全文获取类型
收费全文 | 3681篇 |
免费 | 156篇 |
国内免费 | 19篇 |
专业分类
化学 | 2625篇 |
晶体学 | 26篇 |
力学 | 55篇 |
数学 | 414篇 |
物理学 | 736篇 |
出版年
2023年 | 21篇 |
2022年 | 27篇 |
2021年 | 72篇 |
2020年 | 64篇 |
2019年 | 79篇 |
2018年 | 56篇 |
2017年 | 48篇 |
2016年 | 119篇 |
2015年 | 116篇 |
2014年 | 123篇 |
2013年 | 239篇 |
2012年 | 228篇 |
2011年 | 279篇 |
2010年 | 174篇 |
2009年 | 175篇 |
2008年 | 246篇 |
2007年 | 214篇 |
2006年 | 189篇 |
2005年 | 162篇 |
2004年 | 149篇 |
2003年 | 101篇 |
2002年 | 122篇 |
2001年 | 63篇 |
2000年 | 80篇 |
1999年 | 41篇 |
1998年 | 47篇 |
1997年 | 37篇 |
1996年 | 39篇 |
1995年 | 47篇 |
1994年 | 33篇 |
1993年 | 42篇 |
1992年 | 39篇 |
1991年 | 40篇 |
1990年 | 35篇 |
1989年 | 26篇 |
1988年 | 25篇 |
1987年 | 24篇 |
1986年 | 24篇 |
1985年 | 30篇 |
1984年 | 28篇 |
1983年 | 19篇 |
1982年 | 24篇 |
1981年 | 17篇 |
1980年 | 15篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1970年 | 8篇 |
排序方式: 共有3856条查询结果,搜索用时 390 毫秒
981.
Garcia Y Moscovici J Michalowicz A Ksenofontov V Levchenko G Bravic G Chasseau D Gütlich P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(21):4992-5000
[Fe(hyptrz)3](4-chloro-3-nitrophenylsulfonate)22 H2O (1; hyptrz=4-(3-hydroxypropyl)-1,2,4-triazole) has been synthesized and its physical properties have been investigated by several physical techniques including magnetic susceptibility measurements, calorimetry, and M?ssbauer, optical, and EXAFS spectroscopy. Compound 1 exhibits a spin transition below room temperature, together with a very wide thermal hysteresis of about 50 K. This represents the widest hysteresis loop ever observed for an FeII-1,2,4-triazole spin transition material. The cooperativity is discussed on the basis of temperature-dependent EXAFS studies and of the structural features of a CuII analogue. The EXAFS structural model of (1) in both spin states is compared to that obtained for a related material whose spin transition occurs above room temperature. EXAFS spectroscopy suggests that 1,2,4-triazole chain compounds retain a linear character whatever the spin state of the iron(II). 相似文献
982.
The present paper describes a method for the fluorometric determination of uric acid in blood serum by its reaction with uricase (UOx). The procedure is based on the changes in fluorescence that take place during the enzymatic reaction of UOx with uric acid when the solution is excited at 287 nm and the emission is measured at 330 nm. A mathematical model which relates the analytical signal to the analyte concentration was developed and the model also served to obtain some of the thermodynamic constants of the system (the Michaelis constant and the turnover number). The optimum reaction conditions and its analytical characteristics were studied, linear response range (3x10(-5)-6x10(-4) M) and reproducibility (4%, n=7). The method was applied to the determination of uric acid in three blood serum samples. The results were compared with those obtained by a commercial clinical analyzer and no systematic errors were observed. 相似文献
983.
A procedure for the determination of ammonium is described, based on its transformation into ammonia gas and subsequent measurement by UV-visible molecular absorption spectrometry at 194 nm. Two different procedures for generating the ammonia are proposed, using NaOH solution and solid NaOH, respectively. After generation conditions for both procedures had been optimized, better sensitivity was observed with solid NaOH. In these conditions, the method showed two ranges of linear response: from 1 to 100 g and from 100 to 400 g of ammonium. Finally, we determined ammonium in wine samples. Preliminary studies indicated that the only observable interfering species was ethanol, which could be eliminated by subjecting the sample to vacuum evaporation until nearly dry. This process also increased the sensitivity of the method by as much as ten times. The values obtained from three different wine samples yielded results within the range of the concentration obtained by other authors. The recovery of ammonium added to the wine samples was around 95%. 相似文献
984.
