全文获取类型
收费全文 | 3123篇 |
免费 | 130篇 |
国内免费 | 17篇 |
专业分类
化学 | 2428篇 |
晶体学 | 23篇 |
力学 | 57篇 |
综合类 | 1篇 |
数学 | 224篇 |
物理学 | 537篇 |
出版年
2022年 | 29篇 |
2021年 | 31篇 |
2020年 | 57篇 |
2019年 | 48篇 |
2018年 | 46篇 |
2017年 | 31篇 |
2016年 | 84篇 |
2015年 | 74篇 |
2014年 | 84篇 |
2013年 | 177篇 |
2012年 | 179篇 |
2011年 | 191篇 |
2010年 | 114篇 |
2009年 | 100篇 |
2008年 | 193篇 |
2007年 | 193篇 |
2006年 | 190篇 |
2005年 | 176篇 |
2004年 | 149篇 |
2003年 | 122篇 |
2002年 | 104篇 |
2001年 | 59篇 |
2000年 | 44篇 |
1999年 | 46篇 |
1998年 | 38篇 |
1997年 | 31篇 |
1996年 | 35篇 |
1995年 | 33篇 |
1994年 | 25篇 |
1993年 | 29篇 |
1992年 | 20篇 |
1991年 | 22篇 |
1990年 | 19篇 |
1989年 | 28篇 |
1988年 | 30篇 |
1987年 | 19篇 |
1986年 | 16篇 |
1985年 | 38篇 |
1984年 | 32篇 |
1983年 | 15篇 |
1982年 | 32篇 |
1981年 | 32篇 |
1980年 | 27篇 |
1979年 | 34篇 |
1978年 | 25篇 |
1977年 | 24篇 |
1976年 | 20篇 |
1975年 | 24篇 |
1974年 | 13篇 |
1973年 | 16篇 |
排序方式: 共有3270条查询结果,搜索用时 156 毫秒
31.
H. -L. Li Y. Ujihira S. Tanaka T. Yamashita K. Horie 《Journal of Radioanalytical and Nuclear Chemistry》1996,210(2):543-553
Positron annihilation lifetime measurements of PMMA, PEMA, PiPMA, PnPMA and PnBMA were performed in the temperature range between 15 and 300 K, where , and relaxational transitions occur for these polyalkylmethacrylates. The variations of free volume size and content calculated from the longest lifetime component against temperature are correlated to the results obtained by dielectric, viscoelastic and dynamic mechanical relaxation data of the polymers. The variations of free volume sizes and contents, apparent free volume fraction and size distribution of the polyalkylmethacrylates are well correlated with the rotational transitions of side chains. The location of free volumes, in which positronium annihilates can be estimated near the vicinity of alkyl groups bound to oxygen atom of side ester chains. 相似文献
32.
33.
Chau Jin Hu Ryuichi Oshima Yuji Arai Manabu Sen
《Journal of polymer science. Part A, Polymer chemistry》1988,26(9):2423-2433
Copolymers containing an intramolecular electron donor-acceptor (EDA) complex were synthesized by free radical copolymerization of 2-N-carbazolylethyl acrylate and 2-(3,5-dinitrobenzoyloxy)ethyl methacrylate. Glass transition temperatures show a positive deviation from the weight-average values of copolymers, indicating the presence of the specific EDA interaction in copolymers in the solid state. Photoinduced “memory effect,” which is defined as the percentage of the difference of the surface potential given by corona charging before and after irradiation of light on polymer films, was 30% for the copolymer with 5 mol% of acceptor content. Memory effect increased to 70% for a 8 μm film by doping with 2 wt% of trichloroacetic acid (TCAA), and leveled off at 5 wt% of TCAA content. Memory effect was also enhanced by increasing the thickness of polymer films and irradiation time. The largest value of memory effect (85%) was obtained from the film of the copolymer with 5 mol% acceptor content doped with 1 wt% TCAA and with thickness larger than 14 μm. 相似文献
34.
35.
J. Nishimura Y. Ishida M. Mimura N. Nakazawa S. Yamashita 《Journal of polymer science. Part A, Polymer chemistry》1980,18(7):2061-2070
Polymerization of 1,3-bis(p-vinylphenyl) propane (St-C3-St) was investigated by using radical and anionic initiators. Radical polymerization yielded linear polymer with pendant styryl groups in pertinent conditions without gelation. Anionic polymerization with n-butyllithium and sodium naphthalene produced insoluble polymers that, according to infrared (IR) spectroscopy, had no cyclized units. On the other hand, phenylmagnesium bromide gave soluble polymer in HMPA-benzene mixed solvent. Zero-valent nickel catalyst also gave soluble polymer. The soluble polymers could be analyzed by several spectroscopies, and it was confirmed that those obtained by anionic and coordination polymerization had no [3.3]paracyclophane units in the main chain. From these results it was concluded that cationic propagation could be assumed if the polymer Of St-C3-St contained [3.3]paracyclophane units in the main chain. 相似文献
36.
