全文获取类型
收费全文 | 3504篇 |
免费 | 143篇 |
国内免费 | 289篇 |
专业分类
化学 | 2506篇 |
晶体学 | 14篇 |
力学 | 68篇 |
综合类 | 12篇 |
数学 | 33篇 |
物理学 | 1303篇 |
出版年
2024年 | 8篇 |
2023年 | 78篇 |
2022年 | 30篇 |
2021年 | 53篇 |
2020年 | 52篇 |
2019年 | 45篇 |
2018年 | 42篇 |
2017年 | 46篇 |
2016年 | 86篇 |
2015年 | 66篇 |
2014年 | 90篇 |
2013年 | 243篇 |
2012年 | 210篇 |
2011年 | 231篇 |
2010年 | 149篇 |
2009年 | 232篇 |
2008年 | 205篇 |
2007年 | 223篇 |
2006年 | 244篇 |
2005年 | 218篇 |
2004年 | 158篇 |
2003年 | 190篇 |
2002年 | 100篇 |
2001年 | 114篇 |
2000年 | 75篇 |
1999年 | 105篇 |
1998年 | 88篇 |
1997年 | 95篇 |
1996年 | 47篇 |
1995年 | 68篇 |
1994年 | 44篇 |
1993年 | 44篇 |
1992年 | 27篇 |
1991年 | 48篇 |
1990年 | 33篇 |
1989年 | 29篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1978年 | 4篇 |
1976年 | 6篇 |
1974年 | 4篇 |
1973年 | 2篇 |
排序方式: 共有3936条查询结果,搜索用时 15 毫秒
121.
122.
A new PVC membrane potentiometric sensor that is highly selective to Fe(III) ions was prepared by using 2-[(2-hydroxy-1-propenyl-buta-1,3-dienylimino)-methyl]-4-p-tolylazo-phenol [HPDTP] as a suitable carrier. The electrode exhibits a linear response for iron(III) ions over a wide concentration range (3.5 × 10−6 to 4.0 × 10−2) with a super Nernstian slope of 28.5 (±0.5) per decade. The electrode can be used in the pH range from 4.5 to 6.5. The proposed sensor shows fairly a good discriminating ability towards Fe3+ ion in comparison to some hard and soft metals such as Fe2+, Cd2+, Cu2+, Al3+ and Ca2+. It has a response time of <15 s and can be used for at least 2 months without any measurable divergence in response characteristics. The electrode was used in the direct determination of Fe3+ in aqueous samples and as an indicator electrode in potentiometric titration of Fe(III) ions. 相似文献
123.
单柱离子色谱法测定茶叶中草酸根和硫酸根的研究 总被引:1,自引:1,他引:1
本文研究单柱离子色谱分离测定茶叶中微量草酸根和硫酸根的方法。样品水浸泡,酸化液。上ShimParskIC-A_2柱分离,用2.0mmol·L ̄(-1)H-2C_8O_4/1.9mmol·L ̄(-1)Tri淋洗液洗脱,电导检测器检测,检测灵敏度为1.6S/cm,最低检出限0.05μg/mL;线性范围L;相对标准偏差1.06%;平均回收车98.72%97.88/。该法简便、快速、准确、灵敏、选择性好。 相似文献
124.
Traditionally, the application of supercritical fluid chromatography has been limited to compounds of relatively low polarity.
Thus, we report here that one secondary amine and two quaternary amine salts were successfully eluted from a Deltabond Cyano-bonded
silica column with the addition of sodium alkylsulfonate to the methanol-modified, carbon dioxide-based mobile phase. A possible
ion-pairing interaction between the positively charged analytes and the anionic part of the sulfonate additive has been proposed.
In another set of experiments, the three amine salts readily eluted from both Ethyl-pyridine-bonded silica and Amino-bonded
silica phases without the need of additive although the peak shapes were less than desirable. The addition of sulfonate salt
to the mobile phase again sharpened the peaks. In the presence of a mixture of methanol and CO2, we suggest that these stationary phases are positively charged. We describe here also for the first time the employment
of a strong silica-based anion exchange (SAX) column for supercritical fluid separation of cationic species. Two elution mechanisms
were proposed for the three amine salts depending upon whether the ionic additive was incorporated into the mobile phase.
Separation as the ion-pair was proposed with an ionic additive in the mobile phase; whereas separation of the intact amine
salt from the positively charged basic stationary phases was suggested to be operational without the ionic additive. 相似文献
125.
