全文获取类型
收费全文 | 44172篇 |
免费 | 7256篇 |
国内免费 | 5898篇 |
专业分类
化学 | 31872篇 |
晶体学 | 758篇 |
力学 | 2512篇 |
综合类 | 529篇 |
数学 | 4790篇 |
物理学 | 16865篇 |
出版年
2024年 | 121篇 |
2023年 | 796篇 |
2022年 | 1325篇 |
2021年 | 1474篇 |
2020年 | 1675篇 |
2019年 | 1708篇 |
2018年 | 1396篇 |
2017年 | 1380篇 |
2016年 | 1985篇 |
2015年 | 1991篇 |
2014年 | 2390篇 |
2013年 | 3118篇 |
2012年 | 3949篇 |
2011年 | 3887篇 |
2010年 | 2908篇 |
2009年 | 2827篇 |
2008年 | 3161篇 |
2007年 | 2758篇 |
2006年 | 2703篇 |
2005年 | 2328篇 |
2004年 | 1877篇 |
2003年 | 1504篇 |
2002年 | 1460篇 |
2001年 | 1173篇 |
2000年 | 1050篇 |
1999年 | 993篇 |
1998年 | 737篇 |
1997年 | 615篇 |
1996年 | 609篇 |
1995年 | 532篇 |
1994年 | 513篇 |
1993年 | 454篇 |
1992年 | 344篇 |
1991年 | 323篇 |
1990年 | 289篇 |
1989年 | 204篇 |
1988年 | 164篇 |
1987年 | 105篇 |
1986年 | 125篇 |
1985年 | 100篇 |
1984年 | 56篇 |
1983年 | 55篇 |
1982年 | 49篇 |
1981年 | 37篇 |
1980年 | 18篇 |
1979年 | 18篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1957年 | 7篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
The γcmc values of CTAB-SDS decrease from 63.67 mN/m at 10‡C to 36.38 mN/m at 90‡C, slightly lower than those of either CTAB or SDS.
Correspondingly, the CMC of CTAB-SDS decreases almost by half. The increase of surface activity of CTAB-SDS can be attributed
to the relatively weak electrostatic interaction at high temperature, which is supported by the increase of solubility of
CTAB-SDS with rise in temperature. Catalytic effect on oxidation of toluene derivatives with potassium permanganate follows
the order CTAB-SDS > SDS > CTAB. This is not caused by the dissociative effect of CTAB-SDS with low surface activity at low
temperature, as seen from the fact that almost all oxidative products can be retrieved for different toluene derivatives and
surfactants by mimicking the conditions of reaction. In the emulsifications of toluene derivatives at 90‡C, the time that
turbid water layers of surfactant solutions take to become clear is the same as that of the catalytic effect on oxidation
of toluene derivatives. Thus, it can be inferred that surfactants can improve the oxidation yields of toluene derivatives
by increasing the contact between two reacting phases. 相似文献
2.
3.
Gang Wang Xiulin Zhu Zhenping Cheng Jian Zhu 《Journal of polymer science. Part A, Polymer chemistry》2006,44(9):2912-2921
A series of (di)picolinic acids and their derivates are investigated as novel complexing tridentate or bidentate ligands in the iron‐mediated reverse atom transfer radical polymerization of methyl methacrylate in N,N‐dimethylformamide at 100 °C with 2,2′‐azobisisobutyrontrile as an initiator. The polymerization rates and polydispersity indices (1.32–1.8) of the resulting polymers are dependent on the structures of the ligands employed. Different iron complexes may be involved in iron‐mediated reverse atom transfer radical polymerization, depending on the type of acid used. 1H NMR spectroscopy has been used to study the structure of the resulting polymers. Chain‐extension reactions have been performed to further confirm the living nature of this catalytic system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2912–2921, 2006 相似文献
4.
Incomplete LU factorization preconditioning techniques often have difficulty on indefinite sparse matrices. We present hybrid reordering strategies to deal with such matrices, which include new diagonal reorderings that are in conjunction with a symmetric nondecreasing degree algorithm. We first use the diagonal reorderings to efficiently search for entries of single element rows and columns and/or the maximum absolute value to be placed on the diagonal for computing a nonsymmetric permutation. To augment the effectiveness of the diagonal reorderings, a nondecreasing degree algorithm is applied to reduce the amount of fill-in during the ILU factorization. With the reordered matrices, we achieve a noticeable improvement in enhancing the stability of incomplete LU factorizations. Consequently, we reduce the convergence cost of the preconditioned Krylov subspace methods on solving the reordered indefinite matrices. 相似文献
5.
