全文获取类型
收费全文 | 61741篇 |
免费 | 1888篇 |
国内免费 | 165篇 |
专业分类
化学 | 38345篇 |
晶体学 | 383篇 |
力学 | 1001篇 |
综合类 | 5篇 |
数学 | 11261篇 |
物理学 | 12799篇 |
出版年
2023年 | 388篇 |
2022年 | 523篇 |
2021年 | 850篇 |
2020年 | 897篇 |
2019年 | 920篇 |
2018年 | 1149篇 |
2017年 | 1080篇 |
2016年 | 2069篇 |
2015年 | 1723篇 |
2014年 | 1782篇 |
2013年 | 3814篇 |
2012年 | 3691篇 |
2011年 | 3733篇 |
2010年 | 2504篇 |
2009年 | 2215篇 |
2008年 | 3292篇 |
2007年 | 3098篇 |
2006年 | 2794篇 |
2005年 | 2722篇 |
2004年 | 2299篇 |
2003年 | 1950篇 |
2002年 | 1683篇 |
2001年 | 1292篇 |
2000年 | 1235篇 |
1999年 | 892篇 |
1998年 | 699篇 |
1997年 | 645篇 |
1996年 | 811篇 |
1995年 | 581篇 |
1994年 | 659篇 |
1993年 | 612篇 |
1992年 | 619篇 |
1991年 | 527篇 |
1990年 | 567篇 |
1989年 | 463篇 |
1988年 | 455篇 |
1987年 | 438篇 |
1986年 | 413篇 |
1985年 | 548篇 |
1984年 | 525篇 |
1983年 | 404篇 |
1982年 | 410篇 |
1981年 | 415篇 |
1980年 | 364篇 |
1979年 | 334篇 |
1978年 | 314篇 |
1977年 | 314篇 |
1976年 | 324篇 |
1974年 | 284篇 |
1973年 | 305篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
931.
The performance of a recently developed method to generate representative atomistic models of amorphous polymers has been investigated. This method, which is denoted SuSi, can be defined as a random generator of energy minima. The effects produced by different parameters used to define the size of the system and the characteristics of the generation algorithm have been examined. Calculations have been performed on poly(L,D-lactic) acid (rho = 1.25 g/cm3) and nylon 6 (rho = 1.084 g/cm(3)), which are important commercial polymers. 相似文献
932.
A new method for the speciation of inorganic [Sb(III) and Sb(V)] and organic (Me3SbCl2) antimony species by using a polystyrene-divinylbenzene-based anion-exchange HPLC column (Hamilton PRP-X100) coupled to hydride generation atomic fluorescence spectrometry (HG-AFS) is presented. Several mobile phases were tested for the baseline separation of these three antimony species, investigating in detail experimental parameters such as concentration and pH. The best efficiency and resolution was achieved by using a gradient elution between diammonium tartrate 250 mmol l(-1) pH 5.5 (A) and KOH 20 mmol l(-1) pH 12 (B). The gradient programme used was 100% B for 1.5 min, decreasing to 0% B in 0.1 min and maintained the elution with 100% A for 5.5 min. Analysis time was less than 7 min. Equilibration of the column with the complexing mobile phase was found to be critical in order to avoid Sb(III) double peak formation. Dilution in diammonium tartrate medium was necessary in order to avoid Sb(III) oxidation at microg l(-1) concentration level. Detection limits of 0.06 microg l(-1) for Sb(V), 0.09 microg l(-1) for Me3SbCl2 and 0.04 microg l(-1) for Sb(III) as well as repeatability and reproducibility better than 5% R.S.D. (n = 10) and 9% R.S.D. (n = 30) (for 1 and 5 microg l(-1) of Sb(V) and Sb(III) and 5 and 10 microg l(-1) of Me3SbCl2) were obtained. Accuracy and recovery studies were carried out by analysing one river freshwater sample and two water certified reference materials. The proposed methodology can be considered reliable and straightforward for antimony speciation in fresh water samples. 相似文献
933.
Rubio-Hernández FJ Gómez-Merino AI Ruiz-Reina E 《Journal of colloid and interface science》2000,222(1):103-106
A study on the electroviscous effect of alumina suspensions has been made. At the low volume fraction of the particles studied here only a first-order effect was detected. Ubbelohde-type capillary viscometers have been used. A simple method to determine the hydrodynamic constant k(1) has been proposed. The experimental primary electroviscous coefficients corresponding to different electrolyte concentrations have been compared with two different theoretical approachs (I. G. Watterson, and L. R. White, J. Chem. Soc. Faraday Trans. 2 77, 1115 (1981); F. J. Rubio-Hernández, E. Ruiz-Reina, and A. I. Gómez-Merino, J. Colloid Interface Sci. 206, 334 (1998)) and the results suggest that the presence of a dynamic Stern layer plays a certain role in this effect. Copyright 2000 Academic Press. 相似文献
934.
Optimization of culture medium and conditions for penicillin acylase production by Streptomyces lavendulae ATCC 13664 总被引:1,自引:0,他引:1
Torres-Bacete J Arroyo M Torres-Guzmán R De La Mata I Acebal C Castillón MP 《Applied biochemistry and biotechnology》2005,126(2):119-132
The culture medium for Streptomyces lavendulae ATCC 13664 was optimized on a shake-flask scale by using a statistical factorial design for enhanced production of penicillin acylase. This extracellular enzyme recently has been reported to be a penicillin K acylase, presenting also high hydrolytic activity against penicillin V and other natural aliphatic penicillins such as penicillin K, penicillin F, and penicillin dihydroF. The factorial design indicated that the main factors that positively affect penicillin acylase production by S. lavendulae were the concentration of yeast extract and the presence of oligoelements in the fermentation medium, whereas the presence of olive oil in the medium had no effect on enzyme production. An initial concentration of 2.5% (w/v) yeast extract and 3 microg/mL of CuSO4 x 5H2O was found to be best for acylase production. In such optimized culture medium, fermentation of the microorganism yielded 289 IU/L of enzyme in 72 h when employing a volume medium/volume flask ratio of 0.4 and a 300-rpm shaking speed. The presence of copper, alone and in combination with other metals, stimulated biomass as well as penicillin acylase production. The time course of penicillin acylase production was also studied in the optimized medium and conditions. Enzyme production showed catabolite repression by different carbon sources such as glucose, lactose, citrate, glycerol, and glycine. 相似文献
935.
