全文获取类型
收费全文 | 67151篇 |
免费 | 5982篇 |
国内免费 | 4649篇 |
专业分类
化学 | 28829篇 |
晶体学 | 543篇 |
力学 | 2346篇 |
综合类 | 867篇 |
数学 | 23018篇 |
物理学 | 22179篇 |
出版年
2023年 | 711篇 |
2022年 | 790篇 |
2021年 | 1686篇 |
2020年 | 1505篇 |
2019年 | 1770篇 |
2018年 | 1519篇 |
2017年 | 1468篇 |
2016年 | 1745篇 |
2015年 | 1868篇 |
2014年 | 2652篇 |
2013年 | 4577篇 |
2012年 | 3090篇 |
2011年 | 4067篇 |
2010年 | 3567篇 |
2009年 | 4394篇 |
2008年 | 4699篇 |
2007年 | 4558篇 |
2006年 | 3784篇 |
2005年 | 2698篇 |
2004年 | 2526篇 |
2003年 | 2402篇 |
2002年 | 2230篇 |
2001年 | 1875篇 |
2000年 | 1512篇 |
1999年 | 1427篇 |
1998年 | 1254篇 |
1997年 | 1034篇 |
1996年 | 1178篇 |
1995年 | 1042篇 |
1994年 | 996篇 |
1993年 | 917篇 |
1992年 | 823篇 |
1991年 | 566篇 |
1990年 | 477篇 |
1989年 | 451篇 |
1988年 | 408篇 |
1987年 | 356篇 |
1986年 | 362篇 |
1985年 | 508篇 |
1984年 | 401篇 |
1983年 | 241篇 |
1982年 | 429篇 |
1981年 | 591篇 |
1980年 | 504篇 |
1979年 | 527篇 |
1978年 | 432篇 |
1977年 | 340篇 |
1976年 | 287篇 |
1974年 | 99篇 |
1973年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
Carotenoids are an essential component of cashew and can be used in pharmaceuticals, cosmetics, natural pigment, food additives, among other applications. The present work focuses on optimizing and comparing conventional and ultrasound-assisted extraction methods. Every optimization step took place with a 1:1 (w:w) mixture of yellow and red cashew apples lyophilized and ground in a cryogenic mill. A Simplex-centroid design was applied for both methods, and the solvents acetone, methanol, ethanol, and petroleum ether were evaluated. After choosing the extractor solvent, a central composite design was applied to optimize the sample mass (59–201 mg) and extraction time (6–34 min). The optimum conditions for the extractor solvent were 38% acetone, 30% ethanol, and 32% petroleum ether for CE and a mixture of 44% acetone and 56% methanol for UAE. The best experimental conditions for UAE were a sonication time of 19 min and a sample mass of 153 mg, while the CE was 23 min and 136 mg. Comparing red and yellow cashews, red cashews showed a higher carotenoid content in both methodologies. The UAE methodology was ca. 21% faster, presented a more straightforward composition of extracting solution, showed an average yield of superior carotenoid content in all samples compared to CE. Therefore, UAE has demonstrated a simple, efficient, fast, low-cost adjustment methodology and a reliable alternative for other applications involving these bioactive compounds in the studied or similar matrix. 相似文献
2.
3.
Photocatalytic CO2 reduction to C1 fuels is considered to be an important way for alleviating increasingly serious energy crisis and environmental pollution. Due to the environment-friendly, simple preparation, easy formation of highly-stable metal-nitrogen(M-Nx) coordination bonds, and suitable band structure, polymeric carbon nitride-based single-atom catalysts(C3N4-based SACs) are expected to become a potential for CO2 reduction under visible-light irradiation. In this review, we summarize the recent advancement on C3N4-based SACs for photocatalytic CO2 reduction to C1 products, including the reaction mechanism for photocatalytic CO2 reduction to C1 products, the structure and synthesis methods of C3N4-based SACs and their applications toward photocatalytic CO2 reduction reaction(CO2RR) for C1 production. The current challenges and future opportunities of C3N4-based SACs for photoreduction of CO2 are also discussed. 相似文献
4.
Kento Nishikibe Keisuke Nishikawa Momochika Kumagai Matsumi Doe Yoshiki Morimoto 《化学:亚洲杂志》2022,17(1):e202101137
There are marine cytotoxic bromotriterpenoids, named the thyrsiferol family that are structurally characterized by some tetrahydropyran (THP) and tetrahydrofuran (THF) rings. The thyrsiferol family belongs to natural products that are often difficult to determine their stereostructures even by the current, highly advanced spectroscopic methods, especially in acyclic systems including stereogenic tetrasubstituted carbon centers. In such cases, it is effective to predict and synthesize the possible stereostructures. Herein, to elucidate ambiguous stereostructures and unassigned absolute configurations of aplysiol B, laurenmariannol, and saiyacenol A, members of the thyrsiferol family, we carried out their asymmetric chemical syntheses featuring 6-exo and 5-exo oxacyclizations of epoxy alcohol precursors and 6-endo bromoetherification of a bishomoallylic alcohol. In this paper, we report total assignments of their stereostructures through their asymmetric chemical syntheses and also their preliminary cytotoxic activities against some tumor cells. These results could not have been achieved without depending on asymmetric total synthesis. 相似文献
5.
Dariusz Marek Adam Domaski Joanna Domaska Jakub Szygua Tadeusz Czachrski Jerzy Klamka 《Entropy (Basel, Switzerland)》2021,23(5)
In this article, a way to employ the diffusion approximation to model interplay between TCP and UDP flows is presented. In order to control traffic congestion, an environment of IP routers applying AQM (Active Queue Management) algorithms has been introduced. Furthermore, the impact of the fractional controller and its parameters on the transport protocols is investigated. The controller has been elaborated in accordance with the control theory. The TCP and UDP flows are transmitted simultaneously and are mutually independent. Only the TCP is controlled by the AQM algorithm. Our diffusion model allows a single TCP or UDP flow to start or end at any time, which distinguishes it from those previously described in the literature. 相似文献
6.
Yerko Torres-Nova 《Journal of Pure and Applied Algebra》2021,225(1):106465
Let be integers. A generalized Fermat curve of type is a compact Riemann surface S that admits a subgroup of conformal automorphisms isomorphic to , such that the quotient surface is biholomorphic to the Riemann sphere and has branch points, each one of order k. There exists a good algebraic model for these objects, which makes them easier to study. Using tools from algebraic topology and integration theory on Riemann surfaces, we find a set of generators for the first homology group of a generalized Fermat curve. Finally, with this information, we find a set of generators for the period lattice of the associated Jacobian variety. 相似文献
7.
《中国物理 B》2021,30(5):56501-056501
Thermal expansion control is always an obstructive factor and challenging in high precision engineering field. Here,the negative thermal expansion of Nb F_3 and Nb OF_2 was predicted by first-principles calculation with density functional theory and the quasi-harmonic approximation(QHA). We studied the total charge density, thermal vibration, and lattice dynamic to investigate the thermal expansion mechanism. We found that the presence of O induced the relatively strong covalent bond in Nb OF_2, thus weakening the transverse vibration of F and O in Nb OF_2, compared with the case of Nb F_3.In this study, we proposed a way to tailor negative thermal expansion of metal fluorides by introducing the oxygen atoms.The present work not only predicts two NTE compounds, but also provides an insight on thermal expansion control by designing chemical bond type. 相似文献
8.
10.
Chinese Annals of Mathematics, Series B - In the present article, the authors find and establish stability of multiplier ideal sheaves, which is more general than strong openness. 相似文献