全文获取类型
收费全文 | 62847篇 |
免费 | 1609篇 |
国内免费 | 164篇 |
专业分类
化学 | 38458篇 |
晶体学 | 381篇 |
力学 | 1031篇 |
综合类 | 5篇 |
数学 | 11669篇 |
物理学 | 13076篇 |
出版年
2023年 | 403篇 |
2022年 | 611篇 |
2021年 | 842篇 |
2020年 | 907篇 |
2019年 | 930篇 |
2018年 | 1160篇 |
2017年 | 1094篇 |
2016年 | 2091篇 |
2015年 | 1764篇 |
2014年 | 1800篇 |
2013年 | 3869篇 |
2012年 | 3732篇 |
2011年 | 3757篇 |
2010年 | 2521篇 |
2009年 | 2236篇 |
2008年 | 3288篇 |
2007年 | 3086篇 |
2006年 | 2774篇 |
2005年 | 2693篇 |
2004年 | 2275篇 |
2003年 | 1931篇 |
2002年 | 1670篇 |
2001年 | 1298篇 |
2000年 | 1242篇 |
1999年 | 901篇 |
1998年 | 708篇 |
1997年 | 659篇 |
1996年 | 813篇 |
1995年 | 593篇 |
1994年 | 668篇 |
1993年 | 621篇 |
1992年 | 633篇 |
1991年 | 535篇 |
1990年 | 577篇 |
1989年 | 473篇 |
1988年 | 465篇 |
1987年 | 449篇 |
1986年 | 423篇 |
1985年 | 562篇 |
1984年 | 543篇 |
1983年 | 421篇 |
1982年 | 421篇 |
1981年 | 427篇 |
1980年 | 373篇 |
1979年 | 345篇 |
1978年 | 326篇 |
1977年 | 323篇 |
1976年 | 332篇 |
1974年 | 295篇 |
1973年 | 313篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
991.
Close DM Crespo-Hernández CE Gorb L Leszczynski J 《The journal of physical chemistry. A》2005,109(41):9279-9283
In the present study the ionization energy thresholds (IET's) of uracil and thymine have been calculated (with the B3LYP, PMP2, and P3 levels of theory using the standard 6-31++G(d,p) basis set) with one to three water molecules placed in the first hydration shell. Then (B3LYP) polarizable continuum model (PCM) calculations were performed with one to three waters of the hydration shell included. Calculations show there is a distinct effect of microhydration on uracil and thymine. For uracil, one added water results in a decrease in the IET of about 0.15 eV. The second and third water molecules cause a further decrease by about 0.07 eV each. For thymine, the first water molecule is seen to decrease the IET by about 0.1 eV, while the second and third water molecules cause a further decrease of less than 0.1 eV each. The changes in IET calculated here for thymine with one to three waters of hydration are smaller than the experimental values determined by Kim et al. (Kim, S. K.; Lee, W.; Herschbach, D. R. J. Phys. Chem. 1996, 100, 7933). Preliminary results presented here indicate that the experimental results may involve keto-enol tautomers of thymine. The results of placing the microhydrated structures of uracil and thymine in a PCM cavity was seen to make very little difference in the IET when compared to the IET of ordinary uracil or thymine in a PCM cavity. The implications are that accurate calculations of the IET's of uracil and thymine can be obtained by simply considering long-range solvation effects. 相似文献
992.
García Fernández R Montes Bayón M García Alonso JI Sanz-Medel A 《Journal of mass spectrometry : JMS》2000,35(5):639-646
A novel interface design for coupling gas chromatography and inductively coupled plasma mass spectrometry (GC/ICP-MS) was used to perform mercury speciation in biological tissues. Three derivatization approaches were optimized and compared for this purpose: anhydrous butylation using a Grignard reagent, aqueous ethylation by means of NaEt(4)B and aqueous propylation with NaPr(4)B. The last reagent was synthesized in the laboratory as it is not commercially available. Detection limits obtained by GC/ICP-MS ranged between 100 and 200 fg (as absolute mass) for methylmercury and between 500 and 600 fg for inorganic mercury using a 1 microl injection. Quantification of methyl- and inorganic mercury was carried out by resorting to aqueous calibration, using ethylmercury as internal standard for both propylation and butylation derivatization techniques. For ethylation procedures, a methylpropylmercury solution was used as internal standard. The absence of transmethylation during sample preparation was checked using a 97% enriched (202)Hg inorganic standard. The accuracy of the three derivatization approaches was evaluated by the analysis of the certified reference material DOLT-2 (dogfish liver) from the National Research Council of Canada and certified for methylmercury, with satisfactory results. 相似文献
993.
Ramón Puyané Isabelle Guy Renaud Metz 《Journal of Sol-Gel Science and Technology》1998,13(1-3):575-578
The ceramic microstructure, the chemical homogeneity of specific dopants and the mechanical integrity of a varistor disc are critical parameters in determining the transient voltage suppression features of these devices. The material properties and overall quality of the starting ceramic powders used to produce such components are essential in achieving the desired properties. The present work describes a novel chemical method developed to produce doped zinc oxide powders and an industrial scale manufacturing process for the production of final varistor blocks for surge arrester applications. The results are compared with those obtained when using standard varistor powder made by the mixed oxide route is used. All the fundamental electrical properties of the discs have been determined and correlated with the relevant manufacturing steps. 相似文献
994.
