全文获取类型
收费全文 | 1427篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
化学 | 898篇 |
晶体学 | 16篇 |
力学 | 15篇 |
数学 | 88篇 |
物理学 | 424篇 |
出版年
2021年 | 8篇 |
2020年 | 9篇 |
2019年 | 8篇 |
2017年 | 15篇 |
2016年 | 18篇 |
2015年 | 8篇 |
2014年 | 18篇 |
2013年 | 55篇 |
2012年 | 42篇 |
2011年 | 47篇 |
2010年 | 26篇 |
2009年 | 24篇 |
2008年 | 48篇 |
2007年 | 71篇 |
2006年 | 82篇 |
2005年 | 75篇 |
2004年 | 79篇 |
2003年 | 57篇 |
2002年 | 68篇 |
2001年 | 35篇 |
2000年 | 34篇 |
1999年 | 26篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 30篇 |
1995年 | 14篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 24篇 |
1991年 | 15篇 |
1990年 | 12篇 |
1989年 | 27篇 |
1988年 | 21篇 |
1987年 | 21篇 |
1986年 | 18篇 |
1985年 | 29篇 |
1984年 | 25篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 23篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 25篇 |
1977年 | 18篇 |
1976年 | 13篇 |
1975年 | 10篇 |
1974年 | 21篇 |
1973年 | 24篇 |
1972年 | 12篇 |
1971年 | 8篇 |
排序方式: 共有1441条查询结果,搜索用时 375 毫秒
71.
The room-temperature diffuse-reflectance spectra of compositions within the Li1+xTi2?xO4 spinel system () show three absorption bands in the range 4000 to 48,000 cm?1. Two high-energy absorption bands correspond to charge-transfer transitions from the oxygen-2p valence band to the titanium t2g and σ1 conduction bands, where the σ1 band of eg character has hybridized titanium-3d and titanium-4s parentage. The absorption band arising from promotion of electrons to the empty σ1 band does not alter with composition whereas the absorption band arising from promotion of electrons to the partially filled t2g band narrows as the concentration of conduction electrons in the t2g band decreases. These two high-energy absorption bands fall entirely within the ultraviolet spectral region, and the absorption edge in occurs at 24,300 cm?1 (3.02 eV). A low-energy absorption band is observed in compositions with and in samples of reduced in hydrogen at elevated temperatures. This band straddles the boundary between the visible and infrared spectral regions and shifts toward lower energy as the concentration of conduction electrons in the t2g band decreases. The possible origins of the band are discussed; the argument is in favor of a d-d interband transition from states in the partially filled t2g band to states in the empty σ1 band. 相似文献
72.
We have designed and constructed an atmospheric pressure laser desorption/chemical ionization (AP-LD/CI) source that utilizes a laser pulse to desorb intact neutral molecules, followed by chemical ionization via reagent ions produced by a corona discharge. This source employs a heated capillary atmospheric pressure inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. Preliminary results demonstrate that this technique provides approximately 150-fold increase in analyte ions compared to the ion population generated by atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI). 相似文献
73.
A local hot spot model of gas-surface reactivity is used to investigate the state-resolved dynamics of methane dissociative chemisorption on Pt(111) under thermal equilibrium conditions. Three Pt surface oscillators, and the molecular vibrations, rotations, and the translational energy directed along the surface normal are treated as active degrees of freedom in the 16-dimensional microcanonical kinetics. Several energy transfer models for coupling a local hot spot to the surrounding substrate are developed and evaluated within the context of a master equation kinetics approach. Bounds on the thermal dissociative sticking coefficient based on limiting energy transfer models are derived. The three-parameter physisorbed complex microcanonical unimolecular rate theory (PC-MURT) is shown to closely approximate the thermal sticking under any realistic energy transfer model. Assuming an apparent threshold energy for CH(4) dissociative chemisorption of E(0)=0.61 eV on clean Pt(111), the PC-MURT is used to predict angle-resolved yield, translational, vibrational, and rotational distributions for the reactive methane flux at thermal equilibrium at 500 K. By detailed balance, these same distributions should be observed for the methane product from methyl radical hydrogenation at 500 K in the zero coverage limit if the methyl radicals are not subject to side reactions. Given that methyl radical hydrogenation can only be experimentally observed when the CH(3) radicals are kinetically stabilized against decomposition by coadsorbed H, the PC-MURT was used to evaluate E(0) in the high coverage limit. A high coverage value of E(0)=2.3 eV adequately reproduced the experimentally observed methane angular and translational energy distributions from thermal hydrogenation of methyl radicals. Although rigorous application of detailed balance arguments to this reactive system cannot be made because thermal decomposition of the methyl radicals competes with hydrogenation, approximate applicability of detailed balance would argue for a strong coverage dependence of E(0) with H coverage--a dependence not seen for methyl radical hydrogenation on Ru(0001), but not yet experimentally explored on Pt(111). 相似文献
74.
