全文获取类型
收费全文 | 304736篇 |
免费 | 16906篇 |
国内免费 | 674篇 |
专业分类
化学 | 191490篇 |
晶体学 | 4178篇 |
力学 | 12885篇 |
综合类 | 9篇 |
数学 | 30522篇 |
物理学 | 83232篇 |
出版年
2023年 | 4441篇 |
2021年 | 3980篇 |
2020年 | 6508篇 |
2019年 | 4366篇 |
2018年 | 5383篇 |
2017年 | 3672篇 |
2016年 | 9646篇 |
2015年 | 7828篇 |
2014年 | 8893篇 |
2013年 | 15430篇 |
2012年 | 10920篇 |
2011年 | 10782篇 |
2010年 | 10189篇 |
2009年 | 9921篇 |
2008年 | 10218篇 |
2007年 | 10119篇 |
2006年 | 8950篇 |
2005年 | 8112篇 |
2004年 | 7453篇 |
2003年 | 6764篇 |
2002年 | 6671篇 |
2001年 | 7352篇 |
2000年 | 5670篇 |
1999年 | 4372篇 |
1998年 | 3870篇 |
1997年 | 3881篇 |
1996年 | 3626篇 |
1995年 | 3435篇 |
1994年 | 3554篇 |
1993年 | 3362篇 |
1992年 | 3723篇 |
1991年 | 3723篇 |
1990年 | 3631篇 |
1989年 | 3499篇 |
1988年 | 3630篇 |
1987年 | 3514篇 |
1986年 | 3319篇 |
1985年 | 4402篇 |
1984年 | 4684篇 |
1983年 | 3926篇 |
1982年 | 4072篇 |
1981年 | 4002篇 |
1980年 | 3995篇 |
1979年 | 3966篇 |
1978年 | 4276篇 |
1977年 | 4247篇 |
1976年 | 4335篇 |
1975年 | 4012篇 |
1974年 | 4066篇 |
1973年 | 4155篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Brush S.G. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1992,20(6):577-589
Hannes Alfven's research program in solar system physics provides a valuable case study for the historians and philosophers of science. The response of the scientific community to his remarkable contributions is a severe test of widely held views about how science is or should be done. Seven of those contributions are examined, in the light of two doctrines of scientific methodology, those of Karl Popper (1959) and Imre Lakatos (1978). The seven are: application of the magnetic braking concept to the origin of the solar system; magnetohydrodynamic waves; field-aligned (`Birkeland') currents; critical ionization velocity and the existence of planetary rings; jet streams; electrostatic double layers; and partial corotation 相似文献
992.
993.
Dumbrajs O. Nuisnovich G.S. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1992,20(4):452-457
A theory is presented which describes the dependence of the multimode operation of a gyrotron on the modulation and beam voltages. Nonlinear nonstationary processes are considered. An approximate start-up scenario of a gyrotron is determined which makes it possible to achieve single-mode excitation and high-efficiency operation in a desired mode, as well as to switch the tube to a similar operation in other modes 相似文献
994.
Prof. A. D. Drozdov 《Archive of Applied Mechanics (Ingenieur Archiv)》1995,65(2):99-109
Summary Dynamic stability of an elastic bar with voids is considered. Using the Lyapunov approach some new sufficient stability conditions are obtained and explicit expressions for the critical load are derived. 相似文献
995.
