全文获取类型
收费全文 | 1923篇 |
免费 | 64篇 |
国内免费 | 18篇 |
专业分类
化学 | 1394篇 |
晶体学 | 8篇 |
力学 | 92篇 |
数学 | 251篇 |
物理学 | 260篇 |
出版年
2023年 | 19篇 |
2022年 | 88篇 |
2021年 | 95篇 |
2020年 | 65篇 |
2019年 | 83篇 |
2018年 | 86篇 |
2017年 | 50篇 |
2016年 | 79篇 |
2015年 | 66篇 |
2014年 | 101篇 |
2013年 | 198篇 |
2012年 | 123篇 |
2011年 | 132篇 |
2010年 | 90篇 |
2009年 | 69篇 |
2008年 | 87篇 |
2007年 | 100篇 |
2006年 | 75篇 |
2005年 | 56篇 |
2004年 | 33篇 |
2003年 | 37篇 |
2002年 | 34篇 |
2001年 | 10篇 |
2000年 | 22篇 |
1999年 | 9篇 |
1998年 | 14篇 |
1997年 | 8篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 9篇 |
1991年 | 15篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 14篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 13篇 |
1982年 | 7篇 |
1981年 | 11篇 |
1980年 | 4篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1968年 | 2篇 |
1936年 | 1篇 |
排序方式: 共有2005条查询结果,搜索用时 15 毫秒
991.
Mir Masoud Seyyed Fakhrabadi Mostafa Samadzadeh Abbas Rastgoo Mohammadreza Haeri Yazdi Mahmoud Mousavi Mashhadi 《Physica E: Low-dimensional Systems and Nanostructures》2011,44(3):565-578
This paper presents the molecular mechanics based finite element modeling of carbon nanotubes (CNTs) and their applications as mass sensors. The beam element with elastic behavior is considered as the bond between the carbon atoms and its properties are obtained using equating continuum and molecular characteristics. The first five natural frequencies of CNTs in cantilever and doubly clamped boundary conditions (BCs) and their corresponding mode shapes are studied in detail. Furthermore, a multilayer perceptron neural network is used to predict the fundamental vibration frequencies of the CNTs with different diameters and lengths. In addition, variations of the natural frequencies of the CNTs with distorted cross sections are investigated. Moreover, the effects of some attached masses with various values on the first three natural frequencies of a considered CNT are studied here. 相似文献
992.
We emphasize the close relationship between zeta function methods and arbitrary spectral cutoff regularizations in curved spacetime. This yields, on the one hand, a physically sound and mathematically rigorous justification of the standard zeta function regularization at one loop and, on the other hand, a natural generalization of this method to higher loops. In particular, to any Feynman diagram is associated a generalized meromorphic zeta function. For the one-loop vacuum diagram, it is directly related to the usual spectral zeta function. To any loop order, the renormalized amplitudes can be read off from the pole structure of the generalized zeta functions. We focus on scalar field theories and illustrate the general formalism by explicit calculations at one-loop and two-loop orders, including a two-loop evaluation of the conformal anomaly. 相似文献
993.
In the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by palladium nanoparticles, our previous studies suggested that the phenylboronic acid adsorbs on the nanoparticle surface and then interacts with the iodobenzene that is present in solution. In the present study, FTIR is used to examine the change in the vibrational frequencies of phenylboronic acid in films with and without the addition of palladium nanoparticles. The large change in the B-O stretching frequency of phenylboronic acid from 1348 to 1376 cm(-1) in the presence of sodium acetate and palladium nanoparticles strongly suggests that the mode of binding of phenylboronic acid to the Pd nanoparticle surface involves a B-O-Pd type of bonding. Shifts in the B-C stretching mode and the out-of-plane phenyl C-C ring deformation bands associated with phenylboronic acid provide additional confirmations of the binding process. It is also shown that the phenylboronic acid needs to be in the deprotonated form in the presence of sodium acetate (phenylboronate anion) to bind to the palladium nanoparticle surface. No changes in the characteristic bands of iodobenzene were observed in films made in the presence of the palladium nanoparticles. The FTIR studies provide proof of the mode of binding that occurs in the nanoparticle surface for the first time and also confirms the mechanism of the Suzuki reaction that we proposed previously. 相似文献
994.
