全文获取类型
收费全文 | 3575篇 |
免费 | 218篇 |
国内免费 | 14篇 |
专业分类
化学 | 2702篇 |
晶体学 | 16篇 |
力学 | 76篇 |
数学 | 475篇 |
物理学 | 538篇 |
出版年
2023年 | 50篇 |
2022年 | 41篇 |
2021年 | 65篇 |
2020年 | 136篇 |
2019年 | 146篇 |
2018年 | 64篇 |
2017年 | 42篇 |
2016年 | 148篇 |
2015年 | 143篇 |
2014年 | 132篇 |
2013年 | 187篇 |
2012年 | 289篇 |
2011年 | 294篇 |
2010年 | 157篇 |
2009年 | 109篇 |
2008年 | 219篇 |
2007年 | 166篇 |
2006年 | 153篇 |
2005年 | 178篇 |
2004年 | 119篇 |
2003年 | 93篇 |
2002年 | 79篇 |
2001年 | 70篇 |
2000年 | 56篇 |
1999年 | 47篇 |
1998年 | 44篇 |
1997年 | 34篇 |
1996年 | 50篇 |
1995年 | 47篇 |
1994年 | 34篇 |
1993年 | 29篇 |
1992年 | 33篇 |
1991年 | 29篇 |
1990年 | 13篇 |
1989年 | 21篇 |
1988年 | 23篇 |
1987年 | 19篇 |
1986年 | 27篇 |
1985年 | 24篇 |
1984年 | 18篇 |
1983年 | 15篇 |
1982年 | 13篇 |
1981年 | 14篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1977年 | 15篇 |
1976年 | 7篇 |
1975年 | 14篇 |
1974年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有3807条查询结果,搜索用时 187 毫秒
11.
12.
By reacting platinum with alkali metals (A = K, Rb, Cs) a new family of binary alkali metal platinides has been synthesized and characterized by chemical analysis, X‐ray powder diffraction, thermal analysis (DTA and DSC), and magnetic measurements. All three compounds exhibit similar XRD‐patterns with strong reflections that can be indexed on the basis of a rhombohedral crystal system (KxPt: a = 2.6462(1), c = 17.123(1); RbxPt: a = 2.6415(1) Å, c = 17.871(1) Å; CsxPt: a = 2.6505(1) Å, c = 18.536(1) Å; x < ½. The a lattice constant is independent on the alkali metal used and of value close to the Pt–Pt distance in NaPt2 (2.645Å). The c parameter increases monotonically with the growing atomic radius of the alkali metal. The average structure of the alloys consists of cubic close packed layers of platinum atoms with layers of disordered alkali metals in between. For all compounds besides the strong reflections small satellites are observed which cannot be indexed together with the rhombohedral peaks in any rational 3‐dimensional lattice. However, these satellites can be indexed as incommensurate modulations within the ab plane (found propagation vectors k = (0.1011, 0.2506, 0) for CsxPt, and k = (0.0168, 0.2785, 0) for RbxPt). 相似文献
13.
Thaler F Valsasina B Baldi R Xie J Stewart A Isacchi A Kalisz HM Rusconi L 《Analytical and bioanalytical chemistry》2003,376(3):366-373
beta-Elimination of the phosphate group on phosphoserine and phosphothreonine residues and addition of an alkyldithiol is a useful tool for analysis of the phosphorylation states of proteins and peptides. We have explored the influence of several conditions on the efficiency of this PO(4)(3-) elimination reaction upon addition of propanedithiol. In addition to the described influence of different bases, the solvent composition was also found to have a major effect on the yield of the reaction. In particular, an increase in the percentage of DMSO enhances the conversion rate, whereas a higher amount of protic polar solvents, such as water or isopropanol, induces the opposite effect. We have also developed a protocol for enrichment of the modified peptides, which is based on solid-phase covalent capture/release with a dithiopyridino-resin. The procedure for beta-elimination and isolation of phosphorylated peptides by solid-phase capture/release was developed with commercially available alpha-casein. Enriched peptide fragments were characterized by MALDI-TOF mass spectrometric analysis before and after alkylation with iodoacetamide, which allowed rapid confirmation of the purposely introduced thiol moiety. Sensitivity studies, carried out in order to determine the detection limit, demonstrated that samples could be detected even in the low picomolar range by mass spectrometry. The developed solid-phase enrichment procedure based on reversible covalent binding of the modified peptides is more effective and significantly simpler than methods based on the interaction between biotin and avidin, which require additional steps such as tagging the modified peptides and work-up of the samples prior to the affinity capture step. 相似文献
14.
