全文获取类型
收费全文 | 1042篇 |
免费 | 29篇 |
国内免费 | 7篇 |
专业分类
化学 | 708篇 |
晶体学 | 12篇 |
力学 | 9篇 |
数学 | 162篇 |
物理学 | 187篇 |
出版年
2022年 | 16篇 |
2021年 | 19篇 |
2020年 | 14篇 |
2019年 | 17篇 |
2018年 | 11篇 |
2017年 | 19篇 |
2016年 | 19篇 |
2015年 | 16篇 |
2014年 | 23篇 |
2013年 | 59篇 |
2012年 | 75篇 |
2011年 | 75篇 |
2010年 | 39篇 |
2009年 | 28篇 |
2008年 | 68篇 |
2007年 | 56篇 |
2006年 | 56篇 |
2005年 | 42篇 |
2004年 | 52篇 |
2003年 | 31篇 |
2002年 | 31篇 |
2001年 | 11篇 |
2000年 | 13篇 |
1999年 | 14篇 |
1998年 | 11篇 |
1997年 | 16篇 |
1996年 | 21篇 |
1995年 | 18篇 |
1994年 | 18篇 |
1993年 | 12篇 |
1992年 | 6篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1975年 | 7篇 |
1969年 | 5篇 |
1907年 | 3篇 |
1888年 | 5篇 |
1885年 | 3篇 |
排序方式: 共有1078条查询结果,搜索用时 15 毫秒
1.
Vinyl and diene derivatives of thiolactomycin have been prepared via Horner-Wadsworth-Emmons olefination from protected 5-formyl-3,5-dimethylthiotetronic acid. Several 4-position protecting groups and a variety of phosphonates were evaluated, with MOM protection and β-ketophosphonates yielding the highest ratio of the desired product to deformylated product. 相似文献
2.
Cynthia J. Burrows Richard A. Sauter 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(1):117-121
New synthetic hosts have been designed incorporating two molecules of cholic acid linked by a rigid diamine. Proton NMR studies indicate that the compounds exist in a rigid conformation with the steroid hydroxyl groups intramolecularly hydrogen-bonded. Heat or addition of methanol leads to conformational isomerism due to insertion of methanol into the cavity.A preliminary report of this work was presented at the 4th International Symposium on Inclusion Phenomena, July 20–25, 1986, Lancaster, U.K. 相似文献
3.
4.
Angle-resolved low-energy ion scattering is a valuable technique for examining the topmost surface layers of materials. Using this technique, information about both composition and structure can be obtained. We discuss the physical basis of this technique and present our findings for the fivefold surface of icosahedral (i-) Al–Pd–Mn. Our results clearly show that the exposed surface has a higher Al content than the bulk and can have fivefold periodicity. Information about frequently occurring interatomic distances on the surface can also be obtained by this technique. We discuss the results and compare them to recent scanning tunneling microscopy studies and to bulk structure models. 相似文献
5.
Cynthia Kolb Whitney 《Foundations of Physics》2007,37(4-5):788-812
This paper revisits the historical sequence in which some of the major developments of 20th-century physics occurred, and
explores how theories could have turned out differently, if the sequence of developments had been different. It shows how
a delay in founding special relativity theory until after (1) at least one puzzling problem in electromagnetic theory could
be acknowledged, and (2) sat least some of the experimental observations pertinent to the development of quantum mechanics
had become well known, could have resulted in a larger theory that covers both domains in a manner quite different from that
of any of the theories we use today. The revised theory dispenses with a separate postulate introducing Planck’s constant
h, identifying instead a physical mechanism that implies the constant. Some important aspects of quantum chemistry then follow.
Editor, Galilean Electrodynamics, Proceedings of the Natural Philosophy Alliance; Visiting Industry Professor, Tufts University,
retired 相似文献
6.
Wolfgang Will Gerhard Hoffmann Andreas Zober 《Fresenius' Journal of Analytical Chemistry》1992,342(9):744-745
Summary A simple method is described which allows the quantification of 1-vinyl-2-pyrrolidone (NVP) in human serum. NVP is extracted from serum with diethylether and determined with HPLC/UV-detection. 1-Cyclohexyl-2-pyrrolidone serves as an internal standard. The detection limit is 0.1 mg/l. The method has shown that NVP can enter the organisms of workers occupationally exposed to this substance. 相似文献
7.
