全文获取类型
收费全文 | 3205篇 |
免费 | 153篇 |
国内免费 | 16篇 |
专业分类
化学 | 2594篇 |
晶体学 | 24篇 |
力学 | 54篇 |
数学 | 312篇 |
物理学 | 390篇 |
出版年
2023年 | 41篇 |
2022年 | 104篇 |
2021年 | 144篇 |
2020年 | 111篇 |
2019年 | 102篇 |
2018年 | 71篇 |
2017年 | 59篇 |
2016年 | 146篇 |
2015年 | 149篇 |
2014年 | 116篇 |
2013年 | 218篇 |
2012年 | 262篇 |
2011年 | 279篇 |
2010年 | 161篇 |
2009年 | 146篇 |
2008年 | 203篇 |
2007年 | 176篇 |
2006年 | 152篇 |
2005年 | 123篇 |
2004年 | 121篇 |
2003年 | 98篇 |
2002年 | 73篇 |
2001年 | 32篇 |
2000年 | 35篇 |
1999年 | 24篇 |
1998年 | 17篇 |
1997年 | 19篇 |
1996年 | 15篇 |
1995年 | 10篇 |
1994年 | 14篇 |
1993年 | 9篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 16篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1973年 | 3篇 |
1934年 | 2篇 |
1925年 | 3篇 |
1924年 | 3篇 |
排序方式: 共有3374条查询结果,搜索用时 31 毫秒
1.
João Marcos G. Barbosa Milenna K. Fernandes Rodrigues Lurian C. David Taynara C. e Silva Danielly A. Fortuna Lima Naiara Z. Pereira Emmanuel B. D'Alessandro Anselmo E. de Oliveira Paulo H. Jorge da Cunha Maria Clorinda S. Fioravanti Nelson R. Antoniosi Filho 《Biomedical chromatography : BMC》2020,34(11):e4935
An innovative volatolomic approach employs the detection of biomarkers present in cerumen (earwax) to identify cattle intoxication by Stryphnodendron rotundifolium Mart., Fabaceae (popularly known as barbatimão). S. rotundifolium is a poisonous plant with the toxic compound undefined and widely distributed throughout the Brazilian territory. Cerumen samples from cattle of two local Brazilian breeds (‘Curraleiro Pé-Duro’ and ‘Pantaneiro’) were collected during an experimental intoxication protocol and analyzed using headspace (HS)/GC–MS followed by multivariate analysis (genetic algorithm for a partial least squares, cluster analysis, and classification and regression trees). A total of 106 volatile organic metabolites were identified in the cerumen samples of bovines. The intoxication by S. rotundifolium influenced the cerumen volatolomic profile of the bovines throughout the intoxication protocol. In this way, it was possible to detect biomarkers for cattle intoxication. Among the biomarkers, 2-octyldecanol and 9-tetradecen-1-ol were able to discriminate all samples between intoxicated and nonintoxicated bovines. The cattle intoxication diagnosis by S. rotundifolium was accomplished by applying the cerumen analysis using HS/GC–MS, in an easy, accurate, and noninvasive way. Thus, the proposed bioanalytical chromatography protocol is a useful tool in veterinary applications to determine this kind of intoxication. 相似文献
2.
Fernando Bento Cunha Karina Torres Pomini Ana Maria de Guzzi Plepis Virgínia da Conceio Amaro Martins Eduardo Gomes Machado Renato de Moraes Marcelo de Azevedo e Souza Munhoz Michela Vanessa Ribeiro Machado Marco Antonio Hungaro Duarte Murilo Priori Alcalde Daniela Vieira Buchaim Rogrio Leone Buchaim Victor Augusto Ramos Fernandes Eliana de Souza Bastos Mazuqueli Pereira Andr Antonio Pelegrine Marcelo Rodrigues da Cunha 《Molecules (Basel, Switzerland)》2021,26(6)
Autologous bone grafts, used mainly in extensive bone loss, are considered the gold standard treatment in regenerative medicine, but still have limitations mainly in relation to the amount of bone available, donor area, morbidity and creation of additional surgical area. This fact encourages tissue engineering in relation to the need to develop new biomaterials, from sources other than the individual himself. Therefore, the present study aimed to investigate the effects of an elastin and collagen matrix on the bone repair process in critical size defects in rat calvaria. The animals (Wistar rats, n = 30) were submitted to a surgical procedure to create the bone defect and were divided into three groups: Control Group (CG, n = 10), defects filled with blood clot; E24/37 Group (E24/37, n = 10), defects filled with bovine elastin matrix hydrolyzed for 24 h at 37 °C and C24/25 Group (C24/25, n = 10), defects filled with porcine collagen matrix hydrolyzed for 24 h at 25 °C. Macroscopic and radiographic analyses demonstrated the absence of inflammatory signs and infection. Microtomographical 2D and 3D images showed centripetal bone growth and restricted margins of the bone defect. Histologically, the images confirmed the pattern of bone deposition at the margins of the remaining bone and without complete closure by bone tissue. In the morphometric analysis, the groups E24/37 and C24/25 (13.68 ± 1.44; 53.20 ± 4.47, respectively) showed statistically significant differences in relation to the CG (5.86 ± 2.87). It was concluded that the matrices used as scaffolds are biocompatible and increase the formation of new bone in a critical size defect, with greater formation in the polymer derived from the intestinal serous layer of porcine origin (C24/25). 相似文献
3.
