全文获取类型
收费全文 | 1828篇 |
免费 | 48篇 |
国内免费 | 4篇 |
专业分类
化学 | 1336篇 |
晶体学 | 16篇 |
力学 | 15篇 |
数学 | 80篇 |
物理学 | 433篇 |
出版年
2023年 | 16篇 |
2021年 | 15篇 |
2020年 | 29篇 |
2019年 | 19篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 28篇 |
2015年 | 33篇 |
2014年 | 29篇 |
2013年 | 84篇 |
2012年 | 66篇 |
2011年 | 78篇 |
2010年 | 48篇 |
2009年 | 43篇 |
2008年 | 110篇 |
2007年 | 107篇 |
2006年 | 113篇 |
2005年 | 97篇 |
2004年 | 105篇 |
2003年 | 73篇 |
2002年 | 77篇 |
2001年 | 38篇 |
2000年 | 50篇 |
1999年 | 45篇 |
1998年 | 19篇 |
1997年 | 32篇 |
1996年 | 31篇 |
1995年 | 24篇 |
1994年 | 31篇 |
1993年 | 29篇 |
1992年 | 36篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 19篇 |
1987年 | 23篇 |
1986年 | 16篇 |
1985年 | 23篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1979年 | 22篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1976年 | 15篇 |
1974年 | 13篇 |
1973年 | 13篇 |
1968年 | 8篇 |
排序方式: 共有1880条查询结果,搜索用时 351 毫秒
181.
Summary Dionium cations, polymethylenebis(tributylphosphonium)s (BuCnBPs), were used as ion-pair reagents for ion-pair chromatography of dianionic species, such as maleate, fumarate, phthalates, and naphthalenedisulfonates. The longer the length of methylene chain connecting the two phosphonium cationic ceterns within BuCnBP, the larger was the retention of these dianionic species. The dionium reagents had a higher retention ability compared to conventional monoonium reagents, and enabled a higher resolution among the dianionic species of structural isomers. The extraction behavior of the dianionic species in a loctanol/water system was also investigated. The retention behavior by the use of BuCnBP was discussed in comparison with the extraction behavior. 相似文献
182.
Diastereoselectivity in the photoallylation and photoreduction of 1,1-dicyano-2-methyl-3-phenyl-1-butene by allyltrimethylsilane
in the presence of phenanthrene was dependent on the structures and stoichiometry of the added carboxylic acids. Diastereoselectivity
increased up to 72% by the addition of equimolar amount of l-lactic acid based on the alkene. 相似文献
183.
Chihiro Maeda Pyosang Kim Sung Cho Jong Kang Park Jong Min Lim Dongho Kim Prof. Dr. Josh Vura‐Weis Michael R. Wasielewski Prof. Dr. Hiroshi Shinokubo Prof. Dr. Atsuhiro Osuka Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(17):5052-5061
meso‐Triazolyl‐appended ZnII–porphyrins were readily prepared by CuI‐catalyzed 1,3‐dipolar cycloaddition of benzyl azide to meso‐ethynylated ZnII–porphyrin (click chemistry). In noncoordinating CHCl3 solvent, spontaneous assembly occurred to form tetrameric array ( 3 )2 from meso–meso‐linked diporphyrins 3 , and dodecameric porphyrin squares ( 4 )4 and ( 5 )4 from the L ‐shaped meso–meso‐linked triporphyrins 4 and 5 . The structures of these assemblies were examined by 1H NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the structures of the dodecameric porphyrin squares ( 4 )4 and ( 5 )4 were probed by small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) measurements in solution using a synchrotron source. Excitation‐energy migration processes in these assemblies were also investigated in detail by using both steady‐state and time‐resolved spectroscopic methods, which revealed efficient excited‐energy transfer (EET) between the meso–meso‐linked ZnII–porphyrin units that occurred with time constants of 1.5 ps?1 for ( 3 )2 and 8.8 ps?1 for ( 5 )4. 相似文献
184.
myo-Inositol made from a biomass feedstock was used as an additive for poly (l-lactic acid) (PLLA) which was also made from biomass feedstock. The crystallization and stabilization of PLLA by the addition of myo-inositol were evaluated by the melt injection molding process. While the isothermal crystallization of PLLA at 100 °C had finished over 14 min after melting, that of PLLA with 5 wt% myo-inositol finished within 2 min. The crystal growth of PLLA started when the myo-Inositol crystal was added, and the crystallization was promoted. Furthermore, the molecular weight of PLLA with myo-inositol did not decrease during the melt-mixed at 200 °C, different from that of PLLA without the myo-inositol. myo-Inositol prevented the degradation of PLLA during the thermal melting process. The biomass carbon ratio measured by the accelerator mass spectroscopy method showed that the PLLA with 5 wt% myo-inositol was a fully biobased material. It was demonstrated that myo-inositol was a multi-functional biobased additive for the modification of PLLA without decreasing its mechanical properties. 相似文献
185.
