全文获取类型
收费全文 | 34106篇 |
免费 | 1031篇 |
国内免费 | 973篇 |
专业分类
化学 | 9153篇 |
晶体学 | 288篇 |
力学 | 853篇 |
综合类 | 72篇 |
数学 | 9850篇 |
物理学 | 9170篇 |
综合类 | 6724篇 |
出版年
2023年 | 301篇 |
2022年 | 373篇 |
2021年 | 287篇 |
2020年 | 362篇 |
2019年 | 633篇 |
2018年 | 441篇 |
2017年 | 455篇 |
2016年 | 362篇 |
2015年 | 402篇 |
2014年 | 556篇 |
2013年 | 1413篇 |
2012年 | 964篇 |
2011年 | 1529篇 |
2010年 | 1011篇 |
2009年 | 1498篇 |
2008年 | 1391篇 |
2007年 | 1514篇 |
2006年 | 1347篇 |
2005年 | 1045篇 |
2004年 | 1426篇 |
2003年 | 1468篇 |
2002年 | 2021篇 |
2001年 | 1992篇 |
2000年 | 1855篇 |
1999年 | 2019篇 |
1998年 | 1764篇 |
1997年 | 1250篇 |
1996年 | 1141篇 |
1995年 | 879篇 |
1994年 | 765篇 |
1993年 | 341篇 |
1992年 | 267篇 |
1991年 | 146篇 |
1990年 | 313篇 |
1989年 | 256篇 |
1988年 | 168篇 |
1987年 | 134篇 |
1986年 | 164篇 |
1985年 | 211篇 |
1984年 | 187篇 |
1983年 | 79篇 |
1982年 | 152篇 |
1981年 | 243篇 |
1980年 | 231篇 |
1979年 | 109篇 |
1978年 | 172篇 |
1977年 | 106篇 |
1976年 | 72篇 |
1975年 | 81篇 |
1974年 | 79篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Ajit Kumar Singh Seulgi Ji Baghendra Singh Chittaranjan Das Heechae Choi Prashanth W. Menezes Arindam Indra 《Materials Today Chemistry》2022
Herein, we report a Mott-Schottky catalyst by entrapping cobalt nanoparticles inside the N-doped graphene shell (Co@NC). The Co@NC delivered excellent oxygen evolution activity with an overpotential of merely 248 mV at a current density of 10 mA cm–2 with promising long-term stability. The importance of Co encapsulated in NC has further been demonstrated by synthesizing Co nanoparticles without NC shell. The synergy between the hexagonal close-packed (hcp) and face-centered cubic (fcc) Co plays a major role to improve the OER activity, whereas the NC shell optimizes the electronic structure, improves the electron conductivity, and offers a large number of active sites in Co@NC. The density functional theory calculations have revealed that the hcp Co has a dominant role in the surface reaction of electrocatalytic oxygen evolution, whereas the fcc phase induces the built-in electric field at the interfaces with N-doped graphene to accelerate the H+ ion transport. 相似文献
3.
4.
《自然科学进展(英文版)》2020,30(2):185-191
Metal-organic frameworks(MOFs) have been regarded as promising catalyst materials due to the richness of coordinately unsaturated metal sites on the surface,which can act as catalytic active centres.In this study,a hybrid MOF material composed of Fe-based MOF and Co-based MOF was prepared with the involvement of graphene oxide nanosheets as additive.It was demonstrated that the hybrid MOF materials showed much higher electro-catalytic activity towards oxygen evolution than the single-phase counterparts.To drive current density of 10 mA cm~(-2),the hybrid CoFe-based MOF only needed an over potential of 290 mV in 1 M KOH.The catalytic activity could sustain for a longer time with only slight current density decrease.During oxygen evolution operation,the MOF catalyst evolved into catalytic active species but kept well the microscale sheet-like structure.It is thus believed that this study will provide an avenue for the development of advanced electrocatalysts. 相似文献
5.
《Arabian Journal of Chemistry》2021,14(11):103421
Processing of Carapa guianensis seeds to obtain oil on an industrial scale generates a significant amount of by-product, approximately 66% w/w, which is called cake and is a potential source of biomolecules, including simple phenolic structures. For this reason, studies were carried out on the chemical profiles of hydrolyzed extract from this agro-industrial by-product through High Performance Thin-Layer Chromatography (HPTLC) and Gas Chromatography coupled to Mass Spectrometry (GC–MS). These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product. The sample antioxidant capacity was determined by methods of 2,2-diphenyl-1-picrylhydrazyl (DPPH)and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS+) radicals direct sequestration. The hydrolyzed fraction showed a total of 63.47% in the relative abundance of the total of compounds, standing out: p-hydroxybenzoic acid (39.19%) and protocatechuic acid (3,4-dihydroxybenzoic acid) (5.62%), both from hydroxybenzoic acids and 3-(3,4-dihydroxyphenyl)lactic acid, (7.76%) hydroxycinnamic acids derivatives. In these results, the fraction rich in simple phenolic acids was obtained, attributing the prominent behavior of this matrix antioxidant activity, expressed by (IC50: of 16.42 µg/mL and 6.52 µg/mL for DPPH and ABTS+ radicals, respectively). The research demonstrated an alternative to applicability that involves sustainability from agro-industrial. These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product, generating a process capable of converting biomass into a bioproduct, consisting of bioactive compounds, in addition to adding value to the industrial chain. 相似文献
6.
Jing Yu Jingwei Han Chuanzhong Li 《Mathematical Methods in the Applied Sciences》2020,43(6):3076-3085
For the orthosymplectic Lie superalgebra , we choose a set of basis matrices. A linear combination of those basis matrices presents a spatial spectral matrix. The compatible condition of the spatial part and the corresponding temporal parts of the spectral problem leads to a generalized super AKNS (GSAKNS) hierarchy. By making use of the supertrace identity, the obtained GSAKNS hierarchy can be written as the super bi-Hamiltonian structures. 相似文献
7.
Leila Torkian Mostafa M. Amini Tayebeh Gorji Omid Sadeghi 《Arabian Journal of Chemistry》2019,12(7):1315-1321
In this study, multiwalled carbon nanotube (MWCNT) was modified by the pyridine group using a silane agent and characterized by infrared spectroscopy (IR), thermal analysis (TG/DTA), and elemental analysis (CHN) and scanning electron microscopy (SEM). The application of this sorbent was investigated in determination of lead ions in aqueous samples, using flame atomic absorption spectrometry (FAAS). Through this study, different parameters such as pH and sample flow rate on adsorption process and eluent concentration, volume and flow rate were optimized. The limit of detection (LOD), the relative standard deviation and the recovery of the method were 2 ng mL?1, 1.3% and 99.7%, respectively. Two standard reference materials (NIST 1571 and NIST 1572) were used to verify accuracy of this method. Finally, the sorbent was successfully applied for extraction and determination of low levels of Pb(II) ions in aqueous samples. 相似文献
8.
9.
浅谈乒乓球运动特点和发展规律 总被引:1,自引:0,他引:1
通过对乒乓球运动特点和发展规律的探讨,进一步加深了对本项目特点和发展规律的理解和掌握,为提高科学的教学、训练水平,培养高水平人才提供借鉴。 相似文献
10.
We prove that to most of the known hypercyclic operators A on separable Banach spaces there exist compact (compact convex, compact connected) subsets K of E such that each compact (compact convex, compact connected) subset of E can be approximated with respect to Hausdorff's distance by for suitable .
Received July 8, 1997, in final form October 17, 1997 相似文献