全文获取类型
收费全文 | 388539篇 |
免费 | 11611篇 |
国内免费 | 6884篇 |
专业分类
化学 | 212092篇 |
晶体学 | 6276篇 |
力学 | 18059篇 |
综合类 | 334篇 |
数学 | 42507篇 |
物理学 | 127766篇 |
出版年
2022年 | 3575篇 |
2021年 | 4206篇 |
2020年 | 4592篇 |
2019年 | 4699篇 |
2018年 | 5050篇 |
2017年 | 4897篇 |
2016年 | 7548篇 |
2015年 | 5864篇 |
2014年 | 8159篇 |
2013年 | 18074篇 |
2012年 | 15067篇 |
2011年 | 17662篇 |
2010年 | 12158篇 |
2009年 | 11734篇 |
2008年 | 15164篇 |
2007年 | 14819篇 |
2006年 | 14078篇 |
2005年 | 12829篇 |
2004年 | 11338篇 |
2003年 | 9994篇 |
2002年 | 9727篇 |
2001年 | 10731篇 |
2000年 | 8362篇 |
1999年 | 7016篇 |
1998年 | 5932篇 |
1997年 | 5765篇 |
1996年 | 5561篇 |
1995年 | 5216篇 |
1994年 | 4826篇 |
1993年 | 4792篇 |
1992年 | 4996篇 |
1991年 | 5081篇 |
1990年 | 4759篇 |
1989年 | 4667篇 |
1988年 | 4619篇 |
1987年 | 4516篇 |
1986年 | 4259篇 |
1985年 | 5704篇 |
1984年 | 5987篇 |
1983年 | 4889篇 |
1982年 | 5300篇 |
1981年 | 5056篇 |
1980年 | 5003篇 |
1979年 | 5096篇 |
1978年 | 5278篇 |
1977年 | 5133篇 |
1976年 | 5247篇 |
1975年 | 4902篇 |
1974年 | 4763篇 |
1973年 | 5029篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Using neutron activation the nitrogen and fluorine contents in fossile bones could be measured relative to main constituents
of the inorganic bone material (phosphorus and calcium). By determining the ratio of N and F a relative and absolute age determination
was possible. The method is relatively simple, quick, cheap and nondestructive. 相似文献
962.
A simple gas chromatographic method for the determination of clonazepam in human plasma has been developed. After solvent extraction, the compound is measured by an electron capture detector on an OV-17 column. The electron-capture response is linear for 5-120 ng/ml of plasma. There is no interference from other commonly used anti-epileptic drugs or endogenous substrates. Preliminary data from routin monitoring of epileptic patients shows a 10-fold variation in their clonazepam plasma levels. 相似文献
963.
Summary Thermal density fluctuations in amorphous polymethyl methacrylate, polycarbonate and polyethylene terephthalate were studied by means of small angle X-ray scattering. The measurements were performed in a temperature range from 20 °C up to about 50 °C above the glass transition temperatureT
g
of the individual sample.AboveT
g
the experimental values of the fluctuations are proportional to the isothermal compressibility and the temperature of the sample as predicted by the fluctuation theory for a system in thermodynamic equilibrium.At temperatures belowT
g
this relation is no longer correct. The experimentally determined fluctuations are now proportional to the compressibility of the sample in the equilibrium state atT
g
and to the actual temperature of the glassy sample. By considering a statistical ensemble with exchange of energy, particles and order between the systems of the ensemble an equation can be derived for the fluctuations of the number of particles per given volume which predicts the observed behaviour. The order parameter takes into account the fact that the properties of the glassy state depend on the way by which the state was produced.
Zusammenfassung Es wurden thermische Dichtefluktuationen in amorphem Polymethylmethacrylat, Polycarbonat und Polyäthylenterephthalat mit Hilfe der Röntgenkleinwinkelstreuung untersucht. Die Messungen erstrecken sich über einen Temperaturbereich von 20 °C bis zu etwa 50 °C oberhalb der GlastemperaturT g der jeweiligen Probe.In Übereinstimmung mit den Ergebnissen der Fluktuationstheorie sind die Fluktuationen oberhalb vonT g proportional zur isothermen Kompressibilität und zur Temperatur der Probe.Unterhalb der Glastemperatur ist diese Beziehung nicht mehr erfüllt. Hier ist die Fluktuation proportional zur Kompressibilität der Probe im Gleichgewichtszustand beiT g und zur Temperatur. Auf der Basis einer statistischen Gesamtheit mit der Austauschmöglichkeit von Energie, Partikel und Ordnung zwischen den Systemen der Gesamtheit kann eine Beziehung für die Dichtefluktuation abgeleitet werden, die mit den experimentellen Ergebnissen übereinstimmt. Der zusätzliche Ordnungsparameter berücksichtigt die Tatsache, daß die Eigenschaften im Glaszustand wegabhängig sind.相似文献
964.
In an extension of studies of flameless atomizers for atomic-absorption spectrometry, an electrically heated tungsten-rhenium alloy wire loop was examined. Reduction of metallic salts to ground-state metal atoms was accomplished with the high temperature produced by the loop. Lead and nickel were investigated. Experimental parameters such as wavelength, slit width, atomization temperature and sheathing gas flow rate were optimized. Absolute detection limits of 6.6·10?10 and 1.2·10?10, and absolute sensitivities of 7·10?10 and 8·10?11 g of lead, were established for unenclosed and enclosed cells, respectively. The interferences of twenty cations and sixteen anions were studied; foreign cations generally enhanced the lead absorption by retarding its vaporization, allowing the slow detection system to respond more efficiently. Nickel was investigated as a representative less volatile metal; an absolute detection limit of 1.6·10?9 and an absolute sensitivity of 9·10?10 g of nickel were established. 相似文献
965.
