全文获取类型
收费全文 | 747篇 |
免费 | 21篇 |
国内免费 | 10篇 |
专业分类
化学 | 449篇 |
晶体学 | 6篇 |
力学 | 23篇 |
数学 | 145篇 |
物理学 | 155篇 |
出版年
2024年 | 2篇 |
2023年 | 5篇 |
2022年 | 16篇 |
2021年 | 24篇 |
2020年 | 21篇 |
2019年 | 10篇 |
2018年 | 15篇 |
2017年 | 16篇 |
2016年 | 44篇 |
2015年 | 28篇 |
2014年 | 33篇 |
2013年 | 56篇 |
2012年 | 62篇 |
2011年 | 50篇 |
2010年 | 38篇 |
2009年 | 24篇 |
2008年 | 51篇 |
2007年 | 35篇 |
2006年 | 40篇 |
2005年 | 25篇 |
2004年 | 22篇 |
2003年 | 19篇 |
2002年 | 18篇 |
2001年 | 7篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 13篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1981年 | 4篇 |
1978年 | 3篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 5篇 |
1972年 | 2篇 |
1969年 | 3篇 |
1968年 | 2篇 |
1937年 | 2篇 |
1934年 | 1篇 |
排序方式: 共有778条查询结果,搜索用时 15 毫秒
61.
Piotto M Elbayed K Wieruszeski JM Lippens G 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,173(1):84-89
High resolution magic angle spinning (HRMAS) has become an extremely versatile tool to study heterogeneous systems. HRMAS relies on magic angle spinning of the sample to average out to zero magnetic susceptibility differences in the sample and to obtain resonance linewidths approaching those of liquid state NMR. Shimming such samples therefore becomes an important issue. By analyzing the different sources of magnetic field perturbations present in a sample under MAS conditions, we propose a simple protocol to obtain optimum shim settings in HRMAS. In the case of aqueous samples, we show that the lock level cannot be used as a reliable indicator of the quality of the shims at high spinning speeds. This effect is explained by the presence of temperature gradients imparted by the sample rotation. 相似文献
62.
Omar El-Fallah Emmanuel Fricain Karim Kellay Javad Mashreghi Thomas Ransford 《Constructive Approximation》2016,44(2):269-281
In most classical holomorphic function spaces on the unit disk in which the polynomials are dense, a function f can be approximated in norm by its dilates \(f_r(z):=f(rz)~(r<1)\). We show that this is not the case for the de Branges–Rovnyak spaces \(\mathcal{H}(b)\). More precisely, we exhibit a space \(\mathcal{H}(b)\) in which the polynomials are dense and a function \(f\in \mathcal{H}(b)\) such that \(\lim _{r\rightarrow 1^-}\Vert f_r\Vert _{\mathcal{H}(b)}=\infty \). On the positive side, we prove the following approximation theorem for Toeplitz operators on general de Branges–Rovnyak spaces \(\mathcal{H}(b)\). If \((h_n)\) is a sequence in \(H^\infty \) such that \(\Vert h_n\Vert _{H^\infty }\le 1\) and \(h_n(0)\rightarrow 1\), then \(\Vert T_{\overline{h}_n}f-f\Vert _{\mathcal{H}(b)}\rightarrow 0\) for all \(f\in \mathcal{H}(b)\). Using this result, we give the first constructive proof that, if b is a nonextreme point of the unit ball of \(H^\infty \), then the polynomials are dense in \(\mathcal{H}(b)\). 相似文献
63.
Acta Mathematica Sinica, English Series - In this research article, we shall give some reverse Arithmetic-Geometric mean inequalities for unital positive linear maps on Hilbert space operators... 相似文献
64.
Ashfaque H. Bokhari A. H. Kara M. Karim F. D. Zaman 《International Journal of Theoretical Physics》2009,48(7):1919-1928
In this paper we discuss symmetries of a nonlinear wave equation that arises as a consequence of some Riemannian metrics of
signature −2. The objective of this study is to show how geometry can be responsible in giving rise to a nonlinear inhomogeneous
wave equation rather than assuming nonlinearities in the wave equation from physical considerations. We find Lie point symmetries
of the corresponding wave equations and give their solutions in two cases. Some interesting physical conclusions relating
to conservation laws such as energy, linear and angular momenta are also determined. 相似文献
65.
