全文获取类型
收费全文 | 1406篇 |
免费 | 108篇 |
国内免费 | 142篇 |
专业分类
化学 | 300篇 |
晶体学 | 5篇 |
力学 | 106篇 |
综合类 | 20篇 |
数学 | 860篇 |
物理学 | 365篇 |
出版年
2024年 | 4篇 |
2023年 | 31篇 |
2022年 | 23篇 |
2021年 | 29篇 |
2020年 | 45篇 |
2019年 | 39篇 |
2018年 | 36篇 |
2017年 | 35篇 |
2016年 | 45篇 |
2015年 | 25篇 |
2014年 | 44篇 |
2013年 | 122篇 |
2012年 | 57篇 |
2011年 | 71篇 |
2010年 | 56篇 |
2009年 | 85篇 |
2008年 | 121篇 |
2007年 | 90篇 |
2006年 | 78篇 |
2005年 | 78篇 |
2004年 | 45篇 |
2003年 | 73篇 |
2002年 | 53篇 |
2001年 | 55篇 |
2000年 | 51篇 |
1999年 | 58篇 |
1998年 | 39篇 |
1997年 | 47篇 |
1996年 | 16篇 |
1995年 | 13篇 |
1994年 | 17篇 |
1993年 | 9篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1973年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有1656条查询结果,搜索用时 0 毫秒
101.
A series of(Si_O2/MgO/ID/MgCl_2)·TiClx Ziegler-Natta catalysts for propylene polymerization has been prepared with a new method. These catalysts were synthesized using soluble Mg-compounds as the Mg-source and the preparation progress was relatively simple. The catalyst could copy the spherical shape of the carrier very well. The propylene polymerization results showed that the catalyst revealed the best activity with 9,9-di(methoxymethyl)fluorene(BMMF) as internal donor at 50 °C with the optimal molar ratio Al/Ti = 5, which was much lower than what the industrial polypropylene catalyst used(at least molar ratio Al/Ti = 100), resulting in great cost saving. Additionally, the polymerization kinetics of the catalyst exhibited very stable property after achieving a relatively high value. These catalysts possessed rather high activity and good hydrogen response. The isotactic index(Ⅱ.) value of the PP products could be higher than 98% in the presence of both internal and external electron donors. Moreover, temperature rising elution fractionation method was used to understand the influence of donors and H2 on the properties of the PP products. 相似文献
102.
In this work, we devote to explore excited‐state intramolecular proton transfer (ESIPT) behavior for a novel fluorescent molecule naphthalimide‐based 2‐(2‐hydroxyphenyl)‐benzothiazole (HNIBT) [New J. Chem. 2019, 43, 9152.] in toluene and methanol (MeOH) solvents. Exploring weak interactions, stable HNIBT‐enol, and HNIBT‐MeOH‐enol complex can be found in S0 state via TDDFT/B3LYP/6‐311+G(d,p) level. Given photoexcitation, intramolecular hydrogen bond O1? H2···N3 of HNIBT‐enol and HNIBT‐MeOH‐enol is dramatically enhanced, which offers impetus for facilitates ESIPT reaction. After repeated comparisons, we verify the unavailability of intermolecular hydrogen bonding effects between HNIBT‐enol and MeOH molecules. In view of excitation, HOMO (π) → LUMO (π*) transition and the changes of electronical densities indeed impulse ESIPT tendency. Via constructing potential energy curves (PECs), for both HNIBT‐enol and HNIBT‐MeOH‐enol complex, the ESIPT could only occur along with intramolecular hydrogen bond O1? H2···N3. Through comparison, the potential barrier falls from 4.124 kcal/mol (HNIBT‐enol) to 2.132 kcal/mol (HNIBT‐MeOH‐enol). Therefore, we confirm that the ESIPT of the HNIBT system happens more easily in the MeOH solvent compared with the toluene solvent. 相似文献
103.
Santhanamoorthi Nachimuthu Yi-Hui Kuo Dang Hoai Khanh Zhan-Jun Zhu Jyh-Chiang Jiang 《中国化学会会志》2023,70(3):349-358
The ability of the polymer-based graphitic carbon nitride (g-C3N4) as a gas sensor toward NO, NO2, CO, CO2, SO2, SO3, and O2 gasses is assessed using density functional theory (DFT) calculations in terms of energetic and electronic transport characteristics. In particular, this study is aimed to explore the role of zigzag and armchair edges of the g-C3N4 sheet on sensing performances. The electronic properties of adsorption systems, such as Bader charge analysis, band gaps, work function, and density of states (DOS), are used to understand the interaction between the adsorbed gas molecules and the g-C3N4 sheet. Our calculated results indicate that SOx (SO3 and SO2) gasses have higher adsorption energies on the g-C3N4 sheet than other gasses. Furthermore, the transport properties, such as current–voltage (I-V) and resistance-voltage (R-V) curves along the zigzag and armchair directions are calculated using the non-equilibrium Green's function (NEGF) method to understand the performance of the g-C3N4 sheet as a prominent conductive/resistive sensor. The I-V/R-V results indicate that the zigzag g-C3N4 sheet has excellent sensing ability toward SOx gasses at low applied voltages. However, the presence of H2O degrades the sensing performance of the armchair g-C3N4 sheet. Theoretical recovery time has also been calculated to evaluate the reusability of g-C3N4 sheet-based gas sensors. Our results reveal that the zigzag g-C3N4 sheet-based sensing device has a remarkably high sensitivity (>300%) and selectivity toward SOx gasses and has the potential to work in a complex environment. 相似文献
104.
105.
Let Γ6 be the elliptic curve of degree 6 in PG(5, q) arising from a non-singular cubic curve of PG(2, q) via the canonical Veronese embedding
(1) If Γ6 (equivalently ) has n
GF(q)-rational points, then the associated near-MDS code has length n and dimension 6. In this paper, the case q = 5 is investigated. For q = 5, the maximum number of GF(q)-rational points of an elliptic curve is known to be equal to ten. We show that for an elliptic curve with ten GF(5)-rational points, the associated near-MDS code can be extended by adding two more points of PG(5, 5). In this way we obtain six non-isomorphic [12, 6]5 codes. The automorphism group of is also considered.
相似文献
106.
In this note we study the Landau–Hall problem on a generalized canal surface and classify all uniform magnetic trajectories of a charged particle moving on such a surface under the action of a uniform magnetic field. 相似文献
107.
We prove the existence of fine moduli spaces of simple coherent sheaves on families of irreducible curves. Our proof is based on the existence of a universal upper bound of the Castelnuovo–Mumford regularity of such sheaves, which we provide. 相似文献
108.
Eslam Badr 《代数通讯》2013,41(10):4327-4340
109.
110.
E. Getzler 《Journal of the American Mathematical Society》1997,10(4):973-998
We find a new relation among codimension algebraic cycles in the moduli space , and use this to calculate the elliptic Gromov-Witten invariants of projective spaces and .