, and the detection limits (SIN=3) were below 2 microM except for lincomycin (6.7 microM). The developed method was successfully employed for the simultaneous determination of the five AGs studied in pharmaceutical injections. The feasibility of this method for the simultaneous determination of lincomycin, kanamycin and amikacin in urine sample was also demonstrated.  相似文献   
992.
By the use of differential scanning calorimetry (DSC), a new method to measure the wax content of crude oil has been developed. In this paper, the wax content of a crude oil is proposed and proved to be the Q (total thermal effect of wax precipitation in sample) ratio of the crude oil and its corresponding wax obtained by using standard acetone method, i.e. Qoil/Qwax. For the 14 studied crude oils with the wax content ranging from 1 to 27 wt.%, the wax contents determined by the presented method are in good agreement with those determined by standard acetone method, with an absolute average deviation of only 0.82 wt.%. This method has an advantage over reported DSC methods in which the exact dissolution or precipitation enthalpy of wax is a must. It is also found that the wax contents determined by either of the two methods show good linear relationship with the total thermal effect Qoil, with the correlation coefficients over 0.96. According to the empirical correlations, the wax content of a crude oil can be easily determined by using the DSC total thermal effect Qoil. In addition and more significantly, the new method can be applied to improve the accuracy in determining the amount of precipitated wax in a waxy crude oil at different temperatures.  相似文献   
993.
Poly(malachite green) film modified Nafion‐coated glassy carbon electrodes have been prepared by potentiodynamic cycling in malachite green solution. The pH of polymerisation solution has only minor effect on film formation. Electrochemical quartz crystal microbalance (EQCM) was used to monitor the growth of the poly(malachite green) film. Cyclic voltammogram of the poly(malachite green) film shows a redox couple with well‐defined peaks. The redox response of the modified electrode was found to be depending on the pH of the contacting solution. The peak potentials were shifted to a less positive region with increasing pH and the dependence of the peak potential was found to be 56 mV per pH unit. The electrocatalytic behavior of poly(malachite green) film modified Nafion‐coated glassy carbon electrodes was tested towards oxidation of NADH, dopamine, and ascorbic acid. The oxidation of dopamine and ascorbic acid occurred at less positive potential on poly(malachite green) film compared to bare glassy carbon electrode. In the case of NADH, the overpotential was reduced substantially on modified electrode. Finally, the feasibility of utilizing poly(malachite green) film electrode in analytical estimation of ascorbic acid was demonstrated in flow injection analysis.  相似文献   
994.
Under the background of algae removal and growth inhibition by ultrasonic irradiation, the effects of ultrasonic irradiation on removal of Microcystis, the concentration variation of microcystins (MC) produced by Microcystis in Microcystis suspension, and sonochemical degradation of microcystins in water, were studied in the paper. The results showed that ultrasonic irradiation could efficiently inhibit the growth of Microcystis, and ultrasonic irradiation shorter than 5 min would not introduce the increase of microcystins dissolved in Microcystis suspension simultaneity. Also, microcystins dissolved in Microcystis suspension would not increase as ultrasonic power increasing. Further research showed that microcystins were effectively degraded in ultrasonic fields. After 20 min ultrasonic irradiation at 150 kHz and 30 W, the removal rate of microcystins reached 70%.  相似文献   
995.
The efficient accumulation of dyes in constructed natural polyelectrolyte microshells under moderate conditions, combined with the photo-assisted Fenton reagent, opens a new route for the effective elimination of dye pollutants from waste water.  相似文献   
996.
