全文获取类型
收费全文 | 2040篇 |
免费 | 126篇 |
国内免费 | 4篇 |
专业分类
化学 | 1303篇 |
晶体学 | 13篇 |
力学 | 99篇 |
数学 | 263篇 |
物理学 | 492篇 |
出版年
2024年 | 3篇 |
2023年 | 11篇 |
2022年 | 35篇 |
2021年 | 45篇 |
2020年 | 58篇 |
2019年 | 33篇 |
2018年 | 37篇 |
2017年 | 45篇 |
2016年 | 81篇 |
2015年 | 93篇 |
2014年 | 92篇 |
2013年 | 112篇 |
2012年 | 175篇 |
2011年 | 204篇 |
2010年 | 119篇 |
2009年 | 78篇 |
2008年 | 166篇 |
2007年 | 136篇 |
2006年 | 130篇 |
2005年 | 97篇 |
2004年 | 79篇 |
2003年 | 61篇 |
2002年 | 35篇 |
2001年 | 24篇 |
2000年 | 19篇 |
1999年 | 14篇 |
1998年 | 18篇 |
1997年 | 8篇 |
1996年 | 17篇 |
1995年 | 9篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1975年 | 3篇 |
1974年 | 6篇 |
1973年 | 2篇 |
1972年 | 2篇 |
排序方式: 共有2170条查询结果,搜索用时 0 毫秒
991.
A headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC/MS) method was developed using experimental designs to quantify the flavor of commercial Cheddar cheese and enzyme-modified Cheddar cheese (EMCC). Seven target compounds (dimethyl disulfide, hexanal, hexanol, 2-heptanone, ethyl hexanoate, heptanoic acid, delta-decalactone) representative of different chemical families frequently present in Cheddar cheese were selected for this study. Three types of SPME fibres were tested: Carboxen/polydimethylsiloxane (CAR/PDMS), polyacrylate (PA) and Carbowax/divinylbenzene (CW/DVB). NaCl concentration and temperature, as well as extraction time were tested for their effect on the HS-SPME process. Two series of two-level full factorial designs were carried out for each fibre to determine the factors which best support the extraction of target flavors. Therefore, central composite designs (CCDs) were performed and response surface models were derived. Optimal extraction conditions for all selected compounds, including internal standards, were: 50 min at 55 degrees C in 3M NaCl for CAR/PDMS, 64 min at 62 degrees C in 6M NaCl for PA, and 37 min at 67 degrees C in 6M NaCl for CW/DVB. Given its superior sensitivity, CAR/PDMS fibre was selected to evaluate the target analytes in commercial Cheddar cheese and EMCC. With this fibre, calibration curves were linear for all targeted compounds (from 0.5 to 6 microg g(-1)), except for heptanoic acid which only showed a linear response with PA fibres. Detection limits ranged from 0.3 to 1.6 microg g(-1) and quantification limits from 0.8 to 3.6 microg g(-1). The mean repeatability value for all flavor compounds was 8.8%. The method accuracy is satisfactory with recoveries ranging from 97 to 109%. Six of the targeted flavors were detected in commercial Cheddar cheese and EMCC. 相似文献
992.
Moine B Réhault J Aloïse S Micheau JC Moustrou C Samat A Poizat O Buntinx G 《The journal of physical chemistry. A》2008,112(21):4719-4726
The photophysical and photochemical properties of four 3,3-diphenyl-3H-naphtho[2,1- b]pyrans substituted, via an acetylenic junction, to (thiophene) n oligomers (n = 0-3 units) were investigated by transient absorption in the femtosecond to microsecond time domain and by stationary absorption and fluorescence. The decay of the initially produced excited S1(pi pi*) state is found to occur via three competing processes: fluorescence, intersystem crossing, and a ring-opening reaction leading to a colored merocyanine product, with relative yields varying drastically with n. Whereas ultrafast (sub-picosecond) reaction dynamics and high product quantum yield are observed for n = 0 and 1, the reaction is considerably slowed down on going to the n = 2 (105 ps) compound and does not occur for n = 3. A reaction scheme that accounts for this behavior is proposed and the effect of the oligothiophenic chain length on the photoinduced properties is discussed. It is suggested that increasing the chain length from 1 to 3 thiophene units stabilizes the S1(pi pi*) state by pi conjugation and induces an excited-state potential barrier along the reaction pathway. 相似文献
993.
