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   2089篇
  免费   116篇
  国内免费   238篇
化学   1314篇
晶体学   6篇
力学   64篇
综合类   2篇
数学   525篇
物理学   532篇
  2024年   23篇
  2023年   35篇
  2022年   63篇
  2021年   83篇
  2020年   70篇
  2019年   61篇
  2018年   51篇
  2017年   53篇
  2016年   59篇
  2015年   105篇
  2014年   123篇
  2013年   120篇
  2012年   225篇
  2011年   196篇
  2010年   150篇
  2009年   132篇
  2008年   112篇
  2007年   149篇
  2006年   106篇
  2005年   63篇
  2004年   47篇
  2003年   43篇
  2002年   40篇
  2001年   46篇
  2000年   15篇
  1999年   23篇
  1998年   15篇
  1997年   6篇
  1996年   6篇
  1994年   3篇
  1992年   3篇
  1989年   7篇
  1988年   14篇
  1987年   33篇
  1986年   32篇
  1984年   3篇
  1982年   3篇
  1972年   6篇
  1971年   10篇
  1970年   5篇
  1969年   9篇
  1968年   8篇
  1967年   11篇
  1966年   6篇
  1965年   9篇
  1964年   10篇
  1963年   9篇
  1962年   5篇
  1958年   3篇
  1956年   3篇
排序方式: 共有2443条查询结果,搜索用时 0 毫秒
1.
A Nafion-modified glassy carbon electrode incorporated with tobramycin for the voltammetric stripping determination of Cu2+ has been explored. The electrode was fabricated by tobramycin containing Nafion on the glassy carbon electrode surface. The modified electrode exhibited a significantly increased sensitivity and selectivity for Cu2+ compared with a bare glassy carbon electrode and the Nafion modified electrode. Cu2+ was accumulated in HAc-NaAc buffer (pH 4.6) at a potential of -0.6 V (vs. SCE) for 300 s and then determined by differential pulse anodic stripping voltammetry. The effects of various parameters, such as the mass of Nafion, the concentration of tobramycin, the pH of the medium, the accumulation potential, the accumulation time and the scan rate, were investigated. Under the optimum conditions, a linear calibration graph was obtained in the concentration range of 1.0 x 10(-9) to 5.0 x 10(-7) mol l(-1) with a correlation coefficient of 0.9971. The relative standard deviations for eight successive determinations were 4.3 and 2.9% for 1.0 x 10(-8) and 2.0 x 10(-7) mol l(-1) Cu2+, respectively. The detection limit (three times signal to noise) was 5.0 x 10(-10) mol l(-1). A study of interfering substances was also performed, and the method was applied to the direct determination of copper in water samples, and also in analytical reagent-grade salts with satisfactory results.  相似文献   
2.
The multiplicity results are obtained for solutions of the Neumann problem for nonlinear elliptic equations with unbounded nonlinearity by the Implicit Function Theorem and the Morse theory.  相似文献   
3.
A plant tissue-based chemiluminescence biosensor for ethanol based on using mushroom (Agaricus bisporus) tissue as the recognition element is proposed in this paper. The principle for ethanol sensing relies on the luminol-potassium hexacyanoferrate(III)-hydrogen peroxide transducer reaction, in which hydrogen peroxide is produced from the ethanol enzymatic catalytic oxidation by oxygen under the catalysis of alcohol oxidase in the tissue column. Under optimum conditions, the method allowed the measurement of ethanol in the range of 0.001 - 2 mmol/l with a detection limit (3 sigma) of 0.2 micromol/l. The relative standard deviation (RSD) was 4.14% (n = 11) for 0.05 mmol/l ethanol. The proposed method has been applied to the determination of ethanol in biological fluids and beverages with satisfactory results.  相似文献   
4.
A planar bilayered silver prisms with rounded corner is shown to have a singular optical mode depends of gaps by numerical simulation. Such behavior results from the plasmon resonance in the pairs of nanoprisms both the electric and the magnetic components of light.  相似文献   
5.
We numerically investigate the complete chaotic synchronization mechanism of polarization mode of VCSEL with anisotropic optical feedback. Firstly, the synchronization quality of the x-linear polarization (LP) mode and the y-LP mode are both periodically changed with the angle θ between the polarizer and the light axes. Secondly, in a enough large scale region of the feedback coefficient and the injection current, where each LP mode can obtained good synchronization quality when the angle θ exists in the former half period region and the injection current is fixed at a certain value, which is over the threshold current. Under this condition, their synchronization quality is independent of the polarization states. By contrast, in the latter half period region of the angle θ where each LP mode can be obtained very instable and inferior synchronized quality. At last, the robustness of the synchronization scheme is sensitive to the internal and external parameter mismatch between the transmitter-VCSEL and the receiver-VCSEL.  相似文献   
6.