A FUNDAMENTAL SOLUTION FOR THE LAPLACE OPERATOR ON THE QUATERNIONIC HEISENBERG GROUP 总被引:2,自引:0,他引:2
朱理 《数学物理学报(B辑英文版)》2002,22(3)
In this paper, the author studies the Laplace operator on the quaternionic Heisenberg group, construct a fundamental solution for it and use this solution to prove the Lp-boundedness and the weak (1-1) boundedness of certain singular convolution operators on the quaternionic Heisenberg group. 相似文献
6.
In this work, the natural convection in a concentric annulus between a cold outer square cylinder and a heated inner circular cylinder is simulated using the differential quadrature (DQ) method. The vorticity‐stream function formulation is used as the governing equation, and the coordinate transformation technique is introduced in the DQ computation. It is shown in this paper that the outer square boundary can be approximated by a super elliptic function. As a result, the coordinate transformation from the physical domain to the computational domain is set up by an analytical expression, and all the geometrical parameters can be computed exactly. Numerical results for Rayleigh numbers range from 104 to 106 and aspect ratios between 1.67 and 5.0 are presented, which are in a good agreement with available data in the literature. It is found that both the aspect ratio and the Rayleigh number are critical to the patterns of flow and thermal fields. The present study suggests that a critical aspect ratio may exist at high Rayleigh number to distinguish the flow and thermal patterns. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
7.
Bingxin LiuYoshio Tatamitani Jun ShimadaTeruhiko Ogata 《Journal of Molecular Spectroscopy》2002,211(1):99-106
The microwave spectrum of the 35Cl and 37Cl isotopic species of 1-chloro-1,1,2-trifluoroethane (HCFC-133b) has been investigated in the frequency region 10 to 50 GHz using a Stark modulation microwave spectrometer. A pulsed jet Fourier transform microwave spectrometer was also used for the measurement of hyperfine splittings. A least-squares analysis of the observed b-type Q- and R-branch transition frequencies gave rotational and centrifugal distortion constants and components of the chlorine nuclear quadrupole coupling constant tensors in the principal axes system as follows: A=4625.161 (3) MHz, B=2004.127 (2) MHz, C=1875.813 (2) MHz, ΔJ=0.144 (9) kHz, ΔJK=1.0748 (8) kHz, ΔK=1.57 (1) kHz, δJ=0.01376 (4) kHz, δK=−0.146 (4) kHz, χaa=−57.958 (10) MHz, χbb=21.231 (11) MHz, and χcc=36.727 (11) MHz for 35ClCF2CH2F species, and A=4607.684 (6) MHz, B=1960.565 (2) MHz, C=1834.823 (2) MHz, ΔJ=0.106 (7) kHz, ΔJK=1.022 (3) kHz, ΔK=1.48 (1) kHz, δJ=0.0142 (2) kHz, δK=−0.18 (2) kHz, χaa=−46.268 (11) MHz, χbb=17.319 (13) MHz, and χcc=28.950 (13) MHz for 37ClCF2CH2F species. The structural parameters are calculated from the observed six rotational constants by assuming the partial structure of ab initio calculation. The electronic properties of the C-Cl bond are evaluated from the observed nuclear quadrupole constants of chlorine. These molecular properties are compared with those of other related molecules. 相似文献
8.
9.
The fuzzification of convex sets in median algebras is considered, and some of their properties are investigated. A characterization of finite valued fuzzy convex set is given. 相似文献
10.
Jong Chan Lee Jin Young Park So Young Yoon Yong Hun Bae Seung Jun Lee 《Tetrahedron letters》2004,45(1):191-193
A novel and direct method for the synthesis of α-halocarbonyl compounds using sequential treatment of carbonyl compounds with [hydroxy(tosyloxy)iodo]benzene followed by magnesium halides under solvent-free microwave irradiation conditions is described. 相似文献