全文获取类型
收费全文 | 733篇 |
免费 | 25篇 |
国内免费 | 4篇 |
专业分类
化学 | 363篇 |
晶体学 | 21篇 |
力学 | 25篇 |
数学 | 69篇 |
物理学 | 284篇 |
出版年
2023年 | 8篇 |
2022年 | 15篇 |
2021年 | 16篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 18篇 |
2017年 | 10篇 |
2016年 | 23篇 |
2015年 | 25篇 |
2014年 | 24篇 |
2013年 | 66篇 |
2012年 | 50篇 |
2011年 | 48篇 |
2010年 | 40篇 |
2009年 | 37篇 |
2008年 | 46篇 |
2007年 | 36篇 |
2006年 | 33篇 |
2005年 | 35篇 |
2004年 | 24篇 |
2003年 | 16篇 |
2002年 | 14篇 |
2001年 | 5篇 |
2000年 | 12篇 |
1999年 | 10篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1978年 | 2篇 |
1976年 | 7篇 |
1975年 | 3篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1967年 | 2篇 |
1937年 | 2篇 |
1931年 | 1篇 |
1930年 | 10篇 |
1929年 | 4篇 |
1906年 | 2篇 |
排序方式: 共有762条查询结果,搜索用时 0 毫秒
31.
Ajeet Kumar Sharma Parul Bansal Gaurav Kumar Nim Prasenjit Kar 《Particle & Particle Systems Characterization》2019,36(12)
This work reports the first synthesis of MAPbBr3 perovskite nanocrystals (PNCs) using amino acids as the ligand with excellent optical properties. A variety of amino acids are used to optimize the luminescence properties. A mechanochemical approach has taken lead over conventional colloidal chemistry during synthesis. All morphological and optical studies are performed to characterize the synthesized perovskite nanoparticles. Later, stability studies are investigated through thermogravimetric analysis, temperature‐dependent photoluminescence, time‐dependent X‐ray diffraction, as well as X‐ray photoelectron spectroscopy. In an application, interestingly, these perovskites show high luminescence upon scratching on flexible conducting plates and on plain paper surface. These results suggest that the amino acid–ligated perovskite nanocrystals can be potential materials for optoelectronic application including light‐emitting diodes and imaging. 相似文献
32.
The numerical solution of linear elliptic partial differential equations most often involves a finite element or finite difference
discretization. To preserve sparsity, the arising system is normally solved using an iterative solution method, commonly a
preconditioned conjugate gradient method. Preconditioning is a crucial part of such a solution process. In order to enable
the solution of very large-scale systems, it is desirable that the total computational cost will be of optimal order, i.e.
proportional to the degrees of freedom of the approximation used, which also induces mesh independent convergence of the iteration.
This paper surveys the equivalent operator approach, which has proven to provide an efficient general framework to construct
such preconditioners. Hereby one first approximates the given differential operator by some simpler differential operator,
and then chooses as preconditioner the discretization of this operator for the same mesh. In this survey we give a uniform
presentation of this approach, including theoretical foundation and several practically important applications for both symmetric
and nonsymmetric equations and systems, and some nonlinear examples in the context of Newton linearization.
Dedicated to the memory of Gene Golub for his friendly manner and for his broad interest and significant impact on numerical
analysis. 相似文献
33.
A brief survey is given to show that harmonic averages enter in a natural way in the numerical solution of various variable coefficient problems, such as in elliptic and transport equations, also of singular perturbation types. Local Green’s functions used as test functions in the Petrov-Galerkin finite element method combined with harmonic averages can be very efficient and are related to exact difference schemes. 相似文献
34.
Indrani Mitra Partha Pratim Roy Supratik Kar Probir Kumar Ojha Kunal Roy 《Journal of Chemometrics》2010,24(1):22-33
Validation is a crucial aspect for quantitative structure–activity relationship (QSAR) model development. External validation is considered, in general, as the most conclusive proof of predictive capacity of a QSAR model. In the absence of truly external data set, external validation is usually performed on test set compounds, which are members of the original data set but not used in model development exercise. In the case of small data sets, QSAR researchers experience problem in model development due to the fact that the developed models may be less reliable on account of the small number of training set compounds and such models may also show poor external predictability because the models may not have captured all necessary features required for the particular structure–activity relationships. The present paper attempts to show that ‘true r(LOO)’ statistic calculated based on the model derived from the undivided data set with application of variable selection strategy at each cycle of leave‐one‐out (LOO) validation may reflect external validation characteristics of the developed model thus obviating the requirement of splitting of the data set into training and test sets. This approach may be helpful in the case of small data sets as it uses all available data for model development and validation thus making the resulting model more reliable. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
35.
