全文获取类型
收费全文 | 21372篇 |
免费 | 3793篇 |
国内免费 | 3354篇 |
专业分类
化学 | 16318篇 |
晶体学 | 489篇 |
力学 | 1091篇 |
综合类 | 297篇 |
数学 | 2247篇 |
物理学 | 8077篇 |
出版年
2024年 | 33篇 |
2023年 | 301篇 |
2022年 | 440篇 |
2021年 | 593篇 |
2020年 | 725篇 |
2019年 | 856篇 |
2018年 | 657篇 |
2017年 | 681篇 |
2016年 | 920篇 |
2015年 | 1011篇 |
2014年 | 1135篇 |
2013年 | 1544篇 |
2012年 | 1970篇 |
2011年 | 1944篇 |
2010年 | 1562篇 |
2009年 | 1530篇 |
2008年 | 1715篇 |
2007年 | 1454篇 |
2006年 | 1463篇 |
2005年 | 1333篇 |
2004年 | 1070篇 |
2003年 | 842篇 |
2002年 | 899篇 |
2001年 | 691篇 |
2000年 | 561篇 |
1999年 | 444篇 |
1998年 | 283篇 |
1997年 | 212篇 |
1996年 | 196篇 |
1995年 | 173篇 |
1994年 | 186篇 |
1993年 | 145篇 |
1992年 | 102篇 |
1991年 | 111篇 |
1990年 | 70篇 |
1989年 | 60篇 |
1988年 | 59篇 |
1987年 | 33篇 |
1986年 | 46篇 |
1985年 | 48篇 |
1984年 | 43篇 |
1983年 | 32篇 |
1982年 | 30篇 |
1981年 | 35篇 |
1980年 | 25篇 |
1979年 | 33篇 |
1977年 | 24篇 |
1975年 | 26篇 |
1974年 | 22篇 |
1972年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Two new spirostanol saponins, namely elephanosides G and H ( 1 and 2 , resp.) were isolated from the leaves of Yucca elephantipes (Agavaceae), together with the two known furostanol saponins 3 and 4 and the six known flavonoid O‐ and C‐glycosides 5 – 10 . The new structures were elucidated as (3β,25S)‐spirost‐5‐en‐3‐yl O‐β‐D ‐glucopyranosyl‐(1→3)‐O‐β‐D ‐glucopyranosyl‐(1→4)‐β‐D ‐galactopyranoside ( 1 ) and (3β,5β,25R)‐3‐[(2‐O‐β‐D ‐glucopyranosyl‐β‐D ‐galactopyranosyl)oxy]spirostan‐12‐one ( 2 ) on the basis of detailed spectroscopic analysis and acidic hydrolysis. 相似文献
982.
A new and efficient access to (Z)‐N‐(2‐argio‐1‐(1H‐perimidin‐2‐yl)vinyl)benzamide derivatives from readily available substrates in HOAc is described with aid of microwave irradiation. The results of our study provide a simple, straightforward synthetic route to these interesting classes of 2‐substituted perimidines analogs in excellent yields. 相似文献
983.
Mingming Zou Ying Li Jun Wang Qi Wang Yumei Kong Jingqun Gao Ping Fan 《Journal of solution chemistry》2013,42(4):849-865
The interaction between bovine serum albumin (BSA) and FeIII complexes with three binary organic acid (biorga) ligands, [FeIII(oxa)(H2O)4]+ (oxa = oxalic acid), [FeIII(pra)(H2O)4]+ (pra = propanedioic acid) and [FeIII(sua)(H2O)4]+ (sua = succinic acid), as well as the sonocatalytic damage of BSA in the presence of these three FeIII–biorga complexes under ultrasonic irradiation, were studied by UV–vis and fluorescence spectra. The experimental results show that the fluorescence quenching process of BSA caused by three FeIII–biorga complexes are all static quenching and the corresponding quenching rate constants (K q), equilibrium constants (K A) and the binding site numbers (n) were calculated. The results reveal that, under ultrasonic irradiation, the BSA molecules were obviously damaged by these FeIII–biorga complexes. In addition, the effects of several factors on the damage of BSA molecules were examined. The experimental results demonstrate that the damage degree of BSA increased with an increase of ultrasonic irradiation time, FeIII–biorga complex concentration, and ionic strength. In comparison, [FeIII(pra)(H2O)4]+ exhibited higher sonocatalytic activity than [FeIII(oxa)(H2O)4]+ and [FeIII(sua)(H2O)4]+. Finally, the extent of generation of $ \cdot {\text{O}}_{2}^{ - } $ · O 2 ? and ·OH during sonocatalytic processes was estimated. Perhaps, the results will be significant for promoting sonodynamic treatment (SDT) of tumors at the molecular level. 相似文献
984.
985.
Yao Huang Long‐Jiang Liu Ming‐Shan Zhou Xiao‐Jun Zhou Chao Zhu Fei‐E Fu Yong‐Ming Zhang 《无机化学与普通化学杂志》2013,639(10):1884-1887
Two metal‐organic frameworks, [Zn(dmtrz)(btrc)1/3]n ( 1 ) and [Zn2(dmtrz)2(btec)(H2O)2]n ( 2 ) (dmtrz = 3, 5‐dimethyl‐1‐H‐1, 2,4‐triazole, btrc = 1, 3,5‐benzenetricarboxylate, btec = 1, 2,4, 5‐benzenetetracarboxylate), were synthesized by hydrothermal reaction. The crystal structure analysis reveals that compound 1 is a dense 3D framework with Schläfli symbols of {43}2{46 · 66 · 83}3, which is a loh1 structure. Compound 2 is a 2D network. In addition, the photoluminescence of two compounds were studied in solid state at room temperature, together with their thermal analysis. 相似文献
986.
