全文获取类型
收费全文 | 39136篇 |
免费 | 2703篇 |
国内免费 | 2373篇 |
专业分类
化学 | 19405篇 |
晶体学 | 211篇 |
力学 | 4668篇 |
综合类 | 153篇 |
数学 | 9783篇 |
物理学 | 9992篇 |
出版年
2023年 | 368篇 |
2022年 | 539篇 |
2021年 | 1172篇 |
2020年 | 985篇 |
2019年 | 976篇 |
2018年 | 689篇 |
2017年 | 887篇 |
2016年 | 1005篇 |
2015年 | 1041篇 |
2014年 | 1550篇 |
2013年 | 2564篇 |
2012年 | 1802篇 |
2011年 | 2106篇 |
2010年 | 1795篇 |
2009年 | 2399篇 |
2008年 | 2433篇 |
2007年 | 2670篇 |
2006年 | 2098篇 |
2005年 | 1354篇 |
2004年 | 1169篇 |
2003年 | 1360篇 |
2002年 | 1268篇 |
2001年 | 1265篇 |
2000年 | 876篇 |
1999年 | 758篇 |
1998年 | 714篇 |
1997年 | 562篇 |
1996年 | 616篇 |
1995年 | 581篇 |
1994年 | 552篇 |
1993年 | 534篇 |
1992年 | 499篇 |
1991年 | 318篇 |
1990年 | 271篇 |
1989年 | 189篇 |
1988年 | 174篇 |
1987年 | 197篇 |
1986年 | 206篇 |
1985年 | 319篇 |
1984年 | 235篇 |
1983年 | 146篇 |
1982年 | 298篇 |
1981年 | 463篇 |
1980年 | 423篇 |
1979年 | 467篇 |
1978年 | 370篇 |
1977年 | 280篇 |
1976年 | 239篇 |
1974年 | 75篇 |
1973年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
1.
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
2.
基于有限元法对单面柱局域共振声子晶体进行带隙特性分析,研究了结构参数对该类型声子晶体的影响。结果表明:随着散射体高度的增加,单面柱声子晶体的第一完全带隙的起始频率逐渐降低,带宽逐渐增大;随着基板厚度的增大,单面柱声子晶体的起始频率逐渐升高,截止频率先增大后减小。并且在经典单面柱声子晶体的基础上,组合了两种新型的三组元单面柱声子晶体结构:嵌入式单面柱声子晶体(以下简称结构Ⅰ)和粘接式单面柱声子晶体(以下简称结构Ⅱ)。通过对其带隙特性的分析得出:这两种新结构与经典的单面柱声子晶体相比,都具有更低频的带隙,这对于低频减振降噪是非常有利的。本文的结果将对实际的工程应用提供一定的理论指导。 相似文献
3.
《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
4.
We propose a conjecture on the relative twist formula of l-adic sheaves, which can be viewed as a generalization of Kato—Saito's conjecture. We verify this conjecture under some transversal assumptions. We also define a relative cohomological characteristic class and prove that its formation is compatible with proper push-forward. A conjectural relation is also given between the relative twist formula and the relative cohomological characteristic class. 相似文献
5.
《Journal of Saudi Chemical Society》2022,26(2):101436
Continuous administration of most chemotherapeutic drugs can induce different types of side effects. There has been growing interest in exploring an alternative approach to synthesizing compounds that are most effective and have fewer side effects. We synthesized 29H,31H-Phthalocyanine, and Chloro (29H,31H- phthalocyaninato) aluminum at low temperatures using lithium in the present study with diisopropylamide as the nucleophile. The physical characteristics of 29H,31H-Phthalocyanine, and Chloro (29H,31H- phthalocyaninato) aluminum were confirmed by FT-IR method, XRD, SEM, and the impact of these compounds on human colorectal carcinoma (HCT-116) and human cervical cells (HeLa) was examined. Treatment with 29H,31H-Phthalocyanine significantly decreased cancer cell growth and proliferation, as determined by MTT and DAPI staining analysis. In contrast, Chloro (29H,31H- phthalocyaninato) aluminum treatment did not show any inhibitory action on colon or cervical cancer cells. We also calculated the inhibitory concentration (IC50) of 29H,31H-Phthalocyanine, which was 30 µg/ml (HCT-116) and 33 µg/ml (HeLa cells). The antibacterial effectiveness of 29H,31H-Phthalocyanine, and chloro (29H,31H- phthalocyaninato) aluminum was studied using Enterococcus faecalis (E. faecalis). The CFU (colony frequency unit) assay confirmed significant activity against the test bacterium after treatment with 29H,31H-Phthalocyanine. However, no activity was seen upon treatment with chloro (29H,31H- phthalocyaninato) aluminum against E. faecalis. 相似文献
6.
分子张力作为空间设计的重要组成部分正成为调控有机半导体的重要手段。由于分子内产生的拉伸张力、扭曲/弯曲张力以及空间张力而导致p轨道排布重组和构型构象结构发生变化,最近各种几何与拓扑结构的高张力有机半导体材料相继被报道,这使得高张力有机半导体材料成为有机电子领域研究的焦点。为了进一步梳理分子张力在有机半导体材料中扮演的角色与价值,该综述从分子张力的类型、实验与理论量化以及可视化出发,总结了高张力共轭芳烃的分子设计策略、与其光电性能分子张力之间的关系,以及这类新兴材料在光电领域的应用。最后,对高张力共轭芳烃的研究前景进行了展望,阐述了该类材料所面临的机遇与挑战。 相似文献
7.
