全文获取类型
收费全文 | 90217篇 |
免费 | 759篇 |
国内免费 | 419篇 |
专业分类
化学 | 34151篇 |
晶体学 | 854篇 |
力学 | 6905篇 |
数学 | 33428篇 |
物理学 | 16057篇 |
出版年
2023年 | 107篇 |
2022年 | 447篇 |
2021年 | 503篇 |
2020年 | 282篇 |
2019年 | 331篇 |
2018年 | 10593篇 |
2017年 | 10392篇 |
2016年 | 6432篇 |
2015年 | 1265篇 |
2014年 | 693篇 |
2013年 | 1123篇 |
2012年 | 4567篇 |
2011年 | 11324篇 |
2010年 | 6133篇 |
2009年 | 6536篇 |
2008年 | 7315篇 |
2007年 | 9452篇 |
2006年 | 876篇 |
2005年 | 1891篇 |
2004年 | 2026篇 |
2003年 | 2388篇 |
2002年 | 1421篇 |
2001年 | 402篇 |
2000年 | 418篇 |
1999年 | 259篇 |
1998年 | 301篇 |
1997年 | 240篇 |
1996年 | 321篇 |
1995年 | 200篇 |
1994年 | 158篇 |
1993年 | 169篇 |
1992年 | 112篇 |
1991年 | 137篇 |
1990年 | 103篇 |
1989年 | 130篇 |
1988年 | 120篇 |
1987年 | 106篇 |
1986年 | 109篇 |
1985年 | 109篇 |
1984年 | 124篇 |
1983年 | 79篇 |
1982年 | 107篇 |
1981年 | 96篇 |
1980年 | 101篇 |
1979年 | 90篇 |
1978年 | 88篇 |
1977年 | 56篇 |
1976年 | 48篇 |
1914年 | 45篇 |
1909年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Hiléia K. S. Souza V. R. da Silveira F. M. M. Borges Dulce M. A. Melo H. Scatena Jr. O. A. de Oliveira A. G. Souza 《Journal of Thermal Analysis and Calorimetry》2007,87(2):433-436
Thermal behavior of rare earth nitrate complexes
with 2-azacyclononanone (AZA) with Ln(NO3)3·3(AZA)
composition (where Ln=Gd, Er and Ho) was
analyzed in kinetic point of view. Kinetic parameters were calculated from
thermogravimetric data. All obtained results were similar. The first decomposition
step was representative to the loss of ligand and the residue was essentially
Ln2O3. Furthermore, a reaction
path was proposed for the thermal decomposition of the Ln(NO3)3·3(AZA). 相似文献
962.
Maria ?wita?a-?eliazkow 《Polymer Degradation and Stability》2006,91(6):1233-1239
Thermal degradation processes in copolymers of styrene with two stereoisomers of 1,2-ethylenedicarboxylic acids (i.e. maleic acid (the cis-isomer) and fumaric acid (the trans-isomer)) have been studied by mass spectrometry, FTIR and TG measurements. The influence of the chemical composition on thermal degradation of copolymers with acid by non-isothermal thermogravimetric analysis (TG) was also studied. The stereo-configuration of carboxyl groups in copolymers was analysed by potentiometric titration. It was found that copolymers degrade in two main complex stages. The initial step in the decomposition involves the formation of cyclic five-membered anhydrides. The presence of trans configuration of carboxyl groups hinders the formation of cyclic anhydrides and shifts the decomposition to higher temperatures, making decarboxylation competitive. At higher temperatures anhydride decomposition takes place and finally the degradation of main chains occurs. 相似文献
963.
Xiaodong Zhang Dexu Lin Zhengmin Liu 《Journal of Radioanalytical and Nuclear Chemistry》2005,266(1):149-151
Summary The elastic scattering cross sections of natural vanadium for protons have been measured for beam energies below 2.64 MeV in steps of 10 keV and in the angular 160° and 170°. The overall standard error (ΔσExp/σExp) in the measured cross sections is estimated to be better than ±3% at all the beam energies and angles. The results are presented in graphical and tabular forms that should be useful for those who wish to use the cross sections for material analysis. 相似文献
964.
E. Anklam Maria Rosa Bassani Thomas Eiberger Stefan Kriebel Markus Lipp Reinhard Matissek 《Analytical and bioanalytical chemistry》1997,357(7):981-984
Pyrolysis Mass Spectrometry (Py-MS) was used for the discrimination of cocoa butters from other vegetable fats. Mass spectra
ranging from 50 amu to 250 amu were analyzed by principal component analysis (PCA) and with neural nets. The application of
neural nets leads to a good discrimination between the two classes. Detailed analysis of the nets revealed that only the first
60 masses were used within the net. The use of PCA requires a careful selection of the number of masses included in the calculation.
