全文获取类型
收费全文 | 4737篇 |
免费 | 588篇 |
国内免费 | 455篇 |
专业分类
化学 | 3470篇 |
晶体学 | 50篇 |
力学 | 285篇 |
综合类 | 36篇 |
数学 | 494篇 |
物理学 | 1445篇 |
出版年
2024年 | 19篇 |
2023年 | 77篇 |
2022年 | 132篇 |
2021年 | 175篇 |
2020年 | 157篇 |
2019年 | 173篇 |
2018年 | 152篇 |
2017年 | 133篇 |
2016年 | 221篇 |
2015年 | 185篇 |
2014年 | 234篇 |
2013年 | 333篇 |
2012年 | 331篇 |
2011年 | 371篇 |
2010年 | 253篇 |
2009年 | 247篇 |
2008年 | 279篇 |
2007年 | 256篇 |
2006年 | 254篇 |
2005年 | 183篇 |
2004年 | 177篇 |
2003年 | 148篇 |
2002年 | 180篇 |
2001年 | 134篇 |
2000年 | 106篇 |
1999年 | 111篇 |
1998年 | 98篇 |
1997年 | 85篇 |
1996年 | 53篇 |
1995年 | 72篇 |
1994年 | 60篇 |
1993年 | 52篇 |
1992年 | 52篇 |
1991年 | 32篇 |
1990年 | 36篇 |
1989年 | 17篇 |
1988年 | 18篇 |
1987年 | 12篇 |
1986年 | 29篇 |
1985年 | 26篇 |
1984年 | 17篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1977年 | 4篇 |
1974年 | 7篇 |
1973年 | 7篇 |
1969年 | 4篇 |
排序方式: 共有5780条查询结果,搜索用时 15 毫秒
91.
Different-shaped aluminas were readily prepared via hydrothermal reaction. It was found that the morphology and the electrochemical sensing properties of alumina were heavily dependent on the reaction time. When extending the reaction time from 6 h to 24 h, the obtained alumina samples changed from amorphous bumps to regular microfibers in diameter of 200 nm, as confirmed by scanning electron microscopy. Transmission electron microscopy observation revealed that longer reaction time was beneficial for the formation of porous and uniform fiber-like structures. Electrochemical tests proved that alumina microfibers were more active for the oxidation of amaranth and exhibited much higher enhancement effect, compared with alumina bumps. On the surface of alumina microfibers, the oxidation peak currents of amaranth increased remarkably. The influences of pH value, amount of alumina microfibers, and accumulation time on the signal enhancement of amaranth were discussed. As a result, a novel electrochemical method was developed for the detection of amaranth. The linear range was from 1 to 150 nM, and the detection limit was 0.75 nM after 1-min accumulation. The analytical application in drink samples was investigated, and the results consisted with the values that obtained by high-performance liquid chromatography. 相似文献
92.
Zhenyu Wang Xiaojuan Zhang Jun Yang Zhong Yang Xiaoping Wan Ning Hu Xiaolin Zheng 《Analytica chimica acta》2013
A large number of microscale structures have been used to elaborate flowing control or complex biological and chemical reaction on microfluidic chips. However, it is still inconvenient to fabricate microstructures with different heights (or depths) on the same substrate. These kinds of microstructures can be fabricated by using the photolithography and wet-etching method step by step, but involves time-consuming design and fabrication process, as well as complicated alignment of different masters. In addition, few existing methods can be used to perform fabrication within enclosed microfluidic networks. It is also difficult to change or remove existing microstructures within these networks. In this study, a magnetic-beads-based approach is presented to build microstructures in enclosed microfluidic networks. Electromagnetic field generated by microfabricated conducting wires (coils) is used to manipulate and trap magnetic beads on the bottom surface of a microchannel. These trapped beads are accumulated to form a microscale pile with desired shape, which can adjust liquid flow, dock cells, modify surface, and do some other things as those fabricated microstructures. Once the electromagnetic field is changed, trapped beads may form new shapes or be removed by a liquid flow. Besides being used in microfabrication, this magnetic-beads-based method can be used for novel microfluidic manipulation. It has been validated by forming microscale dam structure for cell docking and modified surface for cell patterning, as well as guiding the growth of neurons. 相似文献
93.