Nitrido-Sodalites. II. Synthesis, Crystal Structure, and Properties of M(6+(y/2)–x)H2x[P12N24]Zy with M = Fe, Co, Ni, Mn; Z = Cl, Br, I; 0 ≤ x ≤ 4; y ≤ 2 The nitrido sodalites M(6+(y/2)–x)H2x[P12N24]Zy with M = Fe, Co, Ni, Mn; Z = Cl, Br, I; 0 ≤ x ≤ 4; y ≤ 2 are obtained by the reaction of HPN2 or [PN(NH2)2]3 with the metal halogenide MZ2 (T = 700°C). The compounds are isotypic to Zn(7–x)H2x[P12N24]Cl2. An increase of the ionic radii of the cations or anions results in an expansion of the lattice which is caused by an increase of the P? N? P angle. The influence of the cation is more dominant than that of the anion. By reacting [PN(NH2)2]3 with metal halogenide (MZ2) hydrogen free, X-ray amorphous products are obtained. The formation of the chloride-containing P? N-sodalite in this reaction begins at temperatures below 450°C. 相似文献
15.
Jan F. Keij Jan Th . M. Jansen Frank W. Schultz Jan W. M. Visser 《Photochemistry and photobiology》1994,60(5):503-509
Abstract-During the development of a photodamage cell sorter several photosensitizers were tested for their ability to photoinactivate more than 90% of the sensitized cells after a brief irradiation with a fluence of 10 kJ/m2 . In pilot experiments, yeast cells sensitized with 10-dodecyl acridine orange (DAO) were effectively photoinactivated after receiving a fluence of 10 kJ/m2 delivered in 8 s. However, when the same fluence was delivered in 3 μ s during passage through a focused laser beam in the cell sorter, all cells survived.
Computer simulations of the relevant photophysical and chemical reactions inside the irradiated cell were used to investigate the cause of this phenomenon. The results indicated that the absence of photoinactivation by DAO, after flash irradiations, was caused by the combined effects of (1) limited oxygen diffusion into the cell and (2) a reduced number of collisions between photosensitizer triplet and oxygen molecules during the irradiation due to saturation of the intracellular photosensitizer triplet concentration. The contributions of triplet-triplet annihilation and triplet quenching by ground state photosensitizer molecules were found to be minimal and not significant. These findings indicate that Type II photosensitizers are incapable of rapid selective photoinactivation in cell sorters. 相似文献
Computer simulations of the relevant photophysical and chemical reactions inside the irradiated cell were used to investigate the cause of this phenomenon. The results indicated that the absence of photoinactivation by DAO, after flash irradiations, was caused by the combined effects of (1) limited oxygen diffusion into the cell and (2) a reduced number of collisions between photosensitizer triplet and oxygen molecules during the irradiation due to saturation of the intracellular photosensitizer triplet concentration. The contributions of triplet-triplet annihilation and triplet quenching by ground state photosensitizer molecules were found to be minimal and not significant. These findings indicate that Type II photosensitizers are incapable of rapid selective photoinactivation in cell sorters. 相似文献
16.