Jinghua Zhang Toshiko Nagahama Hiroki Ohwaki Yasuhiro Ishibashi Yuji Fujita Sunao Yamazaki 《Analytical sciences》2004,20(1):37-43
To understand the cause of discoloration of the sea laver "nori," which is found in the Ariake Sea, the concentrations of pigments and elements in the normal and discolored laver samples were determined. In the discolored samples, a decrease in all of the pigments, chlorophyll a and carotenoids, and proteinous pigments, phycobiliproteins, was clearly observed. This was accompanied by a decrease in the content of Fe, Zn, Mn, Cu, and P. Good correlations between these elements and chlorophyll a, as well as between these elements and phycobiliproteins, were confirmed, indicating that, in addition to the deficiency of nitrogen and phosphorus, the deficiency of trace elements (Fe, Zn, Mn, and Cu), which are specifically required for photosynthesis, could be a reason for the discoloration of nori. The cause of elemental deficiency is also discussed. 相似文献
37.
The enantio-controlled synthesis of trans-4,5-benzhydrindan-1-ones was achieved by means of a stereoselective [4+2] cycloaddition of o-quinodimethanes generated by a thermal cleavage of benzocyclobutene derivatives as a key step. The chiral substrates of the thermal reaction were synthesized by a diastereoselective Grignard addition to the chiral O-isopropylideneglyceroketones connected to a benzocyclobutene ring, which were simply prepared from D-mannitol as a chiral source. This approach can provide a new efficient access to A-nor B-aromatic steroidal compounds. 相似文献
38.
Tadatomi Nishikubo Atsushi Kameyama Jun Yamashita Masao Tomoi Wakichi Fukuda 《Journal of polymer science. Part A, Polymer chemistry》1993,31(4):939-947
The addition reaction of oxiranes ( 26a—e ) with carbon dioxide (CO2) was performed using insoluble polystyrene beads containing pendant quaternary ammonium or phosphonium salts as catalysts under atmospheric pressure. The reaction of 26a—e with CO2 proceeded smoothly catalyzed by 1–2 mol % of the polymer-supported quaternary onium salts to give the corresponding cyclic carbonates ( 27a—e ) in high yields at 80–90°C. In this reaction system, the catalytic activity of the polymer-supported quaternary onium salts was strongly affected by the following factors: degree of ring substitution (DRS) of the onium salt residues to the polymer, degree of crosslinking (DC) of the polystyrene beads, chain length of the alkylene spacer between the polymer back-bone and the onium salt, hydrophobicity of the alkyl group on the onium salts, and kind of onium salts. That is, the polymer-supported quaternary phosphonium salts with low DRS and DC and with long alkylene spacer chain were found to have higher catalytic activity than low molecualr weight quaternary onium salts. The above polymer-supported catalysts can easily be separated at the end of a reaction by filtration and can be reused for at least seven runs. It was also found that the rate of reaction was proportional to the products of catalyst concentration and oxirane concentration. © 1993 John Wiley & Sons, Inc. 相似文献
39.
Labeling experiments using several deuterated lipids were pursued to study the biosynthesis of macrocyclic isoprenoidal lipids of thermophilic methanogenic archaea, Methanothermobacter thermautotrophicus. The isopropylidene terminal of geranylgeranyl group of monomeric precursor appeared to be important for the CC bond formation at the hydrophobic end in the macrocyclic lipids. A mechanism involving a radical trigger at the allylic methyl group is proposed for this CC bond formation. 相似文献
40.
Takeshi SasakiHirotsugu Takizawa Kyota UhedaTakahiro Yamashita Tadashi Endo 《Journal of solid state chemistry》2002,166(1):164-170
Two new high-pressure phases of binary boron-sulfur compounds, B2S3-II and B2S3-III, were synthesized at 3-6.2 GPa. A single crystal of B2S3-III was grown and the structure was determined (tetragonal, space group I41/a, a=16.086(2) Å, c=30.488(4) Å; V=7888(1) Å3, Z=100, R=3.0% and Rw=2.8% for 3047 observed data [I>3.00σ(I)]. The structure of B2S3-III consists of two kinds of macrotetrahedra built up from 20 and 34 BS4-tetrahedra. These macrotetrahedra connect each other to form an interpenetrating zincblende-type structure by sharing BS4-tetrahedra at the corners of those. B2S3-III is anticipated having a rather disordered structure. From the UV-Vis diffuse reflectance spectrum, the optical band gap of B2S3-III was estimated to be 3.7 eV. 相似文献