全文获取类型
收费全文 | 3195篇 |
免费 | 108篇 |
国内免费 | 12篇 |
专业分类
化学 | 1960篇 |
晶体学 | 59篇 |
力学 | 69篇 |
数学 | 193篇 |
物理学 | 664篇 |
无线电 | 370篇 |
出版年
2023年 | 28篇 |
2022年 | 49篇 |
2021年 | 77篇 |
2020年 | 79篇 |
2019年 | 94篇 |
2018年 | 81篇 |
2017年 | 65篇 |
2016年 | 113篇 |
2015年 | 68篇 |
2014年 | 98篇 |
2013年 | 260篇 |
2012年 | 207篇 |
2011年 | 206篇 |
2010年 | 124篇 |
2009年 | 104篇 |
2008年 | 151篇 |
2007年 | 162篇 |
2006年 | 136篇 |
2005年 | 120篇 |
2004年 | 131篇 |
2003年 | 75篇 |
2002年 | 60篇 |
2001年 | 39篇 |
2000年 | 34篇 |
1999年 | 36篇 |
1998年 | 21篇 |
1997年 | 23篇 |
1996年 | 25篇 |
1995年 | 37篇 |
1994年 | 26篇 |
1993年 | 34篇 |
1992年 | 24篇 |
1991年 | 20篇 |
1990年 | 21篇 |
1989年 | 27篇 |
1988年 | 15篇 |
1987年 | 24篇 |
1985年 | 19篇 |
1984年 | 23篇 |
1983年 | 26篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 41篇 |
1979年 | 17篇 |
1978年 | 21篇 |
1977年 | 30篇 |
1976年 | 19篇 |
1975年 | 19篇 |
1973年 | 17篇 |
1970年 | 15篇 |
排序方式: 共有3315条查询结果,搜索用时 31 毫秒
61.
Ramaswamy K Saito H Murakami H Shiba K Suga H 《Journal of the American Chemical Society》2004,126(37):11454-11455
Fx3 is an artificial ribozyme with the ability to aminoacylate various tRNAs with phenylalanine and its nonnatural derivatives. Herein we report a simple strategy to build tRNA specificity into the generic Fx3, by appending to its 3'-end a tRNA-specific sequence (TSS), which is complementary to the acceptor stem of the cognate tRNA. This new designer ribozyme, referred to as Fx10, is able to recognize its cognate tRNA via a 10-base-pair interaction that is formed after the invasion of the tRNA acceptor stem by the TSS. We have demonstrated that Fx10 can aminoacylate its cognate tRNA with a high degree of specificity and also discriminate against the noncognate tRNAs. Because the tRNA specificity can be easily programmed into Fx10, it is a custom-made catalyst to generate nonnatural aminoacyl-tRNAs. 相似文献
62.
Krishna C. Joshi Renuka Jain Anshu Dandia Vandana Sharma 《Journal of heterocyclic chemistry》1986,23(1):97-99
A novel system 2-oxo-1′,2′,4′,5′-tetrahydrospiro[3H-indole-3,3′-1,2,4,5-tetrazine]-6′-thione has been synthesized by the treatment of fluorinated isatins with thiocarbohydrazide in aqueous ethanolic medium. Under exactly similar conditions, N-acetylisatin gave exclusively thiocarbohydrazone. The spiro product, on treatment with acetic acid, gave fluorinated isoindigo. Characterization of these products have been done by elemental analyses, ir, pmr and mass spectral studies. 相似文献
63.
Abstract— Recently, a new technique of 1O2 generation has been described by Midden and Wang. The sensitizer is immobilized on silica gel particles which are attached on one surface of a glass plate which is placed, sensitizer-side down, above a solution leaving a small air space between the sensitizer and the solution. Applying the principle of the technique, i.e. generation of 1O2 in the gas phase above the solution, we first improved the long-term stability of the dye (rose bengal) by using double stock transparent tape instead of rubber cement. Under these conditions, although the initial rate of production was higher in rubber cement, the rate of production reached a plateau after 30 min, whereas with double stick transparent tape, the rate of generation was linear up to 100 min. This result was explained by the fact that silica glass beads are embedded in the cement which could lead to the bleaching reaction of the dye. Different reactions have been investigated, the bleaching of N,N-dimethyl-4-nitrosoaniline (RNO) in the presence of imidazole, destruction of anthracenedipropionic acid, formation of a stable nitroxide from a sterically hindered secondary amine and the influence of pH on the nitroxide yield. The yield was measured to be 5 × 1012 molecules of singlet oxygen produced per second per cm2 from the surface of the plate using the RNO bleaching reaction. This method of generating 1O2 also seems promising for studying the killing of mammalian cells by 1O2 as the sole damaging species. 相似文献
64.
Dimers of the pyrrole amino acid (Paa), 5-(aminomethyl)pyrrole-2-carboxylic acid, and its derivatives having Lys anchored on N- and C-termini bind in the minor groove of DNA with considerable apparent binding affinities. When the Lys unit is attached to the C-terminus, the resulting ligand binds to ds-DNA with twice the affinity, of the order of 105, than the one carrying two positive charges at the same end. 相似文献
65.