The condensation of 3-amino-1H-1,2,4-triazole with benzaldehyde and terephthalaldehyde provides the bidentate and tetradentate Schiff bases 1,2,4-triazolo-3-imino-benzene L1H and 1,4-bis(1,2,4-triazolo-3-imino)benzene L2H2, respectively. The well characterized Schiff bases were allowed to react with cis-Ru(bpy)2Cl2 · 2H2O. Isomers of the mononuclear complexes Ru(bpy)2L1]PF6 · NH4PF6 (1a, N4) and [Ru(bpy)2L1]PF6 · 0.5NH4PF6 (1b, N2), and the dinuclear Ru(II) complexes [Ru(bpy)2L2Ru(bpy)2](PF6)2 · NH4PF6 (2a, N4N4), [Ru(bpy)2L2Ru(bpy)2](PF6)2 · NH4PF6 · 2H2O (2b, N2N2) and [Ru(bpy)2L2Ru(bpy)2](PF6)3 · NH4PF6 (2c, Ru(II)-Ru(III)) were separated by column chromatography and characterized by their elemental analysis, FAB mass and spectral (IR, NMR, UV–Vis) data. The data obtained suggest that the ligands are bound to the metal centre via the N4 and N2 atoms of the triazole moiety along with the N (imine) atom. The complexes display metal-to-ligand charge-transfer (MLCT) transitions in the visible region from the dπ(RuII) → π∗L transition. Highly intense ligand-based π→π∗ transitions are observed in the UV region. A dual emission occurs from the N2 and N2N2 isomers. 相似文献
36.
Anirban Kar 《Tetrahedron》2005,61(22):5297-5302
Starting from citraconic anhydride (13), a simple multistep (9-10 steps) synthesis of naturally occurring butyrolactones maculalactone A (3), maculalactone B (1), maculalactone C (2) and nostoclide I (4) have been described with good overall yields via dibenzylmaleic anhydride (20) and benzylisopropylmaleic anhydride (27). The two anhydrides 20 and 27 were prepared by SN2′ coupling reactions of appropriate Grignard reagents with dimethyl bromomethylfumarate (14), LiOH-induced hydrolysis of esters to acids, bromination of carbon-carbon double bond, in situ dehydration followed by dehydrobromination and chemoselective allylic substitution of bromoatom in disubstituted anhydrides 19 and 26 with appropriate Grignard reagents. The NaBH4 reduction of these anhydrides 20 and 27 furnished the desired lactones 21 and 29, respectively. The lactone 21 on Knoevenagel condensation with benzaldehyde, furnished maculalactone B (1), which on isomerization gave maculalactone C (2). Selective catalytic hydrogenation of 1 gave maculalactone A (3). The conversion of lactone 29 to nostoclide I (4) is known. 相似文献
37.
A simple, rapid, and solvent-free method for quantitative determination of benzene, toluene, and Xylene in exposed Drosophila larvae was developed using headspace solid-phase microextraction (HS-SPME) coupled to GC/MS. Larvae fed on standard Drosophila food mixed with benzene, toluene, and Xylene for 48 h were homogenized in Milli-Q water. Extraction of benzene, toluene, and Xylene was performed at 65 degrees C for 30 min on the SPME fiber (silica-fused). Subsequently, the fiber was desorbed in the GC injection port, followed by GC/MS analysis in the selected-ion monitoring mode. An external calibration curve was used for the quantification of benzene, toluene, and Xylene in the exposed organism. Recoveries were in the range of 78-82% (intraday) and 76-81% (interday) in larvae, and 91-96% (intraday) and 87-92% (interday) in the diet. LOD with an S/N of 3:1 and LOQ with an S/N of 10:1 were in the range of 0.01-0.023 and 0.034-0.077 microg/L, respectively. Percent RSD values for benzene, toluene, and Xylene were in the range of 0.50-0.81 (intraday) and 0.89-1.23 (interday) for retention time, and 2.16--3.85 (intraday) and 2.99-4.95 (interday) for peak concentration, showing good repeatability. This method was sensitive enough to quantitate benzene, toluene, and Xylene in small exposed organisms like Drosophila larvae. The SPME/GC/MS method developed may have wider applications in various in vivo toxicological studies. 相似文献
38.
