全文获取类型
收费全文 | 611篇 |
免费 | 35篇 |
国内免费 | 3篇 |
专业分类
化学 | 460篇 |
力学 | 10篇 |
数学 | 26篇 |
物理学 | 153篇 |
出版年
2022年 | 4篇 |
2021年 | 13篇 |
2020年 | 13篇 |
2019年 | 13篇 |
2018年 | 6篇 |
2017年 | 10篇 |
2016年 | 20篇 |
2015年 | 17篇 |
2014年 | 19篇 |
2013年 | 30篇 |
2012年 | 48篇 |
2011年 | 42篇 |
2010年 | 23篇 |
2009年 | 19篇 |
2008年 | 48篇 |
2007年 | 39篇 |
2006年 | 36篇 |
2005年 | 31篇 |
2004年 | 24篇 |
2003年 | 30篇 |
2002年 | 15篇 |
2001年 | 18篇 |
2000年 | 13篇 |
1999年 | 2篇 |
1998年 | 6篇 |
1996年 | 9篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1967年 | 3篇 |
1966年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有649条查询结果,搜索用时 0 毫秒
1.
This paper investigates second best policies to regulate nitrogen leaching. When the policy maker controls the total nitrogen use, an overall tax is superior to an action equivalent overall quota. When the goal is to regulate the expected level of nitrogen leaching, expected profit of a risk neutral farmer is greater under an overall tax than under an overall quota under a reasonable condition. The welfare ranking of action equivalent taxes and quotas for a risk averse farmer is generally ambiguous. In the numerical analyses, overall taxes turned out to be superior to overall quotas with both targets, despite a very high value of absolute risk aversion, 0.01. These results suggest that an overall tax is probably a superior policy to an overall quota to regulate expected total nitrogen use or expected nitrogen leaching for risk neutral and most of risk averse farmers. 相似文献
2.
We present a review of our recent measurements of total cross sections (Q
T's) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (Q
Ps's) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQ
Ps measurements, our approach involves setting upper and lower limits onQ
Ps using a combination of (1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQ
T's andQ
Ps's for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQ
T's andQ
Ps's for K which also appears in our measurements. The absence of such a peak in our measuredQ
T's andQ
Ps's (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQ
T which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQ
T measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb. 相似文献
3.
The solubility and diffusion coefficient of carbon dioxide in intermediate‐moisture starch–water mixtures were determined both experimentally and theoretically at elevated pressures up to 16 MPa at 50 °C. A high‐pressure decay sorption system was assembled to measure the equilibrium CO2 mass uptake by the starch–water system. The experimentally measured solubilities accounted for the estimated swollen volume by Sanchez–Lacombe equation of state (S‐L EOS) were found to increase almost linearly with pressure, yielding 4.0 g CO2/g starch–water system at 16 MPa. Moreover, CO2 solubilities above 5 MPa displayed a solubility increase, which was not contributed by the water fraction in the starch–water mixture. The solubilities, however, showed no dependence on the degree of gelatinization (DG) of starch. The diffusion coefficient of CO2 was found to increase with concentration of dissolved CO2, which is pressure‐dependent, and decrease with increasing DG in the range of 50–100%. A free‐volume‐based diffusion model proposed by Areerat was employed to predict the CO2 diffusivity in terms of pressure, temperature, and the concentration of dissolved CO2. S‐L EOS was once more used to determine the specific free volume of the mixture system. The predicted diffusion coefficients showed to correlate well with the measured values for all starch–water mixtures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 607–621, 2006 相似文献
4.
H. Fawcett S. Barlag H. Becker E. Belau T. Böhringer M. Bosman V. Castillo V. Chabaud D. Bucholz C. Damerell C. Daum G. De Rijk H. Dietl S. Gill A. Gillman R. Gilmore T. Gooch P. Gras Z. Hajduk E. Higon B. Hyams D. Kelsey J. Kemmer R. Klanner U. Kötz S. Kwan B. Lücking G. Lütjens G. Lutz J. Malos W. Männer E. Neugebauer H. Palka M. Pepé G. Polok J. Richardson M. Rozanska K. Rybicki H. Seebrunner U. Stierlin R. Tapper H. Tiecke M. Turala G. Waltermann S. Watts P. Weilhammer F. Wickens L. Wiggers A. Wylie T. Zeludziewicz ACCMOR Collaboration 《Zeitschrift fur Physik C Particles and Fields》1990,46(4):513-519
The production of the neutralK ? (892) resonances by 200 GeVK ? andπ ? has been studied over the kinematic range 0.0<x f<1.0 andp t 2 <5.0 GeV2. Longitudinal and transverse momentum distributions are presented. In addition the decay angular distributions inK ? fragmentation to \(\bar K^{0*} \) have been investigated. 相似文献
5.