This study aimed to evaluate the effects of ultrasound on Lactobacillus fermentum BT 8633 in parent and subsequent passages based on their growth and isoflavone bioconversion activities in biotin-supplemented soymilk. The treated cells were also assessed for impact of ultrasound on probiotic properties. The growth of ultrasonicated parent cells increased (P < 0.05) by 3.23-9.14% compared to that of the control during fermentation in biotin-soymilk. This was also associated with enhanced intracellular and extracellular (8.4-17.0% and 16.7-49.2%, respectively; P < 0.05) β-glucosidase specific activity, leading to increased bioconversion of isoflavones glucosides to aglycones during fermentation in biotin-soymilk compared to that of the control (P < 0.05). Such traits may be credited to the reversible permeabilized membrane of ultrasonicated parent cells that have facilitated the transport of molecules across the membrane. The growing characteristics of first, second and third passage of treated cells in biotin-soymilk were similar (P > 0.05) to that of the control, where their growth, enzyme and isoflavone bioconversion activities (P > 0.05) were comparable. This may be attributed to the temporary permeabilization in the membrane of treated cells. Ultrasound affected probiotic properties of parent L. fermentum, by reducing tolerance ability towards acid (pH 2) and bile; lowering inhibitory activities against selected pathogens and reducing adhesion ability compared to that of the control (P < 0.05). The first, second and third passage of treated cells did not exhibit such traits, with the exception of their bile tolerance ability which was inherited to the first passage (P < 0.05). Our results suggested that ultrasound could be used to increase bioactivity of biotin-soymilk via fermentation by probiotic L. fermentum FTDC 8633 for the development of functional food. 相似文献
66.
Rubina Abdul‐Karim Syed Ghulam Musharraf Muhammad Imran Malik 《Journal of polymer science. Part A, Polymer chemistry》2017,55(11):1887-1893
Synthesis of novel amphiphilic biodegradable block copolymers based on ethylene carbonate is reported in this study. Polyethylene glycol monomethyl ether (MeO‐PEO) and polyethylene glycol (PEG) of varying molar masses are used as macro‐initiator for ring‐opening polymerization of ethylene carbonate in the presence of sodium stannate trihydrate as a heterogeneous transesterification catalyst. Earlier elution of block copolymer from macro‐initiator in size exclusion chromatography (SEC) indicated the successful synthesis of the block copolymers. Ratios of both types of blocks are varied systematically. Liquid chromatography at critical conditions is used for the analysis of the non‐critical individual blocks, and if there are any critical segments that are not attached to the non‐critical block. To the best of our knowledge, this is the first report on the synthesis of ethylene carbonate‐based amphiphilic block copolymers. Chromatographic critical conditions of the ethylene carbonate polymer are also reported for the first time. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1887–1893 相似文献
67.
In an erbium-doped fiber amplifier(EDFA), erbium ions act as a three-level system. Therefore, much higher pump energy is required to achieve the population inversion in an erbium-doped fiber(EDF). This higher pump energy requirement complicates the efficient design of an EDFA. However, efficient use of the pump power can improve the EDFA performance. The improved performance of an EDFA can be obtained by reducing the doping radius of the EDF. A smaller doping radius increases pump–dopant interactions and subsequently increases the pump–photon conversion efficiency. Decreasing the doping radius allows a larger proportion of dopant ions,which are concentrated near the core, to interact with the highest pump intensity. However, decreasing the doping radius beyond a certain limit will bring the dopant ions much closer and introduce detrimental ion–ion interaction effects. In this Letter, we show that an optimal doping radius in an EDF can provide the best gain performance. Moreover, we have simulated the well-known numerical aperture effects on EDFA gain performance to support our claim. 相似文献
68.
Ben Ali K Lafon O Zimmermann H Guittet E Lesot P 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,187(2):205-215
We describe several homo- and heteronuclear 2D NMR strategies dedicated to the analysis of anisotropic (2)H spectra of a mixture of dideuterated unlike/like stereoisomers with two remote stereogenic centers, using weakly orienting chiral liquid crystals. To this end, we propose various 2D correlation experiments, denoted "D(H)(n)D" or "D(H)(n)C" (with n=1, 2), that involve two heteronuclear polarization transfers of INEPT-type with one or two proton relays. The analytical expressions of correlation signals for four pulse sequences reported here were calculated using the product-operators formalism for spin I=1 and S=1/2. The features and advantages of each scheme are presented and discussed. The efficiency of these 2D sequences is illustrated using various deuterated model molecules, dissolved in organic solutions of polypeptides made of poly-gamma-benzyl-L-glutamate (PBLG) or poly-epsilon-carbobenzyloxy-L-lysine (PCBLL) and NMR numerical simulations. 相似文献
69.
G.W. van Apeldoorn D. Harting D.J. Holthuizen B.J. Pijlgroms M.M.H.M. Rijssenbeek V. Karimäki R. Kinnunen M. Korkea-Aho J. Maalampi J. Tuominiemi G.D. Patel P. Johnson P. Mason H. Muirhead B.M. Whyman 《Physics letters. [Part B]》1982,115(1):55-58
Data on the density of charged particles in rapidity space for annihilation and non-annihilation p?p interactions at 7.3 and 12 GeV/c are reported. It is shown that the central rapidity density for non-annihilation processes is a simple linear function of from these low energies up to p?p collider energies (). Significant differences in density distributions for p?p and e?e+ annihilations at comparable energies are found. 相似文献
70.