U.Murali Krishna 《Tetrahedron letters》2004,45(10):2077-2079
A phosphoramidite approach was utilized for the first time to synthesize cardiolipin. Optically active 1,2-di-O-acyl-sn-glycerol was coupled with 2-O-protected glycerols utilizing mono- and bifunctional phosphitylating agents to yield, after final removal of protecting groups, the title compound. 相似文献
66.
Cellulose synthesis in maize: isolation and expression analysis of the cellulose synthase (CesA) gene family 总被引:7,自引:0,他引:7
Laura Appenzeller Monika Doblin Roberto Barreiro Haiyin Wang Xiaomu Niu Krishna Kollipara Lori Carrigan Dwight Tomes Mike Chapman Kanwarpal S. Dhugga 《Cellulose (London, England)》2004,11(3-4):287-299
Stalk lodging in maize results in significant yield losses. We have determined that cellulose per unit length of the stalk is the primary determinant of internodal strength. An increase in cellulose concentration in the wall might allow simultaneous improvements in stalk strength and harvest index. Cellulose formation in plants can be perturbed by mutations in the genes involved in cellulose synthesis, post-synthetic cellulose alteration or deposition, N-glycosylation, and some other genes with as yet unknown functions. We have isolated 12 members of the cellulose synthase (CesA) gene family from maize. The genes involved in primary wall formation appear to have duplicated relatively independently in dicots and monocots. The deduced amino acid sequences of three of the maize genes, ZmCesA10–12, cluster with the Arabidopsis CesA sequences that have been shown to be involved in secondary wall formation. Based on their expression patterns across multiple tissues, these three genes appear to be coordinately expressed. The remaining genes show overlapping expression to varying degrees with ZmCesA1, 7, and 8 forming one group, ZmCesA3 and 5 a second group, and ZmCesA2 and 6 exhibiting independent expression of any other gene. This suggests that the varying levels of coexpression may just be incidental except in the case of ZmCesA10–12, which may interact with each other to form a functional enzyme complex. Isolation of the expressed CesA genes from maize and their association with primary or secondary wall formation has made it possible to test their respective roles in cellulose synthesis through mutational genetics or transgenic approaches. This information would be useful in improving stalk strength. 相似文献
67.
Muralidhar P Kumar MM Krishna N Rao CB Rao DV 《Chemical & pharmaceutical bulletin》2005,53(2):168-171
Chemical investigation of a soft coral species of the genus Lobophytum has resulted in the isolation of three new sphingolipids--(2S,3S,4R)-2-nonadecanoylamino-octadecane-1,3,4-triol (1), (2S,3R,4E,8E)-[(2'R)-2'-hydroxyheptadecanoylamino]-4,8-octadecadiene-1,3-diol (2), 1-O-(beta-D-glucopyranosyl)-(2S,3R,4E,8E)-2-[(2'R)-2'-hydroxynonadecanoylamino]-9-methyl-4,8-octadecadiene-1,3-diol (3) and a sterol--(24S)-ergost-5-en-3beta,7beta-diol (4) along with the known sphingolipid--(2S,3R,4E,8E)-2-hexadecanoylamino-4,8-octadecadien-1,3-diol (5) which showed cytotoxicity against human peripheral blood mononuclear cells (PBMC). 相似文献
68.
R. Rai 《Zeitschrift für Physik B Condensed Matter》1995,99(3):327-332
The closeness of low-lying T1u and T1g levels of C 60 ? could enable their mixing under an odd parity vibration of (T1 u + T1 g ? (hg + τ1 u)type. In addition, the two levels are susceptible to Jahn-Teller interaction due to five-fold degenerate hg vibrations. This complex problem of (T1u+T1g)?(hg+τ1u) vibronic interaction is transformed to a form similar to T2g ? (εg + τ2g) vibronic problem of octahedral symmetry. The problem is analysed in an infinite coupling model and compared with the experimental spectroscopic results for the C 60 ? radical. The resulting parameters are used to calculate the pair-binding energy and superconducting transition temperature in C 60 n? fullerides. Vibronic mixing with the T1g level is found to be responsible for maximising the pair-binding energy at the doping level n=3. It is also found to be an important source of Tc enhancement. 相似文献
69.
70.
Penmatsa Krishna Kanth Varma Nagesh Kallollu Narayanaswamy 《International Journal of Communication Systems》2023,36(8):e5460
This communication presents a compact field de-correlation lines integrated dual band with dual-polarized (LP & CP) multiple-input multiple-output (MIMO) antenna for the fifth generation (5G) sub-6-GHz wireless communication systems. Dual working bandwidths, smaller interelement gaps, and superior isolation within the MIMO components are the distinguishing characteristics that give the proposed MIMO system an aspect of novelty. The modeled MIMO antenna has compact configurations of 20 × 21 × 0.8 mm3. The unit cell consists of a microstrip feed line with optimized rectangular slots branches etched from the radiated patch. The MIMO module is generated by the antiparallel replication of a single unit cell. To enhance the isolation, two rectangular slots are incorporated on the patch between the unit elements, which act as field de-correlation lines. The MIMO identity is supported by diversity performance calculations in terms of ECC, DG, and TARC. Simulated and measured counterparts are found in the agreement. 相似文献