全文获取类型
收费全文 | 14804篇 |
免费 | 545篇 |
国内免费 | 44篇 |
专业分类
化学 | 10058篇 |
晶体学 | 234篇 |
力学 | 468篇 |
综合类 | 1篇 |
数学 | 1121篇 |
物理学 | 3511篇 |
出版年
2024年 | 66篇 |
2023年 | 173篇 |
2022年 | 443篇 |
2021年 | 431篇 |
2020年 | 472篇 |
2019年 | 531篇 |
2018年 | 488篇 |
2017年 | 466篇 |
2016年 | 672篇 |
2015年 | 477篇 |
2014年 | 763篇 |
2013年 | 1262篇 |
2012年 | 1116篇 |
2011年 | 1157篇 |
2010年 | 764篇 |
2009年 | 593篇 |
2008年 | 721篇 |
2007年 | 728篇 |
2006年 | 567篇 |
2005年 | 491篇 |
2004年 | 378篇 |
2003年 | 313篇 |
2002年 | 256篇 |
2001年 | 155篇 |
2000年 | 130篇 |
1999年 | 99篇 |
1998年 | 71篇 |
1997年 | 99篇 |
1996年 | 94篇 |
1995年 | 73篇 |
1994年 | 68篇 |
1993年 | 93篇 |
1992年 | 98篇 |
1991年 | 78篇 |
1990年 | 68篇 |
1989年 | 71篇 |
1988年 | 49篇 |
1987年 | 45篇 |
1986年 | 42篇 |
1985年 | 63篇 |
1984年 | 62篇 |
1983年 | 53篇 |
1982年 | 47篇 |
1981年 | 37篇 |
1980年 | 45篇 |
1979年 | 62篇 |
1978年 | 48篇 |
1977年 | 53篇 |
1976年 | 41篇 |
1975年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
P. V. Bharatam Amita D. Kaur P. Senthil Kumar 《International journal of quantum chemistry》2006,106(5):1237-1249
The potential energy surface (PES) of thionylimide has been searched using ab initio MO and density functional calculations. The electronic structures of the isomers of HNSO have been studied using the HF/6‐31+G*, MP2(full)/6‐31+G*, and B3LYP/6‐31+G* levels. Final energies of these molecules have been calculated at the high‐accuracy G2 and CBS‐Q levels. The probable pathways of isomerization of thionylimide to its isomers (e.g., thiocyanic acid, HONS, nitrosothiols) have been explored by studying the three‐ or four‐membered transition states. This study identified total eight possible isomers ( 1–8 ) of HNSO, of which four ( 1–4 ) have already been realized experimentally. Of the remaining four ( 5–8 ), at least two ( 5, 7 ) can be generated experimentally. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
92.
Kulwant Singh Kawaldeep Vijay Kumar 《Journal of Radioanalytical and Nuclear Chemistry》1998,231(1-2):45-50
The sum peak at 437 keV (356+81 keV) has been measured in various chemical and physico-chemical environments. The fact that
the intensity ratio of the sum peak to its single peak changes with the circumstances in which the radioactive source is placed
is used to observe the effect of the chemical environment on directional correlation coefficients of 351–81 keV cascade in133Ba−133Cs decay and to detect the variations in the electric field gradient in different barium compounds. The sum peak method has
been found to be useful in describing the chemical influences onG
22 parameters and hence in the study of electric field gradients. 相似文献
93.
The Conversion of Eperuic Acid into Ethers of the enantio-14, 15-Dinorlabdane Series 5 and 6 are strongly odiferous substances of the ambra-type. Their enantiomers 7 and 8 , hitherto unknown, have been synthesized from eperuic acid (4) and their olfactory properties compared with those of 5 and 6 . 4 was esterified by CH2N2 and dehydrogenated with (C6H5Se)2/H2O2 to the α,β-unsaturated ester 9 (61%). Oxidation by KMnO4 in acetone yielded the ketone 3 (60%). Epoxidation followed by treatment with acid converted 3 into the acetals 7 (61%) and 8 (14%). 7 and 8 differ from 5 and 6 in odor intensity, and 6 and 8 show slightly different odor quality. 相似文献
94.
Vijay Kumar Kawaldeep K. Singh 《Journal of Radioanalytical and Nuclear Chemistry》1992,166(2):131-142
The sum peak method has been applied to electron capture probability changes to the 1086 keV level in the decay of152Eu in the different environments, e.g. ethylenediaminetetraacetic acid (EDTA), bovine serum albumin (BSA) and hydrochloric acid (HCl). An attempt is also made to determine the effect of chemical environment on the directional correlation coefficient of the 1409–122 keV - cascade in152Eu 152Sm decay. A change of G22 is found in the different chemical forms. 相似文献
95.
