全文获取类型
收费全文 | 84560篇 |
免费 | 9068篇 |
国内免费 | 4922篇 |
专业分类
化学 | 31348篇 |
晶体学 | 321篇 |
力学 | 8441篇 |
综合类 | 1571篇 |
数学 | 31159篇 |
物理学 | 25710篇 |
出版年
2023年 | 818篇 |
2022年 | 1395篇 |
2021年 | 2565篇 |
2020年 | 2521篇 |
2019年 | 2269篇 |
2018年 | 1976篇 |
2017年 | 2460篇 |
2016年 | 3064篇 |
2015年 | 2703篇 |
2014年 | 3974篇 |
2013年 | 6038篇 |
2012年 | 4497篇 |
2011年 | 4840篇 |
2010年 | 4225篇 |
2009年 | 5359篇 |
2008年 | 5275篇 |
2007年 | 5546篇 |
2006年 | 4497篇 |
2005年 | 3518篇 |
2004年 | 3138篇 |
2003年 | 2923篇 |
2002年 | 2703篇 |
2001年 | 2397篇 |
2000年 | 1942篇 |
1999年 | 1620篇 |
1998年 | 1605篇 |
1997年 | 1224篇 |
1996年 | 1205篇 |
1995年 | 1157篇 |
1994年 | 1062篇 |
1993年 | 1021篇 |
1992年 | 913篇 |
1991年 | 674篇 |
1990年 | 574篇 |
1989年 | 443篇 |
1988年 | 479篇 |
1987年 | 409篇 |
1986年 | 417篇 |
1985年 | 561篇 |
1984年 | 465篇 |
1983年 | 246篇 |
1982年 | 443篇 |
1981年 | 576篇 |
1980年 | 497篇 |
1979年 | 562篇 |
1978年 | 458篇 |
1977年 | 373篇 |
1976年 | 307篇 |
1974年 | 101篇 |
1973年 | 178篇 |
排序方式: 共有10000条查询结果,搜索用时 15 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.
One of the major capacity boosters for 5G networks is the deployment of ultra-dense heterogeneous networks (UDHNs). However, this deployment results in a tremendous increase in the energy consumption of the network due to the large number of base stations (BSs) involved. In addition to enhanced capacity, 5G networks must also be energy efficient for it to be economically viable and environmentally friendly. Dynamic cell switching is a very common way of reducing the total energy consumption of the network, but most of the proposed methods are computationally demanding, which makes them unsuitable for application in ultra-dense network deployment with massive number of BSs. To tackle this problem, we propose a lightweight cell switching scheme also known as Threshold-based Hybrid cEll swItching Scheme (THESIS) for energy optimization in UDHNs. The developed approach combines the benefits of clustering and exhaustive search (ES) algorithm to produce a solution whose optimality is close to that of the ES (which is guaranteed to be optimal), but is computationally more efficient than ES and as such can be applied for cell switching in real networks even when their dimension is large. The performance evaluation shows that THESIS significantly reduces the energy consumption of the UDHN and can reduce the complexity of finding a near-optimal solution from exponential to polynomial complexity. 相似文献
3.
Yi-Xuan Shan 《中国物理 B》2022,31(8):80507-080507
Astrocytes have a regulatory function on the central nervous system (CNS), especially in the temperature-sensitive hippocampal region. In order to explore the thermosensitive dynamic mechanism of astrocytes in the CNS, we establish a neuron-astrocyte minimum system to analyze the synchronization change characteristics based on the Hodgkin-Huxley model, in which a pyramidal cell and an interneuron are connected by an astrocyte. The temperature range is set as 0 ℃-40 ℃ to juggle between theoretical calculation and the reality of a brain environment. It is shown that the synchronization of thermosensitive neurons exhibits nonlinear behavior with changes in astrocyte parameters. At a temperature range of 0 ℃-18 ℃, the effects of the astrocyte can provide a tremendous influence on neurons in synchronization. We find the existence of a value for inositol triphosphate (IP3) production rate and feedback intensities of astrocytes to neurons, which can ensure the weak synchronization of two neurons. In addition, it is revealed that the regulation of astrocytes to pyramidal cells is more sensitive than that to interneurons. Finally, it is shown that the synchronization and phase transition of neurons depend on the change in Ca2+ concentration at the temperature of weak synchronization. The results in this paper provide some enlightenment on the mechanism of cognitive dysfunction and neurological disorders with astrocytes. 相似文献
4.
Xin Liu Lili Chen Yanfeng Zhao Xianhua Song 《Mathematical Methods in the Applied Sciences》2022,45(1):77-92
In this paper, the problem of the uniform stability for a class of fuzzy fractional-order genetic regulatory networks with random discrete delays, distributed delays, and parameter uncertainties is studied. Although there is a portion of literature on using fixed point theorems to study the stability of fractional neural networks, most of them required the fractional order to be in . However, the case of the fractional-order belonging to has not been discussed. To solve it, this work proposes a novel idea of using fixed point theory to study the stability of fuzzy (0,1) order neural networks, the problem of the uniqueness of the solution of the considered genetic regulatory networks is resolved, and a novel sufficient condition to guarantee the uniform stability of above genetic regulatory networks is also derived. Eventually, an example is given to demonstrate that the obtained result is effective. 相似文献
5.
6.
7.
In this paper, a quantum cascade laser (QCL) design is proposed based on GaAs/AlGaAs material system, which simultaneously operates at three widely separated wavelengths ( and ). In the design, all the wavelength radiations are achieved by the engineering of the electronic spectrum via the quantum-well widths and the applied electric field in a single active region within a same waveguide. The mid-infrared (mid-IR) wavelengths are obtained by adoption a dual-upper-state active region, and the proposed design aims to use both the mid-IR radiations as the coherent deriving fields to populate the upper THz lasing state to aid the THz-laser population inversion via optical pumping instead of direct electrical injection. A detailed analysis of electronic transport in the structure is carried out using a multi-level rate-equation model. The results show that the proposed structure offers an alternative approach to room temperature THz generation in QCLs. 相似文献
8.
9.
10.