T. Cserháti 《Fresenius' Journal of Analytical Chemistry》1993,345(7):541-544
Summary The interaction of the herbicides diquat and paraquat with humic acid and the influence of various salts and pH on the strength of interaction was studied by charge-transfer chromatography. Interactions between humic acids and both herbicides, probably by complex formation, were confirmed; they were stronger with paraquat than with diquat. The complex formation makes the herbicides more hydrophilic, facilitating their movement on cellulose surface. Salts in the environment significantly decreased the strength of interaction, indicating the hydrophilic character of the interactive forces. The charge of the cation also influenced the strength of interaction whereas the effect of ion radii was negligible. The effect of pH was lower than that of salt concentration. 相似文献
995.
Bromazepam, in the form of a cationic iron(II) chelate, can be determined spectrophotometrically at 588 nm with a limit of detection of ca. 10-6 M. When this chelate is ion-paired with perchlorate, it can be extracted into organic solvents such as 1,2-dichloroethane and 4-methyl-2-pentanone, and determined by atomic absorption spectrometry with a limit of detection of 1.5 × 10-5 M bromazepam at the iron resonance 248.3-nm line. Ion-pairs involving the Fe(II), Cu(II) and Co(II) chelates and perchlorate can be separated by h.p.l.c. using a C18 reverse-phase column and a mobile phase of 4:1 water—methanol, with a u.v. detector at 242 nm. This approach allowed for the determination of iron(II) ions in aqueous solution with a limit of detection of 10-8 M. The h.p.l.c. method could also be used to quantify bromazepam spiked in plasma in the concentration range 1–10 μg ml-1, following extraction of bromazepam from plasma and subsequent formation of the iron(II) ion-pair. Copper(II) forms a labile chelate with bromazepam in pH 4.8 acetate buffer which, when subjected to differential pulse voltammetry at the hanging mercury drop electrode, gives rise to a catalytic phenomenon which can be utilised for the determination of bromazepam in the concentration range 10-5–10-9 M. 相似文献
996.
Zusammenfassung Die früher beschriebene Halbmikromethode zur Bestimmung von Isonitrosogruppen in organischen Verbindungen wurde in den Mikromaßstab übergeführt. Sie beruht auf der Reaktion der Substanz mit Eisen(III)-sulfat in verd. Schwefelsäure. Dabei wird der Oximstickstoff als Distickstoffoxid frei, das mit Kupfer zu elementarem Stickstoff reduziert und im Mikroazotometer gemessen wird. Für die Berechnung des Oximstickstoffgehaltes aus den abgelesenen Gasvolumina wurden Werte für die konstante und proportionale Volumenkorrektur mit Hilfe der Methode der Regressionsgeraden experimentell festgestellt. Beide Werte sind den bei der N-N-Gruppenbestimmung benützten Volumenkorrekturen fast gleich. Die Bestimmung von Oximgruppen ist durch die Anwesenheit von Nitro- oder Aminogruppen nicht gestört. Diese Gruppen können aus derselben Einwaage durch nachfolgende Oxydation mit Chromsäure bestimmt werden.
XXII. Mitteilung s.1. 相似文献
Analytical aspects of the oxidation of organic nitrogen compounds with chromic acid. XXIII
Summary The semimicro method previously described for the determination of isonitroso groups in organic compounds has been converted to the microscale. It is based on the reaction of the material with iron(III)-sulfate in dilute sulfuric acid. The oxime nitrogen is thus liberated as dinitrogen oxide, which is reduced to elementary nitrogen by copper and measured in the mieroazotometer. To calculate the oxime nitrogen content from the read-off gas volume, values for the constant and proportional volume correction with the aid of the regression lines were established experimentally. Both values are almost equal in the case of the determination of the volume corrections employed in the determination of the N-N-groups. The presence of nitro-or amino groups does not interfere with the determination of oxime groups. These groups may be determined from the same sample by subsequent oxidation with chromic acid.
XXII. Mitteilung s.1. 相似文献
997.
P. Podhájecký P. Beneš A. Gosman 《Journal of Radioanalytical and Nuclear Chemistry》1979,51(2):315-324
Theoretical and experimental analysis of the open-ended capillary diffusion method for study of the state of trace elements
in aqueous solutions is presented. It is shown by theoretical considerations that the use of the discontinuous capillary method
of diffusion and the evaluation of single measurements using the classical Anderson-Saddington equation yields incorrect diffusion
coefficients if two or more forms of the trace element with significantly different diffusion characteristics are simultaneously
present in the solution. A new method for the evaluation of diffusion is suggested, which enables calculation of the abundance
of two different forms of a trace element. The method employs a continuous measurement of diffusion combined with computer
analysis of the data obtained. Verification of the method is carried out for a particular case using trace radiocerium. The
effect of adsorption on the diffusion measurement and interpretation is discussed. 相似文献
998.
M. Colomina P. Jiménez R. Pérez-Ossorio C. Turrión 《The Journal of chemical thermodynamics》1979,11(12):1179-1184
The enthalpies of formation in the crystalline state at 298.15 K of o-, m-, and p-t-butyl-benzoic acids have been determined by static-bomb calorimetry. Vapour-pressure determinations were made by the Knudsen-effusion technique and the sublimation enthalpies at the mean temperatures of the measurement ranges have been derived for the three acids. The values obtained are:
o-t-butylbenzoic acid | 476.2 ± 1.9 | 99.8 ± 0.4 | (at 314.6 K) |
m-t-butylbenzoic acid | 504.3 ± 1.6 | 103.0 ± 0.5 | (at 326.5 K) |
p-t-butylbenzoic acid | 502.9 ± 1.7 | 103.8 ± 0.4 | (at 334.1 K) |