全文获取类型
收费全文 | 72622篇 |
免费 | 5384篇 |
国内免费 | 7255篇 |
专业分类
化学 | 28674篇 |
晶体学 | 350篇 |
力学 | 6555篇 |
综合类 | 1286篇 |
数学 | 25785篇 |
物理学 | 22611篇 |
出版年
2023年 | 607篇 |
2022年 | 1212篇 |
2021年 | 1339篇 |
2020年 | 1419篇 |
2019年 | 1720篇 |
2018年 | 1388篇 |
2017年 | 1640篇 |
2016年 | 1868篇 |
2015年 | 1673篇 |
2014年 | 2447篇 |
2013年 | 4623篇 |
2012年 | 2872篇 |
2011年 | 3242篇 |
2010年 | 2783篇 |
2009年 | 3949篇 |
2008年 | 4395篇 |
2007年 | 4642篇 |
2006年 | 4320篇 |
2005年 | 3694篇 |
2004年 | 3383篇 |
2003年 | 3435篇 |
2002年 | 3119篇 |
2001年 | 2658篇 |
2000年 | 2592篇 |
1999年 | 2324篇 |
1998年 | 2181篇 |
1997年 | 1790篇 |
1996年 | 1616篇 |
1995年 | 1459篇 |
1994年 | 1355篇 |
1993年 | 1137篇 |
1992年 | 1134篇 |
1991年 | 875篇 |
1990年 | 724篇 |
1989年 | 637篇 |
1988年 | 574篇 |
1987年 | 452篇 |
1986年 | 387篇 |
1985年 | 448篇 |
1984年 | 439篇 |
1983年 | 216篇 |
1982年 | 354篇 |
1981年 | 413篇 |
1980年 | 309篇 |
1979年 | 299篇 |
1978年 | 227篇 |
1977年 | 208篇 |
1976年 | 161篇 |
1974年 | 110篇 |
1973年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
GC headspace analyses of various aromatizing herbs have been compared with those relating to the essential oils, obtained by steam distillation, of the same plants. In this way it was possible to establish the most significant differences between the composition of a herb flavor and that of its essential oil. In particular, we observed some very volatile compounds in the headspace samples which were absent from the essential oil; these components may make an important contribution to the herb flavor. The identification of these substances is still in progress. 相似文献
202.
A highly sensitive spectrophotometric method for the determination of nitrite in water and soil has been developed. The reaction of nitrite with acidified potassium iodide to liberate iodine which oxidizes leuco‐crystal violet (LCV) to form crystal violet having absorption maxima at 590 nm forms the bases of this method. In aqueous medium the system obeys Beer's law in the range of 0.1 to 1.0 μg per 25 mL (0.004–0.04 ppm), while in an extractive system the range is 0.025–0.25 μg in 100 mL (0.00025–0.0025 ppm). The molar absorptivity and Sandell's sensitivity were found to be 1.54 × 106 1 mol?1 cm?1 and 44 pg cm?2, respectively. 相似文献
203.
The regions of variation of density, enthalpy of formation, and detonation velocity of all of the formally possible structual isomers were determined for several molecular formulas of C,H,N,O-containing explosives. The histograms of the distribution of the structual isomers over these parameters were constructed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 859–862, May, 1995.The authors are grateful to Mrs. I. D. Solodukhina for the design of the graphical material.The work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 94-03-09323). 相似文献
204.
Julie L. S. Bell David J. Wesolowski Donald A. Palmer 《Journal of solution chemistry》1993,22(2):125-136
The dissociation quotients of formic acid were measured potentiometrically from 25 to 200°C in NaCl solutions at ionic strengths of 0.1, 0.3 1.0, 3.0, and 5.0 mol-kg–1. The experiments were carried out in a concentration cell with hydrogen electrodes. The resulting molal acid dissociation quotients for formic acid, as well as a set of infinite dilution literature values and a calorimetrically-determined enthalpy of reaction, were fitted by an empirical equation involving an extended Debye Hückel term and seven adjustable parameters involving functions of temperature and ionic strength. This regressional analysis yielded the following thermodynamic quantities for 25°C: logK=–3.755±0.002, Ho=–0.09±0.15 kJ-mol–1, So=–72.2±0.5 J-K–1-mol–1, and C
p
o
=–147±4 J-K–1-mol–1. The isocoulombic form of the equilibrium constant is recommended for extrapolation to higher temperatures. 相似文献
205.
Arrhenius parameters values, in non-isothermal kinetic vaporisation processes for a series of compounds with related structures,
have been calculated. This was made using a method of calculation that allows to find the most probable vaporisation mechanisms.
According to this method DTG curves were compared with some theoretical ones reported in literature, whose shape results to
be only a function of the mechanisms. In this way the choice of the mathematical functions which can be inserted in the kinetic
equations, was influenced by the shape of the DTG plots and other thermal analysis signals thus allowing to choose the most
probable mechanisms.
