全文获取类型
收费全文 | 11550篇 |
免费 | 318篇 |
国内免费 | 22篇 |
专业分类
化学 | 6938篇 |
晶体学 | 178篇 |
力学 | 315篇 |
数学 | 899篇 |
物理学 | 3560篇 |
出版年
2023年 | 114篇 |
2022年 | 237篇 |
2021年 | 181篇 |
2020年 | 213篇 |
2019年 | 271篇 |
2018年 | 239篇 |
2017年 | 262篇 |
2016年 | 361篇 |
2015年 | 264篇 |
2014年 | 463篇 |
2013年 | 895篇 |
2012年 | 679篇 |
2011年 | 792篇 |
2010年 | 506篇 |
2009年 | 486篇 |
2008年 | 564篇 |
2007年 | 553篇 |
2006年 | 420篇 |
2005年 | 365篇 |
2004年 | 285篇 |
2003年 | 211篇 |
2002年 | 216篇 |
2001年 | 141篇 |
2000年 | 144篇 |
1999年 | 111篇 |
1998年 | 70篇 |
1997年 | 68篇 |
1996年 | 103篇 |
1995年 | 116篇 |
1994年 | 115篇 |
1993年 | 104篇 |
1992年 | 140篇 |
1991年 | 100篇 |
1990年 | 90篇 |
1989年 | 105篇 |
1988年 | 98篇 |
1987年 | 102篇 |
1986年 | 80篇 |
1985年 | 121篇 |
1984年 | 133篇 |
1983年 | 93篇 |
1982年 | 111篇 |
1981年 | 105篇 |
1980年 | 109篇 |
1979年 | 96篇 |
1978年 | 109篇 |
1977年 | 85篇 |
1976年 | 62篇 |
1975年 | 68篇 |
1974年 | 79篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
151.
Kundu S Pal A Ghosh SK Nath S Panigrahi S Praharaj S Pal T 《Inorganic chemistry》2004,43(18):5489-5491
Ligands with a beta-diketone skeleton have been employed for the first time as reductant to produce ligand stabilized gold nanoparticles of different shapes from aqueous HAuCl(4) solution. Evolution of stable gold nanoparticles follows first order (k approximately equal to 10(-2) min(-1)) kinetics with respect to Au(0) concentration. Growth of particles of different shapes (spherical or triangular or hexagonal) goes hand in hand under the influence of different beta-diketones, which have excellent capping and reducing properties. Chlorine insertion was observed to take place in the beta-diketone skeleton. 相似文献
152.
S. P. Mishra /Miss/ Jyoti Singh 《Journal of Radioanalytical and Nuclear Chemistry》1986,105(2):107-116
Fractional precipitation techniques have been utilized to separate the lower valent and parent forms of56Mn in permanganate targets and an attempt is made to study a few aspects of chemical stabilization of recoil56Mn in permanganates. Ammonium permanganate, recoil behaviour of which has not been studied previously, is chosen as one of the targets along with the potassium permanganate for initial retention and also for isothermal annealing.56Mn initial retentions of about 12% and about 4% are obtained for potassium and ammonium permanganate, respectively, by activation from a Ra–Be neutron source. A usual trend for KMnO4 and the reduction of recoil fragments by ammonium ions in NH4MnO4 are seen through the isothermal annealing study. 相似文献
153.
The yellow complex of Pt(IV) with 1-pyridyl-4,4,6-trimethyl-1H,4H-pyrimidine-2-thiol (PyTPT) which has maximum absorbance at 430 nm, is studied for the spectrophotometric determination of the metal. Molar absorptivity is 5000 liters mol?1 cm?1 and Sandell's sensitivity is 0.039 μg/cm2. The determination of Pt(IV) (2.8–8.4 ppm) in the presence of diverse ions is described. 相似文献
154.
A visible light promoted, photoredox catalysed, green one-pot approach for the alkylation of amine substrates with sp2 carbon has been developed. This eosin Y based organic transformations, can behave as an effective direct hydrogen-atom transfer catalyst for coupling reaction. The proposed strategy includes simple procedure which can make adduct product with sp2 carbon. This eosin Y based photocatalytic hydrogen-atom transfer strategy may hold great potential for diverse functionalization of a wide range of native CH bonds in an economical and sustainable manner. 相似文献
155.
Imidazol-2-ylidenes, a family of N-heterocyclic carbenes (NHC), are efficient catalysts in the transesterification involving numerous esters and alcohols. Low catalyst loadings of aryl- or alkyl-substituted NHC catalysts mediate the acylation of alcohols with enol acetates in short reaction times at room temperature. Commercially available and more difficult to cleave methyl esters react with primary alcohols in the presence of alkyl-substituted NHC to efficiently form the corresponding esters. While primary alcohols are selectively acylated over secondary alcohols with use of enol esters as acylating agents, methyl and ethyl esters can be employed as protective agents for secondary alcohols in the presence of the more active alkyl-substituted NHC catalysts. The NHC-catalyzed transesterification protocol was simplified by generating the imidazol-2-ylidene catalysts in situ. 相似文献
156.
