首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   68721篇
  免费   2930篇
  国内免费   1724篇
化学   34767篇
晶体学   1198篇
力学   3789篇
综合类   126篇
数学   5677篇
物理学   27818篇
  2022年   1021篇
  2021年   1080篇
  2020年   1035篇
  2019年   984篇
  2018年   1032篇
  2017年   912篇
  2016年   1439篇
  2015年   1305篇
  2014年   1714篇
  2013年   3335篇
  2012年   3319篇
  2011年   3947篇
  2010年   2767篇
  2009年   2863篇
  2008年   3466篇
  2007年   3166篇
  2006年   2980篇
  2005年   2579篇
  2004年   2285篇
  2003年   1965篇
  2002年   1858篇
  2001年   3155篇
  2000年   2290篇
  1999年   1666篇
  1998年   1177篇
  1997年   1125篇
  1996年   946篇
  1995年   830篇
  1994年   743篇
  1993年   665篇
  1992年   984篇
  1991年   964篇
  1990年   868篇
  1989年   766篇
  1988年   745篇
  1987年   811篇
  1986年   673篇
  1985年   897篇
  1984年   838篇
  1983年   571篇
  1982年   564篇
  1981年   535篇
  1980年   498篇
  1979年   623篇
  1978年   651篇
  1977年   661篇
  1976年   577篇
  1975年   481篇
  1974年   522篇
  1973年   450篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
Lee O  Koga Y  Wade AP 《Talanta》1990,37(9):861-873
A thermosonimetric study has shown that the Phase II/III polymorphic transition of hexachloroethane emits acoustic signals. This solid-solid phase transition is known to occur by a nucleation-growth process during which a nucleus of the new phase, once formed, grows at the expense of the mother phase to form a complete crystal without fracture. Acoustic emissions from a conditioned multi-crystal sample have been used to study the transition. Acoustic activity correlated well with dilatometric measurements. Frequency analysis on waveforms of many hundreds of individual acoustic emissions revealed marked differences between individual signals. Principal-components analysis on 24 signal features revealed a single dispersed cluster with a highly non-uniform distribution of signals. These experiments provided highly reproducible average power spectra. Time-resolved acoustic power spectra were also generated. These additional types of information cannot be obtained by other techniques.  相似文献   
192.
Recovered polyethylene is used with polyethylene waxes as a new binder for mineral granules. This binder is characterized and optimized mainly on a rheological basis. Its fluid index (I.F.), molecular distribution and crystallinity are especially studied. Its rheograms and tensile properties are compared with those of industrial polyethylene (I.F. = 250). The dimensional shrinkages of such binders during cooling from 190° to ambient temperature are measured.  相似文献   
193.
A. Nohara  T. Umetani  Y. Sanno 《Tetrahedron》1974,30(19):3553-3561
By the application of the Vilsmeier-Haack reaction to various o-hydroxyacetophenone derivatives, 4-oxo-4H-1-benzopyran-3-carboxaldehydes were synthesized in one step. Their IR, NMR and mass spectra were studied. In the mass spectra characteristic fragmentation pathways were observed.  相似文献   
194.
Low-frequency (80-700 cm-1) Qy-excitation resonance Raman (RR) spectra are reported for thin-solid-film aggregates of several chlorophyll (Chl) a and bacteriochlorophyll (BChl) c/d pigments. The pigments include Chl a, pyrochlorophyll a (PChl a), methylpyrochloropyllide a (MPChl a), methylbacteriochloropyllide d (MBChl d), [E,M] BChl cS, [E,E] BChl cF, and [P,E] BChl cF. The BChl c/d's are the principal constituents of the chlorosomal light-harvesting apparatus of green photosynthetic bacteria. Together, the various Chl a's and BChl c/d's represent a series in which the peripheral substituent groups on the chlorin macrocycle are varied in systematic fashion. All of the Chl a and BChl c/d aggregates exhibit rich low-frequency vibrational patterns. In the case of the BChl c/d's, certain modes in the very low-frequency region (100-200 cm-1) experience exceptionally strong Raman intensity enhancements. The frequencies of these modes are qualitatively similar to those of oscillations observed in femtosecond optical experiments on chlorosomes. The RR data indicate that the low-frequency vibrations are best characterized as intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. The coupling of the out-of-plane modes in turn implies that the Qy electronic transition(s) of the aggregate have out-of-plane character. The RR spectra of the BChl c/d's also reveal that the nature of the alkyl substituents at the 8 and 12 positions of the macrocycle plays an important role in determining the detailed features of the low-frequency vibrational patterns. The frequencies of the modes are particularly sensitive to larger substituent groups whose conformations may be more easily perturbed in the tightly packed aggregates. These factors also make aggregates of pigments containing larger substituents more susceptible to structural, electronic, and vibrational inhomgeneities. Collectively, the RR studies of the various pigments delineate the factors which influence the low-frequency vibrational characteristics of chlorosomal aggregates.  相似文献   
195.
