全文获取类型
收费全文 | 4391篇 |
免费 | 140篇 |
国内免费 | 12篇 |
专业分类
化学 | 2825篇 |
晶体学 | 55篇 |
力学 | 125篇 |
数学 | 438篇 |
物理学 | 1100篇 |
出版年
2023年 | 40篇 |
2022年 | 73篇 |
2021年 | 75篇 |
2020年 | 106篇 |
2019年 | 104篇 |
2018年 | 105篇 |
2017年 | 89篇 |
2016年 | 144篇 |
2015年 | 124篇 |
2014年 | 157篇 |
2013年 | 312篇 |
2012年 | 288篇 |
2011年 | 320篇 |
2010年 | 213篇 |
2009年 | 197篇 |
2008年 | 278篇 |
2007年 | 244篇 |
2006年 | 210篇 |
2005年 | 189篇 |
2004年 | 121篇 |
2003年 | 105篇 |
2002年 | 87篇 |
2001年 | 38篇 |
2000年 | 48篇 |
1999年 | 39篇 |
1998年 | 35篇 |
1997年 | 27篇 |
1996年 | 26篇 |
1995年 | 35篇 |
1994年 | 35篇 |
1993年 | 24篇 |
1992年 | 35篇 |
1991年 | 38篇 |
1990年 | 23篇 |
1989年 | 23篇 |
1988年 | 25篇 |
1987年 | 26篇 |
1985年 | 34篇 |
1984年 | 33篇 |
1983年 | 29篇 |
1982年 | 31篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 21篇 |
1978年 | 29篇 |
1977年 | 29篇 |
1976年 | 38篇 |
1975年 | 23篇 |
1974年 | 19篇 |
1973年 | 19篇 |
排序方式: 共有4543条查询结果,搜索用时 562 毫秒
71.
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
72.
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
73.
Photoirradiation at >300 nm of aqueous suspensions of several natural crystal specimens and synthesized crystallites of mercury(II) sulfide (HgS) induced deaminocyclization of optically active or racemic lysine into pipecolinic acid (PCA) under deaerated conditions. This is the first example, to the best of our knowledge, of photoinduced chemical reactions of natural biological compounds over natural minerals. It was found that the natural HgS crystals had activity higher than those of synthesized ones but lower than those of other sulfides of transition metals, e.g., CdS and ZnS, belonging to the same II-IV chalcogenides. In almost all of the photoreactions, decompostion of HgS occurred to liberate hydrogen sulfide (H(2)S) and Hg(2+), and the latter seemed to have undergone in-situ reductive deposition on HgS as Hg(0) after a certain induction period (24-70 h) during the photoirradiation, as indicated by the darkened color of the suspensions. The formation of PCA, presumably through combination of oxidation of lysine and reduction of an intermediate, cyclic Schiff base, could also be seen after a certain induction time of the Hg(0) formation. This was supported by the fact that the addition of small amount of Hg(2+) (0.5 wt % of HgS) increased the PCA yield by almost 2-fold. We also tried to elucidate certain aspects of the plausible stereochemical reactions in relation to the chiral crystal structure of HgS. Although, in some experiments, slight enantiomeric excess of the product PCA was observed, the excess was below or equal to the experimental error and no other supporting analytical data could not be obtained; we cannot conclude the enantiomeric photoproduction of PCA by the natural chiral HgS specimen. 相似文献
74.
Anju Midha Rajender D. Verma K. Brown R. V. Parish 《Monatshefte für Chemie / Chemical Monthly》1983,114(6-7):661-667
Mixed dialkytin(IV) trifluoroacetates,Me EtSn(O2CCF3)2,Et Pr
n
Sn(O2CCF3)2 andPr
n
Bu
n
Sn(O2CCF3)2 have been prepared by metathetical reactions of the corresponding dialkyltin(IV) chlorides with silver trifluoroacetate in CH2Cl2. They are monomeric in benzene and nonconducting inMeNO2 andMeCN. Bidentate trifluoroacetate groups are indicated by their IR spectra.Mössbauer spectra confirmtrans-arrangement of theR-Sn-R moiety.1H,19F NMR and mass spectra are also discussed.
Gemischte Dialkylzinn(IV)-trifluoracetate
Zusammenfassung Die gemischten Dialkylzinn(IV)-trifluoroacetateMe EtSn(O2CCF3)2,Et Pr n Sn(O2CCF3)2 undPr n Bu n Sn(O2CCF3)2 wurden über Metathese-Reaktionen der entsprechenden Dialkylzinn(IV)-chloride mit Silbertrifluoracetat in CH2Cl2 dargestellt. Sie sind monomer in Benzol und nichtleitend inMeNO2 undMeCN. Die IR-Spektren zeigen zweizähnige Trifluoracetat-Gruppen an. DieMössbauer-Spektren bestätigen dietrans-Anordnung derR-Sn-R-Einheit. Die1H-,19F-NMR und die Massenspektren werden ebenfalls diskutiert.相似文献
75.
