全文获取类型
收费全文 | 2835篇 |
免费 | 121篇 |
国内免费 | 7篇 |
专业分类
化学 | 2204篇 |
晶体学 | 9篇 |
力学 | 50篇 |
数学 | 405篇 |
物理学 | 295篇 |
出版年
2023年 | 26篇 |
2022年 | 82篇 |
2021年 | 95篇 |
2020年 | 74篇 |
2019年 | 79篇 |
2018年 | 40篇 |
2017年 | 43篇 |
2016年 | 116篇 |
2015年 | 99篇 |
2014年 | 104篇 |
2013年 | 178篇 |
2012年 | 208篇 |
2011年 | 205篇 |
2010年 | 157篇 |
2009年 | 121篇 |
2008年 | 172篇 |
2007年 | 157篇 |
2006年 | 149篇 |
2005年 | 141篇 |
2004年 | 133篇 |
2003年 | 108篇 |
2002年 | 105篇 |
2001年 | 37篇 |
2000年 | 42篇 |
1999年 | 32篇 |
1998年 | 25篇 |
1997年 | 26篇 |
1996年 | 30篇 |
1995年 | 21篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 21篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1970年 | 2篇 |
排序方式: 共有2963条查询结果,搜索用时 15 毫秒
211.
A four step synthesis of 4(S)-oxazolidineacetic acid, 2-oxo benzyl ester (D-Oxac-OBn) from L-Asp-OH in 45% overall yield is reported. The formation of by-products is completely avoided, by microwave irradiation and by the use of caesium carbonate as base. Moreover the synthesis and IR and 1H NMR conformational analysis of the tetramers Boc-L-Val-D-Oxac-L-Ala-OBn and Boc-L-Val-D-Oxac-Aib-L-Ala-OBn in solution is reported. 相似文献
212.
Daniele Cauzzi Claudia Graiff Chiara Massera Giovanni Predieri Antonio Tiripicchio 《Journal of Cluster Science》2001,12(1):259-271
The reaction of [Ru3(CO)12] with Ph2(pyth)PSe (pyth=5-(2-pyridyl)-2-thienyl) allows to obtain two novel clusters [Ru3(3-Se)2(CO)7{P(pyth)Ph2}2] 1 and [Ru3(3-Se)(-PPh2)(-pyth)(CO)6{P(pyth)Ph2}] 2 in satisfactory yields. The first one exhibits the well-known bicapped, open triangular, 50-electron nido-core, whereas 2, whose crystal structure has been determined, shows the rather rare Ru3Se tetrahedron with the Ph2P and pyth fragments as side-bridging ligands. Morever cluster 2 belongs to the exiguous family of selenido-phosphido clusters not easily achievable by other routes. 相似文献
213.
The core of a projective dimension one module is computed explicitly in terms of Fitting ideals. In particular, our formula recovers previous work by R. Mohan on integrally closed torsionfree modules over a two-dimensional regular local ring.
The first author was partially supported by a Faculty Research Fellowship from the University of Kentucky. The second and third authors gratefully acknowledge partial support from the NSF.Mathematics Subject Classification (2000):Primary 13A30, 13B21; Secondary 13C99 相似文献
214.
Ji-Won Moon Lucas W. Yeary Adam J. Rondinone Claudia J. Rawn Melanie J. Kirkham Yul Roh Lonnie J. Love Tommy J. Phelps 《Journal of magnetism and magnetic materials》2007
The magnetic susceptibility (κRT) and saturation magnetization (MS) of microbially synthesized magnetites were systematically examined. Transition metal (Cr, Mn, Co, Ni and Zn)- and lanthanide (Nd, Gd, Tb, Ho and Er)-substituted magnetites were microbially synthesized by the incubation of transition metal (TM)- and lanthanide (L)-mixed magnetite precursors with either thermophilic (TOR-39) or psychrotolerant (PV-4) metal-reducing bacteria (MRB). Zinc incorporated congruently into both the precursor and substituted magnetite, while Ni and Er predominantly did not. Microbially synthesized Mn- and Zn-substituted magnetites had higher κRT than pure biomagnetite depending on bacterial species and they exhibited a maximum κRT at 0.2 cationic mole fraction (CMF). Other TMs’ substitution linearly decreased the κRT with increasing substitution amount. Based on the MS values of TM- and L-substituted magnetite at 0.1 and 0.02 CMF, respectively, Zn (90.7 emu/g for TOR-39 and 93.2 emu/g for PV-4)- and Mn (88.3 emu/g by PV-4)-substituted magnetite exhibited higher MS than standard chemical magnetite (84.7 emu/g) or pure biomagnetite without metal substitution (76.6 emu/g for TOR-39 and 80.3 emu/g for PV-4). Lanthanides tended to decrease MS, with Gd- and Ho-substituted magnetites having the highest magnetization. The higher magnetization of microbially synthesized TM-substituted magnetites by the psychrotroph, PV-4 may be explained by the magnetite formation taking place at low temperatures slowing mechanics, which may alter the magnetic properties compared to the thermophile, through suppression of the random distribution of substituted cations. 相似文献
215.
