全文获取类型
收费全文 | 2350篇 |
免费 | 49篇 |
国内免费 | 57篇 |
专业分类
化学 | 821篇 |
晶体学 | 10篇 |
力学 | 21篇 |
综合类 | 15篇 |
数学 | 48篇 |
物理学 | 165篇 |
综合类 | 1376篇 |
出版年
2022年 | 4篇 |
2021年 | 6篇 |
2020年 | 3篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 12篇 |
2016年 | 15篇 |
2015年 | 21篇 |
2014年 | 42篇 |
2013年 | 46篇 |
2012年 | 42篇 |
2011年 | 73篇 |
2010年 | 48篇 |
2009年 | 74篇 |
2008年 | 93篇 |
2007年 | 124篇 |
2006年 | 141篇 |
2005年 | 127篇 |
2004年 | 125篇 |
2003年 | 137篇 |
2002年 | 96篇 |
2001年 | 89篇 |
2000年 | 64篇 |
1999年 | 40篇 |
1998年 | 28篇 |
1997年 | 22篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1991年 | 2篇 |
1985年 | 75篇 |
1984年 | 47篇 |
1983年 | 43篇 |
1982年 | 46篇 |
1981年 | 161篇 |
1980年 | 149篇 |
1979年 | 35篇 |
1978年 | 76篇 |
1977年 | 71篇 |
1976年 | 51篇 |
1975年 | 57篇 |
1974年 | 52篇 |
1973年 | 35篇 |
1968年 | 19篇 |
1962年 | 4篇 |
1958年 | 4篇 |
1957年 | 3篇 |
1955年 | 4篇 |
1949年 | 4篇 |
排序方式: 共有2456条查询结果,搜索用时 15 毫秒
11.
A new titrimetric method for determination of tin (4%) in aluminium alloys, tin- and lead-base alloys, solders, white bearing alloys, special bronzes and silver brazing alloys is proposed. HEDTA, Semi-Xylenol Orange and bismuth perchlorate are used as titrant, indicator and back-titrant respectively. Measures are taken to overcome the hydrolysis of Sn(IV). Monochloroacetic acid and ethylene glycol are added as auxiliary agents. The standard deviation of this method was found to be 0.2 mg and its coefficient of variation to vary from 0.25 to 2%, according to amount of tin. A novel method of sample decomposition and a modified method for separating Sn(IV) are also suggested. 相似文献
12.
I.J. Galpin F.E. Hancock B.K. Handa A.G. Jackson G.W. Kenner P. McDowell P. Noble R. Ramage 《Tetrahedron》1981,37(17):3043-3050
The previously synthesised (1–37), (38–75), (76–93), (94–104), (105–117) and (118–129) fragments of the analogue were combined making extensive use of the DCCI/HONSu method. The final coupling involved the (1–75) and (76–129) sub-fragments. Aggregation of the latter fragment caused problems in purification by routine gel filtration methods employing Enzacryl K2 or Sephadex LH60. The fully protected (1–129) product was partially purified by washing, then deprotectcd and purified by gel filtration and ion exchange chromatography. Satisfactory removal of the acetamidomethyl group used for cysteine protection could not be achieved. 相似文献
13.
An universal function for non-bonded interactions, which takes into account the relative orientation of the bonds is considered in calculating the conformational energies of cycloalkenes and cycloalkadienes. A comparison is made with previous results obtained by using usual 6-exp functions for non-bonded interactions. 相似文献
14.
The rg structure of cyclopentene oxide has been determined by the simultaneous least squares analysis of electron diffraction and microwave spectroscopic data. The investigation has reaffirmed previous studies indicating that the molecule prefers a boat conformation. The methylene and epoxide flap angles obtained are 152.3±2.1° and 104.7±1.0° respectively. Other structural parameters determined are: rg (C-H avg.) = 1.120±0.004 Å; rg (C-C avg.) = 1.538±0.002 Å; rg (C-O) = 1.443±0.003 Å, and rg (C-C) = 1.482±0.004 Å for the carbon-carbon bond in the three membered epoxide ring. These results compare favorably with the reported structures of ethylene oxide and cyclohexene oxide. A tentative rationalization of the unusual boat conformation is also offered. 相似文献
15.