Devendra Kumar Uday Razdan Alka D. Gupta 《Journal of polymer science. Part A, Polymer chemistry》1993,31(3):797-804
Heat-resistant polymers which are processable into void-free components and suitable for composite applications have been synthesized by thermal/chemical polymerization of four newly developed bisimido-bisphthalonitriles containing silicon, ether, carbonyl, and hexafluoroisopropylidene groups. Thermal polymerization involving addition reactions was performed at 200–275°C for 2–10 h and then post-curing at 310°C for 10 h. Polymers VI, VII, VIII , and IX were obtained. The thermal polymerization was monitored using infrared spectroscopy. Thermal polymerization was also carried out in the presence of an aromatic diamine. A polyhexasocyclane ( V ) was synthesized by condensation polymerization of ether containing bisimido-bisphthalonitrile with 4,4′-diaminodiphenyl ether in solvent phenol. The synthesized polymers were evaluated for thermal stability using dynamic thermogravimetric analysis (TGA). Polymers VII, VIII, IX , and X showed thermal decomposition temperature in the range of 475–500°C in nitrogen and air atmosphere. The char yield of the polymers was in the range of 60–69% in nitrogen at 800°C. This study indicated that synthesized thermosetting polymers from ether and keto containing bisimido-bisphthalo-nitrile are potential candidates for development of graphite composites. © 1993 John Wiley & Sons, Inc. 相似文献
96.
Sanjib Kumar Saha Ajit Kumar Chaudhuri 《Journal of polymer science. Part A, Polymer chemistry》1987,25(2):519-532
The polymerization of methyl methacrylate was studied in carbon tetrachloride medium with ferric laurate, a metal soap, in combination with n-hexyl amine as the initiator system at 60°C. The rate of polymerization was found to be linear with the monomer concentration and proportional to the square root of both ferric ion and amine concentration. A reaction scheme involving initial complex formation between ferric ion and amine and subsequent reaction of the complex with the solvent molecule to produce free radicals responsible for initiation of polymerization has been postulated to account for the observed results. 相似文献
97.
Amino Acid based cationic surfactants in aqueous solution: physicochemical study and application of supramolecular chirality in ketone reduction 总被引:2,自引:0,他引:2
Roy S Das D Dasgupta A Mitra RN Das PK 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10398-10404
The present study provides a molecular understanding of the origin of the chirality in aqueous micelles and its correlation with the proficiency of stereoselective ketone reduction. The effects of varied headgroup architecture on the surface-active properties as well as on other microstructural parameters were studied and correlated to the structural differences of these naturally occurring amino acid containing surfactants (1-4). Micropolarity sensed by pyrene showed that the micelles prepared using 1-4 are mostly hydrated; particularly large headgroup size surfactant produces more polar environment. A theoretical study was done to quantify the varied spatial dissymmetry for all four surfactants. Asymmetric reduction of prochiral ketones was carried out at the aqueous micellar interface of these chiral amphiphiles by exploiting the supramolecular chirality as evidenced from a circular dichroism study. The enantioselectivity of the reduction process is rationally improved through increase in spatial dissymmetry and steric constraint imposed at the micellar interface by the polar head of surfactants. 相似文献
98.
Benzoxazine-bismaleimide blends: Curing and thermal properties 总被引:4,自引:0,他引:4
A blend of bisphenol A based benzoxazine (Bz-A) and a bismaleimide (2,2-bis[4(4-maleimidophenoxy) phenyl] propane (BMI), was thermally polymerised in varying proportions and their cure and thermal characteristics were investigated. The differential scanning calorimetric analysis, supplemented by rheology confirmed a lowering of the cure temperature of BMI in the blend implying catalysis of the maleimide polymerisation by benzoxazine. FTIR studies provided evidences for the H-bonding between carbonyl group of BMI and -OH group of polybenzoxazine in the cured matrix. The cured matrix manifested a dual phase behaviour in SEM and DMTA with the minor phase constituted by polybenzoxazine dispersed in an interpenetrating polymer network (IPN) of polybenzoxazine and cured BMI. The IPN possessed improved thermal stability over the constituent polybenzoxazine. A benzoxazine monomer possessing allyl functional groups, 2,2′-bis(8-allyl-3-phenyl-3,4-dihydro-2H-1,3-benzoxazinyl) propane (Bz-allyl) was reactively blended with the same bismaleimide in varying stoichiometric ratios (Bz-allyl/BMI), where the curing involved mainly Alder-ene reaction between allyl- and maleimides groups and ring-opening polymerisation of benzoxazine. The rheological analysis showed the absence of catalytic polymerisation of BMI in this case. The overall processing temperature was lowered in the blend owing to the co-reaction of the two systems to form a single-phase matrix. The cured resins of both Bz-A/BMI and Bz-allyl/BMI blends exhibited better thermal stability than the respective polybenzoxazines. The Tg of the IPN was significantly improved over that of polybenzoxazine (Bz-A). However, the co-reaction resulted in a marginal decrease in the Tg of the system in comparison to the polybenzoxazine (Bz-allyl). 相似文献
99.
Chen Luo Chenggang Zhou Jinping Wu T. J. Dhilip Kumar Naduvalath Balakrishnan Robert C. Forrey Hansong Cheng 《International journal of quantum chemistry》2007,107(7):1632-1641
Structures and physical properties of small palladium clusters Pdn up to n = 15 and several selected larger clusters were studied using density functional theory under the generalized gradient approximation. It was found that small Pdn clusters begin to grow 3‐dimensionally at n = 4 and evolve into symmetric geometric configurations, such as icosahedral and fcc‐like, near n = 15. Several isomers with nearly degenerate average binding energies were found to coexist and the physical properties of these clusters were calculated. For several selected isomers, relatively moderate energy barriers for structural interchange for a given cluster size were found, implying that isomerization could readily occur under ambient conditions. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
100.
The chemical synthesis and NMR characterization of the benzo ring tetrahydro- and diol epoxides of the carcinogen benz[c]acridine are described. 相似文献