全文获取类型
收费全文 | 70863篇 |
免费 | 1281篇 |
国内免费 | 305篇 |
专业分类
化学 | 36172篇 |
晶体学 | 1095篇 |
力学 | 3399篇 |
综合类 | 8篇 |
数学 | 6339篇 |
物理学 | 25436篇 |
出版年
2022年 | 595篇 |
2021年 | 589篇 |
2020年 | 637篇 |
2019年 | 625篇 |
2018年 | 693篇 |
2017年 | 620篇 |
2016年 | 1306篇 |
2015年 | 1068篇 |
2014年 | 1451篇 |
2013年 | 3372篇 |
2012年 | 3311篇 |
2011年 | 3883篇 |
2010年 | 2648篇 |
2009年 | 2572篇 |
2008年 | 3572篇 |
2007年 | 3365篇 |
2006年 | 3231篇 |
2005年 | 2874篇 |
2004年 | 2547篇 |
2003年 | 2229篇 |
2002年 | 2072篇 |
2001年 | 3176篇 |
2000年 | 2313篇 |
1999年 | 1648篇 |
1998年 | 1155篇 |
1997年 | 1135篇 |
1996年 | 995篇 |
1995年 | 855篇 |
1994年 | 765篇 |
1993年 | 705篇 |
1992年 | 1016篇 |
1991年 | 994篇 |
1990年 | 896篇 |
1989年 | 787篇 |
1988年 | 769篇 |
1987年 | 833篇 |
1986年 | 712篇 |
1985年 | 965篇 |
1984年 | 895篇 |
1983年 | 604篇 |
1982年 | 618篇 |
1981年 | 590篇 |
1980年 | 557篇 |
1979年 | 668篇 |
1978年 | 702篇 |
1977年 | 699篇 |
1976年 | 623篇 |
1975年 | 509篇 |
1974年 | 549篇 |
1973年 | 476篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
994.
4-Monodeuterated NAD(P)H model compounds (1,4,6,7-tetrahydro-1,6,11-trimethyl-5-oxo-5H-benzo[c]pyrido[2,3-e]az epin; 11Me-MMPAH) have been oxidized with a series of p-benzoquinone and its derivatives in the presence of Mg2+. The models have an axial chirality with respect to the orientation of carbonyl dipole, the dihedral angle of which is larger than 55 degrees out of the plane of dihydropyridine ring. Without Mg2+, the anti- (with respect to the carbonyl dipole) hydrogen is 3 to 32 times more reactive than the corresponding syn-hydrogen, whereas, when Mg2+ is present in the system, the selectivity is shifted toward the syn-preferency. Mg2+ plays the role of a Lewis acid catalyst to control the stereochemistry at the same time as it catalyzes the reaction. 相似文献
995.
Diamond-like carbon (DLC) and tetrahedral amorphous carbon (ta-C) have attracted much attention recently for biomedical and antifouling applications due to their excellent biocompatibility and inherent nonstick properties. It has been demonstrated that the solid surface free energy is a dominant factor in cellular or fouling adhesion. However, few data for the surface free energy of DLC and ta-C coatings at temperatures in the range 37-95 degrees C are available. In this study DLC and ta-C coatings on stainless steel 304 sheets were prepared using an unbalanced magnetron sputtering system and a filtered cathodic vacuum arc system, respectively. The contact angles of water, diiodomethane and ethylene glycol on the coated surfaces at temperatures in the range 20-95 degrees C were measured using a Dataphysics OCA-20 contact angle analyzer. The surface free energy of the coatings and their components (e.g., dispersion, polar or acid/base portions) were calculated using various methods. The experimental results showed that the total surface free energy and dispersive surface free energy of the ta-C coatings, DLC coatings, stainless steel 304 and titanium decreased with increasing surface temperature, while the acid-base SFE component increased with increasing temperature. 相似文献
996.
Burton A Elomari S Chen CY Medrud RC Chan IY Bull LM Kibby C Harris TV Zones SI Vittoratos ES 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(23):5737-5748
The syntheses, structure solutions, and physicochemical and catalytic characterizations of the novel zeolites SSZ-53 and SSZ-59 are described. SSZ-53 and SSZ-59 were synthesized under hydrothermal conditions with the [1-(4-fluorophenyl)cyclopentylmethyl]trimethyl ammonium cation and 1-[1-(4-chlorophenyl)cyclopentylmethyl]-1-methyl azocanium cation, respectively, as structure-directing agents. The framework topology of SSZ-53 was solved with the FOCUS method, and the structure of SSZ-59 was determined by model building. Rietveld refinement of synchrotron X-ray powder diffraction data confirms each proposed model. SSZ-53 and SSZ-59 each possess a one-dimensional channel system delimited by 14-membered rings. Results from transmission electron microscopy, electron diffraction, catalytic experiments (spaciousness index and constraint index tests), and argon and hydrocarbon adsorption experiments are consistent with the proposed structures. 相似文献
997.