Canonical variance analysis was applied to determine the significant masses. Optimal performance of PCA was observed only
using the first 22 significant masses. Most of these masses were different from the ones used by the neural net. It seems
that the mass spectra obtained by Py-MS contain sufficient information for the discrimination of pure cocoa butter from other
vegetable fats, but none of the methods seems to be able to extract all information available. Neural net provides a very
robust method for this task and no prior data selection was necessary.
Received: 13 May 1996 / Revised: 7 August 1996 / Accepted: 7 August 1996 相似文献
965.
Conditions have been found which make possible the determination of thallium and/or lead in cadmium and its salts without preliminary separation. The electrochemical activity of the cadmium, which usually interferes in the determination of thallium, is inhibited by the addition of 0.01% of polyethylene glycol of M.W. 4000. Thallium is determined by electrolysis at ?0.74 V vs. SCE, in 0.1M EDTA solution: 10?1M thallium can be determined in the presence of 0.1M cadmium, while copper and lead at 10?2M and 10?5M respectively do not interfere. Lead is determined in 0.1M acetic acid containing 0.1% cetyltrimethylammonium bromide (CTAB). The addition of CTAB shifts the cadmium peak, as well as the optimum deposition potential for cadmium, to more negative values, making it possible to determine lead in the presence of cadmium as long as the deposition potential lies in the range between ?0.50 and ?0.56 V vs. SCE. Lead can be determined in the presence of ten times as much thallium. 相似文献
966.
An electrochemical method for the determination of the ionophores monensin and lasalocid was developed, based on the polarization of an agar gel/nitrobenzene electrolyte interface. The measured current corresponding to the facilitated ion transfer across this interface is directly proportional to the concentration of an ionophore dissolved in the organic phase. Using cyclic voltammetry in a three-electrode system the detection limit for both ionophores is about 3 × 10?5 M. 相似文献
967.
Peter D van der Wal Maria Skowronska-Ptasinska Albert van den Berg Piet Bergveld Ernst J.R Sudhölter David N Reinhoudt 《Analytica chimica acta》1990
Several polymeric materials were studied as membrane materials for potassium-selective ion-sensitive field-effect transistors (ISFETs) to overcome the problems related with the use of conventional plasticized poly(vinyl chloride) membranes casted on ISFET gate surfaces. Several acrylate materials, such as ACE, Epocryl and derivatives, showed no reproducible results. Three room-temperature vulcanizing (RTV)-type silicone rubbers were tested. The addition-type RTV-2 silicone rubber was not suitable as a membrane material, but the condensation-type RTV-1 and especially the RTV-2 silicone rubber showed good results. ISFETs with a Silopren membrane showed a durability of at least 2 months. 相似文献
968.
Egermayer M Karlberg M Piculell L 《Langmuir : the ACS journal of surfaces and colloids》2004,20(6):2208-2214
Previous work has shown that amylose (AM) can cross-link hydrophobically modified polymers by inclusion complexation, whereby thermoreversible cold-setting gels are formed. Here we investigate the complexation of AM with different samples of hydrophobically modified ethyl(hydroxyethyl) cellulose (HMEHEC), distinguished by differences in the architecture of the hydrophobes (the hydrophobic side chains). All hydrophobes, except one, were based on linear alkyl chains, but with varying chain lengths (C12-C14). In addition, some samples contained short hydrophilic "spacers", consisting of 2-5 ethylene oxide units, between the alkyl chains and the EHEC backbone. Gels of varying strength were obtained for the different AM/HMEHEC samples. The alkyl chain length seemed to be the major factor affecting the gel strength, with longer alkyl chains giving stronger gels. For similar alkyl chain lengths, stronger gels were obtained when a spacer was present. Addition of AM caused a small increase of the cloud points of HMEHECs with C14 hydrophobes in water. Time-dependent effects and effects of the sample preparation procedure were also investigated. The reversibility of the gelation with respect to shear was confirmed. A gel destroyed by added surfactant was shown to reform on removal of the surfactant by dialysis. 相似文献
969.
T. Vlase Gabriela Vlase A. Chiriac N. Doca 《Journal of Thermal Analysis and Calorimetry》2005,80(1):87-90
Summary In order to obtain catalysts, the thermal decomposition of the precursors is a compulsory step. However, kinetic analysis of the decomposition data obtained under non-isothermal conditions lead very seldom to the intimate reaction mechanism. There is also a lack of information because in non-isothermal kinetics, the compensation effect, is rather a rule and unfortunately a source of debate. In order to discriminate between these processes, and the influence of conversion, respectively temperature on the reaction rate, the NPK (non-parametric kinetic - Sempere and Nomen) method was used. This method is based on the singular value decomposition algorithm (SVD) applied on the matrix of reaction rate at corresponding conversion and temperature. This method allows a less speculative determination of the conversion functions and of the kinetic parameters. 相似文献
970.
Ribeiro da Silva MD Gomes JR Gonçalves JM Sousa EA Pandey S Acree WE 《The Journal of organic chemistry》2004,69(8):2785-2792
The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported. 相似文献