全文获取类型
收费全文 | 16991篇 |
免费 | 3212篇 |
国内免费 | 2494篇 |
专业分类
化学 | 8466篇 |
晶体学 | 198篇 |
力学 | 1799篇 |
综合类 | 176篇 |
数学 | 1460篇 |
物理学 | 10598篇 |
出版年
2024年 | 39篇 |
2023年 | 162篇 |
2022年 | 585篇 |
2021年 | 565篇 |
2020年 | 634篇 |
2019年 | 542篇 |
2018年 | 507篇 |
2017年 | 683篇 |
2016年 | 779篇 |
2015年 | 610篇 |
2014年 | 949篇 |
2013年 | 1453篇 |
2012年 | 1083篇 |
2011年 | 1180篇 |
2010年 | 993篇 |
2009年 | 1114篇 |
2008年 | 1177篇 |
2007年 | 1040篇 |
2006年 | 1082篇 |
2005年 | 868篇 |
2004年 | 832篇 |
2003年 | 711篇 |
2002年 | 623篇 |
2001年 | 599篇 |
2000年 | 524篇 |
1999年 | 389篇 |
1998年 | 433篇 |
1997年 | 354篇 |
1996年 | 351篇 |
1995年 | 265篇 |
1994年 | 243篇 |
1993年 | 213篇 |
1992年 | 204篇 |
1991年 | 145篇 |
1990年 | 123篇 |
1989年 | 111篇 |
1988年 | 117篇 |
1987年 | 82篇 |
1986年 | 56篇 |
1985年 | 46篇 |
1984年 | 56篇 |
1983年 | 20篇 |
1982年 | 38篇 |
1981年 | 22篇 |
1980年 | 15篇 |
1979年 | 12篇 |
1978年 | 20篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
301.
Experimental results are presented for electrode erosion on copper cathodes in magnetically rotated arcs in argon, dry air, nitrogen, ammonia, and carbon monoxide as well mixtures of the above with argon. Water-saturated argon was also used. Erosion rates were determined by weight loss after chemical cleaning, and the runs were sufficiently long (between 5 to 60 min) to represent steady-state operation. Arc currents of 100 A and gas pressures of 1.1 atm. were used. Pure argon gave the highest erosion rates and the lowest arc velocities. Small concentrations of any of the diatomic gases in argon greatly increased the arc velocity and decreased the erosion rates. The results suggest that erosion is primarily a thermal phenomenon but that the surface chemistry can greatly influence erosion rates by modifying arc behavior. 相似文献
302.
Y. Sakaiguchi T. Shikata H. Urakami A. Tamura H. Hirata 《Colloid and polymer science》1987,265(8):750-753
Some cationic surfactants such as cetyltrimethylammonium bromide (CTAB) show a very spectacular viscoelasticity in solution, in even very diluted states, with some aromatic substances added, such as salicylic acid. Formerly, the authors established that the remarkable solution behaviour was the result of the entanglement of such enormously elongated giant micelles, based on recognition of the existence of large aggregates, through direct observation of them as substantial images under the electron microscope.In this report, we further confirm the relation between micelle length and solution viscoelasticity after arbitrarily obtaining different size aggregates by altering the media pH, or by admixing shorter chain length surfactants than CTAB, which alone did not induce viscoelasticity. 相似文献
303.
Summary Molecular surface areas accessible to a 4 ? diameter spherical probe were calculated from crystallographic data for five proteins:
α-chymotrypsinogen A, lysozyme, trypsinogen, ribonuclease A and ribonuclease S. The retention factors of various proteins
were measured on stationary phases having polyether- and phenylligates and with aqueous eluents containing (NH4)2SO4, Na2SO4 or NaCl at pH 7.0. The logarithmic retention factors were plotted against the salt molality and the hydrophobic interaction
parameters evaluated from the limiting slopes of the plots at high salt concentrations for the proteins in the chromatographic
systems investigated. The hydrophobic interaction parameters thus obtained were linear in both the molecular surface areas
of the proteins and the molal surface tension increments of the salts. The experimental results obtained with these relatively
simple proteins of known molecular structure, which were available in high purity, support earlier theoretical predictions
for the dependence of the hydrophobic interaction parameter on the surface area of the protein and the surface tension raising
effect of the salt. 相似文献
304.
305.
碳酸钙晶须合成过程中可溶性磷酸盐的作用机理研究 总被引:2,自引:0,他引:2
以可溶性磷酸盐为控制剂,一步碳化法制备了文石相碳酸钙晶须。借助于XRD和FTIR,分析了可溶性磷酸盐在碳酸钙晶须合成过程中的作用机理。研究结果表明:通入CO2进行碳化反应前,可溶性磷酸盐与Ca(OH)2反应生成了热力学上最稳定的磷酸钙化合物——羟基磷灰石;在通入CO2初期,[CO32-(OH)]进入到羟基磷灰石的晶格,部分替代[PO43-],生成碳酸羟基磷灰石,然后以此为结晶中心诱导文石相的异相成核,Ca2+、CO32-不断叠加,进而生长为碳酸钙晶须。 相似文献
306.