CNDO MO calculations are carried out for four acrylates, H2C=C (R1)-COO (R2) with R1=H, Me and R2=Me, Et for dipole moment determination using the Pople-Segal expression.7 Energy minimization for various conformers in each molecular system was achieved by the gradient method. Dipole moments for comparatively stable conformers of each of the molecules considered are reported. Theoretically calculated energy values are used to arrive at the equilibrium geometry and the corresponding dipole moments are compared with the experimental values in each system considered. 相似文献
157.
Kulwant Singh Kawaldeep Vijay Kumar 《Journal of Radioanalytical and Nuclear Chemistry》1998,231(1-2):45-50
The sum peak at 437 keV (356+81 keV) has been measured in various chemical and physico-chemical environments. The fact that
the intensity ratio of the sum peak to its single peak changes with the circumstances in which the radioactive source is placed
is used to observe the effect of the chemical environment on directional correlation coefficients of 351–81 keV cascade in133Ba−133Cs decay and to detect the variations in the electric field gradient in different barium compounds. The sum peak method has
been found to be useful in describing the chemical influences onG
22 parameters and hence in the study of electric field gradients. 相似文献
158.
Summary Thermodynamic effective fixed charge densities of mercuric phosphate and carbonate parchment supported membranes were evaluated by a number of methods particularly those ofTeorell-Meyer-Sievers, Altug andHair andKobatake et al. The value of the permselectivity was obtained for the two membranes based onKobatake et al. procedure. Membrane transport number was calculated using a modified Nernst relation and compared with the values determined by the TMS method. The theoretical predictions for membrane potential usingKobatake et al. equation are borne out quite satisfactorily by our experimental results for both membrane.
With 8 figures and 3 tables 相似文献
Zusammenfassung Es wurden die Dichte der fixierten Ladungen von Quecksilberphosphat und Quecksilberkarbonat-Niederschlagsmembranen nach den Methoden vonTorell-Meyer-Sievers, Altug undHair undKobatake bestimmt. Weiterhin wurden Durchlässigkeit und Transportzahlen ermittelt und mit Werten der TMS-Methode verglichen. Theoretische Voraussagen über das Membranpotential nach den Gleichungen vonKobatake stimmen mit den experimentellen Ergebnissen überein.
With 8 figures and 3 tables 相似文献
159.
Vijay Kumar Kawaldeep K. Singh 《Journal of Radioanalytical and Nuclear Chemistry》1992,166(2):131-142
The sum peak method has been applied to electron capture probability changes to the 1086 keV level in the decay of152Eu in the different environments, e.g. ethylenediaminetetraacetic acid (EDTA), bovine serum albumin (BSA) and hydrochloric acid (HCl). An attempt is also made to determine the effect of chemical environment on the directional correlation coefficient of the 1409–122 keV - cascade in152Eu 152Sm decay. A change of G22 is found in the different chemical forms. 相似文献
160.
In this study, chemometric techniques such as cluster analysis (CA), discriminant analysis (DA), principal component analysis (PCA) and partial least squares (PLS) were used to analyse the wastewater dataset to identify the factors which affect the composition of sewage of domestic origin, spatial and temporal variations, similarity/dissimilarity among the wastewater characteristics of cis- and trans-drains and discriminating variables. Samples collected from 24 wastewater drains in Lucknow city and from three sites on Gomti river in the month of January/February, May, August and November during the period of 5 years (1994-1999) were characterized for 32 parameters. The multivariate techniques successfully described the similarities/dissimilarities among the sewage drains on the basis of their wastewater characteristics and sources signifying the effect of routine domestic/commercial activities in respective drainage areas. Spatial and seasonal variations in wastewater composition were also determined successfully. CA generated six groups of drains on the basis of similar wastewater characteristic. PCA provided information on seasonal influence and compositional differences in sewage generated by domestic and industrial waste dominated drains and showed that drains influenced by mixed industrial effluents have high organic pollution load. DA rendered six variables (TDS, alkalinity, F, TKN, Cd and Cr) discriminating between cis- and trans-drains. PLS-DA showed dominance of Cd, Cr, NO3, PO4 and F in cis-drains wastewater. The results suggest that biological-process based STPs could treat wastewater both from the cis- as well as trans-drains, however, prior removal of toxic metals will be required from the cis-drains sewage. Further, seasonal variations in wastewater composition and pollution load could be the guiding factor for determining the STPs design parameters. The information generated would be useful in selection of process type and in designing of the proposed sewage treatment plants (STPs) for safe disposal of wastewater. 相似文献