C. Caldera N. Caponio C. de Bernardi F. Delpiano G. Destefanis S. Morasca M. Puleo G. Schiavini F. Schiattone A. Stano E. Legros M. Billard 《Optical and Quantum Electronics》1994,26(5):S529-S545
Optical waveguide 3-dB couplers integrated on semicondutors have been studied, designed and fabricated, using both bulk and diluted multi-quantum-well InGaAsP/InP-based materials, grown by MOCVD. The device structure is based on the two-mode interference (TMI) principle and is fully compatible for integration with the optoelectronic and electronic components of a coherent receiver. Bulk material couplers provide an output balanced within 0.05 dB per nanometer and an excess loss of 1.4 dB, compared to a straight guide, while coupling loss to a tapered-lensed single mode fibre is 4 dB.Improved coupling efficiency to single-mode fibres is achieved by use of moderately diluted multi-quantum-well waveguides, which include InGaAsP wells and InP barriers: coupling loss to a tapered-lensed single-mode fibre as low as 0.5 dB and excess loss of 1.8 dB are featured. Couplers fabricated with this waveguide structure have a balance sensitivity of 0.03 to 0.04 dB per nanometer.A moderately diluted multi-quantum-well 3-dB coupler has been permanently pigtailed and butt-coupled to a dual balanced PIN photoreceiver. This hybrid assembly was tested in a coherent transmission system at 155 and 622 Mbits-1 showing sensitivities, for 10-9 BER, of about-38.0 dBm and-28.8 dBm, respectively. 相似文献
996.
G. V. Golubev 《Journal of Mathematical Sciences》1992,58(1):69-72
We consider nonlinear seepage with an initial pressure gradient in a region without stagnant zones. An algorithm based on the method of integral relationships is proposed for the determination of an auxiliary function defined as the square root of the permeability coefficient of the layer. The advantage of this method is that it reduces the problem to a form which can be solved by standard computer subprograms. Applications of the proposed approach are described.Translated from Vychislitel'naya i Prikladnaya Matematika, No. 55, pp. 87–92, 1985. 相似文献
997.
J. P. S. Farinha J. M. G. Martinho H. Xu M. A. Winnik R. P. Quirk 《Journal of Polymer Science.Polymer Physics》1994,32(9):1635-1642
Cyclization of a polystyrene chain (Mn = 10,600; Mw/Mn = 1.09) both ends labeled with 4-(1-pyrenyl)butanoamide groups was studied in cyclohexane between 25 and 95°C. The amide groups (peptide bonds) at both ends can form an intrachain hydrogen bond between the amide hydrogen at one chain end and the carbonyl oxygen at the other. The presence of two sets of conformers, random coils, and chains cyclized through hydrogen bonding, complicates the data analysis. The pyrene excimer kinetics of this polymer is well described by a model composed of two monomers (hydrogen bonded and nonbonded chains) and one excimer, in equilibrium. The cyclization rate constant for hydrogen-bonded chains is larger than the one for nonhydrogen-bonded chains. The pyrene excimer binding energy (ca. 1.6 kcal/mol) is lower than the published value for nonhydrogen-bonded chains (~ 9 kcal/mol), suggesting that intrachain hydrogen bonding hinders the stabilization of the excimer. © 1994 John Wiley & Sons, Inc. 相似文献
998.
999.
Confined thin film melt polymerization (CTFMP) of naphthalene chloride/hydroquinone (NCMQ, 1/1, molar)mixtures at polymerization temperatures (T_p) below ca. 300℃ resulted in relatively thick, elongated crystals. Polymerizationof NC/HQ above 300℃ between glass yielded well-formed lamellar crystals ca. 100 A thick. Phase Ⅰ and Ⅱ [001] EDpatterns were obtained for all T_p, the relative amount of phase Ⅰ increasing with T_p. Polymerization of naphthalenedicarboxylic acid/hydroquinione diacetate 1/1 mixtures at high T_p also yielded lamellar crystals that "curled up" off of thesubstrate. When the high temperature CTFMP polymerization was conducted between mica, aggregates of lamellae on-edgedeveloped but epitaxial growth did not occur. Epitaxial growth of lamellae between mica could be obtained, however, byconfined thin film solution polymerization, with both of the latter samples yielding apparently related ED patterns from adifferent unit cell than phase Ⅰ or Ⅱ. Fiber patterns, obtained from sheared samples, indicated considerably greater crystaldisorder than in the nascent crystals. Refinement of the phase Ⅰ unit cell parameters, based on the [001] and [01 1] EDpatterns, with modeling based on Cerius~2, suggests a monoclinic phase Ⅰ unit cell with a = 7.76, b = 5.71, c = 14.99 A, α = γ= 90°, β= 99.7°, ρ = 1.47 g/cm~3, space group P12_1/al. 相似文献
1000.