全文获取类型
收费全文 | 7022篇 |
免费 | 602篇 |
国内免费 | 583篇 |
专业分类
化学 | 5204篇 |
晶体学 | 93篇 |
力学 | 270篇 |
综合类 | 33篇 |
数学 | 552篇 |
物理学 | 2055篇 |
出版年
2024年 | 9篇 |
2023年 | 50篇 |
2022年 | 111篇 |
2021年 | 102篇 |
2020年 | 145篇 |
2019年 | 145篇 |
2018年 | 129篇 |
2017年 | 129篇 |
2016年 | 177篇 |
2015年 | 182篇 |
2014年 | 227篇 |
2013年 | 567篇 |
2012年 | 408篇 |
2011年 | 305篇 |
2010年 | 262篇 |
2009年 | 402篇 |
2008年 | 442篇 |
2007年 | 438篇 |
2006年 | 385篇 |
2005年 | 360篇 |
2004年 | 335篇 |
2003年 | 272篇 |
2002年 | 560篇 |
2001年 | 240篇 |
2000年 | 225篇 |
1999年 | 177篇 |
1998年 | 187篇 |
1997年 | 143篇 |
1996年 | 129篇 |
1995年 | 122篇 |
1994年 | 113篇 |
1993年 | 99篇 |
1992年 | 98篇 |
1991年 | 77篇 |
1990年 | 57篇 |
1989年 | 47篇 |
1988年 | 48篇 |
1987年 | 37篇 |
1986年 | 44篇 |
1985年 | 35篇 |
1984年 | 28篇 |
1983年 | 12篇 |
1982年 | 24篇 |
1981年 | 26篇 |
1980年 | 23篇 |
1979年 | 25篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1973年 | 7篇 |
排序方式: 共有8207条查询结果,搜索用时 46 毫秒
91.
Addition of a small amount of polar solvent (i.e., modifier) to CO2 in packed column supercritical fluid chromatography (SFC) has shown major improvements in both polar analyte solubility and interaction of the polar analyte with the stationary phase. Recently, the addition of an ionic component (i.e., additive) to the primary modifier by one of us has been shown to extend even further the application of SFC to polar analytes. In this work, the effect of various ionic additives on the elution of ionic compounds, such as sodium 4-dodecylbenzene sulfonate and sodium 4-octylbenene sulfonate, has been studied. The additives were lithium acetate, ammonium acetate, tetramethylammonium acetate, tetrabutylammonium acetate, and ammonium chloride dissolved in methanol. Three stationary phases with different degrees of deactivation were considered: conventional cyanopropyl, deltabond cyanopropyl, and bare silica. The effect of additive concentration and additive functionality on analyte retention was investigated. Sodium 4-dodecylbenzene sulfonate was successfully eluted using all the additives with good peak shape under isocratic/isobaric/isothermal conditions. Different additives, however, yielded different retention times and in some cases different peak shapes. 相似文献
92.
The quenching of the fluorescence of three anthropogenic fulvic acids (FA) provoked by Cu(II) (pH 6.0), Fe(III) (pH 4.0) and UO22+ (pH 3.5), was analyzed by a non-linear method and by Stern-Volmer plots. The FA samples were extracted from composted sewage sludges (csFA), composted municipal wastes (mwFA) and composted livestock wastes (lsFA). Synchronous-scan fluorescence (SyF) spectra were collected as a function of metal ion concentration. Spectral data were treated by a self-modeling mixture analysis method (SIMPLISMA) to detect the SyF spectral band with the strongest quenching and to calculate the corresponding quenching profile. The analysis of these profiles by a non-linear method allowed the estimation of conditional stability constants (K) and of the percentage of non-complexing fluorophores. The same quantitative information was obtained by the modified Stern-Volmer equation taking into account the existence of fluorophores that do not participate in the complexation. Good agreement was found between the results of the two procedures. The log K calculated by the non-linear method were (standard deviation in parenthesis): csFA, Cu(II), 4.22 (5); Fe(III), 5.0 (1); UO22+, 5.2 (2); mwFA, Cu(II), 4.21 (3); Fe(III), 5.6 (2); UO22+, 4.7 (3); lsFA, Cu(II), 4.51 (8); Fe(III), 5.5 (2); UO22+, 3.6 (2). 相似文献
93.
