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101.
The title sterically crowded di-9-anthrylethyne derivative with 3,5-iPr2-phenyl groups at 1-position showed a barrier to rotation about the acetylenic axis of 18.0 kcal mol−1 based on a dynamic NMR study, which is an extremely high value for acyclic diarylethynes. The mechanism of the dynamic stereochemistry and the substituent effect on the rotational barrier are discussed. 相似文献
102.
Josef Janča 《Mikrochimica acta》1994,112(5-6):197-215
The primary field forces can generate spatially oriented gradient of the effective property of a continuum or pseudo-continuum fluid (carrier liquid). When this gradient is coupled with the action of a secondary field of identical or different nature the isoperichoric focused zones of the dispersed species can appear. Consequently, they can be separated according to differences responding to the property gradient of the carrier liquid. This concept can be applied under static (non-flow) conditions in thin layer focusing as well as under dynamic conditions with the elution due to the carrier liquid flow in focusing field-flow fractionation. The gradient established by the action of the primary field and the concentration distribution of the isoperichoric focused zone formed by the coupled effect of the gradient and of the primary or secondary field are described theoretically. The rigorous relationship describing the shape of the focused zone is compared with the approximate solutions. The performances of the proposed principle were evaluated by model calculations. Potential experimental configurations considering the implementation of the static and dynamic conditions are discussed. The generalized isoperichoric focusing theory can be applied to describe the particular processes operating in analytical and preparative focusing separations of the particles of various, but especially of biological origin. 相似文献
103.
104.
Wojciech Paszkowicz 《Analytica chimica acta》2006,566(1):81-98
Genetic algorithms represent a powerful global-optimisation tool applicable in solving tasks of high complexity in science, technology, medicine, communication, etc. The usual genetic-algorithm calculation scheme is extended here by introduction of a quadratic self-learning operator, which performs a partial local search for randomly selected representatives of the population. This operator is aimed as a minor deterministic contribution to the (stochastic) genetic search. The population representing the trial solutions is split into two equal subpopulations allowed to exhibit different mutation rates (so called asymmetric mutation). The convergence is studied in detail exploiting a crystallographic-test example of indexing of powder diffraction data of orthorhombic lithium copper oxide, varying such parameters as mutation rates and the learning rate. It is shown through the averaged (over the subpopulation) fitness behaviour, how the genetic diversity in the population depends on the mutation rate of the given subpopulation. Conditions and algorithm parameter values favourable for convergence in the framework of proposed approach are discussed using the results for the mentioned example. Further data are studied with a somewhat modified algorithm using periodically varying mutation rates and a problem-specific operator. The chance of finding the global optimum and the convergence speed are observed to be strongly influenced by the effective mutation level and on the self-learning level. The optimal values of these two parameters are about 6 and 5%, respectively. The periodic changes of mutation rate are found to improve the explorative abilities of the algorithm. The results of the study confirm that the applied methodology leads to improvement of the classical genetic algorithm and, therefore, it is expected to be helpful in constructing of algorithms permitting to solve similar tasks of higher complexity. 相似文献
105.
Stern HA 《Journal of computational chemistry》2004,25(5):749-761
We review principles of non-Hamiltonian statistical mechanics and present a new set of equations and integration algorithm for isothermal-isobaric dynamics. The chief advantage of the present scheme is that it is somewhat simpler than previous methods. We perform numerical simulations to test the accuracy of the algorithm and compare its stability to that of a "gold standard," a symplectic integrator for Hamiltonian dynamics of the same system. The stability of the isothermal-isobaric algorithm is comparable to the stability of the symplectic integrator. 相似文献
106.
建立了气相色谱过程的动态数学模型,给出了模型的实用算法,并据此对各种操作条件下的色谱流出曲线进行了动态模拟计算。模拟计算结果表明该模型是有效的,能够用于各种程序控制的色谱过程,可得到色谱分离的优化条件。 相似文献
107.
108.
D. J. Burlett 《Journal of Thermal Analysis and Calorimetry》2004,75(2):531-544
The transfer of heat through an elastomeric matrix is important for both the processing of the material and its subsequent
lifetime. Thermal conductivity can be used to evaluate the influence of different polymers and fillers on heat transfer. Additionally,
the dispersion of the filler has an effect on heat transfer and thermal conductivity measurements can be used to provide semi-quantitative
estimations of filler dispersion. The degradation of sulfur-crosslinked elastomer systems has been studied for many years.
The degradation of the crosslinks (changes in sulfur rank) and degradation of the polymer backbone by thermal and/or oxidative
processes have been studied extensively using many techniques including thermal analysis (references). However, the degradation
of the crosslinked-polymer 'network' is less well understood. The relationship of the crosslink network to this degradation
process is a key to both the long term and higher temperature performance of the sulfur-crosslinked elastomer. The changes
in physical properties observed upon exposure of sulfur-crosslinked elastomers can be monitored using dynamic mechanical analysis.
Subsequently, other thermal techniques can be used to monitor the chemistry that is occurring during these degradations. Thermal
desorption/mass spectroscopy and dynamic scanning calorimetry are used to complete the picture of the degradation processes
taking place. Examples of these techniques will be provided to illustrate the utility of the analytical approach, the chemistry
involved in these degradation processes and the effect of changes in the polymer, cure package and other ingredients.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
109.
S. Fakirov Z. Sarac T. Anbar B. Boz I. Bahar M. Evstatiev A. A. Apostolov J. E. Mark A. Kloczkowski 《Colloid and polymer science》1997,275(4):307-314
This second part of a systematic study of the properties of crosslinked-oriented gelatin involves the effects of orientation
and water content on the glass transition temperature T
g and on the melting behavior. The samples were the same as those in the preceding study, and their transition temperatures
were determined by both differential scanning calorimetry and dynamic mechanical thermal analysis. The crosslinked gelatin
which had been room-conditioned showed two transition temperatures: the lower one was attributed to T
g of the water-plasticized gelatin, and the higher one was interpreted as T
g of dried gelatin superimposed by melting. A rather unusual situation arose because of the fact that the T
g and melting temperatures T
m (217 and 230 °C, respectively) are so similar. Using water as plasticizer not only decreases T
g but produces imperfect crystallites which melt below the T
g of the system. The presence of the amorphous phase in the glassy state would presumably make it essentially impossible to
define a melting point or crystallization temperature in the normal manner, as an equilibrium between crystalline and amorphous
phases.
Received: 8 October 1996 Accepted: 2 November 1995 相似文献
110.
HeinzD. Roth 《Helvetica chimica acta》2006,89(12):2847-2860
Chemically induced dynamic nuclear polarization (CIDNP) observed during electron transfer (ET) reactions of tertiary amines such as DABCO ( 1 ) or Et3N ( 2 ) with a wide range of electron acceptors support the involvement of amine radical‐cations (e.g., 1. + or 2. + ) as key intermediates. Radical ions such as 2. + may be deprotonated, generating neutral aminoalkyl radicals (e.g., 2. ). When generated by reaction with an electron acceptor of energetically low triplet state such as naphthalene (1Naph*), the resulting pair 2. + /Naph.? reacts mostly by reverse electron transfer (RET) from triplet pairs populating the naphthalene triplet state. 相似文献