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991.
J. Krägel J. B. Li R. Miller M. Bree G. Kretzschmar H. Möhwald 《Colloid and polymer science》1996,274(12):1183-1187
The surface viscoelastic properties of monolayers of two phospholipids DPPC (L--dipalmitoylphosphatidylcholine) and DMPE (L--dimyristoylphosphatidyl-ethanolamin), at the air-water interface have been investigated. Two techniques were used for the investigation. One involved use of an interfacial shear rheometer (torsion pendulum apparatus ISR1), to provide measurements of the shear viscosity s as a function of surface pressure, and the second, a modified LB trough with an oscillating barrier to generate periodie dilation and compression so as to measure the dilational elastic modulusE as a function of surface area.Results indicate a strong dependence of s andE upon monolayer phases. This suggests that the viscoelastic relaxation of monolayers can be understood as molecular rearrangements, domain exchange and molecular reorientations between different monolayer states. 相似文献
992.
The enzyme catalyzed conversion of R/S-(±)-2-octanol with hexanoic acid to R/S-(±)-2-octyl hexanoate has been studied in
different microenvironments and in the presence of the competing substrate ethanol. The reactions were performed in various
gels made from aqueous gelatin solutions and liposome dispersions or isotropic liquid solutions, with or without oil and ethanol.
The lipase Candida sp. (SP 525) was dissolved in the dispersions or solutions stabilized by the naturally occurring zwitterionic surfactant
soybean lecithin. The sectioned porous gel was immersed in hexane containing 0.33 mol dm-3 of racemic 2-octanol and hexanoic acid. Since ethanol acts both as a substrate and as a part of the gel it is of fundamental
interest to know the phase behaviour of the used systems. Partial phase diagrams for the systems ethanol–water–soybean lecithin
and ethanol/water (7:3)–oil–soybean lecithin were determined at 298.2 K. The oil was either castor oil or hexadecane. The
conversion of R-2-octyl hexanoate was about 0.45 when no or small amounts of ethanol was present, but decreased considerably
with high amounts of ethanol present and ethyl hexanoate became the main product. Hydrolysis of R-2-octyl hexanoate was favoured
in the latter systems and hexanoic acid formed was immediately esterified to ethyl hexanoate. The conversion of R-2-octyl
hexanoate and ethyl hexanoate depends only on the ethanol content present in the systems and is thus independent of the environment
of the enzyme. However, the chiral esters synthesized from racemic 2-octanol and hexanoic acid showed high optical purities
regardless of the ethanol content.
Received: 1 July 1996 Accepted: 30 August 1996 相似文献
993.
The measurement is carried out of aqueous solutions. The burner is supplied with a mixture of acetylene and nitrous oxide. Interfering influences by acid and iron concentrations of the sample solution as well as by organic solvents are shown. The standard deviation is ± 0.001%. The agreement with conventional analysis methods is very good. By this procedure a rapid and safe determination of aluminium will be possible. 相似文献
994.
Eberling J Braun P Kowalczyk D Schultz M Kunz H 《The Journal of organic chemistry》1996,61(8):2638-2646
The selective C-terminal deprotection of O-glycopeptide (methoxyethoxy)ethyl esters is achieved under mild conditions (pH 6.6, 37 degrees C) by enzymatic hydrolysis using papain or lipase M from Mucor javanicus to give building blocks useful for chain-extending glycopeptide synthesis. On the other hand, the selective removal of acetyl protecting groups from the saccharide portion of glycopeptides is accomplished by alternative enzymatic hydrolysis with lipase WG from wheat germ to furnish model substrates for enzymatic glycosyl transfer reactions in order to extend the carbohydrate side chain of these conjugates. 相似文献
995.
James O'Donnell Bruno Rusterholz Barbara Aebersold Denise Rüegg Wilhelm Simon Ernö Pretsch 《Mikrochimica acta》1994,113(1-2):45-52
In an investigation of octamethylene bis(malonic acid diamides) and their selectivities for magnesium, it was found that presence of secondary amides within the particular ionophore played a considerable role in the enhancement of magnesium selectivity. Similar effects in other ionophores, i.e. tris(malonic acid diamides), were thus systematically looked at with the help of selectivity measurements with the hope of optimizing the number of secondary and tertiary amides so as to improve the magnesium selectivity. The syntheses of several investigated tris(malonic acid diamide) isologues are equally reported.Deceased in November 1992 相似文献
996.