Dong‐Jun Yu Jun Hu Yan Huang Hong‐Bin Shen Yong Qi Zhen‐Min Tang Jing‐Yu Yang 《Journal of computational chemistry》2013,34(11):974-985
Understanding the interactions between proteins and ligands is critical for protein function annotations and drug discovery. We report a new sequence‐based template‐free predictor (TargetATPsite) to identify the Adenosine‐5′‐triphosphate (ATP) binding sites with machine‐learning approaches. Two steps are implemented in TargetATPsite: binding residues and pockets predictions, respectively. To predict the binding residues, a novel image sparse representation technique is proposed to encode residue evolution information treated as the input features. An ensemble classifier constructed based on support vector machines (SVM) from multiple random under‐samplings is used as the prediction model, which is effective for dealing with imbalance phenomenon between the positive and negative training samples. Compared with the existing ATP‐specific sequence‐based predictors, TargetATPsite is featured by the second step of possessing the capability of further identifying the binding pockets from the predicted binding residues through a spatial clustering algorithm. Experimental results on three benchmark datasets demonstrate the efficacy of TargetATPsite. © 2013 Wiley Periodicals, Inc. 相似文献
987.
Jun Yang Rong Guo Stig E. Friberg 《Journal of Dispersion Science and Technology》2013,34(3-4):249-261
Addition of alcohol with longer chain length (C6H13OH, C8H17OH, and C12H21OH) caused a reduction the cloud point of a commercial nonionic surfactant, Tesgitol (T15-s-9). The formation of lamellar liquid crystal (LLC) was favored so that isotropic liquid (L1)-LLC two-phase region became wider with increasing temperature at an appropriate weight ratio of surfactant to alcohol. The isotropic liquid phase/liquid two phase transformation was replaced by a two-phase transformation to isotropic liquid/lamellar liquid crystal at the cloud point for the system without alcohol. 相似文献
988.
Phase change material (PCM, octadecane) nanocapsules were successfully prepared by resin fortified emulsion (RFE) polymerization using the alkali soluble resin (ASR) of poly(ethylene‐co‐acrylic acid) (EAA) and poly(styrene‐co‐acrylic acid) (SAA). Stable PCM nanocapsules were obtained by resin fortified emulsion polymerization, which could be attributed to the prevention of Ostwald ripening due to PCM being hydrophobic. Analysis of online FTIR measurements throughout the reaction confirmed that the nanocapsules contained octadecane as a PCM. TEM imaging of the PCM nanocapsules showed spherical and core/shell morphology. The characteristics of PCM nanocapsules can be controlled by process parameters. As a result, the particle size and particle size distribution (i.e., polydispersity index (Dw/Dn)) of the PCM nanocapsules were created by adjusting manufacturing conditions. The PCM nanocapsules exhibited thermal energy storage (~49.8 J/g) and release (~47.9 J/g) behavior. 相似文献
989.
为掌握生物质中钾对生物质超临界水降解过程的影响,选择生物质气化转化过程中生成的重要小分子中间产物甲醛作为研究对象,研究不同工艺条件下(反应温度400~650℃、压力23~29 MPa、停留时间4~12 s),单一钾成分(KHCO3/K2CO3/KCl)或混合钾成分(KHCO3、K2CO3、KCl)对甲醛超临界水降解气体产物的影响。结果表明,KHCO3、K2CO3、KCl、混合钾均降低了气体产物中CO的体积分数,提高了CO2的体积分数,但KCl的影响程度弱于其他三种钾成分。此外,由于不同钾成分均不利于气体产物中CO和H2体积分数的提高,从而使得气体产物的热值降低。KHCO3、K2CO3、KCl、混合钾均降低了H2、CO、CO2的产率以及气化率,其对H2产率以及气化率的抑制程度从大到小依次为:混合钾、KHCO3、K2CO3、KCl,且提高反应温度、延长反应停留时间时抑制气体生成的作用相对越明显。而当反应压力改变时,钾成分对H2产率及气化率的影响程度变化较小。较高反应温度、较高反应压力、较长停留时间时,混合钾中各成分出现协同作用,明显抑制了气体产物的生成。 相似文献
990.
将奎宁与Keggin结构磷钼钒杂多酸结合制备出一种新颖的多相苯羟基化杂化催化剂, 采用傅里叶变换红外光谱、紫外-可见光谱、X射线衍射、扫描电镜、热重、N2吸附-脱附和CHN元素分析等表征手段对催化剂进行了分析. 结果表明, 该催化剂是一种具有较高比表面积和孔体积的半无定形有机杂多酸盐. 在H2O2为氧源的苯羟基化反应中, 杂化催化剂引导了液-固两相催化体系, 表现出较高催化活性和重复使用稳定性. 催化剂中奎宁与杂多阴离子间的氢键和电子相互作用赋予了其高熔点和难溶性, 而高比表面积和因奎宁而改善的杂多阴离子的氧化还原性是其获得优异催化性能的主要原因. 这为多相苯羟基化反应提供了一种新的且制备便捷的基于多金属氧酸盐的高效催化剂. 相似文献