M. V. Pryadeina Ya. V. Burgart M. I. Kodess V. I. Saloutin 《Russian Chemical Bulletin》2005,54(12):2841-2845
Cyclocondensation of 2-arylmethylidene-3-fluoroalkyl-oxopropionates with 2-amino-pyridine occurs at both the polyfluoroacylvinyl
and alkoxycarbonylvinyl fragments to give alkyl 4-aryl-2-polyfluoroalkyl-4H-pyrido[1,2-a]pyrimidine-3-carboxylates and 4-aryl-2-hydroxy-3-polyfluoroacyl-4H-pyrido[1,2-a]pyrimidines, respectively. When treated with copper(II) acetate, the pyrido[1,2-a]pyrimidines yield metal complexes.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2745–2749, December, 2005. 相似文献
94.
The standard enthalpies of formation ΔfHo (liq. or cr.) at the temperature T = 298.15 K were measured using combustion calorimetry for benzophenone (A), 1-indanone (B), -tetralone (C), 9-fluorenone (D), anthrone (E) and dibenzosuberone (F). The standard enthalpies of vaporization ΔvHo or sublimation ΔsHo of A-F and 5,7-dihydro-6H-dibenzo[a,c]cyclohepten-6-one (G) were obtained from the temperature function of the vapor pressure measured in a flow system. Enthalpies of fusion ΔmH of solid compounds were measured by DSC. From the enthalpies of formation of the gaseous compounds of A-G the values of their strain enthalpies were derived and structural effects discussed.
95.
在硼氢化钾碱性溶液中对金属氢化物(MH)电极的表面进行化学还原处理,提高了MH电极的放电容量、活化性能和电催化活性.用其为负极组装的AA型MH-Ni电池进行了封口化成,电池放电容量达到1150mAh,5C下电池的放电容量达到0.2C下容量的80%以上,电池在1C100%DOD(放电深度)充放电条件下,循环寿命由原来的100次左右提高到200次以上 相似文献
96.
The ion product of water and the dissociation constants of carbonic acid have been determined in 0.1, 1.0, 3.0, and 5.0M NaClO4 at 25°C. The ion product of water K
w
'
has been evaluated by emf measurements with a combined glass electrode in NaClO4 solutions containing 0.001–0.1M HCLO4 or NaOH. The product K
H
'
K
l
'
K
2
'
of the Henry constant for CO2 and the dissociation constants for H2CO3 have been determined by titration of carbonate solutions equilibrated with pCO2 =10–3.52 atm, and K
2
'
has been evaluated by potentiometric titration and by measuring the H+ concentration at fixed HCO
3
–
and CO
3
2-
concentrations. The ion interaction (Pitzer) equations are applied to describe the constants K
w
'
, K
2
'
and K
H
'
H
1
'
K
2
'
as a function of the NaClO4 concentration. The experimental data are used to evaluate the mixing parameters
i/ClO
4 and
i/ClO
4
-/Na+ fori = OH
-,HCO
3
-
andCO
3
2- 相似文献
97.
R. Sakamoto 《Colloid and polymer science》1984,262(10):788-792
Solvent effects on the phase separation of poly(-benzyl L-glutamate) to liquid crystal and isotropic solution have been observed in various helicogenic solvents. The temperature-composition phase diagrams have been determined for each solution. The critical concentrations,
2
*
, at which the phase separation occours have been compared in various solvents. In dimethylformamide in which the polymer is molecularly dispersed, the observed
2
*
value has agreed with that calculated by Flory's theory. In some solvents in which the polymer aggregates in a head-to-tail mode such as chloroform, the observed
2
*
values have been considerably small. It is assumed that the polymer aggregates behave as longer particles than the original particles. In dioxane in which the polymer aggregates highly both in a head-to-tail and a side-by-side modes, the
2
*
value has been a little larger than that in chloroform. In this case the relationship between the aggregation and the liquid crystal formation is so complicated that further investigation is necessary. In aromatic solvents such asm-cresol that dissolves the polymer almost molecularly, the
2
*
is smaller than that in dimethylformamide. Therefore, the intermolecular interactions between the phenyl groups in the side groups of the polymer and those in solvent molecules must be considered.The author is grateful to Mr. K. Sano and Mr. M. Watanabe for their observation of the liquid crystal formation. 相似文献
98.
