全文获取类型
收费全文 | 4635篇 |
免费 | 160篇 |
国内免费 | 31篇 |
专业分类
化学 | 2910篇 |
晶体学 | 22篇 |
力学 | 109篇 |
数学 | 671篇 |
物理学 | 1114篇 |
出版年
2023年 | 36篇 |
2022年 | 35篇 |
2021年 | 76篇 |
2020年 | 94篇 |
2019年 | 112篇 |
2018年 | 68篇 |
2017年 | 79篇 |
2016年 | 144篇 |
2015年 | 85篇 |
2014年 | 116篇 |
2013年 | 237篇 |
2012年 | 263篇 |
2011年 | 310篇 |
2010年 | 167篇 |
2009年 | 111篇 |
2008年 | 225篇 |
2007年 | 277篇 |
2006年 | 244篇 |
2005年 | 232篇 |
2004年 | 178篇 |
2003年 | 152篇 |
2002年 | 96篇 |
2001年 | 75篇 |
2000年 | 69篇 |
1999年 | 42篇 |
1998年 | 34篇 |
1997年 | 61篇 |
1996年 | 48篇 |
1995年 | 57篇 |
1994年 | 51篇 |
1993年 | 56篇 |
1992年 | 60篇 |
1991年 | 41篇 |
1990年 | 42篇 |
1989年 | 33篇 |
1988年 | 36篇 |
1987年 | 28篇 |
1985年 | 51篇 |
1984年 | 40篇 |
1982年 | 35篇 |
1981年 | 50篇 |
1980年 | 34篇 |
1979年 | 44篇 |
1978年 | 42篇 |
1977年 | 40篇 |
1976年 | 44篇 |
1975年 | 34篇 |
1974年 | 31篇 |
1973年 | 43篇 |
1972年 | 31篇 |
排序方式: 共有4826条查询结果,搜索用时 18 毫秒
51.
Barry M. Trost Subham Mahapatra Martin Hansen 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(20):6130-6134
A new mode of activation of an imine via a rare aza‐substituted π‐allyl complex is described. Palladium‐catalyzed C(sp3) H activation of the N‐allyl imine and the subsequent nucleophilic attack by the α‐alkyl cyanoester produced the 1‐aza‐1,3‐diene as the sole regioisomer. In contrast, nucleophilic attack by the α‐aryl cyanoester exclusively delivered the 2‐aza‐1,3‐diene, which was employed in an inverse‐electron‐demand Diels–Alder reaction for heterobiaryl synthesis. 相似文献
52.
Detecting changes in arabidopsis cell wall composition using time‐of‐flight secondary ion mass spectrometry 下载免费PDF全文
Alex Yi‐Lin Tsai Robyn E. Goacher Emma R. Master 《Surface and interface analysis : SIA》2015,47(5):626-631
Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) was previously used to characterize lignocellulosic materials, including woody biomass. ToF‐SIMS can acquire both rapid spectral and spatial information about a sample's surface composition. In the present study, ToF‐SIMS was used to characterize the cell walls of stem tissue from the plant model organism, Arabidopsis thaliana. Using principal component analyses, ToF‐SIMS spectra from A. thaliana wild‐type (Col‐0), cellulose mutant (irx3), and lignin mutant (fah1) stem tissues were distinguished using ToF‐SIMS peaks annotated for wood‐derived lignocellulose, where spectra from the irx3 and fah1 were characterized by comparatively low polysaccharide and syringyl lignin content, respectively. Spatial analyses using ToF‐SIMS imaging furthermore differentiated interfascicular fiber and xylem vessels based on differences in the lignin content of corresponding cell walls. These new data support the applicability of ToF‐SIMS peak annotations based on woody biomass for herbaceous plants, including model plant systems like arabidopsis. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
53.
Frontispiece: Hydrogenated Graphenes by Birch Reduction: Influence of Electron and Proton Sources on Hydrogenation Efficiency,Magnetism, and Electrochemistry 下载免费PDF全文
54.