A simple procedure for the determination of methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), ethyl butyl ether (EBE), tert-amyl methyl ether (TAME), benzene, toluene, ethylbenzene, and xylenes (BTEX) in water using headspace (HS) solid-phase microextraction (HS-SPME) was developed. The analysis was carried out by gas chromatography (GC) equipped with flame ionization detector (FID) and 100% dimethylpolysiloxane fused capillary column. A 2 Plackett-Burman design for screening and a central composite design (CCD) for optimizing the significant variables were applied. Fiber type, extraction temperature, sodium chloride concentration, and headspace volume were the significant variables. A 65 microm poly(dimethylsiloxane)-divinylbenzene (PDMS-DVB) SPME fiber, 10 degrees C, 300 g/l, and 20 ml of headspace (in 40 ml vial) were respectively chosen for the best extraction response. An extraction time of 10 min was enough to extract the ethers and BTEX. The relative standard deviation (R.S.D.) for the procedure varied from 2.6 (benzene) to 8.5% (ethylbenzene). The method detection limits (MDLs) found were from 0.02 (toluene, ethylbenzene, and xylenes) to 1.1 microg/l (MTBE). The optimized method was applied to the analysis of the rivers, marinas and fishing harbors surface waters from Gipuzkoa (North Spain). Three sampling were done in 1 year from June 2002 to June 2003. Toluene was the most detected analyte (in 90% of the samples analyzed), with an average concentration of 0.56 microg/l. MTBE was the only dialkyl ether detected (in 15% of the samples) showing two high levels over 400 microg/l that were related to accidental fuel spill. 相似文献
985.
986.
J. M. Garcia de la vega E. San fabian 《International journal of quantum chemistry》1994,52(4):947-956
We have studied symmetry breaking in three open-shell systems: CF (D2d and C2v) and CF (D3h) molecular ions. These different Hartree–Fock solutions are employed as starting points to calculate the correlation energy of these ions with perturbative, configuration interaction, and density functional methods. When symmetry-broken or symmetry-adapted wave functions are used, the correlation energy obtained with each method changes the order of stability of CF for a determined symmetry. Density functional methods produce higher correlation energies although they do not alter the order of stability of Hartree–Fock calculations. The behavior of correlation energy with different methods and the characteristics of the symmetry of wave functions are compared. A study of appearance energies for three different channels of the decomposition reaction of ionized carbon tetrafluoride are considered by using different methods with symmetry-broken or symmetry-adapted wave functions to calculate correlation energies. © 1994 John Wiley & Sons, Inc. 相似文献
987.
Mara E. F. Braibante Gunter Clar Marcos A. P. Martins 《Journal of heterocyclic chemistry》1993,30(4):1159-1160
1-Methyl-5-(trifluoromethyl)-1H-pyrazoles 2, 3 and 4,5-dihydro-1-phenyl-5-(trifluoromethyl)-1H-pyrazol-5-ol 4 were prepared by reaction of 4-alkoxy-1,1,1-trifluoro-3-alken-2-ones 1 and hydrazine, methylhydrazine, and phenylhydrazine, respectively, in good yields. Compound 1 proved to be a versatile building block for the regiospecific construction of pyrazole rings having an 5-trifluoromethyl substituent. 相似文献
988.
Neusa M.T. Pires Fernanda M.B. Coutinho Marcos A.S. Costa 《European Polymer Journal》2004,40(11):2599-2603
This paper reports the study of the dependence of reaction conversion, catalyst activity, polymer microstructure, molecular weight, molecular weight distribution curves and Mooney viscosity on reaction temperature and monomer concentration in the reaction medium used in the synthesis of high cis-polybutadiene. A ternary catalyst system composed by neodymium versatate, trans-butyl chloride and diisobutylaluminum hydride was used in its synthesis. The highest molecular weights were obtained at polymerization temperatures in the range from 70 to 80 °C. The highest content of cis-1,4 repeating units (about 99%) was observed when the polymerization was carried out at the lowest initial monomer concentration (0.56 mol/l). 相似文献
989.
Kotelnikov AI Ortega JM Medvedev ES Psikha BL Garcia D Mathis P 《Bioelectrochemistry (Amsterdam, Netherlands)》2002,56(1-2):3-8
The broad set of nonexponential electron transfer (ET) kinetics in reaction centers (RC) from Rhodopseudomonas sulfoviridis in temperature range 297-40 K are described within a mixed adiabatic/nonadiabatic model. The key point of the model is the combination of Sumi-Marcus and Rips-Jortner approaches which can be represented by the separate contributions of temperature-independent vibrational (v) and temperature-dependent diffusive (d) coordinates to the preexponential factor, to the free energy of reaction DeltaG=DeltaG(v)+DeltaG(d)(T) and to the reorganization energy lambda=lambda(v)+lambda(d)(T). The broad distribution of protein dielectric relaxation times along the diffusive coordinate is considered within the Davidson-Cole formalism. 相似文献
990.
Glaucia R. Martinez Flávia Garcia Jean Cadet Graziella E. Ronsein Marisa H.G. Medeiros 《Tetrahedron》2006,62(46):10762-10770
We report herein the synthesis of a new hydrophilic and non-ionic anthracene derivative, the N,N′-di-(2,3-dihydroxypropyl)-9,10-anthracenedipropanamide. The evaluation of this compound as chemical trap of singlet molecular oxygen by using labeling experiments and HPLC-MS analysis showed that it could be efficiently used in biological investigations. 相似文献