Houillot L Nicolas J Save M Charleux B Li Y Armes SP 《Langmuir : the ACS journal of surfaces and colloids》2005,21(15):6726-6733
The miniemulsion polymerization of styrene has been carried out using two pH-responsive cationic diblock macromonomers as reactive stabilizers. As a comparison, the analogous nonpolymerizable cationic diblock copolymer was also investigated. Each of these three stabilizers based on 2-(diethylaminoethyl)methacrylate and quaternized 2-(dimethylaminoethyl)methacrylate residues were prepared via oxyanionic polymerization and had relatively low polydispersities. It was found that all three copolymers were grafted to the polystyrene latex particles, as judged by X-ray photoelectron spectroscopy, aqueous electrophoresis and FTIR spectroscopy studies. Kinetics studies and colloidal characteristics indicated poorer stabilization properties of the partially quaternized diblock macromonomer and electron microscopy confirmed that the latexes invariably had relatively broad particle size distributions. 相似文献
994.
[reaction: see text] Upon tandem Reformatsky reaction, easily accessible porphyrinic ketones give "locked" chlorinic diester. Both ketones and diesters, as bases or palladium complexes, efficiently generate singlet dioxygen, as demonstrated by trapping with cholesterol. 相似文献
995.
P.?Cladé S.?Guellati-KhélifaEmail author C.?Schwob F.?Nez L.?Julien F.?Biraben 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2005,33(2):173-179
We use Bloch oscillations to accelerate coherently
rubidium atoms. The variation of the velocity induced by this
acceleration is an integer number times the recoil velocity due to
the absorption of one photon. The measurement of the velocity
variation is achieved using two velocity selective Raman -pulses: the first pulse transfers atoms from the hyperfine state
5S1/2,
$" align="middle" border="0">
to 5S1/2,
$" align="middle" border="0">
into a narrow velocity class. After the
acceleration of this selected atomic slice, we apply the second
Raman pulse to bring the resonant atoms back to the initial state
5S1/2,
$" align="middle" border="0">
. The populations in (F=1
and F=2) are measured separately by using a one-dimensional
time-of-flight technique. To plot the final velocity distribution
we repeat this procedure by scanning the Raman beam frequency of
the second pulse. This two -pulses system constitutes then a
velocity sensor. Any noise in the relative phase shift of the
Raman beams induces an error in the
measured velocity. In this paper
we present a theoretical and an experimental analysis of this
velocity sensor, which take into account the phase fluctuations
during the Raman pulses. 相似文献
996.
Julien Marché 《Comptes Rendus Mathematique》2005,340(5):363-368
We study the Gordian graph of all knots in : two knots are adjacent if they differ by a single crossing change. We prove that this graph contains isometrically an infinite countable tree with infinite valency, and that the complement of any finite subset is connected. To cite this article: J. Marché, C. R. Acad. Sci. Paris, Ser. I 340 (2005). 相似文献
997.
A catenane consisting of a large ring threaded through both cyclic units of a handcuff-like compound
The template effect of copper(I) allowed the preparation of a new catenane constructed around a bis-macrocyclic unit; a double ring-closing metathesis reaction afforded the central ring which is threaded through both rings of the bis-macrocycle. 相似文献
998.
Dynamical localization is a localization phenomenon taking place, for example, in the quantum periodically driven kicked rotor. It is due to subtle quantum destructive interferences and is thus of intrinsic quantum origin. It has been shown that deviation from strict periodicity in the driving rapidly destroys dynamical localization. We report experimental results showing that this destruction is partially reversible when the deterministic perturbation that destroyed it is slowly reversed. We also provide an explanation for the partial character of the reversibility. 相似文献
999.
A one-channel time-reversal (TR) experiment is performed inside a rough reverberating cavity in the presence of a rotational flow. The amplitude of the TR wave is plotted versus the distance between the TR channel and the initial source: when they coincide, it exhibits a "time-reversal enhancement" (TRE). With no flow, the TRE is the same as the coherent backscattering enhancement (CBE). But contrary to CBE, the TRE peak is shown to be insensitive to the breaking down of reciprocity due to the flow. This new effect of weak localization is sustained by a diagrammatic approach. 相似文献
1000.
Stimulated emission of surface plasmons at the interface between a silver film and an optically pumped dye solution 总被引:1,自引:0,他引:1
Surface plasmons (SPs) are surface-bound electromagnetic waves supported by metals, offering the possibility of strong spatial confinement of electromagnetic fields on the micro- and nanoscales. They suffer, however, from strong damping caused by internal absorption and radiation losses. Here we demonstrate amplification of SPs by stimulated emission, which marks a possible solution to this problem. We use an attenuated-total-reflection setup to detect stimulated emission of SPs at the interface between a silver film and an optically pumped dye solution acting as the amplifying medium. Clear evidence of stimulated emission is provided by an excellent agreement of the experimental observations with a theoretical analysis. Amplification of SPs can be considered analogous to photon amplification in a laser, thereby suggesting novel approaches in the field of nano-optics. 相似文献