The reactions of metastable [C5H10O]+ ˙ radical cations produced by ionization of 4-penten-1-ol are reported and discussed. These [C5H10O]+ ˙ species undergo mainly ethyl radical loss, with smaller contributions of methyl radical and water expulsion. 2H-Labelling studies reveal different specificities of hydrogen selection in these three fragmentations. The behaviour of these [C5H10O]+ ˙ ions is compared to those reported previously for isomeric radical cations containing linear alkenyl chains and a terminal hydroxyl group. 相似文献
75.
76.
Self-focussing of high-power TEA CO2 laser pulses for a number of 10 m P-band lines is reported in SF6 molecular gas. Application of this effect to estimating the intensity-dependent refractive index of the gas is demonstrated. 相似文献
77.
The development of a sensitive and specific method of determining individually the five tetraalkyllead compounds normally present in ambient air is described. The method is based on collection of the analytes on a porous polymer using a prefilter to destroy ozone and prevent decomposition of the sample during collection and storage. Two-stage thermal desorption, separation by gas chromatography and detection by modified flame atomic absorption spectrometry gives detection limits (3σ) of ca. 0.25 ng Pb m?3 for tetramethyllead and 0.37 ng Pb m?3 for tetraethyllead in an air sample of 80 dm3 collected over a 3–24 h period. Environmental sampling by this method in paralle with a wet chemical (iodine monochloride) method was used to validate the method. During the comparative study, higher organic lead levels were consistently found by the iodine monochloride method; it is postulated that this indicates the presence of vapour-phase tri-(or di)-alkyllead in excess of 1 ng Pb m?3 in rural air. The versatility of the method is demonstrated by the results of atmospheric sampling at two locations, one rural and one at the kerbside in a city centre. 相似文献
78.
Electron spin resonance (ESR) and magnetic-susceptibility measurements on the Li1+xTi2?xO4 spinel system () indicate the presence of two types of localized moments in this material. In both cases, an unpaired electron is trapped as a Ti3+ ion in a crystal field that is predominantly octahedral, but with a strong tetragonal component. This type of crystal field cannot arise in the stoichiometric spinel. We propose two types of defect in the title spinel system: an oxygen vacancy and a hydroxyl ion. Unpaired electrons are trapped as Ti3+ ions adjacent to these defects, and it is argued that the strong tetragonal field is associated with the formation of a static (TiO)+ ion by a displacement of the titanium ion from the defect. Spin relaxation occurs via a thermal ionization of the trapped electron that appears to be associated with a static-dynamic transition in the titanium-ion displacement. 相似文献
79.
Sabatino Nacson Alex. G. Harrison William R. Davidson 《Journal of mass spectrometry : JMS》1986,21(6):317-319
The collision-induced dissociation (CID) mass spectra of protonated cocaine and protonated heroin have been measured using a triple quadrupole mass spectrometer at 50 eV ion/neutral collision energy for protonated molecules prepared by different protonating agents. The CID mass spectra of protonated cocaine using H+(H2O)n, H+(NH3)n and H+((CH3)2NH)n as protonating agents are essentially identical and it is concluded that, regardless of the initial site of protonation, the fragmentation reactions occurring on collisional activation are identical. By contrast, protonated heorin prepared with H+(H2O)n and H+(NH3)n as protonating agents show substantial differences. That formed by reaction of H+(H2O)n shows a much more abundant peak corresponding to loss of CH3CO2H. From a comparison with model compounds, and from a consideration of the three-dimensional structure of heroin, it is concluded that with H+(H2O)n as protonating agent significant protonation occurs at the acetate group attached to the alicyclic ring, leading to acetic acid loss on collisional activation, but that reaction of H+(NH3)n leads to protonation at the nitrogen function. The proton attached to nitrogen cannot interact with the acetate group and, consequently, the probability of loss of acetic acid on collislional activation is greatly reduced. 相似文献
80.
D.J. Buttrey H.R. Harrison J.M. Honig R.R. Schartman 《Journal of solid state chemistry》1984,54(3):407-413
Growth of large single crystals of quasi-two-dimensional Ln2NiO4 (Ln = La, Pr, Nd) has been accomplished using the radio frequency technique of skull melting under controlled oxygen partial pressures. No evidence of intergrowth of other phases such as Ln3Ni2O7 or higher homologs was observed. Determination of the Ni3+ content indicates that the crystals are cation deficient. 相似文献