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951.
The rate constant of the reaction OH + HCl → H2O + Cl was measured in a flow tube over the temperature range 224 to 460°K using resonance fluorescence detection of OH. An Arrhenius expression k1 = (2.0 ± 0.1) × 10?12 exp [?(620 ± 20 cal/mole)/RT] was obtained. Stratospheric and reaction kinetic implications are discussed briefly. 相似文献
952.
Interpretation of the reduction path of TiNb24O62 is complicated by uncertainty about both the stoichiometric ranges of the possible block structures and the formation of TiNb solid solutions. Reduction forms the Me12O29 phase, probably from the outset, with an initial composition close to Ti2Nb10O29, thereby rapidly depleting the Me25O62 phase of titanium. When log pO2 (atm) has dropped to ?9.62, a phase approximately Ti0.95Nb11.05O29 is in equilibrium with titanium-free Nb25O62 at its lower composition limit (NbO2.471). Nb25O62 is then reduced to Nb47O116 without change in the Me12O29. At ?9.62>log pO2 (atm) > ?10.0, niobium is transferred to the Me12O29 phase and Nb47O116 is consumed. A second univariant equilibrium is set up as Nb47O116 is reduced to Nb22O54. This is consumed in turn, to increase the niobium content of the Me12O29 until, at log pO2 close to ?10.8, monophasic Ti0.48Nb11.52O29 is formed. The (Ti,Nb)O2 solid solution then appears and the final product is Ti0.04Nb0.96O2, with the rutile superstructure cell reported for NbO2. 相似文献
953.
A survey of 16 different chiral auxiliaries and a variety of strategies found that an (-)-8-phenylmenthol ester of a glycine derived migrating group can control the absolute stereochemistry of aza-[2,3]-Wittig sigmatropic rearrangements with diastereoselectivities of ca. 3 : 1 with respect to the auxiliary. In two specific examples, ca. 50% yields of enantiomerically pure products were obtained after chromatographic purification. These were synthetically manipulated with no erosion of stereochemistry into intermediates that completed formal asymmetric syntheses of (+)-HyMePro and (-)-kainic acid. 相似文献
954.
[reaction: see text] Stereo-controlled syntheses of two possible C1-C25 diastereomers of spirastrellolide A containing the cis-disubstituted tetrahydropyran and [6,6]-spiroacetal are reported, exploiting boron-mediated asymmetric aldol and allylation methodology. 相似文献
955.
Z. B. Güvenç M. A. Anderson B. H. Choi 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1995,35(1):51-55
A detailed evaluation of the structural and dynamical properties of isolated water dimers and tetramers using the Lemberg, Stillinger and Rahman potential energy surface and microcanonical molecular dynamics simulations are reported. The lowest-energy configurations of these clusters were obtained by simulated thermal quenching. Relative root-mean-square bond length fluctuations were used to characterize phase transitions. Solid-like, pre-melting, liquid-like, and dissociated states were identified. Global-local minimum transitions and the minimum energy path to dissociation of the dimer were also characterized. 相似文献
956.
A renormalization group method is used to construct approximants for the magnetization,m, and the static structure factor,
(q), for the simple cubic Ising model. Using the best values for the thermal critical index, the transition temperature, and the nearest-neighbor correlation function as input, we obtain recursion relations form and
(q) which lead to reasonable results over a wide range of temperatures and wave numbers. 相似文献
957.
A group of dissolved-bioavailable organochlorine (OC) pesticides and inorganic anions in water and total OC pesticides in sediments were measured in the Malheur Watershed, a semi-arid region in the western United States, over a 2-year period. OC pesticide levels were compared with those from a 1990 study of the lower section of the river, the most recent data available. After calculating the dissolved fraction from the 1990, study it seems that DDD and dieldrin levels have decreased in the water by 50-70%, while DDE and DDT have changed little. Although banned nearly 30 years ago, DDT is still persistent throughout the Malheur River basin/watershed because it was found in all water samples tested. All of the OC pesticides tested during the 2-year study are well below the criterion continuous concentration for aquatic community exposure as defined by the U.S. Environmental Protection Agency (EPA). OC pesticides appear to be decreasing, however, at lower Ontario there remains a human health risk (EPA Human Health Risk Water Quality Criteria) for DDT, because this criteria includes daily consumption of water and fish from the river. Overall, although the upper forest watershed sites have lower OC pesticide concentrations, they represent an important contribution to the total DDT load to this watershed, a source not previously acknowledged. The large increase in DDT and sigmaDDT between the Ontario sites may indicate a possible historical point source of contamination or historical preferential deposition of contamination. Normalized sediment (sigmaDDT/organic carbon) strongly correlates with dissolved water sigmaDDT. 相似文献
958.
