全文获取类型
收费全文 | 9720篇 |
免费 | 1778篇 |
国内免费 | 1816篇 |
专业分类
化学 | 7807篇 |
晶体学 | 126篇 |
力学 | 722篇 |
综合类 | 147篇 |
数学 | 1066篇 |
物理学 | 3446篇 |
出版年
2024年 | 9篇 |
2023年 | 190篇 |
2022年 | 248篇 |
2021年 | 348篇 |
2020年 | 452篇 |
2019年 | 446篇 |
2018年 | 354篇 |
2017年 | 379篇 |
2016年 | 488篇 |
2015年 | 531篇 |
2014年 | 598篇 |
2013年 | 775篇 |
2012年 | 901篇 |
2011年 | 967篇 |
2010年 | 695篇 |
2009年 | 674篇 |
2008年 | 683篇 |
2007年 | 571篇 |
2006年 | 514篇 |
2005年 | 475篇 |
2004年 | 370篇 |
2003年 | 315篇 |
2002年 | 386篇 |
2001年 | 355篇 |
2000年 | 247篇 |
1999年 | 226篇 |
1998年 | 189篇 |
1997年 | 138篇 |
1996年 | 124篇 |
1995年 | 102篇 |
1994年 | 119篇 |
1993年 | 81篇 |
1992年 | 74篇 |
1991年 | 76篇 |
1990年 | 45篇 |
1989年 | 31篇 |
1988年 | 31篇 |
1987年 | 19篇 |
1986年 | 21篇 |
1985年 | 19篇 |
1984年 | 9篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1975年 | 1篇 |
1957年 | 1篇 |
1943年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
Xi‐Sha Zhang Yun‐Fei Zhang Zhao‐Wei Li Fei‐Xian Luo Zhang‐Jie Shi 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(18):5568-5572
A rhodium(III)‐catalyzed cross‐coupling of benzyl thioethers and aryl carboxylic acids through the two directing groups is reported. Useful structures with diverse substituents were efficiently synthesized in one step with the cleavage of four bonds (C H, C S, O H) and the formation of two bonds (C C, C O). The formed structure is the privileged core in natural products and bioactive molecules. This work highlights the power of using two different directing groups to enhance the selectivity of a double C H activation, the first of such examples in cross‐oxidative coupling. 相似文献
23.
24.
Using polydimethylsiloxane (PDMS) as a basic matrix to prepare ethanol and butanol permselective pervaporation membranes is a vibrant field. Many studies have verified that the three-dimensional Hansen solubility parameters (HSP) theory offers a valid explanation for the swelling performance of ethanol and butanol in PDMS. Five parameters (δD, δP, δH, δt, and Ra) are defined in HSP theory which can be individually used to explain the interaction strength between a solvent and a polymer. However, for the above five parameters, which one is the most effective parameter for deciding the swelling degree still needs to be determined. In this study, a commonly used hydroxy-terminated PDMS precursor was adopted to prepare the PDMS network. The HSP of the chosen PDMS precursor was measured by an advanced “solubility-rating” method. The special software package HSPiP (4.1.03), purchased from the HSPiP team, was used to process the “solubility-rating” results. The equilibrium swelling degree (Q value) of the PDMS network in water, ethanol, butanol, and toluene was measured and the relationships between the five HSP parameters of the solvents and the logarithmic equilibrium swelling degree, log(Q), were discussed. It was found that the measured polar parameter, δP, of PDMS was 0.12 MPa0.5. The measured hydrogen bonding parameter, δH, was larger than δP, attaining a value of 8.6 MPa0.5, because the hydroxy groups directly contributed to the hydrogen bonding solubility parameter, δH. With respect to the relationships between log(Q) and δD, δP, δH, δt, and Ra, linear relationships existed after plotting log(Q) vs. δP and log(Q) vs. δH. The linear relation degree of the fitted lines was 0.995 and 0.989, respectively. Their standard deviations were 0.149 and 0.232, respectively. Therefore, a better linear relationship existed between log(Q) and δP than the other solubility parameters. This indicated that the polar interaction was the main effect for deciding the swelling degree of the PDMS network in water and alcohol systems. 相似文献
25.
In‐Depth Understanding of the Chemical Properties of Rarely Explored Carbide Cluster Metallofullerenes: A Case Study of Sc2C2@C3v(8)‐C82 that Reveals a General Rule 下载免费PDF全文
Wenting Cai Muqing Chen Lipiao Bao Dr. Yunpeng Xie Prof. Dr. Takeshi Akasaka Prof. Dr. Xing Lu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(8):3449-3454
The chemical properties of carbide‐cluster metallofullerenes (CCMFs) remain largely unexplored, although several new members of CCMFs have been discovered recently. Herein, we report the reaction between Sc2C2@C3v(8)‐C82, which is viewed as a prototypical CCMF because of its high abundance, and 3‐triphenylmethyl‐5‐oxazolidinone ( 1 ) to afford the corresponding pyrrolidino derivative Sc2C2@C3v(8)‐C82(CH2)2NTrt ( 2 ; Trt=triphenylmethyl). Single‐crystal X‐ray crystallography studies of 2 revealed that the reaction takes place at a [6,6]‐bond junction, which is directly over the encapsulated C2 unit and is far from either of the two scandium atoms. On the basis of theoretical calculations and by considering previously reports, we have found that a hexagonal carbon ring on the cage of Sc2C2@C3v(8)‐C82 is highly reactive toward different reagents due to the overlap of high p‐orbital axis vector (POAV) angles and large LUMO coefficients. We propose that this highly concentrated area of reactivity is generated by the encapsulation of the Sc2C2 cluster because this region is absent from the empty fullerene C3v(8)‐C82. Moreover, the absorption and electrochemical results confirm that derivative 2 is more stable than pristine Sc2C2@C3v(8)‐C82, thus illuminating its potential applications. 相似文献
26.