The equilibrium geometrical structures of small AlmFen clusters have been determined through ab initio calculation of the cluster total energy at the UB3LYP/Lanl2dz level. For dimers of iron and aluminum, the dissociation energies,
the equilibrium atomic distances, and the vibrational frequencies were calculated. The agreement between calculations and
experiments is reasonable. The ground stable geometrical structures of Fe4, FeAl3, Fe3Al and Fe2Al2 clusters favor three-dimension configurations, but Al4 tetramers are planar structures. The Al-rich tetramers are more stable than the other two composition tetramers. This is
different from that of bulk alloys. 相似文献
998.
Some chemical transformations in the series of the pyrrolo [3,2-b]azepines and the pyrrolo[3,2-c]azepines are reported: alkylation at the pyrrolo nitrogen and/or the azepino nitrogen, reduction of the lactam group by means of LiAlH4, preparation of lactim ethers and amidines, substitution in position 2 by MANNICH reaction, bromination and condensation with oxalyl chloride or chloral hydrate, and the cyclocondensation with dimethylmalonyl chloride. The structures of the 2-substitution products have been evaluated by NMR.-spectroscopy. 相似文献
999.
Casas JS Castaño MV García-Tasende MS Rodríguez-Castellón E Sánchez A Sanjuán LM Sordo J 《Dalton transactions (Cambridge, England : 2003)》2004,(13):2019-2026
Zinc(II) and cadmium(II) acetates were reacted in methanol under various experimental conditions with thiosemicarbazones derived from beta-keto amides or beta-keto esters (HTSC). Some of these reactions afforded thiosemicarbazonate complexes [M(TSC)2] with IR and NMR spectra compatible with N,S-coordination, but most gave complexes [ML2], where HL is a substituted 2,5-dihydro-5-oxo-1H-pyrazole-1-carbothioamide resulting from cyclization of the HTSC. Some of these pyrazolonates and two of the HL ligands were studied by X-ray diffractometry, and their structures are discussed. Surprisingly, the reactions of zinc(II) acetate with HTSC in 1:1 mol ratio usually gave a third, previously unreported type of complex with a dideprotonated ligand, [Zn(L-H)], which was also formed when [ZnL2] and Zn(OAc)2 interacted at room temperature in 1:1 mol ratio. These L-H complexes are highly insoluble in all common solvents, which hinders their characterization but suggests that they are polymeric in nature. 相似文献
1000.
A software-controlled flow-through optical fiber diffuse reflectance sensor capitalized on the implementation of disk-based solid-phase pre-concentration schemes in a multisyringe flow injection analysis (MSFIA) set-up is proposed for the trace determination of sulfide in environmental waters and wastewaters. The fully automated flowing methodology is based on Fischer's coupling reaction of sulfide with N,N-dimethyl-p-phenylenediamine (DMPD) in the presence of Fe(iii) as oxidizing reagent in a 0.5 M HCl medium. The on-line generated methylene blue dye is subsequently delivered downstream to a dedicated optode cell furnished with an octadecyl-chemically modified (C(18)) disk, while continuously recording the diffuse reflectance spectrum of the pre-concentrated compound. A double regeneration protocol is finally executed to warrant minimum background noise and negligible baseline. Under the optimized chemical and hydrodynamic conditions, the optosensing MSFIA method features coefficients of variation better than 0.7%(n= 10) at 50 microg l(-1) concentration, a linear working range of 20-200 microg l(-1) sulfide, a 3sigma(blank) detection limit of 2.9 microg l(-1) sulfide and an injection throughput of 8 h(-1) for a pre-concentration sample volume of 2.9 ml. The interfacing of the robust and versatile multisyringe flow injection-based optode with a plug-in spectrophotometer furnished with a light emitting diode assures the miniaturization of the overall flow analyzer, which is, thus, readily adaptable to real-time monitoring schemes. The potential of the multisyringe flow method was assessed via the determination of sulfide traces in water samples of different complexity (namely, freshwater, seawater and wastewater). 相似文献