Synthesis and Crystal-Structure of Na2Mn3O7 Single crystals of Na2Mn3O7 have been grown hydrothermally applying high oxygen pressure (p = 2 kbar). The new compound cystallizes triclinic; space group P1 ; a = 6.636(6) Å, b = 6.854(6) Å, c = 7.548(6) Å, α = 105.76(6)°, β = 106.86(6)°, γ = 111.60(6)°; Z = 2. The crystal structure has been solved and refined to R = 7.9% and Rw = 6.2% (diffractometer data, 1044 independent reflexions). The crystal structure consists of Mn3O72? anions with manganese coordinated octahedrally by oxygen. These layered anions are hold together by Na+ ions (coordination numbers 5 and 6). 相似文献
17.
Hydrolysis of Amides of Phospholinic Acid The acid catalysed hydrolysis of (= OAP, R = organic substituent) has been investigated kinetically. Provided the pH remains constant, the hydrolysis is a first order reaction. From the temperature dependance of the rate constant the activation energy Ea as well as the thermodynamic data ΔH* and ΔS* have been calculated. In comparison to the compound with R = H aromatic substituents and additional the t-butyl group enhance Ea whilst aliphatic substituents included the benzyl group diminish Ea. The first step of the reaction is a protonation at the oxygen or nitrogen resulting in the formation of an equilibrium. After the determination of the basicity constant KA of some OAPs it was possible to calculate the respective rate constants of the rate determining step of the reactions and the corresponding thermodynamic data EF, ΔH and ΔS. It is assumed that the mechanism of the hydrolysis takes place according to the same scheme which has been assumed for the hydrolysis of cyclic esters of the phosphinic acid by Ugi [1] and colaborators. 相似文献
18.
Synthesis, Crystal Structure, and Spectroscopic Characterization of P4O6Se3 P4O6Se3 has been prepared for the first time as a pure substance by photochemical selenation of P4O6 with red selenium. The compound crystallizes in a monoclinic system with space group P21/a (a = 973.6(2); b = 846.9(2); c = 1277.7(3) pm; β = 90.56(2)°; Z = 4; 1358 diffractometer data; R1 = 0.044; wR2 = 0.099). Bond lengths and angles within the molecule as well as the arrangement of the molecules within the crystal are discussed; vibrational (IR and Raman) and 31P-n.m.r. (solution) data are given. 相似文献
19.
An optical biosensor for the determination of hydrogen peroxide based on immobilized horseradish peroxidase is described. The fluorescence of the dimeric product of the enzyme catalysed oxidation of homovanillic acid is utilized to determine the concentration of H2O2. The membrane-bound enzyme is attached to a bifurcated fibre bundle permitting excitation and detection of the fluorescence by a fluorometer. The response of the sensor is linear from 1 to 130 M hydrogen peroxide; the coefficient of variation is 3%. The sensor is stable for more than 10 weeks. The operating pH for maximal sensor response is 8.15. This allows the sensor to be used in combination with oxidase reactions producing hydrogen peroxide, as is demonstrated with a co-immobilized lactate oxidase-horseradish peroxidase optode for the determination of L-lactate. The fluorescence intensity of this sensor depends linearly on the concentration of lactate between 3 and 200 M and a throughput of 10 samples per hour is possible. The precision is in the same range as that of the monoenzyme optode. The lifetime of the bienzyme sensor for lactate is considerably shorter than that of the peroxidase sensor; it is limited by the stability of the immobilized lactate oxidase enzyme. The sensor has been applied to the determination of lactate in control serum. 相似文献
20.
Na5AlF2(PO4)2: Synthesis, Crystal Structure and Ionic Conductivity Two different procedures (precipitation from aqueous solution and solid state reaction) for the synthesis of hitherto unknown Na5AlF2(PO4)2 were optimized. The crystal structure was determined using diffractometer data (P3 , a = b = 10.483(1), c = 6.607(1) Å, MoKα, 1080 independent reflections, Rw = 0.025). PO4-tetrahedra and AlO4F2-“octahedra” are connected via common vertices forming a twodimensionally extended heteropolyanion. Sodium is located in interconnected spacings of the [AlF2(PO4)2]-part of the structure. Ionic conductivity as expected because of these structural features was affirmed experimentally. 相似文献