The stoichiometric solubility constant of eitelite (NaMg
0.5
CO
3
+2H+ ⇄ Na++0.5Mg
2+
+CO
2
(g)+H
2
O, log*K
pso
I
=14.67±0.03 was determined at I=3 m (mol kg−1) (NaClO
4
) and 25°C. The stability of magnesium (hydrogen-)carbonato complexes in this ionic medium was explicitely taken into account.
Consequently, trace activity coefficients of free ionic species, calculated from the Pitzer model with ion-interaction parameters
from the literature, were sufficient for an evaluation of the thermodynamic solubility constants and Gibbs energies of formation
for eitelite (−1039.88±0.60), magnesite (−1033.60±0.40), hydromagnesite (−1174.30±0.50), nesquehonite (−1724.67±0.40), and
brucite (−835.90±0.80 kJ-mol
−1
). The increasing solubilities of nesquehonite and eitelite at higher sodium carbonate molalities were explained by invoking
a magnesium dicarbonato complex (Mg2++2CO
3
2−
⇄ Mg(CO3)
2
2−
, log βz = 3.90 ± 0.08). A set of ion-interaction parameters was obtained from solubility and dissociation constants for carbonic
acid in 1 to 3.5 m NaClO
4
media
which reproduce these constants to 0.02 units in log K. The following Pitzer parameters are consistent with the previously
studied formation of magnesium (hydrogen-)carbonato complexes in 3m NaClO
4
. The model and Gibbs functions of solid phases derived here reproduce original solubility data (−log [H+], [Mg
2+
]
tot
) measured in perchlorate medium within experimental uncertainty.
Presented at the XXII International Conference on Solution Chemistry, July 14–19, 1991, Linz, Austria. 相似文献
99.
The overall stability constantsK
1 andK
2 of NdNO
3
2+
and Nd(NO3)
2
+
complexes were determined (K
1=1.77;K
2=1.28) using an extraction method with tri-n-butyl phosphate as the extractant. The ratio 1/2 of the stepwise stability constants is discussed. It was established that the Nd(NO3)
2
+
complex was an outer-sphere ion pair.
Die Assoziation von Nitrat-Ion mit Nd3+
Zusammenfassung Mittels einer Extraktionsmethode wurden die StabilitätskonstantenK 1 undK 2 von NdNO 3 2+ - und Nd(NO3)+-Komplexen bestimmt (K 1=1.77;K 2=1.28; Tri-n-butylphosphat als Extraktionsmittel). Das Verhältnis 1/2 der stufenweisen Stabilitätskonstanten wird diskutiert. Es stellte sich heraus, daß der Nd(NO3)+-Komplex als ein Outer-Shere-Ionenpaar vorliegt.相似文献
100.
E. V. Tulyakova O. A. Fedorova Yu. V. Fedorov G. Jonusauskas A. V. Anisimov 《Russian Chemical Bulletin》2007,56(11):2166-2174
Using 1H NMR spectroscopy and steady-state and time-resolved electronic spectroscopy, the optical properties of mono-and bis(styryl)pyridinium
perchlorates and their complexes with Mg2+, Ba2+ cations were studied. The stability constants of the complexes were determined using spectrophotometric titration. The formation
of inclusion complexes for Mg2+ and sandwich type complexes for Ba2+ results in fluorescence enhancement and increases the lifetimes of the excited states of the initial bis-styryl ligands.
The variation of position of the styryl fragment in the pyridinium aromatic ring gives rise to photochromic crown ethers with
different optical and photophysical characteristics and is also an easy route to bis(crown-ethers) of symmetrical and unsymmetrical
structure.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2092–2100, November, 2007. 相似文献