Sazlinda Kamaruzaman Peter C. Hauser Mohd Marsin Sanagi Wan Aini Wan Ibrahim Salasiah Endud Hong Heng See 《Analytica chimica acta》2013
A simple adsorption/desorption procedure using a mixed matrix membrane (MMM) as extraction medium is demonstrated as a new miniaturized sample pretreatment and preconcentration technique. Reversed-phase particles namely polymeric bonded octadecyl (C18) was incorporated through dispersion in a cellulose triacetate (CTA) polymer matrix to form a C18-MMM. Non-steroidal anti-inflammatory drugs (NSAIDs) namely diclofenac, mefenamic acid and ibuprofen present in the environmental water samples were selected as targeted model analytes. The extraction setup is simple by dipping a small piece of C18-MMM (7 mm × 7 mm) in a stirred 10 mL sample solution for analyte adsorption process. The entrapped analyte within the membrane was then desorbed into 100 μL of methanol by ultrasonication prior to high performance liquid chromatography (HPLC) analysis. Each membrane was discarded after single use to avoid any analyte carry-over effect. Several important parameters, such as effect of sample pH, salting-out effect, sample volume, extraction time, desorption solvent and desorption time were comprehensively optimized. The C18-MMM demonstrated high affinity for NSAIDs spiked in tap and river water with relative recoveries ranging from 92 to 100% and good reproducibility with relative standard deviations between 1.1 and 5.5% (n = 9). The overall results obtained were found comparable against conventional solid phase extraction (SPE) using cartridge packed with identical C18 adsorbent. 相似文献
94.
Dadan Hermawan Izdiani Mohd Yatim Khaulah Ab Rahim Mohd Marsin Sanagi Wan Aini Wan Ibrahim Hassan Y. Aboul-Enein 《Chromatographia》2013,76(21-22):1527-1536
A simple solid phase extraction (SPE) method coupled with high performance liquid chromatography (HPLC) using UV detector and microemulsion electrokinetic chromatography (MEEKC) has been developed and compared for the quantitative determination of miconazole nitrate in pharmaceutical formulation. For HPLC method, two parameters were optimized, namely, the wavelength and the mobile phases. The optimized condition was at the 225 nm wavelength and the mobile phase of ACN:MeOH (90:10 v/v). There are seven MEEKC parameters that were optimized, in this research, which were applied to voltage, temperature, wavelength, sodium dodecyl sulfate (SDS) concentration, buffer pH, buffer concentration and butan-1-ol concentration. The optimum MEEKC condition was obtained using 86.35 % (w/w) 2.5 mM borate buffer pH 9, 0.25 % (w/w) SDS, 0.8 % (w/w) ethyl acetate, 6.6 % w/w butan-1-ol and 6.0 % (w/w) acetonitrile. The combination of SPE using a diol column with HPLC–UV and the MEEKC methods were successfully applied for the determination of miconazole nitrate in a pharmaceutical formulation with the recovery percentage of 98.35 and 92.50 %, respectively. 相似文献
95.
Seung Young Lee Kyeong Wan Woo Chung Sub Kim Dong Ung Lee Kang Ro Lee 《Helvetica chimica acta》2013,96(2):320-325
Three new furofuran lignans, (+)‐4,4′‐O‐diangeloylpinoresinol ( 1 ), (+)‐4,4′‐O‐diangeloylmedioresinol ( 2 ), and (+)‐4,4′‐O‐diangeloylsyringaresinol ( 3 ), together with the known compound (+)‐syringaresinol, were isolated from the MeOH extract of Rudbeckia laciniata. The structure elucidation of these compounds were based on 1D‐ and 2D‐NMR, and HR‐ESI‐MS data. The additional structural evidence was obtained from alkaline hydrolysis of the compounds. 相似文献
96.