Nanostructured LiCoO2 fibers were prepared by the sol-gel related electrospinning technique using metal acetate and citric acid as starting materials. The transformation from the xerogel fibers to the LiCoO2 fibers and the nanostructure of LiCoO2 fibers have been investigated in detail. The LiCoO2 fibers with 500 nm to 2 mum in diameter were composed of polycrystalline nanoparticles in sizes of 20-35 nm. Cyclic voltammetry and charge-discharge experiments were applied to characterize the electrochemical properties of the fibers as cathode materials for lithium-ion batteries. The cyclic voltammogram curves indicated faster diffusion and migration of Li+ cations in the nanostructured LiCoO2 fiber electrode. In the first charge-discharge process, the LiCoO2 fibers showed the initial charge and discharge capacities of 216 and 182 (mA.h)/g, respectively. After the 20th cycle, the discharge capacity decreased to 123 (mA.h)/g. The X-ray diffraction and high-resolution transmission electron microscopy analyses indicated that the large loss of capacity of fiber electrode during the charge-discharge process might mainly result from the dissolution of cobalt and lithium cations escaping from LiCoO2 to form the crystalline Li2CO3 and CoF2 impurities.  相似文献   
997.
Summary The method for the contraction and expansion of graphs is used to treat the subspectrality of benzenoid hydrocarbons in relation to eigenvalues ±1. Counts of benzenoid hydrocarbons together with degeneracies of eigenvalues have been carried out for all species having h 7 hexagons. In addition, twelve homologous series are evaluated, and the closed results for the distribution of eigenvalues ± 1 and degeneracies in terms of the number of repeated units are tabulated. This method is universal and applicable to cases sharing other eigenvalues and to nonbenzenoid systems.Also known as Yuan-sun Kiang  相似文献   
998.
This review aims to provide a summary of the progress in TP small molecule fluorescent probes for enzymes in recent years and displays the main fluorescent mechanisms that have been applied to design probes.  相似文献   
999.
采用INDO方法计算了{Fe[P(OMe)_3]_3(C_8H_(13))}~+的简化离子[Fe(PH_3)_3(C_8H_(13))]~+,将正则分子轨道用Edmiston-Ruedenberg定域化方法变换为定域分子轨道,结果表明:在对应C_1-H_(1A)键的定域分子轨道中,明显包含有铁原子轨道成分(7.8%),Fe-H_(1A)和Fe—C_1键级分别为0.190和0.302。指出C_1-H_(1A)键是以一对成键σ电子配位到铁原子上的。C_8H_(13)环以包含三个碳原子的η~4—共轭体系与铁原子相互作用。铁以二价(d~6-Fe(Ⅱ)的形式存在于该离子中。C_1-H_(1A)键的配位满足了文献[15]提出的Fe(Ⅱ)的共价12价。  相似文献   
1000.
This paper reports an investigation of the effects of solvent system, solution concentration, and applied electrostatic field strength (EFS) on the morphological appearance and/or size of as-spun cellulose acetate (CA) products. The single-solvent systems were acetone, chloroform, N,N -dimethylformamide (DMF), dichloromethane (DCM), methanol (MeOH), formic acid, and pyridine. The mixed-solvent systems were acetone–DMAc, chloroform–MeOH, and DCM–MeOH. Chloroform, DMF, DCM, MeOH, formic acid, and pyridine were able to dissolve CA, forming clear solutions (at 5% w/v), but electrospinning of these solutions produced mainly discrete beads. In contrast, electrospinning of the solution of CA in acetone produced short and beaded fibers. At the same solution concentration of 5% (w/v) electrospinning of the CA solutions was improved by addition of MeOH to either chloroform or DCM. For all the solvent systems investigated smooth fibers were obtained from 16% (w/v) CA solutions in 1:1, 2:1, and 3:1 (v/v) acetone–DMAc, 14–20% (w/v) CA solutions in 2:1 (v/v) acetone–DMAc, and 8–12% (w/v) CA solutions in 4:1 (v/v) DCM–MeOH. For the as-spun fibers from CA solutions in acetone–DMAc the average diameter ranged between 0.14 and 0.37 μm whereas for the fibers from solutions in DCM–MeOH it ranged between 0.48 and 1.58 μm. After submersion in distilled water for 24 h the as-spun CA fibers swelled appreciably (i.e. from 620 to 1110%) but the physical integrity of the fibrous structure remained intact.  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