《Electroanalysis》2006,18(22):2243-2250
This work is focused on the voltammetric examination of the ion exchange properties of a smectite type clay, before and after its modification by the replacement of its native interlamellar cations (Na+, K+, Ca2+) by hexadecyltrimethylammonium cations (HDTMA+). The raw clay and its organically modified form were first characterized by X‐ray diffraction (XRD) and N2 adsorption–desorption isotherms (BET method) that confirmed the modification via an intercalation process. These materials were subsequently coated onto glassy carbon surfaces, and the resulting modified electrodes were evaluated for the uptake of [Ru(NH3)6]3+ and [Fe(CN)6]3? ions used as redox probes. Some experimental parameters affecting the incorporation of the probes within the film, including the ionic strength, the surfactant loading and the solution pH are thoroughly examined, in order to highlight the mechanism of the process. The possibility of using the surfactant‐intercalated clay modified electrode as an electrochemical sensor for [Fe(CN)6]3? is also evaluated. 相似文献
126.
A new approach to the voltammetric investigation of facilitated ion transfer processes is reported. The technique uses a rotating diffusion cell approach to induce laminar flow in the organic phase of a liquid|liquid electrochemical cell. The interface between two immiscible electrolyte solutions (ITIES) was stabilised against rotation with either γ-alumina or a track-etched polyester membrane. The resultant voltammetry is shown to be consistent with the Koutecký–Levich equation enabling kinetic parameters associated with facilitated transfer of sodium by dibenzo-18-crown-6 across the water|1,2-dichloroethane interface to be evaluated. In particular, the use of the more hydrophilic alumina membrane permits the uncertainties regarding the use of the membrane-stabilised ITIES, namely the interfacial position, to be eliminated. 相似文献
127.
H. Krzy
anowska A. von Bohlen R. Klockenkmper 《Spectrochimica Acta Part B: Atomic Spectroscopy》2003,58(12):5698-2067
A new method suitable for depth profiling of shallow layers on different materials is presented. It is based on a soft and planar ion sputtering combined with differential weighing, total-reflection X-ray fluorescence (TXRF) spectrometry and Tolansky interferometry. By means of a stepwise repetition of these techniques it is possible to determine both density/depth and concentration/depth profiles. The respective quantities are expressed in terms inherent only to the sample and traceable to the SI-units or subunits gram, nanometer and mole. It is a unique feature of this method that density/depth profiles can directly be obtained from measurements without any calibration or theoretical approximation. The method is applied to a Si wafer implanted with Co ions of 25 keV energy and a nominal dose of 1×1016 cm−2. The depth resolution is shown to be <3 nm while a total depth of some 100 nm can be reached. The concentration/depth profile is compared with RBS measurements, wet-chemical etching plus TXRF and Monte Carlo simulations. In view of the fact that only similar but not exactly the same samples have been examined by these methods, a good correspondence can be noticed. 相似文献
128.
Sorina M. Ulmeanu Henrik Jensen Zdenek Samec Graldine Bouchard Pierre-Alain Carrupt Hubert H. Girault 《Journal of Electroanalytical Chemistry》2002,530(1-2)
Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES). 相似文献
129.
《Electroanalysis》2004,16(21):1785-1790
Binaphthyl‐based crown ethers incorporating anthraquinone, benzoquinone, and 1,4‐dimethoxybezene have been synthesized and tested for Rb+ selective ionophores in the poly(vinyl chloride) (PVC) membrane. The membrane containing NPOE gave a better Rb+ selectivity than those containing either DOA or BPPA as a plasticizer. The response was linear within the concentration range of 1.0×10?5–1.0×10?1 M and the slope was 54.7±0.5 mV/dec. The detection limit was determined to be 9.0×10?6 M and the optimum pH range of the membrane was 6.0–9.0. The ISE membrane exhibits good selectivity for Rb+ over ammonium, alkali metal, and alkaline earth metal ions. Selectivity coefficients for the other metal ions, log KPot were ?2.5 for Li+, ?2.4 for Na+, ?2.0 for H+, ?1.0 for K+, ?1.2 for Cs+, ?1.6 for NH4+, ?4.5 for Mg2+, ?5.0 for Ca2+,?4.9 for Ba2+. The lifetime of the membrane was about one month. 相似文献
130.