《Journal of Pure and Applied Algebra》2022,226(8):106948
Let be a Henselian discrete valued field with residue field of characteristic , and be the Brauer p-dimension of K. This paper shows that if , for some . It proves that if and only if . 相似文献
8.
《Arabian Journal of Chemistry》2022,15(10):104145
In the present study, novel representatives of the important group of biologically-active, dehydroabietic acid-bearing dithiocarbamate moiety, were synthesized and characterized by 1H NMR, 13C NMR, HR-MS. The in vitro antiproliferative activity evaluation (MTT) indicated that these compounds exhibited potent inhibitory activities in various cancer cell lines (HepG-2, MCF-7, HeLa, T-24, MGC-803). Particularly, compound III-b possessed extraordinary cytotoxicity with low micromolar IC50 values ranging from 4.07 to 38.84 µM against tested cancer cell lines, while displayed weak cytotoxicity on two normal cell lines (LO-2 and HEK 293 T). Subsequently, the potential mechanisms of representative compound III-b were elementarily investigated by Transwell experiment, which showed III-b can inhibit cancer cells migration. Annexin-V/PI dual staining showed that the compound can induce HepG-2 cells apoptosis in a dose-dependent manner. Meanwhile this apoptosis may be related to the upregulated protein expression of cleaved-caspase 3, cleaved-caspase 9, Bax and downregulated of Bcl-2 indicated by Western Blot. Later study further confirmed that ROS levels in HepG-2 cells increased significantly with the rise of concentrations. In addition, through the network pharmacology data analyzing, the core targets and signaling pathways of compound III-b for treatment of liver neoplasms were forecasted. Molecular docking model showed that compound III-b had high affinity with hub targets (CASP3, EGFR, HSP90AA1, MAPK1, ERBB2, MDM2), suggesting that compound III-b might target the hub protein to modulate signaling activity. Taken together, these data indicated that dehydroabietic acid structural modification following the “Molecular hybridization” principle is a feasible way to discover the potential multi-targeted antitumor compounds. 相似文献
9.
《Arabian Journal of Chemistry》2022,15(6):103761
In this study, we report the green synthesis of nontoxic, stable, and small size gold nanoparticle by using chitosan/sodium lignosulfonate hydrogel with capping/reducing ability for the synthesis of CS/NaLS/Au NPs. The prepared bio-nanocomposite were characterized by advanced physicochemical techniques like Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray spectroscopy (EDX) and X-ray Diffraction (XRD) study. It has been established that CS/NaLS/Au NPs have a spherical shape with a mean diameter from 20 to 30 nm. Diabetes was induced by administration of 60 mg/kg of streptozotocin (STZ) intraperitoneally in 100 mature male mice and they were randomly divided into 5 groups. The negative control group received normal saline and treatment groups received glibenclamide with dose 0.5 mg/kg and 10 and 40 μg/kg of CS/NaLS/Au NPs through gavage for 50 days. In addition, one group considered as positive control (in treated-diabetic). On the last day, serum levels of samples blood glucose, urea and creatinine were measured. After tissue processing, 5 μm sections of the kidneys were prepared and they were stained by periodic acid Schiff (PAS) and used for stereological analysis. In the antioxidant test, the IC50 of CS/NaLS/Au NPs and BHT against DPPH free radicals were 117 and 86 µg/mL, respectively. In the cellular and molecular part of the recent study, the treated cells with CS/NaLS/Au NPs were assessed by MTT assay for 48 h about the cytotoxicity properties on normal (HUVEC) cell line. The increased levels of blood glucose and urea were decreased (p < 0.05) significantly in CS/NaLS/Au NPs-treated groups as compared to the untreated diabetic. The kidney weight, kidney volume (Volume of cortex, medulla, glomerulus, proximal and distal tubules, collecting ducts, loop of Henle, interstitial tissues, and vessels) and kidney structures length (length of proximal and distal tubules, collecting ducts, loop of Henle, and vessels) decreased significantly (p < 0.05) after treatment with high dose of CS/NaLS/Au NPs (p < 0.05). According to the obtained results, CS/NaLS/Au NPs can regulates the levels of blood glucose and urea and inhibits from kidney damages in STZ-induced diabetic mice. This study suggested CS/NaLS/Au NPs as an antidiabetic and nephroprotective drug in the developing countries. 相似文献
10.
二维材料MXene纳米片由于具有较大的比表面积和较高的电子迁移率而受到广泛的关注。本文采用基于密度泛函理论的第一性原理计算,对单层MXene纳米片Ti2N电磁特性的过渡金属(Sc、V、Zr)掺杂效应进行了系统研究。结果表明,所有过渡金属掺杂体系结合能均为负值,结构均稳定;其中Ti2N-Sc体系的形成能为-2.242 eV,结构更易形成,且保持稳定;掺杂后Ti2N-Sc、Ti2N-Zr体系磁矩增大;此外,Ti2N-Sc体系中保留了较高的自旋极化率,达到84.9%,可预测该体系在自旋电子学中具有潜在的应用价值。 相似文献