Costa T Miguel Mda G Lindman B Schillén K Seixas de Melo JS 《The journal of physical chemistry. B》2005,109(23):11478-11492
Steady-state and time-resolved fluorescence studies were performed on aqueous solutions of poly(acrylic acid) hydrophobically modified with two very different levels of naphthalene (Np). It is demonstrated that unique information on association phenomena involving hydrophobe-modifed polymers can be obtained from an extended fluorescence study by using data for a less-modified polymer as a reference. For the more highly modified polymer, the presence of excited-state (as well as ground-state) dimers in addition to monomer emission due to locally excited naphthalene gives evidence for hydrophobic association between naphthalene groups. This association becomes, as expected, much less important at higher pH due to the electrostatic repulsion between different chain segments. However, it is noted that even at high pH there is a significant self-association. The coexistence of static and dynamic quenching phenomena of the Np monomer label was also revealed in the time-resolved fluorescence data. The data are compatible with the existence of two types of monomers and one excimer and suggest that the essential contribution to the monomer emission comes from isolated chromophores, whereas excimer formation arises from both a dynamic route (excited Np chromophores able to produce a dynamic excimer) and a static route (excitation of ground-state Np dimers). At room temperature, the dissociative reaction, excimer-to-monomer, can be neglected, and thus the rate constant for excimer formation and decay could be obtained with and without considering the influence of preformed dimers. Temperature has shown to induce different behavior in the polymer photophysics. In the case of the less-labeled polymer, the decays were found to be single-exponential with the fluorescence lifetime decreasing with increasing temperature. From the temperature dependence of the steady-state fluorescence data, the activation energy for excimer formation and the binding energy of the excimer were evaluated at different pH values, through the Stevens-Ban-type plots of the excimer-to-monomer intensity ratio. With the time-resolved data, measured in the temperature range of 5-60 degrees C, it was possible to extract the intrinsic activation energies for excimer formation. The thermodynamic driving force for the intrapolymeric association was found to be dependent on a balance between hydrophobic and electrostatic interactions, which are dependent on the pH, temperature, and hydrophobic content of the polymer. 相似文献
936.
Summary The synthesis and x-ray crystal structure oftrans-[Co(dmgH)2(Et)(1,5,6-Me3Bzm)] where dmgH=dimethylglyoximate(–1), and 1,5,6-Me3Bzm=1,5,6-trimethylbenzimidazole, is reported. The compound C19H26N6O4Co is monoclinic, space group P21/n;a=11.700(4);b=24.205(6);c=8.500(3) Å and =101.63(3)°. D(calcd) 1.299 g cm–3; Z=4 and R=0.066 for 2359 independent reflections. Comparison of Co-N(axial ligand) bond lengths for compounds of general formulaetrans-[Co(dmgH)2(R)(L)], with L=pyridine or 1,5,6-trimethylbenzimidazole and R=CH(CN)Cl, CH2NO2, Me, Et,i-Pr, cyclo-hexyl or adamantyl is made. The Co–N(1,5,6-Me3Bzm) bond lengths of the trimethylbenzimidazole derivatives show a fairly linear relationship with the electronic parameter of the axial R group, derived from the13C-n.m.r. spectra of their pyridine analogues. The influence of steric effects on the properties of these CoIII compounds is discussed. 相似文献
937.
Summary The retention of 22 ring-substituted phenol derivatives on porous graphitized carbon (PGC) (eluents: acetonitrile — water and methanol — water mixtures) and on octadecylsilica (ODS) (eluents: methanol — 0.025 M KH2PO4 mixtures) was determined, and the relationship between retention and physicochemical parameters were evaluated by principal component analysis followed by two-dimensional nonlinear mapping and by cluster analysis as well as by canonical correlation analysis. Calculations proved that marked differences can be detected between the retention characteristics of PGC and ODS columns, and the electronic parameters of phenol derivatives have the highest impact on their retention. The comparison of various multivariate mathematical-statistical methods indicated that principal component analysis followed by two dimensional non-linear mapping is the most appropriate method for the evaluation of large data matrices in RP-HPLC. 相似文献
938.
Oscillatory reaction was observed during liquid-phase oxidation of p-xylene in the presence of certain transition metal oxides as catalysts. Experimental conditions under which oscillations occur and parameters influencing this behavior are described.
, , , . - .相似文献
939.
Pavel Šebesta Viktor Dobal Jaroslav Kubát 《Journal of Analytical and Applied Pyrolysis》1982,3(3):263-269
Pyrolysis—gas chromatography of brown coal exhibits additive properties and it is therefore possible to construct the pyrogram of the original coal from the individual pyrograms of the bitumen, humic acids, lignin and humin fractions. The contents of phenols in the pyrograms are typical for all of the above classes except bitumen and are in agreement with the contents of the individual groups in the original coal. The results suggest that the separation does not bring about significant chemical changes in individual brown coal fractions. 相似文献
940.
Method and electronic circuitry are described for synchronization of analytical signal sampling with pulsation of liquid flow in systems with a peristaltic pump. The technique is illustrated for cathodic stripping voltammetry of manganese. 相似文献