Controlled growth of calcium carbonate by poly(ethylenimine) at the air/water interface 总被引:1,自引:0,他引:1
Two metastable calcium carbonate polymorphs, hemispherical vaterite and needle-like aragonite, are selectively formed at the air/water interface by the mediation of poly(ethyleneimine)(with molecular weights of 25000 and 2000, respectively) dissolved in supersaturated calcium bicarbonate solution. 相似文献
6.
The surface-enhanced Raman scatterings of dimethyl sulfide, diethyl sulfide and dimethyl disulfide have been investigated in silver sol. The dimethyl disulfide molecule decomposes on silver to the corresponding mercaptide implying facile cleavage of its S---S bond. The C---S bond in dialkyl monosulfide appears not to cleave on silver. For diethyl sulfide, the C2 conformation seems to be favorable on silver than other conformations. 相似文献
7.
H. Dijkstra R. Bailey E. Belau T. Böhringer M. Bosman V. Chabaud C. Damerell C. Daum G. de Rijk S. Gill A. Gillman R. Gilmore Z. Hajduk C. Hardwick W. Hoogland B. D. Hyams R. Klanner S. Kwan U. Kötz G. Lütjens G. Lutz J. Malos W. Männer E. Neugebauer H. Palka M. Pepé J. Richardson K. Rybicki H. J. Seebrunner U. Stierlin R. J. Tapper H. G. Tiecke M. Turala G. Waltermann S. Watts P. Weilhammer F. Wickens L. W. Wiggers A. Wylie T. Zeludziewicz ACCMOR Collaboration 《Zeitschrift fur Physik C Particles and Fields》1986,32(3):353-358
The joint production of ? mesons andπ ±,π 0,p, \(\bar p\) ,K s 0 andK ± is investigated using a sample of 600,000 inclusive ? meson events obtained in hadron Be interactions with incidentπ ±,p, \(\bar p\) andK ± beams. Evidence is presented for the joint production of ? mesons and strange particles produced with non-strange incident beams. With incidentK ± beam the number of additional strange particles is suppressed. The results are found to be in agreement with the qualitative predictions of a parton fusion model. The comparison with the Lund model for lowp T processes is fair. 相似文献
8.
Jeong Hyun Kim Seung-Hyun Kang Il Nam Jung Kwan Mook Kim Bok Ryul Yoo Myong Euy Lee 《Journal of Sol-Gel Science and Technology》2004,32(1-3):31-35
Reaction of (triphenylmethyl)silanetriol (1) with cyclopentadienyltitanium trichloride (CpTiCl3) in the presence of triethylamine as a HCl scavenger gave both compounds of a partial open-cage type {[Ph3CSiO(OH)]3(Ph3CSiO3/2)(CpTiO3/2)4} (2) and cube type (Ph3CSiO3/2CpTiO3/2)4 (3). The 1:1 reaction of 1 and CpTiCl3 in toluene solvent at reflux temperature for 3 d afforded compounds 2 (22%) and 3 (36%). When 1 is reacted with a 1.5 fold excess of CpTiCl3 under the same conditions, compound 3 was obtained in high yield (81%) along with 2 in trace quantities. Compounds 2 and 3 were fully characterized by the analyses of 1H, 13C, 29Si NMR, IR, and FAB MS data. The solid-state structure of 3 was determined by a single crystal X-ray diffraction study. Compound 3 had shown to have catalytic activity for the oxidation of alkenes such as 1-octene, cyclooctene, and norbornene with t-butyl hydrogen peroxide. The effect of solvent was observed in this epoxidation reaction. The order of reactivity were decreased as follows: CHCl3 > hexane THF. 相似文献
9.
K. No Hyun J. Lee Ki M. Park Shim S. Lee Kwan H. Noh Sung K. Kim Ji Y. Lee Jong S. Kim 《Journal of heterocyclic chemistry》2004,41(2):211-219
A series of novel 1,3‐altemate calix[4]arene azacrowns having mono and bis crown ethers on the lower rim of the calix[4]arene framework were synthesized. Solid‐state structures confirmed the three dimensional conformation of compounds 1–3. 相似文献
10.
Two highly emissive conjugated polymers with tethered rotaxane repeat units are reported. Hydrogen bonding between acidic alcohols and the N-heteroaromatic groups in the rotaxanes attenuates polymer fluorescence. In addition, the rotaxane groups create precise three-dimensional pockets for metal binding, which results in fluorescence quenching. Exposing thin films of Zn-doped polymers to alcohol vapors reverses the quenching by up to 25%. 相似文献