F. H. Gortemaker M. G. Hansen B. de Cindio H. M. Laun H. Janeschitz-Kriegl 《Rheologica Acta》1976,15(5):256-267
Summary Transient stresses including normal stresses, which are developed in a polymer melt by a suddenly imposed constant rate of shear, are investigated by mechanical measurement and, indirectly, with the aid of the flow birefringence technique. For the latter purpose use is made of the so-called stress-optical law, which is carefully checked.It appears that the essentially linear model of the rubberlike liquid, as proposed byLodge, is capable of describing the behaviour of polymer melts rather well, if the applied total shear does not exceed unity. In order to describe also steady state values of the stresses successfully, one should extend measurements to extremely low shear rates.These statements are verified with the aid of a method which was originally designed bySchwarzl andStruik for the practical calculation of interrelations between linear viscoelastic functions. In the present paper dynamic shear moduli are used as reference functions.
a T shift factor - B ij Finger deformation tensor - C stress-optical coefficient, (m2/N) - f (p jl ) undetermined scalar function - G shear modulus, (N/m2) - G(t) time dependent shear modulus, (N/m2) - G() shear storage modulus, (N/m2) - G() shear loss modulus, (N/m2) - G r reduced shear storage modulus, (N/m2) - G r reduced shear loss modulus, (N/m2) - H() shear relaxation time spectrum, (N/m2) - k Boltzmann constant, (Nm/°K) - n ik refractive index tensor - p undetermined hydrostatic pressure, (N/m2) - p ij ,p ik stress tensor, (N/m2) - p 21 shear stress, (N/m2) - p 11 –p 22 first normal stress difference, (N/m2) - p 22 –p 33 second normal stress difference, (N/m2) - q shear rate, (s–1) - t, t time, (s) - T absolute temperature, (°K) - T 0 reference temperature, (°K) - x the ratiot/ - x position vector of a material point after deformation, (m) - x position vector of a material point before deformation, (m) - 0, 1 constants in eq. [37] - 0, 1 constants in eq. [37] - shear deformation - (t, t) time dependent shear deformation - ij unity tensor - n flow birefringence in the 1–2 plane - (q) non-Newtonian shear viscosity, (N s/m2) - * () complex dynamic viscosity, (N s/m2) - | * ()| absolute value of complex dynamic viscosity, (N s/m2) - () real part of complex dynamic viscosity, (N s/m2) - () imaginary part of complex dynamic viscosity, (N s/m2) - (t — t) memory function, (N/m2 · s) - v number of effective chains per unit of volume, (m–3) - temperature dependent density, (kg/m3) - 0 density at reference temperatureT 0, (kg/m3) - relaxation time, (s) - integration variable, (s) - (x) approximate intensity function - 1 (x) error function - extinction angle - m orientation angle of the stress ellipsoid - circular frequency, (s–1) - 1 direction of flow - 2 direction of the velocity gradient - 3 indifferent direction - t time dependence The present investigation has been carried out under the auspices of the Netherlands Organization for the Advancement of Pure Research (Z. W. O.).North Atlantic Treaty Organization Science Post Doctoral Fellow.Research Fellow, Delft University of Technology.With 11 figures and 2 tables 相似文献
Zusammenfassung Mit der Zeit anwachsende Spannungen, darunter auch Normalspannungen, wie sie sich nach dem plötzlichen Anlegen einer konstanten Schergeschwindigkeit in einer Polymerschmelze entwickeln, werden mit Hilfe mechanischer Messungen und indirekt mit Hilfe der Strömungsdoppelbrechung untersucht. Für den letzteren Zweck wird das sogenannte spannungsoptische Gesetz herangezogen, dessen Gültigkeit sorgfältig überprüft wird.Es ergibt sich, daß das im Wesen lineare Modell der gummiartigen Flüssigkeit, wie es vonLodge vorgeschlagen wurde, sich recht gut zur Beschreibung des Verhaltens von Polymerschmelzen eignet, solange der im ganzen angelegte Schub den Wert Eins nicht überschreitet. Um auch stationäre Werte der Spannungen in die Beschreibung erfolgreich einzubeziehen, sollte man die Messungen bis zu extrem niedrigen Schergeschwindigkeiten ausdehnen.Die gemachten Feststellungen werden mit Hilfe einer Methode verifiziert, die vonSchwarzl undStruik ursprünglich für die praktische Berechnung von Beziehungen zwischen Zustandsfunktionen entwickelt wurde, die dem linear viskoelastischen Verhalten entsprechen. In der vorliegenden Veröffentlichung dienen die dynamischen Schubmoduln als Bezugsfunktionen.