An investigation of the properties of poly(dimethylsiloxane)-bioinspired silica hybrids 总被引:1,自引:0,他引:1
Siddharth V. Patwardhan Vijay P. Taori Nikhil R. Agashe Gregory Beaucage Stephen J. Clarson 《European Polymer Journal》2006,42(1):167-178
Elastomers typically require the incorporation of reinforcing fillers in order to improve their mechanical properties. For commercial silicone systems silica and titania are typically used as fillers. Fumed and precipitated silica are made on an industrial scale for many applications; however, we have shown recently that biological and synthetic macromolecules can generate new silica structures using a bioinspired route. Herein we have incorporated bioinspired silica fillers into poly(dimethylsiloxane) (PDMS) elastomers and investigated their mechanical, morphological and thermal properties as a function of filler loading. The equilibrium stress-strain characteristics of the PDMS-bioinspired silica hybrids were determined as a function of bioinspired filler loading and the Mooney-Rivlin constants (2C1 and 2C2) were calculated. The thermal characteristics, in particular glass transition temperatures (Tg) and melting points (Tm), of the PDMS-bioinspired silica hybrids were characterized using differential scanning calorimetry (DSC). The thermal stability of these hybrid materials were investigated using thermogravimetric analysis (TGA). The morphology of the samples was characterized using scanning electron microscopy (SEM), and the filler dispersion was characterized using ultra small angle X-ray scattering (USAXS) and scanning electron microscopy (SEM). Although spherical silica particles were used here, we have demonstrated elsewhere that this bioinspired synthetic route also enables highly asymmetric silica structures to be prepared such as fibres and sheets. This methodology therefore offers the interesting possibility of preparing new hybrid systems where the properties are highly anisotropic. 相似文献
76.
Solubilities and the solvolytic behaviour of various inorganic compounds, Lewis acids and bases in fused monobromoacetic acid at 60 ± 0.5°C are discussed. Ionic compounds are fairly soluble, iodides and thiocyanates being comparatively more soluble than chlorides and bromides. Tetraalkyl ammonium halides are highly soluble in this solvent. Conductometric and spectroscopic studies of various Lewis acids and bases in fused monobromoacetic acid indicate their solvolytic behaviour and their subsequent ionization. The solvolyses products BBr3·CH2BrCOOH and SbCl5·CH2BrCOOH have been observed to be the strongest Bronsted acids. Auto-ionization of this solvent has been supported by acid/base titrations. 相似文献
77.
Tarducci C Badyal JP Brewer SA Willis C 《Chemical communications (Cambridge, England)》2005,(3):406-408
A substrate-independent method for Diels-Alder chemistry at solid surfaces is described for the first time. 相似文献
78.
Potential applications of nanoporous thermosetting polymers include polyelectrolytes in fuel cells, separation membranes, adsorption media, and sensors. Design of nanoporous polymers for such applications entails controlling permeability by tailoring pore size, structure, and interface chemistry. Nanoporous thermosetting polymers are often synthesized via free radical mechanisms using solvents that phase separate during polymerization. In this work, a novel technique for the synthesis of nanoporous thermosets is presented that is based on the reactive encapsulation of an inert solvent using step-growth cross-linking polymerization without micro/macroscopic phase separation. The criteria for selecting such a monomer-polymer-solvent system are discussed based on FTIR analysis, observed micro/macroscopic phase separation, and thermodynamics of swelling. Investigation of resulting network pore structures by scanning electron microscopy (SEM) and small-angle X-ray scattering following extraction and supercritical drying using carbon dioxide showed that nanoporous polymeric materials with pore sizes ranging from 1 to 50 nm can be synthesized by varying the solvent content. The differences in the porous morphology of these materials compared to more common free radically polymerized analogues that exhibit phase separation were evident from SEM imaging. Furthermore, it was demonstrated that the chemical activity of the nanoporous materials obtained by our method could be tailored by grafting appropriate functional groups at the pore interface. 相似文献
79.
An efficient synthesis of [1,3]oxazino[2,3-a]quinoline derivatives via a three-component reaction of quinoline, DMAD and carbonyl compounds is described. 相似文献
80.
O. M. Maikut R. G. Makitra E. Ya. Pal’chikova 《Russian Journal of General Chemistry》2007,77(12):2198-2202
The solubility of organic liquids in water and of water in organic liquids is primarily determined by the capability of organic liquids for hydrogen bonding. Adequate generalization of the solubility data is possible only with multiparameter equations taking also into account other properties of organic components, including the cohesion and molar volume. 相似文献