全文获取类型
收费全文 | 5950篇 |
免费 | 581篇 |
国内免费 | 494篇 |
专业分类
化学 | 3443篇 |
晶体学 | 22篇 |
力学 | 265篇 |
综合类 | 112篇 |
数学 | 1912篇 |
物理学 | 1271篇 |
出版年
2024年 | 7篇 |
2023年 | 32篇 |
2022年 | 76篇 |
2021年 | 97篇 |
2020年 | 133篇 |
2019年 | 120篇 |
2018年 | 123篇 |
2017年 | 173篇 |
2016年 | 165篇 |
2015年 | 175篇 |
2014年 | 173篇 |
2013年 | 371篇 |
2012年 | 411篇 |
2011年 | 323篇 |
2010年 | 295篇 |
2009年 | 356篇 |
2008年 | 319篇 |
2007年 | 443篇 |
2006年 | 345篇 |
2005年 | 344篇 |
2004年 | 375篇 |
2003年 | 266篇 |
2002年 | 235篇 |
2001年 | 172篇 |
2000年 | 183篇 |
1999年 | 154篇 |
1998年 | 159篇 |
1997年 | 143篇 |
1996年 | 118篇 |
1995年 | 112篇 |
1994年 | 77篇 |
1993年 | 80篇 |
1992年 | 82篇 |
1991年 | 48篇 |
1990年 | 60篇 |
1989年 | 46篇 |
1988年 | 36篇 |
1987年 | 19篇 |
1986年 | 18篇 |
1985年 | 29篇 |
1984年 | 27篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 23篇 |
1980年 | 13篇 |
1979年 | 18篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1974年 | 1篇 |
排序方式: 共有7025条查询结果,搜索用时 31 毫秒
101.
102.
Marek Trojanowicz Joanna Szpunar-Łobińska Zdzislaw Michalski 《Mikrochimica acta》1991,103(3-4):159-169
An original computer-controlled solid-state photometer has been developed and its potential for simultaneous multicomponent flow-injection analysis of binary and ternary mixtures is demonstrated. The device is simple in construction. Its principle of operation is based on rapid sequential measurements of the absorbance of the complexes formed by the analytes with chromogenic reagents at the wavelengths corresponding to the emission maxima of three light emitting diodes (563, 580 and 638 nm). 相似文献
103.
考察氧化物担体对Fe-MnO催化剂反应性能影响的结果表明,担体的类型对Fe-MnO催化剂CO加氢反应性能影响很大,其低碳烯烃选择性相差悬殊,Al_2O_3、SiO_2和MgO担载的Fe-MnO催化剂都不利于低碳烯烃的生成,而TiO_2担载的Fe-MnO催化剂则具有较高的烯烃选择性和催化活性。从担体-金属相互作用本质的差异,研究了担体对金属活性组分化学状态的影响及催化活性相与F-T合成烯烃的关系,发现担体-金属间的电子效应有利于催化剂活性和选择性的提高,其它物理化学效应引起的相互作用则不利于改善催化剂性能;还表明Fe_(?)C是催化活性相,Fe~(2+)物种的存在不利于提高烯烃的选择性。 相似文献
104.
A sequential injection system based on the ABTS (2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulphonic-acid) methodology was developed. The proposed method, incorporating a mixing chamber in the side port of the selection valve, was evaluated to measure the total antioxidant activity of several beverages and foods.The ABTS+ is generated by oxidation of ABTS with potassium persulfate and is reduced in presence of hydrogen-donating antioxidants converting into a colourless product. The applicability of the developed method was tested by measurement of the antioxidant activity of pure compounds as well as by analysing complex food and beverage samples. The antioxidant activity was presented as l(+) ascorbic acid equivalence. The values obtained by this methodology were not significantly different from the results obtained by the original spectrophotometric ABTS assay. For most of the studied antioxidants, antioxidant activity varied with pH and dilution. The proposed SIA system is suitable for screening direct or diluted total antioxidant activity of pure compounds or food samples. 相似文献
105.
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
106.
二乙基二硫代氨甲酸银体系流动注射法测定砷 总被引:1,自引:0,他引:1
本文设计了一种AgDDC流动注射-分光光度法测定砷的系统。该系统采用自制的氢化物发生及吸装置,将液体流路和载气流路结合在一起。 相似文献
107.
In this work total (Si-tot) and ‘soluble’ or reactive (Si-sol) concentrations of silicon in natural and tap waters were sequentially determined by electrothermal atomic absorption spectrometry (ETAAS). First, samples were on-line diluted based on the merging-zone principle in order to allow the determination of Si-tot within the 300–1000 μgSi l−1 range. After the dilution process, a sub-sample was collected in the capillary of a sampling arm assembly (SAA). Thereafter, samples were subject to a precipitation/dissolution process in order to allow the determination of Si-sol within the 280–850 μgSi l−1. Si-sol was precipitated with ammonium chloride and collected on the walls of a knotted coil. The precipitate was dissolved with ammonium molybdate in an acidic medium (HNO3) and a sub-sample was then collected in the SAA. In both cases, 10 μl volumes of the sub-sample were injected into the atomizer with the previous introduction of 20 ng of Eu as chemical modifier (10 μl) by the spectrometer autosampler. The recovery values obtained with natural waters spiked samples were over 46% and the agreement between observed and certified samples values was good. The proportion of Si-sol in comparison with the Si-tot was high (85–95%) in most natural waters. The precision of the method was 2.4–3.5 and 4.5–6.2% (n=10) for the determination of Si-tot and Si-sol, respectively. 相似文献
108.