Activated carbon production from almond shells using phosphoric acid activation agent was achieved by applying both conventional heating and microwave heating in succession. The morphology and surface properties of activated carbon were studied using thermogravimetric and differential gravimetric analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, and Brunauer–Emmett–Teller analysis. A surface area of 1128 m2/g was achieved by optimizing the microwave power (500?W), microwave application time (15?minutes), conventional heating time (45?minutes), conventional heating temperature (500?°C), and the phosphoric acid:sample ratio (0.7:1). An adsorption capacity of methylene blue of 148?mg/g and an iodine value of 791?mg/g was obtained for the prepared activated carbon. 相似文献
4.
5.
In this paper, the influence functions and limiting distributions of the canonical correlations and coefficients based on affine equivariant scatter matrices are developed for elliptically symmetric distributions. General formulas for limiting variances and covariances of the canonical correlations and canonical vectors based on scatter matrices are obtained. Also the use of the so-called shape matrices in canonical analysis is investigated. The scatter and shape matrices based on the affine equivariant Sign Covariance Matrix as well as the Tyler's shape matrix serve as examples. Their finite sample and limiting efficiencies are compared to those of the Minimum Covariance Determinant estimators and S-estimator through theoretical and simulation studies. The theory is illustrated by an example. 相似文献
6.
Sara Negri 《Archive for Mathematical Logic》2002,41(8):789-810
The main result of this paper is a normalizing system of natural deduction for the full language of intuitionistic linear
logic. No explicit weakening or contraction rules for -formulas are needed. By the systematic use of general elimination rules a correspondence between normal derivations and cut-free
derivations in sequent calculus is obtained. Normalization and the subformula property for normal derivations follow through
translation to sequent calculus and cut-elimination.
Received: 10 October 2000 / Revised version: 26 July 2001 / Published online: 2 September 2002
Mathematics Subject Classification (2000): 03F52, 03F05
Keywords or phrases: Linear logic – Natural deduction – General elimination rules 相似文献
7.
A practical access to alkyl- and aryl-substituted (E)-2-(azidomethyl)alkenoates and related azido compounds from the corresponding allylic bromides in aqueous acetone is described. An alternative method to obtain the starting bromides based on heterogeneous catalysis under mild conditions was also investigated. 相似文献
8.
Jeremy R. Lizotte Sara G. Anderson Timothy E. Long 《Journal of polymer science. Part A, Polymer chemistry》2004,42(7):1547-1556
A novel dinitroxide mediating agent that was suitable for stable free‐radical polymerization was synthesized and used in the block copolymerization of styrene and t‐butyl styrene. Quantitative yields of a novel dinitroxide based on 1,6‐hexamethylene diisocyanate and 4‐hydroxy‐2,2,6,6‐tetramethyl‐1‐piperidinyloxy were obtained. Various experimental parameters, including the nitroxide‐to‐initiator molar ratio, were examined, and it was determined that the polymerization was most controlled under conditions similar to those of conventional 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated stable free‐radical polymerization. Moreover, the dinitroxide mediator proved to be a viable route for the facile two‐step synthesis of triblock copolymers of styrene and t‐butyl styrene. However, the dinitroxide mediation process resulted in a higher than expected level of nitroxide decomposition, which resulted in polymers possessing a terminal alkoxyamine and an adjacent hydroxylamine rather than a preferred internal bisalkoxyamine. This decomposition resulted in the formation of diblock copolymer species during the triblock copolymer synthesis. Gel permeation chromatography was used to monitor the chain‐end decomposition kinetics, and the determined observed rate constant (5.89 × 10?5 s?1) for decomposition agreed well with previous studies for other dinitroxide mediating agents. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1547–1556, 2004 相似文献
9.
Sara Faridi 《Journal of Pure and Applied Algebra》2004,190(1-3):121-136
This paper uses dualities between facet ideal theory and Stanley–Reisner theory to show that the facet ideal of a simplicial tree is sequentially Cohen–Macaulay. The proof involves showing that the Alexander dual (or the cover dual, as we call it here) of a simplicial tree is a componentwise linear ideal. We conclude with additional combinatorial properties of simplicial trees. 相似文献
10.
Humberto César Chaves Fernandes Arthur Rodrigo Barros da Rocha 《International Journal of Infrared and Millimeter Waves》2003,24(7):1171-1176
The use of photonic materials has been used in the theory of optical waves. The PBG (Photonic Band Gap) theory and material, was developed recently for optical frequencies and can be easily applied to millimeter waves, microwaves and planar antenna frequencies. The presence of photonic materials as substrate in antennas has some good characteristics such as, supression of light spontaneous emission and suppression of surface waves, allowing the application in planar antenna array. In this work an elaborate analysis using the full wave Transverse Transmission Line - TTL method, that provides efficient and concise results is applied to the planar antennas array with PBG substrate. 相似文献