Mariano Escobar M. Sergio Moreno Roberto J. Candal M. Claudia Marchi Pablo I. Polosecki Silvia Goyanes 《Applied Surface Science》2007,254(1):251-256
The effect of acetylene partial pressure on the structural and morphological properties of multi-walled carbon nanotubes (MWCNTs) synthesized by CVD on iron nanoparticles dispersed in a SiO2 matrix as catalyst was investigated. The general growing conditions were: 110 cm3/min flow rate, 690 °C synthesis temperature, 180 Torr over pressure and two gas compositions: 2.5% and 10% C2H2/N2. The catalyst and nanotubes were characterized by HR-TEM, SEM and DRX. TGA and DTA were also carried out to study degradation stages of synthesized CNTs. MWCNTs synthesized with low acetylene concentration are more regular and with a lower amount of amorphous carbon than those synthesized with a high concentration. During the synthesis of CNTs, amorphous carbon nanoparticles nucleate on the external wall of the nanotubes. At high acetylene concentration carbon nanoparticles grow, covering all CNTs’ surface, forming a compact coating. The combination of CNTs with this coating of amorphous carbon nanoparticles lead to a material with high decomposition temperature. 相似文献
216.
Conditions for bromination at the C3 and C5 positions of a 1,4-dihydropyridine (DHP) were investigated. The effect of base concentration, base type, reaction temperature, and solvent were examined. DHP derivatives with ester and amide moieties were synthesized and brominated. The bromine atoms can be replaced by other substituents, utilizing Suzuki cross coupling. 相似文献
217.
Karla Calfumán María Jesus Aguirre Diego Villagra Claudia Yañez Carmen Arévalo Betty Matsuhiro Leonora Mendoza Mauricio Isaacs 《Journal of Solid State Electrochemistry》2010,14(6):1065-1072
In this work, the modification of a glassy carbon electrode with tetraruthenated porphyrins electrostatically assembled onto
a Nafion film, previously adsorbed on the electrode surface, is reported. This modified electrode was characterized by scanning
electron microscopy–energy-dispersive X-ray, Raman spectroscopy, UV-Vis spectroelectrochemistry, and cyclic voltammetry. The
Nafion film onto the glassy carbon electrode shows a smooth disposition; when the tetraruthenated porphyrin is incorporated
on the Nafion film, the complex is adsorbed in a homogeneous way. The modified electrode catalyzes HSO3− oxidation in water–ethanol solutions and shows an enhanced stability compared with the electrode modified with the dip coating
method. Rotating disk electrode experiments showed a kinetic limitation to the electron transfer controlled by charge propagation
in the film. I/E curves show a Tafel slope of 120 mV/decade corresponding to a first electron-transfer reaction, depending on the potential,
as the determining step. Spectroelectrochemical experiments demonstrated that Ru(II) is the active site for the electrocatalysis. 相似文献
218.
Most of the literature on coded excitation describes the signal-to-noise ratio gain of a coded waveform in terms of the time-bandwidth product. We have shown that in the context of ultrasound imaging, the expression for the SNR gain provided by matched filtering a coded waveform, can be reduced to the total number of chips in the transmit signal. Hence, the SNR gain is independent of both the bandwidth and the duration of a single-chip. This concept is described in detail, clarifying this seeming contradiction. The impact of bandwidth and pulse duration on the SNR, SNR gain and axial resolution is described. Bandwidth requirements and the impact of regulatory peak-power limitations are also addressed. 相似文献
219.
Sunkyu Park David K. Johnson Claudia I. Ishizawa Philip A. Parilla Mark F. Davis 《Cellulose (London, England)》2009,16(4):641-647
The crystallinity index of cellulose is an important parameter to establish because of the effect this property has on the
utilization of cellulose as a material and as a feedstock for biofuels production. However, it has been found that the crystallinity
index varies significantly depending on the choice of instrument and data analysis technique applied to the measurement. We
introduce in this study a simple and straightforward method to evaluate the crystallinity index of cellulose. This novel method
was developed using solid state 13C NMR and subtraction of the spectrum of a standard amorphous cellulose. The crystallinity indexes of twelve different celluloses
were measured and the values from this method were compared with the values obtained by other existing methods, including
methods based on X-ray diffraction. An interesting observation was that the hydration of the celluloses increased their crystallinity
indexes by about 5%, suggesting that addition of water increased cellulose order for all the cellulose samples studied. 相似文献
220.