一维非共轭烷烃链虽不具富电子或少电子特征,但常存在于单分子器件或多肽、蛋白质等生物分子中,对电子传输产生重要影响.为理解这类物质的电子输运特征,本研究设计了一维线性非共轭(CH2)n分子结模型,并利用密度泛函理论结合非平衡态格林函数的方法,对(CH2)n(n=1-12)线性分子链与两个石墨烯电极耦合而成的分子结进行了第一性原理计算.结果表明,CH2分子链随着n值的变化,其电导值表现出明显的奇偶振荡现象,并且随着链长的增加呈指数级的衰减,这一结果与实验研究取得了很好的一致性,为理解和设计性能更加优良的单分子器件提供了重要理论依据. 相似文献
27.
Jiamei Han Dr. Guohai Liang Prof. Da Xing 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(35):8353-8362
Accurate diagnosis of tumor characteristics, including its location and boundary, is of immense value to subsequent therapy. Activatable magnetic resonance imaging (MRI) contrast agents that respond to tumor-specific microenvironments, such as the redox state, pH, and enzyme activity, enable better mapping of tumor tissue. However, the practical application of most reported activatable agents is hampered by problems including potential toxicity, inefficient elimination, and slow activation. In this study, we developed a zwitterionic iron complex (Fe-ZDS) as a positive MRI contrast agent for tumor-specific imaging. Fe-ZDS could dissociate in weakly acidic solution rapidly, accompanied by clear longitudinal relaxivity (r1) enhancement, which enabled the complex to act as a pH-sensitive contrast agent for tumor-specific MR imaging. In vivo experiments showed that Fe-ZDS rapidly enhanced the tumor-to-normal contrast ratio by >40 %, which assisted in distinguishing the tumor boundary. Furthermore, Fe-ZDS circulated freely in the bloodstream and was excreted relatively safely via kidneys owing to its zwitterionic nature. Therefore, Fe-ZDS is an ideal candidate for a tumor-specific MRI contrast agent and holds considerable potential for clinical translation. 相似文献
28.
对石英音叉增强型光声光谱(QEPAS)系统中常用的石英音叉进行了有限元模态计算,获得石英音叉前6阶振型与模态频率,认知了第4阶对称摆动振型为有效振动,利用单因素法分析了石英音叉的音臂长度l1、音臂宽度w1、音臂厚度t、音臂切角θ、音臂圆孔直径d及音臂圆孔高度h对低阶有效共振频率(Fre)的影响,敏感度依次为: l1> w1>d>θ>t>h,考虑实际设计情形,筛选出了l1,w1,d与h四个石英音叉设计变量,采用Box-Behnken实验设计方案与RSM(response surface methodology)方法,以Fre为函数目标,建立l1,w1,d与h的二次回归响应面模型,得到了参数之间的交互作用,利用Design-Expert软件对响应面模型进行设计参数反求,结果表明,在15 000 Hz≤Fre≤25 000 Hz计算区域内误差较小,基本满足QEPAS系统的计算需求,所提出的研究与设计方法具有一定通用性,可为QEPAS系统中石英音叉结构参数设计提供参考。 相似文献
29.
Dr. Sevan D. Houston Dr. Hui Xing Prof. Paul V. Bernhardt Timothy J. Vanden Berg Dr. John Tsanaktsidis Dr. G. Paul Savage Prof. Craig M. Williams 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(11):2735-2739
The scope and limitations of Eaton's rhodium(I)-catalyzed valence isomerization of cubane to cyclooctatetraene (COT) were investigated in the context of functional group tolerability, multiple substitution modes and the ability of cubane-alcohols to undergo one-pot tandem Ley–Griffith Wittig reactions in the absence of a transition metal catalyst. 相似文献
30.
Xiaolin Xing Dr. Rongji Liu Kecheng Cao Prof. Ute Kaiser Prof. Carsten Streb 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(47):11098-11104
The rapid development of renewable-energy technologies such as water splitting, rechargeable metal–air batteries, and fuel cells requires highly efficient electrocatalysts capable of the oxygen-reduction reaction (ORR) and the oxygen-evolution reaction (OER). Herein, we report a facile sonication-driven synthesis to deposit the molecular manganese vanadium oxide precursor [Mn4V4O17(OAc)3]3− on multiwalled carbon nanotubes (MWCNTs). Thermal conversion of this composite at 900 °C gives nanostructured manganese vanadium oxides/carbides, which are stably linked to the MWCNTs. The resulting composites show excellent electrochemical reactivity for ORR and OER, and significant reactivity enhancements compared with the precursors and a Pt/C reference are reported. Notably, even under harsh acidic conditions, long-term OER activity at low overpotential is reported. In addition, we report exceptional activity of the composites for the industrially important Cl2 evolution from an aqueous HCl electrolyte. The new composite material shows how molecular deposition routes leading to highly active and stable multifunctional electrocatalysts can be developed. The facile design could in principle be extended to multiple catalyst classes by tuning of the molecular metal oxide precursor employed. 相似文献