全文获取类型
收费全文 | 1422篇 |
免费 | 59篇 |
国内免费 | 4篇 |
专业分类
化学 | 994篇 |
晶体学 | 3篇 |
力学 | 19篇 |
数学 | 123篇 |
物理学 | 346篇 |
出版年
2023年 | 5篇 |
2022年 | 3篇 |
2021年 | 21篇 |
2020年 | 22篇 |
2019年 | 24篇 |
2018年 | 20篇 |
2017年 | 10篇 |
2016年 | 30篇 |
2015年 | 29篇 |
2014年 | 34篇 |
2013年 | 77篇 |
2012年 | 83篇 |
2011年 | 100篇 |
2010年 | 70篇 |
2009年 | 57篇 |
2008年 | 69篇 |
2007年 | 69篇 |
2006年 | 78篇 |
2005年 | 79篇 |
2004年 | 64篇 |
2003年 | 47篇 |
2002年 | 43篇 |
2001年 | 44篇 |
2000年 | 45篇 |
1999年 | 31篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 19篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 28篇 |
1992年 | 23篇 |
1991年 | 15篇 |
1990年 | 16篇 |
1989年 | 15篇 |
1988年 | 19篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 19篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 3篇 |
1970年 | 3篇 |
1967年 | 2篇 |
排序方式: 共有1485条查询结果,搜索用时 46 毫秒
101.
102.
A spectrofluorimetric method has been developed for the determination of two angiotensin II receptor antagonists (ARA II): Losartan and Valsartan. A fractional factorial design and a central composite design were used. The key factors considered in the optimization process were pH, temperature and emission slit width. Maximum fluorescent intensity was established as response for each experiment. The response surfaces confirmed the robustness of the method. A clean-up procedure was used for urine samples that consisted of a solid-phase extraction using C8 cartridges. The total analysis time was lower than 30 min. This method proved to be accurate (RE, 8%), precise (intra- and inter-day coefficients of variation were lower than 8% and sensitive enough (LOQ c.a. 0.5 μg ml−1) to be applied to the determination of Losartan and Valsartan in urine samples. 相似文献
103.
Alonso S Sendiña-Nadal I Pérez-Muñuzuri V Sancho JM Sagués F 《Physical review letters》2001,87(7):078302
A pacemaker, regularly emitting chemical waves, is created out of noise when an excitable photosensitive Belousov-Zhabotinsky medium, strictly unable to autonomously initiate autowaves, is forced with a spatiotemporal patterned random illumination. These experimental observations are also reproduced numerically by using a set of reaction-diffusion equations for an activator-inhibitor model, and further analytically interpreted in terms of genuine coupling effects arising from parametric fluctuations. Within the same framework we also address situations of noise-sustained propagation in subexcitable media. 相似文献
104.
The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton. 相似文献
105.
Sendina-Nadal I Alonso S Perez-Munuzuri V Gomez-Gesteira M Perez-Villar V Ramirez-Piscina L Casademunt J Sancho JM Sagues F 《Physical review letters》2000,84(12):2734-2737
Spiral chemical waves subjected to a spatiotemporal random excitability are experimentally and numerically investigated in relation to the light-sensitive Belousov-Zhabotinsky reaction. Brownian motion is identified and characterized by an effective diffusion coefficient which shows a rather complex dependence on the time and length scales of the noise relative to those of the spiral. A kinematically based model is proposed whose results are in good qualitative agreement with experiments and numerics. 相似文献
106.
Ultrasonic non-destructive testing systems designed to control huge structures normally use several transducers in the reception stage. To avoid increasing the cost of electronics, a multiplexer is used to send all received signals to the same processing module. Traditionally, transmission of such signals is carried out using copper cables. For special applications (i.e. continuous monitoring of nuclear plants) metallic cables are not suitable because of their high sensitivity to electromagnetic perturbations. Moreover, the multiplexing is made electronically. When the distance between the transducers and the reception unit is large and/or electromagnetic noise is important, signal degradation takes place. The proposed system implements the transmission and multiplexing of ultrasonic electrical signals obtained by means of broadband transducers (up to 1 MHz), using an optical fiber. Optical fibers are made of dielectric materials (silica or plastic) so they are inherently passive to electromagnetic noise. Wavelength division multiplexing is utilized for adding channels to the system by means of fiber optic couplers and different light sources. The wavelengths of the optical signals utilized are located far apart in the optical spectrum in order to avoid serious crosstalk in transmission. The limit to the number of multiplexed channels depends on the optical fiber selected, the spectrum of the light sources and the wavelength division multiplexers or couplers utilized. 相似文献
107.
Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results. 相似文献
108.
Evolutionary spatial 2×2 games between unconditional agents are analysed introducing several variants of cellular automata (CA). 相似文献
109.
Rosa Maria Alonso‐Salces Claude Guillou Luis A. Berrueta 《Rapid communications in mass spectrometry : RCM》2009,23(3):363-383
Liquid chromatography coupled with a photodiode array detector, electrospray ionization, collision‐induced dissociation and tandem mass spectrometry (LC‐DAD/ESI‐CID‐MS/MS) on a triple quadrupole (QqQ) has been used to detect and characterize polyphenols and methylxanthines in green coffee beans: three phenolic acids (caffeic acid, ferulic acid and dimethoxycinnamic acid), three isomeric caffeoylquinic acids (Mr 354), three feruloylquinic acids (Mr 368), one p‐coumaroylquinic acid (Mr 338), three dicaffeoylquinic acids (Mr 516), three feruloyl‐caffeoylquinic acids (Mr 530), four p‐coumaroyl‐caffeoylquinic acids (Mr 500), three diferuloylquinic acids (Mr 544), six dimethoxycinnamoyl‐caffeoylquinic acids (Mr 544), three dimethoxycinnamoyl‐feruloylquinic acids (Mr 558), six cinnamoyl‐amino acid conjugates, three cinnamoyl glycosides, and three methylxanthines (caffeine, theobromine and theophylline). Dimethoxycinnamic acid, three isomers of dimethoxycinnamoyl‐caffeoylquinic acids and another three of dimethoxycinnamoyl‐feruloylquinic acids, as well as the three cinnamoyl glycosides, had not previously been reported in coffee beans. Structures have been assigned on the basis of the complementary information obtained from UV‐visible spectra, relative hydrophobicity, scan mode MS spectra, and fragmentation patterns in MS2 spectra (both in the positive and negative ion modes) obtained using a QqQ at different collision energies. A structure diagnosis scheme is provided for the identification of different isomers of polyphenols and methylxanthines. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
110.