全文获取类型
收费全文 | 171765篇 |
免费 | 2037篇 |
国内免费 | 718篇 |
专业分类
化学 | 92523篇 |
晶体学 | 2464篇 |
力学 | 7610篇 |
综合类 | 7篇 |
数学 | 17759篇 |
物理学 | 54157篇 |
出版年
2021年 | 1003篇 |
2020年 | 1124篇 |
2019年 | 1144篇 |
2018年 | 1159篇 |
2017年 | 1186篇 |
2016年 | 2348篇 |
2015年 | 1934篇 |
2014年 | 2500篇 |
2013年 | 7730篇 |
2012年 | 6320篇 |
2011年 | 7994篇 |
2010年 | 4918篇 |
2009年 | 5064篇 |
2008年 | 7346篇 |
2007年 | 7322篇 |
2006年 | 7066篇 |
2005年 | 6583篇 |
2004年 | 5893篇 |
2003年 | 5077篇 |
2002年 | 4983篇 |
2001年 | 6031篇 |
2000年 | 4568篇 |
1999年 | 3479篇 |
1998年 | 2629篇 |
1997年 | 2621篇 |
1996年 | 2573篇 |
1995年 | 2281篇 |
1994年 | 2165篇 |
1993年 | 2028篇 |
1992年 | 2352篇 |
1991年 | 2310篇 |
1990年 | 2022篇 |
1989年 | 2008篇 |
1988年 | 2031篇 |
1987年 | 1966篇 |
1986年 | 1872篇 |
1985年 | 2780篇 |
1984年 | 2755篇 |
1983年 | 2210篇 |
1982年 | 2391篇 |
1981年 | 2273篇 |
1980年 | 2228篇 |
1979年 | 2177篇 |
1978年 | 2246篇 |
1977年 | 2200篇 |
1976年 | 2130篇 |
1975年 | 2118篇 |
1974年 | 2024篇 |
1973年 | 2107篇 |
1972年 | 1199篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
M. R. Yaftian M. Burgard D. Matt C. Wieser C. Dieleman 《Journal of inclusion phenomena and macrocyclic chemistry》1997,27(2):127-140
The complexing, extracting and mobile carrier properties of the tetra(phosphine oxide)-calix[4]arene 1 and the hybrid diamide-di(phosphine oxide)-calix[4]arene 2 were studied. Both ligands give 1 : 1 complexes with alkali cations in THF as shown by the picrate method. 1H NMR experiments were run to follow encapsulation of sodium and potassium cations. The corresponding spectra indicate C2-symmetrical structure. The observed extraction orders of the alkali picrates were as follows K+>Rb+>Li+>Cs+>Na+ for 1 and Li+>Na+>K+>Rb++ for 2. Transport kinetics was analysed by means of a model which assumes pure diffusion and which allows the evaluation of mass transfer coefficients in all systems. These coefficients and their influences on the transport rate are discussed in terms of size of the transporting species in the liquid membrane. 相似文献
993.
Frogs are known to obtain some of their nutrients (e.g., glucose and sodium) through their skin. However, no studies have been made of the possible absorption of transition metals, which exist in most river water at low-ppb (w/w) levels. Therefore, this research was undertaken to evaluate the use of ion chromatography for such an investigation. Solutions of copper and zinc (20 ppb in each) were chosen for use in a small-scale screening study. Ten live frogs were each placed in individual baths for approximately 50 h. Of interest were the net changes in the concentrations of the metals. These differences were the result of any absorption and/or excretion processes that took place. A Dionex IonPac CS5 column was used to analyze this simulated river water, both before and after frogs had been placed in the solution. Included in this paper are: (1) methodology and calculation formulas; (2) precautions needed to ensure sample integrity; (3) statistical analyses, which indicated that ion chromatography is an accurate, precise technique for quantifying Cu and Zn in these samples; and (4) screening-study results, which were used to test the null hypothesis that frogs do not absorb copper and zinc either onto or through their skin. 相似文献
994.
995.
A technique for following the hydrolysis of pyromellitic acid dianhydride (PMDA) by differential thermal analysis (DTA) is described. On exposure of PMDA to moisture, an endothennic peak is initiated between 190 and 200 degrees . As the degree of hydrolysis increases, this peak becomes more intense and shifts to higher temperatures. The formation of pyromellitic acid (PMA) during the hydrolysis of PMDA can be determined qualitatively by DTA. 相似文献
996.
Modern activation analysis requires the application of efficient data processing for the evaluation of theγ-spectra of samples containing several radionuclides, especially if preceding radiochemical separations are to be largely or completely eliminated. The essential aspects of two programs for quantitative determinations and qualitative identifications are described. These programs are based on the least squares analysis of the total spectrum and the search, arrangement and processing of the photopeaks. With respect to their details the programs are adapted for application on the computer IBM-360/75. 相似文献
997.
Some petroleum products, as paraffin waxes, microcrystalline waxes, lube oils, fuel oils, bitumens, were studied using DSC. DSC makes it possible to determine the wax content of various petroleum fractions, to study the effects of additives, to determine the glass transitions of bitumens; it is a useful technique both for research and for product analysis. 相似文献
998.
Smith K. D. Klasson K. T. Clausen Ackerson E. C. Gaddy J. L. 《Applied biochemistry and biotechnology》1991,28(1):787-796
Applied Biochemistry and Biotechnology - The results of this study support the hypothesis that several organisms that are capable of utilizing CO are also capable of degrading CO-like compounds,... 相似文献
999.
1000.
Stationary phase properties of the organic molten salt ethylpyridinium bromide in gas chromatography
Summary The organic molten salt, 1-ethylpyridinium bromide, is suitable for use as a stationary phase in gas chromatography. It has a usable liquid temperature range of 110 to 160°C. It undergoes a single phase transition at 110°C which corresponds to the bulk melting point. Below the melting point the phase may be used as a selective adsorbent, although column efficiency and peak symmetry deteriorate as the temperature is lowered from the melting point. Above the melting point, ethylpyridinium bromide can be used to separate a wide variety of organic compounds retaining strongly those compounds possessing large dipole or hydrogen bonding functional groups. 相似文献