全文获取类型
收费全文 | 81862篇 |
免费 | 468篇 |
国内免费 | 379篇 |
专业分类
化学 | 27155篇 |
晶体学 | 797篇 |
力学 | 6780篇 |
数学 | 32523篇 |
物理学 | 15454篇 |
出版年
2020年 | 56篇 |
2019年 | 69篇 |
2018年 | 10457篇 |
2017年 | 10291篇 |
2016年 | 6135篇 |
2015年 | 924篇 |
2014年 | 402篇 |
2013年 | 453篇 |
2012年 | 3928篇 |
2011年 | 10673篇 |
2010年 | 5736篇 |
2009年 | 6128篇 |
2008年 | 6728篇 |
2007年 | 8908篇 |
2006年 | 387篇 |
2005年 | 1469篇 |
2004年 | 1687篇 |
2003年 | 2085篇 |
2002年 | 1139篇 |
2001年 | 323篇 |
2000年 | 356篇 |
1999年 | 235篇 |
1998年 | 253篇 |
1997年 | 227篇 |
1996年 | 267篇 |
1995年 | 174篇 |
1994年 | 147篇 |
1993年 | 148篇 |
1992年 | 102篇 |
1991年 | 112篇 |
1990年 | 93篇 |
1989年 | 106篇 |
1988年 | 100篇 |
1987年 | 107篇 |
1986年 | 119篇 |
1985年 | 118篇 |
1984年 | 122篇 |
1983年 | 86篇 |
1982年 | 109篇 |
1981年 | 76篇 |
1980年 | 111篇 |
1979年 | 104篇 |
1978年 | 88篇 |
1977年 | 75篇 |
1976年 | 64篇 |
1975年 | 58篇 |
1974年 | 60篇 |
1973年 | 75篇 |
1972年 | 61篇 |
1914年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
The relativistic ground and low-lying excited state potential energy curves of AgH and AuH in the presence of a cylindrical
harmonic confining potential were calculated using the multi-state multi-reference perturbation theory with the spin-free
no-pair Hamiltonian obtained via the third-order Douglas–Kroll transformation, incorporated with the full two-electron Breit–Pauli
spin–orbit operator. The spectroscopic parameters were obtained for both the scalar- and quasi-relativistic potentials. The
spin–orbit coupling constants were calculated for several strengths of the confining potential, and the effects of the applied
potential on the coupling constants were analyzed using configuration interaction.
This paper is dedicated to Serafín Fraga—colleague, mentor, and friend. 相似文献
972.
The boundary effect on the drag on two identical, nonuniformly structured flocs moving along the axis of a cylindrical tube
filled with a Newtonian fluid is investigated at a small to medium larger Reynolds number. A two-layer model is adopted to
simulate various possible structures of a floc, and the flow field inside is described by Darcy–Brinkman model. The results
of numerical simulation reveal that a convective flow is present in the rear region of a floc when Reynolds number is on the
order of 40. The presence of the tube wall and/or the porous structure of a floc has the effect of reducing that convective
flow. For a fixed level of the volume-average permeability of a floc, the influence of the tube wall on the drag depends upon
floc structure; the influence on a nonuniformly structured floc is more significant than that on a uniformly structured floc.
The more nonuniform the floc structure, the more appreciable the deviation of the drag coefficient–Reynolds number curve from
a Stokes’-law-like relation becomes. The smaller the volume-average permeability of a floc and/or the smaller the separation
distance between the two flocs, the greater is the deviation, but the presence of the tube wall has the effect of reducing
that deviation. 相似文献
973.
Karlheinz Ballschmiter Rainer Bacher Angela Mennel Regina Fischer Ulrich Riehle Max Swerev 《Journal of separation science》1992,15(4):260-270
High resolution gas chromatography, with mass selective detection, has been used for the analysis of PCB on methyl 50 % octyl polysiloxane (SB 50 Octyl), methyl octadecyl polysiloxane, and a smectic polysiloxane (SB Smectic); and for the analysis of polychlorodibenzodioxins and polychlorodibenzofurans with 1 to 8 chlorine substituents on 100 % cyanopropyl siloxane (SP 2331), smectic polysiloxane (SB Smectic), a new polar stationary phase (DB-Dioxin). The analysis has also been performed by column coupling. 相似文献
974.
Antonio Doménech María Teresa Doménech-Carbó María Luisa Vázquez de Agredos Pascual 《Journal of Solid State Electrochemistry》2007,11(9):1335-1346
The preparation of indigo from Indigofera suffruticosa following the procedures attributed to ancient Mayas was electrochemically monitored using the voltammetry-of-microparticles
approach. The mechanism formation of indigotin and indirubin from its precursors, indican and isatan, is discussed. Comparison
of voltammetric profiles for differently prepared and commercial indigos and genuine Maya Blue samples suggests that the preparation
procedure of indigo changed during the Late Classical Maya period.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
975.