The amphiphilic shape-persistent macrocycle 1 containing four phenol-OH groups as polar side groups and four hexyloxy groups as nonpolar side groups in an adaptable arrangement was recrystallized from solvents of different polarity. X-ray crystallography reveals that the conformation of the macrocycle is solvent dependent such that in the pyridine solvate only two of the nonpolar side groups point outward while in the THF solvate all four of them point outward. Moreover, in the latter case the three-dimensional packing leads to the formation of a supramolecular channel structure with a large pore size. 相似文献
997.
- The concentration dependence of theF/A-isotherms (premixed spreading, separate spreading) and the equilibrium spreading pressuresF e of the system dipalmitoylcephaline/dimyristoyl-cephaline were studied atT=298 K. The results were analysed and compared by means of thermodynamics with respect to miscibility of the components.
- The spreading technique (pre-mixed spreading, separate spreading (with and without barrier), does not essentially influence the run of theF/A-isotherm. Alone, the collaps pressuresF K differ.
- The concentration dependence of theF/A-isotherms of premixed spread monolayers is analysed in terms of theA-x σ-diagram. The additivity of molecular areas at all surface pressures are assumed to indicate complete miscibility of the cephalins.
- The phase rule according to Crisp was applied to analyse the spreading diagram in connection with the phase diagram of the mixtures in bulk. The results show that the cephalins are completely miscible in the monolayer.
- The experimental collaps pressuresF K of pre-mixed spread monolayers are very much larger than the equilibrium collapse presuresF K calculated according to Garrett. The difference indicate the strong supercompression of mixed film during the compression.
998.
Fluoride ions and, in about 0.005M concentration, tungstate ions form gelatinous precipitates, poorly soluble in water, with calcium ions. The radiometric determinations
of the above ions are based on these reactions. Accurate radiometric determination can be carried out only if the gelatinous
precipitate formed during titration can be properly separated from the solution. It has been found that in the case of fluoride
ions the application of crystalline, easily filtrable Ca(COO)2·H2O or CaCO3, and in the case of tungstate ions the application of CaWO4 precipitate improves the filtration properties of45CaF2 and45CaWO4, respectively, formed during titration, whereby the radiometric determinations of the above ions become possible. Titrations
were carried out with 0.05M and 0.005M CaCl2 solutions, labelled with45Ca. 相似文献
999.
In a comparative study the in vitro corrosion behavior of a selection of nickel- and cobalt-based alloys for application in
dentistry containing no noble metals was studied with slow scan cyclic voltammetry. The obtained breakthrough potentials,
the repassivation behavior and further typical features of the cyclic voltamograms are correlated with the chemical composition
as measured with electron beam microanalysis. Surface inhomogenities detected with the latter method are discussed with respect
to the electrochemical behavior. For all alloys stabilities in terms of breakthrough potential superior to previously reported
data for nickel-base alloys are found. 相似文献
1000.
[11C]Carbon monoxide at low concentrations, aryl halides and amines were used in the palladium-mediated synthesis of twenty 11C-amides. In the study several approaches to improve the radiochemical yield were explored. Eight of the selected amides were prepared by in situ activation of the amines using lithium bis(trimethylsilyl)amide and the radiochemical yields of these reactions were improved compared to utilising a previous reported method. In the synthesis of 1-[carbonyl-11C]benzoyl-3-methyl-1H-indole (11) from 3-methyl-1H-indole (25), the corresponding organotin-amine was prepared prior to the acylation reaction. In a typical experiment, N-(4-hydroxyphenyl)[carbonyl-11C]acetamide (5) was prepared in 15% radiochemical yield using 4-aminophenol (20) but the yield increased to 63% when the amine was activated by lithium bis(trimethylsilyl)amide. 相似文献