全文获取类型
收费全文 | 1293篇 |
免费 | 126篇 |
国内免费 | 100篇 |
专业分类
化学 | 243篇 |
晶体学 | 8篇 |
力学 | 227篇 |
综合类 | 9篇 |
数学 | 125篇 |
物理学 | 907篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2022年 | 16篇 |
2021年 | 13篇 |
2020年 | 32篇 |
2019年 | 34篇 |
2018年 | 34篇 |
2017年 | 43篇 |
2016年 | 41篇 |
2015年 | 35篇 |
2014年 | 80篇 |
2013年 | 95篇 |
2012年 | 51篇 |
2011年 | 66篇 |
2010年 | 73篇 |
2009年 | 80篇 |
2008年 | 88篇 |
2007年 | 91篇 |
2006年 | 82篇 |
2005年 | 77篇 |
2004年 | 54篇 |
2003年 | 39篇 |
2002年 | 65篇 |
2001年 | 63篇 |
2000年 | 52篇 |
1999年 | 32篇 |
1998年 | 41篇 |
1997年 | 16篇 |
1996年 | 13篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1519条查询结果,搜索用时 15 毫秒
941.
果糖是一种常见于蜂蜜和多种植物中的简单酮糖,属于三种可食用单糖之一,可与葡萄糖结合形成双糖蔗糖。纯净的果糖常温呈白色晶体状,具有甜度高、升糖指数低等优点,广泛应用于食品行业。截止目前,由于针对固相果糖的太赫兹和红外特征吸收谱研究大多局限于单纯的实验测试或者基于单分子理论计算,缺乏较为系统完善的研究,因此从理论和实验上系统研究了固相果糖的太赫兹特征吸收谱及红外特征吸收谱,首次报道了固相果糖在频谱大于3.0 THz以外的特征吸收峰的实验值,采用基于单分子的MP2和B3LYP泛函以及基于晶胞的PBE和PW91交换相关泛函计算获得了固相果糖太赫兹及红外特征吸收谱,并与实验获得的固相果糖太赫兹及红外特征吸收谱进行了比对分析,发现基于晶胞的PBE和PW91交换相关泛函计算结果与实验获得的果糖太赫兹特征吸收峰更相符,表明固相果糖在0.1~4.0 THz的大多数太赫兹特征吸收峰源自分子间相互作用而非分子内相互作用,揭示了果糖分子周围环境对果糖振动模式的显著影响。 相似文献
942.
Recycled polyamide 6 (PA6) from post-industrial waste fibers (PIW) and post-consumer carpet waste (PCW) are characterized in this study. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) indicate the presence of polypropylene (PP) in PCW. Furthermore, measured ash content of PCW is ca. 6 wt.%, while PIW has only 0.5 wt.% inorganic content. X-ray fluorescence (XRF) and X-ray diffraction (XRD) show that inorganic contaminants of PCW and PIW are calcium carbonate (CaCO3) and titanium dioxide (TiO2), respectively. Due to higher inorganic filler content, PCW exhibits higher melt viscosity and higher storage modulus than that of PIW. PIW has 20% higher tensile strength than that of PCW. However, a drastic 70% drop is observed in vibration weld strength of PCW, which is attributed to its PP contamination. The negative effect of PP on the weld strength of recycled PA6 is also confirmed by measuring the mechanical properties of model compounds. 相似文献
943.
《Biomedical chromatography : BMC》2018,32(5)
Lipoic acid (LA) is used extensively as a therapeutic agent for the treatment of various diseases. Many methods have been reported for the determination of LA plasma levels and its metabolites after its supplementation, but available information concerning endogenous plasma levels is still scarce. Studies which directly focused on determining the endogenous plasma levels provided highly controversial results, <4.9 nmol/L or 143.7–197.0 nmol/L. The main aim of this study was to verify the levels of free LA in the plasma of 40 individuals (17 women, 23 men). This group was nonsupplemented with LA and met the conditions for incorporation into the blood donors register. We measured the levels of LA using an HPLC method with very sensitive coulometric detection after previous sample preparation including deproteination and solid‐phase extraction with a Phenyl cartridge. Our limit of detection was 1.85 nmol/L and was better than the values reported in studies that directly focused on determining the endogenous plasma levels of LA: 2.4 and 4.9 nmol/L respectively. However, the levels of free LA in the plasma of nonsupplemented voluntary blood donors were not detectable in all cases. The presented results of our study show that endogenous concentrations of LA are <1.85 nmol/L. 相似文献
944.