Jaewan Yoo  J.S. Lee  B. Kahng 《Physica A》2011,390(23-24):4571-4576
As people travel, human contact networks may change topologically from time to time. In this paper, we study the problem of epidemic spreading on this kind of dynamic network, specifically the one in which the rewiring dynamics of edges are carried out to preserve the degree of each node (called fitness rewiring). We also consider the adaptive rewiring of edges, which encourages disconnections from and discourages connections to infected nodes and eventually leads to the isolation of the infected from the susceptible with only a small number of links between them. We find that while the threshold of epidemic spreading remains unchanged and prevalence increases in the fitness rewiring dynamics, meeting of the epidemic threshold is delayed and prevalence is reduced (if adaptive dynamics are included). To understand these different behaviors, we introduce a new measure called the “mean change of effective links” and find that creation and deletion of pathways for pathogen transmission are the dominant factors in fitness and adaptive rewiring dynamics, respectively.  相似文献   
7.
Binding polyacrylamide (PAM) with some basic phenothiazine dyes such as methylene blue (MB), toluidine blue (TB) or Azure B (AB) etc. can result in a significant enhancement of resonance Rayleigh scattering (RRS). The maximum RRS wavelengths (λmax) appear at 348, 340 and 342 nm for MB, TB, and AB systems, respectively. Accordingly, a new RRS method for the direct determination of PAM at nanogram levels has been established. The optimum conditions of these reactions, the influencing factors have been investigated. The RRS intensity is directly proportional to the concentration of PAM in the range of 0.040–5.0 μg/mL for three systems. The methods exhibit high sensitivities, and the detection limits are 15.9 ng/mL for MB system, 44.0 ng/mL for TB system, and 59.8 ng/mL for AB system. The selectivity of the method is investigated by using MB system owing to the highest sensitivity. Concentrations of PAM in tap water, synthetic water and practical waste water samples are determined satisfactorily. The reaction mechanism and RRS enhancement reasons are discussed.  相似文献   
8.
Resonance Rayleigh scattering (RRS) of cationic surfactants–Eosin Y systems and their analytical application have been studied. In aqueous solution at pH 2∼3, Eosin Y reacts with a monomer of cationic surfactants (CS), such as Zephiramine (Zeph), tetradecylpyridinium bromide (TPB), cetylpyridinium bromide (CPB), cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTMAB), to form an ion associate and a new RRS spectrum appears. The spectral characteristics of the five ion associates are similar and their maximum scattering wavelengths (λmax) are all at 313 nm. The intensity of RRS at λmax of the ion associate is directly proportional to the concentration of CS in the range of 0∼3.0 μg/25 mL. The technique has high sensitivity for the determination of CS; their detection limit is between 5.57 ng/mL and 7.60 ng/mL depending on the CS. In this case, most metal and non-metal ions, NH4 + and some anionic surfactants do not interfere, so that the method has a good selectivity. It can be applied to the determination of trace amounts of cationic surfactants in water samples. Received: 9 September 1998 / Revised: 17 November 1998 / Accepted: 18 November 1998  相似文献   
9.
A novel chemiluminescence (CL) flow-through sensor for the determination of analgin with flow-injection analysis (FIA) based on the auto-oxidation of analgin in the presence of Tween 80 sensitized by Rhodamine 6G immobilized on a cation-exchange column is described. This sensor responds linearly to the analgin concentration in the range of 0.4 ∼ 10 mg/L, with a detection limit (3σ) of 0.15 mg/L. A complete analysis, including sampling and washing, takes 1 min with a relative standard deviation of < 5%. The sensor is stable for over 200 determinations and has successfully been applied to the determination of analgin in pharmaceutical preparations. Received: 1 October 1998 / Revised: 19 April 1999 / Accepted: 6 May 1999  相似文献   
10.
二阶导数同步荧光光谱法同时直接测定厚朴酚及和厚朴酚   总被引:2,自引:0,他引:2  
研究了厚朴酚与和厚朴酚及其混合溶液的二阶导数同步荧光光谱,结果两者的二阶导数同步荧光光谱得到完全分离,消除了彼此间的干扰,据此建立了一种二阶导数同步荧光光谱法同时直接测定混合物中厚朴酚与和厚朴酚的新方法.厚朴酚与和厚朴酚的线性范围分别为2.8~500.0 μg/L和4.3~560.0 μg/L;检出限分别为0.84和1.30 μg/L,回收率分别为94.65%~105.58%和95.09%~104.51%; 相对标准偏差均低于4.08%.本方法用于同时直接测定厚朴药材及其提取物中厚朴酚与和厚朴酚含量,结果令人满意.  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