976.
Burlage Robert S. Palumbo Anthony V. Heitzer Armin Sayler Gary 《Applied biochemistry and biotechnology》1994,45(1):731-740
Applied Biochemistry and Biotechnology - Reporter strains of bacteria were tested using soil samples from several sites near a leaking fuel oil storage facility. The reporter bacteria utilized the... 相似文献
977.
978.
JongHwa Moon SangHoon Kang YongSam Chung OkHee Lee 《Journal of Radioanalytical and Nuclear Chemistry》2007,271(1):155-158
Instrumental neutron activation analysis was used to assess the concentration of the inorganic trace elements in Korean women’s
blood serums. It was found that a high concentration of Na and Cl incurs an analytical interference, but the 12 elements such
as Br, Ca, Cl, Co, Cr, Cs, Fe, K, Na, Rb, Se and Zn could be determined under the condition of an interference minimization.
Serum samples collected from 63 women were analyzed and the concentration level and range of the elements were evaluated.
NIST SRMs were analyzed simultaneously for quality control. The average values of the Na and Cl determined in the serum samples
were 3,365 and 3,533 mg/l, Ca was 96.4 mg/l and K was 191 mg/l. Besides, Br, Se and Zn have a concentration level of 6.46,
0.13 and 0.98 mg/l, respectively. It was found that there is no significant difference between the present values and the
reported values. 相似文献
979.
Rasa Pauliukaite Mariana Emilia Ghica Madalina Barsan Christopher M. A. Brett 《Journal of Solid State Electrochemistry》2007,11(7):899-908
The polymer redox mediator, poly(neutral red) (PNR), has been synthesised and characterised electrochemically to investigate
the best electropolymerisation and mediation conditions for application in enzyme biosensors and to clarify the mechanism
of action. Neutral red was electropolymerised by potential cycling on carbon film electrode substrates by allowing the monomer
to be oxidised during the full 20 cycles of polymerisation or reducing the positive limit of the potential window after the
first 2 cycles to impede monomer oxidation with a view to obtaining longer polymer chains and a lesser degree of branching.
Comparison was made with glassy carbon substrates. The PNR films on carbon film electrodes were characterised using cyclic
voltammetry and electrochemical impedance spectroscopy, as well as in glucose biosensors prepared with PNR. Glucose oxidase
enzyme was immobilised by encapsulation in silica sol-gel and compared with that obtained by cross-linking with glutaraldehyde.
The biosensors were evaluated by chronoamperometry in 0.1 M phosphate buffer saline solution, pH 7.0, and showed evidence
of electron transfer between the enzyme cofactor flavin adenine dinucleotide and PNR dissolved in the enzyme layer competing
with PNR-mediated electrochemical degradation of H2O2 formed during the enzymatic process.
This paper is dedicated to Professor Dr. Algirdas Vaskelis on the occasion of his 70th birthday. 相似文献
980.
The constitution and absolute configuration of the rhoeadine alkaloids (+)-alpinigenine and (+)-cis-alpinigenine. The fundamental structure of the hemi-acetal phenylbenzazepine alkaloid (+)-alpinigenine ( 1 ), isolated from Papaver bracteatum LINDL ., was derived essentially from 1H-NMR.- and mass-spectra of 1 and its derivatives 7, 10 and 14 (cf. Scheme 2). The positioning of the four methoxy groups in the two aromatic rings could be deduced from the 1H-NMR.-spectra of the N-oxides 14 and 15 in which, as a result of favourable sterical and conformational behaviour, an interaction exists between the N-oxide oxygen atom and one of the two ortho protons in ring C. The B/D-trans-fused 1 undergoes isomerization in 1N HCl to cis-alpinigenine ( 16 ). A stereochemical correlation between bases in the trans-and cis-series was enabled via an Emde degradation of the corresponding methylacetal-methyliodides 21 resp. 19 leading to the enantiomeric isochroman derivatives 22 resp. 23 which are achiral at C (2) (Scheme 4). The configuration at C (14) in the hemi-acetals (eg. 1 and 16 ) and the methyl ethers (eg. 7 and 8 ) is discussed in detail (cf. Scheme 7). (+)-Alpinigenine ( 1 ) has the (1S, 2R, 14R) configuration and (+)-cis-alpinigenine ( 16 ), in chloroform or acetone solution, the (1R, 2R, 14R) configuration. 相似文献