全文获取类型
收费全文 | 59109篇 |
免费 | 851篇 |
国内免费 | 295篇 |
专业分类
化学 | 27588篇 |
晶体学 | 1030篇 |
力学 | 3199篇 |
综合类 | 8篇 |
数学 | 4672篇 |
物理学 | 23758篇 |
出版年
2022年 | 475篇 |
2021年 | 475篇 |
2020年 | 452篇 |
2019年 | 430篇 |
2018年 | 572篇 |
2017年 | 497篇 |
2016年 | 898篇 |
2015年 | 640篇 |
2014年 | 991篇 |
2013年 | 2520篇 |
2012年 | 2344篇 |
2011年 | 3017篇 |
2010年 | 2133篇 |
2009年 | 2183篇 |
2008年 | 2767篇 |
2007年 | 2600篇 |
2006年 | 2466篇 |
2005年 | 2196篇 |
2004年 | 2000篇 |
2003年 | 1765篇 |
2002年 | 1660篇 |
2001年 | 3010篇 |
2000年 | 2154篇 |
1999年 | 1557篇 |
1998年 | 1086篇 |
1997年 | 1058篇 |
1996年 | 896篇 |
1995年 | 789篇 |
1994年 | 706篇 |
1993年 | 626篇 |
1992年 | 958篇 |
1991年 | 948篇 |
1990年 | 850篇 |
1989年 | 747篇 |
1988年 | 731篇 |
1987年 | 799篇 |
1986年 | 657篇 |
1985年 | 888篇 |
1984年 | 832篇 |
1983年 | 570篇 |
1982年 | 569篇 |
1981年 | 537篇 |
1980年 | 503篇 |
1979年 | 622篇 |
1978年 | 653篇 |
1977年 | 665篇 |
1976年 | 580篇 |
1975年 | 484篇 |
1974年 | 525篇 |
1973年 | 451篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
781.
Djaoued Y. Badilescu Simona Ashrit P.V. Bersani D. Lottici P.P. Brüning R. 《Journal of Sol-Gel Science and Technology》2002,24(3):247-254
TiO2 nanocrystalline thin films with varying degree of porosity have been prepared using a low temperature method. TiO2 films of the anatase form have been obtained by using a polyethylene glycol (PEG) modified sol-gel method. Densification and crystallization of the films was found to result from the thermal treatment of the dip coated films in boiling water. The films have been characterized by Raman, XRD, FTIR, AFM and optical methods. Highly transparent films with transmission in excess of 85% and porosity as high as 58% are formed predominantly of anatase crystallites of dimensions of the order of 5 nm. Initial results on lithium intercalation into these films resulting in an efficient optical modulation in the visible and near infrared regions demonstrate a good potential of these films for electrochromic applications. 相似文献
782.
Relationships between average degree of transformation and time of dissociation were derived for polydisperse granular materials
with account to the type of grain size distribution. It has been checked, under what conditions the kinetic curves obtained
by numeric solution of those relationships may be described in terms of equations D1 α2=kt
, F2 [1/(1 - α) - 1 =kt] and F3 [1/(1 - α)2 - 1=kt].
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
783.
784.
Burguera JL Quintana IA Salager JL Burguera M Rondón C Carrero P Anton de Salager R Petit de Peña Y 《The Analyst》1999,124(4):593-599
An on-line time based injection system used in conjunction with cold vapor generation atomic absorption spectrometry and microwave-aided oxidation with potassium persulfate has been developed for the determination of the different mercury species in fish-eggs oil samples. A three-phase surfactant-oil-water emulsion produced an advantageous flow when a peristaltic pump was used to introduce the highly viscous sample into the system. The optimum proportion of the oil-water mixture ratio was 2:3 v/v with a Tween 20 surfactant concentration in the emulsion of 0.008% v/v. Inorganic mercury was determined after reduction with sodium borohydride while total mercury was determined after an oxidation step with persulfate prior to the reduction step to elemental mercury with the same reducing agent. The difference between total and inorganic mercury determined the organomercury content in samples. A linear calibration graph was obtained in the range 0.1-20 micrograms l-1 of Hg2+ by injecting 0.7 ml of samples. The detection limits based on 3 sigma of the blank signals were 0.11 and 0.12 microgram l-1 for total and inorganic mercury, respectively. The relative standard deviation of ten independent measurements were 2.8 and 2.2% for 10 micrograms l-1 and 8.8 and 9.0% for 0.1 microgram l-1 amounts of total and inorganic mercury, respectively. The recoveries of 0.3, 0.6 and 8 micrograms l-1 of inorganic and organic mercury added to fish-eggs oil samples ranged from 93.0 to 94.8% and from 100 to 106%, respectively. Good agreement with those values obtained for total mercury content in real samples by electrothermal atomic absorption spectrometry was also obtained, differences between mean values were < 7%. With the proposed procedure, 22 proteropterous catfish-eggs oil samples from the northwestern coast of Venezuela were measured; while the organic mercury lay in the range 2.0 and 3.3 micrograms l-1, inorganic mercury was not detected. 相似文献
785.