We have measured serum ferritin level using double antibody radioimmunoassay kit (Eiken ICL) and evaluated the characteristics of the kit and clinical usefulness. Satisfactory results were observed in standard curve, reproducibility, dilution and recovery test. In clinical evaluation, we have measured in normal subjects and patients with various diseases. The range in normal males and females were 13.0-158.7 ng/ml and 7.3-73.0 ng/ml, respectively. Serum ferritin level was elevated in patients with hepatoma, biliary cancer, lung cancer and other malignant diseases. Measurement of serum ferritin value would be useful in the monitoring of cancer patients.  相似文献   
196.
The effects of various synthetic triamines having a general structure, H2N(CH2)xNH(CH2)yNH2, where x = 2-5 and y = 2-8 (abbreviated, x-y; with 3-4 being spermidine itself), on poly(U)-directed polypeptide synthesis of Escherichia coli and on growth of its polyamine-requiring mutants were examined in comparison with those of spermidine. Except for 2-2 and 2-3, all of the triamines stimulated more or less polypeptide synthesis at suboptimal Mg2+ concentrations, but the Mg2+ concentration required for the maximal stimulatory effect was different for each triamine. The degree of maximal stimulation caused by 3-3 (norspermidine), 4-4 (homospermidine), or 4-5 was nearly comparable with that by spermidine. The acetylspermidines were inactive, however, they inhibited the spermidine-stimulated polyphenylalanine synthesis. Many of the triamines examined reduced the ratio of leucine to phenylalanine incorporation into polypeptides during poly(U)-directed translation, and the degree of this effect did not necessarily correspond with that of the stimulatory effect. Moreover, 2-4, 2-5, 3-3 and 4-4 could stimulate the growth of a polyamine auxotroph of E. coli, MA 261, as effectively as did spermidine. However, 3-3 was the only triamine which could fully replaced spermidine in promoting growth of a mutant strain, KK 101, which is more dependent on polyamines than MA 261. Thus, these results demonstrated that some synthetic triamines were as active as spermidine in eliciting these effects, and also that there were some differences among these effects in the structural requirement for triamine.  相似文献   
197.
If the absorption of a trapped electron can be shown to correspond to bound—free transitions, it should be possible to calculate the potential acting on the trapped electron inversely from its absorption spectrum. In this paper we present the method of calculating the potential acting on a trapped electron from the observed absorption spectrum.  相似文献   
198.
6,7-Dimethoxy-2-naphthylethylamine, prepared by the diborane reduction of 6,7-dimethoxy-2-naphtlialeneacetamide, underwent a Pictet-Spengler cyclization to form 8,9-dimethoxy-1,2,3,4-tetrahydrobenz[h]isoquinoline. This compound is identical with that obtained by reduction of the corresponding dihydrobenzisoquinoline prepared from formamide cyclization. 6,7-Dialkoxy-2-naphthaleneacetic acids, the key intermediates for the preparation of these amides, were obtained from 6,7-dialkoxy-2-acetonaphthones by a modified Willgerodt reaction.  相似文献   
199.
The Kyushu Museum of Energy, funded by the Kyushu Electric Power Company, Inc., was opened in September 1982. The planning and design were done by Dentsu, Inc., and the actual construction was shared among several construction and manufacturing companies. I was retained as the principal scientific advisor for the museum, both prior to its construction, throughhout the planning of the exhibits, and afterwards when it was opened to the public. This is an educational museum, which shows how people have developed and depended on energy and energy sources since the discovery of fire in prehistory. There are four major sections, devoted to fire itself, electricity, nuclear power by fission, and nuclear power by fusion. In addition, the museum will track visitors through the exhibits, in terms of The History of Energy as a beginning point for their tour, lead them through Present Energy Needs and Uses, and thence to Future Energy Developments and Needs. The museum is of a hands-on active display type design, with many large and attractive display panels, working models, pleasant mixes of sound, light, and images, and educational levels that encompass general public interest to semitechnical explanations that will satisfy the undergraduate with a technical, background. Visitors can not only operate the models themselves, but join in a variety of games based on energy consumption, usage, and needs. It is very much a participatory museum, and visitors are exposed to the many facets of day-to-day energy generation and consumption, as well as the estimated needs for the future, and possible ways to supply these needs. To date, there have been well over 200,000 visitors recorded. *** DIRECT SUPPORT *** A0653119 00004  相似文献   
200.
Lu W  Jia C  Kitamura T  Fujiwara Y 《Organic letters》2000,2(19):2927-2930
Simple heteroarenes such as pyrroles and indoles undergo addition reactions to C-C triple bonds in the presence of a catalytic amount of Pd(OAc)(2) under very mild conditions, affording cis-heteroarylalkenes in most cases. The cleavage of aromatic C-H bonds is the possible rate-determining step in CH(2)Cl(2), and the addition of heteroaromatic C-H bonds to C-C triple bonds is in a trans-fashion.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号