James L. Anderson 《General Relativity and Gravitation》1984,16(6):595-601
The method of calculating radiation damping effects by means of energy balance is applied to a simple model problem. The balance equation is derived from the conservation law for energy that is a consequence of the equations of motion for the system by integrating over a four-dimensional space-time region bounded in the wave zone by two future-directed null cones and in the near zone by twot=const surfaces. It is assumed that the sources are in slow motion and the fields are calculated by the method of matched asymptotic expansions. Our principle conclusion is that even though the damping isO(
3) where is an expansion parameter the fields need only be determined toO(1) to calculate the effects of damping by this method.Work supported in part by NSF Grant No. 79-11664-01. 相似文献
959.
Surfaces designed for charge reversal 总被引:2,自引:0,他引:2
Matthews JR Tuncel D Jacobs RM Bain CD Anderson HL 《Journal of the American Chemical Society》2003,125(21):6428-6433
We have created surfaces which switch from cationic at pH < 3 to anionic at pH > 5, by attaching aminodicarboxylic acid units to silica and gold substrates. Charge reversal was demonstrated by monitoring the adsorption of cationic dyes (methylene blue and a tetracationic porphyrin) and an anionic sulfonated porphyrin, at a range of pH using UV-vis absorption and reflection spectroscopy. The cationic dyes bind under neutral conditions (pH 5-7) and are released at pH 1-4, whereas the anionic dye binds under acidic conditions (pH 1-4) and is released at pH 5-7. Gold surfaces were functionalized with two different amphoteric disulfides with short (CH(2))(2) and long (CH(2))(10)CONH(CH(2))(6) linkers; the longer disulfide gave surfaces exhibiting charge reversal in a narrower pH range. Adsorption is much faster on the functionalized gold (t(1/2) = 62 s) than on functionalized silica (t(1/2) = 6900 s), but the final extents of coverage on both surface are similar, for a given dye at a given pH, with maximal coverages of around 2 molecules nm(-)(2). These charge-reversal processes are reversible and can be repeatedly cycled by changing the pH. We have also created surfaces which undergo irreversible proton-triggered charge switching, using a carbamate-linked thiol carboxylic acid which cleaves in acid. These surfaces are versatile new tools for controlling electrostatic self-assembly at surfaces. 相似文献
960.
Molina P Alajarín M Sánchez-Andrada P Carrió JS Martínez-Ripoll M Anderson JE Jimeno ML Elguero J 《The Journal of organic chemistry》1996,61(13):4289-4299
The crystal and molecular structure of carbodiimides 2 (5,6,18,19-tetradehydro-5,12,13,18,25,26-hexahydrotetrabenzo[d,h,m,q][1,3,10,12]tetraazacyclooctadecine) and 3 (8,10,22,24-tetraazapentacyclo[23.3.1.1(3,7).1(11,15).1(17,21)]dotriaconta-1(29),3,5,7(32),8,9,11,13,15(31),17,19,21(30),22,23,25,27-hexadecaene) have been determined. The activation barriers for the racemization of carbodiimides 1 (6,7-dihydrodibenzo[d,h][1,3]diazonine), 2, and 3 have been determined. While 1 presents a relatively high barrier (17.4 kcal mol(-)(1)), 2 and 3 have very low activation barriers (between 5 and 7 kcal mol(-)(1)). We tentatively conclude that open-chain and large-ring carbodiimides racemize by nitrogen inversion or trans-rotation while medium-size cyclic carbodiimides racemize by cis-rotation. 相似文献