Vasanthy Karmegam Chinthaka M. Udamulle Gedara Michael C. Biewer Mihaela C. Stefan 《Journal of polymer science. Part A, Polymer chemistry》2018,56(22):2547-2553
Side chain engineering has been used for tuning the opto-electronic properties of organic semiconductors. In this work, a series of pyrimidine-based donor-acceptor (D-A) conjugated polymers functionalized with electron-withdrawing or electron-donating side chains were synthesized. The opto-electronic properties of the pyrimidine D-A conjugated polymers were investigated focusing on the dependence on the electron withdrawing strength of the acceptor moiety, while maintaining the same donor moiety. Fine-tuning of the energy levels was achieved by introducing electron donating (alkoxy [ OR] and alkylthio [ SR]) or electron withdrawing (alkylsulfinyl [ SOR] and alkylsulfonyl [ SO2R]) side chains onto the acceptor moiety. The effects of side chain modification have been investigated through DFT calculations, UV–vis analysis, and electrochemical measurements. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2547–2553 相似文献
945.
946.
Resonant frequency shift and electrical impedance variation are common phenomena in the application of high power ultrasonic transducers, e.g. in focused ultrasound surgery and in cutting. They result in low power efficiency and unstable vibration amplitude. To solve this problem, a driving and measurement system has been developed to track the resonance of high power transducers and to stabilise their vibration velocity. This has the ability to monitor the operating and performance parameters of the ultrasonic transducers in real time. The configuration of the system, with its control algorithm implemented in LabVIEW (National Instruments, Newbury, UK), ensures flexibility to suit different transducers and load conditions. In addition, with different programs, it can be utilised as a high power impedance analyser or an instantaneous electrical power measurement system for frequencies in the MHz range. The effectiveness of this system has been demonstrated in detailed studies. With it, high transducer performance at high power can be achieved and monitored in real time. 相似文献
947.
Free vibration of cantilever multi-layer graphene nanoribbons (MLGNRs) with interlayer shear effect is investigated using molecular dynamics simulations (MD) and nonlocal elasticity. Because of similarity of MLGNRs to sandwich structures, sandwich formulations are expressed in the nonlocal form. By comparing the first two frequencies of MLGNRs with various layers and lengths obtained using MD simulations with those of the nonlocal sandwich formulation; the nonlocal parameter is calibrated to match the results of two methods. The results reveal that the calibrated nonlocal parameter for predicting the second frequencies is dependent on the number of MLGNR layers, and it increases by increasing the number of layers. 相似文献
948.
《Physics letters. A》2014,378(18-19):1303-1308
949.
采用密度泛函理论(DFT)中的杂化密度泛函B3LYP方法,在6-31+G水平上对甲硝唑、替硝唑和奥硝唑的结构进行几何优化,并在同一水平上计算了三种化合物的红外光谱,分析了三种化合物的稳定结构和红外光谱特征。结果表明:咪唑环基本为平面构型,并且环上有一定的芳香性;三种化合物在4000-400cm-1波数区光谱基本一致,此部分是5-硝基咪唑类化合物的特征光谱,而在400-100cm-1波数区三种化合物光谱区别较大。因此,可用中红外光谱来鉴定硝基咪唑类化合物,用远红外光谱对以上三种化合物进行鉴别。 相似文献
950.
采用包含迭代三激发的耦合簇理论( CC3和CCSDT-3),在aug-cc-pVTZ基组水平上对HOF分子几何构型进行优化。通过解析二阶导数结合有限差分技术获得HOF二阶、完全三阶和半对角四阶力场。通过非谐性分析,得到其基频、旋振相互作用常数、非谐性常数和离心畸变光谱常数。应用二阶振动微扰理论(VPT2)得到HOF多个泛频峰位置。目前计算值与实验及其它文献结果符合良好。 相似文献