Luciana B.O. dos Santos 《Analytica chimica acta》2005,528(1):21-27
This paper describes the development of a sequential injection analysis method to automate the determination of atrazine by square wave voltammetry exploiting the concept of monosegmented flow analysis to perform in-line sample conditioning and standard addition. To perform these tasks, an 800 μL monosegment is formed, composed by 400 μL of sample and 400 μL of buffer/standard solution. To obtain an efficient homogenization, the sample solution is divided in five zones intercalated by four zones of the Britton-Robinson buffer (pH 2.0) in presence of appropriate concentration of NaNO3 and varying atrazine standard concentrations. This mixture zone is isolated from the carrier solution by two 100 μL air bubbles. After homogenization in an auxiliary reaction coil the mixture zone is injected toward the flow cell, which is adapted to the capillary of a hanging drop mercury electrode, at a flow rate of 50 μL s−1. After a suitable delay time, the potential is scanned from −0.5 to −1.2 V versus Ag/AgCl using a frequency of 300 Hz and pulse height of 25 mV. The linear dynamic range is observed for atrazine concentrations between 1.16 × 10−7 and 2.32 × 10−6 mol L−1, obeying the linear equation ip = (−6.91 ± 0.07) × 108[atrazine] + (4 ± 8), with r2 = 0.9996, for which the slope is given in nA L mol−1. The detection and quantification limits of the method are 2.1 × 10−8 and 7.0 × 10−8 mol L−1, respectively. The sampling frequency is 37 h−1, when the standard addition protocol is followed. This frequency can be increased to 42 h−1 if the protocol to obtain in-line calibration curve is used for quantification. The method was applied for determination of atrazine in spiked river water samples and its accuracy was evaluated by comparison with the batch standard addition approach, which revealed that there is no evidence of statistically significant differences between the two methods. 相似文献
109.
Thermal plasma processing involves complex interactions of particulates with plasmas. In previous studies (see Parts I and II of this series), an assessment of different effects has been made considering the dynamics and heat and mass transfer of a single particle immersed into a thermal plasma. The last paper of this sequence is concerned with the simulation of thermal plasma jet reactors and the effects caused by multiparticle injection.A mathematical model is proposed for the simulation of thermal plasma jet reactors, including the mixing phenomena between the jet and the surrounding gases by generalizing the governing equations for simple mixing flows. Also included is the density fluctuation effect by extending the K- model to a four-equation turbulence model combined with a probability density function. This model is internally consistent covering additional physical phenomena which are not covered by existing models. Unfortunately, its expected higher accuracy cannot be proven because of the present uncertainties associated with the input.For multiparticle injection, the simulation repeats calculations for single-particle injection, but with different initial conditions correcting the solutions by considering the coupling effects between particles and the plasma.The results indicate that (i) thermal plasmas show different mixing behavior in different gases; (ii) the density fluctuation effect is important since it causes large differences between the mass-weighted and unweighted time-averaged temperatures of thermal plasma jets; (iii) coupling effects become important when the particle loading rate exceeds half of the plasma mass flow rate; (iv) there are 16 constraints imposed on the modeling work which have to be considered for establishing a base for comparison with future experimental studies. 相似文献
110.
In this work, a downsized flow set up designed based on multicommutation concept for photometric determination of iron(II)/iron(III) and nitrite/nitrate is surface water is described. The flow system network comprised a set of three-way solenoid valves, reaction coil and a double-channel flow cell, which were nested in order to obtain a compact and small-size instrument. To accomplish the downsizing requirement light source (LED) and radiation detection (phototransistor) were coupled to the flow cell. In order to demonstrated the effectiveness of the system, the photometer methods based on Griess reaction and 1-10-phenantroline for nitrite and iron(II) determination, respectively, were selected. Under computer control the set up provided facilities to handle four reagent solutions employing a single pumping channel, thus permitting also the determination of nitrate and iron(III) after its reduction to nitrite and to iron(II), respectively. The overall system performance was demonstrated working several days running standard solution, no significant variation of base line, linear response range and slop (less than 1%) were observed. The usefulness of the downsized system was ascertained by analyzing a set of surface water. Aiming to access the accuracy sample were also analyzed employing reference procedures and no significant difference at 95% confidence level were observed for the four analytes. Other profitable features such as analytical throughput of 40 determination per hour; relative standard deviation of 1%; linear response range between 50 and 300 μg l−1 for nitrite and nitrate, 0.5-6.0 mg l−1 iron(II) and iron(III); low reagent consumption 75 μg for nitrate/nitrite and 0.6 mg for iron(II)/iron(III) per determination; and 2.4 ml waste generation per determination were also achieved. 相似文献