Cynthia M. Allen Joseph M. Weber Johan E. van Lier 《Photochemistry and photobiology》1995,62(1):184-189
Abstract— Transmission of infectious diseases through blood transfusions is well known. Ultraviolet irradiation, solvents and detergents provide a means of sterilizing noncellular blood components. However these harsh methods are not applicable to cellular blood products. Recently, attempts have been made to sterilize biological fluids using photodynamic treatment and phthalocyanine (Pc) dyes have been advanced as photosensitizers for this purpose. We have evaluated a series of water-soluble Pc, chelated with different central metal ions, substituted to different degrees with sulfonato and r-butyl groups, for their effectiveness to reduce virus infectivity in red blood cell suspensions. Vaccinia virus cytopathogenicity was determined by endpoint serial dilutions in the CV-1 cell line. Anti-viral activity increased with the central metal ion in the following order: Ga(III) < Al(III) < Zn(II), and varied inversely with the degree of sulfonation. Furthermore, addition of a t -butyl group onto the trisulfonated dyes (PcS3 [ t -Bul) resulted in a 5–40-fold increase in anti-viral potency, suggesting that amphiphilicity enhances the photodynamic activity of the dye. Strong anti-viral photosensitizing properties cannot be the sole selection criterion. Of equal importance is the preservation of blood component integrity. Accordingly, the photohemolytic activity of the dyes was evaluated using the rate of hemolysis as a parameter and a toxicity index was defined. Among the most active dyes, the AlPcS3 ( t -Bu) complex exhibited the most favorable anti-viral properties combined with a low toxicity index. Our results suggest that trisulfophthalocyanines, bearing an additional t -butyl group to enhance amphiphilicity, are particularly promising dyes for photodynamic blood sterilization. 相似文献
8.
N. F. Will K. Hofmann M. Schulz 《Applied Physics A: Materials Science & Processing》1986,41(2):107-114
Trap centers in the Si-SiO2 interface region of MOS structures doped by ion implantation of gold have been investigated using constant capacitance deep level transient spectroscopy (CC-DLTS). Gold doses of 1012–3 × 1013 cm–2 were implanted into the back surface of the wafers and were then redistributed during a diffusion anneal for 30 min at 1100° or 900° C. Three Au-related trap levels have been observed in the interface region, which were attributed to the Au-donor (E
v
+0.35 eV), the Au-acceptor (E
v
+0.53 eV), and the Au-Fe complex (E
v
+0.45 eV). The trap concentration profiles show that the Si-SiO2 interface affects the Au concentration in a depth range of 1 m from the interface and that gettering of Au occurs at the interface. The interface state density is independent of the Au concentration at the interface even for concentrations of 1015 cm–3. 相似文献
9.
The synthesis of well-defined luminescent organoboron polymers via a novel three-step procedure starting from silylated polystyrene is reported. Highly selective borylation of poly(4-trimethylsilylstyrene) (PS-Si), followed by replacement of the bromine substituents in poly(4-dibromoborylstyrene) (PS-BBr) with substituted thienyl groups (R = H, 3-hexyl, 5-hexyl), and final introduction of the 8-hydroxyquinolato moiety yields a series of new organoboron quinolate polymers in 67-83% isolated yield. The hexyl-substituted polymers are highly soluble and solution-processable yielding thin films that efficiently emit light at 513-514 nm upon excitation at 395 nm. 相似文献
10.
Studies of the vibrational spectra of matrix-isolated M+NO
3
–
ion pairs have been extended to glassy aprotic solvents. The deuterated form of the solvents DMSO, THF, and ACN have windows through the 7- nitrate ionv
3(e) mode infrared region, so it was possible to clearly observe the splitting of the degeneracy of this mode,v
3, produced by the contacting, but solvated, alkali metal cation. Primary attention has been directed to the extent to which this splitting is reduced relative to the argon matrix values. This reduction, which reflects electron-density transfer from the solvating molecules to the ion pairs, is comparable to that observed for H2O and NH3 matrices as the splitting is reduced to 20–35% of the argon-matrix values. The extent of reduction ofv
3 for the different solvents has been related to Gutmann's donicity number scale with the correlation holding well for solvent molecules of comparable size, DMSO, THF and DMF, but breaking down for the smaller linear ACN, apparently because of more molecules in the cation solvation sphere. The matrix data have also been used, through comparison with spectra for saturated liquid solutions of Li+NO
3
–
, to show that the contact ion pair is the dominant species in liquid THF and ACN, whereas the ions are largely solvent separated in DMSO. 相似文献