A facile and efficient synthesis of 14-aryl or alkyl-14H-dibenzo[a,j]xanthenes has been accomplished by treatment of β-naphthol with aryl or alkyl aldehydes catalyzed by 2-1′-methylimidazolium-3-yl-1-ethyl sulfate in ionic liquid [BMIM][BF4] at 80°C. The luminescence of products was studied in detail. 相似文献
97.
A novel and easy synthetic route to diethyl (5-substituted phenyl-1,3,4-thiadiazol-2-ylamino) (substituted phenyl) methylphosphonates has been achieved by the reaction of substituted benzylidene-5-(substituted phenyl)-1,3,4-thiadiazol-2-amines and diethyl phosphite under microwave irradiation. These 1,3,4-thiadiazole aminophosphonates were identified by infrared, 1H NMR, and elemental analyses. The target compounds were obtained in better yields (71–89%) and shorter time (10 min) than with conventional heating.
98.
Ling Zhang Hongqi Wan Lei Chen Huidi Zhou Jianmin Chen 《Journal of Dispersion Science and Technology》2013,34(6):846-850
Sb2S3 nanoparticles surface-modified with S-tetradecyl N, N-dihydroxyethyl dithiocarbamate (C14DTC-Sb2S3) have been synthesized via extraction of Sb2S3 colloidal particles from ethylene glycol into toluene in the presence of C14DTC. The obtained products were characterized by high-resolution transmission electron microscope (HRTEM) and Fourier transformation infrared (FTIR), and their tribological behaviors as an additive in liquid paraffin were investigated using a four-ball tribometer. The results show that C14DTC-Sb2S3 nanoparticles can significantly improve the friction reduction, anti-wear, and load-carrying properties of base oils. The preliminary lubrication mechanism was discussed based on the SEM and XPS investigation of the rubbed surfaces. 相似文献
99.
Zal U’yun Wan Mahmood Mei Wo Yii 《Journal of Radioanalytical and Nuclear Chemistry》2013,298(3):1727-1732
The purpose of this study was to estimate the sedimentation rate in the Sungai Linggi estuary using the constant initial concentration of unsupported or excess 210Pb model and verified with 137Cs method. Five sediment cores were collected on 25 January 2011 using gravity corer with the inner tube of 50 cm length and 7.5 cm diameter. The total 210Pb activities in the sediment cores profile at all sampling stations were varied and upper than those obtained for supported 210Pb i.e. 226Ra, indicated disequilibrium among 210Pb and its grandparents in the 238U decay series. Meanwhile, the lower 137Cs activities were observed at all sampling stations due to no significant sources of 137Cs releases were transferred into Malaysian marine. The estimation of sedimentation rate indicated the agreement of 210Pb and 137Cs method with a general presence of deep mixing in the Sungai Linggi estuary. Therefore, the apparent sedimentation rates calculated from 210Pb profiles generally reflect the true value with the range from 0.70 to 1.97 cmyr?1. High sedimentation rate was observed at some sampling stations which are located in river channel, estuary and closer to mainland. This suggested that land-use development, agriculture activities, channelization etc. introduced a large amount of sediment loaded into those areas. 相似文献
100.
A homologous series of 4,4'-bis(omega-hydroxyalkoxy)-alpha-methylstilbenes, denoted HAMS-n, has been synthesized. Here n is the carbon atom length of the peripheral n-alkyl chain; in this work n = 2 to 8 and 11. The composition, microstructure, thermal properties, and mesomorphism of the homologues were determined using elemental analysis, 1H NMR, FTIR, DSC, wide angle X-ray diffraction, and polarizing optical microscopy with a heating stage. The influence of the length and the even/odd numbers of methylene units of the peripheral chains on the variants of mesomorphism and the stability of mesophases, as well as on their transition temperatures, are discussed. The homologues with n = 8 and 11 were enantiotropic monomorphic, CrG (or CrH). The odd homologues showed monotropic trimorphism on cooling, while the even homologues exhibited enantiotropic trimorphism. A pronounced even/odd effect on the melting temperatures was observed. 相似文献