Two new series of copper(II), nickel(II), cobalt(II), zinc(II), iron(III), chromium(III), vanadyl(IV) and uranyl(VI) complexes
with two bifunctional tridentate Schiff base, H4L1 and H2L2 ligands have been prepared. The Schiff base, H4L1 and H2L2, ligands were synthesized by the condensation of 4,6-diacetylresorcinol with o-aminophenol or o-phenylenediamine. The ligands are either di- or tetra-basic with two symmetrical sets of either OON or NNO tridentate chelating
sites. The ligands and their metal complexes have been characterized by elemental analysis, 1H-n.m.r., FT-IR, mass, electronic, esr spectra and thermal gravimetric analysis and magnetic susceptibility. With the exception
of CoII ion with H2L2 which afforded a trinuclear complex, a variety of binuclear complexes for the rest of the metal complexes were obtained with
the ligands in its di- or tetra-deprotonated forms. The bonding sites are the azomethine and amino nitrogen atoms, and phenolic
oxygen atoms. The metal complexes exhibit different geometrical arrangements such as square planar, tetrahedral, square pyramid
and octahedral arrangement. 相似文献
995.
Gholamhossein Mohammadnezhad Mostafa M. Amini Hamid Reza Khavasi 《Journal of chemical crystallography》2011,41(8):1224-1227
Abstract
A new magnesium alkoxide which posses centrosymmetric tetranuclear structure derived from 1-methoxy-2-propanol, a functionalized alcohol containing asymmetric carbon. X-ray analysis of single crystal shows an open dicubane-like structure with triple-bridging alkoxo groups between alternating five- and six-coordinated magnesium atoms. There are six alkoxo bridging groups in which two of them bridge three metal centers and the other four bridge two. All of the etheric-oxygens are coordinated to magnesium and provide a rigid structure. Two chlorine atoms are bonded to five-coordinated magnesium. This compound crystallizes in the monoclinic system with a = 10.040(6) Ǻ, b = 14.004(9) Ǻ, c = 15.344(5) Ǻ, β = 95.55(9)˚, Z = 2 and V = 2147 Ǻ3 in the space group of P21/n. 相似文献996.
Ahmed S. Saad Mohammed E. Draz Ibrahim A. Naguib Hala E. Zaazaa Adel S. Lashien Fatma F. Abdallah 《Biomedical chromatography : BMC》2022,36(6):e5353
Cyclizine hydrochloride (CYC) and meclozine hydrochloride (MEC) are antihistaminic drugs generally co-formulated with pyridoxine hydrochloride (PYR) to treat nausea and vomiting in pregnancy. Several analytical techniques have been applied for the determination of CYC or MEC with PYR, but determination of CYC impurity; benzhydrol (BEH) or MEC impurity; or 4-chlorobenzophenone (BEP) has not been paid attention to. Therefore, micellar UPLC method is introduced for analysis of ternary mixtures containing PYR together with both CYC and BEH (mixture I) or MEC and BEP (mixture II). Chromatographic separation was achieved using a Hypersil gold C8 column (50 × 2.1 mm, 1.9 μm) using 0.01 M sodium dodecyl sulfate modified to pH 3.5 using phosphoric acid:acetonitrile (45:55 by volume) for mixture I and 0.1% sodium dodecyl sulfate, 0.1% sodium bicarbonate adjusted to pH 2.6 by phosphoric acid:acetonitrile (47:53 by volume) for mixture II as mobile phases. The separated peaks were detected at 230 and 245 nm for mixtures I and II, respectively. The adopted methods were validated in conformance with the International Conference on Harmonization (ICH) recommendations and were properly applied in commercial pharmaceutical formulation analysis. Comprehensive ecological comparison was achieved, confirming a higher ecological value of the presented methods compared to the earlier reported methods. 相似文献
997.
The role of folic acid in inducing of apoptosis by zinc(II) complex in ovary and cervix cancer cells
Yu Chuanrong Heidari Majd Mostafa Shiri Fereshteh Shahraki Somaye Karimi Pouya 《Molecular diversity》2022,26(3):1545-1555
Molecular Diversity - Herein, the synthesis, structure, binding affinity, cytotoxicity, and apoptotic properties of the new Zn(II) complex composed of folic acid and bipyridine ligands are... 相似文献
998.
999.
Haghighi Arman Maleki Akbar El Haj Assad Mamdouh Chen Lingen Alhuyi Nazari Mohammad Safdari Shadloo Mostafa 《Journal of Thermal Analysis and Calorimetry》2022,147(10):5533-5547
Journal of Thermal Analysis and Calorimetry - Heat pipes are broadly applied for thermal management of devices with high heat flux. Due to the dependency of their efficient operating range on the... 相似文献
1000.
Debbouche Nadjette Ouannas Adel Batiha Iqbal M. Grassi Giuseppe 《Nonlinear dynamics》2022,109(1):33-45
Nonlinear Dynamics - Mathematical models based on fractional-order differential equations have recently gained interesting insights into epidemiological phenomena, by virtue of their memory effect... 相似文献