Hassan Sedaira Kamal A. Idriss Elham Y. Hashem Mohamed M. Seleim Abu-Bakr El-Nady 《Journal of solution chemistry》1996,25(1):95-103
The solvation number and hydration constant of Th4+ in ethanol-water medium were determined at 25°C using UV-spectral and electrochemical measurements. A solvate formation equilibrium is demonstrated and characterized. Three molecules of ethanol (S) can bond to the metal cation with strengths comparable to that for H2O to form ThS3(H2O) 3 4+ Formation of thorium monochelate with lawsone (2-hydroxy-1,4-naphthoquinone) eliminates bonding with alcohol molecules. The dissociation constant of the chelating agentsKa and the formation contant of the monochelated metal ionsK f * that are essentially independent of the solution composition are evaluated. Hydration titrations involving thorium-lawsone monochelate are performed and the data obtained from the changes of pH with solvent composition are analyzed. The solution independent constant,sK f * for thorium-lawsone complex formation in mixed aqueous ethanol is given by $$log_s K_f * = vpK_a + log_s K_h - log[LH] - vpH + 3logv$$ where vpKa is the dissociation constant of the chelating agent LH in the solvent system of ν volume fraction of water andsKh is the solution-independent hydration constant of thorium (IV) in the solvent system. Log-values for the constantssKh,sK f * andsK a * are found to be 7.8±0.02, 11.38±0.04 and ?0.753, respectively. 相似文献
786.
Brena B Zhuang GV Augustsson A Liu G Nordgren J Guo JH Ross PN Luo Y 《The journal of physical chemistry. B》2005,109(16):7907-7914
The electronic structure of pure poly(ethylene oxide) (PEO) for four different polymeric chain conformations has been studied by Hartree-Fock (HF) and density functional theory (DFT) through the analysis of their valence band photoelectron spectroscopy (VB-PES), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS). It is shown that the valence band of PEO presents specific conformation dependence, which can be used as a fingerprint of the polymeric structures. The calculated spectra have been compared with experimental results for PEO powder. 相似文献
787.
Price WJ Kuo PK Lee TR Colorado R Ying ZC Liu GY 《Langmuir : the ACS journal of surfaces and colloids》2005,21(18):8422-8428
Nanostructures of self-assembled monolayers (SAMs) are designed and produced using coadsorption and nanografting techniques. Because the structures of these artificially engineered domains are predesigned and well-characterized, a systematic investigation is possible to study the mechanical responses to force modulation under atomic force microscope tips. Force modulation imaging reveals characteristic contrast sensitivity to changes in molecular-level packing, molecule chain lengths, domain boundaries, and surface chemical functionalities in SAMs. By means of actively tuning the driving frequency, the resonances at the tip-surface contact are selectively activated. Therefore, specific surface features, such as the edges of the domains and nanostructures or desired chemical functionalities, can be selectively enhanced in the amplitude images. These observations provide a new and active approach in materials characterization and the study of nanotribology using atomic force microscopy. 相似文献
788.
A novel photothermal phase-shift spectroscopy configuration based on the retro-reflected beam interference has been developed and its operational principle is described. The weak absorption measurement ability of this technique is experimentally proven with a water/ethanol solution of standard Pyronine G dye and the limit of detection is found to be 1.8 x 10(-6) absorbance. Potential applications of the technique are discussed. 相似文献
789.
Dilute aqueous dispersions of colloidal polystyrene latex spheres were flocculated by adding a nonadsorbing polymer sample, poly(acrylic acid). The structural compactness of the flocs thus formed was characterized in terms of their mass fractal dimension using the small-angle static light scattering technique. It was found that with low poly(acrylic acid) concentrations and thus weak depletion attraction forces, the dispersion medium viscosity had a marked effect on the floc structure. An increase in the viscosity led to formation of denser flocs. This was revealed in three sets of depletion flocculation experiments: (a) adjusting the background electrolyte concentration at a fixed level of poly(acrylic acid), (b) using water and 30% (w/w) glycerol as the respective solvents, and (c) inducing latex flocculation with two poly(acrylic acids) of different molecular weights at the respective critical polyacid concentrations. Direct force measurements were made with atomic force microscopy to isolate the influence of viscosity on floc structure from that of interparticle interaction energies. We conclude that the formation of denser flocs with increasing medium viscosity can be attributed to the reduced diffusivity of particles in the solution. The latter resulted in an enhanced rate of floc restructuring (through relaxation of attached particles) relative to floc growth. 相似文献
790.
Based on gaseous compound introduction as 1-phenyl-3-methyl-4-benzoylpyrazolone[5](PMBP) chelate of aluminum by electrothermal vaporization in ICP-AES, a method for determination of trace aluminum was developed. Trace aluminum was vaporized at temperature of 1000 degrees C, and the vaporization behavior of aluminum chelate was detailedly investigated. Under the optimum conditions, the detection limit of aluminum was 0.6 ng ml(-1), and the relative standard deviation for 0.1 mug ml(-1) aluminum was 4.7% (n=8). The proposed method was applied to the determination of trace aluminum in rice flour reference materials, and the results well agreed with the reference values. 相似文献