The kinetic parameters derived from these mechanisms were compared, using statistical analysis, with those obtained from another
method of calculation based on ‘a priori’ vaporisation mechanism chosen for the investigated liquid–gas transition.
The standard deviations of the slope and of the intercept, together with the standard deviation and the square correlation
coefficient (r
2) of the linear regression equations related to the mechanisms of the two methods were calculated. Student t-test, Fisher F-test, confidence intervals (c.i.) and residuals valueswere also given.
Statistical analysis shows that the mechanisms obtained with the former method (diffusive and geometrical models) and the
related Arrhenius parameters result to be more significant (in terms of probability) than the corresponding quantities of
the latter for which a first-order model was chosen.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
206.
Ab initio molecular orbital calculations using both minimal and extended basis sets have been applied to two isoelectronic sets of molecules. One set corresponds to the 18 electron species H3NO, H3CO– and H3COH while the second set contains the 42 electron fluorinated molecules F3NO, F3CO– and F3COH. The geometries of these molecules have been optimized, using both the minimal STO-3G and the extended 4-31G basis sets. These comparative calculations reveal that the 4-31G basis produced structural parameters in much better agreement with experiment. The effect of includingd-orbitals in the basis set was also investigated. For the fluorinated oxides it has been found that the optimized 4-31G structures were only slightly altered by the addition ofd-orbitals. For H3NO, on the other hand, the inclusion ofd-orbitals considerably shortens the N-O bond distance. Both H3NO and CF3OH, which are unknown experimentally, are theoretically predicted to be capable of existence. The electronic structures of these molecules have also been examined using electronic partitioning according to the Mulliken scheme. 相似文献
207.
Ab initio molecular orbital calculations employing a 4-31G basis set have been used to study the minimum energy paths for the formation of HCO+, COH+, and HCOH2+ from CO by protonation. The protonation of N2 to give NNH+ and HNNH2+ and of NO+ to form HNO2+ and NOH2+ have also been investigated. All species formed have linear equilibrium geometries and the minimum energy path for approach of the proton is along the line-of-centers of the heavy atoms. Energy barriers to the formation of the various species are given, where appropriate, and changes in geometry, ordering of molecular orbitals and orbital occupancy are discussed. 相似文献
208.
Solid solutions NaCl—CdCl2 were studied in an interval of CdCl2 concentrations of 0.05—3 mol.% by Raman spectroscopy. The molecular form Na2CdCl4 decomposes: Na2CdCl4 2Na+ + Cd2+ + 2Cl2
– + 2e–; 2Na + 2e– 2Na0. Free sodium atoms form color centers of crystal (F centers) in the region of jumpwise changing the partition coefficient of CdCl2 in NaCl (K jump). 相似文献
209.
2-Trifluoromethyl-4H-thiochromene-4-thione obtained from 2-trifluoromethyl-4H-thiochromen-4-one and P2S5 reacts with aromatic amines, hydrazine hydrate, phenylhydrazine, and hydroxylamine at the C(4) atom of the chromene ring
to give the corresponding anils, azine, hydrazones, and oxime of thiochromone. 2-Trifluoromethyl-4H-thiochromen-4-one is oxidized by hydrogen peroxide in AcOH into 4-oxo-2-trifluoromethyl-4H-thiochromene 1,1-dioxide and reduced by NaBH4 to 2-trifluoromethyl-4H-thiochromen-4-ol or cis-2-(trifluoromethyl)thiochroman-4-ol. When treated with hydrazine hydrate, thiochromen-4-one gives 3(5)-(2-mercaptophenyl)-5(3)-trifluoromethylpyrazole.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 504–509, March, 2006. 相似文献
210.
This work describes the use of a dual-standard analysis approach termed the time-average ratio (TAR) in affinity capillary electrophoresis (ACE) to estimate binding constants of receptors to ligands. In this form of analysis the TAR is the migration time of the receptor divided by the average of the sum of the migration times of two non-interacting standards. This change in TAR as a function of the concentration of ligand yields a value for the binding constant. This concept is demonstrated using three model systems: carbonic anhydrase B (CAB, EC 4.2.1.1) and arylsulfonamides, vancomycin (Van) and ristocetin (Rist) from Streptomyces orientalis and Nocardia lurida, respectively, and d-Ala- d-Ala terminus peptides. Three ACE techniques are used to examine the three systems: standard ACE, flow-through partial-filling ACE (FTPFACE), and on-column derivatization coupled to ACE. The findings described here demonstrate that ACE data analyzed using the TAR form of analysis yield binding constants between receptors and ligands comparable to those estimated using other ACE forms of analysis. A comparison to three other forms of analysis is described. 相似文献