Summary A new, simple, and selective test is described for the detection of nitrite ion with isoperthiocyanic acid (I) as the reagent. The test is based on the reaction of a 5% solution ofI with the nitrite ion solution, a deep orange precipitate being formed. Conversely, the test can be used in the identification ofI itself. Presumably, the coloured product is 3-hydroxy-5-thione-1, 2, 4-dithiazole, formed by the action of nitrous acid (producedin situ by the action of sulphuric acid and nitrite) on the 3-amino group ofI. The limit of detection is 300g of NO2
– in a drop (0.05 ml). Most anions (including IO3
–) do not interfere. Only I–, thiourea, and hydrogen peroxide interfere; S2O3
2– interferes only when present in large amounts.
Zusammenfassung Ein neuer, einfacher und selektiver Nachweis von Nitrit mit Isoperthio cyansäure (I) wurde beschrieben. Er beruht auf der Umsetzung einer 5% igen Lösung von I mit Nitritlösung, wobei ein tief orange gefärbter Niederschlag entsteht. Umgekehrt kann die Reaktion auch zum Nachweis von I dienen. Vermutlich handelt es sich bei dem gefarbten Produkt um 3-Hydroxy-5-thion-1, 2, 4-dithiazol, das durch Einwirkung der salpetrigen Säure auf die 3-Aminogruppe von I entsteht. Die Erfassungsgrenze betragt 300g NO2 –im Tropfen (0,05 ml). Die meisten Anionen (inklusive JO3 –) stören nicht. Nur J–, Thioharnstoff und H2O2 stören; Thiosulfat stört nur in großen Mengen.相似文献
966.
Model compounds have been used in studies at 60° in connection with transfer to polymer during the copolymerization of vinyl acetate with esters of fumaric acid. Succinate esters were used to simulate the fumarate ester units and ethyl acetate the vinyl acetate units in the copolymers. The succinate esters are much more reactive than ethyl acetate in transfer reactions with polyvinyl acetate radicals. Methyl isobutyrate and methyl propionate were also examined to assess the difference in reactivity in transfer of tertiary and secondary hydrogen atoms. It is concluded that branching is a very important reaction in the preparation of high conversion copolymers of fumarate esters with vinyl acetate. 相似文献
967.
A. V. Kazymov L. P. Shchelkina N. G. Kabirova A. F. Vompe 《Chemistry of Heterocyclic Compounds》1971,7(11):1452-1455
Several new polymethine dyes of the cyanine and merocyanine series with 1-methyl-1H-imidazo [4,5-b]pyridine residues were synthesized, and their colors were studied. Replacement of the benzimidazole residue in the cyanine dyes by a l-methyl-1H-imidazo[4,5-b]-pyridine residue leads to deepening of the dye color.See [1] for communication II.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1561–1565, November, 1971. 相似文献
968.
The formation constants, log K(mab), for the reactions MA + B right harpoon over left harpoon MAB [where M = Cu(II), Ni, Zn or Cd, A = terdentate ligand and B = bidentate or terdentate ligand] have been determined. Potentiometric evidence is presented for the stepwise addition of the secondary ligand B to the 1:1 metal iminodiacetate (MA). The formation constants and the free energies of formation (DeltaG) have been calculated at 25 +/- 1 degrees and mu = 0.10. The order in terms of secondary ligands has been found to be ASPA > Gly > Aln and Gly > Aln > ASPA with iminodiacetic and nitrilotriacetic acid as primary ligands respectively (ASPA = aspartic acid, Gly = glycine, Aln = dl-alanine). The plot of log K(mab) against log k(mb)(2) shows a linear relationship between the formation constants of the ternary and 1:2 M(II)secondary ligand complexes. 相似文献
969.
The valence ionization potentials (IP's) of cyclopentadiene and 1-sila-cyclopentadiene-(2,4) are studied by an ab initio many-body approach which includes the effect of electron correlation and reorganization beyond the Hartree-Fock approximation. The Hartree-Fock approximation gives the correct ordering of the IP's for cyclopentadiene but this ordering does not agree with the results of the previous experimental and theoretical studies. The ordering is 1a2(π), 2b1(π), 4b2, 6a1, 5a1, 3b2, 1b1 (π), 4a1, 2b2, 3a1. For sila-cyclopentadiene the ordering of the IP's is: 1a2(π), 4b2, 2b1(π), 6a1, 1b1(π), 5a1, 3b2, 4a1, 3a1, 2b2. The Hartree-Fock approximation is found to be incorrect with respect to the ordering of the 4b2 and 2b1(π) IP's. A number of one-electron properties are calculated in the one-particle approximation and compared with the available experimental data. 相似文献
970.
N-methylaminoalkoxides of titanium of the type Ti(OR)4?n(O · CHR′ · CH2 · NR″R?)n where R = Et and Pr1; n = 1–4; and R′ = R″ = H, R? = Me; R′ = H, R″ = R? = Me; R′ = R″ = R? = Me, synthesized by the reactions of titanium alkoxides with aminoalcohols, show interesting variations in their properties like physical state, volatility and molecular complexity. I.r. and p.m.r. spectra of these derivatives have been recorded. A few interchange reactions with methanol and tert-butanol have also been carried out. These aminoalkoxides get cleaved with acetyl chloride and undergo insertion reactions with phenylisocyanate, thus providing the first examples of insertion reactions in such derivatives. 相似文献