Electrical, magnetic, and electron paramagnetic resonance (EPR) measurements have been made on crystals and powders of several quaternary chalcogenides of the type Cu2AIIBIVX4, where AII = Zn, Mn, Fe, or Co, BIV = Si, Ge, or Sn, and X = S or Se. The electrical properties of these compounds are extrinsic, but their magnetic properties do not appear to be affected by impurities. The magnetic moments of the Cu2MnBX4 compounds decrease with increasing covalency of the MnX bond, and those of Cu2FeGeS4 and Cu2CoGeS4 reflect an orbital contribution to the moment. Both the Weiss constants and magnetic ordering temperatures in these compounds show an evolution from antiferromagnetism to ferromagnetism with increasing separation between the moments. Magnetic measurements on single crystals of Cu2MnGeS4, Cu2CoGeS4, and Cu2FeGeS4 indicate that only the latter is anisotropic. EPR measurements on crystals and powders of Cu2ZnGeS4 doped nominally with 0.1% Mn reveal that Mn2+ experiences an axial distortion and that the bond ionicity is the same as in ZnS. 相似文献
16.
The use of wet and dry ashing procedures to decompose marine biological tissues and to degrade organoarsenic compounds to inorganic arsenic for analysis by zinc-column arsine generation and atomic-absorption spectrophotometry was investigated. Wet ashing with nitric, sulphuric and perchloric acids (10:2:3 v/v) released the largest percentage of arsenic from fish tissue and quantitatively degraded methylated and other organoarsenic compounds to inorganic arsenic. The arsenic concentrations found when standard reference materials were ashed with this acid mixture were in agreement with the certified values. 相似文献
17.
The kinetics of the reaction between organocobaloximes, RCo(DH)2H2O, and iodine have been investigated. They reveal the participation of an RCo(DH)2H2O · I2 intermediate which undergoes intramolecular transalkylation and acts as an electrophile towards a second organocobaloxime molecule. The trend in reactivity as the R group is varied is discussed. 相似文献
18.
The dielectric constant ?' and loss tangent tan δ of chlorobenzene-cis-decalin mixtures have been measured in the temperature range 77 K to 330 K and frequency range 0.1 to 100 kHz. On cooling, ?' increases with decreasing temperature upto about 135 K, after which it drops rapidly with decreasing T followed by a slow decrease. This indicates that the liquid mixture goes to an amorphous phase which transforms to a glass phase of restricted dipole rotation below Tg; however, the peak in ?' is due to relaxation in the amorphous phase (α relaxation) and does not give an exact Tg. On heating, the behaviour of the cooling curve is retraced upto 160 K, after which ?' drops suddenly to a value lower than that at 77 K in the glass phase. This indicates the transition to a crystalline phase in which dipole rotational freedom is completely lost. The crystalline phase changes to a eutectic liquid phase of high ?' at a temperature (200 K) lower than the melting point of chlorobenzene and cis-decalin. Dielectric dispersion is observed only in the glass and amorphous phases. The dielectric relaxation time is independent of the concentration of chlorobenzene. 相似文献
19.
The system copper-tellurium was investigated by DTA, DSC, and X-ray methods. A phase diagram with the phases Cu2?xTe (33.5–36.2 mole% Te), Cu3?xTe2 (40–41 mole%), and CuTe was constructed. In the homogeneity ranges of the nonstoichiometric phases several superstructures were observed. The lattice parameters and d values of some of these phases are given. 相似文献
20.
Isolation and structure elucidation of a C11 mono-carboxylic acid, apparently a nor-sesquiterpene, and, a sesquiterpene diosphenol from the essential oil of Cedrus deodara Loud. are described. 相似文献