Ruili Zhang Tomonori Maeda Sho Kusaka Kei-ichiro Murai 《Journal of solid state chemistry》2010,183(3):620-623
Host lattice Ba3Si5O13−δNδ oxonitridosilicates have been synthesized by the traditional solid state reaction method. The lattice structure is based on layers of vertex-linked SiO4 tetrahedrons and Ba2+ ions, where each Ba2+ ion is coordinated by eight oxygen atoms forming distorted square antiprisms. Under an excitation wavelength of 365 nm, Ba3Si5O13−δNδ:Eu2+ and Ba3Si5O13−δNδ:Eu2+,Ce3+ show broad emission bands from about 400-620 nm, with maxima at about 480 nm and half-peak width of around 130 nm. The emission intensity is strongly enhanced by co-doping Ce3+ ions into the Ba3Si5O13−δNδ:Eu2+ phosphor, which could be explained by energy transfer. The excitation band from the near UV to the blue light region confirms the possibility that Ba3Si5O13−δNδ:Eu2+, Ce3+ could be used as a phosphor for white LEDs. 相似文献
186.
Maeda S Matsuda Y Mizutani S Fujii A Ohno K 《The journal of physical chemistry. A》2010,114(44):11896-11899
The dynamics on the vacuum-ultraviolet one-photon ionization of a formamide-water cluster is investigated by a combination of theoretical reaction-path search and infrared spectroscopic methods. A keto-enol tautomerization of the formamide moiety occurs after photoionization by a catalytic action of the water molecule accompanied with its long-distance migration; the water molecule in the cluster migrates almost one turn around the formamide moiety. During the migration, the water molecule abstracts the proton of CH in the formamide moiety and carries it to the O atom side in the carbonyl group through a "catch and release"-type catalytic action. 相似文献
187.
Takashi Hashimoto Takuma Nishimura Jong Min Lim Prof. Dr. Dongho Kim Prof. Dr. Hiromitsu Maeda 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(38):11653-11661
Bidipyrrin‐bridged macrocycles, prepared from NiII‐bridged dipyrrin‐based nanorings by intramolecular oxidative biaryl coupling reactions, yielded [2+4]‐type ZnII‐assisted stable twisted‐ring dimers comprising two double helices. These [2+4]‐type metal complexes can be optically resolved by chiral HPLC and exhibit tunable electronic and optical properties as a result of spring‐like motions. The double helices behave as glue to connect two macrocycles and as the screws of hinges to form thermally responsive synchronized spring systems. 相似文献
188.
Some factors that may affect human perception thresholds of the vertical whole-body vibrations were investigated in two laboratory experiments with recumbent subjects. In the first experiment, the effects of gender and age of subjects on perception were investigated with three groups of 12 subjects, i.e., young males, young females and old males. For continuous sinusoidal vibrations at 2, 4, 8, 16, 31.5 and 63 Hz, there were no significant differences in the perception thresholds between male and female subjects, while the thresholds of young subjects tended to be significantly lower than the thresholds of old subjects. In the second experiment, the effect of vibration duration was investigated by using sinusoidal vibrations, at the same frequencies as above, modulated by the Hanning windows with different lengths (i.e., 0.5, 1.0, 2.0 and 4.0 s) for 12 subjects. It was found that the peak acceleration at the threshold tended to decrease with increasing duration of vibration. The perception thresholds were also evaluated by the running root-mean-square (rms) acceleration and the fourth power acceleration method defined in the current standards. The differences in the threshold of the transient vibrations for different durations were less with the fourth power acceleration method. Additionally, the effect of the integration time on the threshold was investigated for the running rms acceleration and the fourth power acceleration. It was found that the integration time that yielded less differences in the threshold of vibrations for different durations depended on the frequency of vibration. 相似文献
189.
I. Tsukada M. Hanawa Seiki Komiya A. Ichinose T. Akiike Y. Imai A. Maeda 《Physica C: Superconductivity and its Applications》2011,471(21-22):625-629
The Hall effect is investigated in thin-film samples of iron–chalcogenide superconductors in detail. The Hall coefficient (RH) of FeTe and Fe(Se1–xTex) exhibits a similar positive value around 300 K, indicating that the high-temperature normal state is dominated by hole-channel transport. FeTe exhibits a sign reversal from positive to negative across the transition to the low-temperature antiferromagnetic state, indicating the occurrence of drastic reconstruction in the band structure. The mobility analysis using the carrier density theoretically calculated reveals that the mobility of holes is strongly suppressed to zero, and hence the electric transport looks to be dominated by electrons. The Se substitution to Te suppresses the antiferromagnetic long-range order and induces superconductivity instead. The similar mobility analysis for Fe(Se0.4Te0.6) and Fe(Se0.5Te0.5) thin films shows that the mobility of electrons increases with decreasing temperature even in the paramagnetic state, and keeps sufficiently high values down to the superconducting transition temperature. From the comparison between FeTe and Fe(Se1–xTex), it is suggested that the coexistence of ‘itinerant’ carriers both in electron and hole channels is indispensable for the occurrence of superconductivity. 相似文献
190.
Yasuharu Yoshimi Kosuke Kobayashi Keisuke Nishikawa Kousuke Maeda Tatsuya Itou Minoru Hatanaka 《Tetrahedron letters》2010,51(17):2332-6179
Radical addition to a glyoxylic oxime ether was accomplished under mild conditions using an alkyl radical generated from a free carboxylic acid via photochemical decarboxylation. The photoreaction provided an efficient route to α-substituted α-aminoesters from carboxylic acids and oxime ether. 相似文献