Three new Mn(II) coordination compounds {[Mn(NCNCN)(2)(azpy)]·0.5azpy}(n) (1), {[Mn(NCS)(2)(azpy)(CH(3)OH)(2)]·azpy}(n) (2), and [Mn(azpy)(2)(H(2)O)(4)][Mn(azpy)(H(2)O)(5)]·4PF(6)·H(2)O·5.5azpy (3) (where azpy = 4,4'-azobis(pyridine)) have been synthesized by self-assembly of the primary ligands, dicyanamide, thiocyanate, and hexafluorophosphate, respectively, together with azpy as the secondary spacer. All three complexes were characterized by elemental analyses, IR spectroscopy, thermal analyses, and single crystal X-ray crystallography. The structural analyses reveal that complex 1 forms a two-dimensional (2D) grid sheet motif. These sheets assemble to form a microporous framework that incorporates coordination-free azpy by host-guest π···π and C-H···N hydrogen bonding interactions. Complex 2 features azpy bridged one-dimensional (1D) chains of centrosymmetric [Mn(NCS)(2)(CH (3)OH)(2)] units which form a 2D porous sheet via a CH(3)···π supramolecular interaction. A guest azpy molecule is incorporated within the pores by strong H-bonding interactions. Complex 3 affords a 0-D motif with two monomeric Mn(II) units in the asymmetric unit. There exist π···π, anion···π, and strong hydrogen bonding interactions between the azpy, water, and the anions. Density functional theory (DFT) calculations, at the M06/6-31+G* level of theory, are used to characterize a great variety of interactions that explicitly show the importance of host-guest supramolecular interactions for the stabilization of coordination compounds and creation of the fascinating three-dimensional (3D) architecture of the title compounds. 相似文献
39.
Shubha Pandey Jyotsna R. Kar Amir Azam Siddharth Pandey H. M. Chawla 《Journal of solution chemistry》2010,39(1):107-120
Long chain calix[4]arene ethers have been examined for aggregation in nonaqueous solvents by using UV-vis molecular absorbance
spectroscopy. It has been observed that tetraalkylated (alkyl = hexadecyl and octadecyl, respectively) calix[4]arene ethers
tend to aggregate in chloroform and tetrahydrofuran, possibly via π–π stacking interactions of the phenyl moieties, and the aggregation process appears to be facilitated by the alkyl chains.
The analogous dialkylated compounds do not show any self-aggregation, plausibly due to strong hydrogen bonding between the
–OH and the –O– of calix aryl ether which seems to disrupt the aggregation process. Addition of the anionic surfactant sodium
dodecylsulfate (SDS) appears to hinder the aggregation process in nonpolar chloroform but the same surfactant facilitates
aggregation in the polar tetrahydrofuran. The cationic surfactant (cetyltrimethyl ammonium bromide) and the nonionic surfactant
(Brij-35) have no effect on this aggregation process. Unexpectedly, SDS induces aggregation of dialkylated calix[4]arene ethers
in chloroform. It has been observed that the aggregated form of the tetraalkylated calix[4]arene ethers tend to increase the
dimerization efficiency of cationic dyes (pinacyanol chloride and methylene blue) in chloroform. 相似文献
40.
Ranjit Bag Sourav Kar Suvam Saha Suman Gomosta Beesam Raghavendra Thierry Roisnel Sundargopal Ghosh 《化学:亚洲杂志》2020,15(6):780-786
Triply‐bridging bis‐{hydrido(borylene)} and bis‐borylene species of groups 6, 8 and 9 transition metals are reported. Mild thermolysis of [Fe2(CO)9] with an in situ produced intermediate, generated from the low‐temperature reaction of [Cp*WCl4] (Cp*=η5‐C5Me5) and [LiBH4?THF] afforded triply‐bridging bis‐{hydrido(borylene)}, [(μ3‐BH)2H2{Cp*W(CO)2}2{Fe(CO)2}] ( 1 ) and bis‐borylene, [(μ3‐BH)2{Cp*W(CO)2}2{Fe(CO)3}] ( 2 ). The chemical bonding analyses of 1 show that the B?H interactions in bis‐{hydrido (borylene)} species is stronger as compared to the M?H ones. Frontier molecular orbital analysis shows a significantly larger energy gap between the HOMO‐LUMO for 2 as compared to 1 . In an attempt to synthesize the ruthenium analogue of 1 , a similar reaction has been performed with [Ru3(CO)12]. Although we failed to get the bis‐{hydrido(borylene)} species, the reaction afforded triply‐bridging bis‐borylene species [(μ3‐BH)2{WCp*(CO)2}2{Ru(CO)3}] ( 2′ ), an analogue of 2 . In search for the isolation of bridging bis‐borylene species of Rh, we have treated [Co2(CO)8] with nido‐[(RhCp*)2(B3H7)], which afforded triply‐bridging bis‐borylene species [(μ3‐BH)2(RhCp*)2Co2(CO)4(μ‐CO)] ( 3 ). All the compounds have been characterized by means of single‐crystal X‐ray diffraction study; 1H, 11B, 13C NMR spectroscopy; IR spectroscopy and mass spectrometry. 相似文献