全文获取类型
收费全文 | 67510篇 |
免费 | 15424篇 |
国内免费 | 6397篇 |
专业分类
化学 | 63659篇 |
晶体学 | 922篇 |
力学 | 2645篇 |
综合类 | 387篇 |
数学 | 5713篇 |
物理学 | 16005篇 |
出版年
2024年 | 64篇 |
2023年 | 650篇 |
2022年 | 763篇 |
2021年 | 1226篇 |
2020年 | 2446篇 |
2019年 | 3738篇 |
2018年 | 2065篇 |
2017年 | 1756篇 |
2016年 | 4843篇 |
2015年 | 4952篇 |
2014年 | 5205篇 |
2013年 | 6366篇 |
2012年 | 6005篇 |
2011年 | 5263篇 |
2010年 | 5117篇 |
2009年 | 5034篇 |
2008年 | 4807篇 |
2007年 | 3888篇 |
2006年 | 3576篇 |
2005年 | 3487篇 |
2004年 | 2865篇 |
2003年 | 2434篇 |
2002年 | 3126篇 |
2001年 | 2284篇 |
2000年 | 2056篇 |
1999年 | 1103篇 |
1998年 | 588篇 |
1997年 | 482篇 |
1996年 | 483篇 |
1995年 | 446篇 |
1994年 | 373篇 |
1993年 | 299篇 |
1992年 | 249篇 |
1991年 | 260篇 |
1990年 | 200篇 |
1989年 | 167篇 |
1988年 | 145篇 |
1987年 | 98篇 |
1986年 | 86篇 |
1985年 | 82篇 |
1984年 | 50篇 |
1983年 | 43篇 |
1982年 | 28篇 |
1981年 | 34篇 |
1980年 | 23篇 |
1979年 | 16篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1957年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Abstract Gastrodigenin, also known as 4-hydroxybenzyl alcohol (HBA), is one of the main components of Gastrodia elata, which is a perfect lead compound of natural products. In order to get new active compounds, we modified the structure of HBA through esterification with carboxylic acid, and got a series of derivatives in which 4-hydroxybenzyl alcohol 2-naphthoate (NHBA) showed stronger antidepressant activity than HBA. In this paper, we firstly evaluated the antidepressant activity of NHBA by tail suspension test (TST) and forced swimming test (FST). Then, we carried out the biochemical assay and western blot to determine its mechanism. The results displayed that NHBA could increase the content of serotonin, dopamine, norepinephrine, γ-aminobutyric acid, brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) in mice brain. It suggested that NHBA exhibited an antidepressant-like effect through monoaminergic system, GABAergic system and BDNF/TrkB signaling pathways. 相似文献
82.
以焦炉上升管内壁结焦炭层块为研究对象,采用X射线荧光光谱仪(XRF)、X射线衍射仪(XRD)、傅里叶红外光谱仪(FTIR)和激光共聚焦拉曼光谱仪(Raman)对结焦炭层的元素组成,以及各结焦炭层的矿物组成、组成结构和分子结构进行测试。分析从结焦炭层块外表面向内表面过渡的各结焦炭层的差异性,揭示焦炉上升管内壁结焦机理。结果表明焦炉上升管内粉尘中Fe,S和Cr极易催化荒煤气中蒽、萘等稠环芳烃化合物成炭,在焦炉上升管内壁形成炭颗粒沉积,为焦油凝结挂壁提供载体,在荒煤气温度降至结焦温度时易结焦积碳。结焦炭层均含有芳香层结构,随着结焦炭层从外表面向内表面过渡,各结焦炭层的面层间距(d002)逐渐降低、层片直径(La)先降低后增加、层片堆砌高度(Lc)和芳香层数(N)先稳定后增加。结焦炭层石墨化过程是由结焦炭层内表面向外表面进行,主要包括其片层外缘的羧基和部分C-O结构的降解剥离,从而形成高度规整的共轭结构。结焦炭层块中C元素是以结晶碳与无定型碳的混合物形式存在。以上研究为解决焦炉上升管内壁结焦及腐蚀问题,提高换热器换热效率,有效回收焦炉荒煤气显热,降低焦化企业能耗提供实验基础和理论依据。 相似文献
83.
Two novel 2′-hydroxychalcone derivatives (i.e., M1 and M2) are explored in this work. We mainly focus on investigating the effects of photoexcitation on hydrogen bonds and on the excited-state intramolecular proton transfer (ESIPT) process. On the basis of calculations of electrostatic potential surface and intramolecular interactions, we verify the formation of hydrogen bond O1 H2···O3 in both S0 and S1 states. Exploring the ultraviolet–visible spectra in the liquid phase, our simulated results reappear in the experimental phenomenon. Analyzing molecular geometry and infrared stretching vibrational spectra, we confirm O1 H2···O3 is strengthened for both M1 and M2 in the S1 state. We further confirm that charge redistribution facilitates ESIPT tendency. Constructing potential energy curves, we find the ultrafast ESIPT behavior for M1, which is because of the deficiency of side hydroxyl moiety comparing with M2. This work makes a reasonable affiliation of the ESIPT mechanism for M1 and M2. We wish this paper could facilitate understanding these two novel systems and promote their applications. 相似文献
84.
85.
86.
The γcmc values of CTAB-SDS decrease from 63.67 mN/m at 10‡C to 36.38 mN/m at 90‡C, slightly lower than those of either CTAB or SDS.
Correspondingly, the CMC of CTAB-SDS decreases almost by half. The increase of surface activity of CTAB-SDS can be attributed
to the relatively weak electrostatic interaction at high temperature, which is supported by the increase of solubility of
CTAB-SDS with rise in temperature. Catalytic effect on oxidation of toluene derivatives with potassium permanganate follows
the order CTAB-SDS > SDS > CTAB. This is not caused by the dissociative effect of CTAB-SDS with low surface activity at low
temperature, as seen from the fact that almost all oxidative products can be retrieved for different toluene derivatives and
surfactants by mimicking the conditions of reaction. In the emulsifications of toluene derivatives at 90‡C, the time that
turbid water layers of surfactant solutions take to become clear is the same as that of the catalytic effect on oxidation
of toluene derivatives. Thus, it can be inferred that surfactants can improve the oxidation yields of toluene derivatives
by increasing the contact between two reacting phases. 相似文献
87.
Ji-Dong Lou Chun-Ling Gao Li Li Zhi-Gang Fang 《Monatshefte für Chemie / Chemical Monthly》2006,41(10):1071-1074
A new procedure for the selective oxidation of alcohols to the corresponding aldehydes and ketones with potassium permanganate
supported on aluminum silicate at room temperature under solvent-free conditions and shaking is reported. 相似文献
88.
89.
90.