全文获取类型
收费全文 | 18366篇 |
免费 | 1740篇 |
国内免费 | 2980篇 |
专业分类
化学 | 16918篇 |
晶体学 | 190篇 |
力学 | 529篇 |
综合类 | 127篇 |
数学 | 2673篇 |
物理学 | 2649篇 |
出版年
2024年 | 42篇 |
2023年 | 119篇 |
2022年 | 353篇 |
2021年 | 501篇 |
2020年 | 644篇 |
2019年 | 667篇 |
2018年 | 543篇 |
2017年 | 813篇 |
2016年 | 899篇 |
2015年 | 744篇 |
2014年 | 968篇 |
2013年 | 1760篇 |
2012年 | 1310篇 |
2011年 | 1145篇 |
2010年 | 987篇 |
2009年 | 1069篇 |
2008年 | 1117篇 |
2007年 | 1167篇 |
2006年 | 971篇 |
2005年 | 923篇 |
2004年 | 814篇 |
2003年 | 657篇 |
2002年 | 575篇 |
2001年 | 557篇 |
2000年 | 516篇 |
1999年 | 488篇 |
1998年 | 407篇 |
1997年 | 365篇 |
1996年 | 347篇 |
1995年 | 313篇 |
1994年 | 288篇 |
1993年 | 261篇 |
1992年 | 191篇 |
1991年 | 118篇 |
1990年 | 75篇 |
1989年 | 64篇 |
1988年 | 63篇 |
1987年 | 30篇 |
1986年 | 36篇 |
1985年 | 39篇 |
1984年 | 31篇 |
1983年 | 19篇 |
1982年 | 45篇 |
1981年 | 14篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1973年 | 6篇 |
1972年 | 2篇 |
1969年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
21.
22.
Intramolecular (2+2) photocycloaddition of β-stilbazoles tethered by silyl chains took place with high efficiency. Complexation with dicarboxylic acid or catechol further enhanced both the efficiency and stereoselectivity. 相似文献
23.
在聚合物基体中掺入少量的层状硅酸盐所制备的聚合物/粘土纳米复合材料,其阻隔性能明显地优于纯聚合物及其传统的复合材料。实验及分析结果表明,聚合物/粘土纳米复合材料的微观结构和阻隔性能主要受控于粘土剥离后的径厚比.一简单的重整化群模型被用来评估粘土几何因素(诸如径厚比、取向、剥离程度等)对聚合物/粘土纳米复合材料阻隔性能的影响,所得到的逾渗阈值及最佳粘土含量与实验结果吻合。 相似文献
24.
The usual tool for modelling bond ratings migration is a discrete, time‐homogeneous Markov chain. Such model assumes that all bonds are homogeneous with respect to their movement behaviour among rating categories and that the movement behaviour does not change over time. However, among recognized sources of heterogeneity in ratings migration is age of a bond (time elapsed since issuance). It has been observed that young bonds have a lower propensity to change ratings, and thus to default, than more seasoned bonds. The aim of this paper is to introduce a continuous, time‐non‐homogeneous model for bond ratings migration, which also incorporates a simple form of population heterogeneity. The specific form of heterogeneity postulated by the proposed model appears to be suitable for modelling the effect of age of a bond on its propensity to change ratings. This model, called a mover–stayer model, is an extension of a Markov chain. This paper derives the maximum likelihood estimators for the parameters of a continuous time mover–stayer model based on a sample of independent continuously monitored histories of the process, and develops the likelihood ratio statistic for discriminating between the Markov chain and the mover–stayer model. The methods are illustrated using a sample of rating histories of young corporate issuers. For these issuers the default probabilities predicted by the Markov chain and mover–stayer models are different. In particular for 1–4 years old bonds the mover–stayer model estimates substantially lower default probabilities from rating C than a Markov chain. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
25.
A 2D HgII coordination polymer containing ligands 1,2,4‐triazole (Htrz) and thiocyanate, [Hg(μ3‐trz)(SCN)]n ( 1 ) has been synthesized and characterized by elemental analysis and IR spectroscopy. The single‐crystal X‐ray data show the coordination number of Hg atoms is four and the ligand trz? acts as a three‐fold donor. The thermal stability of compound 1 was studied by thermal gravimetric and differential thermal analyses. The composition and formation of the complex in methanol solution were found to be in support of its solid state structure. 相似文献
26.
《Fiber and Integrated Optics》2007,26(6):335-342
A fiber-optic chemical sensor (FOCS) for detection of nitrogen dioxide (NO2) molecules is reported. The FOCS presents an optropode structure because of the transmission properties of the sensitive material. The NO2 FOCS is activated by using the semiconductor polymer: regioregular head-to-tail poly(3-octylthiophene-2,5-diyl). The operation wavelength of the sensor is 543.5 nm such that a simple LED and detector can be used for the design of this device. The sensor response decreases after each exposure, demonstrating the reduction in sensitivity as well as irreversibility lower than 5%. However, its properties such as rapid response, high selectivity, high sensitivity (0.43 ± 0.01 muW/ppm), hygroscopic properties, and its operation at room temperature make this kind of FOCS a good alternative for NO2 toxic gas detection. 相似文献
27.
The numerical prediction of the fields of inelastic strains (the linear invariant of the tensor of inelastic strains) in thermoset polyester/marble filler composite plates is discussed. A uniformly distributed load is applied to the plates, which lie on a steel base. The strain fields are predicted by means of the boundary element method by using creep test data for the composites and the polyester matrix itself. Identical creep tests were performed for two ages of the materials (1 month and 13 years), which allowed evaluating the aging effect. The study is carried out in two stages. At the first stage, the application of the generalized Maxwell-Gurevich equation to the thermoset matrix/mineral filler composite is demonstrated. The model parameters determined from the experimental creep data is used for the second stage, where the state of inelastic strains in the plates is predicted by applying the boundary element method. The influence of composite formulation (filler content) and physical aging of the polyester matrix on the state of inelastic strains in the plates is shown.__________Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 41, No. 2, pp. 145–156, March–April, 2005. 相似文献
28.
The title compound, [Cd(SeCN)2(pyCN)2]∞, adopts an extended one‐dimensional chain structure in which the neighboring cadmium atoms are bridged by two selenocyanate ions. The central cadmium atom has a distorted octahedral geometry defined by two isonicotinonitrile and four selenocyanate ions in a 4N2Se fashion. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
29.
Ching Sum Hui Lillian Sze Man Lam Chun Yin Wai Kin Chan Aleksandra B. Djurii 《Journal of polymer science. Part A, Polymer chemistry》2003,41(11):1708-1715
Polyurethanes incorporated with rhenium diimine complexes were synthesized. The polymers exhibited interesting morphologies and solution properties. Results from gel permeation chromatography suggested the formation of polymer aggregates in solutions. The polymers could act as probes for metal ions. The addition of metal ions to polymer solutions led to significant changes in the electronic absorption properties of the polymer solutions. This was attributed to the interactions between the polyether moieties and metal ions. The metal complexes could also act as efficient photosensitizers. After doping with charge‐transport viologens, the photoconductivity of the polymers was greatly enhanced. The experimental quantum efficiency was simulated with Onsager's theory. The thermalization distances and the primary yields were typically 12–14 Å and 10?3, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1708–1715, 2003 相似文献
30.