全文获取类型
收费全文 | 17722篇 |
免费 | 1795篇 |
国内免费 | 1410篇 |
专业分类
化学 | 2802篇 |
晶体学 | 145篇 |
力学 | 1403篇 |
综合类 | 261篇 |
数学 | 12698篇 |
物理学 | 3618篇 |
出版年
2024年 | 36篇 |
2023年 | 175篇 |
2022年 | 244篇 |
2021年 | 301篇 |
2020年 | 498篇 |
2019年 | 523篇 |
2018年 | 462篇 |
2017年 | 514篇 |
2016年 | 599篇 |
2015年 | 472篇 |
2014年 | 848篇 |
2013年 | 1612篇 |
2012年 | 875篇 |
2011年 | 977篇 |
2010年 | 852篇 |
2009年 | 1031篇 |
2008年 | 1039篇 |
2007年 | 1100篇 |
2006年 | 977篇 |
2005年 | 874篇 |
2004年 | 786篇 |
2003年 | 732篇 |
2002年 | 750篇 |
2001年 | 626篇 |
2000年 | 641篇 |
1999年 | 562篇 |
1998年 | 466篇 |
1997年 | 456篇 |
1996年 | 296篇 |
1995年 | 219篇 |
1994年 | 190篇 |
1993年 | 115篇 |
1992年 | 138篇 |
1991年 | 99篇 |
1990年 | 75篇 |
1989年 | 73篇 |
1988年 | 65篇 |
1987年 | 50篇 |
1986年 | 46篇 |
1985年 | 79篇 |
1984年 | 77篇 |
1983年 | 38篇 |
1982年 | 58篇 |
1981年 | 52篇 |
1980年 | 50篇 |
1979年 | 54篇 |
1978年 | 33篇 |
1977年 | 30篇 |
1976年 | 30篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
《Arabian Journal of Chemistry》2020,13(10):7289-7301
Black pepper oils have been investigated frequently in the recent years. However, there is a significant variation in physicochemical properties and bioactivity of oils depended on extraction techniques. In this study, the systemic investigation of four various extraction methods was performed to evaluate the physicochemical characterizations, antioxidant and antibacterial activity. The investigation of 1H NMR, FTIR and UV–Vis spectra confirmed presence of non-volatile components in oils extracted through supercritical CO2 and hexane-soaking extractions which induced their typical thermal properties. The isothermal behaviour of extracted oils related to evaporation was within range of 3.2–7.3% (w/w) at 27 °C. The SEM images of the black pepper confirmed different operation manners of mechanism between extractions using the solvents and heating process. The lowest MIC for both essential oils from conventional hidrodistillation and microwave-assisted hidrodistillation against two bacteria including E. coli and B. subtilis were found to be 137 µg mL−1. The non-isothermal decomposition kinetics were investigated on the essential oil of microwave-assisted hydrodistillation extraction. The activation energies and pre-exponent factors of non-isothermal decomposition were found to be in range of 36.5–73.7 KJ mol−1 and 4.98 × 103–1.97 × 108 s−1, respectively, dependent on conversional fractions of the oil. The results revealed that chemical components, physicochemical properties and bioactivity of black pepper essential oils depended on the extraction techniques. 相似文献
992.
Cellulose-based hydrogel materials were prepared and modified with tannic acid and l-methionine using ionic liquid as the solvent. The gels were prepared to develop a sustained release medium for selenourea (SeU). The drug delivery characteristics of selenourea-loaded cellulose (CSeU), selenourea-loaded tannic acid-modified cellulose (CTSeU), and selenourea-loaded L-methionine-modified cellulose (CMSeU) were investigated in aqueous media and simulated gastric fluid (SGF) media. This modified gel beads have been characterized using field emission scanning electron microscope, X-ray energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, thermogravimetry–differential thermal analysis and swelling properties and compared with those of the unmodified ones. We also investigated the inhibitory effects of SeU released from these gels on the activity of mushroom tyrosinase. Out of all the gel materials, CTSeU showed maximum SeU release both in water and SGF media. However, tyrosinase inhibitory action in PBS medium was comparable for all the three gel materials. 相似文献
993.
The electrochemical reduction of antifouling agent 2-thiocyanomethylthiobenzothiazole (TCMTB) was investigated by cyclic and pulse differential voltammetry. The irreversible electrode reduction of TCMTB proceeded by ECEC reaction mechanism by two electrons transfer with one irreversible wave. Upon the basis of electrochemical evidence, the electrodic reaction mechanism was suggested to formation of mercaptobenzothiazole (MTB) in solution.Subsequently, a pulse differential method is described for the formation of TCMTB based on this electroreduction. Having been obtained a detection limit of 1.0 × 10−7 mol L−1 and recovery to 98% to concentration of 2.0 × 10−6 mol L−1. Therefore, the proposed method in this study is practical, sensitive and accurate for the analysis of TCMTB in tannery wastewater samples. 相似文献
994.
A facile cycloaddition reaction of 8,8-dicyanoheptafulvene with styrenes leading to the corresponding [8+2] and [4+2] adducts in excellent yields is described. 相似文献
995.