Sancheeta Kolay Kallol K. Ghosh Amy MacDonald Jonathan Moulins Rama M. Palepu 《Journal of solution chemistry》2008,37(1):59-72
The thermodynamics of micellization and other micellar properties of alkyl- (C10-, C12-, C14- and C16-) triphenylphosphonium bromides in water + ethylene glycol (EG) (0 to 30% v/v) mixtures over a temperature range of 298 to
318 K and cetyltriphenylphosphonium bromide in water + diethylene glycol (DEG) mixtures (0 to 30% v/v) at 298 K have been
studied conductometrically. In all cases, an increase in the percentage of co-solvent results in an increase in the cmc values.
On the basis of these results, the thermodynamic parameters, the Gibbs energy (ΔG
mo), enthalpy (ΔH
mo) and entropy (ΔS
mo) of micellization have been evaluated. In addition to the conductivity measurements, kinetic experiments have also been done
to determine the dependence of observed rate constant for the nucleophilic substitution reaction of p-nitrophenyl acetate and benzohydroxamate ions in the presence of the surfactant cetyltriphenylphosphonium bromide with a
varying concentration of EG and DEG ranging from 0 to 50% v/v at pH=7.9 and 300 K. All of the reactions followed pseudo-first-order
kinetics. An increase in the surfactant concentration results in an increase in the reaction rate and for a given surfactant
concentration, the rate constant decreases as the concentration of co-solvent in the mixture increases. The kinetic micellar
effects have been explained by using the pseudophase model. The thermodynamic and structural changes originating from the
presence of solvents control the micellar kinetic effects. 相似文献
307.
《Journal of polymer science. Part A, Polymer chemistry》2018,56(9):1012-1019
Herein, we report the synthesis, characterization, and field‐effect properties of two cross‐conjugated dithienylmethanone (DMO)‐based alternating polymers, namely, PDMO‐S and PDMO‐Se . Both polymers possess high thermal stability, good solubility, and broad absorption spectra. Their electrochemical properties were investigated using cyclic voltammetry, indicating that PDMO‐Se has higher HOMO/LUMO energy levels of −5.49/−3.49 eV than −5.57/−3.58 eV of PDMO‐S . The two polymers exhibited promising charge transport properties with the highest hole mobility of 0.12 cm2 V−1 s−1 for PDMO‐S and 0.025 cm2 V−1 s−1 for PDMO‐Se . AFM and 2D‐GIXRD analyses demonstrated that the PDMO‐S formed lamellar, edge‐on packing thin film with close π‐π stacking. These findings suggest that cross‐conjugated polymers might be potential semiconducting materials for low‐cost and flexible organic electronics. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1012–1019 相似文献
308.
《International journal of quantum chemistry》2018,118(16)
Theoretical calculations were performed to elucidate the ability of the recently reported bis‐tert‐alcohol‐functionalized crown‐6‐calix[4]arene (BACCA) molecule to promote nucleophilic fluorination of alkyl mesylates with cesium fluoride reagent. It was found that a similar structure, named BACCAt, can separate the cesium fluoride ion pair in tert‐butanol solution. This separation has a free energy cost, even considering the double hydrogen bonds with the fluoride ion. The solvent has an important effect on the stabilization of this complex, due to interaction with the high dipole moment of the separated ion pair. The observed rate acceleration effect involves a structure with double hydrogen bonds between the BACCAt and the centers of negative charges of the SN2 transition state. The predicted free energy barrier of 27.3 kcal mol−1 is in excellent agreement with the estimated experimental value of 26.2 kcal mol−1. 相似文献
309.
Tripti Vimal Shivani Pandey Swadesh K. Gupta Dharmendra P. Singh Kaushlendra Agrahari Govind Pathak 《Liquid crystals》2018,45(5):687-697
Rod-shaped 5 wt.% copper-doped ZnO (ZnO:Cu2+) ferromagnetic nanoparticles (NPs), prepared by hydrothermal method, were dispersed in ferroelectric liquid crystal (FLC) named Felix 17/100. The effect of ferromagnetic NPs on the physical properties of FLC material (Felix 17/100) has been investigated by dielectric, electro-optical and polarising optical microscopic methods. A noteworthy time-dependent memory has been observed in the NPs-dispersed FLC composite attributed to the coupling of magnetic field associated to NPs with the director orientation of FLC. Improvement in spontaneous polarisation and dielectric susceptibility of FLC material has been ensued with the addition of ferromagnetic NPs. Faster electro-optic response, at lower applied voltage, has also been observed in NPs-dispersed FLC composite. These changes are accredited to the magneto-electric dipolar coupling existing due to the interactions between magnetic-dipole and electric-dipole moments of magnetic NPs and FLC material, respectively. The formation of periodic domains capable to show memory effect has been observed in composite. The observed time-dependent memory was confirmed by dielectric and electro-optical methods. FLC material enriched with the properties of ferromagnetic NPs can be utilised in advanced multifunctional optical devices, time-dependent memory-based security devices and computational purposes. 相似文献
310.
The analytical expressions of the various structural units and the average degree of branching for the hyperbranched polymers resulted from AB2 polycondensation with substitution effect were derived by the kinetic mechanism.The reactivity difference between the B group in linear unit and that in terminal group has great effect on the molecular parameters of the products obtained.The concentration of terminal units has a maximum with the increase of the conversion of A groups(x).The higher the reactivity ratio(r) of linear B group to branched one is,the later the maximum appears and the larger it is.The degree of branching of the hyperbranched polymers obtained is controllable by adjusting the parameters of r and x,which increases with increasing both x and r. 相似文献