a T shift factor - B ij Finger deformation tensor - C stress-optical coefficient, (m2/N) - f (p jl ) undetermined scalar function - G shear modulus, (N/m2) - G(t) time dependent shear modulus, (N/m2) - G() shear storage modulus, (N/m2) - G() shear loss modulus, (N/m2) - G r reduced shear storage modulus, (N/m2) - G r reduced shear loss modulus, (N/m2) - H() shear relaxation time spectrum, (N/m2) - k Boltzmann constant, (Nm/°K) - n ik refractive index tensor - p undetermined hydrostatic pressure, (N/m2) - p ij ,p ik stress tensor, (N/m2) - p 21 shear stress, (N/m2) - p 11 –p 22 first normal stress difference, (N/m2) - p 22 –p 33 second normal stress difference, (N/m2) - q shear rate, (s–1) - t, t time, (s) - T absolute temperature, (°K) - T 0 reference temperature, (°K) - x the ratiot/ - x position vector of a material point after deformation, (m) - x position vector of a material point before deformation, (m) - 0, 1 constants in eq. [37] - 0, 1 constants in eq. [37] - shear deformation - (t, t) time dependent shear deformation - ij unity tensor - n flow birefringence in the 1–2 plane - (q) non-Newtonian shear viscosity, (N s/m2) - * () complex dynamic viscosity, (N s/m2) - | * ()| absolute value of complex dynamic viscosity, (N s/m2) - () real part of complex dynamic viscosity, (N s/m2) - () imaginary part of complex dynamic viscosity, (N s/m2) - (t — t) memory function, (N/m2 · s) - v number of effective chains per unit of volume, (m–3) - temperature dependent density, (kg/m3) - 0 density at reference temperatureT 0, (kg/m3) - relaxation time, (s) - integration variable, (s) - (x) approximate intensity function - 1 (x) error function - extinction angle - m orientation angle of the stress ellipsoid - circular frequency, (s–1) - 1 direction of flow - 2 direction of the velocity gradient - 3 indifferent direction - t time dependence The present investigation has been carried out under the auspices of the Netherlands Organization for the Advancement of Pure Research (Z. W. O.).North Atlantic Treaty Organization Science Post Doctoral Fellow.Research Fellow, Delft University of Technology.With 11 figures and 2 tables 相似文献
55.
Hansen Alex Roux Stéphane Roux Aharony Amnon Feder Jens Jøssang Torstein Hardy H. H. 《Transport in Porous Media》1997,29(3):247-279
We present a spatial renormalization group algorithm to handle immiscibletwo-phase flow in heterogeneous porous media. We call this algorithmFRACTAM-R, where FRACTAM is an acronym for Fast Renormalization Algorithmfor Correlated Transport in Anisotropic Media, and the R stands for relativepermeability. Originally, FRACTAM was an approximate iterative process thatreplaces the L × L lattice of grid blocks, representing the reservoir,by a (L/2) × (L/2) one. In fact, FRACTAM replaces the original L× L lattice by a hierarchical (fractal) lattice, in such a way thatfinding the solution of the two-phase flow equations becomes trivial. Thistriviality translates in practice into computer efficiency. For N=L ×L grid blocks we find that the computer time necessary to calculatefractional flow F(t) and pressure P(t) as a function of time scales as N1.7 for FRACTAM-R. This should be contrasted with thecomputational time of a conventional grid simulator N2.3. The solution we find in this way is an accurateapproximation to the direct solution of the original problem. 相似文献
56.
57.
58.
59.
Alex Alberto Silva Huerta Flvio Augusto Sanzovo Fiorelli Otvio de Mattos Silvares 《Experimental Thermal and Fluid Science》2007,31(8):957-966
This work presents the results of an experimental study with pure refrigerants R-134a and R-600a and refrigerant–oil mixtures flowing through capillary tubes in order to analyse the metastable flow. A large number of experiments were carried out to verify the influence of several variables on the underpressure of vaporization, mainly the inlet subcooling, internal diameter and inlet pressure. Capillary tubes with internal diameter of 0.69 mm and 0.82 mm were tested for condensation temperatures between 40 °C and 50 °C and subcooling degrees between 3 °C and 12 °C. Measurements for oil concentrations of 1% and 3% were conducted and compared with those for pure refrigerant R-134a. The oil influence on the metastable flow was tested and the effect on the underpressure of vaporization is addressed for lower oil concentrations. 相似文献
60.
Effective permeability for porous rocks is calculated using mean field theory. We make two simplifying assumptions about the internal conductances in a network representation of the porous rock: (i) Pore space is characterized by a uniform fractal scaling; (ii) the internal conductances depend only on the characteristic pore sizes. Within these approximations, it is possible to derive a simple probability density for the internal conductances which is used for calculating effective permeability. Good agreement between calculations and experimental data of permeability vs. porosity is achieved. 相似文献