Hemocompatibility is an essential aspect of blood contacting polymers. Knowledge of the relationship between polymer structure and hemocompatibility is important in designing such polymers. In this work, the effect of swelling behavior and states of water on the hemocompatibility of poly(acrylonitrile-co-N-vinyl-2-pyrrolidone) (PANCNVP) films was studied. Platelet adhesion and plasma recalcification time tests were used to evaluate the hemocompatibility of the films. Considering the importance of surface properties on the hemocompatibility of polymers, static water contact angles were measured by both sessile drop and captive bubble methods. It was found that, on the film surface of PANCNVP with a higher NVP content, adhered platelets were remarkably suppressed and the recalcification time was longer. The total water content adsorbed on the PANCNVP film was determined through swelling experiments performed at temperatures of interest. Differential scanning calorimetry and thermogravimetric analysis were used to probe the states of water in the films. Based on the results from these experiments, it was hypothesized that the better hemocompatibility of PANCNVP films with higher NVP contents was due to their higher free water content, because water molecule exchange at the polymer/liquid interface, facilitated by a high free water content, is unfavorable for the formation of surface bound water, which causes poor hemocompatibility. [diagram in text]. 相似文献
996.
《Electroanalysis》2005,17(21):1952-1958
The performance of a poly(1,8‐diaminonaphthalene)‐modified electrode for the determination of the Se(IV) ion in an aqueous medium was investigated with anodic stripping voltammetry without the pretreating of the sample. The experimental parameters for the analysis of Se(IV) were optimized and the characteristics of this polymer‐modified electrode were investigated by using cyclic voltammetry. The Se(IV) ions were chemically deposited onto the surface of the pDAN‐Au electrode in an acidic medium. The detection limit employing the anodic stripping differential pulse voltammetry was 9.0×10?9 M for Se(IV) with 4.4 % of RSD. Satisfactory result for the determination of Se(IV) was acquired employing a certified standard urine reference material, SRM's 2670 (trace element in urine) with 4.1 ppb of SD. 相似文献
997.
β-Lactoglobulin-carboxymethyl cyclodextrin (β-LG-CMCyD) conjugates were prepared by using water soluble carbodiimide. Three kinds of CMCyDs differing in molecular mass were used to investigate the effects of different CMCyD contents, net charge and hydrophobicity on the structural changes in β-lactoglobulin. The effect of CMCyDs on the structure of β-lactoglobulin was utilized to investigate the contribution of hydrophobic interactions to the stability of the protein. Spectroscopic studies suggested that the conformation around had not changed in either conjugate but the α-helix content of β-LG-CMCyD conjugates had markedly increased as compared with that of β-lactoglobulin. The differential scanning calorimetry technique confirmed that the addition of one glucose unit in β-LG-CMCyD conjugates, enthalpy change of calorimetry decreased and the denaturation temperature of each conjugate was higher than that of native β-lactoglobulin. The heat contents agreed well with the conformational transition measured by molar ellipticity at 222 nm ([θ]222) and Stoke's radius (RS) values. Therefore, hydrophobic forces play an important role in stabilizing and shielding of the β-LG-CMCyD conjugates. 相似文献
998.
Summary {\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}}
\viewkind4\uc1\pard\f0\fs20 New explanations are given for two types of irregular thermogravimetric (TG) data. A TG relationship between mass and temperature is derived on the basis of migration behavior of bubbles generated in bulk of sample system, and superposed on that derived on the basis of kinetics of the 4\super th\nosupersub order event, which is superposed on the experimental TG data obtained from three reference papers. This suggests that these TG data are reflecting migration of bubbles. A dependence of TG behavior on heating rate, which is contrary to usual that, is shown and is explained in terms of event-rate determination by boiling.
\par } 相似文献
999.
Senatra D. Gambi C. M. C. Carlà M. Chittofrati A. 《Journal of Thermal Analysis and Calorimetry》1999,56(3):1335-1346
The thermal properties of a perfluoropolyether (PFPE) W/O microemulsion were investigated by Differential Scanning Calorimetry
(DSC) both on freezing the liquid samples and upon their melting. PFPE systems as a function of increasing volume fraction
(Φ=water+surfactant/total), were studied along a dilution line with a water/surfactant molar ratio W/S=11. The percolative
nature of these systems emerged directly from the spreading of the exothermic peaks associated with the freezing of the dispersed
phase. This behaviour was found to depend on whether the starting temperature of the DSC measure was at, below or above the
percolative threshold temperature of the given sample. A low temperature 'needle-like' peak was found around 143 K, immediately
after the glass transition due to the oil continuous phase. The low temperature peak was also present in other percolative,
three-component microemulsions. The higher order phase transition at the percolation temperature was also evidenced.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
1000.
Mineral oil type lubricating oils have an important role in reducing engine wear. But after some period, depending on the
oil quality, oil replacement is needed since the oil loses its properties. In this paper, an engine oil was tested in a 1300
cc engine car and the change of the thermal stability of the collected portions of oil at different usage periods were experimented
by TG/DTG and DSC techniques. Results show that, as usage period increases lubricating oil contains more of the light components
that distill at low temperatures. The increase in the peak maximum temperature, Tmax, of the DTG, as usage period increases, proposes that the molecular mass of the oil increase, which presumably changes the
viscosity characteristics. Moreover as the usage period increases some residue type components, which only can be destroyed
at very high temperatures, are produced in oil phase.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献